JPH0651359A - Wavelength conversion element, short wavelength laser device and wavelength variable laser device - Google Patents

Abstract

PURPOSE:To provide the wavelength conversion element with which the higher output and the higher stability are attained by forming a dielectric film on a substrate and forming gratings on this dielectric film. CONSTITUTION:The dielectric film 10 is formed on the substrate 1 consisting of an optical waveguide 5 having periodic polarity inversion layers 4 and further, the gratings 11 are formed on the dielectric film 10. The gratings 11 act only on basic light 6 propagating in the optical waveguide 5 to reflect the basic light of a specific wavelength, thereby stabilizing the wavelength of a semiconductor laser 13 exciting the wavelength conversion element. The SHG light 7 propagating in the waveguide 5 is not affected by the gratings 11 and, therefore, the waveguide loss to the SHG light 7 is lessened and the stable output is obtd. with high efficiency.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Polarization reversal for forcibly reversing the polarization of a ferroelectric substance is performed by forming a periodic polarization reversal layer in the ferroelectric substance, and using an optical frequency modulator utilizing surface acoustic waves or a nonlinear polarization. It is used for a wavelength conversion element and the like utilizing the polarization inversion of the. In particular, if it becomes possible to periodically invert the nonlinear polarization of the nonlinear optical material, it is possible to manufacture a second harmonic generation element (hereinafter referred to as an SHG element) having a very high conversion efficiency. Thus, by converting light from a semiconductor laser or the like, a compact short-wavelength light source can be realized, and it can be applied to the fields of printing, optical information processing, optical application measurement control, and the like, and thus is actively researched. The polarization inversion type SHG element is capable of highly efficient wavelength conversion, and can also perform arbitrary wavelength conversion by changing the periodic structure. However, since it is based on the periodic structure, the wavelength dependence is high and the output fluctuation with respect to the wavelength fluctuation of the fundamental light is very large.

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 waveguide as a wavelength conversion element. (Yamamoto et al., Optics Letters
rs Vol.16, No.15, 1156 (1991)). FIG. 14 shows a schematic configuration diagram of a short wavelength light source using a semiconductor laser and a QPM wavelength conversion element. The light emitted from the semiconductor laser 101 is converted into a parallel beam by the collimator lens 102, the deflection direction is rotated by the λ / 2 plate 103, and NA = 0.6.
The light is focused on the incident end surface 5 of the waveguide by the focusing lens 104. Then, wavelength-converted blue light is obtained. The incident end face 105 is non-reflective coated to avoid returning light to the semiconductor laser, but about 1% of returning light is generated from the end face 105. As a result, 1.1 mW of blue light was obtained for an incident light intensity of 35 mW entering the waveguide of the semiconductor laser. However, the QPM wavelength conversion element has a wavelength tolerance of 0.2n.
The output is stable for only a few seconds because there is only m, the fluctuation of the oscillation wavelength with respect to the temperature change of the semiconductor laser is 0.2 nm / ° C, and the mode hop due to the returning light is about 1 nm. Therefore, wavelength stabilization of the semiconductor laser is indispensable.

On the other hand, there is a report on a wavelength conversion element for performing stable wavelength conversion of a semiconductor laser by a QPM wavelength conversion element. (K. Shinozaki et al. Applied Physics Letters. Vol. 5
9, No. 29, 510-512 (1991)). FIG. 15 shows a configuration diagram of a conventional wavelength conversion element. The harmonic generation (wavelength 6.5 μm) for the fundamental light in the wavelength 1.3 μm band will be described in detail below with reference to the drawings.

As shown in FIG. 15, an optical waveguide 44 is formed on a LiNbO ₃ substrate 41, and a layer 45 (polarization inversion layer) whose polarization is periodically inverted is further formed on the optical waveguide 44. By compensating the mismatch between the propagation constants of the fundamental wave and the generated harmonic with the periodic structure of the polarization inversion layer 45, the harmonic can be generated with high efficiency. When the fundamental wave P1 (43) is incident on the incident surface of the optical waveguide 44, the harmonic P2 (42) is efficiently generated from the optical waveguide 44 and operates as an optical wavelength conversion element. Furthermore, since the polarization inversion layer has a slightly higher refractive index than the substrate, it constitutes a grating by a change in the refractive index. This grating is Distributed B
It was used as a ragg Reflector (hereinafter referred to as DBR Grating).

