DE102004023750B4 - Method for operating a gas laser with a pulsed high-frequency voltage and operated according to this method gas laser - Google Patents

Abstract

Method for operating a gas laser, in particular for operating a stripline laser, the gas discharge path (2) with a pulsed high frequency voltage (HF) is fed, wherein the gas discharge path (2) supplied RF power at a duty cycle v = 1 is a maximum, and wherein the amplitude of the high-frequency voltage (HF) is controlled as a function of the pulse duty factor v such that it increases continuously or stepwise with decreasing duty cycle v at least in a range (B) of the pulse duty factor v or at a pulse duty factor v which is below a predetermined limit value v _g , At least approximately constant and greater than at a duty cycle v, which is above this limit v _g .

Description

The The invention relates to a method for operating a gas laser, in particular for operating a stripline laser whose gas discharge path is fed with a pulsed high-frequency voltage, as well as on a gas laser operated by this method.

For controlling the power of a gas laser in a wide power range, in particular a band conductor laser, as for example in the DE 102 30 522 A1 is disclosed, it is known to feed its gas discharge path with a generated by a high frequency generator, pulsed high-frequency voltage. The control of the power can be effected both by controlling the amplitude of the high-frequency voltage and - alternatively or in addition thereto - by a control of the duty cycle, as is known for example from WO 00/46891 A1. Suitable controllable or controllable high-frequency generators are for example from the EP 0 374 444 B1 or the EP 0 572 856 A1 known. In practice, however, it has been shown that triggering the gas laser over its entire power range - usually between 10 and 100% of the maximum cw power - is problematic.

Of the The invention is based on the object, a method for operating a gas laser whose gas discharge path with a pulsed high-frequency voltage to provide that in a simple way a secure Operation of the gas laser over Allows almost the entire performance range. In addition, the invention is the task is based on a gas laser operable by this method specify.

Regarding the Method, the task is solved with the features of claim 1.

By these measures is a safe operation of the gas laser in the entire power range allows. The invention is based on the consideration that the amplitude the high frequency voltage generated by a high frequency generator - regardless of whether it is a semiconductor generator or a tube generator acts - in small duty cycle to be taller can be considered the maximum allowable Amplitude at large Duty cycle, there this despite larger amplitude at a small duty cycle is loaded with an average power smaller than its maximum allowable average power. In other words, it is the use of one High-frequency generator possible, whose maximum power output can be exploited, and with Nevertheless, the gas laser even in the lower power range still safe can be operated because even with a small duty cycle the the gas laser due to the larger amplitude supplied Power is sufficient to ensure a safe ignition as well as a homogeneous discharge bring about. By such an optimal design of the high-frequency generator are the technical effort and accordingly the cost of the gas laser significantly reduced.

The Amplitude of the high-frequency voltage can in the method according to the invention with decreasing duty cycle stepwise increase. This leaves realize itself in terms of circuitry in a particularly simple manner.

One throughout the performance range as uniform as possible operating behavior of the gas laser is achieved when the amplitude at least in one predetermined range of the duty cycle with decreasing duty cycle in several stages, preferably steadily increasing.

Especially the amplitude is in a range below the given lower one Limit value at least approximately constant.

In A preferred embodiment of the method is for generating the high-frequency voltage uses a free-running tube generator with a grid tube and the amplitude of the high frequency voltage is determined by the amplitude of the to the grid of the grid tube flowing Grid current controlled.

Regarding the Gas laser will solve the problem with the features of claim 5, its advantages as well how the advantages of the subordinate subordinates from the advantages of each associated method claims result.

to further explanation The invention is based on the embodiments referred to the drawing. Show it:

1 a gas laser according to the invention with a free-running tube generator as a high-frequency generator in a schematic diagram,

2 an advantageous control device for controlling the RF power by changing the amplitude and duty cycle of the high frequency voltage also in a schematic diagram,

3 a diagram in which the mean grid current and the amplitude of the grid current (grid current amplitude) of the grid tube used in the tube generator is plotted against the duty cycle respectively in two different operating operations,

4 a diagram in which the pulse power of a gas laser according to the invention and a gas laser according to the prior art is also plotted against the duty cycle,

5 an alternative embodiment of a gas laser according to the invention with a free-running tube generator as a high-frequency generator also in a schematic diagram.

According to 1 a gas laser contains a gas discharge path 2 passing through two spaced electrodes 4 is formed, between which there is a laser gas LG. In the exemplary embodiment, a stripline laser is shown schematically, in which the laser gas LG, usually a CO ₂ or CO-containing gas mixture, between electrodes 4 which are flat and have a distance of a few mm from each other.

