Introduction to photodetectors and applications

Eric J. Lerner

From the lowly photoconductor in a door opener to the most advanced x-ray or far-infrared cell on an astronomical satellite, photodetectors span a vast range of technology.

Photodetectors among the most ubiquitous types of technology in use today. They range from simple devices that automatically open supermarket doors, to receivers on TV and VCR remote controls, to photodiodes in a fiberoptic connection, to the CCD in a video camera, to enormous arrays used by astronomers to detect radiation from the other side of the universe. Photodetectors are present in a huge variety of devices used in commerce, industry, entertainment and research. In fact, the field of photodetector design and use has grown to the point that few practitioners have a complete overview.

For our purposes, photodetectors include any device for registering photons with frequencies above that of radio waves—from far infrared on up to gamma rays. In this article we survey the main types of applications that use photodetectors. Broadly speaking, these applications fall into two general groupings—communications and remote sensing. In communications, the radiation is simply the carrier for an encoded signal, while in the various forms of remote sensing, the radiation is the signal, conveying information about an object or scene.


FIGURE 1: GaInAs PIN detectors are photodiodes commonly used in fiberoptics communications. (Photo courtesy of Courtesy www.CAOGroup.com)
Click here to enlarge image

Perhaps the most ubiquitous communications market is fiberoptic communications systems, in which photodetectors, generally operating in the infrared (IR), pick up high-speed signals. These detectors do not need high sensitivity, since the laser drivers provide plenty of radiation to the fiber, but they must have extremely fast response, exhibit high reliability, and have a low cost. Photodiodes, especially those based on indium gallium arsenide (InGaAs) are the workhorses of optical communications (see Fig. 1), currently achieving data communication rates as high as 2.5 Gbits/s, which is more than 200,000 times the capacity of a single copper telephone wire.

While the detectors used in fiberoptic communications are invisible to the telephone user, those used in common remote control devices are obvious and are found in nearly every American home. Again, photodiodes sensitive to IR wavelengths are the standard, but requirements are far laxer than for fiberoptic devices, since data transmission rates are low, and only a small amount of information is generally conveyed—simple commands to change channels, or switch a VCR or TV on or off. Even remote controls, however, are increasing in sophistication—cordless mouse devices for PCs, for example, use detectors that measure the relative strength of signals to determine mouse orientation and position.

Safety and security
The simplest types of remote sensing applications involve just the detection of the presence or absence of an object or a condition for safety and security monitoring. Such applications typically use photoconductors, the cheapest and most rugged of detector technologies. The most common of such applications is IR-sensitive motion detectors for home security systems. More recently developed examples include such automotive uses as collision detectors, to monitor objects in a driver's "blind spot," and passenger detectors that determine when to activate air bags. In factories, safety detectors sensitive to visible or ultraviolet (UV) wavelengths perform such tasks as electrical arc detection, automatically cutting off the current where arcing occurs.

Process control
The next step up in sophistication comes in process control applications, another large volume consumer application of photodetectors. In many cases, these devices may be as simple as position sensors to check that a work piece is in the proper place or to provide feedback for robotic systems (see Fig. 2). But other systems need accurate comparison of radiation intensity at different wavelengths, generally requiring accurately repeatable detectors such as photodiodes or phototransistors. In recycling plants, for example, the natural fluorescence of certain plastics can be used with optically filtered detectors to sort different materials, while other spectroscopic tests can differentiate different types of glass.


FIGURE 2: Germanium position sensors consist of a single-element photodiode with quadrupole-electrode geometry. These devices provide linear X-Y beam position information for lasers and other infrared beams. (Photo courtesy of Judson Technologies)
Click here to enlarge image

Comparison of radiation intensity at two or more IR wavelengths is used in pyrometry to remotely measure and control high-temperature processes. Similar techniques in the UV are used to monitor and control flames. In some of these applications, in which radiation is intense, ruggedized bolometers—which measure radiation by the heat transferred, rather than by quantum detection of individual photons—are frequently preferred. At the end of the production process, detectors are used as part of nondestructive testing units, using thermal scans to check for physical defects or for surface composition and finish. Increasingly, imaging systems, especially CCDs, are used in machine vision systems that are incorporated into more flexible robots, not only in factories but also, to a limited extent, in mobile devices used in service institutions, such as hospitals.

Process control applications in factories often require detector systems that can endure extreme environmental conditions. In the food industry, for example, sanitizing caustic solutions are sprayed out at high temperature and at pressures up to 1200 psi, necessitating extremely durable photodetector packaging. In metalworking plants, detectors may require Teflon housing to facilitate removal of molten metal splatter from spot welding. Such environments also put a premium on detector materials able to withstand wide ranges in temperature conditions to reduce insulation needs.

A third high-volume detector application is in all forms of video cameras, from the large cameras used by broadcast and cable TV studios to a palm-sized camcorder. While this is a relatively mature field, refinements in light sensitivity, response time, and resolution are continuing in the basic CCD technology that dominates the video camera business (see Fig. 3). The latest development in the field involves the extension of mass-production video camera technology to IR night-vision applications that were previously limited mostly to defense uses.

The cutting edge
The most advanced detectors tend to be made for low-volume specialized fields, such as laboratory research, biomedicine, defense, environmental monitoring, and astronomy. Thermography, measuring the heat radiated by the human body, has long been a routine diagnostic test, requiring IR detectors. Recent developments have allowed research biologists to observe rapid biochemical reactions with ultrahigh-speed CCDs. Cameras used for tracing chemical diffusion in individual cells, for example, typically have read-out rates of several megahertz and frame rates of up to 10,000 per second. In spectroscopic applications, the high sensitivity of photomultiplier tubes (PMTs) makes them the workhorse of laboratory detectors.

