Efficiency of a high pressure CO2laser pumped CH3F Raman laser

An experimental investigation of the conversion efficiency of a high pressure CO2laser pumped CH3F Raman laser is reported. We show that resonance absorption of the CO2laser radiation in the CH3F gas can lead to a severe limitation of the efficiency. At CO2laser frequencies where the stimulated Raman action is strongest, a quantum efficiency for conversion of CO2laser radiation into far infrared radiation of the order of 0.1 is observed.     

Monolithic silicon mosaics for far-infrared imaging

Monolithic detector mosaics were constructed for image sensing at wavelengths from three to 30 µm. The mosaics use impurity photoconduction in silicon to sense infrared radiation. Operation in the 25 to 40°K temperature range is obtained with closed-cycle cooling. Photosensor elements, spaced on 32 mil centers, are formed by solid-state diffusion, and a junction diode is constructed in series with each photoresistor to reduce crosstalk. The integrated 30-by-30 mosaics have crossedX-Yelectrodes for direct-wire readout of picture elements. Performance of individual elements is described. At 25°K, a detectivity of 1 × 10⁸cm Hz^1/2/W per element is found using 10.6 µm CO2laser radiation. The quantum efficiency at 10.6 µm is about 7 percent, and the response time is observed to be less than 0.2 µs. Uniformity and crosstalk data on prototype arrays are presented, and the integration of these arrays with cryogenic amplifier and scanning circuits is discussed.     

Atomic vapor quantum counter: Narrow-band infrared upconverter

A novel device for the upconversion of narrow-band infra-red (IR) radiation in the 1.5-20-µm spectral region is described. The atomic-vapor quantum counter (AVQC) takes advantage of the efficient incoherent upconversion of narrow-band IR radiation in an optically excited atomic vapor. In a 300 K background and a 2π-sr field of view, a D^*is projected that is two orders of magnitude greater than that of conventional semiconductor devices. The device offers the extremely high Q's (>10⁵) associated with heterodyne detection while simultaneously offering a broad tuning range and an unlimited collection aperture. The anticipated performance characteristics of the AVQC are discussed, Experimental results in sodium and potassium have demonstrated the high Q and the tunability of the device. Electric field tuning of the wavelength response to CO2laser wavelengths spanning 23 cm^-1has been accomplished. Applications of the AVQC involving the detection of low level laser radiation and molecular emission are discussed.     

Gain limitations in TE CO2laser amplifiers

The factors which limit the small-signal gain of TE CO2laser amplifiers are investigated with a novel technique based on gain measurements of the sequence, hot, and regular CO2laser bands. This new technique enables us, for the first time, to determine accurately and independently the rotational and vibrational temperatures characterizing the CO2laser system. The gain ratio of the sequence band (00° 2) to the regular band (00° 1) provides a simple and accurate determination of the ν3mode vibrational temperature. It is found experimentally that the ν3mode vibrational temperature saturates at a high input discharge energy. This saturation sets an upper limitation to the gain attainable in TE CO2laser amplifiers. As we can measure all the characteristic temperatures relevant to the gain medium, a detailed comparison between the calculated and experimental gain can be carried out with no variable parameters. The result of such a direct comparison confirms both the validity of the conventional "mode temperature" model for CO2laser dynamics and the validity of our measurement technique for vibrational temperatures.     

Homodyne detection of infrared radiation from a moving diffuse target

Experiments have been performed in which the radiation from a CO2laser was coherently detected after being scattered from a moving diffuse reflector. This is generally the configuration of an infrared laser radar. The power-spectral-density of the heterodyne signal was measured and its width was shown to agree with the calculated value based on a theoretical model for the process. Expressions are obtained for the ratio of heterodyne signal bandwidth to heterodyne frequency for the cases of focused radiation, unfocused radiation, and for a typical radar configuration. In most cases, the heterodyne signal is found to possess a narrow-band character. The probability density of the signal envelope was also measured for a known scatterer (providing Gaussian scattered radiation) and was found to be Rayleigh distributed, as expected. The power-spectral-density and envelope probability distribution provide information about a scattering medium or target which cannot be obtained from average-value measurements of the heterodyne signal-to-noise ratio. This information is useful for communications applications, infrared radar, and heterodyne spectroscopy experiments. Finally, a simple and direct method of obtaining information about the statistics of an incident radiation field (which does not involve photocounting) is discussed.     

