JP2006279067A - Semiconductor laser stimulating solid-state laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To precisely stabilize output of a semiconductor laser stimulating solid-state laser, even if change with respect to time and humidity change occur, in which a solid-state laser crystal is pumped by the semiconductor laser, and a part of the laser beams produced by pumping is split through a partial reflecting mirror, and a quantity of light of the split laser beam is detected for applying APC. <P>SOLUTION: The partial reflecting mirror 44, which splits the laser beam (second harmonic) 19 to lead to a photodetector for APC 25, is a mirror, in which both the front and rear sides becoming reflecting surfaces thereof are non-coated faces. The partial reflecting mirror 44 is arranged so that the laser beam 19 reflected by the front and rear sides respectively, is incident to the photodetector 25 together. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a semiconductor laser-pumped solid-state laser, and more particularly to a semiconductor laser-pumped solid-state laser that improves the accuracy of output stabilization control.

  For example, as shown in Patent Document 1, a solid-state laser that excites a laser crystal to which neodymium (Nd) is added by light emitted from a semiconductor laser is known.

  In this semiconductor laser pumped solid-state laser, in order to obtain a laser beam with a shorter wavelength, an optical wavelength conversion element made of a nonlinear optical material is arranged in the resonator, and the solid-state laser beam is converted into a second harmonic wave or the like. Conversion is also widely performed.

  On the other hand, in many cases, the above-described semiconductor laser-pumped solid-state laser is required to keep the output of the wavelength-converted wave constant. Therefore, conventionally, for example, as shown in Patent Document 2, a light branching means such as a partial reflection mirror that transmits a part of the laser beam and reflects the remainder, a photodetector that detects the light amount of the branched laser beam, A semiconductor laser pumped solid-state laser which is provided with an APC (Automatic Power Control) circuit for controlling the driving current of the semiconductor laser so as to receive the output of the photodetector and to stabilize the output so as to stabilize the output. Has been proposed.

When the wavelength of the solid laser beam is converted as described above, the laser beam after the wavelength conversion is made incident on the optical branching means, and the output of the laser beam after the wavelength conversion is stabilized. .
JP 62-189783 A Japanese Unexamined Patent Publication No. 7-15014

  However, in this conventional semiconductor laser-pumped solid-state laser, there is a problem that the output is likely to fluctuate if there is a change over time or a change in humidity even if output stabilization control is applied.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a semiconductor laser-excited solid-state laser capable of accurately stabilizing the output even when there is a change with time or a change in humidity.

The semiconductor laser excitation 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,
In the semiconductor laser pumped solid state laser that stabilizes the output of the laser beam by providing the partial reflection mirror as the light branching means, the photodetector for detecting the light quantity of the laser beam, and the APC circuit,
As a partial reflection mirror, both the front and back surfaces serving as a reflection surface are used as uncoated surfaces,
The partial reflection mirror is arranged so as to guide both the laser beams reflected by the front and back surfaces to the photodetector.

The partial reflection mirror is preferably arranged so that the optical paths of the laser beams reflected on the front and back surfaces are shifted from each other by 1.5 times or more of the beam diameter (1 / e ² diameter).

  Further, the partial reflection mirror is preferably arranged so that the branched laser beam is incident as p-polarized light.

  On the other hand, in the present invention, the laser beam incident on the partial reflection mirror may be wavelength-converted by an optical wavelength conversion element.

  According to the research of the present inventor, it has been found that the problem in the conventional apparatus described above is caused by the coat for adjusting the reflectivity applied to the partial reflection mirror. That is, the reflectance of this type of coat changes with time and humidity, and when this reflectance change occurs, even if the output is constant, it enters the APC photodetector via the partial reflection mirror. The light quantity of the laser beam will fluctuate. 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.

