Abstract

In Fourier-domain optical coherence tomography, finite acquisition bandwidths can limit the achievable imaging range, axial resolution, and/or imaging speed. Circular ranging (CR) methods were recently demonstrated to overcome these barriers [1,2,3]. By using optical frequency combs (FC), the depth space of the interferometric signal is folded such that extended imaging ranges are compressed into a reduced RF bandwidth. To avoid overlap of artefacts, this folding is performed in a circular manner through the combination of a frequency comb optical source and the use of complex demodulation, i.e., the detection of in-phase (I) and quadrature fringe signals (Q) that resolve the sign of the measured delay. Complex demodulation methods have been explored in Fourier-domain OCT to extend the imaging range by a factor of two through use of the positive and negative delay spaces. Passive methods based on optical quadrature circuits and active methods based on dynamic phase modulation have been demonstrated. In prior CR OCT demonstrations, a passive method was employed [1]. While this provided the required performance, there may be advantages in active methods that reduce digitizer channel counts and avoid the need for environment stability. In this work, we describe the extension of active demodulation schemes to CR OCT systems. In this, we highlight CR-specific alterations that overcome these limitations. We demonstrate the active demodulation methods using a stretched pulse active mode locked (SPML) FC laser at 1.3 μm for the first time.

© 2019 SPIE/OSA