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