Mitigation of spectral broadening in high peak power holmium-doped fiber sources

Simakov, Nikita, Hemming, Alexander, Boyd, Keiron, Davidson, Alan, Daniel, Jae, Carmody, Neil, Swain, Robert, Mies, Eric, Oermann, Michael, Clarkson, W.Andrew, Farley, Kevin, Carter, Adrian and Haub, John (2017) Mitigation of spectral broadening in high peak power holmium-doped fiber sources. CLEO: Science and Innovations 2017, , San Jose, United States. 14 - 19 May 2017. 1 pp . (doi:10.1109/CLEOE-EQEC.2017.8086982).

Abstract

Holmium fibre lasers are required for remote sensing, LIDAR and some medical applications [1]. In addition, pulsed Holmium fibre lasers also offer an attractive power scalable alternative to Ho:YAG and Ho:YLF lasers for pumping mid-infrared optical parametric oscillators (OPOs). In these applications it is necessary to operate at a high peak power (>20 kW) with minimal spectral broadening (<;2 nm bandwidth) [2]. This is challenging due to the onset of nonlinear effects such as modulation instability (MI) and stimulated Brillouin scattering (SBS). In an effort to increase the thresholds for the onset of various nonlinear mechanisms, we implement strategies such as transitioning to a core-pumped configuration, minimizing intensity fluctuations in the master oscillator, operating with a large mode-field diameter [3] and aggressive spectral filtering. The schematic of the high peak power amplifier is shown in Fig. 1(a). The master oscillator produced 5 ns pulses at 2077 nm with a FWHM of 0.5 nm and peak power of 1.5 kW at a 100 kHz repetition rate. These were spectrally filtered and injected into the active fibre via an in-house fabricated wavelength division multiplexer (WDM) device. A single mode 1950 nm thulium pump laser is also injected into this device [4]. Both the 2077 nm master oscillator signal and the 1950 nm pump radiation exit the WDM propagating in a robustly single mode 15 μm, 0.1 NA core. This fibre is then spliced to an adiabatically tapered (from 20/200 μm to 60/600 μm) active holmium-doped fibre with a dopant concentration of 0.5 wt.%. A 2 m length of active fibre is used for this amplifier. The output end of the amplifier is terminated by a CO₂ laser splice 1cm x 1cm x 1cm end cap with an AR coating at 1.9 - 2.1 μm. The output is then collimated and analysed using power meters (Ophir), photodiodes (EOT extended InGaAs), optical spectrum analyser (Yokogawa) and an imaging detector (Pyrocam III).

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