Mode-locking of a Pr3+-doped fibre laser by pump pulse induced cross phase modulation

We report for the first time on a synchronously pumped mode-locked praseo-dymium-doped fluoride fibre laser emitting at~1.3µm. Mode-locking is achieved by a periodic phase modulation, which results from cross-phase modulation induced by the pump–pulses in the active fibre. The pump source used in the experiments is a Nd:YAG laser generating 90 ps pulses at a repetition rate of 82 MHz. The fibre laser cavity length is matched to a multiple of the pump laser cavity length (9 times). The active fibre is an 8-m-long praseodymum-doped ZBLAN fibre with a dopant concentration of 1000 ppm, a core diameter of 4.8 pm, and a cutoff wavelength of~1µm. The absorption at 1064 nm is approximately 0.2 dB/m. The fibre laser typically reaches threshold for ~400 mW of coupled pump power. Despite the fact that the fibre laser is pumped in the extreme long-wavelength wing of the absorption band, a cw output power of 20 mW is obtained for a coupled pump power of ~1 W. Because of the strong (normal) group velocity dispersion at 1.3 pm in the praseodymium-doped fibre, the generated pulses are far from bandwidth limited, when no dispersion compensating elements are used in the cavity. However, the spectral width of these pulses is ~1.2 nm, which shows that pulse durations in the ps range can be obtained using dispersion compensation. Details of the cw and the modelocking characteristics of the fibre laser are presented. The method of modelocking by cross-phase modulation is compared with other mode-locking techniques such as loss modulation using an acousto-optic modulator.