1. The High-finesse Fabry-Perot Cavity
Abdurahim Rakhman
Advisors:
Prof. Paul Souder, Dr. Sirish Nanda
PREx Collaboration Meeting
12/07/2007

2. Outline Compton Upgrade at Jefferson Lab
Principles of the Fabry-Perot Cavity
Cavity Parameters & Prometheus Laser
Mode Matching
Cavity Locking
Conclusions and Future Plans

3. Compton Polarimeter: e-  e-  Z

4. Compton Upgrade: Gain: 7000 15000 λ=1064nm λ=532nm 0.85 8GeV 4GeV 1GeV

5. Fabry-Perot Cavity: L r1 r2 Circulation Incident Transmission
Reflected r1 r2
Previously locking was successful
with a G~2,000
Current Cavity Mirror:
Los Gatos Research, Inc. (LGR)
Type: Plano-Concave
Radius of Curvature: 1 m
Reflectivity: ±0.001%
G=6,000
F=18,000

6. Prometheus Laser: 2 W 0.1W Innolight GmbH, Germany ▪809 nm Pump Diodes
▪1064 nm Nd:YAG
▪532 nm SHG via PPKTP
2 W W

7. Prometheus Laser Profile:
Beam is highly elliptic, circularization is needed
Before After

8. Mode-Matching: laser mode (beam) should match
the cavity resonator mode
beam waist at the center should
match the natural waist of the cavity L3 L2
cavity waist L1
0.5787
TEM00 mode
incident
reflected
mode matched

9. OptoCad: FORTRAN90 program for tracing Gaussian beams
computes the parameters of the optical system
Green cavity optical layout by OptoCad

10. Complete ray tracing by OptoCad (in development):

11. Pound-Drever-Hall (PDH) Locking Scheme:
Detect phase of the resonance from reflected light
Feedback to tunable element to stay “locked” to resonance
Photo detector
Beam Splitter
Cavity
Oscillator
Phase Shifter
Mixer
Low Pass Filter
Tunable Laser
PID-Regulator
Error signal

12. Digital CavLock Electronics (Homemade):4”
previous
current

13. Search / scan / lock logic:
Transmission
Reflection
Fast Scan
Slow Search
Successful locking with low gain cavity
Reflection
Error Signal
Transmission
Signals of LGR cavity

14. Cavity Locking: Feedback loop tuning in progress
Reflection
Transmission
2 sec level lock, only with fast PZT feedback, but unacceptable due to large fluctuations
Reflection
Error Signal
Transmission
reworked electronics, 40 msec level short lock with fast PZT feedback only
Feedback loop tuning in progress

15. Thank you ! Conclusions: Prometheus Laser has been reprofiled
Mode matching of LGR mirror has been accomplished
Complete ray tracing of the system by OptoCad is in development
Cavity feedback loop tuning in progress
Short duration lock has been achieved
Thank you !

16. Cavity Parameters:
Free Spectral Range :
Gain:
Finesse:
Band Width: