The Optimum Coupling Coefficient of Optically Pumped NH3 Far-Infrared Cavity Laser

Based on the semiclassical density matrix theory, the optimum output power reflection coefficient (coupling coefficient) of output coupler for optically pumped NH₃ far-infrared (FIR) cavity laser was calculated by means of iteration method. Experimentally, FIR Fabry-Perot cavity laser was designed and constructed by using inductive metallic meshes as couplers, and the measurement showed that the theoretical calculation were in agreement with experimental results.     

Study of the spectral characteristics and the output coupling of pulse optically pumped cavity SMMW laser

The miniaturization of OPFIRL plays an important role in application of this new wavelength range of electromagnetic wave, especially in that of spaceflight where efficiency and miniature are emphasized.[1][8] So it is important to optimize the operating parameters of OPFIRL. This article will start with density matrix equations, then calculate the FIR output power density under different operating conditions by means of iteration calculation. And then parameters of the laser will be changed in order to study it's spectral characteristic of the FIR laser. We found that, working under the optimum operating gas pressure, FIF output power varies with the output coupling of the cavity OPFIRL. There is an optimum value of reflection coefficient R_o of the output window which makes the output FIR power denity and spectral characteristic of miniature OPFIRL optimum.     

Operating parameter optimization of optically pumped CH3OH far-infrared laser

By solving the density matrix equations of a quantum system, the output power intensity of an optically pumped CH₃OH FIR laser (CH₃OH-OPFIRL) was calculated by means of iteration method, and the spectral characteristics were got. Base on the calculation, optimization of operating parameters including operating gas pressure, pumped power and output coupling coefficient of the CH₃OH cavity laser were systematically studied. Experimental, a series of FIR emissions of the CH₃OH cavity laser pumped by TEA-CO₂ laser with 9P(16) line were measured. The experimental results were in good agreement with theoretical calculations. The project was supported by the Special Research Foundation of Doctorate Station in University of P.R.C.     

High power performances of a CW optically pumped 117 μm/184 μm CH2F2 laser with uniform coupling

This paper reports on power performances, laser gain measurements and pressure behavior of the CH₂F₂ laser operating on the intense lines at 117.7 μm and 184.3 μm. The optimum laser coupling rates are determined for these two transitions. 150 mW output power at 184.3 μm is obtained with a uniform coupling of the FIR cavity. To our knowledge this is the best result obtained in such a condition.     

The optimum pumping power for optically pumped FIR lasers

The relations between the output power of optically pumped FIR laser and the pumping power were studied theoretically by solving the density matrix equation of four-level system by means of the matrix signal flow graph method. The output power density of FIR laser was calculated by iteration method. A set of curves of output FIR power density against pumping power has been obtained. It has found that every of each curve has a maximum point of which the position is different for different pumping detuning. According the these results we predicted that there would exist an optimum pumping power density for an optically pumped FIR laser with certain pumping detuning. This theoretical results would help us to design the optically pumped FIR lasers.     

The energy exchange and the optimum length of the sample tube of opfirl

The energy exchange between the pumping laser and FIR signal in the optically pumped FIR laser (OPFIRL) system was studied. The iteration method has been used to calculate the power density of the pumping and FIR signals at any point along the optical axis of the sample tube. It was found that the power density of the FIR signal did not always increase along the length of the laser tube. There was a maximum FIR power density at the appropriate point in the sample tube. If the distance of maximum power point from the entrance of the OPFIRL tube was taken as the length of sample tube Z_opt, the maximum FIR laser output should be obtained. This was the optimum length of OPFIRL tube. The value of Z_opt was closely related to the parameter of OPFIRL such as the pumping power density, the pumping detuning and the frequency of the FIR laser. The energy-exchange and the maximum output of the fir laser were calculated under the ideal condition.     

Theoretical study of multi-mode optically pumped NH3 FIR laser

The two or multi-mode optically pumped NH₃ FIR laser had been studied theoretically. The NH₃ molecular gas was assumed to be a three-level system and obeyed the time-dependent behavior of the density matrix equations. Considering the situation of playing the two or multi-mode optical pumping and the FIR laser field were same polarized. The gain coefficient and the output FIR power of the system could be calculated by using the iteration method.It had been predicted that the two or multiple longitudinal mode optically pumped NH₃ FIR laser could have a greater output power or higher lasing efficiency than single mode pumped FIR laser under suitable selected operating parameters of laser.Supported by The Education Foundation of PRC.     

