Frequency Doubling with PPLN crystal Copper Heat Sink Copper holder PPLN 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 M 2 of 1.07 at the beginning of the installation
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 7
Cavity Power Calibration Cavity power was recalibrated during Summer 2010 and found that it was under estimated by 40 % during PREx *. 11 * 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 Power Injected (W)~ 1.0 Average Decay Time (μs)13.5 Average Finesse13,000 Average Gain45,00 Average Bandwidth (kHz)13.0 Average Mode Match Coupling0.85 Q-factor4.15 x Free Spectral Range (MHz)176 Transmission (ppm)180 Loss (ppm)< 10 Average Cavity Power (kW)3.5 (after calibration) CIP spot size (μm) 135 ( x ), 154 ( y )
CIP Spot Size vs. Cavity Luminosity 13 I b = 100 μA P cav = 3.5 kW σ e = 50 μm α c = 1.4 degree Wanted to be here But in here Lost 30 % efficiency !! Measured the CIP spot size during summer 2010 and found that average σ γ is ~ 140 µm
Polarized Beam Transport 14 Optical ElementDOLP (%)Angle (deg) PPLN SHG Faraday Isolator (FOI) Half Wave Plate (HWP) Mr Polarized Beam Splitter (PBS) Optical ElementDOCP (%)Angle (deg) Quarter Wave Plate (QWP#1)99.96 (L) (R)45 (L),315 CIP w/o cavity99.57 (L) (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.
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 ) 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. 15
Polarization Measurement w/ Rotating Quarter Wave Plate (Stokes Formalism) Exit Line x x y y Wollaston Prism Integrating Sphere S1 Integrating Sphere S2 +β+β Incoming Polarization Ellipse TE pol. state TM pol. state QWP Slow Stokes Parameters: P 0,P 1,P 2,P 3
Conventions 17 Jones Vector: /4 plate Incoming linear pol. X (slow) Y (fast) LEFT Z DOCP>0 X (slow) Y (fast) RIGHT Z DOCP<0
1st mirror 2nd mirror x y z x y z ++ CIP Exit Line Z is always along photon propagation ++ Rotatable GL Polarizer Rotatable QWP Rotatable GL Polarizer Fast PD in IS Exit Line - Eigen-state generator - Constant DOCP (92%, 97%) - Scan ellipse angle Transfer Function Measurement CIP Polarization Measurement Station Rotatable GL Polarizer Fast PD in IS Rotatable QWP Wollaston S1 S2 TFA TFB
19 Model of Transfer Function Phase shift , Slow axis at Jones matrix of a mirror: Exit line ---> CIP: J CIP =[TF]●J Exit Rotator - measure DOCP and CIP and exit (10 points in each polar state) - measure DOCP and 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
Transfer Function 20 2D Counter view of transfer function maps out CIP polarization from Exit polarization and angle.
21 Cavity Status DOCP L (%) L (deg) DOCP R (%) R (deg) DOCP L (%) L (deg) DOCP R (%) R (deg) Δ DOCP L (%) Δ DOCP R (%) OPEN OPEN OPEN CLOSED Analysis Result Cavity Status DOCP L (%) L (deg) DOCP R (%) R (deg) DOCP L (%) L (deg) DOCP R (%) R (deg) Δ DOCP L (%) Δ DOCP R (%) OPEN OPEN OPEN CLOSED Cavity Status DOCP L (%) L (deg) DOCP R (%) R (deg) DOCP L (%) L (deg) DOCP R (%) R (deg) Δ DOCP L (%) Δ DOCP R (%) OPEN OPEN OPEN CLOSED CLOSED TFA1 TFA2 TFB ExitCIP Measurement Calculation
Preliminary Error Estimation 22 Transfer Function A (%)Transfer Function B (%) DOCP at Exit Line0.02 Theta at Exit Line0.13 Variation in Time0.04 Validation of Transfer Function Cavity Installation Transmission Through Me0.10 Transmission Through Ms0.10 Coupling0.10 Birefringence of Cavity Mirrors?? Total (w/o mirror birefringence) L E F T (%)R I G H T (%) Transfer Function A99.10 ± 0.90(sys) ± 0.10(stat) ± 0.90(sys) ± 0.13(stat) Transfer Function B99.26 ± 0.74(sys) ± 0.10(stat) ± 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 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 1 st draft should be ready by late February.
Acknowledgment Many thanks to those who have contributed, helped and supported to make the green Compton project successful. Special thanks to JLab machine shop. 24