RU2204875C2 - Method for exciting gas laser with pulse-periodic high-frequency voltage - Google Patents

Abstract

FIELD: quantum electronics; electrically excited gas-discharge laser systems. SUBSTANCE: method used in developing high-frequency excited CO₂ laser involves laser pumping with incompletely matched active element impedance of laser and output resistance of high-frequency oscillator operating at fixed frequency in pulse-periodic mode with controlled radio-pulse on-off time ratio Q. Reliable firing of high-frequency gas discharge is ensured by building up high peak power at output of high-frequency oscillator, safe thermal conditions for output transistors being provided due to adequate choice of radio-pulse on-off time ratio Q. In addition radio pulse repetition rate is set at level not to exceed deionization time. Upon firing discharge in radiator discharge gap on-off time ratio Q is controlled to set desired mode of laser radiation. Method provides for simple and effective control of laser radiation modes and simplified matching mode. EFFECT: enhanced efficiency and operating reliability of high-frequency oscillator. 1 cl, 3 dwg

Description

The invention relates to quantum electronics, in particular to gas-discharge laser systems with electric excitation, and can be used in the development and operation of a CO ₂ laser with RF excitation.

Known methods for exciting a CO ₂ laser using a system comprising an RF generator, a power cable, a matching circuit, and a gas discharge tube. In this case, to enter the pump power into the gas gap, the mode of completely matching the impedance of the active element of the laser with the output impedance of the RF generator, which generates voltage at the operating frequency, is used. In addition, the high level of the electric field necessary to initiate the discharge is created due to the resonant increase in the RF voltage in the resonance circuit of the matching circuit.

The transition from the mode of initiation of the discharge to the mode of full matching is carried out by changing the impedance of the matching circuit at a constant pump frequency, as described in the patent for the RF RF pump system [1]. Such a transition is also carried out by changing the frequency of the RF generator voltage at constant parameters of the matching circuit, as shown in the patent for a pump system for gas devices with RF excitation [2] and in the patent for a pump source of a waveguide CO ₂ laser with a frequency sweep [3 ].

 The disadvantages of such methods of excitation are the low efficiency of the RF generator, the difficulty of ensuring full matching and the lack of reliability of the laser due to the difficulty of ensuring the thermal safety of the transistors of the RF generator.

 A known method of excitation at different frequencies, given in autosvid. on a method for exciting a gas laser and a device for its implementation [4], in which the reliability of the laser is increased by introducing into the excitation device means for monitoring the change in the mismatch and means for controlling the frequency of the RF generator. The disadvantages of this method of excitation are the low efficiency of the RF generator and the increased complexity of the excitation device.

 The closest technical solution is the method of excitation of a gas laser by continuous RF voltage of a fixed frequency, described in the patent for a waveguide gas laser with RF pump [5]. The voltage of the RF generator is supplied through a power cable of length λ / 4 to the primary winding of the step-up transformer, the secondary winding of which is included in the resonance matching circuit. An additional increase in the level of RF voltage is obtained through the use of a power cable with a length of λ / 4, the value of the wave impedance of which is higher than the output resistance of the RF generator. Prior to ignition of the discharge, the value of the RF power is set at a level sufficient to ignite the gas medium of the laser and ensuring the thermal safety of the output transistors of the RF generator. After the discharge occurs, the output power level of the RF generator smoothly increases to a maximum value in a time of at least 1 ms.

 The disadvantage of this method of excitation is the low efficiency of the RF generator and the low reliability of the laser due to the difficulty of ensuring the thermal safety of the output transistors of the RF generator.

 The objective of the invention is to increase the efficiency of the RF generator, increasing the reliability of its operation by ensuring the thermal safety of the output transistors, simplifying the matching mode, as well as introducing the possibility of simple and effective control of laser radiation modes.

