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Fourier domain mode-locking (FDML) has been a popular laser design for highspeed optical frequency domain imaging (OFDI) but achieving long coherencelengths, and therefore imaging range, has been challenging. The narrowinstantaneous linewidth of a frequency comb (FC) FDML laser could provide anattractive platform for high speed as well as long range OFDI. Unfortunately,aliasing artifacts arising from signals beyond the principle measurement depthof the free spectral range have prohibited the use of a FC FDML for imaging sofar. To make the enhanced coherence length of FC FDML laser available, methodsto manage such artifacts are required. Recently, coherent circular ranging hasbeen demonstrated that uses frequency combs for imaging in much reduced RFbandwidths. Here, we revisit circular ranging as a tool of making the longcoherence length of an FDML frequency comb laser as well as its use for tissueimaging accessible. Using an acousto-optic frequency shifter (AOFS), wedescribe an active method to mitigate signal aliasing that is both stable andwavelength independent. We show that an FC FDML laser offers an order ofmagnitude improved coherence length compared to traditional FDML laser designswithout requiring precise dispersion engineering. We discuss design parametersof a frequency stepping laser resonator as well as aliasing from a frequencycomb and AOFS in OFDI with numerical simulations. The use of circular rangingadditionally reduced acquisition bandwidths 15-fold compared with traditionalOFDI methods. The FC FDML/AOFS design offers a convenient platform for longrange and high speed imaging as well as exploring signal and image processingmethods in circular ranging.

Extended coherence length and depth ranging using a Fourier domain mode-locked frequency comb and circular interferometric ranging