  On the other hand, in the semiconductor laser pumped solid-state laser of the present invention, a partially reflecting mirror is used in which both the front and back surfaces serving as reflecting surfaces are uncoated surfaces. Since the rate change is completely eliminated or reduced, the output of the semiconductor laser-excited solid-state laser can be stabilized with high accuracy regardless of changes with time and humidity.

  Further, in the semiconductor laser pumped solid state laser of the present invention, since the partial reflection mirror is arranged so as to guide each laser beam reflected on the front and back surfaces to the APC photodetector, the laser beam reflected on one side of the front and back surfaces. The amount of light received by the photodetector can be secured more than when detecting only the light.

  In addition, when using a partially reflecting mirror whose both front and back surfaces are uncoated surfaces, an inexpensive partially reflecting mirror that can be formed by simply cutting a large uncoated flat plate into small pieces can be used. It will be suppressed.

On the other hand, if partial reflection mirrors are formed so that the optical paths of the laser beams reflected on the front and back surfaces are shifted from each other by more than 1.5 times the beam diameter (1 / e ² diameter), the laser beams will be too close and interfere. As a result, the amount of light received by the photodetector can be prevented from changing.

  If the partial reflection mirror is arranged so that the branched laser beam is incident as p-polarized light, the reflectance becomes lower than that when incident as s-polarized light. Therefore, the laser beam can be extracted with a small loss.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, a reference example for explaining the semiconductor laser excitation solid-state laser of the present invention will be described with reference to FIG.

  This semiconductor laser excitation solid-state laser includes a semiconductor laser 11 that emits a laser beam 10 as excitation light, a condenser lens 12 that condenses the laser beam 10 that is diverging light, and a solid-state laser medium doped with neodymium (Nd). YAG crystal (Nd: YAG crystal) 13 and a resonator mirror 14 disposed on the front side of the Nd: YAG crystal 13, that is, on the opposite side of the semiconductor laser 11.

Further, between the Nd: YAG crystal 13 and the resonator mirror 14, in order from the Nd: YAG crystal 13 side, an MgO: LiNbO ₃ crystal (hereinafter referred to as an inversion domain LN crystal) which is a nonlinear optical material having a periodic domain inversion structure. 15) and a calcite crystal 17 tilted 35 'with respect to the optical axis.

  The semiconductor laser 11 is a broad area laser having 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 to the β direction in the figure.

  The condensing lens 12 is composed of, for example, a gradient index lens (Selfoc lens (registered trademark) or the like), and is fixed to the holding member 20 with one surface side subjected to spherical processing facing the Nd: YAG crystal 13. Yes. The Nd: YAG crystal 13 is formed to a thickness of 1 mm, and the condenser lens 12 converges the laser beam 10 at a magnification ratio of 0.8 to 1.0 times at a position 0.5 mm from the incident end face of the crystal 13 in the thickness direction. The position is adjusted so that

  The condensing lens 12 and the semiconductor laser 11 are fixed to the holding member 20 after the relative positions are adjusted so that the condensing point is located at predetermined positions in the α and β directions in the figure. 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: YAG crystal 13 emits light having a wavelength of 946 nm when neodymium ions are excited by the incident laser beam 10. The incident end face 13a of the Nd: YAG crystal 13 reflects light with a wavelength of 946 nm satisfactorily (reflectance of 99.9% or more), while transmitting the excitation laser beam 10 with a wavelength of 809 nm satisfactorily (transmittance of 93% or more). ) There is a coat. On the other hand, the mirror surface 14a of the resonator mirror 14 has a coat that reflects light with a wavelength of 946 nm well (reflectance of 99.9% or more) and transmits light with a wavelength of 473 nm (transmittance of 90% or more). It has been subjected.

  Therefore, the light with a wavelength of 946 nm is confined between the Nd: YAG crystal end face 13a and the mirror face 14a, which is a highly reflective surface, and causes laser oscillation, and a laser beam 18 with a wavelength of 946 nm is generated. This laser beam 18 as a fundamental wave is converted by the inversion domain LN crystal 15 into a second harmonic 19 having a wavelength of 1/2, that is, 473 nm, and the second harmonic 19 is mainly emitted from the resonator mirror 14. To do.

