Holography Offers Eye-Safe IR Networking

Brent D. Johnson

The wireless home and office are upon us, with computers and peripheral components connected by streams of microwaves. However, interference problems and the potential for hackers to intercept these transmissions -- which can penetrate beyond the intended sphere of operation -- have some individuals exploring alternatives. Researchers at the University of Ottawa's School of Information Technology and Engineering have devised a holographic diffuser that would disperse infrared laser radiation over a large area to enable eye-safe transmissions at powers of up to 37.1 mW for wireless networking.

To enable eye-safe infrared networking with higher-power laser emitters, researchers at the University of Ottawa have developed computer-generated holographic diffusers. The simulated annealing approach yielded the optimal pattern, which they transferred to a quartz plate using a laser writing system. In application, the network would incorporate IR and Bluetooth microwave-frequency technologies for the downstream and upstream data links, respectively. Courtesy of Jianping Yao.

The most important advantages offered by IR are the availability of an abundant and unregulated spectrum and the fact that the radiation does not penetrate walls or other opaque barriers, so signals are secure and the bandwidth can be spatially reused. But eye safety limits the effective power at which IR signals can be transmitted to 0.24 mW at 850 nm for a point source, and lower transmission powers equal higher bit-error rates. The researchers believe that they can convert the output of a higher-power point source to be as safe as that of a large-area, or extended, source.

With funding from Nanyang Technological University in Singapore, Jianping Yao and his colleagues created the holographic diffuser using a computer-generated hologram. Unlike typical holograms, the com-puter-generated variety does not require a physical template but is created by means of a mathematical description using the Fraunhofer approximation. Once the optimal holographic image is designed in the computer through an iterative process called simulated annealing, it is displayed on a spatial light modulator and illuminated by a HeNe laser, which writes the pattern onto a piece of quartz.

In an IR wireless networking application, this quartz holographic diffuser would be placed between a collimating lens and a Fourier lens, and the laser source would pass through the optical setup. The diffuser would expand the size of the laser beam by 10 times and spread the radiation pattern uniformly over an area of approximately 4 x 5 m at a distance of 2 m, making it safe to use higher-power sources.

Yao said that they are considering applying the approach in a hybrid arrangement for home networking. In this configuration, the downstream data link would use IR signals, and the upstream link would use microwave technology.