A fundamental way to improve the ability of lidar to detect the small target far away is to increase the light intensity on it, and the coherent combination of laser beams is an effective way to obtain a large intensity on the target. A parameter named combining effect factor is introduced to evaluate the combining effect on small target. On this basis, the factors influencing combining effect are discussed. The combining effect factor becomes oscillatory as the spacing between the waists of laser beams increases, and its amplitude decreases gradually. The distance at which an ideal combining effect can be achieved increases with the increase of the spacing between the waists of laser beams, which is a reference for the determination of the spacing between the waists of laser beams in the design of coherent combining system. The dependence of combining effect on the phase control accuracy is also studied. The combining effect can reach 80 percent of its ideal value when the phase control accuracy reaches /4, and the combining effect decreases to 50 percent of its ideal value when the accuracy is /2. The influence of unparallel polarization direction between beams on combining effect is not very obvious.
A method for multiple sub-beams coherently emitting is put forward to improve the ability of laser to detect small targets in a long distance. The laser beam is split into many sub-beams with equal power parallelly emitted to the target in a certain arrangement. The sub-beams produce interference pattern on the target plane due to the fact that they are from the same laser source and have good coherence. Calculated results show that an interference peak is produced. Ideally, the maximum intensity would be N times as large as the intensity of the single beam emitted directly, where N is the number of the sub-beams. The detection is realized by using the interference peaks, and it will benefit especially to the detection of small targets. Beams divergence should be reduced to let a greater power to the target. In order to control the light intensity instability and the spot distortion, the jitters of the emitting mirrors must be controlled as well. Results show that a position accuracy of 0.1λ and an angle accuracy of 0.2θ are needed to have a stable and ideal interference peak, where λ is the wavelength and θ is the divergence angle of the beam.
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