  In this example, the thickness of the Nd: YAG crystal 13 is 1 mm. The length of the inversion domain LN crystal 15 is 2 mm, the radius of curvature of the resonator mirror 14 is 15 mm, and the distance between the Nd: YAG crystal end face 13a constituting the resonator and the mirror face 14a is about 10 mm. On the other hand, the end face 13a of the Nd: YAG crystal is also coated with the second harmonic wave 19 having a wavelength of 473 nm satisfactorily (reflectance of 95% or more). The second harmonic wave 19 traveling to the Nd: YAG crystal 13 side is reflected to the resonator mirror 14 side to obtain a high second harmonic output.

  The crystal axis of the inversion domain LN crystal 15 is arranged so that the c-axis is in the β direction, as shown in FIG. The period of the domain inversion portion 15a of the inversion domain LN crystal 15 is about 4.7 μm, and is configured to be phase-matched at a temperature at which the center wavelength of the laser beam 10 emitted from the semiconductor laser 11 is 809 nm.

  The calcite crystal 17 is a parallel plate having a thickness of about 340 μm and is cut in the direction of 46.7 ° from the c-axis to the a-axis. Therefore, an a-axis and an axis including c and a are projected on the light incident / exit surface forming the parallel plane. Due to the birefringence of the calcite crystal 17, the optical path differs depending on the polarization of the laser beam 18 having a wavelength of 946 nm in the crystal 17, so that only desired polarization can be oscillated by adjusting the position of the resonator mirror 14.

  The light incident / exit surface of the calcite crystal 17 is coated with a coating having a reflectance of 14.5% with respect to light having a wavelength of 946 nm, and the crystal 17 functions as an etalon for changing the oscillation mode to a single longitudinal mode. In this example, the a-axis of the calcite crystal 17 is substantially aligned in the β direction, and light having a wavelength of 946 nm polarized in the β direction is oscillated.

On the other hand, since the inversion domain LN crystal 15 is arranged so that the c-axis is in the β direction as described above, the second harmonic 19 having a wavelength of 473 nm polarized in the β direction is obtained using the nonlinear optical constant d _33. appear.

  The Nd: YAG crystal 13, the resonator mirror 14, the inversion domain LN crystal 15, and the calcite crystal 17 described above are attached to the holding member 21. Hereinafter, the portion of the holding member 21 is referred to as a resonator portion.

  The second harmonic wave 19 emitted from the resonator part is incident on the APC part composed of components attached to the holding member 22. As shown in FIG. 3, the holding member 22 has end faces 22 a and 22 b that are orthogonal to each other and an end face 22 c that forms an angle of 45 ° with respect to these end faces 22 a and 22 b, as shown in FIG. A light passage hole 22d that is bent at a right angle and opened in three end faces 22a, 22b, and 22c is formed in the inside.

  A filter 23 that absorbs the excitation laser beam 10 and the solid-state laser beam 18 while allowing the second harmonic 19 to pass therethrough 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 through and reflects the remainder toward the end face 22b is attached to the oblique end face 22c. An APC photodetector 25 for detecting the reflected second harmonic 19 is attached to the end face 22b.

  As the filter 23, a filter having a transmittance of 0.01 to 0.001% for the excitation laser beam 10 having a large amount of light is used. The light incident / exit surface is provided with an AR coating having a reflectance of 0.5% or less with respect to light having a wavelength of 473 nm.

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

  On the other hand, the partial reflection mirror 24 as a light branching unit is made of a synthetic quartz flat plate with light incident / exit surfaces almost parallel, and is fixed to the end surface 22c, so that it is inclined by 45 ° with respect to the optical axis (γ axis). It is in the state. The front surface (A surface) 24a of the partial reflection mirror 24 is an uncoated surface, and the rear surface (B surface) 24b is AR coated so that the reflectance with respect to the second harmonic 19 is 0.5% or less. Yes.

