JPH0820655B2 - Optical wavelength conversion element - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion element used in the field of optical information processing using coherent light or the field of optical measurement and control.

Prior Art FIG. 11 shows a configuration diagram of a conventional optical wavelength conversion element. Less than
Harmonic generation for a fundamental wave with a wavelength of 0.84 μm (wavelength 0.42
μm) will be described in detail with reference to the drawings. [T. Taniuchi
and K. Yamamoto, "Second harmonic generation by Che
renkov radiation in proton-exchanged LiNbO ₃ optica
l waveguide ", CLEO '86, WR3, 1986,
reference]. When the fundamental wave P1 is incident on the incident surface of the embedded optical waveguide 2 when the condition that the effective refractive index N1 of the guided mode of the fundamental wave becomes equal to the effective refractive index N2 of the harmonic wave is satisfied, the optical waveguide 2 harmonics P2 in LiNbO ₃ substrate 1 is efficiently radiated, operates as an optical wavelength conversion element from.

Such a conventional optical wavelength conversion element has an embedded optical waveguide as a basic constituent element. A method of manufacturing this embedded optical waveguide will be described. Li which is a ferroelectric substrate
High-refractive index layer (refractive index difference ΔNe = 0.13) with NbO ₃ substrate vapor-deposited with Cr or Al and having slits with a width of several μm opened by photo process and etching and heat-treated in benzoic acid. Had formed. [JL Jackel, C.
E.Rice, and JJ Veselka, “Proton exchange for high-
index waveguides in LiNbO ₃ Applied Physics Letter ”(Appl.Phys.Lett.), Vol41, No.7, pp607-6
08 (1982)] See FIG. 12 shows a perspective view of a conventional method for manufacturing an embedded optical waveguide using a proton exchange method in a solution. The LiNbO ₃ substrate 1 on which the protective mask 4 and the slit 5 are formed is heat-treated in benzoic acid 6 to exchange H ⁺ (proton) in the benzoic acid 6 with Li ⁺ in the LiNbO ₃ substrate 1 immediately below the slit 5. Occurs, and the high refractive index layer 2 made of H _x Li _1-x NbO ₃ (0 ≦ X ≦ 1) is formed. The stripe-shaped high refractive index layer 2 serves as a buried optical waveguide.

Next, the manufacturing process of the light wavelength conversion element will be described in detail with reference to FIG. In FIG. 13A, a protective mask 4 is formed on the LiNbO ₃ substrate 1 by using a normal photo process. The material of this protective mask 4 is Al. Next, as shown in FIG. 3B, heat treatment is performed in benzoic acid (230 ° C.) for 12 minutes to form a buried optical waveguide having a thickness of 0.5 μm. Further, after removing the protective mask 4 in the same figure (c), the surface perpendicular to the embedded optical waveguide 2 is optically polished and the fundamental wave is made incident to extract the harmonic.

The optical wavelength conversion element manufactured by the above benzoic acid treatment has a waveguide thickness of 0.5 μm for the fundamental wave P1 having a wavelength of 0.84 μm.
Shows the maximum conversion efficiency, the length of the waveguide is 6mm, P1 = 40mW
When, the harmonic of P2 = 0.4mW was obtained. The conversion efficiency P1 / P2 in this case is 1%.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention In the optical wavelength conversion element based on the embedded optical waveguide as described above, the refractive index difference in the lateral direction is small, and the light confinement in the lateral direction is weak and the mask width is actually small. There is a problem that the width of the produced optical waveguide expands in the lateral direction and the confinement becomes worse. Therefore, it has been difficult to obtain harmonics of 1 mW or more, which is the practical level of the optical wavelength conversion element.

Means for Solving the Problems The present invention makes it possible to greatly improve the conversion efficiency by adding a new device to the structure of the optical wavelength conversion element based on the optical waveguide. That is, an object of the present invention is to obtain a highly efficient optical wavelength conversion element by adopting a structure that enhances light confinement by using an upwardly convex proton exchange optical waveguide that can propagate only a single mode with respect to a fundamental wave. And

To achieve the above object, the optical wavelength conversion element of the present invention is
LiNb _x Ta _1-x O ₃ (0 ≦ X ≦ 1) Proton exchange optical waveguide having a structure that is convex upward on the substrate and is capable of single mode propagation with respect to the fundamental wave And an emission part of the.

ECR (Electron using C ₃ F ₈ gas as an embodiment example
Cyclotron Resonance) A method of manufacturing an optical wavelength conversion element by etching will be described with reference to the drawings.

