JP2002258340A - Optical wavelength conversion element, method of manufacturing for the same, short wavelength light source and optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the following problems with a proton exchange optical waveguide; the proton concentration distribution in this optical waveguide has a diffusion distribution to maximize the proton concentration on a substrate surface and the high-concentration proton exchange segment near the surface of the proton exchange optical waveguide is the cause for the propagation loss and light loss of the optical waveguide, thus heretofore causing the instability of output and the degradation in conversion efficiency. SOLUTION: This optical wavelength conversion element is provided with a layer partly decreased in the proton concentration near the surface of the proton exchange optical waveguide 3, by which the propagation loss of the optical waveguide is lessened and the conversion efficiency of the optical wavelength conversion element can be greatly improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion element used in the optical information processing and optical communication fields, a method of manufacturing the same, a short wavelength light source, and an optical system.

[0002]

2. Description of the Related Art In the field of optical information processing, a small-sized short-wavelength light source is required to realize a high-density optical disk and a high-definition display. Semiconductor lasers and quasi-phase matching (Quasi-Phas)
e-Match / hereinafter referred to as QPM) optical waveguide type wavelength conversion (Yamamoto et al., Optics Letters Vol. 16, No. 15, 1156)
(1991)) Second harmonic generation using a device (Second-Har
monic-Generation / hereinafter referred to as SHG). QP
The M-SHG element is composed of a periodically poled structure and an optical waveguide, and uses quasi-phase matching using the poled structure,
It is converted into a fundamental wave SHG propagating through the waveguide. The conversion efficiency of this type of optical wavelength conversion element largely depends on the overlap between the light propagating in the optical waveguide and the periodically poled structure. Therefore, in order to realize a highly efficient optical wavelength conversion element, a structure has been proposed in which the thickness of the periodically poled structure is increased and the overlap with the optical waveguide is increased.

As a method for increasing the thickness of the domain-inverted structure, a method has been known in which the crystal axis of the substrate is slightly inclined with respect to the plane of the substrate. Since the domain-inverted portion of the ferroelectric crystal grows along the crystal axis, using a substrate in which the C-axis of the substrate is inclined by 3 ° from the crystal surface, the domain-inverted portion grows toward the inside of the crystal, so A domain-inverted structure can be formed. As a conventional optical wavelength conversion element, Japanese Patent Laid-Open No. 9-2
No. 148431. The substrate is made of Mg-doped LiNbO _3, and the X axis of the substrate crystal is inclined by about 3 ° with respect to the normal to the substrate. By forming a periodic polarization inversion structure on this substrate, deep polarization inversion is formed. By forming an optical waveguide with respect to the periodic domain-inverted structure, it is possible to increase the overlap between the deep domain-inverted structure and the optical waveguide, and a highly efficient optical wavelength conversion element can be configured.

FIG. 9 is a schematic configuration diagram of a short wavelength light source using an optical waveguide type wavelength conversion device. As a semiconductor laser, Distributed Bragg reflector
(Hereinafter, referred to as DBR) tunable DBR semiconductor laser having a region. A tunable semiconductor laser having a DBR region is hereinafter referred to as a tunable DBR semiconductor laser. 30 is a 100 mW class AlGa in a 0.85 μm band.
An active layer region 31 and a DBR
It is composed of an area 32. By varying the injection current into the DBR region 32, the oscillation wavelength can be varied. Optical waveguide type wavelength conversion device 3 which is a wavelength conversion element
Numeral ₃ comprises an optical waveguide 34 formed on an X-plate MgO-doped LiNbO ₃ substrate and periodic domain-inverted regions 35. The laser light obtained from the emission surface of the DBR semiconductor laser 30 is coupled to the optical waveguide 34 of the optical waveguide type wavelength conversion device 33. A laser light of 60 mW was coupled to the optical waveguide for a laser output of 100 mW. The amount of current injected into the DBR region 32 of the wavelength tunable DBR semiconductor laser 30 is controlled to change the oscillation wavelength to the wavelength conversion device 3 of the optical waveguide type.
3, the blue light having a wavelength of 425 nm was obtained at about 10 mW.

When the SHG blue light source is applied to an optical disk, it is mounted on an optical pickup and used. Blue light emitted from the module is collimated by a collimating lens, beam-shaped by a prism pair, and polarized beam splitter (hereinafter referred to as PBS) and λ / 4.
After passing through the plate, it is bent 90 degrees by the rising mirror, and focused on the optical disk by the objective lens. The reflected light from the optical disk is bent by 90 degrees by PBS, and guided to a photodetector (hereinafter, referred to as PD) by a detection lens and a cylindrical lens to detect a signal. By using this optical pickup, reproduction of a high-density optical disk of 10 GB or more is realized.

[0006]

In the proton exchange waveguide, there is a propagation loss of the optical waveguide depending on the proton concentration. For this reason, there is a problem that the conversion efficiency of the optical wavelength conversion element is reduced due to the propagation loss of the optical waveguide. Also,
In an optical wavelength conversion element using an off-cut substrate in which the crystal axis is inclined from the normal line of the substrate, there is a problem that when an optical waveguide is formed by proton exchange, the propagation loss of the optical waveguide greatly increases.

[0007] The polarization axis of the SHG light emitted from the light wavelength conversion element using the off-cut substrate is inclined. Therefore, when a separation optical system using polarized light is used, the separation ratio of polarized light separation decreases due to the inclination of the polarization axis. In the conventional configuration diagram, the light from the light source is reflected on the optical disk surface, so that the polarized light rotates 90 ° reciprocating through the λ / 4 plate, is reflected by the polarizing prism, and is detected by the detector. Since the polarization of the SHG light is inclined, the polarization component increases and noise increases. In the case of an optical disk of a magnetic field modulation type, a signal from the disk is detected based on a rotation angle of polarized light.
For this reason, the inclination of the polarization angle of the light causes deterioration of the signal light.

Therefore, the present invention solves the above-mentioned problems,
It is an object of the present invention to provide an optical wavelength conversion element and a wavelength light source that can realize a short wavelength light source.

