Photoinjector

Synchronous Photoinjection

A B C A B C….

1497 MHz 111ps

Beam

Curr

ent

Extracting DC beam, very wasteful, most of the beam

dumped at chopper. Need ~ 2mA from gun to provide

100uA to one hall. Gun lifetime not good enough…..yet.

RF Chopper

Sets Bunchlength

Laser applications for accelerators

Laser Basics

Yuelin Li X-Ray Science Division Argonne National Laboratory ylli@aps.anl.gov

4

Content

Laser and accelerator history

Map of laser application in accelerators

Laser basics

– Rate equations

– Laser configurations

– Gaussian beam optics and ABCD law

– Laser cavity and laser modes

Laser configurations

– Mode-locking and Q-switch

– MOPA

– CPA and dispersion

Laser materials

Other lasers

– Semiconductor lasers

– Fiber lasers

Frequency conversion and short wavelength lasers

5

Laser

Light Amplification by Stimulated Emission of Radiation

Excited medium

If a medium has many excited molecules, one photon can become

many.

This is the essence of the laser. The factor by which an input beam is

amplified by a medium is called the gain and is represented by G.

Credit: R. Trebino

6

Rate equations for a two-level system

Rate equations for the densities of the two states:

21 2 2( )

dNBI N N AN

dt

12 1 2( )

dNBI N N AN

dt

22 2d N

BI N ANdt

Absorption Stimulated emission Spontaneous

emission

1 2N N N

1 2N N N

If the total number

of molecules is N:

2 1 2 1 22 ( ) ( )N N N N N

N N

2d N

BI N AN A Ndt

2

1

N2

N1

Laser Pump

Pump intensity

Credit: R. Trebino

7

Why inversion is impossible in a two-level system

0 2BI N AN A N

1 / sat

NN

I I

/ 2satI A B

In steady-state:

( 2 )A BI N AN

where:

N is always positive, no matter how high I is!

It’s impossible to achieve an inversion in a two-level system!

2d N

BI N AN A Ndt

/( 2 )N AN A BI

/(1 2 / )N N BI A

Isat is the saturation intensity.

2

1

N2

N1

Laser

Credit: R. Trebino

8

A 3-level system

Assume we pump to a state 3 that

rapidly decays to level 2.

21 2

dNBIN AN

dt

11 2

dNBIN AN

dt

1 22 2d N

BIN ANdt

Absorption

Spontaneous

emission

1 2N N N

1 2N N N

The total number

of molecules is N:

22N N N

d NBIN BI N AN A N

dt

Fast decay

Laser

Transition

Rate: A

Pump

Transition

Rate: BI

1

2

3

Level 3

decays

fast and

so is zero.

12N N N

Credit: R. Trebino

9

Population inversion, 3 level system

1 /

1 /

sat

sat

I IN N

I I

/satI A B

In steady-state:

( ) ( )A BI N A BI N

where:

Now if I > Isat, N is negative!

( ) /( )N N A BI A BI

Isat is the saturation intensity.

d NBIN BI N AN A N

dt

0 BIN BI N AN A N

Fast decay

Laser

Transition

Rate: A

Pump

Transition

Rate: BI

1

2

3

Gain: Ng Credit: R. Trebino

10

Rate equations for a four-level system

Now assume the lower laser level 1

also rapidly decays to a ground level 0.

20 2

dNBIN AN

dt

1 0,N

22 2( )

dNBI N N AN

dt

2N N

Laser

Transition

Pump

Transition

Fast decay

Fast decay

1

2

3

0 As before:

Because 0 2N N N

The total number

of molecules is N :

0 2N N N

d NBIN BI N A N

dt

At steady state: 0 BIN BI N A N Credit: R. Trebino

11

Population inversion in a four-level system (cont’d)

0 BIN BI N A N

/

1 /

sat

sat

I IN N

I I

/satI A Bwhere:

Isat is the saturation intensity.

Now, N is negative—always!

