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High finesse multi-mirror optical cavities with feedback

High finesse multi-mirror optical cavities with feedback. Fabry -Perot cavity in cw mode: feedback & optical issues Comparison with Sapphire parameters Fabry -Perot cavity in pulsed mode Comparison with Sapphire parameters Present R&D on optical cavities at LAL.

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High finesse multi-mirror optical cavities with feedback

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  1. High finesse multi-mirror optical cavities with feedback Fabry-Perot cavity in cw mode: feedback & optical issues Comparison with Sapphire parameters Fabry-Perot cavity in pulsed mode Comparison with Sapphire parameters Present R&D on optical cavities at LAL F. Zomer, 19, march, 2013

  2. Gain=1/(1-R)10000 Fabry-Perot cavity: Principle with continous wave ~10kW e beam ~1W LASER isolateur ~1W JLAB/Saclay Polarimeter, NIMA459(2001)412HERA /Orsay Polarimeter, JINST 5(2010)P06005 WhennLaserc/2Lrésonance • But:Dn/nLaser = 10-11STRONG & ROBUST laser/cavity feedback needed…

  3. Illustration of one issue : the laser cavity feedback CavityresonancefrequencylinewidthDn=c/(LF)~6kHz ! Sapphire Cavityfinesse : F~100 p Optical pathlength : L~150m Dn/n=l/(LF)=~10-11-10-12 Samenumbers as in metrology !!! DL=l/F M. Oxborrow

  4. From a feedback point of view: Locking a ‘150m’ cavityof finesse~ 100 p(‘gain’~100) is the same asLocking 0.2m cavity to 300000 finesse !BUT Into an hyper isolated room The hyper stable smallcavityis ‘hyper’ temperaturestabilised • For Sapphire & Compton machines • ‘Geant’ mechanical structure • Noisyacceleratorenvironment • Pulsed laser beamregime •  1kHz linewidthoscillator • Hugeaverage & peak power ! Put on an hyper stabilisedoptical table And an hyper stable LOW POWER cw laser isused, linewidth 1kHz http://www.innolight.de/index.php?id=mephisto M. Oxborrow

  5. An Optical issue Small laser beam size +stable resonator  2-mirror cavity Laser input e- beam Stable solution: 4-mirrorcavityas in Femto laser technology BUT elliptical& linearlypolarisedeigen-modeswhich are instable because of vibrationsatveryhighfinesse Non-planar 4-mirrorcavity Stable & circularlypolarisedeigenmodes (AO48(2009)6651)as needed for an ILC polarised positron source

  6. Optical issues for ‘focusing’ resonators w0~7µm for sapphire (?) • Mode focusingstrongellipticity/astigmatism • Non-planar 4-mirrorresonator & ‘strong’ focusing • generalastigmatism(Arnaud,Bell Syst. Tech. ( 1970)2311) •  Complex mode structure 50cm FLabaye/LAL Carrefuloptical design optimize mode shapeat the IP optimize modepolarization FLabaye/LAL

  7. Fabry-Perot cavity in pulsed regime Electron beam 1ps Mode lock oscillator Fabry-Perot cavity with Super mirrors Same feedback technics (more complexe) isusedincw & pulsedregime  Wellknown techniques (analog and numerical)

  8. Frequencycomb all the combmust belocked to the cavity  Feedback with2 degreesof freedom : control of the Dilatation (rep. Rate) &Translation(CEP) Pulsed laser/cavity feedback technique T=2p/wr Dfce Specificity  properties of passive mode locked laser beams wn=nwr+w0 n~106 T. Udem et al. Nature 416 (2002) 233

  9. State of the art (Garching MPI) : ~70kW, 2ps pulses @78MHz (F~5600)storedin a cavity(O.L.35(2010)2052)~20kW, 200fs pulses @78MHz From a feedback point of view: Locking a ‘150m’ cavity to finesse~ 100 p(‘gain’~100) @ 350nm isthe sameas Locking a 4m cavity @ 800nm to ~25000 finesse R&D doneat Orsay 2ps Tis:apph 76MHz oscillator (~0.2nm spectrum) cavity finesse ~28000

