Robert Ward NAOJ seminar, March 1, The Caltech 40m Prototype Detuned RSE Interferometer Robert Ward NAOJ seminar March 1, 2006 Osamu Miyakawa, Rana Adhikari, Matthew Evans, Benjamin Abbott, Rolf Bork, Daniel Busby, Hartmut Grote, Jay Heefner, Alexander Ivanov, Seiji Kawamura, Michael Smith, Robert Taylor, Monica Varvella, Stephen Vass, and Alan Weinstein

Robert Ward NAOJ seminar, March 1,  Signal and power enhancement using Fabry-Perot cavity in each arm  Power enhancement using Power Recycling Michelson interferometer as a gravitational wave detector  Gravitational wave detection using Michelson interferometer BS FP cavity Laser PRM BS FP cavity Laser

Robert Ward NAOJ seminar, March 1,  LIGO:Power recycled FPMI »Optical noise is limited by Standard Quantum Limit (SQL)  AdvLIGO:GW signal enhancement using Detuned RSE »Two dips in the quantum noise due optical spring, optical resonance »Has the potential to beat the SQL QND detector »Or allows quantum noise curve to be optimized in the presence of thermal noise Advanced LIGO optical configuration Detuning PRM BS FP cavity Laser GW signal Power

Robert Ward NAOJ seminar, March 1, AdLIGO noise curve Bench Active Seismic Isolation External Seismic Pre- Isolation Quadruple pendulum suspensions 40 kg, fused silica Test Masses 125W Laser Fight the Fundamental Noise Sources: 1)Seismic 2)Thermal 3)Quantum

Robert Ward NAOJ seminar, March 1, Resonant Sideband Extraction and Signal Recycling  Resonant Sideband Extraction(RSE) »Decrease storage time of GW signal in IFO –Allows high finesse arm cavities –Low power recycling, or power recycling not required –Less thermal effects  Signal Recycling(SR) »Increase storage time of GW signal in IFO –Low finesse arm cavities, or arm cavities not required –High power recycling required, so-called dual recycling(DR) –Higher thermal effects GEO600 AdLIGO LCGT LIGO VIRGO TAMA300

Robert Ward NAOJ seminar, March 1, Caltech 40 meter prototype interferometer Objectives  Develop a lock acquisition procedure for suspended-mass detuned RSE interferometer with power recycling, preferably one that will be applicable to Advanced LIGO BS PRM SRM X arm Dark port Bright port Y arm  Characterize and optimize optical configuration (for robust control and sensitivity)  Characterize noise mechanisms  Develop DC readout scheme  Test QND techniques  Extrapolate to AdLIGO via simulation  Prototyping will yield crucial information about how to build and run AdLIGO

Robert Ward NAOJ seminar, March 1, Pre-Stabilized Laser(PSL) and 13m Mode Cleaner(MC)  10W MOPA126  Frequency Stabilization Servo (FSS)  Intensity Stabilization Servo  Pre-Mode Cleaner (PMC)  13m Mode Cleaner MOPA126 FSS VCO AOM PMC 13m MC 40m arm cavity PSL Detection bench Mode Cleaner BS ITMy ITMx South Arm East Arm ETMx ETMy

Robert Ward NAOJ seminar, March 1, LIGO-I type single suspension  Each optic has five OSEMs (magnet and coil assemblies), four on the back, one on the side  The magnet occludes light from the LED, giving position  Current through the coil creates a magnetic field, allowing mirror control

Robert Ward NAOJ seminar, March 1, m Sensitivity Bench Not very likely that we’ll actually detect any gravitational waves here, but hopefully we’ll learn some things about operating interferometers, especially about the quantum noise.

Robert Ward NAOJ seminar, March 1, m DARM Optical Plant The 40m operates in a detuned RSE configuration, which gives rise to two peaks in the DARM transfer function: 1)Optical Resonance 2)Optical Spring UGF

Robert Ward NAOJ seminar, March 1, Detune Cartoon Responses of GW USB and GW LSB are different due to the detuning of the signal recycling cavity. IFO Differential Arm mode is detuned from resonance at operating point DARM Carrier frequency frequency offset from carrier [Hz] Sideband amplitude [a.u.] FWHM USB LSB f sig IFO DARM/CARM slope related to spring constant IFO Common Arm mode is detuned from resonance at intial locking point PRCCARM SRC

