Shifting of Infrared Radiation Using Rotational Raman Resonances in Diatomic Molecular Gases

The present invention pertains generally to infrared lasers 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 laser outputs in the IR spectrum. These methods have included four-wave mixing as disclosed in commonly assigned 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 issued June 13, 1978 and Raman scattering as disclosed in commonly assigned 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 issued Dec. 6, 1977 of which the present invention comprises an improvement.

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 the 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 efficiencies, Raman shifting of a CO.sub.2 laser output provides high overall efficiencies because of 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, T.sub.2, HD, HT, or DT in the infrared requires threshold powers for stimulated Raman scattering which are near the breakdown threshold of the diatomic molecular gas for 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 J. of Quantum Electronics, Vol. QE-12-732-733, November 1976.

The present invention overcomes the disadvantages and limitations of the prior art by providing an improved device for shifting infrared radiation using rotational Raman resonances in a diatomic molecular gas. The invention utilizes a capillary waveguide in combination with a resonator to considerably reduce threshold intensities required by single pass focused geometry. The capillary waveguide also allows use of short focal length lenses for providing maximum intensity without causing damage to the dichroic mirrors or interaction cell windows. Since the resonator allows lower gain parameters due to multiple oscillations in the lasing cavity, the size of the device is reduced substantially and operation is assured within the limitations of the breakdown threshold of the gaseous medium. Further, the invention employs circular polarization to increase Raman gain and restrahl reflectivity to reduce waveguide losses.

In addition, by modelocking the CO.sub.2 laser and matching the lengths of the CO.sub.2 laser cavity and the Raman laser cavity, high peak gain is achieved on synchronously generated Raman Stokes pulses. Modelocking the CO.sub.2 laser and matching cavity lengths also allows double-pass of the CO.sub.2 laser radiation without having an overlap of high peak power pulses within the waveguide.

It is therefore an object of the present invention to provide a device for shifting across a broad range of frequencies in the infrared spectrum.

It is also an object of the present invention to provide a device for shifting across a broad range of IR frequencies which is simple in operation.

Another object of the present invention is to provide a device for IR frequency shifting across a broad range of frequencies which is highly efficient in operation.

Another object of the present invention is to provide a device for stimulated Raman scattering in a diatomic molecular gas.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. A detailed description indicating the preferred embodiment 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 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 it is not intended to limit the scope of the invention as disclosed herein nor is it intended to be used in interpreting or any way limiting the scope or fair meaning of the appended claims.

FIG. 1 discloses the Raman laser of the preferred embodiment of the invention.

FIG. 2 discloses a variation of the preferred embodiment of FIG. 1.

FIG. 3 discloses an alternative preferred embodiment.

FIG. 1 discloses the Raman laser which comprises the preferred embodiment of the invention. CO.sub.2 input radiation 10 is applied to spatial filter 12 to eliminate "hot spots" from the spatial intensity of the beam which prevents possible damage to various mirrors and windows of the Raman oscillator. The spatially filtered beam is reflected by mirror 14 and focused by lens 16 through dichroic mirror 18 into the interaction cell 20. Dichroic mirrors 18 and 30 function to transmit the 10 .mu.m infrared radiation produced by the CO.sub.2 radiation source and reflect nearly all of the frequency shifted radiation generated within the interaction cell 20 produced by stimulated Raman scattering from rotational transitions of a diatomic molecular gas such as H.sub.2, D.sub.2, T.sub.2, HD, HT, DT. Alternatively, mirror 30 can be made highly reflecting at the CO.sub.2 wavelength as well so as to double pass the CO.sub.2 radiation through the interaction cell. By modelocking the CO.sub.2 laser and matching CO.sub.2 laser cavity length and the length between the dichroic mirrors, overlap of high intensity CO.sub.2 pulses during double pass operation is avoided, and high gain is provided for synchronously generated Raman Stokes pulses. A capillary waveguide 22 is positioned within the interaction cell 20 such that the diatomic molecular gas flowing through the interaction cell 20 via gas inlet 26 and gas outlet 28 is contained within the capillary waveguide 22. A liquid nitrogen jacket 24 surrounds the primary length of the capillary waveguide 22 and functions to cryogenically cool the diatomic molecular gas to maintain ground state population. The capillary waveguide 22 is tapered at one end to minimize ablation or sputtering of the waveguide material upon the application of infrared radiation from the infrared CO.sub.2 radiation source. The capillary 22 is fabricated from pyrex or quartz or of either MgO or Al.sub.2 O.sub.3 to reduce losses as a result of restrahl reflectivity of these materials at desired IR frequencies. A LiF restrahl filter 32 reflects the frequency shifted radiation which is focused by lens 34 upon a 14 .mu.m to 17 .mu.m filter 36. A HgCdTe or other infrared type detector 38 is utilized to detect the presence of desired spectral lines.

