Design and Development of the Electro-Optic Hybrid System for Measuring Atmospheric Turbulence Under Low SNR Conditions.

A major problem in atmospheric optics applications is optical-system-performance degradation caused by atmospheric turbulence. Obviously, accurate estimation of the turbulence level along the propagation path plays an important role in the prediction and improvement of atmospheric optical system performance. The objectives of our research are the following: (1) Developing an optical spatial filtering method to measure the strength of the turbulence; (2) Designing and developing an electro-optical system to measure the turbulence profile based on the method developed; (3) Applying the resultant electro-optical system to measure the turbulence strength in a low signal-to-noise ratio environment. Even though a variety of turbulence measurement techniques have been developed, scientists are still seeking more effective methods which can measure the turbulence profile conveniently and accurately. In this project, we develope a new single-ended atmospheric-turbulence remote -sensing method. The first key technique of this method is the optical spatial filtering technique. By blocking the target-induced speckle field, the high-pass optical spatial-filter extracts the turbulence-induced part of the reflected signals from the received laser intensities. As a result, we can use the received signals to estimate the turbulence strength along the laser propagation path. The second key technique is the pseudo-random-code (PRC) modulation method, where transmitted laser signals are modulated by the PRC and the received backscattered signals are demodulated by cross-correlation with the delayed PRC signal. This technique is essential to accomplish range-resolved turbulence detection under low SNR conditions. By coding and decoding the laser signal, the technique can estimate the range -resolved turbulence strength. Additionally, the PRC technique is capable of suppressing the background signal level in the detected signals by increasing the PRC code length. Therefore, the method is superior to the conventional turbulence measurement method in terms of range-resolved turbulence detection and low SNR signal measurement. In this research, we theoretically analyze the proposed method and practically establish the experimental system. The theoretical analysis shows that the filtered laser intensity can be related with the turbulence strength within certain limits of turbulence level. This linear -dynamic range is determined by the spatial filter size, the laser beam size, and the laser propagation distance. But it is not affected by the coherence of the laser source. The developed experimental system is an electro-optic hybrid turbulence detecting system, which consists of an optical subsystem, an electrical subsystem, a fiber-optical subsystem, and a computer control subsystem. Because the compact experimental system is highly stable and fully automated, it can achieve reliable real-time turbulence detection. The system has made range resolve measurements of the backscattered intensity and measured the turbulence strength under both high and low SNR conditions. Experimental results in both range resolution and turbulence measurement verify that the developed method and the experimental system can successfully detect the strength of turbulence under low SNR conditions.