Picosecond fiber laser for thin film
micro-processing
Author: Jaka Petelin
Mentor: doc. dr. Rok Petkovšek
Co-Mentor: dr. Boštjan Podobnik
March 2011
Introduction
Nonlinear Effects
Introduction
Contents:
Motivation
Fiber amplifier
Nonlinear effects
Self-phase-modulation
Stimulated Raman scattering
Stimulated Brillouin scattering
Burst Amplification
Summary
Burst Amplification
Summary
Introduction
Nonlinear Effects
Burst Amplification
Motivation
Material micro-processing applications currently employ
nanosecond laser sources
Shorter pulses: better resolution (less heat diffusion and thus
smaller heat-affected-zone)
Summary
Introduction
Nonlinear Effects
Burst Amplification
Summary
Motivation
However:
Lower energy of pulses
OR
Very high peak power at same pulse energy (nonlinear effects –
especially detrimental in fiber lasers and fiber amplifiers)
For thin film micro-processing (photovoltaics):
P  100 kW
E  10  J
For a 10 ps pulse the peak power would be over 1 MW (nonlinear
effects and fiber damage)
Possible solution: laser bursts
Introduction
Nonlinear Effects
Burst Amplification
Fiber amplifier
Rare-earth-doped optical fiber (eg. Er3+, Yb3+,Nd3+)
High efficiency, compactness, robustness
High beam quality
Double-clad fibers
Summary
Introduction
Nonlinear Effects
Burst Amplification
Fiber amplifier
Tight confinement of light (core diameter ~10 μm)
Long interaction length (> 1 m)
Very high single-pass gain
Nonlinear phenomena
Photonic crystal fibers (PCF) with effective mode
area up to 1000  m 2
Summary
Introduction
Nonlinear Effects
Burst Amplification
Summary
Nonlinear Effects
Maxwell equations:
1  E
2
 E
2
c
t
2
PP P
L
2
 P
2
 0
t
2
NL
The lowest-order nonlinear effects in silica fibers originate from
the third-order susceptibility:
Pi
NL
  0  ijk l E j E k E l
(3)
Intensity-dependent refractive index (self-phase-modulation),
Raman scattering, Brillouin scattering
Introduction
Nonlinear Effects
Burst Amplification
Self-Phase Modulation (SPM)
Intensity dependance of refractive index
3  1111
(3)
n NL  n  n2 j ,
n2 
4 c0 n  0
2
Leads to spectral broadening of pulses
d n NL
dt
d j
 n2
d j
dt
 0, for the leading edge of the pulse
dt
d j
 0, for the trailing edge of the pul se
dt
 t   0 
2 L d n N L

 in our cas e:
dt
  m ax
0
 0   t 
~ 10
4
Summary
Introduction
Nonlinear Effects
Burst Amplification
Summary
Stimulated Raman Scattering (SRS)
QM:
Signal photon generates the frequency-shifted Stokes wave and
an optical phonon
Frequency shift is determined by the vibrational modes of the
medium (for silica fibers around 13.2 THz)
For continuous-wave (CW) signal:
d IR
dz
d IS
gR
 gR IS IR