The DBR is a reflector having wavelength selectivity and reflects only a specific wavelength. When a specific wavelength is fed back to the semiconductor laser by the DBR grating, the wavelength of the semiconductor laser is fixed to the reflection wavelength of the DBR. If the phase matching wavelength by QPM and the reflection wavelength of DBR are matched, stable wavelength conversion can be performed using a semiconductor laser. In the conventional example, the polarization inversion layer was formed in the waveguide, and this was also used as the DBR grating. Therefore, the light emitted from the semiconductor laser is coupled to the optical waveguide through the fiber, but it is partially reflected by the DBR grating and the oscillation wavelength of the semiconductor laser can be fixed. DBR
To match the reflection wavelength of QPM and the phase matching wavelength of QPM,
A polarization-inverted grating with a period of 6.5 μm is formed, and at the wavelength of the fundamental light of 1.327 μm, the first-order, D
In BR, matching can be achieved for the first time with the 43rd-order grating order.

For the fundamental wave P1 having a wavelength of 1.327 μm, the length of the optical waveguide is 2 mm, and the power of the fundamental wave P1 is 6
When the power was set to 0 μW, the power of the harmonic P2 was 0.652 pW. The conversion efficiency at this time is 4.1% / W · c
It was m ² .

[0008]

In the conventional wavelength conversion element, the polarization inversion layer and the DBR grating are formed at the same time. However, in order to make the phase matching wavelength due to the period of the polarization inversion layer and the reflection wavelength of the DBR grating coincide with each other. In addition, it is necessary to control the propagation speeds of the basic light and the SHG light in the waveguide with high accuracy, which is difficult to manufacture, and the wavelength that can be converted by the wavelength conversion element is limited. Therefore, in the conventional wavelength conversion element, it is possible to configure a wavelength conversion element that is stable only with respect to the fundamental light having a wavelength of 1.3 μm, and a short wavelength (wavelength 400 to 500 nm) that can be widely applied to the wavelength conversion element. However, there is a problem that it is difficult to realize the SHG light.

Further, since the polarization inversion layer is used as the DBR grating, there is a problem that the basic light is gradually reflected in the traveling direction and is gradually attenuated to reduce the efficiency of the wavelength conversion element.

Further, the periodic structure of the DBR grating affects not only the basic light but also the generated SHG light, and the conversion efficiency of the wavelength conversion element is lowered due to scattering and reflection of the SHG light by the DBR grating. was there.

Therefore, in view of the above points, the present invention forms a dielectric film and a grating on a nonlinear optical crystal having an optical waveguide and a domain inversion layer to stabilize the light of a semiconductor laser with high wavelength. Wavelength conversion element that can convert
It is an object to provide a short wavelength laser device and a wavelength tunable laser device using the same.

[0012]

In order to solve the above problems, according to the present invention, (1) a substrate made of a non-linear material having an optical waveguide and a polarization inversion layer, and one or more dielectric films formed on the substrate. And a grating formed on the dielectric film, the grating is formed in the propagation direction of the optical waveguide, and the thickness T of the dielectric film is 0 <T <
The wavelength conversion element has a thickness of 0.1 μm. (2) A substrate made of a nonlinear material having an optical waveguide and a domain inversion layer, and a grating formed on the substrate, wherein the grating is formed in the propagation direction of the optical waveguide, and the optical waveguide and the grating are formed. The wavelength conversion elements are adjacent to each other. (3) A substrate made of a non-linear material having an optical waveguide, a domain-inverted layer having a periodic structure formed in the substrate, an electrode formed on a part or all of the domain-inverted layer, and below the electrode. The period Λ of the polarization inversion layer in the propagation direction of the optical waveguide is λ = 2Nω · Λ / q (q = for the effective refractive index Nω of the fundamental light having the wavelength λ propagating in the optical waveguide.
1, 2, 3, ...), which are wavelength conversion elements. (4) A short wavelength laser device comprising the wavelength conversion element and the semiconductor laser described above. (5) An electro-optic substrate having an optical waveguide, a domain-inverted layer having a periodic structure formed in the substrate, an electrode formed on the domain-inverted layer, and a semiconductor laser,
Further, the wavelength tunable laser device is such that the light emitted from the semiconductor laser is incident on the optical waveguide.

