JPH0980497A - Laser light generating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase wavelength conversion light power by changing the polarization direction of an oscillation mode of a semiconductor laser as a basic wave light source. SOLUTION: A laser light generating device is provided with a light source 3 and a non-linear optical element, that is, a non-linear optical crystal element provided with a light waveguide 2, a first end surface 2a and a second end surface 2b. Then, the light source 3 consists of the semiconductor laser, and the propagation of the light waveguide 2 has light polarization dependency, and outgoing light from the semiconductor laser is made incident on the light waveguide 2 from the first end surface 2a, and the polarization of the light source 3 is converted to the orthogonal polarization. That is, the semiconductor laser as the basic wave light source is converted from TE mode oscillation to TM mode oscillation, and by such a manner, the overlap between the TM mode electric field distribution of the semiconductor laser and the same of the waveguide is improved, and connection efficiency of an incident light quantity of a wavelength conversion element to the waveguide is increased remarkably.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser light generator such as a wavelength converter.

[0002]

2. Description of the Related Art In recent years, in order to improve recording density in information recording such as optical recording or reproduction, for example, magneto-optical recording, a short wavelength light source is required as a light source for optical recording or reproduction. . As this short-wavelength light source, a laser light generation device using second harmonic generation (hereinafter referred to as SHG) is drawing attention. This SHG
Is to convert the light of the fundamental wave optical frequency ω into the second harmonic frequency 2ω and shorten the wavelength of the laser light.

As a method of generating this short wavelength light, for example, there is a wavelength conversion element that guides the semiconductor laser light as a fundamental wave by a non-linear optical element and extracts the second harmonic laser light.

[0004]

There is a waveguide type wavelength conversion element as a wavelength conversion element for extracting the second harmonic laser light by the above-mentioned nonlinear optical element.

For example, a semiconductor laser is used as the fundamental wave light source for the wavelength conversion, and the waveguide type wavelength conversion element is, for example, lithium niobate (LiNbO ₃ ) (hereinafter referred to as LNb) which is a non-linear optical element in which a proton exchange waveguide is formed. Is used).

As this waveguide type wavelength conversion element, as shown in FIG. 6, a non-linear optical element 1 using a so-called Z plate LN whose plate surface direction is orthogonal to the Z axis is used, and its X axis direction is set. When the optical waveguide 2 is formed by, for example, a proton exchange waveguide in the waveguide direction, the polarization direction is only the TM mode having an electric field component in the Z direction, as schematically indicated by an arrow in FIG. Is transmitted, TE
The mode is not propagated and the TM mode field distribution is T
It has an elliptical shape that is wide in a direction perpendicular to the electric field direction of the M mode, that is, in the Y direction.

On the other hand, as shown in FIG. 7, the semiconductor laser 3 used as a fundamental wave light source normally oscillates in the TE mode, as shown by the arrow schematically showing the polarization direction thereof,
The field distribution has an elliptical shape that largely spreads in parallel with the electric field direction of the TE mode.

Here, for example, as shown in FIG. 8, when a semiconductor laser 3 as a fundamental wave light source and a Z-plate LN nonlinear optical element 1 in which an optical waveguide 3 as a wavelength conversion element is formed are integrated, The light is guided by bringing the emitting point of the laser 1 close to the end face of the optical waveguide 2 (butt coupling).
At this time, since the polarization directions of the two are orthogonal to each other, they are positioned perpendicular to each other so as to match them, and are coupled.

Since the amount of guided light at this time is substantially determined by the degree of overlap of the mode sizes of both waveguides, the coupling efficiency of both waveguides is not so good in this case.

Therefore, when the input power from the semiconductor laser, which becomes the fundamental wave at the time of wavelength conversion, into the waveguide is reduced, the wavelength-converted light power generated is also small.

In view of the above situation, the present invention makes it possible to increase the coupling efficiency of the amount of incident light between the semiconductor laser as the fundamental wave light source and the waveguide of the wavelength conversion element.

[0012]

That is, in the present invention, the coupling efficiency between the semiconductor laser and the optical waveguide can be improved by changing the polarization direction of the oscillation mode of the semiconductor laser as the fundamental wave light source. By doing so, the wavelength converted light power can be increased.

A laser light generator according to the present invention comprises a light source and a nonlinear optical element having an optical waveguide and having a first end surface and a second end surface, that is, a nonlinear optical crystal element. , The light source is made of a semiconductor laser, the propagation of the optical waveguide has optical polarization dependency, and the light emitted from the semiconductor laser is incident on the optical waveguide from its first end face, and the polarization of the light source is orthogonal. It is to be converted into polarized wave.

