Multistaged stokes injected Raman capillary waveguide amplifier
BACKGROUND OF THE INVENTION 
     The present invention pertains generally to infrared lasers and amplifiers and more particularly to stimulated Raman scattering utilized rotational transitions in a diatomic molecular gas. The present invention comprises an improvement in the inventions disclosed in copending application Ser. No. 802,400 entitled &#34;Shifting of CO 2  Laser Radiation Using Rotational Raman Resonances&#34; filed June 1, 1977, by Norman A. Kurnit, now abandoned in favor of continuation-in-part application Ser. No. 960,409 entitled &#34;Shifting of Infrared Radiation Using Rotational Raman Resonances in Diatomic Molecular Gases&#34; filed Nov. 13, 1978 by Norman A. Kurnit and copending application Ser. No. 948,267 entitled &#34;Stokes Injected Raman Waveguide Amplifier&#34; filed Oct. 3, 1978 by Norman A. Kurnit. These commonly assigned copending applications disclose the basis for stimulated Raman scattering in a capillary waveguide and amplification of an external Stokes injection source in a Raman capillary waveguide amplifier. 
     SUMMARY OF THE INVENTION 
     The present invention provides a multistaged Stokes injected Raman capillary waveguide amplifier. A plurality of optically coupled capillary waveguide amplifiers are injected with an external source of CO 2  laser radiation to improve Raman gain. Progressively larger capillary diameters in successive stages provide greater output power of the amplified Stokes signal. Additionally, multiple stages can be used to provide a high gain Stokes signal for injection into a multifocused Raman gain cell to further increase output power. One or more stages of the capillary waveguide amplifier can employ regenerative amplification. Additionally, capillary waveguide material can be varied throughout the stages to enhance propagation of CO 2  radiation to increase gain in early stages and propagation of Stokes radiation to increase output power in latter stages. 
     It is therefore an object of the present invention to provide a multistaged Stokes injected Raman capillary waveguide amplifier. 
     Another object of the present invention is to provide a multistaged Stokes injected Raman capillary waveguide amplifier in conjunction with a multifocused Raman gain cell to provide high output powers. 
     Another object of the present invention is to provide a multistaged Stokes injected Raman capillary waveguide amplifier for providing high output power Stokes signals. 
     Another object of the present invention is to provide a multistaged Stokes injected Raman capillary waveguide amplifier which is highly efficient in operation. 
     Other objects and further scope of applicability of the present invention will become apparent in the detailed description given hereinafter. The detailed description, indicating the preferred embodiments of the invention, is given by way of illustration since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. The foregoing Abstract of the Disclosure is for the purpose of providing a nonlegal brief statement to serve as a searching and scanning tool for scientists, engineers and researchers and is not intended to limit the scope of the invention as disclosed herein, nor is it intended to be used in interpreting or in any way limiting the scope or fair meaning of the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a multistaged Stokes injected Raman capillary waveguide amplifier comprising the preferred embodiment of the invention. 
     FIG. 2 is a schematic illustration of a multistaged Stokes injected Raman capillary waveguide amplifier in conjunction with a multifocused Raman gain cell. 
     FIG. 3 is a schematic illustration of a reflective coupling device. 
     FIG. 4 is a schematic illustration of a dichroic mirror coupling device for injecting an external source of CO 2  radiation. 
     FIG. 5 is a schematic illustration of a prism coupling device for injecting an external source of CO 2  radiation. 
     FIG. 6 is a schematic illustration of a grating coupling device for injecting an external source of CO 2  radiation. 
     FIG. 7 is a schematic illustration of a Ge Brewster plate coupling device for injecting an external source of CO 2  radiation. 
     FIG. 8 is a schematic illustration of a reflective coupling device utilizing a CO 2  amplifier. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
     FIG. 1 is a schematic illustration of a multistaged Stokes injected Raman capillary waveguide amplifier comprising the preferred embodiment of the invention. According to the present invention, an external source of Stokes radiation from Stokes injection source 10 is combined with CO 2  laser radiation from CO 2  laser source 20 and circularly polarized in opposite circular directions. These radiation signals are applied to a capillary waveguide in the manner disclosed in copending application Ser. No. 948,267 entitled &#34;A Stokes Injected Raman Capillary Waveguide Amplifier&#34; filed Oct. 3, 1978 by Norman A. Kurnit. Capillary waveguide amplifier 36 comprises a capillary waveguide disposed within a Raman gain cell such that the Raman gain medium comprising H 2 , D 2 , HD, HT, DT, or T 2  is contained throughout the capillary. Additionally, the capillary is cooled to cryogenic temperatures by liquid nitrogen or other cooling medium. Partially reflecting mirrors 32 and 34, such as described in the above cited copending applications, can be utilized in one or more stages to create a regenerative amplifier for increasing Raman gain. This is particularly useful in low power stages where greater Raman gain is important. 
     Coupling device 40 couples the Stokes radiation generated by capillary waveguide amplifier 36 to an additional capillary waveguide amplifier stage 46. CO 2  radiation generated by CO 2  laser source 20 can be coupled through coupling device 40 or, alternatively, can be provided by an optional CO 2  input laser source 42. Of course, CO 2  laser radiation from CO 2  laser source 20 can be split and applied to coupling device 40 to eliminate additional CO 2  lasers 42, 52 and 62. Each of the coupling devices 40, 50 and 60 can comprise any of the devices shown in FIGS. 3-8. It should also be noted that each source of CO 2  laser radiation can comprise a multiple frequency beam to enhance generation of a multiple frequency Stokes output signal. Several Stokes injection frequencies corresponding to the several CO 2  frequencies may be necessary to provide sufficient gain on each of the Stokes output wavelengths. However, when sufficient gain is provided on a single Stokes frequency for which a sufficiently intense Stokes injection signal is provided, other output Stokes wavelengths are generated by a four-wave mixing process. 
