A PLASMA-RECOMBINATION LASER HAVING HIGH POWER OUTPUT

Abstract: A recombination type laser of high efficiency. Energy is applied to the laser (Fig. 6, 10, 11-12) in such manner as to a) create a plasma (120-124) and b) maintain the plasma at a high temperature unfavorable to recombination until the electron density has fallen into an optimum range for laser action. The energy can be provided from a single source, e.g., a high voltage, high current source (150) providing a dual pulse, or from multiple sources e.g., first (Fig. 7, 151) and second (152) electrical sources providing differently timed pulses.

Description: A PLASMA-RECOMBINATION LASER HAVING HIGH POWER OUTPUT Background of the Invention The present invention pertains to the field of plasma recombination lasers. The idea of producing a population inversion during plasma recombination was first proposed in an article entitled "Negative Absorption in a Nonequillibrium Hydrogen Plasma", Sov. Phys. JETP, Vol. 18, 1964, pp. 998- 1000 by L. I. Gudzenko and L. A. Shelepin. Experimental observations of lasers of this type were made in 1977. The population inversion mechanism responsible for these lasers has been discussed in an article entitled "Recombination Lasers in Expanding C0 2 Laser-Produced Plasmas of Argon, Krypton, and xenon". Applied Phys. Letts. , Vol. 31, 1977, pp. 334-337 by W. T. Silfvast, L. H. Szeto and 0. R. Wood and in an article entitled "Recombination Lasers Utilizing Vapors of Chemical Elements. I. Principles of Achieving Stimulated Emission Under Recombination Conditions", Sov. J. Quantum Electron., Vol. 7, 1977, pp. 704-708 by V. V. Zhukov, E. L. Latush, V. S. Mikhalevskii and M. F. Sem. PIG. 1 illustrates the population inversion mechanism, where E z+ denotes the ground state of the Z fc " ionization stage of element E and E"- z+ -** )+ denotes the ground state of the (Z+l) 1 -" ionization stage. During the heating phase of the plasma, step 1 in FIG. 1, a highly ionized plasma at high density is formed by an electrical discharge or a laser-produced plasma. At this time, the atoms of element E are excited, preferably within a gaseous medium, and some fraction are ionized into stage E' 2" * *1 - 1 . After formation, the plasma is allowed to expand, illustrated as step 2 in FIG. 1. During this expansion phase, plasma electrons are cooled via collisions with the surrounding gases. As a consequence of this cooling the electron-ion recombination rate is significantly increased. The recombining electrons move downward in energy, via collisions with the free electron continuum, through the highly excited states of E z+ , step 3 in FIG. 1, until a significant gap in the energy levels of that charge state is reached. The reduced collisional decay rate across the gap creates a bottleneck, which bottleneck causes a population inversion to develop with respect to one or more lower levels. Laser action is achieved on transitions across the gap when a high decay rate exists for the lower level of such a transition. The importance of the electron temperature on the electron-ion collisional recombination process is well known. The total three-body recombination rate for hydrogen-like ions (τ-) " - * - is given by the equation found in Physics of Shock Waves and High Temperature Hydrodynamic Phenomena, Vol. I, Academic Press, New York, 1966, by Ya. B. Zeldovich and Yu. P. Raizer (τg) "1 = 8.75 x 10 "27 Z e N2 T 4 ' 5 (1) where Z is the nuclear charge, N e is the electron density and T e is the electron temperature. This collisional recombination process is much more sensitive to temperature T -4 5 than is the radiative recombination process, T e - 75. When a plasma is completely ionized, the plasma will not relax unless the plasma electrons are allowed to cool. An article entitled, "Collisional-Radiative Transfer Between Rydberg States of Helium and Electronic Recombination of He + ", Phys. Rev. A, Vol. 15, 1977, pp. 1502-1512 by J. Boulmer, F. Devos, J. Stevefelt and J. F. Delpech discloses that the emission from a recombining plasma can be quenched by heating the electrons with an external source. An article entitled, "Ultra- High-Gain Laser-produced Plasma Laser in Xenon Using Periodic Pumping", Appl. Phys. Lett., Vol. 34, 1979, pp. 213-215, by W. T. Silfvast, L. H. Szeto and 0. R. Wood discloses the fact that the plasma cooling rate has a dramatic effect on the output from a C0 2 laser-produced plasma recombination laser in xenon. While there is thus known much theoretical information about recombination lasers, a problem is that such lasers are relatively inefficient, and a need exists for increasing the power output thereof. This is accomplished according to this invention. Summary of the Invention In accordance with the present invention, energy is applied to a recombination type laser in such manner as to a) create a