In other words, in the above-described structure of the present invention, the semiconductor laser as the fundamental wave light source changes from the TE mode oscillation to the T mode oscillation.
This can be changed to M mode oscillation, and by doing so, the TM mode electric field distribution of the semiconductor laser and that of the waveguide can be well overlapped, and the coupling efficiency of the amount of light incident on the waveguide of the wavelength conversion element can be greatly increased. You can do it.

Further, as the coupling efficiency increases, the basic waveguide power of the waveguide of the wavelength conversion element also increases, so that the wavelength conversion light power also increases.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described. Prior to this explanation, in order to facilitate understanding of the present invention, the principle of semiconductor laser oscillation from TE mode oscillation to TM mode oscillation will be described with reference to FIG. Will be described with reference to.

Now, for example, a Z plate L is used as the nonlinear optical element 1.
An N substrate is used, and X is used as the optical polarization dependent waveguide 2, for example.
Consider the case of a directional propagation proton exchange waveguide.

The optical waveguide 2 formed by this proton exchange is Z
In addition to the optical waveguide forming portion on the surface of the plate LN substrate, a mask made of, for example, a Ta film is adhered, and the Z plate LN substrate is immersed in high temperature pyrophosphoric acid to form an optical waveguide forming portion on which the mask is not adhered. , The L in the LN is selectively
LN by exchanging i-ions and H-ions in phosphoric acid
A three-dimensional waveguide is formed by forming a portion having a higher refractive index than the substrate.

This Z-plate LN substrate, that is, the optical waveguide 2 formed by a proton exchange in the non-linear optical element has light confinement only in the Z-axis direction, whereby only the TM mode having an electric field in the Z-direction can be excited. , TE mode is not propagated.

If butt coupling is performed so that the TE-mode emitted light from the semiconductor laser is incident on the optical waveguide 2 by the proton exchange of the nonlinear optical element 1, the electric fields of the propagation modes are orthogonal to each other. In the coupling mode as shown in FIG. 8, the electric field field distributions of both guided modes are not so well overlapped for the above-mentioned reason, and the coupling efficiency is low.

Therefore, as shown in FIG. 1, the semiconductor laser is positioned so that the TE mode polarization direction of the semiconductor laser and the TM mode polarization direction of the waveguide are perpendicular to each other.

In this way, the TE-mode emission light emitted from the semiconductor laser 3 is emitted from the LN nonlinear optical element 1.
Since the light is not guided in the waveguide 2 due to the proton exchange, the incidence on this is blocked.

In the semiconductor laser 3, the TE mode oscillates predominantly because the TE mode has a slight gain difference such as a propagation loss difference in the semiconductor laser as compared with the TM mode.

However, in the semiconductor laser, the gain of the mode can be obtained by returning the emitted light into the laser again. Therefore, by returning the TM mode of the semiconductor laser to the semiconductor laser, the TM mode is higher than the TE mode. If the gain is obtained, the semiconductor laser can oscillate in the TM mode.

Therefore, in FIG. 1, the end face (second end) of the optical waveguide 2 of the non-linear optical element 1 opposite to the end face (referred to as the first end face) 2a on the side to which the semiconductor laser 3 is optically coupled is called the second end. 2b, which is referred to as an end surface of the semiconductor laser 3, is coated with a highly reflective film such as a dielectric multilayer film so that light is emitted from the semiconductor laser 3
If the optical waveguide 2 is fed back to the side, the optical waveguide 2 does not guide the TE mode of the semiconductor laser 3 but guides the TM mode, so that its propagation has optical polarization dependence. Therefore, at the first end face 2b. The TM mode is fed back to the semiconductor laser 3 by the high reflection film, whereby T
Since the M mode can obtain more gain than the TE mode, the TM mode oscillation is performed.

When the semiconductor laser 3 oscillates TM,
Since the electric field field distribution of the TM mode has almost the same shape as the TE mode, the TM mode field distribution of the semiconductor laser 3 and the T of the optical waveguide 2 in the butt coupling.
The overlap of the M-mode field distribution becomes good, and the amount of light incident on the optical waveguide 2 increases considerably.