A structural diagram of the first embodiment of the optical wavelength conversion device of the present invention is shown in FIG. In this embodiment, LiNb is used as an optical wavelength conversion element.
A ridge type optical waveguide having a convex structure formed on an O ₃ substrate 1 is used. FIG. 1 (a) is a perspective view of an optical wavelength conversion element, and (b) is a surface perpendicular to the optical waveguide. A cross-sectional view taken along line (c) is a cross-sectional view taken along a plane parallel to the optical waveguide. In FIG. 1, 1 is a LiNbO ₃ substrate of a + Z plate (+ side of the substrate cut out perpendicular to the Z axis), 2 is a high refractive index layer formed by a proton exchange treatment in phosphoric acid, and 10 is a fundamental wave P 1 , And 12 is a higher harmonic wave emitting portion. Further, 2a in the high refractive index layer 2 is a core portion that serves as an optical waveguide through which light propagates, and 2b is a cladding portion. A grating is formed in the harmonic wave emission section 12.

Next, a method of manufacturing this light wavelength conversion element will be described with reference to the drawings. FIG. 2 shows a manufacturing process drawing of the optical wavelength conversion element of the present invention. In FIG. 3A, the LiNbO ₃ substrate 1 is heat-treated (proton exchange treatment) at 230 ° C. for 5 minutes in pyrophosphoric acid, which is a kind of phosphoric acid, to form a high refractive index layer 2 having a thickness of 0.37 μm.
Next, in FIG. 2B, a photoresist 7 having a thickness of 1.2 μm is patterned on the high refractive index layer 2 by a normal photo process. Next, in FIG. 3C, the protective mask 4a made of Ti was vapor-deposited by 0.3 μm by electron beam vapor deposition. Ti is C ₃ F ₈
It has the features that it is difficult to etch by ECR etching using gas and that patterns can be easily formed by lift-off. Next, in FIG. 3D, the photoresist 7 was dissolved in acetone and lifted off to pattern Ti. This results in a width of Ti 1.
A protective mask 4a made of Ti and having a length of 5 μm and a length of 8 mm was formed on the high refractive index layer 2. As described above, when the lift-off method is used for patterning Ti, the side surface of the Ti protection mask pattern becomes smooth and the side surface of the waveguide formed after etching becomes smooth. As a result, the propagation loss can be reduced. Finally, with the mask 4a formed, etching is performed by a reactive ion beam etching apparatus using an ECR ion source to etch the portion other than the mask 4a. Specifically, C ₃ F ₈
Acceleration voltage of 400V, 1 × 10 by ECR etching with gas
Etching was performed at a vacuum degree of ^-4 Torr for 5 minutes. The etching amount of the high refractive index layer 2 is 2000A. Then Ti mask 4
7 a with a mixture of sodium hydroxide and hydrogen peroxide (10: 1)
It was removed at 0 ° C. for 1 minute to form a ridge type optical waveguide having the shape shown in (e). Immediately below the protective mask 4a becomes the core portion 2a of the ridge type optical waveguide, and the etched portion becomes the cladding portion 2b.

FIG. 3 shows the etching characteristics (relationship of etching amount to etching time) between untreated LiNbO ₃ and proton-exchanged LiNbO ₃ (H ⁺ —LiNbO ₃ ) under the above conditions, that is, the high refractive index layer. This shows that the proton-exchanged LiNbO ₃ has a triple etching rate compared to untreated LiNbO ₃ . Therefore, compared with the method of directly etching LiNbO ₃ , the proton exchange treatment significantly improves the etching rate and facilitates fine processing.

