Abstract:
A repetitively pulsed chemical oxygen-iodine laser having a resonant cavity containing a lasing medium in the form of a flowing mixture of excited oxygen and iodine atoms and an iodine absorption region within the resonant cavity. The iodine absorption region includes a source of iodine atoms and a magnetic field associated therewith. Selectively altering the magnetic field results in changing the absorption characteristics of the iodine atoms and therefore effectively pulses the output of the laser.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to chemical oxygen-iodine lasers, and, more particularly to a repetitively pulsed chemical oxygen-iodine laser in which the threshold of the resonator is reduced by the incorporation therein of a scaleable intracavity gas phase Q-switch. 
     It is well recognized in the art that photochemical iodine lasers are capable of generating pulsed emission of very short duration and high energy. Examples of such photochemical iodine lasers can be found in the “Handbook of Chemical Lasers,” edited by R. W. F. Gross and J. F. Bott, John Wiley and Sons, New York, 1976, chapter 12, pages 670-701. In the photochemical iodine laser the population inversion is produced in the flash photolysis of a parent alkyl-iodide, generally C 3 F 7 —I or CF 3 —I. The iodine atom that is produce in the photolysis process is in the upper laser level,  2 P ½ . 
     High energy pulsed lasers of this type require large stores of electrical energy and generally operate at efficiencies of less than a few percent. In recent years a new type of iodine laser, the chemical oxygen iodine laser, COIL, has been under development. In this laser the population inversion is produced by energy transfer from excited molecular oxygen in the O 2 ( 1 Δ) state. In the most recent development stages the emphasis has been placed on supersonic chemical oxygen iodine lasers. In the supersonic mode thermally induced medium effects and their influence on beam quality are much smaller by comparison with subsonic operation. In subsonic operation of the COIL under loaded cavity conditions power extraction with its attendant heat release occurs so rapidly that unacceptable density variations are produced in the flow direction. Supersonic operation of coil lasers, however, requires the generation and transport of 0 2 ( 1 Δ) at high pressure (&gt;10 torr) which is difficult. 
     It would therefore be highly desirable to be able to effectively utilize the low pressure generator technology and subsonic flow in chemical oxygen-iodine lasers in which the aerodynamic problems associated with supersonic flow would be much less critical. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems of severe flu induced density gradients in a continuous wave subsonic cavity of a chemical oxygen-iodine laser by operating the laser in a repetitively pulsed mood through the incorporated therein of a scaleable intracavity gas phase Q-switch. 
     In the present invention, the extraction volume defined by the resonator of the chemical oxygen-iodine laser is filled during the interpulse time with a thermally uniform gain medium. Once the optical mode volume is filled, laser action is triggered by reducing the threshold of the resonator with a scaleable intracavity gas phase Q-switch. For subsonic flow, cavity volumetric exchange times are on the order of milliseconds, whereas pulse extraction times due to the large magnitude of the 0 2 ( 1 Δ)/I transfer rate are on the order of microseconds. During the laser pulse, the medium is essentially stationary with temporal density variations caused by the flux induced temperature rise occurring uniformly over the optical aperture. Thus, by operating the laser at subsonic velocity in a repetitively pulsed mode the single most critical issue, that of medium quality and its effect on beam quality, is substantially reduced. In addition, the short duration, high intensity pulses produced in this particular mode of operation offers significant advantages in terms of propagation, target interaction effects, and the potential for frequency doubling when compared with lower average power CW operation. 
     The peak power enhancements in the repetitively pulsed mode of operation of the chemical oxygen-iodine laser of the present invention come about as a direct result of the relatively long radiative and collisional lifetimes of the  2 P ½  state which permits efficient accumulation of energy within the resonant cavity during the switch off portion of the cycle. The particular intracavity gas phase switch arrangement of this invention is based upon the application of the Zeeman effect. 
     The present invention encompasses several alternative embodiments utilizing the repetitively pulsed Q-switch mode of operation. In each of these concepts the resonant cavity input flow is continuous and an iodine atom absorption region is placed intracavity and collinear with the optical axis of the laser. The absorption regions are configured with magnetic oils that produce fields which are parallel to the optical axis of the laser. When the magnetic field is off the absorption region counteracts the laser region and oscillation does not occur. In all of these concepts O 2 ( 1 Δ), i.e., oxygen, and I 2 , iodine, are supplied continuously to the cavity region. 
     In the preferred embodiment of the present invention the absorbing iodine atoms are produced in a separate chemical oxygen-iodine generator. This absorption region is coupled directly to the chemical oxygen-iodine gain region. Repetitive pulse operation occurs by first allowing the gain region to fill with fresh media when the field is off. Once the extraction volume is filled, a fast rising current pulse is applied to the field coils producing transparency in the absorption region. With the gain medium essentially stationary a short duration high intensity pulse is then extracted from the gain region. The peak intensity and pulse width are determined by the concentrations of O 2 ( 1 Δ) and iodine atoms. 
