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 [email protected]
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
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
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
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
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).
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
Top Related