Polarized monolithic fiber oscillator for pico- and nanosecond operation at 2088 nm

We present a turn-key pulsed polarized Holmium doped fiber source at 2088 nm. In our experimental setup we are utilizing an acousto-optic modulator as a mode-locker or Q-switch. The linear cavity is comprised of a fiber Bragg grating (FBG) pair with central wavelength of 2088 nm in a polarization maintaining passive fiber. The active fiber is polarization maintaining, doped with Holmium, and pumped with an in-house made Thulium doped fiber laser with an emission wavelength of 1940 nm and a maximum output power of more than 10 W. In q-switch operation it was possible to generate stable nanosecond pulses between 5 and 250 ns at repetition frequencies between 10 and 250 kHz. Shaping of the AOM driving signal allows for some modification of output pulse shape and its length. The maximum output energy was measured to be 0.5 µJ. In mode-locked operation the laser emitted pulses at 27.35 MHz repetition rate which corresponds to the optical cavity roundtrip time. The calculated band-width limited pulse width was approx. 13 ps (sech2) and the pulse energy was 0.5 nJ. In both operation regimes the signal quality was very high with optical signal-to-noise ratio greater than 50 dB without amplified spontaneous emission (ASE). Similarly, the polarization extinction ratio (PER) was greater than 20 dB. The oscillator design is fully monolithic and polarization-maintaining. Its compactness, turn-key operation, and all-fiber design makes it a very promising platform for nonlinear frequency conversion.