An optical data storage system includes a light emitting and receiving device (30), a light transmission device (31) having an input port (32) and an output port (33). The input port is disposed adjacent to the light emitting and receiving device. Furthermore, a micro window is provided at the output port, and a diameter of the micro window is in a range of 5 to 70 nanometers. Therefore, in the optical data storage system, the spot size of the light beams is close to the diameter of the micro window. Accordingly, the size of the beam spot is small enough to write information at a higher density with respect to an optical storage medium.