In Cavity Pumping for Infrared Laser

This invention relates to far infrared (FAR) lasers which are optically pumped by CO.sub.2 lasers. Such systems are described in the articles: "c w Submillimeter Laser Action in Optically Pumped Methyl-Fluoride, Methyl Alcohol, and Vinyl Chloride gases" by Chang et al, pp 249-251, in Applied Physics Letters, Volume 17 No 6, Sept. 15, 1970; "Millimeter and Submillimeter Wave Laser Action in Symmetric Top Molecules Optically Pumped via Parallel Absorption Bands." by Chang et al, pp 103-105, in Applied Physics Letter, Volume 19, No. 4, Aug. 15, 1971; "Submillimeter Lasers Optically Pumped Off Resonance" by Fetterman et al, pp 156-159, in Optics Communications, Volume 6, No. 2, Oct. 1972; "New Submillimeter Laser Lines in Optically Pumped Gas Molecules", by Wagner et al, pp 46-47, Volume 8, No. 1, May 1973; "Waveguide Laser for the Far Infrared (FIR) Pumped by a CO.sub.2 Laser", by Hodges et al, pp 252-253, in Applied Physics Letters, Volume 23, No 5, Sept. 1, 1973 and "Low-Threshold, C W Submillimeter and millimeter Wave Laser Action in CO.sub.2 Laser Pumped C.sub.2 H.sub.4 F.sub.2, C.sub.2 H.sub.2 F.sub.2, and CH.sub.3 OH" by Hodges et al, pp 1159-1160, in Journal of Quantum Electronics Volume 9, No 12, Dec. 1973.

In the normal state of the art optically pumped far infrared lasers, the CO.sub.2 laser is separate from the far infrared cavity, and the CO.sub.2 power is focused into the far infrared cavity to excite the molecules by optical energy transfer. The CO.sub.2 power is coupled into the far infrared cavity by means of a hole, in the mirror at one end. A hole in the mirror at the other end is used to extract the far infrared power from the cavity. In such systems, only the CO.sub.2 laser energy that enters the far infrared cavity is available for exciting the far infrared molecules.

According to this invention, the CO.sub.2 laser is located within the optical cavity for the far infrared laser. The problem of reflective optics is not present here since both the CO.sub.2 radiations and the far infrared radiation can be reflected from the same type of mirrors. The far infrared molecules are optically pumped by the resonant circulating CO.sub.2 laser radiation within the optical cavity.

FIG. 1 shows one prior art system for pumping far infrared wavelengths with a CO.sub.2 laser.

FIG. 2 shows an in cavity pumping laser system for a far infrared laser according to the invention.

Reference is now made to FIG. 1 of the drawing which shows a prior art test apparatus for a far infrared laser system 10 wherein a far infrared laser 12 is optically pumped by the CO.sub.2 laser 14. The CO.sub.2 energy is coupled into the far infrared laser by means of a Ba F.sub.2 window 16 and a crystalline quartz window 18 is used for the extraction of the far infrared energy.

In the system of FIG. 2, according to this invention, the CO.sub.2 cell 20 is located in the same optical cavity as the far infrared cell 22. A conventional CO.sub.2 gas mixture, such as CO.sub.2 -He-N.sub.2, 13-53-3, circulates through cell 20 from the supply 24 to the output 26. The far infrared gas circulates through the cell 22 from supply 28 to the outlet 26. Both gases may be pumped by a single pump 30.

A current flow between the electrodes 32 and 34 is used to excite the CO.sub.2 molecules in a conventional manner. Since the far infrared molecules do not enter the space between the electrodes 32 and 34, dissociation of the far infrared molecules does not become a problem. The CO.sub.2 laser radiation flux circulating in the optical cavity between mirrors 36 and 37 excite the far infrared molecules in the far infrared cell 22, by optical energy transfer, in the same manner as in the prior art device of FIG. 1. The mirrors 36 and 37 can be used for both CO.sub.2 laser wavelengths and far infrared laser wavelengths provided the mirrors are front coated with a metal, such as copper, silver or aluminum. Some dielectrically coated mirrors could also be used. The far infrared radiation is extracted through aperture 38, in mirror 37, and crystalline quartz window 40 which filters out the CO.sub.2 laser wavelengths. Other materials than crystalline quartz may be used for the windows 40. Some of these are given on page 100 of "Methods of Experimental Physics," Volume 10 "Physical Principles of Far-Infrared Radiation" by L. C. Robinson.

By locating the CO.sub.2 laser and the far infrared laser in the same optical cavity, the far infrared molecules are pumped by the much larger circulating flux in the CO.sub.2 optical cavity than in prior art devices. In this device, the infrared molecules are not pumped by just a portion of the flux produced in the CO.sub.2 cell as in prior art devices and some of the losses in the optical system for transmitting the CO.sub.2 laser energy to the far infrared laser cell are not present.

With the use of internal mirrors as in the device of FIG. 2, the CO.sub.2 laser device is not wavelength selective, so that a great many far infrared wavelengths will be produced. It would be desirable to use external mirrors and also it would be desirable to use a grating as shown in FIG. 1. This, however, would require a window material that would transmit both the CO.sub.2 wavelengths and the far infrared wave lengths. In an apparatus built and tested, no such material was available. The materials used for the transmission of CO.sub.2 wave lengths were found to absorb too great a portion of the far infrared wave lengths, to permit the device to produce a far infrared output.

Any of the known far infrared molecules where a resonance exists between the CO.sub.2 pump radiation wavelengths and the absorbility transition in the far infrared gas molecules can be used. Some of these known in the art are C.sub.2 H.sub.2 F.sub.2, C.sub.2 H.sub.4 F.sub.2, CH.sub.3 OH, CH.sub.3 Cl, CH.sub.3 CH.sub.2 F, CH.sub.3 CHF.sub.2, CH.sub.3 CHF.sub.3 CH.sub.3 CN, CH.sub.3 CCN, HCOOH, NH.sub.3, H.sub.2 C:CHCl.

While the device shown in FIG. 2 has the CO.sub.2 laser gas flowing in a separate cell from the far infrared gas, these could be mixed and supplied to a single cell. However, if the mixed gases were used in a single cell the flow rate of the gas should be increased to minimize the problem, of molecular dissociation, by molecular replacement.

There is thus provided a far infrared laser which permits the pumping of the far infrared molecules by the large circulating flux in the CO.sub.2 optical cavity.