JP2002076487A - Semiconductor laser stimulating solid-state laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent noise generation by return light to a semiconductor laser. SOLUTION: This semiconductor laser stimulating solid-state laser for solid- state laser medium 13 by a laser beam L1 generated from the semiconductor laser 11 is provided with a photodetector 33 for detecting at least a part of the outputted laser beam L3 and an APC circuit 34 for controlling the driving current of the semiconductor laser 11 so as to fix the output of the laser beam L3 based on the output signal S1 of the photodetector 33. Then, the driving frequency of the APC circuit 34 is turned higher than the frequency of noise by the return light to the semiconductor laser 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser-excited solid-state laser that excites a solid-state laser medium with a laser beam emitted from a semiconductor laser, and more particularly, to the prevention of noise due to light returning to the semiconductor laser. The present invention relates to a semiconductor laser pumped solid-state laser as described above.

[0002]

2. Description of the Related Art As disclosed in, for example, JP-A-7-302946, a semiconductor laser-excited solid-state laser for exciting a solid-state laser medium to which a rare earth element such as neodymium is added by a semiconductor laser (laser diode) is known. ing. In this type of semiconductor laser-excited solid-state laser, in order to obtain a shorter-wavelength laser beam, a nonlinear optical crystal is placed in the resonator, and the solid-state laser beam is converted into a second harmonic, a sum frequency, etc. Is also widely practiced.

In a conventional semiconductor laser-pumped solid-state laser, oscillation of the semiconductor laser becomes unstable due to so-called return light, that is, pumping light that is reflected by an end face of the solid-state laser medium and returns to the semiconductor laser. A problem has been recognized that noise (including output fluctuation) occurs due to fluctuations in the intensity and oscillation wavelength of the oscillation beam. When the semiconductor laser-excited solid-state laser is used, for example, as a recording light source of an optical scanning recording apparatus, the noise causes unevenness in the density of a recorded image.

In order to suppress this return light noise, a wavelength plate or the like is conventionally provided between a semiconductor laser as an excitation light source and an optical member such as a solid laser medium as a source of return light. It has been proposed to insert an isolator for rotating the polarization of semiconductor laser light.

Further, as shown in Japanese Patent Application Laid-Open No. Hei 5-343770 and Japanese Patent Application No. 10-163614, the optical axis of a semiconductor laser as an excitation light source is shifted from the optical axis of a solid-state laser resonator. It is also considered to be arranged in a state.

Further, as disclosed in Japanese Patent Application No. 5-301,037, it has been considered to reduce the influence of return light by making the end face reflectivity of a semiconductor laser relatively high.

[0007]

However, in the case of a solid-state laser pumped with a semiconductor laser using the above-mentioned isolator, an expensive isolator is required, so that the cost of the apparatus is greatly increased, and an increase in the size of the apparatus is inevitable. The problem is also recognized. Furthermore, in this configuration, when the optical member that generates the return light has birefringence, the polarization direction differs depending on the reflection surface, so it is difficult to remove all the effects of the return light from the reflection surface. I have.

On the other hand, in a semiconductor laser pumped solid-state laser in which a semiconductor laser is disposed with its optical axis deviated from the optical axis of a solid-state laser resonator, in order to more reliably suppress noise, It is necessary to set a large deviation between the two optical axes. However, in this case, the mode matching between the semiconductor laser beam and the solid-state laser beam in the solid-state laser medium is deteriorated, and the output and the efficiency are reduced, and at the same time, the beam quality is deteriorated. In particular, in the case of a solid-state laser pumped solid-state laser using a high-output semiconductor laser of a broad guide as an excitation light source for high output, the mode matching is extremely difficult to improve because the beam diameter of the excitation light is large. Was difficult.

Therefore, if the deviation between the two optical axes is set relatively small so as not to cause deterioration of the mode matching,
In this case, the generation of the return light noise cannot be sufficiently suppressed. Specifically, at least about 2% of the return light noise is generated. Generally the same).

Further, when the end face reflectivity of the semiconductor laser is made relatively high, the light density and the drive current value inside the semiconductor laser increase due to the high end face reflectivity. However, the problem that the life and reliability are deteriorated has been recognized.

The present invention has been made in view of the above circumstances, and provides a semiconductor laser-excited solid-state laser capable of reliably preventing noise from being generated by return light to a semiconductor laser without increasing the cost of the apparatus or increasing the size of the apparatus. With the goal.

