JP3264081B2 - Optical wavelength conversion element and short wavelength light generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Light (fundamental wave) is generated by a second harmonic generation element (hereinafter referred to as an SHG element) utilizing a nonlinear optical effect.
Can be converted to a second harmonic having a half wavelength. By converting semiconductor laser light in this way, a small-sized short-wavelength light source can be realized and applied to printing, optical information processing, optical applied measurement control fields, and the like. To realize highly efficient wavelength conversion in the SHG element, the fundamental wave and the second
It is essential to satisfy the phase matching condition 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. However, since the tolerance is generally narrow, precise control of the wavelength of the fundamental wave 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 domain-inverted optical waveguide (Yamamoto et al., Applied Physics Letters). Le
tters, Vol.62, No.21, 2599 (1993)). An optical waveguide having a periodically domain-inverted layer is formed on a LiTaO ₃ substrate, and QPM-
This constitutes an SHG element.

[0004] The SHG element has successfully generated 31 mW of blue light with a conversion efficiency of 21%, but the tolerance of the fundamental wave is 0.1 mm.
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 a semiconductor laser
Other changes with mode hopping by applying current or the like (e.g., temperature change, 0.1 at 0.1 to 0.2 nm / ° C., the applied current
The wavelength changes with a 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.

[0005] Therefore, for example, FIG. 11 shows a short wavelength light generating device by tuning the wavelength of a semiconductor laser using grating feedback as shown in FIG. FIG. 14 shows a schematic configuration diagram of a conventional short-wavelength light generator using a semiconductor laser and a QPM-SHG element. The light emitted from the semiconductor laser 101 is converted into a parallel beam by a collimating lens 102 with NA = 0.55, and λ
The deflection direction is rotated by a half plate 103, and a focusing lens 104 of NA = 0.45 is used to input an incident portion 105 of the optical waveguide.
Is collected. The fundamental wave and the second harmonic emitted from the emission end of the optical waveguide are converted into parallel beams by the collimator lens 106, and then separated by the dichroic mirror 107 into the fundamental wave and the second harmonic.

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

As a grating feedback system, in addition to using an external grating shown in FIG. 14, a type in which a grating is integrated in an SHG element has been reported. As an example of this, for example, a dielectric grating layer is formed on an SHG element, a part of a fundamental wave propagating in an optical waveguide is selected by a grating, and is fed back to a semiconductor laser. (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
When the length of the optical waveguide is 2 mm and the power of the fundamental wave P1 is 60 μW with respect to the m fundamental wave P1, the power of the harmonic wave P2 is 0.652 pW. At this time, the conversion efficiency 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 perform wavelength conversion of the semiconductor laser by the SHG element. For example, there is a method of adjusting the phase matching wavelength by applying a voltage to the optical waveguide and controlling the refractive index of the optical waveguide by an electro-optic effect so that the phase matching wavelength matches the oscillation wavelength of the semiconductor laser. 4-20
No. 4815).

FIG. 15 shows a configuration diagram of a conventional optical wavelength conversion element that adjusts the phase matching wavelength by applying an electric field. In FIG. 15, 110 is a LiTaO ₃ substrate of a -Z plate, 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, description regarding the width of the electrode was not made. When a voltage is applied to the electrode 113, a voltage in the Z direction is applied to the optical waveguide. Therefore, the electro-optical effect having the substrate, via the electro-optic constant r ₃₃ to refractive index with respect to the Z direction of the electric field component of the light is changed. Thereby, the phase matching wavelength of the optical wavelength conversion element is modulated.

[0010]

First, problems of a conventional short wavelength light generator will be described.

In the conventional short-wavelength light generating device using the feedback of the grating, the oscillation wavelength of the semiconductor laser can be controlled by the grating. However, since the oscillation wavelength of the semiconductor laser exists discretely at every 0.1 to 0.2 nm, the wavelength allowable. It is difficult to completely match the phase matching wavelength of an optical wavelength conversion element having a small degree (for example, in the case of a 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 low. There was a problem of instability. Further, since the phase matching wavelength fluctuates depending on the temperature of the optical 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 device with an integrated grating, the oscillation wavelength of the semiconductor laser is fixed to the reflection wavelength of the grating, so it is difficult to precisely match the reflection wavelength of the grating with the phase matching wavelength, resulting in higher efficiency. There was a problem that it was difficult.

