LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

L C L S C O N C E P T U A L D E S I G N R E P O R T
portions of this laser probe is determined according to the dynamic phase retardation induced by
the electric field from the electron bunch. Any transmission attributed to intrinsic (static) phase
retardation (i.e., without an electron bunch) is cancelled using a pair of polarizers with a biasing
optical compensator. Polarizers are positioned up- and down-stream of the electro-optic crystal
and are cross-polarized relative to each other. The electric field of the electron bunch modulates
the transmitted intensity of the laser pulse in the following way. The short-lived bunch field
induces a birefringence that generates phase retardation in the laser pulse. Consequently a
polarization component orthogonal to that of the incident pulse can be transmitted to a
spectrometer during a ‘gated’ time interval. The temporal structure of the bunch-induced
modulation can come from knowledge of the initial wavelength chirp on the laser waveform. A
spectrometer, in this case part of a Frequency Resolved Optical Gating (FROG) device, is used to
measure which part of the band has been ‘gated’ by the electron bunch.
electron bunch
co-propagating
laser pulse
ωl
beam pipe
initial laser chirp
polarizer EO crystal analyzer
spectrometer
t t ωs
bunch charge gated spectral signal
Figure 7.70. The principal of measurement of the electron bunch length by modulation of a chirped
laser pulse in an electro-optic crystal by the electric field of the bunch. In practice only
the EO crystal is in vacuum. All other optical components are located outside a window.
In order for the FROG to adequately record transmission of a single laser pulse from a single
electron bunch a photon flux of 10 µJ is required in the gated spectral signal. This translates into
a peak power requirement of ~200 MW. The chirped laser waveform is stretched to a 10-ps
duration (thereby allowing up to 10 ps of relative timing jitter between laser and beam). The high
peak power and proportionately wide laser bandwidth requirements can be met with a
Ti:Sapphire laser oscillator seeding a regenerative amplifier. The extent of the waveform chirp is
limited by the pulse bandwidth which can exceed 10 nm. At this high peak power, care is
required to remain below the fluence damage threshold near the 1-J·cm −2 level in the optical
materials. The co-propagating geometry is chosen since a transverse light propagation geometry
would require focusing the laser light to micron level spot sizes in the EO crystal in order to
preserve temporal resolution. This would exceed damage thresholds by several orders of
magnitude. One constraint on the co-propagating geometry is the relative slippage between the
I
A C C E L E R A T O R ♦ 7-99