Holmium-doped fiber amplifier for pumping a ZnGeP₂ optical parametric oscillator

We present a holmium-doped all-fiber master oscillator power amplifier (MOPA) system operating at 2108 nm targeting optical frequency conversion applications. The MOPA delivers pulses of 0.52 mJ energy at 10 kHz repetition rate after three amplification stages, with a close to square-shaped temporal profile of 50 ns duration, diffraction-limited beam quality and linear polarization. Challenges with achieving high gain and efficiency in the final amplification stage are discussed and attributed to quenching effects inferred from measurements of non-saturable absorption in the holmium fibers. Using this MOPA, we demonstrate a mid-IR conversion efficiency of 59% by direct pumping of a ZnGeP₂ optical parametric oscillator.