Low frequency characteristics of a quasioptical Schottky diode detector part II: Experimental results

A detailed experimental study of the low frequency (video) response of a quasioptical Schottky diode detector over the microwave and FIR wavelength range is presented. An optimization of the responsivity versus the bias current is proposed and a generalized curve of the saturation power versus the FIR wavelength is given. This curve defines for any antenna point-contact Schottky diode detector, suitable for FIR detection, the power range for a linear detector response. A simple method is also described to calculate the coupling efficiency of the laser radiation into the antenna reception pattern.     

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.     

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.     

Optically pumped FIR-laser with variable Fabry-Perot output coupler

An optically pumped FIR laser was operated with a copper mesh Fabr-Perot output coupler. With this device a Gaussian output beam was obtained andthe degree of coupling could be varied continuously, thus allowing optimum coupling. From the measured properties of the wire mesches grains of two FIR laser lines are calculated and the saturation intensities estimated.     

Transition behaviors in biased asymmetric quantum dot-in-double-well photodetector

A triple-band mid-/far-infrared (MIR/FIR) photodetector tunable by polarity is demonstrated by asymmetric quantum dot-in-double-well (DdWELL) structure that exhibits unique photoresponse (PR) transitions. In contrast to the MIR2 band with no dependence, the two MIR1/FIR PR bands are blue/red-shifted by the bias voltage, and the MIR2-FIR dual-band spectrum changes to a single-band feature due to the polarity. A four-level energy band model is proposed for the transition scheme, and the electric field dependence of the FIR band numerically calculated by a simplified DdWELL structure is in good agreement with the experimental PR spectra.     

Optical rectification as a probe of quantum dynamics in a heterostructure

We have measured the rectification of far-infrared radiation resonant with the lowest intersubband transition of an AlGaAs/GaAs asymmetric coupled double-quantum well in which the subband spacing is 11meV. From these measurements we can extract an intersubband lifetime of 1.2±0.4 ns at low excitation intensity and T=10 K, which appears promising for devices which can operate at low excitation and temperature, such as FIR detectors or mixers. At high intensities saturation of the rectified signal is observed due to saturation of the two level system.     

Some considerations on a Michelson spatial interferometer in the far IR

The principle of Michelson spatial interferometer and considerations relating to its use in the FIR Astronomy are presented. A model of Michelson interferometer is proposed.     

Global Fit of Torsion-Rotational Transitions in the First Three Torsional States of CH(3)OD

A global fit of the observed high-resolution microwave (MW), millimeter-wave (MMW), and Fourier transform far-infrared (FIR) spectra for CH(3)OD has been performed using a reduced torsion-rotational Hamiltonian that is obtained from the one-large-amplitude internal-rotation model. The CH(3)OD data set contains 460 MW and MMW transitions with two or fewer quanta in the torsion, v(t) 相似文献   

Interaction of polarized fields with A resonant medium of NH3 laser

A numerical model of optically pumped FIR laser for arbitrary field polarizations is discussed. A restricted set of linear equations is proposed. Calculated spectra for NH₃ laser pumped by CO₂ laser are presented. The validity and limitations of the model are considered.     

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.     

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.     

Low frequency characteristics of a quasioptical schottky diode detector. Part III: Higher order theory

A higher order theoretical model is presented to describe the behaviour of FIR point contact Schottky diodes for high signal levels and/or high bias currents. This model includes non-linear perturbation on the voltage induced at the Schottky junction. Its fit to experimental results in case of envelope detection is also discussed and compared to the fit determined with our previous non-perturbation theory.     

Application of the opto-acoustic effect to the operation of optically pumped far-infrared gas lasers

A method is reported which allows the operation of optically pumped farinfrared (FIR) gas lasers in a simple way: A spectrophone is used to control and to stabilize the pump laser frequency in order to achieve most efficient optical pumping of the FIR laser gas.     

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.     

Accurate quantification of (1)H spectra: from finite impulse response filter design for solvent suppression to parameter estimation.

