JPH0426230B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a pulse laser device that performs pulse oscillation with a laser output having a high peak output.

[Prior art]

FIG. 1 is a block diagram of a conventional pulse laser device. In FIG. 1, 1 is a laser oscillator, 2 is a power source that supplies discharge energy to the flash lamp 4, 3 is a drive circuit for the Pockels cell 7, 5 is a laser active material excited by the light emission of the flash lamp 4, and 6 is a power source that supplies discharge energy to the flash lamp 4. A polarizer that limits the polarization plane of the laser oscillation light, 8 a first reflecting mirror that takes out the laser output,
9 is a second reflector having a high reflectance; 10 is a delay circuit that delays a synchronization signal synchronized with the light emission start time of the flash lamp 4 until the operating time of the Pockels cell 7; Pulse generation circuit for obtaining pulse signals, 1
2 is a high-voltage pulse generation circuit for obtaining high-voltage pulses from the output signal of the pulse generation circuit 11;

The conventional pulse laser device is constructed as described above, and the flash lamp 4 receives energy from the power source 2 and emits light. The laser active substance 5 is excited by the light emitted from the flash lamp 4 and forms a highly inverted population.

At this time, the reflectance when looking from the laser active substance 5 to the second reflecting mirror 9 side through the polarizer 6 is zero, and laser oscillation cannot occur. In other words, since the plane of polarization of the oscillated light that has traveled back and forth through the Pockels cell 7 is rotated by 90 degrees, it
When the oscillated light that has passed through is reflected by the second reflecting mirror 9 and reaches the polarizer 6 again, the plane of polarization of the oscillated light is 90°.
Since it is rotating, it is prevented from passing through the polarizer 6. From this state, the population inversion of the laser active substance 5 eventually reaches its maximum. At this time, the delay circuit 10
is set in advance at a timing at which the population inversion is at its maximum, the pulse generator 11 and the high-voltage pulse generation circuit 12 operate, and the Pockels cell 5 is driven. When the Pockels cell 5 operates, the reflectance from the laser active substance 5 looking toward the second reflecting mirror 9 through the polarizer 6 rapidly approaches 100%, the gain of the laser oscillator 1 rapidly increases, and the peak output A large pulsed laser oscillation occurs. FIG. 2 shows the emission intensity waveform of the flash lamp 4 and the population inversion of the laser active substance 5 at this time.

In the figure, A shows the emission intensity of the flash lamp 4, and B shows the population inversion of the laser active substance 5.

Population inversion B increases as the emission intensity I increases until just before laser oscillation occurs, but it rapidly decreases once laser oscillation occurs.

Note that the population inversion n before laser oscillation occurs
The time change of can be expressed by the following formula.

αn/αt=ωp(nt-n)-n/τf...(1) Here, ωp is an amount proportional to the emission intensity of the flash lamp 4, and nt is the total number of atoms at the energy level related to laser transition. , τf is the spontaneous emission lifetime. Since τf is sufficiently large in a normal solid-state laser active material, the population inversion n increases in proportion to the emission intensity of the flash lamp 4.

The laser output reaches its maximum when the Pockels cell 7 operates at the time when the population inversion is at its maximum, that is, when Q switching is performed.

Normally, the timing at which this Pockels cell 7 operates is set in advance by a delay circuit 10. In other words, if the energy supplied from power source 2 is constant,
The laser oscillation output is measured and the timing is set to obtain the maximum output.

However, with the above configuration, the optimum operating timing of the Pockels cell changes due to changes in the energy supplied from the power source 2, replacement of the flash lamp 4, deterioration, etc., making it impossible to obtain a stable maximum laser output.

FIG. 3 shows an equivalent circuit of the discharge circuit of the flash lamp 4. In the figure, 13 is a capacitor that stores the discharge energy of the flash lamp 4;
shows a choke coil for shaping the discharge current waveform of the flash lamp 4.

In FIG. 3, when a discharge current starts flowing through the flash lamp 4, the following equation holds true regarding the current i.

Ldi/dt+Koi1/2+1/C∫ ^t _p idt=Vo...(2) However, L is the inductance of the choke coil 14, Ko is the characteristic impedance of the flash lamp 4, C is the capacitance of the capacitor 13, and Vo is the charge of the capacitor 13. It is voltage.

Here, Zo=(L/C) ^1/2 , i=IVo/Zo, τ=
FIG. 4 shows an example of the numerical calculation results for equation (2) where t/T, T=(LC) ^1/2 and α=Ko/(VoZo) ^1/2 . FIG. 4 shows the standard value I of the current i on the vertical axis and the standard value τ of the time t on the horizontal axis, and shows that the time τ at which I becomes maximum changes depending on the value of the parameter α. In the figure, C is the current waveform when α is 0.6, D is the current waveform when α is 0.8, and E is α
The current waveform is shown when is 1.0.

When the discharge current waveform of the flash lamp 4 changes in this way, the emission intensity waveform also changes, and the third
The population inversion shown in the figure also changes. Therefore, the timing at which the laser output reaches its maximum, that is, the timing at which the population inversion B reaches its maximum, also changes.

Note that the parameter α depends on the values of Ko, Vo, and Zo; for example, Vo changes depending on the magnitude of discharge energy to the flash lamp 4, and Ko changes when the flash lamp 4 is replaced or deteriorated. Changes to

As mentioned above, if the operation timing of the Pockels cell 7 is set to a constant value by the delay circuit 10 as described above, when the parameter α changes, that is, when the charging voltage Vo of the capacitor 13 or the flash lamp 4 is replaced, the characteristic impedance Ko The disadvantage is that when the discharge current waveform changes, the maximum possible laser output cannot be obtained.

