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

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

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)

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

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

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

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

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

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:

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

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:

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:

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

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.

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 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:

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):

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

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: ~ 100 - 500 kW

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