A scheme for accurate quantification of (1)H spectra is presented. The method uses maximum-phase finite impulse response (FIR) filters for solvent suppression and an iterative nonlinear least-squares (NLLS) algorithm for parameter estimation. The estimation algorithm takes the filter influence on the metabolites of interest into account and can thereby correctly incorporate a large variety of prior knowledge into the estimation phase. The FIR filter is designed in such a way that no distortion of the important initial samples is introduced. The FIR filter method is compared numerically with the HSVD method for water signal removal in a number of examples. The results show that the FIR method, using an automatic filter design scheme, slightly outperforms the HSVD method in most cases. The good performance and ease of use of the FIR filter method combined with its low computational complexity motivate the use of the proposed method.     

光数字相干接收机中色散补偿模块设计与优化

在高速光纤通信数字相干接收系统中,色散补偿模块的结构和性能将直接决定其是否具有实用价值.设计了比传统的有限脉冲响应(FIR)滤波器结构简单的偶对称型FIR滤波器,并通过数值仿真分析了FIR滤波器长度选取对补偿效果的影响.研究结果表明,在接收端光信噪比(OSNR)代价不高于1 dB的情况下,FIR滤波器长度折衷选取为最大...     

Extinction coefficients of carbon grains in far-infrared

With a simple model it is shown that the wavelength-dependence of the extinction of crystalline and amorphous carbon grains in the FIR region is caused by the vibrational density of states g_c(ω) and g_d(ω), respectively. Both are approximated by the well known expressions of the model of the linear chain and the susceptibilities in the FIR are calculated by the equation of Kubo. The result is that the extinction in FIR of the crystalline carbon grains has a λ⁻²-dependence and the amorphous modification has a λ⁻¹-dependence. The results are compared with laboratory measurements, astrophysical observations and other theoretical models.     

Model independent control of lightly damped noise/vibration systems

Feedforward control is a popular strategy of active noise/vibration control. In well-damped noise/vibration systems, path transfer functions from actuators to sensors can be modeled by finite impulse response (FIR) filters with negligible errors. It is possible to implement noninvasive model independent feedforward control by a recently proposed method called orthogonal adaptation. In lightly damped noise/vibration systems, however, path transfer functions have infinite impulse responses (IIRs) that cause difficulties in design and implementation of broadband feedforward controllers. A major source of difficulties is model error if IIR path transfer functions are approximated by FIR filters. In general, active control performance deteriorates as model error increases. In this study, a new method is proposed to design and implement model independent feedforward controllers for broadband in lightly damped noise/vibration systems. It is shown analytically that the proposed method is able to drive the convergence of a noninvasive model independent feedforward controller to improve broadband control in lightly damped noise/vibration systems. The controller is optimized in the minimum H2 norm sense. Experiment results are presented to verify the analytical results.     

Spectral Characteristics of Raman Emissions from Miniature Pulsed Optically Pumped Far Infrared Laser

A miniature FIR laser with a sample-tube in 10 or 20cm length were constructed, and the laser's cavity consisted of a pair of metallic meshes, with NH3 molecules as active medium pumped by a TEA-CO2 laser, a series of FIR laser lines were observed. By taking account of the properties of the Fabry-Perot cavity and the gain characteristics of Raman transitions in the miniature optically pumped FIR laser, the spectral characteristics of the FIR laser were studied.     

一种计算远红外干涉仪相位差的数值方法

一、引言 远红外(FJR)激光干涉仪广泛用于等离子体电子密度测量。探测光束通过等离子体产生相移,参考光束不通过等离子体,两道信号由TGS热释电探测器分别接收,两者的相位差就是被测物理量,它与线电子密度成正比。目前,国内外此类干涉仪都用模拟或数字电路的相位计过零比较信号相位差。这种硬件相位计性能指标不稳定,制作费用昂贵。我们提出一种数值方法,通过软件计算相位差,这种方法可提高信号的时间分辨率。