[0009]

Means for Solving the Problems To solve the above problems,
Therefore, the light wavelength conversion element of the present invention is made of LiNb_xTa_(1-x)O
_Three(0 ≦ x ≦ 1) formed on the substrate and near the substrate surface
A periodic domain-inverted structure and a substrate formed near the substrate surface.
And a roton exchange waveguide.
At least part of the surface of the proton exchange waveguide near the surface
Light having a part where the proton concentration is lower than the center part
It is a wavelength conversion element.

[0010] The optical wavelength conversion element of the present invention is preferably composed of LiN.
b _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1) substrate, a periodically poled structure formed near the surface of the substrate, and an optical waveguide, wherein the C axis of the substrate is aligned with the surface of the substrate. 0.5 ° to 2
° characterized by tilting.

Further, the method of manufacturing an optical wavelength conversion element according to the present invention is characterized in that a LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1) substrate is heat-treated in an acid and Li in the substrate and Li in the acid are heat-treated. A first proton exchange step of exchanging protons to form a proton exchange layer on the surface of the substrate, and a first step of heat-treating the substrate in a Li salt to ion exchange protons in the proton exchange layer with Li in the Li salt. And a reverse proton exchange step.

[0012] A short wavelength light source according to the present invention includes the optical wavelength conversion element, a semiconductor laser, and a submount.
The semiconductor laser and the optical wavelength conversion element are fixed on the submount, and the light from the semiconductor laser is wavelength-converted by the optical wavelength conversion element.

The optical system according to the present invention includes the short-wavelength light source, a focusing optical system, and a recording medium, and irradiates the recording medium with light from the short-wavelength light source.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A conventional light wavelength conversion device has realized a highly efficient light wavelength conversion device by increasing the thickness of polarization inversion by inclining the crystal axis by 3 ° with respect to the normal line of the substrate. However, it has been clarified that the tilt of the crystal axis with respect to the normal line of the substrate has the following problems in the light wavelength conversion device and the light source using the light wavelength conversion device. The inventor
By examining the characteristics of an optical wavelength conversion element comprising a periodically poled structure and an optical waveguide on a non-linear optical crystal substrate containing LiNbO ₃ as a main component, it was clarified that the following problems existed.

1) Propagation loss increases depending on the proton exchange concentration.

2) Increasing the crystal axis increases the propagation loss of the optical waveguide.

3) In the optical wavelength conversion device, the optical damage strength is deteriorated along with the increase in the waveguide loss of the optical waveguide. The propagation loss of the optical waveguide greatly affects the conversion efficiency. In an optical wavelength conversion element utilizing the second-order nonlinear optical effect, the SHG output increases in proportion to the square of the incident light to be converted. For this reason, when the propagation loss increases, the conversion efficiency decreases in proportion to the propagation loss, which causes the characteristics of the optical wavelength conversion element to be greatly deteriorated. Further, an increase in the propagation loss affects an increase in the absorption of the guided light, and the absorbed light significantly deteriorates the stability of the optical wavelength conversion element due to optical damage and heat distribution. In particular, when obtaining high-output SHG light, light absorption and light damage are serious problems. The propagation loss of the proton exchange waveguide used for the optical wavelength conversion element is about 1 dB / cm,
For a SHG output of about W, there is no major problem. However, experiments have shown that in order to obtain high output light of 10 mW or more, it is necessary to greatly reduce the propagation loss to less than half.

This embodiment solves the above problem by proposing a new waveguide structure for reducing the propagation loss of the optical waveguide.

(Embodiment 1) FIG. 1 shows an optical wavelength conversion element according to the present embodiment. On a Mg-doped LiNbO ₃ substrate 1, a periodically poled region 2 and a proton exchange optical waveguide 3 are formed. A high refractive index cladding layer 4 is formed on the surface of the substrate, and a layer 5 having a low proton concentration is formed near the surface of the waveguide.
Are formed. The C axis of the substrate is 1.5
° tilted. The proton exchange waveguide is formed by proton exchange and annealing, and the concentration of protons decreases in a graded manner from the surface toward the inside of the substrate. It is the point that forms.

The proton exchange waveguide is formed by exposing the LiNbO ₃ substrate to heat in an acid to exchange Li in the substrate with protons in the acid. Thereafter, an annealing process is performed to reduce the proton concentration. Annealing the proton exchange layer makes it possible to form a low-loss proton exchange waveguide, but there is a limit in reducing the proton exchange concentration by the annealing treatment. This is because the proton concentration is proportional to the change in the refractive index of the proton exchange layer. If the proton concentration is too low by the annealing treatment, the change in the refractive index of the optical waveguide becomes small, and it becomes difficult to sufficiently confine the guided light. It is difficult to reduce the proton concentration only to the extent that it exists as an optical waveguide.

When the proton exchange layer is annealed, protons diffuse into the substrate by thermal diffusion. At this time, the proton concentration distribution has a graded distribution shape as shown in FIG. The proton concentration distribution before annealing shown in FIG. 2A is converted into the concentration distribution shown in FIG. At this time, a portion having a high proton concentration is formed in the initial proton exchange portion (a), and the waveguide loss in this portion is particularly large. As can be seen from the figure, the proton concentration distribution decreases as the substrate surface reaches its maximum at the inside. The refractive index distribution of the proton exchange waveguide depends on the proton exchange concentration distribution. That is, in order to maintain a proton concentration distribution that functions as an optical waveguide, it is necessary to make a portion having a considerably high proton concentration exist near the surface of the proton exchange waveguide. This portion has a great effect on the propagation loss of the proton exchange waveguide.

In the present invention, attention is paid to the vicinity of the surface of the optical waveguide having a high proton concentration, and as shown in FIG. 3, by lowering the proton concentration near the surface of the optical waveguide, the propagation loss of the optical waveguide is greatly reduced. Made it possible. Furthermore, by reducing the proton exchange concentration on the surface of the optical waveguide, the light damage resistance was improved, and a high-output optical wavelength conversion device could be manufactured. In particular, it was effective to reduce the proton concentration in the thickness of the portion where the initial proton exchange was performed.

