Optical sampling system for detailed measurement of the longitudinal pulse shape

30. Nov. 200630. Nov. 2006I.Will, G. Klemz, Max Born Institute: Optical sampling systemI.Will, G. Klemz, Max Born Institute: Optical sampling system

Optical sampling system Optical sampling system for detailed measurement for detailed 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)


Present status at PITZ: Pulse shape is measured using a synchroscan streak camera

Present limits:Present limits:• Streak camera signal is noisyStreak camera signal is noisy• Resolution limited:Resolution limited:

• Green light: to 2...4 ps Green light: to 2...4 ps • UV light: to 3…5 psUV light: to 3…5 ps

• Measurement is sensitive to Measurement is sensitive to illumination of the cathodeillumination of the cathode

• space-charge effects in the space-charge effects in the streak tube: streak tube: pulse broadening to 60 ps pulse broadening to 60 ps

• no direct measurement for IR no direct measurement for IR • modification of the pulse modification of the pulse

shape in the amplifier chain shape in the amplifier chain cannot be investigatedcannot be investigated

laser #2

tim

e

Measurement: Nov 03, 2003

resolution limited to 3..4 ps

strong intensity noiseof the streak camera


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


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


Formation of flat-top laser pulsesFormation of flat-top laser pulses

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







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

100 ps(10mm glass plate)


Requirements for a system Requirements for a system measuring the pulse shapemeasuring the pulse shape

Further progress in pulse shaping requires a measurement Further progress in pulse shaping requires a measurement system with the following parameters:system with the following parameters:

Temporal resolution: better 1 psTemporal resolution: better 1 ps Suitable for IR, green and UV lightSuitable for IR, green and UV light Clean signal, reliable measurement especially for:Clean signal, reliable measurement especially for:

• the edgesthe edges

• the flatness of the pulsethe flatness of the pulse

Question: Question: How to build a system that displays the pulse shape in a real-How to build a system that displays the pulse shape in a real-time manner ?time manner ?(complete measurement during each laser shot)(complete measurement during each laser shot)


Different beam shapes depending on the Different beam shapes depending on the alignment of the shaperalignment of the shaper

These images are taken with 5 Hz repetition rate and displayed on These images are taken with 5 Hz repetition rate and displayed on an oscilloscope in a real-time manner an oscilloscope in a real-time manner


How does the optical sampling system workHow does the optical sampling system work

Every sampling measurement system requiresEvery sampling measurement system requires 1.1. periodic signals periodic signals

2.2. a very fast gate a very fast gate3.3. Synchronization of the gate to the signal Synchronization of the gate to the signal

Laser pulses (to be measured):

Pulses of the sampling laser:

Output of the mixing crystal:

Output of a slow photodiode (with low-pass filter):


Sampling measurement systemsSampling measurement systems

Example of a sampling measurement device: Example of a sampling measurement device: Commercial RF Commercial RF sampling oscilloscopessampling oscilloscopes • electrical switchelectrical switch based on very fast switching diodes based on very fast switching diodes

• gating timegating time (=resolution) reached with of today’s sampling oscilloscopes: (=resolution) reached with of today’s sampling oscilloscopes: ~ 15 ps ~ 15 ps

Optical sampling system:Optical sampling system:• utilizesutilizes periodicity of the micropulses in the train periodicity of the micropulses in the train

(requires > 100 micopulses)(requires > 100 micopulses)

• Fast Fast optical gate:optical gate: mixing with short laser pulses mixing with short laser pulses

• Gating time (=Gating time (=resolutionresolution) determined by the) determined by the duration of the pulses from the laser oscillator duration of the pulses from the laser oscillator of the sampling systemof the sampling system (at present:(at present: = 0.5…0.6 ps) = 0.5…0.6 ps)

~ 12 ps FWHM


Sampling system presently used toSampling system presently used to measure the pulse shape in the IR measure the pulse shape in the IR

~ 12 ps FWHM


Stable synchronisation during the scan Stable synchronisation during the scan requires an embedded C++ state machinerequires an embedded C++ state machine


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) ?


Typical pulse shapes measured Typical pulse shapes measured with the optical sampling systemwith the optical sampling system

Present status:Present status: Sampling system operational in the IR (1030 nm)Sampling system operational in the IR (1030 nm) work is ongoing to extend the measurement to the UVwork is ongoing to extend the measurement to the UV

Gaussian pulse flat-top pulsenon-Gaussian pulse

6 ps 25 ps

~ 14 ps FWHM

edges< 2 ps


SummarySummary An optical sampling system is being developed at the MBIAn optical sampling system is being developed at the MBI

• displays the shape of ps pulses on a standard oscilloscopedisplays the shape of ps pulses on a standard oscilloscopein a real-time manner in a real-time manner

• Based on cross correlation between the pulses to be measured and the Based on cross correlation between the pulses to be measured and the femtosecond pulses from a KGW oscillatorfemtosecond pulses from a KGW oscillator

• Temporal resolution: 0.5 psTemporal resolution: 0.5 ps• Linearity in time: ~ 10%Linearity in time: ~ 10%

The system utilizes the periodicity of the pulses of the The system utilizes the periodicity of the pulses of the photocathode lasers used at FLASH and PITZphotocathode lasers used at FLASH and PITZ

Work is ongoing extend the measurement range to the UVWork is ongoing extend the measurement range to the UV

Optical sampling system for detailed measurement of the longitudinal pulse shape

Documents

Transcript of Optical sampling system for detailed measurement of the longitudinal pulse shape