 The problem is achieved, firstly, by introducing an incomplete matching mode, namely by introducing a mismatch between the active component of the impedance of the laser head and the output impedance of the RF generator, which leads to an increase in the efficiency of the RF generator and to simplification of the matching mode. And, secondly, due to the fact that a pulse-periodic RF voltage of a fixed frequency with a controlled duty cycle Q (radio pulses) is used to excite the laser. This allows you to set the peak output power of the RF generator to a level sufficient for reliable ignition of the gas high-frequency discharge, and to ensure the thermal safety of the output transistors, and also makes it possible to enter control of the laser radiation modes after the gas discharge is established.

 It is known that when the laser is in full matching, the RF generator is loaded on the conjugate load, i.e. the active components of the impedance of the laser emitter and the output impedance of the RF generator are equal, and the reactive components are equal and opposite in sign.

 It is also known that the efficiency of the RF generator in full matching is determined by the ratio of the active component of the load resistance (impedance of the active element of the laser) to the sum of the active components of the load resistance and output resistance of the RF generator, and the efficiency in this case is 50%.

 With a constant value of the load resistance, it is possible to obtain an efficiency value above 50% by reducing the output resistance of the RF generator.

 The use of the regime of incomplete matching leads to a simplification of the matching circuit, since now it only matches the reactive components of the impedance of the active element of the laser and the output impedance of the RF generator. As you know, the goal of incomplete coordination is to minimize the possible increase in the standing wave coefficient in order to avoid exceeding the permissible collector voltage level at the output transistors of the RF generator and to prevent their failure.

In addition, it is known that if, before ignition of a gas discharge, the RF generator will operate continuously at a fixed frequency, then in the absence of coordination, the magnitude of the dissipation power at the output transistors P _{pacc1 will} almost double the nominal level. This will cause temperature overheating. In other words, the temperature of the structure of the transistors will exceed a critical value, which means that the breakdown voltage at the collector junction will drop below the supply voltage, which will lead to failure of the output transistors.

In order to avoid temperature overheating, use is made of a pulse-periodic mode of operation of an RF generator with a variable duty cycle of radio pulses Q. In this mode, the value of the temperature of the output transistors T _tr is related to the duty cycle Q inversely proportional to:
T _tr ≅T _o + P _ras / CQ, (1)
where T _about - the value of the ambient temperature; R _rac - the magnitude of the peak power dissipated at the output transistors of the RF generator; C is the average heat capacity of the output transistors (by definition, equal to the ratio of the amount of heat received to the difference between the final and initial temperatures).

T _about - a constant value; C is also a constant value for a particular transistor; P _ras can take two different values: P _ras1 - before the ignition of the gas discharge and P _ras2 - after the occurrence of the discharge, while P _{ras2 is} less than R _ras1 .

Until a discharge occurs in the gas medium of the laser, there are two critical values of duty cycle Q _cr1 and Q _cr2 . Firstly, it can be seen from formula (1) that there is a duty cycle Q _cr1 such that at Q≥Q _{cr1 the} temperature of the output transistors T _Tr will not exceed a critical level. And secondly, for the RF generator there is a value of Q _kr2 such that at Q≥Q _{kr2 the} magnitude of the output peak power is insufficient to ignite the discharge.

It is clear that for the operation of the laser, an RF generator with a high level of output peak power is selected that ensures the fulfillment of the condition Q _kp2 > Q _kp1 . Then the discharge ignition occurs when the duty cycle Q _z satisfying the condition Q _kr1 <Q _s <Q _kr2 .

After ignition of the discharge, the magnitude of the power dissipation at the output transistors decreases to the level P _rac2 and, accordingly, the critical value of the duty cycle Q _{kp1 decreases} to the minimum level Q _min . It is clear that there is also a maximum (threshold) value of the duty cycle Q _pop at which the value of the output peak power of the RF generator becomes insufficient to generate radiation. Thus, the generation of laser radiation occurs when the duty cycle Q _g lying in the range of Q _min <Q _g <Q _pop .

In addition, if the duty cycle is set above the threshold level Q _pop , but below Q _cr2 , then the discharge is on, but there is no radiation at the laser output. Therefore, it can be said that the laser is in standby mode with a duty cycle value Q _w satisfying the condition Q _pore <Q _w <Q _cr2 .

