arXiv subjects

Rana X. Adhikari

Publications and source records attributed to Rana X. Adhikari.

23 records · Page 2Linked to original sources

arXiv2012

A New Bound on Excess Frequency Noise in Second Harmonic Generation in PPKTP at the 10^-19 Level

We report a bound on the relative frequency fluctuations in nonlinear second harmonic generation. A 1064nm Nd:YAG laser is used to read out the phase of a Mach-Zehnder interferometer while PPKTP, a nonlinear crystal, is placed in each arm to generate second harmonic light. By comparing the arm length difference of the Mach Zehnder as read out by the fundamental 1064 nm light, and its second harmonic at 532 nm, we can bound the excess frequency noise introduced in the harmonic generation process. We report an amplitude spectral density of frequency noise with total RMS frequency deviation of 3mHz and a minimum value of 20 μHz/rtHz over 250 seconds with a measurement bandwidth of 128 Hz, corresponding to an Allan deviation of 10^-19 at 20 seconds.

physics.optics↗

arXiv2012

Brownian Thermal Noise in Multilayer Coated Mirrors

We analyze the Brownian thermal noise of a multi-layer dielectric coating, used in high-precision optical measurements including interferometric gravitational-wave detectors. We assume the coating material to be isotropic, and therefore study thermal noises arising from shear and bulk losses of the coating materials. We show that coating noise arises not only from layer thickness fluctuations, but also from fluctuations of the interface between the coating and substrate, driven by internal fluctuating stresses of the coating. In addition, the non-zero photoeleastic coefficients of the thin films modifies the influence of the thermal noise on the laser field. The thickness fluctuations of different layers are statistically independent, however, there exists a finite coherence between layers and the substrate-coating interface. Taking into account uncertainties in material parameters, we show that significant uncertainties still exist in estimating coating Brownian noise.

gr-qc↗

arXiv2012

A Technique for In-situ Measurement of Free Spectral Range and Transverse Mode Spacing of Optical Cavities

Length and g-factor are fundamental parameters that characterize optical cavities. We developed a technique to measure these parameters in-situ by determining the frequency spacing between the resonances of fundamental and spatial modes of an optical cavity. Two laser beams are injected into the cavity, and their relative frequency is scanned by a phase-lock loop, while the cavity is locked to either laser. The measurement of the amplitude of their beat note in transmission reveals the resonances of the longitudinal and the transverse modes of the cavity and their spacing. This method proves particularly useful to characterize complex optical systems, including very long and/or coupled optical cavities, as in gravitational wave interferometers. This technique and the results of its application to the coupled cavities of a 40-meter-long gravitational wave interferometer prototype are here presented.

physics.optics↗

arXiv2012

Global Feed-Forward Vibration Isolation in a km scale Interferometer

Using a network of seismometers and sets of optimal filters, we implemented a feed-forward control technique to minimize the seismic contribution to multiple interferometric degrees of freedom of the LIGO interferometers. The filters are constructed by using the Levinson-Durbin recursion relation to approximate the optimal Wiener filter. By reducing the RMS of the interferometer feedback signals below \sim10 Hz, we have improved the stability and duty cycle of the joint network of gravitational wave detectors. By suppressing the large control forces and mirror motions, we have dramatically reduced the rate of non-Gaussian transients in the gravitational wave signal stream.

physics.ins-det↗

arXiv2012

Multi-color Cavity Metrology

Long baseline laser interferometers used for gravitational wave detection have proven to be very complicated to control. In order to have sufficient sensitivity to astrophysical gravitational waves, a set of multiple coupled optical cavities comprising the interferometer must be brought into resonance with the laser field. A set of multi-input, multi-output servos then lock these cavities into place via feedback control. This procedure, known as lock acquisition, has proven to be a vexing problem and has reduced greatly the reliability and duty factor of the past generation of laser interferometers. In this article, we describe a technique for bringing the interferometer from an uncontrolled state into resonance by using harmonically related external fields to provide a deterministic hierarchical control. This technique reduces the effect of the external seismic disturbances by four orders of magnitude and promises to greatly enhance the stability and reliability of the current generation of gravitational wave detector. The possibility for using multi-color techniques to overcome current quantum and thermal noise limits is also discussed.

astro-ph.IM↗