30 nov. 06 i.will et al., max born institute: long trains of flat-top laser pulses photocathode...

30 Nov. 0630 Nov. 06I.Will et al., Max Born Institute: Long trains of flat-top laser pulsesI.Will et al., Max Born Institute: Long trains of flat-top laser pulses

Photocathode lasers Photocathode lasers generating long trainsgenerating long trains

of flat-top pulsesof flat-top pulses

Ingo Will, Guido KlemzIngo Will, Guido Klemz

Max Born Institute BerlinMax Born Institute Berlin800 s

1 s100 ps

(10mm glass plate)


Optical sampling system Optical sampling system for high-resolution measurement for high-resolution measurement

of the longitudinal pulse shapeof the longitudinal pulse shape

Ingo Will, Guido KlemzIngo Will, Guido Klemz

Max Born Institute BerlinMax Born Institute Berlin

100 ps(10mm glass plate)


The desired pulse trains and pulse energy(superconducting linac, Cs2Te photocathode)

Spacing of the pulses: 1 Spacing of the pulses: 1 ss• In future: 0.2 In future: 0.2 s = 5 MHz (XFEL)s = 5 MHz (XFEL)

and 0.11 and 0.11 s = 9 MHz (option for FLASH)s = 9 MHz (option for FLASH)

Duration of the pulse train: Duration of the pulse train: at least 800 at least 800 s, variables, variable

Very reliable synchronizationVery reliable synchronization Rectangular envelope of the pulse Rectangular envelope of the pulse

trains trains Energy:Energy:

• > 100 > 100 J in the IR J in the IR (I.e. (I.e. = 1047 nm) = 1047 nm)

– corresponds to >100 W power during the pulse corresponds to >100 W power during the pulse traintrain

• 15 15 J in the UV (I.e. J in the UV (I.e. = 262 nm) = 262 nm)

800 s

1 sDesired parameters (according to the requirements specified by DESY):


Desired pulse shape Desired parameters Desired parameters

of the micropulsesof the micropulses• Wavelength: UV (262 nm)Wavelength: UV (262 nm)• Edges: < 2 ps (UV)Edges: < 2 ps (UV)• Noise in the flat-top region: Noise in the flat-top region:

< 10…20 %< 10…20 %• Pulse duration Pulse duration ~ 20 ps ~ 20 ps

Completely remote-Completely remote-controlled laser systemcontrolled laser system

Very reliable Very reliable synchronizationsynchronization

20 ps

< 2 ps < 2 ps


First completely diode-pumped pulse-train laser First completely diode-pumped pulse-train laser operational at PITZ since April 2005operational at PITZ since April 2005

photo-diode

#1

photo-diode

#3

wavelengthconversion

IR -> UV

outputpulses

photo-diode

#2

pulseshaper

boosteramplifier(2 stages)

mainpulse picker

auxiliarypulse picker

preamplifier(4 stages)

modelockedoscillator

Conclusion: The duration of the train in the amplifiers must be much larger (>1.5 ms) than the length of the output train


Pre-compensation of changes of the pulse shape Pre-compensation of changes of the pulse shape during amplification and conversion to the UVduring amplification and conversion to the UV

flat top:17.2 ps

edges(10-90%):

4.9 psFWHM:22.5 ps

time [ps]0 20 40 8060

IR = 1.047 m

green = 0.524 m

UV = 0.262 m

flat top:16.4 ps

edges(10-90%):

6.1 psFWHM:23.3 ps

time [ps]0 20 40 8060

flat top:17.3 ps

edges(10-90%):

6.5 psFWHM:23.8 ps

time [ps]0 20 40 8060


The MBI setup of a generator of long flat-top The MBI setup of a generator of long flat-top pulse trains for the superconducting linacpulse trains for the superconducting linac

photo-diode

#1

photo-diode

#3


IR -> UV

outputpulses

photo-diode

#2

pulseshaper

boosteramplifier

mainpulse picker


preamplifiermodelockedoscillator

100 ps 100 ps


MBI setup of a generator of long flat-top MBI setup of a generator of long flat-top pulse trains for the superconducting linacpulse trains for the superconducting linac

photo-diode

#1

photo-diode

#3


IR -> UV

outputpulses

photo-diode

#2

pulseshaper

boosteramplifier

mainpulse picker



Effect of all components of the laser on the micropulsesEffect of all components of the laser on the micropulsesshould be constant for a duration of 800 should be constant for a duration of 800 ss

