We present and demonstrate a swept source with a large coherence length using a quasi-phase continuous tuning (QPCT) technique. QPCT is a method of minimizing the phase shift per round trip with respect to the tunable filter so that the resonance of lasing becomes high, resulting in high finesse of lasing during a rapid sweep. The demonstrated swept source consists of a fiber ring extended cavity laser with a diffraction grating and a polygon scanner-based tunable filter configuration. The projected beam on the diffraction grating is expanded with a multiple of beam expanders to achieve high finesse of the filter. The source demonstrated an $18\mathrm{nm}$ swept range at $1060\mathrm{nm}$ wavelength, $28\mathrm{mm}$ coherence length, and $6.2\mathrm{mW}$ peak power at a $2.5\mathrm{kHz}$ swept rate. OCT imaging results showed that a coherence length of $28\mathrm{mm}$ enables the measurement of the axial length of a pig's eye with $20\mathrm{mm}$ length in physical size.