Free-Space Optical Communications II

May 10, 2012

A previous article (Free-Space Optical Communications, August 18, 2011) presented a somewhat thorough review of the technology of free-space optical communications. Since technology advances at a rapid rate, we're due for an update. I include, also, a bit more historical background.

Free-space optical communication is the use of light to transmit signals. It has some advantages, such as not requiring a license, although safety regulations must be obeyed if there are high intensity light sources. Today, it's very easy to generate data-modulated light in many wavelengths and at high intensity.

Light-emitting diodes, laser diodes, and fast semiconductor photodetectors, have made free-space optical communication quite easy. In the past, such light sources were not available, so many optical signaling systems were passive; that is, they relied on ambient lighting.

A semaphore is a well known computer programming concept. It's essentially a flag that indicates whether or not some code should be executed. A common example is an operating system's not letting more than one program modify a working file at the same time. Before the digital age, there were semaphores, also, but these were flag signals. You may have seen used for ship-ship signaling in older war films.[1] Flag semaphores are an example of passive optical communications.

A flag semaphore is a method to send alphanumeric characters via hand positions. To make things clear, even at a distance, the hand positions are only at 0, 45, 90, 135, 180, 225, 270, 315 degrees, with each hand at a different angle.[2]

The signaling is done with flags on short sticks to make them more visible, but flags are not needed. It can be seen that there are (8 x 7 / 2) such hand combinations when we don't allow overlaps and remember that the flags are identical, so there are 28 possible characters. Numbers and letters necessarily share codes, but these are generally obvious from the context.

Semaphore code for "TIKALON." (Figure rendered using Inkscape.)

In an early example of a machine replacing man, semaphore lines were larger, mechanical versions of semaphores intended for communication over long terrestrial distances. These were generally mounted atop high towers, or on hills. Since computer scientists are playful people, there was even a demonstration of sending Internet signals using semaphores. Privacy might be problematic for such a system.

Everyone has seen barcodes and the now ubiquitous QR codes.[3] QR codes have found their way onto billboards, print ads, and packaging, as a means of directing people to a relevant Internet URL. You just image these codes on your mobile phone or tablet computer, and the page appears.

QR Code for http://www.tikalon.com (Courtesy of Kaywa.com).

With the diminishing costs of flexible displays and printable electronics, there's no reason why these codes can't be animated to provide some serial, or changeable data; for example, your bathroom scale providing you with a means to log your daily weight. Who needs RFID when your ID card can flash data to a reader using an E Ink display?

When the system requirements call for high speed signaling, we need active systems. For short range optical communications of less than a meter, the common solutions are IrDA (Infrared Data Association) devices that will operate at data rates from a pedestrian, 9.6-115.2 kbit/sec, to 1 Gbit/sec using near-IR emitters of 875 ± 30 nm wavelength. There is recent standardization effort for visible light communications by the IEEE 802.15.7 Visible Light Communication Task Group.[4]

A group of Taiwanese engineers has just demonstrated an inexpensive, high speed, wavelength-division multiplexing free-space optical communications link using common laser pointers.[5-7] They modified red (671 nm, 5 mW) and green (532 nm, 5 mW) laser pointers by substituting a modulation source for the provided battery pack. They were able to modulate each laser at 500 Mbit/sec, for a combined data rate of 1 Gbit/sec, over a ten meter range.[7] The bit-error rate (BER) was less than 10^-9.[5]

This is nearly ten times the data rate of an 802.11n wireless network, such as that found in many Wi-Fi routers.[6] Hai-Han Lu of the Institute of Electro-Optical Engineering, National Taipei University of Technology (Taipei, Taiwan), and principal engineer for this study, is quoted in New Scientist as saying that the system cost was about $600.[7] The laser pointers are inexpensive, but quite a bit of signal processing electronics was involved. Laser links are highly directional, which can be an advantage in some systems, and a disadvantage in others.

Laser pointers. Images like this are produced by blowing smoke into the beam paths. (Modified Wikimedia Commons image).

References:

1. See, for example, the documentary television series, Victory at Sea.
2. Monty Python - The Semaphore Version of Wuthering Heights, YouTube Video.
3. Masahiro Hara, Motoaki Watabe, Tadao Nojiri, Takayuki Nagaya and Yuji Uchiyama, "Optically readable two-dimensional code and method and apparatus using the same," US Patent No. 5,726,435. March 10, 1998.
4. Visible Light Communication Tutorial, ieee802.org Web Site, March 17, 2008.
5. Wen-Yi Lin, Chia-Yi Chen, Hai-Han Lu, Ching-Hung Chang, Ying-Pyng Lin, Huang-Chang Lin and Hsiao-Wen Wu, "10m/500Mbps WDM visible light communication systems," Optics Express, vol. 20, no. 9 (April 23, 2012), pp. 9919-9924.
6. Sebastian Anthony, "1Gbps wireless network made with red and green laser pointers," Extreme Tech, May 2, 2012.
7. Jeff Hecht, "Laser pointers make super-fast 'optical Wi-Fi'," New Scientist, May 2, 2012.