A tunable far infrared laser

A continuously tunable far infrared (FIR) laser has been demonstrated; experimental results are presented. A high-pressure (10-12 atm) continuously tunable CO2TE laser is used to pump Raman transitions in CH3F; the generation of continuously tunable radiation in the250-300 mum wavelength range is reported. Accurate frequency and bandwidth measurements have been made and the FIR bandwidth in superradiant emission isapprox4-5GHz. Consequently, the generation of frequency tunable, subnanosecond pulses in the FIR appears feasible. The generation of tunable laser radiation from 150 to 1000 μm by stimulated Raman scattering should be possible using higher pump intensity and/or other gases.     

Far-infrared raman laser with high intensity laser pumping

Theoretical and experimental results are presented for a pulsed far-infrared (FIR) molecular gas laser with high intensity laser pumping. In these FIR lasers, high intensity pumping is found to produce stimulated Raman emission at very large offsets (up to 30 GHz) from resonance with the intermediate state. A theoretical, density matrix model is developed for these lasers to account for simultaneous Raman emission on rotational levels in the ground and excited vibrational states (double Raman resonance). This theoretical approach is necessary in the case of off-resonant, high intensity pumping. Theory predicts the FIR emission frequency, the FIR laser gain, and the pump threshold intensity as a function of pump laser frequency. Experimental results are obtained onP-,Q-, andR-branch transitions in¹²CH3F and¹³CH3F using a single-mode, grating tuned CO2TEA pump laser with an intensity of up to 40 MW/cm². Good agreement is obtained between theory and experiment for the observed values of FIR emission frequency and pump threshold intensity. These results indicate that a widely tunable (150-1200 mum), pulsed FIR CH3F laser could be constructed with a tunable, multiatmospheric CO2pump laser of modest power (about 2-5 MW).     

Narrow bandwidth emission from a mirrorless, far infrared, 13CH3F laser

Frequency tuning and linewidth measurements are reported for a pulsed, mirrorless, kilowatt-power-level, far-infrared (FIR) ¹³ CH₃F laser operating at 245 GHz. The pump laser is an etalon tunable, single-mode CO₂ TEA laser. The FIR frequency spectrum was measured with 2.5-MHz resolution on individual 100-ns laser output pulses using harmonic mixing techniques. The linewidth of the amplified spontaneous emission was found to be surprisingly narrow, about 15 MHz. Frequency tuning of the FIR laser, as the pump laser frequency is tuned, is nonlinear, possibly due to inhomogeneous broadening of the gain by the K-level substructure of the rotational states. These results indicate that heterodyne receivers capable of single-shot frequency measurements can be important tools for investigating the properties of Raman FIR lasers     

A broadly tunable NH3Raman FIR laser

The generation of broadly tunable FIR laser emission by pumping Raman transitions in NH3with 300 mJ, 50 ns duration pulses from a continuously tunable CO2laser is reported. FIR laser outputs up to 4 mJ and a photon conversion efficiency as high as 16 percent have been obtained.     

FIR emissions from a laser cavity having a zig-zag axis

FIR emissions in three directions from a FIR NH₃ laser cavity having a zig-zag axis were observed when radiation from a pump CO ₂ laser with multiple or single modes was linearly injected into the cavity. The temporal variation and Fabry-Perot interferogram of these emissions showed that the FIR zig-zag emission is dominated by a laser process due to a population inversion, while the other forward and backward emissions are due to a Raman process     

Multipulse operation and limits of the Kerr-lens mode-locking stability

Numerical analysis in combination with experimental data for Cr/sup 2+/:ZnSe and Ti:sapphire lasers reveal the following main mechanisms of multiple-pulse generation for Kerr-lens mode-locked solid-state lasers: 1) continuum amplification due to a spectral loss growth for ultrashort or chirped pulses and 2) a bounded perturbation rise for high-energy pulses. The role of such laser parameters as gain saturation and relaxation, saturable and unsaturable loss, self-phase modulation, Kerr-lensing, and pump intensity is analyzed. This analysis provides basic directions for single-pulse stability enhancement and for multiple-pulse generation control.     

Far-infrared laser emission by microwave-infrared-far-infrared three-photon interaction in NH3

We report low threshold Raman and hyper-Raman far-infrared laser emission associated with the¹⁴NH3,nu_{2}, sQ(5, 4)infrared transition. Using a circular metallic waveguide FIR resonator, which also acts as a microwave resonator, threshold pump powers were 50 W CO2-laser radiation and 150 W CO2, 15 W microwave for Raman and hyper-Raman emission, respectively. The hyper-Raman process is suitable to generate frequency-tunable FIR radiation.     