[0013]

The present invention has the above-described structure, and in the wavelength conversion element based on the optical waveguide and the polarization inversion layer, the basic film propagating through the waveguide is formed by the dielectric film on the substrate and the DBR grating formed on the dielectric film. To the pumping light source, the wavelength of the pumping light source can be fixed, and the generated SHG
Since the light is not affected by the grating, it is possible to prevent reduction due to loss of SHG light, and it is possible to stably perform highly efficient wavelength conversion.

The reason will be described below. Polarization inversion type SH
The wavelength of the semiconductor laser can be fixed by forming a DBR grating in the G element and feeding back the basic light of a specific wavelength to the semiconductor laser.

However, if the DBR grating is formed directly in or on the waveguide of the wavelength conversion element, it affects not only the basic light but also the generated SHG light, causing loss of SHG light such as scattering and reflection. , Which causes a decrease in conversion efficiency of the device. If a dielectric film is formed between the DBR grating and the optical waveguide in order to prevent this, the grating can act only on the fundamental light. As a result, the loss of SHG light can be reduced, and a stable and highly efficient wavelength conversion element can be formed.

Further, according to the present invention, D is provided in a portion adjacent to the waveguide.
By forming the BR grating, the DBR grating acts only on the basic light, and for the reasons described above, the loss of the SHG light can be reduced and a stable and highly efficient wavelength conversion element can be formed.

Further, when an electrode is formed on the periodic domain inversion, an electric field is applied to cause a change in the refractive index of the domain inversion layer due to the electro-optic effect. Therefore, a grating having a periodic change in refractive index can be formed.
When this grating is used as a DBR grating, the wavelength of the semiconductor laser that excites the wavelength conversion element can be stabilized. Furthermore, since the reflected wavelength of the DBR grating can be controlled by the applied voltage, the wavelength of the semiconductor laser can be adjusted to the optimum value of the phase matching condition, and highly efficient wavelength conversion can be performed. The applied electric field also makes it possible to modulate the SHG output.

[0018]

【Example】

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

FIG. 1 is a perspective view showing the structure of a wavelength conversion element according to an embodiment of the present invention. 1 is a -C plate LiTaO ₃ substrate (C of crystalline
(-Side of the plane perpendicular to the axis), 4 is a polarization inversion layer, 5 is a proton exchange waveguide, 6 is basic light with a wavelength of 860 nm, 7 is a wavelength of 4
30 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 layer, 10 is
A film of SiO2, 11 is a grating of Ta2O5, 12 is a converging optical system, 13 is a semiconductor laser, 14 is an incident part, and 15 is an emission part. In FIG. 1, the polarization inversion layer 4 is a portion in which the direction of polarization is reversed with respect to the polarization inversion of the substrate.
In the case of 1 LiTaO ₃ substrate, the polarization direction is + C direction,
On the other hand, the polarization direction of the polarization inversion layer is the −C direction. The period Λ of the polarization inversion layer 4 is the wavelength λ of the fundamental wave 6,
The refractive index and shape of the waveguide 5 differ. Waveguide width is 4μ
m, the depth is 2 μm, and when the wavelength λ of the fundamental wave 6 is 860 nm, the primary period is about 3.6 μm.

Next, the operation principle of the wavelength conversion element of this embodiment will be described. Basic light 6 emitted from the semiconductor laser 13
Is condensed by the condensing optical system 12 and is incident on the incident portion 14. The incident basic light 6 propagates in the 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. Therefore, in the vicinity of the emitting portion, the converted SHG light and the basic light are both propagating. The grating 11 formed in the vicinity of the emitting portion functions as a DBR grating, reflects the basic light 6 having a specific wavelength, and returns to the semiconductor laser 13 through the incident portion 14 and the condensing optical system 12. Semiconductor laser 1
Since the oscillation wavelength of 3 is fixed to this feedback wavelength, the oscillation wavelength of the semiconductor laser can be stabilized.

