RESUMO
Small, highly absorbing points are randomly present on the surfaces of the main interferometer optics in Advanced LIGO. The resulting nanometer scale thermo-elastic deformations and substrate lenses from these micron-scale absorbers significantly reduce the sensitivity of the interferometer directly though a reduction in the power-recycling gain and indirect interactions with the feedback control system. We review the expected surface deformation from point absorbers and provide a pedagogical description of the impact on power buildup in second generation gravitational wave detectors (dual-recycled Fabry-Perot Michelson interferometers). This analysis predicts that the power-dependent reduction in interferometer performance will significantly degrade maximum stored power by up to 50% and, hence, limit GW sensitivity, but it suggests system wide corrections that can be implemented in current and future GW detectors. This is particularly pressing given that future GW detectors call for an order of magnitude more stored power than currently used in Advanced LIGO in Observing Run 3. We briefly review strategies to mitigate the effects of point absorbers in current and future GW wave detectors to maximize the success of these enterprises.
RESUMO
In this Letter, the pump wavelength dependence of the amplified spontaneous emission (ASE) and the threshold of stimulated Brillouin scattering (SBS) in a typical single-frequency continuous wave Er3+:Yb3+-codoped fiber amplifier is investigated numerically. The Er3+:Yb3+ system is modeled as coupled two- and three-level systems, linked by a Förster resonance energy transfer process and described by the corresponding rate equations. The evolution of the pump and signal power along the fiber is modeled by differential equations, taking into account the steady-state population densities. Since the absorption at 976 nm can exceed the Yb3+-to-Er3+ energy transfer rate in high-power operation, unsaturated gain at around 1.0 µm can generate excessive ASE. Off-peak pumping with commercially available pump diodes at 915 or 940 nm spatially distributes the energy over a longer distance. For the studied amplifier configuration, energy transfer bottlenecking is prevented without the onset of SBS.
RESUMO
Next-generation gravitational wave detectors require single-frequency and high power lasers at a wavelength of 1.5 µm addressing a set of demanding requirements such as linearly-polarized TEM00 radiation with low noise to run for long periods. In this context, fiber amplifiers in MOPA configuration are promising candidates to fulfill these requirements. We present a single-frequency monolithic Er:Yb co-doped fiber amplifier (EYDFA) at 1.5 µm with a linearly-polarized TEM00 output power of 100 W. The EYDFA is pumped off-resonant at 940 nm to enhance the Yb-to-Er energy transfer efficiency and enable higher ASE threshold. We also performed numerical simulations to investigate the off-resonant pumping scheme and confirm the corresponding experimental results.