High finesse multimirror optical cavities with feedback 1
High finesse multi-mirror optical cavities with feedback 1. Fabry-Perot cavity in cw mode: feedback & optical issues 1. Comparison with Sapphire parameters 2. Fabry-Perot cavity in pulsed mode 3. Present R&D on optical cavities at LAL F. Zomer, 19, march, 2013 1
Fabry-Perot cavity: Principle with continous wave e beam Gain=1/(1 -R) 10000 ~10 k. W ~1 W LASER isolateur ~1 W JLAB/Saclay Polarimeter, NIMA 459(2001)412 HERA /Orsay Polarimeter, JINST 5(2010)P 06005 When n. Laser c/2 L • But: Dn/n. Laser = 10 -11 feedback needed… résonance STRONG & ROBUST laser/cavity 2
Illustration of one issue : the laser cavity feedback Sapphire Cavity finesse : F~100 p Optical path length : L~150 m Cavity resonance frequency linewidth Dn=c/(LF)~6 k. Hz ! Dn/n=l/(LF)=~10 -11 -10 -12 Same numbers as in metrology !!! DL=l/F M. Oxborrow 3
From a feedback point of view: Locking a ‘ 150 m’ cavity of finesse~ 100 p(‘gain’~100) is the same as Locking 0. 2 m cavity to 300000 finesse ! BUT The hyper stable small cavity is ‘hyper’ temperature stabilised Into an hyper isolated room For Sapphire & Compton machines ü‘Geant’ mechanical structure üNoisy accelerator environment ü Pulsed laser beam regime Put on an hyper stabilised optical table And an hyper stable LOW POWER cw laser is used, ü� 1 k. Hz linewidth oscillator linewidth 1 k. Hz üHuge average & peak power ! http: //www. innolight. de/index. php? id=mephisto M. Oxborrow 4
An Optical issue Small laser beam size +stable resonator 2 -mirror cavity Stable solution: 4 -mirror cavity as in Femto laser technology Laser input BUT elliptical & linearly polarised eigen-modes which are instable because of vibrations at very high finesse e- beam Non-planar 4 -mirror cavity Stable & circularly polarised eigenmodes (AO 48(2009)6651) as needed for an ILC polarised positron source 5
Optical issues for ‘focusing’ resonators w 0~7µm for sapphire (? ) ØMode focusing strong ellipticity/astigmatism ØNon-planar 4 -mirror resonator & ‘strong’ focusing general astigmatism (Arnaud, Bell Syst. Tech. ( 1970)2311) Complex mode structure F Labaye/LAL 50 cm Carreful optical design optimize mode shape at the IP optimize mode polarization F Labaye/LAL 6
Fabry-Perot cavity in pulsed regime Electron beam 1 ps Mode lock oscillator Fabry-Perot cavity with Super mirrors Same feedback technics (more complexe) is used in cw & pulsed regime Well known techniques (analog and numerical) 7
Pulsed laser/cavity feedback technique T=2 p/wr Specificity properties of passive mode locked laser beams Frequency comb all the comb must be locked to the cavity Feedback with 2 degrees of freedom : control of the Dilatation (rep. Rate) & Translation (CEP) Dfce wn= nwr+w 0 n~10 6 T. Udem et al. Nature 416 (2002) 233 8
State of the art (Garching MPI) : ~70 k. W, 2 ps pulses @78 MHz (F~5600) stored in a cavity (O. L. 35(2010)2052) ~20 k. W, 200 fs pulses @78 MHz From a feedback point of view: Locking a ‘ 150 m’ cavity to finesse~ 100 p(‘gain’~100) @ 350 nm is the same as Locking a 4 m cavity @ 800 nm to ~25000 finesse R&D done at Orsay 2 ps Tis: apph 76 MHz oscillator (~0. 2 nm spectrum) cavity finesse ~28000 9
Orsay setup: Picosecond/High Finesse MIRA Driver VERDI 6 W 532 nm M 1 MOTOR AOM EOM M 2 PZT Driver 2 -Mirror Fabry-Perot cavity Finesse ~ 28000 Amplifier AOM +/- Driver grating SLITS PDH #1 Front end Pound-Drever-Hall Scheme Transmission Signal +/- Serial RS 232 Laser Length Control Laser Δφce Control DAQ Feedback PDH #2 Front end TRANS Front-end
