From master-slave to down-conversion optical coherence tomography

We present here advances on the Master Slave (MS) concept, applicable to spectral/Fourier/frequency-domain optical coherence tomography (OCT) technology. Instead of obtaining an A-scan from the sample investigated via a Fourier Transform (FT) or equivalent, the amplitude of the A-scan for each resolvable point along the depth is obtained along a separate output. A multiplier produces the product of the photo-detected signal from the OCT system with that generated by an Electrical or an Optical Master. This allows acquisition at a frequency comparable to that of the sweeping, much inferior to the frequency bandwidth of the channeled spectrum. 3 advantages of the down-conversion method are demonstrated here: (a) real time delivery of an en-face image; (b) axial optical path difference (OPD) range at the level of the source’s dynamic coherence length and (c): tolerance to fluctuations in the sweep of the swept source. The most important advantage of the down-conversion method is that it reduces the signal bandwidth considerably, to the level of the sweeping rate. This facilitates real-time operation. Conventional A-scan production can only be performed real-time if the FT processing is carried out in a time comparable to or less than the sweep time, which depending on the number of sampled points and dynamic range determines a limit of ∼ MHz sweep rate. Before even calculating a FT, acquisition may also be limited by the sampling rate of the digitiser. In conventional SS-OCT, the number of depth points can exceed 1,000, which for a sweeping time of 1 μs would determine signals in the GHz range. Using long coherence length swept sources, this number of depths could be even larger, hence the conventional FT-based method faces a bottleneck due to the time needed to calculate the FT, combined with the need to acquire data at many GS/s.