Wavelength-selective photodetector promises 4x faster Li-Fi

March 09, 2017 //By Julien Happich
Researchers from A*STAR and the National University of Singapore have devised a wavelength-selective photodetector that could boost the speed of white-light Li-Fi from 5 MHz to 20 MHz by focusing on the blue signal emission of white-light phosphors.

Although phosphor-coated white light LEDs have become the mainstream for bright illumination the colour-conversion phosphors used in the mix have largely varying photoluminescence lifetimes. As an example, inspecting the time-resolved yellow emission photoluminescence spectra of a white LED at 450 and 560nm, the researchers revealed a decay time for blue light emission of 1.4ns, much faster than the 54.4ns for the yellow light emission.

This means that when a white-light is modulated to transmit a digital signal, the blue light effectively turns on and off at a faster rate than the yellow phosphor emission, and a broadband Si photodetector ends up getting the slowest common denominator of the signal, slow rise and fall times.

To focus on the fast blue emission, the researchers devised a narrowband, green InGaN LED as the receiver. Their paper "Textured V‑Pit Green Light Emitting Diode as a Wavelength-Selective Photodetector for Fast Phosphor-Based White Light Modulation" published in the ACS Photonics journal details the fabrication of an InGaN LED with a sharp wavelength selectivity at 380nm, using V-pit texturing to improve the peak responsivity of the thin active layer in the InGaN LED.