Lidar frequency modulation vibrometry in the presence of speckle

We report laboratory target vibration measurements that use an easily aligned and adjusted fiber-based 1.5-microm heterodyne lidar. The targets are simple spherically curved retroreflectors with well-controlled vibration frequencies and amplitudes. A rotating ground-glass screen creates Gaussian speckle. We wish to understand the modulated and fast-fading lidar returns seen from real target. We frequency demodulated the recorded laboratory data by phase differencing to provide estimates of dphi/dt, where phi is the phase of the received carrier-plus-noise phasor. Experimental results for signal strength and signal-to-noise ratio, for specific target modulation parameters, agree well with our recently developed dphi/dt correlation-function theory.     

Acoustooptical processor for radio signals with discrete frequency modulation

A study is made of a processor for pulsed radio signals with discrete frequency modulation, where the frequency varies discontinuously within a pulse and takes on N different values. The spectral components are separated with an acoustooptic spectrum analyzer whose spectral plane is imaged onto a linear charged-coupled photodetector by means of a planar (one-dimensional) spatial converter made of optical fibers. In the imaging, the spectral components of the signal injected into the fiber are transposed by a special arrangement (permutation) of the fibers. The charge-coupled detector, which operates in the time-delay and accumulation mode, detects the spectral components and sums them with the required delay, and thereby compresses the signal and separates it from the noise. The compression factor is limited by the value of N and can be several hundred. Results of experimental investigations of a prototype of the processor are presented. Pis’ma Zh. Tekh. Fiz. 23, 12–19 (January 26, 1997)     

激光器自动锁频方法研究综述

介绍了激光自动锁频的意义与目的,阐述了实现激光自动锁频的算法和常用的光学频率参考,分析了不同算法和频率参考的区别和优劣,探讨了在自动领域下实现激光锁频需要解决的问题和方法,指出了有待加强的领域,提出了部分仍待解决的困难,得出了当前激光自动锁频领域的发展方向和改良趋势,为激光器自动锁频的后续研究提供了参考。     

Spectral characterization of integrated acousto-optic tunable filters by means of laser frequency modulation spectroscopy

The spectral characteristics of an integrated acousto-optic tunable filter (AOTF) as well as its responsivity to the rf driving signal and sensitivity to temperature changes are experimentally investigated and quantified using a diode-laser-based interrogation system. A spectroscopic technique, exploiting the rf frequency modulation of the laser beam and the phase-sensitive detection of the AOTF transmission, has been used for this purpose. That allows for the generation of a dispersivelike signal, which serves as a reference for tracking any wavelength change of the filter's peak with high resolution. The possibility of using the integrated AOTF as a spectrum analyzer with this interrogation scheme for fiber Bragg grating (FBG) strain sensing is also discussed.     

Simple method for frequency locking of an extended-cavity diode laser

We have developed an extended-cavity tunable diode laser system that has a small linewidth and a large output power (more than 90% of the free-running power) whose operating frequency can be conveniently locked to a transition line of Rb atoms. Based on flat-mirror feedback and frequency self-locking and with weak feedback, we have achieved a continuous frequency detuning range greater than 900 MHz and a short-time linewidth stability of better than 0.4%. By using a two-step locking procedure we not only can lock the laser frequency but also can detune the frequency to any desired value. The locking is quite sturdy and rugged.     

Amplitude modulation of guided radiation in single-mode fiber lightguides with a low normalized frequency due to seismic signals

The features of the amplitude modulation of guided radiation in single-mode fiber lightguides with a low normalized frequency when acted upon by seismic signals are investigated. It is shown that the sensitivity of sections of such lightguides to seismic shocks can reach 6·10^–4 g. This makes it possible to use them to construct fiber-optical measuring transducers, whose metrological characteristics enable them to be used to solve a wide range of geophysics problems.     

Library of reference signals in the audio frequency range

Multimedia hardware and software of personal computers (PC) can be used to generate various signals in audio-frequency range. These signals are useful in applied metrology for testing and calibrating low-frequency measuring instruments. Typical metrological aspects of these possibilities are discussed in this paper. Translated from Izmeritel'naya Tekhnika, No. 10, pp. 39–42, October, 1999.     

Atmospheric modulation transfer function in the infrared

In high-resolution ultranarrow field-of-view thermal imagers, image quality over relatively long path lengths is typically limited by atmospheric degradation, especially atmospheric blur. We report our results and analyses of infrared images from two sites, Fort A. P. Hill and Aberdeen Proving Ground. The images are influenced by the various atmospheric phenomena: scattering, absorption, and turbulence. A series of experiments with high-resolution equipment in both the 3-5- and 8-13-microm regions at the two locations indicate that, as in the visible, image quality is limited much more by atmosphere than by the instrumentation for ranges even of the order of only a few kilometers. For paths close to the ground, turbulence is more dominant, whereas for paths involving higher average elevation, aerosol modulation transfer function (MTF) is dominant. As wavelength increases, turbulence MTF also increases, thus permitting aerosol MTF to become more dominant. A critical role in aerosol MTF in the thermal infrared is attributed to absorption, which noticeably decreases atmospheric transmission much more than in the visible, thereby reducing high-spatial-frequency aerosol MTF. These measurements indicate that atmospheric MTF should be a basic component in imaging system design and analysis even in the infrared, especially as higher-resolution hardware becomes available.     

A digital method of frequency measurement for harmonic signals

Conclusions A new method and an algorithm are described for determining the frequency or period of a harmonic signal no matter what the period of the signal. The principle can be used in digital measuring systems, particularly at low and infralow frequencies.Translated from Izmeritel'naya Tekhnika, No. 11, pp. 71–73, November, 1973.     

CLAM钢穿孔等离子超声电弧焊接激励频率的优化

为了优化超声电弧焊接激励频率,针对穿孔等离子超声电弧焊接熔池进行了计算机模态分析.根据穿孔等离子弧焊接特点,利用特定的数值分析模型,通过ANSYS软件计算超声电弧焊接熔池模态,分析与模态频率相对应的熔池响应情况.最后,分别施加不同大小的超声电弧频率,以3组4.5 mm厚的中国低活化马氏体(CLAM)钢板为实验材料进行平板对接焊试验.结果表明:在谐振条件下,焊缝区面积增大,组织的细化效果较好,界面棒状碳化物生长得到抑制,同时焊缝区硬化现象得到显著改善;利用这种方法基本达到了超声电弧频率优化的目的.