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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:
- See, for example, the documentary television series, Victory at Sea.
- Monty Python - The Semaphore Version of Wuthering Heights, YouTube Video.
- 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.
- Visible Light Communication Tutorial, ieee802.org Web Site, March 17, 2008.
- 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.
- Sebastian Anthony, "1Gbps wireless network made with red and green laser pointers," Extreme Tech, May 2, 2012.
- Jeff Hecht, "Laser pointers make super-fast 'optical Wi-Fi'," New Scientist, May 2, 2012.
Permanent Link to this article
Linked Keywords: Technology; free-space optical communications; light; signal; Federal Communications Commission; license; Laser safety; data; modulation; modulated; wavelength; light-emitting diode; laser diode; semiconductor photodetector; ambient lighting; semaphore; computer programming; flag; code; operating system; information Age; digital age; flag signal; ship-ship; war; film; alphanumeric characters; hand; degree; flag; combination; number; letter; context; Inkscape; semaphore line; computer scientist; demonstration; Internet; privacy; barcode; QR code; billboard; print ad; packaging; Internet URL; mobile phone; tablet computer; Kaywa.com; flexible display; printable electronics; serial communication; weighing scale; bathroom scale; Radio-frequency identification; RFID; ID card; E Ink; meter; Infrared Data Association; IrDA; data rate; near-IR; nanometer; nm; IEEE 802.15 Task Group 7: visible light communication; standardization effort for visible light communications; IEEE 802.15.7 Visible Light Communication Task Group; Taiwan; engineer; wavelength-division multiplexing; WDM; laser pointer; red; milliwatt; mW; green; battery; bit-error rate; BER; IEEE 802.11n; wireless network; Wi-Fi; router; Hai-Han Lu; Institute of Electro-Optical Engineering; National Taipei University of Technology (Taipei, Taiwan); New Scientist; signal processing electronics; Wikimedia Commons; Victory at Sea; Monty Python - The Semaphore Version of Wuthering Heights.