It has also been found that the dependence of the waveguide loss on the proton concentration appears remarkably in the off-cut substrate. In an off-cut substrate in which the crystal axis of the X plate (the normal to the substrate surface is parallel to the X axis) was slightly inclined, the increase in propagation loss due to the proton concentration was more remarkable than in the ordinary X plate.
In the proton exchange waveguide of the X plate, if the proton concentration is reduced to about 20% or less as compared with that before annealing, the waveguide loss depending on the proton concentration is reduced to a negligible level. On the other hand, even when the proton concentration of the off-cut substrate was 15% or less as compared with that before annealing, considerable waveguide loss was present, and it was found that the proton concentration had to be further reduced. However, further reduction of the proton concentration required a drastic reduction in the change in the refractive index of the optical waveguide, resulting in extremely reduced confinement of the optical waveguide.

Specifically, in the crystal of the X plate, the X and Z axes were rotated by θ ° about the crystal Y axis. A study on the X off-cut substrate. When θ was 0.5 ° or more, a difference was observed in the propagation loss between the off-cut substrate and the X-cut substrate. 1
Above °, the propagation loss increased by 10% or more, and the propagation loss increased with the off-cut angle. That is, in the annealed proton exchange waveguide, a portion having a high proton exchange concentration existing near the surface is a propagation loss of the optical waveguide.

In the off-cut substrate, the propagation loss due to an increase in the proton exchange concentration becomes remarkable, and there is a limit to the reduction in the propagation loss due to the annealing treatment. The two points were found as new issues.

In order to solve these problems, the present invention proposes a new waveguide structure. That is, a layer having a low proton concentration is formed near the surface of the proton exchange waveguide in the off-cut substrate. It has been found that this configuration solves the above two problems. By reducing the concentration of protons near the surface of the optical waveguide, it is possible to maintain the refractive index change of the optical waveguide and to remove a layer near the surface, which is a main cause of propagation loss.
This effect was particularly noticeable for off-cut substrates.

A method of removing a layer having a high proton concentration on the surface by etching or the like can be considered. However, if the surface is removed by etching or the like, damage due to etching remains in the proton exchange waveguide. In particular, since the periodic domain-inverted structure has different etching characteristics between the inverting portion and the non-inverting portion, irregularities are generated according to the domain-inverted structure. As a result, there is a problem that a new propagation loss occurs. Also,
By reducing the proton exchange concentration on the surface, the optical waveguide becomes a buried type, the waveguide mode becomes symmetrical in the depth direction of the substrate, and the efficiency of optical coupling such as coupling by a semiconductor laser, an optical fiber, a waveguide, and a lens is improved. Significantly improved.

As shown in FIG. 2B, in the conventional proton exchange waveguide, since the refractive index distribution is asymmetric in the depth direction, the mode profile of the guided light propagating through the waveguide has an asymmetric structure. Therefore, in coupling with a semiconductor laser, an optical fiber, a lens, or the like having a symmetric electric field distribution, the coupling efficiency is deteriorated due to a mode mismatch. On the other hand, by reducing the protons on the surface of the proton exchange waveguide, the refractive index distribution of the waveguide became closer to symmetric in the thickness direction of the substrate. For this reason, the coupling efficiency can be greatly increased.

Next, a method for manufacturing the optical waveguide of the present invention will be described. It is impossible to reduce the proton concentration near the surface of the proton exchange layer by the conventional method of annealing the proton exchange waveguide. Therefore, a method was introduced in which protons in the proton exchange layer were exchanged with Li in the Li salt by heat-treating the proton exchange layer in the Li salt to reduce the proton exchange concentration on the surface of the proton exchange layer.

As shown in FIG. 4, the manufacturing method is as follows.
A mask pattern 22 of Ta is formed on the surface 21 of the bO ₃ substrate. (B) The substrate is heat-treated in pyrophosphoric acid at 200 ° C. for about 7 minutes, and the proton-exchange layer
Form 3 (C) After removing the Ta mask, annealing is performed at 330 ° C. for about 2 hours. (D) Heat treatment in a Li salt (here, LiNO ₃ is used) at 250 ° C. for about 1 hour to form a portion 24 having a low proton exchange concentration. (E)
Anneal at 330 ° C. for about 1 hour. In the heat treatment (d), protons in the proton exchange are exchanged for Li in the Li salt (reverse proton exchange), so that the proton concentration near the surface of the proton exchange layer can be reduced.

A proton exchange waveguide was manufactured by this method, and the propagation loss was compared. In the off-cut substrate, a large difference was observed in the propagation loss. When a substrate with a 1.5 ° off-cut was used, it was confirmed that the propagation loss was reduced from 2 dB / cm to 1 dB / cm or less and the propagation loss was significantly reduced by reducing the proton exchange layer on the surface. Was. Further, by performing the reverse proton exchange, the crystallinity deteriorated by the proton exchange is recovered, so that the number of determined non-linear optical effects, electro-optical effects, acousto-optical effects, and the like is increased, and the characteristics are improved.

Next, the high refractive index cladding structure will be described. For the purpose of improving the conversion efficiency of the optical wavelength conversion element, a structure in which a clad layer having a higher refractive index than the waveguide layer (hereinafter, referred to as a high refractive index clad) is deposited on the surface of the optical waveguide has been proposed. The high-refractive-index cladding layer enhances the light confinement and increases the overlap between the fundamental wave and the harmonic, thereby improving the conversion efficiency.
However, applying a high refractive index cladding layer to a proton exchange waveguide is not appropriate in terms of propagation loss. As described above, the annealed proton exchange waveguide has a layer having a high proton concentration and a large propagation loss near the surface.

As shown in FIG. 5, the electric field distribution (b) of the guided light in the optical waveguide having the high refractive index cladding layer is different from the electric field distribution (a) in the optical waveguide having no high refractive index cladding. Pull up near the surface with high refractive index to enhance confinement. However, due to the high refractive index cladding,
The ratio of the electric field distribution in the surface layer of the proton exchange waveguide where the propagation loss is large increases, and the waveguide loss increases. In an optical waveguide having a high-refractive-index cladding layer, it is very preferable to reduce the proton concentration near the surface of the proton-exchanged waveguide because it has the advantages of reducing propagation loss and increasing efficiency. In addition, by combining a high refractive index clad with a waveguide having a reduced surface proton concentration, an optical waveguide with strong confinement can be realized.

