A newly developed single-mode fiber-designed optical coherence tomography (OCT) system is built based on low coherence interferometry and optical heterodyne detection. A broadband infrared optical source centered at 840 nm with a bandwidth of 50 nm is used as the system source. A stable high carrier is generated by Fourier-domain rapid scanning optical delay line (RSOD) as phase modulator. With this developed system, two-dimensional (2D) cross sectional images of tissues in vivo are reconstructed. The experiment results indicate that the system has an axial resolution of 6.7 μm, approaching the ideal resolution, and transversal resolution of 4.7 μm in air. The depth imaging range is above 3 mm in air. Although less than 300 μW optical power is incident on the sample, the system sensitivity is above 88 dB. With the same incident power on the sample, an image of the same sample (fresh orange) with a 1310 nm OCT system is also provided. The qualitative comparisons between the two system at different central wavelengths demonstrate the impressive potential of 840 nm OCT system to perform eye posterior structure imaging. The image of the animal retina in vivo is presented.