In operation, the device of FIG. 1 functions as a Raman oscillator in which the capillary waveguide 22 increases the focal interaction length (L) by the length of the capillary 22. Stimulated Raman scattering is initiated by rotational transitions of the diatomic molecular gas. Frequency shifted radiation produced by Raman scattering oscillates within the optical cavity of the Raman laser defined by dichroic mirrors 18 and 30, and a portion of this energy is emitted from the oscillating cavity via partially reflective dichroic mirror 30. The focal interaction length is therefore increased by the number of times the frequency shifted radiation traverses the length of the capillary waveguide. This large increase in the focal interaction length (L) increases the exponential gain factor (e.sup.gL) by an amount sufficient to overcome losses and produce a frequency shifted output signal.

Diatomic molecular gases suitable for operation in such a device comprises H.sub.2, D.sub.2, T.sub.2, HD, HT, DT. Stimulated Raman scattering from rotational transitions of H.sub.2 give coverage throughout the range 13.5 to 18 .mu.m using the 354 cm.sup.-1 S.sub.oo (0) transition and from 20 to 30 .mu.m using the 587 cm.sup.-1 S.sub.oo (1) transition. Rotational transitions of D.sub.2 give coverage from 11 .mu.m to 14 .mu.m using the 179 cm.sup.-1 S.sub.oo (0) transition, 12.6 .mu.m to 16.9 .mu.m using the 298 cm.sup.-1 S.sub.oo (1) transition, and 14.7 .mu.m to 21 .mu.m using the 415 cm.sup.-1 S.sub.oo (2) transition. With a tunable high pressure CO.sub.2 laser utilizing either D.sub.2 or H.sub.2, any wavelength in the range 11 .mu.m to 30 .mu.m can be generated by Raman lasing in the device of the preferred embodiment. With an atmospheric pressure CO.sub.2 laser, tunability is limited to a few GHz near line center of transitions spaced by .about.25 GHz to .about.70 GHz. However, by use of isotopic variants of CO.sub.2 or of rotational transitions of the other hydrogen isotopes (T.sub.2, HD, HT, DT), as disclosed for example by G. Herzberg, "Sprectra of Diatomic Molecules," Van Nostrand, 1950, pp. 530-533, essentially continuous coverage of this range can also be achieved. Other molecular gas lasers such as N.sub.2 O or CS.sub.2 can be used to provide other input radiation frequencies.

FIG. 2 discloses a variation of the preferred embodiment of FIG. 1 in which a Fresnel rhomb .lambda./4 plate 44 is introduced between the spatial filter 12 and focusing optics 16. The Fresnel rhomb .lambda./4 plate 44 functions to circularly polarize the infrared radiation 10 from the infrared CO.sub.2 radiation source. When circularly polarized radiation is applied to the interaction cell 20, it increases Raman gain and reduces anti-Stokes generation in the diatomic molecular gas.

FIG. 3 discloses an alternative embodiment in which CO.sub.2 IR radiation 46 is circularly polarized by Fresnel rhomb .lambda./4 plate 52 and applied to a single pass interaction cell 72. The single pass interaction cell comprises a several meter long capillary 64 positioned within the interaction cell 72 which contains the desired diatomic molecular gas. The capillary is cooled by a liquid nitrogen cooling jacket or trough 68 to maintain ground state population in the diatomic molecular gas. A flat window 60 is utilized rather than a Brewster angle window because of the circular polarization of the IR radiation. The use of circularly polarized light in the embodiment of FIG. 3 is especially necessary to reduce competition from anti-Stokes generation. In the embodiment of FIG. 3, interaction length is provided by extending the length of the capillary 64 rather than by multiple passes in the Raman oscillator as accomplished in the devices of FIGS. 1 and 2. Modelocking of the CO.sub.2 laser in the embodiment of FIG. 3 provides high peak intensity CO.sub.2 pulses for increased Raman gain.

The present invention therefore provides a means for increasing the exponential gain factor (e.sup.gL) sufficiently to overcome losses in the molecular gas and produce stimulated Raman scattered frequency shifted radiation from rotational transitions. The embodiments of the present invention have the advantage of simplicity and single step operation for generating a wide range of frequencies in the infrared spectral region.

Obviously many modification and variations of the present invention are possible in light of the above teachings. For example, multiple Stokes frequencies can be generated by using a multiple frequency CO.sub.2 beam as the source of input radiation. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as described.