JPH03191332A - Production of optical waveguide and optical wavelength converting element - Google Patents

Abstract

PURPOSE:To produce the efficient optical wavelength converting element by partially inverting polarization by a heat treatment and forming a high refractive index layer on a crystal after proton exchange and patterning. CONSTITUTION:An LiNbO3 substrate 1 is heat treated in phosphoric acid to form the proton exchange layer 5. The periodic patterns of an SiO2 mask 6 are formed thereon. The polarization inverting layer 3 is formed right under the SiO2 6 by executing the heat treatment. The proton exchange layer 5 is spread in thickness by the heat treatment and the Li is decreased, by which the Curie temp. is lowered; therefore, the polarization inversion is executed at a low temp. The SiO2 6 is then removed by etching and optical waveguides 2 are formed by using the proton exchange in the polarization inverting layer 3. The optical wavelength converting element having high efficiency is thus formed by the simple stage.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to the field of optical information processing using coherent light;
Alternatively, the present invention relates to a method of manufacturing an optical waveguide and an optical wavelength conversion element used in the field of optical measurement and control.

Conventional technology FIG. 7 shows a configuration diagram of a conventional optical wavelength conversion element.

Harmonic generation for the fundamental wave with a wavelength of 1.06 μm (
The wavelength (0.53 μm) will be described in detail using figures.

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IGWO (1988), as shown in Figure 7, an optical substrate path 2 is formed on a LINbOs substrate, and a layer 3 (polarization an inversion layer) is formed. By compensating for the mismatch between the propagation constants of the fundamental wave and the generated harmonics with the periodic structure of the polarization inversion layer 3, harmonics can be generated with high efficiency. When the fundamental wave P1 is incident on the entrance surface 10 of the optical waveguide 2, the harmonic wave P2 is efficiently generated from the optical waveguide 2, and the optical waveguide 2 operates as an optical wavelength conversion element.

Such a conventional optical wavelength conversion element had a structure in which a polarization inversion layer 3 was formed as a basic component. A method for manufacturing this element will be explained using FIG. 8. In the figure (a), a Ti pattern with a width of several μm was formed on a LiNbO2 substrate 1, which is a nonlinear optical crystal, by lift-off and vapor deposition. Next, as shown in FIG. 4B, heat treatment was performed at a temperature of about 1100° C. to form a polarization inversion layer 3 whose polarization was reversed to that of the LINbOz substrate 1. Next, as shown in the same figure (C), after 30 minutes of heat treatment in benzoic acid (200℃),
An optical waveguide 2 is formed by annealing at .degree.

The optical wavelength conversion element produced by the above benzoic acid treatment has an optical waveguide length of 1 mm5 for the fundamental wave P1 with a wavelength of 1.06 μm5, and when the power of the fundamental wave P1 is 1 mW, the power of the harmonic P2 is 0.5 nW. was obtained. If the fundamental wave is incident at 40 mW, a harmonic output of 800 nW is possible. In this case, the conversion efficiency per IW in a 1 cm element is 5%/W·cm.

Problems to be Solved by the Invention In the optical wavelength conversion element based on a polarization inversion layer as described above, the output is only 1/10 to 100 times lower than the theoretical value. This is because the shape of the domain-inverted layer 3 cannot be controlled due to lateral diffusion occurring during the fabrication of the domain-inverted layer, making it impossible to form the ideal narrow width and deep depth. temperature 113
This is due to the fact that the temperature is near 0°C and the polarization is non-uniformly reversed.

Therefore, the conversion efficiency of the optical wavelength conversion element was extremely low compared to theory. Furthermore, it has been found that a refractive index change occurs between the polarization inversion layer and the non-polation inversion layer, which causes propagation loss and further reduces harmonic output.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides a highly efficient optical wavelength conversion element by adding new ideas to the manufacturing method of an optical wavelength conversion element and waveguide based on a polarization inversion structure. This makes it possible to manufacture

In other words, the present invention includes a step of performing proton exchange in a nonlinear optical crystal and a step of patterning a dielectric film on the substrate, and then a step of partially reversing the polarization by heat treatment, and a step of partially inverting the polarization by heat treatment. An optical waveguide and an optical wavelength conversion element are formed by using a step of forming a refractive index layer.

