JP2009289894A - Laser oscillation device - Google Patents

Abstract

Provided is a laser oscillation device capable of changing the oscillation repetition frequency of laser light without changing laser output characteristics.
An EOQ switching element is switched in synchronization with an oscillation pulse of excitation light thinned out by a predetermined number of pulses with respect to an oscillation pulse of excitation light for exciting a YAG rod, thereby causing laser oscillation. The oscillation repetition frequency of the excitation light is not changed, the heat input condition for the YAG rod 14 that is a laser medium is made constant, and only the driving repetition frequency of the EOQ switch element 16 is changed. The laser light oscillation repetition frequency can be changed without changing the laser output characteristics.
[Selection] Figure 1

Description

  The present invention relates to a laser oscillation device including a Q switch.

  In the fields of laser ablation, laser-induced fluorescence analysis, laser peening and the like, a giant pulse (hereinafter referred to as GP) oscillation laser light having a pulse width of several nsec and a peak power of 1 MW or more is used.

  In the solid-state laser, the upper level lifetime of the laser transition is generally long. Therefore, the laser medium is sufficiently excited, the laser oscillation is suppressed by the Q switch inserted in the laser oscillator, and when the stored energy of the laser medium becomes sufficiently large, the Q switch is switched. Since laser oscillation occurs at that moment in a short time, GP oscillation laser light having a short pulse width and a high peak power can be obtained (for example, see Non-Patent Document 1).

  FIG. 9 shows a laser oscillation device 1 that oscillates GP oscillation laser light. On the central axis of the laser oscillator 2, an output mirror 3, a YAG rod 4 as a laser medium, a Q switch 5, and a rear mirror 6 are arranged, and a flash lamp 7 is arranged facing the YAG rod 4.

  Then, the YAG rod 4 is excited by the excitation light of the flash lamp 7, and the light emitted from the YAG rod 4 enters the Q switch 5. When the Q switch 5 is in a state in which laser oscillation is suppressed by voltage application, no laser oscillation occurs between the rear mirror 6 and the output mirror 3, so that energy is accumulated in the YAG rod 4. When energy storage is started, that is, after a delay time of usually 100 to 300 μsec has elapsed since the excitation of the YAG rod 4 is started, when the voltage application of the Q switch 5 is stopped and laser oscillation is allowed, Laser oscillation occurs between the output mirror 3 and GP oscillation laser light is emitted from the output mirror 3.

  As shown in FIG. 10, the flash lamp 7 oscillates excitation light at a constant repetition frequency by a flash lamp trigger, and the Q switch 5 is generated by a Q switch trigger that is generated with a certain delay time for each oscillation of the excitation light. Is switched to obtain a GP oscillation laser beam.

In addition, as shown in FIG. 11, when the GP oscillation laser light is collected by the incident lens 8 and enters the optical fiber 9 having a core diameter of about 100 microns to 1 mm, the peak power exceeds several MW. The peak power density is very high, on the order of sub GW / cm ^{2 to} GW / cm ^2, and the optical fiber 9 is damaged due to electron avalanche and multiphoton absorption due to the difference in the incident conditions. The fiber 9 is broken and beam transmission becomes difficult.

  Therefore, it is necessary to change the oscillation repetition frequency of the GP oscillation laser light. Conventionally, the oscillation repetition frequency of the excitation light by the flash lamp 7 is changed.

Therefore, when it is necessary to change the oscillation repetition frequency of the GP oscillation laser light, conventionally, the oscillation repetition frequency of the excitation light by the flash lamp 7 is changed. In this case, the YAG rod 4 which is a laser medium is used. Since the laser beam characteristics (beam aperture, beam divergence angle, output, etc.) change due to changes in the heat input to the optical fiber 9, it is necessary to adjust the incident position on the optical fiber 9 again.
Laser Society, “Laser Handbook”, Ohmsha, December 15, 1982, p. 221-223

  When the laser beam is incident on the optical fiber, it is necessary to perform not only the amount of incident light but also the spatial characteristics of the optical fiber such as the incident aperture and the incident angle under a certain condition.

