JPH06102554A - Optical element and production of optical element and production of optical waveguide - Google Patents

Abstract

PURPOSE:To eliminate the surface roughening of an optical waveguide, to lessen loss and to obtain the linearity approximately equal to the linearity of a substrate in the process for production of the optical waveguide and the optical element, such as optical wavelength conversion element. CONSTITUTION:Mask patterns by Ta 6 are formed on an LiTaO3 substrate 1 and thereafter, a high-refractive index layer 5 is formed by executing a proton exchange method in pyrophosphoric acid added with lithium phosphate. This high-refractive index layer 5 is heat treated at 460 deg.C for 10 seconds by an IR heater to restore crystallinity, by which the optical waveguide having high nonlinearity and good confinement is produced. Then, the optical waveguide which is low in loss and high in nonlirear optical effect is obtd. by diluting phosphoric acid with a lithium salt.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element and an optical waveguide used in the field of optical information processing utilizing coherent light, or in the field of optical application measurement control, and a method of manufacturing the same.

[0002]

2. Description of the Related Art FIG. 7 shows a block diagram of an optical wavelength conversion element based on an optical waveguide as a conventional optical element. 0.84 μm or less
The harmonic generation (wavelength 0.42 μm) with respect to the fundamental wave of the wavelength is described in detail with reference to the drawings. (K.Mizuuchi, K.Yama
moto and T. Taniuchi, Applied Physics Letters, Vol 5
See page 8, 2732, June 1991,). As shown in FIG. 7, an optical waveguide 2 is formed on a LiTaO ₃ substrate 1a, and a layer 3 (polarization inversion layer) whose polarization is periodically inverted is further formed on the optical waveguide 2. By compensating for the mismatch of the propagation constants of the fundamental wave P1 and the generated harmonic wave P2 with the periodic structure of the polarization inversion layer 3 and the non-polarization inversion layer 4, the harmonic wave P2 can be generated with high efficiency. When the fundamental wave P1 is incident on the incident surface 10 of the optical waveguide 2, the harmonic P2 is efficiently generated from the optical waveguide 2 and operates as an optical wavelength conversion element.

Such a conventional light wavelength conversion element has an optical waveguide 2 manufactured by a proton exchange method using phosphoric acid as a basic constituent element. A method of manufacturing this element will be described. First, Ta is periodically patterned on the LiTaO ₃ substrate 1a by using a normal photoprocess and dry etching. Next, the LiTaO ₃ substrate 1a on which the Ta pattern was formed was subjected to proton exchange in phosphoric acid at 260 ° C. for 30 minutes to form Ta.
A 0.8 μm-thick proton exchange layer is formed immediately below the slit not covered with. Then, heat treatment is performed at a temperature of 590 ° C. for 10 minutes. The rising rate of heat treatment is 10 ° C./min, and the cooling rate is 50 ° C./min. As a result, the domain inversion layer is formed. Just below the proton exchange layer, Li is reduced and the Curie temperature is lowered, so that polarization inversion can be partially performed. Then, Ta is removed by etching with a 1: 1 mixed solution of HF: HNF ₃ for 2 minutes. Further, an optical waveguide is formed in the polarization inversion layer by using proton exchange. By patterning Ta as an optical waveguide mask in a stripe shape, a slit having a width of 4 μm and a length of 12 mm is formed in the Ta mask. 260 on the substrate covered with this Ta mask
Proton exchange is performed at 16 ° C. for 16 minutes to form a 0.5 μm high refractive index layer. After removing the Ta mask, 10 at 380 ° C
Heat treatment for minutes. The region directly under the slit of the proton-exchanged protective mask becomes the optical waveguide 2 whose refractive index is increased by about 0.03. The optical wavelength conversion device manufactured by this conventional method has a fundamental wave P1 having a wavelength of 0.84 μm, a power of the harmonic wave P2 of 0..0 when the length of the optical waveguide 2 is 9 mm and the power of the fundamental wave P1 is 27 mW. 13 mW and a conversion efficiency of 0.5% were obtained. In this case, the conversion efficiency per 1 W is 18% / W.

Next, proton exchange to a LiNbO ₃ X plate will be described as another conventional example. Optically polished LiNbO ₃ X
The plate was heat-treated in phosphoric acid at 260 ° C. for 1 hour to form a high refractive index layer. As a result, a high refractive index layer having a thickness of 2 μm was formed.

