JP2006237530A - Light stimulation solid-state laser apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light stimulation solid-state laser apparatus stabilizing a laser output independently of a polarization state of the output light. <P>SOLUTION: The light stimulation solid-state laser apparatus 21 is provided with: a semiconductor laser 1 for emitting a stimulated light L1; a solid-state laser medium 2 stimulated by the stimulated light L1 to produce an oscillation light L2; an optical branch element 22 for partially extracting a harmonic L3 outputted from a nonlinear optical element 3; a light receiving element 9 for receiving the harmonic L2 extracted from the optical branch element 22 and converting the harmonic into an electric signal; and an output stabilizing circuit 10 for receiving an output of the light receiving element 9 to constantly control an output of the stimulated light L1. The optical branch element 22 comprises a partial reflection mirror for reflecting part of the output light L3 and transmitting through the rest, and a partial reflection film 24 with reflection factors nearly equal to each other at which a P polarization component and an S polarization component of the output light L3 are reflected. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optically pumped solid-state laser device having an APC (auto power control) function for controlling laser beam output to be constant.

  2. Description of the Related Art Conventionally, as a solid-state laser device, a wavelength conversion solid-state laser device that generates a laser beam having a wavelength multiplied by a fundamental wavelength using a nonlinear optical element has been used in various fields. In this solid-state laser device, an optically pumped solid-state laser device using a semiconductor laser as a pumping source for pumping a solid-state laser medium is widely used.

In order to improve the frequency conversion efficiency of laser light, this type of wavelength conversion type solid-state laser apparatus often employs an internal resonance type structure in which a solid-state laser medium and a nonlinear optical element are arranged between a pair of reflecting mirrors. ing. Crystal elements such as Nd: YAG and Nd: YVO ₄ are used for the solid-state laser medium, and crystal elements such as KTP (KTiOPO ₄ ) and BBO (β-BaB ₂ O ₄ ) are used for the nonlinear optical elements. .

  On the other hand, in the above-described wavelength conversion solid-state laser device, in order to keep the output after wavelength conversion constant, a partial reflection mirror that reflects a part of the output and transmits the remainder, and converts the reflected output into an electric signal Some devices use a light receiving element and an output stabilization circuit that receives an electric signal obtained from the light receiving element and controls a drive current of a semiconductor laser that is an excitation source.

  FIG. 5 shows a schematic configuration of a conventional wavelength conversion type solid-state laser device having an output stabilization function. This wavelength conversion type solid-state laser device 11 has an input side in which a semiconductor laser 1 as an excitation source, a solid-state laser medium 2, a nonlinear optical element 3, and one end face of the solid-state laser medium 2 are formed in a plane on the same axis. A reflecting mirror 4 and an output-side reflecting mirror 5 formed on one end face of the nonlinear optical element 3 are arranged. The input-side reflecting mirror 4 and the output-side reflecting mirror 5 constitute a laser resonator 6.

  The input-side reflecting mirror 4 serves to prevent reflection of the pumping light emitted from the semiconductor laser 1, that is, the pumping light L 1, and an oscillation laser as a fundamental wave obtained by making the pumping light L 1 incident on the solid-state laser medium 2. A mirror coat film that is highly reflective is formed on the light L2 and the harmonic laser light L3 wavelength-converted by the nonlinear optical element 3. On the other hand, the output-side reflecting mirror 5 is formed with a mirror coat film that is highly reflective to the oscillation laser light L2 as the fundamental wave and prevents reflection of the harmonic laser light L3.

  In this wavelength conversion solid-state laser device 11, the pumping light L 1 emitted from the semiconductor laser 1 enters the solid-state laser medium 2, and the input-side reflecting mirror 4 at the end face of the solid-state laser medium 2 and the output-side reflection at the end face of the nonlinear optical element 3. A laser resonator 6 formed with the mirror 5 obtains a fundamental oscillation laser beam L2. Then, the fundamental laser beam L2 passes through the nonlinear optical element 3 to convert the wavelength of the fundamental laser beam L2, and is output from the output-side reflecting mirror 5 as the harmonic laser beam L3.

