Thursday, May 31, 2012


SWIR - Short-Wavelength (or Short-Wave) Infrared, wavelengths at or near the short-wave end of the infrared.

You might think the infrared band stretching from the red end of the visible spectrum to three micrometers (µm) is used widely enough that the it would have a well-accepted definition. Dream on. Everybody has their own take, and they can't even agree whether that part of the spectrum should contain one or two bands.

Wikipedia's "Infrared" entry lists several definitions of SWIR, without trying to resolve the conflicts or explain the differences. It first cites a "commonly used subdivision scheme" that defines SWIR as wavelengths from 1.4 to 3 µm, where water absorption is high, with a separate near-infrared (NIR) band at 0.75 to 1.4 µm where water absorption is low. It's not a bad definition, but why is it referenced to a book titled Unexploded Ordnance Detection and Mitigation?  The International Commission on Illumination makes a similar split at 1.4 µm because shorter wavelengths (the IR-A band) can reach the retina, but longer wavelengths (the IR-B band) are absorbed within the eye. Both divisions make sense from the standpoint of illumination.

Other divisions are based on sensor response. Wikipedia cites a split between NIR extending to the long-wave limit of silicon detectors near 1.0 µm, and SWIR from 1.0 to 3.0 µm, given in Miller and Friedman's Photonic Rules of Thumb. A Raytheon wall chart similarly defines NIR as 0.7 to 1 µm, and SWIR as 1 to 2.7 µm. However, makers of other detectors pick other dividing points. An article by Nova Sensors (Solvang, CA) defines SWIR as 0.9 to 1.7 µm, the response range of the InGaAs sensors used in their SWIR cameras. However, sometimes the two ranges overlap. Headwall Photonics (Fitchburg, MA) lists its NIR hyperspectral imaging sensor as responding to 0.9 to 1.7 µm and its SWIR version as responding to 0.95 to 2.5 µm.

Some don't bother splitting the band at all. The International Organization for Standardization (ISO) 20473 standard calls 0.78 to 3 µm NIR, and the McGraw-Hill Dictionary of Scientific and Technical Terms calls NIR 0.75 to 3 µm. Neither lists SWIR.

I could go in agonizingly nit-picking detail, but the lesson is clear -- SWIR (and NIR) can be useful labels for parts of the infrared, but you need to check the numbers to be sure what they mean.

Tuesday, May 22, 2012


VCSEL - Vertical Cavity Surface-Emitting Laser, a type of semiconductor laser in which the resonant cavity is perpendicular to the junction layer and light is emitted from the surface of the chip.

All early diode lasers oscillated in the plane of the junction or active layer and emitted from the edge of the semiconductor chip. This design is logical because keeps laser oscillation in the plane of the active layer where recombination of current carriers produces a population inversion, so the round-trip gain in the cavity is high. However, because the active layer is very thin, the beams from edge-emitting diode lasers diverge rapidly, particularly in the direction perpendicular to the active layer.

VCSELs oscillate vertically, in a cavity formed by reflective layers on the top and bottom of the chip. Single-pass gain much lower than in edge emitters because only a very thin layer of gain material is between the cavity mirrors but the emitting aperture typically is much wider than the active layer is thick, producing a higher-quality, circular beam. First demonstrated in 1979, VCSELs went through a series of structural refinements to improve their performance and fabrication processes. In current designs, one or often both of the reflectors are multilayer Bragg reflectors containing many the tens of pairs of layers needed to produce the very high reflectivity needed to sustain oscillation with only a thin gain medium.

The short length of VCSEL cavities brings some advantages, including allowing direct current modulation at speeds to 40 gigabits per second (Gbit/s) and without the mode hopping possible in edge emitters. Yet ironically, a commercial attraction of the more complex VCSEL design is that it makes them more economical. All the hard parts of VCSEL production are done by highly automated semiconductor manufacturing techniques. The resulting VCSELs can be tested on the wafer, unlike edge emitters, which can't be tested until the wafer is diced into chips. That combined with their larger emitting area greatly reduces packaging expenses, which account for more of finished product costs than the laser chips. So more complex winds up being cheaper, as well as better for many applications.

And VCSEL types and applications keep growing. Recently, a VCSEL-type cavity was used in optically pumped colloidal quantum dot lasers emitting red, green and blue light.

Complexity in a VCSEL:  Beam Express (Lausanne, Switzerland) makes 1310 nm VCSELs by bonding AlGaAs/GaAs distributed Bragg reflectors on top and bottom of an InAlGaAs/InP gain layer containing strained quantum wells and a tunnel junction because high-contrast Bragg reflectors are not practical in InP-based materials.

Tuesday, May 15, 2012


EUV or XUV:  Extreme Ultraviolet, the short-wavelength or high-energy end of the ultraviolet spectrum, from 120 (or 200) nanometers to about 10 nanometers (nm).

Atmospheric transmission in the ultraviolet decreases sharply with wavelength, and air absorption is so strong that wavelengths shorter than 150 to 200 nm must be studied in a vacuum. The first explorers of the EUV spectrum were astronomers using satellite instruments. The quest to squeeze more and smaller transistors onto semiconductor chips has changed that by shrinking chip features to dimensions on the scale of EUV waves. The semiconductor industry is now testing the first wave of EUV photolithography systems operating at 13.5 nm.

