1

ELI-NPlaser IP cavity

1) Requests 2) Recirculating cavity

1) biblio2) limits

3) Fabry-Perot cavity solutiona) Technical Contraintslimitsb) Possible solution : 2 frequencies

2

. . .

100 PULSES 1J/PULSE

5ns (200MHz)

. . .5ns

1ms (100Hz)

Assume the following Laser request at the Compton IP

100 PULSES 1J/PULSE

l~1µmtmax~10psPpeak=1011W<P>=10kW

3

Recirculating cavity

M. Y. Shverdin et al. High Power Picosecond Laser Pulse Recirculation Opt Lett35(2010)2224

In the 2010 paper: 677mJ @10Hz incident 1µmbeam and 177mJ after freq doubling.Pulse energy measurement turn after turn :

6% loss per cavity round trip

LLNL (& now BNL/AFT) method

50th pulse

4

Recirculating cavity experimental results

Estimated integrated pulse energy : 3J in total : Need 33 times more 677mJ@10Hz : need 10 times more ?Nothing after 50 pulses : need less than 50 pulsesNo information on the laser beam profil quality

5

Technique limitations Numerical estimate in :

Non linear effects Induced in the freq doublerThe highest the efficiency the worse the beam profile

6

~50%Comptonlosses after 20 passes

g spectrum modified after 20 round trips

7

A new démonstration experiment à BNL/ATF (2011)

R&D experiment2-3J total(idem paper 2010)@1-3 HzUpgrade foreseen at ~10J @100 HzBut still an R&D

8

Bibliography Summary Advantage : easy to enter the cavityDrawbacks/issuesNon-linear effectsNb of passes limited (~20)Beam profil not shown & beam

ellipticity not adressed mirror damage issue not adressed

(40cmx40cm cristal for 1J…)Still frar from being a mature techno

visit/contact BNL

9

Puzzlingly LLNL proposes another Techniques for an ILC gg colliderlaser source (laser request not so far fromELI-NP !)

10

Oscillator200MHzOscillator200MHz

AmpliDt~500ns5mJ/pulses100 pulses50W

AmpliDt~500ns5mJ/pulses100 pulses50W

« empty » optical resonatorF~6001J/pulses for 100 pulses : 10kW1 circulating pulse of 1J

« empty » optical resonatorF~6001J/pulses for 100 pulses : 10kW1 circulating pulse of 1J Scheme

Issues1. Laser amplifier cost >3M€

1. Faisable (1rst discussion with Amplitude System, SupOptics laser groupe)

2. Effects of a 1J pulse inside an optical resonator1. Mirror Fluence damage threshold constraint

1. A priori also a problem for the recirculating cavity2. Cavity feedback

1. Thermal load in the mirors 2. Radiation pressure

Fabry-Perot technique

11

Figure 1: Dielectric bulk material, exposed to 0.9 ps laser pulses at l=532 nm, two spots inthe middle of the picture show damage onset. The weaker of those spots wasirradiated with a laser fluence just above the damage threshold.

Results, 0.9 ps pulsesThe same measurements as for 5 ns pulses were carried out in order to compare damagethresholds for different time domains. Single shot results were Fth = 2,37 J/cm2 for bulkmaterial (fused silica), Fth = 0,35 J/cm2 for the untreated HR mirror stack and Fth = 0,25 J/cm2for the dielectric grating. Multishot measurements (n=100) gave Fth = 0,20 J/cm2 for themirror and Fth = 0,26 J/cm2 for the grating, respectively.

For l=532nm

Damage threshold measurements of gold and dielectric coated optical components at 50 fs – 5 nsR. Bödefeld, W. Theobald, J. Schreiber, H. Gessner, E. Welsch, T. Feurer, R. Sauerbrey

12

Depends on the spot sizeAlmost a facteur 10 for this exemple

Depends on l, for 0.5µm : 2 times worse than 1µm !

Depends on nb of pulsesDamage threshold à 200MHz ???

