VELA PHOTOINJECTOR LASER E.W. Snedden, Lasers and Diagnostics Group
Contents Introduction PI laser step-by-step: Ti:Sapphire oscillator Regenerative amplifier Single-pass amplifier Frequency mixing Emphasis on physics of operation, not users guide
VELA PI laser Photoinjector laser 266 nm, 2 mj 10 Hz, 180 fs VELA
Actually Femtosecond oscillator: 800 nm, ~6 nj, 80 MHz, 50 fs Regenerative amplifier: 800 nm, ~3 mj, 10 Hz, 80 fs Single-pass amplifier: 800 nm, ~10 mj, 10 Hz, 80 fs Frequency mixing: 266 nm, ~2 mj, 10 Hz, 180 fs VELA
Femtosecond oscillator: 800 nm, ~6 nj, 80 MHz, 50 fs VELA
Femtosecond oscillator Basic theory: longitudinal modes A very basic laser cavity: Pump (optical, electrical) Gain medium Gain medium (amplification) High reflectivity mirrors (positive feedback) Enclosure
Femtosecond oscillator Longitudinal modes Mirrors: boundary conditions Cavity supports finite number of modes Frequency separation: Inverse time light takes to complete one round trip L
Femtosecond oscillator Self mode-locking Pulsed operation requires fixed phaserelationship between modes Self mode-locking (Kerr Lens modelocking) most popular: Feature of Ti:Sapphire gain medium Refractive index of medium is function of intensity:
Femtosecond oscillator Self mode-locking Introduce slit to modulate losses High intensity: little attenuation Low intensity: large attenuation After time, only one intense pulse in cavity remains (mode-locking in time domain)
Femtosecond oscillator Ti:Sapphire oscillator A very basic laser femtosecond cavity: Pump (optical) Ti:Sapphire Slit Ti:Sapphire (amplification, bandwidth, Kerr Lens) High reflectivity mirrors (positive feedback) Slit (mode-locking)
Femtosecond oscillator Ti:Sapphire oscillator End mirror Ti:Sapphire Prisms for dispersion compensation Slit End mirror
Coherent Vitara: 800 nm, ~6 nj, 80 MHz, 50 fs Sold as a sealed black box, no manual control, emphasis on convenience : Computer crash = no laser Recent downtime due to failure of pump laser (v. rare) Femtosecond oscillator Our system
Femtosecond oscillator: 800 nm, ~6 nj, 80 MHz, 50 fs Regenerative amplifier: 800 nm, ~3 mj, 10 Hz, 80 fs Single-pass amplifier: 800 nm, ~10 mj, 10 Hz, 80 fs VELA
Amplification The need for amplification Oscillator produces high repetition rate but low pulse energies: limited applications Increasing the pulse energy via amplification enhances number of applications: 3 rd / 4 th order harmonic generation Supercontinuum generation Photon energies from 0.01-5 ev now accessible
Amplification Regenerative amplification Ti:Sapphire has low single-pass gain: Solution: ensure many passes by trapping pulse in cavity, take all energy in gain medium Pockels Cell 1: injection Pump (optical) Pockels Cell 2: dump Ti:Sapphire Pockels cell: electro-optic control of polarisation, control injection and dump
Good control of injection/dump times is required: Dump too early inefficient energy extraction Dump too late energy absorbed by ground state of gain medium Amplification Regenerative amplification Response time of electro-optic units limits maximum repetition rate to typically <100 khz
Amplification Regenerative amplification Operation of regen amplifiers can be complicated! Free space optics: very sensitive to alignment. Drift in performance due to temperature changes, seed variation, voltage to Pockels cells Pump Injection cell Compressor for dispersion compensation Ti:Sapphire Dump cell
Amplification Single-pass amplifier For our purposes, the regenerative amplifier does not provide enough pulse energy: use single-pass amplifier for one last boost Pump laser is more powerful to maximise gain Single-pass gain ~ 3
Amplification Our system Coherent Legend Duo HE: Regen: 800 nm, ~3 mj, 10 Hz, 80 fs SPA: 800 nm, ~10 mj, 10 Hz, 80 fs Drift of regenerative amplifier through day: Reduced power, increased pulse duration. Expect basic realignment every 2-3 months to maintain best performance
Femtosecond oscillator: 800 nm, ~6 nj, 80 MHz, 50 fs Regenerative amplifier: 800 nm, ~3 mj, 10 Hz, 80 fs Single-pass amplifier: 800 nm, ~10 mj, 10 Hz, 80 fs Frequency mixing: 266 nm, ~2 mj, 10 Hz, 180 fs VELA
Frequency mixing Basic principles Amplified femtosecond pulse is intense enough to efficiently activate non-linear optical processes. Example: sum frequency mixing: Output electric field proportional to product of incident fields and second-order susceptibility χ (2) :
Frequency mixing Third harmonic generation Third-order non-linear mixing (e.g. ) can be used, but is very weak: χ (3) / χ (2) ~ 1 10-7 Can be more efficient to chain second order processes: 400 nm 800 nm 266 nm 800 nm
Frequency mixing Efficiency and bandwidth Conversion efficiency goes down with increasing non-linear order, limits output power: Harmonic order Energy conversion efficiency / % 2 ~50 3 ~10-20 4 ~1-10 Bandwidth filtering and dispersion in mixing crystals typically result in increase in pulse duration
The end product Measure UV beam using Frequency Resolved Optical Gating Looking to apply similar techniques for characterisation of CLARA FEL output UV pulse parameters: 266 nm, ~2 mj, 10 Hz, 180 fs