Frequency-Dependent Squeezing for Advanced LIGO

The first detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015 launched the era of gravitational-wave astronomy. The quest for gravitational-wave signals from objects that are fainter or farther away impels technological advances to realize ever more sensitive detectors. Since 2019, one advanced technique, the injection of squeezed states of light, is being used to improve the shot-noise limit to the sensitivity of the Advanced LIGO detectors, at frequencies above $\sim 50\mathrm{Hz}$. Below this frequency, quantum backaction, in the form of radiation pressure induced motion of the mirrors, degrades the sensitivity. To simultaneously reduce shot noise at high frequencies and quantum radiation pressure noise at low frequencies requires a quantum noise filter cavity with low optical losses to rotate the squeezed quadrature as a function of frequency. We report on the observation of frequency-dependent squeezed quadrature rotation with rotation frequency of 30 Hz, using a 16-m-long filter cavity. A novel control scheme is developed for this frequency-dependent squeezed vacuum source, and the results presented here demonstrate that a low-loss filter cavity can achieve the squeezed quadrature rotation necessary for the next planned upgrade to Advanced LIGO, known as “$\mathrm{A}+$.”

Figure 1

Schematic overview of the optical and electronic layout for the experiment. (a) The output of a 1064 nm laser is used to produce a 532 nm field via a second harmonic generator (SHG), a local oscillator (LO) for homodyne detection, and two frequency-shifted fields (CLF and RLF) for generating error signals to control the squeeze angle and the filter cavity detuning. The squeezing angle is sensed using the coherent locking field (CLF), and the filter cavity detuning is sensed by the resonant locking field (RLF). The 532 nm field is split into two components, one for generating squeezing as well as controlling the length of the OPO, and the other for controlling the length of the filter cavity. (b) Frequency-independent squeezed vacuum generated by the OPO is injected into the filter cavity, and experiences frequency-dependent rotation upon reflection. (c) The frequency-dependent squeezing is measured on a balanced homodyne detector. (d) How the filter cavity will integrate optically with the LIGO interferometer.

Figure 2

Frequency-dependent squeezing at gravitational-wave detector frequencies. Measured noise (solid) is plotted alongside models (dashed). The 0 db line refers to the shot noise level, representing unsqueezed vacuum fluctuations. Frequency-independent squeezing (brown) shows the performance of our squeezer to low frequencies without the filter-cavity (FC). The action of the cavity on the squeezed vacuum is demonstrated in both squeezed (purple) and antisqueezed (blue) quadratures, as well as intermediate homodyne angles $\varphi$ (orange, green, red). The cavity detunings $\delta$ were selected to be appropriate for gravitational-wave detectors (20–40 Hz). The detuning variations are from a nonlinear relationship between the squeezing angle and operating point of the RLF control scheme. The black line shows the minimum relative quantum noise possible in an interferometer by squeezing after reflection by this cavity (detuned 30 Hz). For a relative comparison, the gray curve models the quantum noise change expected from injecting only the frequency-independent source of this experiment into a matched interferometer [6]. Data coinciding with acoustic peaks have been excluded from the frequency-binned data, but are presented in the faded traces. The turn up in each curve starting at 20 Hz is due to a mechanical resonance of the optical table.