As shown in FIG. 1, the second harmonic 19 transmitted through the partial reflection mirror 24 passes through the opening 33a of the opening plate 33 and then exits from the exit window 34a of the package cap 34 to the outside of the package. The diameter of the opening 33a is, for example, about 1.5 to 2.5 times the beam diameter of the second harmonic 19 (1 / e ² diameter). By passing the second harmonic 19 therethrough, the resonator section and Stray light generated in the APC section is cut.

  In addition, the window glass 35 attached to the exit window 34a of the package cap 34 has a parallelism between the front and back surfaces of about 5 to 30 'in order to prevent the second harmonic output from decreasing due to interference caused by multiple reflections on the front and back surfaces. . These front and back surfaces are coated with a coating having a reflectivity with respect to the second harmonic 19 of 0.5% or less.

  The holding members 20, 21, and 22 holding the excitation unit, the resonator unit, and the APC unit described above are bonded onto the plate-like base plate 30 and bonded to the package base 32 via the base plate 30 and the Peltier element 31. It is fixed.

  Then, the temperature in the resonator is detected by the thermistor 36 attached to the resonator unit, and the current of the Peltier element 31 is adjusted so that the detected temperature becomes a predetermined temperature by a temperature control circuit (not shown), and the resonator The temperature inside 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, 22 and the base plate 30 are formed of a material having high thermal conductivity such as aluminum, copper, or an alloy thereof. Is desirable. In addition, it is desirable that each part is bonded and fixed with a thermal conductivity adhesive or a solder having a high thermal conductivity such as solder. However, even with a general adhesive such as an epoxy adhesive, the adhesive layer is thinned to about 10 μm or less. In this case, the thermal resistance can be reduced, so that it can be used.

  Further, in this example, after all the components have been attached, the package base 32 and the package cap 34 that have been wired with the electrical components are seam welded in a dry nitrogen atmosphere.

  Next, the aforementioned 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 compares the photodetection signal S with a predetermined reference signal, for example. When the former exceeds the latter, the drive current I of the semiconductor laser 11 is reduced, and when the opposite is true, the drive current I is increased. Thereby, the output of the second harmonic 19 is kept at a predetermined constant value.

  Here, since the second harmonic wave 19 polarized in the β direction is incident on the surface 24a of the partial reflection mirror 24 as s-polarized light, the reflectance is about 8.5% from the reflectance formula. This reflectivity is determined only by the refractive index and incident angle of air and synthetic quartz, and the reflectivity changes due to changes over time and humidity, like the partially-reflected mirror with coat used in conventional devices. There is nothing to do. Therefore, the amount of light detected by the APC photodetector 25 does not change due to the change in reflectance, and as a result, a second harmonic output that is stable over a long period of time can be obtained.

  Note that the reflectance of the back surface 24b of the partial reflection mirror 24 is 0.5% or less, and the amount of reflected light from there is originally very small. Therefore, even if this reflected light is detected by the photodetector 25, the change in the detected light amount due to the change in reflectance can be ignored in practice. However, in this example, as shown in FIG. 3, the thickness of the partial reflection mirror 24 is set so that the reflected light from the mirror back surface 24b does not enter the photocathode of the photodetector 25. This prevents the amount of light detected by the photodetector 25 from changing more reliably.

  Next, a semiconductor laser pumped solid state laser according to an embodiment of the present invention will be described with reference to FIG. In FIG. 4, elements that are the same as those in FIG. 1 are given the same numbers, and redundant descriptions thereof are omitted.

Compared with that of FIG. 1, the semiconductor laser pumped solid state laser of FIG. 4 is based on a basic configuration, and instead of the Nd: YAG crystal 13, a YVO ₄ crystal (Nd: Nd: Nd) is a solid laser medium doped with neodymium (Nd). YVO ₄ crystal) 43 is different in that KTP crystal 45 is used instead of inversion domain LN crystal 15 and Brewster plate 47 is used instead of calcite crystal 17. Further, the orientation of the partial reflection mirror 44 in the holding member 42 constituting the APC section is also different from that in FIG.

