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M.Danailov,

February 2007

Winter College on Fibre Optics,

Fibre Lasers and Sensors

1

APPLICATIONS OF FIBRE LASERS

AND FIBRE OPTICS IN FREE

ELECTRON LASER FACILITIES

Miltcho Danailov

M.Danailov,

February 2007



2

OUTLINE

1. Introduction: What is a FEL?

2. Layout of a typical seeded single pass FEL: FERMI@ELETTRA

3. Motivations for fibre lasers and fibre optics use at FEL facilities

4. Fibre-based timing&synchronization system

- Laser master oscillator

- Stabilised fibre link

- Optical-to-optical synchronisation

- Direct seeding of Ti:Sapphire amplifiers by Er-doped fibre lasers (frequency doubling in PPLN and PPKTP)

M.Danailov,

February 2007



3

Introduction: What is an FEL?

Free-Electron Laser (FEL) is a light source exploiting the spontaneous and/or

induced emission of a relativistic electron beam “guided” by the periodic magnetic

field of an undulator

Main Ingredients:

Relativistic electron beam

Undulator

Electromagnetic field co-propagating with

the electron beam and getting amplified getting amplified on the

expense of electrons’ kinetic energy

M.Danailov,

February 2007



4

FEL Wavelength/power

Wavelength (m)

Ti:Sapphire

Present FEL’s

Future FEL’s

Photon energy (eV)

Pe

ak

po

we

r (W

)

Synchrotron radiation

Conventional

lasers

0.11101021031041054·105

HHG gases

Er fibre

M.Danailov,

February 2007



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Storage –ring FEL

Linac based FEL

FERMIproject

Existing linac

Elettra Synchrotron

in Trieste

THE FERMI PROJECT

M.Danailov,

February 2007



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Laser Systems

PI LASER

PUMP-PROBE

LASER 2

LASER

HEATER

LINAC

PUMP-PROBE

LASER 1

RF GUN UNDULATORS

B L 1

B L 2

SEED LASER

Microwave

Master

Oscillator

DIAGNOSTICS

LASER

SYNC SYNC SYNC SYNC

SYNC

The main ‘conventional’ lasers

included in the machine:

Photoinjector laser :

-high energy (0.5 mJ) UV 10 ps long

shaped pulses, flat –top beam,

jitter <200 fs

Laser Heater

-20 µJ range, 10 ps, IR (800 nm) pulses

Jitter <200 fs

Seed Laser

-10 µJ, broadly tuneable UV pulses,

100 fs long, <100 (50) fs jitter

Pump-probe (beam-line) lasers

-Variable energy, tuneable, <50 fs jitter

-Need for timing &synchronization distribution system

with less then 10 fs added jitter

- Need for local laser sync schemes and lasers

with less than 50 fs jitter (in the 10 Hz-10 MHz range)

STABILISED

FIBER LINKLaser Master

Oscillator

M.Danailov,

February 2007



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Optical Timing&Sync Layout

MAIN SUB-UNITS NEEDED

1. Low phase noise laser

master oscillator

2. Stabilised fibre link

3. Low-jitter optical to RF

Conversion

4. Low-jitter scheme for

laser-to-optical clock sync

5. Direct optical seeding of

Ti:Sapphire amplifiers

Laser Master

Oscillator

Low noise

Microwave

Oscillator

Optical-to-RF

conversion

Optical-to-RF

conversion

Optical-to-optical

Locking unit

Direct optical

seeding unit

Local laser

Oscillator

Ti:Sapphire

Amplifier

Stabilised

Fibre Link

M.Danailov,

February 2007



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Hybrid Master Oscillator

M.Danailov,

February 2007



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Laser Master OscillatorRequirements

Wavelength in the 1560 nm window (low

dispersion in fibre propagation,

SH coincides with Ti:Sapphire wavelength)

Low intrinsic phase noise->low jitter

Pulse-length in the few-100 fs range

Rep-rate 50MHz-3GHz range

(better above 100 MHz)

Lockable to external RF

oscillator

High long-term stability

Stable self-starting mode-

locking

Long diodes lifetime

Commercially available

ML Er-doped fibre laser

M.Danailov,

February 2007



10

Er-fibre lasers MIT

Courtesy Franz Kaertner

(MIT)

40 MHz laser

/4

/4/2

M.Danailov,

February 2007



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Stabilised fibre link

Proposed layout of the stabilised fibre link

Features:-Works by cross-correlating a pulse reflected by OC with a later pulse from the MO train

-Error signal drives a PZT fibre stretcher

-Bandwidth limited by 2nL/ c (200 KHz for L=500 m=> not a problem in our case)

M.Danailov,

February 2007



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Optical-to-RF conversion

Spectrum analyser screen with input

signal from a 100 MHz rep rate fibre laser

Seen by a 3 GHz PIN diode

Extraction of RF signal form a train

Of laser pulses

M.Danailov,

February 2007



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Optical-to-optical synchronisation

Local laser

3mmSFM

SFM

Rep.-RateControl

DISP

Timing

Pulse

Timing pulses at 1560 nm coming from the stabilised

fibre link

Pulses from the local laser (Ti:Sapphire, freq. doubled

fibre, etc) at 780 nm

Pulses produced by SFM at 520 nm

Measured 0.3 fs jitter in 10mHz

to 2.3 MHzT. Schibli et al, Opt.

