Abstract
This work uses numerical simulations of a thulium-doped optical fiber amplifier to predict various performance characteristics such as peak temperatures, expected output powers and efficiencies, presence of amplified spontaneous emission (ASE), and transverse mode instability (TMI) onset power thresholds. Single- and two-tone configurations are studied. In the latter case, the two laser sources are separated in frequency by the amount that corresponds to the peak Raman gain, and a few seed ratios at various total seed powers are examined. The goal is to provide the field with pertinent information on what is feasible for this type of amplifier.
Highlights
Typical high-power beam combinable active gain fiber amplifiers are seeded with only one laser source, usually referred to as the signal light
When more than one highly coherent laser source is seeded into the amplifier, a laser gain competition (LGC) between the various laser wavelengths ensues within the active dopant, and this may be advantageous for power scaling purposes [1]
As described previously, one problem with too high of a seed ratio is the onset of amplified spontaneous emission (ASE); this is where having a larger bulk Raman gain coefficient is helpful a mitigating the onset of ASE
Summary
Typical high-power beam combinable active gain fiber amplifiers are seeded with only one laser source, usually referred to as the signal light. The difficulty of lasing and/or amplifying a laser signal in the 2.1-2.2 μm regime is due to the fact that the gain spectrum of the thulium (Tm) dopant rapidly diminishes beyond 2.1 μm, as can be ascertained from Fig. 1 Part of this challenge is that it will be difficult to create a strong seed power for the signal, and combined with the low gain in this wavelength range, one expects to observe significant amounts of amplified spontaneous emission (ASE), and possibly parasitic. For the two-tone configured fibers, the tone wavelength is chosen to be exactly 13.2 THz higher in frequency than the signal wavelength in order to take advantage of the peak Raman gain in a fused silica medium (see Fig. 4) This makes for a hybrid gain amplifier, including the active Tm dopant [13] and gain from the SRS optical nonlinearity [14], with the hope of, at least slightly, improving the suppression of the ASE and reducing the amplifier length that achieves 95% pump absorption. Each of these result subsections quantify aspects of complicated trade-space that pertains to Tm-doped amplifiers operating in the 2.1-2.2 μm wavelength range
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