  Fourier domain mode-locking (FDML) has been a popular laser design for high
speed optical frequency domain imaging (OFDI) but achieving long coherence
lengths, and therefore imaging range, has been challenging. The narrow
instantaneous linewidth of a frequency comb (FC) FDML laser could provide an
attractive platform for high speed as well as long range OFDI. Unfortunately,
aliasing artifacts arising from signals beyond the principle measurement depth
of the free spectral range have prohibited the use of a FC FDML for imaging so
far. To make the enhanced coherence length of FC FDML laser available, methods
to manage such artifacts are required. Recently, coherent circular ranging has
been demonstrated that uses frequency combs for imaging in much reduced RF
bandwidths. Here, we revisit circular ranging as a tool of making the long
coherence length of an FDML frequency comb laser as well as its use for tissue
imaging accessible. Using an acousto-optic frequency shifter (AOFS), we
describe an active method to mitigate signal aliasing that is both stable and
wavelength independent. We show that an FC FDML laser offers an order of
magnitude improved coherence length compared to traditional FDML laser designs
without requiring precise dispersion engineering. We discuss design parameters
of a frequency stepping laser resonator as well as aliasing from a frequency
comb and AOFS in OFDI with numerical simulations. The use of circular ranging
additionally reduced acquisition bandwidths 15-fold compared with traditional
OFDI methods. The FC FDML/AOFS design offers a convenient platform for long
range and high speed imaging as well as exploring signal and image processing
methods in circular ranging.
