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

7

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

8

Beam transport schematics by OptoCad

9

Cavity Performance (Locking Stability)

Reflected

Transmitted

Error

Fast Feedback

10

Cavity Performance (Decay Time)

11

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.

12

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)

13

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

14

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.

15

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

17

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

19

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

20

Transfer Function

2D Counter view of transfer function maps out CIP polarization from Exit

polarization and angle.

21

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

22

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.

24

Acknowledgment

Many thanks to those who have contributed, helped and

supported to make the green Compton project successful.

Special thanks to JLab machine shop.

25

26

Thank you !

27