JPH07270632A - Optical element, optical wavelength converting element and generator of short wavelength light - Google Patents

Abstract

PURPOSE:To stabilize the output of an optical wavelength converting element by controlling the phase matching wavelength. CONSTITUTION:This optical wavelength converting element is produced by forming a polarization reversed layer 4 and a proton-exchanged optical waveguide 5 on a LiTaO3 substrate 1. The electric field applied on the optical waveguide 5 and temp. are controlled with electrodes 10 and a heater 11 to control the refractive index of the optical waveguide 5. By controlling temp., the refractive index can be modulated in a wide range. By combining with controlling of the electric field, fast modulation can be obtd. As a result, the phase matching wavelength of the optical wavelength converting element can be fast controlled in a wide range. Thereby, the phase matching wavelength of the optical wavelength converting element which is changed by environmental temp. or the like can be controlled to obtain stable output.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion element to which a coherent light source is applied and which is used in the fields of optical information processing and optical application measurement and a short wavelength coherent light source using the optical wavelength conversion element.

[0002]

2. Description of the Related Art Light (fundamental wave) is generated by a second harmonic wave generating element (hereinafter referred to as SHG element) utilizing a nonlinear optical effect.
Can be converted to the second harmonic of half the wavelength. By converting the semiconductor laser light by this, a small short-wavelength light source can be realized, and it can be applied to the fields of printing, optical information processing, optical applied measurement control, and the like, and thus is actively researched. In order to realize highly efficient wavelength conversion in the SHG element, the fundamental wave and the second
It is essential that the phase matching condition be satisfied between the harmonics.
The phase matching condition depends on the material characteristics of the SHG element, the wavelength of the fundamental wave, and the like, but since the tolerance is generally narrow, precise control of the fundamental wave wavelength is required to satisfy the condition.

As an example showing this, there is a report of wavelength conversion of a semiconductor laser using a quasi-phase matching (hereinafter referred to as QPM) type polarization inversion optical waveguide (Yamamoto et al. Applied Physics Letters). Le
tters, Vol.62, No.21, 2599 (1993)). An optical waveguide having a periodic domain-inverted layer is formed on a LiTaO ₃ substrate, and QPM-
It constitutes an SHG element.

The SHG element has succeeded in generating a blue light of 31 mW with a conversion efficiency of 21%, but the tolerance of the fundamental wave is 0.
It is only 12 nm, which indicates that the wavelength tolerance of the SHG element is narrow. On the other hand, the oscillation wavelength of the semiconductor laser changes with mode hopping due to temperature or applied current (for example, the temperature change is 0.1 to 0.2 nm / ° C, and the applied current is 0.1
Since the wavelength changes with mode hop of ~ 0.2 nm),
It is not easy to match the oscillation wavelength of the semiconductor laser with the phase matching wavelength of the SHG element.

Therefore, for example, FIG. 11 shows a short-wavelength light generating device by wavelength tuning of a semiconductor laser using grating feedback as shown in FIG. FIG. 14 shows a schematic configuration diagram of a conventional short wavelength light generating device using a semiconductor laser and a QPM-SHG element. The light emitted from the semiconductor laser 101 is converted into a parallel beam by the collimating lens 102 with NA = 0.55, and λ
/ 2 plate 103 rotates the deflection direction, and focusing lens 104 with NA = 0.45 allows incident portion 105 of the optical waveguide.
Is focused on. The fundamental wave and the second harmonic emitted from the emitting end of the optical waveguide are converted into a parallel beam by the collimator lens 106, and then separated by the dichroic mirror 107 into the fundamental wave and the second harmonic.

After the wavelength of the fundamental wave is selected by the grating 108, the lens 106, the optical waveguide and the lens 10 are selected.
It is returned to the semiconductor laser 101 through 4, 102 and the λ / 2 plate 103. The oscillation wavelength of the semiconductor laser 101 can be controlled by the selection wavelength of the grating 108. As a result, the wavelength conversion of the semiconductor laser succeeded in generating a blue light of 8 mW, and the temperature change of the semiconductor laser was reduced to 1
A stable SHG output is obtained between 7 and 35 ° C.

As the grating feedback method, in addition to utilizing the external grating shown in FIG. 14, a type in which a grating is integrated in an SHG element has also been reported. As an example showing this, for example, a dielectric grating layer is formed on an SHG element, a part of the fundamental wave propagating in an optical waveguide is wavelength-selected by a grating, and the selected wavelength is fed back to a semiconductor laser. Is fixed (Japanese Patent Application No. 5-85950). As an integrated grating, a type in which a grating is formed inside an optical waveguide has also been reported (K. Shinozaki et al. Applied Physics Letters. V
ol. 59, No. 29, 510-512 (1991)). Wavelength 1.327μ
For the fundamental wave P1 of m, when the length of the optical waveguide was 2 mm and the power of the fundamental wave P1 was 60 μW, the power of the harmonic wave P2 was 0.652 pW. The conversion efficiency at this time was 4.1% / W · cm ² .

On the other hand, there is a method of controlling the phase matching wavelength of the SHG element in order to convert the wavelength of the semiconductor laser by the SHG element. For example, there is a method in which a voltage is applied to the optical waveguide and the refractive index of the optical waveguide is controlled by the electro-optic effect to adjust the phase matching wavelength to match the phase matching wavelength with the oscillation wavelength of the semiconductor laser. Wishes 4-20
4815).

FIG. 15 shows a configuration diagram of a conventional optical wavelength conversion element for adjusting the phase matching wavelength by applying an electric field. In FIG. 15, 110 is a -Z plate LiTaO ₃ substrate, 111 is a periodically poled layer, 112 is a proton exchange optical waveguide, 113 is an electrode, 114 is a semiconductor laser, and 115 is an incident part. In the conventional light wavelength conversion element, there is no description about the width of the electrode. When a voltage is applied to the electrode 113, a voltage in the Z direction is applied to the optical waveguide. Therefore, due to the electro-optic effect of the substrate, the refractive index of the light with respect to the electric field component in the Z direction changes via the electro-optic constant r ₃₃ . As a result, the phase matching wavelength of the light wavelength conversion element is modulated.

