This paper introduces Fourier domain mode locking (FDML), a new laser operation regime that enables high-speed, narrowband optical frequency sweeps for swept-source optical coherence tomography (OCT). FDML is analogous to active laser mode locking but modulates the spectrum rather than the amplitude of the light field. The FDML laser, constructed using a long fiber ring cavity, a semiconductor optical amplifier, and a tunable fiber Fabry-Perot filter, achieves effective sweep rates up to 290 kHz with a 105 nm tuning range at 1300 nm. The laser outputs 3 mW directly and 20 mW after post-amplification. Using this laser for OCT, sensitivities of 108 dB were achieved, with a 7.5 dB decrease in sensitivity over a 7 mm depth. The system demonstrated OCT imaging with acquisition rates of up to 232,000 axial scans per second, corresponding to 906 frames/second with 256 transverse pixel images and 3.5 volumes/second with a 256x128x256 voxel 3-D data set. The FDML laser is ideal for high-speed, swept-source OCT imaging, enabling large imaging depths and high imaging speeds. The study also highlights the advantages of FDML over conventional methods, including narrow linewidth, high sensitivity, and the ability to achieve high-speed 3-D OCT imaging with minimal motion artifacts. The FDML laser is promising for future high-speed, swept-source OCT applications.This paper introduces Fourier domain mode locking (FDML), a new laser operation regime that enables high-speed, narrowband optical frequency sweeps for swept-source optical coherence tomography (OCT). FDML is analogous to active laser mode locking but modulates the spectrum rather than the amplitude of the light field. The FDML laser, constructed using a long fiber ring cavity, a semiconductor optical amplifier, and a tunable fiber Fabry-Perot filter, achieves effective sweep rates up to 290 kHz with a 105 nm tuning range at 1300 nm. The laser outputs 3 mW directly and 20 mW after post-amplification. Using this laser for OCT, sensitivities of 108 dB were achieved, with a 7.5 dB decrease in sensitivity over a 7 mm depth. The system demonstrated OCT imaging with acquisition rates of up to 232,000 axial scans per second, corresponding to 906 frames/second with 256 transverse pixel images and 3.5 volumes/second with a 256x128x256 voxel 3-D data set. The FDML laser is ideal for high-speed, swept-source OCT imaging, enabling large imaging depths and high imaging speeds. The study also highlights the advantages of FDML over conventional methods, including narrow linewidth, high sensitivity, and the ability to achieve high-speed 3-D OCT imaging with minimal motion artifacts. The FDML laser is promising for future high-speed, swept-source OCT applications.