When the proton concentration is reduced near the surface of the proton exchange waveguide, the effective refractive index of the optical waveguide is greatly reduced, and the confinement of the waveguide is deteriorated. This is because the refractive index distribution in the proton exchange waveguide is such that the layer having a high proton concentration near the surface has the largest refractive index and realizes an increase in the effective refractive index of the optical waveguide. Reducing the surface proton concentration is equivalent to removing this layer, which significantly reduces the effective refractive index of the waveguide. A waveguide structure having a high-refractive-index cladding layer is an optimal structure as a means for compensating for this and further increasing the effective refractive index.

By combining the high refractive index cladding with an optical waveguide having a reduced proton concentration on the surface, it becomes an effective structure capable of realizing an optical waveguide structure with high confinement and low loss. An oxide film containing Nb ₂ O ₅ as a main component is effective as a material for the high refractive index cladding. Nb ₂ O ₅ has excellent transmission characteristics up to a short wavelength range, and has a higher refractive index than LiNbO _{3 as} a substrate. Further, it is very preferable because an optical thin film can be easily deposited by a normal sputtering apparatus. Also, nitride films such as GaN and AlN are preferable because of their excellent transmission characteristics.

Next, the absorption coefficient of the high refractive index cladding layer will be described. By forming the high refractive index cladding layer on the optical waveguide, confinement of the optical waveguide is enhanced. By increasing the power density of the guided light, the conversion efficiency of the optical wavelength conversion element can be significantly improved. However,
It has become clear that reducing the absorption coefficient is a major issue in applying a high refractive index cladding to an optical wavelength conversion device. An optical wavelength conversion element using a periodic polarization inversion structure has a very narrow phase matching wavelength tolerance of about 0.1 nm in full width at half maximum. For this reason, it is necessary to make the temperature uniformity over the entire length of the element less than several degrees Celsius. When the temperature distribution becomes large, the phase matching state becomes non-uniform, and the conversion efficiency is greatly reduced. If absorption exists in the high refractive index cladding layer, a temperature distribution due to guided light occurs. The second harmonic generated in the optical wavelength conversion element has a distance of 2 with respect to the propagation direction.
It increases in proportion to the power.

Accordingly, a distribution of SHG light is generated in the propagation direction that is proportional to the square characteristic of the distance. Therefore, if SHG light is absorbed, a temperature distribution occurs in the propagation direction of the optical waveguide. This temperature distribution becomes a factor of characteristic deterioration of the optical wavelength conversion element. Specifically, what is required for the cladding layer is SH.
Reduction of the absorption coefficient for G light, especially for wavelengths of 450 to 400 nm. The ratio of the electric field distribution of the guided light existing in the cladding layer is several percent in total. SHG due to cladding absorption
When the power attenuation is calculated, the series coefficient of the cladding k = 1
About 50% per 1 mm is absorbed at 0 ^-3 , and 5% or more is absorbed even at k = 10 ^-4 . SHG output 10mW
In the case where the above occurs, in order to reduce the temperature distribution of the optical waveguide due to the absorption of the clad to several degrees Celsius or less, the temperature distribution of the optical wavelength conversion element due to the absorption occurs unless the absorption coefficient is kept at least k = 10 ^-4 or less. However, it was found that characteristic deterioration occurred.

Therefore, what is required for the high-refractive-index cladding is a characteristic in which the SHG light has an absorption coefficient of 10 ^-4 or less for wavelengths of 400 to 450 nm. For that purpose, first, a material having very low absorption in a short wavelength region is used. Also, it is necessary to set conditions for realizing low loss for the film forming conditions. As a material for realizing such low absorption characteristics at a short wavelength, an oxide film containing Nb ₂ O ₅ as a main component is very preferable. Also, nitride films such as GaN and AlN are preferable because of their excellent transmission characteristics.

Next, the light damage resistance will be described. Optical damage is a phenomenon in which the refractive index of an optical waveguide changes depending on the intensity of light, and is noticeable in LiNbO ₃ . Optical damage is a phenomenon that becomes a problem when an optical waveguide is used, and changes in refractive index and refractive index distribution cause instability and deterioration of device characteristics using an optical waveguide. Increasing the power at which optical damage occurs (light damage resistance) is an important issue for the stability of an optical waveguide. By reducing the proton concentration in the surface layer, the light damage resistance can be greatly increased. An increase in the proton concentration causes an increase in the propagation loss of the optical waveguide and an increase in the absorption loss for the propagating light.

The absorption of the propagating light causes an increase in the temperature distribution difference in the waveguide, and further causes a change in the refractive index due to optical damage. In our experiments, when SHG light in the 400 nm band was generated, a stable output operation was demonstrated for a waveguide light of about 10 mW in a normal waveguide, but optical damage was observed for SHG light of 20 mW or more. Causes the waveguide light characteristics to become unstable. On the other hand, when a waveguide structure having a reduced proton concentration near the surface of the proton exchange layer is used, 40 m
Stable waveguide characteristics can be realized even for SHG light of W or more. By reducing the proton concentration in the vicinity of the surface of the proton exchange layer, it was possible to greatly improve the light damage resistance.

Next, the relationship between the proton exchange concentration and the propagation loss, which are the basis of the present invention, will be described. It has been known that the proton exchange concentration affects the propagation loss of an optical waveguide. However, the relationship between proton exchange and propagation loss in an off-cut substrate has not been clarified. In a normal proton exchange waveguide, after performing proton exchange, annealing treatment is performed to reduce the proton concentration, thereby reducing waveguide loss. The propagation loss of the proton exchange waveguide increases in the order of the Z plate, the X plate, and the Y plate.
In the case of a dimensional waveguide, the waveguide loss is determined depending on the propagation direction. An X plate or a Y plate is effective for achieving the coincidence of polarization directions in direct coupling with a semiconductor laser. The X plate is most desirable in consideration of the propagation loss.

