JPH0529695A - Laser apparatus - Google Patents

Abstract

PURPOSE:To obtain a semiconductor laser-excited laser having a stable optical output by lowering a temperature of a heat sink by a temperature corresponding to a product of power consumption change of a light irradiating point due to a variation in a driving current of the semiconductor laser owing to a negative feedback of an optical output of the laser and a thermal resistance between the heat sinks. CONSTITUTION:Part of a laser output light is guided to a photodetector 29, and further fed back to a semiconductor laser. Then, a temperature change DELTATH = a junction voltage VLXa driving current change DELTAILX thermal resistance RTH of a heat sink is calculated by a calculator 37. Further, the DELTATH is subtracted from a temperature set value 38 of the heat sink by a differential amplifier 39, and a set value 42 of new temperature is obtained. The heat sink temperature of the semiconductor laser is obtained from the output of a temperature sensor 22 through an amplifier 40. A difference between the output value 43 of the amplifier 40 and the set value 42 is obtained by a differential amplifier 41, and its output is fed back to the heat sink temperature of the semiconductor laser by a thermoelectric cooling element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control mechanism for always keeping the output light from a laser device constant.

[0002]

2. Description of the Related Art In general, the absorption of light in a laser medium has a wavelength dependency and has a spectrum having a sharp absorption line at a specific wavelength. It is necessary to match the wavelength of the absorption line of the medium. Semiconductor lasers have a high conversion effect from electrical input to optical output, and the spectral line width of their output is narrow, so they are extremely compact when used as an excitation light source for solid-state lasers and liquid lasers that were conventionally excited by high-pressure gas discharge light sources. It is possible to construct a highly efficient laser. However, since the wavelength of the semiconductor laser changes depending on the external temperature and the driving current, in the conventional semiconductor laser pumped laser device, in order to keep the pumping efficiency constant, the temperature of the envelope of the semiconductor laser is kept constant. The output has been stabilized by using a control device and a control circuit for keeping the drive current constant.

[0003]

When a temperature change or mechanical vibration is applied as an external factor to such a laser device excited by a semiconductor laser, the optical system shifts or the effect of the laser medium itself changes. As a result, the optical output fluctuates.
However, the semiconductor laser has a linear relationship between the drive current of the semiconductor laser and the optical output of the laser device because the optical output increases even if the drive current is changed even if the temperature of the envelope is kept constant, but the emission wavelength changes. Not in. Similarly, when the driving current of the semiconductor laser is kept constant and the temperature of the semiconductor laser is changed, the optical output does not show a linear relationship with the temperature.
Therefore, it is not possible to detect the optical output in order to correct the optical output fluctuation of the above laser device and use negative feedback control so that the value becomes constant, and stabilize the optical output of the laser device excited by the semiconductor laser. It was difficult to do.

The present invention solves the above problems by controlling the drive current and temperature of a semiconductor laser according to a certain law, and provides a semiconductor laser pumped laser device with stable optical output.

[0005]

In order to solve the above-mentioned problems, the present invention detects the optical output of a laser device excited by a semiconductor laser and applies a negative feedback to the drive current of the semiconductor laser, and the negative feedback. The temperature of the envelope of the semiconductor laser is lowered by a temperature corresponding to the product of the change in the power consumption at the emission point of the semiconductor laser due to the change of the driving current of the semiconductor laser due to the temperature difference and the thermal resistance between the emission point and the envelope. The means for this is provided.

In the above laser device excited by the semiconductor laser, a thermoelectric element utilizing the Peltier effect is used as a temperature control element of the semiconductor laser.

