Coherence Properties of Multi-Section Semiconductor Frequency Swept Lasers

The development of new Optical Coherence Tomography (OCT) techniques, such as swept-source OCT (SS-OCT), has increased the demand for ultra-fast, broadband and highly coherent swept sources. Among the existing technologies, Fourier Domain Mode-Locked Lasers rely on the periodic modulation of an intra-cavity filter in resonance with the cavity round-trip time [1] while the modulation of the cavity length of a single mode Vertical Cavity Surface Emitting Laser (VCSEL) can provide continuous tuning over 100's of nm [2]. Multi-section semiconductor lasers, such as Sample-Grating Distributed Feedback (SG-DBR) Lasers have been used as widely tunable lasers in the telecom industry for many years and have, more recently, been considered for OCT applications [3]. One of the shortcomings of these lasers, when compared to VCSEL's, is that they cannot have a continuous tuning of a single mode all across a wide spectrum required for OCT application as they exhibit many mode hops during a single sweep.