[0010]

First, the problems associated with the conventional short wavelength light generator will be described.

In the conventional short-wavelength light generator using the grating feedback, the oscillation wavelength of the semiconductor laser can be controlled by the grating, but since the oscillation wavelength of the semiconductor laser exists discretely every 0.1 to 0.2 nm, the wavelength tolerance It is difficult to completely match the phase matching wavelength of the optical wavelength conversion element with a narrow degree (for example, in the case of the QPM-SHG element, the wavelength tolerance is about 0.1 nm in full width at half maximum), it is difficult to achieve high efficiency, and the output is There was a problem of instability. Further, since the phase matching wavelength fluctuates depending on the temperature of the light wavelength conversion element, there is a problem that the SHG output fluctuates due to a change in environmental temperature. Furthermore, in an optical wavelength conversion element with integrated grating, since the oscillation wavelength of the semiconductor laser is fixed to the reflection wavelength of the grating, it is difficult to precisely match the reflection wavelength of the grating with the phase matching wavelength, and high efficiency is achieved. There was a problem that it was difficult.

In view of the above points, the present invention has an object to provide a short-wavelength light generating device capable of high-efficiency conversion and having stable output characteristics against environmental temperature changes.

Next, the problems associated with the optical wavelength conversion element will be described. A method of adjusting the phase matching wavelength of the optical wavelength conversion element by utilizing the electro-optic effect is shown. The change in the refractive index due to the electro-optic effect has a fast response speed and high-speed modulation is possible, but the change in the refractive index that can be modulated is as small as 10 ⁻⁴ , so the range of the phase matching wavelength that can be modulated is narrower than 1 nm. There was a problem of being limited to the range.

On the other hand, although it is possible to control the phase matching wavelength by changing the temperature of the substrate, there is a problem that the modulation due to the temperature change has a slow response speed and high speed modulation cannot be performed.

In view of the above points, an object of the present invention is to provide an optical wavelength conversion element capable of modulating a phase matching wavelength at high speed and modulating a wide range.

[0016]

In order to solve the above problems, according to the present invention, (1) a ferroelectric substrate, an electric field applying means formed on the substrate, and a temperature applying means formed on the substrate. It is an optical element provided with. (2) Non-linear optical substrate, periodic polarization inversion layer formed on the surface of the substrate, optical waveguide formed on the surface of the substrate, electric field applying means formed on the optical waveguide, and the optical waveguide. And a temperature applying unit formed on at least one side of the optical waveguide, and the refractive index of the optical waveguide is controlled by the electric field applying unit and the temperature applying unit. (3) Non-linear optical substrate, periodic domain-inverted layer formed on the substrate surface, optical waveguide formed on the substrate surface, electric field applying means formed on the optical waveguide, and electric field application An optical wavelength control device comprising an electric insulation means formed on the means and a temperature application means formed on the insulation means, wherein the refractive index of the optical waveguide is controlled by the electric field application means and the temperature application means. It is a conversion element. (4) Non-linear optical substrate, periodic domain-inverted layer formed on the surface of the substrate, optical waveguide formed on the surface of the substrate, first stripe-shaped optical waveguide formed on the optical waveguide
And a striped second conductor formed on both sides or one side of the first conductor, and the strap directions of the first and second conductors are substantially parallel to each other, and the first conductor is provided. The optical wavelength conversion element has a potential difference between a conductor and the second conductor, and a current flows through at least one of the first and second conductors. (5) A semiconductor laser, a non-linear optical substrate, a periodic domain inversion layer formed on the substrate surface, an optical waveguide formed on the substrate surface, an optical waveguide formed on the substrate surface, An electric field applying unit formed on the substrate, a temperature applying unit formed on the substrate, and a condensing optical system are provided, and the refractive index of the optical waveguide is controlled by the voltage applying unit and the temperature applying unit. It is a short wavelength light generating device. (6) Wavelength selection means, semiconductor laser, non-linear optical substrate, periodic domain-inverted layer formed on the substrate surface, optical waveguide formed on the substrate surface, and formed on the substrate surface An optical waveguide, an electric field applying means formed on the substrate, a temperature applying means formed on the substrate,
A light-collecting optical system, wherein at least a part of the emitted light of the semiconductor laser is wavelength-selected by the wavelength selecting means, and then the light is returned to the semiconductor laser again, and the refractive index of the optical waveguide is the voltage. It is a short wavelength light generator controlled by an application unit and the temperature application unit. (7) A nonlinear optical substrate having a temperature coefficient of refractive index dn1 / dT (° C. ^-1 ), a periodic domain-inverted layer formed on the surface of the substrate, an optical waveguide formed on the surface of the substrate, and the optical waveguide The waveguide is provided with a grating formed in or on at least a part of the waveguide, a thin film formed on the grating, a semiconductor laser, and a condensing optical system, and the temperature coefficient of refractive index dn2 / dT (° C. ^-1 ) is dn1 /
It is a short-wavelength light generator with dT> dn2 / dT.

[0017]

According to the present invention, by providing the optical element with the voltage applying means and the temperature applying means at the same time, the refractive index of the optical element can be modulated at a high speed over a wide range. This makes it possible to modulate the refractive index over a wide range by applying temperature,
Moreover, high-speed modulation is possible by compensating for the low-modulation temperature application by the electric field application. further,
The coarse wavelength adjustment is performed by modulation by the temperature applying means, and the fine adjustment is performed by the electric field applying means, whereby precise refractive index control can be performed over a wide range.

Further, by forming the electric field applying means and the temperature applying means in a laminated structure with the electrical insulating means sandwiched therebetween and integrating them, voltage and temperature can be efficiently applied to the optical waveguide of the optical wavelength conversion element. Thus, an optical wavelength conversion element capable of modulating the phase matching wavelength with low power consumption can be configured.

Further, when a current is passed through the electrodes, it also serves as a thin film heater, so that it is possible to simultaneously apply voltage and temperature by the electrodes.