The gas discharge path 2 is fed by a high frequency voltage HF from a freely oscillating tube generator 6 is produced. The tube generator 6 includes a grid tube for this purpose 8th In the embodiment, a triode with anode A, cathode K and grid G, in whose anode circuit 10 a parallel resonant circuit consisting essentially of the parallel connection of a resonant circuit capacitance C2 with two resonant circuit inductances L1 and L2 connected in series 12 is switched. One between anode A and parallel resonant circuit 12 switched coupling capacitance C1 serves to filter out the at the anode to supply the grid tube 8th applied DC voltage HV. The for feeding the gas discharge path 2 used high-frequency voltage HF is tapped between the two resonant circuit inductances L1 and L2.

The feedback required for generating a free oscillation to the cathode K via the feedback capacitor C3. Through this feedback, the output power of the grid tube 8th taken the required control power and the cathode K of the grid tube 8th fed.

A heating circuit 14 serves to heat the cathode K. The grid G is grounded across the grid capacitance C4 with respect to the fed back high frequency voltage. The grid G is also via a controllable grid switch 16 connected to ground. With open grid switch 16 is applied to the grid G, a reverse voltage U _S. When closed grid scarf ter 16 the blocking voltage U _S drops at a series resistor R _V and the grid G is coupled to ground.

The amplitude I _{G, amp} of the grid current I _G flowing from the grid G in this case to the cathode K is determined, on the one hand, by the sum of one in the grid circuit 18 connected solid grid resistor R _G and also in series in the grid circuit 18 switched controllable resistor R _S and on the other hand, by the resonant frequency of a cathode of a circle inductance L3 and a cathode circuit capacitance C3, tunable cathode circuit 17 , This increases with a smaller distance to the resonant frequency of the tube generator 6 ,

The grid switch 16 is controlled by means of a pulse width modulated control signal PWM. This control signal PWM is a rectangular signal with a constant clock rate in the kHz range and an adjustable duty cycle v between 0 and 100%. In accordance with this duty ratio v, the tube generator generates 6 high-frequency oscillation pulses of a certain period of time, wherein the gas discharge path 2 supplied average RF power is determined primarily by the duty ratio and at a duty ratio v = 1 (or 100%) is maximum.

If, as is the case in the prior art, the resistance lying between ground and grid is constant and designed such that at maximum duty ratio v = 1 and predetermined value of the cathode circuit inductance L3, ie with a cathode circuit tuned to a fixed resonance frequency 17 the mean grid current I _{G, av} , which is defined as the product of duty cycle v and grid current amplitude I _{G, amp} , one for each grid used 8th given limiting value I _{G, avmax} , a reduction of the duty cycle v results in that the RF power supplied at a low duty cycle v is no longer sufficient to bring about a reliable uniform ignition of the gas laser over the entire electrode surface.

To avoid this, according to the invention is a control circuit 20 provided with the aid of which the grid current amplitude I _G, and thus the amplitude of the high frequency voltage HF is controlled in dependence on the duty cycle v. The control circuit contains this 20 one in the grid circle 18 switched controllable resistor R _S whose resistance depends on the duty cycle v. In a preliminary stage 22 The control circuit is the pulse width modulated control signal PWM determining the duty ratio v in a proportional to the duty cycle v control signal S, in the embodiment, the duty cycle v proportional DC voltage, converted. The control circuit 20 is designed such that at maximum duty ratio v = 1 (100%) of the control resistance R _{S is} maximum and together with the constant grid resistance R _G, the average grid current I _{G, av} on the for the grid tube 8th permissible maximum value I _{G, avmax} limited. In a given area of the Tastver If the ratio v, which can also be between 0% and 100%, the controllable resistance R _S decreases with decreasing duty cycle v, so that in this area the amplitude of the grid current I _{G, amp} increases. The decrease of the controllable resistance R _S can be done either continuously or in stages.