Energy Exchanged Process of Superradiant Optically Pumped Far-Infrared Laser with Buffer Gas

By means of solving the density matrix equations, base on the mechanism model of buffer gas, the output intensity of FIR and the FIR signal gain coefficient Gs and IR pumping signal absorption coefficient Gp were calculated, energy exchanged process of superradiant OPFIRL with buffer gas was analyzed. It was found that after adding buffer gas into the laser medium, the FIR output could be increased, and the active region of a FIR laser was lengthened.     

FIR Laser Efficiency Enhancement by Double Resonance Tuning-Detailed Analysis

In a previous paper by the author (Ref. 1), FIR laser resonators with aperture coupling were shown to have resonance effect at the frequency that could be used to enhance the efficiency of the laser. This paper studies this effect over a wide range of laser parameters such as coupling aperture, cavity length and Fresnel Number. A set of curves are given to help a FIR laser designer optimize the laser.     

Design consideration of miniature OPFIRL. II. Popt of miniature cavity OPFIRL affected by Ro

The influence of the optimum parameters of mini-OPFIRL by the output couple of laser cavity was studied theoretically. The Density Matrix treatment is applied to a three-level system for steady state. The Density Matrix equations were solved under the assumption that Is 1E–13 w/cm² equal to the actual FIR power density at any points in the sample tube. Through numerical calculations, we found that the output coupling of OPFIRL affected seriously the energy exchange between pumping signal and FIR signal the stored energy of FIR signal and the optimum operating gas pressure. These problems are important to design a practical miniature OPFIRL.Supported by national natural science foundation of P. R. C.     

波导自由电子激光振荡器的三维数值模拟

 建立了带波导的自由电子激光振荡器方程组，能够描述光场衍射、孔耦合输出以及电子运动、腔镜倾斜等三维问题。采用整体数值（INEX）模拟方法，研究了中国工程物理研究院（CAEP）远红外自由电子激光装置的三维问题与横模结构。结果表明：激光器的横模品质好，孔耦合输出方式下仍然是基模为主，在腔功率中所占份额约87%。研究了电子束偏入射、斜入射、腔镜倾斜等不同轴（misalignment）因素对激光器的增益、功率、输出、横模结构等的影响，并给出了最大允许值。     

Theoretical Study of Optimum Operation of Two-Longitudinal-Mode Optically Pumped Miniature FIR Laser

Based on the semi-classical density matrix equation, the optimum operation of two-longitudinal-mode optically pumped miniature FIR laser was studied theoretically. The relations and the contributions to FIR laser output power of the laser structural parameters and operating parameters were calculated and discussed.     

Electric field effects on an optically-pumped HCOOH FIR laser and Zeeman laser theory

Electric field effects have been investigated on the output power of six far-infrared (FIR) laser lines from H¹²COOH optically-pumped by a CO₂ laser with its polarization arranged perpendicular to the Stark field. Optoacoustic signals observed on the pump lines were hardly affected by the applied electric field up to 0.6 kV/cm. By neglecting the electric field effects on the pump transitions, Zeeman laser theory has been applied to the FIR laser transitions. Numerical calculation predicts the observed FIR output power as a function of electric field. Experessions for oscillation frequency and intensity in homogeneous limit are given, which may be applicable to any FIR Stark laser so far as the pump transition is free from electric field effects.     

Effective far infrared laser operation with mesh couplers

In the pursuit of increased far infrared (FIR) laser output power and higher beam quality, we have designed, fabricated and applied hybrid metal mesh couplers. Fourier Transform Spectroscopy was performed to evaluate the spectral characteristics of the couplers. With smallest mesh features of 1.0 μm, these couplers, fabricated in a class 100 clean-room, exhibited remarkably wide bandpass characteristics. Application in an Apollo Model 122 FIR laser system resulted in high output powers on many lines in the 100–500 μm range, with only moderate pump powers (10–60 W). A careful analysis of the FIR laser beam showed undiffracted Gaussian beam profiles with low divergence angles.     