[0012]

According to the present invention, there is provided a solid-state laser excited by a solid-state laser, a solid-state laser medium, a semiconductor laser emitting excitation light for exciting the solid-state laser medium, and at least a part of an output laser beam. And an APC (Automati Detector) that controls a driving current of the semiconductor laser based on an output signal of the photodetector so as to stabilize an output of the laser beam.
c power control) circuit, wherein the driving frequency of the APC circuit is set to be higher than the frequency of noise due to return light to the semiconductor laser.

In the present invention, a Nd: YVO ₄ crystal is preferably used as the solid-state laser medium. The driving frequency of the APC circuit is desirably set to 10 kHz or more.

Further, the present invention is preferably applied to a semiconductor laser-excited solid-state laser in which the output of the semiconductor laser for excitation is 1 W or more.

On the other hand, the present invention is also applicable to a semiconductor laser-excited solid-state laser having a configuration for performing the above-described wavelength conversion. In that case, the semiconductor laser pumped solid-state laser of the present invention is provided with a nonlinear optical crystal for wavelength-converting solid-state laser oscillation light in a solid-state laser resonator, and detects a wavelength-converted wave by the nonlinear optical crystal with the photodetector. Thus, the output of the wavelength converted wave is configured to be constant. That is, the laser beam detected by the photodetector in the semiconductor laser-excited solid-state laser of the present invention includes not only the oscillation light of the solid-state laser itself but also the above-mentioned wavelength-converted wave.

Further, in the semiconductor laser-pumped solid-state laser of the present invention, as described above, it is desirable that the pumping semiconductor laser is disposed such that its optical axis is shifted from the optical axis of the solid-state laser resonator.

[0017]

In the semiconductor laser pumped solid-state laser of the present invention, the driving frequency of the APC circuit is set to be higher than the frequency of the noise due to the return light to the semiconductor laser, so that the return light noise is sufficiently suppressed. it can. Specifically, the rising speed of the return light noise is generally less than about 10 kHz at the maximum. Therefore, if the drive frequency of the APC circuit is set to 10 kHz or more, the return light noise can be sufficiently suppressed. Conventionally, the generation of noise of about 2% was unavoidable, but according to the present invention, it can be reduced to about 0.2%.

The above effects can be obtained even when the reflectivity of the end face of the semiconductor laser for excitation, that is, the reflectivity of the end face of the chip is made relatively low. It is not necessary to increase the end face reflectance. In that case, the output of the semiconductor laser can be increased to about 1 W or more without impairing the life and reliability, and the output and reliability of the semiconductor laser pumped solid-state laser can be improved.

If the return light noise can be sufficiently suppressed as described above, in a configuration in which the optical axis of the semiconductor laser for excitation and the optical axis of the solid-state laser resonator are shifted from each other, the amount of deviation of the optical axis should be reduced. Accordingly, the deterioration of the mode matching described above can be minimized, and high beam quality can be ensured.

The response speed of a Nd: YAG crystal or the like, which has been widely used as a solid-state laser medium, is at most several kHz because its fluorescence lifetime is relatively long.
Stays on. If such a solid-state laser medium is used, the response of the solid-state laser medium cannot follow the drive frequency of the APC circuit even if it is set to 10 kHz or more.

On the other hand, the Nd: YVO ₄ crystal has a short fluorescence lifetime, and its response speed is as fast as several tens of kHz. Therefore, this Nd: YVO is used as a solid laser medium.
_{If four} crystals are used, it is possible to follow the above-mentioned driving frequency of 10 kHz or more, so that the APC operation is normally performed and the return light noise is suppressed.

Further, the semiconductor laser-pumped solid-state laser of the present invention can suppress return light noise only by specifying the driving frequency of the APC circuit, and does not require special expensive means or complicated means. Therefore, there is no increase in the cost of the apparatus or increase in the size of the apparatus.

[0023]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a semiconductor laser-pumped solid-state laser (hereinafter, referred to as one embodiment) of the present invention.
FIG. 2 is a partially cut-away side view showing a planar shape and a circuit configuration of a solid-state laser).

The solid-state laser of this embodiment is used as excitation light.
Semiconductor laser that emits all laser beams L1
And the laser beam L1 which is divergent light.
Focusing lens composed of a gradient index rod lens
And a solid-state laser doped with neodymium (Nd)
-YVO as a medium₄Crystal (hereinafter, Nd: YVO)₄ Conclusion
13) and this Nd: YVO₄Front side of crystal 13
(The right side in the figure) and the resonator mirror 14
Mirror 14 and Nd: YVO₄Arranged between crystal 13
KNbO₃Crystal (hereinafter referred to as KN crystal) 15
ing.