On the other hand, the intensity distribution of the light propagating through the optical waveguide 5 in the depth direction differs depending on the wavelength. Therefore, if a dielectric layer is provided on the waveguide, the intensity distribution of the light leaking into this dielectric layer differs between the basic light and the SHG light. For example, when a SiO ₂ dielectric film is provided, the light intensity distribution is as shown in FIG. 2, and the point at which the maximum value is 1 / e ² is
0.1 μm with basic light and 0.04 μm with SHG light from substrate
Seeps into the SiO ₂ film. Therefore, a SiO ₂ film 10 having a thickness of 0.04 μm or more is provided on the optical waveguide 5, and Ta ₂ O ₅ is formed on the SiO ₂ film 10.
If the grating 11 is formed, only the basic light propagating in the waveguide is affected by the grating, and it is possible to prevent the SHG light propagating in the waveguide from being scattered or reflected by the grating. Further, the relationship between the grating period and the reflection wavelength has the relationship shown in FIG. 3 by calculation.
Therefore, the phase matching wavelength of the polarization inversion type SHG element is 860 nm.
Then, when the period of the DBR grating that matches this wavelength is calculated, the 10th DB with the period of 1.98 μm is obtained from FIG.
It can be seen that the basic light is reflected by the R grating. Thus, the phase matching wavelength and DB
Since the reflection wavelength of the R grating can be designed separately,
Any wavelength can be converted depending on the element design.

The results of evaluating the characteristics of the wavelength conversion element formed according to the above embodiment will be shown below. A domain-inverted layer was formed on the entire surface over an element length of 15 mm, and a grating was formed on the surface of the substrate near the emitting portion over a length of 5 mm. The fundamental light 6 from the semiconductor laser having a wavelength of 860 nm was made incident on the incident portion 14 of the optical waveguide 5 using the condensing optical system 12. The output of the semiconductor laser was 100 mW, and the light coupled to the waveguide was 50 mW. The intensity of the SHG light output from the emission part was measured with a power meter. FIG. 4 shows the relationship between the thickness of the SiO ₂ layer provided between the grating and the substrate and the SHG output. When the thickness of SiO ₂ is 0.1 μm or more, neither the fundamental light nor the SHG light is affected by the grating, so that the wavelength of the semiconductor laser cannot be fixed and stable wavelength conversion becomes impossible. When the thickness of SiO ₂ becomes 0.4 μm or less, the SHG light begins to be affected by the grating and gradually attenuates. As a result, Si
The maximum output of 7 mW was obtained by setting the thickness T of the O ₂ film to 0.04 <T <0.1. The fundamental light is reflected by the grating (maximum 15 when the SiO ₂ film thickness is 0).
% Reflected. ) Stable oscillation of the semiconductor laser was obtained, and the SHG light could avoid scattering and reflection loss due to the grating, and highly efficient wavelength conversion could be performed.

Although SiO 2 is used as the dielectric film in this embodiment, other loss such as Ta ₂ O ₅ , Ti ₂ O ₅ , SiN, LiNbO ₃ due to absorption or scattering is not given to the guided light. Any dielectric film can be used.

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. (0 ≦
x ≦ 1) Similar devices can be manufactured using a substrate and KTP. Since LiTaO ₃ , LiNbO ₃ , and KTP all have high nonlinearity, a highly efficient wavelength conversion element can be manufactured. Moreover, since a method of forming a periodic domain-inverted layer has been confirmed for these materials, highly efficient domain-inverted SHG.
An element can be formed.

Next, another embodiment of the present invention will be described. FIG. 5 is a configuration perspective view of a wavelength conversion element of another embodiment of the present invention. 1 is a -C plate LiTaO ₃ substrate, 4 is a polarization inversion layer, 5 is a proton exchange waveguide, 6 is basic light with a wavelength of 860 nm, 7 is SHG light with a wavelength of 430 nm, 11 is a grating of Ta ₂ O ₅ , and 12 is A condenser optical system, 13 is a semiconductor laser, 14 is an incident part, and 15 is an emission part.

