JPH06281981A - Light wavelength conversion element - Google Patents

Abstract

PURPOSE:To facilitate the compensation between charges in the spatial charge electric field and to improve optical damage resistance by providing a plarization inversion layer with a short period in the polarization inversion layer. CONSTITUTION:The polarization inversion layer 104 with a shorter period is formed in the polarization inversion layer 102. This layer 104 is obtained in the same process as the process forming the polarization inversion layer 102 by using a short period mask as a photoprocess mask. The width of the polarization inversion layer 104 is 0.5mum. Then, incident and outgoing end surfaces are ground, and non inversion code is performed for a 0.87mum wavelength to obtain an optical wavelength conversion element with a 9mm length. When a semiconductor laser beam (0.87mum wavelength) is propagated from an incident part as a basic wave, the beam is single mode-propagated, and a harmonics with a 0.435mum wavelength is taken out from an outgoing part to the outside of a substrate at a first mode. At this time, though the power of output beam is flickered due to optical damage when the harmonics with 20mW is emitted in a usual sample, a stable harmonics output is obtained in the optical wavelength conversion element.

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 fields of optical information processing and optical application measurement control.

[0002]

2. Description of the Related Art FIG. 7 is a block diagram of a conventional optical wavelength conversion element based on an optical waveguide.

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

As shown in FIG. 7, an optical waveguide 702 is formed on a LiTaO ₃ substrate 701, and a layer 703 (polarization inversion layer) whose polarization is periodically inverted is formed on the optical waveguide 702. The mismatch of the propagation constants of the fundamental wave P1 and the generated harmonic wave P2 is caused by the polarization inversion layer 703 and the non-polarization inversion layer 70.
By compensating with the periodic structure of 4, the harmonic P2 can be efficiently generated.
Can be issued. When the fundamental wave P1 is incident on the incident surface 705 of the optical waveguide 702, the output end surface 7 of the optical waveguide 702 is
The harmonic P2 is efficiently generated from 06 and operates as an optical wavelength conversion element.

Such a conventional optical wavelength conversion element has an optical waveguide 702 manufactured by a proton exchange method as a basic constituent element. A method of manufacturing this element will be described.
First, Ta is periodically patterned on the LiTaO ₃ substrate 701 by using a normal photo process and dry etching.
Then, the LiTaO ₃ substrate 701 on which the Ta pattern is formed is formed on the 26
Proton exchange is performed at 0 ° C. for 30 minutes to form a proton exchange layer having a thickness of 0.8 μm just below the slit not covered with Ta. Then, heat treatment is performed at a temperature of 590 ° C. for 10 minutes. Heat treatment increase rate is 10 ℃ / min, cooling rate is 50 ℃ /
Minutes. As a result, the domain inversion layer 703 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 702 is formed in the polarization inversion layer 703 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. Substrate 70 covered with this Ta mask
0.5μm after proton exchange for 16 minutes at 260 ℃
To form a high refractive index layer. 38 after removing the Ta mask
Heat treatment is performed at 0 ° C. for 10 minutes. The region immediately below the slit of the proton-exchanged protective mask becomes an optical waveguide 702 having a refractive index increased by about 0.02. The optical wavelength conversion device manufactured by this conventional method has a fundamental wave P1 having 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) has been achieved.

[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 with respect to high-power harmonics. What is photo-induced refractive index change? Electrons in a trapped state are excited by light irradiation and move in the polarization direction by an internal electric field, which causes a new space charge electric field, and the refractive index changes locally due to the electro-optic effect. It happens. Light damage is a phenomenon that is likely to occur in the short wavelength region, and the higher the light intensity, the greater the decrease in the refractive index.

[0007] With the current output of several mW of harmonics, the harmonics can be stably obtained with little influence of optical damage. However, when the incident power is increased and harmonic light of 20 mW or more is obtained, 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 optical damage resistance of the nonlinear crystal that is the substrate. However, although impurities are mixed in the crystal and the growth conditions are variously changed, the good results have not been obtained.

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 the nonlinear optical crystal having the polarization inversion structure as shown in FIG. 8, the obtained space internal charges compensate each other, so that the optical damage can be reduced.

Therefore, an object of the present invention is to provide a light wavelength conversion element used in a short wavelength light source, a measurement light source, etc., which is small in size and resistant to light damage and which is stable and highly efficient.

