Microsoft Research has been working on Project Silica — a glass-like write-once storage device that can store terabytes of data for 10,000 years — for well over five years now, without making any promises about its commercialization or disclosing detailed technical information. This week, the company finally detailed its mediums and end-to-end workflows in Nature, which is meant to confirm that the project is still under development, but without making any promises about commercialization.
The main concern around Project Silica is that Microsoft Research is now working in two different directions that rely on two fundamentally different physical mechanisms, which means that nothing is ready for prime time yet.
Two different writing methods
The first method leverages the original Project Silica write-once read-many (WORM) storage solution and relies on fused silica medium and femtosecond laser pulses to create microscopic data points — voxels — stacked across hundreds of layers. The revamped method relies on a pseudo-single-pulse approach in which each laser pulse is split into seed and data components: the seed pulse initiates a nanovoid, and the following pulse elongates it to produce the final birefringent voxel state. Eight polarization levels are encoded per voxel.
A research-grade Writer used to set the record for high speed data writing into glass. (Image credit: Microsoft)
This method enables a voxel pitch of 0.500 µm × 0.485 µm, 6 µm layer spacing, and 301 layers. Thus, the medium reaches 1.59 Gbit/mm³, which translates to 4.84 TB usable capacity in a 120 mm × 120 mm × 2 mm glass platter after overhead. From a performance point of view, we are talking about 25.6 Mbit/s per beam and 10.1 nJ per bit energy efficiency.
The second method introduces phase voxels, a new storage primitive that modifies the refractive index of the glass to create phase shifts in transmitted light. A phase voxel contains four levels of amplitude modulation and requires only a single femtosecond pulse to modify its phase. More importantly, they can be formed reliably in borosilicate glass, the same material used in cookware and oven doors.
At a 0.5 µm × 0.7 µm pitch, 7 µm interlayer spacing, and 258 layers, density reaches 0.678 Gbit/mm³, which means 2.02 TB per platter with four modulation levels at a 0.92 quality factor. Throughput per beam is 18.4 Mbit/s, and energy efficiency is 8.85 nJ per bit. In addition, by splitting the beam into four independently modulated channels, researchers demonstrated 65.9 Mbit/s without inducing thermal damage, and they believe that 16 or more beams should be feasible.
Two different retrieval methods
As there are two different types of voxels, Silica uses two distinct optical retrieval methods, each corresponding to the physical nature of the stored voxel.
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