By combining the above-mentioned light wavelength conversion element and a semiconductor laser, a compact short wavelength light generating device can be constructed. The light emitted from the semiconductor laser is coupled to the optical waveguide of the optical wavelength conversion element by the condensing optical system, and the temperature application means and the electric field application means are arranged so that the phase matching wavelength of the optical wavelength conversion element matches the oscillation wavelength of the semiconductor laser. When adjusted by, the phase matching wavelength can be modulated at high speed over a wide temperature range, and the light of the semiconductor laser can be wavelength-converted with high efficiency.

Further, even if the phase matching wavelength of the light wavelength conversion element or the oscillation wavelength of the semiconductor laser changes due to environmental changes such as temperature, by modulating the phase matching wavelength by the above means, a constant SHG light is generated. As a result, it is possible to form a short wavelength light generating device having stable output characteristics.

Further, the oscillation wavelength of the semiconductor laser can be fixed and controlled by utilizing optical feedback. This is because the oscillation wavelength of the semiconductor laser is fixed to the selected wavelength when a part of the emitted light of the semiconductor laser is wavelength-selected by a wavelength selection means such as a grating and then returned to the active layer of the semiconductor laser. Is. When this is applied to the short-wavelength light generator, the oscillation wavelength of the semiconductor laser is fixed, and it is possible to prevent the oscillation wavelength of the semiconductor laser from varying due to environmental changes such as temperature. by this,
A short wavelength light generating device having more stable output characteristics can be configured.

[0023]

【Example】

 (Example 1) An example of the present invention will be described below.

FIG. 1 is a perspective view showing the structure of an optical wavelength conversion device according to an embodiment of the present invention. 1 is a -C plate LiTaO ₃ substrate (-side of the plane perpendicular to the C axis of the crystal), 4 is a polarization inversion layer, 5 is a proton exchange optical waveguide, 6 is a fundamental light with a wavelength of 860 nm, 7 is a first with a wavelength of 430 nm. Two harmonics (hereinafter abbreviated as SHG light), 8 is the period Λ of the polarization inversion layer, 9 is the width W of the polarization inversion part, 10 is an electrode, 11 is a thin film heater, and 12 is a SiO ₂ buffer layer. The period Λ of the domain inversion layer 4 differs depending on the wavelength λ of the fundamental wave 6 and the refractive index and shape of the optical waveguide 5. The optical waveguide width is 4 μm, the depth is 2 μm, and the wavelength λ of the fundamental wave 6 is 860.
When nm, the first-order period Λ is about 3.8 μm.

Next, a method of manufacturing the optical wavelength conversion device of this embodiment will be described. A Ta film of 30 nm is deposited on the surface of the LiTaO ₃ substrate of the −C plate by the sputtering method. The periodic pattern for domain inversion is transferred to the Ta film by photolithography and dry etching. The substrate is heat-treated in pyrophosphoric acid at 260 ° C. for 20 minutes to perform proton exchange directly under the unmasked portion. afterwards
Heat treatment is performed at 540 ° C. for 30 seconds to form a periodic domain inversion layer. After removing Ta mask with HF, 420 in oxygen atmosphere
Anneal at 6 ° C. for 6 hours. Next, a slit for an optical waveguide is formed. After depositing Ta to 30 nm on the substrate, an optical waveguide pattern is formed by photolithography and dry etching. Proton exchange is performed by heat treatment in pyrophosphoric acid at 260 ° C for 14 minutes. Then, an optical waveguide is formed by annealing at 420 ° C. for 1 minute. Next, a method of forming the electrodes and the heater will be described with reference to FIG.

(A) A SiO ₂ film is deposited to a thickness of 400 nm on the surface of the substrate on which the domain inversion layer and the optical waveguide are formed. SiO ₂ serves as a buffer layer to prevent the electrode or the thin film heater from increasing the propagation loss of the optical waveguide. When the thickness of SiO ₂ was 200 nm, the propagation loss of the optical waveguide increased, and it was revealed that the thickness of SiO ₂ needs to be 400 nm or more to function as a buffer layer. (B) A resist is deposited to a thickness of 1 μm, and an electrode and a heater are patterned by a photolithography method. (C) Ti is evaporated to a thickness of 200 nm. (D) Ti deposited on the resist by lift-off after cleaning in acetone
Are removed and a pattern of electrodes and heaters is formed. Both ends of the optical waveguide were optically polished to form a device. The produced light wavelength conversion element had a width of 10 mm, a length of 10 mm and a thickness of 0.5 mm. The electrodes consisted of stripes having a width of 4 μm, and the electrode interval was 3 μm.

Next, the results of the characteristic evaluation of the optical wavelength conversion device of this embodiment will be described. Here, the principle of wavelength conversion of the optical wavelength conversion element will be briefly described in the present embodiment. The wavelength conversion is a method of converting a fundamental wave into a harmonic by utilizing a nonlinear optical effect, and here, a fundamental wave is converted into a second harmonic (half the wavelength of the fundamental wave) by utilizing a second-order nonlinear optical effect. . In order to perform wavelength conversion with high efficiency, it is necessary to establish a phase matching condition for matching the phases of the light of the fundamental wave and the light of the second harmonic with respect to the propagation direction. Therefore, in the present embodiment, a quasi-phase matching method that compensates for the phase difference between the fundamental wave and the second harmonic by using a non-linear grating composed of a periodically poled layer is adopted.

This system is characterized in that the phase matching condition can be satisfied for a wide range of wavelengths by changing the grating period, and highly efficient wavelength conversion can be performed.
On the other hand, since a grating is used, there is a problem that the wavelength tolerance of phase matching is as small as about 0.1 nm. Furthermore, there is a feature that the phase matching wavelength changes due to the change of the refractive index of the nonlinear grating. Therefore, we investigated the control of the phase matching wavelength by changing the refractive index of the nonlinear grating.

First, the modulation characteristic of the phase matching wavelength by heating the heater was evaluated. Light from the Ti: Al ₂ O ₃ laser was made incident on the optical waveguide, and the phase matching wavelength at which the SHG output was maximized was measured. The heater has a width of 100 μm, a thickness of 200 nm and a resistance of 500 Ω.
And the distance between the optical waveguide and the heater was 20 μm. FIG. 3 shows the relationship between the applied power and the phase matching wavelength obtained by applying the power to only one heater.

