Disc or slab laser apparatus employing compound parabolic concentrator

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match between emission of flashlamp and absorption of

DISC OR SLAB LASER APPARATUS EMPLOYNG Nd:glass and the low thermal conductivity of glass. COMPOUND PARABOLIC CONCENTRATOR To avoid these problems, advanced developments in the solid state fusion driver art employ as sources of

BACKGROUND OF THE INVENTION 5 pumping radiation arrays or panels of light emitting This invention relates to improvements in laser appa diodes or excimer fluorescors or lasers. These devices ratus. Lasers are devices which generate or amplify, and provide a higher pump efficiency and reduce the emit coherent electromagnetic radiation at higher fre amount of heat deposited in the glass laser slab or disc. quencies than microwave frequencies, usually in the O Pumps of this type emit a generally collimated large infrared and visible portions of the electromagnetic cross section beam normal to a usually plane emitting spectrum called optical radiation. The light emission surface, in contrast to the 4t sterad radiation from long from a laser is characterized by a narrow wavelength and narrow cylindrical light sources provided by flash spread, i.e., it is essentially monochromatic, and by its lamps. However, the reflector designs which provide spatial or in-phase relationship. Lasers may be fabri 15 optimum coupling between flashlamps and lasers are cated from any active medium in which a population optimized for the cylindrical, incoherent emitter, and inversion may be established by suitable pumping. Typi are either unusable or strongly inadequate for use with cal of active laser media are neodymium glass, ruby, the panel type pump sources.

carbon dioxide, and helium-neon mixtures.

While the laser may take a variety of forms, the pres SUMMARY OF THE INVENTION ent invention is concerned with disc or slab type lasers 20 With the foregoing in mind it is a general object of which are characterized by having a thickness dimen the present invention to provide an improved laser sion along a line normal to the major faces thereof apparatus wherein a disc or slab type laser and a pump which is small as compared to the transverse dimension adapted to provide a large cross section beam of gener across the faces thereof. ally collimated pumping radiation are coupled with Emission of coherent radiation from a laser requires 25 maximum flux concentration allowed by phase space population inversion, the condition which exists when a conservation.

substantial number of the possible atomic or molecular Another object of the invention is to provide an im radiating species in the active laser material are raised to proved laser apparatus of the class described wherein a metastable energy state above the ground state of the the coupling of the pump and laser and the flux concen species. When this condition exists, an incident photon tration is provided

by a compound parabolic concentra of laser emission wavelength may stimulate a radiative transition from a metastable level to a lower level, tor.A further object of the invention is to provide an which may or may not be the ground state of the spe improved laser apparatus of the type mentioned in cies. Such radiative transitions are cumulative and self stimulating, resulting in the emission of radiation which 35 which the compound parabolic concentrator may be is coherent and in-phase. Population inversion is either troughlike, conelike, pyramidal or the like in configuration.

achieved, for example, by irradiation of the laser me dium with a high intensity of electromagnetic radiation A more specific object of the invention is to provide at a wavelength of appropriate energy to raise the radi a laser apparatus of the class described in one form of ating specie to a metastable state when the radiation is 40 which the laser is face pumped, and in another form of absorbed thereby. Such inversion-causing radiation is which the laser is edge pumped.

referred to as pumping radiation, and the wavelength of Another object of the invention is to provide an im the pumping of activating radiation is known as the proved laser apparatus of the class described which is pumping wavelength. adaptable for use in either an oscillator or an amplifier The activation or creation of a population inversion, 45 configuration.

essential for coherent electromagnetic radiation, is Other and further objects and advantages of the in achieved by optical pumping, namely the irradiation of vention will become apparent as the description pro the active medium with optical radiation from a source ceeds.

usually referred to as a pump. The amount of inversion created within an active laser medium is a function of 50 BRIEF DESCRIPTION OF THE DRAWINGS the flux density (energy per unit area) of the pumping FIG. 1 is a semidiagrammatic sectional view of one optical radiation incident thereupon. It is therefore de form of laser apparatus constructed in accordance with sirable that means be provided for maximizing the the invention; and amount of pumping radiation from the pump which FIG. 2 is a semidiagrammatic sectional view of a may be incident upon an active surface of the laser. 55 modified form of laser apparatus constructed in accor Neodymium doped glass (Nd:glass) has found fre dance with the invention.

