| Laser heterodyne detection is widely used in the detection of space target andthe discrimination of camouflaged target; Compared with the direct laser detection,it has the advantage of high detection sensitivity and high speed resolution. In recentten years, Lincoln Laboratory has introduced the photon counter into heterodynedetection to achieve the weak local oscillator light, low shot noise heterodyne signaldetection.Two aspects of work are mainly discussed in this paper which are heterodynedetection with the photon counter and the discrimination of the frequency-spectrumof the beat signal. For the first aspect, heterodyne detection is conducted usingphoton counter; the beat frequency is derived by two methods, one is by frequencyspectrum density distribution of the recording photons time and the other is by theprobability density distribution of successive photons time-interval. For the secondaspect, types of targets are detected by laser heterodyne detection, theretro-reflection signal’s Doppler frequency spectrum is identified which is caused bythe movement of the targets, the moving information such as the speed,frequency-spectrum broadening of rotation, two-dimension frequency-spectrumimaging of vibration are derived. Based on the work of the two aspects, we form acomplete understanding of laser heterodyne detection system, deeply explore of number of techniques, and give an optimization design project of the system. In theexperiment, we analyze the noise source, the optical setup, the influence factor andso on. The detail work contents are as follows:(1) We introduce the basic theory of laser heterodyne detection and photonstatistics, and the analytical theory to derive the photon time frequency-spectrum ofheterodyne signal; also analyze the Signal-to-noise ratio of the laser transmission.All the work provides a good theoretical basis for the experiment in the subsequentsection.(2) A multi-pixel single photon counter (MPPC) is used to detect the laserheterodyne signal and to get the beat frequency. In the first method, the frequencyspectrum analysis is used in which a fast Fourier transform is conducted on thephoton arrival time of the beat signal to get the frequency spectrum densitydistribution. With next process, sets of frequency spectrum density distribution areaveraged or multiplied together after normalized to obtain a maximum frequencypeak of1.04MHz. In the second method, the probability density distribution (PDF)of successive photons time-interval is derived, and in the experimental PDF curvewe achieve the beat frequencies of6MHz,8MHz,10MHz and12MHz. We give ananalysis of the upper limit of the frequency the method can derive, and point out thatthe upper frequency is not limited by the Nyquist criterion, and then the frequencyof80MHz is derived in the further experiment.(3) Based on the detailed analysis of the heterodyne laser detection system, wepoint out the source of noise and propose the solutions. The effect of the lasercoherent length on the heterodyne detection is investigated with the conclusion thatwhen the range of laser transmission is far larger than the laser coherent length, thepeak frequency in the frequency spectrum density distribution is broadened to about2times of the laser frequency line width. A relatively mature heterodyne detectionsystem is formed based on large number of research and experiment work.(4) Based on the optimized design system, we achieve the frequency spectrumidentification of moving point target which include the high-resolution real-timevelocity measurement of the harmonic motion and rotation, and achieve the successful detection experiment of cooperative target in the range of8.1km whichtest the effect of laser coherent length on laser heterodyne detection. We propose adetection scheme to improve the detection probability in which a modulation isconducted on the frequency of transmitted laser and local oscillation using thepseudo-random sequence code.(5) We analyze the model of the reflected laser signal frequency-spectrumbroadening for the quadric surface targets in rotation, and the theory is verified bythe experiment in which the rotating cone and cylinder is detected using scanningpoint by point, the frequency-spectrum broadening curve is in consistent with thetheory well. The rotating and vibrating targets are detected by point scanning, andthe two-dimensional space-frequency-spectrum image is formed which could beused to detect the target profile. The technique can identify the camouflage targetwhich the intensity image could not.In this paper, we achieve heterodyne detection with single photon counterwhich is an innovation in the detector; and give a comprehensive analysis of anumber of practical problems encountered in the heterodyne detection systemresulting in some optimization solutions for the impact factor; the frequencyspectrum identification and2D spectrum image is achieved. The paper provides adeep understanding of the laser heterodyne detection system by combination of thetheoretical analysis and experiment and provides some inference for the furtherstudy. |
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