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Free-Space Optical Communications

August 18, 2011

Radio is a really useful communication technology. But one primary feature of radio, the ability to send your signals a great distance, is a disadvantage in some applications. When you intend to send a signal across a room, it's overkill to use a technology that will transmit beyond your room, perhaps interfering with your neighbors. People who have wireless networks have experienced interference from other radio devices in proximity.[1]

The military face the same communications problem. Often, they want their radio signals to travel just a short distance, but not any farther. Some covert systems utilize extremely-high frequency (EHF) signals that are rapidly attenuated by the atmosphere. A oxygen absorption in the range of frequencies from 57–64 GHz attenuates signals strongly. Water vapor in the atmosphere is also a strong absorber. If these extremely-high frequencies are used, the spy satellites aren't able to eavesdrop on your signals.

Radio waves aren't the only electromagnetic waves that are useful for communications. Just a little lower in wavelength than the millimeter waves of the extremely-high frequency band is light. The idea of using light to signal has been around since antiquity, in the form of signal fires and signaling mirrors, called heliographs. Alexander Graham Bell, inventor of the telephone, experimented with transmission of sound using light in a system called the photophone.

Bell's photophone used audio modulations of a flexible mirror to focus and de-focus light to modulate a transmitted beam, as shown in the figure. Reception was done using the photoresistive effect of selenium. The resistance of selenium will decrease when exposed to light. While I was an elementary school student, I used the same effect in a cadmium sulfide (CdS) photocell in one of my first electronic circuits.[2] Bell's selenium detector varied the current in a telephone circuit.

A portion of figure 1 from US Patent 235,199.A portion of figure 1 from US Patent 235,199, "Apparatus for Signaling and Communicating, called Photophone," December 7, 1880, by Alexander Graham Bell.

(Via Google Patents)
[3].

Bell's demonstration of the photophone on April 1, 1880, (or June 3, 1880, according to a commemorative plaque) qualifies as the first wireless audio communication. It covered a distance of 213 meters, or 700 feet, between the window of Bell's laboratory and a nearby school. This was a quarter of a century before Reginald Fessenden made the first audio radio transmission on December 24, 1906, although Fessenden's transmission was received hundreds of miles away.

Light communications in open space, what's now called free-space optical communication to distinguish it from fiberoptic communication, wasn't that popular after the invention of radio, but all that changed with the invention of solid state light sources. The light-emitting diode (LED), the laser diode and associated semiconductor photodetectors, made such communications quite easy.

There was quite a time interval between the invention of the LED and the laser diode, so early free-space optical communications developed around the LED. The ubiquitous television remote controller uses infrared optical signals at the rather pedestrian signaling rate of about 38 kHz. The question naturally arises as to how fast an LED can be modulated. Some simple, back-of-the-envelope calculations answer this question.

Infrared LEDs of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) have a voltage drop of about 2 volts, and they typically withstand pulsed operation at currents of 25 mA. Plugging into Ohm's law gives us an equivalent resistance R of an LED.
R = E/I = 2/0.025 = 80 ohms
Since the capacitance of LEDs can be from about 25-250 pF, depending on the junction area and other factors, we can calculate an RC time constant t for an LED circuit element,
t = (80)(25 x 10-12) = 2 x 10-9
t = (80)(250 x 10-12) = 2 x 10-8
Converting to frequency (f = 1/t), gives approximate switching rates of between 50 MHz and 500 MHz. These rates are realized in practice. You can always get a higher rate by using a parallel array of smaller diodes (with smaller capacitance) driven from separate current sources.

Short distance free-space communications, as between adjacent electronic devices, is made easier by a standard promulgated by the Infrared Data Association and known as IrDA. The IrDA standard specifies just a one meter range using an 875 ± 30 nm infrared optical signal. The speed varies, depending on the application, but it ranges from 2.4 kbit/sec to 1 Gbit/sec.

Perhaps fueled by licensing fees from the MP3 patent,[4] researchers at the Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute, Berlin, Germany, have developed a high speed, medium range free-space communications system that uses LEDs intended for room illumination.[5]

Room lighting needs a white light spectrum, often provided by arrays of red, green and blue LEDs. A mixture of these, plus white LEDs, provides four wavelengths for data transmission, thereby increasing signal rate. Of course, a little digital trickery is required, since white activates the red, green and blue channels simultaneously.

The German research team was able to achieve a transmission rate of 800 Mbit/sec in the laboratory. Their work was part of the European Union Omega Project, which involved technologies for implementing gigabit home networks. The team will demonstrate the technology at the International Telecommunications Fair (Internationale Funkausstellung IFA) in Berlin, September 2-7, 2011.[5]

I'll close this article with an interesting tidbit from the history of using LEDs in optical communications. Forrest Mims is a self-taught inventor, and Mims and I were experimenting with CdS photocells at about the same time. Mims was apparently the first to recognize that LEDs can be used also as detectors of light. According to New Scientist Magazine, Mims approached Bell Labs with his discovery in 1973, but they weren't interested.[6]

Mims published his idea in Popular Electronics, and he was then surprised to learn, in November, 1978, that Bell Labs was claiming his invention. The issue was settled out of court with a cash settlement to Mims.[6] In 1980, Mims demonstrated bi-directional free-space optical communication, and also communication in a hundred meters of optical fiber, at the same site where Bell demonstrated his photophone a hundred years earlier.

One interesting paper, published by Mims in 1985, describes bouncing a laser from a window pane to detect conversations, and possible countermeasures for this.[7] This is a novel variation of what Alexander Graham Bell was doing in 1880.

References:

  1. WiFi Interference Problems in Urban Environments?, Slashdot, February 10 2004.
  2. When I was a small child, I was fascinated by a photocell-activated automatic door at our local supermarket. That's likely the reason why one of my first circuits was a cadmium sulfide photocell-activated relay. The circuit was powered directly from the AC mains, an experiment not recommended for children!
  3. A.G. Bell, "Apparatus for Signaling and Communicating, called Photophone," US Patent No. 235,199, December 7, 1880.
  4. Bernhard Grill, Karl-Heinz Brandenburg, Thomas Sporer, Bernd Kurten and Ernst Eberlein, "Digital Encoding Process, US Patent No. 5,579,430, November 26, 1996.
  5. Data are traveling by light, Research News, Fraunhofer Institute, August 2011 .
  6. Jeff Hecht, "Yarns from the Technological Jungle," New Scientist, vol. 109, no. 1497 (February 27, 1986), pp. 50f.
  7. Forrest M. Mims III, "Surreptitious interception of conversations with lasers," Optics News, vol. 11, no. 11 (1985), pp. 6-12.