Multimode interference filter for tuning of a mode-locked all-fiber erbium laser

We present a mode-locked all-fiber erbium laser that can be tuned in wavelength while in pulsed operation. A low-cost multimode interference bandpass filter based solely on standard fibers was employed in a sigma-shaped cavity design. By bending the fiber filter, the spectrum could be continuously shifted by up to 11.6 nm without interrupting pulsed operation, and output powers of more than 3.3 mW at pulse durations below 350 fs were achieved.