Towards ultrahigh resolution MHz retinal SS-OCT: 187nm section-wise tuning of a FDML laser at 1050nm

Fourier domain mode-locked (FDML) lasers have been the light sources of choice for ultra-high-speed optical coherence tomography (OCT), achieving scan rates well beyond one Megahertz. FDML lasers are used in some of the highest-performing swept-source OCT (SS-OCT) systems with superior noise performance and coherence length. However, the sweep bandwidth of FDML lasers - especially in the 1050nm region - does not support an axial resolution as good as the best spectral domain OCT systems. For this reason, we developed a wider bandwidth 1050nm FDML laser using a cavity design with two multiplexed semiconductor optical amplifier gain media. In the first step, we investigate different laser cavity topologies compatible with our fastest home-built sweeping fiber Fabry Perot tunable filter (FFP-TF). These FFP-TFs are the key element to achieving multi-MHz sweep rates in FDML. In our experimental laser setup during section-wise FDML tuning, we analyze spectral ripples, losses, parasitic lasing, and negative effects caused by polarization mode dispersion. The laser cavity consists of 2 semiconductor optical amplifiers in a parallel geometry. They have a center wavelength of 1020nm and 1130nm and are used inside a fiber ring laser cavity. We use four isolators to suppress parasitic lasing whereby two of them are located only in one of the two parallel branches. In section-wise FDML operation, we achieve a tuning bandwidth of 187nm with a repetition rate of 2x400 kHz, which theoretically would support an axial resolution better than 3μm in tissue.