  The harmonic laser beam L3 thus output passes through the filter 7 that is non-transmissive to the excitation light wavelength and the fundamental wave and transmissive to the harmonic component, and then partially outputs by the partial reflection mirror 8. LR enters the light receiving element 9, and the rest is output as transmitted light L3 ′. The partial reflection mirror 8 reflects a part of the output light L3 using surface reflection by general optical glass. The light receiving element 9 generates an electric signal E proportional to the output of the reflected light LR and outputs it to the output stabilization circuit 10. The output stabilization circuit 10 controls the drive current I supplied to the semiconductor laser 1 so that the electric signal E obtained by the light receiving element 9 is kept constant, and controls the laser light output L3 to be constant. (Contorol) function.

Prior art documents related to the invention of this application are shown below.
JP-A-10-93166 JP 2000-114633 A

  However, the conventional wavelength conversion solid-state laser device 11 has a problem that the output is likely to fluctuate due to the excitation light intensity and temperature change even if output stabilization control is applied.

  This is due to the change in the polarization state of the harmonic output, that is, the polarization ratio and the polarization due to the change in the refractive index of the solid-state laser medium 2 and the nonlinear optical element 3 due to the temperature and the excitation light intensity and the interference of the light output in the laser resonator 6. If the rotation angle fluctuates, the effective reflectivity of the partial reflection mirror 8 changes, and the output of the reflected light LR fluctuates. If the harmonic L3 has a constant light output, the output stabilization circuit This is because the incorrect output control is performed as determined in 10.

  More specifically, the fundamental oscillation laser beam L2 oscillated in the solid-state laser medium 2 oscillates as linearly polarized light. When this oscillation laser light L2 passes through the nonlinear optical element 3, it becomes elliptically polarized light, and when it returns to the solid-state laser medium 2, polarization rotation occurs. The degree of this rotation varies depending on the temperature of the solid-state laser medium 2, the temperature of the nonlinear optical element 3, the excitation input from the semiconductor laser 1 to the solid-state laser medium 2, and the like.

  In addition, harmonics generated in the laser resonator 6 are emitted toward both sides of the input-side reflecting mirror 4 and the output-side reflecting mirror 5. In this case, although the luminous flux is matched between the harmonic component reflected by the input-side reflecting mirror 4 and the harmonic component generated in the direction of the output-side reflecting mirror 5, the phase changes depending on the temperature of the nonlinear optical element 3. The harmonic state of the polarization is not constant. For this reason, in the output of the harmonic L3 incident on the partial reflection mirror 8, the ratio of the P-polarized component and the S-polarized component varies.

  As illustrated in the polarization reflection characteristic on the glass surface in FIG. 6, the reflectance of P-polarized light and S-polarized light is greatly different depending on the reflection angle. Accordingly, when the polarization ratio of the harmonic L3 changes, the effective reflectivity of the harmonic L3 at the partial reflection mirror 8 changes and the output of the reflected light LR fluctuates, so that the output is stabilized even if the actual harmonic output is constant. Since the output is determined to be changed by the circuit 10 and erroneous output stabilization control is performed, the output stabilization function of the harmonic L3 is consequently impaired.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an optically pumped solid-state laser device capable of stabilizing the laser output regardless of the polarization state of the output laser light.

  In solving the above problems, the present invention provides an excitation source that emits excitation light, a solid-state laser medium that generates excitation light by being excited by excitation light from the excitation source, and oscillation light that is output from the solid-state laser medium. A light branching element that partially extracts the light, a light receiving element that receives the oscillation light extracted by the light branching element and converts it into an electrical signal, and an output stabilization that receives the output of the light receiving element and controls the output of the excitation light to be constant In the optically pumped solid-state laser device provided with the optical circuit, the optical branching element is a partial reflection mirror that reflects part of the output light from the solid-state laser medium and transmits the rest, and the reflection surface of the partial reflection mirror includes An optical film that reflects the P-polarized component and the S-polarized component of the output light with substantially the same reflectance is formed.

  In the present invention, the optical film formed on the reflection surface of the partial reflection mirror reflects the P-polarized component and the S-polarized component of the output light with substantially the same reflectance. Regardless of the polarization state of the output light oscillated in, the intensity of the reflected light input to the light receiving element is kept constant so that the output stabilization control can be performed properly.

  As the partial reflection mirror, one having the front and back surfaces as a reflection surface is used, and the optical film can be formed on at least one surface thereof. Or while forming the said optical film in the surface of one side, the antireflection film which becomes reflection prevention with respect to output light can be formed in the surface of the other side.