In the embodiment according to 2 is a control circuit 20 illustrated in which a controllable in several stages resistor R _{S is} provided. This is done at the output of the preliminary stage 22 pending analog voltage signal S in each case to an input of an operational amplifier 26 ₁ . 26 ₂ , ..., 26 _n supplied there and in each case with reference voltages U _{ref, 1} , U _{ref, 2} , U _{ref, n} compared, where U _{ref, 1} > U _{ref, 2} >...> U _{ref, n} . If the voltage signal S exceeds the value of such a reference voltage U _{ref, i} , then via the operational amplifier 26 _i the associated control switch 24 _i opened and the relevant resistor R _i switched. Are all control switches 24 ₁ to 24 _n opened, between the constant grid resistance R _G and ground, the sum of the resistors R ₁ to R _n (ΣR _i ) is connected and the grid current amplitude I _{G, amp} is limited accordingly. The resistors R ₁ to R _n may be equal to each other, but this is not required. In principle, only a single stage, ie a single switchable resistor R ₁ may be provided. However, consistent performance is achieved when a plurality of stages are provided and the range of duty cycle in which the grid current amplitude I _G, increases with decreasing duty cycle v covers a significant portion of the utilized duty cycle range between 10% and 100%.

In the diagram according to 3 are the grid current amplitude I _G, and the average grid current I _{G, av} against the duty cycle v in different operating situations for a CO ₂ -band conductor laser with 6 kW rated power, shown. The grid current amplitude I _{G, amp} is an indirect measure of the amplitude of the high frequency voltage HF. That is, the amplitude of the high frequency voltage HF is a monotonically increasing function of the grid current amplitude I _{G, amp} .

The curve a (crosses) shows the grid current amplitude I _G, and curve b (triangles) the average grid current I _{G, av} in the absence of control circuit and constant grid resistance R _G. It can be seen from the curves that the grid current amplitude I _{G, amp is} approximately constant and the average grid current I _{G, av} increases linearly in accordance with the increasing duty cycle v. The constant grid resistance has to be designed in this case such that it is ensured at a pulse duty ratio v = 1 (100%) that the maximum average grid current I _{G, avmax} , or the maximum grid power loss of the grid tube is not exceeded. In the case of a stripline laser with large discharge areas and / or high operating pressure of the discharge path, the amplitude of the high-frequency voltage, ie the RF power, is too small for a small duty cycle v to produce a homogeneous homogeneous discharge over the whole area. In the event of mains fluctuations, the discharge may contract, which leads to a laser power breakdown or damage to the electrode surface of the discharge path. Even with conventional gas lasers, instabilities of the gas discharge occur under these conditions.

Curves c (diamonds) and d (squares) show a situation as it exists when a control circuit according to the invention, which controls the control resistor R _S either continuously or in at least 10 stages, is used. Up to a lower limit v _{g of} the duty cycle v of about 45%, the grid current amplitude I _{G, amp} (curve c) is limited only by the grid resistance R _G , so that it is constant and significantly larger at a lower duty cycle than in the operating mode according to the curve a. In other words, in the operating mode according to the invention (curve c) the total grid resistance is only given by the constant grid resistance R _G and approximately a factor of 2 less than the sum of constant grid resistance R _G and a maximum control resistance at a duty ratio v <v _g R _{S, max} .

If the entire grid resistance were then exclusively determined by the fixed grid resistance R _G with increasing pulse duty factor v, the maximum permissible average grid current I _{G, avmax} (curve d) would be exceeded at the latest at a pulse duty factor v of approximately 45%. This is now prevented according to the invention in that from a lower limit v _{g of} the duty cycle v of about 45% of the controllable resistor R _S , for example, discontinuously the first resistor R ₁ , is switched on. With further increase of the duty ratio v, either further resistors R _i are successively connected or the resistance value of a continuously variable controllable resistor is steadily increased. The control circuit is designed so that the grid current amplitude I _G, and thus the amplitude of the high frequency voltage HF with increasing duty cycle v is reduced such that the average grid current I _{G, av} the maximum allowable average grid current I _{G, avmax} does not exceed in the exemplary embodiment, the mean grid current I _{G, av} from a duty cycle v = 0.5 (50%) is approximately constant and approximately equal to the maximum permissible average grid current I _{G, avmax} , as shown by curve d (squares). In the illustrated embodiment, the grid current amplitude I _{G, amp takes} in a range B of the duty cycle v between a non-zero lower limit value v _g and an upper limit, which in the embodiment of the maxima len duty cycle (v) corresponds. In principle, however, embodiments are also possible in which the lower limit value v _g = 0 and / or the upper limit value can be less than 1.

