  We demonstrate an all-optical approach for precise characterization of
attosecond extreme ultraviolet pulses. Isolated attosecond pulse is produced
from high order harmonics using intense driving pulse with proper gating
technique. When a weak field is synchronized with the driver, it perturbs the
harmonics generation process via altering the accumulated phase of the electron
trajectories. The perturbed harmonic spectrum can be formulated as a
convolution of the unperturbed dipole and a phase gate, implying the validity
of complete reconstruction of isolated attosecond pulses using conventional
frequency resolved optical gating method. This in situ measurement avoids the
central momentum approximation assumed in the widely used attosecond streaking
measurement, providing a simple and reliable metrology for isolated attosecond
pulse.