Effect: By the method of manufacturing optical waveguides and optical wavelength conversion elements of the present invention, the polarization inversion layer formed in the nonlinear optical crystal is deep and uniform, and the processing temperature can be significantly reduced compared to conventional methods, resulting in polarization inversion. The mask can be easily removed, and the propagation loss of the optical waveguide can be significantly reduced. As a result, a highly efficient optical wavelength conversion element can be manufactured through simple steps.

Embodiment As one of the embodiments, a method for manufacturing an optical waveguide according to the present invention will be explained with reference to the drawings. First, a method for manufacturing an optical waveguide according to the present invention is shown in FIG. Further, FIG. 2 shows a structural diagram of a first example of an optical wavelength conversion element manufactured using the manufacturing method of the present invention. In this example, L is used as a polarization inversion type optical wavelength conversion element.
It uses an optical waveguide fabricated using proton exchange in the INbOa substrate 1. FIG. 2(a) is a perspective view of the optical wavelength conversion element, and FIG. 2(b) is a cross-sectional view taken along a plane parallel to the optical waveguide. It is. In Figure 2, 1 is the LINbOs substrate of the +Z plate (the + side of the substrate cut out perpendicular to the Z axis), 2 is the formed optical waveguide, 10 is the incident part of the fundamental wave P1, and 12 is the harmonic wave P
This is the second output section. This optical waveguide 2 has a periodic structure formed by a polarization inversion layer 3 and a non-polarization inversion layer 4 whose polarization is not inverted. The proton exchange layer 5 is formed by pretreatment such that the Li concentration on the substrate surface is reduced by proton exchange treatment in phosphoric acid and polarization inversion occurs at a low temperature.

In the figure (b), the fundamental wave P1 entering the optical waveguide 2 is converted into a harmonic wave P2 by the polarization inversion layer 3 having a length equal to the phase matching length, and then the next non-polarized wave P2 having the same length L. The harmonic power increases in the inversion layer 4. The harmonic wave P2 whose power has been increased in the optical waveguide 2 in this way is radiated from the output section 12.

Next, a method for manufacturing this optical wavelength conversion element will be explained using figures. In FIG. 1(a), first, a NbO* substrate 1 is heat treated in phosphoric acid to form a proton exchange layer 5. As shown in FIG. The thickness of the proton exchange layer is 0.2 μm.

Next, in the same figure (b), the proton-exchanged LINbO is placed on the substrate 1 as a dielectric film with a thickness of 0. After depositing 2 μm of 5iO2B by sputtering, 5iO2B was deposited by photoetching.
A periodic pattern of the TO2 mask 6 is formed.

Next, heat treatment was performed at 1025°C for 10 seconds in the same figure (C).
A polarization inversion layer 3 with a thickness of 1.4 μm is formed directly under the i 026. The heat treatment ramp rate is 10°C/min, and the cooling rate is 50°C/min. If the cooling rate is slow, non-uniform inversion will occur, so a cooling rate of 7 minutes or more is desirable. At this time, the thickness of the proton exchange layer 5 increases to 1.4 μm due to the heat treatment.