  However, in order to change the oscillation repetition frequency of the GP oscillation laser light, since the oscillation repetition frequency of the excitation light has been changed conventionally, a change in heat input to the YAG rod, which is a laser medium, occurs. Since the spatial characteristics of the light change, it is necessary to adjust the incident position with respect to the optical fiber in order to match the incident beam aperture with respect to the optical fiber.

  In addition, when laser light is incident on the optical fiber, the diverged laser light is incident on the optical fiber after being focused before the optical fiber so that the inside of the optical fiber cannot be focused. If the value is lowered, the effect of the thermal lens effect is weakened and the beam divergence angle is reduced, so that the light condensing property is improved and the focal condensing aperture is reduced. Then, air breakdown occurs due to optical dielectric breakdown of air near the focal position, and laser light cannot be stably transmitted to the optical fiber. For this reason, it may be difficult to transmit the laser light to the optical fiber by changing the oscillation repetition frequency to various values.

  As described above, the change of the beam pattern and the beam divergence angle of the laser beam that occurs when the oscillation repetition frequency is changed changes the degree of the thermal lens effect by changing the heat input of the excitation light to the YAG rod that is the laser medium. Caused by

  The present invention has been made in view of these points, and an object of the present invention is to provide a laser oscillation device capable of changing the oscillation repetition frequency of laser light without changing the laser output characteristics.

  The present invention relates to a laser oscillator, a laser medium provided in the laser oscillator, excitation light oscillation means for oscillating excitation light for exciting the laser medium at a constant repetition frequency, and suppressing laser oscillation of the laser oscillator. A Q switch that switches between a state of oscillating and a state of laser oscillation, and the Q switch in synchronization with the oscillation pulse of the excitation light thinned out by a predetermined number of pulses with respect to the oscillation pulse of the excitation light by the excitation light oscillation means Switching operation means for switching from a state in which laser oscillation is suppressed to a state in which laser oscillation is performed.

  According to the present invention, the laser light beam pattern and beam divergence angle can be changed by changing the number of repetitions of the Q-switch drive repetition frequency by constantly maintaining the heat input condition for the laser medium without changing the oscillation repetition frequency of the excitation light. It is possible to change the oscillation repetition frequency of the laser light without changing the laser output characteristics.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  1 to 4 show a first embodiment.

  As shown in FIG. 1, the laser oscillation device 11 includes an EOQ switch voltage drop operation type laser oscillator 12 that oscillates GP oscillation laser light. In this laser oscillator 12, an output mirror 13, a YAG rod 14 as a laser medium, a polarizer 15, an EOQ (Electro Optical Q) switch element (Pockels cell) 16 and a rear mirror 17 are arranged on a central axis. Yes. A laser resonator 18 is configured between the output mirror 13 and the rear mirror 17. Opposite to the YAG rod 14, a flash lamp 19 is disposed as excitation light oscillation means.

  The EOQ switch element 16 utilizes an electro-optic effect (also called a Pockels effect). When an electric field is applied to a certain type of crystal (for example, a KD * P uniaxial crystal), it is refracted with respect to polarized light having a field direction component. A phenomenon in which the rate changes, and the polarization changes as it passes through the crystal depending on the applied voltage. When a λ / 4 voltage is applied to the EOQ switch element 16, it acts as a λ / 4 plate and can convert linearly polarized light into circularly polarized light. When a λ / 2 voltage is applied, it acts as a λ / 2 plate and is incident. It is possible to rotate the direction of the linearly polarized light 90 degrees. In order to obtain a GP oscillation laser beam having a pulse width of several nsec, Q-switching with quick response is required, and a Pockels cell is used for the EOQ switch element 16.