plasma, and b) maintain it at a high temperature unfavorable to recombination until the electron density has fallen into an optimum range for laser action. The energy can be provided from a single source or from multiple sources, and different kinds of energy sources can be used. Brief Description of the Drawing FIG. 1 illustrates the physical principles upon which a plasma-recombination laser operates; FIG. 2 shows an embodiment of a seg ented- plasma-excitation-recombination (SPER) laser; FIG. 3 shows, in graphical form, the laser output provided by a conventional plasma excitation pulse and the laser output obtained utilizing an excitation pulse generated according to the present invention; FIG. 4 shows, in graphical form, the laser output resulting from an excitation pulse shaped as shown in an insert to the figure; FIG. 5 shows in graphical form, the laser output provided by a different excitation pulse; FIGS. 6-17 show recombination lasers constructed according to the present invention; and FIG. 18 shows a circuit used to generate one excitation pulse according to the invention. Detailed Description FIG. 2 shows a known type segmented-plasma- excitation-reco bination (SPER) laser. A typical device comprises a number of 1 mm thick by 2 mm wide by 12 mm long strips 100-106 of a lasing species of known type positioned end-to-end on an insulating substrate 10 in such a manner as to leave a 2 mm gap between each pair of strips. When a 15kv, 50-150 ampere pulse of 2.5 μsec duration from voltage source 5 is applied to the ends of this series of strips, a high density metal-vapor plasma 120-125 is formed in each gap. Once formed, plasmas 120-125, primarily comprising vaporized strip material, expand spherically away from the gaps and into a resonant cavity filled with helium gas at 2.5-3 Torr. The optic axis of the resonant cavity formed by mirrors 11 and 12 is positioned parallel to and 7 mm above the row of strips. The laser is enclosed in a known type enclosure, not illustrated. Also, while various different materials can be used in such lasers, to obtain different lasing frequencies, such materials are not listed herein as being well known. This embodiment of a SPER laser produces laser action in the ultraviolet, visible and near-infrared at o wavelengths between 2983A and 1.838 μm. Curve 200 in FIG. 3 is an oscilloscope trace of a 160 amp excitation pulse used in the prior art. Curve 201 in FIG. 3 shows an oscilloscope trace of the laser intensity from a SPER laser o operating on the 3008A transition in Inlll produced by the excitation pulse of curve 200. Increasing laser intensity" is in the downward direction. The excitati-on pulse of curve 200 has an approximately 3 μsec duration with a rapidly falling trailing edge. Curve 202 in FIG. 3 shows the oscilloscope trace of a 160 amp excitation pulse generated according to an embodiment of the present invention. Curve 203 shows an oscilloscope trace of the laser intensity obtained from the same SPER laser that generated curve 201. The dramatic increase in power output is directly attributable to the control of the electron recombination rate in the plasma achieved with the specially shaped excitation pulse 202. The specific function of each of the various parts of an excitation pulse used according to another embodiment of the present invention is seen from the results presented in FIG. 4. A pulse 205 was applied to ao cadmium SPER laser operating on a Cdll transition at 5375A. The initial portion of the pulse, 100 amps having a 3 .Psec duration, produces and heats the cadmium plasma. The subsequent portion of excitation pulse 205, having a 10 amp amplitude and a duration variable over a range of 0- 25 μsec, keeps the electron temperature high until the electron density has fallen by plasma expansion to the optimum value for laser action and until uniform mixing of the expanding plasma with the background gas occurs. The end of the current pulse results in the immediate o appearance of laser action at 5378A, indicated by curve 206 in FIG. 4. The energy (or excitation) pulse, shaped according to the present invention, provides a high degree of control over the gain, power and temporal behavior of the cadmium SPER laser because laser action only occurs after a time delay determined by the delay time of the lower-current portion of the excitation pulse. An example of a circuit for generating the pulse 205 is shown in FIG. 18. A Nanofast Model 568 Dual Delay Generator 705 sends a first pulse to a Maxwell Model 40095A Trigger Generator 704. Trigger Generator 704 triggers an N-spark gap 703 so that a 100 amp 3ysec pulse generated by a 0-15kv dc supply 700, 10MΩ resistor 701 and 1500 ft RG-8U coax cable 702 is applied to the SPER laser, i.e. metal strips 100, 101, etc. The Dual Delay Generator 705 sends a second pulse, delayed by 3μsec from the