JPH1172810A - Optical waveguide and its production, optical wavelength conversion element using this optical waveguide, short wavelength light emitting device and optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the output of a second harmonic wave having an excellent light condensing characteristic and to increase the overlap of a basic wave on the second harmonic wave by providing an ion exchange layer with a refractive index larger than the refractive index of a waveguide and specific thickness. SOLUTION: The ion exchange layer (high-refractive index layer) 63 formed on the surface of the waveguide 62 on the surface of a crystalline substrate 61 has the refractive index larger than the refractive index of the waveguide 62 and the thickness at which the basic wave 64 propagating in the waveguide 62 cannot propagate the inside of the ion exchange layer 63 but the harmonic wave of the basic wave 64 can propagate the ion exchange layer 63. Namely, the thickness and refractive index of the high-refractive index layer 63 satisfy a cut-off condition for the basic wave 64. The thickness and refractive index of the high-refractive index layer 63 are merely so selected as to satisfy the cut-off condition for the basic wave 64 and a waveguide condition for the SHG wave in order to embody the wavelength conversion element of the high refractive index in the waveguide 62 having such high-refractive index layer 63.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide and a wavelength conversion element using a coherent light source and used in the fields of optical information processing and optical applied measurement. Further, the present invention relates to a light generating device and an optical pickup using such an optical wavelength conversion element.

[0002]

2. Description of the Related Art Optical waveguides are used in a wide range of fields such as communication, optical information processing, and measurement for controlling light waves. Particularly, application of an optical waveguide to an optical wavelength conversion element has been actively studied in order to realize a small-sized short-wavelength light source by wavelength conversion of a semiconductor laser.

For example, Japanese Unexamined Patent Publication No. Hei 5-273624 discloses an optical wavelength conversion element in which a nonlinear deterioration layer is provided near the surface of an optical waveguide to achieve high efficiency and stability, as shown in FIG. Have been. In FIG. 20, 201 is Li
TaO ₃ substrate, 202 is a waveguide layer, 203 is a domain-inverted layer,
204 is a nonlinear deterioration layer. Fundamental wave 205 of TM00 mode incident on waveguide layer 202 of the optical wavelength conversion element
Is converted into a TM10 mode harmonic 206 in the waveguide layer 202. The thickness of the nonlinear deterioration layer 204 is about 0.45 μm, and the thickness of the waveguide layer 202 is 1.8 μm. As shown in FIG. 21, the harmonics of the TM10 mode
It has almost the same peak output as the fundamental wave of the TM00 mode. That is, the overlap between the fundamental wave of the TM00 mode and the harmonic wave of the TM10 mode is increased, and high efficiency is achieved.

[0004] For example, Japanese Patent Laid-Open No. 4-254834
According to the report, a high refractive index layer with a higher refractive index than the waveguide layer
Has been reported. This optical wavelength conversion element
22 is shown in FIG. This light wavelength conversion element is composed of Li
NbO_ThreeBy proton exchange formed on the substrate 301
On the surface of the optical waveguide layer 302, a polarization inversion layer 303 and TiO _Two
Is formed. TiO
_TwoIs the refractive index of the waveguide layer 302 due to proton exchange.
The higher refractive index layer 304, which is larger than the
By forming it on 302, the confinement of the fundamental wave is enhanced.
Thus, the efficiency of the optical wavelength conversion element is improved.

Further, Japanese Patent Application Laid-Open No. 1-238631 discloses an optical wavelength conversion element employing a ridge type optical waveguide structure in order to enhance the confinement of the optical waveguide.

[0006]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. 5-237.
The light wavelength conversion element described in Japanese Patent No. 624 has a problem that the emitted TM10 mode light has a two-peak intensity distribution, and the light-collecting characteristics deteriorate when the light is collected. Further, in this method, since the harmonics are dispersed into the peaks of the higher-order modes in order to increase the light damage resistance, the output harmonic has two peaks of almost equal magnitude. Therefore, there is a problem that a special optical system is required to condense the harmonic output, and the optical system becomes complicated and miniaturization is difficult. Further, in order to converge the light to the diffraction limit, it is necessary to considerably shape the beam, and there is a problem that the utilization efficiency of the output light is reduced to 50% or less. In addition, since the nonlinear deterioration layer does not have a function of enhancing the confinement of the fundamental wave, the power density of the fundamental wave cannot be increased, and there is a limit to high efficiency.

In addition, the optical wavelength conversion element described in Japanese Patent Application Laid-Open No. 4-254834 has a problem in that a dielectric film having a high refractive index is used as a high refractive index layer. The high refractive index layer on the waveguide layer has a large effect on the effective refractive index of the waveguide layer, and high accuracy is required for the uniformity of the film thickness over the entire waveguide layer. For example, in the case of an optical wavelength conversion element, the phase matching condition over the entire length of the waveguide layer is strictly dependent on the effective refractive index of the waveguide layer. Therefore, if the effective refractive index of the waveguide layer changes slightly in the propagation direction, the conversion efficiency will increase. Is extremely reduced. Therefore, strict uniformity has been required for controlling the thickness of the high refractive index layer. In addition, when a material different from that of the substrate is deposited on the surface of the waveguide layer, there is a problem that waveguide loss easily occurs at the interface between the waveguide layer and the high refractive index layer. Further, there is a problem that the waveguide layer is distorted due to a difference in expansion coefficient from the substrate, and the effective refractive index of the waveguide layer has a distribution in the propagation direction of the waveguide layer.

Further, it has been found that the propagation loss of the high refractive index layer to light propagating through the waveguide layer becomes a serious problem. The propagation loss of the waveguide layer, which deteriorates the characteristics of the optical wavelength conversion element, is classified into a harmonic wave and a fundamental wave. The high-refractive-index layer made of a dielectric material has a relatively small propagation loss with respect to the fundamental wave, and there is no problem. However, since the wavelength of the higher harmonic wave is short, the propagation loss provided by the conventional high-refractive-index film is considerably large. In the experiments of the present inventors, various kinds of dielectric films having a high refractive index were tried.
For each of the higher harmonics in the band, a large propagation loss of −several dB / cm was observed, and it was found that the conversion efficiency of the optical wavelength conversion element was reduced to 以下 or less. Furthermore, since the high refractive index layer does not have a function of enhancing the confinement of the fundamental wave, the power density of the fundamental wave cannot be increased, and there has been a limit in improving the efficiency.

Further, this optical wavelength conversion element aims at improving the overlap between the fundamental wave of the fundamental mode propagating in the waveguide layer and the second harmonic. For this reason, since the distribution of the waveguide modes in the waveguide layer is greatly different due to 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. It has been difficult to significantly improve efficiency. Further, since the portion where the fundamental wave and the second harmonic do not overlap is large, it has been difficult to achieve an improvement in the light damage resistance.

In the optical wavelength conversion device described in Japanese Patent Application Laid-Open No. 1-238631, the conversion efficiency is improved by using a ridge-type optical waveguide. In this case, the conversion efficiency is improved by the increase in power density due to the confinement effect of the optical waveguide. However, for the fundamental wave, the increase in the confinement effect by the ridge waveguide is limited to the lateral direction, and confinement in the depth direction is not improved. Accordingly, an increase in the overlap between the fundamental wave and the second harmonic having the greatest effect on the conversion efficiency (particularly, an increase in the depth direction) cannot be achieved by the ridge structure, and it is difficult to greatly improve the conversion efficiency. there were. Also, for the loaded optical waveguide, the overlap between the fundamental wave and the second harmonic could not be increased as in the ridge type.

Accordingly, an object of the present invention is to provide an optical wavelength conversion element capable of outputting a second harmonic having excellent light condensing characteristics and having a large overlap between a fundamental wave and a second harmonic. And It is another object of the present invention to provide an optical waveguide used for the optical wavelength conversion element and a method of manufacturing the same, and a light generating device and an optical pickup using the optical wavelength conversion element.

[0012]

In order to solve the above problems, a first optical waveguide of the present invention comprises a crystal substrate, a waveguide formed on the surface of the crystal substrate, and a waveguide formed on the surface of the waveguide. Wherein the ion exchange layer has a refractive index greater than the refractive index of the waveguide, and the fundamental wave propagating in the waveguide cannot propagate in the ion exchange layer, but the fundamental wave The harmonic is characterized by having a thickness capable of propagating in the ion exchange layer.

Further, a second optical waveguide of the present invention comprises a crystal substrate, a waveguide formed on the surface of the crystal substrate, and a waveguide formed on the surface of the waveguide and having a higher refractive index than the waveguide. A high-refractive-index layer, wherein a fundamental wave propagating through the waveguide cannot propagate in the high-refractive-index layer, but a harmonic of the fundamental wave can propagate in the high-refractive-index layer. It is characterized by having a thickness.

A third optical waveguide according to the present invention includes a crystal substrate, a waveguide formed on a surface of the crystal substrate, and an ion exchange layer formed on a surface of the waveguide. The layer has a refractive index larger than the refractive index of the waveguide, a width smaller than the width of the waveguide, and a depth not more than half the depth of the waveguide.