  The configuration of the optical film is not particularly limited. For example, the optical film can be configured by a multilayer film in which a plurality of tantalum oxide films and silicon oxide films are alternately stacked.

  Further, the output light incident on the partial reflection mirror is not limited to the oscillation laser light of the solid-state laser medium, and may be a harmonic laser light obtained by converting the wavelength. In this case, a wavelength conversion element (nonlinear optical element) is provided on the output end side of the solid-state laser medium.

  As described above, according to the optically pumped solid-state laser device of the present invention, the intensity of the reflected light input to the light receiving element can be kept constant regardless of the polarization state of the output light. It is possible to obtain a stable laser beam output that is not affected by the excitation power.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows an embodiment of the present invention. In the present embodiment, an example in which the optically pumped solid-state laser device of the present invention is applied to a wavelength conversion solid-state laser device 21 will be described.

  The wavelength conversion solid-state laser device 21 of this embodiment includes a semiconductor laser 1 as an excitation source, a solid-state laser medium 2 that outputs fundamental laser light, a nonlinear optical element 3 as a wavelength conversion element, and a solid-state laser medium 2. The flat input side reflecting mirror 4 disposed on the light incident side end face and the flat output side reflecting mirror 5 disposed on the light emitting side end face of the nonlinear optical element 3 are provided.

  The semiconductor laser 1 is composed of an element that oscillates laser light having a predetermined wavelength when a voltage is applied, for example, a gallium-aluminum arsenic (GaAlAs) laser diode element. In the present embodiment, a laser element that oscillates laser light L1 having a wavelength of 808 nm is used as excitation light. The oscillation laser light L1 may be continuous light or pulsed light.

For example, Nd: YVO ₄ is used as the solid-state laser medium 2 and, for example, potassium phosphotitanate (KTiOPO ₄ ; KTP crystal) is used as the non-linear optical element 3. : YAG, other crystal materials such as BBO (β-BaB ₂ O ₄ ) can be used as the nonlinear optical element 3. The solid-state laser medium 2 and the nonlinear optical element 3 may be configured as a combination crystal integrated with each other by bonding or the like.

  The input-side reflecting mirror 4 and the output-side reflecting mirror 5 constitute a laser resonator 6 by sandwiching the solid-state laser medium 2 and the nonlinear optical element 3. The laser resonator 6 generates a fundamental laser beam L2 having a wavelength of 1064 nm by the solid-state laser medium 2 when the laser beam (excitation light L1) emitted from the semiconductor laser 1 is incident thereon. Green laser light L3 of the second harmonic (wavelength 532 nm) of the fundamental wave is generated and emitted to the outside.

  The input-side reflecting mirror 4 is formed with a mirror coat film that is highly reflective with respect to the fundamental wave L2 and the harmonic wave L3 and is antireflective (AR) with respect to the excitation light L1. On the other hand, the output-side reflecting mirror 5 is formed with a mirror coat film that is highly reflective for the fundamental wave L2 and prevents reflection of the harmonic wave L3.

  Further, in the wavelength conversion solid-state laser device 21 of the present embodiment, only the harmonic laser light L3 is transmitted to the emission end side of the laser resonator 6 without transmitting the excitation light L1 and the fundamental laser light L2. The optical filter 7 and the optical branching element 22 according to the present invention are arranged in order.

  The optical branching element 22 is a partial reflection mirror composed of an optical glass 23 having front and back surfaces as light reflecting surfaces, and a partial reflection film 24 formed on the front surface side (optical filter 7 side) of the optical glass 23. It is said that. The optical branching element 22 reflects a part (approximately 2 to 10%) of the harmonic laser beam L3 incident through the optical filter 7 by the partial reflection film 24 and transmits the rest.

  The partial reflection film 24 is composed of an optical film that reflects the P-polarized component and the S-polarized component of the harmonic output light L3 incident on the light branching element 22 with substantially the same reflectance. Specifically, the partial reflection film 24 is configured such that the difference in reflectance between the P-polarized component and the S-polarized component is within 0.5%.

A specific example of the configuration of the partial reflection film 24 is shown in FIG. The partial reflection film 24 has a multilayer structure in which a plurality of tantalum oxide (Ta ₂ O ₅ ) films 24 A and silicon oxide (SiO ₂ ) films 24 B are alternately stacked on the optical glass 23.