Undoubtedly the biggest drivers of high-technology detectors, especially in the IR, have been defense applications. For decades, the armed forces have used IR detectors on heat seeking missiles, thus setting off a long-term arms race, as decoys such as magnesium flares confused these detectors, prompting the development of more sophisticated devices to distinguish the decoys from real targets. This has led to the development of tunable IR detectors that can pick out jet exhaust spectra, satellite-borne staring array detectors to isolate warm targets against a warm background, and continuous improvements in IR video cameras for night vision. Infrared scanning from aircraft reveals the location of land mines, which show up as regions of warmer soil. The ultimately futile missile defense program, the Strategic Defense Initiative (SDI), while it never arrived at a practical way to shoot down ICBMs, did lead to the development of sensitive far-IR detectors for wavelengths longer than 5 µm, which subsequently found use in astronomy, among other places.

With the scaling back of SDI and the end of the Cold War, many military technologies are moving out into civilian applications. Work is well advanced on adapting military night vision IR detector systems to enhance driver vision beyond the reach of headlights. Workhorses of low-light detection such as avalanche photodiodes first emerged from Pentagon-funded research.

Environmental sensing
Environmental monitoring today uses a broad range of photodetectors from the UV to the IR. Typically, signals are low intensity, so the primary detectors are PMTs and avalanche photodiodes (solid-state photomultipliers). Pollution detection generally relies on UV spectroscopy, with detectors measuring the strength of absorption lines for such pollutants as sulfur dioxide, nitrous oxides, and ozone. Fluorescence spectroscopy allows detection of extremely small amounts of pollutants such as benzene, toluene, xylene and sulfur oxides, which can be measured at a parts-per-billion level. Another sensitive technique is chemiluminescence, in which a reactant is introduced that selectively combines with a pollutant, producing a faint luminescence in the process. Large active areas and low noise levels are required for detectors in such applications.

More straightforward techniques are used for monitoring solid particulates in air and water. Here, the amount of light scattered by the particles is a good continuous measure of pollution levels. Ample light is often available, so photodiodes can be used, taking advantage of their robustness and fast response times.

For monitoring of pollution over large areas, lidar scattering techniques are increasingly used. In this method, a laser sends out light pulses at a repetition rate of about 50 Hz; these pulses scatter off pollutants in the air. Sensitive detectors, generally PMTs, record the amount of light scattered and the time of arrival of the scattered light. In this way, the distance to the pollutant can be measured. As the lidar scans across a region, the distribution of a given pollutant can be mapped.

Space-based environmental monitoring, both for tracking pollutants and for weather and climate analysis make greater demands on detectors, both for sensitivity and for compactness. Satellite-mounted IR spectrometers can measure ozone concentrations from space, sometimes using pyroelectric detectors to convert heat into electric impulses. Space-based IR detectors for temperature monitoring are generally mercury-cadmium-telluride IR arrays that were first developed for defense purposes.

Astronomy
Without doubt the most challenging applications for photodetectors come in the field of astronomy, where the range of wavelengths studied extends from the far-IR at hundreds of microns to cosmic ray photons with 1020 eV of energy and wavelengths of 10-20 µm, a range of 22 orders of magnitude. At the long wavelength end, the European Space agency is making plans for FIRST, the far-infrared and submillimeter space telescope, which will open up the one spectral band not yet surveyed—from 60 to 670 µm. The design will use arrays of germanium gallium photoconductors for 100 to 200 µm and arrays of bolometers cooled to 0.1 K for the rest of the band. At near-IR wavelengths, focal plane arrays of indium antinomide, mercury cadmium telluride, and platinum silicide, all initially developed for the military, are becoming routing accessories for both ground- and space-based telescopes. Miniaturized IR and CCD cameras on board the spacecraft Clementine enabled it to survey the moon at wavelengths ranging from the UV to 9.5 µm in the far-IR.


FIGURE 3: Ultraminiature, high-resolution CCD color board video camera measures 2.2 cm and can be used in video surveillance. (Photo courtesy of Courtesy www.rock2000.com)
Click here to enlarge image

At UV wavelengths, necessarily viewed from space because the atmosphere is mostly opaque to UV, advances in microchannel-plate PMTs have led to major improvements in sensitivity, evident in such instruments as those used in the ASTRO platform on board the space shuttles. At still higher energies, x-ray telescopes such as ROSAT, launched by the European Space Agency, are opening new windows into the most energetic processes in the universe. X-ray detectors generally are based on approaches different from those used for longer wavelength radiation. In gas-filled detectors, electrons are knocked out by x-rays, and electron multiplication occurs in the gas as electrons avalanche toward a high-voltage wire. However, techniques borrowed from other parts of the spectrum, such as microchannel plates for electron multiplication, can be used by x-ray detectors as well. For soft x-rays, below 10 keV, specially designed CCDs are finding greater application. Higher energy detectors often employ calorimeters, which measure the total heat liberated when x-rays are absorbed. And research is currently developing fundamentally new types of detectors based on superconducting tunnel junctions.

Finally, at the highest energies, huge arrays of optical detectors can indirectly respond to ultrahigh-energy cosmic rays entering the top of Earth's atmosphere. The Fly's Eye detector at the University of Utah (Salt Lake City, UT) has a thousand sensors spread over a quarter square kilometer watching for the synchronized flashes of light that signal the cascade produced by cosmic rays colliding with atmospheric atoms. Possibly even more exotic are the arrays of visible light photodetectors immersed in millions of gallons of ocean water, intended to detect the extremely rare interactions of ultrapenetrating neutrinos with hydrogen nuclei in water (see Laser Focus World, October 2000, p. 77).

ERIC J. LERNER is a physicist, freelance book author, and contributing editor to Laser Focus World; e-mail: elerner@igc.org.

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