picosecond fiber laser for thin film micro-processing picosecond fiber laser for thin film...
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Picosecond fiber laser for thin filmmicro-processing
Author: Jaka Petelin
Mentor: doc. dr. Rok PetkovšekCo-Mentor: dr. Boštjan Podobnik
March 2011
Introduction
Contents:
Motivation
Fiber amplifier
Nonlinear effectsSelf-phase-modulation
Stimulated Raman scattering
Stimulated Brillouin scattering
Burst Amplification
Summary
Introduction Nonlinear Effects Burst Amplification Summary
Material micro-processing applications currently employ nanosecond laser sources
Shorter pulses: better resolution (less heat diffusion and thus smaller heat-affected-zone)
MotivationIntroduction Nonlinear Effects Burst Amplification Summary
However:Lower energy of pulsesORVery high peak power at same pulse energy (nonlinear effects – especially detrimental in fiber lasers and fiber amplifiers)
For thin film micro-processing (photovoltaics):
For a 10 ps pulse the peak power would be over 1 MW (nonlinear effects and fiber damage)
Possible solution: laser bursts
MotivationIntroduction Nonlinear Effects Burst Amplification Summary
100kW
E 10 J
P
Rare-earth-doped optical fiber (eg. Er3+, Yb3+,Nd3+)
High efficiency, compactness, robustness
High beam quality
Double-clad fibers
Fiber amplifierIntroduction Nonlinear Effects Burst Amplification Summary
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
Fiber amplifierIntroduction Nonlinear Effects Burst Amplification Summary
21000 m
Nonlinear Effects
Maxwell equations:
The lowest-order nonlinear effects in silica fibers originate from the third-order susceptibility:
Intensity-dependent refractive index (self-phase-modulation), Raman scattering, Brillouin scattering
2 22
02 2 2
1
c t t
E P
E
L NL P P P
(3)0 ijk
NLi l j k lP E E E
Introduction Nonlinear Effects Burst Amplification Summary
Self-Phase Modulation (SPM)
Intensity dependance of refractive index
Leads to spectral broadening of pulses
Introduction Nonlinear Effects Burst Amplification Summary
(3)1111
2 2 20 0
3,
4NLn n n j nc n
2
0 0
4
0
d d
d d
d0, for the leading edge of the pulse
dd
0, for the trailing edge of the pul
d
d
in our cas
sed
2
e: ~ 10
NL
NL
max
nL
n
t tt
jn
t t
j
tj
t
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:
Threshold for CW signal:
d
dd
d
S R
S SS R
RR
RR
Ig I I
zI
g I Iz
1310 m/W (at 1 )mRg
16 70kWefftr
R eff
AP
g L
Introduction Nonlinear Effects Burst Amplification Summary
effective mode-field area
effective interaction length
eff
eff
A
L
Stimulated Raman Scattering (SRS)
For pulsed signal we consider the following characteristic lengths:2
0 ~ 50ps, ~ 100kW ~ 700 m, ~ 1m, :amplifier effT P L A
2
0
1 10
2
~ 100km
~ 100m
1~ 10cm
1~ 10cm
DS
W
NL
S R
R
S
R
TL
TL
v v
Lj
Lg j
Dispersion length:
Walkoff length:
Nonlinear length:
Raman - gain length:
Introduction Nonlinear Effects Burst Amplification Summary
2 group velocity dispersion parameter
, signal and Raman group velocity
nonlinear parameterR
s
Sv v
Stimulated Raman Scattering (SRS)
In fiber amplifiers we can achieve power levels beyond the calculated Raman threshold:
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
Brillouin gain is reduced for broad-band signal by a factor:
Brillouin gain is strongly reduced for pulse durations:
21 300 Wefftr
B eff
AP
g L 113 51 m· 0 /WBg
1 /S B
0 10nsT
Introduction Nonlinear Effects Burst Amplification Summary
Avoiding NL effects
Chirped-pulse amplification:1. Pulse is stretched in a dispersive element (reduces peak power)2. Stretched pulse is amplified3. Pulse is recompressed
For picosecond pulses, chirped-pulse amplification requires impractically large amounts of dispersion
Another solution: burst amplification
Introduction Nonlinear Effects Burst Amplification Summary
Burst AmplificationIntroduction Nonlinear Effects Burst Amplification Summary
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.
Burst AmplificationIntroduction Nonlinear Effects Burst Amplification Summary
The leading edge of the burst is amplified more than the trailing edge, because of population inversion depletion
Burst AmplificationIntroduction Nonlinear Effects Burst Amplification Summary
Burst can be aproximated by a square pulse (if repetition rate is high)
If initial population inversion is homogeneous, ie.:
then the density of photons at the end of the amplifier equals:
0, 0x t
00
0
/ 02, 0 /
, 1 1
0, otherwise
gt L vLgcn
nt L v T
n z L t e e
0 density of photons in the i
stimulated emission cross-
nitial square pulse
section
group velocitygv
n
Burst AmplificationIntroduction Nonlinear Effects Burst Amplification Summary
For a 20 dB fiber amplifier, where burst = 20 pulses with FWHM 50 ps and peak power 1 kW at 100 MHz repetition rate:
Burst AmplificationIntroduction Nonlinear Effects Burst Amplification Summary
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):
SetupIntroduction Nonlinear Effects Burst Amplification Summary
Two stage fiber amplifier setup:
SetupIntroduction Nonlinear Effects Burst Amplification Summary
First amplifier:Yb-doped photonic crystal fiber with 16 μm mode-field-diameterExpected gain: ~ 30 dBExpected peak output power: ~ 10 kW
Second amplifier:Yb-doped photonic crystal fiber with ~ 30 μm mode-field-diameterHigher nonlinear thresholdsExpected gain: ~ 10 dBExpected peak output power: ~ 100 - 500 kW
SummaryIntroduction Nonlinear Effects Burst Amplification 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:
Possible application in thin film micro-processing
1 W
10 J
00kP
E