MIR - ACRONYM

MWIR or MIR - Mid-Wave-InfraRed or Mid-InfraRed, a range of wavelengths nominally in the middle part of the infrared spectrum, defined in a variety ways.

The terms hiding behind acronyms sometimes can be more confusing than the acronyms themselves. The diverse definitions for "mid-infrared" illustrate the problem. The name clearly implies that MWIR/MIR should be  the middle of the infrared spectrum, but the infrared is a vast region, sprawling from 700 nm at the long end of the visible range to around 1 mm at the upper end of the radio/microwave band. What is the middle?

On a logarithmic scale, it seems simple enough--10 to 100 µm fits right in the middle. But nobody uses that definition. Wikipedia, today's great arbiter of popular culture, defines the MWIR as 3 to 8 µm, but that section of the "infrared" article bears a warning dating from July 2006 that it needs to be cleaned up! The 3–8 µm definition follows the recommendations of the International Commission on Illumination, which considers the MWIR as the short end of the IR-C band from 3 µm to 1 mm.

However, other definitions abound. In its tabulation of optical spectrum bands, ISO, the International Standardization Organization, defines MWIR as 3-50 µm. NASA's Infrared Processing and Analysis Center at Caltech defines the astronomical mid-IR as stretching from 5 µm to 25 or 40 µm. Military system designers traditionally define 3–5 µm as MIR, the window used by heat sensors on missile guidance systems. The McGraw-Hill Dictionary of Scientific and Engineering Terms does not list mid-infrared, but defines "intermediate infrared radiation" as from 2.5 to 50 µm.

Who's right? It depends on your viewpoint. In my recent mid-infrared lasers and applications webcast, I spoke from the laser industry viewpoint set the boundaries as 2 to 12 µm--starting beyond the telecommunications band and extending to include the carbon-dioxide laser band, as shown in the image. But when I wrote about uncooled infrared cameras in the April issue, I wrote from the detector viewpoint, and called the thermal imaging band from 7.5 to 14 µm "long-wave infrared."  In truth, detectors used in the 3–5 µm and 7.5- to 14-µm bands differ much more than the laser used. But on reflection I have to wonder why the spectral bands should differ between the light source and the detector.  

APD - ACRONYM

An avalanche photodiode (APD) is a semiconductor photodetector in which incident light generates a photocurrent, which is then multiplied by an avalanche process to give a stronger signal.

An avalanche photodiode is a single device that incorporates two distinct semiconductor stages. The first is a photodiode detector, in which light with energy above the bandgap of a semiconductor delivers enough energy to valence electrons for them to enter the conduction band. The electron leaves behind a hole in the valence band, which also functions as a current carrier. Application of a voltage across the device pulls the electrons and holes in opposite directions.

The second stage applies a strong reverse bias across the semiconductor to accelerate electrons. When electrons reach high enough velocities, they can ionize other atoms in the semiconductor, producing an avalanche of electrons. This multiples the original photocurrent and produces a much stronger response than a simple photodiode. Typically, silicon APDs are biased with about 100 V to multiply photocurrent by around a factor of 100. Further increasing the reverse voltage increases the dark current as it approaches the ionization threshold of the semiconductor. Breakdown occurs at a reverse bias of about 150 to 200 V for silicon, depending on device design, and at different voltages in other semiconductors.

You can think of APDs as solid-state counterparts of photomultiplier tubes (PMTs), but they are far from plug-in replacements. As you expect from solid-state devices, APDs are much smaller and their bias voltages are much lower--about a tenth of those in PMTs. However, PMTs are less subject to noise, their gain does not depend as strongly on bias voltage, and they can be engineered to respond to different wavelengths than APDs, so they are among the few survivors of the vacuum-tube era.

New designs and new materials are extending the range of APDs for applications ranging from single-element fiber-optic detectors to focal-plane arrays for imaging. Germanium/silicon ADPs have reached a gain-bandwidth product of 105 GHz, attractive for high-speed optical interconnects. APD elements designed for single-photon counting can be assembled in arrays to make a single-photon counting camera.

Gain or multiplication factor of an APD increases sharply as voltage approaches breakdown, but so does dark current, limiting usable gain. (From Jeff Hecht, Understanding Fiber Optics: 5th edition [Pearson Prentice-Hall, 2006])

PMT - ACRONYM

PMT - Photomultiplier Tube, a vacuum tube light sensor in which input photons cause a cathode to emit electrons that are amplified through a chain of electron amplifiers called a multiplier. Invented in 1934, PMTs are still in use. They offer high gain, low noise, reasonably fast response, and a large collecting area, all important for detecting faint signals.

First observed in 1887 by Heinrich Hertz, the light-induced emission of electrons became an important puzzle because classical physics could not explain why electron emission occurred only for wavelengths shorter than a threshold value rather than depending on light intensity. Albert Einstein won the Nobel prize in 1921 for showing that the photoelectric effect was due to photons needing to have a threshold energy to free electrons from atoms.

The first photoemissive detectors were vacuum photodiodes, in which light illuminated a metal cathode, freeing electrons collected by an anode when a voltage was applied across the tube. Alkali metal cathodes were used to detect visible light. In the days before semiconductor electronics, these devices were called photodiodes or photocells. They were too insensitive for use in early electronic television cameras, so engineers added amplification stages, called dynodes, inside the tube; electrons collided with the dynodes, producing additional or secondary electrons. A series of acceleration stages and dynodes multiplied the photocurrent, thus earning the name photomultiplier. Developed in the 1930s, PMTs became the detectors of choice for applications demanding high sensitivity, low noise, and high speed.

This high performance has made the PMT a remarkably durable technology, one of the last vacuum tubes that is still a standard product in the age of solid-state photonics. Continuing refinements in design and packaging have adapted PMTs for modern applications such as single-photon counting. PMTs continue to new challenges, such as silicon photomultipliers, containing a hundred to several thousand tiny avalanche photodiodes (APDs) connected in parallel for single-photon detection, also called multichannel APDs. But PMTs just keep plugging along.

Modern metal-channel dynode PMT, shown in cutaway. (Image courtesy of Hamamatsu)