Stokes injected Raman capillary waveguide amplifier

A device for producing stimulated Raman scattering of CO.sub.2 laser radiation by rotational states in a diatomic molecular gas utilizing a Stokes injection signal. The system utilizes a cryogenically cooled waveguide for extending focal interaction length. The waveguide, in conjunction with the Stokes injection signal, reduces required power density of the CO.sub.2 radiation below the breakdown threshold for the diatomic molecular gas. A Fresnel rhomb is employed to circularly polarize the Stokes injection signal and CO.sub.2 laser radiation in opposite circular directions. The device can be employed either as a regenerative oscillator utilizing optical cavity mirrors or as a single pass amplifier. Additionally, a plurality of Raman gain cells can be staged to increase output power magnitude. Also, in the regenerative oscillator embodiment, the Raman gain cell cavity length and CO.sub.2 cavity length can be matched to provide synchronism between mode locked CO.sub.2 pulses and pulses produced within the Raman gain cell.

BACKGROUND OF THE INVENTION 
The present invention pertains generally to infrared oscillators and 
amplifiers and more particularly to stimulated Raman scattering utilizing 
rotational transitions in a diatomic molecular gas. 
Various methods have been disclosed for shifting frequencies of 
conventional lasers in the IR spectrum. These methods have included 
four-wave mixing as disclosed in application Ser. No. 787,415 filed Apr. 
14, 1977 by Richard F. Begley et al. entitled "Resonantly Enhanced 
Four-Wave Mixing" now U.S. Pat. No. 4,095,121 and Raman scattering, as 
disclosed in application Ser. No. 466,583 filed May 2, 1974 by C. D. 
Cantrell et al. entitled "Infrared Laser System now U.S. Pat. No. 
4,061,921. In each of these systems and other previous systems for IR 
frequency shifting to a broad range of frequencies, simplicity and overall 
efficiency are important factors for economic utilization of the device. 
By minimizing the steps required for frequency shifting, such as 
elimination of the Raman spin-flip laser, as set forth in the above 
disclosed application Ser. No. 466,583, the device can be simplified to 
reduce problems inherent in more complex systems. 
Since the stimulated Raman effect can be produced in a single step with 
high conversion efficiency, Raman shifting of CO.sub.2 laser radiation 
provides high overall efficiencies due to the high efficiencies and well 
developed technology of CO.sub.2 lasers. However, Raman gain in gaseous 
media such as H.sub.2, D.sub.2, HD, HT, DT or T.sub.2 requires powers 
which are near the breakdown threshold of these diatomic molecular gases 
for a single pass focused geometry, such as suggested by Robert L. Byer, 
in an article entitled "A 16 .mu.m Source for Laser Isotope Enrichment" 
published in IEEE Journal of Quantum Electronics, Vol. QE12, pp. 732-733, 
November 1976. 
Other devices have also used rotational Raman gain to generate Stokes 
signals such as disclosed in copending application Ser. No. 802,400 
entitled "Shifting of CO.sub.2 Laser Radiation Using Rotational Raman 
Resonances" filed June 1, 1977 by Norman A. Kurnit, of which the present 
invention comprises an improvement. The device of the above disclosed 
invention relies upon spontaneous generation of the desired Stokes signal. 
Such a system, of course, requires high power densities and long focal 
interaction lengths to ensure spontaneous generation of the desired Stokes 
signal. 
SUMMARY OF THE INVENTION 
The present invention overcomes the disadvantages and limitations of the 
prior art by providing a Stokes injected Raman waveguide amplifier. The 
device of the present invention uses an external Stokes signal injected in 
a capillary waveguide amplifier which Raman scatters CO.sub.2 laser 
radiation by rotational states of a diatomic molecule such as H.sub.2, 
D.sub.2, HD, HT, DT or T.sub.2. The Stokes injection signal reduces the 
required field strength of the CO.sub.2 laser radiation and eliminates the 
necessity for spontaneous generation of Stokes radiation within the 
capillary amplifier. 
It is therefore an object of the present invention to provide a Stokes 
injected Raman capillary waveguide amplifier. 
It is also an object of the present invention to provide a Stokes injected 
Raman capillary waveguide regenerative amplifier. 
Another object of the present invention is to provide a Stokes injected 
Raman capillary waveguide amplifier having high output powers. 
Another object of the present invention is to provide a Stokes injected 
Raman capillary waveguide regenerative amplifier having high output 
powers. 
Other objects and further scope of applicability of the present invention 
will become apparent from the detailed description given hereinafter. The 
detailed description, indicating the preferred embodiments of the 
invention, is given only 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 
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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
FIG. 1 is a schematic illustration of a Stokes injected Raman waveguide 
amplifier. As outlined in the Background of the Invention, the approach of 
copending application Ser. No. 802,400, filed June 1, 1977 by Norman A. 
