Study On Heterodyne Receiving Technology For 2 Micrometer Laser With Multiple Single Mode Fibers

Wind lidar is a kind of technical means to detect wind velocity, wind direction and wind shear. Its capacity to detect clear air and vector wind filed is strong, so it can be used to supplement microwave radar. Wind lidar has a broad and bright prospect in the application of detection of wind turbulence, detection of aircraft wake, and protection of aircraft taking off and landing. The studying of 2μm coherent wind lidar is a hot area of research in the world. Focusing on key theoretical problems of multiple single-mode fibers heterodyne detection and optical design, this paper models, simulates and designs multiple single-mode fibers heterodyne receiving system. The main work of this paper includes the following aspects.First of all, based on spatial coherence theory and temporal coherence theory, this paper analyses and models the heterodyne efficiency, which is the main evaluation parameter of coherent detection system. For signal beam is Airy disk, local oscillator beam are uniform beam and Gaussian beam respectively, this paper analyses the changing of heterodyne efficiency in the presence of slope angle and the changing of heterodyne efficiency in the presence of both slope angle and deviation.Secondly, since Mie scattered light is the main target of detection of coherent wind lidar, for the scattered situations of different sizes of particles, this paper analyses the functional relationship between scattered light intensity and scattered angle, proves the necessity and feasibility to receive scattered signal with multiple single-mode fibers. It analyses the distribution of light filed of oblique incident signal light on the receiving plane. It establishes the model of heterodyne efficiency of multiple single-mode fibers receiving system, and gives the results of simulation.Thirdly, based on multiple single-mode fibers coherent detecting theory and simulation results, this paper designs a multiple single-mode fibers receiving system. It designs image space telecentric lens to enhance the coupling efficiency of single-mode fibers. It designs six-sided conical prism to split the local beam according to the slope angle needed. This paper gives the example of one local beam splitting into seven beams, and one beam’s direction is the incident direction, six beams’ direction has a slope angle with the incident direction,and their distribution is a plum flower petal shape. This paper uses multiple single-mode fibers and 2×1 single-mode fiber coupler to realize the collimated output of local beam and signal beam, and heterodyne signal is generated when local beam and signal beam are mixed on the photosensitive surface of the photodetector.