Method for generating laser pulses by means of a gas laser and apparatus for carrying out the method
This is a continuation, of application Ser. No. 648,323, filed Jan. 12, 1976 and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     It is known that transversally excited gas lasers, containing carbon dioxide at atmospheric pressure, make it possible to generate laser beams of high energy. However, the use of such gas lasers is impossible in many cases because the energy is not uniformly radiated. The uniformity of radiation depends on the homogeneity of the electrical discharge. It is known to improve the homogeneity of the discharge by using a pre-ionization of the gaseous laser medium. Among various possibilities of pre-ionization, a photo-pre-ionization has been found highly effective. 
     A direct photo-ionization of carbon dioxide, nitrogen or helium molecules contained in the gas laser is conditional upon the photons having energies exceeding 14 eV. Such photons, at atmopsheric pressure, have a penetration depth limited to 30 microns, so that a space pre-ionizatin is not possible. In order to overcome this difficulty, supplementary substances with a low ionization potential can be used. This method has the great advantage that penetration depth of the photons is independent of the gas pressure and that, accordingly, a genuine space pre-ionization is possible at high gas pressures, too. 
     The choice of such a supplementary substance is, however, greatly limited by the following requirements: 
     The ionisation potential of the supplementary substance must be smaller than about 7 eV. 
     The supplementary substance must have a high vapor pressure at room temperature. 
     The supplementary substance and its possible decomposition products must not react with the laser gas and must not cause a reduction of the excited level. 
     The supplementary substance and its possible decomposition products must be transparent to the laser light at its wavelength and must not disturb the electrical discharge. 
     Furthermore, a light source of high power is needed which has powerful emission near the ionization energy of the supplementary substance. 
     Various supplementary substances have already been proposed. However, their ionization potentials are too high to permit a single-stage hoto-ionization. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for generating laser pulses by means of a gas laser, in which method a gas mixture containing carbon dioxide, nitrogen and helium is exposed at every pulse to an electrical discharge and wherein, to achieve a homogeneous discharge, a supplementary substance, pre-ionized by photo-ionization, is added to the gas mixture. 
     In accordance with the present invention a method is created, which combines a high vapor pressure of a supplementary substance with a low ionization potential and which also fulfills the above-mentioned requirements. 
     The process of the invention uses the vapor of a metallocene which is added to the gas mixture. Then the gas mixture intermixed with the metallocene vapor is irradiated, prior to at least a first discharge, by an ultra-violet light pulse. 
     The invention further provides an apparatus for carrying out the method defined above comprising a gas discharge tube containing the gas mixture, in which tube, in order to generate a homogeneous electrical field, two elongated electrodes are positioned at a distance from and parallel to the tube axis, which electrodes are connected to a controllable electrical pulse circuit and are also connected to resonators containing two mirrors situated along the tube axis, one each on each side of the gas discharge tube, of which one mirror is designed as a de-coupling mirror for the laser beam. According to the invention this apparatus contains at least one tubular ultra-violet light source which is placed in the gas discharge tube parallel to the electrodes, which light source is connected for its ignition to a further electrical pulse circuit. Further, means are provided in order to put the supplementary substance in contact with the gas mixture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a diagrammatic longitudinal cross-section of a gas discharge tube used, by way of example, for carrying out the method according to the invention. 
     FIG. 2 shows a cross-section through the gas discharge tube along the line II-- II in FIG. 1. 
     FIG. 3 shows a longitudinal cross-section of a low-pressure air discharge lamp used as an ultra-violet light source. 
     FIG. 4 shows a diagrammatic, partly perspective illustration of a gas discharge gap, of the ultra-violet light sources and of the associated pulse circuits. 
     FIG. 5 shows a cross-section through apparatus for the purpose of adding the supplementary substance to the gas mixture flowing into the gas discharge tube. 
    
    
     Referring specifically to the drawing, examples of embodiments of the method according to the invention as well as examples of embodiments of the apparatus for carrying out the method are explained. 
