Description
Single-frequency, thulium fiber master-oscillator power amplifiers (MOPAs) operating in the 2 μm spectral region are promising high-power, low-noise sources for future-generation gravitational-wave detectors. Understanding how pump and seed power fluctuations couple to amplifier output power and frequency is essential in designing active stabilisation schemes. This has been investigated for in-band and out-of-band pumped 2 μm amplifiers, but multi-wavelength pumping schemes remain unexplored.
We investigate a thulium-doped silica fiber amplifier employing a 793/1550 nm multi-wavelength pumped scheme. The amplifier is seeded by a high-power, single-frequency thulium fiber laser developed in-house to ensure seed gain saturation. 793 nm pumping provides high amplifier gain, while 1550 nm pumping exploits the simpler dynamics of in-band pumping. The amplifier power and frequency response to seed and 1550 nm pump power modulation is characterised through transfer function (TF) measurements, which constitute the system transfer matrix (TM).
The simple dynamics of the in-band 1550 nm pump permits the derivation of fully analytic TFs from the thulium rate-equation model, accurately predicting the TM of the hybrid-pumped amplifier. Singular value decomposition (SVD) is applied to the measured and predicted TMs to orthogonalise the system response. This identifies weighted combinations of seed and pump modulation that preferentially drive output power or frequency. The resulting orthogonal basis is then used to determine operating conditions that maximise the frequency range over which amplifier output power and frequency remain decoupled.
These results establish a framework for designing highly decoupled power and frequency stabilisation schemes for MOPAs, thus supporting the development of laser sources that meet the stringent noise requirements of future-generation gravitational-wave detectors.
| I am the presenting author | Yes |
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