  The film thickness of the first tantalum oxide film 24A is 127.77 nm, the film thickness of the second silicon oxide film 24B is 161.59 nm, the film thickness of the third tantalum oxide film 24A is 173.73 nm, The thickness of the fourth-layer silicon oxide film 24B is 89.56 nm, and the incident angle of the harmonic output light L3 is about 24 ° with respect to the normal direction of the reflecting surface. The reflectance of the P-polarized component at (532 nm) is 6.9%, and the reflectance of the S-polarized component is 7.1%.

  The reflected light LR of the harmonic output light reflected by the light branching element 22 enters the light receiving element 9. The light receiving element 9 generates an electric signal E proportional to the output intensity of the reflected light LR, and supplies the generated electric signal E to an output stabilization circuit (APC circuit) 10. The output stabilization circuit 10 has an APC function for controlling the drive current I supplied to the semiconductor laser 1 and controlling the laser light output L3 constant so that the electric signal E obtained by the light receiving element 9 is kept constant. Do.

  In the wavelength conversion solid-state laser device 21 of the present embodiment configured as described above, the pumping light L1 emitted from the semiconductor laser 1 enters the solid-state laser medium 2, and the input-side reflecting mirror 4 and the output-side reflecting mirror. 5 is obtained by the laser resonator 6 formed by the step 5. Then, the fundamental laser beam L2 passes through the nonlinear optical element 3 to convert the wavelength of the fundamental laser beam L2, and is output from the output-side reflecting mirror 5 as the harmonic laser beam L3.

  The harmonic laser beam L3 output in this way passes through the optical filter 7 that is non-transparent to the excitation light L1 and the fundamental wave L2 and transmissive to the harmonic L3, and then is output by the optical branching element 22. The reflected light LR is incident on the light receiving element 9, and the rest is output as transmitted light L3 ′. The light receiving element 9 generates an electric signal E proportional to the output of the reflected light LR and outputs it to the output stabilization circuit 10. The output stabilization circuit 10 controls the drive current I supplied to the semiconductor laser 1 so that the electric signal E obtained by the light receiving element 9 is kept constant, thereby controlling the laser light output L3 to be constant.

  At this time, in the conventional wavelength conversion solid-state laser device 11 (FIG. 5), since the partial reflection mirror 8 is made of general optical glass (uncoated surface), if the polarization ratio of the harmonic L3 changes, There was a change in the output of the reflected light LR from the reflecting mirror 8.

That is, the output P of the harmonic L3 is represented by the sum of the output Pp of the P polarization component and the output Ps of the S polarization component.
P = Pp + Ps (1)
On the other hand, since the partial reflection mirror 8 has polarization dependency in reflection / transmission characteristics, assuming that the reflectance for the P-polarized component is Rp and the reflectance Rs for the S-polarized component, the output Pr of the reflected light LR is as follows: Become.
Pr = Rp · Pp + Rs · Ps (2)
Therefore, from the equations (1) and (2),
Pr / P = Rp (Pp / P) + Rs (Ps / P) (3)
Thus, the fluctuation of the polarization ratio of the harmonic output P directly causes the fluctuation of the reflected light output Pr.

On the other hand, in the wavelength conversion solid-state laser device 21 of the present embodiment, the P-polarized component and the S-polarized component of the harmonic output are reflected with substantially the same reflectance by the partial reflection film 24 of the optical branching element 22. Therefore, assuming that the reflectance for the P-polarized component and the reflectance for the S-polarized component are both Rps, the above equation (2) is
Pr = Rps · (Pp + Ps) (2) ′
It becomes. This
Pr / P = Rps {(Pp / P) + (Ps / P)} = Rps (3) ′
Thus, the reflected light output Pr is constant without being affected by the polarization ratio of the harmonic output P.

  Therefore, according to the present embodiment, the reflected light intensity Pr input to the light receiving element 9 can be kept constant regardless of the polarization state of the harmonic L3, so that the temperature and excitation power of the solid-state laser medium 2 etc. It is possible to obtain a stable harmonic output P that is not affected by.

  3A to 3C show other embodiments of the optical branching device 22 according to the present invention.

  The configuration shown in FIG. 3A shows an example in which the partial reflection film 24 having the same configuration is formed not only on the front surface side but also on the back surface side of the optical glass 23. Thereby, the reflected light which does not depend on the polarization state of the harmonic output reflected on the back surface side of the optical glass 23 is obtained, and the stability of the harmonic output control can be further enhanced.

  The configuration shown in FIG. 3B shows an example in which a partial reflection film 24 is formed on the front surface side of the optical glass 23 and an antireflection film 25 that prevents reflection of harmonic output is formed on the back surface side. 3C shows an example in which an antireflection film 25 is formed on the front side of the optical glass 23 and a partial reflection film 24 is formed on the back side. According to these configuration examples, since a part of the harmonic output is reflected only on the one side surface on which the partial reflection film 24 is formed, there is an influence due to mutual interference of light reflected on both sides. Without receiving, stable output control of harmonics can be realized.

  The embodiment of the present invention has been described above. Of course, the present invention is not limited to this, and various modifications can be made based on the technical idea of the present invention.

  For example, in the above embodiment, the example in which the present invention is applied to the wavelength conversion solid-state laser device that converts the wavelength of the laser light L2 oscillated in the solid-state laser medium 2 into the second harmonic by the nonlinear optical element 3 and outputs it is described. However, the present invention is not limited to the wavelength conversion type, and the present invention can also be applied to a solid-state laser device that directly uses the laser light L2 oscillated by the solid-state laser medium 2 as output light.

  As shown in FIG. 4, as the optical branching element 26, the partial reflection film 24 is formed on at least one surface of the optical filter 27 that is non-transmissive to the excitation light L1 and the fundamental wave L2 and transmissive to the harmonic L3. Even if the one formed with is used, the same effect as in the above-described embodiment can be obtained. In this case, it is more preferable to dispose the optical filter 7 having the same configuration as that of the above embodiment between the light branching element 26 and the light receiving element 9.

It is the schematic explaining the structure of the wavelength conversion solid-state laser apparatus 21 by embodiment of this invention. It is sectional drawing which shows one structural example of the partial reflection film 24 which comprises the light branching element 22 which concerns on this invention. FIG. 6 is a diagram illustrating another configuration example of the optical branching element 22. It is a schematic block diagram which shows the modification of embodiment of this invention. It is a schematic block diagram of the conventional wavelength conversion solid-state laser apparatus. It is a figure which shows the polarization reflectance characteristic in the glass surface.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser (excitation source), 2 ... Solid laser medium, 3 ... Nonlinear optical element (wavelength conversion element), 4 ... Input side reflecting mirror, 5 ... Output side reflecting mirror, 6 ... Laser resonator, 9 ... Light receiving element DESCRIPTION OF SYMBOLS 10 ... Output stabilization circuit, 21 ... Wavelength conversion solid-state laser device (light excitation solid-state laser device), 22 ... Optical branching element, 23 ... Optical glass, 24 ... Partial reflection film, 24A ... Tantalum oxide film, 24B ... Silicon oxide film 25: Antireflection film.

Claims (5)

An excitation source that emits excitation light;
A solid-state laser medium that generates oscillation light when excited by excitation light from the excitation source;
An optical branching element for partially extracting oscillation light output from the solid-state laser medium;
A light receiving element that receives the oscillation light extracted by the light branching element and converts it into an electrical signal;
In an optically pumped solid-state laser device comprising an output stabilizing circuit that receives the output of the light receiving element and controls the output of the pumping light to be constant,
The optical branching element is a partially reflecting mirror that reflects part of the output light from the solid-state laser medium and transmits the rest.
An optical film that reflects the P-polarized component and the S-polarized component of the output light with substantially the same reflectance is formed on the reflection surface of the partial reflection mirror. The optically pumped solid-state laser device according to claim 1, wherein the partial reflection mirror has front and back surfaces as the reflection surface, and the optical film is formed on at least one surface of the partial reflection mirror. The partial reflection mirror has front and back surfaces as the reflection surface, the optical film is formed on one surface, and an antireflection film that prevents reflection of the output light is formed on the other surface. The optically pumped solid-state laser device according to claim 1, wherein the optically pumped solid-state laser device is formed. The optically pumped solid-state laser device according to claim 1, wherein the optical film is a multilayer film in which a plurality of tantalum oxide films and silicon oxide films are alternately stacked. 2. The optical excitation according to claim 1, wherein a wavelength conversion element is provided on an output end side of the solid-state laser medium, and the output light wavelength-converted by the wavelength conversion element is incident on the optical branching element. Solid state laser device.

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