  10. Orsay setup: Picosecond/High Finesse MIRA 2-Mirror Fabry-Perot cavity Finesse ~ 28000 Driver M1 MOTOR VERDI 6W AOM 532nm AOM EOM M2 PZT +/- Driver Driver Amplifier Driver grating SLITS PDH #1 Front end PDH #2 Front end TRANS Front-end Pound-Drever-Hall Scheme Serial RS232 +/- Transmission Signal Laser Length Control Laser Δφce Control DAQ Feedback

  11. Welocked the laser to the cavity • But weobservedstrong free running laser/cavitycoupling variations (Finesse~28000) Laser/cavitycoupling 25% coupling variationover ~15min • Stacked power variations up to ~60% • ‘noisy’ Stacked power (~7%) • Feedback bandwidth ~100kHz • BW up to a few MHz on the rep. Rate. needed to reduce the noise

  12. CEP effectsmeasurement in picosecond/high finesse regime CELIA, LAL, SZEGED Univ. • 2ps Ti:Sapph (75MHz) Locked to a~28000 finesse cavity • No control of the CEP drift in the • feedback loop • Numerical feedback loop • BW=100-200kHz • BW ~1MHz needed • CEP measuredwithSzegedinterferometer CEOLiTmeasurement of  Carrier  Envelope Offset Phase Drift by a Linear Transmission Ring

  13. Weobservedstrong free running laser/cavitycoupling variations (Finesse~30000) CEP measurement Laser/cavitycoupling Fit: Frequencycomb +Dfce variations Only3 free parameters in the fit: a normalisation, an offsetthe Finesse 25% coupling variationover ~15min Here 80% of the laser power iscoupled highqualitywave front needed

  14. Variation of the pump power laser/cavitycouplingmeasurementeffective enhancement factor CEP measurement F=45000 Measuredenhancementfactor Freq. Combfit (0.2nm width !) WithF~28000 F=15000 F=3000 • 60% enhancement factor variation if CEP phase [0,2p] for 2ps & ~28000 Finesse • CEP phase must bealsocontroled in high Finesse/picosecondregime • Feedback loop BW must be>100kHz

  15. Some laser oscillator issues • Atpresentincreasing the average power @ frep>2MHz rep rate  Yb fibertechnology • Need to find/build a low noise laser oscillator • CEP and rep. Rate lockingrequired • Possible feedback BW imitations using a 2MHz laser oscillator ( R&D on the oscillator & opticalreference) • Present R&D with Yb fiberoscillators (frep>100MHz) • CELIA-LAL R&D • 2 commercial Yb (fiber) lasers • Fullyconnectorised (robust) fiber amplifier • 50W(100W) atpresent(200W for ThomX, seeA.Variola)

  16. Bordeaux-Orsay R&D Stable oscillator(Origami Onefive) 0.2W, 1030nm Dt~0.2ps frep=178.5MHz ~10W 50W 100W 4-mirror Fabry-Perotcavity Gain~1000 Amplifier(s) photonicfiberYb Doped AOM 8000 1 piezo 1piezos1 temp. Ctrl. 1 AOM Mixed Analog-Numericalfeedback Highly ‘tunable’ oscillator(Orange Menlo) 0.02W, 1030nm Dt~0.2ps frep=178.5MHz 1piezo 1 EOM 1 trans. Stage 1AOM 1 pumpcurrent ATF clock • Setup required feedback (10kHz10MHz BW) • Setup a robustfiber amplifier • Study noise induced by the amplifier • Push the cavitystored power at maximum 10MHz feedback bandwidthneeded…

  17. 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 >600kW)

  18. Laser/cavity numerical feedback development Clk = 100 MHz 8x ADC 14 bits 8x DAC 14 bits => Filtering => 18 bits / 400 kHz FPGA Virtex II Rétroaction on laser frequency 18

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