In view of the above, an object of the present invention is to provide a short-wavelength light generator capable of high-efficiency conversion and having stable output characteristics with respect to environmental temperature changes.

Next, problems concerning the optical wavelength conversion element will be described. A method of adjusting a phase matching wavelength of an optical wavelength conversion element using an electro-optic effect is disclosed. The change in refractive index due to the electro-optic effect has a fast response speed and enables high-speed modulation. However, since the change in refractive index that can be modulated is as small as 10 ^-4 , the range of the phase matching wavelength that can be modulated is as narrow as 1 nm or less. There was a problem that it was limited to the range.

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

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

[0016]

In order to solve the above problems, the present invention provides: (1) a ferroelectric substrate, an optical waveguide formed on the substrate, a periodically poled structure formed on the substrate, A first conductor having a stripe shape formed on a substrate; and a second conductor having a stripe shape formed on both sides or one side of the first conductor, in a stripe direction of the first and second conductors. Are substantially parallel to each other, have a potential difference between the first and second conductors, and a current is flowing in at least one of the first and second conductors, and the first and second conductors are An optical wavelength conversion element in which an electric field applied to the optical waveguide gradually changes in the propagation direction by changing a potential difference between the optical waveguides in the propagation direction of the optical waveguide. (2) a semiconductor laser, the light wavelength conversion element, and a condensing optical system, wherein a phase matching wavelength of the light wavelength conversion element is:
The current applied to the first or second conductor formed in the optical wavelength conversion element and the potential difference between the conductors are adjusted to match the oscillation wavelength of the semiconductor laser. It is a short wavelength light generator.

[0017]

By the action of the present invention has been described above method, since the play when an electric current is applied to the electrode is also the role of the thin-film heater, Ri Do can play a voltage application and temperature simultaneously applied by the electrodes, the optical wavelength
The tolerance of phase matching wavelength of the conversion element can be expanded.
Therefore, a stable SHG output can be obtained.

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

Even if the phase matching wavelength of the optical wavelength conversion element or the oscillation wavelength of the semiconductor laser fluctuates due to a change in environment such as temperature, the above means modulates the phase matching wavelength so that a constant SHG light is always generated. As a result, a short-wavelength light generator having stable output characteristics can be formed.

[0020]

EXAMPLES described in Reference Example 1 (Reference Example 1) following the present invention.

[0021] FIG. 1 is a configuration perspective view of the optical wavelength conversion device of Reference Example 1 of the present invention. 1 is a LiTaO ₃ substrate of a -C plate (− side of a plane perpendicular to the C axis of the crystal), 4 is a domain-inverted layer, 5 is a proton exchange optical waveguide, 6 is fundamental light having a wavelength of 860 nm, and 7 is a fourth light having a wavelength of 430 nm. 2 is a harmonic (hereinafter abbreviated as SHG light), 8 is the period of the domain-inverted layer, 9 is the width W of the domain-inverted portion, 10 is an electrode, 11 is a thin film heater, and 12 is a SiO ₂ buffer layer. The period Λ of the domain-inverted layer 4 differs depending on the wavelength λ of the fundamental wave 6 and the refractive index and the 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.
In the case of nm, the first-order period Λ is about 3.8 μm.

