Study of the saturation effect and output power of optically pumped FIR laser

In this paper, the density matrix equation of a six-level system has been solved, in case of the FIR wave signals in the system were not small enough to be neglected. The output power density of the laser system has been calculated by means of the iteration method. The saturation effect of the system has been revenled by the numerical calculation. Several sets of curves of laser output power vs, the length of laser with defferent pumping detuning and the optimum operating pressure under certain condition have been obtained. These calculated values were in good agresment with the data provided in the literatures.     

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.     

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.     

Diffusion and vibrational bottleneck in optically pumped submillimetre lasers

By observing the time dependence of the onset of the submm laser power in the millisecond time scale, just after the switching on of the pump laser, the vibrational bottleneck due to diffusion and vibrational relaxation has been clearly exhibited in CH₃F, D₂CO and HCOOH submm lasers. Experimental results are compared with those deduced from a simplified rate equation model. Both show a diffusion limited regime and a collisional regime.     

Highly efficient optically pumped cesium vapor laser

We have demonstrated an optically pumped continuous wave Cs vapor laser with an input power to output power slope efficiency of 81%. The laser power was 0.35 W for an input power of 0.57 W, yielding an overall optical efficiency of 63%.     

Theoretical study of miniature optically pumped cavity NH3 Far-infrared laser

Miniature optically pumped cavity NH3 Far-infrared laser was studied theoretically, with the length of the FIRL cavity as short as 1.5cm, the dependences of FIRL output power and laser lineshape on operating gas pressure and output coupling condition, and on the effect of laser line saturation broadening were calculated.Supported by the National Natural Science Foundation of China.     

Laser induced line narrowing and saturation broadening effects in OPFIRL

The variation of the laser linewidth within an optically pumped NH3 FIR laser has been studied theoretically on the basis of three-level quantum system approach. It has been shown that the activated medium within the sample tube of an optically pumped ASE FIR laser can be divided into three regions: A). Laser induced line narrowing region, in which the FIR laser linewidth is less than Lorentz linewidth; B). Saturation broadening region, in which the power density of the FIR. signal becomes large enough to make saturation effect important and the linewidth broadens; C), Self-absorption narrowing region, in which the pumping energy density is below the threshold, so self-absorption dominates and the linewidth decreases. The necessary linewidth of the OPFIRL can be obtained by appropriate selection of the length of the sample tube.     

Piezoelectric transducers as monitors for optically pumped fir lasers

The power and frequency of an optically pumped far infrared laser can be monitored by piezoelectric transducers, reliable solid state devices which detect chopped absorbed radiation by the opto-acoustic effect.Work partially supported by ONR     

光泵亚毫米波激光的理论模型及计算方法(Ⅱ)--多能级分子系统的激光增益系数

为了便于对光泵亚毫米波激光进行理论计算,从量子系统的密度矩阵方程出发,分别详细推导了三能级和四能级分子系统的激光增益系数表达方式以及多组独立三能级系统的增益迭加原理.     

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.     

Predicted new optically pumped FIR molecular lasers

A total of nine cw FIR laser lines are reported from two new FIR laser molecules; CH₂CHF and S¹⁸O₂. These two are from a list of twelve candidate molecules that were predicted using a set of selection criteria. The candidate D₂ ¹⁸O was tested, but did not lase. Combining these results with independent studies on other candidates, brings the number of proven laser molecules to 4 out of 5 that were tested. These results confirm the value of the selection criteria as a guideline for predicting new optically pumped FIR laser molecules.Work supported by the Department of Energy     

The dispersion of the lasing medium and optimum length of the sample tube of OPFIRL

The dispersive characteristic of the optically pumped FIR laser (OPFIRL) system had been studied theoretically. It was found that the refractive index of the lasing medium was not a constant in the OPFIRL, and the approximate value n=1 must be corrected, otherwise, it will lead to a large error for theoretical calculation of the optimization of the operating parameters of the OPFIRL. The value of n as function of (Ip, Is, x, y, z, P) was calculated theoretically. The additional losses due to the energy exchange between the lasing system and the molecules populated in the energy levels except the appointed three-level system was estimated. The calculated optimum length of the sample tube of OPFIRL with the corrected theory was in pretty good agreement with the experimental result.Supported by national science foundation of PRC     

New FIR laser lines and frequency measurements for optically pumped CD3OH

Twenty new cw FIR laser lines in CD₃OH, optically pumped by a CO₂ laser, are reported. The frequencies of 39 of the stronger laser lines were measured relative to stabilized CO₂ lasers with a fractional uncertainty, as determined by the reproducibility of the FIR frequency itself, of 2 parts in 10⁷.Contribution of the U.S. Government, not subject to copyright.     