Laser

Transition

Pump

Transition

Fast decay

Fast decay

1

2

3

0

( / ) /(1 / )N BIN A BI A

/( )N BIN A BI

( )A BI N BIN

Credit: R. Trebino

12

How to build a laser

R = 100% R < 100%

I0 I1

I2 I3 Laser medium

I

• Laser medium

• Depends on wavelength, pulse duration, power

• Pump it: ASE

• Multimode in time and space

• Add resonator: laser oscillation

• Mode selection

Credit: R. Trebino

13

Gaussian optics: summary

.2

,2

,

,1)(

,)(

exp)(

),(

0

0

2

0

0

2

0

0

2

2

0

0

zb

w

wz

z

zwzw

zw

r

zw

wEzrE

w0: beam waist

z0: Rayleigh range

b: confocal parameter

• Gaussian distribution is the solution of paraxial Helmholtz equation

• TM00 mode

14

Cavity: Optical resonators

Credit: W. Koechner: Solid State Laser engineering,

Credit: Wekipedia

15

Stability of laser resonators

.1

,1

,2110

2

2

1

1

21

R

Lg

R

Lg

R

L

R

L

Credit: W. Koechner: Solid State Laser engineering,

Credit: Wekipedia

16

A cavity and laser oscillator

Resonant condition for cavity: L=n

Energetic electrons in a glow

discharge collide with and excite

He atoms, which then collide

with and transfer the excitation

to Ne atoms, an ideal 4-level

system. Credit: R. Trebino

Credit: Cundiff, UCB

17

Content

Laser and accelerator history

Map of laser application in accelerators

Laser basics

– Rate equations

– Laser configurations

– Gaussian beam optics and ABCD law

– Laser cavity and laser modes

Laser configurations

– Mode-locking and Q-switch

– MOPA

– CPA and dispersion

Laser materials

Other lasers

– Semiconductor lasers

– Fiber lasers

Frequency conversion and short wavelength lasers

18

Mode locking: what

Credit: Cundiff, UCB

19

Mode locking: how

Introduce amplitude or phase modulation/control

Active mode locking

• Acousto-optic modulator, driven by RF

Passive mode locking

• Saturable absorption

• Nonlinear lensing + aperture

• Nonlinear polarization rotation + polarizer

Mode locking: result

Shorter pulse, high intensity, larger bandwidth

Single mode

Accurate timing at round trip time

20

Ti: Sapphire oscillator: an example

Femtolasers: Fusion (28”x12”x3”)

21

Q-switch

LTQ

2

Q factor of a resonator

T: round trip time; : optical frequency;

I: fraction power loss per round trip

Q switch: reducing the loss

Active: accousto-optical, electro-

optical, and mechanical

Passive: saturable absorbers

22

Compressing,

Shaping,

Conversion,

diagnosing

filtering

stretching,

shaping,

filtering

A MOPA system Master Oscillator – Power Amplifier

An oscillator usually does not have enough energy, thus needs

amplification

A MOPA is expected to carry over the characteristics of an OSC

Pulse duration is limited at 10 ps due to damage

Master

Oscillator Pre-amp Power amp

Post

processing

23

A MOPA example: Flash lamp drive laser

24

Content

Laser and accelerator history

Map of laser application in accelerators

Laser basics

– Rate equations

– Laser configurations

– Gaussian beam optics and ABCD law

– Laser cavity and laser modes

Laser configurations

– Mode-lokcing and q-switch

– MOPA

– CPA and dispersion

Laser materials

Other lasers

– Semiconductor lasers

– Fiber lasers

Frequency conversion and short wavelength lasers

25

Laser materials

What we care

– Lasing mechanism: four-level systems is always preferred

– Lasing wavelength: tunable is better

– Lasing bandwidth: bigger is better but not always

– Pump requirement: visible preferred

– Gain lifetime: longer is better

– Damage threshold: higher is better

– Saturation flux: higher is better

– Heat conductivity: as high as possible

– Thermal stability: as small as possible

– Form: solid is always preferred

26

Laser materials

Host + active ions

Host

– Crystalline solids (Sapphire, Garnets, Fluoride, Aluminate, etc.)

• Difficult to grow to large size

• Narrow line width thus lower lasing threshold, and narrow absorption

band

• Good thermal conductivity

– Glass (property varies by make, and processing)

• Easy to make in large size and large quantities

• No well defined bonding field thus larger line width and higher lasing

threshold, large absorption band

• Lower thermal conductivity thus severe thermal birefringence and

thermal lensing, lower duty cycle

Active ions

– Rare earth ions: No. 58-71, most importantly, Nd3+, Er3+…

– Transition metals: Ti3+, Cr3+,

27

Laser material: Ti: Saaphire (Al2O3:Ti3+)

• Giving shortest pulse so far

• Wonderful tunability

• Good thermal properties

• Short gain lifetime (has to be

pumped by a ns-pulsed green laser)

28

Laser material (Nd:YAG)

• High saturation flux

• Narrow bandwidth (0.15 nm), thus

long pulse (>10 ps), high gain

• Good thermal properties

• Long gain lifetime (diode pump)

Absorption spectrum

Fluorescent spectrum

29

Laser material: (Nd:Glass)

• High saturation flux

• large bandwidth (20 nm), thus short

pulse (<1 ps),

• Poor thermal

• Long gain lifetime (diode pump)