Next, a method for manufacturing the optical wavelength conversion element of the first embodiment will be described. A Ta film is deposited to a thickness of 30 nm on the surface of a LiTaO ₃ substrate of a C plate by a sputtering method. Periodic pattern for polarization reversal using photolithography and dry etching
Transfer to film. The substrate is heat-treated in pyrophosphoric acid at 260 ° C. for 20 minutes to perform proton exchange just below the unmasked portion. Thereafter, heat treatment is performed at 540 ° C. for 30 seconds to form a periodic domain-inverted layer. After removing the Ta mask with HF, 420
Anneal at 6 ° C. for 6 hours. Next, an optical waveguide slit is formed. After depositing 30 nm of Ta on the substrate, an optical waveguide pattern is formed by photolithography and dry etching. Heat exchange in pyrophosphoric acid at 260 ° C for 14 minutes to perform proton exchange. Thereafter, annealing is performed at 420 ° C. for 1 minute to form an optical waveguide. Next, a method of forming an electrode and a heater will be described with reference to FIG.

(A) A 400 nm SiO ₂ film is deposited on the surface of the substrate on which the domain-inverted layer and the optical waveguide have been formed. SiO ₂ serves as a buffer layer so that the electrode or the thin-film heater does not increase 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 became clear that the thickness of SiO ₂ was required to be 400 nm or more to function as a buffer layer. (B) A resist having a thickness of 1 μm is deposited, and the electrodes and the heater are patterned by a photolithography method. (C) Ti is deposited to a thickness of 200 nm. (D) Washed in acetone and deposited on resist by lift-off
The pattern of the electrode and the heater is formed by removing Ti.
Both ends of the optical waveguide were optically polished to form an element. The manufactured light wavelength conversion element had a width of 10 mm, a length of 10 mm, and a thickness of 0.5 mm. The electrodes were composed of stripes having a width of 4 μm, and the interval between the electrodes was 3 μm.

Next, the results of the evaluation of the characteristics of the optical wavelength conversion element of the first embodiment will be described. Here, the principle of wavelength conversion of the optical wavelength conversion element will be briefly described. The wavelength conversion is a method of converting a fundamental wave into a harmonic using a nonlinear optical effect. Here, the wavelength is converted into a second harmonic (half the wavelength of the fundamental wave) using a second-order nonlinear optical effect. . In order to perform wavelength conversion with high efficiency, it is necessary to satisfy a phase matching condition for matching the phases of the fundamental wave and the light of the second harmonic with the propagation direction. Therefore, in the first embodiment , the nonlinear grating composed of the periodically poled layers is used.
A quasi-phase matching method for compensating for a phase difference between the fundamental wave and the second harmonic is employed.

This method has the advantage that the phase matching condition can be satisfied for a wide range of wavelengths by changing the grating period, and wavelength conversion can be performed with high efficiency.
On the other hand, since the grating is used, there is a problem that the wavelength tolerance of the phase matching is as small as about 0.1 nm. Further, there is a feature that the phase matching wavelength changes due to a change in the refractive index of the nonlinear grating. Therefore, control of the phase matching wavelength by changing the refractive index of the nonlinear grating was studied.

First, the modulation characteristics of the phase matching wavelength due to heater heating were evaluated. Light from a Ti: Al ₂ O ₃ laser was incident on the optical waveguide, and the phase matching wavelength at which the SHG output was maximized was measured. The heater is 100μm wide, 200nm thick and the resistance is 500Ω
The distance between the optical waveguide and the heater was 20 μm. Power is applied to only one heater, and the relationship between the applied power and the phase matching wavelength is determined and is shown in FIG.

[0027] mark pressurized Then phase matching wavelength in heater power of 3W has changed over the 4nm. When calculated from the temperature coefficient of the optical waveguide, the temperature of the optical waveguide has increased to about 150 ° C.
It was found that the phase matching wavelength could be controlled over a wide range by heating the heater. Further, to measure the response time of the temperature mark pressurized. The modulation characteristics of the phase matching wavelength were measured by periodically changing the voltage (40 V) applied to the heater. As a result, it was found to have a response speed on the order of several Hz.
By reducing the area and thickness of the substrate, the heat capacity of the substrate can be reduced by 1 /
With a setting of 10, a response speed on the order of several tens of Hz was obtained.

Next, the modulation characteristics of the phase matching wavelength by the electrodes were evaluated. LiTaO ₃ is a material having an electro-optic effect. The electro-optic effect is an effect in which the refractive index of the substrate is changed by applying a voltage, and the direction and magnitude of the changing refractive index are changed with respect to the crystal direction by the direction and magnitude of the applied electric field.
It is decided uniquely. 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, the parallel electrode is
The optical waveguides were arranged so that an electric field in the z-axis direction was applied to the optical waveguides. FIG. 4 shows the result of applying a DC voltage to the parallel electrodes and determining the relationship between the applied voltage and the phase matching wavelength. When a voltage of ± 50V is applied, the phase matching wavelength becomes 0.6n
m modulation was possible. When the modulation response speed was measured,
It had a response speed of 00 MHz or more.

As described above, modulation by heater heating has a wide modulation range but a low response speed, while modulation by voltage can realize a high response speed, but has a problem that the modulation range is narrow. Therefore, a voltage was simultaneously applied to the heater and the electrode, and the modulation characteristics were measured. FIG. 5 shows the time dependency of the phase matching wavelength change.

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

Although the three parallel electrodes are used as the electrode structure in the first embodiment , an electric field in the Z direction can be applied to the optical waveguide with two parallel electrodes.

In the first embodiment , the parallel electrodes are used. However, if a comb-shaped electrode is used, the phase matching wavelength can be modulated more efficiently. The reason is described below. When an electric field is applied by the parallel electrodes, an electric field in the same direction (Z-axis direction) is uniformly applied to the optical waveguide. However, when the optical wavelength conversion element is of a quasi-phase matching type including a periodically poled layer as shown in FIG. 1, the optical waveguide has a structure in which the polarization is periodically reversed. sign of the electro-optical constant r ₃₃ is also reversed. Therefore, when a voltage is applied by the parallel electrodes, the increase / decrease of the refractive index change is reversed between the polarization inversion portion and the non-inversion portion of the optical waveguide. Therefore, the refractive index that changes by applying a voltage is canceled out 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.
As shown in (2), 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 the refractive index shows the same increase and decrease in both the inverted portion and the non-inverted portion. For this reason, the refractive index could be changed efficiently by applying a voltage. For example, when a voltage of ± 50 V is applied to the comb-shaped electrode, the phase matching wavelength can be modulated over 1 nm, and the phase matching wavelength can be modulated about twice that of the parallel electrode. The period of the teeth of the comb of the comb-shaped electrode is (2m-1) Λ (m = 1, 2, 3
...), The refractive index change can be given efficiently.

In the first embodiment , a Ti thin film is used for the heater, but any other high-melting metal such as Cr, Ta, or W can be used. In addition, any method can be used as long as the temperature can be applied to the substrate, such as resistance heating, infrared heating, and a Peltier element.

In the first embodiment , Ti is used for the electrode. However, other materials such as 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 generated when the electrode directly contacts the optical waveguide is small, so that the buffer layer 12 can be omitted, which is effective.

In the first embodiment , the buffer layer is made of SiO ₂
However, other dielectric materials such as Ta ₂ O ₅ and SiN, and semiconductors such as Si can be used. In particular, a semiconductor is effective because an electric field due to a pyroelectric effect generated by a change in the substrate temperature can be prevented and the temperature characteristics can be improved.

In the first embodiment , a LiTaO ₃ substrate was used as the substrate. Alternatively, LiTaO ₃ doped with MgO, Nb, Nd, or the like, or LiNbO ₃ or a mixture thereof, LiTa _(1-x) Nb _x O ₃ (0
≦ x ≦ 1) substrate, other KTP (KTiOPO ₄₎ any similar device can be fabricated. Since LiTaO ₃ , LiNbO ₃ , and KTP all have high nonlinearity, a highly efficient optical wavelength conversion element can be manufactured. In addition, since a method of forming a periodically poled layer has been confirmed for these materials, a highly efficient optical wavelength conversion element can be formed.

[0038] (Reference Example 2) The following reference examples 2 of the optical wavelength conversion device of the present invention will be described.

