A two-stage optical parametric amplifier for femtosecond fiber laser generation at 920 nm

We demonstrate a cascaded, two-stage, fiber-based optical parametric amplifier (OPA) in which the signal pulses are generated in the first stage via spontaneous degenerate four-wave mixing (DFWM) mechanism and subsequently are used as the seed pulse for the stimulated DFWM in the second stage. A compact femtosecond fiber laser provides the pump pulse with 220-fs duration at 20-MHz repetition rate. Large-mode area (LMA) photonic crystal fibers (PCFs) with zero dispersion wavelength at 1054 nm and a dispersion slope of 0.20 ps/km/nm² provide the parametric gain. Pulses with 2-mW average power in spectral range from 850 to 1000 nm are obtained from the first-stage OPA, and they are further amplified to 23.4 mW in the second stage OPA, corresponding to 10.7-dB parametric gain. The output pulse duration and spectral bandwidth are closely related to the temporal delay between the signal and pump pulses in the second stage. With an optimized time delay, pulses with >1 nJ energy and <200 fs duration are obtained at the spectral range from 850 to 1000 nm.