Components of the laser should work Components of the laser should work with 1 MHz rep. rate during the trainwith 1 MHz rep. rate during the train

High average power during the train of the final amplifiers: High average power during the train of the final amplifiers: PPtraintrain > 100 W > 100 W


Selected pulse shaping techniques: Selected pulse shaping techniques: suitability for single pulses and pulse trainssuitability for single pulses and pulse trainsTypeType MethodeMethode schemescheme effecteffect Single Single

pulses pulses Pulse Pulse trainstrains

(bursts)(bursts)

Linear Linear shaping shaping techniquestechniques

Grating pulse Grating pulse shaper:shaper:

Spectral shaperSpectral shaper

Spectral decomposition

ok ok.ok.

Grating pulse Grating pulse shaper:shaper:

Direct Direct space-to-timespace-to-time

Diffraction on a grating

ok okok

Acoustooptic filterAcoustooptic filter(i.e. DAZZLER)(i.e. DAZZLER)

Diffraction on a sound wave

package

ok Not Not suitablesuitable

Birefringent filterBirefringent filter filter with a transmission

sin()/

ok okok


Selected pulse shaping techniques/effects: Selected pulse shaping techniques/effects: suitability for single pulses and pulse trainssuitability for single pulses and pulse trainsTypeType MethodMethod schemescheme effecteffect Single Single

pulses pulses Pulse Pulse trainstrains

(bursts)(bursts)

Nonlinear Nonlinear shaping shaping techniquestechniques

Fiber shaperFiber shaper Self-phase modulation in a

fiber

ok ok.ok.

Nonlinear amplifying Nonlinear amplifying loop mirror (NALM)loop mirror (NALM)

Nonlinear phase shift in a fiber

ok okok

Fast optical power Fast optical power limiterlimiter

Nonlinear phase shift in a bulk

medium

ok Limited, Limited, depends depends

on NL on NL mediummedium

Nonlinear interaction Nonlinear interaction in crystalsin crystals

(SHG, FHG, OPA)(SHG, FHG, OPA)

Nonlinear interaction in

3 crystals

ok okokoutputpulses

UV


ddapertureaperture = 9 mm = 9 mm

FWHMFWHM = 75 ps = 75 ps





output pulses recorded with a streak camera:

Flat-top laser pulsesFlat-top laser pulses• generate electron bunches

with a flat-top shape in z-direction

• -> improved brightness -> improved brightness of the electron beamof the electron beam

knifeedge

knifeedge

flat-topoutput pulses

slit

Gaussianinput pulses

grating

toamplifier

chain

master oscillator

synchroscanstreak camera

(3 ps resolution)

short-pulseNd:YLF oscillator = 7 ps FWHM

Simple DST shaper forming flat-top laser pulsesSimple DST shaper forming flat-top laser pulses


Simple DST shaper forming flat-top laser pulsesSimple DST shaper forming flat-top laser pulses







output pulses recorded with a streak camera:

Flat-top laser pulsesFlat-top laser pulses• generate electron bunches

with a flat-top shape in z-direction

• -> improved brightness -> improved brightness of the electron beamof the electron beam


Amplification of flat-top pulses from an Yb:YAG oscillator

Record of flat-top pulses with a synchroscan streak camera (Optronis, ~3...4 ps resolution) at 515 nm wavelength

Parameters of the pulses Parameters of the pulses shown:shown:• length of the train: 1.5 ms length of the train: 1.5 ms

(1500 pulses)(1500 pulses)• Energy in the train: 27 mJEnergy in the train: 27 mJ• Energy per micropulse: 18 Energy per micropulse: 18 J J

(at 1030 nm)(at 1030 nm)• Streak camera measurement Streak camera measurement

taken taken with SHG (at 515 nm)with SHG (at 515 nm)

Energy is ~ 4…5 times Energy is ~ 4…5 times smaller than in the present smaller than in the present Nd:YLF phothocathode laserNd:YLF phothocathode laser

Increasing this energy is a Increasing this energy is a major challenge to the laser major challenge to the laser designerdesigner



The MBI setup of a generator of long flat-top The MBI setup of a generator of long flat-top pulse trains for the superconducting linacpulse trains for the superconducting linac

photo-diode

#1

photo-diode

#3


IR -> UV

outputpulses

photo-diode

#2

pulseshaper

boosteramplifier

mainpulse picker



100 ps 100 ps


Some amplification techniques: Some amplification techniques: suitability for single pulses and pulse trainssuitability for single pulses and pulse trainsTypeType MaterialMaterial Single pulses Single pulses Amplifiers for pulse trainsAmplifiers for pulse trains

