The High-finesse Fabry-Perot Cavity

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
0.85
Gain 7000 15000
5
Fabry-Perot Cavity
Circulation
Incident
Transmission
L
Reflected
r1
r2
Previously locking was successful with a
G2,000 Current Cavity Mirror Los Gatos
Research, Inc. (LGR) Type Plano-Concave Radius
of Curvature 1 m Reflectivity
99.9870.001 G6,000 F18,000
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Prometheus Laser
Innolight GmbH, Germany ?809 nm Pump
Diodes ?1064 nm NdYAG ?532 nm SHG via PPKTP
2 W 0.1W
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Prometheus Laser Profile
Beam is highly elliptic, circularization is
needed Before
After
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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
cavity waist
L2
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
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Complete ray tracing by OptoCad (in development)
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Pound-Drever-Hall (PDH) Locking Scheme
Detect phase of the resonance from reflected
light Feedback to tunable element to stay
locked to resonance
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
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Cavity Locking
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
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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