JP2904189B2 - Optical parametric oscillator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical parametric oscillator for performing wavelength conversion of laser light using optical parametric oscillation, and more particularly to a semiconductor laser pumped solid-state laser as a pump source and an optical parametric oscillation inside the resonator. Resonator type optical parametric oscillator provided with a resonator for the same.

[0002]

2. Description of the Related Art Conventionally, as one method of converting the wavelength of laser light, when a nonlinear optical crystal is placed in an optical resonator in which signal light, idler light, or both resonate and pump light is incident, Optical parametric oscillation that outputs one of signal light and idler light is used.

An optical parametric oscillator generates signal light or idler light upon incidence of pump light. At this time, the power density of the pump light has a threshold value for starting optical parametric oscillation. In order to improve the wavelength conversion efficiency of laser light, it is necessary to make pump light having a high power density exceeding the above threshold value incident on the resonator of the optical parametric oscillator.

In general, in a laser oscillator, the power density inside the resonator is higher than the power threshold of the laser beam than the output extracted from the output mirror. From this,
By arranging the resonator of the optical parametric oscillation inside the resonator of the laser for generating the pump light, it is possible to easily obtain the power density of the pump light exceeding the threshold value. Such a method of arranging a resonator for optical parametric oscillation in a laser resonator for generating pump light is called an internal resonator type optical parametric oscillation.

[0005] The above-mentioned internal resonator type optical parametric oscillator is described in J. Falk et al.
Paper by IEE Journal of Quantum Electronics, Volume QE-7, Volume 7
No. 359, 1971 (IEEE Journal of Quantum E
Electronics, Vol. QE-7, No. 7, p. 359, 1971).

In the above document, a flash lamp is used to excite a solid-state laser which is a source of pump light. However, in the case of a flash-lamp-excited solid-state laser, the pulse repetition frequency is limited to several tens Hz. In recent years, research and development of an internal resonator type optical parametric oscillator using a solid-state laser pumped by a semiconductor laser as a pump source have been conducted due to the difficulty of miniaturizing the device and the short life of the flash lamp. .

[0007]

A pulse switch having a large peak value can be obtained by providing a Q switch in the internal resonator type optical parametric oscillator and instantaneously increasing the Q value in the optical resonator. However, a multi-pulse in which a plurality of pulses are generated by one Q-switch operation may occur. The principle of generation of this multipulse is described in detail in the above-mentioned literature by Joel Fork et al.

[0008] The generation of multi-pulses may be unsuitable for certain laser applications. As an example,
There is a measurement application in which the distance to the target is obtained from the round-trip time of the Q-switch laser pulse. Of the multi-pulses generated by one Q-switch operation, the second and subsequent pulses may cause erroneous measurement. . In such measurement applications, it is desirable that the Q-switched laser pulse has a higher peak value. However, it is also preferable that the energy that can be taken out as an output be dispersed to the second and subsequent pulses due to the generation of a multi-pulse. Absent.

In order to convert this multi-pulse into a single pulse, the time width of the Q-switch used for the Q-switch operation of the pump light generating laser in a low loss state, that is, the opening time is narrowed, and the pulses after the second pulse are reduced. When the laser light is generated, the number of pulses generated by one Q-switch operation can be reduced to one by making the laser cavity for generating pump light a high loss.

On the other hand, the characteristics of the single-pulsed laser output with respect to the opening time of the Q-switch exhibit a single-peak characteristic which is maximum when both the time-average output and the pulse peak value are at the optimum values of the opening time. When the time is shortened, the peak value of the pulse also decreases because a loss occurs during the generation of the single pulse.

Further, even if the opening time of the Q switch is optimally set in a specific state, the excitation intensity of the solid-state laser, the repetition frequency of the Q switch, the optical adjustment state of the laser oscillator may change, or Due to a change in environmental conditions such as temperature, a change in the diameter of the device, or the like, the output decreases or a multi-pulse occurs again.

The present invention relates to an internal cavity type optical parametric oscillator using a semiconductor laser pumped solid-state laser as a pump light source, which outputs a single pulse laser using an acousto-optic Q switch. It is another object of the present invention to prevent a decrease in output due to a change in an optical adjustment state of a laser oscillator or an aging of a device and a re-generation of a multi-pulse, and to improve the stability of an output of an optical parametric oscillator.

[0013]

SUMMARY OF THE INVENTION The present invention monitors a laser output in order to stabilize the output of an internal resonator type optical parametric oscillator using a solid-state laser pumped by a semiconductor laser as a pump source, and monitors the output of the laser. It is characterized in that corresponding control information is fed back to means for causing the solid-state laser to perform a Q-switch operation.

More specifically, the present invention provides at least an internal resonator having a resonator for optical parametric oscillation including a nonlinear optical crystal inside a semiconductor laser-pumped solid-state laser resonator having a means for Q-switch operation. In the optical parametric oscillator, means for changing the opening time during which the loss of the solid-state laser resonator is reduced by the Q switch in accordance with the input value, and means for periodically changing the opening time of the Q switch. Monitoring the time average of the laser output light to determine whether to increase or decrease the aperture time in order to maximize the laser output, and providing feedback to the means for changing the aperture time of the Q switch according to the input. Characterized by having means for giving

In this optical parametric oscillator, the means for determining whether to increase or decrease the aperture time in order to bring the aperture time closer to the optimum value includes a time-average AC component of the laser output and the aperture time. Means is provided for obtaining a product with a signal from a means for giving a periodic minute fluctuation, and means for giving feedback by discriminating increase or decrease of the opening time by the sign of the value of the product is included.

Further, the means for changing the opening time in accordance with the input value includes a comparator which compares a value obtained by integrating the output of the rectangular wave generating circuit with a reference signal including a feedback signal, and outputs the output of the rectangular wave generating circuit. And the output of the comparison circuit.

[0017]

Embodiments of the present invention will be described with reference to the drawings. Referring to FIG. 1, a semiconductor laser 1 for exciting a solid-state laser crystal 4 is arranged. Output light of the semiconductor laser 1 is condensed by lenses 2 a and 2 b and is incident on an end face mirror 3. The end face mirror 3 is provided with a coating that has low reflection with respect to the wavelength of the semiconductor laser 1 and high reflection with respect to the oscillation wavelength of the solid-state laser crystal 4. On the right side of the end face mirror 3, a solid-state laser crystal 4 is arranged. Lens 2
The output light of the semiconductor laser 1 condensed by a and 2b passes through the end face mirror 3 and then enters the solid-state laser crystal 4. The space between the end face mirror 3 and the output mirror 8 forms a resonator of the solid-state laser.

On the right side of the solid-state laser crystal 4, an acousto-optic Q switch 5 is arranged. This acousto-optic Q switch 5
As a result, a solid-state laser having a laser resonator between the end face mirror 3 and the output mirror 8 is Q-switched. The output mirror 8 is coated so as to have a high reflectance with respect to the oscillation wavelength of the solid-state laser crystal 4 and a partial transmission with respect to the wavelength of the output light of the optical parametric oscillation.

A nonlinear optical crystal 17 is arranged on the left side of the output mirror 8. The nonlinear optical crystal 7 is cut so as to obtain an optical parametric oscillation output of a desired wavelength when the light of the oscillation wavelength of the solid-state laser crystal 14 is used as pump light. On the left side of the nonlinear optical crystal 17, the internal mirror 6 is arranged. The internal mirror 6 is provided with a coating having a low reflectance with respect to the oscillation wavelength of the solid-state laser crystal 14 and a high reflectance with respect to the wavelength of signal light generated by optical parametric oscillation. Internal mirror 6
Between the output mirror 8 and the output mirror 8 is a resonator for optical parametric oscillation. Output light of the optical parametric oscillation is extracted from the output mirror 8. This is the part that becomes the laser head by the internal resonator type optical parametric oscillator.

Next, a portion for driving the acousto-optic Q switch 5 will be described with reference to FIGS. The rectangular wave generating circuit 10 generates a periodic rectangular pulse giving a repetition frequency of the Q switch operation. The repetition frequency needs to be set to a repetition frequency that is shorter than the laser upper level lifetime of the solid-state laser crystal 4.
In the case of a crystal to which d ³⁺ is added, the frequency is approximately 10 kHz or more. The output of the square wave generation circuit 10 is a pulse width modulation circuit 24.
And pulse width modulated. Pulse width modulation circuit 24
Outputs a periodic pulse whose repetition frequency is the same as that of the rectangular wave generation circuit 10 and whose pulse width is proportional to the output value of the addition circuit 26.

The high-frequency generation circuit 11 generates high-frequency power having a frequency of several tens of MHz when the input is 0 V, and stops the generation when the input is 5 V. The time width during which the high-frequency power is turned off and the loss given to the laser resonator by the Q switch becomes low is called the opening time of the Q switch. The high frequency power input to the acousto-optic Q switch 5 is reduced from several hundred ns to several μs.
When the solid state laser is turned off instantaneously during the opening time of, the solid-state laser oscillates in a Q-switch, and a signal light output of the internal resonator type optical parametric oscillator is generated. Normally, multi-pulse oscillation is performed as described above, but by shortening the opening time of the Q switch, it is possible to suppress the generation of the second and subsequent pulses among the optical pulses generated by one Q switch operation. it can.

When the repetition period of the Q-switch operation is a high repetition period shorter than the laser upper-level life of the solid-state laser crystal 4, not only the single pulse is formed by controlling the aperture time, but also the pulse peak power of the laser. Both the time averaged output increases compared to when a multi-pulse is generated.

As the aperture time is further reduced, the intensity of a single pulse is reduced. There is an optimum value for the opening time. The laser output with respect to the aperture time has a single-peak characteristic that is maximized when the aperture time reaches the optimum value for both the time average output and the pulse peak value. Therefore, the output voltage of the DC bias circuit 17 is adjusted in order to optimally initialize the aperture time so that the laser output is maximized.

Next, a feedback circuit for obtaining control information for controlling the opening time of the Q switch will be described with reference to FIG. The sine wave generation circuit 22 generates a minute sine wave signal having a frequency of several Hz to several hundred Hz, and the output of the sine wave generation circuit 22 is applied to a control circuit 25 and an addition circuit 26. In the addition circuit 26,
The DC bias voltage for optimally initializing the opening time of the Q switch and the sine wave signal are added and input to the pulse width modulation circuit. It changes periodically according to the output. Therefore, the laser output also changes periodically.

A part of the laser output light is extracted by the partial reflecting mirror 9 and received by the photodetector 18. The photodetector 18 is capable of responding satisfactorily to a change in the output of the repetition frequency of the sine wave generation circuit 22, but cannot respond to the repetition frequency of the laser, and outputs a voltage proportional to the time average output. Therefore, an output corresponding to the repetition frequency of the sine wave generation circuit 22 is obtained from the photodetector 18.

The control circuit 25 determines from the signal from the photodetector 18 and the signal from the sine wave output circuit 22 whether to increase or decrease the opening time to control the opening time of the Q switch to an optimum value. An output obtained by discriminating the result by a positive voltage and a negative voltage is applied to an adding circuit 26. By this feedback, even when the set value of the opening time of the Q switch by the DC bias circuit 17 does not match the optimum value, the pulse time is automatically corrected so that the laser output is maintained at the maximum value. The DC bias voltage output to the width modulation circuit 24 is controlled.

[0027]

FIG. 3 is a diagram showing an embodiment embodying the present invention. In FIG. 3, a semiconductor laser 1 for exciting a solid-state laser crystal is of an optical fiber coupling type, and an excitation laser beam is output from the other end of an optical fiber 27 connected to the semiconductor laser 1. Is done. The semiconductor laser 1 has a wavelength of 810 nm and an optical fiber end output of 10 W. Lens 2 has a focal length of 10
mm, the distance from the optical fiber end of the semiconductor laser 1 is about 20 mm. The focused diameter of the semiconductor laser light is about 0.5 mm.

The end mirror 3 has a flat surface on the lens 2 side and a concave surface having a radius of curvature of -5 m on the opposite side. The surface of the end mirror 3 on the lens 2 side has a reflectance of 0.5% at a wavelength of 810 nm, and the opposite surface has a reflectance of 0.5%.
The reflectance is 5% at a wavelength of 810 nm and the wavelength is 10
Each coating is applied so that the reflectance is 99% for 64 nm. For the solid-state laser crystal, an Nd: YVO ₄ crystal 4 having a length of 3 mm is used.

The Nd: YVO ₄ crystal 4 has an a-axis cut, and is arranged such that its c-axis is perpendicular to the optical axis of the laser resonator. In the oscillation by the Nd: YVO ₄ crystal 4, the electric field component is parallel to the c-axis of the crystal, and the oscillation wavelength is 1064 nm. The acousto-optic Q switch 5 uses a tellurium dioxide (TeO ₂ ) crystal as a medium, and has a length of 20 mm in the direction in which the laser light propagates.

The internal mirror 6 has a convex surface on the Q switch 5 side,
The opposite side is concave and their radius of curvature is 0.2 m. The surface of the internal mirror 6 on the side of the Q switch 5 has a reflectance of 0.5% at a wavelength of 1064 nm.
The opposite surface has a reflectance of 5% for a wavelength of 1064 nm and a reflectance of 9 for a wavelength of 1573 nm.
Each coating is applied to 9%. The thickness of the internal mirror 6 is 5 mm.

