  An optical fast-scan delay exploiting the near-collinear interaction between
a train of ultrashort optical pulses and an acoustic wave propagating in a
birefringent crystal is introduced. In combination with a femtosecond Er:fiber
laser, the scheme is shown to delay few-fs pulses by up to 6 ps with a
precision of 15 as. A resolution of 5 fs is obtained for a single sweep at a
repetition rate of 34 kHz. This value can be improved to 39 as for multiple
scans at a total rate of 0.3 kHz.
