Investigating The Effect Of Optics Factors On The Performance Of Laser Heterodyne Detection System

Laser heterodyne detection is a sensitive optical detection method. Compared with the direct detection, laser heterodyning has many advantages, this detection technique can be applied in numerous fields such as laser radars, laser communications, laser monitoring of the atmosphere, target detection and classification and other areas. Along with the research deepening and expanding unceasingly, laser heterodyne detection is moving in the direction of long-range target detection and short-range Doppler imaging. There are many systems have been achieved, such as Firepond, HI-CLASS and other similar set-ups in foreign countries. And a series of experiments have been carried out to verify the tracking and detection abilities of heterodyne detection on space targets at a range up to thousands of kilometers. Based on laser heterodyning, Polytec provides different vibration measurement equipments demonstrating that heterodyne detection techniques have wide application prospect in noncontact vibration measurement. Taking into account of the advantages of laser heterodyne detection technique and national demand for this detection method, the research of laser heterodyne detection technique is of great significance. Laser coherent detection in our country, whether to achieve long-range target tracking and detection or perform high accuracy noncontact vibration measurement, is necessary to make the preparation works on the basic knowledge and technical accumulation including laser techniques, optical alignment, processing method for high-frequency signal and so on.Design and implementation of a high-performance heterodyne detection system needs plenty of prophase research to guide the optimization of detection system. In this paper, the effects of optics factors on the performance of laser heterodyne detection system are deeply investigated through theoretical analysis, numerical simulation and experimental research, the main works are as follows:(1) This paper introduces the basic theories of heterodyne detection, turbulence, partially coherent beam and stochastic electromagnetic beam in detail, which provides the theoretical basis for subsequent research works.(2) The change in characteristics of partially coherent beams propagating in turbulence is studied through theoretical analysis and numerical simulation. Based on the obtained results, the effects of light sources parameters, turbulence intensity and other factors on the performance of heterodyne system are discussed for partially coherent beams in detail. We have innovatively introduced the turbulence spectrum of refractive-index fluctuations for performance analysis of the heterodyne detection system. Through theoretical analysis, the analytical expressions obtained in this work can be conveniently used to study the influence of misalignment angle and other factors on the heterodyne detection system.(3) According to the theory of stochastic electromagnetic beams, the change in polarization of stochastic electromagnetic beam propagating in free space or turbulence is studied by theoretical analysis and numerical simulation. Based on the obtained results, the mechanism governing the polarization change of an electromagnetic beam on propagation is elucidated. The innovative work on the performance analysis of heterodyne system employing stochastic electromagnetic beams is performed. After that, the effects of light sources parameters, turbulence, detector size and other factors on the performance of detection system are investigated through numerical simulation.(4) The operation of lasers exists lots of random factors, which results in the optical spectral spread. A single frequency laser is commonly thought to be quasi-monochromatic with a linewidth. Based on statistical theory, the effect of laser linewidth on the performance of heterodyne detection is investigated in this article. For a heterodyne detection system, the frequency measurement of intermediate frequency signal is discussed with different linewidth according to the analytical expressions, and the effect of noise on the frequency measurement is discussed as well. In order to check the correctness of theoretical results, an experiment is carried out by using a laser with a linewidth of 1 MHz, which has an 8.1km channel path. The experimental results show that the linewidth of laser is not the determinant limiting on the detection range of heterodyne detection systems.(5) Random fluctuation of atmospheric turbulence cause phase disturbance of the optical field, and lead to irradiance fluctuations, beam spreading, and beam wander, among other effects. The research of the influence of turbulence on the performance of heterodyne detection system can guide the solution to inhibit and improve the effects caused by turbulence. Based on the Rytov approximation, we theoretically analyze the effects of turbulent atmosphere on the heterodyne detection system. However, the theory of turbulence is valid in the weak and moderate fluctuation regime. For stong-turbulence effects, the theoretical analyses show poor agreement with experimental results. In order to investigate the performance of heterodyne detection system under strong turbulence conditions, the Fourier series based atmospheric phase screens are used to perform simulation and analysis. And the turbulent atmosphere simulation system with static phase plate is established in the laboratory. With the turbulence simulation system, the experiment is carried out to investigate the performance of heterodyne detection system under turbulence conditions.(6) Generally, optical systems inevitably exist aberrations. It is of great significance to analyze the effects of different aberrations on heterodyne detection system, which can guide the design of optical systems for heterodyne detection systems. According to aberration theory, we have studied the performance of heterodyne detection system under the assumption that the optical fields with different aberrations are various combinations of uniform and Gaussian distributions. The results obtained demonstrate that spherical aberration and defocus have greater impact on the performance of detection system. Considering that turbulent atmosphere and aberration in essence cause the wavefront distortion of optical fields, the effects of different aberration types on the heterodyne detection system in turbulence are investigated through numerical simulation.In this paper, we have generally studied the effects of optics factors on the performance of laser heterodyne detection system including three aspects: laser source, transmission environment and optical system. A great number of studies are performed through theoretical analysis, numerical simulation, and experimental study to provide the necessary technical reserves for the further development of laser heterodyne detection techniques.