Upconverted light for GW detector control and readout

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Abstract:: The LIGO detectors, which are designed to measure the incredibly small disturbances caused
by passing gravitational waves, must maintain a stable locked configuration. LIGO and its
40m prototype locks and stabilizes the primary laser to the arm cavity by using an auxiliary
(AUX) laser as a reference. Noise budgeting catalogs all relevant noise sources to explain the
observed noise, enabling targeted improvements to enhance the interferometerâ€™s stability and
sensitivity. The first phase of the project aims to identify contributors to the noise spectrum
through noise budgeting.
The LIGO Voyager upgrade aims to enhance high-frequency sensitivity by using high-power
operation and highly squeezed vacuum states, with a strict squeezing loss budget requiring
quantum efficiency (QE) of at least 99% at the 2 Âµm operating wavelength. Sum-frequency
generation (SFG) offers an alternative to improve photodetection QE by upconverting 2 Âµm
fields to 700 nm, allowing the use of traditional photodetectors like InGaAs or Si with QE
of ≥ 99%. The second phase of the project is to demonstrate a single pass of SFG using a
PPLN crystal and a pump of 1064 Âµm.

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LIGO Document T2400259-v1