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention An optical wavelength conversion element based on an optical waveguide formed by the above-mentioned proton exchange method is used at the time of proton exchange for producing an optical waveguide.
The crystallinity of LiTaO ₃ was impaired and the nonlinear optical effect disappeared. FIG. 8 shows a cross-sectional view of the manufactured light wavelength conversion element. The first proton-exchanged high refractive index layer 5 loses the nonlinear optical effect. The high-refractive index layer 5 expands to become the optical waveguide 2 by the annealing treatment, but the nonlinearity is still lost even in that state. Therefore, the conversion efficiency is 1 /
There was a problem that it decreased to about 6. on the other hand,
If the annealing time is lengthened to, for example, about 1 hour, the nonlinearity is recovered, but the optical waveguide to be manufactured has a thickness of 5 μm or more, resulting in poor confinement and a practical problem.

Further, when the LiNbO ₃ X plate was subjected to proton exchange for a long time, surface roughness occurred. Therefore, the propagation loss increases in the optical waveguide and the transmission loss occurs in the Fresnel lens.

Therefore, an object of the present invention is to provide a method of manufacturing an optical element and a method of manufacturing an optical waveguide which eliminates surface roughness and reduces loss.

[0008]

In order to solve the above problems, the present invention provides a method for manufacturing an optical element which is free from deterioration of the nonlinear optical effect by adding a new device to the method of manufacturing an optical element. Is provided. That is, in the present invention, LiNb _x Ta _1-x O ₃ (0 ≦ x ≦
1) A means for heat-treating the substrate to form a high refractive index layer on the LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) substrate.

[0009]

[Function] According to the method of manufacturing an optical element of the present invention, by diluting phosphoric acid with a lithium salt, damage to LiNbO ₃ or LiTaO ₃ is significantly mitigated, and the surface is not roughened and the nonlinear optical effect is not deteriorated. Optical elements can be formed. Further, according to the method of manufacturing an optical waveguide of the present invention, an optical waveguide that is well confined without loss can be manufactured.

[0010]

EXAMPLE A first example of the method for manufacturing an optical waveguide of the present invention will be described with reference to FIG. 1 is LiNb which is a ferroelectric substrate
The O ₃ substrate 6 is a Ta film which is a protective mask formed on the + Z surface of the LiNbO ₃ substrate 1 and has a Ta film thickness of 30 nm. Reference numeral 13 is a slit formed on the protective mask 6 by a photo process and etching. The LiNbO ₃ substrate 1 is immersed in an acid 8 containing phosphoric acid as a main component. Phosphoric acid is orthophosphoric acid (H ₃ PO
₄ ) and pyrophosphoric acid (H ₄ P ₂ O ₇ ) are generic names, and by the way, 215 ℃
At the above temperatures, phosphoric acid 8 is a mixture of orthophosphoric acid (PKa = 2.2) and pyrophosphoric acid (PKa = 1.7). In addition to phosphoric acid (100 g), 20 g of lithium phosphate, which is a kind of lithium salt, is mixed in the acid 8. As a result, the acidity of phosphoric acid is weakened so that the LiNbO ₃ substrate 1 is not damaged. The quartz beaker 15 containing the acid 8 is always kept at a temperature of Tc = 250 ° C. by a heater 16 with a temperature controller. The LiNbO ₃ substrate 1 is heat-treated for 4 minutes, extracted, and washed with running water. In this way the thickness is 0.
A high-refractive index layer 7 having a thickness of 5 μm is formed, and this high-refractive index layer 7 serves as an optical waveguide.

By the treatment with acid 8, a high refractive index layer having Δn = 0.14 can be formed. Further, since phosphoric acid is soluble in water, washing after proton exchange is easy. Furthermore, phosphoric acid is inexpensive. Further, even at a high temperature of 300 ° C. or higher, phosphoric acid has little evaporation and its melting point is below room temperature, so even if it evaporates, it solidifies and adheres to the surroundings to prevent work. On the contrary,
Other organic acids such as benzoic acid not only hinder the work by solidifying the evaporate and adhering to the surroundings, but they are insoluble in water and cannot be washed with water, which poses a problem.

Next, a second embodiment of the optical element forming method of the present invention will be described with reference to FIG. The second embodiment shows a method of forming a microlens as an optical element. FIG. 2 is a cross-sectional view showing a microlens manufacturing process, in which the slit 3 on the LiNbO ₃ substrate 1 in FIG. 1 is changed to a circular portion 9 which becomes a lens. The X plate is used in this embodiment. Proton exchange is carried out in phosphoric acid as in the first embodiment. FIG. 3 shows the relationship between the proton exchange time and the proton exchange depth. From Figure 3, 6.3 at temperature T = 280 ℃
By performing heat treatment for a time, the high refractive index layer 5 having a depth of 10 μm can be obtained. On the other hand, it takes almost 100 hours at T = 230 ° C to obtain the depth of 10 μm, and it can be seen that the high temperature treatment greatly contributes to the reduction of the manufacturing time. In this example, no surface roughness was observed on the LiNbO ₃ substrate 1, and scattering due to it was not generated. That is, by adding the lithium salt to phosphoric acid, it became possible to perform proton exchange for a long time without chemical damage on the + x plane or the −x plane. It can also be used to form Fresnel lenses and grating lenses.

By the way, if the temperature for heat treatment is 170 ° C. or lower, the diffusion constant is low, which is an industrial problem. Further, when the temperature for heat treatment is 300 ° C. or higher, the acid state is apt to change greatly and the reproducibility deteriorates.

A third embodiment of the optical waveguide manufacturing method of the present invention will be described with reference to the drawings. FIG. 4 shows a process chart of the method for manufacturing an optical waveguide of the present invention. In Fig. 4 (a), the Li of Z plate
Ta6 is patterned into stripes on the TaO ₃ substrate 1a as an optical waveguide mask, and Ta6 has a width of 4 μm and a length of 1
A 2 mm slit was formed. Next, in FIG.
℃, 20 minutes, add 30% lithium phosphate in pyrophosphoric acid
Proton exchange was performed on the LiTaO ₃ substrate 1a. As a result, the high refractive index layer 5 having a thickness of 0.6 μm was formed. Next, after removing the Ta mask 6 in the same figure (c), heat treatment was performed at 460 ° C. for 10 seconds using an infrared heating device. The heating rate is 10 ° C./sec. If the temperature rising rate is slow, the high refractive index layer will spread, so 100 ° C./min or more is desirable. The annealing process reduces the loss and restores the high refractive index layer 5 with nonlinearity. The region immediately below the slit of the proton-exchanged protective mask becomes the high refractive index layer 2 having a refractive index increased by about 0.03. The light propagates through the high refractive index layer 2 and becomes the optical waveguide 2. The nonlinear optical constant of the optical waveguide 2 was 26 pm / v, which is the same as that of the LiTaO ₃ substrate 1a. The thickness is 1.6 μm.
Since it is thin, the optical waveguide 2 with good confinement can be formed. Furthermore, the propagation loss was 0.2 dB / cm, which was a significant improvement over the conventional 1 dB / cm.

Next, a method of manufacturing an optical element according to the present invention will be described as a fourth embodiment with reference to the drawings. Here, an optical wavelength conversion element is taken as an optical element. FIG. 5 is a manufacturing process diagram thereof. In the same figure (a), first, Ta6 was applied to the LiTaO ₃ substrate 1a by using a normal photoprocess and dry etching.
Pattern a in a periodic pattern. Next, in FIG.
26 on the LiTaO ₃ substrate 1a on which the pattern of 6a is formed.
Proton exchange is performed in pyrophosphoric acid containing lithium phosphate at 0 ° C. for 30 minutes to form a 0.8 μm-thick proton exchange layer immediately below the slit, and then heat treatment is performed at a temperature of 590 ° C. for 10 minutes. The rising rate of heat treatment is 10 ° C./min, and the cooling rate is 50 ° C./min. Thereby, the domain inversion layer 3 is formed. If the cooling rate is slow, non-uniform inversion occurs, so 3
0 ° C./min or more is desirable. Since the Li content of the proton exchange layer is reduced and the Curie temperature is lowered, polarization inversion can be partially performed. Next, Ta6a is removed by etching with a 1: 1 mixed solution of HF: HNF ₃ for 2 minutes. Next, the optical waveguide 2 is formed on the polarization inversion layer 3 by using proton exchange. In the same figure (c), Ta is patterned into a stripe shape as an optical waveguide mask, and then a Ta mask having a width of 4 μm is formed.
260 with m and 12 mm long slits
Proton exchange was carried out in phosphoric acid containing lithium phosphate at 16 ° C. for 16 minutes. As a result, the high refractive index layer 5 having a thickness of 0.45 μm is formed. Next, after removing the Ta mask in the same figure (d), heat treatment was performed at 420 ° C. for 30 seconds using an infrared heating device. The heat treatment uniformizes the loss and reduces the loss, and the high refractive index layer 5 returns to the non-linearity. By the heat treatment, the high-refractive index layer 5 becomes the high-refractive index layer 2 in which the widening refractive index is increased by about 0.03. The light propagates through the high refractive index layer 2 and becomes the optical waveguide 2. Finally, 3000 A of SiO ₂ was added by vapor deposition.

