JP3052654B2 - Optical wavelength conversion element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art FIG. 7 shows a configuration diagram of a conventional optical wavelength conversion device based on an optical waveguide.

Hereinafter, generation of harmonics (wavelength 0.42 μm) with respect to a fundamental wave having a wavelength of 0.84 μm will be described in detail with reference to the drawings. (K. Mizuuchi, K. Yamamoto and T. Taniuchi, Appli
ed Physics Letters, Vol 58, p. 2732, June 1991, see).

As shown in FIG. 7, an optical waveguide 702 is formed on a LiTaO ₃ substrate 701, and a layer 703 (a domain-inverted layer) having periodically inverted polarization is formed on the optical waveguide 702. The mismatch between the propagation constant of the fundamental wave P1 and the generated harmonic wave P2 is determined by the polarization inversion layer 703 and the non-polarization inversion layer 70.
High efficiency harmonic P2 by compensating with the periodic structure of
Can be issued. When the fundamental wave P1 is incident on the incident surface 705 of the optical waveguide 702, the emission end face 7 of the optical waveguide 702
The harmonic P2 is efficiently generated from 06 and operates as an optical wavelength conversion element.

[0005] Such a conventional optical wavelength conversion element has an optical waveguide 702 produced by a proton exchange method as a basic component. A method for manufacturing this device will be described.
First, Ta is periodically patterned on the LiTaO ₃ substrate 701 using a normal photo process and dry etching.
Next, the LiTaO ₃ substrate 701 having the Ta pattern
Proton exchange is performed at 0 ° C. for 30 minutes to form a 0.8 μm-thick proton exchange layer immediately below the slit not covered with Ta. Next, heat treatment is performed at a temperature of 590 ° C. for 10 minutes. The rate of heat treatment is 10 ° C / min and the cooling rate is 50 ° C / min.
Minutes. Thus, a domain-inverted layer 703 is formed.
Immediately below the proton exchange layer, Li is reduced and the Curie temperature is lowered, so that domain inversion can be partially performed.
Next, Ta is removed by etching with a 1: 1 mixed solution of HF: HNF ₃ for 2 minutes. Further, an optical waveguide 702 is formed in the domain-inverted layer 703 by using proton exchange. By patterning Ta in a stripe shape as an optical waveguide mask, a slit having a width of 4 μm and a length of 12 mm is formed in the Ta mask. The substrate 70 covered with this Ta mask
1 was subjected to proton exchange at 260 ° C. for 16 minutes, and 0.5 μm
Is formed. After removing the Ta mask 38
Heat treatment is performed at 0 ° C. for 10 minutes. The area immediately below the slit of the proton-exchanged protective mask becomes the optical waveguide 702 whose refractive index has increased by about 0.02. The optical wavelength conversion device manufactured by this conventional method has a wavelength of 0.84 μm, a length of the optical waveguide 702 of 9 mm, and a power of the fundamental wave P1 of 40 mW. 6mW
(162% / W).

[0006]

The nonlinear optical crystal is Li
In the case of another dielectric crystal substrate such as Nb _x Ta _{1 -x} O ₃ (0 ≦ x ≦ 1) or KTP, a photo-induced refractive index change (optical damage) occurs for a high-power harmonic. The light-induced change in refractive index means that electrons trapped by light irradiation are excited and move in the direction of polarization due to the internal electric field, and a new space charge electric field is generated. It will happen. Optical damage is likely to occur in a short wavelength region, and is a phenomenon in which the higher the light intensity, the more the refractive index decreases.

With the current harmonic output of about several mW, it is possible to stably obtain a harmonic with little influence of optical damage. However, when higher harmonic power of 20 mW or more is obtained by increasing the incident power, the output of the obtained harmonic becomes unstable due to the influence of optical damage.

In order to stably obtain high-output harmonic light,
It is necessary to improve the light damage resistance of the nonlinear crystal as the substrate. However, although impurities have been mixed into the crystal and various growth conditions have been changed, very good results have not been obtained.

[0009] The mechanism of photodamage is caused by the space charge electric field generated by the movement of charges by the internal electric field of electrons. Therefore, in a nonlinear optical crystal having a domain-inverted structure as shown in FIG. 8, the obtained space internal charges compensate each other, so that optical damage can be reduced.

It is an object of the present invention to provide a light wavelength conversion element that is resistant to optical damage and that is small, stable, and efficient, and that is used in a short wavelength light source, a measurement light source, and the like.

