JP2001194694A - Optical waveguide, optical wavelength conversion element and method for manufacturing the same as well as short wavelength generator, optical information processor, coherent light generator and optical system using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make output stable by realizing the higher efficiency and light damage resistant strength of an optical wavelength conversion element of a waveguide type. SOLUTION: The optical waveguide has a nonlinear optical crystal, a first ion exchange region formed near the surface of this nonlinear optical crystal and a second ion exchange region formed near the surface of the first ion exchange region. The second ion exchange region has a refractive index higher than the refractive index of the first ion exchange region and the refractive index distribution of the second ion exchange region has a step-like graded shape.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide used in the field of optical information processing and applied optical measurement using a coherent light source, an optical wavelength conversion element using the same, and a structure using the same. The present invention relates to a short-wavelength light generation device and an optical information processing device to be used, and a manufacturing method thereof. The present invention further provides an optical wavelength conversion element and a coherent light generator used in the optical information processing field and the optical applied measurement field,
And an optical system using the coherent light generator.

[0002]

2. Description of the Related Art Optical waveguides have been applied in a wide range of fields such as communication, optical information processing, and measurement as light wave control techniques. Above all, studies on application of an optical waveguide to an optical wavelength conversion element have been actively conducted in order to realize a small-sized short-wavelength light source by wavelength conversion of a semiconductor laser.

First, a conventional optical wavelength conversion element 600
Will be described with reference to FIGS. 24 (a) and 24 (b). FIG. 24A is a perspective view showing a configuration example of a conventional optical wavelength conversion element 600, and FIG.
FIG. 6A is a diagram schematically illustrating a state in which a fundamental wave P1 incident on the wavelength conversion element 600 in FIG.

In the conventional optical wavelength conversion element 600 shown in FIG. 24A, a non-linear deterioration layer is provided near the surface of an optical waveguide as taught in, for example, Japanese Patent Application Laid-Open No. Hei 5-273624. High efficiency and stabilization of the operation of the optical wavelength conversion element are achieved. Specifically, in FIG. 24A, 601 is a LiTaO ₃ substrate, 602 is an optical waveguide,
603 is a domain-inverted region, and 605 is a nonlinear deterioration layer.

The fundamental wave P1 of the TM00 mode incident on the optical waveguide 602 of the optical wavelength conversion element 600 shown in FIG.
As shown in FIG. 24B, while propagating inside the optical waveguide 602 in which the domain-inverted regions 603 are periodically formed, the wavelength is converted to the harmonic P2 of the TM10 mode. Typically, the thickness of the nonlinear degradation layer 605 is about 0.45 μm, and the thickness of the optical waveguide 602 is 1.8 μm.
m.

The harmonic P2 in the TM10 mode is shown in FIG.
As shown in (b), the peak output has substantially the same magnitude on the + E side and the −E side. By increasing the overlap between the harmonic wave P2 of the TM10 mode having such an intensity distribution and the fundamental wave P1 of the TM00 mode, higher efficiency of the wavelength conversion operation is achieved.
Further, the harmonic P2 obtained by the wavelength conversion is converted to TM10
By setting the mode, the power density can be dispersed. As a result, the occurrence of optical damage can be suppressed even for a high harmonic output.

As another configuration of the conventional optical wavelength conversion element, a configuration in which a high refractive index layer having a higher refractive index than the optical waveguide is formed on the optical waveguide is disclosed in, for example, JP-A-4-25.
4834. FIG. 25A shows a conventional optical wavelength conversion element 64 having such a high refractive index layer.
FIG. 25B is a perspective view showing the configuration of FIG.
It is a figure which shows typically the mode of confining in the optical waveguide the fundamental wave P1 which injects into the wavelength conversion element of (a).

An optical wavelength conversion element 640 shown in FIG.
The fundamental wave P1 incident on one end face 645 of the proton exchange optical waveguide 642 formed on the LiNbO ₃ substrate 641 propagates in the optical waveguide 642 in which the domain-inverted regions 644 are periodically provided. The wavelength is converted during the operation, and the harmonic P is output from the other end face 646 of the optical waveguide 642.
2 is output.

Further, in the configuration of FIG.
42, a TiO ₂ high refractive index layer 643 is formed. The refractive index of TiO ₂ constituting the high refractive index layer 643 is larger than the refractive index of the proton exchange waveguide 642,
Thus, the high refractive index layer 643 having a high refractive index is
As shown in FIG. 25B, by confining the fundamental wave P1 in the optical waveguide 642 (more strictly, the domain-inverted region 644 therein) as shown in FIG. High efficiency of the wavelength conversion operation of the device has been achieved.

As another conventional optical waveguide structure of an optical wavelength conversion element, an optical wavelength conversion element employing a ridge-type optical waveguide structure for enhancing confinement of the optical waveguide is disclosed in, for example, Japanese Patent Laid-Open No. 1-238631. It is reported in the gazette.

Alternatively, a high-refractive-index cladding layer having a higher refractive index than the optical waveguide is provided on the surface of the optical waveguide, and the phase between the fundamental wave of the fundamental mode propagating in the optical waveguide and the higher-order mode harmonics is increased. A configuration of a wavelength conversion element that achieves high-efficiency wavelength conversion by increasing the overlap between guided lights by matching has been proposed.

In Japanese Patent Application Laid-Open No. 9-281536, a second proton exchange region is further formed on the surface of a proton exchange optical waveguide formed by a proton exchange treatment and an annealing treatment, so that a fundamental wave propagating through the optical waveguide is converted to a higher order. A method of converting the wavelength to a second harmonic having a guided mode has been reported.

The optical wavelength conversion element utilizing the nonlinear optical effect is intended to expand the range of generation of coherent light, such as second harmonic generation, parametric generation, sum frequency generation and difference frequency generation by wavelength conversion of light. It is applied in many fields because it becomes possible. In particular, an optical wavelength conversion element using an optical waveguide has advantages such that a high power density can be realized by a light confinement effect and a long interaction length can be obtained. Because these advantages enable highly efficient wavelength conversion, many optical devices have been proposed as optical wavelength conversion elements using optical waveguides.

In an optical wavelength conversion device using an optical waveguide, optical damage has been a serious problem. The optical damage described here is a change in the refractive index induced by light. In a part where light is strongly confined, such as an optical waveguide, the refractive index of the waveguide is changed by light due to the high power density of light. (Light damage) occurs. When this optical damage occurs, the phase matching state changes due to a change in the refractive index, and the output of the optical wavelength conversion element fluctuates. This is a problem that hinders an increase in the output of the optical waveguide type optical wavelength conversion element. In particular,
In a substrate such as LiNbO ₃ or LiTaO ₃ having a large nonlinear optical constant, occurrence of optical damage is remarkable, and optical damage occurs for an output of several mW to several tens mW. In order to reduce the optical damage, it is effective to reduce impurities in the crystal and reduce the loss of the optical waveguide.

Conventionally, as a method for solving the problem of optical damage, a method has been proposed in which a LiNbO ₃ substrate is doped with a metal element such as Zn, Mg, Sc or In to improve the light damage resistance of the substrate itself. ing. According to this method, by adding about 5 mol of metal to the substrate,
The light damage resistance of the crystal itself constituting the substrate is improved by one digit or more.

On the other hand, in an optical wavelength conversion element using an optical waveguide (optical waveguide type optical wavelength conversion element), in order to improve the conversion efficiency of wavelength conversion of a fundamental wave to a harmonic, the power of the propagating fundamental wave is required. It is essential to increase the density and increase the overlap between the converted harmonic and the electric field distribution of the fundamental wave.

As an optical wavelength conversion element using an optical waveguide, an optical wavelength conversion element 160 using a plurality of optical waveguides as shown in FIG. 26 has been proposed. In this optical wavelength conversion element 160, two optical waveguides 151 adjacent to the surface of the LiNbo ₃ substrate are formed by proton exchange, and a fundamental wave P _w 152 that propagates through the two optical waveguides 151 and a harmonic that emits Cherenkov radiation. The phase is matched with the wave P2 _w 153. Since the central portion between the two optical waveguides 151 is not proton-exchanged, it has a high non-linear optical effect, which makes it possible to form a highly non-linear optical waveguide. As a result, the conversion efficiency can be increased in the Cherenkov radiation type optical wavelength conversion element.

Furthermore, the configuration of an optical wavelength conversion element utilizing the coupling between a plurality of optical waveguides is disclosed in, for example,
No. 261924 and Japanese Patent Application Laid-Open No. 5-188420. In this configuration, a polarization-reversed structure, an optical waveguide through which a fundamental wave propagates, and an optical waveguide through which a harmonic propagates are formed on the substrate surface, and light propagation that is different between the fundamental wave and the harmonics is performed, so that light resistance is improved. It has improved damage strength and conversion efficiency.

[0019]

Problems to be solved by the conventional optical wavelength conversion element as described above will be described below.

FIG. 24 shows a conventional optical wavelength conversion element 600.
In the configuration shown in (a), as described above, the nonlinear degradation layer 605 is provided near the surface of the optical waveguide 602, and
Fundamental wave P1 of M00 mode and harmonic wave P of TM10 mode
2 to achieve a high efficiency of the wavelength conversion operation and an improvement of the light damage resistance. However, in this conventional configuration, as shown in FIG.
Since the emitted harmonic light P2 of the TM10 mode has an intensity distribution having two peaks on the + E side and the -E side,
There is a problem that the light-collecting characteristic deteriorates when the light is collected.

In this conventional method, the output harmonic P2 has two peaks of substantially equal magnitude because:
This is because the harmonic P2 is dispersed in the peak of the higher-order mode in order to increase the light damage resistance. However, since a special optical system is required to collect the harmonic output having such an intensity distribution, the optical system is complicated, and it is difficult to reduce the size of the optical system. Further, in order to converge to the diffraction limit, it is necessary to considerably shape the beam, and there is also a problem that the utilization efficiency of output light is reduced to 50% or less.

Further, the nonlinear deterioration layer 605 includes the fundamental wave P1
Since there is no function to increase the confinement of the fundamental wave P1, the power density of the fundamental wave P1 cannot be increased, and there is a limit to high efficiency.

The conventional optical wavelength conversion element 640 having a configuration in which the high refractive index layer 643 having a higher refractive index than the optical waveguide 642 is formed to enhance the confinement of the optical waveguide 642 and improve the efficiency. There is a problem in that a high-refractive-index dielectric film is used as the high-refractive-index layer 643 formed on the surface of the optical waveguide 642. That is, the high refractive index layer 643 on the optical waveguide 642 has a large effect on the effective refractive index of the optical waveguide 642, and high accuracy is required for the uniformity of the film thickness over the entire area of the optical waveguide 642. For example, in the case of an optical wavelength conversion element, since the phase matching condition over the entire length of the optical waveguide is strictly dependent on the effective refractive index of the optical waveguide, if the effective refractive index of the optical waveguide changes even slightly in the propagation direction, the wavelength conversion efficiency becomes extremely high. Decrease. Therefore, strict uniformity is required for controlling the thickness of the high refractive index layer 643.

In the above configuration, since a material different from that of the substrate 641 is deposited on the surface of the optical waveguide 642, a waveguide loss easily occurs at the interface between the optical waveguide 642 and the high refractive index film 643. Further, when the optical waveguide 642 is distorted due to a difference in expansion coefficient between the high refractive index film 643 and the substrate 641, the effective refractive index of the optical waveguide 642 has a distribution in the propagation direction.

Further, it is becoming clear that the propagation loss of the high refractive index layer 643 to the light propagating through the optical waveguide 642 becomes a serious problem.

In general, the propagation loss of an optical waveguide that deteriorates the characteristics of an optical wavelength conversion element includes a loss for a harmonic and a loss for a fundamental wave. The dielectric high refractive index layer 643 is
Although the propagation loss given to the fundamental wave is relatively small and there is no problem, the conventional high refractive index film 6 is used for harmonics having a short wavelength.
It has become clear that 43 gives a rather large propagation loss. For example, specific experiments have revealed that various dielectric films having a high refractive index have a large propagation loss of −several dB / cm with respect to harmonics in a wavelength band of 400 nm. Thereby, it was found that the wavelength conversion efficiency of the optical wavelength conversion element was reduced to 以下 or less.

Further, in the conventional configuration having the nonlinear deterioration layer, the nonlinear deterioration layer does not have a function of enhancing the confinement of the fundamental wave, so that the power density of the fundamental wave cannot be increased, and the efficiency is limited. .

The configuration of the conventional optical wavelength conversion element described above aims at improving the overlap between the fundamental wave of the fundamental mode propagating in the optical waveguide and the second harmonic.
However, the distribution of the guided modes in the optical waveguide is significantly different from the difference in the refractive index dispersion between the fundamental wave and the second harmonic.
There is a limit to the increase in the overlap between the two modes, and it is difficult to greatly improve the wavelength conversion efficiency. Furthermore, since there is a large portion where the fundamental wave and the second harmonic do not overlap, it is difficult to achieve an improvement in the light damage resistance.

In a conventional optical wavelength conversion element which uses a ridge type optical waveguide to improve the wavelength conversion efficiency, the power density is increased due to the confinement effect of the optical waveguide.
The wavelength conversion efficiency has been improved. However, the increase in the confinement effect of the ridge waveguide with respect to the fundamental wave is limited to the lateral direction (the width direction of the ridge), and confinement in the depth direction is not improved. Therefore, an increase in the overlap between the fundamental wave and the second harmonic having the greatest influence on the wavelength conversion efficiency (particularly, an increase in the depth direction) cannot be achieved by the ridge structure, and the wavelength conversion efficiency is greatly improved. Have difficulty.

Further, in the loaded optical waveguide, as in the ridge type, the overlap between the fundamental wave and the second harmonic cannot be sufficiently increased.

In any of the conventional methods, blue light of about 10 mW can be generated from the viewpoint of light damage resistance. However, when the output is 10 mW or more, it is difficult to obtain a stable output for a long time. .

Further, as a configuration of a conventional optical wavelength conversion element, there is a structure that realizes highly efficient wavelength conversion characteristics by forming a high refractive index cladding on the surface of an optical waveguide. This is a configuration in which a cladding layer having a higher refractive index than that of the optical waveguide is provided on the surface of the optical waveguide. As the cladding layer, a dielectric film having a high refractive index or a layer formed by ion exchange is used. . Although high wavelength conversion efficiency and excellent optical damage characteristics can be obtained with this configuration, observation of long-term output stability also reveals that this configuration has a new problem of unstable output. Was.

As described above, in the conventional light wavelength conversion element, a method of adding a metal element has been proposed as a method for improving the light damage resistance of a substrate. However, in this method, when the optical waveguide is formed by proton exchange or the like, there is a problem that the optical damage resistance of the waveguide cannot be sufficiently improved.

The conventional optical wavelength conversion element 16 shown in FIG.
In the case of No. 0, since the generated harmonics are in the radiation mode, the light-gathering characteristics are poor, and it has been difficult to apply them to an optical system or the like that requires light-gathering to the diffraction limit. In addition, since the phase matching condition between the fundamental wave propagating in the guided mode and the harmonic propagating in the radiation mode is satisfied in a wide range, it is difficult to control the propagation mode of the harmonic, and high efficiency is obtained. There is a problem that it is difficult to select a phase matching relationship that allows wavelength conversion.

Further, in a configuration in which a fundamental wave and a higher harmonic wave propagate in different optical waveguides as disclosed in JP-A-3-261924 and JP-A-5-188420, the fundamental wave propagating through each optical waveguide is And the harmonics, the overlap of the electric field distribution is small. For this reason, there has been a problem that it is difficult to increase the conversion efficiency.

The present invention has been made in consideration of the above-mentioned problems, and has the following objects. (1) A second light-emitting device having excellent light damage resistance and long-term output stability. An optical waveguide capable of outputting a higher harmonic wave and realizing an optical wavelength conversion element in which an overlap between a fundamental wave and a second harmonic wave is increased. (2) Light configured using the optical waveguide. A wavelength conversion element, (3) a short wavelength light generation device and an optical information processing device configured by using the light wavelength conversion device,
And (4) a method for producing the same.

Further, the present invention has been made in order to solve the above-mentioned problems of the prior art, and provides an optical wavelength conversion element, a coherent light generating device, and an optical system having excellent light damage resistance. With the goal.
In addition, the present invention also provides a
An optical waveguide-type optical wavelength conversion element that can convert wavelengths to sum frequency, difference frequency, etc. with high efficiency, improve beam characteristics, and generate converted wavelength light with excellent light collection characteristics And a coherent light generator and an optical system.

[0038]

An optical waveguide according to the present invention comprises a nonlinear optical crystal, a first ion exchange region formed near the surface of the nonlinear optical crystal, and a surface of the first ion exchange region. A second ion exchange region formed in the vicinity of
In the second ion exchange region, the region in which the ion exchange rate changes in the depth direction is 0.02 to 0.2 μm, thereby achieving the above object.

In the second ion exchange region, the region where the amount of change in the ion exchange rate is 5 to 50 μm ⁻¹ is 0.02 to 0.2 μm.
m.

[0040] In the first ion exchange region, the amount of change in the ion exchange rate may be 0.06 µm ^-1 or less.

In the second ion exchange region, the region where the ion exchange rate changes in the depth direction is 0.03-0.1 μm.
m.

The second ion exchange region may have an ion exchange rate change of 10 to 30 μm ⁻¹ .

The nonlinear optical constant of the first ion exchange region is 90% or more of the nonlinear optical constant of the crystal, and the nonlinear optical constant of the second ion exchange region is 60% or less of the nonlinear optical constant of the crystal. It may be.

The surface refractive index change Δn of the first ion exchange region is 0.02 or less for light having a wavelength of 633 nm.
The change in the surface refractive index Δn of the second ion exchange region is 0.
It may be 11 or more.

The optical waveguide of the present invention comprises a nonlinear optical crystal,
A first ion exchange region formed near a surface of the nonlinear optical crystal; and a second ion exchange region formed near a surface of the first ion exchange region; The ion exchange region has a higher refractive index than the first ion exchange region, and the refractive index distribution of the second ion exchange region has a step-like graded shape. Achieved.

The optical waveguide of the present invention comprises a nonlinear optical crystal,
A first ion exchange region formed near a surface of the nonlinear optical crystal; and a second ion exchange region formed near a surface of the first ion exchange region; The ion exchange region has a higher ion concentration than the first ion exchange region, and the ion concentration distribution in the second ion exchange region has a step-like graded shape. Achieved.

The first ion exchange region and the second ion exchange region may have different graded shapes obtained by annealing at different temperatures.

[0048] The depth of the first ion exchange region may be increased by an annealing process to eight times or more the value before the annealing process is performed.

[0049] The depth of the second ion exchange region may be increased to 1.2 times or more the value before the execution of the annealing process by the annealing process.

[0050] The depth of the second ion exchange region may be increased by annealing to more than twice the value before the annealing.

The surface refractive index of the second ion exchange region may be substantially equal to the value of the surface refractive index before the annealing.

The surface ion concentration in the second ion exchange region may be substantially equal to the value of the surface ion concentration before performing the annealing.

The step-like graded shape of the refractive index distribution of the second ion-exchange region has the following relationship (1) and (2):

[0054]

(Equation 7)

(However, C (k, t): ion exchange concentration,
k: depth (μm), t: annealing time (hour), C0:
Initial ion exchange concentration, Erf []: error function, h:
Initial ion exchange depth (μm), and Dp: diffusion constant of ion by annealing (μm ² / hour))

[0056]

(Equation 8)

It may be formed by an annealing process that satisfies the conditions.