The wavelength of the semiconductor laser 11 is 809 nm.
Which emits the laser beam L1 of the laser beam. N
The d: YVO ₄ crystal 13 emits light having a wavelength of 1064 nm when neodymium ions are excited by the incident laser beam L1. End face of Nd: YVO ₄ crystal 13 on excitation light incident side
At 13a, light having a wavelength of 1064 nm is well reflected (reflectance
99.9%), 809 nm wavelength excitation laser beam L1
Has a coating that allows good transmission (transmittance of 99% or more). On the other hand, light having a wavelength of 1064 nm is reflected well on the mirror surface 14a of the resonator mirror 14 and has the following wavelength 532 nm.
A coating for transmitting light of nm is provided.

In this example, the Nd: YVO ₄ crystal 13
And a resonator mirror 14 constitute a Fabry-Perot resonator, and light having a wavelength of 1064 nm is emitted from the surfaces 13a and 14a.
The laser beam L2 is confined in between and causes laser oscillation. The laser beam L2 thus generated is converted by the KN crystal 15 into a second harmonic L3 having a wavelength of 1/2, that is, 532 nm, and the second harmonic L3 is mainly converted into a resonator mirror. Emitted from 14.

The semiconductor laser 11 and the condenser lens 12 are fixed to a holder 20, and the holder 20 is fixed to a reference plate 22. The reference plate 22 is made of copper or the like that has a high thermal conductivity and does not easily cause a temperature gradient, and is fixed on a Peltier element 24 fixed on the device mounting surface 26. Further, a thermistor 40 for temperature detection is fixed to the reference plate 22. The thermistor 40 includes a semiconductor laser 11 and a condenser lens 12.
The temperature is detected by the temperature control circuit.
Entered in 42. The temperature control circuit 42 is a feedback-type temperature control circuit, and drives and controls the Peltier element 24 to maintain the semiconductor laser 11 and the condenser lens 12 at the reference temperature T1 set by the sequence controller 45.

On the other hand, Nd:
The YVO ₄ crystal 13, the KN crystal 15, and the resonator mirror 14 are fixed to another holder 21, and this holder 21 is fixed to another reference plate 23. The reference plate 23 is also made of copper or the like that has a high thermal conductivity and hardly causes a temperature gradient, and is fixed on another Peltier element 25 fixed on the device mounting surface 26. Further, a thermistor 41 for temperature detection is fixed to another reference plate 23. The thermistor 41 detects the temperature of the resonator, and its output is input to the temperature control circuit 43. The temperature control circuit 43 is a feedback-type temperature control circuit, and drives and controls the Peltier element 25 to maintain the resonator at the reference temperature T2 set by the sequence controller 45.

Next, the solid-state laser output APC will be described. On the device mounting surface 26, a holder 30 is fixed separately from the reference plates 22, 23 and the Peltier elements 24, 25,
A dichroic filter 31 and a partially transmitting mirror 32 are fixed to the holder 30. The dichroic filter 31 is a resonator mirror with the second harmonic L3.
The weak laser beams L1 and L2 emitted from 14 are cut. The partial transmission mirror 32 arranged at an angle to the traveling direction of the second harmonic L3 transmits most of the second harmonic L3 as the use light L3A, and reflects part of the second harmonic L3 as the detection light L3B. The holder 30 has a hole for passing the second harmonic L3 before being branched by the partially transmitting mirror 32 and the detection light L3B after being branched.
30a are formed.

The detection light L3B is detected by an APC photodetector 33 fixed on the reference plate 23, and its output signal
S1 is input to the APC circuit 34. The APC circuit 34 controls the drive current of the semiconductor laser 11 so that the output signal S1 is stabilized at the reference value R1 set by the sequence controller 45. As a result, the optical output of the laser beam L1 is controlled, and the output of the second harmonic L3 is stabilized at a constant value.

Here, when the laser beam L1 as the excitation light is reflected on the end face of the Nd: YVO ₄ crystal 13 and returns to the semiconductor laser 11, the oscillation of the semiconductor laser 11 becomes unstable, and the intensity of the oscillation beam becomes unstable. And the oscillation wavelength fluctuates, causing noise (including output fluctuation). The rising speed of this noise, that is, the return light noise is generally at most 10 kHz.
Less than about. On the other hand, the driving frequency of the APC circuit 34 is set to 10 kHz, which is higher than the rising speed of the return light noise. As a result, in this embodiment, the amount of return light noise can be suppressed to 0.2%.