Revision on the calculation of optically pumped FIR laser spectrum with Doppler broadening and pressure broadening effects

By means of iteration method and also taking into account Doppler broadening and pressure braodening effect of IR pumping laser, the output power density of optically pumped NH₃ molecules FIR laser is calculated by solving density matrix equations of three-level system. The results of theoretical calculation shows that, in comparison with the FIR laser under the assumption of ideal monochromatic IR pumping, the output power density of FIR laser in the case of Doppler broadening and pressure broadening IR pumping decreases much more. Meanwhile, via the revision of Doppler broadening and pressure broadening effect, the theoretical value of optimum operating gas pressure reduces, which is in better agreement with the experimental value. Further study shows that the revision of Doppler broadening effect is more fit for the conventional FIR laser with a 2m sample tuble and the revision of pressure broadening effect is much more suitable for the miniature FIR laser with a short sample tube of 5 to 20cm in length.Supported by the National Natural Sciences Foundation of P.R.China and the NSF of Guangdong Province.     

Optically pumped far infrared lasers

The far-infrared (FIR) region of the electromagnetic spectrum is commonly thought of as the wavelength region ≈ 30μm-≈2 mm. Thus, the FIR wavelength region is located between the more familiar areas of microwaves and optics. Primarily due to the lack of FIR sources and detectors, the FIR region is difficult to access and therefore relatively unexplored and unused. The FIR source problem is presently under attack from neighbouring disciplines; from the microwave side by extending the frequency operating range of classical electron tube oscillators (e.g. backward wave oscillators) and semiconductor devices (e.g. IMPATT and quantum well oscillators) and from the optical side primarily by optically pumped molecular gas lasers.The FIR technology evolution accelerated in the mid 60's with the discovery of the discharge pumped hydrogen cyanide laser, lasing at a handful of lines located at about 330μm wavelength. However, the most important step towards a useful coherent FIR source was the discovery of the optically pumped FIR laser in 1970. In optically pumped FIR lasers a molecular gas (e.g. methyl fluoride methyl alcohol, formic acid) is pumped by an external laser, usually a carbon dioxide laser. The FIR laser transitions typically takes place between adjacent rotational levels in an excited vibrational state. Today, optically pumped FIR lasers cover the full FIR region by more than one thousand discrete laser lines observed in hundreds of FIR laser media. FIR output powers on the order of 1–100 mW are available from a vast number of laser transitions.Despite the rapid development of semiconductor FIR oscillators the optically pumped FIR laser is still the only practical unit that bridge the full frequency-gap between microwaves and optics. The fact that FIR lasers are considered as local oscillators in space born applications, indicate that FIR laser technology has matured considerably.This survey paper discusses optically pumped FIR lasers from the engineer's point of view: principles of operation, design and characteristics.     

High power FIR NH3 laser using a folded resonator

A compact FIR laser cavity having a couple of folding mirrors was designed. By inserting this FIR cavity into the pump TE CO₂ laser resonator, we observed 19 FIR NH₃ laser lines of which 4 new lines were included. The maximum output power was over 500 W at 90 m.     

Effects of Buffer Gas on Spectra of Optically Pumped Far-Infrared Cavity Laser

By developing a mechanism model of buffer gas, the comprehensive relaxation time of laser operation substance was calculated. By means of solving the density matrix equations, the effects of buffer gas N₂ on the spectrum characteristics of NH₃ FIR laser line with buffer gas N₂ were studied. It was found that by adding proper buffer gas, FIR laser output power could be increased, and spectra widened because of the molecular collision. This work made it possible to develop high output and wide range tunable FIRL.     

FIR Laser Efficiency Enhancement by Double Resonance Tuning

FIR Resonators with aperture coupling are shown to have resonance effects at the pumping frequency. Detailed calculations show that these effects can be enhanced by proper choice of the Gaussian parameters of the input pumping laser. The result should be an increase in efficiency of the FIR laser. A procedure is given to tune the cavity to both the FIR and pumping frequency simultaneously.     

Relaxation oscillations in cw optically pumped CD3OD and15NH3 lasers

We report the results of heterodyne measurements of the mode quality and tuning range of a cw CO₂ laser designed for optically pumping a far-infrared (FIR) molecular laser, and describe the implications of multi-mode operation. In particular we report the observation of relaxation oscillations in the output of a FIR waveguide laser.