Abstract:
A compact RF excited annular laser system has a stable resonator with a high quality output beam. The stable resonator has internal axiconical and annular mirrors to conform with the annular lasing medium, and to convert the annular beam into a compact cylindrical beam. Mode control is achieved by a combination of diffractive effects in both the annular beam and the compact beam.

Description:
FIELD OF THE INVENTION 
     The invention relates to improved stable optical resonators which are of special value in laser systems. According to the invention there is provided a stable resonator for an annular configuration of the lasing medium in a laser system, in which there is used as an intracavity element a two stage waxicon or axicon. The novel system has a number of advantages, one of these being the achievement of stable operation with a compact, cylindrical, non-annular high-quality output beam. 
     DESCRIPTION OF THE BACKGROUND 
     In a variety of lasers an annular cylindrical configuration of the lasing medium is used. This has pronounced advantages especially in high power lasers (chemically, electrically or optically pumped). The annular configuration makes possible a compact design, efficient gas cooling, gas renewal, a pumping uniformity, etc. The annular configuration has the drawback of complicating the generation of single low order transverse mode operation where a compact beam region is required where the mode originates on the axis. By feedback, the mode in the compact region controls the radiation in the annular region. 
     One way of obtaining a central beam region where a low order single transverse mode may be attained is to use an additional intracavity element of the axicon or waxicon type which transforms the annular beam into a compact cylindrical one, or vice versa. Unstable resonators, in which there are used axicons or waxicons as intracavity elements, are well known. Such unstable resonators are very sensitive to optomechanical instabilities and to misalignments of optical elements. This sensitivity is a serious problem, especially with industrial lasers which are operated under severe environmental conditions. Hitherto one of the means of overcoming this sensitivity was to replace the flat feed-back mirror by a corner retroreflector, which reduces the optical alignment sensitivity by one or two orders of magnitude. With industrial lasers of the CO 2  type, which have a large volume and a high power output, the use of such a corner retroflector may be expensive. For a relatively short-gain medium unstable resonators are not well suited. The low output coupling coefficient which is required causes two problems: 
     (i) Degraded transverse mode discrimination, and thus a loss of some of the advantages of unstable resonators; 
     (ii) Generation of a thin annular output beam which is a disadvantage for industrial applications. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a laser system which has very low sensitivity to optomechanical instabilities. 
     It is another object of this invention to provide a laser system which has very low sensitivity to optomechanical instabilities without requiring a corner reflector. 
     It is another object of this invention to provide a laser system with a large modal volume and high power in a short length. 
     It is another object of this invention to provide a relatively compact laser system with a very low transverse mode output beam. 
     The invention relates to stable optical resonators which are of use in a variety of laser systems. The invention relates to a stable resonator for an annular configuration of the lasing medium in a laser system, which comprises as an intracavity element a two stage waxicon or axicon. The resulting laser system is one which gives a high performance and which is of rather compact configuration. 
     A laser system according to a preferred embodiment of the invention comprises an annular gain region between two concentric tubular coaxial electrodes. The two cylindrical electrodes are held in place by suitable insulating mechanical members. The cylindrical electrodes are of hollow, double-walled construction, thus providing an inner space through which a suitable cooling medium can be circulated. At one of the ends of the annular gain region means are provided for the introduction of a gas or gaseous mixture, and an exit for such gas or gaseous mixture is provided at the other end of the annular gain region. At both ends of the annular gain region there are provided circular windows of a suitable material, such as ZnSe which may be replaced by bellow-sealed mirror mounts. Means are provided to apply RF power to the two electrodes, thus establishing a discharge in the gas mixture in the gain region. On both sides of the gain region there are provided respectively a flat annular feedback mirror perpendicular to the axis of the cylinders, and at the other end, also perpendicular to the axis, a two stage waxicon. At the central region defined by the annular flat mirror there is provided a concave output coupler. The laser beam coming from the active medium has an annular shape and it leaves through the first of the ZnSe windows and reaches the two stage waxicon, where it is converted to a cylindrical beam. The beam is reflected towards the output coupler, from which part of the beam leaves the system as output beam, while part is reflected backwards towards the two stage waxicon, which again converts its shape to an annular one, being directed towards the flat annular feedback mirror. The beam undergoes amplification in the annular gain region, being again reflected back, undergoing further amplification in the gain region, and being directed again at the two stage waxicon, where the shape is again changed to a cylindrical one. The discharge and light amplification take place in the lasing region (gain region). The compact region where the beam is cylindrical is the region where the mode control is performed. In this region, and by virtue of diffraction phenomena, the beam is controlled so as to achieve a low order mode of high quality. This process is enhanced by the diffraction filtering properties of the annular aperture of the active lasing medium. The optical resonator of this system may be looked on as a folded resonator, in which the folding element is the two stage waxicon. In the following there are also provided results of a wave propagation analysis of this system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a configuration of an unstable resonator which comprises an intracavity two stage Waxicon, which is highly sensitive to optomechanical instabilities and to slight misalignments of optical elements of the system. 
