  We present a method of producing single attosecond pulses by high-order
harmonic generation with multi-cycle nonlinear chirped driver laser pulses. The
symmetry of the laser feld in several optical cycles near the pulse center is
dramatically broken, and then the photons which cover a much broad spectrum
burst almost only in one optical cycle. So an ultra-broad continuum spectrum
appears in the high order harmonic spectrum, from which an isolated sub-50
attosecond pulse could be obtained. The results are almost independent of the
length and chirp form of the driver laser pulse.
