Motivated by rapid advances in solar blind ultraviolet (UV) light emitting diodes (LEDs), filters and photomultiplier
tubes (PMTs), together with unique UV atmospheric propagation characteristics, a non-line-of-sight (NLOS) UV
communication test-bed has been recently built and utilized for extensive experimental evaluation of performance of
NLOS UV links in outdoor environments. Towards this end, key link components are first characterized and their
limitations are identified. The tradeoffs among communication range, received number of photons, and bit-error-rate are
revealed via field measurement results. Wavelength diversity is achieved by utilizing combinations of sources and
detectors centered at different wavelengths in the solar blind band. It is demonstrated that signals can be reliably
transmitted to their destinations of dozens of meters away through an NLOS channel. Although all reported results in
this paper are based on open field experiments, it is found that reflections from surrounding objects such as trees and
buildings can enhance the received signal strength, up to an order of magnitude increase in the received number of
photons in some cases, thus significantly improving link performance.
An new optical correlator containing a tapped delay line with thousands of taps is described. This enables ultra-high resolution correlation. We apply this to monitoring quality-of-signal by correlating the received, degraded bits with and un-degraded signal. The strength of the correlation signal, which is all optical, is proportional to the quality. Dispersion and attenuation can be evaluated in less than 100 ps at 40Gb/s, and jitter and noise in less than 100 ns. This is a significant improvement over minutes or even hours for bit-error-rate measurements. Simulations show good correspondence to eye-diagram measurements, the conventional (but slow) way to measure signal quality. If a network node can know the quality of all its links in real-time, it can re-route signals around poor links, and provide restoration and protection as well. The key to all this is an optical correlator with a very large number of taps in its internal tapped delay line. Our device uses a White cell and a fixed micro-mirror array. In a White cell, light bounces back and forth between three spherical mirrors. Multiple beams circulate in the same cell without interfering and are each refocused to a unique pattern of spots. We make the spots land on the micro-mirror array to switch between cells of slightly different lengths. Our current design provides 6550 possible delays for thousands of light beams, using only ten mirrors, a lens, and the micro-mirror array. We have developed two routing and protection protocols to exploit having this real-time information available to the network.