The 90 ° photomixer coherent optical communication
|School||University of Electronic Science and Technology|
|Keywords||Coherent optical communication Optical phase-locked loop 90 ° light mixer Relative difference Insertion Loss|
Laser is mainly used in the field of communication fiber optic communications and free space (including the atmosphere, underwater, space) communication in two forms, widely used intensity modulation / direct detection method does not give full play to the long coherence length of the laser beam, small divergence angle , advantages of resource-rich band, local oscillator light and signal light coherent mixer coherent optical communication technology has high sensitivity, wavelength selective advantages of higher bandwidth efficiency, in particular in the field of inter-satellite communication compared to existing RF communication has a huge advantage. Homodyne coherent optical communication sensitivity of the quantum limit can be achieved, in the communication mode, to maintain the signal light and the local oscillator phase of the light lock, and makes the optical phase-locked loop being applied to the inter-satellite receiver of the coherent optical communication. Comprehensive comparison of balanced phase-locked loop, the decision-making drive ring bit synchronous phase-locked loop and oscillation phase locked loop optical phase-locked loop performance found lower Costas phase-locked loop laser linewidth requirements, the technology is mature, easy to implement, and is used at the LO / signal light is phase-locked control. 2 × 4-port 90 ° optical mixer is one of the key components in the Costas PLL, compared with other optical mixer, the best overall performance phase noise suppression this paper focuses on the mixer The theoretical and experimental studies. This paper introduces the principle of a space optical structure, the use of spectral polarization transformation phase delay to achieve the 2 × 4-port 90 ° optical mixer. Analysis the coherent mixing principle of the structure, established phase error theoretical analysis model, analysis results show that the inphase / quadrature branch of the optical path length difference causes relative difference deviation does not exceed 0.628mrad, the direction of the fast (slow) axis of the wave plate error has no effect on the relative difference between the adjacent two, but will affect the relative difference between the I / Q branch. The calculated P light, optical coupling efficiency of the optical system is 37.8%, S 42.2%, the light. Herein chosen dimensions of 12.7mm optical element structures 90 ° light mixer and tested, with a laser wavelength of 1064 nm. The four beams of the output light by the experimentally measured relative difference with the theoretical value of 0 °, 90 °, 180 ° and 270 °, the average deviation of 4 °, the maximum does not exceed 8 °, for BPSK encoding a Costas phase locked loop experiment. In the case of the signal / local oscillator optical power are of 10mW measured coupling efficiency of 31%, and the analysis result basically consistent, superior to similar commercial product.