(bursts)(bursts)

Optical-parametric Optical-parametric amplifiersamplifiers

(OPA)(OPA)

BBO, LBO, KTPBBO, LBO, KTP Ok OkOk(implemented in the FLASH (implemented in the FLASH

pump/probe laser)pump/probe laser)

Laser amplifiers:Laser amplifiers: Nd:YLFNd:YLF Ok OkOk

Edges limited to > 4...5 psEdges limited to > 4...5 ps(see present PITZ (see present PITZ

photocathode laser)photocathode laser)

Ti:SaTi:Sa Ok

(regenerative amplifier required)

Unknown:Unknown:- very strong thermal lensvery strong thermal lens

- Sophisticated pump laser Sophisticated pump laser neededneeded

Yb-doped mediaYb-doped media(Yb:KGW, (Yb:KGW, Yb:YAG,Yb:YAG,

Yb:CaF)Yb:CaF)

Ok(regenerative

amplifier required)

Under development,Under development,- moderate thermal lensmoderate thermal lens

- regen required, but too low regen required, but too low saturation during a single saturation during a single

micropulse micropulse


Part 2: OPCPA stage generating femtosecond pulses

output pulse train which output pulse train which contains 700 micropulsescontains 700 micropulses

OPCPA: OPCPA: Optical Parametric Chirped-Pulse amplification Generates femtosecond pulses Generates femtosecond pulses

150 fs FWHM 150 fs FWHM pulse energy available at present : pulse energy available at present :

EEmicromicro = 100 = 100 J (before J (before compressor)compressor)

EEmicromicro = 50 = 50 J (behind J (behind compressor)compressor)

Available wavelength: Available wavelength: • = 790…830 nm= 790…830 nm• on request: on request: = 395…415 nm = 395…415 nm

G ~ 20

Ti:Sa oscillator grating stretchergrating

compressor

outputpulse trains800 s long, = 790 ...

830 nm

three-crystalOPA

Piezo

photodiode

mixer1.3 GHz

master clockf = 1.3 GHz

primarysynchronization loop

G > 5 000

= 15 ps

picosecond-pulseoutput channel:

pulse trains, 800 s long

= 150 fs (FWHM)Emicro = 50...100 J

@ f= 1 MHz

synchronizedNd:YLF Burst-Mode laser

pumping the OPA = 523 nm

= 12 ps(FWHM)

100 fs

pulsepicker


= 14 ps (FWHM)Emicro = 1.2 mJJ @ f = 1 MHz

Esingle pulse > 8 mJ

G ~ 20

fastcurrent

controller

fastcurrent

controller

Ti:Sa oscillator

gratingstretcher

gratingcompressor

outputpulse trains800 s long, = 790 ...

830 nm

three-crystaloptical-parametric amplifier

(OPA)

Piezo

photodiode

mixer1.3 GHz

master clockf = 1.3 GHz

primarysynchronization loop

G > 5 000 100 fs = 15 ps

diode-pumpedNd:YLF oscillator

AOM

fround trip = 27 MHz

EOMAOM

Faradayisolator

pulsepicker1 MHz

pulsepicker

two-stage diode-pumpedNd:YLF amplifier

fastcurrent

controller

SHGcrystal

pumpdiodes

pumpdiodes

three-stageflashlamp-pumpedbooster amplifier

outputpulse trains800 s long, = 523 nm

= 12 ps (FWHM)Emicro = 600 J @ f = 1 MHz

Esingle pulse = 4 mJ

= 150 fs (FWHM)Emicro = 50...100J

@ f= 1 MHz

OP

CP

A

sta

ge

Pu

mp

lase

rScheme of the Pump-Probe laser


Regenerative amplifiers can be made to Regenerative amplifiers can be made to work at 1 MHz repetition ratework at 1 MHz repetition rate

Specialty in burst mode:Specialty in burst mode:Each micropulse can extract only a small Each micropulse can extract only a small fraction (~ 0.2%) of the stored energy fraction (~ 0.2%) of the stored energy • Low stability (2% fluctuation during the Low stability (2% fluctuation during the

train)train)• Reduced efficiency (~50%) in Reduced efficiency (~50%) in

comparison to single pulsescomparison to single pulses• Failure in the trigger will damage the Failure in the trigger will damage the

amplifieramplifier– sophisticated software solution, (present sophisticated software solution, (present