ØWe locked the laser to the cavity ØBut we observed strong free running laser/cavity coupling variations (Finesse~28000) Laser/cavity coupling 25% coupling variation over ~15 min Stacked power variations up to ~60% ‘noisy’ Stacked power (~7%) Feedback bandwidth ~100 k. Hz BW up to a few MHz on the rep. Rate. needed to reduce the noise 11
CEP effects measurement in picosecond/high finesse regime CELIA, LAL, SZEGED Univ. ü 2 ps Ti: Sapph (75 MHz) Locked to a ~28000 finesse cavity üNo control of the CEP drift in the feedback loop Numerical feedback loop üBW=100 -200 k. Hz üBW ~1 MHz needed üCEP measured with Szeged interferometer CEOLi. T measurement of Carrier Envelope Offset Phase Drift by a Linear Transmission Ring 12
We observed strong free running laser/cavity coupling variations (Finesse~30000) CEP measurement Laser/cavity coupling Fit: Frequency comb +Dfce variations 25% coupling variation over ~15 min Here 80% of the laser power is coupled high quality wave front needed Only 3 free parameters in the fit: a normalisation, an offset the Finesse 13
Variation of the pump power laser/cavity coupling measurement effective enhancement factor CEP measurement F=45000 Measured enhancement factor F=15000 Freq. Comb fit (0. 2 nm width !) With F~28000 F=3000 60% enhancement factor variation if CEP phase � [0, 2 p] for 2 ps & ~28000 Finesse CEP phase must be also controled in high Finesse/picosecond regime 14 Feedback loop BW must be>100 k. Hz
Some laser oscillator issues üAt present increasing the average power @ frep>2 MHz rep rate Yb fiber technology üNeed to find/build a low noise laser oscillator üCEP and rep. Rate locking required üPossible feedback BW imitations using a 2 MHz laser oscillator ( R&D on the oscillator & optical reference) üPresent R&D with Yb fiber oscillators (frep>100 MHz) üCELIA-LAL R&D ü 2 commercial Yb (fiber) lasers üFully connectorised (robust) fiber amplifier ü 50 W(100 W) at present ( 200 W for Thom. X, see A. Variola) 15
Bordeaux-Orsay R&D Stable oscillator (Origami Onefive) 0. 2 W, 1030 nm Dt~0. 2 ps frep=178. 5 MHz ~10 W 50 W 100 W Amplifier(s) AOM photonic fiber Yb Doped 1 p 1 t iezos e 1 A mp. OM Ctr 1 piezo 4 -mirror Fabry-Perot cavity Gain~1000 8000 l. Highly ‘tunable’ oscillator (Orange Menlo) 1 p 1 E iezo 0. 02 W, 1030 nm 1 t OM r Dt~0. 2 ps frep=178. 5 MHz 1 AO ans. S tag 1 p M e u cur mp ren t 10 MHz feedback bandwidth needed… Mixed Analog-Numerical feedback ATF clock o. Setup required feedback (10 k. Hz 10 MHz BW) o. Setup a robust fiber amplifier o. Study noise induced by the amplifier o. Push the cavity stored power at maximum 16
Summary • Fabry-Perot cavity – Advantages • • Very high gain (eventually) ‘easy’ laser-electron synchronization Stable transverse & longitudinal modes Though painful, laser/cavity feedback techniques are well know – Disadvantages for Sapphire • Very long cavity – technical noise (? ) – Tight feedback as difficult as a highest finesse table top experiment… » (BW may be limited by the laser frep) • Very small laser waists & circ. Polar. (? ) careful optical design of the geometry and mirror shape • Optical issues – High peak power • coating damage threshold large mirrors – Large average power: thermal load effects • Thermal lens in the coupling mirror (cf VIRGO upgrade with >600 k. W) 17
Laser/cavity numerical feedback development Rétroaction on laser frequency Clk = 100 MHz 8 x ADC 14 bits 8 x DAC 14 bits => Filtering => 18 bits / 400 k. Hz FPGA Virtex II 18
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