Abstract
Advanced LIGO (aLIGO) requires multiple frequency sidebands to disentangle all of the main interferometer's length signals. This paper presents the results of a risk reduction experiment to produce two sets of frequency sidebands in parallel, avoiding mixed 'sidebands on sidebands'. Two phase modulation frequencies are applied to separate Electro-Optic Modulators (EOMs), with one EOM in each of the two arms of a Mach-Zehnder interferometer. In this system the Mach-Zehnder's arm lengths are stabilized to reduce relative intensity noise in the recombined carrier beam by feeding a corrective control signal back to the Rubidium Titanyl Phosphate (RTP) EOM crystals to drive the optical path length difference to zero. This setup's use of the RTP crystals as length actuators provides enough bandwidth in the feedback to meet arm length stability requirements for aLIGO.
Highlights
The advanced Laser Interferometer Gravitational-Wave Observatory consists of two detectors located in Hanford, WA and Livingston, LA
In addition to the arm cavities (which are formed by the input test mass (ITM) and end test mass (ETM) mirrors) LIGO uses a power recycling mirror (PRM) to increase the power circulating in the arm cavities; advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) adds a signal recycling mirror (SRM) to form the signal recycling cavity, which adds the ability to tune the frequency response of the detector [7]
Where ω0 is the laser carrier frequency, m j is the modulation depth, Ω j is the modulation frequency applied in Electro-Optic Modulators (EOMs) j, and φL j is the phase gained by the beam traversing its respective arm
Summary
The advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) consists of two detectors located in Hanford, WA and Livingston, LA. The gravitational wave signal is read out as the differential arm length changes (DARM) in the interferometer. This signal is enhanced by using Fabry-Perot resonant cavities as the interferometer’s arms. The aLIGO interferometer uses a phase modulation/demodulation scheme [8,9,10] to produce the necessary control signals for feedback. A scheme for producing two sets of frequency sidebands by phase modulating the laser field in the two arms of a Mach-Zehnder (MZ) interferometer has been explored [11], along with the effect of carrier-sideband relative phase noise (due to MZ arm length noise) on a LIGO heterodyne RF-readout scheme. This paper discusses a novel method for stabilizing MZ arm lengths in a parallel phase modulation scheme as well the effect of MZ differential arm length noise on laser power noise in the context of the requirements in aLIGO
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