[0011]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention does not contribute to the phase matching of the optical wavelength conversion element.
Providing a second domain inversion layer or a proton exchange layer
Cormorant is to provide a highly efficient convertible optical wavelength conversion device with improved optical damage resistance by adding a new twist. In other words, the present invention provides: (1) In a device in which an optical waveguide is formed in a nonlinear optical crystal having a domain-inverted layer, by providing a domain-inverted layer having a short period in the domain-inverted layer, compensation of charges of the space charge electric field is achieved It is intended to provide a high-efficiency optical wavelength conversion element which is liable to occur and has excellent light damage resistance. (2) In an element in which an optical waveguide is formed in a nonlinear optical crystal having a domain-inverted layer, a domain-inverted layer having a domain inversion cycle shorter than the domain-inverted layer cycle at the top, bottom, or inside of the optical waveguide. Another object of the present invention is to provide a high-efficiency optical wavelength conversion element that easily compensates for the electric charges of the space charge electric field and has excellent light damage resistance. (3) In an element in which an optical waveguide is formed in a nonlinear optical crystal having a domain-inverted layer, since the domain-inverted layer is asymmetric on the left and right with respect to the traveling direction of the optical waveguide, compensation of the space charge electric field is likely to occur. Another object of the present invention is to provide a high-efficiency optical wavelength conversion element having excellent light damage resistance. (4) In a device in which an optical waveguide is formed in a nonlinear optical crystal having a domain-inverted layer, since the domain-inverted layer is provided inside the optical waveguide in parallel to the traveling direction of the optical waveguide, compensation of electric charges in the space charge electric field is performed. It is intended to provide a high-efficiency optical wavelength conversion element which is liable to occur and has excellent light damage resistance. (5) In a device in which an optical waveguide is formed in a nonlinear optical crystal having a domain-inverted layer, since the polarization directions inside the domain-inverted layer are multiplied, the charges of the space charge electric field are likely to be compensated, and light damage is prevented. An object of the present invention is to provide a high-efficiency optical wavelength conversion element having excellent performance. (6) In a device in which an optical waveguide is formed in a nonlinear optical crystal having a domain-inverted layer, a proton exchange layer is provided inside the optical waveguide in parallel to the traveling direction of the optical waveguide.
It is an object of the present invention to provide a high-efficiency optical wavelength conversion element that easily compensates for electric charges in a space charge electric field and has excellent light damage resistance.

[0012]

According to the light wavelength conversion element of the present invention, the space charge distribution of electrons generated by light irradiation is compensated for by the domain inversion structure (c
A short-period domain-inverted layer is formed in a domain-inverted layer or in an optical waveguide to generate high-power harmonic light with excellent light damage resistance.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a polarization inversion layer of an optical wavelength conversion device of the present invention,
FIG. 1 shows an optical wavelength conversion element having a short-period domain-inverted layer as a second domain-inverted layer that does not contribute to phase matching . A method for forming an element will be described with reference to FIG.

Reference numeral 102 denotes a domain-inverted layer. First, Ta is periodically patterned on the LiTaO ₃ substrate 101 using a normal photo process and dry etching. The period of the primary polarization reversal with respect to the fundamental wave having a wavelength of 870 nm is 4.0 μm. L with pattern formed by Ta
Proton exchange is performed on the iTaO ₃ substrate 101 at 260 ° C. for 20 minutes to form a proton exchange layer having a thickness of 0.8 μm immediately below the slit, and then heat-treated at a temperature of 540 ° C. for 30 seconds. The rate of heat treatment is 80 ° C / sec and the cooling rate is 50 ° C / sec.
Minutes. Thereby, the domain-inverted layer 102 is formed.
If the cooling rate is slow, non-uniform reversal occurs, so that the cooling rate is preferably 30 ° C./min or more. In the proton exchange layer, since Li is reduced and the Curie temperature is lowered, polarization inversion can be partially performed. Ta is removed by etching with a 2: 1 mixture of HF: HNF ₃ for 2 minutes.

Reference numeral 103 denotes an optical waveguide formed on the domain-inverted layer 102. The optical waveguide 103 is the domain-inverted layer 1
02 was formed using proton exchange. After patterning Ta in a stripe shape as an optical waveguide mask, proton exchange was performed in pyrophosphoric acid at 260 ° C. for 16 minutes on a Ta mask having a slit having a width of 4 μm and a length of 12 mm. Thus, a proton exchange layer having a thickness of 0.45 μm is formed. After removing the Ta mask, heat treatment was performed at 420 ° C. for 1 minute using an infrared heating device. The heat treatment makes the waveguide uniform and reduces the loss, thereby forming a waveguide having a depth of 1.9 μm.