A ridge-type optical waveguide was manufactured by the above-mentioned process. In this optical waveguide, the thickness h of the clad portion 2b is 0.17 μm,
The core portion 2a has a thickness d of 0.37 μm and a ridge ratio h / d of 0.46.
Has become. In addition, the side surface 2c of the optical waveguide formed by etching is extremely smooth with ± 200 A or less.
The core portion 2a of the high refractive index layer 2 serves as an optical waveguide. Part of this optical waveguide 2 is partially heated to form an incident portion 10 having a larger thickness than the core portion 2a. Increasing the thickness of the incident part 10 (shown in FIG. 1) increases the coupling efficiency with the fundamental wave. A surface perpendicular to the optical waveguide 2 is optically polished to form an incident surface 11 on the incident portion 10. Finally, a photo process is applied to the emitting section 12.
The optical wavelength conversion device shown in FIG. 1 (a) can be manufactured by forming a grating using ECR etching. The length of this element is 6 mm. (In Fig. 1 (c), a semiconductor laser beam (wavelength 0.84
m) was guided from the incident part 10 and propagated in a single mode, and a harmonic wave P2 having a wavelength of 0.42 μm was extracted from the emitting part 12 to the outside of the substrate. This harmonic P2 is emitted in the optical waveguide 2
It is perpendicular to the surface 30 on which is formed. This is because the harmonics emitted from the substrate at an angle of 16 degrees have their directions changed by the grating formed on the emitting portion 12. The period of this grating is 0.2 μm. As a result, the optical path length of each part of the harmonic becomes equal, and astigmatism at the time of focusing is eliminated.

A harmonic of 1 mW (wavelength 0.42 μm) was obtained with an input of 40 mW of fundamental wave. The conversion efficiency in this case is 2.5%. The conversion efficiency is greatly improved compared to the conventional optical wavelength conversion device using the embedded optical waveguide. The propagation loss was almost the same as that of the embedded optical waveguide. It is thought that this is because the proton exchange treatment facilitated the fine processing of LiNbO ₃ to reduce the unevenness on the side surface. Optical waveguide thickness d
FIG. 4 shows the conversion efficiency with respect to. Harmonics are 0.3 to 0.42
A large value is obtained in the range of μm, which is 1.05 to 1.5 times the cutoff thickness d _cut of the optical waveguide. This cut-off thickness d _cut is obtained by the formula shown below.

Here, n _s is the refractive index of the LiNbO ₃ substrate 1, and n _f is the refractive index of the high refractive index layer 2. Further, k ₀ is a wave number, and when the wavelength of the fundamental wave is λ, k ₀ = 2π / λ. The values used in this embodiment are substituted into the above equation. The wavelength λ of the fundamental wave is 0.84 μm,
The refractive index of the LiNbO ₃ substrate 1 at that wavelength is 2.17, and the refractive index of the high refractive index layer is 2.28. From this, d _cut is 0.285μ
m, the peak is present between 1.05 and 1.5 times this value. The maximum refractive index difference between the high refractive index layer 2 formed by the pyrophosphoric acid treatment used in this example and the LiNbO ₃ substrate 1 is 10% or more higher than the value obtained by the solution treatment of benzoic acid. Therefore, confinement of light is also a factor in improving conversion efficiency. The relationship between the ridge ratio h / d and the conversion efficiency is shown in FIG. The closer h / d is to 0, the better the confinement of light, but at the same time, the propagation loss increases, so the conversion efficiency does not seem to be large, and a peak appears when h / d is around 0.4.

Note that the output of the harmonics is unstable in multimode propagation with respect to the fundamental wave, which is not practical.

Next, a second embodiment of the optical wavelength conversion device of the present invention will be described with reference to the drawings. FIG. 6 shows a configuration diagram of the light wavelength conversion element. FIG. 7 shows a manufacturing process diagram of the optical wavelength conversion element including the annealing process and the protective film forming process. Figure 7 (a)
In, the LiNbO ₃ substrate 1 was annealed after the proton exchange. Specifically, proton exchange is performed in pyrophosphoric acid at 230 ° C. for 4 minutes to form a high refractive index layer 2 having a thickness of 0.3 μm,
Next, an annealing treatment was performed in air at 190 ° C. for 20 minutes. The thickness of the high refractive index layer 2 was expanded to 0.35 μm by the annealing treatment. This annealing process further homogenized the optical waveguide and reduced the propagation loss. After that, Ti protective mask 4a
Was patterned. Next, ECR etching was performed as shown in FIG. The condition is an acceleration voltage of 400 V with C ₃ F ₈ gas, 1 ×
Etching was performed for 10 minutes at a vacuum degree of 10 ⁻⁴ Torr. The etching amount is about 4000 A (0.4 μm). Finally, as shown in FIG. 3C, SiO ₂ was formed as the protective film 8 on the optical waveguide by sputter deposition. The thickness of SiO ₂ 8 is 4000A. S as a protective film
It is preferable to use iO _{2 because} it does not cause scattering and absorption in the protective film. This protective film can prevent deterioration of the characteristics of the light wavelength conversion element due to dirt on the surface. No and SiO ₂ if yet there is annealed with a SiO ₂ in FIG. 8, yet shows a relationship between the optical waveguide width and the propagation loss when no annealing.
A significant reduction in propagation loss is seen. The ridge ratio h / d of the optical wavelength conversion element manufactured by the above process is 0. With this optical wavelength conversion element, a fundamental wave P1 with a wavelength of 0.78 μm was obtained as a harmonic wave of 10 mW with 100 mW input, and the conversion efficiency was 10%. Note the fundamental wave incident from the incident part 10 is emitted to be converted into higher harmonic waves P2 in the optical waveguide 2a LiNbO ₃ substrate 1 in FIG. 6 (b). This harmonic P2 is generated by the Al
The direction is changed by the grating coated with the film 13 and the light is emitted in the direction perpendicular to the optical waveguide 2.