Normally, in the case of the X-plate, the reduction of the propagation loss is completed only by annealing the proton exchange concentration to less than half of that after the proton exchange, and the subsequent annealing treatment has no effect on the reduction of the propagation loss. Was known. However, it has been found that in a so-called off-cut substrate that is slightly tilted from the X-axis of the crystal axis, this characteristic is greatly different. The substrate used was 3 ° off-cut substrate Mg.
It was doped LiNbO ₃ . 0.2μ proton exchange
m, and the annealing treatment was performed at 330 ° C. for 210 minutes to expand the proton exchange layer to about 2 μm. Protons are thermally diffused into the substrate by the annealing treatment, and the concentration distribution is attenuated in a graded manner from the surface toward the inside of the substrate. Proton concentration on the surface is 10% immediately after proton exchange
It had decayed below.

However, in the off-cut substrate, the proton concentration was extremely lowered by the annealing treatment. It has been discovered that even in such cases, there is a propagation loss dependent on the proton concentration. In particular, it has been found that there is a remarkable increase in propagation loss due to chemical damage in a region of about 0.2 μm on the surface where the initial proton exchange was formed. It has been found that it is important for the off-cut substrate to reduce the propagation loss near the proton exchange surface. further,
As a method of greatly reducing the propagation loss in the surface layer, it has been found that a method of reducing the proton exchange concentration near the surface is very effective. That is, by reducing the proton concentration in the vicinity of the surface of the proton exchange layer with respect to the concentration inside the proton exchange layer, the propagation loss of the optical waveguide can be significantly reduced.

The waveguide structure of the optical wavelength conversion device of the present invention has a low proton concentration over the entire surface of the optical waveguide by performing reverse proton exchange on the entire surface of the proton exchange exchange waveguide as shown in FIG. Although the structure was such that an inverse proton exchange layer was formed, if the inverse proton exchange layer was further formed only at the center of the waveguide as shown in FIG. 6, a more efficient optical wavelength conversion device could be configured. Becomes The main factor of the propagation loss of the optical waveguide is the proton exchange portion before annealing, and the high proton concentration portion of the proton exchange waveguide expanded by the annealing process, that is, the central portion of the waveguide. Therefore, by performing reverse proton exchange on this portion, it is possible to reduce the loss of the waveguide. Further, by limiting the reverse proton exchange portion to the center portion of the waveguide, it is possible to minimize the reverse proton exchange area. Inverse proton exchange reduces the refractive index change due to proton exchange, so that the effective refractive index of the waveguide is reduced and the confinement of the waveguide is deteriorated.

Therefore, by limiting the area of the reverse proton exchange to the central part of the waveguide, the area of the reverse proton exchange can be limited, and the decrease in the refractive index of the proton exchange layer can be suppressed. As a result, an optical waveguide having stronger confinement can be formed, and a highly efficient optical wavelength conversion element can be formed. In the fabrication process, alignment-free fabrication becomes possible by applying a proton exchange mask to reverse proton exchange. Specifically, it is almost the same as the manufacturing process shown in FIG. The point to be changed is that in the annealing step (c), annealing is performed without removing the Ta selection mask. The proton exchange layer diffuses twice or more in the depth direction and width direction of the waveguide. Thereafter, when heat treatment is performed in a Li salt, an inverse proton exchange layer (a layer having a low proton concentration) is formed near the center of the waveguide surface.

(Embodiment 2) Here, the structure of another optical wavelength conversion element of the present invention will be described. An off-cut substrate is used to increase the efficiency of the light wavelength conversion element. The use of an off-cut substrate increases the thickness of the domain inversion and increases the overlap with the optical waveguide, so that the conversion efficiency can be improved. Assuming that θ when the X and Z axes are rotated by θ ° around the crystal Y axis in the crystal of the X plate is the offcut angle, the thickness of the domain-inverted layer increases with the offcut angle. In order to form a thick domain inversion structure, a crystal having a large off-cut angle is required. Usually, an off-cut substrate of about 3 ° is used. However, when the proton exchange waveguide is used as described above, a significant increase in the propagation loss was observed with an increase in the off-cut angle. Further, when the high-refractive-index clad and the proton exchange waveguide are used together, the result is that the waveguide loss is further increased. I checked off-cut substrates. When θ was 0.5 ° or more, a difference was observed in the propagation loss between the off-cut substrate and the X-cut substrate. In the case of a crystal having θ of 3 ° or more, the propagation loss was 2 dB / cm or more. Therefore, when the off-cut angle is set to 1.5 °, the waveguide loss can be reduced to 1 dB / cm or less.

On the other hand, from the viewpoint of forming a deep polarization inversion, the thickness of the polarization inversion formed becomes smaller as θ decreases. When θ is 0.5 ° or less, only a domain-inverted thickness equivalent to that of a normal X plate can be obtained. Therefore, in consideration of the domain-inverted thickness, an off-cut angle of 0.5 ° or more is required. Further considering the propagation loss, the off-cut angle of the substrate is 0.5
~ 2 ° is most desirable. In order to realize high efficiency of the light wavelength conversion element by increasing the inversion thickness, 1 ° to 2 ° is more preferable. When the off-cut angle is set to 2 ° or less, the light damage resistance is improved. This is the result of suppressing the occurrence of optical damage by reducing the propagation loss of the optical waveguide.

Further, when the proton concentration on the surface of the proton exchange waveguide was reduced, the waveguide loss was further reduced. When an off-cut substrate is used as described above, the influence of the high refractive index cladding is large. When an off-cut substrate is used, the electric field distribution of the guided mode is attracted to the substrate surface. As a result, the propagation loss when using the off-cut substrate is greatly increased. It is very important to set the off-cut angle to 2 ° or less when using a high refractive index clad.

In the present invention, the X off-cut substrate in which the X and Z axes of the X plate whose crystal X-axis is perpendicular to the substrate surface is slightly inclined about the Y-axis (parallel to the substrate) has been described. This is because the X plate can form a low-loss proton exchange waveguide with less chemical damage during proton exchange than the Y plate.
On the other hand, the use of a Y off-cut substrate in which the YZ axis of the Y plate crystal is slightly inclined about the X axis was also studied. In the Y-offcut substrate, the crystal surface and the X-axis of the crystal are in a parallel relationship. The major difference between the Y-offcut substrate and the X-offcut substrate lies in the domain-inverted structure formed. The polarization inversion of the Y off-cut substrate can be formed deeper than that of the X off-cut substrate, and the cross-sectional shape is close to a symmetrical rectangular shape. On the other hand, the polarization inversion formed on the X off-cut substrate has a triangular cross section and has left-right asymmetry.