dz
S
R
10
 13
m /W (at   1  m )
gR IS IR
Threshold for CW signal:
Ptr  16
Aeff
g R L eff
70 kW
Aeff  effective m ode-field area
L eff  effective interaction length
Introduction
Nonlinear Effects
Burst Amplification
Summary
Stimulated Raman Scattering (SRS)
For pulsed signal we consider the following characteristic lengths:
T0 ~ 50 ps,
P ~ 100 kW ,
L am plifier ~ 1 m ,
Aeff ~ 700  m :
2
2
D isp ersio n len gth :
LD 
W a lko ff len gth :
LW 
N o n lin ea r len gth :
R a m a n - ga in len gth :
LNL 
LR 
T0
~ 100 km
 2S
T0
v
1
S
v
1
S j
1
1
R
~ 100 m
~ 10 cm
~ 10 cm
gR j
 2  group velocity dispersion param eter
v S , v R  signal and R am an group velocity
 s  nonlinear param eter
Introduction
Nonlinear Effects
Burst Amplification
Stimulated Raman Scattering (SRS)
In fiber amplifiers we can achieve power levels beyond the
calculated Raman threshold:
Summary
Introduction
Nonlinear Effects
Burst Amplification
Summary
Stimulated Brillouin Scattering (SBS)
Signal photon generates the frequency-shifted Stokes wave and
an acoustical phonon
In fibers, SBS occurs only in the backward direction
Lower frequency shift (~10 GHz) and bandwidth (~10 MHz)
Much lower threshold for narrow-bandwidth CW signal
Ptr  21
Aeff
g B L eff
300 W
gB
3  5·1 0
 11
m /W
Brillouin gain is reduced for broad-band signal by a factor:
1   S /  B
Brillouin gain is strongly reduced for pulse durations:
T0  10 ns
Introduction
Nonlinear Effects
Burst Amplification
Avoiding NL effects
Chirped-pulse amplification:
1. Pulse is stretched in a dispersive element (reduces peak power)
2. Stretched pulse is amplified
3. Pulse is recompressed
For picosecond pulses, chirped-pulse amplification requires
impractically large amounts of dispersion
Another solution: burst amplification
Summary
Introduction
Nonlinear Effects
Burst Amplification
Summary
Burst Amplification
Why bursts?
To avoid nonlinear effects
The energy of the burst is high and easily scalable with the
number of pulses in burst
The peak power of the individual pulse is lower (nonlinear
effects) but still high-enough to reach material micro-processing
thresholds.
Faster risetime of the burst envelope in comparison to a single
nanosecond pulse.
A good energy/peak power/duration compromise for material
processing.
Introduction
Nonlinear Effects
Burst Amplification
Summary
Burst Amplification
The leading edge of the burst is amplified more than the trailing
edge, because of population inversion depletion
Introduction
Nonlinear Effects
Burst Amplification
Burst Amplification
Burst can be aproximated by a square
pulse (if repetition rate is high)
If initial population inversion is
homogeneous, ie.:   x , t  0    0
then the density of photons at the end
of the amplifier equals:
n0

,


2

cn
t

L
/
v


 L
0
g
n  z  L, t   1  1  e 0 e

0,



0  t  L / v g  T0
otherw ise
  stim ulated em ission cross- section
n 0  density of photons in the i nitial square pulse
v g  group velocity
Summary
Introduction
Nonlinear Effects
Burst Amplification
Summary
Burst Amplification
For a 20 dB fiber amplifier, where burst = 20 pulses with FWHM
50 ps and peak power 1 kW at 100 MHz repetition rate:
Introduction
Nonlinear Effects
Burst Amplification
Summary
Burst Amplification
The leading edge of the burst is amplified more than the trailing
edge, because of population inversion depletion
Possible solution – amplitude modulation of the seed laser
(and modulation of pump light):
Introduction
Nonlinear Effects
Setup
Two stage fiber amplifier setup:
Burst Amplification
Summary
Introduction
Nonlinear Effects
Burst Amplification
Setup
First amplifier:
Yb-doped photonic crystal fiber with 16 μm mode-field-diameter
Expected gain: ~ 30 dB
Expected peak output power: ~ 10 kW
Second amplifier:
Yb-doped photonic crystal fiber with ~ 30 μm mode-field-diameter
Higher nonlinear thresholds
Expected gain: ~ 10 dB
Expected peak output power: ~ 100 - 500 kW
Summary
Introduction
Nonlinear Effects
Burst Amplification
Summary
Summary
Fiber lasers have many advantages over bulk solid state lasers
Nonlinear effects are the main limitation of fiber lasers
Picosecond fiber lasers are rarely used in material micro-processing
today
Proposed solution: burst amplification with seed amplitude
modulation
Expected output:
E  10  J
P  1 00 k W
Possible application in thin film micro-processing