The step-like graded shape of the ion concentration distribution in the second ion exchange region has the following relationship (1) and (2):

[0059]

(Equation 9)

(However, C (k, t): ion exchange concentration,
k: depth (μm), t: annealing time (hour), C0:
Initial ion exchange concentration, Erf []: error function, h:
Initial ion exchange depth (μm), and Dp: diffusion constant of ion by annealing (μm ² / hour))

[0061]

(Equation 10)

It may be formed by an annealing process that satisfies the conditions.

The nonlinear optical crystal is LiNb _x Ta _1-x O
_It may be _three crystals (0 ≦ x ≦ 1).

The ion exchange processing for forming the first ion exchange region and the second ion exchange region may be a proton exchange treatment, respectively.

The surface of the nonlinear optical crystal made of LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) is formed by ion exchange, and the molar concentration ratio of Li in the optical waveguide is 40 mol.
% Or more.

The non-linear optical crystal is made of LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) and has a metal element added at least in the vicinity of the surface. The molar concentration ratio Y of Li and the molar concentration Z of the metal element may be Y + Z ≧ 45 mol%.

An optical wavelength conversion element according to the present invention includes the optical waveguide according to any one of the above, and a periodic polarization inversion structure, wherein the optical waveguide comprises a fundamental wave having a wavelength λ and a wavelength λ / 2. The second harmonic can be guided, and the refractive index and the depth of the second ion exchange region included in the optical waveguide satisfy the waveguide condition for the second harmonic. In addition, the above problem is achieved by satisfying the cutoff condition for the fundamental wave.

In the optical waveguide, a fundamental wave of a fundamental mode and a second harmonic of a higher-order mode may be phase-matched.

The optical wavelength conversion element of the present invention has an optical waveguide group composed of a plurality of optical waveguides adjacent to the surface of a nonlinear optical crystal, and converts a fundamental wave incident in the optical waveguide group to a different wavelength. An optical wavelength conversion element for converting light into light, wherein the fundamental wave propagates in the optical waveguide group in a single mode, and the converted wavelength light propagates in an optical waveguide constituting the optical waveguide group in a waveguide mode. Alternatively, the above problem is achieved by the single-mode propagation of the converted wavelength light through the optical waveguide group and the fundamental wave propagating through the optical waveguides constituting the optical waveguide group.

The group of optical waveguides may be composed of a plurality of optical waveguides having different propagation directions.

The optical waveguide may satisfy a cutoff condition for one of the fundamental wave and the converted wavelength light, and may satisfy a waveguide condition for the other light.

At least one of the optical waveguides is
It may have a different propagation constant from other optical waveguides.

The optical waveguide group satisfies a single-mode propagation condition with respect to one of the fundamental wave and the converted wavelength light, and the one of the optical waveguide and the one of the optical waveguides. The phase may be matched with the other light guided through one line.

The optical waveguide group may be composed of an odd number of optical waveguides, and may have a structure symmetric with respect to the center optical waveguide.

The optical waveguide groups have substantially the same propagation direction.
The optical waveguide in the center has a different propagation constant from the optical waveguides on both sides, and one of the fundamental wave and the converted wavelength light propagating through the optical waveguide group in a single mode. Phase matching may be performed between the light and the other light propagating through the central optical waveguide of the optical waveguide group.

In the optical waveguide group, the number of the optical waveguides may be different in at least one of the vicinity of the entrance and the exit.

The light wavelength conversion element of the present invention is composed of LiNb _x T
an optical waveguide formed by ion exchange on the surface of a crystal made of a _1-x O ₃ (0 ≦ x ≦ 1), and Li in the optical waveguide
Is 40 mol% or more,
The above object is achieved.

The light wavelength conversion element of the present invention is a LiNb _x T
a _1-x O ₃ (0 ≦ x ≦ 1), having an optical waveguide formed by ion exchange on the surface of a crystal to which a metal element is added at least in the vicinity of the surface; m
ol concentration ratio Y and mol concentration Z of the metal element are Y + Z ≧ 4
When the content is 5 mol%, the above object is achieved.

The metal elements are Mg, Zn, Sc and I
Any one of n may be used, or a mixture of two or more of n may be used.

A short-wavelength light generating device according to the present invention includes a semiconductor laser and any one of the above-described light wavelength conversion elements, and the wavelength of light emitted from the semiconductor laser is controlled by the light wavelength conversion element. By being converted to a predetermined harmonic,
The above object is achieved.

The waveguide mode of the harmonic obtained by the conversion by the optical wavelength conversion element is a higher-order mode, and a main peak having the maximum intensity is included among a plurality of peaks in the intensity distribution of the higher-order mode of the harmonic. The width of one of the other sub-peaks may be smaller than the diffraction limit for the harmonic.

The higher-order mode of the harmonic may be a first-order mode.

The higher-order mode of the harmonic may be a TE10 mode.

The optical information processing apparatus according to the present invention includes the short-wavelength light generator according to any one of the above, and a light-converging optical system, and the short-wavelength light emitted from the short-wavelength light generator is provided. The above object is achieved by being configured to condense light by the light condensing optical system.

The method for manufacturing an optical wavelength conversion element according to the present invention comprises the steps of: forming a periodic domain-inverted structure in the nonlinear optical crystal; forming a first ion-exchange region in the domain-inverted structure; Performing a first annealing process on the first ion exchange region, forming a second ion exchange region on the surface of the first ion exchange region, and performing a second annealing on the second ion exchange region Performing a process;
The above object is achieved when the first annealing temperature at which the first annealing process is performed is different from the second annealing temperature at which the second annealing process is performed.

[0086] The first annealing temperature may be 300 ° C or higher, and the second annealing temperature may be 250 ° C or lower.

The step of forming the domain-inverted structure may include a step of heat-treating the formed domain-inverted structure at a temperature of 400 ° C. or higher.

The second annealing temperature is 130 ° C. to 20 ° C.
It may be in the range of 0 ° C.

The second annealing is performed by the following (1)
And the relationship of (2):

[0090]

[Equation 11]

(However, C (k, t): ion exchange concentration,
k: depth (μm), t: annealing time (hour), C0:
Initial ion exchange concentration, Erf []: error function, h:
Initial ion exchange depth (μm), and Dp: diffusion constant of ion by annealing (μm ² / hour))

[0092]

(Equation 12)

[0093] This is an annealing process that satisfies the above conditions, and the refractive index distribution of the second ion exchange region formed by the annealing process may have a step-like graded shape.

The nonlinear optical crystal is LiNb _x Ta _1-x O
_It may be _three crystals (0 ≦ x ≦ 1).

The ion exchange processing for forming the first ion exchange area and the second ion exchange area may each be a proton exchange treatment.

A coherent light generating apparatus according to the present invention includes any one of the above-described light wavelength conversion elements and a semiconductor laser, and converts the wavelength of the light of the semiconductor laser by the light wavelength conversion element. The task is achieved.

An optical system according to the present invention includes the above-described coherent light generator and a condensing optical system, and converges light emitted from the light wavelength conversion element by the condensing optical system. The above object is achieved.

The operation of the present invention will be described below.

The present invention relates to an optical wavelength conversion element for converting a fundamental wave propagating through an optical waveguide using a nonlinear optical effect into a harmonic, parametric, sum frequency, difference frequency or the like.
In order to improve the light damage resistance, increase the wavelength conversion efficiency, and improve the beam characteristics of the emitted light, the optical waveguide constituting the optical wavelength conversion element has a special structure different from the conventional one. is there.

As will be described later in a seventh embodiment and an eighth embodiment, an optical waveguide group composed of a plurality of adjacent optical waveguides is formed on the surface of a nonlinear optical crystal, and this optical waveguide group is used. From fundamental to harmonic, parametric,
A wavelength conversion to a sum frequency or a difference frequency is realized. In this configuration, by controlling the phase matching state, it becomes possible for the optical waveguide group to give different propagation states to the fundamental wave and the converted wavelength light. For example, as shown in the seventh embodiment, a fundamental wave can be propagated in a group of optical waveguides in a single mode, and a harmonic and a sum frequency can be propagated in a waveguide mode in a waveguide mode. Alternatively, as shown in the eighth embodiment, the fundamental wave can be propagated in the optical waveguide in single mode, and the parametric or difference frequency can be propagated in the optical waveguide group in single mode. With this optical waveguide structure, it is possible to reduce the area of the surface of the optical waveguide by reducing the area for proton exchange, metal diffusion, and the like,
Absorption and scattering in the optical waveguide are greatly reduced. Therefore, in the present invention, of the fundamental wave and the converted wavelength light,
Although the wavelength is short, optical damage caused by absorption can be significantly reduced. In addition, by controlling one propagation mode of the fundamental wave and the converted wavelength light by the waveguide and controlling the other by the waveguide group, the electric field distribution of the fundamental wave and the converted wavelength light in the optical waveguide is controlled. It is possible to increase the overlap and increase the efficiency of wavelength conversion. further,
By appropriately selecting the propagation state of the converted wavelength light, it is possible to control the aspect ratio of the waveguide mode and realize emission light having excellent light-collecting characteristics.

When the group of optical waveguides is composed of a plurality of optical waveguides having different propagation directions, the propagation constant of the group of waveguides can have a distribution, and the tolerance of the wavelength conversion element can be increased. . In addition, a tapered waveguide (a waveguide whose shape becomes narrower or wider in a tapered shape with respect to the propagation direction) can be manufactured.

When the wavelength of the converted light is shorter than the fundamental wave (for example, a harmonic or a sum frequency), if the width of the optical waveguide is set so as to satisfy the cutoff condition with respect to the fundamental wave, a plurality of wavelengths can be obtained. Can be propagated in a single mode as a single waveguide.
The cut-off condition is set by the waveguide width and the thickness. Here, the cut-off condition is set by the waveguide width. At this time, for the wavelength light converted from the fundamental wave, the optical waveguide can satisfy the waveguide condition. Therefore, it is possible to easily control the propagation mode of the converted wavelength light as compared with the conventional optical wavelength conversion element in which the harmonics are guided in the radiation mode, and to easily control the phase matching between the fundamental wave and the converted wavelength light. It is. When the wavelength of light converted with respect to the fundamental wave is long (for example, parametric or difference frequency), the fundamental wave propagates through any one of the optical waveguides, and the converted wavelength light propagates in the optical waveguide group in a single mode. . In this case, the propagation mode of the fundamental wave can be easily controlled, and the phase matching between the fundamental wave and the converted wavelength light can be easily controlled.

When the wavelength of the converted light is shorter than the fundamental wave (for example, a harmonic or a sum frequency), the propagation constant of at least one of the optical waveguides is set to be different from that of the other optical waveguides. By setting to a constant, the wavelength light converted from the fundamental wave selectively guides the optical waveguide,
It is possible to improve conversion efficiency and stabilize output light. Here, the propagation constant is also set by the waveguide width. When the wavelength of the light converted from the fundamental wave is long (for example, parametric or difference frequency), the fundamental wave is selectively guided in the optical waveguide to improve the conversion efficiency and stabilize the output light. Becomes possible. On the other hand, in a normal waveguide, when the wavelength of light converted with respect to the fundamental wave is long (for example, parametric or difference frequency), if the waveguide is designed according to the waveguide condition of the converted light, the fundamental wave , It becomes a multimode waveguide, and the conversion efficiency is greatly reduced.

If the group of optical waveguides is asymmetrical, the beam shape of the emitted light becomes asymmetrical and the light-collecting characteristics deteriorate, so that it is preferable to have a structure symmetrical about the center optical waveguide. When the number of optical waveguides is odd,
The center of the power density is concentrated on the central optical waveguide for both the fundamental wave and the converted wavelength light, and the overlap between the fundamental wave and the converted wavelength light becomes high, so that high conversion efficiency can be obtained, which is preferable. For example, if the wavelength of the converted light is shorter than the fundamental wave (for example, a harmonic or a sum frequency),
The group of optical waveguides is composed of three optical waveguides whose propagation directions are substantially equal, the central optical waveguide has a different propagation constant from the optical waveguides on both sides, and a fundamental wave that propagates the optical waveguide group in a single mode and an optical waveguide. A configuration may be adopted in which phase matching is performed between the converted wavelength light propagating through the optical waveguide at the center of the group of waveguides. When the wavelength of the light converted with respect to the fundamental wave is long (for example, parametric or difference frequency), the converted wavelength light that propagates in the optical waveguide group in a single mode and the light that propagates in the central optical waveguide of the optical waveguide group. A configuration in which the phase is matched with the fundamental wave can be adopted. When the number of optical waveguides is large, it is conceivable that the size of the group of waveguides increases, the power density decreases, and the conversion efficiency decreases. Therefore, it is preferable to form the optical waveguide group with the three optical waveguides having the least number of the symmetrical structures.

Furthermore, by changing the number of optical waveguides constituting the optical waveguide group in at least one of the vicinity of the entrance and the exit of the optical waveguide, shaping of the beam shape and improvement of the coupling efficiency are achieved. Is possible.

By the way, conventionally, LiNbO_ThreeBoards and
LiTaO_ThreeIf a proton exchange layer is formed on the substrate,
It is reported that the light damage resistance is improved in the Roton exchange layer.
I was However, according to the study of the present inventors,
Proton exchanged LiNbO_ThreeThe light damage resistance is sufficient
No, for example, LiNbO_ThreeAbout an order of magnitude higher for substrates
High light damage resistance, but Mg-doped LiNbO_Three
It is clear that the light damage resistance is reduced by a factor of
It became clear. This is caused by the doping material.
Light damage resistance is reduced by proton exchange
It is thought that there is. Therefore, in the second invention,
Is the light damage resistance and the Li concentration in the proton exchange layer
Focusing on the relationship, as shown in a ninth embodiment described later
And LiNb _xTa_1-xO_Three(0 ≦ x ≦ 1) on the surface of the substrate
In an optical waveguide formed by proton exchange, Li
The molar concentration ratio is set to 40 mol% or more. Or
Metal such as Mg, Zn, Sc, In, etc. at least near the surface
LiNb with element added_xTa_{1 -x}O_Three(0 ≦ x ≦ 1) groups
Optical waveguide formed by proton exchange on plate surface
And the molar concentration ratio Y of Li and the molar concentration Z of the metal element
It is set so as to satisfy the relationship of Y + Z ≧ 45 mol%.
This reduces crystal defects in the proton exchange layer and reduces light
It is possible to improve the light damage resistance of the waveguide.

The coherent light generating device of the present invention can realize high output and stable output characteristics by using the optical wavelength conversion element of the present invention having excellent light damage resistance.

Further, the optical system of the present invention can reduce noise by using the coherent light generator of the present invention which has a small output fluctuation and a high output.

[0109]

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical wavelength conversion element utilizing the second harmonic generation by a nonlinear optical effect, in a fundamental mode fundamental wave and a higher mode second harmonic (SHG).
The present invention proposes a new configuration in which the overlap is increased by utilizing the phase matching with the above to increase the wavelength conversion efficiency to SHG, and the light damage resistance is dramatically improved. Specifically, in a configuration in which a first ion exchange region formed by ion exchange is enlarged by annealing and a second ion exchange region is formed near the surface of the first ion exchange region, The inventors of the present application have found that the addition of the annealing treatment significantly reduces the propagation loss of the optical waveguide and further improves the light damage resistance. The principle will be described below.

[0110] As the ion exchange treatment performed in the present invention with the above-mentioned meaning, for example, a proton exchange treatment can be carried out. In that case, the formed first and second ion exchange regions are the first and second proton exchange regions, respectively. In the description of the present invention in the present specification, a proton exchange process is described as a specific ion exchange process, and a case where a proton exchange region is formed by the proton exchange process is particularly described. However, the present invention is not limited to this, and even if an ion exchange region other than the proton exchange region is formed by other similar ion exchange treatments, the same effect as that described in the present specification is obtained. be able to.

First, an example in which the operation characteristics of an optical wavelength conversion element have been improved by using an optical waveguide utilizing proton exchange, which has been conventionally reported, and a problem newly found in the conventional structure will be described.

A conventional optical wavelength conversion element 300 will be described with reference to FIG.

In FIG. 1, an X-plate LiNbO ₃ substrate 3
An optical waveguide 305 in the form of a stripe is formed near the surface of the optical waveguide 01, and a high refractive index layer 310 is formed near the surface of the optical waveguide 305. LiNbO ₃ substrate 301
Has a periodically poled structure 304 for phase matching. Fundamental wave 30 incident on optical waveguide 305
6 is converted into a second harmonic 307 and emitted from the optical waveguide 305.

FIG. 2A shows the structure of a conventional optical waveguide device 300 in which an optical waveguide 2 is formed on a substrate 1 and a high refractive index layer 4 is formed near the surface of the optical waveguide 2. FIGS. 2B and 2C are diagrams schematically showing refractive index distributions in the depth direction in the configuration of FIG. 2A, and fundamental waves and harmonics propagating in the configuration of FIG. Is an electric field distribution in the depth direction. Specifically, a fundamental wave having a wavelength of 850 nm is incident in the TE00 mode, and quasi-phase-matches with the TE10 mode harmonic in the optical waveguide 2.

The optical waveguide 2 is an optical waveguide formed by performing an annealing process on a proton exchange region. By this annealing process, the shape of the proton exchange region changes from a step shape to a graded shape, and the proton concentration also decreases. I do. On the other hand, the high-refractive-index layer 4 is a high-refractive-index layer formed by proton exchange, but has not been subjected to annealing treatment, so that the distribution of protons is step-shaped.

In the optical wavelength conversion element 300, the confinement of the fundamental wave is strengthened by the high refractive index layer 4, and the overlap between the fundamental wave of the fundamental mode and the harmonic of the higher order mode can be increased. And high wavelength conversion efficiency can be achieved.

However, the conventional optical wavelength conversion device 300
As a result of various investigations on the output characteristics of the above, the following problems were clarified.

First, a phenomenon in which the phase matching wavelength changes with time was found. As a result, there arises a problem of fluctuation of the phase matching wavelength that the output of the second harmonic becomes unstable.

Second, there is a problem with the life of the device that the characteristics of the optical wavelength conversion device are deteriorated in the high temperature test.

Third, the propagation loss of the optical waveguide is large.

Each problem will be described in detail below.

Regarding the first problem, it has been observed that the phase matching state is slightly changed by increasing the output of the generated second harmonic. Although the cause will be described later, optical damage occurring at the boundary between the high refractive index layer 4 and the optical waveguide 2 is related.

Conventionally known optical damage is a phenomenon in which the output of the second harmonic is reduced due to the collapse of the phase matching state over the entire optical waveguide.
This is a phenomenon in which the phase matching wavelength shifts while maintaining the phase matching curve. Since the wavelength conversion efficiency does not decrease, but the phase matching wavelength changes, if the wavelength of the fundamental wave is fixed, the output of the second harmonic gradually decreases. Therefore, unless the phase matching wavelength is always adjusted to the optimum value, a stable output cannot be obtained. When the phase matching wavelength changes, the phase matching wavelength tolerance of the optical wavelength conversion element is very narrow at 0.1 nm or less.
The output of the second harmonic becomes unstable.