  Here, the semiconductor laser 11 is a broad area laser having an active layer width of about 100 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 to a direction that forms an angle of 45 ° with respect to the α and β directions in the figure. This polarization direction and the like are shown in FIG.

The Nd: YVO ₄ crystal 43 is formed to a thickness of 1 mm, and the condenser lens 12 converges the laser beam 10 at a magnification ratio of 0.8 to 1.0 times at a position about 0.4 mm in the thickness direction from the incident end surface of the crystal 43. The position is adjusted so that

The Nd: YVO ₄ crystal 43 emits light having a wavelength of 1064 nm when neodymium ions are excited by the incident laser beam 10. The incident end face 43a of the Nd: YVO ₄ crystal 43 reflects light with a wavelength of 1064 nm well (reflectance of 99.9% or more), while transmitting the excitation laser beam 10 with a wavelength of 809 nm well (transmittance of 93%). Above) A coat is given. The mirror surface 14a of the resonator mirror 14 has a coating that reflects light with a wavelength of 1064 nm well (reflectance of 99.9% or more) and transmits light with a wavelength of 532 nm as described below (transmittance of 90% or more). Yes.

Therefore, the light having a wavelength of 1064 nm is confined between the Nd: YVO ₄ crystal end face 43a and the mirror surface 14a, which is a highly reflective surface, and causes laser oscillation, and a laser beam 18 having a wavelength of 1064 nm is generated. The laser beam 18 as the fundamental wave is converted by the KTP crystal 45 into a second harmonic 19 having a wavelength of 1/2, that is, 532 nm, and the second harmonic 19 is mainly emitted from the resonator mirror 14.

In this example, the Nd: YVO ₄ crystal 43 is a-axis cut and has a thickness of 1 mm. The length of the KTP crystal 45 is 5 mm, the radius of curvature of the resonator mirror 14 is 50 mm, and the distance between the Nd: YVO ₄ crystal end face 43a constituting the resonator and the mirror surface 14a is about 11 mm.

  The Brewster plate 47 is composed of a parallel plate of quartz, and realizes single longitudinal mode oscillation by controlling the polarization direction of the laser beam 18 oscillating in the resonator. That is, the Brewster plate 47 is tilted at a Brewster angle (about 54 °) with respect to a wavelength of 1064 nm around the rotation axis shown in FIG. As a result, the light having a wavelength of 1064 nm polarized in the direction of the rotation axis is incident on the Brewster plate 47 as s-polarized light having a high reflectance, so that the loss increases, and the wavelength of 1064 nm with a small loss having a polarization component in the direction perpendicular thereto. Only the light of oscillates.

  Further, in order to perform wavelength conversion efficiently while realizing single longitudinal mode oscillation, the crystal axes of the crystals 43 and 45 and the polarization directions of the laser beams 10 and 18 are set as shown in FIG. The polarization direction of the generated second harmonic 19 is the β direction.

  The partial reflection mirror 44 is composed of a synthetic quartz flat plate whose light incident / exit surfaces are substantially parallel, and is inclined by 45 ° with respect to the optical axis (γ axis). Both the front surface 44a and the back surface 44b of the partial reflection mirror 44 are uncoated surfaces, and the second harmonic wave 19 having a wavelength of 532 nm polarized in the β direction enters the surfaces 44a and 44b as p-polarized light. From the reflectance formula, the reflectance is about 0.7%.

  This reflectivity is determined only from the refractive index and incident angle of air and synthetic quartz, and the reflectivity does not change due to changes over time or humidity. Therefore, also in this example, the amount of light detected by the APC photodetector 25 does not change due to this change in reflectance, and as a result, a second harmonic output that is stable over a long period of time can be obtained.

