KEYWORDS: Fiber couplers, Receivers, Antennas, Signal detection, Digital signal processing, Optical transmission, Phase compensation, Telecommunications
The combination of numerous signals for a spatial diversity Free-Space Optical Communication (FSOC) framework can be actualized based on fiber-optic devices or digital signal processing. The optical combination process is normally achieved in binary tree structure to cascade multiple signals, so the number of input laser beams is subject to binary condition. In this article, we coherently combined an arbitrary number of laser beams by using a modified scheme based on the fiber couplers with specific coupling ratios. The theoretical research of three fibers combination is expressed and the practical combining efficiency of 98.3% is obtained in the experiment, which surpasses the theoretical greatest combining efficiency of customarily using 3-dB couplers. For a particular spatial diversity antenna that consists of 19 apertures, we designed its optical combination module based on specific fiber couplers. In the condition of phase differences among 19 input beams are compensated with SPGD algorithm, the average output power rises 25 to 40 times in the closed loop; the experimental combining efficiency reaches 56.5%, which is very close to the transmission efficiency of 59.3% where the combined module serves as a laser splitter without phase compensation. It means that overlooking the devices’ insert losses, the coherent combining efficiency reaches 95.3%. This study shows that optical combination provides an effective alternative for the digital signals combining process in spatial diversity FSOC.
High-power and high-quality pulsed fiber lasers with low repetition frequency are widely applied to various fields ranging from basic science to industrial applications. Coherent beam combining (CBC) is a significant method to obtain that beam, but few methods used for large-scale CBC of pulsed fiber lasers with low repetition frequency were presented. To realize it, a new method based on a continuous carrier was designed, where the continuous wave worked as the beacon signal, and the stochastic parallel gradient descent algorithm was employed for phase locking and tilt correction. The beam combining experiment revealed that the combining efficiency of two lasers with a repetition frequency of 15 kHz and a pulse width of 100 ns was 95%, and the fringe contrast in the center of the far-field spot was improved about three times. This method promises to be furtherly applied to combine the pulsed lasers with lower repetition frequency and narrower pulse width. These results pave the way for large-scale CBC of high-power and high-quality pulsed fiber lasers.
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