Key metrics
We evaluate the Silica storage system against the following metrics:
Voxel quality, Q = B / n V , is an average (bit per voxel) calculated from the number of user bits, B , stored in a large number of voxels, n V . Q < Q w , where Q w is total number of bits per voxel, because redundant bits are added to correct for errors. We define q = Q / Q w as the quality factor (see section ‘Redundancy optimization and error correction’). All other metrics except lifetime are dependent on Q .
Data density, ρ = Q / V is the voxel quality that can be stored in a volume V of glass (Gbit mm − 3 ), where V is computed as the product of the x pitch, y pitch and the effective z pitch (that is, the total 2 mm thickness of glass divided by the number of layers written).
Usable capacity is the total number of user bits that can be stored in a single glass platter 120 mm square and 2 mm thick, reduced by a factor of 0.747 to account for engineering overheads, measured in TB per platter.
Write throughput, θ = f N L Q , where f is the laser repetition rate and N L is the number of beam lines, is the speed at which data can be written to the glass, measured in bit s −1 . It is defined as a peak throughput.
Write efficiency, η = E / Q measures the energy consumed to write each user bit (nJ per bit). E is measured after the objective. A lower η represents better write efficiency and so is preferred as it allows more bits to be written in parallel for the same laser pulse energy.
Lifetime is an experimental estimate of the lifetime of the data stored in the glass (see section ‘Lifetime’).
Write
Figure 2a shows the key building components of the data writing system. The source is an amplified femtosecond laser (Amplitude Systems, Satsuma HP3 with harmonic generation, 516 nm central wavelength (second harmonic), and tunable pulse duration from 300 fs to 1,000 fs). We measured the pulse duration before the objective lens using a Gaussian fit of the autocorrelator signal (APE, Carpe). The output laser pulse train at 10 MHz passes through a tunable attenuator, quartz half-wave plate on a motorized rotary stage and Glan linear polarizer.
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