Such a sudden technological interest in a long-neglected part of the spectrum is a great recipe for muddied definitions of spectral bands. Astronomers consider the 121 nm Lyman alpha line of hydrogen to be the major landmark in the EUV spectrum, so they picked 120 nm as the long-wave end of the EUV band. However, the major technological landmark for the semiconductor industry is the 13.5 nm lithography wavelength, so they often define EUV as having a wavelength of 13.5 nm. The laser community uses a broader definition of 10 to 120 or 200 nm as it explores a broader range of applications, enabled by new techniques such as high-harmonic generation.

The EUV largely overlaps the older vacuum-ultraviolet (VUV) band, usually defined as 10 to 200 nm. A draft document from the International Standardization Organization (developed for space observations) also defines two other overlapping bands, the far-ultraviolet (FUV) at 122 to 200 nm and the germicidal Ultraviolet C (UVC) band at 100 to 280 nm.

XUV often is an alternative abbreviation for extreme ultraviolet, substituting the fashionable X for the relatively drab E, but the ISO lists it as an abbreviation for soft X rays at 0.1 to 10 nm. A quick Google search gives the impression XUV is the more popular form, with 5.6 million hits compared to 3.2 million for EUV and 1.8 million for VUV. But that's misleading because XUV also is shorthand for "crossover utility vehicle," a sport-utility vehicle based on a car rather than a truck. That usage may be popular on the Internet, but it was new to me--and for years I've been driving a Toyota RAV4, which is classed as an XUV.

Monday, May 7, 2012


FIR - Far InfraRed, the long-wavelength end of the infrared spectrum. As is typical of infrared bands, definitions vary.

When Arthur Schawlow and Charles Townes proposed extending the maser principle to frequencies well above the 22 gigahertz (GHz) of the ammonia microwave maser reached in 1954, they targeted frequencies three orders of magnitude higher, near 300 terahertz (THz), corresponding to one micrometer (1 µm) in the near-infrared.  Their choice reflected the technological reality of the time, the long wavelength end of the infrared was a terra incognito of the electromagnetic spectrum, little explored because few sources and detectors were available. One reason that wavelengths longer than about 15 µm came to be called "far-infrared" in the early days of lasers probably was that that part of the spectrum seemed far out of reach.

Technology has come a long way since then, but the far-infrared remains beyond the well-developed parts of the infrared; although, as with other parts of the infrared, the specified wavelengths differ among definitions.

Wikipedia's Infrared article lists multiple definitions of the far-infrared. The first is 15 to 1000 µm, putting it thermal infrared where blackbody emission peaks around room temperature, longer than the mid-infrared and some definitions of the long-wavelength IR. It also cites the International Standardization Organization's definition of 50 to 1000 µm, a definition also used in the McGraw-Hill Dictionary of Scientific and Technical Terms. The infrared article notes that astronomers have their own definition, with 25 to 40 µm the short end and 200 to 350 µm the long end.

Oddly, the bands specified for "far-infrared lasers" differ. Wikipedia says their wavelengths range from 30 to 1000 µm, close to the 40 to 1000 µm I used in The Laser Guidebook. The McGraw-Hill Dictionary lists a far more limited range, from "well above 100 µm" to 500 µm.

A couple decades ago, such inconsistent definitions didn't matter much, because wavelengths longer than 30 µm were a sparsely inhabited part of the spectrum, largely absorbed by air, hard to detect, and even harder to use. Now new technology is upscaling the unfashionable far-infrared neighborhood and redefining it as the terahertz band, which Wikipedia defines as from 100 to 1000 µm, or 300 GHz to 3 THz.

Plot of atmospheric opacity shows the strength of atmospheric absorption in the far-infrared. (Wikipedia art, modified)

Wednesday, May 2, 2012


LWIR - Long-Wavelength InfraRed: an infrared band at wavelengths longer than the mid-infrared. One common definition is from 8 to 15 micrometers, also known as the thermal infrared, but there is no generally accepted standard.

The infrared spectrum sprawls from the edge of the visible, nominally 0.7 µm, to about one millimeter, such a broad range that it demands subdivision. The definitions of mid- and long-wavelength bands may have grown from the atmospheric transmission windows at 3-5 µm, and 8-14 µm. Those bands generally require different detectors, and also were a handy division between the blackbody peaks of "hot" objects and those of "body-temperature" objects. The strongly absorbed wavelengths in between didn't matter much as long as the infrared was mostly used for looking through the air. Longer wavelengths were lumped as the "far-infrared," a vast region extending to about one millimeter that seemed of little use because atmospheric transmission was spotty and instrumentation was poor.

Atmospheric transmission has not changed, but new infrared detectors and sources have opened up previously little-used regions of the infrared, and satellites have opened the whole infrared spectrum to astronomers. New applications have emerged, such as LWIR monitoring of beehives. That has made drawing dividing lines problematic, particularly on the ends of the LWIR. Should the ends be defined by the atmospheric windows or at some other points? One suggestion was to define each band as an octave wide, spanning a factor of two in wavelength or frequency, but that logical idea failed a crucial practical test because it could not fit both the 3-5 µm and 8-14 µm bands in adjacent octaves. So we're stuck with informal definitions that depend on things like atmospheric windows, and detector ranges, and differ between fields like lasers, astronomy, and night vision.

It could be worse. Geologists built their time scale for the Earth's history on the boundaries between solid rocks, then found that their calendar changed every time a better way was found to date the rocks.