Impact of backscattered light in a squeezing-enhanced interferometric gravitational-wave detector

Squeezed states of light have been recently used to improve the sensitivity of laser interferometric gravitational-wave detectors beyond the quantum limit. To completely establish quantum engineering as a realistic option for the next generation of detectors, it is crucial to study and quantify the noise coupling mechanisms which injection of squeezed states could potentially introduce. We present a direct measurement of the impact of backscattered light from a squeezed-light source deployed on one of the 4 km long detectors of the Laser Interferometric Gravitational Wave Observatory (LIGO). We also show how our measurements inform the design of squeezed light sources compatible with the even more sensitive advanced detectors currently under construction, such as Advanced LIGO. PACS numbers: 95.55.Ym, 42.50.Lc, 42.25.Fx Submitted to: Class. Quantum Grav. Impact of backscattered light in a squeezed interferometric gravitational-wave detector 2 Laser-interferometric gravitational wave detectors, such as those of the Laser Interferometer Gravitational Wave Observatory (LIGO), are the most sensitive position meters yet made, able to measure length variations of order 10 m over a multikilometer baseline. The Advanced LIGO detectors currently under construction aim to achieve even greater sensitivities, of the order of 10 m/ √ Hz at 200 Hz [1]. The LIGO interferometers are limited by quantum noise down to 150 Hz, and the Advanced LIGO detectors are expected to be limited by quantum noise across their entire measurement band. In the last decade, the injection of squeezed states of light (or squeezing) has been established as a promising technique to reduce quantum noise [2–8], providing an opportunity to improve the detector sensitivity even further [9,10]. Due to the sub-attometer sensitivity Advanced LIGO aims to achieve, the interferometer needs to be carefully isolated from the outside world. To establish squeezing as a technology compatible with advanced gravitational wave detectors, it is critical to understand and quantify any potential noise coupling mechanism that could arise from squeezing injection. One of the most pernicious enemies of gravitational-wave detectors operating at the quantum limit is scattered light [11–15], i.e. light that scatters from a moving surface and reaches the interferometer readout photodetector. Depending on the scattered optical power and the scattering-object motion, scattered light can degrade the interferometer sensitivity, typically in the audio frequency region between 50 Hz and 300 Hz that is especially important for several astrophysical sources [16]. Backscattering noise is generally difficult to model as it depends on several variables which are not known a priori, such as the seismic motion transfer function to various optics and components of the interferometers. Many measurements of backscattered light impact have been made, focussing on the arm cavity beam tubes [17, 18], light baffles [19], and in-air optical benches used for interferometer control [20]. Squeezed state enhancement is achieved by injecting squeezed light into the output port of the Michelson [21], a separate location within an interferometer to the abovelisted areas, and in the opposite propagation direction to outcoming interferometer optical beams. In the presence of squeezed state injection, the squeezing source itself (the Optical Parametric Oscillator (OPO)), becomes a scattering surface, causing scattered light to co-propagate with the squeezed vacuum state back towards the gravitational wave photodetector [15,22,23]. Further, any scattered light circulating within the OPO is power-amplified by the optical parametric process that generates the squeezed state, of which the level of amplification is an unknown time-varying quantity. This makes an a priori estimate for the amount of backscattered light power and noise reaching the interferometer readout photodetector even more difficult. In this paper, we report on the direct measurement of the impact of backscattered light from a squeezed-light source deployed on the 4 km LIGO H1 detector located in Hanford, WA, during the LIGO Squeezed Light Injection Experiment [24]. We also provide an analytical expression for the bidirectional scattering distribution function (BSDF) of an OPO. The techniques adopted to perform these measurements and the Impact of backscattered light in a squeezed interferometric gravitational-wave detector 3