Abstract:
LIGO (Laser Interferometry Gravitational-Wave Observatory) detectors help detect gravi- tational waves (GW) that Albert Einstein first predicted in the early 20th century. These detectors on earth face an unprecedented challenge of detecting modulation in space-time that is trillionth the wavelength of the laser used by the detectors. These disturbances in the fabric of space-time originate from collisions of stellar mass objects like black hole mergers about a million light-years away. LIGO observatories adopt perpendicular Michelson Interferometry arms with arm lengths of about 4 kilometers and an ultra-stable high-power laser system to reach the sensitivity requirements necessary to enable these detections. A high laser power is necessary to increase the signal to quantum noise ratio at high Fourier frequencies. Besides quantum noise, the technical noise in laser coupling into interferometer arms is a prominent noise source that requires attention. Technical noise is alleviated by exploiting passive and active laser stabilization schemes. Designing optical cavities that exploiting symmetry is another avenue towards the reduction of laser noise.
During this Master’s project, an attempt is made to address frequency noise of a 1064 nm laser (Make: Coherent, Model: Mephisto- Non-Planar Ring Oscillator (NPRO), Power: 1W). We attempt absolute frequency locking and relative frequency locking. Absolute fre- quency locking addresses frequency drift of a laser, while relative frequency locking addresses frequency noise at finite Fourier frequencies (or equivalently the laser linewidth). To reduce laser frequency drift, a ro-vibrational transition line in molecular cesium is used as a frequency reference for the laser. Three cesium vapour cells are prepared. Characterization of the cell(s) is done using a 852 nm laser diode. The standard Saturation Absorption Spec- troscopy (SAS) is done to probe sub-Doppler lines which can be later exploited to stabilize laser frequency. All the three vapour cells were lost to contamination of vacuum before any effort of stabilizing the laser could be attempted. On the other hand, a high finesse triangular optical resonator is developed (Courtesy: Prof. Rana Adhikari, Caltech, USA) to address laser linewidth reduction. This is a fixed length optical resonator and the mirrors are clamped on a stainless steel spacer that is locally fabricated. This resonator is also aimed at addressing spatial profile filtering and temporal filtering of the laser beam. The laser beam is phase modulated using an EOM (Electro-Optic Modulator) and PDH (Pound-Drever-Hall) technique is adopted to extract an error signal from the resonator. The error signal is exploited to lock laser to the cavity and also cavity to the laser. An absolute frequency reference in conjunction with an optical resonator can be shown to be a promising combination for laser frequency stabilization.
