Most high-speed networking is done using optical fibers. The hardware on each end of these fibers has to convert the optical signals to electronic ones in order to figure out a packet's destination and will often return it to optical form before sending it on toward its destination.

Researchers at the Japanese telecom NTT find all that converting a bit wasteful and are working on ways to avoid it. They've recently published a paper that includes a description of a working 115-bit optical Random Access Memory device, made of a carefully structured series of photonic crystals, each of which can store light of a different wavelength.

Photonic crystals are made of layered semiconductors, with the precise structure (the thickness and spacing of the layers) determining how they interact with light—it's possible to make photonic crystals that selectively block or transmit a narrow frequency range.

In the past, specialized versions have also been used to "store" light. In essence, a photonic crystal tuned to a specific wavelength will absorb a pulse of light and remain in a high-energy state as long as a weak light of the same wavelength is supplied (termed the "bias light"). A more intense pulse will cause the original light to be emitted as the photonic crystal returns to its low energy state.

It's possible to use that process to create an optical bit. If a pulse of light is stored, the result will be strong emissions when the second pulse is applied, allowing a "1" state to be read. If no pulse is stored, the second pulse will have little effect, providing a "0" reading. Turning the bias light off simply resets the bit.

The researchers beyond the current paper had been using photonic crystals to store multiple bits by placing an optical switch upstream of them, which could direct light pulses to read and write different bits to the appropriate locations. But this is pretty limiting since you could only use as many bits as you could accurately direct the incoming light to. So the researchers decided to restructure their hardware.

Since photonic crystals can be tuned to store different wavelengths and are transparent to any others, it's possible to arrange them in a series, where each crystal only pays attention to a specific wavelength. Photons sent down this array (the researchers call it a bus) will travel unimpeded until they run into the crystal corresponding to their wavelength. Once they do, they can be absorbed, stored, and read out again later.

The team started out using layers of indium-gallium-arsenic-phosphorous and indium-phosphorous. This allowed them to create 31 bits, each storing a wavelength separated from its neighbors by 0.9 nanometers. Once they got this working, they switched to silicon layers, which allow finer control over the wavelengths involved. This allowed a separation of only 0.23 nanometers and allowed them to store 105 optical bits in a single device.

That's pretty limited, but the team thinks it can be combined with the optical switching they had been using previously to create a two-dimensional array of bits, allowing them to expand the data being stored. The processes of reading and writing are pretty fast as well, since they can be done with light pulses on the order of 100 picoseconds.

The bad news is that it's still pretty energy intensive. Between the read and write light pulses and the bias light, the researchers estimate that a 28-bit memory takes about 150 micro-watts to store, which can add up pretty fast as you try to add additional bits. Unless this energy cost can be brought down, it will be very difficult to make this competitive for anything but rare cases.

Nature Photonics, 2014. DOI: 10.1038/NPHOTON.2014.93 (About DOIs).