The method of converting the basic light into the SHG light by the wavelength conversion element and the wavelength stabilization of the semiconductor laser light source by the DBR grating are as described above. When the DBR grating is formed on the side portion of the optical waveguide as shown in FIG. 5, only the basic light propagating through the optical waveguide can be affected by the grating. The intensity distribution of the light propagating through the optical waveguide in the direction parallel to the substrate is shown in FIG.
As shown in, it depends on the wavelength. Waveguide width 4 μm,
In the waveguide having a depth of 2 μm, the widthwise expansion of the intensity distribution of the fundamental light having a wavelength of 860 nm is 4.3 μm, and the wavelength is 430 n.
The width of the intensity distribution of SHG light of m is 3.9 μm.
Met. Therefore, the grating of Ta ₂ O ₅ (thickness 0.1 μm, period 1.
98 μm) was formed. Since only the basic light is affected by the grating according to the intensity distribution of light, the basic light is reflected by the grating and stable oscillation of the semiconductor laser is obtained, and the SHG light causes scattering and reflection loss by the grating. It was possible to avoid wavelength conversion with high efficiency.

(Second Embodiment) A second embodiment of the present invention will be described below.

FIG. 7 is a perspective view showing the configuration of the wavelength conversion element according to the embodiment of the present invention. 1 is a -C plate LiTaO ₃ substrate, 4 is a polarization inversion layer, 5 is a proton exchange waveguide, 6 is a wavelength of 860 nm
Basic light, 7 is SHG light having a wavelength of 430 nm, 17 is a polarization inversion layer, 12 is a condensing optical system, 13 is a semiconductor laser, 16
Is an electrode, 14 is an entrance part, and 15 is an exit part. In FIG. 7, the polarization inversion layer 4 is a portion in which the polarization direction is reversed with respect to the polarization inversion of the substrate. In the case of the No. 1 LiTaO ₃ substrate, the polarization direction is the + C direction, whereas the polarization direction of the polarization inversion layer is the −C direction. The period Λ of the polarization inversion layer differs depending on the wavelength λ of the fundamental wave and the refractive index and shape of the waveguide 5. The waveguide width is 4 μm and the depth is 2 μm. When an electric field is applied to the polarization inversion layer, the refractive index changes due to the electro-optic effect.

Here, the electro-optical effect will be described. The electro-optic effect is a phenomenon in which the refractive index changes with an electric field, and the direction, magnitude, and electro-optical constant (tensor) of a substance
The amount of change in the refractive index is determined by. In the case of LiTaO ₃ , the electro-optic constant has a large value in the r33 direction (coefficient when the refractive index in the z-direction changes due to the electric field in the z-direction), and the electro-optic constant is not present in the part where polarization is inverted. Is ±
Takes the opposite value. Therefore, as shown in FIG. 8, when an electric field in the C direction is applied, when the refractive index changes by Δn in the part where the polarization is not inverted, the refractive index changes by −Δn in the part where the polarization is inverted, and the grating due to the refractive index distribution is changed. It can be formed (FIG. 8B).

The operating principle of the wavelength conversion device of this embodiment will be described below. The basic light 6 emitted from the semiconductor laser 13 is condensed by the condensing optical system 12 and enters the incident portion 14.
The incident basic light 6 propagates in the 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. When an electric field is applied to the electrodes, the refractive index of the domain-inverted layer 17 under the electrodes changes, forming a refractive index grating. The basic light 6 having a specific wavelength is reflected by this grating, and the incident portion 14 and the condensing optical system 1 are reflected.
The oscillation wavelength of the semiconductor laser 16 is fixed to this feedback wavelength for returning to the semiconductor laser 13 via 2

In this embodiment, since the element length is 15 mm and the wavelength of the fundamental wave (wavelength: 860 nm) is converted, the period 3.
A 6 μm periodic domain-inverted layer was formed over 10 mm from the incident portion 15. An optical waveguide having a depth of 2 μm and a width of 4 μm was formed in the polarization inversion layer in the vertical direction. The electrode 16 is formed over 5 mm from the emitting portion 15 and has a period of 1.98 μm under the electrode.
A domain-inverted layer of m is formed over the entire surface.

When a voltage of 50 V was applied to the electrodes, the inversion layer and the non-inversion layer in the waveguide caused a change in refractive index of about ± 10 ^-4 to form a grating. When a voltage of 10 V or higher was applied, the grating caused feedback to the semiconductor laser, and the SHG output was stable. The output of the SHG light could be switched by switching the voltage applied to the electric field. The SHG output was 4 mW and the extinction ratio due to switching was -30 dB.

