Post on 14-Dec-2015
Compton Laser and Systematics for
PREx
Abdurahim Rakhman
Syracuse University
PREx Collaboration Meeting, JLab
January 30, 2011
Outline
Laser & Cavity Performance Optical Setup SHG Green Beam Characteristics Mode Matching Cavity Performance Cavity Characterization
Laser Polarization Polarized Beam Transport Polarization Measurement Transfer Function Polarization Analysis
Summary
Optical Setup
IR(1064 nm) seed laser -> Fiber Amp ->Single Pass PPLN SHG (532 nm) -> High-Finesse FP cavity -> Feedback to seed laser PZT to lock
4
Optical Setup in
Beamline
Frequency Doubling with PPLN crystal
Copper Heat Sink
Copper holderPPLN
Copper plate
Two layer TEC
Teflon Cover
In foil
MgO doped PPLN crystal (3 x 0.5 x 50 mm), QPM period 6.92 um
Quasi-phase matched to 1064 nm beam from Fiber Amplifier
TEC based temperature controller gives good power stability
6
Routinely achieved ~30 % SHG conversion efficiency while setting up the optics in the tunnel
Measured a total M2 of 1.07 at the beginning of the installation
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Beam Transport & Mode Matching
Laser mode (beam) should match the cavity resonator mode
Beam waist at the center should match the natural waist of the cavity
The amount of primary power actually amplified in the fundamental mode
d
0ω2
0ω′2
2
20
2
0 211
d
MM
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Beam transport schematics by OptoCad
9
Cavity Performance (Locking Stability)
Reflected
Transmitted
Error
Fast Feedback
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Cavity Performance (Decay Time)
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Cavity Power Calibration
Cavity power was recalibrated during Summer 2010 and found
that it was under estimated by 40 % during PREx*.
0 500 1000 1500 2000 2500 3000 3500 4000 45000
1000
2000
3000
4000
5000
6000
f(x) = 1.41401099154422 x + 56.2905000922581R² = 0.998791141686604
Cavity Power Calibration Curve
Power Reported by EPICS System(Watt)
Mea
sure
d P
ow
er S
cale
d b
y M
irro
r T
ran
smis
sio
n (
Wat
t)
* Used Thorlabs S140A integrating head, measured transmitted power above M3 while cavity was open and compared it with EPICS power reading.
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Cavity Characterization
Vacuum (Torr) 2 x 10-9
Power Injected (W) ~ 1.0Average Decay Time (μs) 13.5Average Finesse 13,000Average Gain 45,00Average Bandwidth (kHz) 13.0Average Mode Match Coupling 0.85Q-factor 4.15 x 1012
Free Spectral Range (MHz) 176Transmission (ppm) 180Loss (ppm) < 10Average Cavity Power (kW) 3.5 (after
calibration)CIP spot size (μm) 135 (x), 154 (y)
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CIP Spot Size vs. Cavity Luminosity
Ib = 100 μA
Pcav = 3.5 kW
σe = 50 μm
αc = 1.4 degree
40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 1902.8E+05
3.3E+05
3.8E+05
4.3E+05
4.8E+05
5.3E+05
5.8E+05
6.3E+05
6.8E+05
7.3E+05
7.8E+05
8.3E+05
Luminosity vs. CIP spot size
σγ (μm)
Lu
min
osi
ty (
1(b
arn
*s)) Wanted to be
here
But in here
Lost 30 % efficiency !!
cehc
cavPeeIcL
sin1
221
22)cos1(
Measured the CIP spot size during summer 2010 and found that average σγ is ~ 140 µm
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Polarized Beam Transport
Optical Element DOLP (%) Angle (deg)PPLN SHG 99.88 -89.9Faraday Isolator (FOI) 99.98 -45Half Wave Plate (HWP) 99.99 0.1Mr1 99.20 0.0Polarized Beam Splitter (PBS) 99.99 0.0
Optical Element DOCP (%) Angle (deg)Quarter Wave Plate (QWP#1)
99.96 (L) -99.98 (R)
45 (L),315 (R)
@ CIP w/o cavity 99.57 (L) -98.07 (R)
50 (L),310 (R)
There is 1.5 % asymmetry in optimized DOCP at CIP in Left/Right states, not understood !!
