Gas laser excited with pulsed microwave energy
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a gas laser excited with pulsed microwave energy of the type having a discharge tube terminated with Brewster windows conducted through the waveguide. 
     2. Description of the Prior Art 
     A gas laser is described in an article &#34;Laser Generation by Pulsed 2.45 GHz Microwave Excitation of CO 2 ,&#34; Handy et al., Journal of Applied Physics, Vol. 49 (1978), pages 3753-3756. According to this article, a standing microwave discharge is generated in a waveguide, with a glass tube penetrating the waveguide in the transverse direction and serving as a discharge channel for the laser discharge. Profiled wedges of electrically conducting material, which influence the field distribution, are arranged along its longitudinal direction. Excitation of the gas laser in this manner yields relatively low intensity values, because the glow discharge required for the excitation of the laser can be produced only in a relative short region of the waveguide. The achievable discharge zone is shorter than a half-wave of the exciting microwave energy in the waveguide. This is also the reason that the discharge channel in this known device is arranged transversely relative to the waveguide. An attempt is made in this structure to maintain the discharge in the region of the waveguide by specially shaping the wedges. This embodiment can realize only an extremely short excitation zone for the laser because of the limitations in the dimensions of the waveguide which must be observed. The laser energy obtainable in this manner is therefore extremely low. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a gas laser excited with pulsed microwave energy having a gas discharge tube extending through the laser waveguide which achieves an increase in the laser power without changing the dimensions of the waveguide. 
     This and other objects are achieved in accordance with the principles of the present invention in a gas laser wherein the discharge tube extends in a longitudinal direction of the waveguide, with the gas discharge tube being connected to a gas supply at ports disposed in the proximity of Brewster windows which respectively close each end of the discharge tube. The waveguide has a substantially straight region and is terminated at one end by a short-circuit element and has an input for the microwave energy at an opposite end. The distance between the short-circuit element and the microwave input is greater than one-half of the wave length of the microwave signal. An igniter is disposed in the straight region of the waveguide, at the end at which the short-circuit element is disposed. The distance between the igniter and the short-circuit element is selected so that the igniter is disposed in the region of high electrical field strength of the microwave energy reflected at the short-circuit element. This insures that ignition of the glow discharge will always take place in the region of the igniter. 
     In the structure disclosed herein, a glow discharge is ignited in the discharge tube at the end thereof closest to the short-circuit element. This discharge migrates through the discharge tube and remains standing in the proximity of the microwave feed until the intensity of the microwave energy has decreased below a threshold for the gas discharge. 
     As a consequence of the dimensioning of the components of the laser, and also dependent upon the gas selection, the excited condition of the laser gas can be maintained after the gas discharge zone has migrated away. The laser discharge ignites by self-excitation as soon as the required length of the discharge tube is filled with excited gas, and does not expire until after the microwave discharge has expired. 
     In one embodiment of the invention, the waveguide has end sections disposed at opposite ends of the straight portion with the end sections being joined to the straight portions at angle. The waveguide may have a rectangular profile, and the discharge tube is conducted through a wall of each end section. A microwave feed in a known manner and the application of a short-circuit slide to the respective end faces are thereby possible in a simple manner. In this embodiment, the gas discharge is formed in the entire region of the discharge tube disposed inside the waveguide. The conditions necessary for the appearance of a glow discharge are not satisfied outside of the discharge tube. The igniter is thus preferably disposed in the straight portion of the waveguide in proximity to the location where the straight portion is joined to the end section in which the short-circuit element is disposed. The igniter is preferably in the form of a screw or tapered tip which projects into the waveguide. 
     Outside of the end sections, the discharge tube is surrounded with a metal tube shield over a portion of its length, the metal tube being joined to the waveguide in HF-tight fashion. This shields the microwave energy from the environment. 
     The discharge tube is preferably attached to the surface of a ridge which faces toward the symmetry axis of the waveguide. The laser is preferably a CO 2  laser, because CO 2  stays in its excited condition for a relatively long timespan. As used herein, the term &#34;CO 2  laser&#34; includes those lasers having CO 2  and the standard additives of further gases, such as nitrogen and helium. 