Robert Ward NAOJ seminar, March 1, Signal Extraction Scheme  Arm cavity signals are extracted from beat between carrier and f 1 or f 2.  Central part (Michelson, PRC, SRC) signals are extracted from beat between f 1 and f 2, not including arm cavity information.  Only +f 2 sideband resonates in combined PRC+SRC Double demodulation Central part information f1f1 -f 1 f2f2 -f 2 Carrier Single demodulation Arm information PRM

Robert Ward NAOJ seminar, March 1, DOF for length control : L  =( L x  L y ) / 2 : L  = L x  L y : l  =( l x  l y ) / 2 =2.257m : l  = l x  l y = 0.451m : l s =( l sx  l sy ) / 2 =2.15m Por t Dem. Freq. LL LL ll ll l s SPf1f APf2f SP f1  f2f1  f AP f1  f2f1  f PO f1  f2f1  f Signal Extraction Matrix (in-lock, DC) Common of arms Differential of arms Power recycling cavity Michelson Signal recycling cavity Laser ETMx ETMy ITMy ITMx BS PRM SRM SP AP PO lxlx lyly l sx l sy L x =38.55m Finesse=1235 L y =38.55m Finesse=1235 Phase Modulation f 1 =33MHz f 2 =166MHz 40m

Robert Ward NAOJ seminar, March 1, Disturbance by sidebands of sidebands  Sidebands of sidebands are produced by two series EOMs.  Beats between carrier and f 2 +/-  f 1 disturb central part. Original concept Real world f1f1 -f 1 f2f2 -f 2 Carrier f 1 =33MHz-f 1 f 2 =166MHz-f 2 Carrier PortDem. Freq. LL LL ll ll l s SPf1f APf2f SP f1  f2f1  f AP f1  f2f1  f PO f1  f2f1  f MHz 133MHz

Robert Ward NAOJ seminar, March 1, Mach-Zehnder interferometer on 40m PSL to eliminate sidebands of sidebands Series EOMs with sidebands of sidebands EOM2 EOM1 Mach-Zehnder interferometer with no sidebands of sidebands PD EOM2 EOM1 PZT PMC trans To MC Locked by internal modulation f1f1 f2f2 f1f1 f2f2 PMC transmitted to MC

Robert Ward NAOJ seminar, March 1, Real Time Digital Control System Suspension controllers Length Sensing and Control Interface to Operators Data Acquisition Fiber communication network 16kHz Sampling Rate Also have an extensive slow controls network (EPICS)

Robert Ward NAOJ seminar, March 1, Digital length sensing and control system AP166 A/D mixer D/A Provides great flexibility to try out new control/locking schemes Easy to optimize control matrix

Robert Ward NAOJ seminar, March 1, Digital Controller  Flexibility »digital filtering »smooth signal handoff  Diagnostics »arbitrary transfer functions  Reconfigurability »construct new control links, servos rapidly »quickly change MIMO servo filter  Optimization »can do automatic matrix diagonalization  Automation

Robert Ward NAOJ seminar, March 1, Automation of Routine Tasks  Using the digital control system, we use shell scripts to automate routine tasks. Restores alignment of ETM, ITM, mis-aligns other optics, sets up loop gains and control flow, and engages lock acquisition routine. Steers ITM to center transmitted beam on QPD, dithers input beam and ETM alignment, servos to minimize dither signal in power level.

Robert Ward NAOJ seminar, March 1, Control Setting Log (conlog) Constantly records all digital control settings (thousands of channels). Log files accessible through a simple web interface Useful for operators, commissioners, and data analysts.

Robert Ward NAOJ seminar, March 1, m Goal #1: Develop a Lock Acquisition procedure for AdLIGO  Version 1.0: »Basically a “Brute Force” technique »All optics are aligned, all DOFs are swinging. »Do some fast normalization, switching on/off of feedback: try to grab control without pumping too much energy into the system »Works much better with large available signals, strong actuators. »Suitability of procedure for AdLIGO to be extrapolated via simulation.

