Multicolor cavity metrology

Abstract

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 4 orders of magnitude and promises to greatly enhance the stability and reliability of the current generation of gravitational-wave detectors. The possibility for using multicolor techniques to overcome current quantum and thermal noise limits is also discussed.

A new bound on excess frequency noise in second harmonic generation in PPKTP at the 10^−19 level

“…The obvious correlated mechanisms (temperature and intensity noise coupling) are likely insignificant compared to this as discussed in section 3. This is low enough to not limit the lock acquisition [9,10] scheme for Advanced LIGO and other gravitational-wave detectors. Additionally, we have shown that there is no excess noise process at a level which is of interest to those doing precision atomic spectroscopy [7], using frequency combs to transfer optical harmonics [4,5,21] and other tests of fundamental physics.…”

“…In order to simplify this problem, auxiliary fields will be generated by SHG and used to form independent signals for two of the cavities. This technique is being developed using some small scale prototypes [9,10]. To understand the limits of such a scheme, we need to bound the frequency noise introduced in the second harmonic generation process.…”

Calibration of the Advanced LIGO detectors for the discovery of the binary black-hole merger GW150914

“…The frequency of each is independently locked to one of the arm cavities with a control bandwidth of several kilohertz. Therefore, the frequency fluctuations of each green laser are proportional to the length fluctuations of the corresponding arm cavity through the relation Δν g =ν g ≈ ΔL=L, where ν g is the frequency of either of the auxiliary lasers [39]. Beams from these two lasers are interfered and measured on a photodetector, producing a beat-note close to 158 MHz.…”

Guided lock of a suspended optical cavity enhanced by a higher-order extrapolation

“…Moreover, the effective actuator force can be smaller than the aforementioned value for fast signals due to the frequency response of the electronics and can therefore further deteriorate lock acquisition. Several approaches have been proposed to make lock acquisition less stochastic [7][8][9][10][11]. An intriguing approach among them is guided lock, which virtually increases the signal range ΔL max by extrapolating the motion of the cavity [12].…”