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