Since 1996, the transverse beam sizes at the SLC interaction point (IP) can be determined with a 'laser wire', by detecting the rate of Compton-scattered photons as a function of the beam-laser separation in space. Nominal laser parameters are: 350 nm wavelength, 2 mJ energy per pulse, 40 Hz repetition rate, and 150 ps FWHM pulse length. The laser system is presently being modified to enable measurements of the longitudinal beam profile. For this purpose, two laser pulses of slightly different frequency are superimposed, which creates a travelling fringe pattern and, thereby, introduces a bunch-to-bunch variation of the Compton rate. The magnitude of this variation depends on the beat wavelength and on the Fourier transform of the longitudinal distribution. This laser heterodyne technique is implemented by adding a l-km long optical fibre at the laser oscillator output, which produces a linearly chirped laser pulse with 4.5-A linewidth and 60-ps FWHM pulse length. Also, the pulse is amplified in a regenerative amplifier and tripled with two nonlinear crystals. Then a Michelson interferometer spatially overlaps two split chirped pulses, which are temporally shifted with respect to each other, generating a quasi-sinusoidal adjustable fringe pattern. This laser pulse is then transported to the Interaction Point.
