Optical computing is tempting but unwieldy technology.
In principle it can be inherently parallel – a wavefront is effectively a 2D matrix of intensity (or phase, or polarisation) values, and if this is processed through an optical system, every point on that matrix is processed at the same time – a second order level of parallisation.
And some optical components naturally perform mathematical functions with virtually no energy requirement – simple lenses can perform Fourier transforms, for example.
However, what is easily possible can differ widely from what computation is needed, these systems don’t necessarily lend them selves to miniaturisation, and reprogramming is an issue.
The Tokyo technique is a multi-function optical technique dubbed ‘diffraction casting’, where an input image passes though a stack of processing layers, diffractive in nature, together forming a diffractive neural network which performs logical operations on the image.
“In the 1980s, researchers in Japan explored an optical computing method called shadow casting, which could perform some simple logical operations,” said Tokyo Information Photonics Lab scientist Ryoichi Horisaki. “But their implementation was based on relatively bulky geometric optical forms. They worked in principle, but lacked flexibility and ease of integration to make something useful.”
“Shadow casting is based on light rays interacting with different geometries,” Horisaki went on the explain, “whereas diffraction casting is based on properties of the light wave itself, which results in more spatially efficient, functionally flexible optical elements that are extensible in ways you’d expect and require for a universal computer.”
The technique has been simulated, using 16 x 16 pixel monochrome input images, “performing the 16 basic logic operations at the heart of much information processing”, said fellow researcher Ryosuke Mashiko.
It “yielded very positive results”, said Horisaki.
Diffraction casting is not enough for an entire optical computer.
“It might be best to think of it as an additional component rather than a full replacement of existing systems,” said Mashiko. “There’s also scope for extending our system into quantum computing. Much work has to be done on the physical implementation, which, although grounded in real work, has yet to be constructed.”
Above is only the briefest skim through the technology. For those wanting more, the paper ‘Diffraction casting’ is available in full without payment from the SPIE digital library‘