     In an alternate embodiment of the present invention the iodine atoms are produced in a heated cell from thermal dissociation of I 2 . The heated cell is configured with a solenoid that produces an axial magnetic filed. The heated cell is then repetitively pulsed modulating the absorption and producing a train of laser pulses in the manner set forth above. In a further alternate embodiment of the present invention, the natural build-up and decay of the subsonic gain profile is utilized. The gain region is folded within the resonator through the absorption region. In the absorption region the subsonic flow passes through the solenoid field coils. When the gain is positive in the upstream section the absorption in the downstream section is reduced by applying a current pulse to the solenoid. Net gain is established in the resonator and the energy in the upstream section of the flow is extracted in a short duration laser pulse. 
     It is therefore an object of this invention to provide a repetitively pulsed Q-switched chemical oxygen-iodine laser. 
     It is another object of this invention to provide a repetitively pulsed Q-switched chemical oxygen-iodine laser which incorporates therein the use of a magnetic gas phase optical switch. 
     It is a further object of this invention to provide a repetitively pulsed Q-switched chemical oxygen-iodine laser in which the absorption region can be coupled directly to the gain region without the addition of material windows. 
     It is still a further object of this invention to provide a repetitively pulsed Q-switched chemical oxygen-iodine laser in which pulse power requirements are minimal. 
     It is an even further object of this invention to provide a repetitively pulsed Q-switched chemical oxygen-iodine laser which is economical to produce and which utilizes conventional, currently available components that lend themselves to standard mass producing manufacturing techniques. 
     For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of the repetitively pulsed Q-switched chemical oxygen-iodine laser of this invention; 
     FIG. 2 is a schematic illustration of an alternate embodiment of the repetitively pulsed Q-switched chemical oxygen-iodine laser of this invention; and 
     FIG. 3 is a schematic illustration of a further alternate embodiment of the repetitively pulsed Q-switched chemical oxygen-iodine laser of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1 of the drawings which represents schematically the laser  10  of the preferred embodiment of this invention. In laser  10  the resonant cavity  12  is depicted as having a pair of spaced apart mirrors  14  and  16 , one of which,  14 , is partially transmissive so as to enable the output of the laser beam  18  to pass therethrough. Oxygen, O 2 ( 1 Δ), is continuously fed into the resonant cavity  12  from any suitable source (not shown) and as indicated by arrow  20 . Iodine, I 2 , is injected into the resonant cavity  12  by means of a conventional injector  22  connected to any suitable iodine source  23 . Arrows  24  are indicative of the iodine flow. Within resonant cavity (or gain region)  12  the excited oxygen and iodine atoms combine to form the lasing or gain medium. 
     Laser  10  of the present invention is operated in the repetitively pulsed mode. In this type of operation the extraction volume defined by the resonant cavity  12  is filled during the interpulse time with the thermally uniform gain medium. Once the optical mode volume is filled, laser action is triggered by reducing the threshold of the resonant cavity  12  by the use of a scalable intracavity gas phase switch  26 . The intracavity gas phase switch  26  defines an iodine absorption region situated within a pulsed magnetic filed and is placed intracavity and collinear with the optical axis  25  of the resonant cavity  12 . As illustrated in FIG. 1 of the drawings the absorbing iodine atoms are produced in a separate oxygen-iodine generator  28  by the interaction of molecular iodine and excited oxygen. From oxygen-iodine generator the absorbing iodine atoms are fed into the absorption region  30 . The absorption region  30  has encompassed therearound a series of magnetic oils  32  that produce fields which are parallel to the optical axis  25  of laser  10 . Any suitable power source such as conventional pulsed power modulator  33  is interconnected to coils  32 . When the magnetic field is off the absorption region  30  counteracts the laser gain and oscillation within the resonant cavity does not occur. 
     More specifically, for subsonic flow, cavity volumetric exchange times are in the order of milliseconds, whereas pulse extraction times due to the large magnitude of the O 2 ( 1 Δ)/I transfer rate are on the order of microseconds. During the laser pulse the gain medium is essentially stationary with temporal density variations caused by the flux induced temperature rise occurring uniformly over the optical aperture. Thus, by operating subsonic in a repetitively pulsed mode the single most critical issue, that of medium quality and its effect on beam quality is eliminated. In addition, the short duration, high intensity pulses produced in this mode of operation may offer significant advantages in terms of propagation, target interaction effects, and the potential for frequency doubling when compared with lower average power CW operation. The peak power enhancements and repetitively pulsed mode of operation are a direct result of the relatively long radiative and collisional lifetimes of the  2 P ½  state which permits efficient accumulation of energy within the cavity during the switch-off portion of the cycle. 