Continuously tunable optically pumped high-pressure DF → CO2transfer laser

The operational characteristics of a continuously tunable DF → CO2transfer laser optically pumped with radiation from a pulsed DF laser are experimentally and theoretically studied. The pump radiation is absorbed by DF in a high-pressure DF/CO2/He gas mixture, and subsequent V-V energy transfer to the CO2ν3mode provides the CO2laser population inversion. Continuous tuning of the CO2laser frequency between five CO2line centers from 29.14 to 29.30 THz has been demonstrated, using a 12 atm gas mixture. The maximum pulse energy was about 0.8 mJ. In experiments with a two-mirror CO2laser resonator, pulse energies up to 6 mJ and 35 percent slope quantum efficiency have been obtained at 10 atm gas pressure. The gas mixture typically contained 0.5 percent DF, 5 percent CO2, and 94.5 percent He, but this was not critical. Computer simulations based on a rate equation model of the laser have given results which are in reasonable agreement with those obtained experimentally.     

Up-conversion of 10.6 µ radiation to the visible and second harmonic generation in HgS

The frequency mixing of infrared signals with visible laser light in nonlinear crystals (here referred to as up-conversion) has been suggested as a possible means of detecting infrared signals by use of sensitive and fast visible wavelength detectors. Preliminary experiments are reported of the up-conversion of the CW 10.6 μ CO2laser by phase-matched difference mixing in HgS with the CW 0.6328 μ, He-Ne laser to produce 0.6729 μ which is detected with anS-20 photomultiplier. Reasonable agreement with theory is obtained. No additional sources of noise due to up-conversion were observed, although the actual performance was limited by pump fluctuations. No attempt was made to produce a practical device competitive with existing photoconductive detectors. In principle this is possible but would require various improvements, which are outlined. As a preliminary to the above experiment we report the measurement of the nonlinear coefficientd_{11}= (1.2 pm 0.4) times 10^{-7}ESU in HgS for second harmonic generation (SHG) of the 10.6 μ laser to produce 5.3 μ.     

Heterodyne measurements of submillimeter laser spectrometer frequencies

The frequencies of 46 CW laser lines commonly used for submillimeter spectroscopy, with wavelengths between 0.1 and 0.7 mm, have been measured by heterodyne methods. All the fines are optically pumped by a CO2laser, with threshold pump powers of 3 W or less. The precision of measurement, limited by the laser linewidth, is typically ± 1 part per million.     

RF-pumped infrared laser using transverse gas flow

A transverse gas flow configuration has been developed utilizing RF discharge waveguide technology for several infrared lasers. Two potential applications have been identified: pulsed chemical laser and CW CO2laser. In the 3.8 μm DF laser, the flowing gas device provides rapid gas replenishment to maintain high electrical efficiency at high repetition rates. An average power of 0.6 W was achieved at 1 kHz. An order of magnitude power improvement can potentially be developed in a closed cycle system. In the CW CO2laser, the flowing gas provides efficient cooling so that high output power per unit gain length can be achieved. A 16 W output in a 20 cm gain length device, corresponding to a record 0.8 W/cm output has been demonstrated. This system can be developed into a 20-60 W laser with a 20-50 cm gain length.     

Pulse-probe measurements in the v3band of SF6at low temperature and low pressure

To improve understanding of multiple-photon absorption by polyatomic molecules, we have performed a series of experiments wherein a pulsed CO2laser irradiated 0.02-0.08 torr samples of SF6at 145 K. A CW probe laser monitored the time response of the induced absorption or transmission at many CO2laser lines that are in or near the ν3absorption band of SF6. The experiments covered a 40-fold fluence range and probe times out to 4 ms. We conclude that the absorbed laser radiation produces a nonthermal vibrational-energy distribution and that intermolecular vibrational-energy transfer is important at early times in redistributing the absorbed energy. We also discuss the influence of other processes on the induced spectrum.     

Nonlinear properties of cuprous halides

A study of the nonlinear properties of cuprous halides in the near infrared (YAG Nd³⁺laser) and in the medium infrared (CO2laser) is presented. The second-order susceptibilities have been compared to those of quartz and GaAs. Coherence lengths were measured at 1.06 μ and for the four principal transition lines of CO2laser. In addition some linear parameters such as refractive indices in the visible and absorption coefficients from 0.3 to 25 μ were determined. The large values of Miller's δ14of those compounds are consistent with Levine's bond-charge model.     

Photon-drag germanium detectors for CW CO2lasers

CW CO2laser radiation is detected by employing the photon-drag effect in a p-type germanium bar. Design, fabrication, and performance of such detectors are discussed.     

CO2pulsed laser radiation tellurium detectors

Subnanosecond detectors and beam monitors for pulsed CO2laser radiation using the photon drag and optical rectification effects in tellurium are presented. Tellurium devices at 10.6 μ have a response of the order of 80 μV/kW . cm^-2with an NEP of8 times 10^{-4}W . Hz^1/2andD*of 500 cm . Hz^1/2/W which is superior to commercially available photon drag detectors and monitors. It is also expected and experimentally demonstrated that the responsivities of large area monitors can be multiplied by cutting the original area into multi-element strips and electrically connecting them in series.     