quent application in laser developments, and the pump DETAILED DESCRIPTION OF THE ing means conventionally used in such developments PREFERRED EMBODIMENT have been flashlamps. A flashlamp represents a long and narrow cylindrical radiation source which radiates into Referring more particularly to FIG. 1 of the drawing, 47 sterad, and reflector designs have been developed there is illustrated therein a disc or slab type laser me which provide optimum coupling of this pump source dium body 5, a pump source 6, and a concentrator 7 to disc or slab lasers. coupling the source 6 to the laser medium body 5. Flashlamp pumped Nd:glass discs or slab lasers are Source 6 preferably takes the form of a panel compris not suitable for certain applications, for example as 65 ing an array of light emitting laser diodes or excimer drivers in inertial confinement fusion systems, because fluorescors or lasers. The source 6 is provided with an of low conversion efficiency and limited repetition rate inner surface 8 and is adapted to emit from said surface capability. Both limitations arise because of a very poor a substantially collimated beam of radiation of relatively

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large cross section, which beam is directed generally trated in FIG. , is similar in construction to the laser toward the laser medium body 5 and normal to the oscillator in that a body of laser material is optically surface 8, the latter being preferably planar. pumped, the primary distinction being that in the ampli The concentrator 7 is of the compound parabolic fier an unamplified laser beam generated by a laser type described in Solar Energy, 1974, Vol. 16, pg. 89, oscillator is incident upon the face 10 of the laser me Roland Winston. Both the troughlike and conelike ge ometries of concentrators described therein can usefully dium body 5, and upon a double pass through the laser material (i.e. reflection from the totally reflective coat be embodied in the present invention. The compound ing on surface parabolic concentrator 7 provides optimum coupling of through the face9)10.anSince amplified laser beam is emitted the amplifier is of the nonres the source 6 to laser medium body 5 by virtue of the fact 10 onant type, the face 10 is either uncoated, or preferably that it provides the maximum concentration of the pumping radiation flux permitted by phase space con is antireflection coated.

servation. The large area of the faces 9 and 10 of the disc or slab The compound parabolic concentrator is a nonimag efficient shaped laser medium body 5, in addition to providing an ing light funnel that derives its characteristic optical 15 pumping geometry, permits the generation of properties from the specific shape of the external wall, high output of laser energy while maintaining the en which is made specularly reflecting. In FIG. 1 the pa ergy density within the laser material below the level rabola BA has its focus at C and its axis (which passes destructive of the laser material. The combination of a through C) parallel to the line BD. It has a focal length laser medium body 5, optical pumping panel or array 6 equal to BC/2 (1--sin 8). Parabola CD is the same, 20 (and power supply therefor), and compound parabolic except that its focus is at B and its axis is parallel to the concentrator 7 forms what may be defined as a laser line CA. Such devices achieve the following concentra module. Laser modules can be combined in multi-stage tion ratios (X) of entrance area/exit area: systems such as high energy laser oscillators and power amplifiers. Depending upon the application, the laser

Xtrough = n/sin 8max 25 medium body 5 may be disc shaped, i.e. with surfaces 9

Xcone= n2/sin2 6max and 10 circular, in which case the concentrator 7 is preferably conelike in shape. Alternatively the laser medium body 5 may be rectangular in shape, i.e. slab