Robert Ward NAOJ seminar, March 1, Transmitted light is used as 40m Lock Acquisition part I: Off-resonant lock scheme for a single cavity Off-resonant Lock point Resonant Lock 10x higher finesse than LIGO

Robert Ward NAOJ seminar, March 1, m Lock acquisition procedure (v 1.0) Start with no DOFs controlled, all optics aligned. ITMy ITMx BS PRM SRM SP DDM 13m MC 33MHz 166MHz SP33 SP166 AP DDM AP166 PO DDM

Robert Ward NAOJ seminar, March 1, m Lock acquisition procedure (v 1.0) DRMI + 2arms with CARM offset ITMy ITMx BS PRM SRM SP DDM 13m MC 33MHz 166MHz SP33 SP166 AP DDM AP166 PO DDM Average wait : 3 minute (at night, with tickler) T =7% I Q 1/sqrt(TrY) 1/sqrt(TrX) MICH: SP33Q PRC: SP33I SRC PO133I XARM: DC lock YARM DC lock Less than 1% of maximum circulating power

Robert Ward NAOJ seminar, March 1, m Lock acquisition procedure (v 1.0) ITMy ITMx BS PRM SRM SP DDM 13m MC 33MHz 166MHz SP33 SP166 AP DDM AP166 To DARM PO DDM AP166 / sqrt(TrX+TrY) CARM DARM + + Short DOFs -> DDM DARM -> RF signal CARM -> DC signal 1/sqrt(TrX)+ 1/sqrt( TrY) CARM -> Digital CM_MCL servo All done by script, automatically

Robert Ward NAOJ seminar, March 1, m Lock acquisition procedure (v 1.0) Reduce CARM offset: 1. Go to higher ARM power (10%) 2. Switch on AC-coupled analog CM servo, using REFL DC as error signal. 3. Switch to RF error signal (POX) at half-max power. 4. Reduce offset/increase gain of CM. ITMy ITMx BS PRM SRM SP DDM 13m MC 33MHz 166MHz SP33 SP166 AP DDM AP166 To DARM REFL DARM PO DDM AP166 / (TrX+TrY) script 1900W

Robert Ward NAOJ seminar, March 1, DARM Optical response with fit to Buonanno & Chen formula DARMin1 / DARMout XARMin1 / XARMout times arm cavity pole. Yields optical response, taking out pendulum, analog & digital filtering, etc. XARM TF is understood semi-quantitatively. Offset-locked CARM also has optical spring peak, also well modeled. Anti-spring TF also well modeled.

Robert Ward NAOJ seminar, March 1, The DARM anti-spring  With SRM detuned in the wrong direction, will see an anti- spring in DARM  This is equivalent to resonating the –f2 RF sideband in SRC.  Oddly, this is also easier to lock

Robert Ward NAOJ seminar, March 1, Detune Cartoon Responses of GW USB and GW LSB are different due to the detuning of the signal recycling cavity. IFO Differential Arm mode is detuned from resonance at operating point DARM Carrier frequency frequency offset from carrier [Hz] Sideband amplitude [a.u.] FWHM USB LSB f sig IFO DARM/CARM slope related to spring constant IFO Common Arm mode is detuned from resonance at intial locking point PRCCARM SRC

Robert Ward NAOJ seminar, March 1, Simple picture of optical spring in detuned RSE Move arms differentially, X arm longer, Y arm shorter from operating point  Power X arm down, Y arm up X arm down, Y arm down X arm up, Y arm down  Radiation pressure X arm down, Y arm up X arm down, Y arm down X arm up, Y arm down  Spring constant Negative(optical spring) N/A Positive(no optical spring) DARM (Lx-Ly) Power(W) BRSE Correct SRM position Wrong SRM position X arm Y arm

Robert Ward NAOJ seminar, March 1, Changing the DARM quadrature 1.Lock IFO with CARM offset 2.Handoff DARM to RF 3.Adjust RF demodulation phase 4.Reduce CARM offset 5.This changes the quadrature of the signal. As we are not compensating for this by adjusting the demod phase, the shape of the response changes. demodulation phase b1 b2 Unbalanced Sideband Detection:

Robert Ward NAOJ seminar, March 1, Changing the DARM quadrature Squares are data, solid lines are from Optickle. Optickle results are generated by measuring response in a single quadrature while changing the CARM offset. This should be analogous to how the data was taken (reducing the CARM offset while always measuring with the same RF demod phase). Blackboard

Robert Ward NAOJ seminar, March 1, CARM optical springs Solid lines are from TCST Stars are 40m data Max Arm Power is ~80 Also saw CARM anti-springs, but don’t have that data