In the diagram according to 4 is the pulse power P (power per cm ² discharge area) of a laser against the duty cycle shown in two different operating situations. Curve a shows the pulse power P in the absence of control circuit 20 , Curve b the pulse power P using the control circuit, which also the curves c and d the 3 underlying. It can be seen from the figure that the power available in one pulse when the control circuit is inactivated is significantly lower at small duty cycles v than when the control circuit is activated. This shows that the gas laser at low duty cycles v increasingly enters an operating situation in which a safe and uniform ignition of the gas discharge is no longer guaranteed.

In the alternative embodiment according to 5 the grid resistance R _{G has} a constant value and it is a control circuit 40 provided at the cathode circuit 17 is detuned as a function of the duty cycle v that with decreasing duty cycle v, the grid current amplitude I _{G, amp} increases and thus the amplitude of the high frequency voltage at the gas discharge gap HF 2 is raised. The necessary for this control signal S is also here from the pulse width modulated control signal PMW with a precursor 42 generated. The dynamic detuning of the cathode circuit 17 , ie the control of its resonant frequency, can be achieved by changing the capacitance value of the cathode circuit capacitance C5 (as in FIG 5 shown) or alternatively by changing the inductance of Kathodenkreisinduktivität L3 done.

2

Gas discharge path

4

electrodes

6

tube generator

8th

mesh tube

10

anode circuit

12

Parallel resonant circuit

14

heating circuit

16

grid switch

17

cathode circuit

18

grid circuit

20 40

control circuit

22 42

preliminary stage

24

control switch

26

operational amplifiers

v

duty cycle

v _g

limit

B

Area

PWM

pulsewidth modulated control signal

S

control signal

U _S

Grid reverse voltage

R _S

controllable resistance

R _G

grid resistor

R _V

dropping resistor

I _{G, amp}

Grid current amplitude

I _{G, avmax}

 medium, maximum grid current

LG

laser gas

HF

RF voltage

HV

DC

A

anode

K

cathode

G

grid

L1, L2

resonant

L3

Kathodenkreisinduktivität

C1

coupling capacitance

C2

Resonant circuit capacitance

C3

Feedback capacitance

C4

grid capacity

C5

Cathode circuit capacity

Claims (8)

Method for operating a gas laser, in particular for operating a stripline laser whose gas discharge path ( 2 ) is supplied with a pulsed high-frequency voltage (HF), wherein the gas discharge path ( 2 ) is maximum at a duty ratio v = 1, and in which the amplitude of the high frequency voltage (HF) is controlled in dependence on the duty cycle v such that it at least in a range (B) of the duty cycle v with decreasing duty ratio v steadily or increases stepwise or at a duty cycle v, which is below a predetermined limit v _g , at least approximately constant and greater than at a duty cycle v, which is above this limit v _g . Method according to Claim 1, in which, to generate the high-frequency voltage (HF), a free-running tube generator ( 6 ) with a grid tube ( 8th ) is used, and in which the amplitude of the high-frequency voltage (HF) generated by the free-running tube generator by the amplitude (I _{G, amp} ) of the grid (G) of its grid tube ( 8th ) flowing grid current (I _G ) is controlled. Method according to Claim 2, in which the amplitude (I _{G, amp} ) of the grid current (I _G ) is _fed through one into the grid circuit ( 18 ) controlled controllable resistance (R _S ) is controlled. Method according to Claim 2, in which the amplitude (I _{G, amp} ) of the grid current (I _G ) is determined by detuning a cathode _{resonant circuit} ( 13 ) is controlled. Gas laser, in particular stripline laser, with a high-frequency generator ( 6 ) for feeding its gas discharge path ( 2 ) with a clocked high Frequency voltage (HF), wherein the gas discharge path ( 2 ) supplied RF power at a duty ratio v = 1 is maximum, as well as with a control circuit ( 20 . 40 ) for controlling the amplitude of the high-frequency voltage (HF) as a function of the pulse duty factor v such that it increases continuously or stepwise with decreasing duty cycle v at least in a range (B) of the pulse duty factor v or at a pulse duty factor v below a predetermined limit value v _g is at least approximately constant and greater than at a duty cycle v, which is above this limit v _g . Gas laser according to claim 5, comprising a free-running tube generator ( 6 ) for generating the high frequency voltage (HF) at which the control circuit ( 20 . 40 ) the amplitude (I _{G, amp} ) of the grid (G) of its grid tube ( 8th ) flowing grating current (I _G ) controls. Gas laser according to Claim 6, in which the control circuit ( 20 ) one in the grid circle ( 18 ) controlled controllable resistor (R _S ) controls. Gas laser according to Claim 6, in which the control circuit ( 40 ) a cathode circuit ( 17 ) detuned.