Next, the polarization in the inversion layer 102 of the resultant optical wavelength conversion element, a polarization inversion layer 104 in a shorter period, the phase
It is formed as a second domain-inverted layer that does not contribute to matching . If a short-period mask is used as a mask for the photo process, it can be obtained in the same process as the process for manufacturing the domain-inverted layer 102. However, here, EB (electron beam scanin)
g). The width of the obtained domain-inverted layer 104 was 0.5 μm.

The input and output end faces are polished to a wavelength of 0.87 μm.
Is subjected to a non-reflection code to obtain a light wavelength conversion element having a length of 9 mm. Semiconductor laser light (wavelength 0.
(87 μm) was guided from the incident part, propagated in single mode, and a harmonic having a wavelength of 0.435 μm was extracted from the emission part to the outside of the substrate in the primary mode. A harmonic of 3 mW (wavelength 0.42 μm) was obtained by inputting a fundamental wave of 40 mW. T
Using an i: Sapphire laser, a harmonic output of 25 mW was obtained for a fundamental wave of 100 mW. In a conventional sample, the power of output light fluctuated due to optical damage when a harmonic of 20 mW was emitted, but a stable harmonic output was obtained with the wavelength conversion element of the present invention.

Schematic Configuration In the optical wavelength conversion element shown in FIG. 1, the width of the domain-inverted layer 104 formed in the domain-inverted layer 102 is set to 0.
It was 5 μm and the depth was about 1.9 μm. Preferably, the width of the domain-inverted layer 104 is 1/100 of the period of the domain-inverted layer 102.
10 (0.4 μm) or less is desirable for high output stabilization.

Further, as shown in FIGS. 2 (a) and 2 (b), a second portion which does not contribute to phase matching is provided at the top, bottom and inside of the optical waveguide .
Even when the domain-inverted layer 204 was formed as the domain-inverted layer, an effect was obtained. In the case of forming the domain-inverted layer 204 on the upper part, a process similar to that of a normal manufacturing process of the domain-inverted layer 202 is performed. When the period of the photo process mask was set to about 0.4 μm, a domain-inverted layer 204 having a depth of about 0.4 μm was formed above the optical waveguide. Even when only proton exchange was performed without forming a domain-inverted layer, improvement in light damage was observed. This is because the proton exchange layer has a high electron mobility and thus is excellent in charge compensation.

On the other hand, when manufacturing the domain-inverted layer 205 inside or at the bottom of the optical waveguide, the manufacturing method is slightly different. Formation of a normal polarization inversion layer, first LiTaO ₃ substrate using conventional photo process and dry etching to pattern the Ta to periodic, Ta LiTaO ₃ substrate 1 to 260 ° C. which a pattern is formed by a proton 20 minutes After performing exchange and forming a proton exchange layer having a thickness of 0.8 μm immediately below the slit,
Heat treatment at a temperature of 540 ° C. for 30 seconds. Since the formation of the domain inversion starts from the proton-exchanged bottom, when forming a domain-inverted layer inside the optical waveguide, annealing is performed before the heat treatment, and the proton exchange layer is extended to the inside. Annealing is performed at 350 ° C. for about 10 minutes, and then performed for 57 minutes.
Heat treatment is performed at 0 ° C. for 30 seconds to form domain inversion 205. When a domain-inverted layer is formed inside, electric charge is compensated for at a place where the electric field distribution is large, that is, at a place where many electrons are generated, so that the effect is large.

Schematic configuration As shown in FIG.
However, the light wavelength conversion element whose right and left are asymmetrical with respect to the traveling direction of the optical waveguide 303 also has improved light damage resistance. The manufacturing method is different only in the shape of the photomask used when manufacturing Ta on the LiTaO3 substrate 301. Schematic Configuration As shown in FIG. 3, by forming the domain-inverted layer 302 so that each domain-inverted layer easily compensates for electric charges, improvement in light damage resistance can be seen.