Robert Ward NAOJ seminar, March 1, Relationship between the CARM and DARM springs at the 40m  With the 40m Lock Acquisition scheme, we only see a CARM spring if there’s also a DARM spring. XarmYarmDARMCARM ++xx xx -++- Using the DC-locking scheme for the arms, there are, prima facie, four locking points corresponding to the four possible gain combinations, but only two will acquire lock. XarmYarmDARMCARM xx xx Correct SRM position Incorrect SRM position

Robert Ward NAOJ seminar, March 1, Will it lock? NO YES x-axis: EY position y-axis: signal blue:X err green: Y err black: DARM red: CARM modeled with FINESSE (open loop)

Robert Ward NAOJ seminar, March 1, Compensating the resonances 4kHz >> UGF no compensation AdLIGO: 180 Hz ~ UGF 40Hz < UGF no compensation AdLIGO: 70Hz? 1kHz -> 100Hz ~ UGF dynamic compensationcompensation 0->100Hz ~ UGF Not yet coherently compensated Compensation Filters for the two resonances associated with the signal cavity: OpticalOpto-mechanical DARM CARM UGFs ~ 250Hz

Robert Ward NAOJ seminar, March 1, Dynamic compensation filter for CARM servo Optical gain of CARM Open loop TF of CARM Optical gain (normalized by transmitted arm power) shows moving peaks due to reducing CARM offset. We have a dynamic compensative filter having nearly the same shape as optical gain except upside down. Designed using FINESSE. Open loop transfer function has no phase delay in all CARM offset.

Robert Ward NAOJ seminar, March 1, Mode healing/injuring at Dark Port Negative spring constant with optical spring Positive spring constant with no optical spring Repeatable The same alignment quality Carrier power at DP is 10x smaller

Robert Ward NAOJ seminar, March 1, Next steps  Stable operation and noise hunting  More lock acquisition schemes More lock acquisition schemes  Modeling/E2E simulation for AdLIGO Modeling/E2E simulation for AdLIGO  DC readout with Output Mode Cleaner DC readout with Output Mode Cleaner  Squeezed Vacuum in the Dark Port Squeezed Vacuum in the Dark Port  Active Alignment control with wave front sensors  LF RF modulation scheme  Alternatives to Mach-Zender Alternatives to Mach-Zender

Robert Ward NAOJ seminar, March 1, Other Lock Acquisition Schemes Alternative Locking Schemes are on the way!  Deterministic Locking: »Locking occurs in stages, with each stage having robust control »Each stage can (and should) lock on the first ‘fringe’, or be robust to fringes. »Transitions between stages are smooth and robust: example-PR-FPMI  Advantages: »Easier to diagnose problems »Should require less actuation potential –If we can lock a single arm cavity, we can lock the IFO. »No statistical characterization (i.e., mean-time-to-lock). 40M: 7 mN 1.3 kg test mass f/m = 5 AdLIGO 200 µN 40 kg test mass f/m =5e-3

Robert Ward NAOJ seminar, March 1, Deterministic Locking: PRFPMI  Example Procedure to lock the PRFPMI: »Mis-align PRM,SRM »Lock ARMs with DC-signal (offset) –normalized by power in recycling cavity »Lock MICH with DC-signal (offset) –dark port power normalized recycling cavity power »Slowly re-align the PRM –Stable in this stage (power in IFO fluctuates as PRM swings, but the other optics are not disturbed as this power is normalized out) »Lock PRC with SP33I »Reduce MICH offset, handoff to SP33Q »Reduce ARM offset (not done yet)

Robert Ward NAOJ seminar, March 1, e2e SIMULATION: 4Om/AdvLIGO package Comparison between real data (black) and e2e simulated data (red) of the transmitted light for both the arms (full IFO): the mirror velocities used in E2E simulation are the values obtained fitting the real data Real data have been used to estimate relative mirror velocity for both the arms: V xarm = (0.35 ± 0.13) μm/s V yarm = (0.26 ± 0.13) μm/s E2E real data Tr X Tr Y

Robert Ward NAOJ seminar, March 1, E2E DARM TF to I and Q 5W Input Arms controlled with POX, POY (no DARM) no MICH control Hiro Yamomoto

Robert Ward NAOJ seminar, March 1, Optickle: Frequency Domain IFO Simulation  Optickle is a new frequency domain IFO modeling tool: »Written in Matlab –Matlab allows easy integration to other modeling efforts (a frequency- domain e2e, like LinLIGO) –Easily Extensible –Uses Matlab classes for generality »Uses the methods outlined in T. Corbitt et al: “Mathematical framework for simulation of quantum fields in complex interferometers using the two-photon formalism” ( LIGO-P R ) to calculate the IFO opto-mechanical frequency response. »Designed for concrete units (Watts, meters, Hz)

Robert Ward NAOJ seminar, March 1, Optickle example: detuned FP cavity  Response of front mirror to back mirror ‘excitation’  1 nm detune  finesse ~ 1200

Robert Ward NAOJ seminar, March 1, Optickle Example: AdLIGO  Easy to create a frequency dependent coupling matrix, useful for, e.g., estimating the contribution of loop noise to DARM.  This plot is Open Loop. Closed loop coming soon!

Robert Ward NAOJ seminar, March 1, DC Readout Quantum Noise: Heterodyne vs Homodyne Quantum noise curves plotted using formulas in: A. Buonanno, Y. Chen, N. Mavalvala, “Quantum noise in laser-interferometer gravitational-wave detectors with a heterodyne readout scheme” PHYSICAL REVIEW D 67,

Robert Ward NAOJ seminar, March 1, What is DC Readout and how does it relate to Homodyne Detection? DC Readout is Homodyne detection, using light circulating in the interferometer as a local oscillator. Advantage: LO light has been filtered by the <1Hz coupled cavity pole, Spatially perfectly matched to signal sidebands Disadvantage: limited ability to control homodyne phase OMC

Robert Ward NAOJ seminar, March 1, RF vs DC oPhase modulate the input light oRF sidebands act as local oscillator for GW signal, after passing through (unstable) recycling cavity(ies) oGW signal is an audio frequency sideband of RF photocurrent oMix GW signal down to near- DC oAcquire GW signal at DC with ADC Eliminate the RF sidebands at Dark Port with an Output Mode Cleaner Eliminate junk light at the Dark Port with Output Mode Cleaner Carrier light acts as a local oscillator GW signal is an audio frequency photocurrent Acquire GW signal at DC with ADC

Robert Ward NAOJ seminar, March 1, Technical noise sensitivity Noise SourceRF readoutDC readout Laser frequency noise ~10x more sensitive Less sensitive since carrier is filtered Laser amplitude noise Sensitivity identical for frequencies below ~100 Hz; both driven by technical radiation pressure x more sensitive above 100Hz Carrier is filtered Laser pointing noiseSensitivity essentially the same Oscillator phase noise -140 dBc/rtHz at 100 Hz NA

Robert Ward NAOJ seminar, March 1, Making the DC local oscillator  Two components »Carrier field due to loss differences (not controllable? TCS?) »Carrier field due to dark fringe offset (controllable) »An output mode cleaner should take care of the rest. (RF sidebands, junk light)  Loss mismatch component »Average arm round trip loss: 200 ppm »Difference between arms: 50 ppm »Output power due to mismatch: 20 µW  Detection angle, β »Tuned by adjusting fringe offset –Can tune from 0-80 deg with 0-10pm of DARM offset –1 mW LO »Angle of GW is frequency dependent in detuned RSE Loss mismatch fringe offset β LIGO I GW parallel to DC offset Detuned RSE: GW signal gets f- dependent phase shift in SRC No slope Some linear component

Robert Ward NAOJ seminar, March 1, OMC Properties The Output Mode Cleaner filters the light coming out of the dark port, cleaning away the junk and transmitting the GW-signal containing TEM00 mode of the carrier

Robert Ward NAOJ seminar, March 1, Output mode cleaner, DC PD OMC: Monolithic, copper, 4-mirror design. DCPD: Monolithic, electronics in vacuum nipple

Robert Ward NAOJ seminar, March 1, Output Optic Chamber Mike Smith from SRM to AS RF beamline (roughly 1/3 of AS power) also a convenient path for autocollimator beam, for initial alignment in air to OMCR beamline to OMCT beamline from PSL to IMC IMCR, IMCT, and SP beamlines 2 nd PZT steering mirror PZT steering mirrors and their controls are duplicates of a pair that we have already installed and commissioned for steering from IMC to main IFO (in-vac); controls are fully implemented in the ASC system (by Rolf). Similar systems can be used for “LIGO I.V”. Piezosystem Jena PSH 5/2 SG-V, PZT tilting mirror mount with strain gauge, and associated drivers and power supplies Existing in-vac seismically isolated optical table (OOC) Mike Smith has designed a compact, monolithic MMT, similar to our input MMT, using spherical mirrors. 4-mirror monolithic OMC. Pair of DC PDs with in-vac electronics on monolithic base.