Next, the modulation of the reflection wavelength will be described. When a voltage is applied to the element, the refractive index changes of the inversion layer and the non-inversion layer occur in opposite directions. These are defined as ± Δn. When the width of the inversion layer is W and the period is Λ, the average refractive index change ΔN = (ΔnW-Δn (Λ-W)) / Λ under the electrode is obtained, and as W departs from 0.5Λ, the optical waveguide due to the application of an electric field. The average refractive index change ΔN of is large. When the period Λ is constant and the average refractive index of the waveguide changes, the propagation constant of the waveguide changes, so the wavelength reflected by the grating is approximated by λ = 2Λ (Neff + ΔN) / q (Neff is the waveguide's Effective refractive index, q is the order of the grating). A DBR grating can be formed by applying an electric field.
Furthermore, by modulating W ≠ 0.5 Λ applied voltage, DB
The wavelength reflected by the R grating can be controlled.

The results of evaluating the characteristics of the wavelength conversion element formed according to the above embodiment will be shown below. Wavelength 8
The experiment was conducted using a semiconductor laser of 60 nm (output 100 mW). The coupling loss of the condensing optical system was 50%, and 50 mW of light was incident on the optical waveguide. Below the electrode (length 5 mm)
A domain-inverted layer having a domain-inverted layer width W of 1.7 μm and a period of 1.98 μm was formed. Other configurations are similar to those of the above wavelength conversion element. The relationship between the electric field applied to the electrodes and the SHG output is shown in FIG. The voltage allows the wavelength of the semiconductor laser to be continuously changed, which makes it possible to continuously modulate the SHG output of the wavelength conversion element.
Furthermore, as a result of adjusting the voltage applied to the electrodes so that the SHG output becomes maximum while monitoring the SHG output from the wavelength conversion element, a very stable SHG output can be obtained as shown in FIG. did it.

In this embodiment, the width W of the domain-inverted layer under the electrode is set so that the relationship of the period Λ is W ≠ 0.5Λ, but the same effect can be obtained if the depth of the domain-inverted layer is less than the depth of the optical waveguide. It is obtained by If the depth of the domain inversion layer is shallower than the optical waveguide, the average refractive index of the waveguide can be controlled by the applied voltage, and thus the SHG output can be controlled by the applied voltage.

(Embodiment 3) A short wavelength laser light source of the present invention will be described.

FIG. 11 shows a short wavelength laser light source in which a semiconductor laser having a wavelength of 0.8 μm and the wavelength conversion element produced in Example 1 are combined. The light of the semiconductor laser 31 is condensed by the condensing optical systems 34 and 35, and the incident portion 12 of the wavelength conversion element 12
It came from. When these were integrated into a module, a very small wavelength light source could be formed. Since the oscillation wavelength of the semiconductor laser is fixed by returning the basic light from the wavelength conversion element, fluctuations in the oscillation wavelength due to external temperature changes, semiconductor laser drive current changes, etc. are suppressed, and a very stable SHG output is obtained. Was given. This light source has an output of 2 mW
With its extremely high output, small size, and low noise, it can be applied to light sources such as optical disks. With this short wavelength light source, the storage capacity of the optical disk can be significantly increased, and a very small device can be manufactured.

(Embodiment 4) A tunable laser according to the present invention will be described.

FIG. 12 is a perspective view showing the structure of the wavelength tunable laser according to the embodiment of the present invention. 1 is a -C plate LiTaO ₃ substrate, 4 is a polarization inversion layer, 5 is a proton exchange waveguide, 13 is a semiconductor laser, 16 is an electrode, 14 is an incident part, and 15 is an emission part. In FIG. 12, the polarization inversion layer 8 is a portion in which the direction of polarization is reversed with respect to the polarization inversion of the substrate. 1 Li
In the case of the TaO ₃ substrate, the polarization direction is + C direction, whereas the polarization direction of the polarization inversion layer 17 is −C direction.
The period Λ of the polarization inversion layer differs depending on the wavelength of the fundamental wave and the refractive index and shape of the waveguide 5. Waveguide width 4μm, depth 2μ
m.

