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We present a broadly tunable single-frequency ytterbium-doped fiber laser. Its broadband tunability is made possible by low resonator losses. The wavelength is determined by a grating filter, while the single-frequency regime is achieved by filtering the longitudinal modes in a fiber ring resonance filter. We obtained a tuning range from 1023 nm to 1107 nm. A feedback loop driving a fiber stretcher actuated by a piezo-element prevents mode hopping. Based on the coherent delayed self-heterodyne interferometry, the laser linewidth is estimated to be 600 Hz with feedback open and 11.7 kHz with feedback closed.

We milled a sub-wavelength diffraction grating on the facet of a large mode area fiber. The diffraction grating had different reflectivities for TE and TM polarized light. It was tested in a thulium-doped fiber laser where it functioned as a low reflectivity output mirror integrated with an intracavity polarizer. Compared to the laser with a perpendicularly cleaved output fiber, the laser with diffraction grating had a slightly increased threshold power and the same slope efficiency. The beam quality factor M 2 was not impaired. Polarization extinction ratios of about 20 dB that were observed at low laser powers dropped to 10 dB at high powers.

The simulated annealing method is used for retrieving the amplitude and phase from cross-phase modulation spectrograms. The method allows us to take into account the birefringence of the measurement fiber and resolution of the optical spectrum analyzer. The influence of the birefringence and analyzer resolution are discussed.