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

We have fabricated bandpass filters based on pi -shifted long-period gratings for application in actively mode-locked erbium. fiber lasers. Introducing 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 inserted in an actively mode-locked erbium. fiber sigma laser, solitonlike pulses are generated, with a power-dependent duration of approximate to3-5 ps, at a 3-GHz repetition rate. These all-fiber filters have the advantages of low insertion loss (<0.5 dB) and a wide bandwidth (10-20 nm), and they do not require that a circulator be inserted into the laser cavity. (C) 2001 Optical Society of America.