When the electric power of 3 W was applied to the heater, the phase matching wavelength changed over 4 nm. When calculated from the temperature coefficient of the optical waveguide, the temperature of the optical waveguide has risen to about 150 ° C.
It was found that the phase matching wavelength can be controlled over a wide range by heating the heater. Furthermore, the response speed of temperature application was measured. The voltage (40V) applied to the heater was periodically changed, and the modulation characteristic of the phase matching wavelength was measured. As a result, it was found to have a response speed of the order of several Hz.
Reduce the heat capacity of the board to 1 / by reducing the area and thickness of the board.
When set to 10, a response speed on the order of several tens Hz was obtained.

Next, the modulation characteristic of the phase matching wavelength by the electrode was evaluated. LiTaO ₃ is a material having an electro-optical effect. The electro-optic effect is an effect of changing the refractive index of the substrate by applying a voltage, and the direction and the magnitude of the changing refractive index are different from the crystallographic direction depending on the direction and the magnitude of the applied electric field.
It is uniquely determined. In the case of LiTaO _3, an electric field is applied in the z direction of the crystal, and the electro-optic constant of r ₃₃ at which the refractive index in the z direction changes is as large as 30 pm / V. Therefore, a parallel electrode is shown in FIG.
And the z-axis direction electric field was applied to the optical waveguide. A DC voltage is applied to the parallel electrodes, and the relationship between the applied voltage and the phase matching wavelength is obtained. The result is shown in FIG. When a voltage of ± 50V is applied, the phase matching wavelength is 0.6n
I was able to modulate m. When measuring the response speed of modulation,
It had a response speed of 00 MHz or more.

As described above, the modulation by the heater heating has a wide modulation range but a slow response speed. On the other hand, the voltage modulation can achieve a high response speed, but has a problem that the modulation range is narrow. Therefore, the voltage was applied to the heater and the electrodes at the same time, and the modulation characteristics were measured. FIG. 5 shows the time dependence of the phase matching wavelength change.

(A) shows the case where the electric power of 2 W is applied to the heater, and (b) shows the case where the electric power of 2 W is applied to the heater and the voltage is applied to the electrodes at the same time. In the case of (a), the wavelength changes by 2 nm, but it takes about 2 until the phase matching wavelength reaches the set wavelength.
It took a second. This is because it takes time for the element temperature to reach a constant temperature. Therefore, in order to compensate the change in the refractive index until the temperature reaches a steady state by applying an electric field,
As shown in (b), voltage was applied to the electrodes simultaneously with temperature control. The control of the refractive index by the electric field has a narrow control range but enables high-speed control. Therefore, as shown in FIG. 5B, the refractive index of the element can reach the set value in 0.5 seconds or less. The combined use of temperature control and electric field control enabled high-speed refractive index control over a wide range.

In this embodiment, the electrode structure is parallel 3
Although this electrode was used, an electric field in the Z direction can be applied to the optical waveguide even with two parallel electrodes.

Although parallel electrodes are used in this embodiment,
In addition, if a comb-shaped electrode is used, the phase matching wavelength can be modulated more efficiently. The reason will be described below. When an electric field is applied by the parallel electrodes, the electric field in the same direction (Z-axis direction) is uniformly applied to the optical waveguide. However, in the case where the optical wavelength conversion element is a quasi phase matching type composed of a periodic polarization inversion layer as shown in FIG. 1, the optical waveguide has a structure in which the polarization is periodically inverted, and in the portion where the polarization is inverted. The sign of the electro-optical constant r ₃₃ is also reversed. Therefore, when a voltage is applied to the parallel electrodes, the increase / decrease in the refractive index change is reversed between the polarization inversion part and the non-inversion part of the optical waveguide. Therefore, the refractive index that changes with the application of a voltage is canceled and becomes small when averaged over the entire optical waveguide.

On the other hand, when a comb-shaped electrode pair having the same period as the domain inversion period Λ shown in FIG. 6A is formed, FIG.
As shown in, since the direction of the electric field applied to the domain-inverted layer in the optical waveguide is reversed between the domain-inverted portion and the non-inverted portion, the change in refractive index shows the same increase and decrease in both the inverted and non-inverted portions. Therefore, it was possible to efficiently change the refractive index by applying a voltage. For example, when a voltage of ± 50 V is applied to the comb-shaped electrodes, the phase matching wavelength can be modulated over 1 nm, and the phase matching wavelength modulation about twice that of the parallel electrodes is possible. The period of the comb teeth of the comb-shaped electrode is (2m−1) · Λ (m = 1, 2, 3
..), it is possible to efficiently change the refractive index.

Although a Ti thin film is used for the heater in this embodiment, any other refractory metal such as Cr, Ta or W may be used. Also, any method such as resistance heating, infrared heating, and Peltier element can be used as long as the temperature can be applied to the substrate.

In this embodiment, Ti is used as the electrode, but in addition, a transparent electrode or Al, Au, Ag, Cr, Ni, Cu, Ta,
Any metal such as Fe can be similarly used. In particular, when a transparent electrode is used, the propagation loss of the optical waveguide caused by the direct contact of the electrode with the optical waveguide is small, so that the buffer layer 12 can be omitted, which is effective.

Although SiO ₂ is used for the buffer layer in this embodiment, other dielectrics such as Ta ₂ O ₅ and SiN, and semiconductors such as Si can be used. Particularly, the semiconductor is effective because it can prevent the generation of an electric field due to the pyroelectric effect generated by the change in the substrate temperature and can improve the temperature characteristics.

In this embodiment, a LiTaO ₃ substrate was used as the substrate, but other LiTaO ₃ doped with MgO, Nb, Nd, etc., or LiNbO ₃ or a mixture thereof LiTa _(1-x) Nb _x O _{3 was used.} (0 ≦
A similar device can be manufactured by using x ≦ 1) substrate and KTP (KTiOPO ₄ ). Since LiTaO ₃ , LiNbO ₃ , and KTP all have high nonlinearity, a highly efficient optical wavelength conversion element can be manufactured. Moreover, since the method of forming the periodically poled layer is confirmed for these materials, a highly efficient optical wavelength conversion element can be formed.

