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Laser and Optical Issues in Laser and Optical Issues in

Laser and Optical Issues in - PowerPoint Presentation

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Laser and Optical Issues in - PPT Presentation

Gatling Gun Development Brian Sheehy June 28 2012 I Laser description for Phase I experiments II Scaling Issues for multiple cathodes synchronization transport III Other long term optical issues ID: 276160

phase laser edfa pulse laser phase pulse edfa cathodes reference 780 pulser 1560 beam gun signal transport poled design mhz periodically power

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Slide1

Laser and Optical Issues in Gatling Gun DevelopmentBrian Sheehy June 28, 2012

I. Laser description for Phase I experiments

II. 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 cathodesSlide2

parameter

unit

spec

commentwavelengthnm780repetition ratekHz70414.07 MHz / 20 cathodes pulse energy at photocathodeuJ2.8assuming QE=0.2% & 3.5 nC bunch chgaverage laser power needed at cathodeW2assuming QE=0.2%avg laser power outputW4pulse widthnsec1.5Gaussian FWHMjitterpsec10rmsamplitude stability1.00E-03requiresnoise-eatercontrast1.00E-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 EDFA

10 W

1560 nm

Periodically – poled LiNbO

3

4W

780 nm

CW DFB laser

Accelerator RF refSlide3

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 needed1.5 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 nmContrast -30 dB in the fundamental, -60 dB at 780 nmSynchronization 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 photocathodeSlide4

1560 nm Laser schematic. Abbreviations: MZI, Mach-Zender Interferometer, ER extinction ratio, EDFA erbium-doped fiber amplifier, ABC automatic bias control.

Optilab

EDFA laserSlide5
Slide6

Optilab

EDFA test results continued

Using 2.8nsec pulse @352 kHz Slide7

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 Slide8

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 entranceReference = pulser RFσ = 1.3mV = 700 fsecReference = Pulser + δf (calibration)Slide9

signal generator 2 (for calibration)

signal generator

Picosecond pulser

Low-pass filter 2 MHzSplitter703.5 MHz bandpass filterlow noise preampFast PhotdiodeAculight LaserMonitorMixer Digital Scope or DAQ systemPhase Stability Measurement Layoutrefsignal 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.Slide10

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 specsSlide11

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 Slide12

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 slowSlide13

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.