The second problem was discovered during the life test of the optical wavelength conversion device. Specifically, when a high-temperature test was performed at a temperature of 80 ° C. for several tens of hours, phenomena such as a large change in the phase matching wavelength of the optical wavelength conversion element and a deterioration in wavelength conversion efficiency were observed. After examining the cause,
It became clear that the high refractive index layer 4 changed in the high temperature test. The high refractive index layer 4 is a proton layer that is not subjected to an annealing process, has a high proton concentration, and has a step-shaped proton concentration distribution. Therefore, since the proton concentration is high and the concentration difference from the peripheral portion is large, the thermal diffusion constant is large. Further, since the thermal diffusion of the proton exchange region of the high refractive index layer 4 greatly affects the characteristics of the optical waveguide 2, the change in the proton exchange region of the high refractive index layer 4 during the high temperature test causes It has been clarified that the characteristics of the above deteriorate.

[0125] The third problem is mainly a problem of the first proton exchange region. In the optical waveguide structure in which the high refractive index layer 4 is formed on the surface, since the confinement of light is strengthened, the propagation loss of the optical waveguide 2 tends to increase. Further, the propagation loss of the optical waveguide 2 affects the characteristics of the first proton exchange region where the presence of the electric field distribution is greatest. As a result of the study by the present inventors, it has been clarified that there is a close relationship between the proton concentration in the first proton exchange region and the propagation loss of the optical waveguide.

Hereinafter, some specific embodiments of the present invention achieved in consideration of the above-described study results will be described with reference to the accompanying drawings.

(First Embodiment) In this embodiment, a new optical waveguide structure is proposed in order to solve the problem in the conventional optical waveguide using proton exchange, in consideration of the above-described results of the study. I do.

As described above, the inventors of the present application have examined the problems in the optical wavelength conversion device using the optical waveguide having the high refractive index cladding layer, and as a result, have found that the high refractive index layer formed on the optical waveguide has a high refractive index. A problem has been found at the boundary between the waveguide and the optical waveguide. In particular, it has been found that the problem is that the refractive index changes in a step shape at this boundary.

Therefore, as a result of various studies on the shape of the proton exchange region forming the high refractive index layer, it was found that the first and second proton exchange regions were formed by the proton exchange treatment and the annealing treatment. It has been found that the characteristics of the optical waveguide are significantly improved in the special refractive index distribution shape of the proton exchange region.

FIG. 3A is a cross-sectional view showing the structure of a proton exchange waveguide (optical waveguide) according to the present embodiment, and shows a first proton exchange region (optical waveguide) formed on the substrate 1 by proton exchange and annealing. Part that functions as a wave path)
5 and a second proton exchange region (a portion functioning as a high refractive index layer) 6 formed near the surface of the first proton exchange region 5. FIG. 3B shows the refractive index distribution in the depth direction in the configuration of FIG. FIG. 2 (b)
As apparent from comparison with the optical waveguide of the present embodiment, unlike the conventional optical waveguide, the refractive index distribution of the second proton exchange region 6 has a step-like graded shape due to annealing. Have.

Conventionally, high refractive index layer (high refractive index cladding layer)
A film having a step-like refractive index distribution, such as a non-annealed proton exchange region or a dielectric film having a high refractive index, has been used as the second proton exchange region functioning as the second proton exchange region. However, according to the study by the inventors of the present application, by changing the refractive index distribution of the second proton exchange region, which is a high refractive index layer, from a stepped shape to a step-like graded shape, the light damage resistance of the optical waveguide is significantly increased. Was found to increase.

The reason will be described.

To realize a highly efficient optical wavelength conversion element,
Using the first proton exchange region 5 as a waveguide layer and the second proton exchange region 6 as a high-refractive-index cladding layer, an increase in the overlap between a fundamental wave and a harmonic using the high-refractive-index cladding layer is reduced. Aim. In order to function effectively as a high refractive index cladding layer, a layer having a higher refractive index than the waveguide layer and having a step-shaped refractive index distribution is required. However, it has been clarified that a phase matching wavelength shift occurs due to optical damage in a structure having a high refractive index cladding layer. We conducted various studies on this issue,
The cause was clarified. That is, since the refractive index changes stepwise at the boundary between the high refractive index layer and the waveguide layer, a sharp change in the power density of the harmonic occurs, as shown in FIG. ing. Optical damage is a phenomenon in which an impurity order is excited by light excitation to generate an internal electric field, thereby causing a change in refractive index due to an electro-optic effect. The difference in the intensity of the internal electric field increases depending on the difference in the intensity distribution of the light, causing optical damage. If the harmonic power density changes rapidly at the boundary between the high refractive index layer and the waveguide layer, refraction occurs at that portion. The rate change is greater and the effect of photodamage is increased. Furthermore, changes in physical properties, refractive index, and power density of light may occur simultaneously at the boundary,
This is a factor that further increases the effect of light damage.

In order to alleviate this state, it was considered effective to reduce the rate of change of the refractive index distribution and enlarge the portion where the boundary exists. Therefore, the first proton exchange region 5
When the boundary between the first and second proton exchange regions 6 was enlarged, it was found that there was a structure capable of greatly reducing the occurrence of optical damage. That is, the area of the boundary between the first and second proton exchange regions 5 and 6 is increased by changing the refractive index distribution of the second proton exchange region 6 from stepwise to step-like graded. In addition, the occurrence of optical damage can be greatly reduced. However, if the boundary between the proton exchange regions 5 and 6 is too large, the wavelength conversion efficiency of the optical wavelength conversion device will be greatly reduced. That is, in order to simultaneously achieve high efficiency of wavelength conversion efficiency and high light damage resistance, it is necessary to appropriately control the refractive index distribution of the second proton exchange region 6.

Next, a step-like graded shape will be described.

The refractive index distribution in the proton exchange region depends on the proton diffusion distribution. Generally, the diffusion distribution of protons in the proton exchange region has a stepped shape at the end of the proton exchange process, but changes to a graded distribution when annealing is performed thereafter. Here, the relationship between the proton concentration distribution and the annealing time is represented by the following equation (1).

[0137]

(Equation 13)

However, in the above equation (1), C (k,
t): proton exchange concentration, k: depth (μm), t: annealing time (hour), C0: initial proton exchange concentration, E
rf []: error function and h: initial proton exchange depth (μm). Dp is the diffusion constant (μm ² / hour) of protons due to annealing, and its temperature dependence is shown in FIG.

The refractive index distribution having a step-like graded shape in the present specification means a step-like refractive index distribution obtained by a step-like proton concentration distribution and a graded refractive index distribution obtained by a graded-like proton concentration distribution. In a state intermediate with the refractive index distribution in the shape of a circle, in which the value of the proton exchange concentration on the surface is almost the same as the initial value C0, that is, C (0, t) is almost C0.
Is realized in the range where the value of This condition is actually
It can be defined by the value of h / 2 / (Dp × t) ^1/2 included in equation (1). Specifically, a refractive index distribution having a step-like graded shape is realized within a range satisfying the following inequality expression (2).

[0140]

[Equation 14]

For example, the initial proton exchange depth h = 0.
In the case of 18 μm, the range of (2) for realizing a refractive index distribution having a step-like graded shape is a range of about 2 hours to about 50 hours as an annealing time when the annealing temperature is 140 ° C. If the temperature is 180 ° C., the annealing time ranges from about 10 minutes to about 5 hours.

As described above, the proton concentration distribution in the proton exchange region generally has a step shape before annealing treatment after proton exchange.
Protons are thermally diffused and become graded from step-like to Gaussian-like distribution. Since the refractive index of the proton exchange region is proportional to the proton concentration, the refractive index distribution shape changes from a step shape to a graded shape as the proton concentration distribution shape changes due to the annealing treatment.

FIGS. 5A to 5C show how the shape of the refractive index distribution changes as the proton concentration distribution changes due to the annealing treatment.

As shown in the figure, the annealing treatment changes the refractive index distribution shape of the proton exchange region from the step shape (a) to the step-like graded shape (b), and further to the graded shape (c) close to the Gaussian distribution. ). Among these, the step-like graded shape is an intermediate shape between the step shape and the graded shape, and the proton diffusion distribution shape almost maintains the step-like shape as long as the proton exchange concentration on the surface does not decrease. I have.

The change in the refractive index distribution more strictly depends on the proton exchange depth before annealing, the annealing temperature, and the annealing time. FIG. 6 shows a change in the refractive index distribution due to the annealing treatment. In FIG. 6, the spread of the refractive index distribution is defined by the width of the portion where the amount of change in the refractive index is 1 / e ² of the maximum value. When the expansion of the proton exchange region by annealing becomes about twice or more, the refractive index distribution changes from a step-like shape to a shape close to a Gaussian distribution. In order to obtain a step-like refractive index distribution, it is necessary to keep the depth of the proton exchange region from being expanded by about 1.1 times to about 2 times by the annealing process.

FIG. 6 shows that the surface refractive index immediately after proton exchange is maintained while having a step-like refractive index shape. However, when the refractive index distribution approaches a Gaussian distribution and the width of the refractive index distribution approaches twice,
The surface refractive index starts to decrease gradually. The value of the surface refractive index in the proton exchange region where the step-like refractive index distribution was maintained was about 90% or more of the value before the annealing treatment.

From the above points, the phenomenon that the wavelength conversion efficiency of the light wavelength conversion element decreases when the annealing treatment of the second proton exchange region is performed for a long time is caused by the decrease in the surface refractive index of the second proton exchange region. It became clear that this was due to the decline. That is, the reduction in the surface refractive index of the second proton exchange region weakens the confinement of the guided light, resulting in a decrease in the wavelength conversion efficiency. Therefore, it is desirable to perform the annealing of the second proton exchange region to such an extent that the surface refractive index does not decrease. In addition, a high surface refractive index is required for increasing the efficiency of the optical wavelength conversion element, and a step-like refractive index distribution is required for the second proton exchange region 6 in the optical waveguide having the high refractive index cladding layer. Therefore, it is preferable that the spread of the second proton exchange region 6 be suppressed to twice or less.

According to the experimental results by the present inventors, in order to achieve both high wavelength conversion efficiency and good light damage resistance, the refractive index distribution of the second proton exchange region 6 must have a step shape and a Gaussian distribution. It has been found that it is necessary to take an intermediate refractive index distribution (see FIG. 5 (b)) between a perfect graded shape close to. This is referred to as a step-like graded shape in the present specification. on the other hand,
The first proton exchange region 5 is a waveguide layer having a small propagation loss, and needs to have a completely graded shape (error function shape) in order to realize the improvement of the overlap by the high refractive index cladding layer. is there.

That is, it is highly likely that the first proton exchange region 5 and the second proton exchange region 6 have graded refractive index distributions of different shapes (ie, graded proton concentration distributions of different shapes). To realize an optical wavelength conversion device that is efficient and has excellent light damage resistance,
It turned out to be a necessary condition.

The refractive index distribution (proton concentration distribution) in the proton exchange region easily changes to a graded shape close to a Gaussian distribution by annealing. In order to control the refractive index distribution of the second proton exchange region 6 to have a step-like graded shape, it is necessary to precisely control the annealing conditions for the second proton exchange region 6.
According to the present invention, it has been clarified that the characteristics of the optical wavelength conversion element can be improved by accurately controlling the step-like graded shape before changing to the graded shape.

FIG. 7 shows the distribution of the proton exchange rates in the first and second proton exchange regions. The LiNbO ₃ crystal becomes Li _(1-x) H _x NbO ₃ by proton exchange. The proton exchange rate X shown here indicates the exchange rate between Li and H in the crystal, and X = H / (L
i + H). In a crystal that has not undergone proton exchange, the proton exchange rate X is 0, and the value of the proton exchange rate X becomes about 1 when Li and H are completely exchanged. In an actual proton exchange layer, the proton exchange rate immediately after proton exchange without annealing treatment slightly changes depending on the proton exchange temperature, the acid used for proton exchange, and the proton exchange time. Typically, X = 0.67 ± in the case of benzoic acid, which is a typical acid used for proton exchange.
0.02, X = 0.76 ± 0.0 when proton exchanged with pyrophosphoric acid capable of forming a proton exchange layer having a high refractive index.
About 2. About 70% of Li in the crystal is exchanged with protons. Since a proton exchange layer having a high refractive index can be formed, proton exchange is preferably performed with pyrophosphoric acid.

FIG. 7A shows the distribution of the proton exchange rate in the waveguide composed of the first and second proton exchange layers, which was produced using pyrophosphoric acid. The proton exchange rate in the second proton exchange layer is about 0.76 near the surface.
It is. On the other hand, the proton exchange rate in the first proton exchange layer is about 0.1 near the surface. By providing a large difference in the proton exchange rates, a sufficient difference in the refractive index between the second proton exchange layer and the first proton exchange layer is obtained.
Further, as can be seen from the figure, in order to form the second proton exchange layer, an annealing process must be performed under conditions that minimize the diffusion of protons. It is necessary to control the temperature and time so that the proton concentration distribution does not collapse and does not affect the first proton exchange layer. Therefore, the temperature for forming the second proton exchange is set to 200
C. or lower, and formed while controlling the time so that the diffusion distribution was not lost.

The distributions of the first and second proton exchange rates are enlarged in the distribution diagrams of the proton exchange rates shown in FIGS. 7B and 7C, respectively. Is represented. As shown in the figure, it has a completely different shape. The distribution of the proton exchange rate in the first proton exchange layer is completely graded by the annealing treatment. On the other hand, the second proton exchange layer has a step-like graded shape. The major difference between the first and second proton exchange layers is the amount of change in the proton exchange rate in the depth direction. In the first proton exchange layer, the proton concentration on the surface is reduced, and the amount of change in the proton exchange rate X in the depth direction is very small. On the other hand, in the second proton exchange layer, the proton exchange rate changes rapidly in the depth direction. Change in the proton exchange rate X in the depth direction (μm
^-1 ) is defined as follows. The amount of change in the proton exchange rate = | ΔX / Δdepth | and the annealing state of the proton exchange can be represented by the absolute value of the change in the proton exchange rate. First, the amount of change in the proton exchange rate in the first proton exchange layer is about 0.04 μm ⁻¹ . To lower the proton concentration of the proton exchange layer and sufficiently recover the nonlinear optical constant, the amount of change in the proton exchange rate should be at least 0.1 μm ^−1.
The following is required, and 0.06μ
m ^-1 or less is desired.

On the other hand, in the second proton exchange layer,
The size of the region where the proton concentration changes in the depth direction becomes important. As described above, the boundary between the first and second proton exchange layers is a region where the proton concentration is changed, and by forming this region by annealing, an optical waveguide having excellent light damage resistance is formed. Formation is possible.
Specifically, as shown in FIG. 7C, the area where the proton exchange rate changes is about 0.05 μm in the second proton exchange area. Immediately after proton exchange, this region is 0.01 μm or less. It is desirable that the region where the proton exchange rate is changed by the annealing treatment of the second proton exchange region is expanded to 0.02 to 0.2 μm. Furthermore, in order to realize high-efficiency conversion, 0.03-
It is desirable to keep the thickness within about 0.1 μm. The major difference from the first proton exchange layer is the size of this region. In the first proton exchange region, since the proton distribution is completely graded, the region where the proton exchange rate changes reaches several μm. In contrast,
Since the second proton exchange region has a graded shape in which the distribution of protons is step-like, the region where the proton exchange rate changes is significantly different from 0.02 to 0.2 μm. Further, the difference between the first and second proton exchange regions is the amount of change in the proton exchange rate. Since the first proton exchange region has a completely graded proton exchange rate, the rate of change of X is about 0.04 μm ⁻¹ . On the other hand, the second proton exchange region is 15 μm.
It is as large as about m ^-1 . In order to realize a step-like graded shape capable of high-efficiency conversion, it is desirable to set the amount of change in the proton exchange rate to about 5 to 50 μm ⁻¹ . To further increase the light damage resistance, the amount of change is 1
It is more desirable to set it to about 0 to 30 μm ⁻¹ .

In the first and second ion exchange layers, the phase relationship between the fundamental wave and SHG is reversed (see FIG. 2).
An increase in the nonlinear optical effect in the second ion exchange layer results in a decrease in the conversion efficiency. Therefore, the lower the nonlinear optical constant of the second ion exchange layer, the higher the conversion efficiency. It is desirable that the nonlinear constant be reduced to 60% or less at a high proton exchange concentration.

Next, the results of a more detailed study of the proton concentration distribution shape of the second proton exchange region 6 forming the optical waveguide having excellent light damage resistance will be described.

First, the manufacturing conditions under which a preferable refractive index distribution in the second proton exchange region 6 can be realized were studied.

FIGS. 10 and 11 show the manner in which the second proton exchange region 6 is enlarged when the annealing process is performed, and the depth of the second proton exchange region 6 (the full width at half maximum of the proton exchange region). The wavelength conversion efficiency and the surface refractive index of the optical wavelength conversion element (FIG. 10), the light damage resistance (FIG. 11), and the second proton exchange layer 6 are represented by parameters (horizontal axis). The relationship between the depth and the depth. However, the horizontal axis indicates a value standardized by the depth of the second proton exchange region 6 before the start of the annealing process. Therefore, the values of the wavelength conversion efficiency and the surface refractive index (FIG. 10) of the optical wavelength conversion element and the light damage resistance (FIG. 11) at the intersection with the vertical axis indicate values before the start of the annealing process.

As shown in FIG. 11, the light damage resistance shows that the second proton exchange region
When the depth was increased to 1.2 times or more the depth before the annealing treatment, it rapidly increased, and when the annealing treatment was further performed, the light damage resistance gradually increased. This is because the boundary between the first and second proton exchange regions 5 and 6 expands as the refractive index distribution of the second proton exchange region 6 approaches a graded shape with the continuation of the annealing process. That's why.

Further, a new phenomenon was discovered during this study.
That is, it has been found that when the second proton exchange region 6 is annealed, the wavelength conversion efficiency of the light wavelength conversion element is greatly increased as shown in FIG.

Specifically, the wavelength conversion efficiency increases with an increase in the second proton exchange region 6 (an increase in the depth), and reaches a maximum when the depth becomes approximately 1.5 times that before annealing. became. At this point, the wavelength conversion efficiency was close to twice the value obtained before the annealing treatment, and a significant improvement in the wavelength conversion efficiency was confirmed. However, when the annealing treatment is further continued, the wavelength conversion efficiency starts to decrease when the depth of the second proton exchange region 6 becomes about twice the value before annealing, and
It decreased with continued annealing. This is shown in FIG.
This is because the surface refractive index of the second proton exchange region 6 starts to decrease around this point.

There are two possible reasons why the wavelength conversion efficiency is improved by annealing the second proton exchange region 6.

First, the annealing process increases the depth of the second proton exchange region 6, but in a range where the refractive index distribution has a step-like graded shape and the decrease in the surface refractive index is small. , The effective refractive index increases. For this reason, the confinement of the guided light in the optical waveguide is strengthened and the power density of the guided light is increased, so that the wavelength conversion efficiency is improved.

Second, the effect of improving the uniformity of the second proton exchange region 6 can be considered. That is, when the annealing process is not performed, the second proton exchange region 6 has a crystal distortion near the boundary with the first proton exchange region 5, and this crystal distortion occurs in the optical waveguide (first proton exchange region). This causes the non-uniformity of the region 5). The non-uniformity of the optical waveguide (the first proton exchange region 5) has a large effect on the wavelength conversion efficiency and causes a reduction in efficiency. On the other hand, when annealing is applied to the second proton exchange region 6, crystal distortion is reduced, and as a result, the uniformity of the optical waveguide (first proton exchange region 5) is increased. Thereby, the wavelength conversion efficiency is significantly improved.

