LAUSR

Abstract In the present work, we investigated the polarization-imbalanced non-linear optical loop mirror (NOLM) for compressing the soliton spectrum as well as removing the sidebands. In the passive mode-locked soliton fiber laser, the spectra of solitons are usually accompanied with dispersive waves that can form spectral sidebands. The sidebands are the result of a constructive interference between the soliton and dispersive waves that take place at frequencies in which the phase difference is 2π. These sidebands may contain a significant part of the energy of the output spectrum, it is important first clean the spectrum by removing the sidebands and then compress the spectrum to have an optimal soliton profile. Our NOLM consists of a nearly symmetrical coupler with a 51/49 coupling ratio, and a fiber loop which consists of two 50-m pieces of SMF-28 fibers twisted in opposite directions. The maximum transmission of the NOLM at high power was 34%. At the NOLM output we reached a spectral compression of more than 50% for different values of input power. Although the sidebands around 1555 nm and 1570 nm were totally cleaned, nearly 5% of the sidebands were remaining around 1560 nm and 1567 nm. In order to get stronger spectral compression, the signal at the NOLM output was first amplified then sent into 1 km of SMF-28. For minimum input power the full width at half maximum (FWHM) bandwidth of the spectrum decreased from 5.5 nm to 0.67 nm. For higher input power the FWHM decreased from 4.23 nm to 0.59 nm.