     The waveguide is preferably a fundamental mode waveguide with a rectangular cross-section, with a distance between the microwave input and the short-circuit element being greater than or equal to λ h , with λ h  being the wavelength of the microwave radiation in the waveguide. 
     An especially high energy exploitation and a quasi-continuous operation is achieved in an embodiment wherein the laser gas having a long timespan in its excited condition is used, for example, CO or CO 2 , and wherein the pulse width and pulse amplitude of the microwave radiation are selected so that the running time of the microwave gas discharge through the waveguide is shorter than the timespan of the excited condition of the laser gas. In this embodiment, the entire length of the waveguide is exploited for the excitation of the laser gas, and the glow discharge is immediately quenched after the glow discharge has migrated through the waveguide, so that a very fast re-ignition by a new pulse is possible. The laser discharge continues even after expiration of the glow discharge, and ignition of the new pulse is still possible before the quenching of the laser discharge, thus achieving a quasi-continuous laser operation. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side schematic view, partly broken away, of a gas laser constructed in accordance with the principles of the present invention. 
     FIG. 2 is a sectional view of the gas laser of FIG. 1 taken along line II--II. 
     FIGS. 3 through 6 are graphs illustrating the operation of the waveguide of FIG. 1. 
     FIG. 7 is a sectional view of a further embodiment of a gas laser constructed in accordance with the principles of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a waveguide 1 has a central section 2 which is substantially straight, and two end sections 3 and 4 proceeding at an angle form the straight region 2. The end sections 3 and 4 have respective end faces 11 and 12. The end face 11 is formed by a short-circuit slide 6. A connector or input 5 for microwave feed is attached to the end face 12, and is also connected to a microwave generator 13. 
     A discharge tube 7 extends through the waveguide 1 in the longitudinal direction of its straight portion 2, and emerges from the waveguide 1 through respective walls 16 and 17 of the angled end sections 3 and 4. Electrically conductive tube sections 15 are disposed adjoining the walls 16 and 17 and envelope the discharge tube 7. The tube sections 15 are connected RF-tight to the waveguide 1, and function to shield the microwave energy from the environment. 
     The discharge tube 7 is closed at both ends with Brewster windows 8. Resonator mirrors 9 arranged coaxially with the discharge tube 7, complete the laser resonator. Gas connections 10 and 14 enable the constant feed of fresh laser gas, with gas deliver preferably ensuing via the connection 10 at the same side of the microwave feed. This results in a particularly good efficiency of the laser. A mixture of He, N 2  and CO 2  in the ratio of 75:15:10 is suitable as the laser gas. 
     An igniter 21 is disposed in the straight portion 2 of the waveguide 1 at the side nearest the short-circuit element 6. The igniter 21 is preferably in the form of a sharply tapered element or screw which projects into the waveguide 1. The igniter 21 may have any shape edge or tip as long as the shape causes an increase in the field strength in this region which is greater than any increase in the field strength at other locations in the waveguide, for example at the bend 25 at the connection of the other end section 4. The short-circuit element 6 is adjusted so that the igniter 21 lies in the region of a field maximum of the microwave radiation generated in the waveguide. The igniter 21 insures that the glow discharge, when the high-frequency pulse is applied, always ignites in the region closest to it. This insures that ignition will occur at the end of the waveguide 1 farthest away from the microwave feed. 
     The glow discharge initially occurs in a volume which is smaller than half the wave length of the microwave radiation in the waveguide. After ignition of the glow discharge, energy is taken from the microwave radiation and the ignition behaviour is modified such that the discharge detaches from the region of the igniter 21 after a slight rise in the momentary field strength, and moves toward the other end of the waveguide 1, i.e., toward the connector 5 for the microwave feed. Given a corresponding gas selection, the portion of the gas volume no longer subjected to the gas discharge because of this motion, remains in an excited condition. The excited gas column grows, and the intensification for stimulated emission grows as well, until after a certain minimum length is covered, laser emission occurs. This laser emission consumes a portion of the excited gas for a relatively long time, so that the laser intensity constantly increases upon passage of the glow discharge through the waveguide 1, until the gas discharge arrives at the opposite end of the waveguide 1 or, dependent upon dimensioning a saturation is reached. 
