The Application Of Laser Coherent Detection In Frequency-spectrum Discrimination For Moving Targets

Laser coherent detection has large potential in the detection of weak signal incomplicated background due to its high gain coefficient and anti-jamming ability.This method of detection requires narrow-linewidth and frequency-stable lasersource, if the system works in far distance, the output power will be much higher.Fortunately, the development of frequency-stable laser becomes faster recently, andthe laser source problem in the long-range detection will be solved in a few years.Laser coherent detection has two main applications: long-range target detection andtracking, short-range target imaging. In the area of long-range application, someexperimental system in America can realize1000km above target tracking andhundreds of kilometers Doppler imaging, but the research in China is so few becauseof the high-stability, long-coherence,high-power laser source. In the area ofshort-range application, there are many research reports in landmine detection andcamouflage discrimination abroad, and the main application in our country is focuson range-imaging for3-D recognition. Therefore, no matter the long-rangedetection/tracking or the short-range imaging, the coherent detection technique is thesignificant developing orientation in the future.The paper introduces the combination of simulation and experiment methods inthe moving target frequency-spectrum discrimination, and the main content are shown as follows:(1). Analyze the research status about optical coherent detection at home andabroad, and summarize the key technology in different applications. At last, theprincipal problems need to be solved are put forward while developing thistechnology in our own country, and give some guidance for the subsequent research.(2). Discuss the heterodyne efficiency and Signal to Noise Ratio (SNR) of theoptical coherent detection, and give the simulated calculation corresponding to thephysical model in order to supply some theoretical foundation for the system design.At the same time, the acting factors of the coherent detection ability are analyzed.(3). The traditional interference experiment based on strong local oscillator (LO)was completed indoor, outdoor100m,360m,1000m,7000m respectively. We havegrasped the experimental requirement about the long-range laser interference.Moreover, the Si-APD detector was used in the velocity measurement with linearand rotatable moving target, and the velocity error is0.06m/s.(4). The Multi-Pixel Photon Counter(MPPC) module was applied in the photonstatistic for coherent and pseudo-thermal light field, the Fano factor and symmetryparameter can be obtained during the experiment, the cross-talk probability and gaincoefficient can be extracted through the curve fitting of the experimental data. At theend, the saturation threshold was measured as1200kcps while at the condition ofcontinuous light injecting on the active area of MPPC module.(5). The MPPC module was used in the weak LO laser coherent detection basedon rotating diffuser indoor. The results showed that the SNR would be increased withthe method of Power Spectral Density (PSD) averaging. In addition, if the longerdata segment was cut into many shorter sections (named data segment fractionizing),then averaging the PSD for the total shorter sections, the SNR will be inceased at thesame level compared with no data cutting while just have the1/10data volume inthe experiment above.Without doubt, the method of data segment fractionizing andPSD averaging also have some restrictions during in the course of SNRimprovement, such as the influence of DC or low frequency noise. (6). The MPPC module was used in the laser interference with outdoor diffuserat the distance of30m,45m and60m. Because the echo photons were disturbed bythe atmosphere, the interference signal was not stable for a long time. So the PSDaveraging and matched filtering were applied to enhance the SNR, the resultsshowed that the SNR raising5.90times at the distance of45m, and2.73times at thedistance of60m.(7). The oscilloscope was employed in the data acquirement of target vibrationdetection with laser coherent detection, the amplitude,frequency and velocity of thevibrating object will be extracted from this technique. The time span of each datasegment is200ms, and dividing this data into200parts equally, so every short datasegment has1ms time span. In order to ensure that each short data segment haveadequate data volume for FFT algorithm, the sampling rate is setting as5MHz. Theresults showed that the vibrating frequency is83.3Hz, the maximal amplitude is5μm,the maximal velocity is0.6mm/s. In addition, the vibrating frequency-spectrum wasachieved for several typical vibrating objects.(8). The frequency spectrum analyzer and high speed data acquisition card wereapplied in the intermediate frequency (IF) signal recording and memorizing, and thescanning galvanometer was used in the target surface well-distributed latticescanning. At first, the frequency spectrum analyzer was adopted as the dataaquisition system, but the efficiency of data recording was not so satisfy. So the highspeed data acquisition card was selected, and the maximal sampling rate is80MHz,combining with FFT algorithm, we can obtain frequency spectrum of the rotatingdiffuser, and realize the2-D frequency spectrum discrimination preliminarily.(9).The application of coherent detection will expand into the detection oflong-range and fast-moving target, so the data processing system requires widebandwidth near GHz level, and the method of FFT can not satisfy this requirement,but the Method of heterodyne spectrum analyzing will be a strong candidate for highfrequency and radio frequency (RF) signal processing. In order to save manpowerand material resources, we seek cooperation with a domestic company to develop and manufacture the signal processing module, and the main parameter valves are asfollows: input bandwidth is1GHz, frequency extracting and memorizing speed is20Hz, the frequency spectrum sweep time is less than50ms under100MHzfrequency span and100kHz resolution bandwidth.To sum up, we state and resolve the main difficulty of coherent detectionthrough the theoretical and experimental research in the paper, and give sometechnical storage for the long-range laser coherent tracking or the short-range2-Dfrequency spectrum discrimination.