The speed of sound is 100000 times slower than the speed of light. Thus, by transferring the digital information carried in light waves into sound waves, slowing it down drastically, the researchers temporarily “parked” the data in an integrated circuit (or microchip). This transfer of data from the optical to the acoustic domain and back is a critical feat in developing photonic integrated circuits.
Chip-integrated coherent photonic-phononic memory
Around the world, large data centers — such as those of Google, Microsoft, Amazon and Facebook — are running up against the challenge of high energy consumption and immense heat production. “Our vision is to replace the electronic interconnects between different processors and computing machines with photonic ‘wires.’ So light transmission will be used instead of electronic connections,” says project supervisor Birgit Stiller from the University of Sydney, whose team aims to replace electronic connections between different processors with light connections in order to provide high speed, broad bandwidth and low heat production.
The speed of light, however, can also become “a nuisance,” when it comes to synchronization, according to the expert, which is why an optical solution for a short-term "parking" (memory) is needed. “Our approach is a solution for a light memory that is entirely controlled by light pulses,” she says. “Therefore, all advantages of the photonics are preserved.”
Storing light as sound — a first-time feat
“Light waves and sound waves are of a completely different nature. Sound waves are density waves, which is the reason why our ear is sensing vibrations of the air and transferring it to the brain,” explains Stiller. “Light waves are electro-magnetic waves. They also have completely different frequencies. Making these two completely different types of waves talk to each other and transfer information coherently between them is very fascinating for me.”
Stiller and her colleagues have, in fact, stored light as sound for the very first time on a photonic chip. Furthermore, they have shown that this storage preserves the coherent and frequency information of light. She notes that ten years ago, there had been another proof of principle in optical fibers (in Durham/Rochester), but not for the entire information and not on a photonic chip.
The process behind coherent photonic-phononic memory
“There is a nonlinear optical effect — called Stimulated Brillouin Scattering — that links light and sound waves under specific conditions,” says Stiller. “If a data stream (light) that travels at the speed of light encounters a control light pulse (write), the information can be transferred to an acoustic wave. This means the information travels further at the speed of sound, which is 100000 times slower than the speed of light. By reversing the process with another control light pulse (read), the information is transferred back to a data stream carried by light. Therefore, it has been ‘stored’ for a while in the acoustic wave.”
The waveguides the scientists in Australia are currently using in their light-to-sound experiments are made out of softglass. “One step forward has already been shown recently in our group in integrating these softglass waveguides on a silicon chip, which can be industrially fabricated,” Stiller says, noting, however, that this integration is still on a research level. “Other next steps would be to engineer the system further, and make it ready for a prototype.”
The study, titled “A chip-integrated coherent photonic-phononic memory,” is published in the journal Nature Communications.
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today