Moreover, if a substrate doped with MgO is used, optical damage can be prevented even for light of a short wavelength, and there is no fluctuation in the output of harmonics. By forming the proton exchange optical waveguide in the shape of a triangle, the shape of the propagation mode of the fundamental wave is less likely to change, and the structure is resistant to optical damage.

Next, an example in which the optical wavelength conversion element of the present invention is applied to the optical wavelength conversion of a YAG laser having a fundamental wave wavelength of 1.06 μm will be described as a third embodiment. The basic structure is based on that shown in FIG. 1 (a), and the manufacturing method is based on the first embodiment. However, in this example, after forming the protective mask of Ti, wet etching was performed to further smooth the side surface of the protective mask. Specifically, after forming a protective mask of Ti of 3500A, wet etching is performed at 25 ° C for 1 minute in a mixed solution of HF, HNO ₃ and H ₂ O (mixing ratio 1: 1: 800) to obtain Ti of 500A. Etched. As a result, the unevenness on the side surface was reduced to ± 100 A or less.

The thickness of the core 2a of the manufactured optical wavelength conversion element is 0.55μ.
m, the thickness of the cladding portion 2b is 0.22 μm, and the ridge ratio is 0.
4 The core thickness d is 1.3 times the cutoff thickness d _cut . The conversion efficiency of the generated 0.53 μm wavelength harmonic was 8.5% at 100 mW input. In this way, if the wavelength of the fundamental wave changes, the thickness can be designed according to the above-mentioned formula to convert the harmonics into higher harmonics with high efficiency.

The generation of harmonics by the optical wavelength conversion element was confirmed using a fundamental wave having a wavelength of 0.65 to 1.6 μm.

Next, an example in which a ridge type optical waveguide is embedded inside will be described as a fourth embodiment of the optical wavelength conversion device of the present invention. Ninth
A sectional view is shown in FIG. As a manufacturing method, the low refractive index layer 9 was formed by heat-treating the manufactured ridge type optical waveguide in stearic acid containing lithium stearate. The refractive index of the low refractive index layer 9 is LiNb
It is 0.01 higher than the refractive index of the O ₃ substrate 1. The thickness of the low refractive index layer 9 is
0.1 μm, the thickness of the high refractive index layer 2 is 0.4 μm, and the width is 2 μm
m. The propagation loss for a wavelength of 0.8 μm was 3 dB / cm. By thus embedding the optical waveguide inside, the propagation loss due to scattering in the proton exchange optical waveguide was significantly reduced.

In FIG. 9B, reference numeral 11 denotes an incident surface formed on the incident portion 10 of the fundamental wave, and SiO ₂ is formed on the incident surface 11 as the antireflection film 30. This increases the coupling efficiency of the fundamental wave P1 to the optical waveguide 2 by 15%. Further, the emission surface 12 is polished perpendicularly to the radiated harmonic P2, and the harmonic P2 goes out of the LiNbO ₃ substrate 1 vertically as it is. According to this configuration, it is unlikely to be affected by the change in the emission angle of the harmonic due to the change in temperature.