In addition, since Li is reduced and the Curie temperature is lowered by about 100"C compared to the one without proton exchange treatment, polarization can be reversed at low temperature. The polarization can be reversed only directly under SiO*8 in the LINbO3 substrate 1. This is thought to be because Li is diffused to 5tOt, which makes it easier to invert because there is less Li than in the non-poling inversion layer 4.The length of the polarization inversion layer 3 is 1.5 μm.Next, in Figure Cd) HF: HN
Etched for 20 minutes with a 1=1 mixture of Fa and S i
Remove Oa6. Next, an optical waveguide is formed in the polarization inversion layer 3 using proton exchange. After patterning Tag's into a stripe shape as a mask for an optical waveguide, a Ta206 mask with a width of 6 I is shown in the same figure (e).
I ffh Proton exchange was performed at 230° C. for 2 minutes in a sample in which a slit with a length of 10 mm was formed. Finally, after removing the mask, annealing was performed at 350° C. for 1 hour. The annealing process uniformizes the layer, reduces loss, and restores nonlinearity to the proton exchange layer. The region immediately below the slit of the protective mask that has undergone proton exchange becomes a high refractive index layer 2 whose refractive index has increased by about 0.03. Light propagates through the high refractive index layer 2, which becomes an optical waveguide.

An optical waveguide was manufactured through the steps described above.

The thickness d of this optical waveguide 2 is 1. The thickness is 2 μm, which is smaller than the thickness of the polarization inversion layer 3, which is 1.4 μm, and the wavelength can be converted effectively. A graph showing the relationship between the processing temperature and the thickness of the polarization inversion layer is shown in FIG. It can be seen that according to the method of the present invention, polarization inversion occurs at a lower temperature of 80° C. compared to the conventional method without proton exchange treatment, and the depth of the polarization inversion layer is about twice that of the conventional method. A deep polarization inversion layer can be formed at a processing temperature of 1000°C or higher and 1040°C or lower. Moreover, there is no change in the refractive index of the non-poling inversion layer 4 and the polarization inversion layer 3 of this optical waveguide 2, and the propagation loss when light is guided is small. A surface perpendicular to the optical waveguide 2 was optically polished to form an input section 10 and an output section 12. In this way, the optical wavelength conversion element shown in FIG. 2 can be manufactured. Further, the length of this element is 8 mm. In FIG. 2(b), when a semiconductor laser beam (wavelength 0.84 μm) is guided as a fundamental wave P1 from the input section 10, it propagates in a single mode, and a harmonic P2 with a wavelength of 0.42 μm is transmitted from the output section 12 to the outside of the substrate. It was taken out. The propagation loss of the optical waveguide 2 was as small as 1 dB/cta, and the harmonic P2 was effectively extracted. One of the reasons for the reduction in loss is that a uniform optical waveguide is formed using phosphoric acid. Fundamental wave 40m
A harmonic of 2 mW (wavelength 0.42 μm) was obtained by inputting W. The conversion efficiency in this case is 5%. The conversion efficiency is 2,000 times higher than that of conventional polarization-inverted optical wavelength conversion elements, and about 20 times higher when compared with the same length. Note that in multimode propagation, the harmonic output is unstable compared to the fundamental wave, making it impractical.

Incidentally, generation of biharmonic waves by the present optical wavelength conversion element was confirmed using a fundamental wave having a wavelength of 0.65 to 1.6 μm.

Although Tag's was used as the mask for forming the optical waveguide, Ta
Even 5W etc. can perform proton exchange processing and are effective.

The method for manufacturing an optical waveguide of the present invention is applicable not only to the optical wavelength conversion element shown in FIG. 1.2 but also to other devices using optical switches such as directional couplers or optical waveguides.

Next, a second embodiment of the method for manufacturing an optical wavelength conversion element of the present invention will be described. The configuration of the optical wavelength conversion element is the same as in the first embodiment. In this example, MgO-doped LINbOs, which is more resistant to optical damage than an LfNbOs substrate, was used and heat-treated at 1100° C. to form a polarization inversion layer. The reason why the processing temperature is higher than that of LINbOa is because the Curie temperature is about 80° C. higher due to MgO doping. In addition, a proton exchange optical waveguide was used as the optical waveguide, which can be treated at a lower temperature than the heat treatment temperature when forming the polarization inversion layer. The conversion efficiency in this example was 4% at 40 mW input, and the output was also very stable.

