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Photonic Jets Shown to Increase Optical Disk Capacity

by June 10, 2008

A recent study released by Northwestern University demonstrated the possibility of using photonic jets to further increase the storage density of optical disc media.

Traditional methods of storing data on optical media are bounded by certain physical limitations that current generation technologies are now pushing.  Photonics, which involves directly using a stream of photons rather than light as a transmission medium, may be a way to push those physical limits out further.

Data storage on optical media is accomplished using a two dimensional array of microscopic pits of various sizes whose measurements are correlated to the data that is being stored.  A laser, a collimated beam of light, is focused through a lens onto the reflective surface of the disc to read these dimension and generate a data stream for recovery.  The physical limitation on readable pit size, and thereby the data storage density, rests with the laser frequency and the optics.  The ability of the lens to refract light sets the limit, which is characterized by the refractive index of the material.  The tightest possible focus for a given frequency of light is set by the Abbe Diffraction Limit, a function of the wavelength of light (lambda) and the numerical aperture (n sin alpha):

This limit is a theoretical bound based on a perfect lens that the highest quality real world optics could only approach.  The limit places optical resolution very near the wavelength of the light source and this sets the lower bound of pit size, spacing, and track pitch that theoretically can be read.

For Blu-ray using a blue-violet laser frequency at 405 nm, the laser focus is to a 480 nm spot with pits that are 250 nm x 150 nm (W x L) minimum at a 400 nm track pitch.  As a fraction of the wavelength, the pits are 0.62 x 0.37 wavelengths with an area of 0.23 square wavelengths.

The evolution of the optical disc from CD to DVD to Blu-ray had been the laser frequency as reliable miniature lasers have been developed at higher frequencies.  The compact disc utilized a near infrared laser, DVD used a red laser, and BD uses a blue laser, allowing for smaller pit dimensions and tighter spacing at each iteration in turn.  Layering was also added to increase capacity, but this is limited by decreasing signal to noise ration with every partially reflective layer added to a disc.

What all this means is that under most circumstances the wave behavior of light limits optical disc storage density.  However, light and the universe have a dual nature.  Quantum mechanics is the basis for understanding the dual nature of energy, particles, and of matter.  Visible light is a range of frequencies of electromagnetic radiation that is detectable by the human eye, a waveform.  That waveform consists of a stream of particles, well known as photons.  Under most common conditions, light is detectable as a wave and subject to the physical behavior of wave mechanics, but under certain conditions, the wave form can be revealed as the underlying particles.

Recent discoveries have shown that light focused through a glass sphere will produce a jet of photons for a very short distance out of the backside of the sphere before an expansion back into a waveform.  That distance is again related to the frequency of light as a wave and for blue frequency lasers, is approximately a micrometer.  Within that distance the stream of photons, or photonic jet, is much narrower than a beam of light.

What the Northwestern research team has done is demonstrate the feasibility of using photonic jets to retrieve data using an up scaled optical disc model to simplify the experimental setup and avoid the significantly tighter tolerances that a blue laser system would require.  The experiment consisted of focusing microwaves with a 10 mm wavelength through an acrylic sphere.  Data storage was simulated using an aluminum disc with scaled pits configured to match the geometry currently used in Blu-ray based storage that were machined into the surface and a scaled equivalent of the protective coating found on standard optical discs was applied to the surface.

The experimental results showed that pits with a significantly smaller dimension that the wavelength of the beam could be read.  Scaled to microwave frequencies, the photonic jet was able to read pits that were 0.125 x 0.20 (W x L) wavelengths, with lateral area of 0.025 square wavelengths, at a signal to noise ratio 28 dB greater than with a lens system.

This first experiment in photonic data storage alone has shown an estimated storage density increase by nearly a factor of ten in terms of the readable pit area, assuming spacing and track pitch scale equally.  Additionally, the experiment also showed that the peak power ratio for pit to non-pit was sensitive to pit depth allowing for the potential of storing data three dimensionally, giving rise to further increases in disc capacity.

A minimum tenfold increase in storage capacity for optical discs would certainly go a long way to prolonging the useful lifespan of the technology.

About the author:
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Professionally, David engineers building structures. He is also a musician and audio enthusiast. David gives his perspective about loudspeakers and complex audio topics from his mechanical engineering and HAA Certified Level I training.

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