The nonlinear optical crystal has a KT having a length of 30 mm.
A P (KTiOPO ₄ ) crystal 7 is used. The KTP crystal 7 has an x-axis cut with a crystal orientation θ = 90 ° and φ = 0 °, and is arranged so that its y-axis is parallel to the c-axis of the Nd: YVO ₄ crystal 4. With this arrangement, the oscillation light having a wavelength of 1064 nm, which becomes the pump light of the optical parametric oscillation, enters the KTP crystal 7 such that its electric field component is parallel to the y-axis of the KTP crystal 7. At this time, the KTP crystal 7 has a wavelength of 1573 nm and an electric field component of the KTP crystal 7.
, And idler light having a wavelength of 3.3 μm and an electric field component parallel to the z-axis of the KTP crystal 7 are generated.

The output mirror 8 is a plane having an infinite radius of curvature. The surface of the output mirror 8 on the KTP crystal 7 side has a reflectance of 99% at a wavelength of 1064 nm and a reflectance of about 90% at a wavelength of 1573 nm.
The opposite surface has a reflectance of 0.1 at a wavelength of 1573 nm.
Each is coated so as to be 5%. The space between the end face mirror 3 and the output mirror 8 serves as a resonator of the Nd: YVO ₄ laser, and the space between the internal mirror 6 and the output mirror 8 serves as a resonator for optical parametric oscillation.

Next, a portion for driving the acousto-optic Q switch 5 will be described with reference to a time chart of FIG.
The rectangular wave generating circuit 10 generates a periodic rectangular pulse giving a repetition frequency of the Q switch operation. The repetition frequency is set to 100 kHz and the pulse width is set to 1 μs. One of the outputs of the rectangular wave generation circuit 10 is input to the integration circuit 13, and the integration circuit 13 outputs a sawtooth wave. The output of the integrating circuit 13 and the output of the adding circuit 15 are compared by a comparing circuit 14, and a voltage of 5 V is output when the output of the integrating circuit is larger, and conversely, when the output of the adding circuit 15 is larger. Outputs a voltage of 0V.

The output of the comparison circuit 12 and the output of the rectangular wave generation circuit 10 are ANDed by the AND circuit 12, and a 1 V output is generated and a 0 V output is generated when the output is 0. The integration circuit 13, the comparison circuit 14, and the AND circuit 12 apply the rectangular wave pulse of the rectangular wave generation circuit 10 to the addition circuit 1.
The pulse width modulation circuit modulates the pulse into a pulse having a pulse width proportional to the input from No. 5.

The high frequency generating circuit 11 generates a high frequency of 80 MHz when the input is 0 V, and stops generating the high frequency when the input is 5 V. High-frequency power generated by the high-frequency generation circuit 11 enters the acousto-optic Q switch 5, and the laser head operates as a Q switch. In the laser head according to the present embodiment, the optimum value of the opening time of the Q switch at which the laser output is maximized is about 130 ns. In order to initialize the opening time to the optimum value, the output voltage of the DC bias circuit 17 is adjusted in advance.

Next, a feedback circuit for obtaining control information for controlling the opening time of the Q switch and its operation will be described with reference to FIGS. Note that the time chart of FIG. 5 shows the operation when the opening time of the Q switch is longer than the optimum value.

The sine wave generation circuit 22 has a frequency of 20 Hz
5 (1) shown in FIG. 5, the output of the sine wave generation circuit 22 is supplied to the addition circuit 16 and the multiplication circuit 2
Added to 1. In the addition circuit 16, the DC bias voltage of the DC bias circuit 17 and the sine wave signal are added and input to the pulse width modulation circuit having the above configuration.
Changes periodically in accordance with the output of. Therefore, the laser output also changes periodically.

Considering the case where the opening time of the Q switch by the DC bias circuit 17 is shifted to a direction longer than the optimum value, at this time, the laser output also shows a periodic change due to the sine wave signal. The phase of the output of the sine wave generation circuit 22 is shifted by 180 °. A part of this laser output is taken out by the partial reflection mirror 9 for monitoring and supplied to the photodetector 18.

As the photodetector 18, a sine wave generating circuit 22
Although it can respond to the output change of the repetition frequency, but cannot respond to the repetition frequency of the laser and uses a narrow band characteristic that outputs a voltage proportional to the time average output, the photodetector 18 outputs Output in response to a periodic change in the laser output due to the sine wave signal (FIG. 5 (2))
Is obtained.

The output of the photodetector 18 is input to a DC cut filter 19, where only the AC component is extracted, further amplified by an amplifier 20, and then input to a multiplication circuit 21.

In the multiplication circuit 21, the output of the amplifier 20 (FIG. 5 (3)) and the output of the sine wave generation circuit 22 (FIG.
(1)) is multiplied, and the output is a periodic wave (FIG. 5 (4)) which always takes a negative value. The output of the multiplying circuit 21 is input to a low-pass filter 23,
Output only a DC component having a negative value (FIG. 5 (5)).