Transporting a circularly polarized light is tough.
Transporting a linearly polarized light is easy.
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Polarization Measurement w/ Rotating Linear Polarizer
Measurement based on rotating GL linear polarizer to measure the change in
light intensity w.r.t. rotation angle. (Extinction ratio 10-6)
Fully automated measurement station, fast photodiode mounted inside an
integrating sphere reads out I(θ) vs. θ for a full rotation with a step size of 5
deg.
Read-out power normalized to laser power fluctuations upstream to cancel
systematic error. )(sin)(cos)( 2min
2max III
minmax
minmaxIIIIDOLP
2/12)1( DOLPDOCP
Polarization Measurement w/ Rotating Quarter Wave Plate (Stokes Formalism)
0
0
1
1
2sin2sin2cos2cos2
132
2101 PPPPS
0
0
1
1
2sin2sin2cos2cos2
132
2102 PPPPS
Exit Lin
e
x
x
y
y
Wollaston Prism
Integrating Sphere S1
Integrating Sphere S2
+β
Incoming Polarization Ellipse
TE pol. state
TM pol. state
QWP
Slow
9.808.80
Stokes Parameters: P0,P1,P2,P3
0
3
PP
DOCP
Conventions
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Jones Vector:
J AxAye
i
/4 plate
Incoming linear pol.
X (slow)
Y (fast)
JLeft a
ib
LEFT
Z
DOCP>0
X (slow)
Y (fast)
JRight a
ib
RIGHT
Z
DOCP<0
1st mirror
2nd mirror
x
y
z
x
y
z+
CIP
Exit LineZ is always along photon propagation
+
Rotatable GL Polarizer
Rotatable QWP
Rotatable GL Polarizer
Fast PDin IS
Exit Line
- Eigen-state generator- Constant DOCP (92%, 97%)- Scan ellipse angle
Transfer Function Measurement
CIP PolarizationMeasurement Station
Rotatable GL Polarizer
Fast PDin IS
RotatableQWP
Wollaston S1
S2
TFA
TFB
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Model of Transfer Function
cossin
sincos
2sincos
2cos
2sin2sin
2sin2sin
2sincos
2cos
,,ii
iiM
Phase shift , Slow axis at
Jones matrix of a mirror:
122221111 ,,,, MMTFExit line ---> CIP: JCIP=[TF]●JExit
Rotator
- DOCP@CIP=92%, measure DOCP and angle @ CIP and exit (10 points in each
polar state)
- DOCP@CIP=97%, measure DOCP and angle @ CIP and exit (10 points in each
polar state)
- Nominal DOCP, 1 point in each polar state
- Use the 92% data points to fit the 6 parameters of the transfer function
- Validate the result with the 97% and nominal data points
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Transfer Function
2D Counter view of transfer function maps out CIP polarization from Exit
polarization and angle.