From the viewpoint of the light damage resistance, the depth of the second proton exchange region 6 is desirably 1.2 times or more the value before annealing. Furthermore, the value of 1.
When it is 5 times or more, the light damage resistance is further improved.
More preferred. On the other hand, from the viewpoint of improving the wavelength conversion efficiency, the depth of the second proton exchange region 6 is desirably about 1.2 to 2.5 times the value before annealing. Further, when the value before annealing is about 1.5 times to 2 times, the wavelength conversion efficiency is 2 times.
It is more preferable to increase by a factor of two.

Further, it has been clarified that the time-dependent change of the light wavelength conversion element is eliminated by making the refractive index distribution of the second proton exchange region 6 into a step-like graded shape. This will be described below.

As described above, the characteristics of the optical wavelength conversion element formed by the proton exchange optical waveguide are deteriorated by a high-temperature reliability test at about 80 ° C. Investigations by the inventors of the present application have revealed that an increase in the depth of the second proton exchange region 6 with the passage of time is a main cause of the above-described characteristic deterioration. This is because the thickness of the proton exchange region where the annealing is not performed is easily changed due to aging, and even if the thickness of the step-like refractive index distribution is slightly changed, the effect on the light wavelength conversion element is large. In particular, it is caused. Specifically, in the experiment conducted by the inventors of the present invention, the temperature of the sample not annealed was 8 ° C.
A high temperature reliability test at 0 ° C. for about 100 hours showed that the phase matching wavelength changed by about 0.2 nm to about 0.6 nm. In contrast, for the annealed sample,
Even after a high-temperature reliability test at a temperature of 80 ° C. for about 1000 hours, the shift of the phase matching wavelength was about 0.02 nm or less, and showed no substantial change.

On the other hand, when the second proton exchange region 6 was annealed to have a graded shape according to the present invention, there was almost no change with time. This is because the change with time in the characteristics, particularly the change in the refractive index distribution at a high temperature, is significantly reduced by accelerating the change with time by adding the annealing treatment. Further, since the refractive index distribution of the second proton exchange region 6 changes from step-like to step-like graded, the boundary between the first and second proton exchange regions 5 and 6 is a constant region. Therefore, it is considered that the effect on the optical wavelength conversion element when the refractive index distribution changes in the depth direction can be significantly reduced.

Specifically, the relationship between the expansion of the proton exchange region and the change with time of the characteristics is examined. As a result, the change with time is observed until the thickness of the proton exchange is increased to about 1.1 times by the annealing treatment. However, when the magnification was increased to 1.3 times or more, deterioration in the high-temperature test characteristics was not observed.

On the other hand, the first proton exchange region 5 has a close relationship with the propagation loss of the optical waveguide. By expanding the first proton exchange region 5 by annealing,
The propagation loss of the optical waveguide can be greatly reduced. For example, 33
When the annealing treatment was performed at 0 ° C. for 3 hours, when the distribution of protons in the first proton exchange region 5 (depth of the first proton exchange region 5) was expanded to about eight times that before the annealing, the photoconduction was started. The propagation loss of the waveguide was 1 dB / cm or less, and exhibited the lowest level. However, even if the annealing treatment was further continued, further reduction of the waveguide loss was not recognized.

Thus, in order to form a low-loss optical waveguide, it is necessary to increase the depth of the first proton exchange region 5 to at least eight times by annealing.

Next, a process for forming an optical waveguide by proton exchange will be described.

Referring to FIGS. 8A to 8E, a method for manufacturing an optical waveguide will be described. FIGS. 8A to 8E are schematic cross-sectional views for explaining each step, and further, FIGS.
Regarding (e), the refractive index distribution shape in the depth direction in the shape obtained by each step is also shown.

As the substrate 1, for example, an X-plate MgO-doped LiNbO ₃ substrate 1 is used. First, as shown in FIG. 8A, a stripe-shaped mask pattern 11 for an optical waveguide is formed on the surface of the substrate 1 using, for example, tantalum (Ta). Next, as shown in FIG.
The first proton exchange region 5 is formed in a region of the substrate 1 which is not covered by the mask pattern 11 by performing a heat treatment in pyrophosphoric acid at a temperature of ℃. Subsequently, as shown in FIG. 8C, after removing the Ta mask pattern 11, the first proton exchange region 5 is thermally diffused by a first annealing process. Further, as shown in FIG. 8D, a second proton exchange region 6 is formed on the surface of the substrate 1 by using pyrophosphoric acid at 200 ° C. Subsequently, as shown in FIG. 8E, a second annealing process is performed to thermally diffuse the second proton exchange region 6.

Depth (thickness) of first proton exchange region 5
Is typically about 0.17 μm to about 0.25 μm, which is a condition under which a wavelength near 850 nm can be propagated as a fundamental wave in a single mode condition after a high refractive index cladding is deposited.

The temperature of the first annealing treatment for annealing the first proton exchange region 5 having the depth (thickness) as described above was studied.

By the first annealing, the first proton exchange region 5 functioning as an optical waveguide needs to be expanded to a depth of 2 μm or more. The first reason is that the refractive index distribution in the optical waveguide section is completely graded to reduce the propagation loss. Second, since the nonlinearity of the substrate 1 is greatly reduced due to the proton exchange, the nonlinearity is recovered. In order to realize a high-efficiency optical wavelength conversion element, it is necessary to make the nonlinearity of the optical waveguide sufficiently high. By reducing the proton exchange ratio to 1/8 or less by annealing, the proton exchange region is reduced. The nonlinearity can be restored to a value close to that of the substrate 1. A third reason is to form an optical waveguide structure having a high refractive index cladding layer. By reducing the surface refractive index of the first proton exchange region 5 and increasing the refractive index difference between the first proton exchange region 5 and the second proton exchange region 6, a high refractive index clad layer by proton exchange can be realized.

To satisfy the above conditions, the temperature at which the first proton exchange region 5 is annealed is preferably 300 ° C. or higher. If the annealing temperature is set to 300 ° C. or lower, the required refractive index distribution cannot be formed even if the annealing is performed for several tens of hours, so that the process efficiency is extremely reduced. Further, the reduction of the propagation loss of the optical waveguide due to the annealing can be more efficiently achieved by setting the annealing temperature for the first proton exchange region 5 to 300 ° C. or higher.

The thermal diffusion and the propagation loss of the proton exchange region due to the annealing process depend on the annealing temperature, and the higher the annealing temperature, the lower the loss of the same level of proton diffusion can be realized. For this reason,
In order to form a lower loss optical waveguide, high-temperature annealing is required. When the relationship between the annealing temperature and the propagation loss was determined, the propagation loss could be reduced to 1 dB / cm or less by setting the temperature of the annealing treatment for the first proton exchange region 5 to 300 ° C. or higher.

On the other hand, the depth (thickness) of the second proton exchange region 6 is preferably about 0.15 μm to about 0.21 μm for a fundamental wave having a wavelength of about 850 nm. This is a region where the light of the fundamental wave is cut off in the second proton exchange region 6.

Further, according to the study by the inventors of the present application, the depth of the second proton exchange region 6 is determined by the following reason.
It has been found that it is more preferable to set the depth to 90% or less with respect to the depth of the boundary that is cutoff with respect to the fundamental wave (the boundary between the waveguide condition and the cutoff condition).

The effective refractive index of the second proton exchange region 6 is increased by the annealing. In this case, the second proton exchange region 6 is located near the boundary of the cutoff condition.
Is set, the effective refractive index of the second proton exchange region 6 is increased by the annealing treatment on the second proton exchange region 6, and the fundamental wave is guided in the second proton exchange region 6. It becomes possible. According to the study by the inventors of the present application, it has become clear that as a result of the above, a significant decrease in the wavelength conversion efficiency occurs. In order to prevent this, the depth of the second proton exchange region 6 is set to 90% or less with respect to the depth of the boundary of the cutoff condition, so that the second proton exchange region 6 can be formed even after annealing. It is necessary that the depth of the region 6 keeps the cutoff condition with respect to the fundamental wave.

As a second annealing condition for annealing the second proton exchange region 6, it was found that the annealing temperature had to be set to 250 ° C. or less. The two reasons are controllability of the refractive index distribution and reduction of the degree of decrease in the surface refractive index.

In the second annealing process, it is necessary to control the refractive index distribution of the second proton exchange region 6 into a step-like graded shape. For this reason, it is necessary to precisely control the time of the annealing process. At the same time, in order to obtain the uniformity of the optical waveguide (first proton exchange region 5) after the annealing, high uniformity is also required for the distribution of the annealing in the surface of the light wavelength conversion element.
Since the thermal diffusion rate of the proton exchange region due to the annealing increases with the temperature, the annealing temperature is set to 250 ° C.
Above this, the annealing time is about several seconds, and precise temperature time control becomes difficult. In addition, when the high-temperature processing was performed for a short time, a phenomenon was observed in which the distribution of the propagation constant of the optical waveguide was formed due to the temperature distribution in the substrate, and the uniformity of the optical waveguide was deteriorated.

Therefore, in view of the above, in order to accurately form a uniform refractive index distribution required for the second proton exchange region 6, it is preferable to perform annealing at a low temperature for a long time. Specific annealing temperature for the second proton exchange region 6 is 150 ° C. for 5 to 6 hours or more,
It takes several hours at 0 ° C. and several minutes at 200 ° C. or higher.

For example, the second proton exchange region 6
When annealing was performed at 0 ° C., the conditions under which the optimum step-like graded distribution was obtained were that the annealing time ranged from 2 hours to 20 hours. In particular, when the annealing treatment was performed for 3 hours or more, a great improvement in the light damage resistance was observed. On the other hand, when annealing was performed at 180 ° C., a good distribution shape was obtained by annealing for 3 hours or less. However, if the annealing treatment is performed at an annealing temperature of 180 ° C. for more than 10 hours, the second proton exchange region 6 becomes too wide and becomes a completely graded shape,
A phenomenon in which the wavelength conversion efficiency was reduced was also observed.

From these facts, regarding the second annealing treatment for the second proton exchange region 6, an annealing treatment temperature of 250 ° C. or less is required in order to control the refractive index distribution required for the optical waveguide with good control. In order to achieve the uniformity of the optical waveguide, the temperature is preferably 200 ° C. or less.

At the same time, it was found that the decrease in the surface refractive index of the second proton exchange region 6 due to the annealing treatment increased in proportion to the annealing temperature. In order to realize a highly efficient light wavelength conversion element, it is necessary to make the surface refractive index of the second proton exchange region 6 as high as possible. Specifically, a value substantially equal to the surface refractive index before annealing is desirable. However, as the annealing temperature rises, the refractive index of the crystal surface constituting the second proton exchange region 6 decreases. This is presumably because the annealing temperature changes the crystal structure of the proton exchange region. Looking at the relationship between the annealing temperature and the surface refractive index, the annealing temperature is about 25%.
It was found that when the temperature exceeds 0 ° C., the decrease in the surface refractive index becomes large.

From the above results, the annealing temperature of the second proton exchange region 6 (the temperature of the second annealing process) is 2
It is preferable to set the temperature to 50 ° C. or lower.

The lower limit of the annealing temperature of the second proton exchange region 6 was also examined. When the annealing temperature was 130 ° C. or lower, the change in the refractive index distribution was small no matter how much annealing was performed. It was found that a step-like graded shape could not be achieved. Therefore, the second annealing temperature for the second proton exchange region 6 is preferably 130 ° C. or higher.

In this embodiment, an X-plate Mg
O: LiNbO ₃ substrate is used. Alternatively, the same effect can be obtained by using a substrate having a different crystal orientation such as a Z plate or an 87-degree cut Z plate. In particular, the use of a substrate whose crystal surface is inclined from the crystal axis is effective. In the case of using a substrate in which the crystal axis and the substrate surface are tilted, for example, an X plate (a substrate cut out on a plane perpendicular to the X axis of the crystal) is
It has been reported that a substrate whose axis is tilted by several degrees allows a deeper domain-inverted structure to be formed and a more efficient optical wavelength conversion element to be formed.

In a substrate having a tilted crystal axis, it is important to reduce the propagation loss of an optical waveguide formed by proton exchange. It has been found that in a substrate with a tilted crystal axis, the propagation loss greatly depends on the annealing treatment, and the propagation loss increases more easily than in a normal X-plate. Anneal the proton exchange region 5 by 300
It must be performed at a temperature of at least ℃. Further, as shown in the present embodiment, the second proton exchange region 6 is annealed,
Proton exchange concentration distribution shape (refractive index distribution shape)
Has been found to be very effective to change from a step shape to a step-like graded shape.

As the substrate 1, instead of the Mg-doped LiTaO ₃ substrate mentioned above, a substrate made of another material (non-linear optical crystal), for example, a Zn or In-doped LiTaO ₃ substrate, a LiNbO ₃ substrate, a LiTaO ₃ substrate,
₃ Substrate, and further, LiNb _x Ta _1-x
Substrate made of O ₃ crystal (0 ≦ x ≦ 1)
A substrate made of a material doped with g, Zn, In, or the like can be used similarly. Proton exchange is possible in any of these substrates, and the same effect is obtained because the proton exchange region has the same annealing characteristics.

(Second Embodiment) Next, an optical wavelength conversion element of the present invention will be described with reference to FIGS. 9 (a) and 9 (b).

FIG. 9A is a perspective view of an optical wavelength conversion element 60 according to the present embodiment, and FIG. 9B is a cross-sectional view of an optical waveguide included therein. FIG. 2B also shows the refractive index distribution in the depth direction.

In FIG. 9A, the LiNbO ₃ of the X plate was used.
A first proton exchange region 5 in the form of a stripe is formed near the surface of the substrate 1, and a second proton exchange region 6 is formed near the surface of the first proton exchange region 5. On the LiNbO ₃ substrate 1, a periodically poled structure 7 is formed for phase matching. The first proton exchange region 5 has a width W1 = about 5 μm and a depth D1 = about 2.5 μm.
m, and the second proton exchange region 6 has a depth D2 of about 0.22 μm.

The first proton exchange region 5 is made of LiNbO
₃ is formed by annealing after proton exchange, has a composition represented by Li _1-x H _x O ₃ (0 <x <1), and a part of Li in the LiNbO ₃ crystal is H
Has been replaced. Specifically, in the first proton exchange region 5, first, a proton exchange region is formed with a depth of about 0.2 μm, and this is annealed at 330 ° C. for 3 hours.
Enlarged to 2.5 μm.

The second proton exchange region 6 is also formed by proton exchange. In the second proton exchange region 6, a proton exchange region is formed at a depth of about 0.18 μm, which is annealed at 180 ° C. for 1 hour to produce a 0.22 μm.
It is enlarged to μm and has a step-like refractive index distribution.

The light wavelength conversion element 6 thus obtained
In the configuration of No. 0, the first proton exchange region 5 functioning as a waveguide portion and the second proton exchange region 6 functioning as a high refractive index cladding layer are different from each other as shown in FIG. The shape has a graded refractive index distribution. This optical wavelength conversion element has a wavelength of 850 nm.
Is incident in the TE00 mode, and TE
The phase is quasi-matched with the harmonics of 10 modes.

A description will be given of the principle that the optical wavelength conversion element 60 can convert a wavelength into a harmonic with high efficiency.

High refractive index layer (second proton exchange region 6)
In the optical waveguide having the above, the overlap between the fundamental wave and the harmonic wave increases, the confinement of the guided light is strengthened, and highly efficient wavelength conversion becomes possible. This is because the electric field distribution of the fundamental wave is drawn to the surface of the substrate 1 by the high refractive index layer, and confinement in the optical waveguide is strengthened.
At the same time, the power density of light also increases. On the other hand, as for the generated second harmonic, a higher-order TE
By selecting 10 modes, there is no influence of the enhancement of confinement in the optical waveguide by the high refractive index cladding layer. As a result, the overlap between the fundamental wave with enhanced confinement and the second harmonic obtained by wavelength conversion is increased. Due to such an increase in the power density of the fundamental wave and an increase in the overlap between the fundamental wave and the second harmonic, a great improvement in the wavelength conversion efficiency is realized.

Furthermore, by forming the second proton exchange region 6 forming the high refractive index cladding layer into a step-like graded shape by annealing, the light damage resistance can be greatly improved. Became. As a result, a blue light output of 10 mW or more can be stably output for a long time of several hundred hours or more. Further, there was no change with time of the light wavelength conversion element, and no change in the characteristics was observed even in a standing experiment for several tens of thousands hours at room temperature and for several thousand hours in an environment of 60 ° C.

As a result, it has become possible to realize an optical wavelength conversion element that can be put to practical use.

In order to realize highly efficient operation characteristics (wavelength conversion characteristics) by using a high refractive index clad structure, an optical waveguide (first proton exchange region 5) must be provided in the second proton exchange region 6. Is cut off, while the second harmonic needs to satisfy the condition that enables the waveguide. Since the fundamental wave is cut off in the second proton exchange region 6, the fundamental wave can be guided through the waveguide portion (the first proton exchange region 5). In addition, the harmonic wave is a high refractive index layer (second proton exchange region 6).
Under the condition that the light can be guided, the higher-order guided mode of the harmonic can be used for highly efficient wavelength conversion.

On the other hand, also from the viewpoint of the light-collecting characteristics, the refractive index between the high refractive index layer (second proton exchange region 6) and the waveguide portion (first proton exchange region 5) has a predetermined relationship. Must be satisfied.

When condensing guided light having a subpeak with a lens system, the light condensing characteristics are degraded due to the size of the subpeak. In order to make the converging spot a single spot and obtain a converging spot equal to or less than the diffraction limit, it is necessary to reduce the sub-peak of the waveguide mode to the diffraction limit of the converging lens or less. The width of the sub-peak is the high refractive index layer (second
Is defined by the width of the proton exchange region 6) and the refractive index thereof, but is practically substantially equal to the depth of the high refractive index layer.
Therefore, to keep the width of the subpeak below the diffraction limit,
Depth D2 of high refractive index layer (second proton exchange region 6)
Must be suppressed below the diffraction limit of harmonics. Thus, the expansion of the depth D2 of the second proton exchange region 6 by the annealing process must be suppressed to at least twice the value before annealing.

When the depth D2 of the second proton exchange region 6 is about 0.5 μm, the light-collecting characteristics are considerably improved,
When 2 <0.4 μm, light-collecting characteristics below the diffraction limit were obtained. The numerical aperture NA of the condenser lens is 1 and the wavelength is 425.
When the diffraction limit is calculated assuming the value of nm, the value is about 0.
34 μm, and the depth D2 of the second proton exchange region 6
Is smaller than the diffraction limit, it is possible to obtain an outgoing beam having excellent focusing characteristics.

In this embodiment, an X-plate substrate is used as the substrate 1, but a Y-plate, a Z-plate, or a substrate whose crystal axis is inclined from the surface may be used instead. For example, the use of a Z plate or a substrate whose crystal axis is inclined from the substrate surface is promising because a deep domain-inverted structure can be easily formed and high efficiency can be achieved.

In this embodiment, the polarization direction of the TE mode is used. This is because the polarization direction of light emitted from a normal semiconductor laser and the polarization direction of the waveguide are matched. By using an optical waveguide having the same polarization direction as the semiconductor laser, the waveguides can be coupled with low loss. However, an optical waveguide of TM mode polarization can also be used, and in the optical waveguide of TM mode, the coupling loss can be reduced by controlling the polarization direction by a λ / 2 plate.