Schematic Configuration As shown in FIG. 4, the phase adjustment is performed inside the optical waveguide 403 in parallel with the traveling direction of the optical waveguide 403.
Domain inversion layer 40 as a second domain inversion layer that does not contribute to
Also in the light wavelength conversion element in which No. 4 was formed, improvement in light damage resistance was observed. Schematic configuration It is obtained by a process similar to that of the device of FIG. The width of the domain inversion layer 404 was 0.5 μm, and domain inversion was performed using EB. In this case, the charge compensation was sufficiently performed as compared with the elements shown in the schematic configuration diagrams of FIGS. 1 to 3, so that the improvement of the light damage resistance was large. Although the effect is small, improvement in light damage resistance was observed even when a proton exchange layer was formed as shown in FIG. This is because the proton exchange layer has a high electron mobility and thus is excellent in charge compensation. 1 to 4 contribute to phase matching
Although the second domain-inverted layer is not formed in the optical waveguide,
The same effect can be obtained by disposing the ton exchange layer as shown in FIGS.
Is obtained.

Schematic structure As shown in FIG.
Even when is a multi-domain, its light damage resistance was improved.

In this embodiment, the wavelength conversion element using the LiTaO3 substrate has been described, but LiNbO3 or KTP (KTiOPO4)
Similar effects can be obtained in a domain-inverted element using another inorganic nonlinear optical crystal or organic nonlinear optical crystal.

[0025]

According to the present invention, in order to facilitate charge compensation in an optical wavelength conversion element having a domain-inverted structure, a domain-inverted structure is further formed on the optical waveguide, inside or inside the domain-inverted layer. The light damage resistance of the conversion element is improved, and highly efficient and stable wavelength conversion resistant to light damage is realized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an optical wavelength conversion element of the present invention.

FIG. 2 is a schematic configuration diagram of an optical wavelength conversion element of the present invention.

FIG. 3 is a schematic configuration diagram of an optical wavelength conversion element of the present invention.

FIG. 4 is a schematic configuration diagram of an optical wavelength conversion element of the present invention.

FIG. 5 is a schematic configuration diagram of an optical wavelength conversion element of the present invention.

FIG. 6 is a schematic configuration diagram of an optical wavelength conversion element of the present invention.

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

FIG. 8 is an explanatory diagram of a charge compensation mechanism in an optical wavelength conversion element having a domain-inverted structure.

[Explanation of symbols]

101 LiTaO ₃ substrate 102 domain-inverted layer 103 optical waveguide 104 domain-inverted layer 201 LiTaO ₃ substrate 202 domain-inverted layer 203 optical waveguide 204 domain-inverted layer 205 domain-inverted layer 301 LiTaO ₃ substrate 302 domain-inverted layer 303 optical waveguide 401 LiTaO ₃ substrate 402 Polarization inversion layer 403 Optical waveguide 404 Polarization inversion layer 501 LiTaO ₃ substrate 502 Polarization inversion layer 503 Optical waveguide 601 LiTaO ₃ substrate 602 Polarization inversion layer 603 Optical waveguide 701 LITaO ₃ substrate 702 Optical waveguide 703 Polarization inversion layer 704 Non-polarization inversion layer 705 Incident Surface 706 Emission surface 801 LITaO ₃ substrate 802 Domain-inverted layer 803 Optical waveguide

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Makoto Kato 1006 Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/377 JICST File (JOIS)

Claims (5)

(57) [Claims] 1. A device forming an optical waveguide in a nonlinear optical crystal having a polarization inversion layer to satisfy the phase matching period in each of the polarization inversion layer does not contribute to the short rather phase matching than the polarization inversion layer An optical wavelength conversion element having a domain-inverted layer. 2. An element in which an optical waveguide is formed in a nonlinear optical crystal having a domain-inverted layer that satisfies phase matching, wherein the domain of the domain-inverted layer has a period of domain inversion at the top, bottom, or a part of the inside of the optical waveguide. tank position than the period
An optical wavelength conversion element having a domain-inverted layer that does not contribute to phase matching . 3. An element in which an optical waveguide is formed in a nonlinear optical crystal having a domain-inverted layer that satisfies phase matching , wherein the domain inversion does not contribute to the phase matching inside the optical waveguide in parallel to the traveling direction of the optical waveguide. An optical wavelength conversion element comprising a layer. 4. An element in which an optical waveguide is formed in a nonlinear optical crystal having a domain-inverted layer satisfying phase matching , wherein a proton exchange layer is provided inside the optical waveguide in parallel to a traveling direction of the optical waveguide. Characteristic light wavelength conversion element. 5. The nonlinear optical crystal is LiNbxTa1-xO.
3. The substrate according to claim 1, wherein the substrate is a 3 (0 ≦ x ≦ 1) substrate.
5. The optical waveguide according to any one of items 4 to 4 .