(Embodiment 2) Another embodiment of the optical wavelength conversion device of the present invention will be described below.

FIG. 7 is a perspective view showing the construction of an optical wavelength conversion device according to another embodiment of the present invention. 1 is a -C plate LiTaO ₃ substrate (-side of the plane perpendicular to the C axis of the crystal), 4 is a polarization inversion layer, 5 is a proton exchange waveguide, 6 is a fundamental light with a wavelength of 860 nm, and 7 is a first with a wavelength of 430 nm. 2 harmonics (hereinafter abbreviated as SHG light), 8 is the period Λ of the polarization inversion layer, 9 is the width W of the polarization inversion part, 10 is an electrode, 11 is a thin film heater, 12 is a SiO ₂ buffer layer, and 13 is SiO. ₂ buffer layers. Optical waveguide width is 4μ
m, the depth is 2 μm, and the period Λ of the domain inversion layer is about 3.8 μm.
Is. The method of manufacturing the light wavelength conversion element is almost the same as that of the first embodiment. However, after forming the electrode, the SiO ₂ buffer layer is
m was deposited and a heater was formed on it.

Next, the operation principle of the optical wavelength conversion device of FIG. 7 will be described. A thin film heater was formed on the electrodes with an insulating film sandwiched therebetween. By forming the insulating film, the electrode and the thin film heater are electrically separated from each other and integration is possible. Also,
By integrating the heater and the electrode in the laminated structure, the thermal conductivity of the temperature by the heater is improved, so that the response speed of the refractive index modulation by the temperature application is about twice that of the optical wavelength conversion element of the first embodiment. I got faster. As in Example 1, when a voltage was applied to the heater and the electrodes to modulate the refractive index of the nonlinear optical element, it was possible to achieve a desired refractive index in 0.3 seconds. By integrating the heater and the electrodes in layers, a faster phase-matched wavelength modulation became possible. Furthermore, by arranging the heater and the electrode in a laminated structure, the area occupied by the element is reduced, and it is possible to form a wavelength conversion element of about 3/2 times in the substrate having the same area.

(Embodiment 3) FIG. 8 is a perspective view showing the configuration of an optical wavelength conversion device according to another embodiment of the present invention. 1 is -C plate Li
TaO ₃ substrate (-side of the plane perpendicular to the C axis of the crystal), 4 is a polarization inversion layer, 5 is a proton exchange optical waveguide, and 6 is a wavelength of 860 nm.
Fundamental light, 7 is the second harmonic of wavelength 430 nm (hereinafter, S
Abbreviated as HG light. ), 8 is the period Λ of the domain inversion layer, 9 is the width W of the domain inversion portion, 14 is the Ti thin film that is the first conductor, and 15
Is a Ti thin film which is the second conductor, and 12 is a SiO ₂ buffer layer. The optical waveguide width is 4 μm, the depth is 2 μm, and the period of the domain inversion layer is about 3.8 μm.

Next, the operation principle of the optical wavelength conversion device of FIG. 8 will be described. When a voltage of V1 is applied to both ends of the Ti thin film 14 which is the first conductor and a voltage of V2 is applied to both ends of the second conductor 15, a current is caused to flow, whereby a thin film heater can be operated and temperature can be applied to the optical waveguide. At the same time, if a potential difference V is provided between the Ti thin films 14 and 15, an electric field can be applied to the optical waveguide. An electric field and temperature can be applied simultaneously by the pair of electrodes.
The modulation characteristics of the phase matching wavelength of the optical wavelength conversion element were almost the same as those of the type in which the electrode and the heater were integrated in a laminated structure. Further, this light wavelength conversion element has a simple structure, and since the step of forming a buffer layer and the step of forming a thin film heater, which are necessary for manufacturing the element of the second embodiment, can be omitted, it is easy to manufacture.

As the electrode structure, the same structure can be obtained even with the three stripe structure or the comb-shaped electrode structure shown in the first embodiment. In particular, the comb-shaped electrode is effective because the change in the refractive index due to electric field control is large.

Next, the result of studying the expansion of the phase matching tolerance of the optical wavelength conversion element using the structure of the optical wavelength conversion element of FIG. 8 will be described.

In the structure of the optical wavelength conversion device shown in FIG. 8, a current was passed through the first conductor 14 and the second conductor 15 was set to a constant voltage (ground, voltage 0 in this case). In the experiment,
When a voltage of 40 V was applied in the stripe direction of the first Ti thin film, the Ti thin film generated heat due to resistance heating and acted as a heater. Further, the electric field between the fourteenth conductor and the second conductor 15 is 0 to 40 V / V over the propagation direction of the optical waveguide.
Increase to 3 μm. Therefore, the refractive index of the optical waveguide is
Due to the electro-optic effect, as shown in FIG. 9, it gradually increases in the traveling direction of the optical waveguide. As a result, the phase matching wavelength of the optical wavelength conversion element changes linearly in the traveling direction, and the phase matching wavelength tolerance of the entire element is increased from 0.1 nm to 1 nm by 10 times as shown in FIG. I was able to expand. With this optical wavelength conversion element,
A stable SHG output was obtained with respect to the wavelength fluctuation of the fundamental wave.

(Embodiment 4) By forming the temperature applying means and the electric field applying means in the optical element, the element characteristics can be stabilized. Here, an optical switch will be described as an optical element having the same configuration as that of the second embodiment (FIG. 8). By forming the electrode structure shown in FIG. 8 on the optical waveguide, an optical switch having stable characteristics can be constructed.

The operating principle of the optical switch will be described with reference to FIG. The light emitted from the laser 23 is reflected by the half mirror 24.
, And one of the light beams is incident on the optical waveguide of the optical switch 25 by the condensing optical system 26. The light passing through the optical waveguide is collimated, combined with the branched light, and detected by the detector 27. When a voltage is applied to the electrodes formed on the optical waveguide, the refractive index of the optical waveguide changes, the optical path difference changes, and the combined light interferes to operate as an optical switch. Light output 10
It was possible to modulate at a frequency of 0 MHz. However, element 2
When the temperature of No. 5 changes, the refractive index of the optical waveguide changes due to the temperature change, and the optical path length changes, so that the characteristics of the switch become unstable. Therefore, apply a current to the heater electrode,
By keeping the temperature of the optical element constant, the stability of the output could be achieved and the output fluctuation could be suppressed to 2% or less. Since the temperature control and the electric field control can be performed simultaneously by the electrodes, a switch with stable operation can be constructed.

