JPS61168979A - Pulse laser device - Google Patents

Abstract

PURPOSE:To stabilize laser output by a method wherein a Pockels cell control circuit, which controls change of impressed voltage to the Pockels cell, is related to generation time of high pulse depending upon the magnitude of exciting energy of a laser rod. CONSTITUTION:A laser rod 1, a polarizer 2, a Pockels cell 3 and a flash lamp 4 are provided and a resonator is constituted of a whole reflecting mirror 5 and an output mirror 6. The impressed voltage to the Pockels cell 3 is controlled so that loss of a Pockels cell control circuit 13 for laser light in the resonator is large at the time that high pulse generates in case of large exciting energy of the laser rod 1 and loss thereof for laser light in the resonator is small at the time that high pulse generates in case of small exciting energy of the laser rod 1. Laser output of high pulse can be almost constant even if exciting energy of the laser rod 1 is changed in the range of practical use by means that the time constant of impressed voltage change is set suitably.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a pulse laser device that performs gigantic pulse oscillation, and particularly to stabilization of its laser output.

[Conventional technology]

As a Q-switch element necessary for gigantic pulse oscillation, a combination of a Pockels cell using a crystal having an electro-optic effect and a polarizer is widely used in practical use because of its high switching speed and good extinction ratio.

FIG. 7 shows an example of the configuration of a pulse laser device in this case. In the figure, il+ is a laser rod, (
2) is a polarizer, (3) is a Pockels cell, and (4) is a 7-lash lamp. (5) has a reflectance of practically 10 (1
% is a total reflection mirror, (6) is an output mirror, and these constitute a resonator. (7) is a flash lamp (4
) is a reflector tube that effectively focuses the energy from the laser rod (1''
This is a power source for supplying the discharging force of the flash lamp (4) that excites the flash lamp (4), and charges the capacitor of the discharging circuit (9). The electric power charged in this capacitor is supplied to a flash lamp (4), and is discharged with a large current in a short time in response to a high voltage pulse signal from a flash lamp trigger pulse generation circuit. This will cause the flash lamp (4)
emits light. fill is a Pockels cell drive circuit, which applies voltage stepwise to the Pockels cell (3), which is a Q-switch element, to control the transmittance of the laser source in the resonator to change instantaneously. . The a beach is a charging source (8), a flash lamp trigger pulse generation circuit +1 (1), and a Pockels cell drive circuit (control i1 circuit that controls the operation of the ID).

During the pumping of the laser rod Il), the magnitude of the voltage applied to the Pockels cell (3) is such that the two orthogonal components constituting a linearly polarized wave in the direction determined by the polarizer (2) are mutually /2(4) A phase difference (phase delay) of only 1/1 wavelength) is generated.

At this time, the wave exiting to the left from the left end face of the Pockels cell (3) is a circularly polarized wave. Total reflection mirror on the left (
The wave reflected at step 5) passes through the Pockels cell (3) again, and at this time, a phase delay of /2 is added to υ, resulting in a total delay of π, and the wave moves from the right end face of the Pockels cell (3) to the right. The wave exiting in the direction becomes linearly polarized in a direction perpendicular to the linear polarization direction in the laser resonator determined by the polarizer (2).

The light is blocked by the polarizer (2). Therefore, when a voltage is applied to the Pockels cell (3), the loss within the resonator increases and one oscillation is prevented. If the timing of the Q-switch is adjusted to the time when the population inversion within the laser rod (1 unit) is maximum, and the voltage applied to the Pockels cell (3) is removed, the phase delay becomes zero, and the linearly polarized wave becomes the polarizer (2). )
・9 oscillations are possible because it can pass through without loss. As a result, a temporary gigantic pulse with a slight delay is generated from the Q switch.

FIG. 8 shows the time course of the above operation. In Figure 8, (a) shows the waveform of the voltage applied to the d-Pockels cell (3), and (b) shows the temporal change in the loss inside the resonator (corresponding to the transmittance to the laser source). , (C) shows the laser output waveform of a giant pulse oscillated with a delay of Ta after the q switch time.