Further, in this embodiment, the optical wavelength conversion device using the optical waveguide has been described, but the optical waveguide of the present invention is also effective for other optical waveguide devices. By forming the high refractive index layer (second proton exchange region 6) on the optical waveguide (first proton exchange region 5), the electric field distribution of the guided light propagating through the optical waveguide is strongly attracted to the vicinity of the surface. Therefore, the effect of the planar electrode and the grating element formed on the optical waveguide can be strongly given to the guided light, and a highly efficient modulation and diffraction effect can be obtained.

(Third Embodiment) Here, the configuration of another optical wavelength conversion element will be described.

In the optical wavelength conversion element of the present invention, the X plate
O: On a LiNbO ₃ substrate, a periodic polarization inversion and an optical waveguide are formed. MgO: LiNbO ₃ substrate
The substrate is cut out so that the Z axis of the crystal is tilted 3 degrees with respect to the surface of the substrate. When a substrate having an inclined crystal axis is used, a deep domain-inverted structure can be formed, so that a highly efficient optical wavelength conversion element can be formed. This is because the polarization inversion is formed along the Z axis of the crystal, and is thus formed from the substrate surface to the inside of the substrate.

Therefore, an optical wavelength conversion element was formed utilizing this property. In the obtained optical wavelength conversion element, the domain-inverted region is formed inside the substrate, and a periodic polarization is formed from the portion of the surface near the boundary between the first proton exchange region and the second proton exchange region toward the inside of the substrate. The second proton exchange region has an inversion structure and is formed so as not to have a polarization inversion structure. Thereby, the efficiency of the wavelength conversion operation in the optical wavelength conversion element can be improved.

[0214] The wavelength conversion efficiency of the optical wavelength conversion element depends on the overlap between the domain-inverted structure and the guided light (fundamental and harmonic) propagating through the optical waveguide. However, in order to improve the efficiency, generally a higher-order guided mode (TE
10 mode), the electric field distribution of the harmonic wave is determined by the high refractive index layer (second proton exchange region 6) and the waveguide portion (first proton exchange region 6).
At the boundary with the proton exchange region 5), the phase of the harmonic in the electric field strength is reversed. Therefore, the electric field intensity of the harmonic becomes zero at the boundary between the second proton exchange region 6 forming the high refractive index cladding layer and the first proton exchange region 5 forming the waveguide portion. When the intensity of the harmonic wave suddenly changes at the boundary between the two, a power distribution of the harmonic wave is formed, which causes optical damage. In order to prevent this, it has been effective to expand the boundary distribution by annealing the second proton exchange region 6 to form a graded shape.

On the other hand, an optical waveguide (first proton exchange region 5) formed in a periodically poled structure has a higher propagation loss than an optical waveguide formed on a substrate having no poled structure. This is considered to be because an internal electric field is generated when a periodic polarization inversion is formed by the application of an electric field, thereby causing a change in the refractive index in the domain-inverted portion.
Therefore, the optical waveguide is affected by the periodic change in the refractive index due to the polarization reversal, thereby increasing the waveguide loss.

Therefore, in order to reduce the propagation loss in the domain-inverted structure, a study was made to remove the internal electric field existing in the domain-inverted structure 7 and reduce the change in the refractive index.

An internal electric field is generated when charges are trapped in the order of impurities. Therefore, by performing heat treatment,
An attempt was made to reduce the internal electric field by exciting the impurity order forming the internal electric field to release charges. Specifically, before forming the optical waveguide (the first proton exchange region 5), the heat treatment of the domain-inverted structure 7 was studied. As a result, they found that the internal electric field can be reduced by performing heat treatment at a high temperature.

A change in the propagation loss of the optical waveguide was observed by performing the heat treatment while changing the heat treatment temperature.
At 50 ° C. or lower, there was almost no change in the propagation loss, and it was found that heat treatment at 400 ° C. or higher was necessary to reduce the propagation loss. When the heat treatment temperature exceeded 400 ° C., the propagation loss was reduced in proportion to the heat treatment time. Further, in order to completely remove the change in the refractive index, a heat treatment at 500 ° C. or more is preferable. However, LiNbO ₃ substrate or MgO: LiN
In the case of a bO ₃ substrate, if the heat treatment temperature exceeds 800 ° C.,
It was found that the characteristics of the optical wavelength conversion element deteriorated because the domain-inverted structure was reduced and the domain-inverted thickness was reduced.

From the above points, the annealing temperature for the domain-inverted structure performed for the above purpose is 400 ° C. to 80 ° C.
It has been found that 0 ° C. is preferable, and 500 ° C. to 800 ° C. is more preferable to further reduce the propagation loss of the optical waveguide.

Further, in the structure having the high refractive index cladding layer, since the confinement of the fundamental wave is strengthened, the occurrence of waveguide loss due to the change in the refractive index increases. In particular, the fundamental-harmonic overlap at the high refractive index layer portion (second proton exchange region 6) reduces the wavelength conversion to harmonics. To prevent this, it is necessary to prevent the overlap between the fundamental wave and the higher harmonic wave in the high refractive index layer from affecting the wavelength conversion.

Therefore, by eliminating the periodic domain-inverted structure in the second proton exchange region 6, it is possible to selectively prevent the phase matching condition in the high refractive index layer from being satisfied. Thereby, high efficiency of the optical wavelength conversion element is realized.

In this embodiment, a crystal whose crystal axis is inclined by 3 degrees is used.
It can be used because a deep domain-inverted structure can be formed. In particular, a crystal having a crystal axis inclination of 3 degrees to 0.5 degrees is promising because the polarization inversion inclination is small and the used area increases.

(Fourth Embodiment) In this embodiment, a short-wavelength light generator (short-wavelength light source) constituted by using the optical wavelength conversion element of the present invention will be described with reference to the accompanying drawings.

FIG. 12 shows the configuration of a short-wavelength light generating device 400 according to this embodiment.

This short-wavelength light generating device 400 includes a semiconductor laser 421 and a light wavelength conversion element 422, and a fundamental wave P1 emitted from a light emitting region 423 of the semiconductor laser 421.
Is incident on an optical waveguide (first proton exchange region) 402 formed in the optical wavelength conversion element 422. On the surface of the optical waveguide 402, a second proton exchange region 403 is formed. Each component is fixed to the Si mount 420.

The fundamental wave P incident on the light wavelength conversion element 422
1 propagates in the waveguide 402 in the TE00 mode and is converted to the TE10 mode, which is a higher-order mode of higher harmonics. This harmonic P2 is emitted from the light wavelength conversion element 422 and is used as a short wavelength laser beam.

Attention is paid here to the optical wavelength conversion element 42.
Of the harmonic P2 propagating in the second optical waveguide 402
This is a mode profile of the 0 mode. Since the TE10 mode is a higher-order guided mode, it has two peaks as an intensity distribution. Here, the larger one of the two peaks is called a main peak, and the smaller one is called a sub-peak.

In general, the TE10 mode having a sub-peak becomes a condensed spot having the same sub-peak as that of the waveguide mode, which is condensed by the condensing optical system, and is used as condensed light of a single peak near the diffraction limit. It becomes a problem in cases.
Therefore, the inventors of the present application have newly found a method of substantially eliminating a sub-peak which is a problem when focusing the harmonic P2. According to this, the width of the sub-peak in the state of the waveguide mode is set to a value sufficiently smaller than the diffraction limit of the condensing optical system used for condensing. Specifically, by suppressing the sub-peak to a width equal to or less than the resolution of the condensing optical system, the influence of the sub-peak on the condensed spot can be eliminated.

In the experiment, harmonics having a wavelength of 425 nm were collected using a condenser lens having a numerical aperture of NA = 0.95. At this time, the diffraction limit of light in the air is about 0.34 μm. When the optical waveguide 402 of the optical wavelength conversion element 422 was designed so that the width of the sub-peak was 0.32 μm or less, no sub-peak was observed in the obtained condensed spot. That is, if the width of the sub-peak is not more than the diffraction limit of light in air (0.8 · λ / NA), the effect of the higher-order guided mode having the sub-peak on the condensed spot at the time of condensing is small. It turned out that it could be used effectively. For this reason, it is possible to improve the utilization efficiency of the output that is reduced by the beam shaping to 80% or more, which is effective.

Further, it was found that the use of guided light having a subpeak having a width equal to or less than the diffraction limit can provide a super-resolution effect capable of converging a beam below the diffraction limit of the lens. When light in the TE00 mode was condensed, a value close to the diffraction limit (0.8 · λ / NA) of the lens was obtained as the width of the condensed beam. On the other hand, a higher-order TE having a sub-peak
When the light of 10 modes is condensed, the beam shape on the side having the sub-peak becomes steep, and the light can be condensed up to about 90% of the diffraction limit as the width of the condensed spot, and has a super-resolution effect. I understood that. According to experiments, by setting the width of the sub-peak to be equal to or less than the diffraction limit (= about 0.8λ) of the light having the wavelength λ in the air, the width of the condensed spot can be reduced by the lens (numerical aperture: NA) used Diffraction limit (0.8λ /
NA). As described above, by condensing a guided mode having a subpeak having a width equal to or smaller than the diffraction limit, a smaller condensed spot can be obtained.

Further, the relationship between the width of the sub-peak and the converging spot was examined in detail. (1) The width of the sub-peak: <0.8 × λ The condensing characteristics (super-resolution effect) below the diffraction limit can be obtained. (2) Subpeak width: 0.8 × λ × (1 + 0.2 (1
/ NA-1)) to 0.8 × λ × (1 + 0.5 (1 / NA)
-1)) A condensed spot almost equivalent to the diffraction limit is obtained, and no deterioration of the condensing property due to the subpeak is observed. (3) Subpeak width: 0.8 × λ × (1 + 0.7 (1
/ NA-1))-A result was obtained in which a sub-peak appeared in the condensed spot and the condensing property was deteriorated.

That is, in order to prevent deterioration of the light condensing characteristic due to the sub-peak, the width of the sub-peak is set to 0.8 × λ
X (1 + 0.5 (1 / NA-1)). Further, when the waveguide is designed so that the width of the sub-peak is 0.8 × λ or less, a condensed spot smaller than the diffraction limit of the condensing lens is obtained, which is very effective. Also, the diffraction limit of the lens used (0.8 × λ / N
If the width of the converging spot is larger than A), the converging spot has substantially the same shape as the waveguide mode, becomes a two-peak converging spot having a subpeak, and a single-peak converging spot is obtained. Absent.

The size of the sub-peak of the guided light depends on the refractive index and the depth of the second proton exchange region 6, and as the depth of the second proton exchange region 6 increases, the width of the sub-peak also increases. Further, when the surface refractive index of the second proton exchange region 6 decreases, the difference in the refractive index from the first proton exchange region 5 decreases, and the width of the subpeak increases. In the case where the second proton exchange region 6 is made to have a step-like graded shape by the annealing process, unless the increase in the width of the subpeak is limited, the light-collecting characteristic of the guided light deteriorates as described above. In order not to deteriorate the light collection characteristics,
It is preferable that the surface refractive index of the proton exchange region 6 is not reduced by the annealing treatment. Furthermore, it was found that it is more preferable to suppress the expansion (increase in depth) of the second proton exchange region 6 due to the second annealing treatment to 1.8 times or less of that before annealing.

In the present embodiment, when the short-wavelength light source converts the wavelength of the fundamental wave P1 into a higher harmonic wave P2, the fundamental wave P1 is converted into a higher-order mode having a subpeak having a width smaller than the diffraction limit. The same effect can be obtained by condensing the guided light having the sub-peak. For example, by providing a periodic grating structure on a waveguide, converting TE00 mode guided light into TE10 mode guided light, and condensing this, a condensed spot below the diffraction limit due to the super-resolution effect is obtained. Obtainable.

In the above description, the sub-peak is one TE
Although ten modes are used, the same effect can be obtained even in a higher-order waveguide mode if the width of the sub-peak is the diffraction limit. For example, in the case of the TE20 mode having sub-peaks on both sides of the main peak, the super-resolution effect is further enhanced, and a smaller spot diameter is obtained, which is effective. In addition, T having a subpeak not only in the depth direction but also in the width direction
E01 mode or TE02 mode, and TE11 mode or TE having subpeaks in both the width and depth directions
22 is also effective because a smaller focused spot is obtained.

Further, in the above description of the present embodiment, the TE mode is dealt with. However, similar effects can be obtained by using the TM mode.

(Fifth Embodiment) In this embodiment, another short-wavelength light generator (short-wavelength light source) 15 using the optical wavelength conversion element of the present invention will be described with reference to FIG.

With the configuration of the optical wavelength conversion device according to the above-described embodiment, a highly efficient and stable optical wavelength conversion device can be realized. Therefore, an attempt was made to produce a short wavelength light source using the present optical wavelength conversion device. As shown in FIG. 13, the short-wavelength light source 15 includes a semiconductor laser 21 having a wavelength of 800 nm, condensing optical systems 24 and 25, and a light wavelength conversion element 22. These semiconductor lasers 21, condensing optical systems 24 and 2
5 and the light wavelength conversion element 22 are mounted on a support member 20 having an appropriate shape.

In this configuration, the light P1 emitted from the semiconductor laser 21 is condensed on the end face of the waveguide 2 of the light wavelength conversion element 22 via the condensing optical systems 24 and 25 to excite the waveguide mode. I do. Then, the wavelength-converted second harmonic light (S) is input from the other end face of the waveguide 2 of the optical wavelength conversion element 22.
HG light) P2 is emitted.

Since the optical wavelength conversion element 22 having high wavelength conversion efficiency is realized by the present invention, 20 mW blue SHG light P2 of 20 mW can be obtained by using the semiconductor laser 21 having an output of about 100 mW in the configuration of FIG. . Further, the used optical wavelength conversion element 22 was excellent in light damage resistance and obtained a stable output, so that the output fluctuation could be suppressed to 2% or less, and a stable output was obtained. This short wavelength light source 15
The wavelength in the 400 nm band obtained by the method is desired in a wide range of application fields such as printing plate making, bioengineering, special measurement fields such as fluorescence spectroscopy, and optical disc fields. The short wavelength light source of the present embodiment using the optical wavelength conversion element of the present invention satisfies the requirements in these application fields from the viewpoint of both output characteristics and stability.

In the configuration shown in FIG. 13, the light P1 from the semiconductor laser 21 is coupled to the optical waveguide 2 of the wavelength conversion element 22 using the focusing optical systems 24 and 25. (Wavelength conversion element 22). For example, if an optical waveguide that propagates the TE mode is used, the waveguide mode of the semiconductor laser and the electric field distribution of the guided light in the optical waveguide can be equalized, so that high-efficiency coupling can be achieved without a condenser lens. Is achieved. In experiments, it was confirmed that direct coupling was possible with a coupling efficiency of 80%, and that coupling characteristics almost equivalent to coupling via a lens were obtained. With direct joins,
A small and inexpensive short wavelength light source can be realized and is promising.

(Sixth Embodiment) In the present embodiment, an optical information processing apparatus according to the present invention will be described with reference to FIG.

In the configuration of the optical information processing apparatus 500 shown in FIG. 14, the semiconductor laser 52 described in the fifth embodiment is used.
1 and an output of 10 mW from a short wavelength light generator (short wavelength light source) 520 having a configuration including the optical wavelength conversion element 522.
Is collimated by the collimator lens 540, and then passes through the beam splitter 541.
The optical disk 5 which is an information reproducing medium by the lens 542
43. The reflected light from the optical disk 543 is
Conversely, the light is collimated by the lens 542, is reflected by the beam splitter 541 in the direction of the detector 545, and is condensed on the detector 545 by the lens 544. In the detector 545, a signal is read from the collected light.

Further, information can be written to the optical disk 543 by intensity-modulating the output of the short-wavelength light generator 520. The output from the short-wavelength light generator 520 is TE10 mode light having a sub-peak,
When this light is condensed, a condensed beam having a small spot diameter smaller than the diffraction limit of the lens 542 is obtained. As a result, a small spot diameter due to the super-resolution effect can be obtained in addition to the light-collecting characteristics of short-wavelength light.
It is possible to improve it twice.

As described above, the optical wavelength conversion device of the present invention
Since it is excellent in light damage resistance and can generate high-output blue light, it can be used not only for reading (reproducing) information from an optical disk but also for writing information to the optical disk. Furthermore, since the element does not deteriorate in characteristics even in a long-time high-temperature test, the temperature characteristics of the optical information processing device can be improved. In addition, by using a semiconductor laser as a light source for a fundamental wave, the overall size is significantly reduced, and the semiconductor laser can be used in a small-sized consumer optical disk reading / recording device.

(Seventh Embodiment) FIG. 15 is a perspective view showing the configuration of an optical wavelength conversion device 100 according to a seventh embodiment. This optical wavelength conversion element 100 is made of MgO: LiNbO.
_{3. In the} vicinity of the surface of the substrate 101 (made of magnesium-doped lithium niobate), for example, four parallel domain-inverted regions 102 are arranged at regular intervals to form a periodic domain-inverted structure. Further, in the vicinity of the surface of the substrate 101, an optical waveguide group 103 composed of, for example, three optical waveguides 103a, 103b, and 103c arranged in a direction crossing (here, orthogonal to) the domain-inverted region 102 is formed. I have. The fundamental wave 104 incident on the optical waveguide group 103 propagates through the optical waveguide group 103 and is converted into a harmonic or a sum frequency 105 in the optical waveguides 103a, 103b, and 103c. Emitted from the part.

This light wavelength conversion element 100 can be manufactured, for example, as follows. First, a comb-shaped electrode is formed on a substrate, and a high-voltage is applied to the comb-shaped electrode to form a periodic domain-inverted structure partially having a domain where the domain is inverted. Then, after forming a stripe-shaped mask pattern for forming an optical waveguide, the substrate is treated in an acidic solution to perform proton exchange to form an optical waveguide.

In the optical wavelength conversion element 100 of the present embodiment thus configured, it is possible to reduce the absorption loss of the optical waveguide and to improve the light damage resistance, and to convert the wavelength from the fundamental wave with high efficiency. , And the light collection characteristics of the converted wavelength light can be improved. First, the improvement of the light damage resistance will be described.

The cause of the optical damage is that the generated harmonic or sum frequency causes a change in the refractive index induced by light. This change in the light-induced refractive index depends on the light absorption of the optical waveguide. On the other hand, when an optical waveguide is formed by proton exchange, annealing can be performed after proton exchange to reduce light absorption.However, since the initial proton exchange causes chemical damage to the substrate surface, light scattering near the surface and light scattering Light absorption is likely to occur. Therefore, it is desirable to reduce the initial proton exchange area. However, when the width of the optical waveguide is narrowed to reduce the proton exchange area, the fundamental wave is easily cut off, and the condition in which the overlap of the fundamental wave and the harmonic or the sum frequency in the optical waveguide is optimal cannot be obtained. . According to the experiments performed by the inventors of the present application, for example, from the fundamental wave having a wavelength of 850 nm to the second wave having a wavelength of 425 nm,
When converting to a harmonic, the optimum optical waveguide width is 4 μm to 6 μm.
The result was that the conversion efficiency was significantly reduced at 3 μm or less. Thus, as in the present embodiment, by using an optical waveguide group including a plurality of optical waveguides, the width of the optical waveguide and the optical waveguide group can be freely designed, and the proton exchange area can be reduced. it can.