Where 6 is the angular acceptance (half-angle) and n is shaped the index of refraction of the collector relative to the in which case the concentrator 7 may be trough surrounding medium. This concentration is the maxi 30 the like in configuration with flat end walls, or it may take mum permissible by physical principles. form of a truncated pyramid having parabolic side The laser medium body 5 is preferably formed of walls. As used herein, a conelike compound parabolic con commercially available neodymium doped glass, for example 4% neodymium doped glass available from centrator is one having the shape of a truncated cone American Optical Company of Pittsburgh, PA as 35 which is symmetric with respect to and spaced from its AOLux No. 1689. As shown in FIG. 1 the body 5 is of optic axis and which, in diametric cross section, exhibits the face pumped type, the right hand face 9 thereof on each side thereof a profile curve in the shape of a being coated with dielectric which is totally reflective parabola whose focus is at the opposite edge of the exit at laser energy wavelength and highly transmissive at aperture, and whose axis is inclined at the angle 6max optical radiation wavelengths. The edge surface of the (see FIGS. 1 and 2) with respect to said optic axis. As body 5 is preferably coated with dielectric which is used herein, a troughlike compound parabolic concen highly absorptive at laser energy wavelength to prevent trator is one having the shape of a trough defined by internal reflections from said surface. The body 5 also opposite sidewalls which are bilaterally symmetrically has its left hand face 10 coated with dielectric which is spaced with respect to an optic axis plane. In cross partially transmissive at laser radiation wavelengths, 45 section transverse to said plane the troughlike concen uncoated, or antireflection coated. In any of these three trator exhibits, on each side thereof, the same profile cases the laser medium body 5 may be employed in laser curve exhibited by the conelike concentrator in diamet oscillator configuration. ric cross section, i.e. the focus is at the lower edge of the When used in a laser oscillator apparatus, the pumped opposite sidewall and the axis is inclined at the angle surface 9 of the laser medium body 5 may define one 50 6max with respect to said optic axis plane. end of an optical resonant cavity. The single end surface FIG. 2 illustrates a form of the invention in which a pumped, single end surface laser beam emitting disc or slab or disc type laser medium body 5a is edge pumped, laser medium body 5 may also be utilized in a two-pass rather than face pumped, there being one or more pump laser amplifier configuration wherein, as shown by the sources such as the panels 6a and 6b coupled to the edge broken arrows in FIG. 1, an unamplified beam is inci 55 portions of the body 5a by compound parabolic concen dent upon the surface 10 at an angle relative to the trators 7a and 7b.

normal thereof and is emitted therefrom in amplified In one form of the invention shown in FIG. 2, the form at the conjugate angle. The laser amplifier may laser medium body 5a has planar rectangular faces 9a include a plurality of serially optically coupled active and 10a and planar rectangular edge surfaces 11 and 12, mirrors or modules of the type shown in FIG. 1 60 in which case the concentrators 7a and 7b may be whereby the laser beam emitted therefrom has an ex troughlike in shape with flat end walls, or they may be tremely high energy, especially in pulse mode operation in the shape of truncated pyramids having parabolic of the laser medium bodies. walls. If desired, a pair of pump panels and concentra Many applications require laser energy outputs tors which are higher than that obtained from a laser oscilla 65 withsimilar to those shown in FIG. 2 may be associated the rectangular planar edge surfaces of the laser tor alone, and for such cases a laser power amplifier is medium body 5a (not shown) which are normal to the optically coupled to the output of the oscillator. A laser amplifier in accordance with the invention, as illus surfaces 11 and 12. Alternatively, the body 5a may be in disc form, having circular faces 9a and 1.0a and a cylin

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drical edge face, in which case the concentrator may portion, each of said at least one additional pump panels form part of a toroid having annular parabolic sidewalls being coupled to a separate one of said at least one which extend between the cylindrical edge face and the additional pumping surface portions by a compound panel pump source, which source may take the form of parabolic concentrator.

a cylindrical annular array of light emitting diodes or 5 9. The combination of claim which includes at least excimer fluorescors or lasers. one additional pump panel and wherein said pumping When the form of the invention shown in FIG. 2 is surface portion is an edge surface portion of said laser employed in laser medium body oscillator configura medium body, said laser medium body has at least one tion, the surface 10a of laser 5a is coated with dielectric additional pumping surface portion which is an edge which is totally reflective at laser energy wavelength, 10 surface portion thereof, and each of said at least one whereas surface 9a is coated with dielectric which is additional pumping panels is coupled to a separate one partially transmissive at laser radiation wavelengths, of said at least one additional edge surface portions by a said coatings providing the opposite ends of an optical compound parabolic concentrator.

