1. Fabry-Perot cavity R&D at Orsay
Introduction: high finesse cavity in pulsed regime
Results on cavity locking
R&D goals for
Alghero, 07/09/2008, F. Zomer

2. Introduction: Pulsed_laser/cavity feedback technique
T=2p/wr
Specificity  properties of passive mode locked laser beams
Frequency comb  all the comb must be locked to the cavity
 Feedback with degrees of freedom :
control of the Dilatation & translation
wn= nwr+w0
n~106
T. Udem et al. Nature 416 (2002) 233

3. Technical constraints
First technical constraint: laser phase noise
For all comb components wn=nwr+w0 to be locked to a cavity of finesse F
Ti:sa oscillator
phase noise measurement
free
1/PLL_gain
Possible with mode locked lasers
Ex.: almost no phase noise above ~10kHz in Ti:sa oscillators
locked
noise floor
Ivanov et al. IEEE Trans Ultr, 50(2003)355

4. State of the art: Second technical constraint:
Chromatic dispersion of the cavity mirror coating gives a limit on laser pulse width
 No effect for ~1ps pulses
Third technical constraint:
coating damage
  10MW average power for ps Pulses (see Urakawa san’s talk)
High finesse cavity could be operated in ps regime as in cw regime up to the MW average power regime
State of the art:
Loewen (PhD), gain 6000 for ~30ps pulse width (See R. Ruth’s talk)
KEK/ATF cavities, gains ~1000 for ps lasers (see Urakawa san’s talk)
Femto comb stabilisation, low finesse ~200 (Jones et al., PRL86(2001) 3288)

5. R&D during 2006 2008 LAL/Orsay Frame:
ILC KEK scheme for e+ polarised source (Omori san & Variola talks)
Goals:
locking of Ti:sa oscillator to a (up to ) finesse cavity
Reduce laser spot size inside cavity

6. Experimental setup Pound-Drever-Hall locking technique
MIRA
Driver M1
MOTOR
VERDI 6W
AOM
532nm
EOM
AOM M2
PZT
+/-
Driver
Amplifier
Driver
Driver
grating
5MHz
OSC +
Phase
Adjust
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
Flexibility of the Feedback strategy needed to reach highest cavity finesses
 digital feedback system was chosen

7. C++ GUI LYRTECH DFS : 8 ADC channels Sampling @ 105 MS/s
14 bits resolution
Virtex-II FPGA : XC2V8000
60ns latency
8 DAC channels
Conversion 125 MS/s
C++ GUI

8. Cavity locked (low gain ~1200)
Digital feedback (5k VHDL lines of code)
Already Dfrep/frep~10-10  Dfrep~76mHz for frep~76MHz
Cavity locked
With gain 1200

9. Phase noise of the Ti:sa locked to the 1200 gain cavity
-50
-150
-250
-350
Reconstructed free cavity running
dBc/Hz
Cavity locked
f/Hz
Integrated residual noise rms ~ 8mHz on frep We are presently working on the locking of a cavity finesse
Ex. of improvement: the photodiode readout noise is a little bit too high…

10. Small laser spot size Laser input e- beam
Small laser spot size &2 mirrors cavity  unstable resonator (concentric resonator)
e- beam
Laser input
Stable solution: 4 mirror cavity as in Femto lasers
BUT astigmatic & linearly polarised eigen-modes
Non-planar 4 mirrors cavity
Astigmatism reduced &
Stable circularly polarised eigenmodes

11. Prototype of nonlanar 4 mirrors resonator (low finesse)
Check the general astigmatism mode shape/propagation (Arnaud, Bell Syst. Tech. ( 1970)2311)
 ok
Ellipse intensity
profile ‘turning’
Kogelnik, Apl. Opt. 8(1969)1687
50cm

12. We observed funny aberrations of the cavity mode for cavity waists 40µm (20µm spot size) in our geometrical configuration
Calculation quite challenging
We have ordered 2 inchs mirrors to study higher divergent configuration

13. Continuation of the R&D will start 2009 2011
CELIA / Bordeaux (Laser Lab.)
KEK/ATF (see Urakawa san’s talk)
LAL/Orsay
LMA/Lyon (Mirror coating Lab.)

14. Continuation of the R&D will start 2009 2011
1. Setup the following system at Bordeaux/Orsay
>100W-200W
Oscillator
=1.7W, 1030nm
Dt~0.9ps frep=178.5MHz
From Onefive compagny
Amplifier
Rod type photonic fiber Yb Doped
Fabry-Perot cavity
Gain~10000
Goal: to reach the MW average power
Numerical feedback
2. Study thermal effects Lyon/Bordeaux (a priori dominated by thermal length in the mirror substrat)
3. Installation of the system at ATF/KEK, in collaboration with ATF group

15. The laser amplification R&D E. Cormier (CELIA)
Max published
OneFive laser, 1030 nm
Dt=0.9ps, MHz, 1.7W
F. Röser et al. Optics letters, 30, p2754, 2005
1rst phase noise measurements up to 80W show no extra phase noise
Laser Diode
Possible average power
100W-300W (simulation)
Doped rod type photonic fibre:
Large core diameter 80mm& monomode
Gold grating-based strectcher gives negative chirp for spectral compression

16. 4 mirror cavity for KEK e- ATF beam pipe: 5mm slit…
2 flat mirrors
2 spherical mirrors
Angle laser / e- beam= 8° e-
laser/beam
Interaction point
Injection laser

17. Mirror positioning system
4 mirror cavity for KEK
Mirror positioning system
2 flat mirrors
2 spherical mirrors e-
Injection laser

18. Vacuum vessel for KEK e-
Injection laser

19. Implantation at ATF Δz Δx

20. Summary
We are on the way to lock a 1ps Ti:sa oscillator to a finesse cavity
We started the study of fiber amplifications of Ytterbium oscillator
Looks promising from phase noise point of view
We studied low finesse tetrahedron resonator
Still some work needed to describe highly divergent modes in such resonator (for waists below 40µm)
Started the design for an installation at ATF/KEK