Optical sampling system for detailed measurement of the longitudinal pulse shape

1. Optical sampling system for detailed measurement of the longitudinal pulse shape Ingo Will, Guido Klemz Max Born Institute Berlin 100 ps (10mm glass plate) I.Will, G. Klemz, Max Born Institute: Optical sampling system

2. Present status at PITZ: Pulse shape is measured using a synchroscan streak camera • Present limits: • Streak camera signal is noisy • Resolution limited: • Green light: to 2...4 ps • UV light: to 3…5 ps • Measurement is sensitive to illumination of the cathode • space-charge effects in the streak tube: pulse broadening to 60 ps • no direct measurement for IR • modification of the pulse shape in the amplifier chain cannot be investigated strong intensity noise of the streak camera time laser #2 resolution limited to 3..4 ps Measurement: Nov 03, 2003

3. Pre-compensation of changes of the pulse shape during amplification and conversion to the UV IR l = 1.047 mm green l = 0.524 mm UV l = 0.262 mm

4. Emicro = 3 mJ First regen Compensation of the drop by the drive current of the pump diodes, but the „pumping“ of the beam diamter remains! Emicro = 15 mJ Second regen 2ms (2000 pulses) Two-stage regenerative amplifier concept • Thermal lens in the power regen leads to a drop of the intensity to 50% during 2000 pulses • The two- or three-stage regen concept may enable us to apply advanced amplifier techniques (i.e. thin-disk amplifiers) Yb:KGW oscillator Yb:YAG regen Emicro = 3 mJ First regen DST shaper Drop due to thermal lensing Yb:YAG power regen Emicro = 15 mJ Second regen 2ms (2000 pulses)

5. Formation of flat-top laser pulses • Flat-top laser pulses • generate electron bunches with a flat-top shape in z-direction • -> improved brightness of the electron beam output pulses recorded with a streak camera:

6. Simple DST shaper forming flat-top laser pulses • Flat-top laser pulses • generate electron bunches with a flat-top shape in z-direction • -> improved brightness of the electron beam output pulses recorded with a streak camera:

7. Amplification of flat-top pulses from an Yb:YAG oscillator 100 ps (10mm glass plate) • Parameters of the pulses shown: • length of the train: 1.5 ms (1500 pulses) • Energy in the train: 27 mJ • Energy per micropulse: 18 mJ (at 1030 nm) • Streak camera measurement taken with SHG (at 515 nm) • Energy is ~ 4…5 times smaller than in the present Nd:YLF phothocathode laser • Increasing this energy is a major challenge to the laser designer Record of flat-top pulses with a synchroscan streak camera (Optronis, ~3...4 ps resolution) at 515 nm wavelength

8. Requirements for a system measuring the pulse shape Further progress in pulse shaping requires a measurement system with the following parameters: • Temporal resolution: better 1 ps • Suitable for IR, green and UV light • Clean signal, reliable measurement especially for: • the edges • the flatness of the pulse Question: How to build a system that displays the pulse shape in a real-time manner ?(complete measurement during each laser shot)

9. Different beam shapes depending on the alignment of the shaper These images are taken with 5 Hz repetition rate and displayed on an oscilloscope in a real-time manner

10. How does the optical sampling system work • Every sampling measurement system requires 1.periodic signals 2. a very fast gate3. Synchronization of the gate to the signal

11. Sampling measurement systems • Example of a sampling measurement device: Commercial RF sampling oscilloscopes • electrical switch based on very fast switching diodes • gating time (=resolution) reached with of today’s sampling oscilloscopes: t ~ 15 ps • Optical sampling system: • utilizes periodicity of the micropulses in the train(requires > 100 micopulses) • Fast optical gate: mixing with short laser pulses • Gating time (=resolution) determined by theduration of the pulses from the laser oscillator of the sampling system(at present: t= 0.5…0.6 ps) ~ 12 ps FWHM

12. ~ 12 ps FWHM Sampling system presently used to measure the pulse shape in the IR

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

14. Shortest pulses and bandwidth of this amplifier combination Emicro = 3 mJ • Output pulses of the KGW oscillator: t = 0.5 ps • Output pulses of the regen combination: t = 1.8 ps • Can pulses of this duration efficiently be transferred to the UV(forth harmonics, l = 258 nm) ? Yb:KGW oscillator 2ps Yb:YAG regen 12ps Emicro = 2x0.3 mJ Yb:YAG power regen Emicro = 2x7 mJ

15. edges < 2 ps ~ 14 ps FWHM Typical pulse shapes measured with the optical sampling system • Present status: • Sampling system operational in the IR (1030 nm) • work is ongoing to extend the measurement to the UV 6 ps 25 ps non-Gaussian pulse Gaussian pulse flat-top pulse

16. Summary • An optical sampling system is being developed at the MBI • displays the shape of ps pulses on a standard oscilloscopein a real-time manner • Based on cross correlation between the pulses to be measured and the femtosecond pulses from a KGW oscillator • Temporal resolution: 0.5 ps • Linearity in time: ~ 10% • The system utilizes the periodicity of the pulses of the photocathode lasers used at FLASH and PITZ • Work is ongoing extend the measurement range to the UV