     FIG. 2 is a schematic of an unstable resonator with an intracavity two stage Waxicon, which is less sensitive to optomechanical instabilities and to slight misalignments. 
     FIG. 3 is a schematic of a stable resonator with an intracavity two stage Waxicon which is substantially less sensitive to optomechanical instabilities and to certain misalignments of the optical elements because of the concave mirror. 
     FIG. 4 is a sectional side-view of a laser system of the invention. 
     FIG. 5 depicts beam shapes that result from a wave propagation analysis of the invention, including gain saturation of an RF excited CO 2  --N 2  --He mixture. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying schematic drawings, wherein: 
     In FIG. 1, A is a flat feedback mirror, B is an annular lasing medium, C is a Waxicon, D is a flat folding annular mirror, E is a scraper mirror, F is a convex mirror, O is an annular output beam at a low order transverse mode of a prior art device. 
     In FIG. 2, the elements illustrated are a corner cube retroreflector A, an annular lasing medium B, a Waxicon C, a flat folding mirror D, a scraper mirror E, a Convex mirror F, and an annular output beam O of low order transverse mode of a prior art device. 
     The system of FIG. 3 displays a flat feedback mirror A, an annular lasing medium B, a Waxicon C, a flat folding mirror D, a concave output mirror F, and a low order transverse mode order output beam O. 
     As illustrated with reference to FIG. 4, a laser system of the invention comprises in combination a pair of concentric metal electrodes 1 and 2 between which there is established gain region 5, said concentric electrodes being held by the insulated ring shaped centering members 12 and 3. The two electrodes 1 and 2 are hollow, and thus in each there is provided a channel 4 and 6, respectively, through which a cooling fluid is circulated. The electrodes 1 and 2 are insulated from each other by insulating rings 3 and 12. The lasing gas is admitted to region 5 via entry port 8, and leaves this region via the exit port 13. There are provided two flat ZnSe windows, perpendicular to the axis of the system, one, 14, at the one end, and the other, 15, at the other end of the laser system. These are held in place by window mounts 11, thus isolating the gain region from the outside atmosphere. Means are provided for applying RF power to the eletrodes 1 and 2, establishing a discharge in the gaseous medium in region 5. The lasing process takes place between the annular flat feedback mirror 20 and the concave output coupler 17 through the two stage waxicon 18. 
     The beam coming from the active medium in region 5 has an annular shape, it passes ZnSe window 15 and reaches the two stage Waxicon 18 at surface 21, from where it is reflected to surface 22, and to output coupler 17. After the two reflections at surface 21 and surface 22, the beam assumes the shape of a cylindrical beam 16. The region along the optical path between surface 22 and output mirror 17 is termed &#34;the compact region&#34;, whereas the region along the optical path between surface 21 and mirror 20 is termed &#34;the annular region&#34;. The cylindrical beam is directed at the output coupler 17, where at output mirror 17 part of the beam is transmitted and constitutes the output laser beam while part of said beam is reflected backwards towards the two stage Waxicon 18, where it undergoes two reflections, at surface 22 and at surface 21, reverting to the annular shape, which annular beam is propagated in the direction of the feedback mirror 20, passing the annular gain region 5 where it undergoes optical amplification. The beam passes via the second ZnSe window 14, onto feedback mirror 20. The annular beam is again reflected by mirror 20 and passes through region 5, undergoing again optical amplification, being propagated towards the two stage Waxicon 18, where the beam is again transformed fo a compact cylindrical shape. Discharge and light amplification take place in the annular lasing region 5, the region along the optical path between surface 22 and mirror 17 being where the mode control is performed. In region along the optical path between surface 22 and mirror 17, by virtue of diffraction phenomena, the beam is controlled to retain the shape of a low order transversal mode beam. 
     This process is enhanced by the diffraction filtering properties of the clear aperture of the active medium. 
     The optical resonator of FIG. 4 may be considered as folded resonator, the folding element being the two stage Waxicon 18. 
     FIG. 5 demonstrates the result of a wave-propagation analysis, including gain saturation of an RF excited CO 2  --N 2  --He mixture.