  The laser oscillation device 11 includes a pulse generator 21 such as a 2-channel output digital delay generator with a delay function. The pulse generator 21 generates a flash lamp trigger pulse and a pulse counter input trigger that is synchronized with the flash lamp trigger pulse and delayed by an EOQ drive delay time of about 100 to 300 μsec. The flash lamp trigger pulse is input to the flash lamp power supply 22, and the flash lamp power supply 22 supplies a flash lamp driving voltage to the flash lamp 19 to cause the flash lamp 19 to emit light. The pulse counter input trigger is input to the pulse counter 23 as a switching operation means, and the pulse counter output trigger (EOQ switch trigger) of the pulse counter 23 is input to the EOQ switch driver 24, and the EOQ switch driver 24 drives the EOQ switch. A voltage is supplied to the EOQ switch element 16 to drive the EOQ switch element 16 on / off.

  The GP oscillation laser light oscillated from the laser oscillator 12 is collected by the incident lens 26 and is incident on the optical fiber 27.

  As shown in FIG. 2 (a), symbol a in FIG. 2 is the light traveling direction, symbol b is linearly polarized light parallel to the paper surface, symbol c is linearly polarized light perpendicular to the paper surface, and symbol d is circularly polarized clockwise. E indicates a circularly polarized light counterclockwise.) When the YAG rod 14 is excited by the excitation light of the flash lamp 19, the linearly polarized light component of the light emitted from the YAG rod 14 is extracted by the polarizer 15. The light enters the EOQ switch element 16. When the λ / 4 voltage is applied to the EOQ switch element 16, the EOQ switch element 16 performs the same polarization operation as the λ / 4 plate, and the light that has passed through the EOQ switch element 16 changes to circularly polarized light. When this circularly polarized light is reflected by the rear mirror 17, the rotation of the polarized light is reversed. Therefore, the light incident on the EOQ switch element 16 again and returned through the EOQ switch element 16 has a polarization direction with respect to the outgoing light. It becomes linearly polarized light orthogonal to 90 degrees. For this reason, it cannot pass through the polarizer 15 and cannot oscillate, so that it functions as an optical shutter. At this time, energy is accumulated in the YAG rod 14.

  As shown in FIG. 2 (b), the EOQ switch element is normally after a delay time of 100 to 300 μsec has elapsed since the energy accumulation in the YAG rod 14 is started, that is, after the excitation of the YAG rod 14 is started. When the application of the λ / 4 voltage of 16 stops, the function of the EOQ switch element 16 as the λ / 4 plate stops, and the light that has passed through the polarizer 15 passes through the EOQ switch element 16 as linearly polarized light. Reflected by the rear mirror 17, it becomes the return light as it is linearly polarized light in the same direction, and again passes through the EOQ switch element 16 and passes through the polarizer 15 as it is, so that laser oscillation is possible. With this operation, GP oscillation laser light can be obtained.

  By the way, even when the oscillation repetition frequency of the GP oscillation laser light is variously changed, the spatial characteristic of the laser light needs to be constant as described above in order to perform stable optical fiber transmission.

  When the oscillation repetition frequency of the GP oscillation laser light is changed, if the oscillation repetition number of the excitation light is changed, the heat input to the YAG rod 14 that is a laser medium changes, so that the laser output characteristics change. In particular, the spatial performance is greatly affected, and the change of the beam pattern and the beam divergence angle becomes remarkable. This is because the refractive index distribution in the circumferential direction of the YAG rod 14 changes due to a change in the thermal lens effect of the YAG rod 14.

  Therefore, as shown in FIG. 3, the voltage application frequency of the EOQ switch element 16 is changed as means for changing the oscillation repetition frequency of the GP oscillation laser light. That is, in the pulse counter 23, the pulse that is thinned out by the set number of pulses with respect to the pulse of the pulse counter input trigger fetched from the pulse generator 21 is sent to the EOQ switch driver 24. The number of pulses thinned out by the pulse counter 23 can be variably adjusted.

  As shown in FIG. 4, for example, when the pulse counter 23 is subtracted for one pulse so as to generate a pulse counter output trigger (EOQ switch trigger) pulse for each pulse of the pulse counter input trigger for 10 Hz oscillation, The frequency of the pulses output to the EOQ switch driver 24 is 5 Hz. When 3 pulses are thinned out, the frequency is 2.5 Hz, and when 9 pulses are thinned out, the frequency is 1 Hz. Assuming that the original oscillation repetition frequency is f, the number of pulses to be thinned out is n, and the oscillation repetition frequency obtained is f ', these are related by the following equations.

f ′ = f / (n + 1)
In this way, by controlling only the pulse applied to the EOQ switch element 16 and changing the number of times of generation of the GP oscillation laser light (= oscillation repetition frequency) and keeping the heat input to the YAG rod 14 constant, the GP oscillation laser The oscillation repetition frequency of the GP oscillation laser light can be varied while maintaining the laser output characteristics such as the light beam pattern and the beam divergence angle. In addition, since the energy of the pulse thinned out with respect to the oscillation repetition frequency of the excitation light is lost, the oscillation of the GP oscillation laser light is not affected.

  Therefore, there is no change in the laser output characteristics, and adjustment of the incident position of the optical fiber 27 when changing the oscillation repetition frequency of the GP oscillation laser light can be made unnecessary.

  Next, FIGS. 5 to 7 show a second embodiment.

  An EOQ switch voltage application operation type laser oscillator 12 is used. In this laser oscillator 12, one λ / 4 plate 31 is added between the EOQ switch element 16 and the rear mirror 17 with respect to the EOQ switch voltage drop type. As shown in FIG. The EOQ switch voltage application operation type oscillates a GP oscillation laser beam when a λ / 4 voltage is applied to the.

  In this voltage application method (see FIG. 7), the time during which a high voltage is applied to the EOQ switch element 16 is shorter than in the voltage drop method (see FIG. 3). It is possible to reduce the applied damage.

  In the configuration of this embodiment, the repetition frequency of the GP oscillation laser light set to 0, 1, 3, 4, 9 is set to 10 Hz, 5 Hz, 2.5 Hz, 2 Hz, 1 Hz. A GP oscillation laser beam having a laser pulse width of 7 nsec and a pulse energy of 100 mJ (peak power 15.7 MW≈110 mJ / 7 nsec) is arranged behind the focal point using a plano-convex lens having a focal length f = 40 mm as an incident lens 26. In addition, when a test was performed in which an optical fiber (step index type quartz material) 27 having a core diameter of φ1000 μm was incident at an incident aperture of φ750 μm at an incident angle of 0.1 rad, GP oscillation laser light was optical fiber at each oscillation repetition frequency. 27 was able to transmit stably.

  Next, FIG. 11 shows an embodiment in which the laser oscillator 11 is used in a laser-induced fluorescence analyzer system.

  In the laser-induced fluorescence analyzer system, an irradiation optical system 41 is connected to the emission side of the optical fiber 27 that transmits the GP oscillation laser light oscillated from the laser oscillation device 11, and the GP oscillation laser light is transmitted from the irradiation optical system 41. The sample 43 is irradiated with 42. The sample 43 is set on a drive stage 44 for variably adjusting the analysis position.

  Adjacent to the irradiation optical system 41, a fluorescence light guiding optical system 46 for detecting the fluorescence 45 generated in the sample 43 is disposed. An optical fiber 47 for transmitting the fluorescence 45 is connected to the fluorescence light guiding optical system 46, a monochromator 48 for spectrum measurement is connected to the optical fiber 47, and a CCD camera 49 for photographing the spectrum is disposed on the monochromator 48. ing. The CCD camera 49 is connected to a data collection computer 50 that collects the spectrum photographed by the CCD camera 49 and analyzes the elements contained in the sample 43.

  When the sample is irradiated with the GP oscillation laser light 42 and analyzed, the flash lamp 19 of the laser oscillator 12 is caused to emit light from the pulse generator 21 and the flash lamp power source 22, and the signal from the pulse generator 21 is A desired oscillation repetition frequency is set by the received pulse counter 23, this trigger signal is given to the EOQ switch driver 24, and the EOQ switch element 16 is switched to oscillate the GP oscillation laser light 42.

