Observation of squeezed light in the 2 $\mathrm{μm}$ region

Abstract: We present the generation and detection of squeezed light in the 2 $\mathrm{\mu m}$ wavelength region. This experiment is a crucial step in realising the quantum noise reduction techniques that will be required for future generations of gravitational-wave detectors. Squeezed vacuum is generated via degenerate optical parametric oscillation from a periodically-poled potassium titanyl phosphate crystal, in a dual resonant cavity. The experiment uses a frequency stabilised 1984 nm thulium fibre laser, and squeezing is detected using balanced homodyne detection with extended InGaAs photodiodes. We have measured $4.0 \pm 0.1$ dB of squeezing and $10.5 \pm 0.5$ dB of anti-squeezing relative to the shot noise level in the audio frequency band, limited by photodiode quantum efficiency. The inferred squeezing level directly after the optical parametric oscillator, after accounting for known losses and phase noise, is 10.7 dB.