Suppression of Kelly Sidebands and Compression of Soliton Spectrum Using a Polarization Imbalance Nonlinear Loop Mirror
Cleaning and compressing soliton spectrum is an important part of the all-optical processing systems. In the present work, we report the suppression of Kelly sideband as well as compressing the soliton spectrum through a polarization-imbalanced nonlinear loop mirror (NOLM). The polarization-imbalanced NOLM has zero transmission at low power signals, which can make it a valuable mechanism for cleaning of soliton by elimination of noise and Kelly sidebands which are always present at the output of fiber soliton lasers. Polarization imbalance in the NOLM can be produced by inserting the quarter wave retarder (QWR) in the loop. Rotation of the angle of the QWR allows adjustment of the characteristics of the NOLM that can be used to choose the features that are most adequate to clean solitons with different pulse duration. As an input source, we used a mode-locked fiber ring laser with 0.6 ps duration and 1550 nm central wavelength. The results show more than 95% suppression of Kelly sidebands and two times compression of bandwidth. The maximum transmission of NOLM was 20%. Cleanup of soliton spectrum is possible using a NOLM as intensity filter without dependence of wavelength.
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