Unequally spaced four levels phase encoding in holographic data storage

Holographic data storage system is a candidate for the information recording due to its large storage capacity and high transfer rate. We propose an unequally spaced four levels phase encoding in the holographic data storage system here. Compared with two levels or three levels phase encoding, four levels phase encoding effectively improves the code rate. While more phase levels can further improve code rate, it also puts higher demand for the camera to differentiate the resulting smaller grayscale difference. Unequally spaced quaternary level phases eliminates the ambiguity of pixels with same phase difference relative to reference light compared to equally spaced quaternary levels. Corresponding encoding pattern design with phase pairs as the data element and decoding method were developed. Our encoding improves the code rate up to 0.875, which is 1.75 times of the conventional amplitude method with an error rate of 0.13 % according to our simulation results.