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Cavity Status
DOCPL
(%)
L (deg)
DOCPR (%)
R (deg)
DOCPL (%)
L(deg)
DOCPR (%)
R (deg)
ΔDOCPL
(%)
ΔDOCPR (%)
OPEN 99.77 19.10 -98.33 102.85 97.22 140.20 -97.29 65.34 -- --
OPEN 99.77 19.10 -98.33 102.85 97.40 137.18 -97.75 62.63 0.18 -0.46
OPEN 99.61 13.84 -97.81 101.00 97.22 140.20 -97.29 65.34 -0.16 0.52
CLOSED
99.13 70.10 -96.44 162.03 96.69 117.92 -95.26 14.79 -0.64 1.89
Analysis Result
Cavity Status
DOCPL
(%)
L (deg)
DOCPR (%)
R (deg)
DOCPL (%)
L(deg)
DOCPR (%)
R (deg)
ΔDOCPL
(%)
ΔDOCPR (%)
OPEN 99.77 19.10 -98.33 102.85 96.96 140.41 -97.89 64.35 -- --
OPEN 99.77 19.10 -98.33 102.85 97.43 136.43 -97.84 62.05 0.47 0.05
OPEN 99.61 11.02 -97.97 105.02 97.22 140.20 -97.29 65.34 -0.16 0.36
CLOSED
99.10 76.32 -97.17 157.92 95.90 118.95 -96.53 16.21 -0.67 1.16
Cavity Status
DOCPL
(%)
L (deg)
DOCPR (%)
R (deg)
DOCPL (%)
L(deg)
DOCPR (%)
R (deg)
ΔDOCPL
(%)
ΔDOCPR (%)
OPEN 99.57 31.40 -98.07 109.35 98.33 135.00 -98.46 64.20 -- --
OPEN 99.57 31.40 -98.07 109.35 98.38 135.23 -98.49 64.80 0.05 -0.03
OPEN 99.59 31.55 -98.13 109.16 98.33 135.00 -98.46 64.20 0.02 -0.06
CLOSED
99.33 75.98 -96.84 166.34 96.69 117.92 -95.26 14.79 -0.24 1.23
CLOSED
99.26 83.52 -97.59 162.50 95.90 118.95 -96.53 16.21 -0.31 0.48
TFA1
TFA2
TFB
ExitCIP Measurement
Calculation
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Preliminary Error EstimationTransfer Function
A (%)Transfer Function B
(%)
DOCP at Exit Line 0.02 0.02
Theta at Exit Line 0.13 0.13
Variation in Time 0.04 0.04
Validation of Transfer Function
0.30 0.12
Cavity Installation
Transmission Through Me 0.10 0.10
Transmission Through Ms 0.10 0.10
Coupling 0.10 0.10
Birefringence of Cavity Mirrors
? ?
Total (w/o mirror birefringence)
0.37 0.24
L E F T (%) R I G H T (%)
Transfer Function A
99.10 ± 0.90(sys) ± 0.10(stat)
-97.17 ± 0.90(sys) ± 0.13(stat)
Transfer Function B
99.26 ± 0.74(sys) ± 0.10(stat)
-97.59 ± 0.74(sys) ± 0.13(stat)
- Saclay estimated 0.05 % error for the IR cavity mirror birefringence.
- We still need more study to figure out the birefringence of our mirrors as well as the
vacuum stress induced birefringence of vacuum window.
- Following is a hand-waving estimate of systematic errors.
Summary
Green Cavity installation and commissioning was painful but successful.
Successfully recovered from two major accidents and ran fairly smoothly.
Cavity vacuum level was stable and reached as low as 2 x 10-9 Torr.
Cavity stability was a major concern, but it turned out to be quite good, long term monitoring is still needed.
Cavity turning mirror post stability is poor, needs major redesign.
PPLN doubling setup can be professionally redesigned. Restoring and aligning was not easy.
Cavity birefringence should be studied very carefully. It is important for error study.
Thin polarizer with high power density and high extinction ratio should be pursued.
Mirror mount should be redesigned so that there should be zero stress to the cavity mirror.
More exit polarization scan should have been done to monitor any change in polarization.
Exit line QWP based polarization monitoring scheme can be replaced with an LP scan system. It is simpler than Stokes formalism and seems to give better accuracy.
There are lots of new ideas among the growing Compton community at JLab on laser system and polarization systematics. Pulsed laser idea is quite appealing.
Working on a NIM paper, the 1st draft should be ready by late February.
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Acknowledgment
Many thanks to those who have contributed, helped and
supported to make the green Compton project successful.
Special thanks to JLab machine shop.
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Thank you !
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