2. Information Photonics Research Center, College of Photonic and Electronic Engineering, Fujian Normal University, Fuzhou, Fujian 350117, China
Overview: Based on the principle of the volume holographic data storage, the technologies and its main systems of holographic data storage are reviewed in this paper. The studied holographic storage system is set to collinear holographic storage system due to its compact and anti-interference. The principle, system structure and coding method of the collinear holographic data storage system are introduced and analyzed. In the traditional holographic storage system, only pure amplitude information of light is used for carrying information, which is a kind of waste of multiple modulation properties of light. In order to improve the encoding rate and storage density of holographic storage, the utilization of phase modulation is considered. In spite of using phase modulation can efficiently improve the encoding rate and storage density, the recognized encoding method of phase modulation is still unknown. In the absence of mature phase encoding principle, research group of Beijing Institute of Technology proposed two kinds of new phase encoding of phase-modulated collinear holographic data storage systems. Unlike traditional encoding principle of considering single pixel as a data point, the unequal interval four-order-phase encoding and equal interval four-order-phase encoding use pairs of pixels as one data point and use pure phase modulation to the data page. These two methods take advantage of the relationship between the two pixels of one pair to represent the information rather than the value of each pixel. Compared with the traditional amplitude modulation data encoding, the method using unequally spaced quaternary phase encoding can effectively improves the encoding rate of a data page and controls the error rate in a relatively low level. In addition, combing the unequal interval four-order-phase encoding method, the group improved this phase encoding method and introduced the equal interval four-order-phase encoding method. The coding range is extended from the original 0~π to 0~2π. Besides the pure phase modulation, the group also studied the feasibility of complex amplitude modulation. A multilevel complex amplitude modulated collinear holographic storage system is proposed, which can efficiently reduce material consumption and increase storage capacity. It locks the data page and the corresponding phase-retrieval interference beam together at the same location with sequential recording process, and makes the system more compact and phase retrieval easier. In addition, the method was experimentally evaluated with two SLMs. For further improving data storage density, an orthogonal reference encoding multiplexing method at same position of medium is proposed. Although the encoding rate is improved by using unequal and equal interval four-order-phase ending method and the then storage density is improved by complex amplitude holographic storage system, more reasonable coding method, appropriate modulation and suppression of noise, which are still urgent problems in the researching of holographic data storage technology.