JPH06265951A - Optical wavelength converter - Google Patents

Abstract

PURPOSE:To improve the yield of semiconductor laser selection to reduce the cost of an optical wavelength converter by extending the temperature allowable range for phase matching between a fundamental wave and a wavelength conversion wave with respect, to the optical wavelength converter consisting of a semiconductor laser, an optical wavelength converting element, and a temperature control means common to them. CONSTITUTION:A periodic domain inversion structure 20 is provided as an optical wavelength converting element 13 in the optical wavelength converter consisting of a semiconductor laser 10 as a fundamental wave light source, the optical wavelength converting element 13 which constitutes a monolithic external resonator to allow a laser beam 11 emitted from the semiconductor laser 10 to resonate and converts the wavelength of the laser beam 11, and a common temperature control means which controls the temperatures of the optical wavelength converting element 13 and the semiconductor laser 10. This periodic domain inversion structure 20 is introduced to use a diagonal term of a nonlinear optical constant, thus extending the temperature allowable range for phase matching.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength converter for converting a fundamental wave into a second harmonic wave, and more particularly, an optical wavelength converter comprising a combination of a semiconductor laser and an optical wavelength converter. It is about.

[0002]

2. Description of the Related Art Conventionally, nonlinear optical materials have been used to
Wavelength conversion of laser beam to second harmonic etc. (shorter wavelength)
Various attempts have been made. As a light wavelength conversion element that performs wavelength conversion in this manner, specifically, a bulk crystal type, an optical waveguide type, and the like are known. Further, this type of optical wavelength conversion element is often used in combination with a semiconductor laser in order to convert the wavelength of a laser beam emitted from the semiconductor laser.

As one of the optical wavelength conversion devices in which the semiconductor laser as the fundamental wave light source and the optical wavelength conversion element are combined in this way, the optical wavelength conversion element is a monolithic device that resonates the laser beam emitted from the semiconductor laser. It is known that an external resonator is formed and the temperature of the light wavelength conversion element and the semiconductor laser are adjusted by a common temperature adjusting means. In such an optical wavelength conversion device, conventionally, the non-diagonal terms of the nonlinear optical constants of the optical wavelength conversion element have been used. Therefore, in order to perform phase matching between the fundamental wave and the wavelength-converted wave, the crystal of the nonlinear optical material forming the optical wavelength conversion element is cut out in a specific orientation to achieve so-called angular phase matching, or the optical wavelength conversion element is used. The temperature was adjusted to a specific temperature to achieve so-called temperature phase matching.

[0004]

However, the allowable temperature range for achieving these phase matching is very narrow, and therefore, as described above, the common wavelength adjusting means is used for the optical wavelength conversion element and the semiconductor laser. When adjusting, it was difficult to match the oscillation wavelength of the semiconductor laser with the phase matching wavelength. Due to such circumstances, conventionally,
Since the semiconductor laser whose oscillation wavelength matches the phase matching wavelength is selected and used, the manufacturing cost of this type of optical wavelength conversion device is considerably high.

The present invention has been made in view of the above circumstances, and in a light wavelength conversion device including a semiconductor laser, a light wavelength conversion element, and a temperature adjusting means common to them, a fundamental wave and a wavelength conversion wave. It is an object of the present invention to widen the temperature allowable range for phase matching of the semiconductor lasers, improve the yield of selection of semiconductor lasers, and thereby reduce the cost of the optical wavelength conversion device.

[0006]

An optical wavelength converter according to the present invention comprises a semiconductor laser as a fundamental wave light source and a monolithic external resonator for resonating a laser beam emitted from the semiconductor laser as described above. Also, in the optical wavelength conversion device comprising an optical wavelength conversion element for converting the wavelength of the laser beam and a common temperature adjusting means for adjusting the temperature of the optical wavelength conversion element and the semiconductor laser, a periodic domain inversion is performed as the optical wavelength conversion element. It is characterized in that a material having a structure was used.