Further, in the method of manufacturing an optical waveguide according to the present invention, a step of forming an ion-exchange resistant mask so as to secure a non-masked portion of a line on a surface of a crystal substrate; Forming a first ion-exchange part in the crystal substrate by exchanging; expanding the first ion-exchange part into a waveguide by heat-treating the substrate; A second ion-exchange portion having a higher refractive index than the waveguide is formed in the waveguide by performing ion exchange.

An optical wavelength conversion element according to the present invention includes the optical waveguide described above, and further includes two or more domain-inverted portions formed so as to periodically cross the waveguide.

Specifically, a first optical wavelength conversion element according to the present invention comprises a crystal having a non-linear optical effect, a waveguide formed on a surface of the crystal, and a waveguide which periodically crosses the waveguide. And two or more domain-inverted portions formed on the waveguide, and an ion exchange layer formed on the surface of the waveguide, wherein the ion exchange layer has a refractive index larger than the refractive index of the waveguide; A fundamental wave having a thickness that cannot propagate through the ion exchange layer but has a thickness that allows a harmonic of the fundamental wave to propagate.

Further, the second optical wavelength conversion element of the present invention is formed of a crystal having a non-linear optical effect, a waveguide formed on the surface of the crystal, and formed so as to periodically cross the waveguide. Including two or more domain-inverted portions and an ion exchange layer formed on the surface of the waveguide, wherein the ion exchange layer is
It is characterized by having a refractive index larger than the refractive index of the waveguide, a width smaller than the width of the waveguide, and a depth equal to or less than half the depth of the waveguide.

The third optical wavelength conversion element of the present invention is formed such that a crystal having a nonlinear optical effect, a waveguide formed on the surface of the crystal, and a periodic crossing of the waveguide. Two or more domain-inverted portions, and a high refractive index layer formed on the surface of the waveguide and having a higher refractive index than the waveguide, wherein the high refractive index layer has a fundamental wave propagating through the waveguide. It is characterized in that it has such a thickness that it cannot propagate in the high refractive index layer but a harmonic of the fundamental wave can propagate in the high refractive index layer.

Further, a light generating apparatus according to the present invention includes the above-described light wavelength conversion element and a semiconductor laser, and the wavelength of light emitted from the semiconductor laser is converted by the light wavelength conversion element.

An optical pickup according to the present invention is an optical pickup including the above-described light generating device, a condensing optical system, and a recording medium, wherein the light emitted from the light generating device is recorded by the condensing optical system. Light is condensed on a medium.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to an optical wavelength conversion element utilizing the second harmonic generation due to a nonlinear optical effect, in a fundamental mode fundamental wave and a higher mode second harmonic (SHG).
By using the phase matching with the wave, the overlap is increased, and the conversion efficiency to the SHG wave is improved.

The principle of the increase of the overlap will be described below. Here, the principle of achieving phase matching between the fundamental wave of the fundamental mode and the SHG wave of the higher-order mode to increase the overlap between the two modes and improve the conversion efficiency will be described.

First, light wavelength conversion in a general optical waveguide will be described. Usually, the cross section of the optical waveguide is shown in FIG.
As shown in (a), the substrate 51 (refractive index: ns), the waveguide layer 52 (refractive index: nf), and an outer layer (omitted in FIG. 1) (refractive index: nc) have a refractive index of nf>ns> nc
The electric field distribution of the guided mode is such that the SHG waves of the first-order mode 54 and the second-order mode 55 with respect to the fundamental wave 53 of the fundamental mode are as shown in FIG.

As shown in FIG. 1B, the electric field overlap between the fundamental wave 53 and the SHG waves 54 and 55 is greatest between the fundamental modes, and increases as the order of the SHG wave guided mode increases. It tends to decrease. That is, since the conversion efficiency of the SHG wave is proportional to the mode overlap, the phase matching between the fundamental modes is the conversion with the highest conversion efficiency.

Next, a case where an outer layer having a refractive index higher than nf is formed will be described. When an outer layer having a high refractive index is provided, the mode distribution of the fundamental wave is biased toward the high refractive index layer, and is strongly confined in the vicinity of the surface of the waveguide. Conventional optical wavelength conversion elements utilize such strong confinement of fundamental waves,
The conversion efficiency was improved.

However, since the fundamental wave and the harmonic have different wavelengths and the refractive index dispersion also exists, the dispersion of the guided mode differs (for example, the second harmonic is stronger than the fundamental wave and has a higher refractive index layer). The increase in overlap between modes was limited. Certainly, if a high refractive index layer with a higher refractive index than the waveguide layer is used as the outer layer, the confinement of the fundamental wave can be strengthened.However, it is necessary to increase the thickness of the high refractive index layer to strengthen the confinement of the fundamental wave. Then, the SHG wave having a short wavelength is confined inside the high refractive index layer, and the conversion efficiency of the element is extremely reduced.

However, even with the same SHG wave, this problem can be solved by using a higher-order mode. This method will be described with reference to FIG. 2 showing the electric field distribution in the depth direction of the cross section of the waveguide layer. FIG. 2 shows a waveguide layer 6 formed on a substrate 61.
3 shows a state of a waveguide mode when the thickness of the high refractive index layer 63 is changed.

FIG. 2A shows a case where the high refractive index layer is not formed, and shows basically the same state as FIG. FIG.
(b) shows the case where the high refractive index layer 63 is relatively thin. FIG.
In (b), the high refractive index layer 63 has a fundamental wave 64, SHG
The cut-off condition for the wave 65 (the condition that there is no trapped waveguide mode in the layer) is satisfied. At this time, both the fundamental wave 64 and the SHG wave 65
Since only light cannot be guided, the light propagates in the waveguide layer 62. However, the electric field distribution of the fundamental wave 64 and the SHG wave 65 is drawn to the vicinity of the surface by the high refractive index layer 63. FIG. 2B shows a waveguide state of an optical waveguide using a conventional high refractive index layer.

FIG. 2C shows a state in which the high refractive index layer 63 is formed thicker than that shown in FIG. In this state, the high-refractive-index layer 63 satisfies the cutoff condition for the fundamental wave 64, but can propagate a low-order mode (in this case, the fundamental mode SHG wave 65) for the SHG wave 66. It has become. At this time, the confinement of the fundamental wave 64 is shown in FIG.
It becomes stronger than (b). However, the basic mode S
Since the HG wave 65 is confined inside the high refractive index layer 63, the conversion efficiency from the fundamental wave 64 to the fundamental mode SHG wave 65 is extremely reduced.

However, as shown in FIG. 2C, the SHG wave of a mode whose order is one higher than the mode of the mode that can be guided to the high refractive index layer 63 (here, the SHG wave 6 of the first order mode).
6) mostly propagates through the waveguide layer 62. Notably, the electric field distribution of the SHG wave 66 in the waveguide layer 62 (FIG. 2)
(c)) SH in the basic mode when the high refractive index layer 63 is not provided
This is almost the same as the electric field distribution of the G wave (FIG. 2A). On the other hand, the guided mode of the fundamental wave 64 is strongly attracted to the high refractive index layer 63 (FIG. 2C). That is, while the fundamental wave 64 of the fundamental mode becomes a guided mode with strong confinement, the SHG wave 66 of the higher-order mode forms the high refractive index layer 6.
2 is almost the same as the case in which No. 3 does not exist.
As shown in (c), the overlap between the two modes dramatically increases. By using this, the fundamental wave
The conversion efficiency to HG waves can be greatly improved.

FIG. 2D shows a state in which the high refractive index layer 63 is thicker than that of FIG. 2C, and the fundamental wave 64 can be guided through the high refractive index layer 63. When the fundamental wave 64 can be guided through the high refractive index layer 63, as shown in FIG.
Since the overlap between the fundamental wave 64 and the SHG wave 65 within 2 is extremely reduced, the conversion efficiency is greatly reduced.

That is, in the waveguide layer 62 having the high refractive index layer 63 having a high refractive index, by adjusting the thickness of the high refractive index layer so as to obtain the state shown in FIG. The overlap between the fundamental wave 64 and the higher-order mode SHG wave 66 increases, and highly efficient wavelength conversion becomes possible.

However, in order to realize the state shown in FIG. 2C, it is necessary to satisfy some conditions.

First, the fundamental wave 64 is applied to the high refractive index layer 63.
Must be in a state in which the light cannot be guided. That is, the thickness and the refractive index of the high refractive index layer 63 satisfy the cutoff condition with respect to the fundamental wave 64. When the high refractive index layer 63 satisfies the waveguide condition of the fundamental wave 64, FIG.
And the efficiency of wavelength conversion is reduced.

Second, the high-order mode SHG wave can be guided through the high refractive index layer 63. In the higher-order mode, the main peak of the electric field distribution of the SHG wave propagates through the waveguide layer 62, and the sub-peak satisfies the condition that it propagates through the high refractive index layer 63. When the SHG wave 65 satisfies the cut-off condition in the high refractive index layer 63, FIG.
And the efficiency of wavelength conversion is reduced. In order to realize the state shown in FIG. 2C, the condition for guiding the SHG wave in the high refractive index layer 63 must be satisfied. In a state where the condition is satisfied, if the SHG wave 66 of the higher order mode than the order mode in which the light can be guided is selectively guided in the high refractive index layer 63, the main peak of the waveguide layer 62 is generated. It has been confirmed that the waveguide mode has the following.

