Ultra-narrow interval angular control signal for holographic data storage system

A precise and fast method for controlling the reference-beam angle of an angular-multiplexed holographic data storage system (HDSS)—to achieve larger capacity and faster data-transfer rate—is proposed. The reference beam is first controlled by a galvanometer mirror (GM) with an “angular control signal (ACS)” applied to its zero-cross angle (which differs by a certain offset angle from the target angle). The offset angle is then eliminated by referring to the output from a rotary encoder inside the GM, and the optimum angle for the reference beam is obtained. Next, a servo beam is used for the ACS, and the ACS value is obtained as a differential signal between the beam intensities of the diffracted reference beam and the diffracted servo beam. The servo beam is orthogonally polarized in regard to the reference beam and has a slightly different incident angle. A reference-beam angular error lower than 3.3 mdeg was confirmed in simulations and experiments.