(Embodiment 5) The short wavelength light generating apparatus of this embodiment will be described below.

FIG. 12 is a perspective view showing the structure of the short wavelength light generating device of the present invention. In FIG. 12, 1 is -C plate LiTaO ₃
Substrate (-side of the plane perpendicular to the C axis of the crystal), 4 is a polarization inversion layer, 5 is a proton exchange optical waveguide, 6 is a fundamental light with a wavelength of 860 nm, 7 is a second harmonic of 430 nm, and 8 is a polarization inversion. The layer period Λ, 9 is the width W of the polarization inversion portion, 10 is the electrode, 1
Reference numeral 1 is a thin film heater, 12 is a SiO ₂ buffer layer, 20 is a semiconductor laser, and 21 is a focusing optical system. Optical waveguide width is 4μ
m, the depth is 2 μm, the wavelength λ of the fundamental wave 6 is 860 nm, 1
The next period Λ was 3.8 μm.

The basic light 6 emitted from the semiconductor laser 20 is condensed by the condensing optical system 21 and enters the optical waveguide 5. The incident basic light 6 propagates in the optical waveguide 5, but as it propagates, the periodic polarization inversion layer 4 gradually converts it into SHG light having a half wavelength of the basic light. The oscillation wavelength of a semiconductor laser generally fluctuates due to changes in temperature, drive current and the like. For example, in a 0.8 μm Fabry-Perot type semiconductor laser, it changes at a rate of 0.1 to 0.2 nm / ° C.
For this reason, in an ordinary optical wavelength conversion element, the phase matching wavelength tolerance is only about 0.1 nm, and therefore the SHG output cannot be obtained unless the temperature control is controlled to 1 ° C. or less.

However, when the optical wavelength conversion element of Example 1 is used, the phase matching wavelength can be modulated over a maximum of 4.6 nm. Therefore, the phase matching wavelength is adjusted even when the semiconductor laser temperature changes by ± 10 ° C. By doing so, the SHG output could be kept stable. Furthermore, by detecting the SHG output,
A feedback circuit was added to adjust the voltage applied to the heater and the electrodes so that the SHG output was maximized. Since the wavelength range of 2 nm can be controlled by the heater and the electrode at the modulation speed of several tens of Hz, the feedback circuit enables the modulation of the response speed, which is relatively fast. As a result, SHG
It has become possible to suppress the output fluctuation to 10% or less with respect to the temperature change of the semiconductor laser of ± 10 ° C. However, when the semiconductor laser caused a mode hop, the oscillation wavelength of the semiconductor laser fluctuated greatly (0.1 to 0.2 nm), so that the output temporarily decreased by nearly 50%. Other than that, the fluctuation could be suppressed to 10% or less.

(Embodiment 6) A short wavelength light generating device using grating feedback in order to suppress mode hopping and wavelength fluctuation of a semiconductor laser will be described.

FIG. 13 is a perspective view showing the structure of the short wavelength light generating device of the present invention. In FIG. 13, 1 is LiTaO ₃ of -C plate
Substrate (-side of plane perpendicular to C axis of crystal), 4 polarization inversion layer, 5 proton exchange optical waveguide, 22 grating, 6 basic light of wavelength 860 nm, 7 wavelength 430 nm
Second harmonic (hereinafter abbreviated as SHG light), 8 is the period Λ of the domain inversion layer, 9 is the width W of the domain inversion part, 10 is the electrode, 1
Reference numeral 1 is a thin film heater, 12 is a SiO ₂ buffer layer, 20 is a semiconductor laser, and 21 is a focusing optical system. The grating 22 formed near the emitting portion functions as a DBR grating, the basic light 6 of a specific wavelength propagating through the optical waveguide 5 is reflected, and returns to the semiconductor laser 20 through the condensing optical system 21. Since the oscillation wavelength of the semiconductor laser 20 is fixed to this feedback wavelength, the oscillation wavelength variation of the semiconductor laser can be suppressed and stabilized, and the wavelength variation due to mode hopping is also eliminated.

In the present short wavelength light generator, wavelength fluctuations due to the temperature of the semiconductor laser and the driving current can be suppressed. However, it is necessary to match the phase matching wavelengths of the stabilized semiconductor laser and the optical wavelength conversion element.
When the environmental temperature changes, the phase matching wavelength changes due to the temperature characteristics of the optical wavelength conversion element. Therefore, in order to stabilize the SHG output, it is necessary to control the phase matching wavelength of the optical wavelength conversion element. Therefore, the phase matching wavelength was controlled by the heater and the electrode integrated in the optical wavelength conversion element. Example 1
As mentioned above, since the wavelength range of 2 nm can be controlled in about 0.5 seconds, the feedback circuit is relatively fast,
The response speed can be modulated. The output of the semiconductor laser was 100 mW, and the light coupled to the optical waveguide was 70 mW. The conversion efficiency at this time is 14%, and S of 10 mW
HG output was obtained. Further, it has become possible to suppress the fluctuation of the SHG output to 5% or less with respect to the fluctuation of the environmental temperature of ± 20 ° C. Since the wavelength fluctuation and the mode hop of the semiconductor laser can be reduced, it is possible to manufacture an optical wavelength conversion element with stable output.

As described above, the short-wavelength light generating apparatus of this embodiment provided stable and high-output blue coherent light (SHG light) with respect to changes in ambient temperature. as a result,
It can be applied to light sources such as optical disks and laser printers.
With this short wavelength light source, the storage capacity of the optical disk can be greatly increased, and a very small device can be manufactured.

In this embodiment, the light wavelength conversion element shown in the first embodiment is used as the light wavelength conversion element constituting the short wavelength light generating device, but the light shown in the second or third embodiment is used. A wavelength conversion element can be used as well. The light wavelength conversion element shown in the second embodiment is effective because it can modulate the light efficiency more and can configure a short wavelength light generating device with low power consumption.