As described above, in order to realize a highly efficient wavelength conversion element in the waveguide layer 62 having the high refractive index layer 63, the thickness and the refractive index of the high refractive index May be selected so as to satisfy the cutoff condition and the waveguide condition for the SHG wave 66.

Further, in order to increase the efficiency, the refractive index of the high refractive index layer 63 was examined. In order to increase the conversion efficiency of the element, it is necessary to concentrate the electric field distribution of the SHG wave on the waveguide layer 62. That is, it is desired to reduce the electric field distribution of the high refractive index layer 63 as much as possible. In the electric field distribution of the SHG wave, the ratio of the electric field distribution of the high refractive index layer (cladding) to the electric field distribution of the waveguide layer (core) depends on the ratio of the respective refractive indexes. For example, as the refractive index of the high refractive index layer with respect to the waveguide layer increases, the electric field distribution of the high refractive index layer decreases.

Therefore, in order to make the electric field strength of the high refractive index layer less than, for example, 1/10 of the electric field strength in the waveguide layer,
The refractive index nc of the high refractive index layer may be set to nc> 1.01 nf with respect to the refractive index nf of the waveguide layer (the maximum refractive index nfmax of the waveguide layer in the case of having a graded refractive index distribution). .

Although a single high refractive index layer has been described here, the same effect can be obtained with a high refractive index layer formed of a multilayer film. When a multilayer film is used, the refractive index distribution in the high-refractive-index layer can be controlled, so that the degree of freedom in element design is increased and an element with a high manufacturing tolerance can be manufactured.

Since the mode of the SHG wave propagating in the waveguide layer can be uniquely selected by selecting the wavelength of the fundamental wave, phase matching with a higher-order mode having a large overlap can be selectively performed. Thus, highly efficient wavelength conversion can be performed.

Further, it was confirmed that the above-mentioned structure of the waveguide layer was very effective in improving the light damage resistance. As the overlap between the fundamental wave and the SHG wave in the waveguide layer increases, the electric field in the portion where the SHG wave and the fundamental wave do not overlap decreases. Optical damage is caused by the excitation of impurities in the crystal by short-wavelength light (SHG light) to generate an internal electric field. When a fundamental wave is interposed, a level for trapping impurities is excited, and the internal electric field due to the optical damage is generated. Tend to be fixed. This is remarkable in a portion where the fundamental wave near the electric field distribution of the SHG wave exists alone. However, if the above configuration is adopted, the portion where the fundamental wave and the SHG wave do not overlap is extremely reduced, so that the occurrence of optical damage can be greatly reduced.

The structure of the waveguide layer described above is effective not only for an optical wavelength conversion element but also for an optical waveguide that simultaneously guides two or more lights having different wavelengths. That is, the structure is effective for increasing the overlap of the light of different wavelengths propagating through the optical waveguide to increase the interaction.

Further, since the light distribution is attracted to the vicinity of the surface, it is possible to increase the influence of the elements such as electrodes integrated on the optical waveguide on the guided light, thereby providing an optical integrated circuit element with high efficiency. realizable.

As a specific structure of the optical waveguide, an embedded waveguide structure in which a high refractive index layer is provided near the surface of the waveguide is preferable. The optical waveguide having such a structure has a light-collecting property equal to or less than the diffraction limit, enhances lateral light confinement, reduces the propagation loss of the waveguide layer, and enables higher efficiency. The effect that the tolerance for the optical waveguide shape error is large can be obtained. Further, the use of ion exchange provides a manufacturing advantage that the manufacturing process is facilitated.

Hereinafter, such an optical waveguide, an optical wavelength conversion element having the optical waveguide, a light generator, and an optical pickup will be described in detail.

(Embodiment 1) In this embodiment, an example of the configuration of a short wavelength light generating device will be described. As shown in FIG. 3, this short-wavelength light generating apparatus includes a semiconductor laser 71 and the optical wavelength conversion element 72, the fundamental wave P ₁ emitted from the semiconductor laser 71 through the lenses 73 and 74 light The light enters the waveguide layer 72a formed on the wavelength conversion element 72. Each component is fixed to the mount 75.

The incident fundamental wave P ₁ is transmitted through the waveguide layer 72a by T
TE that propagates in E00 mode and is a higher-order mode of higher harmonics
Converted to 10 modes. These harmonics are radiated from the light wavelength conversion element 72 and used as a short wavelength laser beam. Attention here is a mode profile of the harmonic TE10 mode propagating in the waveguide layer 72a. T
Since the E10 mode is a higher-order waveguide mode, the intensity distribution has two peaks as shown in FIG. 4B, for example. Hereinafter, the larger one of the two peaks is called a main peak, and the smaller one is called a sub-peak.

When the TE10 mode having a sub-peak is condensed by the condensing optical system, a condensed spot having the same sub-peak as that of the waveguide mode is formed. It becomes a problem.

Therefore, a new method has been found for substantially eliminating the sub-peak which is a problem when collecting harmonics.
It is the width of the sub-peak in the state of the guided mode,
The value is set to a value sufficiently smaller than the diffraction limit of the light collecting optical system used for light collection. That is, by suppressing the sub-peak to a width smaller than the resolution of the condensing optical system, the influence of the sub-peak on the condensed spot is eliminated.

Here, harmonics having a wavelength of λ = 425 nm were condensed using a condensing lens with NA = 0.95. At this time, the diffraction limit of light in air is about 0.34 μm. When the waveguide layer 72a was designed to have a subpeak width of 0.32 μm or less, no subpeak was observed in the obtained condensed spot. The higher-order guided mode having a sub-peak has an effect on a condensed spot when condensing if the width of the sub-peak is less than the diffraction limit of light in air (0.8 × λ / NA). It was confirmed that it could be used effectively. For this reason, the utilization efficiency of the output, which is reduced by the beam shaping, can be improved to 80% or more.

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. FIG. 4A shows a case where the light is focused in the TE00 mode.
FIG. 2B shows the shape of the converging spot when the TE10 mode is converged. When the light was focused in the TE00 mode, a value (0.8 × λ / NA) close to the diffraction limit of the lens was obtained.
On the other hand, when a higher-order TE10 mode having a sub-peak is converged, the beam shape on the side having the sub-peak becomes steep, and the width of the converged spot is 90% of the diffraction limit.
It was possible to condense light to a certain extent, and it was confirmed that it had a super-resolution effect.

According to experiments, by setting the width w of the sub-peak to be equal to or less than the diffraction limit (up to 0.8 × λ) of the light having the wavelength λ in the air, the width of the condensed spot is changed to the lens used (numerical aperture: NA). ) Is smaller than the diffraction limit (0.8 × λ / NA). As described above, it has been clarified that a smaller focused spot can be obtained by focusing a waveguide mode having a subpeak having a width equal to or smaller than the diffraction limit.

Further, the relationship between the width of the sub-peak and the focused spot was investigated in more detail. 1) The width of the sub-peak: ≤0.8λ Light-collecting characteristics (super-resolution effect) less than the diffraction limit can be obtained. 2) Subpeak width: 0.8λ (1 + 0.2 (1 / NA-1)) to 0.8λ
(1 + 0.5 (1 / NA-1)) A light-collecting spot almost equivalent to the diffraction limit is obtained, and deterioration of the light-collecting characteristics due to the sub-peak is not observed. 3) Subpeak width:> 0.8λ (1 + 0.7 (1 / NA-1)) A subpeak appears in the converging spot, and the condensing characteristics deteriorate. The result was obtained. That is, in order to prevent deterioration of the light-collecting characteristics due to the sub-peak, it is preferable to suppress the width of the sub-peak to at least about 0.8 × λ (1 + 0.5 (1 / NA−1)) or less.

Further, if the waveguide layer is designed so that the width of the sub-peak is 0.8λ or less, a condensed spot smaller than the diffraction limit of the converging lens can be obtained, which is very effective. On the other hand, if the width of the converging spot is larger than the diffraction limit (0.8 × λ / NA) of the lens used, the converging spot has a shape substantially equal to the waveguide mode, and is a collection of two peaks having sub-peaks. It becomes a light spot, and a converged spot with a single peak cannot be obtained.

In the present embodiment, as a short-wavelength light source, when the wavelength of the fundamental wave is converted to a higher harmonic, the guided mode is converted to 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 layer, 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. be able to.

In this embodiment, the TE10 mode having one sub-peak is used. However, a similar effect can be obtained even in a higher-order waveguide mode if the width of the sub-peak is diffraction-limited. 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 to the depth direction, the TE01 and TE02 modes having subpeaks in the width direction and the TE11 and TE22 having subpeaks in both the width and depth directions are effective because smaller condensed spots can be obtained. In addition,
Although the present embodiment deals with the TE mode, a similar effect can be obtained in the TM mode.