     FIG. 3 shows the chronological curve of the forward microwave power at the beginning of the waveguide 1 given pulsed excitation. FIG. 4 shows the reflected power of the same pulse excitation sequence. One can immediately see that a considerable part of the excitation power has been absorbed in the glow discharge. The light intensity P was measured at various locations of the waveguide 1, which is one meter long. The curves for the light intensity P over time are shown in FIG. 5 for three measuring locations. These measuring locations are at a distance of 25 cm, 50 cm and 75 cm from the end face 11 of the waveguide 1. The first measuring location is thus reached at a time t2, the second measuring location is reached at a time t3, and the third measuring location is reached at a time t4. It can clearly been seen from these diagrams that the glow discharge moves from the end face 11 of the waveguide to the other end face 12 and covers only a small portion of the waveguide. The intensity of the glow discharge remains approximately constant. 
     After the excitation of a portion of the gas in the discharge tube which is sufficient to cause spontaneous laser emission, the laser discharge beings at a time ta. The laser intensity P Laser  increases steadily until the discharge reaches the end face 12 of the waveguide at time t5. The laser discharge then decays, but continues to exist after the disappearance of the exciting microwave pulse. This is clearly seen in FIG. 6, in which the curve of the laser power extends beyond the value t6 on the time axis, i.e., the end of the excitation pulse, with a considerable remaining intensity. 
     The glow discharge expires as soon as the microwave power falls below a threshold. After expiration of the glow discharge, it can be re-ignited in the region of the igniter 21, and the procedure begins again. When the laser gas having a long timespan in its excited condition (for example CO or CO 2 ) is used, and when the pulse width (t6 to t0) and the pulse amplitude of the microwave power (P HF ) are selected such that the running time of the glow discharge through the waveguide 1 is shorter than the timespan of the excited condition of the laser gas, then the entire length of the waveguide 1 is exploited for lasing. 
     A quasi-continuous operation of the laser is achieved if approximately square-wave microwave pulses are used, and wherein their pulse width (t6-t0) is selected less than or equal to the running time of the glow discharge through the waveguide 1, and wherein the pulse repetition rate is selected sufficiently high that an expiration of the gas discharge barely occurs at the time interval between the pulse, i.e., between t6 of the first pulse and t0 of the second pulse. Given pulses that rise relatively flatly, this condition can also be satisfied by pulses which slightly overlap. A waveguide having a cross-section as shown in FIG. 2 can be used as the waveguide 1. This waveguide may be a conventional waveguide having the designation R26, i.e., the dimensions of the cross-section inside the waveguide are 86.36 mm×43.18 mm. Such a waveguide enables oscillation in the fundamental mode given a microwave frequency of 2.45 GHz. A discharge tube consisting of Al 2  O 3  ceramic having an inside diameter of 17 mm is preferably used with such a waveguide, with the distance from the inside wall 19 of the discharge tube 7 to the wall interior 18 of the waveguide 1 is approximately 5 mm. The discharge tube 7 is supported by a ridge 20 within the waveguide 1. The pulse width of the microwave energy is 6 msec. The curves shown in FIGS. 3 through 6 can be obtained with such a waveguide having a length of about one meter. 
     An adaptation of the field distribution in the propagation direction of the wave is not required given the inventive principal of running discharge. For example, it is possible to use a waveguide having a cross-section as shown in FIG. 7. This waveguide providing a significantly more uniform field destruction and better heat elimination than the arrangement shown in FIG. 2. In the embodiment of FIG. 7, a rectangular discharge channel consisting of non-conductive material, preferably Al 2  O 3  ceramic, is situated between two ridges 22 and 23 attached to opposite sides of the waveguide 1. 
     Although modifications and changes may be suggested by those skilled in the art it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.