Lett. 28, 947, 2003.

M.Danailov,

February 2007



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Direct Seeding of All Ti:Sapphire Amplifiers

PI LASER

PUMP-PROBE

LASER 2

LASER

HEATER

LINAC

PUMP-PROBE

LASER 1

STABILISED

FIBER LINK

RF GUN UNDULATORS

B L 1

B L 2

SEED LASER

Microwave

Master

Oscillator

DIAGNOSTICS

LASER

SYNC SYNC SYNC SYNC

SYNC

AMP

HG

AMP

HG

AMP

HG

AMP

HG

AMP

HG

Laser Master

Oscillator

Idea: replace all local laser oscillators

by a fibre amplifier + frequency doubler

M.Danailov,

February 2007



15

Direct Seeding of Ti:Sapphire amplifiers

~100 fs, 780 nm

Er-doped

fiber

input pulse

pump diodes

single-mode

fiber

SHG

PPLN

nonlinear pulse shaping

(amplify & compress)

Si prisms

Problems:A. Amplification to 2 nJ range without strong pulse distortion and addition of jitter

B. Efficient frequency doubling to 780 nm without pulse lengthening

A: Jitter added by the fibre amplifier

Result from

O.Ilday, Bilkent Univ.

M.Danailov,

February 2007



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Seeding of Ti:Sapphire amplifiers

• Frequency doubling requirements

a. Energy per pulse >0.4 nJ for stable operation of commercial amplifiers . Max commercially available ~150 mW , i.e. efficiency >30% needed !

b. For pulse duration of 100 fs GVM needs to be taken into account

c. Quality and bandwidth of the doubled spectrum

Solution: periodically poled crystals (PPLN, PPKTP)

Test experiment: TC1550 (Menlo systems) Fundamental

Power 140 mw

Pulse duration ~100fs

Rep rate 110 MHz

Spectrum width ~80 nm FWHM

Time-bandwidth product >2 T.L.

-500 -400 -300 -200 -100 0 100 200 300 400 500

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

a.u

.

time delay, fs

FWHM=151 fs

M.Danailov,

February 2007



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Frequency doubling in PPLN and PPKTP

Crystals:

PPLN : 1 mm length , period 18.2 µm-19

µm

Results: P775 nm= 42 mW (30% efficiency)

Pulse length 100 fs (sech2)

Bandwidth 6.5 nm

TBWP~1xTL

PPKTP: 1.5 mm length , period 25.2 µm

Focusing: >2 times stronger then Boyd-

Kleinman condition

Results:

P775 nm~20 mW (15 % efficiency)

Pulse length 150 fs (sech2)

Additional interesting effect:

Simultaneous generation of 2nd, 3rd and 4th

harmonic

M.Danailov,

February 2007



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Frequency doubling in PPLN and PPKTP

762 764 766 768 770 772 774 776 778 780 782 784 786 788 790

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FWHM=6.3 nm

Wavelength nm-500 -400 -300 -200 -100 0 100 200 300 400 500

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

a.u

.

Time Delay, fs

FWHM=152.8

Results with PPLN

Spectrum Autocorrelation

20 40 60 80 100 120 140 16

0

5

10

15

20

25

30

35

40

P2

, m

W

Pfund

, mW

R1 T=150 C f=11mm

Future work: - tests with 1.5 mm PPLN

- tests of double-pass scheme

Power dependence

M.Danailov,

February 2007



19

Jitter comparison

Comparison of integrated timing jitter

as a function of frequency for a fibre

laser (MENLO TC1550, red

line) and a Ti:Sapphire laser (Coherent

Mira, blue line)

a. Unlocked

b. Locked to external RF generator Movies by Graeme Hirst

P1010112.mov

M.Danailov,

February 2007



20

AKNOWLEDGEMENTS

• Elettra Team :

Alexander Demidovich , Rosen Ivanov,

Paolo Sigalotti, Mario Ferianis…

• MIT: Franz Kaertner

• DESY (Hamburg) : Axel Winter

• Bilkent University (Ankara)

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