FIG. 7 is a configuration perspective view of an optical wavelength conversion element according to Embodiment 2 of the present invention. 1 is a LiTaO ₃ substrate of a -C plate (− side of a plane perpendicular to the C axis of the crystal), 4 is a domain-inverted layer, 5 is a proton exchange waveguide, 6 is fundamental light having a wavelength of 860 nm, and 7 is a fourth light having a wavelength of 430 nm. 2 is a harmonic (hereinafter abbreviated as SHG light), 8 is the period of the domain-inverted layer Λ, 9 is the width W of the domain-inverted portion, 10 is an electrode, 11 is a thin film heater, 12 is a SiO ₂ buffer layer, and 13 is SiO. _Two buffer layers. Optical waveguide width is 4μ
m, the depth is 2 μm, and the period 分 極 of the domain-inverted layer is about 3.8 μm.
It is. The manufacturing method of the light wavelength conversion element is almost the same as that of the first embodiment . However, after forming the electrodes, the SiO ₂ buffer layer
m, and a heater was formed thereon.

Next, the operation principle of the optical wavelength conversion device shown in FIG. 7 will be described. A thin film heater was formed on the electrode with an insulating film interposed. By forming the insulating film, the electrode and the thin film heater are electrically separated, and integration becomes possible. Also,
By integrating the heater and the electrodes in a 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 application of the temperature is about twice that of the optical wavelength conversion element of Reference Example 1. It got faster. As in Reference Example 1, when a voltage was applied to the heater and the electrode to modulate the refractive index of the nonlinear optical element, the desired refractive index could be modulated in 0.3 seconds. By integrating the heater and the electrodes in layers, it has become possible to modulate the phase matching wavelength at a higher speed. Further, by arranging the heater and the electrodes in a laminated structure, the area occupied by the element is reduced, and a wavelength conversion element about 3/2 times can be formed in a substrate having the same area.

[0041] (Embodiment 1) FIG. 8 is a configuration perspective view of the optical wavelength conversion element of the real施例of the present invention. 1 is a LiTaO ₃ substrate of a -C plate (− side of a plane perpendicular to the C axis of the crystal), 4 is a domain-inverted layer, 5 is a proton exchange optical waveguide, 6 is fundamental light having a wavelength of 860 nm, and 7 is 430 nm.
(Hereinafter abbreviated as SHG light), 8 is the period of the domain-inverted layer, 9 is the width W of the domain-inverted portion, 14 is the Ti thin film as the first conductor, and 15 is the second conductor. Ti thin film, 1
Reference numeral 2 denotes an SiO ₂ buffer layer. The optical waveguide width is 4 μm, the depth is 2 μm, and the period of the domain-inverted layer is about 3.8 μm.

Next, the operation principle of the optical wavelength conversion device shown in FIG. 8 will be described. Both ends indicia pressure to the voltage V1 of the Ti film 14 is a first conductor, the voltage of V2 across the second conductor 15 passing a <br/> mark pressurized Shi current, it acts as a thin-film heater, the optical waveguide Temperature can be applied. At the same time, when a potential difference V is provided between the Ti thin films 14 and 15, an electric field can be applied to the optical waveguide. With a pair of electrodes, an electric field and a temperature can be simultaneously applied.
As the modulation characteristics of the phase matching wavelength of the optical wavelength conversion element, the characteristics were almost the same as those of the type in which the electrode and the heater were integrated in a laminated structure. Further, the light wavelength conversion element has a simple structure and can be easily manufactured because the buffer layer forming step and the thin film heater forming step required for manufacturing the element of the second reference example can be omitted.

The same structure can be obtained with the three stripe structure and the comb-shaped electrode structure shown in Reference Example 1 as the electrode structure. In particular, a comb-shaped electrode is effective because the change in the refractive index due to electric field control is large.

Next, a description will be given of the result of a study on the expansion of the phase matching tolerance of the optical wavelength conversion device using the structure of the optical wavelength conversion device shown in FIG.