• Only for big lasers now (PW or MJ)

Absorption spectrum

30

Laser Material: Nd:YLF

• High saturation flux

• Narrow bandwidth (0.15 nm), thus

long pulse (>10 ps), high gain

• Good thermal properties

• Long gain lifetime (diode pump)

• Difficult to handle

31

Laser materials: summary

Ti:Sa

Al2O3:Ti

Nd:YAG

Y3.0-xNdxAL5O12

Nd:Glass (Kigre Q-

88)

Y3Al5O12:Nd

Nd:YLF

LiY1.0-xNdxF4

Fluorescence life time (ms) 3.2 230 330 485

Peak wavelength (nm) 780 1064 1054 1047,1053

Line width (nm) 220 0.15 22 1

Emission cross section (10-19

cm2)

3 6.5 0.4 1.8

Saturation flux

(J/cm2)

0.9 0.6 4.5 .43

Thermal conductivity

( w cm-1 K-1)

0.5 0.14 0.0084 0.06

Thermal expansion coef

(10-6/C)

7.5 10 10

n 1.76 1.8 1.55 1.5

dn/dT (10-6/C) 7.3 -0.5

32

Content

Laser and accelerator history

Map of laser application in accelerators

Laser basics

– Rate equations

– Laser configurations

– Gaussian beam optics and ABCD law

– Laser cavity and laser modes

Laser configurations

– Mode-lokcing and q-switch

– MOPA

– CPA and dispersion

Laser materials

Other lasers

– Semiconductor lasers

– Fiber lasers

Frequency conversion and short wavelength lasers

33

Semiconductor laser: laser diode

Credit: http://www.olympusmicro.com/

• Convert current into light, can be tuned by junction temperature

• Used mostly for pumping other lasers, also CD and DVD players

• VCSELs and VECSELs (virtical cavity surface emission

lasers and virtical exteranl cavity surface emission lasers)

• Also for seeding pulse fiber lasers (next page)

34

Mode-locked semiconductor lasers: (using V)

• Can achieve sub picosecond pulse duration

• 500 mW power

• Widely tunable

• Very high rep rate, up to 100 GHz

Opt. Express 16, 5770 (2008)

35

Pump for lasers

Flash lamps:

– Converts electrical power to light

– Cheap, low pumping efficiency, and poor

stability.

– Still opted for situations when high energy

capacity is the key (NIF), suitable for ruby

laser, Nd:Glass, Nd:YAG, Nd:YLF, etc.

Laser diodes:

– converts electrical current into light

– High efficiency, high stability especially in CW

mode.

– Opted now for most off the shelf KHz system

and fiber lasers

Pump lasers (Nd:YAG or Nd: YLF)

– Both pumped by flash lamps and diodes

– Normally for Ti: Sapphire system.

36

Fiber lasers

Power progress in fiber laser sources

Teodoro et al, Laser Focus World, Nov. 2006

Peak Power over 1 MW

Average power over 2 kW

limited by available pump

Credit: David Richardson

37

Laser configuration: Fiber lasers

Can be a straight MOPA or a CPA

Low threshold, high efficiency, high rep rate, high power

No thermal problem, good stability

Relatively large bandwidth for CPA fiber

Credit: David Richardson

J. Limpert et al., 'High-power ultrafast fiber laser systems,' IEEE Xplore 12, 233 (2006).

38

Fiber laser wavelength

Credit: David Richardson

39

Fiber laser capabilities

Credit: David Richardson

40

Fiber laser architecture

Credit: David Richardson

41

A short pulse MOPA fiber laser example 1 GHz, 20 ps, 321 W average and 13 kW peak power

Dupriez et al, http://www.ofcnfoec.org/materials/PDP3.pdf

42

A CPA fiber laser example 73 MHz, 220 fs, 131 W average and 8.2 MW peak power

Roeser et al., Opt. Lett. 30, 2754 (2005)

J. Limpert et al., 'High-power ultrafast fiber laser systems,' IEEE Xplore 12, 233 (2006).

1 mm, 1 mJ, 1 ps, 50 kHz has been achieved, F. Roeser et al, Opt. Lett. 32, 3294 (2007)

Synchronous Photoinjection

A B C A B C….

1497 MHz 111ps

Beam

Curr

ent

Extracting DC beam, very wasteful, most of the beam

dumped at chopper. Need ~ 2mA from gun to provide

100uA to one hall. Gun lifetime not good enough…..yet.