Next, the operation principle of the wavelength tunable laser according to the embodiment of the present invention will be described. When an electric field is applied to the polarization inversion layer, the refractive index changes due to the electro-optic effect. When the electrode 16 is formed in a portion where the polarization direction is periodically inverted and a voltage is applied, an electric field in the z direction is applied. The electric field causes a difference in refractive index between the polarization inversion layer and the non-inversion layer to form a grating. When the grating period is Λ, the grating reflects wavelengths of wavelength λ = 2 · Λ · Neff / q (Neff is the effective refractive index of the waveguide, q = 1, 2, 3 ...). Since the wavelength of the semiconductor laser is fixed to this reflection wavelength, the oscillation wavelength of the semiconductor laser can be regulated by the period of the grating. When the change in refractive index due to application of voltage is Δn, the width of the inversion layer is W, and the period is Λ, the average refractive index change under the electrode ΔN = (ΔnW−Δn (Λ−W)) / Λ
Therefore, when W ≠ 0.5Λ, ΔN can be changed by the refractive index Δn that changes with the applied electric field. When the period Λ is constant and the average refractive index of the waveguide changes, the propagation constant of the waveguide changes, so the wavelength reflected by the grating is λ˜2Λ (Neff + ΔN) / q. (Neff
Is the effective refractive index of the waveguide and q is the order of the grating).
In other words, when W ≠ 0.5Λ, the application of the electric field causes the DBR
The grating can be formed, and the wavelength reflected by the grating can be controlled by the applied electric field.
Therefore, the oscillation wavelength of the semiconductor laser can be changed by the applied voltage. Moreover, the wavelength of the semiconductor laser is
Since it is fixed by the DBR grating, very stable and low noise output can be realized.

The results of evaluating the characteristics of the wavelength conversion element formed according to the above embodiment will be shown below. Under the electrode (5 mm in length), the width W of the domain inversion layer is 1.7 μm and the period is 1.
A domain inversion layer of 98 μm was formed. Other configurations are similar to those of the above wavelength conversion element. The relationship between the electric field applied to the electrodes and the output wavelength is shown in FIG. The wavelength of the semiconductor laser could be continuously changed by the voltage.

In this embodiment, the width W of the domain-inverted layer under the electrode is set such that the relationship of the period Λ is W ≠ 0.5Λ, but the same effect can be obtained when the depth of the domain-inverted layer is less than the depth of the optical waveguide. It is obtained by If the depth of the domain inversion layer is shallower than the optical waveguide, the average refractive index of the waveguide can be controlled by the applied voltage, and thus the SHG output can be controlled by the applied voltage.

[0043]

As described above, a waveguide is formed by forming a dielectric layer on the surface of a wavelength conversion element composed of an optical waveguide having a periodically poled layer and forming a grating on the dielectric layer. It is possible to reflect the propagating basic light of a specific wavelength. As a result, the oscillation wavelength of the semiconductor laser exciting the wavelength conversion element can be stabilized and the output of the wavelength conversion element can be stabilized. Furthermore, since it is possible to prevent the influence (scattering and reflection) on the SHG light propagating through the optical waveguide, which is caused by the grating, it is possible to perform wavelength conversion with extremely high efficiency, and its practical effect is great.

Further, in a wavelength conversion element composed of an optical waveguide having a periodic domain inversion layer, if a grating is formed adjacent to the waveguide, it is possible to reflect the basic light of a specific wavelength propagating through the optical waveguide. As a result, the oscillation wavelength of the semiconductor laser exciting the wavelength conversion element can be stabilized and the output of the wavelength conversion element can be stabilized. Furthermore, since it is possible to prevent the influence (scattering and reflection) on the SHG light propagating through the optical waveguide, which is caused by the grating, it is possible to perform wavelength conversion with extremely high efficiency, and its practical effect is great.

Further, in the wavelength conversion element composed of the optical waveguide having the periodically poled layer, the period Λ = qλ / 2Nω (q =
1, 2, 3, ..., Nω: Effective polarization index of the optical waveguide with respect to the fundamental light) By forming a polarization inversion layer and an electrode formed on the polarization inversion layer, a voltage is applied to the electrodes. Since the grating can be formed by changing the refractive index of the polarization inversion layer under the electrode and the reflection wavelength of the grating can be controlled by the applied voltage, the SHG output can be modulated and the phase matching condition can be adjusted to the optimum position. Therefore, highly efficient wavelength conversion becomes possible, and its practical effect is great.

[Brief description of drawings]

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

FIG. 2 is a diagram showing the intensity distribution of light in an optical waveguide.