In this embodiment, the DBR grating formed on the optical waveguide is used for wavelength stabilization of the semiconductor laser, but there is another method using the external grating described in the conventional embodiment. The use of the external grating is effective in that the second harmonic generated is not lost in the grating formed on the optical waveguide, so that a highly efficient short wavelength light generating device can be constructed.

In this embodiment, the DBR grating formed on the optical waveguide is used for stabilizing the wavelength of the semiconductor laser, but a narrow band filter can be used.
By inserting a narrow band filter into the optical system that focuses the semiconductor laser light into the optical waveguide, the specific wavelength is excited into the optical waveguide, and the reflected light from the optical waveguide is returned to the semiconductor laser. Stabilization can be achieved. The use of a narrow band filter is effective because a mechanically stable short wavelength light generating device can be constructed.

(Embodiment 7) An optical wavelength conversion element having stable output characteristics with respect to external temperature changes will be described here.

In the short wavelength light generator of FIG. 13, the temperature characteristic of the output was measured while changing the temperature of the short wavelength light generator. As a result, the output had a maximum value at 25 ° C, and the full width at half maximum of the temperature was about 10 ° C. The cause was analyzed as follows as a result of measuring the characteristics of the light wavelength conversion element. Temperature coefficient of phase matching wavelength in optical wavelength conversion device (dn / d
T) is 0.038 nm / ℃, and dn / dT of the grating reflection wavelength is
It was 0.028 nm / ° C. Therefore, the difference between the phase matching wavelength of the optical wavelength conversion element and the oscillation wavelength of the semiconductor laser (which can be the reflection wavelength of the grating) has a temperature coefficient of 0.01 nm / ° C. Since the full width at half maximum of the phase matching wavelength tolerance of the optical wavelength conversion element is about 0.1 nm, the temperature coefficient of the wavelength generating device shorter than 0.1 nm / (0.01 nm / ° C.) has a temperature tolerance of about 10 ° C.

As described above, the temperature coefficient of the short wavelength light generating device is determined by the dn / d of the phase matching wavelength and the reflection wavelength of the grating.
Determined by the difference in T. Therefore, if it is possible to make the phase matching wavelength and the dn / dT of the reflection wavelength of the grating equal, it is possible to configure a short wavelength light generating device having stable characteristics with respect to external temperature changes.

Therefore, by depositing a film having a temperature coefficient larger than that of the substrate on the grating, the dn / dT of the reflection wavelength of the grating is increased, and the difference of dn / dT between the phase matching wavelength and the reflection wavelength of the grating is reduced. Therefore, the temperature characteristic of the output was improved. The configuration is the same as that of the short wavelength light generation device shown in FIG. 13, but PLZT ((Pb _(1-x) La _x ) (Zr _y Ti _(1-y) ) is formed on the grating 22.
_{A (1-x / 4)} O ₃ ) film was deposited to a thickness of 200 nm. PLZT has a refractive index of about 2.
At 5, the temperature change dn / dT of the refractive index is 10 × 10 ^-5 . LiTaO ₃
Substrate dn / dT: By depositing PLZT, which has a large refractive index change compared to 6.8 × 10 ⁻⁵ / ° C., on the grating,
The temperature coefficient of the reflection wavelength of the grating increased, and as a result, the temperature coefficient of the reflection wavelength dn / dT increased to 0.036 nm / ° C. As a result, the difference in dn / dT between the phase matching wavelength and the grating reflection wavelength can be reduced to 0.002 nm / ° C, and the temperature tolerance of the output is increased by 5 times to 50 ° C.
An element having stable output characteristics with respect to external temperature changes could be constructed.

In this embodiment, the grating is
Although PLZT is deposited, any film may be used as long as it has a dn / dT larger than the temperature change dn / dT of the substrate and does not give a loss to the light guided through the optical waveguide. The temperature coefficient of the optical waveguide can be controlled to some extent by changing the film thickness.

In this example, PLZT was deposited on the grating in order to match the dn / dT of the phase matching wavelength and the grating reflection wavelength. However, dn / dT smaller than dn / dT of the substrate was formed on the portion other than the grating. When a film having is deposited, the phase matching wavelength dn / dT is reduced, and the same result is obtained. For example, Ta ₂ O ₅ , SiO ₂ (dn / dT: about 0.5 × 10 ^-5 /
An oxide film such as (° C.) can be used. In addition, TiO ₂ (dn / d
T: -4 to -7 x 10 ^-5 / ° C), PbMoO ₄ (-4 to -7 x 10 ^-5 / ° C)
Is effective because it has the opposite sign to dn / dt of LiTaO ₃ substrate.

[0068]

As described above, by integrating the electrode and the heater in the light wavelength conversion element, the phase matching wavelength of the light wavelength conversion element can be modulated at a high speed over a wide range. The practical effect is great.

By stacking the electrode and the heater on the light wavelength conversion element, it is possible to reduce the power consumption for modulating the phase matching wavelength of the light wavelength conversion element. In addition, the device can be downsized, and its practical effect is great.

Further, by simultaneously operating the electrodes as an electric field and as a heater, it is possible to reduce the power consumption of the phase matching wavelength modulation of the optical wavelength conversion element, simplify the manufacturing process, and reduce the size of the element. As it is realized, its practical effect is great.

Further, by constructing a small short-wavelength coherent light source with a light wavelength conversion element capable of modulating the phase matching wavelength at high speed and a semiconductor laser, stable blue coherent light can be obtained, which is suitable for optical disks and laser printers. Can be applied, and its practical effect is great.

[Brief description of drawings]

FIG. 1 is a perspective view of the configuration of an optical wavelength conversion element of the present invention.

FIG. 2 is a manufacturing process diagram of an optical wavelength conversion element of the present invention

FIG. 3 is a characteristic diagram showing the relationship between the power applied to the heater of the optical wavelength conversion element and the phase matching wavelength.

FIG. 4 is a characteristic diagram showing the relationship between the voltage applied to the electrode of the optical wavelength conversion element and the phase matching wavelength.

