  We demonstrate a new and efficient laser-locking technique that enables
making large frequency jumps while keeping the laser in lock. A diode laser is
locked at a variable offset from a Doppler-free spectral feature of rubidium
vapor. This is done by frequency shifting the laser before sending the light to
a spectroscopy cell with an acousto-optic modulator (AOM). The frequency of the
locked laser is switched quasi-instantaneously over much more than the width of
the spectral features, i.e. the usual locking range. This is done by
simultaneously switching the AOM frequency and applying feed-forward to the
laser current. The advantage of our technique is that power loss and beam walk
caused by the AOM do not affect the main output beam, but only the small
fraction of light used for the spectroscopy. The transient excursions of the
laser frequency are only a few MHz and last approximately 0.2 ms, limited by
the bandwidth of our locking electronics. We present equations that describe
the transient behavior of the error signal and the laser frequency
quantitatively. They are in good agreement with the measurements. The technique
should be applicable to other types of lasers.