     The gas discharge tube 1 shown in FIG. 1 has a jacket 4 which is closed except for an inlet socket 2 and an outlet socket 3 for the gas mixture containing the carbon dioxide, nitrogen and helium. The jacket 4 consists, for example, of a synthetic material such as &#34;Plexiglas&#34; (registered trade mark). At one end face of the jacket, which has an aperture, a fully reflecting mirror 6 is attached via a bellows 5. The mirror belongs to the optical resonator of the gas laser. The other end face of the jacket 4 is provided with a window 7 made, for example, of germanium. A further mirror 8 is placed outside the window 7 in the longitudinal axis of the gas discharge tube 1. The mirror 8 is partly transparent to light in order to decouple the generated laser beam. Mountings for the gas discharge tube 1 and the mirrors 6 and 8 are not shown in the diagrammatic illustration of FIGS. 1 and 2. Preferably, supports are used for this purpose which are arranged on an optical bench. 
     Two elongated electrodes 9 and 10 are placed inside the gas discharge tube 1, in parallel with and symmetrical to the longitudinal axis of the tube. The electrodes are attached to the jacket 4. Mountings 11 and 12 serve simultaneously as electrical connections to the pulse circuit, described later. The electrodes 9 and 10 have a shape, particularly a cross-sectional shape, such that, when a voltage is applied across the electrodes, the electrical field generated between the electrodes is as uniform as possible. Furthermore, two tubular ultra-violet light sources 13 and 14 are arranged inside the gas discharge tube 1, symmetrical to the electrodes 9 and 10. The mountings and electrical contacts 15 and 16 for the ultra-violet light sources are also fixed on the jacket 4. The supplementary substance 17 added, according to the invention, to the gas mixture, is injected in powdered form into the lower part of the jacket 4. A bowl-shaped shade 18 is provided in this arrangement. 
     The ultra-violet light sources may, for example, be tubes of fused synthetic quartz, filled with xenon, having a 50% transmission at 1800 A . For gas discharge tubes of larger dimensions, it has been found advantageous to provide low-pressure air discharge lamps instead of xenon lamps, because the air discharge lamps are suitable for generating short pulses of abundant energy and, furthermore, they can be easily adapted to the length of the electrodes 9 and 10. These lamps are, in essence, black body emitters at a temperature between 20,000° and 30,000° K. 
     According to FIG. 3, a low-pressure air discharge lamp may consist of a quartz tube 21, with an inside diameter of, for example, 4 mm, which is provided on each side with an electrode 23 or 24, respectively, consisting, for example, of steel, attached via two short elastic cuffs 22, for example, rubber cuffs. One electrode 23 is formed as a cylindrical bolt and the other electrode 24, as a hollow cylinder. A pipeline 25, connected to a vacuum pump, is coupled to the hollow electrode. The quartz tube 21 is filled with air and evacuated down to a pressure of about 5 Torr. The electrodes 23 and 24 are connected, via electrical leads 26 and 27, to a pulse circuit which will be discribed below. 
     In FIG. 4, the electrodes 9 and 10 and the ultra-violet light sources 13 and 14 of the gas discharge tube 1 of FIG. 1, as well as one pulse circuit 31 and 32 each for feeding the electrodes and light sources, respectively, are illustrated diagrammatically. One electrode 10 is connected to the earth or frame 33 in the first pulse circuit 31. One terminal of a controllable spark gap 34 is also connected to earth 33. One terminal of a storage capacitor 35, which is connected, via a charging resistor 36, to a high-voltage source 37, not shown in detail, is also connected to the other terminal of the spark gap 34. The other terminal of the storage capacitor 35 is connected to the other electrode 9 and is also connected to earth 33 via a resistor 38. 
     The second pulse circuit 32 is built up in the same manner and thus comprises a controllable spark gap 39, common to both light sources 13 and 14, a charging resistor 41, connected to a high voltage source 40, as well as a capacitor 42 or 43, respectively, for each light source. The capacitors 42 and 43 are each connected to earth 33 via a resistor 44 or 45, respectively. 
     In order to ignite the spark gaps 34 and 39, one 25 kV trigger circuit 46 or 47, respectively, is provided for each circuit. To trigger off the trigger circuits 46 and 47, a common trigger pulse generator 48 is available, the output of which is directly connected to the spark gap 39 for the light sources 13 and 14 and is also connected, via an adjustable delay circuit 49, to the spark gap 34 for the electrodes 9 and 10. A trigger pulse from the generator 48 thus triggers off the trigger circuit 47 which ignites the spark gap 39 by a 25 kV pulse, so that the capacitors 42 and 43 discharge via the light sources 13 and 14 and, for a short time, ignite this spark gap. The same trigger pulse triggers off, after an adjustable delay, the trigger circuit 46, which, in turn, ignites the spark gap 34 by a 25 kV pulse, so that the capacitor 35 discharges, via the electrodes 9 and 10, i.e. causes an electrical discharge between the electrodes, which generates a laser pulse in the resonator 6, 8 of FIG. 1. 