     The spin orbit states of atomic iodine are characterized by the spectroscopic term symbols  2 P J  with J=½ for the upper state and J={fraction (3/2)} for the lower state. The quantum number J defines the electronic angular momentum of the atom. At the same time, the  127 I nucleus exhibits a nuclear spin of I={fraction (5/2)}. The total angular momentum vector (quantum number of the atom, F) is formed by the quantum mechanical vector addition of the quantum numbers J and I as follows: 
     
       
           F=I+J, I+J− 1,  . . . I−J    (1)  
       
     
     For the upper laser level J=½, there are then two possible values, F=3,2. For the lower laser level, however, there are four possible values, F=4, 3, 2, 1. The optical selection rules governing transitions between different hyperfine levels are given by ΔF=0,±1, and ΔM F =0,±1. Six separate transitions are then possible. In the presence of a magnetic field, however, the degeneracies of the hyperfine energy levels are lifted and each is replaced by 2F+1 spatially quantized components according to the magnetic quantum number M F . The dominant F=3→4 transition then splits into 21 components. The splitting of the levels causes a dilution of the level population densities. This effect, coupled to the lower transition moments computed for transitions between Zeeman components, relative to those observed for zero field transitions between hyperfine levels, forms the basis of the Q-switching approach to the respectively pulsed operation of the chemical oxygen-iodine laser of the present invention. 
     During the field on phase of the switch cycle the absorption region  30  becomes largely transparent to the laser radiation and oscillations will occur, however, during the field off-phase, the absorption region  30  acts as a loss element which precludes oscillation, thereby allowing energy storage within the gain medium. The time between pulses is given by the ratio of extraction width, W, and flow velocity, V. The extraction width is limited by the width of the gain envelope, W MAX . The minimum achievable pulse repetition frequency is then given by the following formula: 
     
       
           PRF   MIN   =V/W   MAX    (2)  
       
     
     Stated more succinctly, during operation, the oxygen, O 2 ( 1 Δ), and iodine, I 2 , are supplied continuously to the resonant cavity  12 . The absorbing iodine atoms are produced in the separate oxygen-iodine generator  28  and fed into the absorption region  30  of the intracavity gas phase switch  26 . The dwell time between the point of I 2  injection into the primary oxygen stream is made sufficiently long such that most of the atoms are in the absorbing  2 P ⅔  state by the time the flow reaches the magnetic field region. The absorption region  30  is then coupled directly to the oxygen-iodine gain region  12  as shown in FIG. 1 of the drawings. Repetitive pulsed operation then occurs by first allowing the gain region to fill with fresh media with the field off. Once the extraction volume is filled a fast rising current pulse is applied by means of field coils  32  to the absorption region  30 . With the gain medium essentially stationary a short duration high intensity output pulse  18  is then extracted from the resonant cavity  12 . 
     The peak intensity and pulse width are determined by the concentrations of O 2 ( 1 Δ) in the iodine atoms. After the laser pulse  18  has been extracted, the magnetic field current is turned off and the resultant absorption prevents oscillation from occurring while the spent media is exhausted and fresh media enters the resonant cavity  12 . The whole process is then repeated with a train of short duration high intensity pulses being extracted from the continuous flow of oxygen and iodine within the resonant cavity  12  of laser  10 . 
     An alternative embodiment of the present invention is illustrated in FIG. 2 of the drawings. Since many of the components utilized with the laser  10 ′ of FIG. 2 are the same as utilized with laser  10  like numerals will be utilized therein to represent identical elements to those found in FIG. 1 of the drawings. In FIG. 2 of the drawings the laser is represented by  10 ′ and incorporates therein an iodine absorption region  30 ′. Within the embodiment of FIG. 2, a heated cell  40  is utilized to produce the iodine atoms through a conventional procedure of thermal dissociation of I 2  (molecular iodine). The heated cell  40  is configured with a solenoid  42  therearound that produces an axial magnetic field. The heated cell  40  is then repetitively pulsed modulating the absorption and producing a train of output laser pulses  18  as discussed with respect to the operation of laser  10  shown in FIG. 1 of the drawings. 
     A further alternate embodiment of the present invention is depicted schematically in FIG. 3 of the drawings. In the configuration set forth in FIG. 3 of the drawings, the natural build-up and decay of the subsonic gain profile is utilized. In FIG. 3 of the drawings the laser is donated by numeral  10 ″ and containing a gain or amplification region  50  and an absorption region  52 . The gain region of  50  is folded through the absorption region  52 . In the absorption region  52 , the subsonic flow passes through a solenoid field coil  54 . When the gain is positive in the upstream section, the absorption in the downstream section is reduced by applying a current pulse to solenoid  54 . Net gain is established in the resonator bounded by mirrors  14  and  16  and mirrors  56  and  58  similar to the operation of lasers  10  and  10 ′ and the energy in the upstream section of the flow is extracted in a short duration laser pulse  18 . The gain and absorption region  50  and  52 , respectively, are then refilled and then the whole process is repeated similar to the operation of lasers  10  and  10 ′ in FIGS. 1 and 2, respectively, of the drawings. 
     All of the three embodiments of the present invention involve the use of a scalable gas phase magnetic switch based on the Zeeman effect in atomic iodine. In all three cases a train of short duration high intensity pules are extracted from a continuous flow of oxygen and iodine by modulating intracavity atomic iodine absorption with a magnetic field. 
     Although this invention has been described with reference to particular embodiments, it will be understood that this invention is also capable of further and other embodiments within the spirit and scope of the appended claims.