When the conversion efficiency was calculated from the overlap between the optical waveguide and the domain-inverted structure, it was found that the use of the Y-offcut domain-inverted structure provided 1.5 times the conversion efficiency as compared with the X-offcut domain-inverted structure. . Considering the crystal structure, the crystal structure of the X-off cut is a symmetrical structure, and it is thought that the domain-inverted structure also forms a symmetrical domain-inverted structure. It has been found that a highly efficient inversion structure is formed in the substrate. Therefore, a more efficient light wavelength conversion element could be formed by using the Y off-cut substrate. A problem when using a Y off-cut substrate is the propagation loss of the optical waveguide. When manufacturing an optical waveguide type wavelength conversion element using a Y off-cut substrate, formation of a low-loss optical waveguide becomes an issue.

This is because it is difficult to form a proton exchange waveguide having a lower loss in the Y plate than in the X plate. In order to form low-loss proton exchange on the Y plate, it is necessary to reduce chemical damage during proton exchange. Pyrophosphoric acid, benzoic acid, and the like, which are usually used for proton exchange, have a large dissociation constant during proton exchange, and thus increase waveguide loss due to chemical damage. Therefore, weak acidity (small dissociation constant)
Acid is required. It is preferred to use at least three weak acids in the proton exchange acid. Also, chemical damage can be reduced by forming an optical waveguide by a method other than proton exchange. For example, by using a waveguide forming method by processing a crystal and using the surface itself as an optical waveguide, the crystal itself can be used for the optical waveguide without using proton exchange, so that a low-loss optical waveguide can be formed. .

Since the Y off-cut substrate is more susceptible to chemical damage than the X off-cut substrate, an increase in the propagation loss depending on the proton concentration also appears remarkably. For this reason, the low loss effect of the waveguide due to the reduction of the proton concentration near the surface of the optical waveguide appeared more remarkably. Normally, the waveguide formed on the Y-offcut plate is 1.5 times smaller than the waveguide formed on the X-offcut substrate.
Although the waveguide loss is twice or more, it was confirmed that a waveguide having a smaller waveguide loss than a normal X off-cut substrate can be formed by reducing the proton concentration near the surface by the reverse proton exchange treatment. As a result, a low-loss waveguide can be formed even on a Y off-cut substrate, and a high-output SH
The G element can be realized.

(Embodiment 3) The characteristics of the optical wavelength conversion element shown in Embodiment 1 will be described. The optical wavelength conversion device shown in FIG. 1 was manufactured, and its characteristics were evaluated. The fabrication method was such that a periodic polarization inversion of about 2.8 μm was formed by an electric field application method. After removing the electrodes, a mask pattern for an optical waveguide is formed, and a heat treatment is performed in pyrophosphoric acid to a depth of 0.2.
A proton exchange layer of about μm is formed. After performing an annealing treatment to expand the proton exchange layer to about 1 μm, heat treatment is performed in a Li salt to form a reverse proton exchange layer of about 0.2 μm. By forming the reverse proton exchange layer more than the thickness of the initial proton exchange, the propagation loss of the optical waveguide can be significantly reduced. Further, the optical waveguide is annealed to form an optical waveguide having a depth of about 2 μm. An Nb ₂ O ₅ film is formed on the waveguide in a 0.
Deposit about 2 μm. Optical input and output portions of the optical waveguide were formed by optically polishing both end surfaces of the optical waveguide.

The characteristics of the manufactured light wavelength conversion device were evaluated. When a fundamental wave having a wavelength of 820 nm was input to the optical wavelength conversion element, the wavelength could be converted into SHG light having a wavelength of 410 nm. 30mW SHG for 60mW fundamental wave input
Output was obtained and conversion efficiency of 50% was achieved. The manufactured optical wavelength conversion element can reduce the propagation loss of the fundamental wave from 1 dB / cm to 0.5 dB / cm. S
As a result of greatly reducing the propagation loss with respect to HG light, the conversion efficiency of the optical wavelength conversion element could be increased 1.5 times. further,
Improvements in the optical damage characteristics were also observed. Due to the structure of the optical wavelength conversion element of the present invention, SHG having a wavelength of 410 nm
The propagation loss has been greatly reduced. As a result, the absorption of SHG light is reduced, and a temperature distribution is generated in the optical waveguide propagation direction,
And the occurrence of optical damage was dramatically reduced. In the conventional optical wavelength conversion device, output instability occurred for SHG light of 10 to 20 mW or more, but in the optical wavelength conversion device of the present invention, the output was unstable even for high output SHG light of 40 mW or more. Output was obtained.

(Embodiment 4) Here, a method for manufacturing an optical wavelength conversion element of the present invention will be described. By heat-treating the proton-exchanged LiNbO ₃ substrate in a Li salt, reverse proton exchange in which protons in the crystal are exchanged with Li becomes possible. A method for manufacturing a high-efficiency optical wavelength conversion device utilizing this phenomenon will be described.

First, a description will be given of how to increase the efficiency by increasing the nonlinear optical constant. Ordinary LiNbO ₃ crystals are crystals having a congruent composition that facilitates crystal growth. this is,
The ratio of Li / Nb in the crystal is not 1/1 of the perfect crystal,
Crystals having a ratio of about 48/52 are easily grown. However, recently, with the progress of crystal growth technology, Li / N
It is possible to grow a perfect crystal having a b ratio of 1/1, and its nonlinear optical constant is changed to that of a conventional congruent composition LiNbO _3.
It has been confirmed that it is about 1.2 times higher than that of. However, it is difficult to grow a perfect crystal, and it is currently impossible to obtain a large substrate.

