Archive for 'March 2012'
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.
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.
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, Y3 Al5 O12 , 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 A3 B2 (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.