As described above, in the conventional method for suppressing the return light noise, it is difficult to reduce the return light noise amount to 2% or less. The effect can be said to be very significant.

The Nd: YVO ₄ crystal 13 has a short fluorescence lifetime and a sufficiently high response speed of several tens of kHz.
By using the Nd: YVO ₄ crystal 13 as the solid-state laser medium, it is possible to follow the above-mentioned driving frequency of 10 kHz or more, and the APC operation is normally performed, and the return light noise is reliably suppressed.

In this embodiment, the semiconductor laser 11
As shown in FIG. 2, the optical axis is disposed in a state where its optical axis is shifted upward with respect to the optical axis of the resonator of the solid-state laser. Although the effect of suppressing the return light noise can be obtained in this manner, if the deviation amount between the two optical axes is excessively increased in order to obtain this effect remarkably, the Nd: YVO ₄ crystal 13
Semiconductor laser beam L1 and solid state laser beam L2
And the output and efficiency are reduced, and at the same time, the beam quality is deteriorated.

However, in the present embodiment, the amount of return light noise can be reduced only by setting the driving frequency of the APC circuit 34. Therefore, it is not necessary to make the amount of deviation between the two optical axes too large. Deterioration of mode matching can be avoided.

It should be noted that if the reflectivity of the end face of the semiconductor laser 11, that is, the reflectivity of the end face of the chip, is set to be relatively high to prevent return light noise, and the output of the semiconductor laser 11 is increased to about 1 W or more, as described above. As a result, the light density and the drive current value inside the semiconductor laser are increased, and the life and reliability thereof may be deteriorated. However, in the present embodiment, the end face reflectance of the semiconductor laser 11 is set to a relatively low value of 25% or less. In other words, A
Since this is performed by setting the drive frequency of the PC circuit 34, it is not necessary to increase the end face reflectance so much. In that case, the problems of the above-mentioned life and reliability deterioration do not occur.

[Brief description of the drawings]

FIG. 1 is a schematic plan view of a semiconductor laser pumped solid-state laser according to an embodiment of the present invention.

FIG. 2 is a partially cutaway side view showing a main part of the semiconductor laser pumped solid-state laser of FIG. 1;

[Explanation of symbols]

L1 Laser beam (excitation light) L2 Solid laser beam L3 Second harmonic 11 Semiconductor laser 12 Condensing lens 13 Nd: YVO ₄ crystal 14 Resonator mirror 15 KN crystal 20,21 Holder 22,23 Reference plate 24,25 Peltier element 26 Device mounting surface 30 Holder 31 Dichroic filter 31a, 31b Light passing end face of dichroic filter 32 Partially transmitting mirror 33 APC photodetector 34 APC circuit 40,41 Thermistor 42,43 Temperature control circuit 45 Sequence controller

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2K002 AB12 CA02 HA20 5F072 AB20 HH02 JJ05 KK01 KK12 KK15 KK30 PP07 QQ02 TT15 TT28 TT29 5F073 AB27 BA09 EA27 FA25 FA30 GA02 GA12 GA14 GA23

Claims (6)

[Claims] 1. A solid-state laser medium, a semiconductor laser emitting excitation light for exciting the solid-state laser medium, a photodetector for detecting at least a part of an output laser beam, and an output signal of the photodetector. A semiconductor laser-excited solid-state laser comprising: an APC circuit that controls a drive current of the semiconductor laser so as to stabilize the output of the laser beam. The drive frequency of the APC circuit returns to the semiconductor laser. A semiconductor laser-pumped solid-state laser, wherein the frequency is set higher than the frequency of noise due to light. 2. The method according to claim 1, wherein the solid-state laser medium is Nd: YVO.
_2. The semiconductor laser-pumped solid-state laser according to claim 1, wherein the semiconductor laser is _four crystals. 3. The driving frequency of the APC circuit is 10 kHz.
The semiconductor laser-pumped solid-state laser according to claim 1 or 2, wherein: 4. The semiconductor laser pumped solid-state laser according to claim 1, wherein an output of said semiconductor laser is 1 W or more. 5. A nonlinear optical crystal for wavelength-converting solid-state laser oscillation light is provided in a solid-state laser resonator. A wavelength-converted wave by the nonlinear optical crystal is detected by the photodetector, and an output of the wavelength-converted wave is output. 5. The semiconductor laser-excited solid-state laser according to claim 1, wherein the solid-state laser is configured to make constant. 6. The semiconductor laser pump according to claim 1, wherein the semiconductor laser is arranged so that its optical axis is shifted from the optical axis of the solid-state laser resonator. Solid state laser.