(Embodiment 2) Next, a short wavelength light generator
The optical wavelength conversion element, which is a basic component of
You. As shown in FIG. 5, this short-wavelength light generator includes:
X-plate LiNbO_ThreeStriped near the surface of substrate 76
Waveguide 77 is formed. This LiNbO _Three76
A polarization inversion section 79 is provided on the substrate for phase matching.
7 are formed so as to periodically cross the same. In addition,
A high refractive index layer 78 is formed near the surface of the wave path 77.
You. The width W of the waveguide 77₁Is 4 μm, depth D₁Is 2.5 μm
And the high refractive index layer 78 has a width W_TwoIs 3 μm, depth D_TwoIs
0.2 μm. As shown in FIG. 5A, a high refractive index
Layer 78 is formed along and approximately in the center of waveguide layer 77.
Has been established.

[0060] waveguide 77 is formed by annealing after proton exchange LiNbO ₃ crystal, LiNbO ₃
Some of the Li in the metal is exchanged for H, and Li _(1-x) H _x NbO
₃ (0 <x <1). Where x
Is the exchange rate of proton exchange. By annealing treatment,
Preferably, the value of x is reduced to 0.3 or less.

The high refractive index layer 78 is also formed by proton exchange. In order to obtain a higher refractive index than the waveguide 77, the high refractive index layer 78 has a higher proton exchange rate x than the waveguide 77. Specifically, it is preferable that x is 0.6 or more. In this optical wavelength conversion element, for example, a fundamental wave having a wavelength of 850 nm is incident in the TE00 mode, and quasi-phase-matched in the waveguide 77 with a harmonic in the TE10 mode.

The principle by which the wavelength can be converted to a harmonic with high efficiency in this optical wavelength conversion element will be described. As described above, in the waveguide layer having the high-refractive-index layer, the overlap between the fundamental wave and the harmonic wave is increased, the confinement of the guided light is enhanced, and the wavelength conversion with high efficiency can be performed. However, according to the configuration of the present embodiment, this effect is further emphasized, and higher conversion efficiency can be obtained.

FIG. 6 shows the distribution of the waveguide mode in the optical wavelength conversion device of the present embodiment. FIG. 7A shows, for comparison, a mode distribution in a waveguide 81 having a slab-like high refractive index layer 82 on the surface, and FIG. 7B shows a waveguide 77 having a stripe-like high refractive index layer 78. 5 shows the distribution of the guided mode in FIG. Although the electric field distribution in the depth direction is almost the same in both FIGS. 7A and 7B, the electric field distribution in the width direction of the waveguide has a smaller electric field distribution in FIG. I have.

Further, as a result of the enhanced confinement, FIG.
The overlap between the fundamental wave and the higher harmonic wave in (b) is also improved, and the conversion efficiency is about 1.5 times that of the waveguide having the slab-like high refractive index layer shown in FIG. As a result, the conversion efficiency was more than doubled as compared with the optical wavelength conversion element having a slab-like high refractive index layer.

Next, the mode profile of higher harmonics will be described. The TE10 mode has two intensity peaks in the depth direction of the waveguide, and has a subpeak on the high refractive index layer side. In the high refractive index layer, since the proton exchange rate is high and the nonlinearity is small, the electric field distribution at the subpeak does not contribute to the conversion from the fundamental wave to the harmonic. Therefore, it is preferable to restrict the existence of the electric field distribution at the sub-peak as much as possible. Therefore, as described above, the refractive index n of the high refractive index layer
It is preferable that the magnitude of c is 1.01 times or more the refractive index nf of the waveguide.

Next, the thickness of the high refractive index layer will be described.
With increasing thickness of the high refractive index layer, the amount of the electric field distribution in the high refractive index layer is increased, it is preferable to limit the thickness D ₂ of the high refractive index layer. In order to achieve high efficiency, it is preferable that the thickness D ₁ of the waveguide is at least twice the thickness D ₂ of the high refractive index layer. When D ₁ <2 × D ₂ , the conversion efficiency decreases to １ ／ or less of the maximum value.

Next, the relationship between the refractive index of the high refractive index layer and the refractive index of the waveguide will be described from the viewpoint of the light-collecting characteristics. When condensing guided light having a sub-peak with a lens system, the light condensing characteristic is degraded due to the size of the sub-peak. As described above, in order to make the condensed spot a single spot and obtain a condensed spot equal to or less than the diffraction limit, it is necessary to reduce the sub-peak of the guided mode to the diffraction limit of the condensing lens or less. Therefore, it is preferable to define the thickness D ₂ of the high refractive index layer. The width of the sub-peak is determined by the thickness and the refractive index of the high refractive index layer, and is substantially equal to the thickness of the high refractive index layer. Therefore, in order to keep the width of the sub-peak below the diffraction limit, it is necessary to keep the thickness of the high refractive index layer below the diffraction limit of harmonics.

Specifically, when the thickness of the high refractive index layer is larger than 0.6 to 0.7 μm (D ₂ > 0.7 μm to 0.6 μm) with respect to a harmonic having a wavelength of 425 nm, NA = 0. .6
The use of a lens having a certain degree deteriorated the light collecting characteristics. D ₂ =
At about 0.5 μm, the light-collecting characteristics are considerably improved, and D ₂
In the case of <0.4 μm, light-collecting characteristics below the diffraction limit were obtained. When the diffraction limit is calculated on the assumption that NA = 1 and the wavelength is 425 nm, the diffraction limit is about 0.34 μm. Therefore, it can be seen that an emission beam having excellent light-collecting characteristics can be obtained if D ₂ is smaller than the diffraction limit.

Next, the width W ₂ of the high refractive index layer will be described. Higher efficiency can be achieved by improving the overlap between the waveguide modes and the confinement effect. However, in order to achieve higher efficiency, the width W ₂ of the high refractive index layer must be smaller than the width W _{1 of the} waveguide layer. Is desirable. This is because the confinement of the waveguide in the lateral direction is strengthened, and high efficiency can be achieved. When W ₂ is equal to or larger than W _1, the electric field distribution of the waveguide mode propagating through the waveguide is almost equal to that of the slab-shaped waveguide having a high refractive index, and the efficiency is not improved. When W ₂ <0.9 × W _1, an improvement in the conversion efficiency is confirmed. When W ₂ <0.8 × W ₁ , the conversion efficiency is 1.
It has improved more than 5 times.

The structure in which the width of the high-refractive-index layer is smaller than the width of the waveguide is not limited to the method of forming the high-refractive-index layer by proton exchange in the waveguide by proton exchange, but also the method of forming the high-refractive-index layer on the surface of the waveguide. It has been confirmed that it works effectively even in a structure in which a dielectric material having a refractive index is deposited.

Next, the refractive index of the high refractive index layer will be described. The proton exchange moiety is Li _(1-x) H _x O ₃ (0 <x <
1), and part of Li in the LiNbO ₃ crystal has been exchanged for H. x is the exchange rate of proton exchange.

The waveguide and the high refractive index layer are both formed by proton exchange, but have different proton exchange rates. The x value immediately after proton exchange is as high as about 0.9. The value of x can be adjusted by annealing, and it is necessary to reduce the exchange rate to 0.5 or less in order to recover the nonlinearity. The exchange rate is proportional to the refractive index of the proton exchange layer, and the lower the exchange rate, the lower the refractive index.

The waveguide of the optical wavelength conversion element requires strong confinement and high nonlinearity. However, if the nonlinearity is recovered by annealing, the refractive index decreases and the confinement is weakened. To recover nonlinearity to about 80% of the substrate, x
Must be reduced to about 0.3. In this state, the refractive index of the waveguide decreases, and it becomes difficult to sufficiently confine the fundamental wave. However, when the high refractive index layer is formed on the waveguide layer, the confinement of the waveguide layer is strengthened, and it is possible to form an optical waveguide with strong confinement and high nonlinearity. In order to enhance the confinement of the waveguide layer, it is necessary to increase the ion exchange concentration of the high refractive index layer, and it is preferable that the ion concentration be twice or more (x> 0.6) that of the waveguide layer.

In this embodiment, the proton exchange layer is used as the high refractive index layer. However, the same effect can be obtained by using a dielectric material having a high refractive index. In the case of proton exchange, a high-refractive-index layer was formed inside the waveguide layer. However, even when a dielectric film is deposited to form a high-refractive-index layer, the layer is selectively deposited on the surface of the waveguide layer to be guided. Preferably, a corrugated layer is formed.

As a dielectric, Nb ₂ O ₅ having a higher refractive index than LiNbO ₃ was used. Nb ₂ O ₅ having a thickness of 200 nm was deposited on the surface of the waveguide by a sputtering method, and formed at the center of the waveguide along the waveguide. However, when the high refractive index layer was processed into the ridge shape by patterning the waveguide surface, slight irregularities were formed on the side surfaces of the ridge shape. As a result, since the propagation loss of the optical waveguide was increased, it was found that the waveguide layer formed by ion exchange was superior in that the propagation loss of the guided light was smaller.

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

The reason why the polarization direction of the TE mode is used in the present embodiment is to make it coincide with the polarization direction of the waveguide of an ordinary semiconductor laser. 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. In the TM mode optical waveguide, 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 this embodiment is also effective for other optical waveguide devices. By forming the high refractive index layer on the optical waveguide, the electric field distribution of the guided light propagating through the optical waveguide is strongly attracted to the vicinity of the surface. For this reason, a planar electrode formed on the waveguide,
The effect of the grating element can be strongly exerted on the guided light, and a highly efficient modulation and diffraction effect can be obtained.

