The VIRGO detection system Paolo La Penna European Gravitational Observatory

VIRGO interferometer

Dark fringe (improved shot noise S/N ratio) DETECTION SYSTEM Aim: Detecting the gravitational wave signal by measuring accurately the output power of the interferometer. Principle: Dark fringe (improved shot noise S/N ratio) Schnupp technique (partially transmitted sidebands (6.26 MHz) beating with the carrier) Spatial filtering (isolate the TEM00 mode with the Output Mode Cleaner): Much smaller photodiode dynamics (1 W, 16 photodiodes) Increase the contrast of the interferometer to reach the optimal sensitivity. Additional functions: Interferometer locking Automatic alignment

WHY AN OUTPUT MODE CLEANER Interferometer on the dark fringe to improve the shot noise S/N But: contrast C < 1 (mirror imperfections, different Rcurv, misalignments, …) If contrast C < 1  modulation index m has to become larger to contain S/N losses m larger  more transmitted power (more sidebands T ) In order to keep the losses (S/N)LOSS <10%, with C 10-2 , G Rec.Gain=50  m 1.1  1.4  PTRANS  25%  75% PIN  2.5  7.5 W Limit on PMAX on photodiodes: too many photodiodes are necessary The contrast C has to be improved  Output Mode Cleaner (OMC)

Output Mode Cleaner

VIRGO OMC requirements FP cavity for spatial filtering: C improvement of a factor 100  F  50 Triangular (no feedback back into the ITF) Transmit TEM00 for carrier and sidebands (6.26 MHz) either very long (two contiguous Airy peaks) or very short cavity (same Airy peak) Chosen a short cavity (about 5 cm): more compact,monolithic (less alignment problems)

VIRGO OMC TOWER

VIRGO BEAM SCHEME Four optical benches: The suspended detection bench The external detection bench, The north external bench, The west external bench. Six beams: B1 (OMC transmitted beam), B1p (before entering the OMC), B1s (OMC reflected beam), B5 (beam reflected by the BS second face), B7, B8 (beam transmitted by NE and WE mirrors).

Suspended and External Detection Benches Air Vacuum 10-6 mbar On ground Suspended

Suspended Bench Local Control The OMC suspended bench is controlled locally: Four leds send light to four mirrors on the bench from four different directions The mirrors send the reflections to a CCD camera The CCD software selects the four zones of the spots and reconstructs the bench movements Corrections are sent to the bench coils The leds are leaning on the tower frame (on ground) The CCD is leaning on ground

Suspended and External Benches Layout B1: OMC transmission (DF signal) B1s: OMC reflection B1s B1 B1p B5 B1p: DF before OMC (lock acq) B5: PR cavity power

Suspended Bench Layout (OMC bench) Telescope (L1+L2+M1+M2+prism): separation of the 2 input beams (B1 and B5), adaptation of the spot size (from 2 cm to 1 mm), bench alignment Quadrant photodiodes (DQ1, DQ2): bench alignment with the input beam B5 Lens L3: adaptation of the spot size B1 before entering into the OMC (from 1 mm to 140 μm) OMC: Output Mode Cleaner Faraday: prevents back reflections

B1s B1 B1p Material: fused silica Length  2 cm Optical path  7.5 cm OMC Characteristics B1s Material: fused silica Length  2 cm Optical path  7.5 cm Finesse: 50 FSR  2 GHz  40 MHz RCURV = 30 cm Waist = 140 mm Obtained matching  94% Losses  1% B1 B1p

The cavity is monolithic: OMC Characteristics The cavity is monolithic: The input beam has to be elliptical to match the cavity. Matching prisms are used to match the cavity waist

OMC Locking OMC locking on TEM00: Via temperature control. Three signals are used to reach and keep a stable lock: The spot shape (a non-gaussian shape means that other modes are transmitted), The power transmitted by the OMC (the power is maximum when the cavity is resonant for the TEM00), The Pound-Drever signal (this signal is achieved thanks to a piezo excitation of the cavity at 28 kHz, on the upper face).

OMC assembling

TEM00 OMC temperature is scanned (2°=1 FSR in 1000 sec) OMC Locking TEM00 OMC temperature is scanned (2°=1 FSR in 1000 sec) Peak detection: a CCD compares the image with the expected TEM00 2 test is performed (10 times/sec) When 2 is below a threshold, starts the linear temperature feedback (less than 0.1 Hz bandwidth)

Alignment and matching is performed once OMC alignment Alignment and matching is performed once Q1 and Q2 quadrants are used as a reference with B5 beam Alignment is performed automatically when Q1 and Q2 asymmetry exceeds a threshold

The External Detection Bench

Final External Detection Bench 16 photodiodes, 1 CCD camera for B1 2 quadrant photodiodes for automatic alignment 2 photodiodes, 1 CCD camera for B1p/B1s/B5

Beams with recycled interferometer Present EDB Setup Beams with recycled interferometer Input power  700 mW (10% attenuation of Pin) B5 3.18 mW B1p 11.9 mW B1 2.7 mW B1s 0.18 mW

Locking procedure: OMC is locked after the ITF locking: OMC and ITF control Locking procedure: OMC is locked after the ITF locking: B1p (before OMC , 1% of Dark Fringe) is used for locking acq After ITF locking, the OMC is locked on the TEM00 (beam B1) After OMC locking, the ITF control is moved to B1 (OMC transmission) OMC has been locked with The Second Stage of Frequency Stabilization (ITF common mode feedback to laser frequency) PR locking Full hierarchical control

The detection system is very reliable and robust: Performances The detection system is very reliable and robust: The lock is acquired within 10 minutes (usually a few minutes) Switch to B1 control does not cause any problem Automatic procedures have been implemented for realignment and relocking when golden state condition is lost or close to be lost Lock is robust: no OMC unlock happens Length precision: about l/60,000 (10-11 m, rms on 1 h ; requirements is l/3,000)

Difference with lock on OMC VIRGO is not limited by shot noise yet: some HF difference is due to the fact that B1p 0.5% of the dark fringe (more amplification, more electronic noise) Sensitivity with switch on B1 (Feb 21 2004) Sensitivity with switch on B1p (Feb 21 2004)

Low frequency B1 and B1p spectrum are different at low frequencies for several reasons: B1p contains higher order modes The low frequency angular motions couple to B1 through the OMC and are reinjected by the control as length noise L : This length noise slightly cancels the effect of angular noise on B1 This length noise is measured by B1p B1_ACp might not measure the real L: B1p can be better estimate of L The real L must be between B1 and B1p estimate B1p_ACp B1_ACp

Summary The OMC of VIRGO is a small triangular monolithic cavity; The cavity is mounted in vacuum on a suspended bench; The bench is locally aligned The OMC is locked with thermal actuators; The OMC has worked satisfactory in the CITF and in VIRGO; The recycled VIRGO has been locked using the OMC transmission; No major problem with the OMC has been detected yet: the system is robust and reliable, and it is operated routinely with automatic procedures; Since VIRGO is still limited essentially by control noise and not by shot noise, no significant improvement in using the OMC locking is visible yet; However, the signal and the beam are clearly cleaner and the cleaning effect of the OMC is evident.