 The use of a repetitively pulsed RF signal to excite a laser is due to the fact that there is a residual ionization effect and an afterglow effect in the active gas medium of the laser emitter. A necessary condition for the occurrence of a discharge is that the period of the radio pulses should not exceed the deionization time. Laser radiation can be quasicontinuous, pulsed, or pulsed-periodic. In particular, a quasi-continuous regime of laser radiation is obtained in the case when the repetition period of the excitation radio pulses does not exceed the afterglow time.

In addition, the value of the average radiation power P _rad in quasi-continuous mode is proportional to the average excitation power, which, in turn, is equal to the ratio of the peak excitation power P _n to duty cycle Q, i.e., it can be written that:
P ~ P _{nak rad} / Q. (2)
It is known that when the laser is in the operating mode, the level of P _nak remains almost unchanged. Then it follows from formula (2) that, by setting the duty cycle of radio pulses ranging from a threshold Q _pop to a minimum Q _{min, an} average value of the laser output power P _rad in the range from threshold to maximum P _{max is} obtained.

Thus, the introduction of control of the value of Q allows you to set different modes of laser radiation: standby, pulsed, pulse-periodic and quasi-continuous with the desired value of the radiation power P _rad .

The essence of the invention lies in the fact that the excitation of the laser is carried out under conditions of incomplete coordination between the impedance of the active element of the laser and the output impedance of an RF generator operating at a fixed frequency in a pulse-periodic mode with a controlled duty cycle of radio pulses Q. Reliable ignition of a gas high-frequency discharge is achieved by creating high values of peak power at the output of the RF generator, while the safety of the thermal operation of the output transistors is carried out due to the choice of the duty cycle of the radio pulses Q _s . In addition, the magnitude of the repetition period of radio pulses is set at a level not exceeding the deionization time. After ignition of the discharge in the gas gap of the emitter, the control of the duty cycle Q is used to establish the desired laser radiation mode.

 Figure 1 shows a diagram of a laser excitation device.

 In FIG. Figure 2 shows the parallel connection of high-power transistors in the output circuit of an RF generator using LC filters.

In FIG. Figure 3 shows diagrams explaining the operation of the device in quasi-continuous mode:
a) the amplitude of the voltage at the electrodes of the laser emitter U _el depending on the time t, where U _s , U _p are the amplitudes of the radio pulses on the electrodes before ignition and after ignition, respectively; τ _s , τ - the duration of the radio pulses before ignition and after ignition, respectively; t _p - the period of the following radio pulses;
b) the duty cycle of radio pulses Q depending on time t, where Q _s is the duty cycle before ignition of the discharge, Q _t is the duty cycle corresponding to the required laser radiation power, Q _min is the minimum duty cycle; t _s - the moment of ignition of the gas medium of the laser emitter; t _t - the moment of installation of the required value of the laser radiation power;
c) the radiation power of the laser P _rad depending on the time t, where P _beg - the initial level of laser radiation power (Q = Q _s ); P _t - the required radiation power (Q = Q _t ); P _max - the maximum level of radiation power (Q = Q _min );
d) the temperature at the output transistors T _Tr , depending on the time t, where T ₁ , T ₂ , T _t is the temperature of the transistors before the discharge is ignited, after it is established and at the required laser radiation power level R _t, respectively; T _nom - nominal temperature transistors _(rad at P = P _max); T _about - the ambient temperature.

 The proposed method is illustrated by a diagram of a laser excitation device shown in FIG. 1. The device consists of series-connected circuits for controlling the duty cycle of pulses 1, a pulse generator 2, an RF generator 3, a power cable 4 of length λ / 4, a matching circuit 5, and a laser emitter 6.

