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Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 J.F. Schmerge, SLAC

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Page 1: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

Lecture 13:Lasers

High Brightness Electron Injectors for Light Sources

January 14-18, 2007

J.F. Schmerge, SLAC

Page 2: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

2

Outline• Laser Parameters

– Cathodes– Laser and Cathode

• Laser Components– Oscillator– Amplifier– Frequency Conversion– Transverse Pulse Shaping– Temporal Pulse Shaping– Transport Line– Diagnostics

Page 3: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

3

Cathodes

Type QE Lifetime WorkFunction

εthermal

Metal ≈ 10-4 Month ≈ 4 eV >0.5 μm/mm

Semi-conductor

≈ 10-2 Hours ≈ 2 eV >0.8 μm/mm

Page 4: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

4

Laser Cathode System

• Cathode choice determines laser parameters

• Laser choice determines operation of cathode

Page 5: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

5

Parameters• QE 5 10-5 requires 100 μJ

of laser energy to generate 1 nC

• 100 Hz repetition rate requires 10 mW of power but 1 MHz requires 100 W

• 4.5 eV work function requires 275 nm photon but 2.2 eV work function requires 550 nm photons

electrons

photons

electrons

laser photons

laser laser

nQEn

Q enhc hcQE n

eQEhcQRP REeQE

λ λ

λ

=

=

= =

= =

Page 6: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

6

Specs at the Cathode

• Wavelength• Energy• Temporal Pulse Shape• Beam Size• Transverse Beam Shape• Repetition Rate

Page 7: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

7

LCLS Injector Drive Laser

CW laser532 nm

fs Mode-LockedOscillator

Q-switched Laser532 nm, 100 mJ, 30 Hz

Q-switched Laser532 nm,100 mJ,30 Hz

Amplifier2-pass Bowtie

CompressorTHG

Pre-Amp4-pass Bowtie

Regen Amp

Stretcher AOPDF

15 mJ30 Hz

75 mJ30 Hz

60 mJ30 Hz

119MHz, 4 nJ20 nm@764 nm

4.5 W

>3mJ252 nm

EnergyController

Pulse Picker

LINAC RFClock

2nm@755 nm

20 mJ756 nm

SHG

UV generator

Lock to Clock loop

> 200 ps

9 ps (adjustable)

0-30 Hz adjustable

Transport System to the

RF GUN

Spectrumshaping

26 mJ

15 mJ

30 Hz, 1 mJ

Spatial shaping

Courtesy P. Hering

Page 8: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

8

Oscillator• Operates at sub-harmonic of RF frequency

– Typical round trip time is 10 ns– ≈100 MHz pulse rate which is 28th sub-harmonic of S-band

• Phase lock laser to RF system– Require < 1° of jitter at RF frequency– Minimize jitter by locking harmonic rather than the laser

fundamental frequency• Pulse energy roughly 1 nJ

– Requires significant amplification for injector application• Oscillator must provide sufficient bandwidth for pulse

shaping in later stages– Generally produce ≈ 100 fs pulse length

Page 9: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

9

Amplifier• Total gain required is approximately 7 orders of

magnitude• Use a regenerative amplifier, to amplify to mJ level and

external amplifiers for final amplification• Chirped Pulse Amplification (CPA)

– Stretch pulse in time– Minimizes non-linear effects such as self focussing, self phase

modulation– Allows increased amplification– Pulse is compressed after amplification

• Diode pumped amplifier minimize energy fluctuations• Thermal effects limit rep rate and energy per pulse

Page 10: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

10

Frequency Conversion• Non-linear crystal with second order susceptibility• Process commonly called Second Harmonic Generation

(SHG)• May require multiple stages to reach final photon

energy• Since process depends on E2 or I, the final pulse length

is often smaller than the input pulse length

2 31 2 3P E E Eχ χ χ= + + + ⋅⋅ ⋅

Page 11: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

11

Spatial Shaping

• Transverse shape of the laser is reproduced on the electron beam assuming the QE is independent of position

• Methods– Aperture Clipping– Aspheric Lens– Adaptive Optics

Page 12: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

12

Laser output beam Gaussian shape

Zoom telescopeAperture

Square shape

UV Laser beam profile ;The aperture is imaged and focused on the cathode (no diffraction rings)

Spatial Shaping With an Aperture

Courtesy P. Hering

Page 13: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

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Energy Loss WithAperture Clipping

