Local readout enhancement for detuned signal-recycling interferometers

Abstract

High power detuned signal-recycling interferometers currently planned for second-generation interferometric gravitational-wave detectors (for example Advanced LIGO) are characterized by two resonances in the detection band, an optical resonance and an optomechanical resonance which is upshifted from the suspension pendulum frequency due to the so-called optical-spring effect. The detector's sensitivity is enhanced around these two resonances. However, at frequencies below the optomechanical resonance frequency, the sensitivity of such interferometers is significantly lower than non-optical-spring configurations with comparable circulating power; such a drawback can also compromise high-frequency sensitivity, when an optimization is performed on the overall sensitivity of the interferometer to a class of sources. In this paper, we clarify the reason for such a low sensitivity, and propose a way to fix this problem. Motivated by the optical-bar scheme of Braginsky, Gorodetsky, and Khalili, we propose to add a local readout scheme which measures the motion of the arm-cavity front mirror, which at low frequencies moves together with the arm-cavity end mirror, under the influence of gravitational waves. This scheme improves the low-frequency quantum-noise-limited sensitivity of optical-spring interferometers significantly and can be considered as an incorporation of the optical-bar scheme into currently planned secondgeneration interferometers. On the other hand it can be regarded as an extension of the optical-bar scheme. Taking compact binary inspiral signals as an example, we illustrate how this scheme can be used to improve the sensitivity of the planned Advanced LIGO interferometer, in various scenarios, using a realistic classical-noise budget. We also discuss how this scheme can be implemented in Advanced LIGO with relative ease.

Gravitational radiation detection with laser interferometry

“…In addition to the squeezing input, the variational readout, and the speed meter, there are a host of other possibilities for QND upgrades: optical ''levers'' (Khalili, 2002), multiwavelength optical springs (Rehbein et al, 2008), and multiwavelength ''xylophones'' (Rehbein et al, 2007), etc. The issues with most of the previous intercomparisons is that they do not include losses in a realistic way.…”

Double optical spring enhancement for gravitational-wave detectors

“…Differently from Ref. [16], in which the second carrier does not enter the arm cavity at all, in this proposal, the second carrier resonates in the arm cavity but is subject to different SR detuning and SR mirror reflectivity. Nevertheless, similarly to Ref.…”

“…We shall assume these carriers have different polarizations (as in Ref. [16]), so that there is no direct coupling between the two fields (although they both directly couple to the mirrors). Differently from Ref.…”

“…Nevertheless, similarly to Ref. [16], a homodyne detection must be performed at the dark port around each of the two carriers, with the two outputs combined with appropriate filters. The main purpose of this second carrier is to create a second optical spring that forms a stable optical spring together with the first one-even though each individual optical spring, acting alone, would be unstable.…”

Shot-noise-limited control-loop noise in an interferometer with multiple degrees of freedom

“…The gravitational-wave signal appears mainly in x 2 , so the signal-to-noise ratio in the second line of x cal would be the one to be considered [9,10]. In fact, we usually use only the y 2 term at the calibration process for simplicity.…”