[0007]

The above-mentioned periodic domain inversion structure is a structure in which the spontaneous polarization (domain) of a ferroelectric substance having a nonlinear optical effect is periodically inverted, and such a periodic domain inversion structure is provided. The method of wavelength conversion of the fundamental wave to the second harmonic wave using the optical wavelength conversion element has already been described.
Proposed by Ombergen et al. (Phys. Rev., vol.
127, No. 6, 1918 (1962)). In this method, the period Λ of the domain inversion part is represented by Λc = 2π / {β (2ω) -2β (ω)} (1) where β (2ω) is the propagation constant 2β (ω) of the second harmonic. Is set to be an integral multiple of the coherent length Λc given by the propagation constant of the fundamental wave, so that the fundamental wave and the second harmonic can be phase-matched (so-called pseudo-phase matching). When wavelength conversion is performed using a bulk crystal of a nonlinear optical material, the phase-matching wavelength is limited to a specific wavelength peculiar to the crystal, but according to the above method, a period that satisfies (1) for any wavelength By selecting Λ, phase matching can be efficiently achieved.

In general, even if the nonlinear optical material is capable of angular phase matching or temperature phase matching between the fundamental wave and the wavelength converted wave by utilizing the non-diagonal term of the nonlinear optical constant, the above-mentioned period When the domain inversion structure is introduced and the diagonal term of the nonlinear optical constant is used, the temperature allowable range for achieving phase matching becomes wide. This is because when the diagonal term of the nonlinear optical constant is used, the fundamental wave and the wavelength-converted wave have the same polarization, and therefore the temperature coefficient of the refractive index of the optical wavelength conversion element with respect to each of them.

[0009]

[Equation 1]

This is because it is difficult for the phase mismatch due to the difference of 1 to occur.

As described above, if the allowable temperature range for phase-matching the fundamental wave and the wavelength-converted wave is widened, the semiconductor laser whose oscillation wavelength matches the phase-matched wavelength is in a wider oscillation wavelength range. You can use the one in. If so, the yield of selection of the semiconductor lasers is improved, and thus the cost of the optical wavelength conversion device is reduced.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the embodiments shown in the drawings. 1 and 2 are respectively the first of the present invention.
3A and 3B show a plan shape and a side surface shape of an optical wavelength conversion device according to an example. This optical wavelength conversion device comprises a semiconductor laser 10 as a fundamental wave light source, a condenser lens 12 for converging a laser beam (fundamental wave) 11 emitted in a divergent light state from the semiconductor laser 10, and a bulk crystal type light. It is composed of a wavelength conversion element 13, a Peltier element 14 on which the above elements 10, 12 and 13 are placed and fixed, and a heat sink 15 for heat radiation to which the Peltier element 14 is attached.

The semiconductor laser 10 has an oscillation wavelength λ ₁ = 860 n
m single longitudinal and transverse mode laser, the output is 100 mW. On the other hand, the light wavelength conversion element 13 is made of a crystal of LiTaO ₃ (LT), which is a ferroelectric non-linear optical material, and has a length of 7 m as shown in the detailed plan view and side view of FIGS. 3 and 4.
m, thickness 2.0 mm, width 3.5 mm, and the light entrance end face 13a and the light exit end face 13b have a radius of curvature of 5 mm.
And the side end surface 13c is polished to be a flat surface. Further, the optical wavelength conversion element 13 is provided with a periodic domain inversion structure 20 having a period Λ = 3.8 μm.

The light wavelength conversion element 13 having the above structure can be produced, for example, as follows. First, an LT single crystal substrate (z plate) that has been monopolarized is prepared, and an electron beam is irradiated onto the LT single crystal substrate to draw a predetermined periodic pattern. The spontaneous domain is inverted to form the periodic domain inversion structure 20. Then, the end faces 13a, 13b and the end face 13c are polished into the above-described shape, and the end face 13a and the end face 13b have a wavelength of 860
nm wavelength and 430 nm light are coated with the following characteristics to obtain the light wavelength conversion element 13.