In the configuration of the optical wavelength conversion element shown in FIG. 8, a current was applied to the first conductor 14, and the second conductor 15 was set to a constant voltage (in this case, ground, voltage 0). 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 / 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 by a factor of 10 from 0.1 nm to 1 nm as shown in FIG. Could be expanded. Using this optical wavelength conversion element,
A stable SHG output was obtained with respect to the wavelength fluctuation of the fundamental wave.

(Example 2 ) By forming a temperature applying means and an electric field applying means on an optical element, the element characteristics can be stabilized. Here, the first embodiment
An optical switch will be described as an optical element having the same configuration as that of FIG. When you form <br/> the electrode structure shown in FIG. 8 on the optical waveguide, an optical switch with stable characteristics can be constructed.

The operation principle of the optical switch will be described with reference to FIG. The light emitted from the laser 23 is transmitted to the half mirror 24
And one is made 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, multiplexed with the branched light, and detected by the detector 27. Then voltage application pressure to the electrodes formed on the optical waveguide, the refractive index of the optical waveguide is changed, the optical path difference is changed, it operates as an optical switch by the interference of the multiplexed light. Light output 10
Modulation was possible 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, a current is passed through the heater electrode,
By keeping the temperature of the optical element constant, output stability could be achieved and output fluctuation could be suppressed to 2% or less. Since temperature control and electric field control can be performed simultaneously by the electrodes, a switch with stable operation can be configured.

(Embodiment 3 ) Hereinafter, a short wavelength light generating apparatus according to the present embodiment will be described.

FIG. 12 is a perspective view showing the configuration of a short wavelength light generator according to the present invention. In FIG. 12, reference numeral 1 denotes LiTaO ₃ of a −C plate.
Substrate (negative side of the plane perpendicular to the C axis of the crystal), 4 is a domain-inverted layer, 5 is a proton exchange optical waveguide, 6 is fundamental light having a wavelength of 860 nm, 7 is a second harmonic having a wavelength of 430 nm, and 8 is domain-inverted The layer period Λ, 9 is the width W of the domain-inverted portion, 10 is the electrode, 1
1 is a thin film heater, 12 is an 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,
The next period Λ was 3.8 μm.

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

However, when the optical wavelength conversion element of Reference Example 1 is used, the phase matching wavelength can be modulated over a maximum of 4.6 nm, so that the phase matching wavelength can be adjusted even when the semiconductor laser temperature changes by ± 10 ° C. By doing so, the SHG output could be kept stable. Further, 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 at a modulation speed of several tens of Hz by the heater and the electrode, a relatively fast response speed can be modulated by the feedback circuit. As a result, SHG
The output fluctuation can be suppressed 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), and the output temporarily decreased by nearly 50%. In other cases, the fluctuation could be suppressed to 10% or less.

[0052] (Reference Example 3) in order to suppress the mode hopping and the wavelength fluctuation of the semiconductor laser, Reference Example 3 using a grating feedback
It will be described short-wavelength light generating apparatus.

FIG. 13 is a perspective view showing the configuration of a short wavelength light generating device according to the present invention. In FIG. 13, reference numeral 1 denotes a LiTaO ₃
Substrate (negative side of the plane perpendicular to the C axis of the crystal), 4 is a domain-inverted layer, 5 is a proton exchange optical waveguide, 22 is a grating, 6 is fundamental light having a wavelength of 860 nm, and 7 is 430 nm.
(Hereinafter abbreviated as SHG light), 8 is the period of the domain-inverted layer, 9 is the width W of the domain-inverted portion, 10 is the electrode, 1
1 is a thin film heater, 12 is an SiO ₂ buffer layer, 20 is a semiconductor laser, and 21 is a focusing optical system. The grating 22 formed in the vicinity of the emission part functions as a DBR grating, and the fundamental light 6 of a specific wavelength propagating in 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 the feedback wavelength, the oscillation wavelength fluctuation of the semiconductor laser can be suppressed and stabilized, and the wavelength fluctuation due to mode hop is eliminated.

