I am doing Project on Wireless Optical Communication(Visible Light Communication). I am using Nicha NSPW510 DS White LED at transmitter and Vishay BPW34 PIN photodiode at receiver. The problem is with LED TURNOFF TIME. It is about 30 usec s. and i am not able to increase my speed beyond 10 kbps.(distance 1 - 2 meters). Could u please suggest any LED type, which supports upto 1 Mbps. ( I have tried to reduce turnoff time of LED by driving it into reverse bias region, but it did'nt helped me.)
White LEDs are the wrong choice for communication. This is because they are a two-part system. Inside is a blue or near UV LED and a mix of phosphors. The LED excites the phosphors, which then emits various colors of light. The result is balanced to look roughly white.
The reason for the lag is that the phosphors will continue to re-emit the energy for a short time after the LED is turned off.
Use anything that is a bare LED. This also has the advantage of all the light being within a very narrow spectral band. That makes it possible to filter out much of the ambient light at the receiver.
IR LEDs are usually used for this since the ambient level at near IR tends to be lower than the visible level, IR LEDs are efficient, and materials are available that block most visible light but let the IR pass. Wavelengths are usually in the 900-1000 nm range.
In addition to switching to bare LEDs rather than phosphor-coated, you may want to have a look into the literature on drive circuits. Here's an example to get you started High-Speed LED Driver for ns-Pulse Switching of High-Current LEDs Halbritter et al. (from Osram, so they should know a thing or two about their LEDs).
And here's another A LED-based calibration system for the external strings of the NT-200+ deep-water neutrino telescope on Lake Baikal, Vasiliev et al.
Both of these are about high-brightness LEDs and shape the current pulse. I may want to apply these techniques to driving reactions in an experiment.
Either the latter paper or another I can't find for now found significant variation in the speed of even nominally identical LEDs in the ns regime. Fast photodiodes are necessary but quite easy in comparison.