FIG. 3 is a characteristic diagram showing the relationship between the grating period and the reflection wavelength.

FIG. 4 is a characteristic diagram showing the relationship between the thickness of a dielectric film and SHG output.

FIG. 5 is a configuration perspective view of a wavelength conversion element according to an embodiment of the present invention.

FIG. 6 is a diagram showing the intensity distribution of light in an optical waveguide.

FIG. 7 is a perspective view showing the configuration of a wavelength conversion element according to an embodiment of the invention.

8A is a diagram showing a cross-sectional view of a wavelength conversion element, and FIG. 8B is a diagram showing a refractive index distribution in an optical waveguide.

FIG. 9 is a characteristic diagram showing the relationship between applied voltage and SHG output.

FIG. 10 is a characteristic diagram showing time variation of SHG output.

FIG. 11 is a sectional view showing the structure of a short wavelength laser light source according to the present invention.

FIG. 12 is a perspective view showing the configuration of a wavelength tunable laser according to the present invention.

FIG. 13 is a characteristic diagram showing a relationship between an applied electric field and an output wavelength.

FIG. 14 is a configuration diagram of a conventional wavelength conversion element.

FIG. 15 is a configuration diagram of a conventional wavelength conversion element.

[Explanation of symbols]

1 LiTaO ₃ Substrate 4 Polarization Inversion Layer 5 Proton Exchange Optical Waveguide 6 Basic Light 7 SHG Light 8 Period Λ 9 Mask Width W 12 Condensing Optical System 13 Semiconductor Laser 14 Incident Port 15 Emitting Portion 16 Electrode 17 Polarization Reversing Layer 27 Detector-30 Al frame 31 Semiconductor laser 33 Quartz plate 34 Collimator lens 35 Focus lens 36 Beam splitter 41 LiNbO3 substrate 42 Second harmonic wave 43 Fundamental wave 44 Proton exchange waveguide 45 Polarization inversion layer 101 Semiconductor laser 102 Collimator lens 103 λ / 2 plate 104 Focusing Lens 105 AR coat

Claims (9)

[Claims] 1. A substrate made of a non-linear material having an optical waveguide and a polarization inversion layer, one or more dielectric films formed on the substrate, and a grating formed on the dielectric film, and the grating. Is formed in the propagation direction of the optical waveguide, and the thickness T of the dielectric film is 0 <T
<0.1 μm wavelength converter. 2. A substrate made of a non-linear material having an optical waveguide and a polarization inversion layer, and a grating formed on the substrate, wherein the grating is formed in the propagation direction of the optical waveguide and A wavelength conversion element, wherein the gratings are adjacent to each other. 3. A substrate made of a non-linear material having an optical waveguide, a polarization inversion layer having a periodic structure formed in the substrate, and an electrode formed on a part or all of the polarization inversion layer, and The period Λ of the polarization inversion layer under the electrode in the propagation direction of the optical waveguide is λ = 2Nω · Λ / q with respect to the effective refractive index Nω of the fundamental light having the wavelength λ propagating in the optical waveguide.
A wavelength conversion element having a relationship of (q = 1, 2, 3, ...). 4. The wavelength conversion element according to claim 1, wherein the substrate is LiTa _(1-x) Nb _x O ₃ (0 ≦ x ≦ 1). 5. The wavelength conversion element according to claim 3, wherein the width Wa of the polarization inversion layer under the electrode is Wa ≠ Λ−Wa with respect to the period Λ of the polarization inversion layer. 6. A short-wavelength laser device comprising the wavelength conversion element according to claim 1, 2 or 3 and a semiconductor laser. 7. An electro-optical substrate having an optical waveguide, a domain-inverted layer having a periodic structure formed in the substrate, an electrode formed on the domain-inverted layer, and a semiconductor laser. A wavelength tunable laser device in which light emitted from the semiconductor laser is incident on an optical waveguide. 8. The wavelength tunable laser device according to claim 7, wherein the width Wa of the domain-inverted layer is Wa ≠ Λ−Wa with respect to the period Λ of the domain-inverted layer. 9. The wavelength tunable laser device according to claim 7, wherein the depth Da of the domain inversion layer is Da <Dw with respect to the depth Dw of the optical waveguide.