[Summary of the invention]

This invention was made with the aim of improving this drawback, and by observing the spontaneous emission light of the laser active substance 5 and measuring the timing at which the spontaneous emission light intensity reaches its maximum, the population inversion of the laser active substance 5 is always at its maximum. The present invention proposes a pulse laser device equipped with a drive circuit 3 that operates the Pockels cell 7 at the timing.

[Embodiments of the invention]

FIG. 5 is a block diagram showing an embodiment of the present invention, in which 1 to 9 and 11 to 12 are the same as or equivalent to the conventional device described above, and 15 is a light beam for observing the spontaneous emission light intensity of the laser active substance 5. Detector, 16 is an amplifier circuit that amplifies the output signal of the photodetector 15, 17 is a differentiation circuit that differentiates the output signal of the amplifier circuit, 18 is a zero value detection circuit that detects the timing when the output of the differentiation circuit 17 becomes zero. , 19 is a gate circuit that enables the drive circuit 3 to operate only within a certain period of time after the flash lamp 4 emits light, 20 is a beam splitter for guiding a part of spontaneously emitted light to the photodetector 15, and 21 is a This is an arrow indicating the emission direction of spontaneous emission light.

In the pulsed laser device configured as described above, the waveform or time change of the spontaneous emission light intensity of the laser active substance 5 is observed by the photodetector 15, and the output signal of the photodetector 15 is amplified by the amplifier 16. . The differentiating circuit 17 obtains a signal whose magnitude is proportional to the time change of the output signal of the amplifier 16, and when the time change of the output signal of the amplifier 16 becomes zero, the output of the differentiating circuit 17 also becomes zero. That is, at the timing when the light emission intensity waveform of the flash lamp 4 obtained from the photodetector 15 becomes maximum, the output of the differentiating circuit 17 becomes zero. Zero value detection circuit 18
A trigger signal is generated at the timing when the output of the differentiating circuit 17 becomes zero, and upon receiving this trigger signal, a certain delay is further applied, and a pulse is generated from the pulse generating circuit 11. The high-voltage pulse generation circuit 12 is operated by the output of the pulse generation circuit 11 and generates high-voltage pulses to control the Pockels cell 7.
to drive.

Note that the photodetector 15 has wavelength selectivity for detecting only the laser oscillation wavelength.

The spontaneous emission light is determined by the spontaneous emission lifetime τf and the population inversion n, as shown in the second term on the right side of equation (1), and the intensity of the spontaneous emission light increases in proportion to the size of the population inversion n. Therefore, when the spontaneous emission light intensity is maximum, the population inversion is also maximum, and the maximum laser output can be extracted.

For this purpose, in order to change the discharge energy of the flash lamp 4, the charging voltage Vo of the capacitor 13 is
Even if the waveform of the emission intensity of the flash lamp 4 changes when the intrinsic impedance Ko changes by changing the flash lamp 4 or replacing the flash lamp 4, the Pockels cell is always set at the timing when the population inversion of the laser active substance 5 is at its maximum. 7 will work. Therefore, it is possible to always obtain the maximum laser output.

In the above embodiment, the beam splitter 20 was used to guide the spontaneously emitted light to the photodetector 15, but the same effect can be obtained even if spontaneously emitted light due to reflection or leakage of the polarizer 6 is detected without using the beam splitter. You can expect this behavior.

〔Effect of the invention〕

As explained above, this invention observes the spontaneous emission light intensity waveform of the laser active substance 5, detects the timing at which the population inversion of the laser active substance 5 is maximum in the drive circuit 3 of the Pockels cell 7, and activates the Pockels cell 7 at this timing. By driving,
Even when the discharge energy of the flash lamp 4 changes or the waveform of the emission intensity of the flash lamp 4 changes due to replacement or deterioration of the flash lamp, the maximum laser output can always be extracted.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of a conventional pulse laser device, Figure 2 is a diagram showing the emission intensity of the flash lamp 4 and temporal changes in the population inversion of the laser active substance, and Figure 3 is a diagram showing the discharge circuit of the flash lamp. equivalent circuit diagram,
FIG. 4 is a diagram showing the change in discharge current of a flash lamp over time, and FIG. 5 is a diagram showing an embodiment of the present invention. In the figure, 1 is a laser oscillator, 2 is a power supply, 3 is a drive circuit, 4 is a flash lamp, 5 is a laser active material, 6 is a polarizer, 7 is a Pockels cell, 8 is a first reflecting mirror, and 9 is a second mirror. Reflector, 10 is a delay circuit, 11 is a pulse generation circuit, 12 is a high-voltage pulse generation circuit, 13 is a capacitor, 14 is a chiyoke coil, 15 is a photodetector, 16 is an amplifier circuit, 17 is a differentiation circuit, 18 is a zero value detection 19 is a gate circuit, and 20 is a beam splitter. Note that the same reference numerals in each figure indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A flash lamp as an excitation light source, a laser active material excited by the excitation light source, a polarizer and a Pockels cell that limit the plane of polarization of oscillated light, and two reflecting mirrors forming a laser resonator.
a power source for supplying discharge energy to the flash lamp; a photodetector for observing the spontaneous emission of the laser active substance; a differential circuit connected to the output end of the photodetector; A pulsed laser device comprising: a zero value detection circuit connected thereto; and a Pockels cell drive circuit having a function of varying the timing at which the Pockels cell is operated in accordance with changes in the output of the photodetector.