A tunable laser and laser tuning method, based on the interaction of a spectrally
dependent beam distortion and a spatial filter within a laser cavity. One embodiment
of this laser is an external cavity semiconductor laser in which broad tunability
is obtained by the insertion of an acousto-optic tunable filter (AOTF) into the
laser cavity such that the intra-cavity laser beam passes through the AOTF in zeroth order.