We demonstrate quasi-supercontinuum (quasi-SC) generation around the $1.3\mathrm{\mu m}$ wavelength region using ultrahigh-speed, wavelength-tunable, femtosecond soliton pulses based on an ultrashort-pulse laser system operating at a wavelength of $1.0\mathrm{\mu m}$ . The wavelength tuning range was from 1.0 to $1.9\mathrm{\mu m}$ , and the scanning speed was up to $1.3\mathrm{MHz}$ . A Gaussian-like quasi-SC with a bandwidth of $220\mathrm{nm}$ was generated at $1220\mathrm{nm}$ . The generated quasi-SC was used in an optical-coherence tomography system. High axial resolutions of $5.1\mathrm{\mu m}$ in air and $3.7\mathrm{\mu m}$ in tissue were obtained. A maximum sensitivity of $100\mathrm{dB}$ was achieved, and ultrahigh-resolution images of a hamster cheek pouch were observed.