1. Photoinjector Lasers for Ultra-Bright Electron Sources
Graeme Hirst
STFC Central Laser Facility

2. Background The CTF3 photoinjector laser
The ERLP photoinjector laser
(with Marta Divall,
Gary Markey and
Fay Hannon 2005)
The CTF3 photoinjector laser
(developed with Marta Divall and Ian Ross - picture shows Ian Musgrave and Gabor Kurdi, 2006)

3. Low-Emittance PI Laser Requirements
Laser beams can be characterised in terms of four parameters:
WAVELENGTH (photon energy, tunability)
POLARISATION (may be dictated by technology choices)
TEMPORAL PROFILE (pulse shape, time-structure of pulse train, timing jitter)
TRANSVERSE PROFILE (intensity distribution, ‘pointing’ stability, could, perhaps, be dynamic ?)

4. Low-Emittance PI Laser Requirements
WAVELENGTH
Cs:GaAs
Cs2Te Mg Cu 1 2 3 4 5
Photon energy (eV)
Photon energy should exceed the photocathode work function by as little as possible.
OPA
and full tunability via OPA.
NLO allows energy
multiplication (2w, 3w ...) Nd Yb
Ti:S
But it adds complexity, reduces efficiency and can compromise stability, beam quality and reliability.
POLARISATION
Doesn’t affect photoelectron production so is, in principle a free parameter. But in practice NLO is polarisation-sensitive and cathode absorption may be too.

5. Low-Emittance PI Laser Requirements
TEMPORAL PROFILE
Repetitive picosecond/femtosecond pulses are generated by phase-locking the discrete frequency-domain modes of an optical cavity.
Fourier relates the pulse shape to the individual modes’ amplitudes and phases which are limited by the laser medium’s gain profile but which are also independently controllable. n
Rapid changes in the pulse need broad spectral bandwidth from the laser.
Low emittance electron bunches may need unusually short drive laser pulses.
Mode control hardware can be complex and challenging but is effective for pulse shaping, even if NLO is involved.
Data from M. Danailov, 2007
On a picosecond timescale pulse shaping by division, delay and stacking is also effective.

6. Low-Emittance PI Laser Requirements
TEMPORAL PROFILE
Pulse shaping systems are now becoming commercially available.
Dazzler AO phase and amplitude modulator from Fastlite
Coherent’s ‘Silhouette’
provides feedback control of spectral amplitude and phase.

7. Low-Emittance PI Laser Requirements
TEMPORAL PROFILE
But in the end there is no point in temporally shaping the laser pulse on timescales much faster than the response time of the photocathode.

8. Low-Emittance PI Laser Requirements
TRANSVERSE PROFILE
Laser oscillators tend to deliver Gaussian profile beams
Amplifier saturation tends to ‘square off’ beams in the near field provided:
Gain media are uniform and pumping is stable and well-profiled
Transport optics and media are good-quality and clean
Diffraction is managed
NLO self-focusing is managed
NLO frequency conversion efficiency is sensitive to beam direction which may
be rapidly varying near a high-intensity focus and tends to reverse the squaring
Data from M. Danailov, 2007
Optical squaring (refractive shaping or simple clipping) must balance inefficiency, limited depth of field, chromaticity and sensitivity to input beam variations

9. Low-Emittance PI Laser Requirements
TRANSVERSE PROFILE
Laser oscillators tend to deliver Gaussian profile beams.
Amplifier saturation tends to ‘square off’ beams in the near field provided:
Gain media are uniform and pumping is stable and well-profiled
Transport optics and media are good-quality and clean
Data from C.S. Chou et al, 2009
Diffraction is managed
NLO self-focusing is managed
NLO frequency conversion efficiency is sensitive to beam direction which may
be rapidly varying near a high-intensity focus and tends to reverse the squaring
Optical squaring (refractive shaping or simple clipping) must balance inefficiency, limited depth of field, chromaticity and sensitivity to input beam variations
Transporting sharp-edged beams requires large numerical aperture and benefits from e.g. adaptive optics and image-relaying

10. Low-Emittance PI Laser Requirements
TRANSVERSE PROFILE
Laser oscillators tend to deliver Gaussian profile beams.
Amplifier saturation tends to ‘square off’ beams in the near field provided:
Gain media are uniform and pumping is stable and well-profiled
Transport optics and media are good-quality and clean
Diffraction is managed
NLO self-focusing is managed
Data from D.H.Dowell et al, FEL09,
Paper WEOA
NLO frequency conversion efficiency is sensitive to beam direction which may
be rapidly varying near a high-intensity focus and tends to reverse the squaring
Optical squaring (refractive shaping or simple clipping) must balance inefficiency, limited depth of field, chromaticity and sensitivity to input beam variations
Transporting sharp-edged beams requires large numerical aperture and benefits from e.g. adaptive optics and image-relaying

11. Low-Emittance PI Laser Requirements
TRANSVERSE PROFILE
Laser oscillators tend to deliver Gaussian profile beams.
Amplifier saturation tends to ‘square off’ beams in the near field provided:
Gain media are uniform and pumping is stable and well-profiled
Transport optics and media are good-quality and clean
Diffraction is managed
NLO self-focusing is managed
Data from D.H.Dowell et al, FEL09,
Paper WEOA
NLO frequency conversion efficiency is sensitive to beam direction which may
be rapidly varying near a high-intensity focus and tends to reverse the squaring
Optical squaring (refractive shaping or simple clipping) must balance inefficiency, limited depth of field, chromaticity and sensitivity to input beam variations
Transporting sharp-edged beams requires large numerical aperture and benefits from e.g. adaptive optics and image-relaying