Next, a third embodiment of the method for manufacturing an optical wavelength conversion element of the present invention will be described. The configuration of the optical wavelength conversion element is the same as in the first embodiment. In this example, LI
TaOa was used as a substrate. LITaOa has a low Curie temperature of 630° C., and polarization inversion processing can be performed at low temperatures. The manufacturing method is shown in FIG. In the same figure (a), LiTa0a
A pattern of 5iCh8 was formed on the substrate 1a, which was heat-treated in phosphoric acid to form a proton exchange layer 5. Next, the same figure (
In b), heat treatment was performed at a temperature of 570° C. to form a polarization inversion layer 3 having a thickness of 1.8 μm. Thereafter, an optical waveguide was formed by proton exchange using phosphoric acid. The thickness of the optical waveguide is 1.5
It is μm. The optical waveguide fabricated on the LITaO1 substrate 1a by proton exchange has large nonlinearity and therefore does not need to be annealed. The conversion efficiency in this example is 40
The mW input was 1%, and since LfTaOl was used, there was no optical damage and the harmonic output was very stable.

Next, a fourth embodiment of the method for manufacturing an optical waveguide according to the present invention will be described with reference to the drawings. In this embodiment, proton exchange is performed after forming a pattern. FIG. 5 shows a manufacturing process diagram of the optical waveguide of this example. In the figure (a), after Ta is evaporated to a thickness of 200 Å on a LINbOs substrate 1, a pattern of Ta 8a slits 7 is formed by ordinary photoetching. Next, in FIG. 4B, proton exchange treatment is performed in phosphoric acid to form a proton exchange layer 5 having a thickness of 0.1 μm directly below the Ta slit 7. Next, in the same figure (C), 1 in the air
A polarization inversion layer 3 is formed by heat treatment at 130°C. During the heat treatment, Ta8a is oxidized and changed to Ta20s6a. Further, the proton exchange layer 5 spreads to a thickness of 1.3 μm. Li directly below the slit 7 is diffused out into the air, and Lt in that portion is reduced. Also, unlike SiO2, Ta2's contains Li
is difficult to spread.

Therefore, the polarization directly below the slit 7 where Li is reduced is reversed. This thickness was 1.3 μm. Then Ta5os
After removing , an optical waveguide was formed by proton exchange.

Next, as a fifth embodiment, an example in which the optical wavelength conversion element manufactured according to the present invention is applied to reading an optical disc will be described. Figure 6 shows its configuration. Semiconductor laser 16
The fundamental wave P1 emitted from the light beam P1 is made into parallel light by the collimator lens 17, and then coupled to the optical wavelength conversion element 15 using the focusing lens 18. This fundamental wave P1 is converted into a harmonic wave P2 by the optical wavelength conversion element 15 and radiated to the outside of the substrate.

This harmonic P2 is beam-shaped by a collimator lens 19 so that it becomes parallel light. This parallel harmonic P2 passes through the polarizing beam splitter 20, is focused by the focusing lens 21, and is focused onto the optical disk 22 by 0.6
Connect the μm spots.

This reflected signal passes through the polarizing beam splitter 20 again and then enters the light receiver 23. Wavelength 0.84μm, output 80
40m as fundamental wave P1 using mW semiconductor laser 16
W was coupled to the optical wavelength conversion element 15. As a result, 2m
A harmonic P2 of W was radiated.

In this way, by using the optical wavelength conversion element of the present invention, the spot can be narrowed down to half that of the conventionally used optical disc reading system using a semiconductor laser in the 0.8 μm band, and the recording density of the optical disc can be reduced to 4. It can be improved twice. Furthermore, in the present invention, a spot free of astigmatism can be easily obtained by emitting harmonic waves from an optical waveguide.

In the examples, LINbOa and L are used as nonlinear optical crystals.
Although ITaOa was used, ferroelectric materials such as KNbOs, MNA
It is also applicable to organic materials such as

As described in detail, according to the method of manufacturing an optical waveguide and an optical wavelength conversion element of the present invention, proton exchange is performed as a pretreatment to reduce Li in the substrate, thereby lowering the Curie point of that portion and lowering the temperature. It is possible to form a deep and uniform domain-inverted layer. By periodically arranging this domain-inverted layer and compensating for phase mismatch, a highly efficient optical wavelength conversion element can be fabricated.