A device for implementing the method works as follows. When you turn on the device, the pulse generator 2 with the duty cycle control circuit 1 performs amplitude modulation of the high-frequency signal of the RF generator 3 by rectangular pulses with a duty cycle Q _s . The resulting pulse-periodic high-frequency voltage from the output of the RF generator 3 (radio pulses) through the power cable 4 with a wave impedance of 50 Ohms and a length λ / 4 through the matching circuit 5 is supplied to the electrodes of the laser emitter 6. The peak power of the RF generator 3 is sufficient for ignition discharge in a laser gas environment. At the same time, the set value of the duty cycle of the radio pulses Q _s provides a safe thermal mode of operation of the output transistors of the RF generator 3. In addition, the established period of the following radio pulses t _p does not exceed the deionization time. After the occurrence of a high-frequency discharge, the change in the radiation regimes and the control of the output power of the laser P _{rad are} performed using the duty cycle control circuit 1.

 The RF generator power sufficient to ignite the discharge is obtained by parallel connection of identical, powerful output transistors. Figure 2 shows an example of such an inclusion. The operation of an RF generator at a fixed frequency simplifies the addition of transistor powers. Combining transistor collectors using matching circuits, implemented, for example, in the form of LC filters with selected parameters, the required output resistance of the RF generator is obtained.

 Figure 3 shows diagrams explaining the operation of the device when the laser is in quasi-continuous radiation mode.

After the device is turned on, radio pulses with an amplitude U _z sufficient to ignite the discharge and with a duty cycle Q _z providing thermal safety of the output transistors of the RF generator 3 are supplied to the electrodes of the laser emitter 6. The period of the radio pulses t _p does not exceed the deionization time. The temperature of the output transistors takes a value of T ₁ not exceeding the nominal level T _nom . There is no radiation at the laser output.

At time t _of the gas discharge is ignited in the laser medium and the amplitude of the radio pulses at the electrodes abruptly decreases to the operating voltage level U _p, which further has a substantially constant value. At the laser output, radiation appears with an initial power P _beg , corresponding to the duty cycle of the radio pulses Q _s . The temperature of the output transistors of the RF generator 3 after a while decreases to the value of T ₂ .

From the time t _t establish the required level of laser radiation power P _t A decrease in the duty cycle of radio pulses Q to the level of Q _t cause an increase in the radiation power to the required value of R _t . The temperature of the output transistors of the RF generator 3 in this operating mode of the laser in all conditions does not exceed the nominal value of T _nom .

 In the implemented laser design, after the initiation of a high-frequency discharge, the output resistance of the RF generator is ~ 20 Ohms, the load resistance at the maximum pump power is ~ 50 Ohms. Thus, under the conditions of incomplete matching, the value of the efficiency of the RF generator 50 Ohm / (50 + 20) Ohm ≈ 70% is obtained.

 The technical efficiency of the method of excitation of a gas laser, in which a pulse-periodic mode of operation of an RF generator with a controlled duty cycle of pulses Q is used under conditions of incomplete matching, consists in high efficiency of the RF generator, ease of matching, high reliability of operation, simplicity and efficiency of control of radiation modes laser.

Sources of information
1. US patent 4451766, cl. H 01 J 61/56, 1984.

 2. US patent 4748634, CL. H 01 S 3/097, 1988.

 3. US patent 5150372, cl. H 01 S 53/00, 1992.

 4. Auth. testimonial. USSR 1785058 A1, cl. H 01 S 3/097, 1993.

 5. US patent 4493087 C. H 01 S 3/03, 1992.

Claims (1)

The method of excitation of a gas laser, which consists in applying to the active element of the emitter through a matching device a high-frequency signal from an RF generator operating at a fixed frequency, the output impedance of which is less than the wave impedance of the power cable, mainly a quarter-wavelength, characterized in that the excitation is carried out in an incomplete matching mode the impedance of the active element of the laser and the output impedance of an RF generator operating in a pulse-periodic mode IU with controllable magnitude duty cycle RF pulse Q, wherein prior to ignition of the gas frequency discharge laser set repetition period of radio pulses not exceeding the time deionization, and the value of the duty ratio Q _3, which provides a secure heat mode output transistors HF generator, and after ignition of the discharge control value of the duty ratio Q radio pulses are used to control laser radiation modes.

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