In this embodiment, LiNbO is used as the substrate.
_Three substrates were used, but LiNbO ₃ doped with MgO, Nb, Nd or the like, or LiTaO ₃ or a mixture thereof, LiTa _(1-x) Nb _x O ₃ (0 ≦ x ≦ 1) substrate, and KTP (KTiOPO ₄ ) can produce a similar device. Since LiTaO ₃ , LiNbO ₃ , and KTP all have high nonlinearity, a highly efficient optical wavelength conversion element can be manufactured.

(Embodiment 3) Next, as another example of the configuration of the optical wavelength conversion element of the present invention, a case where a high refractive index layer is formed by ion exchange will be described. FIG. 7 shows a configuration diagram of the optical wavelength conversion element of the present embodiment. As shown in FIG. 7, a striped waveguide 84 is formed near the surface of an X-plate LiNbO ₃ substrate 83, and a high refractive index layer 85 formed by proton exchange is formed near the surface of the waveguide 84. Is formed. On the LiNbO ₃ substrate 83, a domain inversion portion 86 is formed so as to periodically cross the waveguide for phase matching.

High refractive index layer 85 formed by ion exchange
Has excellent in-plane uniformity of film thickness and can suppress in-plane variation to ± 0.1% or less. ・ Excellent in film thickness controllability and control of film thickness to ± 0.01 μm or less. It is excellent in that: ・ Propagation loss with respect to harmonics and fundamental wave are both small; ・ Distortion at the boundary with the waveguide layer is small and propagation loss and in-plane distribution are not given.

The high-refractive-index layer is required to have a higher refractive index than the substrate, but is also required to have low propagation loss with respect to fundamental waves and harmonics. Especially, the harmonic is 400
Since the wavelength is as short as about nm, the propagation loss significantly increases. For example, Nb ₂ O ₅ , TiO _2, or the like is usually used as a high-refractive-index dielectric, but it may approach the absorption edge of the dielectric, and the propagation loss for short-wavelength light tends to increase. Therefore, an attempt was made to form a high refractive index layer by ion exchange. The ion-exchange layer is advantageous in that propagation loss for short-wavelength light is substantially equal to the transmission characteristics of the substrate and is small.

The ion exchange can be easily applied to the ferroelectric crystal used in the present embodiment. For example, when proton exchange, which is a kind of ion exchange, is performed on LiNbO ₃ , a change in the refractive index is as large as about 0.1, so that a layer having a high refractive index can be easily formed.

In addition, ion exchange is characterized in that the depth can be easily controlled and a highly uniform layer can be formed. For example, when a high refractive index layer is formed by depositing a dielectric material by a sputtering method or the like, the in-plane uniformity is about ± 1 to 3% in an ordinary apparatus. On the other hand, since the ion exchange is formed by diffusion, the thickness variation can be reduced to 0.01 μm or less by controlling the diffusion temperature and time. Further, the uniformity in the plane can be controlled equivalent to the control of the thickness, and an accuracy higher by two digits or more can be achieved as compared with a method of film formation such as sputtering. Specifically, when the characteristics of the device using the Nb ₂ O ₅ sputtered film and the characteristics of the optical wavelength conversion device by proton exchange were compared, the device formed by the proton exchange was compared with the device using the Nb ₂ O ₅ sputtered film. , The uniformity was higher by one digit or more, indicating that the SHG conversion efficiency was more than twice as high.

Next, a method for making the above-mentioned sub-peak of the waveguide mode equal to or less than the diffraction limit of the guided light will be described. The width of the sub-peak of the waveguide mode is determined by the thickness and the refractive index of the high refractive index layer. However, since the width of the sub-peak is only about the thickness of the high-refractive-index layer or more, the width of the high-refractive-index layer must be suppressed at least to the width of the diffraction limit of the guided light. Further, it is preferable that the refractive index of the high refractive index layer be 1.01 times or more the refractive index of the waveguide layer.

When the width of the high-refractive-index layer is smaller than the width of the waveguide layer, and when the refractive index of the high-refractive-index layer is less than 1.01 times that of the waveguide layer, the width of the subpeak becomes larger than the thickness of the high-refractive-index layer. This greatly increased the light-condensing characteristics of the harmonic output, and made it difficult to condense light to the diffraction limit.

Further, the relationship between the width of the sub-peak and the light condensing property was examined by controlling the refractive index and the thickness of the high refractive index layer. As a result, the same result as that described in Embodiment 1 was obtained. Similarly, it was confirmed that a beam having excellent light-collecting characteristics was obtained by setting the width of the sub-peak to be equal to or less than the diffraction limit of the guided light.

From the above, it was found that by setting the thickness of the high refractive index layer to be equal to or less than the diffraction limit of the guided light, it is possible to obtain an outgoing beam having excellent light-collecting characteristics.

Next, the point that the optical wavelength conversion element of the present invention is excellent in tolerance for manufacturing errors will be described. The optical wavelength conversion element needs to satisfy a phase matching condition between the fundamental wave and the harmonic when converting the fundamental wave into the harmonic in the waveguide. The phase matching condition is a condition in which the phase velocities of the fundamental wave and the harmonic wave are made uniform, and it is necessary to control the effective refractive index between the fundamental wave and the harmonic wave.

When an optical waveguide is used, a phase matching condition must be satisfied over the entire length of the optical waveguide. However, the tolerance for the phase matching condition is quite narrow, and when forming an actual optical waveguide type optical wavelength conversion element, it is necessary to manage the waveguide width, refractive index fluctuation, and the like to the utmost. For example, in the case of a quasi phase matching type optical wavelength conversion element using LiNbO ₃ , the waveguide width is 4 mm over a waveguide length of 10 mm.
It is necessary to control the thickness in μm to ± 0.1 μm or less, which causes a reduction in the production yield.

The reason for this is that, as shown in FIG. 8A, the fluctuation of the phase matching wavelength with respect to the width of the waveguide is large. The tolerance of the phase matching wavelength of the optical wavelength conversion element is about 0.1 μm when the element length is 10 mm. For example, when the width of the waveguide shifts by 0.1 μm, the tolerance of the phase matching wavelength deviates from the tolerance. When the width of the waveguide changes slightly in the propagation direction, the phase matching wavelength differs in each part, and when a fundamental wave of a specific wavelength is incident, the waveguide having a width that satisfies some phase matching conditions is obtained. Harmonics are generated only in the part of the wave path. For this reason, the domain-inverted structure formed over the entire waveguide length only partially contributes to optical wavelength conversion,
Conversion efficiency decreases. Also, the optical waveguide in which the high-refractive-index layer was formed in a slab shape on the waveguide surface exhibited the same characteristics, and the fabrication error of the optical waveguide was as severe as ± 0.1 μm or less.

However, in the optical wavelength conversion device having the optical waveguide structure of the present embodiment, as shown in FIG. 8B, it was found that the variation of the phase matching wavelength with respect to the width of the waveguide was very small. Was done. That is, it was found that the tolerance of the width of the waveguide layer satisfying the tolerance of the phase matching wavelength was increased to ± 0.5 μm, which was expanded to five times the tolerance of the related art. For this reason, the conversion efficiency of the optical wavelength conversion element, whose characteristics have been deteriorated due to the manufacturing error of the optical waveguide, can be improved to twice or more.

In this embodiment, an X-plate substrate is used. Alternatively, a Y-plate, a Z-plate, or a substrate whose crystal axis is inclined from the surface may be used. When a Z plate or a substrate whose crystal axis is inclined from the substrate surface is used, 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, because the polarization direction of the waveguide layer of the ordinary semiconductor laser is matched. By using a waveguide layer having the same polarization direction as the semiconductor laser, the waveguide layers can be coupled with low loss. However,
Waveguide layers for TM mode polarization are also available. The coupling loss of the waveguide layer in the TM mode can be reduced by controlling the polarization direction using 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 on the optical waveguide, the electric field distribution of the guided light propagating through the optical waveguide is strongly attracted to the vicinity of the surface.
For this reason, the influence of the planar electrode formed on the waveguide layer and the grating element can be strongly given to the guided light, and a highly efficient modulation and diffraction effect can be obtained.

In this embodiment, the substrate is LiNbO
_{Although three} substrates were used, LiNbO ₃ doped with MgO, Nb, Nd, or the like, or LiTaO ₃ or a mixture thereof, a LiTa _(1-x) Nb _x O ₃ (0 ≦ x ≦ 1) substrate, and KTP (KTiOPO ₄ ) can produce a similar device. Since LiTaO ₃ , LiNbO ₃ , and KTP all have high nonlinearity, a highly efficient optical wavelength conversion element can be manufactured.

(Embodiment 4) In this embodiment, another configuration of the optical wavelength conversion element will be described. As shown in FIG. 9A, this optical wavelength conversion element is composed of an X-plate MgO: LiN
domain-inverted portions 94 formed periodically on a bO ₃ substrate 91
And a waveguide 92 are formed. MgO: LiNbO
_{The three} substrates 91 are cut out such that the Z axis of the crystal is inclined by 3 ° with respect to the surface of the substrate. When a substrate having an inclined crystal axis is used, a deep polarization inversion portion 94 can be formed, so that a highly efficient optical wavelength conversion element can be configured. This is the polarization inversion unit 9
This is because 4 is formed along the Z axis of the crystal and grows from the substrate surface to the inside of the substrate. Therefore, an optical wavelength conversion element as shown in FIG. 9 was manufactured by utilizing this property.