Robert Ward NAOJ seminar, March 1, Squeezing Tests at the 40m  Audio frequency squeezed sources now available at MIT  Time to take steps toward eventual implementation on long baseline interferometers »Homodyne detection along with ifo signals and noise couplings –Most interesting and relevant for complex ifo configurations »A few interferometer configurations possible –narrow- or broadband RSE, DRMI, FPMI »Noise coupling studies possible »LIGO-like control systems for eventually porting squeezing technology to long baseline ifos 1kHz 10kHz 100kHz 1 10

Robert Ward NAOJ seminar, March 1, Interface to ongoing 40m experiments

Robert Ward NAOJ seminar, March 1, Possible Squeezing layout

Robert Ward NAOJ seminar, March 1, Exploring alternatives to the Mach-Zender  With two EOMs in series to generate PM sidebands at f 1 and f 2, get “sidebands on sidebands” at f 1 ± f 2  We use MZ to generate sidebands in parallel f1f1 f2f2

Robert Ward NAOJ seminar, March 1, Synthesize MZ?  The MZ adds complexity and noise. Can we do it with one EOM?  The PM waveform that must be input to a single EOM to produce f 1 and f 2 with no f 1 ± f 2 :  required waveform: Synthesized:

Robert Ward NAOJ seminar, March 1, The synthesized waveform does the job!  BUT it can’t be done with pure PM; it involves complex coefficients (AM).

Robert Ward NAOJ seminar, March 1, Can it be done with pure PM?  Using only real sines and cosines:  Doesn't work: -4f and -6f are suppressed, but +4f and +6f are not.

Robert Ward NAOJ seminar, March 1, Carrier 33MHz 166MHz ITMy ITMx BS PRM SRM OSA DDM PD DRMI lock using double demodulation with unbalanced RF sideband in SRC Carrier 33MHz Unbalanced 166MHz Belongs to next carrier Belongs to next carrier Belongs to next carrier OSA

Robert Ward NAOJ seminar, March 1, L- optical gain with RSE peak Measured in June 2005 Optical gain of L- loop DARM_IN1/DARM_OUT,divided by pendulum transfer function No offset on L- loop 150pm offset on L+ loop Optical resonance of detuned RSE can be seen around the design RSE peak of 4kHz. Q of this peak is about 6. Design RSE peak ~ 4kHz

Robert Ward NAOJ seminar, March 1, Double Demodulation  Double Demodulation used for l +, l -, and l s  Demodulation phases optimized to suppress DC and to maximize desired signal Demodulation Phase of f2 Locking point

Robert Ward NAOJ seminar, March 1, Laser Intensity Noise  calculated using rsenoise  10 pm DARM offset for DC  1e-13m residual L- RF: noise sidebands of RF sidebands beat against residual length offset DC: dark port power proportional to input power Radiation pressure effects not included

Robert Ward NAOJ seminar, March 1, Laser Frequency Noise  calculated using rsenoise  10 pm DARM offset for DC  1e-13m residual L- RF: frequency noise sidebands of RF sidebands beat against static carrier contrast defect DC: Arm cavity pole imbalance couples carrier frequency noise to dark port Radiation pressure effects not included

Robert Ward NAOJ seminar, March 1, How much power inside arm? DesignMeasured(estimated) Cavity reflectivity93%85%(X arm 84%, Yarm 86%) PRM reflectivity93%92.2% Loss in PRC0%2.3% Achievable PRG CouplingOver coupledUnder coupled Input power0.1W1W Power in one arm560W1900W Optical spring23Hz41Hz

Robert Ward NAOJ seminar, March 1, GW readout, Systems Noise SourceRF readoutDC readout Laser frequency noise ~10x more sensitive Less sensitive since carrier is filtered Laser amplitude noise Sensitivity identical for frequencies below ~100 Hz; both driven by technical radiation pressure x more sensitive above 100Hz Carrier is filtered Laser pointing noiseSensitivity essentially the same Oscillator phase noise -140 dBc/rtHz at 100 Hz NA DC rather than RF for GW sensing »Requires Output Mode-Cleaner to reject RF »Offset ~ 1 picometer from dark fringe can tune from 0 to 80 deg with mW of fringe offset power Loss mismatch fringe offset β