Squeezed states of light generation for Short Noise limited Interferometric measurements in the next generation of Gravitational Waves Detectors

The advanced generation of ground based Gravitational Waves (GW) detectors are mainly limited by Quantum Noise (QN) in almost the entire detection bandwidth i.e., 10 Hz - 10 kHz. The coherent vacuum field entering through the output port of the interferometer causes the phase and intensity fluctuations of the circulating light field inside the detector and as a result, the interferometeric measurements are shot noise limited at high frequencies (above 100 Hz up to 10 kHz) and radiation pressure noise limited at low frequencies (below 100 Hz) of the detection bandwidth. The impact of shot noise in GW detectors appears as fluctuating intensity of the output laser light field and radiation pressure noise impinge fluctuating momentum on the test masses, thus displacing the test masses around their equilibrium positions. These noise components obey the Heisenberg uncertainty principle. Since the very basic principle of GWs detection involves the test masses to be quieter than the signal (GW signal) of interest, therefore all noise sources influencing the suspended test masses have to be below the GWs signal magnitude. Caves first proposed the concept of squeezed states of light in GW detectors, to improve the QN limited strain sensitivity. The idea is to replace the coherent vacuum field by the injection of phase squeezed vacuum field into the interferometer’s output port, that improves the shot noise limited interferometric measurements by reducing variance in the phase quadrature on the cost of increased variance in the amplitude quadrature of the vacuum field. Squeezed states of light are generated in degenerate Optical Parametric Amplification (OPA) process in our experiment, where the second order nonlinearity of a nonlinear crystal is exploited. At European Gravitational Observatory (EGO), where the Advanced Virgo (AdV) detector is situated, we have laboratory facility to produce frequency independent squeezed states of light for input laser field at 1064 nm as a test facility for future frequency dependent squeezed states of light generation experiment. The aim of the experiment is to produce measured squeezing level 13 dB. Part of my thesis work concerned the experimental demonstration of squeezed states of light generation and we achieved more than 4 dB of measured squeezing in the Radio Frequency (RF) bandwidth for input laser field at 1064 nm. Along with the squeezing activity, I demonstrated experimentally the single pass Second Harmonic Generation (SHG) in Periodically Poled Potassium Titanyl Phosphate (PPKTP) and Periodically Poled Lithium Niobate (PPLN) nonlinear crystals, that can be used as squeezing pump beam and auxiliary lasers for AdV. In particular, the SHG in PPLN crystal is performed using a fibered amplified Infrared (IR) laser source at 1064 nm, that eliminates the need for dedicated IR laser unit. All these experimental activities contribute towards the ongoing R&D activities in optical systems at AdV.