CALVA : A test facility for Lock Acquisition

1. CALVA : A test facility for Lock Acquisition F.Cavalier1 on behalf of CALVA team : M.-A.Bizouard1, V.Brisson1, M.Davier1, P.Hello1, N.Leroy1, N.Letendre2, V.Loriette3, I.Maksimovic3, A.Masserot2, C.Michel4, B.Mours2, L.Pinard4, F.Robinet1, M.Vavoulidis1, M.Was1 1) LAL, Université Paris-Sud, IN2P3/CNRS, F-91898 Orsay, France 2) Laboratoire d'Annecy-le-Vieux de Physique des Particules (LAPP), IN2P3/CNRS, Université de Savoie, F-74941 Annecy-le-Vieux, France 3) ESPCI, CNRS, F-75005 Paris, France 4) Laboratoire des Matériaux Avancés (LMA), IN2P3/CNRS, F-69622 Villeurbanne, Lyon, France • Principle and Layout • Status of the infrastructure • First lock acquisition with the short cavity • Perspectives

2. CALVA motivation • The acquisition of lockfor advanced detectors will be a crucial problem : • more coupled dofs with Signal Recycling • higher finesse for FP cavities • effect of radiation pressure • question about the maximal possible force, acquisition vs sensitivity problem • Set-up a middle-scale infrastructure dedicated to Locking R&D for Advanced Virgo and beyond (CALVA stands for “Cavité(s) pour l’Acquisition du Lock de Virgo Avancé”)

3. CALVA Principle Lock the long cavity using an auxiliary laser with a different wavelength and bring it to the main laser resonance in a deterministic way • The mirror reflectivity seen by the auxiliary laser can be much lower than for the main laser • the cavities have a lower finesse • easier lock acquisition • requires less force (FMax F ) • the cavities are much less coupled

4. Infrastructure status FP1 FP2 Vacuum Pipe 5.5 m Optical Table Vacuum tanks for mirrors 7 m Room 2 Room 1 5.5 m 45 m 10 m • Two rooms operational: • Cleanliness between 10000 and 100000 • Class 10 air flux in each room • 1o C stability • Vacuum tanks: • Connected by a 25-cm diameter tube • Reached pressure: 10-6mbar • Housing a 80x80 cm2 breadboard

5. CALVA set-up • Lasers from Innolight: • 1 W @ 1064 nm  same radiation pressure effect than in AdV • 100 mW @ 1319 nm • Electronics & Software: • LAPP components built for Virgo+ (ADC, Timing system, Optical Links, Control software and DAQ) • Other parts homemade or commercial • Control loops running at 10 kHz • Mirrors coated by LMA

6. Suspensions and Local control performances Local control laser Local control bench • Motion of free mirror in quiet conditions: • z ~ 1-2 microns • q ~ 10-20 microradians • Local Controls Range: • z ~ 400 microns • q ~ 0.5 milliradians • Local controls sensitivity: • z ~ few tens of nanometers • q ~ fraction of microradians

7. Lock of the short high finesse cavity • Parameters: • L = 5 m • ROC1=ROC2 = 33 m (mirrors foreseen for the long cavity) • Reflectivities at 1064 nm: • R1 = 0.9909 • R2 = 0.99974 • Reflectivities at 1319 nm: • R1= 0.3 • R2 = 0.55 • 1064-nm laser operated at low power (~200 mW) in order to avoid radiation pressure effects  F= 680  F= 3.3

8. Lock sequence: • cool angular motion with local controls • cool longitudinal motion with local controls • release local controls on z and switch on the cavity lock using DC (reflected or transmitted) for 1319 nm laser (with F= 3.3, DC and Pound-Drever signals are quite similar) • When locked on 1319 nm, typical motion for the cavity is about 3 nm (supposing that error signal is due to mirror motion) • No coherence has been seen with laser power or angular motion of the mirrors • Coherence with frequency noise has to be evaluated • Resonance crossing time for 1064-nm laser is about 100 ms Transmitted DC power @ 1319 nm M1 z-motion M2 z-motion

9. Lock sequence (cont’d): • Wait for resonance crossing for 1064-nm laser due to natural frequency drifts of the two lasers (trigger on transmitted power). Controlled search for the resonance to be implemented acting on DC offset or 1319-nm laser frequency • Switch on 1064-nm laser Pound-Drever signal Reflected DC power @ 1319 nm Reflected PD signal @ 1064 nm Transmitted DC power @ 1064 nm Trigger level

10. Lock routinely obtained with this procedure even starting with very excited mirrors • Error signal amplitude corresponds to a motion of 100 picometers • Force for lock acquisition has the same order of magnitude as the force needed to keep the lock on 1064-nm laser. More statistics must be acquired • Error signal spectrum to be understood

11. Coherence seen with angular motion and laser power • Coherence with frequency noise to be evaluated

12. Next steps • Characterization of lock and power increase of the main laser (up to June) • Installation of the 50-m cavity • Lock of long cavity with same optical parameters (this summer) • Addition of the short cavity (F~ 15) • Lock of coupled cavities (this autumn) Needs for Advanced Virgo • Check that it is still mandatory for Adv (recent change of finesse) • Frequency of auxiliary laser has to be stabilized. Two paths under evaluation : • rigid cavity • fiber ITF

13. Conclusion • CALVA infrastructure has been set-up and is working properly • First lock has been acquired using an auxiliary laser on a short (5 meters) cavity with a finesse about 700 for the main laser and 3 for the auxiliary laser • Longer cavity and coupled cavities will be locked before the end of the year • Application to AdV under reevaluation • Use of Laguerre-Gauss beams as possible next step in collaboration with APC