Like WiFi, the light-fidelity (LiFi) technology that uses both visible and near-visible light is used for free-space communication. While microwaves used in WiFi technology to transmit signals can pass through walls, visible and near-visible light that carry the LiFi signal cannot, thus making the network more secure. Now, researchers from Bengaluru’s Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) have added another layer of security to LiFi.
Light bounces off from walls and falls on the receiver. So wall boundaries can be used effectively for reflecting signals so that communication is maintained even without line-of-sight communication between the signal source and receiver. Detectors can receive both direct and reflected signals. “There is no distortion when light bounces of walls but the signal can get attenuated. We can reduce the attenuation to some extent by using more light bulbs, keeping the walls reflective and having large receivers,” says Prof. K.S. Narayan from the Molecular Electronics Lab at JNCASR who led the team of researchers.
So the researchers studied the effect of walls painted with fluorescent and phosphorescent paints. Both fluorescent and phosphorescent paints absorb and then emit light with marginal loss. “This leads to interesting possibilities,” he says.
Jamming the signal
The researchers set out to do the complete opposite of what other researchers have been trying to do. White LED is obtained by combining blue LED with phosphors. While others have been trying to reduce the excitation life time of phosphors to improve the signal bandwidth, the JNCASR researchers purposely added phosphors to introduce noise in the signal to make the network more secure.
“While the fluorescent material absorbs and emits a photon before the next photon can fall on it, the time taken to emit is longer in the case of phosphorescent paint. The phosphorescent paint randomly emits photons, which ultimately scrambles the signal by superimposing a larger noise component to the signal,” says Anaranya Ghorai from the Molecular Electronics Lab at JNCASR and first author of a paper published in the Journal of Optics .
“The phosphorescent paint adds more noise to signal and in the process reduces the signal-bandwidth. This results in slower speeds. So we can selectively restrict the LiFi usage by choosing an area where distortion is needed by painting that part of the wall with phosphorescent paint,” says Prof. Narayan. “The signal is corrupted by the noise and we can sort of jam the signals to inconvenience the user and even prevent usage.”
Harnessing leftover light
On absorbing light, phosphorescent paint remains in an excited state and continues to emit light even several hours after the original source of light has been switched off.
“We can now use the light emitted by the phosphorescent paint as a source of signal by using an appropriate modulator,” says Prof. Narayan. This would mean that signal transmission can continue for hours even when light source has been turned off. “This was not a targeted research. This idea just came about and we had some fun. But it has opened up an interesting area of research,” says Prof. Narayan.
