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Michelson interferometer with diffractively-coupled arm resonators in second-order Littrow configuration

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Abstract

Michelson-type laser-interferometric gravitational-wave (GW) observatories employ very high light powers as well as transmissively-coupled Fabry-Perot arm resonators in order to realize high measurement sensitivities. Due to the absorption in the transmissive optics, high powers lead to thermal lensing and hence to thermal distortions of the laser beam profile, which sets a limit on the maximal light power employable in GW observatories. Here, we propose and realize a Michelson-type laser interferometer with arm resonators whose coupling components are all-reflective second-order Littrow gratings. In principle such gratings allow high finesse values of the resonators but avoid bulk transmission of the laser light and thus the corresponding thermal beam distortion. The gratings used have three diffraction orders, which leads to the creation of a second signal port. We theoretically analyze the signal response of the proposed topology and show that it is equivalent to a conventional Michelson-type interferometer. In our proof-of-principle experiment we generated phase-modulation signals inside the arm resonators and detected them simultaneously at the two signal ports. The sum signal was shown to be equivalent to a single-output-port Michelson interferometer with transmissively-coupled arm cavities, taking into account optical loss. The proposed and demonstrated topology is a possible approach for future all-reflective GW observatory designs.

© 2012 Optical Society of America

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Figures (4)

Fig. 1
Fig. 1 (a) Michelson-type laser interferometer with conventional (transmissively-coupled) Fabry-Perot arm cavities, consisting of the coupling mirrors CM1,2 and the highly reflective end mirrors EM1,2. (b) Interferometer with diffractively-coupled arm cavities. Because of the second-order Littrow configuration, the signal is distributed into two ports that both need to be monitored to obtain the full signal strength.
Fig. 2
Fig. 2 (a) Signal response of a single-ended standing-wave Fabry-Perot (FP) cavity. Both, signal and carrier light fields are back-reflected towards the laser source. (b) Signal response of a single-ended three-port grating cavity with reflection ports C1 and C3. While the carrier interferes destructively at C3, the signal is distributed equally among the two ports. (c) Phase-quadrature readout at the FP output port and at the ports C1/C3. The dotted line shows the sum of the C1- and C3-signals, being identical to the FP reference. As cavity parameters, values realisable in LIGO were chosen: L = 4 km, ρ 0 2 = 1 2 η 1 2 = 99.5 %, ρ 1 2 = 1 τ 1 2 = 99.5 %, η2 = η2min. The two cavities were tuned to resonance, the intra-cavity power was identical.
Fig. 3
Fig. 3 Schematic of the prototype experiment. The main interferometer was operated ”close to the dark fringe” at the detection port, and thus almost all carrier light was back-reflected to the laser. To generate phase-modulation signals in the arm cavities, two electro-optical modulators (EOMs) were used. The EOMs were driven by phase-locked frequency generators, the signal frequency was 13.7 MHz. The signals were brought to interference at the beam splitters BS1 and BS2 and recorded at the photo detectors PD1 and PD2, respectively. The photo detector output was analyzed with a R&S® FSP spectrum analyzer.
Fig. 4
Fig. 4 (a,b) 13.7 MHz signal generated by EOM1,2 and detected by PD1. In these measurements, no signal was generated simultaneously by the other EOM. The modulation depth was adjusted to generate −60 dBm-signals. (c) EOM1 and EOM2 were operated simulteneously, the relative phase between the driving electric field was adjusted for maximally destructive interference. (d) same, the relative phase was shifted by π to obtain maximally constructive interference. The signal amplitudes add up coherently, leading to a signal power increase by 6 dB. (e–h) same as (a–d), but detected by PD2. For a discussion of signal strengths and loss see text.

Equations (3)

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g FPI ( ± Ω ) = i τ 1 exp [ i ( Φ ± Ω L / c ) ] 1 ρ 1 ρ 2 exp [ 2 i ( Φ ± Ω L / c ) ] .
g C 1 ( ± Ω ) = g C 3 ( ± Ω ) = η 1 exp [ i ( ϕ 1 + Φ ± Ω L / c ) ] 1 ρ 0 ρ 2 exp [ 2 i ( Φ ± Ω L / c ) ] ,
G ( Ω ) = g ( + Ω ) g * ( Ω ) ,
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