High efficiency long pulse FIR Raman conversion in ammonia

Generally the quantum efficiency of FIR Raman generation in molecular gases is below the theoretical value. This disagreement is particularly evident in Raman scattering from pulsed sources with short pulse length, when the vibrational saturation limits the power conversion. Other nonlinear processes can overcome this saturation, but they require a very intense pump power for too short a time to use the FIR radiation in high sensitive experiments with heterodyne detection. This is the case, for instance, of RFWM. In this work we describe the experimental conditions to achieve a high efficiency 90 micron Raman conversion in ammonia, with both short and long pulses.     

Very short far-infrared pulses from optically pumped CH3OH/D, CH3F, and HCOOH lasers using an EQ-switched CO2laser as a pump source

Very short far-infrared (FIR) pulses were generated from optically pumped CH3F, CH3OH/D, and HCOOH lasers using aQ- switched, current pulsed (200 mA, 100 μs), low pressure (20 torr) CO2laser as a pump source. Values of 20 ns for the rise time and 50 ns for the decay time of the FIR pulses have been observed. The dependence of the FIR pulse shape parameters, i.e., rise time, decay time, and pulse buildup time, on the width of the pump pulse and the pressure of the molecular gas have been investigated experimentally. Due to the regular pulse shape, high repetition rate (350 Hz), high peak power (≳1 W), and broad spectral range (lambda = 100-500 mum), the pulses are very useful for purposes of solid-state and molecular time resolved spectroscopy.     

Theoretical gain spectrum of far-infrared Raman lasers

The four-level density matrix theory of the Raman FIR laser is extended to include arbitrary pump and emission field strengths and arbitrary pump frequency offsets from resonance. The results accommodate the full range of spectral variation that is observed when CH₃F Raman FIR lasers are operated at widely differing values of the rotational quantum number J. The extended theory is also useful for optimizing the temperature and pressure of these broadly tunable lasers to achieve maximum spectral coverage     

Strong-field four-wave mixing in an optically-pumped methylfluoride laser

Continuously tunable coherent radiation in the 9.8-10.1 μm region has been obtained by pumping ¹²CH₃F and ¹³CH₃F with a 10-atmosphere CO₂ laser. Pulse energies up to 2.5 mJ were observed. The experimental conditions were similar to those used for optically-pumped Raman FIR lasers, and simultaneous emission of tunable FIR radiation was also observed. Under some circumstances, a fixed-frequency mid-infrared component was also present. A detailed theoretical analysis of the RFWM process that produces the mid-infrared emission is presented. It is based on a six-level density matrix model. The importance of FIR cascade and refilling transitions, as well as double-Raman transitions, is demonstrated. Contributions to the MIR gain resulting from both degenerate and nondegenerate parametric processes are analyzed. The pressure dependence of the MIR emission was studied, both theoretically and experimentally, and the possibility of pressure switching between tunable and fixed-frequency operating modes is discussed     

385 µmD2O laser linewidth measurements to -60 dB

First linewidth measurements over a 60 dB dynamic range of a pulsed, high-power, optically pumped far infrared laser are presented. These measurements were made possible by using a 385 μmD2O laser with an N2O absorption filter and a sensitive heterodyne receiver. Studying a 385 μmD2O laser oscillator we find that the low-level linewidth (<-20 dB) can be explained by the ac Stark effect due to the high-power pump field. Also, we have not observed any frequency pulling of the main Raman emission frequency due to the strong pump and FIR laser fields.     

基于氢气填充空芯光子晶体光纤的全光纤型气体拉曼光源特性

理论和实验研究了调Q光纤激光脉冲抽运基于氢气填充空芯光子晶体光纤气体腔的全光纤型气体拉曼光源的特性。抽运光脉冲波长为1064.7nm时,产生的Stokes频移光波长为1135.7nm。理论和实验结果均表明,产生的Stokes频移光脉冲宽度远小于抽运光脉冲,并且,Stokes频移光脉冲宽度随抽运光脉冲能量的提升而增加。此外,减小抽运光脉冲宽度,可以降低拉曼阈值抽运能量、提高Stokes频移光的转换效率。在重复频率为5kHz、脉冲宽度为125ns的调Q光纤激光脉冲抽运下,实验测得拉曼阈值抽运能量和拉曼阈值点处转换效率分别为2.13μJ和9.82%。     

Rate analysis of the pulsed collinearly pumped far-infrared amplifier

A simple rate theory is presented that satisfactorily describes several aspects of the collinear optically pumped far-infrared (FIR) amplifier. The use of the rate model is justified, and its limitations discussed. The theory, when fitted to the experimental results of other workers, yields an FIR emission cross section of4.75 times 10^{-17}cm². The dependence of optimum amplifier pressure on column length emerges from the equations. The influence of pump and FIR intensities and of amplifier pressure on efficiency and gain are discussed. Some guidelines are enunciated for designing a multistage high power FIR amplifier.     

