This paper reports the theoretical analysis and experimental results of a far-field focus sensor. This system utilizes moving interference fringes at the far-field caused by the one-dimension uniform-pitch grating embedded inside a disc. The intensity of the moving fringes was sampled and processed by a digital signal processor and synchronization circuits including a phase lock loop. A well-shaped and low-offset focus error signal was observed all over the disc rotation, as well as, the focus serve operation being confirmed. Finally, how this sensor can be adopted to the collinear hologram data storage system is discussed.