Frequency Comb Swept Lasers for Optical Coherence Tomography

We demonstrate a frequency comb (FC) swept laser and a frequency comb Fourier domain mode locked (FCFDML) laser for applications in optical coherence tomography (OCT). The fiber-based FC swept lasers operate at a sweep rate of 1kHz and 120kHz, respectively, over a 135nm tuning range centered at 1310nm with average output powers of 50mW. A 25GHz free spectral range frequency comb filter in the cavity of swept lasers causes the lasers to generate a series of well defined frequency steps, which are equally spaced in frequency-space (linear in k-space). The narrow bandwidth (0.015nm) of the frequency comb filter enables a ~-1.2dB sensitivity roll off over ~3mm range, compared to conventional swept source and FDML lasers which have -10dB and -5dB roll offs, respectively. Measurements at very long ranges are possible with minimal sensitivity loss. The filtered laser output can be used to self-clock the OCT interference signal sampling, enabling direct fast Fourier transformation of the fringe signals, without the need for fringe recalibration procedures. The design and operation principles of FC swept lasers are discussed and interferometric measurement applications are proposed.