FIG. 9B is a sectional view of the substrate cut along the waveguide layer 92. As shown in FIG. 9B, the domain-inverted portions 94 are formed inside the substrate, and are formed from the vicinity of the boundary between the high refractive index layer 93 and the waveguide layer 92 on the surface to the inside of the substrate. The inside of the high refractive index layer 93 is formed so that the domain inversion structure does not exist. by this,
High efficiency of the optical wavelength conversion element can be achieved. Hereinafter, the reason will be described.

As described above, the conversion efficiency of the optical wavelength conversion element depends on the overlap between the structure of the domain inversion section 94 and the guided light (fundamental wave, harmonic) propagating through the waveguide 92. However, in order to improve the efficiency, the higher harmonic wave uses a higher-order waveguide mode (TE10 mode). Therefore, as the electric field distribution, the higher harmonic wave has an inverted electric field phase between the high refractive index layer 93 and the waveguide 92. ing. For this reason, the overlap in the high refractive index layer reduces the conversion to harmonics.

To prevent this, it is necessary to prevent the overlap in the high refractive index layer 93 from affecting the wavelength conversion. Therefore, as shown in FIG.
By eliminating the periodic polarization inversion structure in 3, it is possible to selectively prevent the phase matching condition from being satisfied only in the high refractive index layer 93. As a result, the efficiency of the optical wavelength conversion element could be improved.

In this embodiment, a crystal whose crystal axis is inclined by 3 ° is used. However, a crystal whose inclination is 5 ° or less can be used because a deep domain-inverted structure can be formed. Especially 0.5
The inclination of from 3 ° to 3 ° is preferable because the inclination of polarization inversion is small and the used area increases.

(Embodiment 5) Next, a method of manufacturing an optical waveguide in an optical wavelength conversion element will be described. here,
The formation of an optical waveguide using proton exchange, which is a type of ion exchange, will be described with reference to FIG.

FIG. 10A shows a step of depositing a Ta film 96 as an ion-exchange resistant mask on the surface of a LiNbO ₃ substrate 95 and forming a 4 μm-wide waveguide pattern using photolithography and dry etching. It is. After this step, as shown in FIG. (B), heat treating the substrate 95 of 200 ° C. in a pyrophosphoric acid, replacing the protons (H ⁺⁾ in Li and acid unmasked partially crystalline, first Is formed. Next, as shown in FIG. 3C, the proton exchange section is annealed to increase the volume of the ion exchange section to form a waveguide. As shown in FIG. 3D, heat treatment is again performed in pyrophosphoric acid to form a second proton exchange portion 98, that is, a high refractive index layer in the non-mask portion. In addition, 160 ° C
The thickness of the high-refractive-index layer was finely adjusted by annealing at the following low temperature to form an optimal waveguide layer structure.

The first proton exchange unit 97 was made to have a thickness of about 0.2 μm by performing proton exchange at 200 ° C. for about 6 minutes. By annealing this at 300 ° C. for about 60 minutes, the exchange layer depth becomes about 2.5 μm,
The width expanded to 5 μm or more. The proton exchange rate of the waveguide thus formed was reduced by the annealing treatment, and the nonlinearity was restored.

Further, by using Ta, which has excellent oxidation resistance, as a mask, a change in film quality due to the subsequent annealing as well as the proton exchange was not observed. Therefore, when proton exchange is performed again using the same mask, a high refractive index layer is formed inside the waveguide.

Since the width and depth of the waveguide are widened by the annealing process, the high refractive index layer formed by the second ion exchange can be a layer smaller in width than the optical waveguide.

Further, it has been found that by annealing the formed waveguide layer at a low temperature of 160 ° C. or lower, the propagation loss can be significantly reduced without changing the characteristics of the waveguide. By annealing at a low temperature, diffusion of the first and second ion exchange portions was slightly suppressed, and at the same time, propagation loss of the waveguide due to annealing was reduced.

Next, the characteristics of the manufactured optical waveguide were examined.
A fundamental wave having a wavelength of 850 nm was incident, and the mode distribution of light guided was examined. The waveguide mode was measured by observing the NFP (near field image) of the guided light with the waveguide width being 4 μm and the depth being 2 μm. The measurement was made by the method described above.
(1) An optical waveguide having a striped (linear) high refractive index layer, (2) an optical waveguide having no high refractive index layer, and (3) an optical waveguide having a slab-shaped high refractive index layer, respectively. did. The results are shown in the table below. However, the width of the mode in the table indicates the width at which the intensity distribution becomes 1 / e ² .

[Μm] Shape of High Refractive Index Layer (1) Strife ゜2) None (3) Slab-shape --- --- --- --- --- --- --- -------- The thickness of the waveguide mode 1.7 2 3.5 1.8 Waveguide mode width 3 4 3.5 3.5 ----------------------------------------------------------------------------------------------------------------

From the above results, it was found that, by providing a stripe-shaped high refractive index, confinement of the optical waveguide was enhanced in the depth direction and the width direction, and an optical waveguide having a high power density could be formed.

Further, when a stripe-shaped high refractive index layer is used, there is a characteristic that the electric field distribution of the waveguide mode can be changed according to the propagation direction of the waveguide. For example, it is possible to form an incident taper for improving the coupling efficiency of the optical waveguide of the laser light at the entrance of the optical waveguide, and to perform mode shaping for shaping the aspect ratio of the output beam to 1: 1.

First, the incident taper is shown in FIG.
This will be described with reference to FIG. In FIG. 11, a waveguide layer 102 is formed on a LiNbO ₃ substrate 101, and a linear high refractive index layer 103 is formed near the surface of the waveguide layer 102. The width of the high-refractive-index layer 103 is tapered in the vicinity of the incident portion. The waveguide layer 102 having the high refractive index layer 103 is
As shown in the above table, confinement is strengthened by the high refractive index layer 103, and the distribution of the guided mode is reduced. Therefore, the high refractive index layer is processed into a tapered shape in the vicinity of the incident portion of the waveguide layer 102, and the width of the tapered portion is reduced toward the incident portion. The confinement is weakened, and the sectional area of the waveguide mode can be increased.

More specifically, the width of the high refractive index layer 103 is set to 4 μm.
When the aperture was narrowed down from 0.5 m to 0.5 μm, the electric field distribution of the waveguide mode became equal to the case where the high refractive index layer 103 was not provided and expanded. The size of the waveguide mode is 1.
7 μm and a width of 3 μm, but the incident part has a depth of 2.5 μm.
μm and 4 μm in width. As a result, the coupling efficiency with the semiconductor laser can be improved to 1.3 times that of the related art, and the tolerance of the coupling accuracy has been improved to 1.5 times.

There are other methods for obtaining the above effects. For example, there is a method in which a stripe-shaped high-refractive-index layer is divided into islands in the vicinity of the incident portion. The effective confinement of the waveguide can be changed by changing the ratio between the portion where the high refractive index layer is formed and the portion where the high refractive index layer is not formed at the incident portion. Specifically, the rectangular period was set to 2 μm, and the ratio of presence / absence was reduced to 100% to 10%, and the ratio of the rectangular high refractive index layer was gradually reduced as approaching the incident part of the waveguide. Also in this case, the same effect as in the tapered shape was obtained, and a tapered waveguide having high coupling efficiency could be formed. As described above, in the optical waveguide, it is preferable that the high refractive index layer be formed such that the ratio of the high refractive index layer per unit length in the waveguide direction decreases as approaching the end.

Similarly, the shape of the output beam can be changed. By changing the width of the high-refractive-index layer 103 at the emission part, the aspect ratio of the emission beam could be made closer to 1: 1.

In this embodiment, an X-plate substrate is used.
However, in addition to the above, a table inclined from the Z or Y plate substrate or crystal axis
It can be used regardless of the crystal axis, such as a substrate with a plane. Ma
In this embodiment, the substrate is LiNbO_ThreeUsing a substrate
But LiNbO doped with MgO, Nb, Nd, etc.
_ThreeOr LiTaO_ThreeOr a mixture thereof, LiTa
_(1-x)Nb_xO_Three(0 ≦ x ≦ 1) Substrate, other KTP
(KTiOPO_Four) Can produce a similar device. Li
TaO_Three, LiNbO_Three, KTP both have high nonlinearity
Therefore, a highly efficient optical wavelength conversion element can be manufactured.

(Embodiment 6) In this embodiment, another example of a short wavelength light generator using the optical wavelength conversion element of the present invention will be described. The configuration includes a semiconductor laser in a wavelength band of 800 nm, a condensing optical system, and a light wavelength conversion element. Excitation of the guided mode.

The wavelength-converted SHG light is emitted from the other waveguide end face of the light wavelength conversion element. Since an optical wavelength conversion element having high conversion efficiency was realized, 10 mW blue SHG light was obtained using a semiconductor laser having an output of about 100 mW. In addition, the used optical wavelength conversion element has excellent light damage resistance,
Since a stable output was obtained, a stable output having an output fluctuation of 2% or less was obtained. A wavelength in the 400 nm band is desired in a wide range of application fields such as optical disks for special measurement of printing plate making, biotechnology, fluorescence spectral characteristics, and the like. The short-wavelength light source using the optical wavelength conversion element of the present invention can be put to practical use in these application fields in terms of both output characteristics and stability.

In this embodiment, the light of the semiconductor laser is coupled to the optical waveguide by using the focusing optical system. However, the semiconductor laser and the optical waveguide can be directly coupled.
When an optical waveguide for propagating the TE mode is used, the waveguide mode of the semiconductor laser and the electric field distribution can be made equal, so that coupling can be performed with high efficiency without a condenser lens. In experiments, it was confirmed that direct coupling was possible with a coupling efficiency of 80%, and that coupling characteristics almost equivalent to lens coupling were obtained. With direct coupling, a small, inexpensive light source can be realized.

(Embodiment 7) In this embodiment, an optical pickup will be described. FIG. 12 shows an example of the optical pickup of the present invention. In FIG. 12, Embodiment 1
10 mW output from the short wavelength light generator 105 shown in FIG.
Is transmitted through a beam splitter 106 and is irradiated by a lens 107 onto an optical disk 108 as an optical information recording medium. The reflected light from the optical disk 108 is conversely collimated by the lens 107, and the beam splitter 1
The signal is reflected at 06 and the signal is read by the detector 109. If the optical disc 108 can record data by intensity-modulating the output of the short-wavelength light generator, information can be written.

The output from the short-wavelength light generator 105 is a TE10 mode having a sub-peak. When this is condensed, a condensed beam having a small spot diameter smaller than the diffraction limit of the lens is obtained. As a result, a small spot diameter due to the super-resolution effect was obtained in addition to the light condensing characteristic with the short wavelength light, so that it was possible to improve the recording density to 1.2 times that of the related art.

Furthermore, since high-output blue light can be generated, not only reading but also writing of information on an optical information recording medium is possible. By using a semiconductor laser as a fundamental wave light source, the semiconductor laser becomes very small, so that it can also be used for a small consumer optical disk reading and recording device.

Further, the optical wavelength conversion element can optimize the aspect ratio of the output beam by optimizing the optical waveguide width. For example, by adopting a waveguide layer structure having a high refractive index layer narrower than the waveguide layer width on the optical waveguide, it becomes possible to make the aspect ratio of the output beam close to 1: 1.

In order to improve the light-collecting characteristics of the optical pickup, a beam shaping prism and the like are not required, and high transmission efficiency, excellent light-collecting characteristics, and low cost can be realized. further,
The noise of the scattered light generated during beam shaping can be reduced, and the pickup can be simplified.

(Embodiment 8) In this embodiment, another optical wavelength conversion element using an ion exchange layer will be described.
As shown in FIG. 13, an ion exchange layer 34 and a waveguide layer 33 are formed on a surface of a crystal substrate 31 having a nonlinear optical effect. As described above, the ion exchange layer 34 can be formed by proton exchange and is a high refractive index layer having a higher refractive index than the substrate 31. Further, a domain-inverted portion 32 is formed on the surface of the substrate so as to periodically cross the waveguide layer 33. In this substrate 31, since the polarization direction and the substrate surface have an angle of 1 ° to 5 °,
The domain inversion section 32 also has an angle with the substrate surface in this range.

In such an optical wavelength conversion element, the relationship between the depth of the domain-inverted portion in the optical waveguide and the wavelength conversion efficiency was investigated. As shown in FIG. 14, the depth of the domain-inverted portion is indicated by the distance d between the substrate surface and the upper end of the domain-inverted portion 32 at the center of the width of the waveguide layer 33.
Also. The high refractive index layer was formed by proton exchange. The thickness of the domain-inverted portion was about 2 μm, and the thickness of the waveguide layer was about 3 μm. The substrate 31 used was MgO-doped LiNbO ₃ whose polarization direction was inclined about 3 ° with respect to the substrate surface.

As a result, as compared with the case where d = 0, the conversion efficiency when d was 0.2 to 0.5 μm was approximately doubled. When d was 0.2 to 0.3 μm, the conversion efficiency was further improved. On the other hand, if d> 1 μm, the conversion efficiency is d = 0.
It was almost the same as in the case of However, since the preferable range of d depends on the depth of the optical waveguide, it is preferable that d is determined so that the overlap between the optical waveguide and the domain-inverted portion increases according to the depth of the optical waveguide.

Further, optical damage in the optical wavelength conversion device was investigated. This is because, in the above investigation, when d is a preferable value, the conversion efficiency is improved, but the optical damage intensity is sometimes reduced. It is considered that the optical damage occurred on the surface of the optical waveguide where no domain-inverted portion was present. Then, when the relationship between d and the thickness D of the high refractive index layer was adjusted, it was found that the optical damage was improved when D> d. On the other hand, when D <d, the effect of the high refractive index layer was not remarkably recognized. This is probably because the high refractive index layer, which is a proton exchange layer, suppressed the occurrence of optical damage due to the deterioration of the electro-optic constant and the improvement of the electric conductivity. Also,
By forming the high refractive index layer on the surface of the optical waveguide, the overlap of the guided light was increased, and the conversion efficiency was improved.

The high refractive index layer may be formed on the entire substrate surface as shown in the figure, but may be formed only on the substrate surface near the surface of the optical waveguide.

Hereinafter, a method for forming a domain-inverted portion so as to have a certain angle with respect to the substrate surface as shown in the figure will be described. Such a domain-inverted portion uses a substrate (for example, a ferroelectric crystal substrate (LiNbO ₃ doped with MgO)) in which the polarization direction of the crystal is inclined with respect to the surface,
The crystal can be formed by arranging an insulating film between two electrodes spaced apart on the surface of the crystal and applying a voltage.

In this case, a comb-shaped electrode 192 and a rod-shaped electrode 193 are formed on the surface of the substrate 191 and a voltage is applied between both electrodes as shown in FIG. A method of extending the domain-inverted portion from the comb-shaped electrode can also be employed. However, it is preferable to use the insulating film 195 formed on the substrate surface in order to form the domain-inverted portions uniformly and at a depth equal to or greater than a certain value.

In this case, the method shown in FIGS. 16 to 18 can be employed, but it is preferable to form the rod-shaped electrode 193 on the insulating film 195 as shown in FIGS. By doing so, the uniformity of the partial pole reversal structure can be improved. Further, as shown in FIG. 17, the rod-shaped electrode 192 serving as a starting point of polarization inversion is
It is more preferable to cover with.

The insulating film 195 is not particularly limited. For example, an SiO ₂ film formed by a sputtering method can be used. When the method as shown in FIG. 17 is adopted, there is almost no effect when the film thickness is less than 100 nm, and when it is 200 nm or more, a uniform and deep domain-inverted portion 194 can be formed as shown in FIG. However, there is no significant change in the reversal characteristics even when the thickness is 1 μm or more. Therefore, it is preferable that the thickness of the insulating film be 200 nm to 1 μm.

[0134]

As described above, according to the present invention,
It is possible to provide an optical wavelength conversion element capable of outputting a second harmonic having excellent light condensing characteristics and having a large overlap between a fundamental wave and a second harmonic. Particularly, in a waveguide layer structure having a high refractive index layer, by forming the high refractive index layer by ion exchange, the loss of the high refractive index layer can be reduced,
Significant improvement in uniformity has become possible. Further, by suppressing the width of the sub-peak of the higher-order guided mode to the diffraction limit of light or less, the output higher-order mode harmonics have a super-resolution effect, and the light-collecting characteristics are dramatically improved. Therefore, its practical effect is great.

Further, by forming the high refractive index layer having a high refractive index into a stripe structure narrower than the width of the waveguide layer, the confinement of the waveguide layer in the width direction can be strengthened. The efficiency of the device can be further improved.

Further, according to the light generating device of the present invention, the light-collecting characteristics of the output beam can be greatly improved. At the same time, short wavelength light with high output can be obtained by the optical wavelength conversion element with high conversion efficiency, so that its practical effect is great.
In addition, the electric field distribution can be controlled by the high refractive index layer,
The controllability of the mode profile propagating in the waveguide layer is improved. As a result, the aspect ratio of the radiation pattern of the second harmonic output can be made close to 1, and the light use efficiency is greatly improved, so that the practical effect is large. This light generation device is suitable for an optical pickup, for example.

Further, by performing ion exchange using the same ion-exchange resistant mask used for forming the waveguide layer and the high refractive index layer, a stripe-like high refractive index layer is formed on the surface of the waveguide layer by self-alignment. Therefore, the practical effect is great.

[Brief description of the drawings]

FIG. 1A is a cross-sectional view of an optical waveguide, and FIG. 1B is a diagram showing an overlap between a fundamental wave and an SHG wave in the optical waveguide.

