JPH1093165A - Semiconductor laser for exciting solid-state laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To precisely stabilize output despite the change of a temperature by arranging optical parts arranged in an optical path from a solid-state laser resonator to a photodetector, in such a way that they are not parallel to the light-receiving face of the photodetector on an optical axis. SOLUTION: A second harmonic 19, emitted from a resonator, is made incident on an APC part fitted to a holding member 22. The holding member 22 has an end face 22a inclined by 5 deg. with respect to a resonator axis, an end face 22b parallel to the resonator axis and an end face 22c making an angle of 45 deg. with respect to the resonator axis. A filter 23 absorbing the solid laser beam 18 and allowing the second harmonic 19 pass through is fitted to the end face 22a. A partial reflecting mirror 24, allowing a part of the second harmonic 19 pass through and reflecting the remainder on the side of the end face 22c, is fitted to the end face 22c, and a photodetector for APC 25 for detecting the reflected second harmonic 19 is fitted to the end face 22b. The photodetector 25 is inclined by 5 deg. with respect to the optical axis and the photoelectric face 25a of the photodetector 25 is not parallel to the filter 23 on the optical axis.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser-pumped solid-state laser, and more particularly to a semiconductor laser-pumped solid-state laser with improved output stabilization control.

[0002]

2. Description of the Related Art As shown in, for example, Japanese Patent Application Laid-Open No. Sho 62-189783, a solid-state laser in which a laser crystal doped with neodymium (Nd) is excited by light emitted from a semiconductor laser is known.

In this semiconductor laser pumped solid-state laser, in order to obtain a laser beam having a shorter wavelength, an optical wavelength conversion element made of a non-linear optical material is arranged in the resonator, and the solid-state laser beam is converted into a second harmonic. Wavelength conversion is widely performed.

On the other hand, in many cases, there is a demand for the above-mentioned semiconductor laser pumped solid-state laser to keep the output of the converted wavelength wave constant. Therefore, conventionally, as shown in, for example, Japanese Patent Application Laid-Open No. H7-154014, a light branching unit such as a partial reflection mirror that transmits a part of a laser beam and reflects the remainder, and a light that detects the amount of the branched laser beam A detector for controlling the drive current of the semiconductor laser so as to receive the output of the photodetector and to stabilize the output
There has been proposed a semiconductor laser-excited solid-state laser in which a PC (Automatic Power Control) circuit is provided to stabilize the output.

When the wavelength of the solid-state laser beam is converted as described above, the laser beam after the wavelength conversion is incident on the optical branching means, and the output of the laser beam after the wavelength conversion is stabilized. Be transformed into

[0006]

However, in this conventional semiconductor laser-pumped solid-state laser, there is a problem that even if the output stabilization control is performed, the output tends to fluctuate when there is a temperature change. .

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a semiconductor laser-pumped solid-state laser capable of stabilizing the output with high accuracy even when there is a temperature change.

[0008]

A semiconductor laser-pumped solid-state laser according to the present invention has a basic structure for exciting a laser crystal with excitation light emitted from a semiconductor laser, as well as a means for branching a part of the laser beam described above. A photodetector for detecting the amount of laser beam, and an AP
In a semiconductor laser-pumped solid-state laser that stabilizes the output of a laser beam by providing a C circuit, an optical component arranged in an optical path from the solid-state laser resonator to the photodetector is positioned with respect to a light-receiving surface of the photodetector. It is characterized by being arranged non-parallel on the optical axis.

As the above-mentioned optical component, specifically, a filter or the like that selectively allows only light in a desired wavelength range to pass is exemplified.

[0010] It is preferable that the light receiving surface of the photodetector is arranged to be inclined with respect to the traveling direction of the laser beam.

The photodetector further includes a light beam that directly enters the light receiving surface and a light beam that is reflected by the window glass of the detector after being reflected by the light receiving surface and re-incident on the light receiving surface. It is desirable to dispose them at least twice the diameter (1 / e ² diameter).

[0012]

According to the study of the present inventor, the problem in the conventional device described above is that the amount of light received by the photodetector becomes unstable due to light interference between optical components. Do you get it. In particular, if the light receiving surface (photoelectric surface) of the photodetector with high reflectivity is parallel to other optical surfaces (that is, the light entrance / exit surface of the filter or the window glass of the photodetector), the light is affected by interference. The amount of light received by the detector becomes extremely unstable.

For example, when the ambient temperature changes, even if the temperature of the resonator portion is adjusted to some extent, the APC unit located far from the temperature detection sensor has a temperature of 0.1 mm. Temperature changes by several degrees Celsius. When such a temperature change occurs, the phase of the interference changes, and the amount of light received by the APC photodetector fluctuates even if the laser output is constant. Then, the drive current of the semiconductor laser is increased or decreased accordingly, and the output of the semiconductor laser pumped solid-state laser fluctuates.