In this short-wavelength light generator, wavelength fluctuations due to the temperature of the semiconductor laser, the driving current, and the like can be suppressed. However, it is necessary to match the phase matching wavelength between 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 a heater and electrodes integrated in the optical wavelength conversion element. Reference Example 1
As described above, since the wavelength range of 2 nm can be controlled in about 0.5 seconds, the feedback circuit is relatively fast,
Modulation of response speed became possible. The output of the semiconductor laser was 100 mW, and the light coupled to the optical waveguide was 70 mW. At this time, the conversion efficiency is 14%, and the Sm of 10 mW
HG output was obtained. Further, it becomes 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 were reduced, it was possible to manufacture an optical wavelength conversion element having a stable output.

As described above, the short-wavelength light generator of the present embodiment 3 provided stable and high-output blue coherent light (SHG light) with respect to changes in environmental 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.

[0056] In the present embodiment 3, as the optical wavelength conversion element constituting the short-wavelength light generating apparatus, it is used an optical wavelength conversion element shown in Reference Example 1, Reference Example 2 or Example 1
The light wavelength conversion element shown by the symbol can be used in the same manner. Reference example 2
The light wavelength conversion element indicated by (1) can modulate the light efficiency more effectively, and can effectively constitute a short-wavelength light generator with low power consumption.

In the third embodiment , the DBR grating formed on the optical waveguide is used for stabilizing the wavelength 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 because the second harmonic generated in the grating formed on the optical waveguide is not lost, and a more efficient short-wavelength light generator can be configured.

In the third embodiment , the DBR grating formed on the optical waveguide is used for stabilizing the wavelength of the semiconductor laser. Alternatively, 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, a specific wavelength is excited into the optical waveguide, and the reflected light from the optical waveguide is fed back to the semiconductor laser, so that the wavelength is reduced . Stabilization can be achieved. The use of a narrow band filter is effective because a mechanically stable short-wavelength light generator can be configured.

Reference Example 4 Here, an optical wavelength conversion element of Reference Example 4 having a stable output characteristic with respect to an external temperature change will be described.

In the short-wavelength light generator of FIG. 13, the temperature characteristics of the output were measured while changing the temperature of the short-wavelength light generator. As a result, the output reached its 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 optical wavelength conversion element. Temperature coefficient of phase matching wavelength (dn / d
T) is 0.038 nm / ° C, and the grating reflection wavelength dn / dT is
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 (determined by 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 light generator having a wavelength 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 generator is represented by 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 equal to dn / dT of the reflection wavelength of the grating, a short wavelength light generating device having a stable characteristic against an external temperature change can be constituted.

Therefore, by depositing a film having a higher temperature coefficient than the substrate on the grating, the dn / dT of the grating reflection wavelength is increased, and the difference between the phase matching wavelength and the dn / dT between the grating reflection wavelength is reduced. Thus, the output temperature characteristics were improved. The configuration is the same configuration as the short-wavelength light generating apparatus shown in FIG. 13, on the grating _{22, PLZT ((Pb (1} -x) La x) (Zr y Ti (1-y))
A film of _{(1-x / 4)} O ₃ ) 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
By depositing PLZT having a large refractive index change on the grating as compared with dn / dT of the substrate: 6.8 × 10 ⁻⁵ / ° C.,
The temperature coefficient of the reflection wavelength of the grating increased, and as a result, the temperature coefficient dn / dT of the reflection wavelength increased to 0.036 nm / ° C. As a result, the difference between dn / dT between the phase matching wavelength and the reflection wavelength of the grating can be reduced to 0.002 nm / ° C., and the temperature tolerance of the output has increased to 50 ° C., which is five times.
An element having stable output characteristics with respect to an external temperature change could be constructed.

In the fourth embodiment , PLZT is deposited on the grating. However, the PLZT has a larger dn / dT than the temperature change dn / dT of the substrate and has a loss in light guided through the optical waveguide. Any film may be used as long as it is not provided. By changing the film thickness, the temperature coefficient of the optical waveguide can be controlled to some extent.

