Variable reflectivity signal-recycling mirror and control Stefan Goßler for the experimental team of The ANU Centre of Gravitational Physics

Outline Tuned and detuned signal recycling Introduction of VRSM Layout of the initial experiment Error signals First results Carrier/Subcarrier offset phase-locking control scheme Summary David Rabeling, Malcolm Gray, David McClelland Control and VRSM

Signal Recycling I Signal recycling can be used to enhance SNL sensitivity of gravitational wave detectors within certain bandwidth: Tuned SR: SRC is tuned on resonance with carrier Requires broadband mode: low finesse of SRC Detuned SR: SRC is detuned from resonance with carrier Enables narrowband mode: high finesse of SRC Microscopic tuning of SRM position alters the tuning Changing the reflectivity of SRM alters the bandwidth Control and VRSM

Signal Recycling II Control and VRSM

125 W input Detuned quant T=0.1% Detuned total T=0.1% Tuned total T=7% 10 -25 -24 -23 -22 -21 -20 h(f) [1/sqrt Hz] 1 2 3 4 Detuned total T=0.1% Tuned total T=7% Tuned quant T=7% Detuned quant T=0.1% Frequency [Hz]

Complication Rotational stage Cantilever springs 360 µm Coilholder for 180 mm 280 mm 420 mm 360 µm Upper masses Coilholder for local control Activ stack isolator 200 µm Steel wire loops 162 µm Intermediate masses Mirror 2,9 kg 150 mm Magnet-coil actuator 3 mm

Complicaton: Solution: A mirror with a reflectivity Changing the reflectivity of the signal-recycling mirror causes a down-time of the GWD for a substantial period of time! A mirror with a reflectivity that can be varied on demand: a VRSM Solution: Control and VRSM

VRSM Can be realised by: Fabry-Perot cavity Thermal tuning of an etalon Kawabe, Hild et al. Downside: Slow and lateral thermal gradients the substrate Michelson interferometer deVine, Shaddock, McClelland Downside: Very complex system Fabry-Perot cavity Strain, Hough Downside: Complex reflectivity Control and VRSM

Simplification of Layout Signal-recycling mirror Power-recycling Arm-cavity Michelson interferometer `Michelson´ Variable reflectivity signal-recycling Dual recycled Michelson interferometer with arm cavities Control and VRSM

Simplification of Layout Signal-recycling mirror Power-recycling Arm-cavity Michelson interferometer `Michelson´ Variable reflectivity signal-recycling Collapse Michelson interferometer into one mirror Control and VRSM

Variable reflectivity Introduction of VRSM Signal-recycling mirror Power-recycling Arm-cavity Michelson interferometer `Michelson´ Variable reflectivity signal-recycling Control and VRSM

Experimental Layout ~ Control of SRC via in-phase error signal shifter PBS R1 R2 R3 190 MHz Local Oscillator Laser l/2 RF output SRC FSR = 380 MHz V RSM 30 GHz Signal ~ Quadrature errror signal Lowpass filter Servo Isolator In-phase modulator l/4 Control of SRC via in-phase error signal Control of VRSM via in-quardature error signal Control and VRSM

Error Signals Control and VRSM Amplitude and phase response of a narrow linewidth cavity Amplitude and phase response of a broad linewidth cavity 1.0 1.0 0.8 Amplitude 0.8 0.6 Amplitude Phase 0.6 0.4 Phase 0.4 0.2 0.2 [Arb. units] 0.0 0.0 [Arb. units] -0.2 -0.2 -0.4 -0.4 -0.6 -0.6 -0.8 -0.8 -1.0 -1.0 -200 -150 -100 -50 50 100 150 200 -200 -150 -100 -50 50 100 150 200 Frequency [MHz] Frequency [MHz] PDH error signal for a narrow linewidth cavity PDH error signal for a broad linewidth cavity 0.5 0.4 0.4 In quadrature 0.3 In quadrature 0.3 In phase 0.2 In phase 0.2 0.1 0.1 [Arb. units] 0.0 [Arb. units] 0.0 -0.1 -0.1 -0.2 -0.2 -0.3 -0.4 -0.3 -0.5 -0.4 -200 -150 -100 -50 50 100 150 200 -200 -150 -100 -50 50 100 150 200 Frequency [MHz] Frequency [MHz] Control and VRSM

Experimental layout ~ Control of SRC via in-phase error signal shifter PBS R1 R2 R3 190 MHz Local Oscillator Laser RF output SRC FSR = 380 MHz V RSM 30 GHz Signal ~ Quadrature errror signal Lowpass filter Servo Isolator In-phase modulator l/2 l/4 Control of SRC via in-phase error signal Control of VRSM via in-quardature error signal Control and VRSM

First results... ...of a linear three mirror coupled cavity when varying the reflectivity of the VRSM from R=93% over R=87% to R=63% Theoretical frequency response Experimental frequency response 1.0 1.0 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 T/Tmax 0.5 T/Tmax 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0.0 370 375 380 385 390 395 400 405 410 370 375 380 385 390 395 400 405 410 Frequency [MHz] Frequency [MHz] Control and VRSM

Carrier/Subcarrier ~ VRSM cavity Phase modulator Power recycled Michelson interferometer Carrier laser VRSM cavity ~ In quadrature Frequency offset In phase Output modecleaner Sub-carrier laser Phase modulator Readout Control and VRSM

Frequency Offsets Control and VRSM VRSM reflectivities for some carrier-subcarrier offsets Norm. Refl. Effect on signal cavity Norm. Trans. Control and VRSM

Frequency Offsets II Control and VRSM Norm. SRC reflection 1.0 0.9 0.8 Norm. SRC reflection 0.7 Norm. VRSM reflection 0.6 0.5 0.4 0.3 0.2 0.1 Narrowband mode Broadband mode Norm. refl. Norm. refl. Control and VRSM

Summary VRSM and Control The use of a VRSM eases the change of bandwidth, avoiding long down-times of the detector A new way to control the SRC including a VRSM by using an auxiliary laser that is offset phase locked to the carrier laser This control scheme is compatible with the injection of squeezed light First experiments using a linear three mirror cavity are carrried out A more complex experiment using a dual-recycled Michelson interferometer with arm cavities is in preparation Control and VRSM