FIG. 5 is a diagram showing the time dependence of phase matching wavelength change, (a) is a characteristic diagram when electric power is applied only to the heater, and (b) is a characteristic diagram when voltage is simultaneously applied to the heater and the electrode.

FIG. 6 is a diagram showing an electric field distribution in the light wavelength conversion element, (a) is a cross-sectional view of the light wavelength conversion element, and (b) is a characteristic diagram showing an electric field applied to an optical waveguide and a change in refractive index.

FIG. 7 is a configuration perspective view of another optical wavelength conversion element of the present invention.

FIG. 8 is a perspective view showing the configuration of another optical wavelength conversion element of the present invention.

FIG. 9 is a characteristic diagram showing a change in the refractive index of the optical waveguide of the optical wavelength conversion element.

FIG. 10 is a diagram showing phase matching characteristics.

FIG. 11 is a diagram showing a measurement optical system of the optical element of the present invention.

FIG. 12 is a perspective view showing the configuration of a short wavelength light generating device according to the present invention.

FIG. 13 is a perspective view showing the configuration of another short wavelength light generating device according to the present invention.

FIG. 14 is a perspective view showing a configuration of a conventional light wavelength conversion element.

FIG. 15 is a perspective view showing the configuration of a conventional short wavelength light generating device.

[Explanation of symbols]

1-C plate LiTaO ₃ substrate 4 polarization inversion layer 5 proton exchange optical waveguide 6 fundamental light 7 second harmonic 8 period Λ 9 width W 10 electrode 11 heater 12 buffer layer 13 buffer layer 14 electrode A 15 electrode B 20 semiconductor laser 21 Condensing Optical System 22 Grating 23 Laser 24 Half Mirror 25 Optical Switch 25 26 Condensing Optical System 27 Detector 101 Semiconductor Laser 102 Collimating Lens 103 λ / 2 Plate 104 Focusing Lens 105 Incident Point 106 Collimating Lens 107 Dichroic Mirror 108 Grating 110 LiTaO ₃ substrate 111 polarization inversion part 112 proton exchange optical waveguide 113 electrode 114 semiconductor laser 115 incident part 116 emission part

Claims (15)

[Claims] 1. An optical element comprising a ferroelectric substrate, an electric field applying unit formed on the substrate, and a temperature applying unit formed on the substrate. 2. A nonlinear optical substrate, a periodic domain-inverted layer formed on the surface of the substrate, an optical waveguide formed on the surface of the substrate, an electric field applying means formed on the optical waveguide, An optical wavelength conversion element, comprising: a temperature applying unit formed on at least one side of the optical waveguide, wherein the refractive index of the optical waveguide is controlled by the electric field applying unit and the temperature applying unit. 3. A nonlinear optical substrate, a periodic domain-inverted layer formed on the surface of the substrate, an optical waveguide formed on the surface of the substrate, an electric field applying means formed on the optical waveguide, An electric insulating unit formed on the electric field applying unit and a temperature applying unit formed on the insulating unit are provided, and the refractive index of the optical waveguide is controlled by the electric field applying unit and the temperature applying unit. An optical wavelength conversion element characterized by the above. 4. The electric field applying means comprises a first conductor formed on the optical waveguide and a second conductor formed on at least one side of the first conductor film. Optical wavelength conversion element. 5. The optical wavelength conversion element according to claim 2, wherein the electric field applying means is a comb-shaped electrode. 6. A nonlinear optical substrate, a periodic domain-inverted layer formed on the surface of the substrate, an optical waveguide formed on the surface of the substrate, and a stripe-shaped first optical waveguide formed on the optical waveguide. A conductor and a stripe-shaped second conductor formed on both sides or one side of the first conductor,
And the strap directions of the second conductor are substantially parallel to each other,
An optical wavelength conversion element, wherein there is a potential difference between the first conductor and the second conductor, and a current flows through at least one of the first and second conductors. 7. The optical wavelength conversion element according to claim 6, wherein at least one of the first and second conductors has a comb shape. 8. A grating is formed on the surface or inside of at least a part of the optical waveguide.
3. The light wavelength conversion element according to any one of 3 and 6. 9. A semiconductor laser, an optical wavelength conversion element according to any one of claims 2, 3 and 6, and a condensing optical system, wherein the phase matching wavelength of the optical wavelength conversion element is that of the semiconductor laser. A short-wavelength light generating device characterized in that the electric field applying means and the temperature applying means of the optical wavelength conversion element are adjusted so as to match the oscillation wavelength. 10. A wavelength selection means, a semiconductor laser, an optical wavelength conversion element according to any one of claims 2, 3 and 6, and a condensing optical system, wherein the wavelength selection means outputs the semiconductor laser. After at least a part of the emitted light is wavelength-selected, it is fed back to the semiconductor laser again, and the phase matching wavelength of the optical wavelength conversion element matches the oscillation wavelength of the semiconductor laser, A short-wavelength light generating device characterized in that the electric field applying means and the temperature applying means are adjusted. 11. A nonlinear optical substrate having a temperature coefficient of refractive index dn1 / dT (° C. ^-1 ), a periodic domain-inverted layer formed on the surface of the substrate, and an optical waveguide formed on the surface of the substrate. The optical waveguide includes a grating formed in or on at least a part of the optical waveguide, a thin film formed on the grating, a semiconductor laser, and a condensing optical system, and the temperature coefficient of refractive index dn2 / dT of the thin film. (℃ ^-1 )
Is a dn1 / dT> dn2 / dT. 12. The short wavelength light generation device according to claim 10, wherein a grating formed on at least a part of the surface or inside of the optical waveguide of the optical wavelength conversion element is used as the wavelength selection means. 13. The short wavelength light generating device according to claim 10, wherein a flat plate grating is used as the wavelength selecting means. 14. The short wavelength light generating device according to claim 10, wherein a narrow band filter is used as the wavelength selecting means. 15. A short-wavelength light generation device in which the thin film has a temperature coefficient dn2 / dT at which a reflection wavelength temperature coefficient of the grating and a phase matching wavelength temperature coefficient of the light wavelength conversion element are equal.