If the set temperature of the temperature control is changed in accordance with a long-term change of the resonator, the temperature of the APC unit changes directly. The output of the solid-state laser fluctuates.

On the other hand, in the semiconductor laser pumped solid-state laser of the present invention, the solid-state laser
Since the optical components arranged in the optical path leading to the photodetector for C are arranged non-parallel on the optical axis with respect to the light receiving surface of the photodetector, the light receiving surface of the photodetector and the light Light interference with the passing surface can be prevented.

In such a case, the phase change of the interference due to the temperature change disappears.
The variation in the amount of light received by the photodetector can be prevented, and the output of the solid-state laser excited by the semiconductor laser can be stabilized with high accuracy.

Further, if the light receiving surface of the APC photodetector is arranged at an angle to the traveling direction of the laser beam,
Light interference with a light-passing window glass normally arranged in front of the light receiving surface can also be prevented, whereby the output of the semiconductor laser-excited solid-state laser can be stabilized more accurately.

The APC photodetector separates the light directly incident on the light receiving surface and the light reflected on the light receiving surface after being reflected on the light receiving surface and re-incident on the light receiving surface. If arranged so as to be displaced from each other by at least twice the beam diameter (1 / e ² diameter), light interference between the light receiving surface of the APC photodetector and the light-passing window glass as described above. Is reliably obtained.

[0019]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a side view of a semiconductor laser-excited solid-state laser according to one embodiment of the present invention.

This semiconductor laser pumped solid-state laser
A semiconductor laser 11 that emits a laser beam 10 as excitation light, a condenser lens 12 that collects a laser beam 10 that is divergent light, and a YVO ₄ crystal (Nd: Nd: a solid laser medium doped with neodymium (Nd)) YVO ₄ crystal)
13 and a resonator mirror 14 arranged in front of the Nd: YVO ₄ crystal 13, that is, on the side opposite to the semiconductor laser 11.

Nd: YVO ₄ crystal 13 and resonator mirror
Between the Nd: YVO ₄ crystal 13 side and Mg, which is a nonlinear optical material having a periodic domain inversion structure,
O: a LiNbO ₃ crystal (hereinafter referred to as an inverted domain LN crystal) 15, a calcite crystal 17, and an etalon 16 tilted 45 ′ with respect to the optical axis are provided.

The semiconductor laser 11 has an active layer width of about 50 μm.
And emits a laser beam 10 having a center wavelength of 809 nm. The linear polarization direction of the laser beam 10 is set in the β direction in the figure.

The condensing lens 12 is, for example, a gradient index lens (trade name: Selfoc lens), and is fixed to the holding member 20 with one side subjected to spherical processing facing the Nd: YVO ₄ crystal 13. Have been. The Nd: YVO ₄ crystal 13 is formed to a thickness of 1 mm, and the condenser lens 12 is placed at a position of 0.3 ± 0.1 mm in the thickness direction from the incident end face of the crystal 13 at a magnification of 0.8 to 1.0 times. The position has been adjusted to converge 10.

The condenser lens 12 and the semiconductor laser 11 are
After being adjusted so that the light-converging point is located at a predetermined position in the α and β directions in the figure, it is fixed to the holding member 20. Note that the portion of the holding member 20 to which the semiconductor laser 11 and the condenser lens 12 are attached is hereinafter referred to as an excitation unit.

The Nd: YVO ₄ crystal 13 emits light having a wavelength of 1064 nm when neodymium ions are excited by the incident laser beam 10. On the incident end face 13a of the Nd: YVO ₄ crystal 13, light having a wavelength of 1064 nm is reflected well (reflectance of 99.9% or more), while the excitation laser beam 10 having a wavelength of 809 nm is transmitted well (transmittance of 93%).
Above) A coat has been applied. On the other hand, the mirror surface 14a of the resonator mirror 14 is provided with a coating that reflects light having a wavelength of 1064 nm well (reflectance of 99.9% or more) and transmits light of the following wavelength 532 nm (transmittance of 90% or more). ing.

Therefore, the above-mentioned light having a wavelength of 1064 nm is a highly reflecting surface against the end face of the Nd: YVO ₄ crystal.
The laser beam is confined between the mirror 13a and the mirror surface 14a to cause laser oscillation, and a laser beam 18 having a wavelength of 1064 nm is generated. This laser beam 18 as a fundamental wave has a wavelength of 1/2,
is converted to a second harmonic 19 of nm, and the second harmonic 19 mainly exits from the resonator mirror 14.