12. Projected emittance (microns rms)
Low-Emittance PI Laser Requirements
TRANSVERSE PROFILE
Laser oscillators tend to deliver Gaussian profile beams.
Amplifier saturation tends to ‘square off’ beams in the near field provided:
Gain media are uniform and pumping is stable and well-profiled
Transport optics and media are good-quality and clean
Diffraction is managed
NLO self-focusing is managed
Data from D.H.Dowell et al, FEL09,
Paper WEOA
Projected emittance (microns rms)
NLO frequency conversion efficiency is sensitive to beam direction which may
be rapidly varying near a high-intensity focus and tends to reverse the squaring
Optical squaring (refractive shaping or simple clipping) must balance inefficiency, limited depth of field, chromaticity and sensitivity to input beam variations
Transporting sharp-edged beams requires large numerical aperture and benefits from e.g. adaptive optics and image-relaying

13. Projected emittance (microns rms)
Low-Emittance PI Laser Requirements
TRANSVERSE PROFILE
Laser oscillators tend to deliver Gaussian profile beams.
Amplifier saturation tends to ‘square off’ beams in the near field provided:
Gain media are uniform and pumping is stable and well-profiled
Transport optics and media are good-quality and clean
Diffraction is managed
NLO self-focusing is managed
Data from D.H.Dowell et al, FEL09,
Paper WEOA
Projected emittance (microns rms)
If required laser designers can generate transverse profiles which are better controlled than the ‘standard’ commercial product
But in the end there is no point in spatially shaping the laser pulse to make it much more uniform than the QE profile of the photocathode.

14. Requirements for Practical PI Lasers
RELIABILITY AND UPTIME
Favours design simplicity, mature technologies, commercial laser systems,
over-specification, low photon energy, high thermal efficiency
AVERAGE POWER
Proportional to average beam current and to photocathode QE, affects cost and reliability, removing heat from the cathode may be an issue 1mA with 1% QE requires 6×1017 ph/s which is 0.25W (green) or 0.5W (UV) ~10W (IR) short pulse lasers are commercially available Militates against the use of low-QE cathodes
STABILITY AND CONTROL
For low emittance the laser must stay within a very small parameter space,
requiring high intrinsic stability plus a multi-parameter feedback control system
(timing jitter, temporal pulse shaping, adaptive beam shaping and pointing, environmental control e.g. temp, vibration, utilities (power, cooling, gas purge)) applied to the whole optical transport system, not just the laser. Individual FCS’s are commercially available but integrated suites are not.

15. Laser System Options Nd:crystal (YAG, YLF, YVO4 ...)
Pros: High power, mature, commercially available, diode or flash pumped,
compatible with fibre systems
Cons: Slow temporal response (<1ps), thermal beam quality issues, low hn
Nd:YLF photoinjector lasers are in use e.g. at FLASH, PITZ and CERN CTF3 and Nd:YVO4 at ALICE
FLASH amplifier chain
PITZ

16. Laser System Options Ti:S
Pros: Fastest temporal response of conventional lasers (~10fs), mature, commercially available, some tunability, higher hn
Cons: Complex, thermally inefficient, laser pumped, noisy (broad bandwidth, sensitive modelocking, short tupper), needs CPA
Ti:S photoinjector lasers are in use e.g. at SPARC, LCLS and
SPARC
LCLS
FERMI

17. Laser System Options Yb:glass, crystal (YAG, SFAP ...) or ceramic
Pros: High power, diode pumped, can be largely fibre-based, efficient, quite fast temporal response (~100fs)
Cons: Less mature but with some commercial availability, low hn, may need cryo cooling, may need CPA
Commercial Yb:fibre laser 20Wave, 100mJ/pulse, 250fs FWHM Recently deployed at HZB BESSY (but not for photoinjection)
Clark-MXR Impulse
Yb:fibre photoinjector laser is in
use at Cornell ERL test facility
and Yb:YAG is in use with SC Pb cathode at HZB (T Kamps)

18. Laser System Options OP(CP)A driven by Nd or Yb or Ti:S
Pros: Tunable hn (real-time tuning not generally required)
Cons: Inefficient, complex, can be noisy, can be prone to optical damage, temporal control is less mature, low QE may demand very high power
STFC CLF ULTRA
RIKEN
OPA systems are in wide use for spectroscopy and selective processing and are being sold into industry and medicine

19. Conclusions
Photoinjector lasers have been developed over many years and are now driving guns with state-of-the-art electron bunch brightness
Further improvements to increase the brightness are likely to involve control of at least three of the lasers’ fundamental parameters:
Wavelength (fine-tuned close to work function)
Temporal profile (shortened and/or shaped)
Spatial profile (smoothed and shaped to reduce electrons’ transverse momentum)
There is ‘headroom’ left to do this
Keeping the laser’s performance inside the necessarily small parameter space will require both high intrinsic stability and tight feedback control
As well as delivering the technical advances photoinjector laser scientists need to satisfy demanding operational needs. The inevitable conflict complicates the choice between commercial and non-commercial vendors