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For 10ps : damage fluence ~3 times % 1ps

Threshold Fmax~0.6J/cm2 for 100pulses@1kHz Assume Fmax=0.1J/cm2

2

max

0.1

/10

EF Jcm

E

spotsize

spotsizp se

eJ ul

spotsize of the beam on the cavity mirror

For Emax=1J, spotsize=10cm2 !

d1

d3

1er waist

2ème waist

d4 d2

q ≠ 0 :•2 plans à considérer

• Plan tangentiel(=plan cavité)f=Rcosq/2

• Plan sagittal (plan cavité) f=R/(2cosq)

•Plus (q h) grand plus le mode propre de la cavité est elliptique•Condition de stabilité :

INTRODUCTIONBow-tie cavity : basic paraxial expressions

R f=R/2

Normal incidence q = 0 :

h2q

L=d2+d3+d4Ltot=d1+L

pour

1

pour

1

1

1 1

cos2cos 2

ou

cos

cos cos

2( ) 2 2

22

2R Rf f

R RLd

Ld d Lf f

d L

fLf d

L Rf L

électrons

electrons

Example : ThomXLtot=8.000799920m, d1=2m

q = atan(h/d1)/2=0.6°, qcross = atan(2h’/d1) = 1.7°

Waists in µm Radii on mirrors in mm

Elliptical mode

d1

d3

d4

d2 h2q

1er et 2ième waist au même endroit

électrons

Drawback : beam pipe cut

Autre géométrie, plus astucieuse :(M. Lacroix pour ThomX)

X rays

We can make use of the ellipticity :The highest h the highest the ellipticipty h must be as small as possibleThe X-angle can be minimized with concave mirror with rectangular edges

•A minima : • Diamètre du miroir = 6wM,min ~ 6*[1,3]mm

Rmirror = [3,9]mm• Soit hmin = 2[3,9]mm = [6,18]mm• X-angle ~(Rmirror+Rbeam pipe)/(d1/2)

~[9,15]/250~[2°,3.5°]

2

2

max

1

ma

1

x

21

1[ ] [

0.1 / for 0.5 factor 2 ]

; 1

f

0

2

1

or

mm

E J spot cF J

mEF spot

spot

F spotd

c

F spot d

m m

1

effmax 1

1 Δd=100nm

1 ∂spotE = spot- Δd

10 ∂d

m12ωm22ω

d1=e+R/cos(q)

Ltot=c/frep 1.5m pour frep=200MHz

m1ωm2ω

d1

d3

d4

d2

2q

L=d2+d3+d4

Ltot=d1+Ld1=e+R/cos(q)

For l=532nm

Non-paraxial region

For l=532nm

~300mJ/pulsefor l=1µm100MHz

Pushing the parameters

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Feddback issues

Cavity finesse Fp/(1-r1*r2*r3*r4) ‘phase matching’ r1=r2*r3*r4 Fp/(1-r1^2) p/T1power/energy enhencement factor F/pCavity resonance linewidth : FWHM=2DL=2 /l F if the cavity length is shifted by DL= /l F half of the power is lost

Pound-Dever-Hall feedback methodeLinear error signal if cavity length variation < l/F Laser resonance frequency n nc/L, Feedback control accuracy :

9

1 , 1000, 110

L m F m

L

L LF

At LAL we have already achieved

But here we see 2 difficulties related to the high pic power

1210

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1rst problem :To fill all the pulses within a time interval < feedback bandwidth (1MHz AT MOST !) The pulse stacking must not change the cavity length by more than ~l/F

2nd problemIf the cavity has been correctly filled : no perturbation on the cavity length > ~l/F must beinduced by the circulation of the very high energy pulse

What can change the cavity length within Dt>1/MHz ?

•The radiation pressure : • for a pulse of 10ps & 1J

stength=2P/c~700 N equivalent to a weight of 70kg falling on the mirrors

each 5ns !• Stress wave propagates ~ at the sound speed (~6km/s in glass)

•The thermo-elastic coupling• Absorption of the mirror coating layers ~1ppm • But very fast mechanism can occur with 10ps pulses…

• E.g.