[0007]

The function of the present invention is used as a semiconductor laser and has an oscillation wavelength of 8
An AlGaAs semiconductor laser having a wavelength of around 00 nm is used, and a Nd-doped YAG crystal (hereinafter referred to as Nd-YAG) is used as a laser medium.
Will be described as an example. FIG. 2 shows an absorption spectrum of Nd-YAG near a wavelength of 800 nm. The absorption spectrum has a bimodal peak in the wavelength range of 800 nm to 810 nm. FIG. 3 shows the oscillation spectrum of the semiconductor laser. The oscillation wavelength of the laser has a wavelength width of 0.01 nm or less in the single mode oscillation, but in the multimode oscillation as shown in this example, several oscillation lines appear simultaneously and have a width of several nm as a whole. FIG. 4 shows the temperature dependence of the oscillation wavelength of the semiconductor laser when the light output is constant. The wavelength usually changes with a coefficient of 0.2 to 0.3 nm / ° C. This is because the band structure of the semiconductor laser has a temperature dependence, and is specific to the material forming the semiconductor laser. FIG. 5 shows changes in the oscillation wavelength when the driving current of the semiconductor laser is changed while keeping the temperature constant. The optical output of the semiconductor laser increases almost linearly with the drive current, but the oscillation wavelength shifts to a longer wavelength as the drive current increases. Therefore, in the absorption spectrum of Nd-YAG, the excitation efficiency of the semiconductor laser changes depending on the correlation of the oscillation wavelength of the semiconductor laser. When the drive current and temperature of the semiconductor laser are changed, the oscillation wavelength changes at the same time as the light output, so the relationship between the temperature and drive current of the semiconductor laser and the light output of the Nd-YAG laser cannot be uniformly determined. FIG. 6 shows the configuration of such a semiconductor laser pumped Nd-YAG laser. The semiconductor laser 1 is driven by a semiconductor laser driving current source 7. The optical output of the semiconductor laser 1 is N
It is incident on d-YAG2 and outputs Nd-YAG laser light 3
Is obtained. In order to stabilize the light source 3, for example, a part of the light beam 3 is taken out by the half mirror 4, the light output intensity is detected by the photodetector 5, and the light output intensity is kept constant. If the wavelength is prevented from changing even if the current value of the current source 1 of the semiconductor laser 1 is changed by some method, the output of the photodetector 5 is changed to the driving current source of the semiconductor laser via the amplifier 6. The optical output 3 can be kept constant by applying the negative feedback.

FIG. 7 is a schematic diagram showing a state of mounting a semiconductor laser chip on an envelope. A semiconductor laser chip 8 is fixed to a heat sink 10 via a submount 9. The heat sink is a metal having a high thermal conductivity, and plays a role of finally transmitting the heat generated in the semiconductor laser chip to the envelope 11 and radiating it outside. When the temperature of the envelope is controlled, the temperature detecting element 12 is usually mounted on the heat sink. There is a thermal resistance between the light emitting point 13 of the semiconductor laser chip and the heat sink 10. Even if the temperature of the heat sink is kept constant by the temperature detecting element 12, if the drive current is increased, the temperature of the heat sink 13 is reduced by the thermal resistance. As a result, the wavelength shift increases as shown in FIG. FIG. 8 shows a state in which the heat flow in the vicinity of the semiconductor laser chip is replaced with an electric circuit.
The heat generated at the light emitting point 17 of the chip is indicated by the heat source 14, and the generated heat reaches the heat sink 18 via the thermal resistance 15. Further, the heat sink is a cooling device 1 for the outside air 19.
6 controls the temperature. Here, the heat generation amount W _L of the heat source 14 is represented by the product I _L × V _L of the semiconductor laser drive current I _L and the junction voltage V _L of the semiconductor laser. V _L is wide I _L
The heat generation amount W _L is approximately proportional to the drive current I _L of the semiconductor laser, since it is substantially constant in the range. The temperature T _L of the light emitting point 17 inside the chip increases by a temperature corresponding to the product of the amount of heat generation and the thermal resistance R _TH with respect to the heat sink temperature T _H.
That is, T _L = T _H + W _L × R _TH = T _H + V _L · R _TH × I _L
And the emission point temperature T _L rises in proportion to the drive current I _L of the semiconductor laser. Since the oscillation wavelength of the semiconductor laser depends on the temperature as shown in FIG. 4, the semiconductor laser oscillation wavelength depends on the driving current due to the change of the emission point temperature. Here, according to the principle of the present invention, when the drive current is increased, the temperature rise of the chip light emitting point due to the increase of the heat generation amount due to the increase is calculated in advance from the product of the power consumption of the chip and the thermal resistance, and the temperature T of the heat sink 18 is _By lowering _H by the cooling device 16, the temperature of the light emitting point is always kept constant irrespective of the drive current, and the wavelength fluctuation does not occur. That is, when the light output decreases due to some cause of the system operating in the steady state, the current of the semiconductor laser is increased by ΔI _L to increase the light output. In that case, the heat sink 1 is cooled by the cooling device 16.
By lowering the temperature of 8 by V _L · R _TH × ΔI _L
L is always constant regardless of ΔI _L , and the oscillation wavelength of the semiconductor laser does not fluctuate. Under such conditions, a linear relationship is maintained between the optical output of the semiconductor laser pumped laser and the driving current I _L of the semiconductor laser, and in FIG. 6, the output of the photodetector 5 drives the semiconductor laser. Current source 7
Negative feedback can be applied to, and stabilization of the optical output 3 is achieved.