In this example, the Nd: YVO ₄ crystal 13
A-axis cut is used and its thickness is 1m
m. The length of the inverted domain LN crystal 15 is 2 m.
m, the radius of curvature of the resonator mirror 14 is 50 mm, Nd forming a resonator: distance between the YVO ₄ crystal end face 13a and the mirror surface 14a is about 11 mm. On the other hand, the end face 13b of the Nd: YVO ₄ crystal 13 on the side of the resonator mirror 14 has a second wavelength of 532 nm.
Harmonic 19 is reflected well (95% or more reflectance), and the wavelength
The 1064 nm laser beam 18 is coated so as to transmit well (transmittance of 99.8% or more), so that the Nd: YVO ₄ crystal 13 is generated in the inverted domain LN crystal 15.
The second harmonic 19 that has propagated to the side is reflected toward the resonator mirror 14 so that a high second harmonic output can be obtained.

As shown in FIG. 2, the crystal axes of the Nd: YVO ₄ crystal 13 and the inverted domain LN crystal 15 are arranged such that the c axis is in the β direction. The period of the domain inversion portion of the inversion domain LN crystal 15 is about 7.0 μm, and the phase is matched at a temperature at which the center wavelength of the laser beam 10 emitted from the semiconductor laser 11 becomes 809 nm.

The calcite crystal 17 is a parallel plate having a thickness of about 0.8 mm and is cut in a direction of 46.7 ° from the c-axis to the a-axis. Therefore, a light input / output surface forming a parallel plane has a
There is an axis and an axis including c and a. This calcite crystal
Due to the birefringence of 17, the optical path varies depending on the polarization of the laser beam 18 having a wavelength of 1064 nm in the crystal 17, so that only the desired polarization can be oscillated by adjusting the position of the resonator mirror 14.

In this example, as shown in FIG. 2, the a-axis of the calcite crystal 17 is aligned substantially in the β direction, and light having a wavelength of 1064 nm polarized in the β direction is oscillated. The light entrance / exit surface of the calcite crystal 17 has a wavelength of 1064 nm and 532 nm.
A coat that gives a reflectance of 14.5% for m light is applied. FIG. 2 also shows the polarization direction of each light.

On the other hand, as described above, the inverted domain LN crystal
15 is arranged so that the c-axis is in the β direction, so that the wavelength 532 n polarized in the β direction using the nonlinear optical constant d _33.
The second harmonic 19 of m is generated.

Etalon 16 for realizing single longitudinal mode oscillation
Is made of a synthetic quartz plate having a thickness of 0.3 mm, and its light entrance / exit surface has a reflectivity of 28% for a light having a wavelength of 1064 nm and a wavelength of 532 nm.
An AR coating having a reflectance of 0.5% or less with respect to light of nm is provided.

The Nd: YVO ₄ crystal 13, resonator mirror 14, inverted domain LN crystal 15, etalon 16 and calcite crystal 17 described above are mounted on the holding member 21. Hereinafter, the portion of the holding member 21 is referred to as a resonator portion.

The second harmonic 19 emitted from the resonator section
Is incident on the APC section composed of the components attached to the holding member 22. The holding member 22 has an end face 22a inclined at 5 ° to the resonator axis, an end face 22b parallel to the resonator axis, and a resonator axis, as shown in FIGS. And an end face 22c at an angle of 45 ° with respect to the three end faces 22a, 22b and 22c which are bent at right angles.
A light passing hole 22d is formed.

A filter 23 for absorbing the excitation laser beam 10 and the solid-state laser beam 18 and passing the second harmonic 19 is attached to the end face 22a facing the resonator section. Further, a partial reflection mirror 24 that allows a part of the second harmonic 19 to pass therethrough and reflects the remainder toward the end face 22b is attached to the oblique end face 22c. And the end face
22b includes an AP for detecting the reflected second harmonic 19 described above.
A photodetector 25 for C is attached.

The filter 23 has a transmittance of 0.01 to 0.001 for the excitation laser beam 10 having a large light amount.
% Is used. In addition, on the light input / output surface,
A that has a reflectance of 0.5% or less for light having a wavelength of 532 nm
R coat is applied. And this filter 23
By being attached to the end face 22a, the laser beam 18 and the second harmonic wave 19 are inclined by 5 ° with respect to the traveling direction, so that these lights 18 and 19 reflected on the back surface of the filter return to the resonator. As a result, unstable operation of the resonator is prevented.

