Model-based wavefront measurement and compensation in atmospheric turbulence.

The model-based method can measure phase aberration without special wavefront detectors. However, the influence of non-uniform beam intensity distribution was not considered, leading to non-negligible system errors. Moreover, no experiments were employed to verify its capability and practicability in atmospheric turbulence. This paper proposes the aberrated pupil method, which can enhance the accuracy of phase recovery by introducing an aberrated pupil function into the traditional model-based method. The effectiveness of the model-based phase measurement and compensation method was experimentally investigated and verified in the application of free-space optical communication. Compared with the traditional model-based method, the aberrated pupil method can reduce the coupling power loss caused by turbulence from 2.59 to 0.87 dB, resulting in a reduction of 1.72 dB. With coherent communication experiments, the model-based phase recovery method significantly improved the communication power budget by about 13.5 dB. The proposed method can measure and compensate for the phase aberration to significantly improve the communication quality of free-space optical communication.

Precise pointing angle deviation measurement for beaconless laser communication.

How to measure the pointing angle precisely without the beacon light is crucial for beaconless laser communication. The conventional intensity method directly measures the intensity of a part of the communication signal beam, which has low sensitivity. We propose the characteristic signal method by superimposing a low-frequency sinusoidal signal on the communication signal to promote the measuring sensitivity. Simultaneously, a fast cyclic cross-correlation algorithm is used to reduce operational complexity. Compared with the experimental results of the direct intensity method, the proposed method can improve the measuring sensitivity about 9.17 dB and increase the power budget for communication about 1.96 dB.

Short-distance equivalent test of acquisition, pointing, and tracking process for space laser communication.

The function and performance of acquisition, pointing, and tracking (APT) for the space laser communication terminals must be characterized on the ground before the launching. However, short working distance causes the terminals to not be viewed as a point, leading to the challenge for the whole-process equivalent test. A novel equivalent test method is proposed, to the best of our knowledge, by using a narrow laser beam and the imaging relationship between two terminals. The communication terminal is transformed to a virtual point by establishing an imaging relationship between the antenna of the transmitter and the tracking detector of the receiver. Thus, the deviation angle between the optical axis of the receiver and the centroid line of two terminals can be measured accurately. Furthermore, the acquisition process of searching the small-size antenna in the receiver with an extensive coverage beam in the transmitter is replaced by searching a large receiver surface with a narrow beam. This point-searching-surface method is equivalent to the surface-searching-point method in actual space communication. With this method, two terminals spaced 5 m apart were employed to construct the short-distance verification platform, and the APT function verification and performance test were completed.