Raman pulse compression of excimer lasers for application to laser fusion

Application of efficient ultraviolet excimer lasers such as the 248 nm KrF laser to laser fusion requires that long laser pulses be efficiently converted to short pulses at high intensity. The backward Raman amplifier is shown to be a promising candidate for this application. Gain, saturation, and limits to amplifier performance are described. It is shown that pump beams of poor spatial quality may be converted to output beams of high spatial quality. Several common gaseous vibrational Raman scatterers are discussed, and it is shown that a simple KrF-pumped backward Raman amplifier using methane at atmospheric pressure will have a saturation fluence near 1 J/cm²and can produce an output five times as intense as the pump in a ten times shorter pulse with an efficiency of about 50 percent. Design tradeoffs and possible techniques for further improving the performance of such amplifiers are discussed.     

Methanol and the optically pumped far-infrared laser

New results on the generation and spectroscopic analysis of optically pumped far-infrared (FIR) laser emission from CH₃OH have been obtained as part of a systematic study of methanol isotopomers as FIR laser sources utilizing the extended line coverage available from a recently developed high-resolution CO₂ laser of high efficiency. For normal CH₃OH, six new short-wavelength lines have been found using a 2 m long Fabry-Perot FIR laser cavity. Accurate heterodyne frequency measurements are reported for 14 CH₃OH FIR laser lines, nearly all above 100 cm^-1, as well as accurate frequency offsets for most of the corresponding CO₂ pump lines. Spectroscopic assignments are presented for nine high-frequency FIR laser lines in four pump systems     

High-efficiency pulse compression with intracavity Raman amplifiers

High-efficiency pulse compression using intracavity Raman amplifiers has been computed. The energy of a pump laser stored in a lossless cavity is extracted at the Stokes frequency by means of Raman amplification of an input Stokes pulse. Calculations are made for both long and short duration input Stokes pulses for different lossless cavities. As an example, we use a hydrogen-argon mixture as the Raman medium and 1.5 J/cm²energy fluence stored in the lossless cavity at the ruby frequency. By comparing amplified Stokes pulses to a 30 ns pulse duration conventional ruby laser delivering the same energy fluence, pulse shortening factors larger than 20 are computed with quantum conversion efficiencies higher than 80 percent. These values compare favorably to backward Raman amplification. Moreover, this technique is proved to be able to provide a pulse compression rate larger than 14, even for a broad-band laser, which is impossible with backward Raman amplification. This technique could be used with any laser, even with absorbing laser media (excimer lasers) provided pump energy is stored in the lossless cavity by shifting of the laser frequency with any nonlinear process.     

New FIR laser lines from fully deuterated dichloromethane:completion of perpendicular/parallel emission pairs

We report 16 new far-infrared (FIR) laser emissions of CD₂ Cl₂. These lines are obtained by optically pumping the active molecule by means of a large-tunability waveguide CO₂ laser. Our tunability of 290 MHz around each CO₂ laser emission allows for the excitation of large-offset absorption transitions of CD₂Cl₂, not reachable by conventional CO₂ Lasers. While the overwhelming majority of the known FIR laser emissions of CD₂Cl₂ are polarized parallel to the polarization of the pump CO₂ line, 13 of the 16 new lines reported in this paper are polarized perpendicularly to the corresponding CO₂ pump line. The presence of both perpendicular and parallel lines is extremely important for the assignment of laser transitions     

Stimulated Raman scattering of picosecond light pulses in hydrogen, deuterium, and methane

Experimental results are presented on stimulated Raman scattering of short pulses of approximately 100 ps duration in H2, D2, and CH4, both in capillary waveguides and in a tight focusing geometry. Experimentally determined thresholds are in good agreement with calculation. Low thresholds (< 20 muJ) are observed in CH4and preliminary results using a mode-locked dye laser as pump indicate a useful source of tunable short pulse radiation in the near infrared.     

New CH3OH laser lines pumped with a fine-tuned high-power CO2-TEA laser

Joule-level pulses from an etalon tuned CO₂-TEA laser with an ～ 4GHz intraline tuning range have been used to pump a large number of new CH₃OH FIR laser lines. FIR wavelength measurements, pump laser offsets which produced FIR laser lines and pump laser offsets at which CH₃OH absorption lines were observed are reported. Identifications are proposed for most of the absorption lines, and an attempt to identify the FIR laser lines is described.