Abstract
We report the influence of higher order modes (HOMs) in large mode fibers operation in Q-switched oscillator configurations at ~2 μm wavelength. S(2) measurements confirm guiding of LP(11) and LP(02) fiber modes in a large mode area (LMA) step-index fiber, whereas a prototype photonic crystal fiber (PCF) provides nearly single-mode performance with a small portion of light in the LP(11) mode. The difference in HOM content leads to a significant difference in Q-switched oscillator performance. In the step-index fiber, the percentage of cladding light increases by 20% to >40% with increasing pulse energy to ~250 µJ. We accredit this degradation to saturation of the gain in the fundamental mode leading to more light generated in the HOMs, which is eventually converted into cladding light. No such degradation is seen in PCF laser system for >400 µJ energies.
Highlights
Thulium based fiber technology has matured in the last decade and is the basis for a wide range of commercial systems providing amplified spontaneous emission (ASE), as well as CW and pulsed laser sources [1–4]
We report the influence of higher order modes (HOMs) in large mode fibers operation in Q-switched oscillator configurations at ~2 μm wavelength
Combining S2 imaging with studies of the performance of 2 μm large mode fibers in Qswitched fiber laser oscillators reveals the impact of HOMs on the laser performance
Summary
Thulium based fiber technology has matured in the last decade and is the basis for a wide range of commercial systems providing amplified spontaneous emission (ASE), as well as CW and pulsed laser sources [1–4]. Several notable laser performance results have been achieved using conventional step-index Tm-doped fibers operating in the 2 μm wavelength regime. Goodno et al demonstrated CW single-frequency output with 600 W [5] with M2 ~1.05 and Imeshev et al achieved 230 kW peak power in the fiber while amplifying ultrashort laser pulses [6]. Regardless of operational wavelength, large mode area (LMA) fiber designs are required to suppress nonlinearities in order to generate and guide pulses with high peak power. The most extensively investigated design for achieving ultra large mode area with quasi single-mode beam quality is photonic crystal fiber (PCF) [9]. A 100 μm core rod type PCF has been used to generate 4.3 MW peak power ns pulses with single mode and linearly polarized output [10]. We investigate and compare the modal properties of PCF and stepindex LMA fibers in the 2-μm wavelength regime and utilize these results to explain the Qswitched oscillator performance of thulium doped fiber lasers
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