Communication via vortices?

by Guest Blogger - Greg Gbur

By Greg Gbur, Skullinthestars.com

This is the second in a series of posts about the upcoming OSA Frontiers in Optics meeting in Orlando. This post covers research related to the presentation FM3F.1: Alan E. Willner, Multiplexing Information-Carrying Orthogonal Beams using Orbital Angular Momentum States. To be (hopefully) cross-posted at the Frontiers in Optics blog.

Do you think your internet is too slow? If you’re like me, you probably do, even though the speed of data transmission has exploded over the past 15 years. When I was in graduate school, dial-up modems that could download 56 kbit/s (56 thousand bits per second) were state of the art, whereas today some broadband business connections* can download 400 Mbit/s (400 million bits per second)! But even still I want more data, and at a faster rate — and I’m not alone!
Fortunately, it is possible that the rate of telecommunications may increase dramatically in the near future. Over the past couple of years, researchers have demonstrated the possibility of transmitting data at a rate of over a terabit/second (one million million bits per second), in both free space** and an optical fiber***, by giving the light that carries the data a “twist” before transmission!

So what is this “twist,” and how does it allow us to increase the rate of data transmission? To understand this, we should first take a rough look at how we currently transmit data.
The most overt illustration of how information is transferred can be seen in commercial radio. Different radio stations broadcast at different frequencies: one of my twitter friends is a DJ at California’s Z-Rock FM, which broadcasts at a frequency of 106.7 MHz (106.7 million cycles/second). What, exactly, does this mean? It means that the broadcast antenna emits waves with a carrier frequency of 106.7 MHz, and that the audio signal is added to this carrier frequency as a variation of the frequency (frequency modulation). For AM (amplitude modulation) radio stations, the audio signal is encoded by varying the amplitude of the carrier frequency.
Humans can hear sounds up to a range of about 20 kHz (thousands of Hertz). Roughly speaking, this means that the actual radio signal has its frequency band broadened by about 20 kHz, and radio stations must have their carrier frequencies separated by at least this amount to avoid overlapping their broadcasts. As the designated FM radio frequency band is between 88 MHz and 108 MHz, only a finite number of stations can broadcast in the same area. Because radio signals decay as they travel, different cities can use the same frequencies; however, if you’ve ever traveled between two such cities, you know that in the “frontier” the stations can overlap.
What can we do if we want to transmit more data? One option is to broadcast using a higher carrier frequency. Today, our landline phone and internet data is transmitted using visible light, which has a frequency on the order of $10^{15}$ Hz, or a million billion cycles per second! If we imagine sending audio signals with a bandwidth of about 20 kHz, the bandwidth is almost negligible compared to the carrier frequency, and we can transmit many signals simultaneously in a single optical fiber.
I’ve described the basics of fiber optics on this blog before. In short, a glass fiber can trap light traveling within it via the process of total internal reflection, and transmit it more or less cleanly through to the other side.

See caption below

A ray of light bouncing in an optical fiber