FPA - ACRONYM

FPA - focal-plane array, an array of light detectors placed in the focal plane of a lens or optical system to record images, such as the sensor chip that takes the place of film on a camera. Often refers to infrared detector arrays, but also can be applied to arrays which respond to other parts of the spectrum, particularly visible and radio bands.

The concept of a focal-plane array is a natural one in the age of digital photography, but a Google Book search finds 10 references dating from the 1960s, mostly referring to infrared detectors for military or astronomical applications. It may have originated in the military; one reference was to a 1963 government document on the defense budget. Extending the search to the start of 1980 found another 450 publications that used the term, too many to examine in detail.

FPA wasn't a common term even then. It's not listed in the index of my 1978 edition of The Infrared Handbook. The only sub-entries under "array" are "silicon diode vidicon," "staring CCDE signal processing," and "in scanners." But those entries do show how imaging technology was evolving toward the FPA concept.

The vidicon was an imaging tube used in analog television, which recorded images as charges on a photoconductor and read them out with an electron beam. By 1978, arrays of silicon photodiodes were available, and had begun to be used for electronic imaging, leading to odd hybrid terms to describe how silicon diode arrays had come to replace television-like cameras in the near infrared.

Scanning and staring refer to different ways of recording images with a limited number of detectors. Today pixels are cheap, so it's natural to put a chip containing a million or more light-detecting elements into the focal plane, where it stares at the scene being imaged. In the 1970s detectors were much more expensive, especially in the infrared, so electronic images often were recorded by scanning a linear array of detectors across the image plane (or by scanning the image across the detector array).

Both technologies live on today. Scanning arrays are standard in flatbed scanners, which drag a linear sensing array across a page. Staring arrays have gone much farther. They are standard in cameras for applications including high-resolution scientific imaging in the near infrared. A gigapixel FPA with response in three bands stretching from 250 to 1100 nm will be launched next year on the European Space Agency's Gaia satellite.

FPA comprised of Geiger-mode avalanche photodiodes was packaged in a ceramic case with microlenslet array, readout circuit, and thermoelectric cooler by Princeton Lightwave (Cranbury, NJ).

LWIR - ACRONYM

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.

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)