second set of modes or wavelengths is illustrated by dashed lines these are offset by a frequency offset that is substantially the same for all corresponding modes or wavelengths of the two sets this offset enables more sophisticated signal detection techniques which includes but is not limited to coherent heterodyne techniques in an alternative embodiment depth scanning can be accomplished by varying the temporal relationship between the captured scattered signal and the reference signal by modifying the coherence phase offset between the first and second mode locked lasers this can be done by for example having both lasers mode locked at substantially the same frequency and varying the phase relation between rf radio frequency signals electronically mode locking the lasers typical outputs of the lasers in such an embodiment are illustrated in fig6 where the first laser output 601 and the second laser output 602 have substantially the same repetition rate indicated by the periods 603 and 604 the phase coherence between the two pulse trains 605 is aligned so the pulse trains are substantially 180 degrees out of phase when the signals are combined interferometrically scanning is accomplished by varying this phase relationship by an amount for example indicated by 606 the scanning frequency is determined by the frequency with which the phase coherence is modulated the scanning range is determined by the magnitude of the phase coherence variation 606 it is understood that the above description is intended to be illustrative and not restrictive many of the features have functional equivalents that are intended to be included in the.