Thursday, March 29, 2012


WDM: Wavelength-division multiplexing, transmission of separate signals simultaneously at separate wavelengths through the same transmission medium, usually an optical fiber.

Multiplexing combines two or more signals for simultaneous transmission through the same medium. It was first used to increase capacity of 19th century telegraph wires. Later, frequency-division multiplexing divided the radio spectrum into separate broadcast channels. Each station was assigned a fixed transmission frequency, and listeners tuned the frequency of their receiver to select a station. Frequency-division multiplexing let cable television networks pack many video channels into frequency slots for transmission through copper cable.

Optics specialists think of wavelength rather than frequency, so when Bell Labs tried sending multiple laser wavelengths through the same hollow light pipe in the 1960s, they called it wavelength-division multiplexing. The appeal was understandable, and the Bell System tried WDM again in 1980 when it designed its first high-capacity fiber-optic system along the Northeast Corridor from Boston to Washington. But that system had a fatal flaw; it used multimode fibers, so every seven kilometers it needed a repeater to separate the wavelengths, detect the signals separately, amplify each one electronically, and drive separate transmitters that had to be multiplexed together. Single-mode fiber won hands down.

Wideband erbium-doped fiber amplifiers (EDFAs) revived interest in WDM because they could amplify many separate laser signals near 1550 nanometers. For long-haul, high-speed transmission, optical channels were packed close together for dense-WDM or DWDM systems. However, a committee from the International Telecommunications Union, apparently dominated by radio engineers, specified DWDM channels in frequency units. Typically 50 gigahertz wide, those channels initially transmitted 2.5 or 10 gigabits per second, and now can carry up to 100 GHz using coherent transmission.

Coarse-WDM (CWDM) came later, allowing the use of lower-cost multiplexing and demultiplexing optics in lower-speed, shorter-distance WDM systems, such as dividing 10-Gigabit Ethernet traffic among four lower-speed CWDM channels. Optical engineers apparently won on the ITU committee handling CWDM, because they specified CWDM channels in wavelength, setting center wavelengths every 20 nm from 1270 to 1610 nm.

CWDM and DWDM grids. The wavelengths for the DWDM grid are approximate; the specification are defined in terahertz.

Friday, March 23, 2012


CPA - Chirped Pulse Amplifier, an optical amplifier that generates very high peak powers in very short pulses by stretching the pulse duration before amplification, then compressing the pulse after amplification. Chirped pulse amplification can produce peak powers in the terawatt range from small systems, and is vital in building petawatt lasers.

Nonlinear effects inherently limit the amount of optical amplification possible in a gain medium. Effects such as Brillouin scattering reduce gain, and effects such as self-focusing can cause optical damage. These effects are proportional to the peak power in the medium, putting an upper limit on the gain possible.

Chirped pulse amplification circumvents this limit by spreading the energy in the pulse over a longer period of time, thus reducing the peak power throughout the longer pulse. This is done by sending the input pulse through a medium with a high wavelength dispersion, such as a pair of gratings or prisms, or a length of dispersive optical fiber. The pulse that emerges from the dispersive medium is chromatically dispersed, with the short wavelengths at one end and the long wavelengths at the other. The degree of dispersion depends both on the medium and the spectral width of the pulse. In practice, chirped pulse amplification works best with pulses lasting tens to hundreds of femtoseconds, which are inherently broadband.

The longer dispersed pulse is amplified in a broadband gain medium, then passed through a medium with dispersion of the opposite sign, so the wavelengths that passed first through the amplifier are delayed and those that passed through the amplifier later in the pulse can catch up. The pre-amplification and post-amplification dispersion do not have to cancel each other out, although the minimum pulse duration still depends on the spectral bandwidth.

Chirped pulse amplification also can be used in optical parametric amplifiers, which have broader bandwidth than laser oscillators and thus can be chirped more strongly to generate higher peak powers. Optical parametric chirped-pulse amplification (OPCPA) will be used in Europe's Extreme Light Infrastructure.

How a CPA works. A pair of gratings that delay the blue end of the spectrum stretches input pulses about a factor of 1000 in duration. Those pulses then pass through a broadband amplifier, and the higher-power output is compressed by a second pair of gratings that delay red wavelengths to produce a high-energy ultrashort pulse.

Friday, March 16, 2012


LIDAR or LADAR: LIght Detection And Ranging or LAser Detection And Ranging, sometimes called "laser radar," the optical counterpart of radar, which measures the distance to an object by timing how long a pulse of light takes to make a round trip between the transmitter and the object.

As acronyms go, LIDAR and LADAR are a rarity--near-identical twins with essentially the same meaning. They were coined to describe the same concept, using pulses of laser light instead of radio waves to measure distance. Radar itself is an acronym for RAdio Detection And Ranging, coined by the U.S. Navy in 1941, so it was logical to replace the radio part of the acronym with an optical term. However, some people replaced the radio with light to make LIDAR and others replaced it with laser to make LADAR. Both terms are still used--although Google searches put LIDAR far in the lead, with 19.8 million hits compared to a mere 503,000 for LADAR.

