KEYWORDS: Sensors, Fiber amplifiers, Optical amplifiers, High power fiber amplifiers, Beam splitters, Step index fibers, Standards development, Signal generators, Polarization
Gravitational wave detectors require single-frequency laser sources with challenging requirements regarding beam quality and noise properties. We developed a reliable single-frequency fiber amplifier architecture based on standard step-index large-mode-area fibers and coherently combined two high power beams to enable further power scaling. A combined power of 391 W could be achieved with a combining efficiency of approx. 92 %. The TEM00-mode content of the combined beam was analyzed and a higher-order-mode content of 6.8 % was measured. This yields 365 W linearly polarized output power in the TEM00-mode that is usable for the application.
Gravitational wave detectors require single-frequency laser sources with challenging properties regarding beam quality, polarization, and noise properties. We developed a single-frequency fiber amplifier engineering prototype based on standard step-index polarization maintaining fibers and characterized the 200 W output beam with the complete set of measurements necessary to evaluate the system's performance with respect to the application requirements. The output beam has a TEM00-mode content of 94:8% at 200W and a polarization extinction ratio of 19 dB. In the crucial frequency range from 1 Hz to 100 kHz the frequency noise, relative power noise, and relative pointing noise measurements demonstrated low noise properties. In addition, the pointing noise below 100 Hz is the lowest reported for single-frequency amplifiers with 200W output power. SBS-free operation was demonstrated by monitoring the relative power noise in the MHz frequency range. The system was operated above 200W for 695 h and evaluated again after 650 h of operation. No signs of photodarkening were found.
Single-frequency Yb3+ fiber amplifiers operating at 1064 nm are promising candidates to fulfill the challenging requirements for laser sources of the next generation of interferometric gravitational wave detectors. We present the current development progress of a fiber amplifier engineering prototype and compare the optical and thermal performance to the solid-state-laser source of advanced LIGO. The fiber amplifier system consists of two monolithic fiber amplifier stages which currently deliver more than 110 W (functional prototype demonstrated 215 W [9,11]) of output power. The fiber amplifier output beam has one to two orders of magnitude lower relative beam pointing and relative power noise in the lower frequency range of 1 Hz to 100 Hz compared to the solid-state-laser system. It also has a polarization extinction ratio above 21 dB and a TEM00-mode content of more than 97.8 % ±0.6 % at 110 W output power. Besides the optical properties, repair and maintenance procedures are improved by a modular design of the system. Each of the modules can separately be maintained and repaired or easily be replaced by a preassembled module; it therefore minimizes laser downtimes. Another advantage is the lower heat load of approximately 500 W compared to the SSL, which produces more than 4500 W of heat, both at an optical output power of 200 W. The lower heat load simplifies cooling and reduces the complexity of the modules.
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