We used the reverse proton exchange step to convert crystals close to perfect crystals into LiNbO ₃ having a congruent composition.
We found that it can be made from crystals. The production process is a method in which proton exchange is performed on the LiNbO ₃ crystal, and further, the proton exchange layer undergoes reverse proton exchange. When proton exchange is performed, excess protons are injected into the crystal.
By subjecting this to reverse proton exchange, it was possible to increase the Li concentration in the crystal. As a result, a layer having a high Li concentration was formed on the crystal surface, and the composition of this layer was close to a perfect crystal, so that a crystal surface layer having a large nonlinear constant could be formed.
Further, by repeating the proton exchange step and the reverse proton exchange step, a surface layer closer to a perfect crystal can be formed, and the thickness of the layer can be increased in the depth direction. Also, by performing proton exchange on a layer close to the formed perfect crystal to form an optical waveguide, an optical wavelength conversion element having a high nonlinear optical constant can be manufactured.

By forming a perfect crystal layer on the crystal surface,
It becomes possible to form a highly efficient optical wavelength conversion element using this layer as a waveguide layer. By completely crystallizing the substrate surface on which the domain inversion has been formed by the method of the present invention, making this into a thin film and using it as a waveguide structure, a complete crystal L having a periodic domain inversion structure
iNbO ₃ can be formed. By making the optical waveguide three-dimensional, a highly efficient optical wavelength conversion element can be formed.
Furthermore, the use of an off-cut substrate as the substrate crystal facilitated the formation of a periodically poled structure.

(Embodiment 5) FIG. 7 is a cross-sectional view of a short wavelength light source of the present invention in which a short wavelength light source is manufactured using the light wavelength conversion element described in the second embodiment. Si submount 10
Above, an AlGaAs-based tunable DBR semiconductor laser 1
1 and the optical wavelength conversion element 12 are fixed. The optical wavelength conversion element 12 is formed of a periodically poled structure 2 and a proton exchange waveguide 3, and the proton concentration distribution of the proton exchange waveguide is small near the surface.

The electric field distribution of the guided light of the semiconductor laser has a symmetric structure in both the depth direction and the thickness direction. In contrast,
A proton exchange waveguide formed by ordinary proton exchange and annealing has a graded refractive index distribution in the depth direction. Therefore, the electric field distribution of the guided light guided through the proton exchange waveguide is asymmetric in the depth direction. When a semiconductor laser having an electric field distribution having a symmetric structure and a proton exchange waveguide having an electric field distribution having an asymmetric structure are directly coupled, the propagation loss of light was 1 dB or more at the coupling portion. On the other hand, by reducing the proton exchange concentration on the surface of the proton exchange waveguide, the electric field distribution of the waveguide can be made closer to a symmetric structure. As a result,
The coupling loss can be significantly reduced to 0.5 dB or less, and the output characteristics of the optical wavelength conversion element can be improved by 1.2 times or more by high efficiency coupling. In addition, the manufactured optical wavelength conversion element has high efficiency and excellent damage resistance.
The above-described high-output short-wavelength light source can be realized.

The off-cut angle of the substrate constituting the light wavelength conversion element also has a significant effect on the characteristics of the short wavelength light source. When an off-cut substrate is used, the polarization axis of emitted light is inclined in proportion to the off-cut angle. Therefore, another polarized light component is included in the polarized light axis of the emitted light. For example, 3
When a substrate having an off-cut angle of not less than ° is used, when an optical system is designed based on the optical axis of a beam, an orthogonal polarization component of 5% or more is included as a polarization component. In an optical system using polarized light, the light of the orthogonal component becomes a noise component. On the other hand, when a substrate having an off-cut angle of 2 ° or less is used, the light of the orthogonal polarization component becomes 3% or less, and the noise component can be greatly reduced.

Further, by reducing the proton concentration near the surface of the proton exchange waveguide, the mode profile of light propagating through the optical waveguide can be made closer to a structure symmetrical with respect to the waveguide depth direction. . As a result, the light beam emitted from the short-wavelength light source using the light wavelength conversion element of the present invention becomes a beam close to the center symmetry, and the light-collecting characteristics have been greatly improved. In the conventional configuration, it was necessary to improve the beam quality by beam shaping or the like.
The light-gathering characteristics close to the diffraction limit were obtained without using beam shaping. This is because the quality of the output beam has been greatly improved.

In joining the optical wavelength conversion element and the submount, heat conduction has a great effect on the stability of the light source. As described above, for stabilizing the output of the optical wavelength conversion element, it is necessary to make the temperature distribution of the crystal uniform. When bonding the optical wavelength conversion element to the submount, the gradient of the temperature distribution of the optical wavelength conversion element could be reduced by using an adhesive having higher thermal conductivity than the optical wavelength conversion element. As a result, a short-wavelength light source having stable output characteristics even for high-output SHG light was realized.

(Embodiment 6) An optical system according to the present invention will be described with reference to FIG. FIG. 8 shows an optical information processing apparatus according to the present invention. In FIG. 8, the short-wavelength light source 40 described above is used.
The beam having an output of 40 mW that has passed through the polarizing beam splitter 51 passes through the λ / 4 polarizing plate 55 and then passes through the lens 5.
2 irradiates an optical disc 53 as an information reproducing medium. Conversely, the reflected light is collimated by a lens 52, reflected by a polarizing plate 55 and a polarizing beam splitter 51, and a signal is read by a photodetector 54. Further, the output of the coherent light generation device is intensity-modulated so that the optical disk 5
3, information can be written. Since the optical wavelength conversion element of the present invention has high efficiency and high output characteristics, it is possible to generate 40 mW SHG light using a 100 mW semiconductor laser. For this reason, information could be easily written on the optical disk using the short wavelength light source.

The short-wavelength light source of the present invention has excellent beam symmetry, so that light-gathering characteristics close to the diffraction limit, which requires beam shaping, were obtained. Also, by suppressing the off-cut angle of the substrate of the optical wavelength conversion element to 2 ° or less,
The inclination of the polarization of the output beam can be suppressed to 2 ° or less, and the generation of noise due to the mixing of other polarization components can be suppressed. Furthermore, since high-output blue light can be generated, not only reading but also writing of information on an optical disc 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.

[0066]

As described above, according to the present invention, in a light wavelength conversion device using a proton exchange waveguide, a layer having a low proton concentration is partially formed on the surface of the proton exchange waveguide to thereby provide an optical waveguide. The loss can be reduced, and the light damage resistance can be greatly improved. As a result, it is possible to increase the efficiency and output of the optical wavelength conversion element, and the practical effect is great.