By the way, in the Pockels cell drive circuit +Ill of the conventional pulse laser device as described above, a high-speed switching element (switching speed of 29 n5ec or less) such as a klytron or an avalanche transistor is used to drive the Pockels cell /I/(3
The voltage applied to the resonator 1 is changed stepwise at high speed so as to respond faster than the generation time of the giant pulse, and as a result, the loss inside the resonator also changes at high speed as shown in FIG. Therefore, in this case, the input/output characteristics (the relationship between the excitation energy of one laser rod and the laser output energy) of the pulsed laser device that emits a huge pulse oscillates almost directly as the excitation energy increases, as shown in Figure 9. The output energy increases.In FIG. 9, the excitation energy on the horizontal axis is normalized to the excitation energy at the oscillation threshold.

[Problem that the invention seeks to solve]

In the conventional pulse laser device as described above, the laser output energy changes in proportion to the change in the excitation energy of the laser rod (1) as shown in FIG.
8) When the above excitation energy changes from the initial setting 111 (variation range ΔE1) due to fluctuations in the output voltage of 8), fluctuations in the emission intensity of the flash lamp (4), 9 fluctuations in the reflection characteristics of the reflector tube (7), etc. The laser output energy is initially (direct E)
There was a problem that the laser output was unstable due to a large change (variable vibration width ΔKo) from O.

This invention was made to solve these problems, and provides a pulsed laser that can stabilize the laser output energy or laser output power even if the excitation energy of the laser rod +11 varies within the range of practical use. The purpose is to obtain equipment.

[Means for solving problems]

The pulse laser device according to the present invention appropriately sets the time constant for changing the voltage applied to the Pockels cell in a stepwise manner, so that when the excitation energy of the laser rod is large and the huge pulse is generated, the pulse laser device The loss to the laser source in the resonator is large (transmittance is low), and the loss to the laser source in the resonator is small (transmittance is high) at the time when a huge pulse is generated when the excitation energy to the laser rod is small. It is equipped with a Pockels cell control circuit like this.

[Effect]

Figure 2 shows an example of the relationship between the excitation energy of the laser rod and the delay time until a giant pulse is generated after the Q-switch time.As the excitation energy increases, the time from after the Q-switch time until a giant pulse is generated the time interval becomes shorter. In Fig. 2, the generation times of giant pulses corresponding to excitation energies E, K2 are Tdl and T, respectively.
(Denoted as L2. In the present invention, as shown in FIG. 3, the voltage applied to the Pocket Shift λ is varied with an appropriate time constant at the side stage during the Q switch.

The loss within the resonator is also varied in the same way. As a result, the loss inside the resonator after the Q-switch time decreases over time, and as mentioned above, the excitation energy to the laser rod is small, and the time interval from after the Q-switch time until the huge pulse is generated is long. In the case of Tdl in Fig. 3, the giant pulse laser source oscillates with a small loss in the resonator at the time of giant pulse generation, and the excitation energy to the laser rod is large, causing a huge drop from the Q-switch time sickle. If the time interval until the pulse occurs is short (
In the case of Ta2 in FIG. 3), the giant pulse laser source oscillates in a state where the loss inside the resonator is large at the time when the giant pulse is generated.

Therefore, the laser output energy or laser output power of the giant pulse can be kept constant within a certain range of excitation energy to the laser rod.

〔Example〕

FIG. 1 shows an embodiment of the present invention.

113 to α1 and α2 are the same as those in the conventional device. As shown in FIG. 3, when the excitation energy of laser rod 111 is large, the loss to the laser source in the cavity is large, and when the excitation energy to laser rod IIJ is small, a3 is This is a Pockels cell control circuit that controls the voltage applied to the Pockels cell (3) so as to reduce the loss to the laser source in the resonator at the time when the huge pulse of .

In a pulsed laser device configured as described above, the loss inside the resonator at the time of generation of a giant pulse is large when the excitation energy to the laser rod IIJ is large, and when the excitation energy is small, the loss in the resonator is large. By appropriately setting the time constant of the change in the voltage applied to the Pockels cell (3) after the Q-switch time in order to cause the pulsed laser to oscillate.

The gigantic pulse laser output can be kept constant within a range in which the excitation energy to the laser rod 11) is practically used (a range around twice the normal oscillation direct energy).

Figure 4 shows the input/output characteristics in this case.The laser output energy is constant within a certain range of excitation energy, and within this range due to fluctuations in the emission intensity of the flash lamp (4), etc. The change in output energy is small with respect to the fluctuation in excitation energy at , and the laser output can be stabilized.