  The GP oscillation laser light 42 from the laser oscillator 12 is transmitted to the optical fiber 27 by the incident lens 26, and the sample 43 is irradiated by the irradiation optical system 41 on the emission side of the optical fiber 27.

When the GP oscillation laser beam 42 having a peak power of several MW is irradiated to the sample 43 with a diameter of several hundreds μm by the irradiation optical system 41, the irradiation power density becomes several to several tens GW / cm ² . The sample 43 is turned into plasma in an instant. In response to the energy of the plasma, each element present in the sample 43 emits a unique fluorescence spectrum. The fluorescence spectrum is captured by the fluorescence light guiding optical system 46, guided to the monochromator 48 by the optical fiber 47, and photographed by the CCD camera 49. At this time, since the fluorescence spectrum is emitted with a delay of several μsec to several hundred μsec from the plasma emission, the CCD camera 49 which has received the trigger signal of the oscillation repetition frequency of the GP oscillation laser beam 42 from the pulse counter 23 is provided therein. The timing adjustment mechanism provides a delay and a gate in the measurement time so that only the necessary fluorescence spectrum can be measured. The measurement results are collected by the data collection computer 50, and the elements contained in the sample 43 are analyzed.

  Further, in the case where the analysis is performed by irradiating the GP oscillation laser beam 42 to various positions of the sample 43, the GP oscillation laser beam 42 is used in consideration of the moving speed of the driving stage 44 and the data capturing speed of the CCD camera 49. There are cases where it is easier to use if the oscillation repetition frequency is lowered.

  By incorporating the laser oscillation device 11 having the function of changing the oscillation repetition frequency of the GP oscillation laser light 42 into the laser-induced fluorescence analyzer system, the optical fiber incident optical system can be adjusted without changing the characteristics of the GP oscillation laser light 42. The oscillation repetition frequency of the GP oscillation laser light 42 can be varied without the necessity.

It is a block diagram of the laser oscillation apparatus which shows the 1st Embodiment of this invention. The operation of the laser oscillation device is shown, wherein (a) is an explanatory diagram of a state in which laser light oscillation is suppressed, and (b) is an explanatory diagram of a state in which laser light is oscillated. It is a timing chart of the laser beam oscillation of a laser oscillation apparatus same as the above. It is a wave form diagram which shows the relationship between the pulse number thinned out from the pulse output from the pulse counter of a laser oscillation apparatus, and a repetition frequency. It is a block diagram of the laser oscillation apparatus which shows the 2nd Embodiment of this invention. The operation of the laser oscillation device is shown, wherein (a) is an explanatory diagram of a state in which laser light oscillation is suppressed, and (b) is an explanatory diagram of a state in which laser light is oscillated. It is a timing chart of the laser beam oscillation of a laser oscillation apparatus same as the above. It is a block diagram of the laser induced fluorescence analyzer system using a laser oscillation apparatus same as the above. It is explanatory drawing explaining a general laser oscillation apparatus. It is a timing chart of the laser beam oscillation of a laser oscillation apparatus same as the above. It is explanatory drawing explaining the incident method of the laser beam to an optical fiber from a laser oscillation apparatus same as the above.

Explanation of symbols

11 Laser oscillator
12 Laser oscillator
14 YAG rod as laser medium
16 EOQ switch element as Q switch
19 Flash lamp as excitation light oscillation means
23 Pulse counter as switching operation means

Claims (2)

A laser oscillator;
A laser medium provided in the laser oscillator;
Excitation light oscillation means for oscillating excitation light for exciting the laser medium at a constant repetition frequency;
A Q switch for switching between a state of suppressing laser oscillation of the laser oscillator and a state of causing laser oscillation;
Switching operation from the state of suppressing the laser oscillation to the state of laser oscillation of the Q switch in synchronization with the oscillation pulse of the excitation light thinned out by the predetermined number of pulses with respect to the oscillation pulse of the excitation light by the excitation light oscillation means And a switching operation means. The laser oscillation apparatus according to claim 1, wherein the switching operation unit varies the number of pulses to be thinned out.

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