FIG. 2 is a diagram showing a relationship between the structure of an optical waveguide and an electric field distribution of a waveguide mode, and an electric field distribution diagram without a cladding layer.
(a), electric field distribution diagram when the high refractive index layer is thin (b), electric field distribution diagram when the high refractive index layer has an appropriate thickness (c), electric field distribution diagram when the high refractive index layer is thick (d).

FIG. 3 is a cross-sectional view illustrating a configuration example of a short-wavelength light generation device according to the present invention.

FIG. 4 is an intensity distribution diagram of a converging spot of an output beam, and FIG.
FIG. 7B is a focused spot intensity distribution diagram (b) of the E10 mode.

FIG. 5 is a perspective view of a configuration example of an optical wavelength conversion element of the present invention.
FIG. 3A is an electric field distribution diagram of a waveguide mode in a cross section of a main part of the optical wavelength conversion element.

FIG. 6 is a view for explaining the waveguide mode of the optical wavelength conversion element, and shows the mode of the waveguide layer having a slab-like high refractive index layer on the surface.
FIG. 3A is a mode distribution diagram and FIG. 3B is a mode distribution diagram of a waveguide layer having a stripe-like high refractive index layer on the surface.

FIG. 7 is a perspective view showing a configuration example of another optical wavelength conversion element of the present invention.

FIG. 8 is a diagram illustrating a relationship between an optical waveguide width and a phase matching wavelength.

FIG. 9 is a perspective view (a) and a cross-sectional view (b) of a main part of a configuration example of an optical wavelength conversion element of the present invention.

FIG. 10 is a view showing a manufacturing process of the optical wavelength conversion device of the present invention, wherein a process of forming a pattern on a substrate (a), a first proton exchange process (b), an annealing process (c), and a second process are performed. It is a figure which shows each proton exchange process (d).

FIG. 11 is a perspective view illustrating a configuration example of an optical waveguide of the present invention.

FIG. 12 is a perspective view illustrating a configuration example of an optical information processing apparatus according to the present invention.

FIG. 13 is a perspective view showing a configuration example of a light wavelength conversion element of the present invention.

FIG. 14 is a cross-sectional view of a main part showing another configuration example of the optical wavelength conversion element of the present invention.

FIG. 15 is a cross-sectional view showing an example of arrangement of a pair of electrodes for forming a domain-inverted portion.

FIG. 16 is a cross-sectional view illustrating a positional relationship between a pair of electrodes and an insulating film for forming a domain-inverted portion.

FIG. 17 is a cross-sectional view illustrating a positional relationship between a pair of electrodes and an insulating film for forming a domain-inverted portion.

FIG. 18 is a cross-sectional view illustrating a positional relationship between a pair of electrodes and an insulating film for forming a domain-inverted portion.

FIG. 19 is a sectional view showing a domain-inverted portion formed by the method shown in FIG. 17;

FIG. 20 is a diagram showing a configuration of a conventional light wavelength conversion element.

21 is a diagram illustrating a cross section of a main part of the optical wavelength conversion element in FIG. 20 together with electric field distributions of a fundamental wave and a harmonic.

FIG. 22 is a diagram illustrating a configuration of another conventional optical wavelength conversion element.

[Explanation of symbols]

Reference Signs List 61 substrate 62 waveguide layer 63 high refractive index layer 64 fundamental wave 65 SHG wave 71 semiconductor laser 72 optical wavelength conversion element 72a waveguide layer 73, 74 lens 76 LiNbO ₃ substrate (X plate) 77 waveguide layer 78 high refractive index layer 79 Polarization inversion part 95 LiNbO ₃ substrate 96 Ta film 97 First proton exchange part 98 Second proton exchange part 105 Short wavelength light generator 106 Beam splitter 107 Lens 108 Optical disk 109 Detector

Claims (23)

[Claims] 1. A crystal substrate, a waveguide formed on a surface of the crystal substrate, and an ion exchange layer formed on a surface of the waveguide, wherein the ion exchange layer has a refractive index of the waveguide. And a thickness such that a fundamental wave propagating in the waveguide cannot propagate in the ion exchange layer but a harmonic of the fundamental wave can propagate in the ion exchange layer. An optical waveguide. 2. The optical waveguide according to claim 1, wherein the thickness of the ion exchange layer is equal to or less than a diffraction limit of the harmonic. 3. The optical waveguide according to claim 1, wherein the width of the ion exchange layer is smaller than the width of the waveguide. 4. The optical waveguide according to claim 1, wherein the fundamental wave of a fundamental mode and the higher harmonic wave can be phase-coupled in the waveguide. 5. The optical waveguide according to claim 1, wherein a refractive index of the ion exchange layer is larger than 1.01 times a refractive index of the waveguide. 6. The optical waveguide according to claim 1, wherein the waveguide is a layer formed by ion exchange, and an ion exchange rate of the waveguide is 50% or less of an ion exchange rate of the ion exchange layer. 7. The optical waveguide according to claim 1, wherein in the crystal substrate, the polarization direction of the crystal and the surface form an angle of 0.5 ° to 5 °. 8. The optical waveguide according to claim 1, wherein a width of the high refractive index layer is tapered at an end of the waveguide. 9. The optical waveguide according to claim 1, wherein the crystal substrate has a nonlinear optical effect. 10. A crystal substrate, a waveguide formed on the surface of the crystal substrate, and a high refractive index layer formed on the surface of the waveguide and having a higher refractive index than the waveguide, The refractive index layer has a thickness such that a fundamental wave propagating in the waveguide cannot propagate in the high refractive index layer, but a harmonic of the fundamental wave can propagate in the high refractive index layer. An optical waveguide. 11. A crystal substrate, comprising: a waveguide formed on a surface of the crystal substrate; and an ion exchange layer formed on a surface of the waveguide, wherein the ion exchange layer has a refractive index of the waveguide. An optical waveguide, comprising: a refractive index larger than the optical waveguide, a width smaller than the width of the waveguide, and a depth equal to or less than half the depth of the waveguide. 12. A step of forming an ion-exchange resistant mask so that a non-masked portion of a line is secured on the surface of the crystal substrate, and ion-exchanged through the non-masked portion of the surface to form a second ion-exchange mask in the crystal substrate. A step of forming the first ion-exchange section, a step of expanding the first ion-exchange section into a waveguide by heat-treating the substrate, and a step of ion-exchanging through the non-mask portion to form the inside of the waveguide. Forming a second ion exchange portion having a higher refractive index than the waveguide. 13. A light comprising: the optical waveguide according to claim 1; and two or more domain-inverted portions formed so as to periodically cross the waveguide. Wavelength conversion element. 14. A crystal having a non-linear optical effect, a waveguide formed on a surface of the crystal, two or more domain-inverted portions formed so as to periodically cross the waveguide, and the waveguide. An ion exchange layer formed on the surface of the, the ion exchange layer has a refractive index larger than the refractive index of the waveguide, the fundamental wave propagating in the waveguide can not propagate in the ion exchange layer, but the An optical wavelength conversion element, wherein a harmonic of a fundamental wave has a thickness capable of propagating. 15. A crystal having a non-linear optical effect, a waveguide formed on a surface of the crystal, two or more domain-inverted portions formed so as to periodically cross the waveguide, and the waveguide. An ion exchange layer formed on the surface of the waveguide, wherein the ion exchange layer has a refractive index larger than the refractive index of the waveguide, a width smaller than the width of the waveguide, and a depth of the waveguide. An optical wavelength conversion element having a depth of less than half. 16. The optical wavelength conversion device according to claim 14, wherein the domain-inverted portion is formed in the waveguide but is not formed in the ion exchange layer. 17. The optical wavelength conversion element according to claim 14, wherein a depth of the ion exchange layer is deeper than a depth from a surface of the crystal at a center of a width of the waveguide to an upper end of the domain-inverted portion. . 18. A crystal having a non-linear optical effect, a waveguide formed on a surface of the crystal, two or more domain-inverted portions formed so as to periodically cross the waveguide, and the waveguide. A high-refractive-index layer having a higher refractive index than the waveguide, the fundamental wave propagating through the waveguide cannot propagate through the high-refractive-index layer, An optical wavelength conversion element, wherein a harmonic of a fundamental wave has a thickness capable of propagating in the high refractive index layer. 19. The optical wavelength conversion device according to claim 18, wherein the domain-inverted portion is formed in the waveguide but is not formed in the high refractive index layer. 20. An optical wavelength conversion device according to claim 13 and a semiconductor laser, wherein light emitted from said semiconductor laser is wavelength-converted by said optical wavelength conversion device. Light generator. 21. The light generator according to claim 20, wherein wavelength conversion is performed to a higher-order mode harmonic including a subpeak of 0.8λ or less. Here, λ is the wavelength of the harmonic. 22. A light-collecting optical system that collects wavelength-converted harmonics, wherein the numerical aperture NA of the light-collecting optical system and the thickness LD of the ion exchange layer or the high refractive index layer are: The light generator according to claim 20, wherein the following relational expression is satisfied. LD <0.8λ / NA where λ is the wavelength of the harmonic. 23. An optical pickup including the light generating device according to claim 20, a light collecting optical system, and a recording medium, wherein the light emitted from the light generating device is recorded on the recording medium by the light collecting optical system. An optical pickup characterized by focusing light on the top.