  Moreover, since the reflectance of the partial reflection mirror 44 is about 1.4% even when the front and back surfaces are combined, most of the second harmonic 19 is transmitted through the partial reflection mirror 44 and effectively used for the original purpose. It becomes like this.

  In the present embodiment, both the second harmonic wave 19 reflected by the surface 44a of the partial reflection mirror 44 and the second harmonic wave 19 reflected by the back surface 44b are incident on the photoelectric surface of the APC photodetector 25. The thickness of the partial reflection mirror 44 is set so that the amount of light detected by the photodetector 25 is sufficiently secured.

Further, the thickness of the partial reflection mirror 44 is set such that the optical paths of the second harmonics 19 reflected by the front surface 44a and the back surface 44b are shifted from each other by 1.5 times or more of the beam diameter (1 / e ² diameter). By doing so, it is possible to prevent the two second harmonics 19 directed toward the photodetector 25 from being too close and interfering with each other, thereby changing the amount of light received by the photodetector 25. Further, with such a thickness, the second harmonic 19 that is reflected by the back surface 44b and further reflected by the front surface 44a and proceeds to the exit window 34 side is transmitted to the second harmonic 19 that is transmitted through the partial reflection mirror 44. It can be cut away by the aperture plate 33 with sufficient separation from the aperture plate 33.

The partially broken side view which shows the semiconductor laser excitation solid-state laser of the reference example with respect to this invention The perspective view which shows a part of said semiconductor laser excitation solid-state laser Partially cutaway plan view showing the part of the partially reflecting mirror of the semiconductor laser pumped solid state laser The partially broken side view which shows the semiconductor laser excitation solid-state laser which is one Embodiment of this invention Schematic explaining the polarization direction of the laser beam in the semiconductor laser pumped solid state laser of FIG.

Explanation of symbols

10 Laser beam (pumping light)
11 Semiconductor laser
12 Condensing lens
13 Nd: YAG crystal
14 Resonator mirror
15 Inversion domain LN crystal
17 Calcite crystal
18 Solid-state laser beam (fundamental wave)
19 Second harmonic
23 Filter
24 partial reflection mirrors
24a Surface of partially reflecting mirror
24b The back of the partial reflection mirror
25 Photo detector for APC
26 APC circuit
30 Base plate
31 Peltier element
32 Package base
33 Opening plate
34 Package cap
36 thermistor
43 Nd: YVO ₄ crystal
44 Partial reflection mirror
44a Surface of partially reflecting mirror
44b The back of the partially reflective mirror
45 KTP crystal
47 Brewster plate

Claims (4)

Laser crystals,
A semiconductor laser emitting excitation light for exciting the laser crystal;
A partially reflecting mirror that transmits a part of the laser beam obtained by this excitation, reflects the remainder, and branches the laser beam;
A photodetector for detecting the light quantity of the branched laser beam;
In a semiconductor laser pumped solid state laser having an APC circuit that receives the output of the photodetector and controls the driving current of the semiconductor laser so as to make the output constant,
As the partial reflection mirror, both the front and back surfaces serving as a reflection surface are used as uncoated surfaces,
A semiconductor laser-excited solid-state laser, wherein the partial reflection mirror is arranged so as to guide both the laser beams reflected by the front and back surfaces to the photodetector. The partially reflecting mirror, the optical path of the laser beam reflected respectively on the front and back surfaces, according to claim 1, characterized in that it is formed so as to shift from each other more than 1.5 times the beam diameter (1 / e ² diameter) Semiconductor laser pumped solid state laser.   3. The semiconductor laser excitation solid-state laser according to claim 1, wherein the partial reflection mirror is arranged so that a branched laser beam is incident as p-polarized light.   4. The semiconductor laser-excited solid laser according to claim 1, wherein the laser beam incident on the partial reflection mirror is wavelength-converted by an optical wavelength conversion element.

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