In order to obtain this effect remarkably, the filter 23 reflects the laser beam 18 and the second harmonic 19 which have been reflected there and returned to the resonator at all positions within the resonator. It is desirable to dispose it at least twice the beam diameter (1 / e ² diameter).

On the other hand, a partial reflection mirror as a light branching means
Reference numeral 24 denotes a flat plate made of synthetic quartz whose light entrance and exit surfaces are substantially parallel to each other, and is fixed to the end surface 22c, so that the optical axis (γ
Axis) at 45 °. The second harmonic 19 polarized in the β direction is applied to the surface 24 of the partial reflection mirror 24.
The light enters a as s-polarized light. The surface (A surface) 24a and the back surface (B surface) 24b of the partial reflection mirror 24 have a reflectance of 12.5% with respect to the second harmonic 19 incident as s-polarized light, respectively.
An AR coat of 0.5% or less is applied. The second harmonic 19 reflected on the mirror surface 24a enters the APC photodetector 25.

As the APC photodetector 25, a photodetector having sensitivity to visible light, such as a Si photodiode or a SiPIN photodiode, can be appropriately selected and used.
It is desirable to use one having relatively low sensitivity to the excitation laser beam 10 and the solid-state laser beam 18. The light detection signal S of the light detector 25 is the same as the APC described above, that is, the second signal.
This is used for the control for stabilizing the output of the harmonic 19, which will be described later.

The second harmonic 19 transmitted through the partial reflection mirror 24
After passing through the opening 33a of the opening plate 33 as shown in FIG. 1, the light exits out of the package through the exit window 34a of the package cap 34. The diameter of the opening 33a is the second harmonic.
For example, the beam diameter (1 / e ² diameter) of 19 is about 1.5 to 2.5 times, and by passing the second harmonic 19 therethrough, stray light generated in the resonator section and the APC section is cut.

The window glass 35 attached to the exit window 34a of the package cap 34 has a parallelism of 5 to 30 'on the front and back surfaces in order to prevent the lowering of the second harmonic output due to interference by multiple reflections on the front and back surfaces. There is a degree. The front and back surfaces are coated so that the reflectance for the second harmonic 19 is 0.5% or less.

Exciting section, resonator section and AP described above
The holding members 20, 21 and 22 holding the portion C are adhered on a plate-shaped base plate 30, and are adhered and fixed to the package base 32 via the base plate 30 and the Peltier element 31.

The temperature in the resonator is detected by a thermistor 36 attached to the resonator, and the current of the Peltier element 31 is adjusted by a temperature control circuit (not shown) so that the detected temperature becomes a predetermined temperature. The temperature in the resonator is maintained at a predetermined temperature.

Here, it is desirable that the temperature of each element on the base plate 30 is uniform, so that the holding members 20, 21,
Preferably, the base plate 22 and the base plate 30 are formed of a material having a high thermal conductivity, such as aluminum, copper, or an alloy thereof. In addition, it is preferable that the fixing of each part is made of a material having a high thermal conductivity such as a heat conductive adhesive or solder. However, even with a general adhesive such as an epoxy adhesive, the adhesive layer is thinned to about 10 μm or less. If it does so, it can be used because the thermal resistance is reduced.

In this embodiment, after all the components have been mounted, the package base 32 and the package cap 34, to which the electric components have been wired, are seam-welded in a dry nitrogen atmosphere.

Next, the above-mentioned APC will be described in detail. As shown in FIG. 3, the light detection signal S of the light detector 25
Is input to the APC circuit 26. The APC circuit 26, for example, compares the light detection signal S with a predetermined reference signal. If the former exceeds the latter, the driving current I of the semiconductor laser 11 is reduced, and if the latter is the opposite, the driving current I is increased. As a result, the output of the second harmonic 19 is kept at a predetermined constant value.

In this apparatus, as shown in an exaggerated manner in FIG. 4, the photodetector 25 is disposed at an angle of 5 ° with respect to the optical axis. It is different from the direction of inclination with respect to the optical axis of 23. Therefore, the photocathode 25a of the photodetector 25 and the filter 23 are not parallel to each other on the optical axis. Note that "non-parallel to each other on the optical axis" means that the directions of the two with respect to the optical axis do not match. That is, when the optical axis is broken, it means that both are mechanically non-parallel in a state where the optical axis is developed so as to be in a straight line.

If the photocathode 25a and the filter 23 are non-parallel on the optical axis as described above, the first portion between the photocathode 25a and the filter 23 or the portion between the window glass 25b of the photodetector 25 and the filter 23 will be described. The interference of the second harmonic 19 can be prevented. If this is the case, the phase change of the interference due to the temperature change is eliminated, so that it is possible to prevent the fluctuation in the amount of light received by the photodetector 25 due to the phase change of the interference, and to stabilize the output of the semiconductor laser pumped solid-state laser with high accuracy. it can.