These operations can be explained by analysis using a rate equation that describes the characteristics of a Q-switched laser.

This rate equation can be expressed as follows after individual normalization.

dψ −−(n−r)ψ ・・・・・・(ln(
it n ---nψ (2) i where ψ is the photon density, n is the population inversion density, γ is the loss within the resonator, and t is time. Considering that the loss r is expressed by the transmittance TP when a Pockels cell and a polarizer are combined, a case will be considered in which the loss r is changed in a linear manner.

Here, τ is the time constant of the voltage applied to the Pockels cell.

Further, the output energy E of the giant pulse laser source can be expressed as ut. where V is the mode volume, N is 0
〇-The active substance density in the rod, hν, indicates the rate of energy loss of photons.

The above formula fi+, +21, (4), +51
Figures 5 and 6 show examples of the results of numerical calculations using
As shown in the figure.

Figure 5 shows the relationship between the excitation energy of the laser rod 111 and the laser output energy of the gigantic pulse source, in the case where the time constant τ of the Pockels cell applied voltage in the curve equation shown by the solid line 0) is 70 n5ec. can be.

It is an input-output characteristic of the pulse laser device according to this invention.

According to this, the excitation energy (standard value) is 2 (oscillation + fl
When fc has a 10% fluctuation around l (corresponding to twice the excitation energy), the fluctuation in output energy is less than 2.5 h, and the conventional system with the input/output characteristics shown by the broken line ←) The fluctuation of the output energy in the same case as above in the pulsed laser device is 15%, which is significantly stabilized.

Figure 6 shows the relationship between the excitation energy of the laser rod [IJ and the laser output power (pulse peak 1i]i) of the giant pulse source under the same conditions as the zS diagram. This is the input/output characteristic of the pulsed laser device. Similarly to the above, when the excitation energy (standard value) varies by 10% around 2, the variation in laser output power is less than 5.5, and the variation in the conventional device shown by the broken line is 28ts. It can be seen that it is significantly stabilized compared to .

〔Effect of the invention〕

As explained above, this invention detects the change in the voltage applied to the Pockels cell after the Q-switch time.

By providing a Pockels cell control circuit that performs control in relation to the generation time of a gigantic pulse that depends on the magnitude of the excitation energy of the laser rod, it is possible to stabilize the laser output against fluctuations in the excitation energy.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing the relationship between excitation energy and giant pulse generation time, and FIG.
The figure shows the voltage waveform applied to the Pockels cell, the time change of the loss in the resonator, and the laser output waveform, and the diagrams illustrating the present invention in detail, FIGS. 4, 5, and 6 are the apparatus according to the present invention. FIG. 7 is a diagram showing the configuration of a conventional pulse laser device, and FIG. 8 is a diagram showing the same time characteristics as FIG. 3 and explaining the operation of the conventional device. FIG. 9 is a diagram showing the input/output characteristics of a conventional device. In the figure, (1) is a laser rod, and (2) is a polarizer. (3) is Pockelsel, (4) is Flashlump.”
l (51 is a total reflection mirror, (6 is an output mirror, (7) is a reflector tube, (8) t is a charging power source, (9) is a discharge circuit, (
II is a flash lamp trigger pulse generation circuit, 11υ
is the Pockels cell drive circuit. a3 is a control circuit, (11 is a Pockels cell control circuit). Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] A resonator consisting of an output mirror and a total reflection mirror, a laser rod as a source of laser light disposed within the resonator, a polarizer, a Pockels cell, a flash lamp for optically exciting the laser rod, and the Pockels cell. and a Pockels cell drive circuit that controls the voltage applied to the resonator to be changed stepwise to instantaneously change the loss to the laser light in the resonator from a high state to a low state, and the pulse laser generates a huge pulse. In the device, by appropriately setting a time constant for stepwise changing the voltage applied to the Pockels cell, the amount of energy in the resonator can be increased at the time when a huge pulse is generated when the excitation energy of the laser rod is large. The voltage applied to the Pockels cell is controlled so that the loss to the laser beam in the resonator is small at the time when a huge pulse is generated when the loss to the laser beam is large and the excitation energy to the laser rod is small. A pulse laser device characterized by: