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

TL;DR

This paper demonstrates that using a frequency comb FDML laser combined with circular ranging and an acousto-optic frequency shifter significantly extends coherence length and imaging depth in optical frequency domain imaging, reducing bandwidth requirements.

Contribution

It introduces an active, wavelength-independent method to mitigate aliasing artifacts in FC FDML lasers, enabling long-range, high-speed imaging with simplified system design.

Findings

Achieved an order of magnitude longer coherence length than traditional FDML lasers.

Reduced acquisition bandwidths by 15-fold using circular ranging.

Demonstrated stable, wavelength-independent aliasing mitigation with AOFS.

Abstract

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.