DOOCS not save) DOOCS not save) – NL limiterNL limiter– Fast repair technology Fast repair technology

Pulse traveling in the resonator

Output pulses

20 s1ms (1000 pulses)

SHG limiter

fiber-coupledpump diode

"switch-out"Pockels cell

"switch-in"Pockels cell

Yb:YAG

Faradayisolator

Faradayisolator

outputbeam

inputbeam

polarizer

polarizer

M1

M4

M2

M3

50 ns


Two-stage regenerative amplifier conceptTwo-stage regenerative amplifier concept

Thermal lens in the power regen Thermal lens in the power regen leads to a drop of the intensity to leads to a drop of the intensity to 50% during 2000 pulses50% during 2000 pulses

The two- or three-stage regen The two- or three-stage regen concept may enable us to apply concept may enable us to apply advanced amplifier techniques advanced amplifier techniques (i.e. thin-disk amplifiers) (i.e. thin-disk amplifiers)

First regen

Secondregen

Emicro

= 15 J

Emicro

= 3 J

2ms (2000 pulses)

Yb:KGW oscillator Yb:YAG regen

Yb:YAG power regen

DST shaper

Drop due to thermal lensing

First regen

Secondregen

Compensation of Compensation of the drop by the the drop by the drive current of the drive current of the pump diodes, pump diodes, but the but the „pumping“ „pumping“ of the beam diamter of the beam diamter remainsremains!!

2ms (2000 pulses)

Emicro

= 15 J

Emicro

= 3 J


Amplification of long pulse trains for the Amplification of long pulse trains for the cold linac by an Yb:YAG boostercold linac by an Yb:YAG booster

Stable pulse train: control of the ramp of the current of the Stable pulse train: control of the ramp of the current of the pump diodespump diodes

Stable beam diameter: Stable beam diameter: beam-shaping aperture at the outputbeam-shaping aperture at the output• Can the beam-shaping aperture in the beamline play this role? Can the beam-shaping aperture in the beamline play this role?

Technology for lossless stabilisation of the beam diameter: Technology for lossless stabilisation of the beam diameter: fast deformable mirrorfast deformable mirror

Emicro

= 15 J

Emicro

= 3 JYb:YAG

fiber-coupledpump diodes

photodiode

Yb:YAG

inputfrom regen

outputbeam

Energy per Energy per micropulse:micropulse: ~ 15 ~ 15 JJ

Amplification Amplification (two stages): (two stages): G = 5...8 G = 5...8

5 ms(5000 pulses)

Without compensatio

nby pump current

5 ms(5000 pulses)

with compensationby pump current


Shortest pulses and bandwidth of this Shortest pulses and bandwidth of this amplifier combinationamplifier combination

Output pulses of the KGW oscillator: Output pulses of the KGW oscillator: = 0.5 ps = 0.5 ps Output pulses of the regen combination: Output pulses of the regen combination: = 1.8 ps = 1.8 ps

Can pulses of this duration efficiently be transferred to the UVCan pulses of this duration efficiently be transferred to the UV(forth harmonics, (forth harmonics, = 258 nm) ? = 258 nm) ?

Emicro

= 2x7 J

Emicro

= 3 JYb:KGW oscillator Yb:YAG regen

Yb:YAG power regenEmicro

= 2x0.1 J

12ps

2ps

d =1.2mm


Shortest pulses and bandwidth of this Shortest pulses and bandwidth of this amplifier combinationamplifier combination

Emicro

= 2x7 J

Emicro

= 3 JYb:KGW oscillator Yb:YAG regen

Yb:YAG power regen

Emicro

= 2x0.3 J

12ps2ps

Output pulses of the KGW oscillator: Output pulses of the KGW oscillator: = 0.5 ps = 0.5 ps Output pulses of the regen combination: Output pulses of the regen combination: = 1.8 ps = 1.8 ps

Can pulses of this duration efficiently be transferred to the UVCan pulses of this duration efficiently be transferred to the UV(forth harmonics, (forth harmonics, = 258 nm) ? = 258 nm) ?