[0015] A laser beam 11 which is a fundamental wave emitted from the semiconductor laser 10 is condensed by a condenser lens 12 so as to be converged inside the light wavelength conversion element 13, and is incident on the end surface 13a into the light wavelength conversion element 13. To do. At this time, the direction of the linearly polarized light of the laser beam 11 is aligned with the z-axis direction of the LT crystal in order to utilize d ₃₃ , which is one of the diagonal terms of the nonlinear optical constant of LT. Further, the semiconductor laser 10 and the light wavelength conversion element 13 are temperature-controlled to a predetermined temperature by a common Peltier element 14 and its drive circuit (not shown).

Wavelength λ ₁ incident on the optical wavelength conversion element 13 =
A laser beam 11 of 860 nm is emitted from the end face 13 of this device 13.
The light is sequentially reflected by b, 13c, and 13a, follows the ring-shaped optical path, and passes through the periodic domain inversion structure 20 in a state where it resonates and becomes high in intensity. The laser beam 11 by the optical wavelength conversion element 13 is converted to the wavelength _{λ 2 = λ 1/2 =} 430 second harmonic 21 nm. This second harmonic 21 and laser beam
11 is phase-matched (quasi-phase matched) in the periodic domain inversion structure 20. The second harmonic 21 obtained in this way
Indicates that the direction of the linearly polarized light coincides with the z-axis direction of the LT crystal, and most of them (95%) pass through the end face 13b and go out of the light wavelength conversion element 13. In this example, when the output of the semiconductor laser 10 was 100 mW, the second harmonic wave 21 of 20 mW was obtained.

The optical wavelength conversion element 13 constitutes a monolithic ring resonator as described above, but in this ring resonator, the counterclockwise ring light is generated by the reflection in the periodic domain inversion structure 20 and the like. Since this reverse ring light returns to the semiconductor laser 10, so-called optical feedback is applied, and its oscillation wavelength is locked to the resonance wavelength of the ring resonator.

In this embodiment, an optical wavelength conversion element for phase-matching the second harmonic wave 21 and the laser beam 11 is used.
The allowable temperature range of 13 is ± 3 ° C. Therefore, if the set temperature of the semiconductor laser 10 can be adjusted within a range of approximately ± 3 ° C. and the temperature dependence of the oscillation wavelength is 0.3 nm / ° C., the oscillation wavelength of this semiconductor laser 10 is 860
Those in the range of nm ± 0.9 nm can be widely used. If so, the yield of selection of the semiconductor laser 10 is improved, and thereby the cost of the optical wavelength conversion device is reduced.

Next, a second embodiment of the present invention will be described with reference to FIG. In FIG. 5, the same elements as those in FIGS. 1 and 2 are designated by the same reference numerals, and the duplicated description thereof will be omitted. The optical wavelength conversion device of the second embodiment is different from the device of the first embodiment only in that a waveguide type optical wavelength conversion device 30 is used in place of the bulk crystal type optical wavelength conversion device 13. . As shown in detail in FIG. 6, this waveguide type optical wavelength conversion element 30 is formed by forming a channel waveguide 31 extending in the x-axis direction on an LT crystal substrate 32 on which the periodic domain inversion structure 20 is formed.

The channel waveguide 31 can be manufactured, for example, as follows. A periodic domain inversion structure is formed on the LT crystal substrate 32 in the same manner as in the first embodiment.
After forming 20, the metal Ta is formed on the −z surface of the LT substrate 32.
Is sputtered to form a Ta thin film having a thickness of 50 nm, and then a mask pattern having a width of 4 μm is formed by photolithography and dry etching. Next, above L
The T substrate 32 was subjected to a proton exchange treatment in pyrophosphoric acid at 230 ° C. for 15 minutes, and the Ta mask was treated with NaOH and H ₂ O.
After removing with the mixed etching solution of No. ₂ , it is annealed at 300 ° C. for 5 minutes to form the channel waveguide 31. Then, the light incident end face 30a and the light emitting end face 30b of the channel waveguide type optical wavelength conversion element 30 thus created are edge-polished. Then, on these end faces 30a and 30b, the wavelength 860
A coating having the following characteristics is applied to the light of wavelengths of nm and 430 nm.