first pulse, to a Cober Model 606 High Power Pulse Generator 707. Pulse -δ^3RE__A O PI generator 707 generates a pulse having the shape of the lower amplitude portion of curve 205 in FIG. 4. The pulse produced by pulse generator 707 is sent to a Radiation Laboratory Pulsed Modulator Model 9, 1" megawatt for amplification. The amplified pulse is applied to the SPER laser. The result is the excitation pulse 205 of FIG. 4 where the delay time of the lower amplitude portion is variable by varying the pulse width output from pulse generator 707. Curve 207 in FIG. 5 shows a graph of the spontaneous emission from a SPER laser operating on the o 3008A transition in mill. Note that the particular excitation pulse 208 used "rings", i.e. repeatedly falls to a 0 current value. The ringing is observed to give rise to a sequence of spontaneous emission pulses. This observation, that line emission can repeatedly be switched on and off, provides direct evidence that the plasma- recombination mechanism can be externally controlled. As described hereinabove, energy is applied to the laser in a manner which first produces and heats a plasma and which then adds energy to the plasma to lower the electron-plasma recombination rate for a sufficient length of time to allow the plasma to expand to an extent which provides optimal laser action. The delay is possible because a completely ionized plasma will not recombine until the plasma electrons are allowed to cool or the plasma hits the walls of an enclosing chamber. It is useful to keep the electron temperature high until the electron density has fallen by plasma expansion to the optimum value for laser action, and until uniform mixing of the expanding plasma with the background gas has occurred and refractive index gradients, shock waves, etc. have ceased to exist. The use of external control in accordance with the present invention has been used to store energy in the form of ions for periods of up to 25 μsec and then to utilize the stored energy in a controlled manner. Thus ions can be created slowly and energy can be stored therein for periods of time which are long compared to their untrapped radiative lifetime. This stored energy can thereby be made available when needed. Further embodiments of the present invention are described hereinbelow for providing the desired excitation pulse. FIG. 6 shows a SPER laser having metal segments 100-105 affixed to insulating substrate 10. High voltage- high current pulse source 150 applies current pulse 160 to strip 100 and strip 105. The application of pulse 160 produces plasmas 120-124, which plasmas expand into the laser cavity formed by mirrors 11 and 12. The first, high current portion of pulse 160 produces plasmas 120-124 and the lower current portion of pulse 160 heats plasmas 120- 124 for a sufficient length of time so that the expansion of the plasmas into the laser cavity provides the appropriate electron density for recombination and efficient laser action. In the FIG. 7 embodiment, excitation source 151 applies a high voltage, high current pulse to metal strips 100 and 105 to generate plasmas as in the FIG. 6 embodiment. In this and subsequently described embodiments, however, a separate energy source is used to control the plasma temperature. Thus, in this embodiment, pairs of control electrodes 130-139 contact the substrate 10 in close proximity to the gaps between the metal strips. Control source 152 applies a current pulse across each gap which current pulse has a different amplitude than the amplitude of the current pulse produced by excitation source 151. The length of current pulse 152 is longer in time duration than the current pulse produced by excitation source 151 and serves to control the plasma temperature. The combination of excitation source pulse 151 and control source pulse 152 provides the composite excitation input as described hereinabove. In the FIG. 8 embodiment, plasmas 120-124 are also created by excitation source 151. Rf source 153 applies a signal to coil 155 to produce an O PI rf electromagnetic field. Energy from the rf field is absorbed by plasmas 120-124. The excitation current pulse provided by excitation source 151 and the energy from the rf field produced by rf source 153 combine to form the requisite excitation pulse according to the present invention as described hereinabove. In the FIG. 9 embodiment, a microwave source 156 applies energy to microwave waveguide 157. Microwave waveguide 157 is disposed about the laser cavity so as to encompass the plasmas 120-123 generated by excitation source 151. Microwave energy generated in microwave waveguide 157 is absorbed by plasmas 120-123. The excitation current pulse provided by excitation source 151 and the excitation provided by microwave source 156 combine to form the requisite excitation pulse. In the FIG. 10 embodiment, target