Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012
description
Transcript of Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012
![Page 1: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/1.jpg)
June 28, 2012
Brian Sheehy
Laser and Optical Issues in Gatling Gun Development
Brian Sheehy June 28, 2012
I. Laser description for Phase I experimentsII. Scaling Issues for multiple cathodes
• synchronization• transport
III. Other long term optical issues• XHV windows with minimal birefringence• minimizing stray light & beam halo• homogeneity of bunch charge across 20
cathodes
![Page 2: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/2.jpg)
June 28, 2012
Brian Sheehy
parameter unit spec comment
wavelength nm 780
repetition rate kHz 70414.07 MHz / 20 cathodes
pulse energy at photocathode uJ 2.8
assuming QE=0.2% & 3.5 nC bunch chg
average laser power needed at cathode W 2
assuming QE=0.2%
avg laser power output W 4
pulse width nsec 1.5 Gaussian FWHM
jitter psec 10 rms
amplitude stability 1.00E-03requiresnoise-eater
contrast 1.00E-06
Phase I Laser System
• 10 W Erbium doped fiber amplifier (EDFA) system at 1560 nm, frequency doubled in periodically-poled LiNBO3
• Continuous Wave distributed feedback laser (CW DFB) + electro-optic modulation for pulse source• control of pulse shape, low jitter
• Frequency double to 780 nm in periodically poled material (40% efficiency)
• Design allows flexibility in pulse parameters
Electro-opticmodulator
Pulser with Phase-locked loop
4 stage EDFA10 W
1560 nm
Periodically – poled LiNbO3
4W780 nm
CW DFB laser
Accelerator RF ref
![Page 3: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/3.jpg)
June 28, 2012
Brian Sheehy
Laser Requirements
• 14 uJ energy per pulse in the 1560 nm fundamental (9 kW peak, 10W avg power)• we will frequency-double to 780 nm in periodically-poled LiNbO3 (PPLN)
• expect 40% conversion => 5.6 uJ at 780 nm• for 3.5 nC charge at 0.2% QE, 2.8 uJ is needed
• 1.5 nsec FWHM Gaussian pulses • EO modulated CW DFB laser for front end
• 704 kHz (14.07 MHz/20) • i.e average power is 9.8 W @1560 nm, 3.9 W @ 780 nm
• Contrast -30 dB in the fundamental, -60 dB at 780 nm
• Synchronization jitter with respect to RF reference: 10 psec rms• beam dynamics requirement not determined, but probably between 10-100 psec
• Amplitude stability• will need 10-3 to 10-4 in the photocathode pulse for eRHIC. Expect maybe 10-2
from EDFA amplifier and polarization extinction ratio, and use noise-eater before the photocathode
![Page 4: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/4.jpg)
1560 nm Laser schematic. Abbreviations: MZI, Mach-Zender Interferometer, ER extinction ratio, EDFA erbium-doped fiber amplifier, ABC automatic bias control.
Optilab EDFA laser
![Page 5: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/5.jpg)
June 28, 2012
Brian Sheehy
![Page 6: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/6.jpg)
June 28, 2012
Brian Sheehy
Optilab EDFA test results continued Using 2.8nsec pulse @352 kHz
![Page 7: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/7.jpg)
Frequency doubling module
• EDFA module has been tested on site at Vendors and will ship in July
• Vendor progress on the doubling module has been very slow. We will implement that ourselves at BNL
![Page 8: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/8.jpg)
June 28, 2012
Brian Sheehy
Scaling to multiple Cathodes: Synchronization
The EO-modulated fiber laser design is extremely stable against timing jitter: no cavity lengths to stabilize, very little is introduced in the pulser electronics. We have tested this with open loop measurements of jitter in a green laser of similar design (Aculight), using a phase detector method (mix reference RF with filtered photodiode signal).- can add fast feedback through the RF driving the pulser, no mechanical
components- detectors placed near gun entrance
-0.05 0 0.05-6
-4
-2
0
2
4
6x 10
-3
time (sec)
sign
al (V
)
Trace 2
0.1 0.105 0.11 0.1150
200
400
600
800
1000
1200
signal (V)
coun
ts
Trace 2
-0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 0.05-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
time (sec)
sign
al (V
)
Trace 3
Reference = pulser RF σ = 1.3mV = 700 fsec Reference = Pulser + δf (calibration)
![Page 9: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/9.jpg)
June 28, 2012
Brian Sheehy
signal generator 2 (for calibration)
signal generator
Picosecond pulser
Low-pass filter 2 MHzSplitter
703.5 MHz bandpass filter
low noise preamp
Fast Photdiode
Aculight Laser
Monitor
Mixer
Digital Scope or DAQ system
Phase Stability Measurement Layout
ref
sign
al
• Extract RF from laser pulse train using fast photodiode + bandpass filter• Mix with reference RF, output • to calibrate (red), drive reference & signal arms with slightly different
frequencies• introduces constantly varying phase which yields sinusoidally varying output,
the amplitude of which gives the calibration.
)cos(
![Page 10: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/10.jpg)
June 28, 2012
Brian Sheehy
Problems in Scaling to multiple Cathodes: Transport
• How to manage 20 transport lines to Gun Platform• use large mode area fibers
• 15 um core photonic crystal fibers commercially available now
• peak intensity at our pulse specs ~ 2 GW/cm2
• larger cores possible• may need less energy than current specs
![Page 11: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/11.jpg)
June 28, 2012
Brian Sheehy
Problems in Scaling to multiple Cathodes: Transport
• Space limitations on Gun Platform table• minimize optics on the table
• refractive shaper• relay lenses• pickoff for sampling• l/4 plate• dump
• difficult but not impossible
![Page 12: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/12.jpg)
June 28, 2012
Brian Sheehy
Other long term optical issues• XHV windows with minimal birefringence
• using zero-degree sapphire for Phase I• will test depolarization
• with wedge/tilt for stray light reduction• pursuing other materials with vendors
• stray light reduction• AR coatings capable of withstanding bakeout temperature can
be made with ion beam deposition (MPF Products Inc)• working on tilted entry design and dumping window-reflected
beam in vacuum• primary reflected beam can be coupled out of chamber
• Homogeneity of bunch charge across 20 cathodes• adjustment is easy: laser intensity• need some method of non-destructive charge measurement in
the electron beam• use signals from BPM’s, FCT?
• inter-cathode variation less problematic than fluctuations from one cathode
• each ion bunch “talks” to only one cathode• QE decay is slow
![Page 13: Laser and Optical Issues in Gatling Gun Development Brian Sheehy June 28, 2012](https://reader036.fdocuments.us/reader036/viewer/2022081515/5681691c550346895de03f23/html5/thumbnails/13.jpg)
June 28, 2012
Brian Sheehy
Summary
• Phase I laser is under development, 1560 nm section near completion• custom commercial EDFA + in house doubling
module
• Addressing problems with extrapolation to full 20 cathode gun• Phase I system will be a useful testbed (eg fiber
transport, synchronization, noise-eater)• problems are daunting, but not insurmountable.