Further, chemical damage due to proton exchange is:
It increases in proportion to the proton exchange volume. This is a phenomenon caused by crystal distortion. When the proton exchange volume increases, cracks occur on the crystal surface due to the magnitude of the distortion.
To prevent this, by dividing the optical waveguide into an optical waveguide group including a plurality of optical waveguides, the proton exchange area and volume can be reduced, and chemical damage to the crystal can be significantly reduced.

In this embodiment, an optical waveguide 103b having a width of 1.4 μm is formed at the center of the substrate 101, and 1 μm is formed on both sides thereof.
Optical waveguides 103a and 103c having a width of 1 μm with an interval of m
Was formed. Thereby, the fundamental wave 10 having a wavelength of 850 nm can be obtained.
In No. 4, the cut-off condition was satisfied for each optical waveguide width, and the three optical waveguides 103a, 103b, and 103c could be propagated in a single mode as one optical waveguide. Further, the proton exchange area was significantly reduced, and the propagation loss of the optical waveguide was reduced to about half from 1.5 dB / cm to 0.8 dB / cm. As a result, the light absorption of the optical waveguide is reduced, and the light damage resistance can be greatly increased to about twice.

Next, the improvement of the conversion efficiency from a fundamental wave to a harmonic or a sum frequency will be described. Here, the case where the second harmonic or the sum frequency is generated will be described.

When generating the second harmonic, the wavelength λ
A harmonic having a wavelength λ / 2 is generated with respect to the fundamental wave. When a sum frequency is generated, the wavelength λ1 and the wavelength λ2
A harmonic having a wavelength of λ3 is generated from this light. In any case, the wavelength of the fundamental wave is longer than the wavelength of the converted wavelength light. In these cases, what is required of the optical waveguide group is a characteristic that a fundamental wave propagates in a single mode in a state where a plurality of optical waveguides are combined. For example, in the case of an optical waveguide group including three optical waveguides, the optical waveguides constituting the optical waveguide group may be configured such that one or two optical waveguides do not satisfy the waveguide condition with respect to the fundamental wave. Need to be designed. If the waveguide group does not satisfy the condition of single mode propagation with respect to the fundamental wave, it is difficult to perform stable wavelength conversion because the waveguide condition of the fundamental wave may change during propagation. . That is, as shown in FIG. 16A, the fundamental wave propagating through the optical waveguide group is guided by a single mode electric field distribution using the optical waveguide group 103 as one optical waveguide. On the other hand, the converted harmonic or sum frequency propagates, for example, in a waveguide mode electric field distribution that selectively propagates through the central optical waveguide 103b, as shown in FIG.

Therefore, in the present embodiment, the optical waveguides are designed so that: a fundamental wave propagates in a group of waveguides in a single mode; and the converted wavelength light is designed to propagate through the optical waveguide. A phase matching condition is established between the fundamental wave and the converted wavelength light. The harmonic or sum frequency guided mode can be realized, for example, by controlling the wavelength of the fundamental wave.

By using the optical waveguide configured as described above, it was possible to realize an optical wavelength conversion element capable of wavelength conversion with high efficiency. The first reason is that the overlap between the electric field distribution of the fundamental wave and the converted wavelength light can be greatly increased. Conventionally, it has been difficult to independently control the electric field distribution of the fundamental wave guided mode and the harmonic guided mode electric field distribution.
On the other hand, in the present embodiment, the propagation mode of the fundamental wave is controlled by the waveguide group, and the propagation mode of the converted wavelength light such as the harmonic or the sum frequency is controlled by the optical waveguides constituting the waveguide group. be able to. As a result, it has become possible to increase the overlap between the fundamental wave and the converted wavelength light. The second reason is that the confinement of the converted wavelength light such as a harmonic or a sum frequency is enhanced. In a conventional optical waveguide, a waveguide through which a fundamental wave can propagate has a multi-mode propagation with respect to a harmonic, so that it is difficult to sufficiently confine the harmonic. On the other hand, according to the optical waveguide structure as in the present embodiment, single mode propagation of the optical waveguide can be performed even for the converted wavelength light. As a result, the power density of the harmonics or the sum frequency can be greatly improved. As described above, the conversion efficiency can be significantly improved by increasing the overlap between the electric field distributions of the fundamental wave and the converted wavelength light and improving the confinement of the light of the harmonic or the sum frequency.

As another feature, improvement of the light-collecting characteristics of the converted wavelength light will be described. In the conventional optical waveguide structure, multi-mode propagation is performed for higher harmonics, so that the output harmonic output beam has an aspect ratio of 1: 3 or more. In order to condense such outgoing light, it was necessary to perform beam shaping to make the beam shape close to 1: 1. On the other hand, according to the optical waveguide structure of the present embodiment, the converted wavelength light can be propagated in the single mode, so that the lateral spread of the output beam can be suppressed, and the aspect ratio of the output beam can be reduced. Also 1:
It became possible to improve to 1.5. As a result, the light-collecting characteristics could be significantly improved.

In order to further improve the light collecting characteristics, a structure as shown in FIG. 17 was effective. This structure has a configuration in which the number of optical waveguides constituting the optical waveguide group 103 of the optical wavelength conversion element is made different in the vicinity of the emission section 107 to form one optical waveguide. Since the optical waveguide 103b alone satisfies the cutoff condition with respect to the fundamental wave 104, the fundamental wave 104 is radiated in a radiation mode near the emission unit 107. For this reason, the fundamental wave component is not mixed in the emitted beam, and the problem that the fundamental wave emitted from the emission end returns to the light source and the output of the light source becomes unstable can be solved. Further, by using a single optical waveguide in the vicinity of the emission part 107, it is possible to further improve the aspect ratio of the emission beam to 1: 1.3.

Further, as a result of the inventors of the present invention conducting experiments on the optical waveguide structure of this embodiment, it became clear that the following points need to be noted.

First, it has been found that attention must be paid to the propagation characteristics of each optical waveguide constituting the optical waveguide group. Propagation constants (= (2π / λ) where both optical waveguides are equal)
(× N, λ: wavelength, N: effective refractive index with respect to λ), the phase matching wavelength of the fundamental wave propagating through the optical waveguide group and the harmonic or sum frequency propagating through each optical waveguide coincide. For this reason, it is difficult to selectively generate a harmonic or a sum frequency propagating through a specific optical waveguide, and the conversion efficiency is reduced. It became. In addition, when the propagation constants of the respective optical waveguides are close to each other, a phenomenon was observed in which coupling between the optical waveguides occurred, and a harmonic or a sum frequency propagating in one optical waveguide was transferred to another optical waveguide. In this case, the propagation state of light changes, and there is a problem that the output light is not stable.Therefore, the propagation constant of the optical waveguide for generating a harmonic or a sum frequency is greatly different from that of another optical waveguide. Is desirable. In order to solve this problem, it is necessary to design the waveguide so that the propagation constant of the optical waveguide is different. For example, as in the present embodiment, 3
In the case where the optical waveguides are composed of two optical waveguides, it is desirable to design the optical waveguides at the center and the optical waveguides at both ends so that the propagation constants are different by about 0.1%. For example, by changing the width, the refractive index, and the depth of the optical waveguide, optical waveguides having different propagation constants can be easily formed.

Second, it has been found that it is necessary to provide a sufficient space between the optical waveguides constituting the optical waveguide group. The formation of the optical waveguide is generally performed by a method such as proton exchange or metal diffusion. In any of the methods, the optical waveguide is formed by a method of diffusing protons or metals into the inside of the substrate. For this reason, even if an optical waveguide pattern is formed by the photolithography method, there is a problem that the optical waveguides come into contact with each other as they diffuse inside. When the present inventors examined several values as the spacing of the optical waveguide,
When the thickness is 0.5 μm or less, the adjacent optical waveguides are completely adhered to each other to form a single optical waveguide.

As described above, according to the optical wavelength conversion device of the present embodiment, the resistance to light damage is improved by the reduction of chemical damage on the surface. The loss of the optical waveguide can be reduced. The conversion efficiency is improved by the improvement of the overlap between them. ・ Because the beam shape of the emitted light can be optimized, effects such as improvement of the light condensing characteristics can be obtained.

In order to realize a high conversion efficiency, it is preferable that the structure of the optical waveguide group is symmetric with respect to the center and that the number of the optical waveguides constituting the optical waveguide group is an odd number. The reason why it is preferable to make the optical waveguide centrally symmetric is because the propagation mode of the guided light (converted wavelength light and fundamental wave light) has a left-right symmetric structure. If the propagation mode of the guided light has an asymmetric structure, the beam shape of the emitted light becomes asymmetric, and the light condensing characteristics deteriorate. In addition, since the asymmetry between the fundamental wave and the converted wavelength light is different, the overlap is reduced, and the conversion efficiency is reduced. The reason why the number of optical waveguides is preferably odd is that high conversion efficiency can be obtained when a harmonic or a sum frequency is generated in the central optical waveguide of the optical waveguide group. The highest overlap between the fundamental wave propagating through the group of optical waveguides and the converted wavelength light propagating through the optical waveguide is when a harmonic or sum frequency propagates through the optical waveguide at the center of the group of waveguides. . This is because a portion having a high power density is concentrated on the central optical waveguide together with the fundamental wave and the converted wavelength light. For this reason, the highest conversion efficiency is obtained when an odd number of optical waveguides are formed and the wavelength light converted from the fundamental wave is propagated to the optical waveguide near the center.

Further, by gradually changing the width of the optical waveguide or the interval between the optical waveguides with respect to the light propagation direction, a plurality of optical waveguides having different propagation directions can be obtained. A tapered waveguide as shown in FIG. 18B (a waveguide whose shape narrows or widens in a tapered shape with respect to the propagation direction) or an optical waveguide whose propagation constant has a distribution in the propagation direction. A wave path can be formed. By giving the optical waveguide a distribution in the propagation direction in this way, it is possible to form an optical waveguide in which the phase matching wavelength has a distribution in the propagation direction. Thereby, for example, in the optical wavelength conversion element, it is possible to realize a structure that expands the tolerance of the phase matching wavelength. In an optical wavelength conversion element utilizing the nonlinear optical effect, the tolerance of the phase matching wavelength is narrow, so that it is necessary to precisely control the phase matching conditions. Therefore, it is very useful that the propagation constant of the optical waveguide has a distribution and the tolerance of the phase matching wavelength is expanded to relax the phase matching condition and realize stable output characteristics.

In this embodiment and an eighth embodiment to be described later, as a method of forming an optical waveguide, proton exchange capable of realizing an improvement in light damage resistance is an effective means. In addition, for example, Ti diffusion, Zn diffusion, I diffusion
The present invention is also applicable to optical waveguides formed by metal diffusion such as n diffusion and Sc diffusion. Also in metal diffusion, the problem of substrate surface roughness is serious, and reduction of light damage resistance due to surface roughness is a problem. Since such surface roughness increases in proportion to the diffusion area, it is possible to greatly reduce the surface roughness by reducing the surface area on which metal is diffused, and also to reduce the surface roughness area. it can. For this reason, light absorption, which causes light damage, can be reduced, and the light damage resistance can be greatly improved.

The optical wavelength conversion element of the present invention is applicable to an optical waveguide structure that generates a difference frequency, parametric, and the like. In this case, since the wavelength of the fundamental wave is shorter than that of the generated light, the design of the optical waveguide is the most important problem. When a difference frequency or a parametric is generated, the optical waveguide needs to have a structure capable of transmitting both the fundamental wave and the converted wavelength light. However, an optical waveguide that satisfies this condition works as a multimode with respect to the fundamental wave. For this reason, the fundamental wave excited in the optical waveguide is divided into several propagation modes (multi-modes) and propagates, and the power is dispersed and the conversion efficiency is greatly reduced. Therefore, in the following eighth embodiment, an example in which the present invention is applied to an optical wavelength conversion element that generates parametrics will be described.

(Eighth Embodiment) FIG. 19 is a perspective view showing a configuration of an optical wavelength conversion element 110 according to an eighth embodiment. This light wavelength conversion element 110 is made of MgO: LiNbO.
_{In the} vicinity of the surface of the substrate 101 made of ₃ , a periodic domain-inverted structure (having a period different from that of the seventh embodiment) is formed as in the tenth embodiment. Further, the substrate 10
In the vicinity of the surface of the optical waveguide 1, an optical waveguide group 10 including, for example, three optical waveguides 103 a, 103 b, and 103 c disposed in a direction intersecting (here, orthogonal to) the domain-inverted region 102.
3 are formed. In the vicinity of the incident part, the optical waveguide 103b
Only one optical waveguide 103 is formed in the middle.
a and 103c are formed. This optical waveguide 103b
When the fundamental wave 104 enters the optical waveguide 103, the fundamental wave 104 propagates through the optical waveguide 103b, is converted into a parametric or difference frequency 105a that propagates through the optical waveguide group 103, and is emitted from the emission portion opposite to the incidence portion. This light wavelength conversion element can be manufactured in the same manner as in the seventh embodiment.

Next, the optical wavelength conversion element 110 of this embodiment
Will be described. In this optical wavelength conversion element, the fundamental wave 104 is excited in the optical waveguide 103b,
The wavelength is converted into a parametric or difference frequency 105a propagating through the optical waveguide group 103. At this time, the optical waveguide 103
b so that the fundamental wave excited by b does not propagate to the waveguides 103a and 103c.
a and 103c are designed to have different propagation constants. For example, by changing the width of the optical waveguide, optical waveguides having different propagation constants can be easily formed.
In this embodiment, the optical waveguide 103b having a width of 1.4 μm is formed in the center of the substrate 101, and the optical waveguides 103a and 103c having a width of 1 μm are formed on both sides of the optical waveguide 103 with an interval of 1 μm.
As a result, the fundamental wave is propagated in the single-mode
03b, and the optical waveguide group 103
Is converted into a parametric or difference frequency that propagates through the optical waveguide group 103. Since the phase matching between the fundamental wave and the parametric wave or the fundamental wave and the difference frequency that propagate together in the optical waveguide in the single mode can be performed, the parametric or difference frequency can be generated with high efficiency. Further, also in the present embodiment, similarly to the seventh embodiment, since the proton exchange area can be reduced, it is possible to reduce the surface chemical damage and improve the light damage resistance. In the present embodiment, the reason why the optical waveguides 103a and 103c are not formed in the vicinity of the incident portion is to prevent the light from being excited in the adjacent optical waveguide when the fundamental wave is excited in the optical waveguide 103b. .

Furthermore, by using an optical waveguide group composed of a plurality of optical waveguides as an entrance and an exit of the optical waveguide, it is possible to use the optical waveguide for improving the coupling efficiency of the optical waveguide and shaping the exit beam. In this case, by forming an optical waveguide group in the vicinity of the entrance or the exit, and connecting the optical waveguide to a single optical waveguide, the beam shape of the entrance or the exit can be controlled. Since the coupling efficiency between a single optical waveguide and a group of optical waveguides can be increased by designing the optical waveguide, it can be used as an optical waveguide structure capable of performing beam shaping with low loss.

For example, in the case where an optical waveguide group is used for the incident part, an incident taper structure that expands the propagation mode of the incident part can be obtained by dividing the optical waveguide in the propagation direction. In this case, the incident part is divided into a plurality of optical waveguides by forming an optical waveguide group including a plurality of optical waveguides near the incident part of the linear optical waveguide. As a result, the area where proton exchange is performed is reduced, so that the amount of change in the refractive index is reduced, and the mode distribution is broadened and increased as the optical waveguide approaches the cutoff condition. By utilizing this, the propagation mode of the incident portion can be expanded, and the incident taper can be formed. In addition, when the optical waveguide group is used for the emission unit, the beam aspect ratio of the optical waveguide is improved as described above,
The aspect ratio of the guided mode is improved, and the output beam can be shaped.

Further, the present inventors have found new crystal characteristics of a proton exchange waveguide for the purpose of improving the light damage resistance. Therefore, in the following ninth embodiment, another example of the present invention in which the light damage resistance is improved will be described.

(Ninth Embodiment) The cause of optical damage when an optical wavelength conversion element using LiNbO ₃ is used is as follows.
Defects in the crystal. That is, electric charges are moved by an electric field excited by light, and the electric charges are trapped in crystal defects to maintain an internal electric field, which induces a change in the refractive index through the electro-optic effect and causes optical damage. To cause it to occur. LiNbO ₃ is crystal-grown by the Czochralski method and has a congruent composition with stable growth conditions. The congruent composition means that the mol concentration ratio of Li is 48 mol% and the mol of Nb is mol
This is a state in which the concentration ratio is about 52 mol%, and the crystal composition ratio slightly deviates from the perfect crystal.

In the proton exchange layer, the molar ratio of Li in the proton exchange layer is determined by the concentration of H ₂ O, Li ₂ O, and Nb ₂ O _{5 in the} crystal.
Is represented by the following formula for each mole number of Li molar concentration ratio (%) = Li ₂ O / (Li ₂ O + H ₂ O)
+ Nb ₂ O ₅ ) * 100 (%).

When a metal additive is added in an amount of Zmol to 1 mol of LiNbO ₃ , the molar concentration of the metal element becomes Zmol * 100 (%).

The term “mol concentration ratio” used herein means LiNbO
₃ represents the ratio of the molar concentration of Li and Nb present in the crystal, and the value obtained by adding the molar concentration ratio of Li and the molar concentration of Nb in the crystal is 100 mol%. In the case of a perfect crystal, since Li and Nb exist in the same mole number,
Each mol concentration ratio is 50 mol%. For this reason, many defects exist in the crystal structure of the congruent composition. Although several theories have been proposed for the crystal defect model of LiNbO ₃ , a Li defect model is a particularly strongly supported model. This crystal model is a model in which Nb deviating from the stoichiometric composition exists at the Li site in the crystal, and further, vacancy defects at the Li site exist.

Conventionally, there has been proposed a method in which a specific metal element such as Mg, Zn, Sc, In or the like is added to LiNbO ₃ crystal to fill crystal defects with the added metal, thereby improving the light damage resistance. . When these additional metals are added to the crystal, the vacancies existing at the Li site are first filled with the additional metal, and the Nb existing at the Li site
Is replaced with an additional metal, thereby eliminating crystal defects (vacancies) and improving the light damage resistance. The light damage resistance sharply increases from the point in time when the amount of the metal element added exceeds the amount that eliminates the vacancy of the Li site. This value is 5 for LiNbO _{3 having} a congruent composition.
This corresponds to an amount of Mg or Zn added of about mol%.

The present inventors have added such a metal element.
LiNbO added_ThreeProton exchange was performed on the crystal
In some cases, it has been found that crystal defects increase. Sand
And LiNbO_ThreeReduces crystal defects by adding metal
However, proton exchange exchanges Li in the crystal with protons.
Method greatly increases the number of defects at the Li site.
Let This is because the exchange amount of Li by proton exchange is
0%, protons are exchanged at the correct position on the Li site.
Conversion rate is low, and LiNbO _ThreeIs 3
While having a tetragonal crystal structure, HNbO_ThreeIs cubic
Crystal structure of the proton exchange layer
It is due to two points. Therefore, the proton exchange layer
In order to improve the light resistance of the
Need to calm protons to the correct Li site
is there. The present inventors performed various experiments and performed annealing treatment.
That these problems can be solved
Proton concentration after Neal is less than a specified amount (for example,
%) Significantly increases the light damage resistance.
I found that. When the annealing process is performed, the proton
Thermal diffusion lowers proton concentration, resulting in significant crystal distortion
To be reduced. In addition, proton treatment
Can be left at the Li position where it should exist.

However, it was found that when the proton concentration in the crystal was high, crystal distortion remained, and as a result, defects at the Li site were not sufficiently removed. According to the experiments performed by the inventors of the present application, in the case of a proton exchange layer that is not subjected to the annealing treatment, the electro-optic constant is greatly reduced, so that the refractive index does not change due to the electro-optic effect. However, when the proton concentration is lowered by the annealing treatment, the optical damage becomes remarkable by the annealing, and the optical damage is reduced by continuing the annealing. In particular, in a proton exchange waveguide used for an optical wavelength conversion element, annealing is indispensable in order to enhance the nonlinear optical effect, so that optical damage is remarkable. When an optical waveguide formed by proton exchange is applied to a nonlinear optical element, it is necessary to ensure that the optical damage resistance is sufficiently high when used as an optical waveguide. Then, the inventors of the present application examined characteristics of a proton exchange optical waveguide having excellent light damage resistance as follows.

First, the relationship between the proton exchange concentration in the proton exchange optical waveguide and the light damage resistance was examined. M to LiNbO ₃ of congruent composition
Using a substrate doped with 5 mol% of g, the proton exchange time and the annealing time were controlled, and the relationship between the proton exchange concentration in the optical waveguide and the light damage resistance was measured. FIG. 20 shows the result. Immediately after the proton exchange, m
Although the ol concentration ratio has dropped to around 20 mol%, the light damage resistance at this time is very high. However, if no annealing treatment is performed, the nonlinear constant is reduced.
Conversion efficiency decreases. Then, when the Li concentration increases due to the decrease in the proton concentration due to the annealing treatment, the light damage resistance gradually decreases. Further, an annealing process is performed to
When the i concentration is 30 mol% or more, the light damage resistance increases again. This is considered to be a result of a decrease in crystal defects due to a decrease in proton concentration. Therefore, it was found that it is necessary to reduce the proton concentration in order to increase the light damage resistance, and it is preferable to restore the mol concentration ratio of Li to 40 mol% or more. Further, when the molar concentration ratio of Li is 43 mol% or more, the light damage resistance becomes 90% or more of the crystal before proton exchange, and therefore, it is more preferable.

As described above, by annealing the proton exchange layer to sufficiently reduce the concentration of protons,
It was found that the light damage resistance could be restored to almost the same level as the crystal before proton exchange. However, the annealed proton exchange layer has a small change in the refractive index due to the low proton concentration, and the problem remains that it is difficult to increase the confinement of the optical waveguide. In order to solve this problem, a structure that can reduce the defect density in the crystal is required even when the amount of protons in the proton exchange layer is large. The present inventors consider from the defect model described above, in order to completely eliminate the crystal defects in the proton exchange layer, Li or an amount of additive metal capable of sufficiently filling crystal defects increased by proton exchange, By pre-existing in the crystal forming the proton exchange layer,
We thought that this problem could be solved. Conventionally, the amount of the added metal is considered to be optimally about 5 mol in the case of, for example, Mg.
It has been reported that the absorption resistance increases at the same time as the crystal defects increase, so that the light damage resistance decreases. However,
This is the amount relative to the LiNbO ₃ crystal itself,
It is not for a proton exchange optical waveguide formed in iNbO ₃ . According to experiments performed by the present inventors, in order to improve the light damage resistance of the proton exchange optical waveguide, it is necessary to use LiN.
It has been clarified that it is necessary to add an additive at a concentration higher than the optimum value in the bO ₃ crystal to form a structure free from crystal defects when the proton exchange layer is formed.

In the experiment, crystals having different Li concentrations and added metal concentrations were pulled up, and the light damage resistance of the layer subjected to proton exchange was measured. As a result, the light damage resistance of the proton exchange layer is expressed as mol of Li in the proton exchange layer.
It was found that the value strongly depends on the value Y + Z obtained by adding the concentration ratio Y and the molar concentration Z of the added metal. The molar concentration ratio of Li is the ratio of Li and Nb in the crystal. In the case of an ideal crystal, the molar concentration ratio of Li and Nb is 50 m
ol%. In a normal crystal (congruent composition), the mol concentration ratio of Li is 48 mol% and the mol of Nb is mol.
The concentration ratio is about 52 mol%. The mol concentration of the added metal is a percentage of the mol of the added metal contained in 1 mol of LiNbO ₃ crystal.

FIG. 21 is a graph showing the relationship between the value of Y + Z and the light damage resistance. When the value of Y + Z is 20 mol% or less, the electro-optic effect of the crystal is almost eliminated, so that the light damage resistance greatly increases. Y + Z is 34 mol% or more
At 40 mol%, the light damage resistance decreases, and 45 mol%
Showed almost the same light damage resistance as LiNbO ₃ doped with 5 mol% of Mg. Although the light damage resistance slightly differs depending on the Li concentration and the added metal concentration, the value of Y + Z is 48 m.
ol% or more of the crystals, all have the same light damage resistance as a LiNbO ₃ substrate doped with 5 mol% of Mg,
It was confirmed that it had strong resistance. Therefore, it is more preferable that the value of Y + Z in the proton exchange layer is 48 mol% or more. This is presumably because crystal defects caused by proton exchange are supplemented by the added metal and Li. Since Li in the crystal is replaced by protons by proton exchange, defects increase at Li sites in the crystal. By heat-treating this, the exchanged protons move to the optimal position in the crystal, but cannot completely fill the Li crystal defects. Therefore, in a state where the proton exchange layer is heat-treated and protons are moved to the optimal position, Li and an additional metal are present in the proton-exchanged crystal portion in advance in an amount sufficient to prevent the occurrence of crystal defects. In addition, crystal defects of the proton exchange layer can be reduced as much as possible. This value is Li mol
The sum of the concentration ratio Y and the molar concentration Z of the added metal is 45 mol%.
It is a value that is 〜50 mol% or more.

By the way, it is difficult to produce a target crystal from a commercially available crystal having a congruent composition doped with 5 mol% of MgO. The reason is as follows. In commercially available crystals, the value obtained by adding the mol concentration ratio of Li and the mol concentration of Mg is about 53 mol%, and this value is reduced to about 20 mol% when proton exchange is performed. Even if this is annealed, the concentration of the proton exchange layer is 4% because the peripheral concentration is as low as 53 mol%.
It was difficult to exceed 8 mol%. This is because the sum of the molar concentration ratio of Li and the molar concentration of the added metal is 48 mol.
%, The refractive index difference between the optical waveguide and the substrate is reduced, so that it is difficult to form an optical waveguide having a small cross-sectional area and strong confinement.

In order to solve this, the following method can be considered. The first is a method for improving the molar concentration of the added metal. For example, a congruent crystal (Li
In the case where 7 mol of Mg is doped to a concentration ratio of 48 mol%, the sum of the mol concentration ratio of Li and the mol concentration of Mg is 55.
mol%. When the sum of the mol concentration ratio of Li and the mol concentration of Mg in the proton exchange layer was set to 48 mol% by proton exchange and annealing, an optical waveguide excellent in light resistance damage could be formed. A second method is to increase the Li concentration of the crystal. For example, LiN which is a perfect crystal
This is a crystal obtained by adding 5 mol% of Mg to bO ₃ (the mol concentration ratio of Li is 50 mol%). The sum of the mol concentration ratio of Li and the mol concentration of the added metal is 55 mol%. By performing an annealing process after performing the proton exchange, the sum of the molar concentration ratio of Li and the molar concentration of the added metal can exceed 50 mol%, and a proton exchange optical waveguide having a strong light damage resistance can be obtained. Forming became possible. In either case, the substrate itself has a large deviation from the optimum composition, so the light damage resistance is lower than the optimum value.However, by taking the optimum structure in the proton exchange layer, the optical waveguide itself has the light damage resistance. Has improved significantly. Therefore, in the optical waveguide subjected to the proton exchange, it was possible to realize a crystal structure having excellent light damage resistance. By further increasing the molar concentration ratio of Li or the molar concentration of the added metal, it was possible to form an optical waveguide having high light damage resistance even when the annealing treatment was small (that is, when the proton concentration was high). In this case, since the change in the refractive index between the optical waveguide and the substrate can be made large, it is possible to form an optical waveguide with further confinement, and to form a highly efficient optical wavelength conversion element. The molar concentration ratio of Li is 50 m
ol% or less, and if it exceeds this, crystal defects increase.

When an optical waveguide having a high Li mole concentration ratio was used, the nonlinear optical constant of the optical waveguide was increased, so that a highly efficient optical wavelength conversion element could be formed. The reason is as follows. The molar concentration ratio of Li in the proton exchange optical waveguide affects the nonlinear optical constant. L
When the mol concentration ratio of i is 45 mol% or more, 40 m
The nonlinear optical constant was about 1.05 times and the conversion efficiency was 1.1 times higher than that of the optical waveguide set to be less than ol%. Furthermore, when the sum of the molar concentration of the metal additive and the molar concentration of Li becomes 50 mol% or more due to the addition of the metal,
The nonlinear optical constant is about 1.1 times, and the conversion efficiency is 1.
Improved by a factor of two. Controlling the Li concentration ratio in the proton exchange waveguide is very effective in increasing the efficiency.

In the present embodiment, LiNbO_Threesubstrate
Has been described, but LiTaO_ThreeOr LiNbO_Three
And LiTaO_ThreeLiNb is a mixture of_xTa_1-xO_Three(0
≦ x ≦ 1), the same effect was obtained. In addition, metal addition
Additives may be Mg, Zn, In, Sc, or a plurality of gold
It was effective to add the genus elements at the same time. LiTa
O _ThreeAnd LiNbO_ThreeAnd LiTaO_ThreeIs a mixture of
LiNb_xTa_1-xO_Three(0 ≦ x ≦ 1) is LiNbO_ThreeWhen
Has similar congruent composition and light damage resistance properties,
Changes in photodamage characteristics due to proton exchange have similar characteristics.
I do. In other words, the light damage resistance is increased due to the increase in crystal defects.
It is a characteristic that it decreases. Therefore, the proton exchange layer
Li or metal addition to prevent Li defects
By adding a large amount of material in advance,
An increase in crystal defects can be prevented.

As a method for increasing the Li concentration in the crystal, there is a method for thermally diffusing Li. LiN
The bO ₃ substrate was placed in a powder containing Li at 1000 ° C. to 1 ° C.
When heat-treated at a temperature of about 100 ° C., Li becomes LiNbO
₃ diffuse to the surface, it is possible to form a high Li concentrations crystals near the surface. Since the area used for the optical waveguide is several μm in the vicinity of the substrate surface, the proton exchange layer formed on the substrate obtained by this method exhibited strong light damage resistance.

Further, as a method of increasing the amount of metal additive in the crystal, there is a method of thermally diffusing the metal in addition to increasing the amount of addition when pulling the crystal. For example, Z
In the case of n, a ZnO film is deposited on LiNbO ₃ and heat-treated at about 1000 ° C., whereby Zn diffuses into the crystal, and a layer having a high metal additive concentration is formed near the surface of the substrate. . By forming a proton exchange optical waveguide in this layer, an optical waveguide having excellent light damage resistance can be formed.

The same effect can be obtained by adding a single metal element or by simultaneously adding a plurality of the above metal elements.

Although the above-described examination of the Li concentration and the metal addition concentration was performed on a single linear optical waveguide, a plurality of optical waveguides as described in the seventh and eighth embodiments were used. By applying the present invention to an optical waveguide structure using an optical waveguide group composed of waveguides, it was confirmed that the light damage resistance was further improved. That is, by using the optical waveguide group, the proton exchange area can be reduced to reduce chemical damage, and further, by controlling the Li concentration and the metal addition concentration, the light damage resistance of the optical waveguide itself can be improved. Was completed. As a result, an optical wavelength conversion element having low loss and high output characteristics was realized.

(Tenth Embodiment) In the present embodiment, a coherent light generation device including a semiconductor laser and an optical wavelength conversion element and capable of always achieving stable output characteristics will be described. A feature of the coherent light generation device of the present invention is that stable output characteristics can be realized by using the optical wavelength conversion element of the present invention having excellent light damage resistance.

In the conventional light wavelength conversion device, the light damage resistance was not sufficient. Therefore, the wavelength of 425 nm
When about 10 mW of blue light was output, a phenomenon that the phase matching wavelength fluctuated due to optical damage was observed. For this reason, there has been a problem that the output of the coherent light generator fluctuates gradually. In order to stabilize this, it is necessary to control the oscillation wavelength of the semiconductor laser so that the SHG (second harmonic) output always takes a constant value, and an output stabilizing circuit is required. Was. On the other hand, since the optical wavelength conversion element of the present invention has excellent light damage resistance, a coherent light generation device using the same does not require an output stabilizing circuit and can obtain a stable output with a simple configuration. be able to.

FIG. 22 is a perspective view showing the configuration of the coherent light generation device 120 of the present embodiment. This coherent light generation device 120 is different from the coherent light generation device 120 in FIG.
The optical wavelength conversion element 121 and the semiconductor laser 1 as shown in FIG.
The light wavelength conversion element 121 has an incident part 123 and an output part 124. The semiconductor laser 122 is, for example, a DBR having a DBR grating structure integrated on an LD.
A semiconductor laser having a function of changing the emission wavelength, such as a semiconductor laser or a semiconductor laser whose wavelength is changed by a temperature or an external grating.By adjusting the emission wavelength to a wavelength that satisfies the phase matching condition of the optical wavelength conversion element, The output of the coherent light generator is stabilized.

The coherent light generator 120 has the following characteristics in addition to the improvement of the light damage resistance of the optical wavelength conversion element. Optical wavelength conversion element 12
As 1, an optical waveguide group having three optical waveguides and converting a fundamental wave having a wavelength of 850 nm into a second harmonic having a wavelength of 425 nm is used. In the optical wavelength conversion element 121, the fundamental wave propagates through the optical waveguide group, and the second harmonic (SHG) propagates through the central optical waveguide. Since the SHG propagates through the central optical waveguide, the aspect ratio of the output beam was about 1: 1.5. Therefore, when the emitted light is condensed by the condensing optical system, 90% or more of the light can be captured and condensed, and the light use efficiency has been greatly improved. In the conventional structure, the aspect ratio of the emitted light is 1:
Since there were three or more, a prism optical system or the like was required to improve the aspect ratio, and the light use efficiency was reduced.

Further, in the wavelength tunable coherent light generator utilizing parametric oscillation, since the waveguide type device is small, the semiconductor laser and the wavelength conversion element are directly coupled as shown in FIG. Miniaturization is possible. For example, parametric oscillation is possible by using an optical wavelength conversion element having the same periodic domain-inverted structure and a laser light source as in the eighth embodiment. In the parametric oscillation, when a fundamental wave λ3 is input, a signal light λ2 and an idler λ1 satisfying a relationship of light wavelength 1 / λ3 = 1 / λ1 + 1 / λ2 are generated, and each generated light guides one group of optical waveguides. Single mode propagation as a wave path. Therefore, light having a wavelength satisfying the above conditions can be output from the fundamental wave of λ3 while changing the wavelength, and a coherent light generation device with variable wavelength can be realized. By using the optical wavelength conversion element of the present invention capable of wavelength conversion with high efficiency in this coherent light generation device, it is possible to output harmonics on the order of mW even by wavelength conversion of a semiconductor laser, and to obtain a wavelength integrated with the semiconductor laser. A variable coherent light generator can be realized. By adjusting the temperature of the wavelength conversion element using, for example, a heater, a Peltier element, or the like, the phase matching wavelength was modulated, and the output wavelength could be changed. In addition, in the case of the generation of the sum frequency and the generation of the difference frequency, two lights are multiplexed outside and are incident on the optical waveguide.

(Eleventh Embodiment) In the above-described embodiment, an optical wavelength conversion element capable of high efficiency and stable output can be realized. Thus, in the present embodiment, an attempt was made to produce a short wavelength light source using the optical wavelength conversion element of the present invention. Using a semiconductor laser having a wavelength of 850 nm, a condensing optical system, and an optical wavelength conversion element as shown in FIG. 15 in the seventh embodiment, the light emitted from the semiconductor laser is condensed by the condensing optical system. Light is condensed on the end face of the waveguide to excite the waveguide mode. Then, the wavelength-converted SHG light is emitted from the other waveguide end face of the optical wavelength conversion element. Here, since an optical wavelength conversion element having high conversion efficiency is used,
Blue SHG light of 10 mW was obtained using a semiconductor laser with an output of about 100 mW. The wavelength in the 400 nm band is desired in a wide range of application fields such as printing plate making, special measurement such as bio and fluorescence spectroscopy, and optical disks. The short-wavelength light source using the optical wavelength conversion element according to the present embodiment can be put to practical use in these application fields in both output characteristics and stability.

In this embodiment, the light of the semiconductor laser is coupled to the optical waveguide by using the condensing optical system. However, the semiconductor laser and the optical waveguide can be directly coupled. T
E-mode propagation (Transverse Electric
When the optical waveguide of c) is used, the waveguide mode of the semiconductor laser can be made equal to the electric field distribution, so that coupling can be performed with high efficiency without a condenser lens. According to experiments performed by the inventors of the present application, it was confirmed that direct coupling was possible with a coupling efficiency of 80%, and coupling characteristics almost equivalent to lens coupling were obtained. The use of direct coupling is promising because a small and inexpensive light source can be realized.

(Twelfth Embodiment) In the present embodiment, an optical system 140 of the present invention will be described. FIG. 23 is a cross-sectional view illustrating a configuration of an optical system 140 according to the present embodiment. This optical system includes the coherent light generator 145 according to the tenth embodiment. A beam having an output of 10 mW emitted from the optical system is condensed by a lens 146 and transmitted through a beam splitter 147 and a λ / 4 plate 157. The light is irradiated on the optical disc 150 as the information reproducing medium by the objective lens 149. The reflected light passes through the objective lens 14
9, reflected by the beam splitter 147, and read by the photodetector 148. Further, by modulating the intensity of the output of the coherent light generator, information can be written on the optical disk 150.

In this optical system, the tolerance of the phase matching condition of the optical wavelength conversion element is expanded, so that the output can be stabilized, and the output can be reduced to 5% or less against external temperature change. It was possible to suppress the fluctuation.

Further, since high-output blue light can be generated, it is possible to not only read information on an optical disk but also write information on the optical disk. Also,
By using a semiconductor laser as a fundamental light source, the size can be reduced, so that it can also be used for small optical disc readers and recording devices for consumer use.

By the way, in order to write information on the optical disk, it is necessary to modulate the output of the coherent light generator. In the optical system of the present embodiment, the output from the coherent light generator can be modulated by modulating the output intensity of the semiconductor laser.

Furthermore, by optimizing the width of the optical waveguide of the optical wavelength conversion element, the aspect ratio of the output beam can be optimized. For example, in a group of optical waveguides composed of three optical waveguides, a structure is adopted in which harmonics are propagated to the central optical waveguide. The aspect ratio of the output beam can be made close to 1: 1 by employing the optical waveguide structure that is cut as described above. This eliminates the need for a beam shaping prism or the like for improving the light-collecting characteristics of the optical pickup, thereby achieving high transmission efficiency, excellent light-collecting characteristics, and low cost. Further, noise of scattered light generated during beam shaping can be reduced, and simplification of the optical pickup can be realized.

[0302]

As described above, according to the present invention, in the optical waveguide structure having the high refractive index layer, the first proton exchange region (first ion exchange region) is annealed to perform a graded process. After the shape, a second proton exchange region (second ion exchange region) is formed, and by forming this into a step-like graded shape, the light damage resistance is greatly improved, the wavelength conversion efficiency is improved, This makes it possible to greatly extend the life at high temperatures and to improve the characteristics by reducing the waveguide propagation loss, and the practical effect is great.

Further, according to the short wavelength light generating device of the present invention, the wavelength of the light of the semiconductor laser is converted in the optical waveguide having the high refractive index layer having a step-like graded refractive index distribution. A short-wavelength light source with high output and high reliability is realized, and its practical effect is great.

Further, as an optical waveguide manufacturing method, an annealing process for forming a high refractive index cladding layer is performed.
By performing the annealing process on the first proton exchange region (first ion exchange region) and the annealing process on the second proton exchange region (second ion exchange region) at different temperatures, the second proton exchange region is obtained. The graded refractive index distribution required for the (second ion exchange region) can be uniformly and accurately formed, and the characteristics of the optical wavelength conversion element can be greatly improved. large.

According to the present invention, by using the optical waveguide structure composed of the optical waveguide group, the loss of the optical waveguide can be reduced, and the light damage resistance can be greatly improved.
Further, by optimizing the structure of the optical waveguide group, it is possible to improve the conversion efficiency and the light-collecting characteristics, and the practical effect is extremely large.

Further, according to the present invention, in an optical wavelength conversion device using a proton exchange optical waveguide, L
By controlling the i-concentration, the light damage resistance can be greatly improved, and its practical effect is very large.

Further, according to the present invention, a coherent light generating device having high output and stable output characteristics can be realized by using the optical wavelength conversion element of the present invention having excellent light damage resistance. , Its practical effect is very large.

Further, according to the present invention, an optical system with less noise can be realized by using the coherent light generating device of the present invention with high output and small output fluctuation, and its practical effect is great.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a configuration of a conventional optical wavelength conversion element.

2A is a diagram schematically showing a configuration of a conventional optical waveguide element. FIG. 2B is a diagram showing a refractive index distribution in a depth direction in the configuration of FIG. Diagram showing electric field distribution in the depth direction of propagating fundamental wave and harmonic wave

FIG. 3A is a sectional view showing a configuration of a proton exchange waveguide (optical waveguide) according to the first embodiment of the present invention. FIG. 3B shows a refractive index distribution in a depth direction in the configuration of FIG. Figure

FIG. 4 is a diagram showing the annealing temperature dependence of the diffusion constant of protons in the annealing process.

5 (a) to 5 (c) are views showing a state of a change in a refractive index distribution shape accompanying a change in a proton concentration distribution due to an annealing treatment. Specifically, FIG. 5 (a) is a step shape, and FIG. Step-like graded shape, (c)
Is graded shape

FIG. 6 is a diagram showing a change in a refractive index distribution due to an annealing process.

FIG. 7 (a) Distribution of proton exchange rate in a waveguide composed of first and second proton exchange layers (b) Enlarged view of distribution of first proton exchange layer in a distribution diagram of proton exchange rate (c) Enlarged view of the distribution of the second proton exchange layer in the distribution diagram of the proton exchange rate

FIGS. 8A to 8E are schematic cross-sectional views for explaining each step of a method for manufacturing an optical waveguide according to the present invention. FIGS. 8B to 8E show depth directions in shapes obtained by the respective steps. Refractive index profile

9A is a perspective view of an optical wavelength conversion element according to the present invention, and FIG. 9B is a cross-sectional view of an optical waveguide included therein and a diagram showing a refractive index distribution in a depth direction.

FIG. 10 is a diagram showing the relationship between the wavelength conversion efficiency and the surface refractive index of the light wavelength conversion element, and the depth of the second proton exchange layer.

FIG. 11 is a diagram showing the relationship between the light damage resistance of the optical wavelength conversion element and the depth of the second proton exchange layer.

FIG. 12 is a diagram showing a configuration of a short wavelength light generation device according to a fourth embodiment.

FIG. 13 is a diagram showing a configuration of a short wavelength light generation device according to a fifth embodiment.

FIG. 14 is a diagram showing a configuration of an optical information processing apparatus according to the present invention.

FIG. 15 is a perspective view illustrating a configuration of an optical wavelength conversion element according to a seventh embodiment.

FIG. 16 is a cross-sectional view of an optical waveguide and a characteristic factor diagram showing an electric field distribution of light propagating through the optical waveguide in the optical wavelength conversion element according to the seventh embodiment. (A) Fundamental wave (b) Converted wavelength light

FIG. 17 is a perspective view showing another configuration of the optical wavelength conversion element according to the seventh embodiment.

FIG. 18A is a plan view showing another optical waveguide configuration in the optical wavelength conversion device according to the seventh embodiment. FIG. 18B is a plan view showing another optical waveguide configuration in the optical wavelength conversion device according to the seventh embodiment.

FIG. 19 is a perspective view showing a configuration of an optical wavelength conversion element according to an eighth embodiment.

FIG. 20 is a characteristic factor diagram showing the relationship between the light damage resistance and the Limol concentration ratio in the proton exchange layer in the optical wavelength conversion device according to the ninth embodiment.

FIG. 21 shows the light damage resistance of the optical wavelength conversion device according to the ninth embodiment and the Limol concentration ratio Y in the proton exchange layer.
And characteristic factor diagram showing the relationship between the additive metal mole concentration Z and the sum Y + Z

FIG. 22 is a perspective view showing a configuration of a coherent light generation device according to a tenth embodiment.

FIG. 23 is a cross-sectional view illustrating a configuration of an optical system according to a twelfth embodiment.

24A is a perspective view showing an example of the configuration of a conventional optical wavelength conversion element. FIG. 24B is a schematic view showing how a fundamental wave P1 incident on the wavelength conversion element shown in FIG. Figure shown

FIG. 25 (a) is a perspective view showing the configuration of another conventional optical wavelength conversion element. FIG. 25 (b) schematically shows how the fundamental wave P1 incident on the wavelength conversion element shown in FIG. Diagram shown

FIG. 26 is a perspective view showing a configuration of a conventional optical wavelength conversion element.

[Explanation of symbols]

Reference Signs List 1 substrate 2 optical waveguide 4 high refractive index layer 5 first proton exchange region 6 second proton exchange region 7 domain-inverted region (domain-inverted structure) 15 short-wavelength light generator (short-wavelength light source) 20 support member 21 semiconductor laser Reference Signs List 22 light wavelength conversion element 24 light collection optical system 25 light collection optical system 60 light wavelength conversion element 101 substrate 102 domain-inverted region 103 optical waveguide group 103a, 103b, 103c, 103d, 103e,
103f Optical waveguide 104, 152 Fundamental wave 105 Harmonic or sum frequency 105a Parametric or difference frequency 106, 123 Incident part 107, 124 Emitting part 121 Optical wavelength conversion element 122 Semiconductor laser 145 Coherent light generator 146 Lens 147 Polarizing beam splitter 148 Light Detector 149 Objective lens 150 Optical disk 151 Optical waveguide 153 Harmonic wave 300 Conventional optical wavelength conversion element 301 Substrate 304 Polarized inversion region (polarized inversion structure) 305 Optical waveguide 306 Fundamental wave 307 Second harmonic 310 High refractive index layer 400 Short wavelength Light generating device 420 Si mount 402 Optical waveguide 403 Second proton exchange region 421 Semiconductor laser 422 Optical wavelength conversion device 500 Optical information processing device 520 Short wavelength light generating device 521 Semiconductor laser 522 Light wavelength conversion element 540 Collimator lens 541 Beam splitter 542 Lens 543 Optical disk 544 Lens 545 Detector 600 Conventional light wavelength conversion element 640 Conventional light wavelength conversion element

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasuo Kitaoka 1006 Kazuma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. Terms (reference) 2H047 KA04 KA13 LA00 LA02 PA12 PA13 QA03 2K002 AA01 AA04 AB12 CA03 DA06 EA04 FA24 FA28 HA20 5D119 AA01 AA43 HA64 JA29 JA36 NA05

Claims (48)

[Claims] 1. A nonlinear optical crystal, a first ion exchange region formed near a surface of the nonlinear optical crystal, and a second ion exchange region formed near a surface of the first ion exchange region.
The optical waveguide, wherein the second ion exchange region has a region where the ion exchange rate changes in the depth direction is 0.02 to 0.2 μm. 2. The method according to claim 1, wherein the second ion-exchange region has a change in the ion-exchange rate of 5 to 50 μm ^{−1 in} a range of 0.02 to 0.2.
The optical waveguide according to claim 1, wherein the optical waveguide has a length of μm. 3. The optical waveguide according to claim 1, wherein the amount of change in the ion exchange rate of the first ion exchange region is 0.06 μm ⁻¹ or less. 4. The second ion-exchange region has a region where the ion-exchange rate changes in the depth direction, from 0.03 to 0.1.
The optical waveguide according to claim 1, wherein the optical waveguide is μm. 5. The optical waveguide according to claim 1, wherein the second ion exchange region has an ion exchange rate change of 10 to 30 μm ⁻¹ . 6. The nonlinear optical constant of the first ion exchange region is 90% or more of the nonlinear optical constant of the crystal,
The optical waveguide according to claim 1, wherein a nonlinear optical constant of the second ion exchange region is 60% or less of a nonlinear optical constant of the crystal. 7. The change in surface refractive index Δn of the first ion exchange region is 0.02 or less for light having a wavelength of 633 nm, and the change in surface refractive index Δn of the second ion exchange region is 0.11 or more. The optical waveguide according to claim 1, wherein 8. A nonlinear optical crystal, a first ion exchange region formed near a surface of the nonlinear optical crystal, and a second ion exchange region formed near a surface of the first ion exchange region.
Wherein the second ion exchange region has a higher refractive index than the first ion exchange region, and the second ion exchange region has a step-like gray refractive index distribution. An optical waveguide having a did shape. 9. A nonlinear optical crystal, a first ion exchange region formed near a surface of the nonlinear optical crystal, and a second ion exchange region formed near a surface of the first ion exchange region.
Wherein the second ion exchange region has a higher ion concentration than the first ion exchange region, and the ion concentration distribution of the second ion exchange region is a step-like gray. An optical waveguide having a did shape. 10. The first ion exchange region and the second ion exchange region.
10. The optical waveguide according to claim 8, wherein the ion exchange region has a different graded shape obtained by annealing at different temperatures. 11. 11. The depth of the first ion exchange region is:
The optical waveguide according to any one of claims 8 to 10, wherein the value of the optical waveguide is increased to eight times or more the value before the annealing process is performed by the annealing process. 12. The depth of the second ion exchange region is:
The optical waveguide according to any one of claims 8 to 11, wherein the value of the optical waveguide is increased to at least 1.2 times the value before the annealing process is performed by the annealing process. 13. The depth of the second ion exchange region,
The optical waveguide according to any one of claims 8 to 11, wherein the value of the optical waveguide is increased to twice or more the value before performing the annealing process by the annealing process. 14. The optical waveguide according to claim 8, wherein a surface refractive index of the second ion exchange region is substantially equal to a value of a surface refractive index before performing an annealing process. . 15. The optical waveguide according to claim 9, wherein a surface ion concentration of the second ion exchange region is substantially equal to a value of the surface ion concentration before performing an annealing process. 16. The step-like graded shape of the refractive index distribution of the second ion exchange region has the following relationship (1) and (2): (However, C (k, t): ion exchange concentration, k: depth (μ
m), t: annealing time (hour), C0: initial ion exchange concentration, Erf []: error function, h: initial ion exchange depth (μm), and Dp: ion diffusion constant by annealing treatment ( μm ² / hour)), The optical waveguide according to claim 8, wherein the optical waveguide is formed by an annealing process that satisfies the following conditions. 17. The step-like graded shape of the ion concentration distribution in the second ion-exchange region,
The relationship between the following (1) and (2): (However, C (k, t): ion exchange concentration, k: depth (μ
m), t: annealing time (hour), C0: initial ion exchange concentration, Erf []: error function, h: initial ion exchange depth (μm), and Dp: ion diffusion constant by annealing treatment ( μm ² / hour)), The optical waveguide according to claim 9, wherein the optical waveguide is formed by an annealing process that satisfies the following conditions. 18. The method according to claim 18, wherein the nonlinear optical crystal is LiNb _x Ta.
The optical waveguide according to claim 8, wherein the optical waveguide is a _1-x O ₃ crystal (0 ≦ x ≦ 1). 19. The optical waveguide according to claim 8, wherein each of the ion exchange processes forming the first ion exchange region and the second ion exchange region is a proton exchange process. . 20. LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1)
Is formed by ion exchange on the surface of the nonlinear optical crystal made of, and the molar concentration ratio of Li in the optical waveguide is 40 m.
The optical waveguide according to claim 1, wherein the optical waveguide is at least ol%. 21. LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1)
And formed on the surface of the nonlinear optical crystal to which a metal element is added at least in the vicinity of the surface by ion exchange, wherein the molar concentration ratio Y of Li and the molar concentration Z of the metal element in the optical waveguide are Y + Z ≧ The optical waveguide according to claim 1, wherein the amount is 45 mol%. 22. An optical waveguide according to claim 1, comprising a periodically poled structure, wherein the optical waveguide comprises a fundamental wave having a wavelength λ and a second wave having a wavelength λ / 2. The waveguide of the second harmonic is possible, the refractive index and the depth of the second ion exchange region included in the optical waveguide satisfy the waveguide condition for the second harmonic, and An optical wavelength conversion element that satisfies the cutoff condition for waves. 23. A phase matching between a fundamental wave of a fundamental mode and a second harmonic of a higher-order mode in the optical waveguide.
An optical wavelength conversion element according to claim 22. 24. An optical wavelength converter having an optical waveguide group composed of a plurality of optical waveguides adjacent to the surface of a nonlinear optical crystal, and converting a fundamental wave incident in the optical waveguide group into light of a different wavelength. An element, wherein the fundamental wave propagates in the optical waveguide group in a single mode, and the converted wavelength light propagates in an optical waveguide constituting the optical waveguide group in a waveguide mode, or the converted wavelength light Is an optical wavelength conversion element that propagates in a single mode through the group of optical waveguides and the fundamental wave propagates through the optical waveguides that constitute the group of optical waveguides. 25. The optical wavelength conversion device according to claim 24, wherein the optical waveguide group is composed of a plurality of optical waveguides having different propagation directions. 26. The optical waveguide, wherein one of the fundamental wave and the converted wavelength light satisfies a cutoff condition, and the other wave satisfies a waveguide condition. The optical wavelength conversion element according to claim 24 or 25. 27. The optical waveguide according to claim 24, wherein at least one of the optical waveguides has a different propagation constant from other optical waveguides.
27. The optical wavelength conversion device according to any one of items 26 to 26. 28. The optical waveguide group satisfies a single mode propagation condition with respect to one of the fundamental wave and the converted wavelength light, and the one of the optical waveguides 28. The optical wavelength conversion element according to claim 24, wherein phase matching is performed between the other light guided through one of the light and the other light. 29. The optical wavelength conversion device according to claim 24, wherein the optical waveguide group is composed of an odd number of optical waveguides, and has a structure symmetric with respect to a center optical waveguide. 30. The group of optical waveguides is composed of three optical waveguides whose propagation directions are substantially equal, the central optical waveguide has a different propagation constant from the optical waveguides on both sides, and the optical waveguide group is transmitted in a single mode. 30. The phase matching between any one of the fundamental wave and the converted wavelength light and the other light propagating through the central optical waveguide of the group of optical waveguides. An optical wavelength conversion element according to any one of claims 1 to 7. 31. The optical wavelength conversion element according to claim 24, wherein the number of the optical waveguides in the group of optical waveguides is different in at least one of the vicinity of the incident part and the vicinity of the emission part. 32. LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1)
Having an optical waveguide formed by ion exchange on the surface of a crystal made of, wherein the molar concentration ratio of Li in the optical waveguide is 40 m.
ol% or more. 33. LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1)
And an optical waveguide formed by ion exchange on the surface of the crystal to which the metal element is added at least in the vicinity of the surface, wherein the molar concentration ratio Y of Li and the molar concentration Z of the metal element in the optical waveguide are An optical wavelength conversion element in which Y + Z ≧ 45 mol%. 34. The optical wavelength conversion device according to claim 33, wherein the metal element is one of Mg, Zn, Sc, and In, or a mixture of two or more thereof. 35. A semiconductor laser and a semiconductor laser according to claim 22.
And a light wavelength conversion element according to any one of the above, wherein the wavelength of light emitted from the semiconductor laser is converted into a predetermined harmonic by the light wavelength conversion element. 36. A waveguide mode of the harmonic obtained by the conversion by the optical wavelength conversion element is a higher-order mode, and has a maximum intensity at a plurality of peaks in an intensity distribution of the higher-order mode of the harmonic. 36. The short-wavelength light generation device according to claim 35, wherein a width of one of the sub-peaks other than the main peak is smaller than a diffraction limit for the harmonic. 37. The short-wavelength light generator according to claim 36, wherein the higher-order mode of the harmonic is a first-order mode. 38. The short-wavelength light generator according to claim 36, wherein the higher-order mode of the harmonic is a TE10 mode. 39. A short-wavelength light generator according to claim 35, and a condensing optical system, wherein the short-wavelength light emitted from the short-wavelength light generator is collected. An optical information processing device configured to collect light by an optical optical system. 40. A step of forming a periodically poled structure in the nonlinear optical crystal, a step of forming a first ion exchange region in the poled structure, and a first annealing for the first ion exchange region. Performing a process; forming a second ion exchange region on the surface of the first ion exchange region; and performing a second annealing process on the second ion exchange region. A method for manufacturing a light wavelength conversion element, wherein a first annealing temperature for performing the first annealing process is different from a second annealing temperature for performing the second annealing process. 41. The method according to claim 40, wherein the first annealing temperature is 300 ° C. or higher and the second annealing temperature is 250 ° C. or lower. 42. The step of forming the domain-inverted structure,
42. The method for manufacturing an optical wavelength conversion device according to claim 40, further comprising a step of heat-treating the formed domain-inverted structure at a temperature of 400 ° C or higher. 43. The second annealing temperature is from 130 ° C.
43. The method according to claim 40, wherein the temperature is in a range of 200C. 44. The second annealing treatment includes the following relationship (1) and (2): (However, C (k, t): ion exchange concentration, k: depth (μ
m), t: annealing time (hour), C0: initial ion exchange concentration, Erf []: error function, h: initial ion exchange depth (μm), and Dp: ion diffusion constant by annealing treatment ( μm ² / hour)), 44. The light according to claim 40, wherein the annealing process satisfies the following condition, and the refractive index distribution of the second ion exchange region formed by the annealing process has a step-like graded shape. A method for manufacturing a wavelength conversion element. 45. The non-linear optical crystal is made of LiNb _x Ta.
_{45. A 1-x} O ₃ crystal (0 ≦ x ≦ 1).
The method for manufacturing an optical wavelength conversion element according to any one of the above. 46. The light wavelength according to claim 40, wherein each of the ion exchange processes forming the first ion exchange region and the second ion exchange region is a proton exchange process. A method for manufacturing a conversion element. 47. A coherent light generation device comprising: the optical wavelength conversion element according to claim 24; and a semiconductor laser, wherein light of the semiconductor laser is wavelength-converted by the optical wavelength conversion element. 48. An optical system comprising: the coherent light generating device according to claim 47; and a condensing optical system, wherein the light emitted from the light wavelength conversion element is condensed by the condensing optical system.