Kurnit was to use a dielectric waveguide to provide a long focal 
interaction length to overcome rapid divergence and low Raman gain at 
infrared wavelengths [proportional to Stokes frequency 
(g.perspectiveto.10.sup.-4 cm.sup.-1 /MW/cm.sup.2) for S.sub.oo (0) of 
para-H.sub.2 at 77.degree. at approximately 1 atmosphere of pressure using 
circularly polarized 10 .mu.m radiation]. The cryogenically cooled 
capillary extends the focal interaction length sufficiently, as disclosed 
in the above referenced copending application, to generate spontaneous 
emission of the desired Stokes signal. 
According to the present invention, an external Stokes injection source 10 
is provided to reduce the required gain and the focal interaction length 
necessary to convert a large fraction of the CO.sub.2 pump radiation to 
the Stokes frequency. As shown in FIG. 1, Stokes injection source 10 is 
collimated by lens 12 and directed to Ge Brewster plate 18 via 10 .mu.m 
rejection filter 14 and LiF reflector 16. Ge Brewster plate 18 combines 
the Stokes injection signal and a CO.sub.2 laser radiation signal from 
CO.sub.2 laser source 20 into A Single coaxial path. The output of the 
CO.sub.2 laser is spatially filtered and defined in direction by passing 
it through an evacuated waveguide (not shown) and recollimated before 
being passed through Ge Brewster plate 18. The Stokes radiation reflected 
from Ge Brewster plate 18 has a polarization orthogonal to the CO.sub.2 
laser radiation. When applied to KBr Fresnel rhomb circular polarizer 22, 
the CO.sub.2 radiation and Stokes radiation are circularly polarized in 
opposite circular directions as shown at 24 and 26. The dispersion of KBr 
is sufficiently small that a rhomb designed to give quarter-wave 
retardation at 10 .mu.m also gives nearly quarterwave retardation at 16 
.mu.m. Opposite circular polarization of these signals provides the 
largest Raman gain and, additionally, eliminates anti-Stokes coupling. 
CO.sub.2 radiation feedback to CO.sub.2 laser source 20 is decoupled by 
Fresnel rhomb circular polarizer 22 which rotates the polarization of 
feedback radiation striking the Ge Brewster plate by 90.degree..CO.sub.2 
feedback radiation to Stokes injection source 10 is decoupled by 10 .mu.m 
rejection filter 14, LiF restrahl reflection plate 16 and, if necessary, a 
gas absorption cell (not shown). The Fresnel rhomb circular polarizer 22 
can be fabricated from KBr, KCl, CsI. NaCl, ZnSe, or other media 
transparent at the pump and Stokes wavelengths. 
CO.sub.2 laser source 20 can also be designed according to conventional 
methods to provide multiple frequency beam of preselected frequencies to 
enhance generation of a multiple frequency Stokes output signal. Several 
Stokes injection frequencies, corresponding to several preselected 
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 high intensity Stokes 
injection signal is provided, other Stokes output wavelengths are 
generated by a four-wave mixing process. 
Stokes injection source 10 can comprise any one of a number of sources 
which provide the specified Stokes frequency which is determined as 
disclosed in the above referenced copending application Ser. No. 802,400 
filed June 1, 1977 by Norman A. Kurnit, or any combination of plurality 
thereof for providing a multifrequency Stokes injection signal. Suitable 
Stokes injection sources include tunable diode lasers, optical parametric 
oscillators, electrical discharge lasers including bending mode lasers, 
various optically pumped lasers, nonlinear mixing including difference 
frequency generation or four-wave mixing as disclosed in above referenced 
copending application Ser. No. 787,415, or any other source of coherent 
radiation providing the desired frequency signal. Microwave frequency 
shifters can also be used, if necessary, to shift a fixed frequency laser 
to the desired Stokes frequency. Additionally, high pressure tunable 
CO.sub.2 lasers can be used as element 20 to provide a tunable Stokes 
output frequency. 
The oppositely circularly polarized Stokes radiation signal and CO.sub.2 
radiation signal are reflected from reflectors 28 and 30 and are focused 
by optics 32 on the cryogenically cooled capillary 36 disposed within 
Raman cell 38 containing the desired diatomic molecular gas, e.g., 
D.sub.2, H.sub.2, HD, HT, DT, or T.sub.2 via window 34. The cryogenically 
cooled capillary 36 functions to extend the focal interaction length of 
the Stokes radiation and CO.sub.2 radiation within the Raman gain medium. 
Depending on the length of the capillary 36, gains of e.sup.6 to e.sup.60 
or greater can be achieved. The CO.sub.2 signal and amplified Stokes 
signal are emitted from the Raman gain cell via output window 40 and 
collimated by optics 42 after a single pass through the capillary 36. 