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One can ‘solve’ the 2nd problem using two wavelengths with high/low finesseCALVA (LAL R&D)/VIRGO upgrade, see next slide

To look at the 1rst problem a possible experiment at the LASERX facilitiy could help (Ti:sapph 0-2J/pulse @10Hz, 30fs,…100ns)

(R. Chiche & LaserX ‘young’ group, K. Cassou, Guillebaud, S. Kazamias)

O

L~10cm

R R

ND:YAGcw <1W laser ~

modulation

X1GHzsynthetiser

demodulation

pdiode

Error Signalcavity length variation induced by high pulse power

bandwidth ~1GHzDynamic range :l/F~20nm if F=50

High energy pulse

Yb Oscillateur ~200MHz>8nm bandwidthStabilisable20mW. Inside :Steper motorPztEOMDouble wedgepump modulation

Freqdoublerpreamplifier

4-mirror cavity

Optical switch+Ampli :

DT~500ns-1µs5mJ/pulses100 pulses

50W

Servofeedback

grating

Cavity round trip lengthL=c/200MHz=1.5m

Fibre connectorised ?

Errorsignals

Reflected signals

transmited signals

2 frequencies solution2nd harmonic

Freqdoubler

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Error signal

Linearity rangeCorresponds to

DL=l/F

Using the frequency doubling :F /2l ~1000 for l precise feedback BUT DL<0.5nmF l ~50 for l DL<20nm can recover the locking after the macro pulse pass (1-2µs)

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Recent improvement from VIRGO

Linearity rangeincreased by a factor ~10

DL~l/(10F)‘just’ by dividing

the error signal by the

transmited signal

Using the frequency doubling :F /2l ~1000 for l precise feedback BUT DL<5nmF l ~50 for l DL<200nm recover the locking after the macro pulse pass (1-2µs)starts to be feasible with the doubled frequency find the optimum for Fl/F /2l

Yb Oscillateur ~200MHz>8nm bandwidthStabilisable20mW. Inside :Steper motorPztEOMDouble wedgepump modulation

Frequencydoubler

preamplifier

4-mirror cavity

Optical switch+Ampli :

DT~500ns-1µs5mJ/pulses100 pulses

50W

Servofeedback

grating

Cavity round trip lengthL=c/200MHz=1.5m

Fibre connectorised ?

Errorsignals

Reflected signals

transmited signals

2 frequencies solution1rst harmonicl~1µm

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summary

Damage threshold limits the max pulse energy in a ‘ring cavity’

We have some experience in high finesse cavity locking

A ‘burst’ regime should work but one must estimate the effects of a ‘huge’ circulating pulse energy

30

LMA designed mirrors with F=50 at l/2

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Il faut faire tourner du code aux elements fini !calculer l’evolution temporelle des deformations

induite par la pression de radiationinduites par la diffusion de la chaleur

Et penser à une manipe auprès d’une source de laser intense …

• On pose h=15mm, d1=R/cosq, R=0.5m, • On tilt tous les miroirs de dx,y=(-1,0,1)µrad 38=6561

combinaisons de désalignements– Pour chaque combinaison on applique le principe de Fermat pour

trouver l’axe optique (on itère trois fois précision Matlab)– On calcul le déplacement de l’axe optique sur tous les miroirs par

rapport aux centres (alignement parfait)– Tolérance = le plus grand déplacement parmi les 38 combinaisons

• Résultats : – 2µm sur les miroirs sphériques– 1µm sur les miroirs plans– 2µm au point de croisement laser électron

0.5’

~13nm

1µrad

2ème ‘bonne propriété’ d’une cavité 4 miroirs :tolérances mécaniques

• On translate tous les miroirs de Dx,y,z=(-1,1)µm 212=4096 combinaisons

• on obtient– 3µm sur les miroirs sphériques– 5µm sur les miroirs plans– 1µm au point de croisement laser électron

Calculation of the cavity eigenmodes

Linear polarisation is preserved for 1µm, 1µrad mirror motionsAnd 1mm, 1mrad missalignments