CF.sub.4 Laser

The present invention pertains generally to infrared lasers and more particularly to optically pumped infrared lasers.

Applications of laser photochemistry have produced a need for simple and efficient lasers for producing coherent radiation at specified wavelengths near 16 .mu.m. Of the various methods and devices utilized for producing these specified wavelengths, including stimulated Raman scattering of selected gases and various types of solid state devices for either producing specified wavelengths or shifting frequency, such as the diode laser and various germanium substrates, respectively, the gas lasers are the most attractive candidates for providing these specified wavelengths because of their simplicity, efficiency, and scalability, in both power and repetition rate. Currently, however, conventional gas lasers, such as CO.sub.2 lasers, are incapable of producing specified frequencies of interest. Although the CO.sub.2 laser has been useful in producing near 16 .mu.m radiation by the various methods disclosed above, these systems have been extremely complex with low overall efficiency, rendering these approaches somewhat impractical for uses outside the laboratory.

With the advent of optically pumped lasers, the highly developed technology of CO.sub.2 lasers has been applied in an optical pumping scheme for producing transitions in another molecular gas causing inversions in the optically pumped gas at desired wavelengths near 16 .mu.m. The problem, however, in producing such a system is selecting a gas having an absorption spectrum which falls within the operating range of a line tunable CO.sub.2 laser and which, according to selection rules, inverts between quantum states to produce the desired wavelengths. Additionally, conversion gains between quantum levels must necessarily be considered to produce an optically pumped laser having sufficient gain.

The present invention overcomes the disadvantages and limitations of the prior art by providing a near 16 .mu.m CF.sub.4 laser. The CF.sub.4 laser of the present invention utilizes a line tunable CO.sub.2 laser as an optical pumping source for exciting the combination vibrational state .nu..sub.2 +.nu..sub.4 of the molecular CF.sub.4 gas which falls within the operating range of the line tunable CO.sub.2 laser. An optical resonant cavity induces oscillations of coherent radiation emitted by the transition .nu..sub.2 +.nu..sub.4 .fwdarw..nu..sub.2 of the CF.sub.4 gas to produce a near 16 .mu.m coherent output signal.

It is therefore an object of the present invention to provide an optically pumped CF.sub.4 laser.

It is also an object of the present invention to provide an optically pumped CF.sub.4 laser capable of producing near 16 .mu.m radiation.

Another object of the present invention is to provide an optically pumped CF.sub.4 laser which is simple in operation.

Another object of the present invention is to provide an optically pumped CF.sub.4 laser which is efficient in operation.

Other objects, advantages, and novel features of the present invention will become apparent from the detailed description of the invention when considered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of the apparatus of the preferred embodiment of the invention.

FIG. 2 is a graph of the absorption spectrum of the .nu..sub.2 +.nu..sub.4 vibrational absorption spectrum of molecular CF.sub.4 gas.

FIG. 3 is a table of CF.sub.4 laser frequency lines obtained and corresponding experimental data.

FIG. 4 is a graph of experimental results of the CF.sub.4 laser output at various gas temperatures.

Referring to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, FIG. 1 schematically discloses the preferred embodiment of the invention. As shown in FIG. 1, a line tunable CO.sub.2 TEA laser 10 is utilized as an optical pumping source. Dichroic mirrors 12 and 14 are spaced approximately 300 cm apart to form the laser optical resonant cavity. Dichroic mirror 12 is transmissive to the near 10 .mu.m radiation produced by the CO.sub.2 laser while almost totally reflective for the near 16 .mu.m radiation produced by stimulated emission from the CF.sub.4 gas. Dichroic mirror 14 is reflective to the near 10 .mu.m radiation while partially reflective to the 16 .mu.m radiation.

The optically pumped cell 16 is constructed of 20 mm inner-diameter pyrex tubing about 280 cm in length and is sealed at both ends with either KBr or CsI window mounted at the Brewster angle. Cooling is accomplished in cooling trough 18 by either liquid nitrogen boil off or various slushes. Cell temperatures as low as 155.degree. K. can be obtained by the liquid nitrogen boil off technique.