> 75%of totalenergy

25%of totalenergy

thirdharmonics

tophotocathode

moderate power, large bandwidth Yb:KGW channel

sum frequencygeneration

pulse shaper(1% transmission)

rectangularly shapedmicropulses

E = 50 J, P = 50 W = 1038 nm

long micropulseswith long edge

E = 150 J, P = 150 W = 349 nm output:

rectangularly shapedmicropulses

with short edges = 260 nm

mask

pulsepicker

pulsepicker

large power, small bandwidth Nd:YLF channel

Mixing stageDiode-pumped

short-pulse oscillator < 1 ps, f = 54 MHz

amplifier chain(one regenerative andone linear amplifier)

Diode-pumpedshort-pulse oscillator = 50 ps, f = 27 MHz

Nd:YLF amplifier chain

Two-channel mixing scheme: reduced energy Two-channel mixing scheme: reduced energy requirements to the broadband laser amplifierrequirements to the broadband laser amplifier

75% of the total laser energy delivered by the Nd:YLF long-75% of the total laser energy delivered by the Nd:YLF long-pulse system pulse system

only 25% need to be delivered by broadband channelonly 25% need to be delivered by broadband channel

Broadband pulse:from Yb:KGW laser

- sharp edges- Emicro = 20J- =1038 nm

Narrowband pulse

from Nd:YLF laser

- slow edges

- Emicro = 100J

- = 349 nm

BBOcrystal

UV output pulse- sharp edges- Emicro > 20J- = 260 nm

tophotocathode

beam

stop

beamstop


transmission < 20 %losses > 80 %

Photocathodelaser

<--- 10...25 m --->

photo-cathode

beamline telescopeor spatial filterwith pinhole

Gaussianlaser beamD = 2 mm

overfilledbeam-shaping

apertureD = 1...4 mm

presentscheme

three times more energythan without pre-shaping

transmission ~ 70 %losses ~ 30 %

"pre-shaped"beam

asphericlens pair

Photocathodelaser withflat-top

pump profiles

<--- 10...25 m --->

photo-cathode

beamline telescopeor spatial filterwith pinhole

Gaussianlaser beamD = 2 mm

overfilledbeam-shaping

apertureD = 1...4 mm

pre-shapingby an asphericalLens pair

Pre-shaping the beam of the photocathode laser Pre-shaping the beam of the photocathode laser may significantly reduce losses in the beamlinemay significantly reduce losses in the beamline


Summary: Long trains of flat-top laser pulses Summary: Long trains of flat-top laser pulses for the superconducting linacfor the superconducting linac Pulse shaping techniques: Only minor limitationsPulse shaping techniques: Only minor limitations

Most linear pulse-shaping techniquesMost linear pulse-shaping techniques (gratings, filters etc) (gratings, filters etc) workwork for long trains for long trains Techniques based on travelling Techniques based on travelling acoustic waves (i.e. DAZZLER) cannot be usedacoustic waves (i.e. DAZZLER) cannot be used Nonlinear techniques: limited duration of the pulse train (100...500 micropules) Nonlinear techniques: limited duration of the pulse train (100...500 micropules)

(only for nonlinear techniques using bulk materials)(only for nonlinear techniques using bulk materials)

Amplifiers: Amplifiers: 1.1. Optical-parametric amplifiers (OPA): work without restrictions, Optical-parametric amplifiers (OPA): work without restrictions,

but large pump laser requiredbut large pump laser required

2.2. For laser amplifiers: The broadband laser materials require regenerative amplifiers in the For laser amplifiers: The broadband laser materials require regenerative amplifiers in the first stages. These regens can work at 1 MHz for Yb:YAG, Yb:KGW:first stages. These regens can work at 1 MHz for Yb:YAG, Yb:KGW: With somewhat reduced stability and with slightly less less efficiency (~ 50%) than for single With somewhat reduced stability and with slightly less less efficiency (~ 50%) than for single

pulses pulses Reason: low saturation during each micropulse (0.2% energy extraction per pulse)Reason: low saturation during each micropulse (0.2% energy extraction per pulse)

3.3. Linear power amplifiers: work well with pulse trains Linear power amplifiers: work well with pulse trains Some problems arise from theSome problems arise from the thermal lens, that drifts during the train thermal lens, that drifts during the train

Solution: Solution: Dynamic correction with fast deformable mirrorsDynamic correction with fast deformable mirrors

4.4. Ti:Saphire: No solutions for long, intense pulse trains availableTi:Saphire: No solutions for long, intense pulse trains available

We have made the correct choice for the laser material: We have made the correct choice for the laser material: Diode-pumped Yb:KGW and Yb:YAG instead of Ti:SaphireDiode-pumped Yb:KGW and Yb:YAG instead of Ti:Saphire

30 nov. 06 i.will et al., max born institute: long trains of flat-top laser pulses photocathode...

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