Next, an embodiment of the present invention will be described. FIG. 2 is a schematic plan view showing the basic structure of an example of the present invention. As shown in FIG. 2, for example, the present invention has a light source, that is, a semiconductor laser 3 and an optical waveguide 2, and a first end face 2
a and a second end face 2b, such as LiTa _x N
b _1-x O ₃ (0 ≦ x ≦ 1).

As described above, the optical waveguide 2 has the Li ion implantation and H
An optical waveguide having an optical polarization dependence on propagation is constituted by an optical waveguide by proton exchange in which ions are exchanged to increase the refractive index or an annealed proton exchange waveguide subjected to high temperature annealing after proton exchange.

Further, the nonlinear optical element 1 is provided with a wavelength converter 4. The wavelength conversion unit 4 is composed of an optical harmonic generating element such as SHG. This wavelength converter 4
Can be, for example, an SHG in which a quasi phase matching structure (QPM) such as a periodically poled structure is formed, or a Cherenkov radiation type SHG structure.

Then, the emitted light which becomes the fundamental wave from the semiconductor laser 3 as the fundamental wave light source is made incident on the optical waveguide 2 from the first end face 2a. At this time, the nonlinear optical element 1
The positional relationship between the semiconductor laser 3 and
It is selected so that the M mode can be guided in the optical waveguide 2.

On the first end face 2a of the optical waveguide 2 on the laser light incident side from the semiconductor laser 3, a low reflection film 11 for the wavelength ω of the fundamental wave, that is, the laser light is deposited and formed.
The second end surface 2b on the opposite side has a high reflectance with respect to the fundamental wave ω, and has a low reflectance with respect to the wavelength converted wavelength, for example, the second harmonic 2ω, in comparison with this. 12
Is formed.

At this time, as described above regarding the principle of the present invention, TM mode oscillation is performed in the semiconductor laser by the TM mode feedback to the semiconductor laser, and the fundamental wave of the TM mode is converted into the wavelength conversion unit 4. A harmonic wave is generated by guiding the wave to. The higher harmonics wavelength-converted by the wavelength conversion unit 4, that is, the higher harmonics generated by the wavelength conversion unit 4, have the output end face coated with the reflection film 12 exhibiting low reflection with respect to the wavelength conversion light described above, that is, the second harmonic. The light is directly emitted from the end face 2b.

At this time, since the semiconductor laser 3 oscillates TM, the incidence efficiency of the LN nonlinear optical element 1 on the optical waveguide 3 is increased and the fundamental wave guided power is increased and the harmonic generation power is increased for the reason described above. To do.

In the case where the harmonic generation efficiency of the wavelength conversion unit 4 has a fundamental wavelength dependence, for example, a quasi phase matching structure SHG, the fundamental wavelength is controlled in order to generate the harmonic with high efficiency, that is, the semiconductor laser. It is necessary to control the wavelength of the oscillated TM mode.

A semiconductor laser 3 is shown in FIG.
An embodiment is shown in which M mode oscillation is performed and the TM mode oscillation wavelength is also controlled.

In the embodiment shown in FIG. 3, the highly reflective film 1 for the fundamental wave on the output end face 2a of the optical waveguide 2 shown in FIG.
In place of 2, the low reflection film 11 is provided with the wavelength selective reflection element 5 capable of reflecting the fundamental wavelength with high efficiency, for example, a diffraction grating.

By adopting this structure, the semiconductor laser 3
By feeding back only the specific wavelength of the TM mode to the semiconductor laser 3, the semiconductor laser 3 oscillates in the wavelength-controlled TM mode, and higher harmonics can be generated with higher efficiency.

In the embodiment shown in FIG. 4, the optical waveguide 2
The wavelength selective reflection element 5 such as a diffraction grating is provided on the semiconductor laser 3 side, and the wavelength conversion unit 4 is provided on the optical waveguide 3 on the opposite side. Of a semiconductor laser 3 whose wavelength is controlled by reflecting light
This is a case where a harmonic is generated by oscillating in a mode and guiding the rest to the wavelength conversion unit 4.

In the embodiment shown in FIG. 5, the wavelength selective transmission element 6 is provided on the optical waveguide 2 of the nonlinear optical element 1 between the semiconductor laser 3 and the wavelength conversion section 4.

Also in this example, if the central transmission wavelength of the wavelength selective transmission element 6 is set to the fundamental wave wavelength for generating a harmonic with high efficiency, the semiconductor laser 3 is oscillated in the wavelength-controlled TM mode also in this example. It is possible to generate harmonics with high efficiency.

