The High-finesse Fabry-Perot Cavity
Abdurahim Rakhman
Advisors:
Prof. Paul Souder, Dr. Sirish Nanda
PREx Collaboration Meeting12/07/2007
Outline
• Compton Upgrade at Jefferson Lab
• Principles of the Fabry-Perot Cavity
• Cavity Parameters & Prometheus Laser
• Mode Matching
• Cavity Locking
• Conclusions and Future Plans
Compton Polarimeter:
Z
e- e-
thennnn APPA exp
Compton Upgrade:
Gain: 7000 15000
8GeV4GeV
1GeV
λ=1064nm
8GeV4GeV
1GeV
λ=532nm
0.85
Fabry-Perot Cavity:
Incident
Reflected
Circulation
Transmission
r1 r2
L
Previously locking was successfulwith a G~2,000
Current Cavity Mirror:
Los Gatos Research, Inc. (LGR)Type: Plano-ConcaveRadius of Curvature: 1 mReflectivity: 99.987±0.001%G=6,000F=18,000
Prometheus Laser:
2 W 0.1W
Innolight GmbH, Germany▪809 nm Pump Diodes▪1064 nm Nd:YAG▪532 nm SHG via PPKTP
Prometheus Laser Profile:
y = 0.0015x + 0.2761
R2 = 0.998
y = 0.0028x + 0.0765
R2 = 0.9996
0.75
0.85
0.95
1.05
1.15
1.25
1.35
1.45
1.55
1.65
350 400 450 500 550
Z (mm)
Dia
ma
ete
r (m
m)
Sagital/Horizontal
Tangential/Vertical
Linear (Sagital/Horizontal)
Linear (Tangential/Vertical)
y = 0.0122x - 2.9607
R2 = 0.9998
y = 0.0125x - 3.0584
R2 = 0.9999
1.2
1.7
2.2
2.7
3.2
3.7
4.2
350 400 450 500 550
Z (mm)
Dia
ma
ete
r (m
m)
Sagital/Horizontal
Tangential/Vertical
Linear (Tangential/Vertical)
Linear (Sagital/Horizontal)
Beam is highly elliptic, circularization is needed
Before After
Mode-Matching:
incident
reflected
mode matched
• laser mode (beam) should match the cavity resonator mode
• beam waist at the center should
match the natural waist of the cavity
TEM00 mode
0.5787
cavity waist
2
1
2
1
0 22
LRLd m
L1
L2
L3
OptoCad:
FORTRAN90 program for tracing Gaussian beamscomputes the parameters of the optical system
Green cavity optical layout by OptoCad
Complete ray tracing by OptoCad (in development):
Pound-Drever-Hall (PDH) Locking Scheme:
Photo detector
Beam Splitter Cavity
Oscillator
Phase Shifter
MixerLow Pass Filter
0
Tunable Laser
PID-Regulator
Error signal
Detect phase of the resonance from reflected lightFeedback to tunable element to stay “locked” to resonance
Digital CavLock Electronics (Homemade):
4”
current
previous
Search / scan / lock logic:
Successful locking with low gain cavity
Transmission
Reflection
Fast Scan
Slow Search
Signals of LGR cavity
Reflection
Error Signal
Transmission
Cavity Locking:
2 sec level lock, only with fast PZT feedback, but unacceptable due to large fluctuations
reworked electronics, 40 msec level short lock with fast PZT feedback only
Feedback loop tuning in progress
Reflection
Error Signal
Transmission
Reflection
Transmission
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 !
Cavity Parameters:
Free Spectral Range :
Gain:
Finesse:
Band Width:
d
cFSR
2
GFWHM
FSRF
F
FSR
21
4
1
21
1 RR
RRG
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