Further, since the photocathode 25a of the photodetector 25 is arranged to be inclined with respect to the traveling direction of the second harmonic 19, as shown in FIG. The optical path of the harmonic 19 and the optical path of the second harmonic 19 after being reflected by the photocathode 25a are separated from each other. Therefore, the interference of the second harmonic 19 between the photocathode 25a and the window glass 25b can also be prevented, whereby the output of the solid-state laser excited by the semiconductor laser can be stabilized more accurately.

In order to prevent the interference of the second harmonic 19 between the photocathode 25a and the window glass 25b, the second harmonic 19 directly incident on the photocathode 25a and the photocathode 25a 25a
The second harmonic 19, which is reflected by the window glass 25b after being reflected by the window glass 25b and re-enters the photocathode 25a, has a beam diameter (1/1) on the photocathode 25a.
It is desirable to dispose the photodetectors 25 so as to be displaced from each other at least twice as large as (e ² diameter).

Furthermore, if the photodetector 25 having a relatively low degree of parallelism of the window glass 25b is used, it is possible to prevent the second harmonic 19 from interfering between the front and back of the window glass 25b. If the thickness of the window glass 25b is about 0.3 mm or less, even if the parallelism is relatively high, a temperature difference of, for example, about 20 to 35 ° C. is required to change the interference state. If the temperature is adjusted as described above, no particular problem occurs.
Also, when the set temperature of the temperature adjustment is changed in accordance with the change with time, the temperature change width is set to 0. Because it is around a few degrees Celsius to 1 degree Celsius,
It doesn't matter.

[Brief description of the drawings]

FIG. 1 is a partially broken side view showing a semiconductor laser pumped solid-state laser according to one embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating a polarization direction of a laser beam in the semiconductor laser pumped solid-state laser.

FIG. 3 shows an APC of the solid-state laser excited by the semiconductor laser.
Plan view showing a part of a photodetector

FIG. 4 is a partially broken elevational view showing a part of the photodetector.

[Description of Signs] 10 Laser beam (pumping light) 11 Semiconductor laser 12 Focusing lens 13 Nd: YVO ₄ crystal 14 Resonator mirror 15 Inversion domain LN crystal 16 Etalon 17 Calcite crystal 18 Solid state laser beam (fundamental wave) 19 2 harmonics 23 Filter 24 Partial reflection mirror 24a Partial reflection mirror surface 24b Partial reflection mirror back surface 25 APC photodetector 25a Photodetector photodetector (light receiving surface) 25b Photodetector window glass 26 APC circuit 30 Base plate 31 Peltier device 32 Package base 33 Opening plate 34 Package cap 36 Thermistor

Claims (7)

[Claims] 1. A laser crystal, a semiconductor laser emitting excitation light for exciting the laser crystal, means for splitting a part of a laser beam obtained by the excitation, and detecting a light amount of the split laser beam. A semiconductor laser-excited solid-state laser comprising: a photodetector; and an APC circuit that receives an output of the photodetector and controls a driving current of the semiconductor laser so as to stabilize the output. A semiconductor laser-excited solid-state laser, wherein optical components arranged in an optical path to a detector are arranged non-parallel on an optical axis with respect to a light receiving surface of the photodetector. 2. The semiconductor laser-pumped solid-state laser according to claim 1, wherein the optical component is a filter that selectively allows only light in a desired wavelength range to pass. 3. The semiconductor laser-excited solid-state laser according to claim 1, wherein a light-receiving surface of the photodetector is arranged to be inclined with respect to a traveling direction of the laser beam. 4. A photodetector according to claim 1, wherein the light directly incident on the light receiving surface and the light reflected on the window glass of the detector after being reflected on the light receiving surface and re-entering the light receiving surface are reflected on the light receiving surface. the semiconductor laser excitation solid-state laser according to claim 3, characterized in that it is arranged to displace each other more than twice the beam diameter (1 / e ² diameter). 5. The semiconductor laser-excited solid-state laser according to claim 1, wherein the temperature of the photodetector is controlled. 6. A semiconductor laser-excited solid-state laser according to claim 1, wherein the laser beam received by said photodetector has been wavelength-converted by an optical wavelength conversion element. . 7. The laser beam reflected by the filter and returned to the solid-state laser resonator has a resonator axis and a beam diameter (1 / e) at all positions in the resonator.
The semiconductor laser-excited solid-state laser according to any one of claims 2 to 6, wherein the semiconductor laser is disposed so as to be separated by at least two times ( ² diameters).