RF Chopper

Sets Bunchlength

Gain-switched Diode Laser and Fiber

Amplifier

Attenuator

PC WP LP

Shutter

Rotatable GaAs

Photocathode

V-Wien Filter

Vacuum

Window

15° Dipole

PZT

Mirror

IHWP

RHWP

Pockels

Cell

Delayed

Helicity Fiber

HV Supply

(0 – 4 kV)

HV Supply

(0 – 90 V)

CEBAF

Hall

T-Settle

Fiber

Charge Feedback (PITA)

Electron

Beam

Helicity Fiber

Charge Feedback (IA)

LP HWP LP

IA

Target

BCM

BPMs

5 MeV

Helicity Magnets

Parity

DAQ

nHelicity

Fiber

Position

Feedback

Helicity

Generator

H-Wien Filter

Spin Solenoids

MOPA - with Gain Switched Diode

Diode seed laser

Diode amplifier

0.5 GHz

1 GHz

1.5 GHz

2 GHz

• Easy to phase lock to accelerator

• Components readily available at

780nm and 850nm but…..

• Not easily wavelength tunable and

• Not much power, only ~ 100mW

Ti-Sapphire Lasers at CEBAF

Homemade harmonic

modelocked Ti-Sapphire

laser. Seeded with light

from gain switched diode.

No active cavity length

feedback. It was a bit

noisy….

• Needed more laser power for

high current experiments

• Diode lasers out, ti-sapphire

laser in

• Re-align Ti-Sapphire lasers

each week

Commercial laser with 499MHz rep rate from Time Bandwidth Products

TJNAF 12 Month Time Accounting by System

0

24

48

72

96

120

144

168

192

216

240

264

288

312

336

360

384

408

Bea

m S

tudi

es 5

.9%

FSD

Trip

s 3.

8%O

ptic

s U

nSch

ed 2

.9%

DC

Pow

er 2

.5%

Ops

2.1

%G

un 1

.9%

RF

1.7%

Vac

uum

1.3

%D

iag

1.2%

Con

trols

1.0

%R

AD

0.9

%S

W 0

.7%

Oth

er 0

.6%

SR

F 0.

5%C

ryo

0.4%

Faci

lity

Mng

0.3

%M

PS

0.3

%P

SS

0.3

%M

ag 0

.0%

Inje

ctor

0.0

%

June'05 55.4%

358/648hrs. July'05 65.2%

227/570hrsAug'05 82.0%

155/742hrsSept'05 84.6%

146/560hrsOct'05 82.7%

123/473hrsNov'05 75.2%

130/552hrsDec'05 85.2%

80/535hrsFeb'06 90.0%

117/571hrsMarch'06 79.9%

268/699hrsApril'06 81.2%

125/705hrsMay'06 84.1%

113/413hrs

1% line

Hours Lost

Accelerator Downtime FY05Q4 – FY06Q3

Realign Ti-Sapphire lasers each week

Fiber-Based Drive Laser

Gain-switched seed

ErYb-doped fiber amplifier

Frequency-doubler

1560nm

780nm

Fiber-Based Drive Laser

e-bunch

~ 30 ps

autocorrelator trace

CEBAF’s last laser!

Gain-switching better than

modelocking; no phase lock

problems, no feedback

Very high power

Telecom industry spurs growth,

ensures availability

Useful because of superlattice

photocathode (requires 780nm)

Ti-sapp

Accelerator Downtime FY06Q4 – FY07Q3

TJNAF 12 Month Time Accounting

024487296

120144168192216240264288312336360384408432

Optic

s U

nSch

ed 6

.4%

Beam

Stu

die

s 3.

7%

FS

D T

rips

3.5

%R

F 3

.0%

DC

Pow

er

2.3%

Ops

1.9

%S

W 0

.9%

Oth

er 0.8

%V

acu

um

0.7

%P

SS

0.6

%G

un 0

.6%

Faci

lity

Mng

0.

5%

Dia

g 0.5

%M

PS

0.5

%C

ryo

0.4

%C

ontrols

0.4

%R

AD

0.2

%M

ag 0

.1%

Inje

ctor

0.1%

SR

F 0

.0%

July'06 63.1%179/304hrsAug'06 74.3% 182/732hrsSept'06 78.3%108/382hrsOct'06 83.1% 139/699hrsNov'06 86%97/558hrsDec'06 84.1%117/496hrsJan'07 85.5%104/549hrsFeb'07 85.8%104/528hrsMarch'07 76.2%184/595hrsApril'07 81.9%189/569hrsMay'07 70.5%252/739hrs

8/06 Capture Regulation

10/06 MARC7A SCR problems

5/07 MARC8A overtemp

~1% line for 12 months

Hours

Fiber-based lasers reduce downtime