By using a substrate having an off-cut angle of 0.5 to 2 °, the loss of the proton exchange waveguide can be reduced, and the practical effect is large.

Further, the LiNbO ₃ substrate is subjected to proton exchange,
By performing reverse proton exchange thereafter, a crystal structure close to a perfect crystal can be formed on the substrate surface, and a highly nonlinear crystal material can be easily formed.

Further, a short-wavelength light source having a high output can be realized by a short-wavelength light source in which the above-mentioned optical wavelength conversion element is modularized with a semiconductor laser, and the practical effect is large.

By configuring an optical system using this short wavelength light source, an optical recording apparatus using a high output short wavelength light source can be realized, and its practical effect is great.

[Brief description of the drawings]

FIG. 1 is a configuration perspective view of an optical wavelength conversion element of the present invention.

FIG. 2 is a characteristic factor diagram showing a proton concentration distribution in a proton exchange waveguide.

FIG. 3 is a characteristic factor diagram showing a proton concentration distribution in the optical waveguide of the present invention.

FIG. 4 is a diagram showing a method for manufacturing an optical wavelength conversion element of the present invention.

5A is a characteristic factor diagram showing an electric field distribution of guided light in a conventional optical waveguide. FIG. 5B is a characteristic factor diagram showing an electric field distribution of guided light in an optical waveguide of the present invention.

FIG. 6 is a configuration perspective view of another optical wavelength conversion element of the present invention.

FIG. 7 is a configuration perspective view of a short wavelength light source according to the present invention.

FIG. 8 is a configuration diagram of an optical system according to the present invention.

FIG. 9 is a configuration diagram of a conventional short wavelength light source.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 Mg-doped LiNbO ₃ substrate 2 Periodically domain-inverted region 3 Proton exchange optical waveguide 4 High refractive index cladding layer 5 Low proton concentration layer 10 Si submount 11 DBR semiconductor laser 12 Optical wavelength conversion element 21 Substrate 22 Ta 23 Proton exchange layer 24 Portion with Low Proton Exchange Concentration 30 DBR Semiconductor Laser 31 Active Layer Region 32 DBR Region 33 Optical Waveguide Wavelength Conversion Device 34 Optical Waveguide 35 Periodic Polarization Inversion Region 40 Short Wavelength Light Source 51 Polarization Beam Splitter 52 Lens 53 Optical Disk 54 Detector 55 λ / 4 polarizing plate

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Akihiro Morikawa 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture F-term in Matsushita Electric Industrial Co., Ltd. (reference) 2K002 AA01 AA02 AB12 BA01 CA03 DA06 EA08 FA26 FA27 HA20

Claims (19)

[Claims] 1. A LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1) substrate, a periodically poled structure formed near the substrate surface, and a proton exchange waveguide formed near the substrate surface An optical wavelength conversion element comprising: a portion having a lower proton concentration than a central portion of the proton exchange waveguide in at least a part near the surface of the proton exchange waveguide. 2. The method according to claim 1, wherein the C axis of the substrate is 0.
The light wavelength conversion device according to claim 1, wherein the light wavelength conversion device is inclined by 5 ° or more. 3. The method according to claim 1, wherein the C axis of the substrate is set at 0.
2. The light wavelength conversion device according to claim 1, wherein the light wavelength conversion device is inclined by 5 to 2 degrees. 4. A substrate comprising: a LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1) substrate; a periodically poled structure formed in the vicinity of the substrate surface; and an optical waveguide. An optical wavelength conversion element, which is inclined by 0.5 ° to 2 ° with respect to the substrate surface. 5. The light wavelength conversion element according to claim 1, wherein said substrate surface is parallel to an X axis of said crystal. 6. The optical wavelength conversion device according to claim 1, further comprising a cladding layer on the surface of the substrate, wherein a refractive index of the cladding layer is larger than a refractive index of the optical waveguide. 7. The optical wavelength conversion device according to claim 4, wherein said optical waveguide is a proton exchange waveguide. 8. An optical wavelength conversion element according to claim 7, further comprising a layer having a low proton concentration near the surface of said proton exchange waveguide. 9. The optical wavelength conversion element according to claim 6, wherein an absorption coefficient of the cladding layer for a harmonic propagating through the optical waveguide is 10 ^-4 or less for light having a wavelength of 400 to 430 nm. 10. The optical wavelength conversion element according to claim 6, wherein said high refractive index film is a cladding layer containing Nb ₂ O ₅ as a main component. 11. LiNb _x Ta _(1-x) O ₃ (0 ≦ x ≦ 1)
A first proton exchange step of exchanging Li in the substrate with protons in the acid by heat treating the substrate in an acid to form a first proton exchange layer on the substrate surface; A first reverse proton exchange step of ion-exchanging protons in the proton exchange layer with Li in a Li salt to form a first reverse proton exchange layer. 12. The method according to claim 11, wherein the C-axis of the substrate is inclined at least 0.5 ° with respect to the surface of the substrate. 13. The method of manufacturing an optical wavelength conversion element according to claim 11, wherein a C axis of said substrate is inclined by 0.5 ° to 2 ° with respect to said substrate surface. 14. The method for manufacturing an optical wavelength conversion device according to claim 11, further comprising a second proton exchange step of forming a second proton exchange layer in addition to said step. 15. The method according to claim 11, wherein the depth of the first proton exchange layer is substantially equal to the depth of the first reverse proton exchange layer. 16. The first proton exchange step and the first proton exchange step.
And alternately repeating the reverse proton exchange step.
2. A method for manufacturing the optical wavelength conversion element according to 1. 17. An optical wavelength conversion device according to claim 1, comprising: a semiconductor laser; and a submount, wherein the semiconductor laser and the optical wavelength conversion device are fixed on the submount. A short-wavelength light source for converting light from the semiconductor laser by the optical wavelength conversion element. 18. The short-circuit according to claim 16, wherein the clad layer of the optical wavelength conversion element and the submount are bonded via an adhesive, and the adhesive has a higher thermal conductivity than the optical waveguide. Wavelength light source. 19. An optical system comprising: the short-wavelength light source according to claim 17; a light-collecting optical system; and a recording medium, and irradiating the recording medium with light from the short-wavelength light source.