The optical waveguide 2 having a large non-linearity was manufactured by the steps as described above. The nonlinear optical constant is about the same as that of LiTaO ₃ substrate 1. The thickness d of this optical waveguide 2 is 1.8 μm.
m, which is smaller than the thickness of the domain inversion layer 3 of 2.0 μm, and the wavelength is effectively converted. The period of the domain inversion layer 3 is 10.8μ
m and operates for a wavelength of 0.84 nm. Further, there is no change in the refractive index of the non-polarization inversion layer 4 and the polarization inversion layer 3 of the optical waveguide 2, and the propagation loss when light is guided is small. A plane perpendicular to the optical waveguide 2 was optically polished to form an incident portion and an emission portion. In this way, the light wavelength conversion element can be manufactured. The length of this element is 9 mm. Fundamental wave P1
As a result, when a semiconductor laser light (wavelength 0.84 μm) is guided from the incident portion, single mode propagation is performed, and the wavelength 0.4
The harmonic P2 of 2 μm was taken out of the substrate from the emitting portion. The propagation loss of the optical waveguide 2 is as small as 1 dB / cm and the harmonic wave P2
Was effectively taken out. One of the causes of low loss is that phosphoric acid forms a uniform optical waveguide. Input of 40 mW of fundamental wave, harmonic of 1 mW (wavelength 0.42μ
m) was obtained. The conversion efficiency in this case is 2.5%, which is 5 times higher than the conventional one. Figure 6
Shows the relationship between the heat treatment time at a temperature of 420 ° C. and the thickness of the optical waveguide. When the heat treatment time is within 4 minutes, an optical waveguide having a good confinement of 3 μm or less can be formed, which is practical. Also, 1
It is possible to manufacture an optical waveguide having a thickness of 2 μm or less by performing heat treatment within a minute, which is the most suitable method for manufacturing an optical waveguide for an optical wavelength conversion element. An infrared heating device capable of rapid heating is suitable for performing such a short-time treatment.

In the examples, lithium phosphate was used as the lithium salt, but other lithium salts such as lithium benzoate may be used. As phosphoric acid, 100% pyrophosphoric acid or 100% orthophosphoric acid can also be used.

[0018]

As described above, according to the optical element forming method of the present invention, LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) is obtained by performing proton exchange using phosphoric acid diluted with a lithium salt. On the board,
A high refractive index layer having a large amount of change in refractive index can be formed without loss.

Further, according to this method, since the high refractive index change layer can be formed without surface roughness, an optical waveguide with better confinement and a microlens with a large amount of phase change can be formed.

Further, according to the method of manufacturing an optical waveguide of the present invention, it is possible to form an optical waveguide having large non-linearity and good confinement.

[Brief description of drawings]

FIG. 1 is a process diagram of a method of manufacturing an optical waveguide according to a first embodiment of the present invention.

FIG. 2 is a process drawing of a method for manufacturing a Fresnel lens according to a second embodiment of the present invention.

FIG. 3 is a characteristic diagram showing the relationship between heat treatment time and optical waveguide thickness.

FIG. 4 is a process chart of a method for manufacturing an optical waveguide according to a third embodiment of the present invention.

FIG. 5 is a process chart of a method for manufacturing an optical element according to a fourth embodiment of the present invention.

FIG. 6 is a diagram showing the relationship between heat treatment time and optical waveguide thickness.

FIG. 7 is a configuration diagram of an optical wavelength conversion element which is a conventional optical element.

FIG. 8 is a sectional view of an optical wavelength conversion device manufactured by a conventional method.

[Explanation of symbols]

1 LiNbO ₃ substrate 2 optical waveguide 3 polarization inversion layer 5 high refractive index layer 6 protective mask 8 acid P1 fundamental wave P2 harmonic

Claims (7)

[Claims] 1. LiNb _x Ta _{1-x in} phosphoric acid containing lithium salt
An optical device comprising a high refractive index layer formed by heat-treating an O ₃ (0 ≦ x ≦ 1) substrate. 2. LiNb _x Ta _{1-x in} phosphoric acid containing lithium salt
The O ₃ (0 ≦ x ≦ 1) substrate is heat-treated, and the LiNb _x Ta _1-x O ₃ (0 ≦ _x
x ≦ 1) A method of manufacturing an optical element, which comprises forming a high refractive index layer on a substrate. 3. LiNb _x Ta _{1-x in} phosphoric acid containing lithium salt
The O ₃ (0 ≦ x ≦ 1) substrate is heat-treated to obtain the LiNb _x Ta _1-x O
₃ (0 ≦ x ≦ 1) After forming a high refractive index layer on a substrate, the substrate is annealed, and a method of manufacturing an optical waveguide. 4. The method of manufacturing an optical element according to claim 2, wherein the + X plane or the −X plane of a LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) substrate is used. 5. The method of manufacturing an optical waveguide according to claim 3, wherein a heat treatment at a temperature of 400 ° C. or higher for 4 minutes or less is performed as the annealing treatment. 6. The lithium salt is lithium phosphate.
A method for manufacturing the optical element described in claim 1. 7. The lithium salt is lithium phosphate.
A method for manufacturing the optical waveguide described.