The laser output is coupled from the optical resonant cavity by a beam splitter 20 to a total reflector 22 and filters 24 for frequency selection, detection and recording, as schematically shown at 30. Detection is performed by HgCdTe and Ge:Cu detectors in conjunction with a monochromator, e.g., 0.25 m Jarrell Ash or 1 m Spex. The laser output can also be detected via total reflector 26 in filters 28 in the manner schematically shown in FIG. 1.

In operation, the line tunable CO.sub.2 laser 10 produces an optical pumping signal which falls within the range of the absorption spectra of the combination vibrational states .nu..sub.2 +.nu..sub.4 of the molecular CF.sub.4 gas. As shown in FIG. 2, various lines of the P and R branches of the CO.sub.2 laser produced by inversion from the 001 to the 020 state, fall within the CF.sub.4 absorption spectrum corresponding to approximately 1050 wavenumbers (near 9.5 .mu.m) to 1080 wavenumbers (near 9.26 .mu.m). Experimental data confirm that (001)-(020) R(12) is absorbed most strongly at a measured absorption coefficient of 1.1.times.10.sup.-4 cm.sup.-1 torr.sup.-1.

Since the line tunable CO.sub.2 laser is capable of exciting various rotational states of the .nu..sub.2 +.nu..sub.4 vibrational state of the molecular CF.sub.4 gas, various frequencies can be obtained in the near 16 .mu.m range from the transition .nu..sub.2 +.nu..sub.4 .fwdarw..nu..sub.2. Selection rules determine allowable inversions and therefore wavelengths which can be produced. A summary of the CF.sub.4 laser lines which have been produced, their assignments, the pumped laser lines utilized, the relative intensity of the CF.sub.4 output, and the operating pressure of the CF.sub.4 gas are summarized in a table shown in FIG. 3. As is clear from the table, CF.sub.4 laser emissions from approximately 653 cm.sup.-1 to 612 cm.sup.-1 are obtained as the CO.sub.2 laser is tuned from the P(12) to the R(22) line. The relative intensities shown range from strong, to medium strong, to medium by a ratio of 9:3:1, where strong is approximately 4 mJ.

FIG. 4 illustrates various CF.sub.4 laser outputs within a range of cell temperatures. The minimum achievable cell temperature using liquid nitrogen boil off in a 350 cm cell has been 155.degree. K. As the graph illustrates, it is important to cool the medium to produce greater outputs since the lower laser level is close to the ground state, i.e., approximately 435 cm.sup.-1 for CF.sub.4.

Optimum operating pressure differs for various CF.sub.4 laser transitions, and is dependent to some extent on the magnitude of the pump energy. Experimental results show that the optimum lasing pressure increases with increasing pump energy. On stronger laser lines, the optimum operating pressure ranges from tenths of a torr to a few torr. Weaker laser lines tend to work well only at concentrations below 1 torr.

Pump thresholds for the CF.sub.4 laser transitions are relatively high. This threshold level, however, is fairly hard to determine since the CF.sub.4 laser output fluctuates dramatically from shot to shot at low pump inputs. This erratic behavior is due to the very narrow gain bandwidth of the 16 .mu.m laser, i.e., 20 MH.sub.z FWHM, in comparison to the longitudinal mode spacing, i.e., 50 MH.sub.z. Estimates of threshold pump energy are 150 mJ for stronger lines and 200-250 mJ for weaker lines.

The present invention therefore provides a method and device for producing near 16 .mu.m radiation which is simple and efficient in operation. Since the absorption characteristics of the .nu..sub.2 +.nu..sub.4 vibrational states of the CF.sub.4 gas fall within the near 9.5 .mu.m wavelength region, a line tunable CO.sub.2 laser can be utilized to excite the desired .nu..sub.2 +.nu..sub.4 vibrational quantum state. The transition .nu..sub.2 +.nu..sub.4 .fwdarw..nu..sub.2 is enhanced by reducing cell temperatures and inducing oscillations of near 16 .mu.m radiation in an optical resonant cavity. Output signal tunability is achieved by varying the input pump laser lines and cell temperature. In this manner, a wide range of output frequencies can be achieved.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. For example, any line tunable pump laser could be used in the present device as an optical pumping source as long as it produces output signals falling within the absorption spectrum of the CF.sub.4 gas such as shown in FIG. 2. Further, dichroic mirror 14 could be replaced with a total reflector, thereby eliminating total reflector 26 and filters 28. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.