[0021] The waveguide type optical wavelength conversion element 30 manufactured as described above
The laser beam 11 emitted from the semiconductor laser 10
Input as the fundamental wave. Ie laser beam
The polarization direction of 11 is aligned with the z-axis direction of the channel waveguide 31, is condensed by the condenser lens 12, and is converged on the end face of the channel waveguide 31. As a result, the laser beam 11 enters the channel waveguide 31 and is guided there.

The laser beam 11 resonates between the element end faces 30a and 30b forming the Fabry-Perot type external resonator to have a high intensity and is phase-matched by the periodic domain inversion structure 20 in the channel waveguide 31. The wavelength is converted into the second harmonic 21. This second harmonic wave 21 also propagates in the channel waveguide 31 in the guided mode and is efficiently emitted from the end face 30b.
Since the polarization direction of the output second harmonic wave 21 is also in the z-axis direction, it means that the nonlinear optical constant d ₃₃ having the largest LT is used. Here, the output of the semiconductor laser 10
The second harmonic output was 20 mW when the length of the waveguide type optical wavelength conversion element 30 was 100 mm and the length was 1 mm.

In the waveguide type optical wavelength conversion element 30 as described above, the power density of the laser beam 11 becomes higher than that of the bulk crystal type optical wavelength conversion element 13 used in the first embodiment. Even if the element length is only 1 mm, the second harmonic wave 21 of high output can be obtained.

In the second embodiment, the allowable temperature range of the optical wavelength conversion element 30 for phase matching the second harmonic wave 21 and the laser beam 11 is ± 15 ° C. Therefore, the temperature dependence of the oscillation wavelength of the semiconductor laser 10 is 0.3 n.
If it is m / ° C, this semiconductor laser 10
Those having an oscillation wavelength in the range of 860 nm ± 4.5 nm can be widely used. If so, laser diode
The yield of 10 selections is remarkably improved, thereby reducing the cost of the optical wavelength conversion device.

In the bulk crystal type optical wavelength conversion element 13 of the first embodiment, if the element length is set to 1 mm, the wavelength conversion efficiency is lowered, but the second harmonic wave 21 and the laser beam are emitted.
The allowable temperature range for phase matching 11 and 11 is ± 15 ° C. as described above, and the oscillation wavelength range of the semiconductor laser 10 is also 860 nm ± 4.5 nm.

The oscillation wavelength of the semiconductor laser 10 can be adjusted by changing the drive current value, and the present invention can be applied to control the oscillation wavelength by adopting such a method. Is.

The non-linear optical material used in the present invention is not limited to the above-mentioned LT, and other materials such as LiNbO ₃ (LN), MgO-LT and MgO- can be used.
LN, KNbO ₃ , KTP, BBO, LBO and the like may be used.

[Brief description of drawings]

FIG. 1 is a plan view of an optical wavelength conversion device according to a first embodiment of the present invention.

FIG. 2 is a side view of the device of the first embodiment.

FIG. 3 is a plan view of a bulk crystal type optical wavelength conversion device used in the device of the first embodiment.

FIG. 4 is a side view of the bulk crystal type optical wavelength conversion device.

FIG. 5 is a side view of an optical wavelength conversion device according to a second embodiment of the present invention.

FIG. 6 is a perspective view of a waveguide type optical wavelength conversion element used in the device of the second embodiment.

[Explanation of symbols]

10 Semiconductor laser 11 Laser beam (fundamental wave) 12 Condenser lens 13 Bulk crystal type optical wavelength conversion element 13a, 13b, 13c End surface of bulk crystal type optical wavelength conversion element 14 Peltier element 15 Heat sink 20 Periodic domain inversion structure 21 Second harmonic Wave 30 Waveguide-type optical wavelength conversion element 30a, 30b End facet of waveguide-type optical wavelength conversion element 31 Channel waveguide

Claims (1)

[Claims] 1. A semiconductor laser as a fundamental wave light source,
An optical wavelength conversion element that constitutes a monolithic external resonator that resonates a laser beam emitted from the semiconductor laser and that wavelength-converts the laser beam, and a common temperature adjustment that adjusts the temperature of the optical wavelength conversion element and the semiconductor laser In the optical wavelength conversion device including the means, an optical wavelength conversion device having a periodic domain inversion structure is used as the optical wavelength conversion element.