Decoherence Effect Of Target Roughness In Optical Heterodyne Detection Signal

Optical heterodyne detection is recognized as the coherent detection system with quantum detection limit. It is also a holographic detection, which realizes the amplitude, frequency and phase of carrying information from the echo signal. In weak signal detection, it has incomparable advantages compared with direct detection. Optical heterodyne detection finds numerous applications in laser radar, coherent optical communication, remote sensing, industrial ultra precision measurement, laser Doppler vibrometer, ranging and velocity measurement, and many other fields. However, the realization conditions of optical heterodyne detection are very strict. Besides the wavefront matching, the wave vector matching and the pattern matching, the rough characteristics of a target surface seriously affects the efficiency of the echo heterodyne detection in fact. Our main work in this paper are as follows. 1. For the light wave, most real target surfaces are rough. It is particularly important to quantization the degree of the rough target surface, in order to do quantitative analysis of the influence on the optical heterodyne detection signal. Based on the Monte Carlo method, one-dimensional and two-dimensional Gauss random rough surfaces models are established. There are two parameters: the height of root mean square and the correlation length to describe such roughness. The height of root mean square describes "vertical" fluctuation of the surface. The correlation length describes "horizontal" fluctuation. The actual rough target surface topography is simulated to study the relationship between the roughness of target surface and the two parameters. The results obtained could be useful for a rough target optical heterodyne performance analysis. 2. Targets based on Gaussian random rough surface model are developed. One and two dimensional wavefront of the echo signal in the optical heterodyne detection system are simulated by use of the Monte Carlo method. The dismatching phenomenon between the signal beam and the local oscillator beam is discussed. Finally, by use of digital waveform analyzers, the wavefront of source beam, the wavefront of local oscillator beam, the wavefront of signal beam from smooth surface target and rough surface target, respectively, are tested. It is shown that the results tested are in agreement well with the simulating ones. There is serious distortion of the wavefront of the echo signal modulated by the rough target surface. 3. In order to discuss the "decoherence" effect caused by the rough target surface, the detection process of the echo signal from Gaussian random rough surface targets is simulated by use of the Monte Carlo method, the effect of the target surface roughness on the IF(Intermediate Frequency) signal is examed. One-dimensional and two-dimensional normalized IF current distribution in photosensitive surface of the detector are discussed. In addition, two optical heterodyne detection experiments of the rough surface and a smooth surface are designed to verify that "decoherence" effect on the IF signal of a rough target surface is serious. The results show that: the results tested are in agreement well with the simulating ones. Under the same external conditions, there is serious "decoherence" effect caused by the rough target surface. 4. If the signal wavefront is modulated by different rough surfaces from various objectives, the actual "decoherence" effect is also different. The relationships between the "decoherence" effect and the two parameters: the height of root mean square and the correlation length of a rough surface are studied in detail. Through a number of IF signal of different rough targets simulation, the quantitative relationship curve between IF signal and the two parameters of rough target are given. Finally, using roughness comparison specimens as targets, the curve in optical heterodyne detection experiments, are made to verify the simulated results. 5. How to accurately set the detection threshold, and to decide whether there is a signal are difficult. Because the rough target surface leads to random fluctuation of the optical heterodyne signal. Traditionally, one uses a simple model, such as Gaussian distribution, describes the optical heterodyne detection echo signal of the target, which often leads to judgment errors. In this paper, we present a more accurate method to describe the probability distribution of the echo signal by use of polynomial fitting. The echo signals from a surface coating specimen of armored car are measured, the statistical analysis shows that the probability of detection is higher and the probability of false alarm is lower, when the method of polynomial fitting to determine the decision threshold is used, rather than a simple Gaussian distribution. The results show that with the method of polynomial fitting on statistical histogram of measured IF signal, the precision of setting detection threshold will be improved, the probability of detection rises by 6.02% and the probability of false alarm reduces by 7.7%. 6. Vibration characteristics measurement of the target is not only a key technology in target recognition and laser radar system designing, but also it is an important application in optical heterodyne detection technique. According to the theoretical analysis of measurement principles of the target vibration characteristics, a system of laser Doppler vibration measurement is designed. The vibration characteristics of a speaker in the laboratory and a car outdoor are successfully measured, and the corresponding vibrational spectrum is obstained. This system could also be used to measure the vibration characteristic of other targets(such as aircraft, tanks, ships, and so on.). It is significant to precisely strike military objectives and precise guidance. In this paper, the optical heterodyne detection of rough targets and its applications are discussed. The fluctuation of the IF signal caused by the roughness of targets is studied in detail. The experiments of the optical heterodyne detection are designed to verify the curve of decoherence effect from different rough targets. In addition, a more accurate method is presented. The result of the measurement of a surface coating specimen shows it is useful to improve the probability of detection. Thirdly, the vibration spectrum of a car outdoor is successfully obtained based on the optical heterodyne detection. The results obtained provide quantitative reference for designing the rough surface echo heterodyne detection system and also provide a useful tool for evaluating the performance of the heterodyne detection system.