Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography

TLDR

FDML is a laser regime that modulates the spectrum of the light field, analogous to active mode locking for short pulse generation. The study introduces FDML as a new frequency‑swept laser technique and applies it to swept‑source OCT imaging. FDML lasers generate high‑speed, narrowband frequency sweeps by synchronizing the sweep period with the cavity round‑trip time in a long fiber ring cavity that incorporates a semiconductor optical amplifier and a tunable fiber Fabry‑Perot filter. FDML achieves sweep rates up to 290 kHz over a 105 nm range at 1300 nm, outputs 3 mW (20 mW post‑amplified), delivers 108 dB sensitivity with only 7.5 dB loss over 7 mm depth, and supports swept‑source OCT imaging at 232,000 axial scans per second (906 fps, 3.5 volumes/s), proving it ideal for high‑speed, deep imaging.

Abstract

We demonstrate a new technique for frequency-swept laser operation--Fourier domain mode locking (FDML)--and its application for swept-source optical coherence tomography (OCT) imaging. FDML is analogous to active laser mode locking for short pulse generation, except that the spectrum rather than the amplitude of the light field is modulated. High-speed, narrowband optical frequency sweeps are generated with a repetition period equal to the fundamental or a harmonic of cavity round-trip time. An FDML laser is constructed using a long fiber ring cavity, a semiconductor optical amplifier, and a tunable fiber Fabry-Perot filter. Effective sweep rates of up to 290 kHz are demonstrated with a 105 nm tuning range at 1300 nm center wavelength. The average output power is 3mW directly from the laser and 20 mW after post-amplification. Using the FDML laser for swept-source OCT, sensitivities of 108 dB are achieved and dynamic linewidths are narrow enough to enable imaging over a 7 mm depth with only a 7.5 dB decrease in sensitivity. We demonstrate swept-source OCT imaging with acquisition rates of up to 232,000 axial scans per second. This corresponds to 906 frames/second with 256 transverse pixel images, and 3.5 volumes/second with a 256x128x256 voxel element 3-DOCT data set. The FDML laser is ideal for swept-source OCT imaging, thus enabling high imaging speeds and large imaging depths.