In Reference Example 4 , PLZT was deposited on the grating in order to match the phase matching wavelength and dn / dT of the grating reflection wavelength. However, dn / dT smaller than the dn / dT of the substrate was applied to portions other than the grating. Is deposited, the phase matching wavelength dn / dT decreases, and the same result is obtained. For example, Ta ₂ O ₅ , SiO ₂ (dn / dT: about 0.5 × 10 ⁻⁵ /
C.) or the like. TiO ₂ (dn / d
T: -4 to -7 × 10 ^-5 / ° C), PbMoO ₄ (-4 to -7 x 10 ^-5 / ° C)
Are effective because they have the opposite sign to dn / dt of the LiTaO ₃ substrate.

[0065]

As described above, by simultaneously using the electrode as an electric field and as a heater, the power consumption of the phase matching wavelength modulation of the optical wavelength conversion element can be reduced, and the tolerance of the phase matching wavelength can be increased. Therefore , the practical effect is large.

Further, by constituting a small short-wavelength coherent light source with the above-mentioned light wavelength conversion element and semiconductor laser, stable blue coherent light can be obtained, and application to optical disks and laser printers becomes possible. The practical effect is great.

[Brief description of the drawings]

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

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

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

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

5A and 5B are diagrams showing the time dependence of a phase matching wavelength change. FIG. 5A is a characteristic diagram when power is applied only to a heater. FIG. 5B is a characteristic diagram when voltage is applied to a heater and an electrode at the same time.

6A and 6B are diagrams illustrating an electric field distribution in an optical wavelength conversion element, wherein FIG. 6A is a cross-sectional view of the optical wavelength conversion element, and FIG. 6B is a characteristic diagram illustrating an electric field applied to the optical waveguide and a change in refractive index.

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

FIG. 8 is a configuration perspective view 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 generator according to the present invention.

FIG. 13 is a configuration perspective view of another short-wavelength light generator according to a reference example of the present invention.

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

FIG. 15 is a configuration perspective view of a conventional short-wavelength light generator.

[Explanation of symbols]

1-C plate LiTaO ₃ substrate 4 domain-inverted layer 5 proton exchange optical waveguide 6 fundamental light 7 second harmonic 8 period ９ 9 width W 10 electrode 11 heater 12 buffer layer 13 buffer layer 14 electrode A 15 electrode B 20 semiconductor laser DESCRIPTION OF SYMBOLS 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 part 106 Collimating lens 107 Dichroic mirror 108 Grating 110 LiTaO ₃ substrate 111 domain inversion section 112 proton exchange optical waveguide 113 electrode 114 semiconductor laser 115 incidence section 116 emission section

Claims (4)

(57) [Claims] A ferroelectric substrate; an optical waveguide formed on the substrate; a periodically poled structure formed on the substrate; a stripe-shaped first conductor formed on the substrate; A second conductor in a stripe shape formed on both sides or one side of the first conductor, wherein the stripe directions of the first and second conductors are substantially parallel to each other, and between the first and second conductors. A potential difference, a current is flowing through at least one of the first and second conductors, and a potential difference between the first and second conductors changes in a propagation direction of the optical waveguide. An optical wavelength conversion element in which an electric field applied to the optical waveguide gradually changes in a propagation direction. 2. The optical wavelength conversion device according to claim 1, wherein at least one of said first and second conductors is a comb-shaped electrode. 3. The optical wavelength conversion device according to claim 1, wherein a grating is formed on at least a part of the surface or inside of the optical waveguide. 4. A semiconductor laser, an optical wavelength conversion device according to claim 1, and a condensing optical system, wherein a phase matching wavelength of the optical wavelength conversion device is an oscillation of the semiconductor laser. A short-wavelength light generating device wherein a current applied to a first or second conductor formed on the optical wavelength conversion element and a potential difference between the conductors are adjusted to match a wavelength. .