Abstract
Laser interferometers are the core measurement tool in gravitational wave observatories. An important factor that can limit the performance is the relative power instability of the laser, a problem often called relative intensity noise (RIN). But exactly how this influences the interferometer performance is not completely understood. Therefore in this paper we analyze laser RIN coupling into the phase readout in balanced and unbalanced heterodyne interferometers. We describe the coupling theoretically, then simulate and finally measure it. Our results reveal a combination of RIN contributions from the heterodyne frequency and twice the heterodyne frequency in the interferometric phase readout. We also show that when an additional, correlated reference measurement is subtracted the combined coupling factor depends on the differential phase between the two measurements and thus can be minimized. Our results have implications for noise models in future space-based gravitational wave observatories like Laser Interferometer Space Antenna, where RIN-to-phase coupling arises directly and is modulated via spacecraft jitter, testmass position and orientation.
Highlights
Optical interferometers are commonly used in highprecision metrology and they form, for example, the core measurement tool in gravitational wave observatories [1,2,3,4] and are used for inter-satellite ranging [5]
Our results reveal a combination of relative intensity noise (RIN) contributions from the heterodyne frequency and twice the heterodyne frequency in the interferometric phase readout
Since RIN is only injected in one beam, the expected phase noise equations take the form of uncorrelated RIN with expected residuals at the minimum due to imperfectly matched beam amplitudes in the two interferometers
Summary
Optical interferometers are commonly used in highprecision metrology and they form, for example, the core measurement tool in gravitational wave observatories [1,2,3,4] and are used for inter-satellite ranging [5]. In a homodyne readout the interfering beams share the same frequency, whereas in heterodyne detection the two beams are shifted in frequency such that they create a time-varying beat signal through interference at the difference frequency This signal can be processed with phase demodulation techniques to measure a differential length change [6]. The resulting phase signal that corresponds to the arm length difference, m − r, is measured in both output ports, A and B, with photodiodes (PDs), which see the signal with a π phase shift between them due to energy conservation at the BS In this case, fm corresponds to the measurement beam with a time-varying phase (the observable of interest being the distance change of the TM), while the propagation path of fr is assumed constant and termed the reference beam. Note that radiation pressure noise on the TM due to the effects of low frequency RIN are not detailed in this paper
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