Further, the optical wavelength conversion element manufactured by this manufacturing method can extract harmonic waves from the optical waveguide, and a spot without astigmatism can be easily obtained, and its practical effects are extremely large.

[Brief explanation of drawings]

1(a) to 1(e) are process cross-sectional views of the first embodiment using the optical waveguide manufacturing method of the present invention, FIG. 2(a),
(b) is a perspective view and a cross-sectional view of the structure of the optical wavelength conversion element of the present invention, FIG. 3 is a diagram showing the depth dependence of the polarization inversion layer on processing temperature, and FIG. 4 (a). (b) is a cross-sectional view of the manufacturing process of the third embodiment of the optical waveguide of the present invention, and FIGS. 5(a) to (c) are the fourth embodiment of the optical waveguide of the present invention.
FIG. 6 is a schematic diagram of an optical disk system to which the optical wavelength conversion element manufactured according to the present invention is applied;
FIGS. 7(a) and 7(b) are a perspective view and a cross-sectional view of the structure of a conventional optical wavelength conversion element, and FIGS. 8(a) to (C) are cross-sectional views of the manufacturing process of the conventional optical wavelength conversion element. 1... LINbO, substrate, 2... optical waveguide, 3...
・Polarization inversion layer, 4... Non-polarization inversion layer, 5... Proton exchange layer, Pl... Fundamental wave, P2... Harmonic wave

Claims (10)

[Claims] (1) After performing a step of performing proton exchange in a nonlinear optical crystal and a step of patterning a dielectric film on the crystal, a step of partially reversing polarization by heat treatment and a high refractive index layer on the crystal. 1. A method of manufacturing an optical waveguide, comprising the steps of: forming an optical waveguide. (2) After performing a step of exchanging protons in a nonlinear optical crystal and patterning a dielectric film on the crystal, a step of partially reversing polarization by heat treatment and forming an optical waveguide in the crystal. 1. A method of manufacturing an optical wavelength conversion element, comprising the steps of forming a fundamental wave entrance part and a harmonic output part. (3) Nonlinear optical crystal is LiNb_xTa_1_-_x
The method for manufacturing an optical waveguide according to claim 1 or the method for manufacturing an optical wavelength conversion element according to claim 2, wherein the substrate is an O_3 (0≦X≦1) substrate. (4) The method for manufacturing an optical waveguide according to claim 1 or the method for manufacturing an optical wavelength conversion element according to claim 2, wherein the polarization directly under the dielectric film is reversed by heat treatment. (5) The method for manufacturing an optical waveguide according to claim 1 or the method for manufacturing an optical wavelength conversion element according to claim 2, wherein the optical waveguide is a proton exchange optical waveguide. (6) The method for manufacturing an optical waveguide according to claim 1 or 2, wherein the dielectric film is SiO_2.
A method for manufacturing an optical wavelength conversion element as described in 2. (7) Cooling of the nonlinear optical crystal during the heat treatment process
The method for manufacturing an optical waveguide according to claim 1, or the method for manufacturing an optical wavelength conversion element according to claim 2, characterized in that the manufacturing method is carried out at a rate of at least .degree. C./min. (8) The method for manufacturing an optical waveguide according to claim 1 or the method for manufacturing an optical wavelength conversion element according to claim 2, wherein the temperature of the heat treatment is 1000°C or more and 1040°C or less. (9) The method for manufacturing an optical waveguide according to claim 1 or the method for manufacturing an optical wavelength conversion element according to claim 2, characterized in that proton exchange is performed using phosphoric acid. (10) The method for manufacturing an optical wavelength conversion element according to claim 2, wherein the optical waveguide propagates in a single mode.