An endoscopic swept-source Fourier Domain optical coherence tomographic system (FDOCT system) for imaging of tissue structure includes a Fourier Domain mode locked (FDML), high speed, narrow line-width, wavelength swept source, an OCT interferometer having a sample arm, a reference arm, a detection arm, and a source arm coupled to the swept source, an endoscopic probe coupled to the sample arm, and a data processing circuit coupled to the detection arm. The swept source includes a long optic fiber functioning as a cavity, a high optical gain lasing module, and a tunable narrow bandwidth bandpass filter for wavelength selection combined to form a unidirectional ring laser cavity, where the tunable narrow bandwidth bandpass filter is driven synchronously with the optical round-trip time of a propagating light wave in the cavity.