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.
We fabricated and reported a pedestal fiber with Yb/Ce-codoped aluminosilicate (Al2O3-SiO2) core and germanosilicate (GeO2-SiO2) pedestal. This newly-optimized chelate precursor doping technique enables us to make homogeneous large-core pedestal fiber with strong pump absorption from Yb3+ ions about 3.66dB/m at 915nm. The fiber core was homogeneously doped with 4450ppm Yb3+, 11600ppm Al3+ and 1800ppm Ce3+, and surrounded by pedestal layers with 25000ppm Ge4+. The results indicate all-gas-phase chelate precursor doping technique is highly competitive for the fabrication of pedestal fiber towards narrow-linewidth fiber laser.
Based on a master oscillator power amplifier configuration, laser performance of commercial Nufern-20/400-8M Ybdoped aluminophosphosilicate ternary laser fiber was investigated. Pumped by 976 nm laser diodes, 982 W laser output power was obtained with a slope efficiency of 84.9%. Spectrum of output was centered at 1066.56nm with 3dB bandwidth less than 0.32 nm, and the nonlinearity suppression ratio was more than 39dB. Beam quality of Mx2 and M2y were 1.55 and 1.75 at 982 W, respectively. The laser performance indicated that Nufern-20/400-8M Yb-doped aluminophosphosilicate ternary laser fiber is highly competitive for industry fiber laser use.
To investigate the laser performance of Ce/ Yb-codoped aluminosilicate (Al2O3-SiO2) binary glass fiber, we took commercial Nufern-20/400-9M fiber as a research object. 0.95 kW laser output power at 1066 nm with an optical-to-optical efficiency of 83.3% was achieved at fiber laser amplifier stage. Beam quality of Mx2 and My2 is 1.56 and 1.68 at 0.95 kW, respectively. The results indicate Nufern-20/400-9M fiber may be suitable
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