Bandpass filters based on pi-shifted long-period fiber gratings for actively mode-locked erbium fiber lasers

We have fabricated bandpass filters based on pi -shifted long-period gratin gs for application in actively mode-locked erbium. fiber lasers. Introducin g the pi -phase shift in the middle of the grating opens a bandpass within the core-cladding mode resonance peaks. With a 22-nm bandwidth filter inser ted in an actively mode-locked erbium. fiber sigma laser, solitonlike pulse s are generated, with a power-dependent duration of approximate to3-5 ps, a t a 3-GHz repetition rate. These all-fiber filters have the advantages of l ow insertion loss (<0.5 dB) and a wide bandwidth (10-20 nm), and they do no t require that a circulator be inserted into the laser cavity. (C) 2001 Opt ical Society of America.