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, "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:
- WiFi Interference Problems in Urban Environments?, Slashdot, February 10 2004.
- 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!
- A.G. Bell, "Apparatus for Signaling and Communicating, called Photophone," US Patent No. 235,199, December 7, 1880.
- Bernhard Grill, Karl-Heinz Brandenburg, Thomas Sporer, Bernd Kurten and Ernst Eberlein, "Digital Encoding Process, US Patent No. 5,579,430, November 26, 1996.
- Data are traveling by light, Research News, Fraunhofer Institute, August 2011 .
- Jeff Hecht, "Yarns from the Technological Jungle," New Scientist, vol. 109, no. 1497 (February 27, 1986), pp. 50f.
- Forrest M. Mims III, "Surreptitious interception of conversations with lasers," Optics News, vol. 11, no. 11 (1985), pp. 6-12.
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