Signal modulation for holographic memories.

다국어 초록 (Multilingual Abstract)

Holographic memories use a photosensitive medium to record the optical interference pattern of a reference beam and a signal beam as a volumetric refractive index distribution. The modulation method used to impart the digital information onto the signal beam can greatly impact the efficiency and uniformity of media consumption, crosstalk among the recorded data, and ultimately, the storage capacity and speed achievable by the device. Previous holographic storage efforts have overwhelmingly concentrated on binary amplitude shift keying (ASK) for signal modulation owing to the considerable difficulties in detecting alternative signal qualities such as phase or frequency with a conventional optical intensity detector.
The main thrust of this thesis is to motivate and enable the use of phase shift keying (PSK) for holographic signal modulation. The first chapters are devoted to theory and analysis. Chapter 2 reviews the well-known problem of intensity inhomogeneity caused by the D.C. component of an ASK signal near the Fourier plane, and introduces the issue of intra-signal modulation noise. Both are sharply alleviated for PSK signals when compared to ASK signals. Chapter 3 addresses the topic of medium consumption, and develops a method for relating this to the signal strength of complex data storage holograms. Chapter 4 is about metrology, and presents a novel phase-sensitive microscope with a transfer function suitable for independently measuring the volumetric index distributions written by holographic signal beams.
The final chapters are concerned with the technical implementation of signal detection. In Chapter 5, a sub-Nyquist detection method is presented that allows ASK data to be recovered from an arbitrarily-aligned, slightly oversampled hologram detector array. This method is currently used in InPhase Technologies, Inc.'s first-generation holographic storage drive. Finally, Chapter 6 develops three different methods for the detection of PSK signals. The third, quadrature homodyne detection, extends the resampling method of Chapter 5 in a natural way in order to provide an evolutionary path towards a PSK-based second-generation holographic storage device.