[0009]

FIG. 1 shows an embodiment of the present invention. The semiconductor laser 20 oscillates at a wavelength near 808 nm which is the Nd-YAG excitation wavelength. The temperature of the case of the semiconductor laser is controlled by the thermoelectric cooling element 21 utilizing the Peltier effect.
The optical output of the semiconductor laser is Nd- by the focusing optical system 23.
It is focused on the YAG laser rod 24 and excites Nd ions in YAG. The YAG laser oscillates by the optical resonator formed by the reflecting thin film 25 and the spherical mirror 26 provided on the end surface of the YAG laser rod, and becomes the YAG laser light output 27 with a wavelength of 1.06 μm. Part of the laser output light is guided to the photodetector 29 by the half mirror 28. The output of the photodetector is converted into a voltage signal by the amplifier 30. An error signal between the output of 30 and the optical output setting value 32 of the Nd-YAG laser is obtained by the differential amplifier 31. The output of the differential amplifier becomes a driving current signal of the semiconductor laser through the integrating circuit 33, and is fed back to the semiconductor laser by the semiconductor laser current driving amplifier 34. Here, in the integrating circuit 33, the time constant of the current feedback loop is set to be larger than the time constant of the temperature control loop including the cooling device 21 for the semiconductor laser, and the entire control system becomes in an oscillating state so that the driving current of the semiconductor laser becomes excessive. This is to prevent it from becoming. The drive current signal of the semiconductor laser is set to the drive current setting value 3 by the differential amplifier 36.
The current change value ΔI _L from 5 is obtained. Temperature change of the heat sink by the calculator 37 from ΔI _L ΔT _H = V
_L · ΔI _L × R _TH is calculated. In this case, since the junction voltage V _L and the thermal resistance R _TH of the semiconductor laser are approximately constant even if the current of the semiconductor laser is changed, 37 is a mere linear amplifier. The differential amplifier 39 subtracts ΔT _H from the heat sink temperature set point 38 to obtain a new temperature set point 42. The heat sink temperature of the semiconductor laser is obtained from the output of the temperature sensor 22 via the amplifier 40. The difference between the output value 43 of the amplifier 40 and the set temperature value 42 is obtained by the differential amplifier 41, and the thermoelectric cooling element feeds back the heat sink temperature of the semiconductor laser at the output of 41.

FIG. 8 shows another embodiment of the present invention. In this embodiment, a nonlinear optical element KTP44 is installed inside the resonator of a YAG laser as an optical system, and a reflecting mirror 45 for optical output is used.
High reflectivity for 1.06 μm, second harmonic 530
530 nm by applying a reflective film with a low reflectance for nm
The SHG solid-state laser configured to effectively extract light is used, and the same control system as that of the first embodiment is used. In such an embodiment, the light output is stabilized as in the first embodiment. Achieved Further, as a laser medium, Nd-YA
The stabilization of the optical output can be achieved by applying a similar control system to the other solid-state laser medium of G and the dye laser medium. FIG. 10 is an example showing the effect of the present invention. Here, in the first embodiment, the change in the YAG laser output when the temperature of the laser resonator was changed as an external factor was measured. When the optical output of the YAG laser is set at an ambient temperature of 25 ° C and the temperature is changed in the range of 15 ° C to 35 ° C, the output fluctuation without any feedback control is about 15% as shown in 46. Is. On the other hand, in the embodiment of FIG. 1, when the feedback current is applied only to the driving current of the semiconductor laser and the temperature feedback is not applied by the differential amplifier 36, the output fluctuation has an extremely narrow control range, and as shown by 47, a large change occurs. Show. FIG. 1 of the present invention
When the current feedback and the temperature feedback are used together as in the embodiment shown in FIG. 5, the output fluctuation is suppressed to 5% or less as shown by 48.

