Fabry-Perot cavity for the Compton polarimeter

Goal: 10-100 J/pulse @ 5MHz repetition rate

& small diameter ≈ 50m (c.f. P. Schuler’s talks)

Fabry-Perot cavity: Principle (HERA cavity, cw laser)

When Laser =0 c/2L resonance

e beam

Polar.Lin.

Polar.Circ.

L

•But : /Laser = 10-11 for Gain=104 laser/cavity feedback

•Done by changing the laser frequency

Gain 10000

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

Ti:sa oscillator500 fs-1ps Pulse laser

≈ 5MHz / ≈10 nJ/pulse

Fabry-Perot cavitywith Super mirrors

Electron beam

Proposal: Cavity filled with a pulsed laser fora Compton polarimeter at FLC

•A priori impossible because the laser frequency width ≈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

t

t=1ps

Fourier transform →superposition of N longitudinal laser mode – in phase

~1012 Hz=1/(1ps)

≈10 ns

Mode lock laser

If F.P. cavity length = laser cavity length all modes are also resonant modes of the FP cavity

Available laser pulse energy: 1-10nJ cavityGain ≈104

• Pulse width limited by dispersion in the super-mirror coatings (Nb round trips=F/(2) ≈ 5000 for F=30000 Gain ≈10000): circulating pulse gets broader and broader power loss when overlapped to the incoming pulses (constructive interferences reduced)

Cavity gain

Width :300fs-1psfor gain=104

R.J. Jones et al. Opt. Lett. 27 (2003) 1848

Reduction of the laser beam size at the IP

• To get a 50 m laser beam size at the electron-laser beam IP– Use of a quasi-concentric cavity (mirror curvature radius ≈ half cavity length)– BUT, mechanical tolerance m & rad needed

on relative mirror positions– Active feedback on relative mirror position

needed (c.f. LIGO & VIRGO where nm tolerances are reached)

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 a Finesse ≈ 30000 in a first step– And using a quasi-concentric FP cavity in a

second step

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 …

Optique input ligne

ellipsometer

HERA CAVITY

4 motorised miroirs

bellow

2003 installationshielding (3 mm pb)

HERA CAVITY

Conclusion• Proposal: a high finesse FP cavity filled with a pulse laser

to produce 100J/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)

• 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 (both pulse-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

Aservissements