     The composition of the waveguide material can be varied in successive stages to enhance generation of the desired signal. For example, initial stages such as 36 and 46 can contain capillaries fabricated from BeO which have low waveguide losses for CO 2  laser radiation in the 10 μm spectral region. Although BeO does not transmit Stokes radiation as well as Al 2  O 3 , the high transmission characteristics of the CO 2  radiation provides high gain which much more than offsets any losses due to lower transmission characteristics of the relatively low powered Stokes signals in the initial stages. However, in later stages, where the Stokes signal has a relatively high power magnitude, the capillary is constructed of Al 2  O 3  or similar material that has low waveguide losses at the Stokes frequency. Although this provides less gain due to the lower transmission characteristics of the CO 2  radiation, lesser gain is required in the power stages compared to the desirability of transmitting the maximum amount of Stokes frequency radiation, which is particularly important since the waveguide losses are proportional to λ 2 . Similarly, the capillary waveguide diameter and length can be increased in successive stages to provide greater power handling capability by supplying a larger quantity of Raman gain medium to produce the desired power output levels. Of course, focusing optics 44 and 54 are designed to provide successively larger focal areas such that the breakdown threshold of the Raman medium is not exceeded. 
     FIG. 2 is a schematic illustration of a device which utilizes a multifocused Raman gain cell in the output power amplification stage. 
     An example of a multifocused Raman gain cell 72 is disclosed in an article entitled &#34;Tunable, Millijoule Radiation Extending to the 16 μm Region&#34; by Robert Frey et al., in Optics Communications 22, p. 355 (1977). An alternative multipass Raman gain cell for use with the embodiment of FIG. 2, utilizes the spherical interferometer disclosed in an article entitled &#34;Off Axis Paths in Spherical Mirror Interferometers&#34; by D. Herriott et al., in Applied Optics 3, p. 523 (1964), with an appropriate room temperature Raman gain medium such as H 2  as suggested by Robert L. Byer and W. R. Trutna at the Tenth International Quantum Electronics Conference, May 31, 1978 in Atlanta, Georgia. Again, the techniques described in FIG. 1 can be utilized for obtaining a suitable Stokes signal for injection into multifocused Raman gain cell 72. 
     FIGS. 3-8 illustrate various coupling devices which can be used in FIGS. 1 and 2. FIG. 3 illustrates a reflective coupling device wherein both the Stokes and CO 2  radiation are transmitted from one stage to another. This type of reflective coupling device is useful when sufficient CO 2  energy is transmitted through the capillary waveguide to provide proper gain. 
     FIG. 4 is a schematic illustration of a dichroic mirror coupling device wherein an additional source of CO 2  radiation is coupled into a successive stage by means of dichroic optics 76 which function to totally reflect the Stokes frequency signal and transmit the CO 2  input frequency signal. 
     FIG. 5 schematically illustrates a prism coupling device wherein additional CO 2  frequency radiation is coupled into a successive capillary waveguide amplifier stage by utilizing the refraction characteristics of prism 78 as illustrated in FIG. 5. 
     FIG. 6 is a schematic illustration of a grating coupling which makes use of the different reflective angles obtainable from a grating for different signals. Additional CO 2  input signals are applied to the grating to align with the reflected Stokes signal. 
     FIG. 7 is a schematic illustration of a Ge Brewster plate coupling device which couples additional CO 2  radiation through a Ge Brewster plate 82. The CO 2  Stokes radiation obtained from a preceeding stage is linearly polarized in Fresnel rhomb 84. The Stokes radiation is axially aligned with additional CO 2  radiation by Ge Brewster plate 82. The additional CO 2  radiation and Stokes radiation are then circularly polarized in opposite circular directions in Fresnel rhomb 86. 
     FIG. 8 is a schematic illustration of a coupling device which utilizes flat reflectors 90 and 92 to reflect both the Stokes and CO 2  radiation from one stage to another, in the manner disclosed in the coupling device of FIG. 3. Additionally, a CO 2  amplifier 94 is included within the coupling device to increase the gain of the CO 2  radiation transmitted through the capillary waveguide of the preceding stage to provide amplified CO 2  radiation for the succeeding stage. The Stokes radiation is also transmitted through the CO 2  amplifier. The Stokes radiation is transmitted without absorption and reflected into a succeeding amplification stage simultaneously with the CO 2  radiation. Each of the coupling devices 40, 50, and 60 illustrated in FIGS. 1 and 2 can comprise a coupling device as illustrated in FIG. 3 so as to eliminate the need for any of the optional CO 2  inputs 42, 52, or 62. 
     The present invention therefore provides a multistaged Stokes injected Raman capillary waveguide amplifier capable of producing high output power Stokes signals. The use of various coupling devices allows for high Raman gain and high powers in a series of successive stages. The use of successively larger capillary waveguides and various materials in waveguide fabrication also allows for increased power in successive amplification stages. Use of a multifocused Raman gain cell, additionally, provides a high output power amplification stage in which gas flow can be provided, if necessary, for greater power handling capability. 
     Obviously many modifications and variations of the present invention are possible in light of the above teachings. For example, the external source of Stokes radiation can be eliminated if sufficient gain is present in the initial stage to spontaneously generate the Stokes frequency signal. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described and that subject matter disclosed and not claimed herein shall not be precluded from being later claimed in the present application or a continuation, continuation-in-part, or reissue application.