1. 1 Fabry-Perot cavity & pulsed laser J. Bonis, V. Brisson, J.N. Cayla, R. Chiche, R. Cizeron, J. Colin, Y. Fedala, G. Guilhem, M. Jacquet-Lemire, D. Jehanno, L. Losev, R. Marie, K. Moenig, V. Soskov, C. Sylvia, A. Variola and F. Zomer LAL-Orsay & Lebedev Inst.-Moscow Outline Introduction: laser needs for Compton scattering at ILC Fabry-Perot cavity, in pulsed regime R&D : description & status

2. 2 e laser  (E ≈ 1eV) EE Photon detector Photon diffusé Electron detector E e, q e dipole e diffusé Polarimeter e+ polarised source : KEK scheme Compton scattering e + laser → e +  Fit E  and/or E e, q e → e beam polarisation

3. 3 Laser characteristics Polarimeter – 1ps pulsed laser – ~ 10-100  J/pulse @ ~5MHz repetition rate –small beam waist  0 ≈ 100  m ( → spot size  0 ≈50  m) e+ polarised source (KEK + K. Moenig schemes) – 1ps pulsed laser – ~ 0.5J/pulse @ 100-350MHz repetition rate –beam waist as small as possible: few tens of  m Solution:  Fabry-Perot resonator in pulsed regime [cw laser off by oders of magnitudes…]

4. 4 Fabry-Perot cavity: Principle (CBAF & HERA cavities,cw laser) When Laser = 0  c/2L  resonance e beam L But:  / Laser = 10 -11 for Gain=10 4  laser/cavity feedback Done by changing the laser frequency Gain  10000

5. 5 Existing FP cavities in HEP (Compton experiments) Continuous laser beam –CEBAF/polarimeter - gain ≈10 4 Falleto et al. (NIMA459(2001)412) –HERA/polarimeter - gain ≈10 4 Pulsed laser beam –25ps pulses & gain ≈ 3000 Loewen (Compton X-ray source, Slac-R-632) –7ps @350MHz (laser wire) R&D in progress (Compton X-ray source & e+ polarised source) –Hiroki Sato and Kazuyuki Sakaue (POSIPOL06: http://posipol2006.web.cern.ch/Posipol2006/Links.html)

6. 6 HERA Laser cavity Cw Nd:YaG laser Cavity gain  10 4 In operation even after 3 years with strong radiations conditions

7. 7 1ps Pulsed laser Fabry-Perot cavity with Super mirrors Electron beam A priori impossible because of the laser frequency width: Dn ≈1/(1ps)=1THz for picosecond laser (c.f. 3kHz cavity banwidth for a gain of 10 4 ) In fact possible with mode-locked lasers Fabry-Perot cavity filled with a pulsed laser

8. 8 t  t=1ps Fourier transform → superposition of N longitudinal laser mode – in phase  ~1THz=1/(1ps) T=1/f rep Mode-locked laser If F.P. cavity length = laser cavity length → all modes are also resonant modes of the FP cavity = frequency comb f rep

9. 9 f 0 =f rep D  Jitter D f 0 ≈ 1 MHz → [f 0 or D  ] & f rep must be controlled even for 1ps pulses if the cavity finesse is very high Feedback bandwidth : subHz-40kHz [Femtosecond optical frequency comb technology, Ye&Cundiff, Springer 2005] Feedback for mode-locked laser beam

10. 10 Eurotev R&D at LAL/Orsay: pulsed cavity for a polarimeter [1ps, 100  J/pulse@76MHz] Locking of a Ti:sa laser (MIRA-Coherent pumped by a 6W VERDI) to a high finesse cavity (=2 spherical mirrors ): –Feedback difficult & never done for 1ps pulses + very high finesse cavity (F=30,0000-300,000) Schedule –STEP 1: Gain=10 4 → 10 5 Start: Sept. 2005  2007 –STEP 2: Reduction of the beam waist Start (thanks to LAL/IN2P3 PhD. & postdoc): Sept. 2006  2007(2008)

11. 11 3.5 m Status: optics Optics in mounting process Acquisition of a large class 10 air flow Up to now main activities: laser noise measurement Necessary for the feedback

12. 12 Laser amplitude noise measurement Just received (LAL financial support)

13. 13 Use of 23 GHz bandwidth photodiode+spectrum analyser Laser phase noise measurement (of the periodic pulsed) Quartz oscillator ordered

14. 14 Feedback status : electronics 8 ADC: 14 bits 105Msps 8 DAC : 14bits 125 Msps fpga latency=60ns Tests of hardware & programming tools : almost finished Ready to be inserted in the optical bench

15. 15 STEP 1 : 2m long stable cavity (confocal) → optomécanics (LAL): GIMBAL miroir mount in vacuum Status: end of cleaning process Installation: end of may Mechanics: status

16. 16 optical axis c c 1mm 10 mm 10  rad R = 1m laser R ≈ > L/2 ±1/e laser enveloppe R ≈ >L/2 L STEP 2: Reduction of the laser beam waist with 2 mirrors  Concentric cavity BUT for:  0 ≈200  m High qulaity coatings are good at the centre …

17. 17  X Y Y X Z 11 22 11 22 plane mirror spherical mirror plane mirror spherical mirror  =0  2D cavity.   0  3D cavity  reduction of astigmatism & light polarisation changes V1V1 V2V2 laser  0  0 when R  L 2D Ring cavity: Tolerance: spot size shift of 1mm for 100  rad, 100  m L R R 2m long concentric cavity: IF  0 =50  m, =800nm  spot size shift of 30mm on the mirrors !!! for an axial and angular mirror misalignment of 1  m and 1  rad.  Mechanical constraints very strong … A mechanical solution: Four mirrors cavity BUT problems due to: Non zero incident angles Astigmatism (spherical mirrors) Polarisation (Fresnel coeffs.)

18. 18  Numerical description: done  1rst experimental setup: low finesse with a cw laser beam width mode shape resonance frequencies Start mechanical setup in autumn 2006  High finesse cavity (inside vacuum) with pulsed laser in 2007

19. 19 Summary R&D EUROTEV at LAL/Orsay : 2005-2007 –Fabry-Perot cavity for a polarimeter 1.1ps/100fs pulses of energy 100  J/pulse@76MHz –Moderate input Ti:sa laser beam power but very high cavity finesse 30000-300000 –Development of a numeric feedback system  The experiment is being mounted 2. Ring cavity to reduce the laser beam size  1 rst exp. studies will start in september 2006 (thanks to an extra support from LAL/IN2P3) If this R&D gives satisfaction –After 2007 a new laser source (e.g doped fiber amplifiers) should be considered to match the laser power required for an e+ polarised source study (100mJ/pulse@~300MHz…) Collaboration started (april 2006) with KEK (ATF2)

20. 20 Maximum Cavity Gain achievable in pulsed regime: limited by the dispersion (=pulse time width broadening) & chromatic dependence of the reflection coefficient of the cavity mirror coatings No effect for a pulse width of 1ps: gain up to 10 5 can - a priori - be envisaged Multilayer coating model