material 400- 402 is affixed to insulating substrate 10. Laser source 410 produces laser beam 420. Laser beam 420 impinges upon partially reflecting, partially transmitting mirrors 421-423 to form laser beams 431-433. Laser beams 431-433 are focused by lenses 441-443 onto targets 400-402, respectively. This produces plasmas 451- 453. Plasmas 451-453 expand into the laser cavity formed by mirrors 11 and 12. Pairs of control electrodes 130-135 are disposed on insulating substrate 10 in the region of targets 400-402. Control source 152 applies a current pulse to pairs of electrodes 130-135, which current pulse adds energy to plasmas 451-453. The combination of the application of laser radiation and the current pulse from control source 152 forms the requisite excitation pulse. The FIG. 11 and 12 embodiments are similar to the FIG. 10 embodiment in that laser source 410 provides an excitation pulse which forms plasmas 451-453. In FIG. 11, rf source 153 and rf coil 155 provide the same functions as has been described hereinabove with respect to the embodiment shown in FIG. 8. In FIG. 12, microwave source 156 and microwave waveguide 157 provide the same functionsas has been described hereinabove with respect to the embodiment shown in FIG. 9. The waveguide 157 has apertures 461-463 allowing passage of the laser radiation into the waveguide. In the FIG. 13 embodiment, similar to that of FIG. 7, the portion of the excitation pulse applied to the gaps by source 170 is delayed by delay means 501-503. The delay means are arranged so that delay (501) > delay (502) > delay (503) . This arrangement of delay times is such that laser action is formed in a travelling wave across the laser cavity. Source 170 may be a high voltage, high current source when the control electrodes are used to apply the second portion of the excitation pulse, or it may be an rf source as shown in FIG. 8, or a microwave source as shown in FIG. 9. The FIG. 14 embodiment is similar to that of FIG. 13 but wherein the plasmas are formed by the use of laser pulses such as in the FIG. 10 embodiment. In the FIG. 15 embodiment, plasmas 120-123 are formed by the application of a high voltage, high current pulse from pulse source 5. Laser radiation beam 620 emitted by long wavelength laser 600 is passed through beam splitters 631-634 and directed to impinge upon plasmas 120-123, respectively. The laser radiation provides the controlled absorption of energy in accordance with the present invention. FIG. 16 shows an embodiment similar to that shown in FIG. 10 in that plasmas 451-453 are formed by the application of laser radiation from laser source 410. Laser radiation beam 620 emitted by long wavelength laser 600 is passed through beam splitters 631-633 and directed to impinge upon plasmas 451-453, respectively, as in the FIG. 15 embodiment. FIG. 17 shows an amplified-spontaneous emission embodiment similar to that shown in FIG. 14 in that plasmas 451-453 are formed by the laser radiation from laser source 410. A laser radiation beam emitted by long OMPI wavelength laser 600 is passed through beam splitters 631- 633 and directed to impinge upon plasmas 451-453 respectively. Beam splitters 631-633 and targets 400-402 are spaced at intervals so that the radiation from the beams 621, 623, and 625 reach the respective plasmas 453, 452, 451 in successive intervals. This arrangement of delay times is such that laser action is formed in a travelling wave across the laser cavity. OMP

Claims: Claims 1. A plasma-recombination laser which comprises a resonant laser cavity (11, 12); means for producing a plasma laser medium in said resonant laser cavity; characterized in that said means for producing a plasma laser medium comprises a material (FIG. 6, 100-105), and means (150) for applying energy to said material in at least two time segments such that the energy during a first time segment is sufficient to produce a plasma from said material, which plasma expands into said resonant laser cavity, and the energy during a second time segment maintains the electron temperature of said plasma high enough to prevent substantial electron-ion recombination in said plasma until recombination is desired. 2. A plasma-recombination laser in accordance with claim 1 wherein said material is disposed in at least one series of strips positioned end-to-end on an insulating substrate (10) so as to leave a gap between at least one pair of strips. 3. A plasma-recombination laser in accordance with claim 2 wherein said means for applying energy comprises a pulse generator for applying a high voltage- high current electrical pulse to said at least one series of strips. 