In each of the illustrated examples, the nonlinear optical element 1
The optical coupling between the optical waveguide 2 and the semiconductor laser 3 is a direct optical coupling between the emission point of the semiconductor laser 3 and the first end face 2a of the optical waveguide without interposing a lens. The optical coupling may be performed via a lens.

According to the configuration of each of the above-described embodiments of the present invention, the semiconductor laser of the light source is changed from the TE mode oscillation to the TM mode oscillation, whereby the TM mode electric field distribution of the semiconductor laser and that of the optical waveguide are well overlapped. This makes it possible to significantly increase the coupling efficiency of the amount of incident light to the optical waveguide of the wavelength conversion element. Also, since the fundamental wave guided power of the optical waveguide of the wavelength conversion element increases as the coupling efficiency increases, the wavelength converted optical power can be increased.

The non-linear optical crystal forming the non-linear optical element 2 is not limited to LN, and may be KTP, for example.
The present invention is not limited to the above-mentioned examples, such as the configuration of (KTiOPO ₄ ), and various modifications and changes can be made.

[0044]

As described above, according to the present invention, the wavelength of the semiconductor laser light of the fundamental wave can be extracted as the wavelength-converted high-output harmonic light, for example, the second harmonic laser light. Therefore, it can be used as a light source device that requires various short-wavelength laser beams, and for example, can be used as a light source for optical recording or reproduction in which high-density recording is performed, and reliable optical recording or reproduction operation can be performed. , Its industrial profit is enormous.

[Brief description of drawings]

FIG. 1 is a schematic perspective view showing a configuration of an example of a laser light generator according to the present invention.

FIG. 2 is a schematic plan view showing the configuration of an example of a laser light generator according to the present invention.

FIG. 3 is a schematic plan view showing the configuration of an example of a laser light generator according to the present invention.

FIG. 4 is a schematic plan view showing the configuration of an example of a laser light generator according to the present invention.

FIG. 5 is a schematic plan view showing a configuration of an example of a laser light generator according to the present invention.

FIG. 6 is an explanatory diagram of a mode electric field distribution of an optical waveguide used for explaining the present invention.

FIG. 7 is an explanatory diagram of a mode electric field distribution of a semiconductor laser used for explaining the present invention.

FIG. 8 is a configuration diagram of a conventional butt coupling.

[Explanation of symbols]

 1 Nonlinear Optical Element 2 Optical Waveguide 3 Semiconductor Laser (Light Source) 4 Wavelength Converter 5 Wavelength Selective Reflector 6 Wavelength Selective Transmitter 11 Low Reflection Film 12 Reflective Film (High Reflection of Fundamental Wave-Low Wavelength Converted Light)

Claims (13)

[Claims] 1. A laser light generator comprising a light source and a nonlinear optical element having an optical waveguide and having a first end surface and a second end surface, wherein the light source is a semiconductor laser and Propagation of the waveguide has optical polarization dependency, and light emitted from the light source is incident on the optical waveguide from the first end face to convert the polarized light of the light source into orthogonal polarized waves. Laser light generator. 2. The laser light generator according to claim 1, further comprising a wavelength converter. 3. The laser light generator according to claim 1, further comprising a wavelength selective reflection element. 4. The laser light generator according to claim 3, wherein the wavelength selective reflection element is a diffraction grating. 5. The laser light emitting device according to claim 1, further comprising a wavelength selective transmission element. 6. The non-reflective film is provided on the first end face, and the second end face is provided with a film having high reflection with respect to the wavelength of the light source. Laser light generator. 7. The laser beam generator according to claim 3, wherein the first and second end surfaces are provided with a non-reflection film. 8. The light emitted from the light source is incident on the optical waveguide from the first end face by direct optical coupling by bringing the emission point of the semiconductor laser and the end face of the optical waveguide close to each other. The laser light generator according to claim 1, wherein 9. The laser light generation according to claim 1, wherein the light emitted from the light source is incident on the optical waveguide from the first end face by optical coupling through a lens. apparatus. 10. The laser light generator according to claim 1, wherein the optical waveguide is a proton exchange optical waveguide. 11. The nonlinear optical element is LiTa _x Nb.
The laser light generator according to claim 1, wherein _{1-xO 3} (0 ≦ x ≦ 1). 12. The laser light generator according to claim 2, wherein the wavelength converter is an optical harmonic generator. 13. The laser light generator according to claim 2, wherein the wavelength conversion unit is a wavelength conversion element having a quasi phase matching structure.