Extended Coherence Length and Depth Ranging Using a Fourier-Domain Mode-Locked Frequency Comb and Circular Interferometric Ranging

Abstract

Fourier-domain mode locking 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 linewidth of a Fourier-domain mode-locked (FDML) frequency-comb (FC) laser could provide an attractive platform for high-speed as well as long-range OFDI. Unfortunately, aliasing artifacts arising from signals beyond the principal measurement depth of the free spectral range have prohibited the use of an FDML FC laser for imaging so far. To make the increased coherence length of an FDML FC laser available, methods to manage such artifacts are required. Recently, coherent circular ranging that uses frequency combs for imaging in much-reduced rf bandwidths has been demonstrated. Here we revisit circular ranging as a tool for making the long coherence length of an FDML FC laser and 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 FDML FC laser increases the coherence length by an order of magnitude compared with 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 reduces acquisition bandwidths 15-fold compared with traditional OFDI methods. The FDML FC–AOFS design offers a convenient platform for long-range and high-speed imaging as well as for exploring signal- and image-processing methods in circular ranging.