[0011]

The present invention provides an optical wavelength conversion element capable of highly efficient conversion with improved light damage resistance by adding a new device to the optical wavelength conversion element in order to solve the above problems. It is a thing. That is, the present invention is as follows: (1) Compensation for charges in a space-charge electric field by providing a polarization inversion layer having a short period in an element in which an optical waveguide is formed in a nonlinear optical crystal having a polarization inversion layer. The present invention provides a high-efficiency light wavelength conversion element that is easy to cause and has excellent light damage resistance. (2) In an element in which an optical waveguide is formed in a nonlinear optical crystal having a polarization inversion layer, a polarization inversion layer having a period of polarization inversion shorter than that of the polarization inversion layer is provided at the top, bottom or inside of the optical waveguide. The present invention provides a highly efficient optical wavelength conversion element that is easy to compensate for charges in a 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 polarization inversion layer, the polarization inversion layer is asymmetrical with respect to the traveling direction of the optical waveguide, so that compensation of charges in a space charge electric field is likely to occur. The present invention provides a high-efficiency light wavelength conversion element having excellent light damage resistance. (4) In an element in which an optical waveguide is formed in a non-linear optical crystal having a polarization inversion layer, since the polarization inversion layer is parallel to the traveling direction of the optical waveguide inside the optical waveguide, compensation of charges in a space charge electric field is performed. The present invention provides a high-efficiency light wavelength conversion element that is easy to cause and has excellent light damage resistance. (5) In an element in which an optical waveguide is formed in a non-linear optical crystal having a polarization inversion layer, the polarization directions inside the polarization inversion layer are multi, so that compensation of charges in a space charge electric field is likely to occur, resulting in light damage resistance. Provided is a highly efficient light wavelength conversion element having excellent properties. (6) In the element in which the optical waveguide is formed in the nonlinear optical crystal having the polarization inversion layer, since the proton exchange layer is parallel to the traveling direction of the optical waveguide inside the optical waveguide,
It is intended to provide a high-efficiency optical wavelength conversion element which is easily compensated for charges of a space charge electric field and has excellent light damage resistance.

[0012]

The light wavelength conversion element of the present invention compensates the space charge distribution of electrons generated by light irradiation by the polarization inversion structure (c
By forming a domain-inverted layer having a short period in the domain-inverted layer or in the optical waveguide, it is possible to generate high-power harmonic light with excellent light damage resistance.

[0013]

EXAMPLE FIG. 1 shows an optical wavelength conversion element having a polarization inversion layer of a short period in the polarization inversion layer of the optical wavelength conversion element of the present invention. Schematic Structure A method of forming an element will be described with reference to FIG.

Reference numeral 102 is a polarization inversion layer. The domain inversion layer is first formed by periodically patterning Ta on the LiTaO ₃ substrate 101 using a normal photo process and dry etching. The period of the first-order polarization inversion with respect to the fundamental wave having a wavelength of 870 nm is 4.0 μm. L with a Ta pattern
Proton exchange is performed on the iTaO ₃ substrate 101 at 260 ° C. for 20 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 540 ° C. for 30 seconds. The heat treatment rate is 80 ℃ / sec, the cooling rate is 50 ℃ / sec.
Minutes. As a result, the domain inversion layer 102 is formed.
If the cooling rate is slow, nonuniform inversion occurs, so 30 ° 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. 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 inversion layer 102. The optical waveguide 103 is the polarization inversion layer 1
02 using proton exchange. After Ta was patterned into a stripe shape as an optical waveguide mask, a Ta mask having slits of 4 μm in width and 12 mm in length was subjected to proton exchange in pyrophosphoric acid at 260 ° C. for 16 minutes. As a result, 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 uniformizes the loss and reduces the depth of the waveguide to 1.9 μm.

Next, a polarization inversion layer 104 having a shorter period is formed in the polarization inversion layer 102 of the obtained light wavelength conversion element. If a short-period mask is used as the mask for the photo process, it can be obtained in the same process as the process for producing the domain inversion layer 102. However, here, EB (electron beam scan)
ing) was used. The width of the obtained domain-inverted layer 104 was 0.5 μm.

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

Schematic Structure In the light wavelength conversion device of FIG. 1, the width of the domain inversion layer 104 formed in the domain inversion layer 102 is 0.
It was 5 μm and the depth was about 1.9 μm. The width of the domain inversion layer 104 is preferably 1 / the period of the domain inversion layer 102.
10 (0.4 μm) or less is desirable for high output stabilization.

Further, the effect can be obtained even if the domain inversion layer 204 is formed on the upper part, the bottom part and the inside of the optical waveguide as shown in FIG. 2 (a) (b). When the domain-inverted layer 204 is formed on the upper portion, the same process as that of the normal domain-inverted layer 202 is performed. Photo process mask cycle 0.4 μm
The polarization inversion layer 20 has a depth of about 0.4 μm.
4 was formed on the optical waveguide. Without creating a domain inversion layer,
Even if only the proton exchange was performed, the improvement of the light damage was observed. This is because the proton exchange layer has a high electron mobility and is excellent in charge compensation.