Next, as a fifth embodiment, an example in which the optical wavelength conversion element of the present invention is applied to reading an optical disk will be described. First
Figure 10 shows the configuration. The fundamental wave P1 emitted from the semiconductor laser 16 is collimated by the collimator lens 17 and then coupled to the incident portion 10 of the LiNbO ₃ substrate 1 using the focusing lens 18. This fundamental wave P2 is converted into a higher harmonic wave P2 by the optical waveguide 2, is radiated into the LiNbO ₃ substrate 1, and is formed on the emission part 12
The direction is changed by the grating that has a reflective film.
The light is emitted from above the NbO ₃ substrate 1 as parallel light in the traveling direction of the optical waveguide 2 and as divergent light in the width direction. The harmonic P2 is beam-shaped by the cylindrical lens 19 so that the diverging light side becomes parallel light, and both sides are made parallel light. After passing through the polarization beam splitter 20, the harmonic P2 made into the parallel light is condensed by the focusing lens 21 and forms a spot of 0.6 μm on the optical disc 22. This reflected signal passes through the polarization beam splitter 20 again and then enters the light receiver 23. A semiconductor laser 16 having a wavelength of 0.84 μm and an output of 60 mW was used to couple 50 mW as the fundamental wave P1 into the optical waveguide 2. This radiated 1.4 mW of harmonic P2. Of these, a harmonic wave of 1 mW was emitted in the direction perpendicular to the surface on which the optical waveguide 2 was formed, and this was used for reading the optical disc.

As described above, by using the optical wavelength conversion element of the present invention, the spot can be narrowed down to a half of the reading system of the optical disc using the 0.8 μm band semiconductor laser which has been conventionally used, and the recording density of the optical disc can be quadrupled. Can be improved. Also, by emitting the harmonics perpendicularly to the surface on which the optical waveguide is formed, a spot without astigmatism can be easily obtained.

Note that the above-mentioned configuration in which harmonics are emitted perpendicularly to the surface on which the optical waveguide is formed using the grating is applicable not only to the ridge type optical waveguide but also to various optical waveguide structures such as embedded type and loaded type. is there.

EFFECTS OF THE INVENTION As described above, according to the optical wavelength conversion element of the present invention, the protons that are convex on the LiNb _x Ta _1-x O ₃ substrate and have a structure capable of single-mode propagation with respect to the fundamental wave By using the exchange optical waveguide, the light confinement is increased, and the conversion efficiency of the optical wavelength conversion element is significantly improved.

Further, a spot without astigmatism can be easily obtained by taking out the higher harmonic wave in a direction perpendicular to the surface on which the optical waveguide is formed, and its practical effect is extremely large.

[Brief description of drawings]

FIG. 1 (a) is a perspective view showing the structure of an optical wavelength conversion element according to an embodiment of the present invention, and FIGS. 1 (b) and (c) are cross sections taken along a plane perpendicular to and parallel to the optical waveguide of the element of FIG. 1 (a). Fig. 2 (a)-
(E) is a process cross-sectional view of an example of a method for manufacturing an optical wavelength conversion device of the present invention, FIG. 3 is a diagram showing a relationship between etching time and etching amount, and FIG. 4 is conversion efficiency dependence on proton exchange waveguide thickness. FIG. 5 is a diagram showing the relationship between the ridge ratio h / d and the conversion efficiency, and FIGS. 6 (a) and 6 (b) are perpendicular to the optical waveguide of another embodiment of the optical wavelength conversion device of the present invention. Parallel views, FIGS. 7 (a) to 7 (c) are process cross-sectional views of the third embodiment of the present invention, FIG. 8 is a graph showing the relationship between optical waveguide width and propagation loss, and FIG. 9 (a). , (B) are sectional views perpendicular to and parallel to the optical waveguide of another embodiment of the present invention, FIG. 10 is a configuration diagram showing an application example of the optical wavelength conversion element of the present invention, and FIG. 11 is a conventional optical wavelength. FIG. 12 is a perspective view of a conversion element, a cross-sectional view, FIG. 12 is a diagram showing a method of manufacturing a conventional optical wavelength conversion element, and FIGS. A granulation step sectional views. 1 …… LiNbO ₃ substrate, 2 …… High refractive index layer, 2a …… Jp part, 10
…… Injection section, 12 …… Exit section.

Claims (3)

[Claims] 1. A proton exchange having a convex upward shape on a LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) substrate and having a structure capable of single-mode propagation with respect to a fundamental wave. An optical wavelength conversion element characterized in that an optical waveguide is formed, the optical waveguide is provided with an incident portion of the fundamental wave and an emission portion of a harmonic, and the fundamental wave is converted into a harmonic by the optical waveguide. . 2. The ratio (h / d) of the thickness d of the core portion of the proton exchange optical waveguide to the thickness h of the cladding portion is in the range of 0 ≦ h / d ≦ 0.7. Optical wavelength conversion element. 3. The thickness d of the core portion of the proton exchange optical waveguide.
Is 1.05 to 1.5 times the cut-off thickness, The optical wavelength conversion element according to claim 1, wherein