FIG. 2 is a schematic illustration of a Stokes injected regenerative 
amplfier. The Stokes injection signal is generated by Stokes injection 
source 44 which is focused via lens 46 and passed through 10 .mu.m 
rejection filter 48 and reflected by LiF reflector 50 and Brewster plate 
52 in the same manner as shown in FIG. 1. The CO.sub.2 laser source 54 
comprises a typical tunable CO.sub.2 laser having an output coupling 
mirror 58, a discharge cavity 56, a tunable grating 60, and, when desired, 
a mode locking element 61. Mode locking element 61 can comprise a 
saturable absorber such as a p-type germanium or an acousto-optic or 
electro-optic modulator. The CO.sub.2 laser 54 is designed to have a total 
optical resonant cavity length equal to L. The combined signals are 
circularly polarized in Fresnel rhomb circular polarizer 62 and reflected 
from reflectors 68 and 70 and focused on the cryogenically cooled 
capillary 80 in the same manner as shown in FIG. 1. Partially reflecting 
mirrors 74 and 76 are placed around the Raman gain cell 78 to form an 
optical resonant cavity also having a length L identical to the length of 
the optical resonant cavity of the CO.sub.2 laser 54. 
With the CO.sub.2 laser source 54 operating in a mode locked configuration, 
CO.sub.2 pulses transmitted through the Raman gain cell 78 and 
cryogenically cooled capillary 80 are reflected from partially reflecting 
mirror 76 for a second pass through the capillary 80 without overlapping 
incoming CO.sub.2 laser pulses within the focal region. Elimination of 
possible overlapping of input and reflected pulses in the cryogenically 
cooled capillary excludes the possibility of exceeding gas threshold 
breakdown levels, locally destroying the Raman gain effect. The optical 
resonant cavity of the CO.sub.2 laser 54 is matched in length to the 
optical resonant cavity surrounding the Raman gain cell to ensure that the 
recirculating Stokes pulses coincide with synchronous gain pulses. 
Multiple passes of CO.sub.2 mode locked pulses through the cryogenically 
cooled capillary 80 are accomplished by fabricating partially reflecting 
mirror 74 to be transmissive to 10 .mu.m radiation, and partially 
reflecting mirror 76 to be reflective to 10 .mu.m radiation. This allows a 
large portion of the CO.sub.2 laser radiation energy to be applied to the 
Raman gain cell for at least two passes through the cryogenically cooled 
capillary 80. Similarly, partially reflecting mirrors 74 and 76 are 
fabricated to be partially reflecting to Stokes injection radiation to 
generate multiple passes of the Stokes injection radiation through the 
Raman gain cell. Although this reduces the magnitude of the Stokes 
radiation injected in the capillary waveguide regenerative amplifier, it 
significantly increases achievable gain of the output amplified Stokes 
signals since each of the multiple passes through the Raman waveguide 
regenerative amplifier provides high gain. The Raman gain signal is 
subsequently transmitted through partially reflecting mirror 76 and 
collimated by optics 82. 
FIG. 3 is a detailed schematic diagram of a Raman waveguide capillary 
amplifier employing a plurality of capillary waveguides 84 joined together 
in a single axial path. The plurality of capillary waveguides 84 are 
joined together by capillary junction supports 86 and surrounded by 
thermally conducting capillary support 88 such as a copper braid or 
similar material. A straight metal tube 90 surrounds the thermally 
conducting capillary support 88 and provides stability and straightness to 
the combined structure. This combined structure is surrounded by a 
refrigerant container 92 which cryogenically cools the capillary structure 
to low temperatures. Liquid nitrogen is used as the cryogenic cooling 
medium which is placed within the refrigerant container 92. Gas inlet and 
outlet ports 94 and 96 provide a supply of Raman gain medium comprising a 
diatomic molecule such as H.sub.2, D.sub.2, HD, HT, DT, or T.sub.2. 
Radiation signals are transmitted through the capillary waveguide via 
input and output windows 98 and 100. 
A typical arrangement, such as shown in FIG. 3, employs three or four one 
meter waveguides connected in series having a 1.6 mm inner diameter. Using 
alumina ((Al.sub.2 O.sub.2), strong restrahl reflection from below 11 
.mu.m to beyond 18 .mu.m provides a very low loss waveguide having 
approximately 90% throughput at 944 cm.sup.-1 P(20) in a three meter long 
waveguide. Other materials such as MgO, BeO and a mixture of MgO and 
Al.sub.2 O.sub.3 are suitable for fabricating capillary waveguides with 
high transmission characteristics in the infrared spectral region. 
The present invention therefore provides a Stokes injected Raman waveguide 
amplifier and Stokes injected Raman waveguide regenerative amplifier 
capable of producing high gain Stokes output signals. This is accomplished 
according to the present invention utilizing a capillary waveguide 
amplifier or capillary waveguide regenerative amplifier which has high 
efficiency in converting CO.sub.2 laser radiation energy to Stokes 
frequency radiation energy. The application of an external Stokes 
injection signal reduces required gain in the capillary waveguide 
amplifier or capillary waveguide regenerative amplifier to generate the 
amplified Stokes signal. 
Obviously, many modications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims, the invention may be 
practiced otherwise than as specifically described.