A novel all-fiber tunable spectral shaper for pre-compensating gain-narrowing based on nonlinear amplifying loop mirror

A novel spectral shaping mechanism for pre-compensating gain-narrowing is investigated numerically and experimentally. The spectral shaping is realized based on the mapping between the frequency and time domain, via the modulation of intensity in time domain can be casted to frequency domain. The optical ultrashort pulse is injected to a nonlinear amplifying loop mirror (NALM) and modulated both in the frequency and time domain. The pulse spectrum can be modulated in saddle-shape, and the sinking depth of spectrum can be tuning with the variation of the pump power of the NALM, which can pre-compensate the gain-narrowing and sustain a broadband spectrum in later high-gain amplifier. A proof-of-principle experiment is carried out. The spectral bandwidth of amplified optical pulse increases from about 10 nm to 19 nm, and pulse duration is shorten from 669 fs to 342 fs with the implementation of NALM spectral shaper, which shows a significant suppressing on the spectral gain-narrowing. This tunable NALM spectral shaper is employed with an all-polarization-maintaining (PM) and all-fiber structure, providing a flexible and practical solution to overcome gain-narrowing in high-gain amplifications.