resonant cavity, with laser radiation being emitted from 10. The combination of claim 1 wherein said laser the surface 9a in the direction of the arrow shown. On 15 medium body has a generally cylindrical edge pumping the other hand, when the apparatus shown in FIG. 2 is surface portion, said pump panel is generally of annular employed as a laser amplifier, an unamplified laser beam cylindrical configuration and coaxially surrounds said generated by a laser oscillator is incident upon the face body, and said pump panel is coupled to said cylindrical 10a of disc 5a, and upon passage through the laser mate edge pumping surface portion by an annular compound rial, an amplified laser beam is emitted through the face 20. parabolic concentrator forming with said panel and 9a. Since the amplifier is of the nonresonant type, the surface portion a generally toroidal configuration. face 10a is either uncoated or preferably is antireflection 11. The combination of claim 1 wherein the pumping coated. surface portion of said laser medium body is overlaid While several embodiments of the invention have with a coating which is reflective at laser energy wave been shown and described, it will be apparent the many 25 lengths and transmissive at optical radiation wave modification and changes therein may be made without lengths.

departing from the spirit of the invention. All of such 12. The combination of claim 1 wherein one of said changes are contemplated as may come within the major faces is overlaid with a coating which is reflec scope of the appended claims. tive at laser energy wavelengths, said laser medium What is claimed as the invention is: 30 body being adapted to permit passage therethrough of a 1. In combination, a laser medium body having a pair laser beam incident upon the other of said major faces, of similar generally parallel dpposite planar major faces such beam being reflected by said reflective coating and having a thickness dimension between said faces back through said body and being emitted in amplified which is small as compared to a transverse dimension of form from said other major surface. said faces, said body also having a pumping surface 35 13. The combination of claim 1 wherein both major portion; a laser pump panel having a surface portion faces of said laser medium body are transmissive at laser from which it is adapted to emit a relatively large cross energy wavelengths, said laser medium body being section beam of generally collimated laser pumping adapted to permit passage therethrough between said radiation; and flux concentrating coupling means for major faces of a laser beam incident upon one of said said pump and laser medium body, said means compris faces, whereby said beam is amplified during such pas ing a compound parabolic concentrator having an inlet sage and is emitted from the opposite face of said body. end opening across which said surface portion of the 14. The combination of claim 1 wherein said concen pump panel extends for direction of pumping radiation trator comprises opposing parabolic sidewalls bilater thereinto, and having an outlet end opening of reduced ally symmetrically spaced from an optic axis plane. area across which the pumping surface portion of said 45 15. The combination of claim 1 wherein said concen laser medium body extends for receipt of concentrated trator has a sidewall which is generally circular in cross radiation from said pump panel. section in planes normal to an optic axis, and in a plane 2. The combination of claim 1 wherein said pump including said optic axis exhibits in cross section oppos comprises an array of light emitting diodes. ing parabolic wall portions which are bilaterally sym 3. The combination of claim 1 wherein said pump 50 metrically spaced from said axis.

comprises an array of excimer lasers. 16. The combination of claim 1 wherein said pumping 4. The combination of claim 1 wherein said pump surface portion is a planar edge surface portion of said comprises an array of GaAs or GaAlAs light emitting laser medium body, and said concentrator comprises diodes. opposing parabolic sidewalls bilaterally symmetrically 5. The combination of claim 1 wherein said pumping 55 spaced from an optic axis plane.

surface portion is an edge surface portion of said laser 17. The combination of claim 1 wherein said pumping medium body. surface portion is a face surface portion of said laser 6. The combination of claim 1 wherein said pumping medium body, and said concentrator has a sidewall surface portion is a face surface portion of said laser which is generally circular in cross section in planes medium body. normal to an optic axis, and in a plane including said 7. The combination of claim 1 wherein said pumping optic axis exhibits in cross section opposing parabolic surface portion is a face surface portion of said laser wall portions which are bilaterally symmetrically medium body and is overlaid by a mirror transmissive spaced from said axis.

of optical pumping radiation wavelengths and reflective 18. The combination of claim 1 wherein said pumping of laser energy radiation wavelengths. 65 surface portion is a planar face surface portion of said 8. The combination of claim 1 which includes at least laser medium body, and said concentrator comprises one additional pump panel and wherein said laser me opposing parabolic sidewalls bilaterally symmetrically dium body has at least one additional pumping surface spaced from an optic axis plane.

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19. The combination of claim 1 which includes an by an additional compound parabolic concentrator, additional pump panel and wherein said pumping sur- each of said concentrators comprising opposing para face portion is a planar edge surface portion of said laser w medium body, said laser medium body has an additional bolic sidewalls bilaterally symmetrically spaced from an planar edge surface portion, and said additional pump 5 optic axis plane.

panel is coupled to said additional edge surface portion x k k