# Presentation: 000326

## EO sampling technique for femtosecond beam characterization

- Jinhao Ruan
- A0 Photon Injector
- Fermi lab

## Outline

- Motivation
- Current EO Techniques and Limitation
- Principle
- Characteristics
- Current status and limitation
- Our plan

## Motivation

-   LUX: 30 fs FWHM @ 3 GeV

**Long Term goal:**

**Short Term goal:**

- Bunch length @ A0 right now   4-5 ps (rms)

- Emittance exchange experiment by Tim Koeth

- Bunch length less than 1 ps (rms)

- New Muon lab’s design will produce 300 μm bunch length (~ 1 ps rms)

- ILC’s design may require 150 μm bunch length (~ 0.5 ps rms)

## Principle of EOS

**Pockel’s effect (ZnTe)**

- Z(110)

- (001)

- p

- p

- By detecting this phase shift  we will know the electrical field

- By detecting the optical pulse we are hoping to get electron bunch information

-  Scanning Delay sampling
-  Spectral decoding
-  Temporal decoding
-  Spatial decoding

## Scanning Delay (SD) sampling

- The bunch profile is sampled by changing the delay between e-bunch and a femptosecond laser pulse
- Commonly used in THz spectroscopy (pump probe)
- Technically simple, highest resolution

- M. J. Fitch et al PRL 87, 34801, 2001

- J. Van Tilborg et al PRL 96, 14801, 2006

## Scanning Delay (SD) sampling

- From G. Berden, DESY

- No bunch length measurable due to  jitter (energy jitter from bunch compressor, Laser synchronization etc)

- In order to see the bunch structure  the jitter between pump and probe must be very small

## Scanning Delay sampling

- Very recently J. Tilborg in LBNL  is able to resolve a 50 fs electron bunch produced by laser acceleration.

- J. Tilborg et al PRL 96, 14801, 2006

## Spectral Decoding

- The laser pulse is stretched spectrally (chirped), the longitudinal structure is therefore encoded in the spectrum
- Single shot experiment
- The instantaneous bandwidth of the chirped pulse needs to be sufficient to represent the e-bunch structure

- I. Wilke et al PRL 88, 124801, 2002

## Spectral Decoding

***I. Wilke et al PRL 88, 124801, 2002***

- fundamental time resolution limit, *T**min* = √*t**o **t**c*
-       e.g.#1   *t**o **= 30 fs, t**c **= 20 ps,**   **T**min** = 770 fs *
-       e.g.#2   *t**o **= 4 fs,   t**c **= 3 ps,**      **T**min** = 110 fs*

***J. Fletcher Optics Express  10, 1425, 2003***

## Temporal Decoding

- The chirped laser pulse behind the EO crystal is measured by another short laser pulse using single shot cross correlation technique
- 1 mJ laser pulse energy necessary

- G. Berden et al PRL 93, 114802, 2002

## Temporal Decoding

***G. Berden et al PRL 93, 114802, 2004***

- FELIX, Holland

- Spectral decoding

- temporal decoding

## Temporal Decoding

- DESY, Germany

## Temporal Decoding

- From B. Steffen's talk at FLS’s workshop, DESY’s situation

## Temporal Decoding

- Time (ps)

- EO at first bunch and LOLA at second bunch

- Compressed

- Time (ps)

- ACC 1° overcompression

- Time (ps)

- ACC 2° overcompression

- Time (ps)

- ACC 3° overcompression

- Preliminary unpublished data by G. Steffen in DESY

## Spatial Decoding

- The femtosecond laser pulse is focused as a line image to the crystal and passes the crystal at an angle
- The bunch length is transferred to the spatial structure of the laser

- A. J. Cavalieri et al PRL 94, 114801, 2002

## Spatial Decoding

- A. J. Cavalieri et al PRL 94, 114801, 2002

- SLAC, USA

## Comparison

|  | SD sampling | Spectral decoding | Temporal decoding | Spacial decoding |
| --- | --- | --- | --- | --- |
| Pros | Simple laser system
Arbitrary time windows
High resolution | Simple laser system
Single shot measurement
High repetition rate
 | Large time window
High resolution (110fs)
Single shot measurement
 | Simple laser system
Single shot measurement
high resolution (160 fs)
High repetition rate |
| Cons | No single shot measurement
Very high requirement on Jitter between e-bunch and laser | Limited resolution (400 fs)
Distorted signal for e-bunches < 200fs
 | Complex laser system (mJ laser pulse energy)
Low repetition rate
 | More complex imaging optics
Good for clocking but tough to get the e-bunch information
 |

## Current effort

|  | Method | Laser source | Crystal used | Best resolution achieved |
| --- | --- | --- | --- | --- |
| DESY, Germany | EO Sampling
Spectral decoding
Temporal decoding
Spacial decoding | Ti:sapphire
Amplified Ti:sapphire | ZnTe
GaN | 110 fs |
| FELIX, Holland | EO Sampling
Spectral decoding
Temporal decoding | Ti:sapphire
Amplified Ti:sapphire | ZnTe | 110 fs |
| SLAC | Spacial decoding | Ti:sapphire | ZnTe | 300 fs |
| BNL | Spectral decoding | Ti:sapphire | ZnTe | 730 fs |
| LBNL | EO Sampling | Ti:sapphire | ZnTe | 50 fs |
| ANL | EO Sampling
Spectral decoding
Temporal decoding | Ti:sapphire
Amplified Ti:sapphire | ZnTe | No real e-bunch test yet. Off line Eo experiment ~ 280 fs |
| Fermi | EO Sampling
 | Nd:YAG 
glass | LiTaO3 | Unable to resolve direct field |

## Our Plan

- Jinhao Ruan (A0)
- Jamie Santucci (A0)
- Cheng-yan Tan (AD)
- Vic Scarpine (Instrumentation)
- RandyThurman-Keup (Instrumentation)

- Yuelin Li (APS)
- John Power (AWA)

- FNAL

- NIU

- P. Piot
- Tim Maxwell

- ANL

- Spatial Decoding setup

- Initial off-line EO experiment  setup

| Stage | Plan | Time | How | Goal |
| --- | --- | --- | --- | --- |
| 1 | Background study; 
optical layout;
Safety training | 1 month | Email communication | Everything ready for EO experiment |
| 2 | Reproduce Dr. Li’s setup  in AWA laser room  | 2-3 month | At least 1 day per week in AWA | EO experiment successful done on laser induced THz signal |
| 3 | Getting Ti-sa pulse into cave in AWA
EO experiment on OTR signal
EO experiment on e-beam (out side the Vac) | 2-3 month | 1 day per week in AWA
 | EO experiment done for e-beam outside the vacuum |

- Our Plan

## EO setup in DESY