1. Picosecond fiber laser for thin film micro-processing
Author: Jaka Petelin
Mentor: doc. dr. Rok Petkovšek
Co-Mentor: dr. Boštjan Podobnik
March 2011

2. Introduction Contents: Motivation Fiber amplifier Nonlinear effects
Burst Amplification
Summary
Introduction
Contents:
Motivation
Fiber amplifier
Nonlinear effects
Self-phase-modulation
Stimulated Raman scattering
Stimulated Brillouin scattering
Burst Amplification
Summary

3. Introduction
Nonlinear Effects
Burst Amplification
Summary
Motivation
Material micro-processing applications currently employ nanosecond laser sources
Shorter pulses: better resolution (less heat diffusion and thus smaller heat-affected-zone)

4. Motivation However: For thin film micro-processing (photovoltaics):
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):
For a 10 ps pulse the peak power would be over 1 MW (nonlinear effects and fiber damage)
Possible solution: laser bursts

5. Fiber amplifier Rare-earth-doped optical fiber (eg. Er3+, Yb3+,Nd3+)
Introduction
Nonlinear Effects
Burst Amplification
Summary
Fiber amplifier
Rare-earth-doped optical fiber (eg. Er3+, Yb3+,Nd3+)
High efficiency, compactness, robustness
High beam quality
Double-clad fibers

6. Fiber amplifier Tight confinement of light (core diameter ~10 μm)
Introduction
Nonlinear Effects
Burst Amplification
Summary
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

7. Nonlinear Effects Maxwell equations:
Introduction
Nonlinear Effects
Burst Amplification
Summary
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

8. Self-Phase Modulation (SPM)
Introduction
Nonlinear Effects
Burst Amplification
Summary
Self-Phase Modulation (SPM)
Intensity dependance of refractive index
Leads to spectral broadening of pulses

9. Stimulated Raman Scattering (SRS)
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:
Threshold for CW signal:

10. Stimulated Raman Scattering (SRS)
Introduction
Nonlinear Effects
Burst Amplification
Summary
Stimulated Raman Scattering (SRS)
For pulsed signal we consider the following characteristic lengths:

11. Stimulated Raman Scattering (SRS)
Introduction
Nonlinear Effects
Burst Amplification
Summary
Stimulated Raman Scattering (SRS)
In fiber amplifiers we can achieve power levels beyond the calculated Raman threshold:

12. Stimulated Brillouin Scattering (SBS)
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:

13. Avoiding NL effects Chirped-pulse amplification:
Introduction
Nonlinear Effects
Burst Amplification
Summary
Avoiding NL effects
Chirped-pulse amplification:
Pulse is stretched in a dispersive element (reduces peak power)
Stretched pulse is amplified
Pulse is recompressed
For picosecond pulses, chirped-pulse amplification requires impractically large amounts of dispersion
Another solution: burst amplification

14. Burst Amplification Why bursts? To avoid nonlinear effects
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.

15. 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

16. Introduction
Nonlinear Effects
Burst Amplification
Summary
Burst Amplification
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:

17. 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:

18. 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):

19. Setup Two stage fiber amplifier setup: Introduction Nonlinear Effects
Burst Amplification
Summary
Setup
Two stage fiber amplifier setup:

20. Setup First amplifier: Second amplifier:
Introduction
Nonlinear Effects
Burst Amplification
Summary
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: ~ kW

21. Summary Fiber lasers have many advantages over bulk solid state lasers
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:
Possible application in thin film micro-processing