     The supplementary substance introduced into the gas discharge tube 1 according to FIG. 1, which, according to the invention, is a metallocene, may advantageously be a ferrocene with an ionization energy of 6.8 eV or a nickelocene with an ionization energy of 6.8 eV. Both these energies are below the cut-off threshold of fused synthetic quartz, which may serve as a window material for the ultraviolet light source. Both ferrocene and nickelocene do not react with the carbon dioxide, nitrogen and helium containing laser gas mixture and, in their vapor state, have no measurable absorption at the wavelength of the laser light. At room temperature, their vapor pressure is about 0.5 Torr. 
     pre-ionization measurements were made with the gas discharge tube 1 illustrated in FIG. 1 and the pulse circuits 31 and 32 illustrated in FIG. 4. In these measurements, a single tubular xenon light source was positioned parallel to the electrodes 9 and 10, each of which was 20 cm long. The gas discharge tube 1 was filled with a mixture of carbon dioxide, nitrogen and helium in the ratio of 2--2-3. In the pulse circuit 32 of FIG. 4, the capacitors 42 and 43 had a capacity of 1 micro-farad each and the high voltage 40 amounted to 2.5 kV, whereby light pulses of 4 microseconds duration could be achieved. 
     With this apparatus, energy densities of up to 300 J/l could be achieved in the gas discharge tube at atmospheric pressure without arc formation, provided the xenon light source was ignited with an input energy of 3 J and provided ferrocene or nickelocene were added to the laser gas mixture mentioned above. If, instead of those supplementary substances, a supplementary substance hitherto considered advantageous was used, e.g. xylene or tri-n-propylamine, unavoidable arc discharge occured. 
     By providing the mirrors 6 and 8 of FIG. 1, reproducable laser pulses of the transversal electromagnetic (TEM oo )-mode with a duration of 75 nano-seconds were observed. The de-coupling energy density reached amounted to 30 J/l. The intensity distribution over the beam cross-section of the laser pulses can be measured by traversing with a pyro-electric detector. The measurement of the amplification for small signals by the threshold reflection yielded a value of about 3%/cm. If the apparatus of FIG. 1 is arranged in a high-pressure container, a laser emission can be observed at pressures of up to at least 5 atmospheres. 
     Instead of injecting the supplementary substance on to the inside wall of the gas discharge tube, it can also be added to the gas mixture flowing into the gas discharge tube. The device illustrated in FIG. 5 can be used for this purpose. This device is constructed essentially in the manner of a wash bottle. A container 51, e.g. a glass container, has an aperture closed by a plug 52. A first tube 53, which extends in the container 51 down to its bottom, and a second tube 54, which opens out into the container immediately behind the plug 52, are led through the plug 52. A quantity of powdered supplementary substance 55 is injected on to the bottom of the container. The gas mixture flowing through the tube 53 into the container 51 traverses the supplementary substance 55 and absorbs its vapor. The gas mixture intermixed with the vapor of the supplementary substance 55 flows via the tube 54 into the gas discharge tube, which may be shaped, for example, as in FIG. 1 and which no longer contains the supplementary substance indicated in FIG. 1. 
     If the supplementary substance provided for is liquid at room temperature, its vapor can also be intermixed with the gas mixture by means of the device according to FIG. 5. A gaseous supplementary substance can be injected into the gas mixture directly before its entry into the gas discharge tube. 
     It has been found that, other supplementary substances than those mentioned above may be added to the gas mixture advantageously, for example, pentacene according to the formula: ##STR1## which has an ionization potential of 6.7 eV and is usually available in the form of crystal needles, which can be pulverised. 
     Furthermore, p-phenylene-diamine according to the formula: ##STR2## with an ionization potential of 6.75 eV, available in the form of solid crystals, has been found suitable. 
     In addition, tetra-methyl-p-phenylene-diamine, according to the formula: ##STR3## with an ionization potential of 6.6 eV, normally available in the form of flakes which can be pulverised, may be used as a supplementary substance.