Thursday, June 21, 2012


POF - Plastic Optical Fiber (also Polymer Optical Fiber): An optical fiber made from a polymer or plastic.

Plastic light guides have a surprisingly long history. Soon after its invention in 1928, the transparent plastic polymethyl methacrylate (PMMA) began replacing quartz in a variety of applications, including bent rods used as dental illuminators. Early developers of fiber-optics tested transparent plastics as well as glass in the 1950s, and plastic was used as a cladding on some of the first clad optical fibers.

For many applications, plastic had important advantages over glass. Plastic is lightweight, inexpensive, and flexible rather than brittle. Thin sheets of clear plastic, like thin sheets of window glass, seem quite transparent. But reducing the attenuation of plastic proved far more difficult than improving the clarity of glass, and plastic was left in the dust when the first low-loss silica fibers were demonstrated in 1970s.

Developers of plastic fibers turned to other light-guiding applications where fiber loss was less important than in telecommunications. By the early 1970s, bundles of plastic fibers were being used in decorative lamps, with the fiber ends splayed out to sparkle with light at their ends. Fiber-optic pioneer Will Hicks strung plastic fibers through a plastic Christmas tree, which he hoped to sell for holiday decoration until it failed an impromptu fire test at a New York trade show. Despite such reverses, plastic fiber-optic decorations live on.

POF communications also survives in short-distance applications where the low cost and ease of termination of plastic fibers offsets their high attenuation. One example is the Media Oriented System Transport (MOST) network for automobiles, which red LED transmitters to transmit signals through up to 10 meters of POF linking electronic systems in cars. Auto mechanics don't need an expensive fusion splicer to connect the large-core step-index fibers. Japanese researchers have developed graded-index POFs with bandwidths high enough to transmit 4.4 gigahertz up to 50 m at 670 to 680 nm, which developers hope could lead to applications in home networking.

Plastic optical fiber does have plenty of competition for the acronym POF, including polymer optical fiber and another optical term, "plane of focus." lists 34 possible definitions ranging from the journal Physics of Fluids and the Pakistan Ordnance Factory to the dating site "Plenty of Fish" and "pontificating old fart".  But plastic fiber fans can take heart -- the site ranks Plastic Optical Fiber ranks as the most-used definition of POF.

Modern decorative fiber-optic lamp, courtesy of Keck Observatory.

Monday, June 11, 2012


QCL: Quantum Cascade Laser, a semiconductor laser lacking a junction in which an electron passes through a series of quantum wells. In each quantum well, the electron emits a photon on an inter-subband transition before tunneling through to the next quantum well. QCLs are important sources in the mid- and far-infrared, including the terahertz band.

Semiconductor and diode lasers were long considered synonymous after demonstration of the first semiconductor diode laser in 1962, although other types had been proposed and lasing had been demonstrated in semiconductors without junctions that were pumped optically or with electron beams.

Russian physicists Rudolf Kazarinov and R. A. Suris took the first step toward the QCL in 1971 by suggesting electrons in a superlattice could tunnel between adjacent quantum wells, but the technology needed to make them was not yet available. The development of molecular beam epitaxy (MBE) revived interest in such complex semiconductor structures, and in 1986 Federico Capasso, Khalid Mohammed, and Alfred Cho of Bell Labs suggested that electrons tunneling through stacks of quantum wells might be used to make infrared lasers. 

Their 1986 paper clearly shows the basic idea, but demonstrating QCLs took eight years, as long as it took to go from the first pulsed cryogenic diode laser to room-temperature operation. Not until 1994 did Jerome Faist, Capasso, Cho, and Deborah Sivco report  the first QCL in a Science paper where they coined the evocative phrase "quantum cascade" to describe its operation; a Google Book search fails to find any earlier use of the phrase. Their device produced 8 milliwatt (mW) pulses at 4.2 micrometers (┬Ám), but like the first diode lasers it required cryogenic cooling, with highest power at 10 degrees Kelvin (K), and operation at up to 90 K with a threshold of 14 kiloamperes (kA) per square centimeter (cm2), comparable to the threshold of the first diode lasers.

Today, QCLs are in the mainstream of laser technology, operating continuous-wave at room temperature with multiwatt output in the mid-infrared. Available commercially, QCLs operate through much of the infrared all the way to the terahertz band.

Electron emits a cascade of photons as it tunnels through a series of quantum wells in this simplified view of a QCL.

Monday, June 4, 2012


DWDM: Dense Wavelength Division Multiplexing: Wavelength-division multiplexing with signals closely spaced in frequency.

CWDM: Coarse Wavelength Division Multiplexing: Wavelength-division multiplexing with signals broadly spaced in wavelength.

The division of the radio spectrum into broadcast channels made the idea of wavelength-division multiplexing (WDM) obvious to any serious student of communications by the time the laser was invented. But how to divide the optical spectrum was far from obvious. In the early 1980s AT&T picked three widely spaced channels for the first commercial system linking Boston to Washington, GaAlAs lasers at 825 and 875 nm, and an InGaAsP LED at 1300 nm. But single-mode fiber transmission quickly eclipsed its capacity and WDM was largely abandoned.

Invention of the erbium-doped fiber amplifier (EDFA) in the late 1980s revived interest in WDM because it could amplify multiple signals across a range of wavelengths with little crosstalk. The question quickly became how tightly wavelengths could be packed across the 1550 nm erbium-fiber gain band. That required developing new filter technology to slice the spectrum finely. By 2000, channel spacing was down to about 0.4 nm or 100 GHz.

To make systems compatible, the International Telecommunications Union (ITU) defined a standard dense frequency grid spanning the erbium gain band. Each DWDM was 100 GHz wide, with the standard specifying channels in frequency units, such as 193.10, 193.20, and 193.30 THz, although optical optical engineers translated them into wavelengths (1552.52, 1551.72, and 1550.92 nm, respectively).

DWDM was designed for expensive high-performance long-haul systems, but WDM also could enhance capacity of shorter fiber systems, if costs could be cut by using cheaper optics with less-demanding specifications. That led ITU to develop a "coarse" grid, for which they specified CWDM channels in wavelength units, spaced 20 nm apart from 1271 to 1611 nm, used in metro and access networks

That's the official CWDM grid, but it hasn't stopped designers from multiplexing other combinations of widely-spaced WDM signals, such as cable-television or fiber to the home (FTTH) systems transmitting downstream at 1550 and (sometimes) 1480 nm, and upstream at 1310 nm. 

So these divisions of the spectrum do have standard meanings. 

Comparison of CWDM and DWDM spacing.