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Fabry-Perot cavity for the Compton polarimeter

The goal of this project is to achieve 10-100 mJ per pulse at a 5 MHz repetition rate using a small diameter Fabry-Perot cavity. The cavity, based on the HERA design, will provide a gain of approximately 10,000. Advantages include compactness, precise frequency control, and laser power concentration at the interaction point. The proposal involves a pulsed laser filled FP cavity with super mirrors. Mode-locked lasers enable resonance with the cavity, facilitating energy gain. Research and development aim to enhance cavity finesse and explore quasi-concentric cavities. Operating cavities at CEBAF and HERA demonstrate feasibility for Compton polarimetry. The high-finesse cavity aims to improve polarization measurements. Laser/cavity feedback control and stabilization systems are essential for effective operation.

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Fabry-Perot cavity for the Compton polarimeter

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  1. Fabry-Perot cavity for the Compton polarimeter Goal: 10-100 mJ/pulse @ 5MHz repetition rate & small diameter ≈ 50mm (c.f. P. Schuler’s talks)

  2. Gain 10000 Fabry-Perot cavity: Principle (HERA cavity, cw laser) e beam L Polar. Circ. Polar. Lin. When nLaser =n0c/2Lresonance • But :Dn/nLaser = 10-11 for Gain=104 laser/cavity feedback • Done by changing the laser frequency

  3. Some of the advantages of using a FP cavity • Compact (& cheap) system compared to a laser of same power (500W in average) • Laser power small outside the cavity: full power only at the electron-laser IP • no thermal effects producing parasitic birefringence & high quality frequency controlled beam accurate control of the laser beam polarisation

  4. Proposal: Cavity filled with a pulsed laser for a Compton polarimeter at FLC ≈5MHz / ≈10 nJ/pulse Electron beam Ti:sa oscillator 500 fs-1ps Pulse laser Fabry-Perot cavity with Super mirrors • A priori impossible because the laser frequency width • Dn ≈1/(1ps)=1012Hz for picosecond laser (c.f. 3kHz cavity banwidth) • In fact possible with mode lock lasers • Jones et al. Opt. Lett. 27 (2003) 1848, Jones at al. Phys. Rev. Lett. 15 (2001) 3288, • Hood et al. Phys. Rev. A64 (2004)033804, Potma et al. Opt. Lett. 28 (2003)1835

  5. Mode lock laser Dt=1ps ≈10 ns t Fourier transform→superposition of N longitudinal laser mode – in phase Dn~1012 Hz=1/(1ps) n Available laser pulse energy: 1-10nJ cavity Gain ≈104 If F.P. cavity length = laser cavity length all modes are also resonant modes of the FP cavity

  6. Cavity gain R.J. Jones et al. Opt. Lett. 27 (2003) 1848 • Pulse width limited by dispersion in the super-mirror coatings (Nb round trips=F/(2p) ≈ 5000 for F=30000  Gain ≈10000): circulating pulse gets broader and broader power loss when overlapped to the incoming pulses (constructive interferences reduced) Width : 300fs-1ps for gain=104

  7. Reduction of the laser beam size at the IP • To get a 50 mm laser beam size at the electron-laser beam IP • Use of a quasi-concentric cavity (mirror curvature radius ≈ half cavity length) • BUT,mechanical tolerance mm & mradneeded on relative mirror positions • Active feedback on relative mirror position needed (c.f. LIGO & VIRGO where nm tolerances are reached)

  8. Present status of FP cavities filled with fs pulses • Power amplification ≈ 120 and cavity Finesse ≈ 300 for pulse width 2-3ps (Potma et al. Opt. Lett. 28 (2003)1835 ) • Proposed R&D: • Reach aFinesse ≈ 30000in a first step • And using a quasi-concentric FP cavity in a second step

  9. Cavities in operation (for Compton polarimetry) • CEBAF (N. Falletto, NIM A459(2001)412): F≈24000 • HERA (upstream the HERMES experiment): F≈30000 • Installation: 2003 summer • Laser & controllers dismounted after synch. rad. damages (huge, generated by 2 new dipoles in HERMES) • Presently: strong shielding and re-mounting • after 1 year of radiation, cavity finesse is still the same and locked again …

  10. ellipsometer 4 motorised miroirs bellow Optique input ligne HERA CAVITY

  11. 2003 installation shielding (3 mm pb) HERA CAVITY

  12. Conclusion • Proposal: a high finesse FP cavity filled with a pulse laser to produce 100mJ/pulse @5MHz • Will contribute to a high precision on the polarisation measurement • This proposition make sense if the polarisation is to be measured bunch by bunch • If not, commercial laser with low rep. rate & high pulse energy do exist • But, this R&D may also be useful for other applications related to FLC (e.g. polarised positrons)

  13. Laser/cavity feedback • similar to cw laser case (Jones et al., Opt. Comm.175(2000)409) • Stabilisation channels, e.g. MIRA (Coherent) Ti:sa oscillator • 3 channels: 2 PZT mounted 2 mirrors & output coupler mounted on translation stage • High frequency correction signal by an EOM if required • Phase velocity & group velocity must be matched to the cavity (bothpulse-round-trip/pulse-repetition matching and frequency matching are required) • A priori not a problem for 0.3-1ps pulse width but precise feedback techniques are known if needed

  14. Aservissements

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