4. A plasma-recombination laser in accordance with claim 2 wherein said means for applying energy comprises first electrical means (FIG. 7, 151) for applying a high voltage-high current electrical pulse to said at least one series of strips; a multiplicity of pairs of electrodes (130-139) , each of said pairs of electrodes having a gap therebetween, said gap being disposed in the vicinity of the gap between said strips; and second electrical means (152) for applying a second electrical pulse to said multiplicity of pairs of electrodes during said second time segment. -^DRE ^ O P 5. A plasma-recombination laser in accordance with claim 2 where said means for applying energy comprises first electrical means for applying a high voltage-high current electrical pulse to said at least one series of strips; and means (FIG. 8, 153, 155) for generating an rf field to surround said at least one series of strips during said second time segment, whereby energy is transferred during said second time segment from said rf field to said plasma. 6. A plasma-recombination laser in accordance with claim 2 wherein said means for applying energy comprises first electrical means for applying a high voltage-high current electrical pulse to said at least one series of strips; a microwave waveguide (FIG. 9, 157) surrounding said at least one series of strips; and a microwave source (156) for generating a microwave field in said waveguide during said second time segment, whereby energy is transferred during said second time segment from said microwave field to said plasma. 7. A plasma-recombination laser in accordance with claim 1 wherein said material comprises at least two spaced apart strips; said means for applying energy comprises a laser source (FIG. 10, 410) of laser radiation; means (421-423) for forming separate beams from said laser radiation and directing (441-443) said beams to impinge upon said at least two strips; a multiplicity of pairs of electrodes (130-135) , each of said pairs of electrodes having a gap therebetween, said gap being disposed in the vicinity of said strips; and means (152) for applying an electrical pulse to said multiplicity of pairs of electrodes during said second time segment. 8. A plasma-recombination laser in accordance with claim 1 wherein said material comprises at least two spaced apart strips; said means for applying energy comprises a laser source (FIG. 11, 410) of laser radiation; means (421-423) for forming separate beams from said laser radiation and directing (441-443) said beams to impinge upon said at least two strips; and means (153, 155) for generating an rf field surrounding said at least two strips during said second time segment, whereby energy is transferred during said second time segment from said rf field to said plasma. 9. A plasma-recombination laser in accordance with claim 1 wherein said material comprises at least two spaced apart strips; said means for applying energy comprises a laser source (FIG. 12, 421) of laser radiation; means (421-423) for forming separate beams from said laser radiation and directing (441-443) said beams to impinge upon said at least two strips; a microwave waveguide (157) surrounding said at least two strips; and a microwave source (156) for generating a microwave field in said waveguide during said second time segment, whereby energy is transferred during said second time segment from said microwave field to said plasma. 10. A plasma-recombination laser in accordance with claim 2 wherein said means for applying energy comprises electrical means (FIG. 15, 5) for applying a high voltage-high current electrical pulse to said at least one series of strips to form plasmas in said gaps; a laser source (500) of laser radiation; and means (631-634) for forming separate beams from said laser radiation and directing said beams to impinge upon said plasmas during said second time segment. 11. A plasma-recombination laser in accordance with claim 1 wherein said material comprises at least two spaced apart strips; said means for applying energy comprises a first laser source (FIG. 16, 410) of laser radiation; -^JREX OMPI means (421-423) for forming separate beams from said first laser radiation and directing (441-443) said beams to impinge upon said at least two strips to form plasmas; a second laser source (600) of second laser radiation; and means (631-633) for forming separate beams from said second laser radiation and directing said beams to impinge upon said plasmas during said second time segment. 12. A plasma-recombination travelling wave laser which comprises at least two spaced apart strips of material; means (FIG. 14, 410, 421-423, 441-443) for producing at least two plasmas from the material of said strips in a first segment; means (170) for adding energy to said two plasmas in a predetermined time sequence (501-503) . OMPI

Inventors: SILFVAST WILLIAM THOMAS, SZETO LEO HUNG, WOOD II OBERT REEVES
Applicants: WESTERN ELECTRIC CO, WESTERN ELECTRIC CO INC
Application year: 1982
Main classification: H01S3
IPC codes: H01S3