Therefore, when the opening time of the Q switch is set longer than the optimum value by the DC bias circuit 17, the output of the low-pass filter 23 is added to the adding circuit 23, so that the opening time of the Q switch is It is controlled so as to approach the shorter direction, that is, the optimum value.

When the opening time of the Q switch by the DC bias circuit 17 is set to be shorter than the optimum value, the time average of the laser output and the time average of the output of the photodetector 18 become a sine wave. It has the same phase as the output of the generation circuit 22. Therefore, the output of the multiplying circuit 21 has a periodic output waveform having a positive value, and the output of the low-pass filter has a DC component having a positive value. Therefore, a positive DC component is added to the output voltage of the DC bias circuit 17, and the opening time of the Q switch is increased so as to approach the optimum value.

When the output voltage of the DC bias circuit 17 is adjusted and the opening time of the Q switch is set to an optimum value, the laser output is a time-average output when the output of the semiconductor laser 1 is 10 W and the pulse repetition frequency is 50 kHz. And the pulse energy is about 20 μJ. Even if the output voltage of the DC bias circuit 17 is fixed in this state and the pulse repetition frequency is changed between 30 kHz and 150 kHz, or the output of the semiconductor laser 1 is changed between 7 W and 10 W, The opening time can always be kept at the optimum value.

[0045]

According to the present invention, since a means for controlling the opening time of the Q switch is provided for an internal resonator type optical parametric oscillator in which multi-pulses are easily generated, a single pulse can be easily formed.

Further, the laser output is monitored by giving a periodic minute change to the aperture time, and an appropriate signal for bringing the aperture time close to the optimum value is fed back. Even when the repetition frequency of the Q switch changes, the opening time of the Q switch can always be set to the optimum value.

[0047]

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a time chart illustrating an operation of the exemplary embodiment of the present invention.

FIG. 3 is a block diagram showing an embodiment of the present invention.

FIG. 4 is a time chart showing the operation of the embodiment of the present invention.

FIG. 5 is a time chart showing the operation of the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2a, 2b lens 3 End face mirror 4 Solid-state laser crystal 4a Nd: YVO4 crystal 5 Acousto-optic Q switch 6 Internal mirror 7 Nonlinear optical crystal 7a KTP crystal 8 Output mirror 9 Partial reflection mirror 10 Square wave generation circuit 11 High frequency generation circuit REFERENCE SIGNS LIST 12 AND circuit 13 integration circuit 14 comparison circuit 15, 16, 26 addition circuit 17 DC bias circuit 18 photodetector 19 DC cut filter 20 amplifier 21 multiplication circuit 22 sine wave generation circuit 23 low-pass filter 24 pulse width modulation circuit 25 control circuit 27 Optical fiber

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/11-3/127 G02F 1/39 H01S 3/108-3/109

Claims (5)

(57) [Claims] 1. An internal-cavity optical parametric oscillator having a resonator for optical parametric oscillation including a nonlinear optical crystal inside a semiconductor laser-pumped solid-state laser resonator having means for Q-switch operation, An internal-cavity optical parametric oscillator comprising means for automatically controlling an opening time during which a loss of a solid-state laser resonator is reduced by a Q switch so as to maximize a laser output. 2. An internal-cavity optical parametric oscillator having a resonator for optical parametric oscillation including a nonlinear optical crystal inside a semiconductor laser-pumped solid-state laser resonator having means for Q-switch operation. Means for changing the aperture time during which the loss of the solid-state laser resonator is reduced by the Q switch, means for periodically varying the aperture time of the Q switch, and monitoring the time average of the laser output light. An internal resonator type optical parametric oscillator comprising: means for giving a control signal to means for changing the aperture time so that the laser output is maximized. 3. The means for providing the control signal includes means for obtaining a product of an AC component of a time average value of a laser output and a signal from a means for providing a small periodic change in the aperture time. 3. The internal resonator type optical parametric oscillator according to claim 2, wherein a control signal is output in which the direction of increase or decrease of the opening time is distinguished by the sign of the product value. 4. The means for changing the aperture time comprises pulse width modulation means for pulse width modulating a periodic rectangular pulse giving a repetition frequency of the Q switch operation by a signal including the control signal. 3. The internal resonator type optical parametric oscillator according to claim 2, wherein: 5. The pulse width modulation means according to claim 1, wherein said pulse width modulation means compares a signal obtained by integrating said rectangular pulse with a signal including said control signal, and ANDs an AND of said rectangular pulse and an output of said comparison circuit. The internal resonator type optical parametric oscillator according to claim 4, further comprising a circuit.