The earliest and simplest lidars were laser rangefinders, which used laser pulses to measure the distance to a military target or some other fixed object. Lidars also can measure speed by firing a series of pulses and calculating how fast the measured distance changes, an approach used in police laser radars because it's simpler than Doppler measurements.

More advanced lidars scan the beam across a target area to measure the distance to points across its field of view, producing a three-dimensional profile. This technique has a wide range of uses. Lidars looking down from aircraft or satellites have profiled terrestrial terrain, and the laser altimeter on the Mars Global Surveyor spacecraft similarly profiled the surface of Mars. Combining lidar profiles of terrain before and after an earthquake can reveal changes caused by the tremor. Lidars can map archeological dig sites or dinosaur trackways too large or too fragile to record in any other way.

Specialized lidars make other measurements. Differential absorption lidar can profile the abundance of water vapor in the atmosphere. Doppler lidars measure changes in the spectrum of pulses scattered by the air to determine wind speed and turbulence.

Tuesday, March 6, 2012


Yttrium aluminum garnet (YAG) is a synthetic crystal doped with neodymium or other rare earths for use in bulk solid-state lasers. Although neodymium lasers were first demonstrated in a calcium tungstate host, YAG has long been the most common host for solid-state lasers. YAG also has been used in jewelry, and may be doped with other rare earths for use in lasers or phosphors.

YAG can be a puzzling acronym to decode if you think of crystals as chemical compounds. The Y stands for yttrium and the A for aluminum, but G is for garnet, which is a class of minerals with a particular cubic crystalline structure, not an element. In fact, the chemical formula of YAG, Y3Al5O12, does not fit the usual definition of garnet. Dictionaries define a garnet as a silicate mineral consisting of three SiO4 groups plus three atoms of a divalent metal (A) and two atoms of a trivalent metal (B), with a chemical formula A3B2(SiO4)3. Yet YAG contains no silicate groups, and no divalent atoms. Both yttrium and aluminum are trivalent, but they combine with a dozen oxygen atoms to form a unit cell containing the same number of atoms as a unit cell of a standard garnet, producing a crystal with a garnet-like structure.

The optical and mechanical properties that make YAG attractive for laser use include high thermal conductivity, high energy storage, and long fluorescence lifetime. For use in Nd:YAG lasers, YAG is doped with a molar concentration of roughly 1% neodymium atoms, which replace yttrium atoms in the crystal. Other rare earths including ytterbium, erbium, holmium, and thulium also can be doped into YAG to make lasers, and additional dopants may be added to aid energy transfer. An important emerging application for cerium-doped YAG is as a yellow-emitting phosphor used with blue LEDs to produce white light.

Traditionally, YAG laser rods have been fabricated from crystalline boules, limiting their size. New processes can produce ceramic YAG in much larger sizes, for use as laser slabs, disks, or rods. Ceramic slab lasers have reached 100-kilowatt powers in experimental military lasers.

FIGURE. 10-centimeter-square slab of ceramic Nd:YAG glows in 808-nm pump light during tests of the Lawrence Livermore National Laboratory's Heat Capacity Laser.

Sunday, March 4, 2012


A fiber Bragg grating (FBG) is a fiber-optic device that strongly reflects a narrow band of wavelengths and transmits other light, like a thin-film mirror. A Bragg grating consists of thin layers of two dielectric materials, one with high refractive index and the other with a lower index, with each layer a quarter-wave thick at the wavelength to be reflected. Reflection from layer junctions a half-wavelength apart produces constructive interference at the selected wavelength. The more layers, the more reflective the structure becomes at the selected wavelength, and the narrower the reflected band. Other wavelengths are transmitted.

Multilayer thin-film mirrors fabricated on bulk optics are Bragg gratings, but their reflective behavior depends on the angle of incidence as well as the layer thicknesses. In a fiber Bragg grating, the layers are normal (perpendicular) to light propagating along the fiber axis, fixing the angle of incidence and limiting reflection to a single narrow band. Fiber gratings are fabricated by illuminating special fibers made of light-sensitive glass with light from an ultraviolet laser, which passes through a light-scattering mask to form a series of light and dark interference fringes along the length of the fiber. The ultraviolet light breaks chemical bonds illuminated by the light fringes, changing the refractive index of the glass to create the grating within the fiber. A new process allows fabrication of fiber Bragg gratings continuously on freshly drawn fiber.

The simplest fiber Bragg gratings have uniformly spaced layers along their length so they have high reflectivity in a narrow band, making them valuable as cavity mirrors in fiber lasers. Narrow-band fiber gratings also can be used in telecommunications systems, where they select one wavelength from a signal containing several optical channels and transmit the others, as shown in the figure. Another communications application is wavelength-selective time delays, with the grating chirped in spacing so different wavelengths are reflected at different points along the grating. Fiber Bragg gratings also can be used in sensing applications such as oil-well monitoring, where the peak reflected wavelength changes with temperature and strain.