On the other hand, when the polarization inversion layer 205 is formed inside or on 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 exchanging and forming a 0.8 μm thick proton exchange layer directly under the slit,
Heat treatment is performed at a temperature of 540 ° C. for 30 seconds. Since the formation of the polarization inversion starts from the bottom portion where the proton exchange is performed, when forming the polarization inversion layer inside the optical waveguide, annealing is performed before the heat treatment to expand the proton exchange layer to the inside. Annealing is performed at 350 ° C. for about 10 minutes, and then 57
The heat treatment is performed at 0 ° C. for 30 seconds, and the polarization inversion 205 is formed. When the domain-inverted layer is formed inside, the effect is large because the charge is compensated in a place where the electrolytic distribution is large, that is, in a place where a large number of electrons are generated.

Schematic Structure As shown in FIG. 3, the polarization inversion layer 302
However, the optical damage resistance of the optical wavelength conversion element whose left and right sides are asymmetric with respect to the traveling direction of the optical waveguide 303 was also improved. The manufacturing method is different only in the shape of the photomask used for manufacturing Ta on the LiTaO3 substrate 301. Schematic Configuration As shown in FIG. 3, the polarization inversion layer 302 is formed so that each polarization inversion layer can easily compensate for the electric charge, so that the optical damage resistance can be improved.

Schematic Structure As shown in FIG. 4, an optical wavelength conversion element in which a polarization inversion layer 402 is formed inside the optical waveguide 403 in parallel with the traveling direction of the optical waveguide 403 is also improved in optical damage resistance. Schematic structure It is obtained by the same process as the device of FIG. The width of the domain inversion layer 404 is 0.5 μ.
m, and polarization reversal was performed using EB. In this case, the charge compensation was sufficiently performed as compared with the devices shown in FIGS. 1 to 3 of the schematic configuration, and thus the improvement of the light damage resistance was large. Further, although the effect is small, the light damage resistance was improved even when the proton exchange layer was formed as shown in FIG. This is because the proton exchange layer has a high electron mobility and is excellent in charge compensation.

Schematic Structure As shown in FIG. 6, a domain inversion layer 602 is provided.
The light damage resistance was improved even when the domain was multi-domain.

Although the wavelength conversion element using the LiTaO3 substrate has been described in the present embodiment, LiNbO3 or KTP (KTiOPO4) is used.
Similar effects can be seen in the polarization inversion element using another inorganic nonlinear optical crystal or organic nonlinear optical crystal as described above.

[0025]

EFFECTS OF THE INVENTION In order to facilitate the compensation of charges in an optical wavelength conversion device having a polarization inversion structure, a polarization inversion structure is further formed in the upper part of the optical waveguide, inside the polarization inversion layer, and 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 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 light wavelength conversion element.

FIG. 8 is an explanatory diagram of a charge compensation mechanism in an optical wavelength conversion device having a polarization inversion structure.

[Explanation of symbols]

101 LiTaO ₃ substrate 102 polarization inversion layer 103 optical waveguide 104 polarization inversion layer 201 LiTaO ₃ substrate 202 polarization inversion layer 203 optical waveguide 204 polarization inversion layer 205 polarization inversion layer 301 LiTaO ₃ substrate 302 polarization inversion 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 incidence Surface 706 Emitting surface 801 LITaO ₃ substrate 802 Polarization inversion layer 803 Optical waveguide

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Makoto Kato 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (7)

[Claims] 1. A device in which an optical waveguide is formed in a nonlinear optical crystal having a polarization inversion layer, wherein each of the polarization inversion layers has a polarization inversion layer having a period shorter than that of the polarization inversion layer. Wavelength conversion element. 2. A device having an optical waveguide formed in a non-linear optical crystal having a polarization inversion layer, wherein the polarization inversion period is shorter than the period of the polarization inversion layer at the top, bottom or part of the inside of the optical waveguide. An optical wavelength conversion device having an inversion layer. 3. An optical wavelength conversion element in which an optical waveguide is formed in a non-linear optical crystal having a polarization inversion layer, and the polarization inversion layer is asymmetric with respect to the traveling direction of the optical waveguide. 4. A device having an optical waveguide formed in a nonlinear optical crystal having a polarization inversion layer, wherein the polarization inversion layer is provided inside the optical waveguide in parallel to the traveling direction of the optical waveguide. Conversion element. 5. An optical wavelength conversion element in which an optical waveguide is formed in a non-linear optical crystal having a polarization inversion layer, wherein the polarization directions inside the polarization inversion layer are multiple. 6. A device having an optical waveguide formed in a non-linear optical crystal having a polarization inversion layer, wherein a proton exchange layer is provided inside the optical waveguide in parallel to a traveling direction of the optical waveguide. Conversion element. 7. A nonlinear optical crystal is LiNb _x Ta _1-x O ₃ (0
≦ x ≦ 1) Substrate, The light wavelength conversion element according to any one of claims 1 to 6.