[0011]

As described above, according to the present invention, it is possible to construct a control system for keeping the light output constant in a laser device excited by a semiconductor laser, and obtain a light source having a stable light output. It will be possible.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a laser device according to an embodiment of the present invention.

FIG. 2 shows a light absorption spectrum of Nd-YAG.

FIG. 3 shows a typical example of an oscillation spectrum of a GaAlAs semiconductor laser.

FIG. 4 shows temperature dependence of a semiconductor laser oscillation wavelength.

FIG. 5 shows the driving current dependency of the semiconductor laser oscillation wavelength.

FIG. 6 is a general configuration diagram of a semiconductor laser pumped YAG laser.

FIG. 7 is a schematic diagram of a mounted state of a semiconductor laser chip.

FIG. 8 shows a heat flow in the vicinity of a semiconductor laser chip.

FIG. 9 is a configuration diagram of an optical portion of a laser device according to another embodiment of the present invention.

FIG. 10 is a graph showing changes in the optical output of the YAG laser with respect to changes in the temperature of the laser resonator, showing the effects of the present invention.

[Explanation of symbols]

1 Semiconductor laser 2 Nd-YAG 3 Nd-YAG laser output light 4 half mirror 5 Photodetector 6 amplifier 7 Semiconductor laser drive current source 8 Semiconductor laser chip 9 submount 10 heat sink 11 envelope 12 Temperature detection element 13 Light emitting point of semiconductor laser chip 14 Semiconductor laser chip light source at the emission point 15 Heat resistance 16 Cooling system 17 light emitting point 18 heat sink 19 Outside air 20 Semiconductor laser 21 Thermoelectric cooling element 22 Temperature sensor 23 Focusing optical system 24 Nd-YAG laser rod 25 Reflective thin film 26 Spherical mirror 27 YAG laser light output 28 half mirror 30 amplifier 31 differential amplifier 32 light output setting 33 integrating circuit 34 Semiconductor laser current drive amplifier 35 Drive current setting value 36 differential amplifier 37 calculator 38 Temperature setting of heat sink 39 Differential amplifier 40 amplifier 41 Differential amplifier 42 New temperature setting value 43 Output value of amplifier 40 44 Non-linear optical element KTP 45 Optical output reflector 46 Output fluctuation without feedback control 47 Output fluctuation when feedback is applied only to drive current 48 Output fluctuation when using both current feedback and temperature feedback

Claims (2)

[Claims] 1. A semiconductor laser capable of controlling the temperature of an envelope to be constant irrespective of environmental temperature, another laser medium having an absorption line in the vicinity of a wavelength of output light of the semiconductor laser, and the semiconductor. An optical system for utilizing laser output light as a light source for exciting the laser medium, and a laser device having an optical resonator for outputting laser light from the laser medium, detecting output light intensity of the laser light. Then, the driving current of the semiconductor laser is negatively fed back according to the value, and the change of the power consumption of the semiconductor laser due to the change of the driving current of the semiconductor laser due to the negative feedback, the light emitting point of the semiconductor laser, and the envelope. A control circuit for keeping the optical output of the laser light always constant by lowering the envelope temperature of the semiconductor laser at a temperature corresponding to the product of thermal resistance between Laser device. 2. A laser device according to claim 1, wherein a thermoelectric element utilizing a Peltier effect is used as a temperature control element of the semiconductor laser.