00.10.20.30.40.50.60.70.80.9

1

0 0.2 0.4 0.6 0.8 1

ΔI/I0

E/E 0

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High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

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Aspheric Lens• Lens shaped to produce flat top output for a

Gaussian input with a specific beam size• Transmission near 100%• Developed by IBM for lithography application• Now commercially available from Newport

Gaussian Input Flat-top Output

Page 15: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

15

Aspheric Lens Measurement

Input Gaussian Output Flat-top

Page 16: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

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Effect of Input Position Jitter

Page 17: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

17

Adaptive Optics

• Deformable mirror used to generate nearly arbitrary transverse pulse shape

• In principle could be combined with e-beam transverse shape measurement to compensate for QE spatial variations and generate a flat e-beam transverse shape

Page 18: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

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Dazzler

• Acousto-Optic Programmable Dispersive Filter (AOPDF)

• Birefringent TeO2 Bragg diffracts different wavelengths at different depths – Optical path, or phase, is wavelength

dependent– The phase delay of each wavelength is

adjustable– Can generate approximate flat-top pulse

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High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

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Temporal Pulse Shaping

• Dazzler (AOPDF)• Frequency Domain• Time Domain

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High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

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Acousto-Optic Programable Dispersive FilterDazzler

Input pulse

Acoustic wave

Output pulse

TeO2 crystal

RF transducer

Diffracted beam

Courtesy P. Hering

Page 21: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

754 755 756 757 758 759 760 761 762

Wavelength (nm)

Spec

rtum

Inte

nsity

(a.u

.)

Super GaussianshapingDazzler outputspectrum

2 nm

Temporal profile obtained in the UV :The pulse duration is fixed by the compressor

The time shape via the spectrum shape is fixed by the Dazzler

9 ps

0

200

400

600

800

1000

1200

730 735 740 745 750 755 760 765 770 775 780 785 790 795 800Wavelength (nm)

Spec

trum

inte

nsity

(a.u

.)

Oscillator spectrum Dazzler shaping

18 nm

Temporal Shaping with Dazzler

Courtesy P. Hering

Page 22: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

22

Frequency Domain Pulse Shaping

Amplitudeand phase

mask

diffractiongrating

E(ω) E'(ω)

ω + δωω

input pulse lens

( ) ( ) ( ) ( ) ( )2

j tE E T I t E e dωω ω ω ω ω∞

−∞

′ ′= = ∫

Can generate nearly arbitrary pulse shapes limited by input bandwidth and available masks

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High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

23

Time Domain Pulse Shaping

• Split beam into multiple pulses• Adjust delay of each pulse and recombine• Gaussian pulse with delay between pulses

approximately equal to the rms width produce approximately flat-top pulse

• Interference between pulses problematic– Rotate polarization between pulses minimizes

interference– Elliptical polarization requires normal incidence due to

QE variation with polaration

Page 24: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

24

Time Domain Pulse Shaping

Beam Splitter

Retro-reflectors

Input

2 Output beams

Page 25: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

25

Transport System• Typically on the order of 10 m between the laser

room and the gun• Transport system must maintain all laser

parameters with minimal loss• Typically relay image the desired plane in the

laser room to the cathode– Maintains transverse beam shape– Minimizes positional jitter

• Some system use a hard aperture a few cm before the cathode to produce a final

Page 26: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

26

Diagnostics

• Monitor Spectrum• Pulse Length

– Streak Camera– AutoCorrelator– Cross Correlator

• Pulse Shape– CCD camera at virtual camera location

• Energy per pulse

Page 27: Lecture 13: Lasers - USPASuspas.fnal.gov/materials/08UCSC/HBl13_Lasers.pdf · Lecture 13: Lasers High Brightness Electron Injectors for Light Sources January 14-18, 2007 ... • Since

High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

27

Losses

• Transmission After Amplifier– Compression – 50%– Frequency Conversion – 10%– Temporal Pulse Shaping – 50%– Transverse Pulse Shaping – 25%– Transport – 50%– Total Transmission – 0.3%

• For 100 μJ at cathode requires 30 mJ from the amplifier

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High Brightness Electron Injectors for Storage Rings - January 14-18 2007

Lecture 13D.H. Dowell, S. Lidia, J.F. Schmerge

28

Summary

• Laser System very complex and usually requires dedicated operators

• Laser specs highly dependent on choice of cathode

• Laser is a significant fraction of the cost of the injector