Transmittance investigation on capacitive mesh on thick dielectric substrates as output windows for optically pumped terahertz lasers

This paper reports that an output window for optically pumped terahertz (THz) laser has been fabricated by depositing a capacitive nickel-mesh on a thick high-resistivity silicon substrate (approximating to 5 mm thick).Unlike the conventional process of depositing a gold film approximating to 100 nm on negative photoresist using electron-beam evaporation,a nickel film approximating to 1.5 μm thick is directly deposited on the clean surface of dielectric substrate using magnetron sputtering and then a positive photoresist is spun onto the nickel metal surface at 6000 r for 60 s.A transmittance spectrum of the output window in a certain frequency range (say,from zero to 1 THz) has been obtained by using THz time domain spectroscopy.Moreover a transmittance spectrum simulated numerically has also been estimated with respect to the output window using the transmission-line model (TLM) containing attenuation component from dielectric substrate.The simulation results show that the TLM can explain well the experimental curve in a certain frequency range from zero to 1 THz.Thus it is demonstrated that the improved optical component can be efficiently used as both output coupler and output window for optically pumped THz lasers.     

FIR Stark spectroscopy on optically pumped CH3OH laser

An extensive theoretical and experimental analysis of the absorption and emission spectrum of a CH₃OH FIR-laser excited by a conventional CO₂ laser is presented. Particular interest is devoted to the Stark shifts of the pump and lasing lines and to the electric field dependence of the Fir-laser output of the various lines. The offsets with respect to the exciting radiation and the Stark shifts of the IR absorption (pump) lines are measured by means of the transferred Lamb dip technique. The theoretical behaviours of the Stark patterns are calculated for several choices of the quantum numbers and selection rules involved in the transitions. A large variety of experimental results are reported and compared to theory. Non-linear Stark shifts have been observed for the 37.5m FIR laser line and for the IR-pump transitions excited by the 9-P(38) and 10-R(38) CO₂ laser Lines. Line assignments are proposed and new FIR laser lines are reported.     

Frequencies of cw FIR laser lines from optically pumped CH2F2

The frequencies of 48 optically pumped cw FIR CH₂F₂ laser lines have been measured relative to stabilized CO₂ lasers. Uncertainties are estimated to be about 5 parts in 10⁷.     

Silicon Fabry-Perot interferometer as an optical FIR component: Application to variable couplers with uniform transmission of FIR optically-pumped lasers

The far-infrared characteristics of the double silicon etalon Fabry-Perot (FP) reflector are investigated. Measurements are reported at short FIR wavelengths. The low IR and FIR absorption and the high optimum reflectivity of this device allow convenient optimization of the coupling conditions of CW optically-pumped FIR lasers. Three sets of Si F. P. reflectors when chosen to cover complementary regions of the spectrum are adequate to cover between them most of the FIR range Using such a coupler respective output powers of 70 mW and 85 mW are obtained with the 70 m and 118 m CH₃OH lasers operating on the EH₁₁ mode for 16 W pump power injected in the FIR cavity. The variable reflector is also used to determine the unsaturated laser gain.Work supported by the Groupement de Recherches coordonnées du CNRS n^o 11 (Greco Micro ondes).     

Spectral characteristic study of Raman transition interaction in optically pumped FIR laser

The spectral characteristic of an optically pumped NH3 molecules FIR laser has been studied by a F-P interferometer and a Michelson interferometer. It was verified that when NH3 molecules were pumped by CO2-9R(16) line, a emission by Raman transitions of two photons was produced. When the gas pressure of NH3 increased, the interaction of the Raman transitions rose, then the emission was enhanced and the width of spectral lines were broadened. The experimental results are in good agreement with the theoretical calculations.Authors gratifully acknowledge to Mrs. Chen Baoqiong and Prof. Qiu Bingsheng for their technical assistance and helpful discussions.     

光泵亚毫米波激光的理论模型及计算方法(Ⅰ)——半经典密度矩阵理论

该作者曾对光泵亚毫米波激光作过系统深入地的研究。重点论述在小型光泵亚毫米波激光的理论研究中所应用的量子系统的半经典密度矩阵理论。     

Analysis on thermal characteristics of annular beam pumped vertical-external-cavity surface-emitting lasers by finite element method

The annular beam as a pump light of VECSELs is presented for the first time in this study. The thermal characteristics of VECSELs were calculated and simulated by finite element method, and the influence of the annular beam's annulus radius, the annulus width, and the pumping power to the VECSELs’ thermal characteristics was analyzed in detail. The results indicated that the annular beam can improve the VECSELs’ thermal characteristics efficiently, and provide a favorable term to generate laser in the central part of the annular. The theoretical results provided theoretical reference and experimental study for the design of high-power optically pumped VECSELs.