J. Henningsen

Assignment of laser lines in optically pumped CH 3 OH

A number of far-infrared laser emission lines from CH3OH, pumped by a CO2laser, have been assigned to specific energy levels of the rotation-internal rotation spectrum in the first excited CO stretch state. The assignments are based on comparison with the established model for the vibrational ground state, and on criteria concerning internal consistency, polarization, and line strength.

The optically pumped far-infrared laser: Rate equations and diagnostic experiments

A four-level rate-equation model for an optically pumped far-infrared (FIR) laser is presented, and an expression is derived for the output power as a function of pump power and gas pressure. The theory is compared with measurements on the 570.5-μm line of CH 3 OH, pumped by a CO 2 laser. The width of the gain curve is measured and is shown to go below the Doppler width of the laser transition at low pressure. This indicates that the use of a monochromatic pump source leads to a velocity selective pumping process.

Photoacoustic detection of NH3 in power plant emission with a CO2 laser

The paper describes a photoacoustic spectrometer for detection of NH3 in power plant emission with a detection limit below 1 ppm. The radiation source is a tunable CO2 waveguide laser, and detection is performed at reduced pressure, where the vibration-rotation lines of NH3 are essentially Doppler broadened. Immunity against interference is ensured by recording a characteristic spectral profile, and problems associated with the high concentration of CO2, and the associated line center absorption are eliminated by utilizing the effect of kinetic cooling on the photoacoustic phase. A computerized spectrometer has been constructed and tested under realistic conditions at a Danish power plant operating a test facility for selective non-catalytic reduction of NOx. Results of this field test are given.

New FIR laser lines from optically pumped CH 3 OH: Measurements and assignments

26 new FIR laser lines have been observed in CO 2 laser pumped CH 3 OH, and together with some previously observed lines their wavelengths have been measured with a relative accuracy of 3 times 10^{-4} by using the FIR resonator as a scanning Fabry-Perot interferometer. Based on the internal consistency of the data, it is suggested that a number of the lines originate from combination bands involving simultaneous excitation of the CO stretch mode and a different vibrational mode.

Mode structure of hollow dielectric waveguide lasers

The mode structure of hollow dielectric waveguide lasers with free space sections and flat mirrors is studied theoretically and experimentally. The study covers the fundamental mode and the three most important higher order modes, and graphs are given which identify regions of high mode discrimination in the parameter space. Calculated coupling losses are verified experimentally by detailed studies of the output power of CO2 lasers as a function of resonator geometry. The intensity profile inside and outside the resonator is calculated, and the profile outside the resonator is compared with experiments for the fundamental mode as well as for the higher order modes. It is shown that in general the fundamental mode is non-Gaussian, and that drastically different output characteristics are obtained for different choice of output plane. The paper identifies design criteria for obtaining single line and single mode oscillation over a wide tuning range, even in the densest region of the CO2 laser line spectrum, and this is exemplified by a spectroscopic application.

Far-Infrared Laser Lines from Optically Pumped CH3OH

Methyl alcohol illustrates in a remarkable way the potential of the optical pumping technique. It was recognized early as a source of strong FIR lasing by Chang et al. (1970). Since then, hundreds of lines have been generated by the various isotopic modifications of the molecule. At the present time, normal CH3OH has generated more than 300 lines, ranging from 19 μm to 1223 μm, in CW or quasi-CW operation. About 200 of these lines have been pumped by conventional CO2 lasers using the normal isotopic modification of CO2 (Tables la and II). Information on wavelength, relative polarization, relative power, and pressure is generally available. Also, precise FIR frequency measurements are at our disposal for 98 FIR lines (Petersen et al., 1980 and unpublished). This is helpful for the assignment of the lines and provides a comb of precise reference FIR frequencies from 245 GHz to 8 THz, using a single laser source.

Photoacoustic study of kinetic cooling

Abstract Kinetic cooling is studied through photoacoustic CO 2 laser monitoring on gas mixtures containing CO 2 in quantities typical of smoke. The photoacoustic response is recorded in the time domain as well as in the frequency domain, and results are given for the change in responsivity induced by small quantities of H 2 O. The importance of phase information is illustrated for a number of cases.

Far infrared laser lines in CH 3 OH, optically pumped to CO stretch states and to strongly interacting vibrational states

A commented table is presented, containing 90 assigned FIR emission lines of the CH 3 OH laser, which can be dixectly or indirectly related to the torsional ground state of the CO stretch. Based on a model, which empirically takes into account interactions with other vibrational modes, the table compares measured and calculated pump and emission frequencies. For 14 pump transitions with J leq 26 , the rms deviation is 0.005 cm-1. For 26 FIR emission transitions involving J leq 26 , where the accuracy of the model can be checked against frequency measurements, the rms deviation is 0.004 cm-1. Pump transitions to a strongly perturbing mode are identified, and accurate energy level parameters for this mode are given.

Second-harmonic and sum-frequency generation in AgGaSe2 with tunable waveguide CO2 lasers

A tunable source is described, giving extensive coverage of a spectral region around 2000 cm−1. A frequency self-calibration routine allows for the recording of Doppler-limited absorption profiles with an absolute accuracy of better than 10 MHz. The applicability of the source is demonstrated through measurements on the12C16O,12C17O, and12C18O isotopomers. Collision-broadening parameters and line strengths are determined, some of the data showing evidence of motional narrowing. Sensitivity limits are determined for linear absorption measurements as well as for photoacoustic absorption, and the feasibility of monitoring all three isotopomers with ppm sensitivity is demonstrated.

Observation and assignment of large offset farinfrared laser lines in CH 3 OH optically pumped by a CO 2 waveguide laser

Four new FIR laser lines at 62.98, 48.77, 224.53, and 190.65 μm are reported from CH 3 OH, optically pumped by the 10R34 and 9R10 lines of a waveguide CO 2 laser. Measurements performed by several techniques confirm that the pump offsets are outside the tuning range of conventional CO 2 lasers. Two of the lines are assigned as high J transitions within the torsional ground state of the CO stretch.