Compact, high-power FIR NH3laser pumped in a CO2laser cavity

A compact CO2-NH3FIR laser system, with an NH3laser cavity inserted in the pump CO2laser cavity, was designed. Temporally smooth and reproducible single mode NH3laser pulses with an energy of 5 mJ (20 kW peak power) were obtained at the 152 μm line when optically pumped by the injection-locked single-mode CO2laser with a pulse energy of 0.3 J.     

New SMMW laser transitions optically pumped by a tunable CO2waveguide laser

Optical pumping of a submillimeter wave (SMMW) laser with a relatively compact RF-excited CW CO2laser is described. The increased frequency tunability of the waveguide pump laser has resulted in new low threshold SMMW emissions in C2H2F2, CDF3, and CD2F2by pumping into absorption lines which are beyond the tuning range of a conventional CO2laser. Frequency offsets and some assignments obtained with the aid of a tunable diode laser heterodyne spectrometer are reported.     

New techniques for determining vibrational temperatures, dissociation, and gain limitations in CW CO2lasers

We have developed an accurate method of determining vibrational temperatures and populations in CO2laser discharges. Our technique involves the use of both the regular 00 ° 1 and sequence 00 ° 2 laser transitions as probes of a CO2laser amplifier. We have been able to separately investigate the quantitative effects of gas heating, dissociation, and ν3mode excitation efficiency on the small-signal gain in typical CW CO2lasers. In general we find that the maximum gain attained in a typical flowing gas CW CO2laser is limited by dissociation of CO2at high discharge currents. To investigate the more fundamental limitations on the gain, we used a short discharge tube with fast flow rates. Contrary to many previous results, we find that thermal effects play a somewhat secondary role in the discharge dynamics, and that the lower laser level populations are small under all discharge conditions. Our results show that the chief limitation on gain in CW CO2lasers is the "saturation" of the ν3mode vibrational temperature T3at high discharge currents. This saturation effect is observed for a wide range of gas mixtures and pressures, and has been studied in detail. Gain coefficients as high as 3.3 percent/cm have been obtained in a conventional 1-cm bore CW discharge tube. We also report preliminary results of an experiment which uses a tunable diode laser to measure gain on a large variety of transitions in a CO2discharge. The diode laser measurements give a striking confirmation of the results described above, and provide the first direct experimental evidence that a Boltzmann distribution exists in the vibrational modes of discharge excited CO2.     

Continuous operation of a long-lived CO2laser tube

A sealed-off CO2laser tube was operated with continuous, almost constant, power output at 10.6 μm for more than 1000 hours. A Ni cathode that was maintained at 300°C acted as a catalyst to regenerate CO2back into the tube from the oxygen and carbon monoxide formed in the discharge. By the time 1100 hours of operation had passed, 92 percent of the original CO2concentration was still available in the tube.     

A 16-µm radiation source utilizing four-wave mixing in cooled parahydrogen gas

Improvements in a 16-μm radiation source based upon the combined effects of stimulated rotational Raman scattering and resonantly enhanced four-wave mixing in parahydrogen gas are described. For this source, the input waves of which are provided by temporally and spatially coincident pulsed beams from a ruby and CO2TEA laser, it was found that cooling the parahydrogen gas from 300 K to <100 K, at constant molecular density, increased the output at 16 μm by roughly a factor 4. The maximum output was measured to besim 40 muJ/pulse, which is near the theoretical limit for the 2.5 MW CO2laser intensities that were applied.     

Theoretical and experimental studies of a multiline TEA CO2laser with hot CO2as an intracavity absorber

The multiline behavior of a ballast resistance helical TEA CO2laser incorporating hot CO2as an intracavity absorber has been studied. Simultaneous laser oscillation onP(16), P(18), andP(20)lines of 10.6 μm has been obtained reliably. A rate equation model has been developed for the hot CO2absorber and is incorporated with the model for the multiline TEA CO2laser for calculating laser intensities onP(16), P(18), P(20), andP(22)lines of the 10.6 μm band in the presence of the absorber. The theoretical calculations agree quite well with the experimental observations. These studies show that a hot CO2column of Proper length and temperature inside a TEA CO2laser cavity can produce simultaneous laser oscillation on at least three rotational linesP(16), P(18), andP(20)of the 10.6 μm band with almost equal intensity.     

Sealed, miniaturized, corona-preionized, high-repetition-rate TEA CO2laser using hydrogen buffered gas mixtures

Sealed-off operation of a compact, corona-preionized, TEA CO2laser capable of efficient laser action at pulse repetition frequencies of up to 1 kHz is reported. By adding hydrogen to the basic gas mixture we have been able to maintain discharge stability and constant average output power for more than 20 million laser shots at a 400 Hz repetition frequency with peak powers in excess of 350 kW.