Photodetectors are one of the most common types of optoelectronic technology in use today. Examples range from the simple devices that automatically open supermarket doors and respond to TV and video-cassette-recorder (VCR) remote controls to photodiodes in fiberoptic connections, charge-coupled devices (CCDs) in video cameras, sensitive cameras used in medical imaging (see Fig. 1), and enormous arrays used by astronomers to detect radiation from the other side of the universe. Photodetectors are present in a variety of devices used in commerce, industry, entertainment, and research. The field of photodetector design and use has grown to the point that few practitioners have a complete overview.
To provide the essential elements of that overview, Laser Focus World begins this month a new series of Back to Basics articles on photodetectors. These articles will offer a guided tour of photodetector technology and applications. For our purposes, we define a photodetector as any device that registers photons with frequencies above those of radio waves and with energies from far-infrared to gamma rays; most will be on the low-frequency end of that range—infrared (IR), visible, and ultraviolet (UV).
We begin with a survey of the main applications of photodetectors. In the coming months, we will look at the basic technology of visible, UV, and IR detectors and then concentrate on some important types of detectors, such as CCDs, photomultiplier tubes (PMTs), and IR arrays, as well as special advanced-technology topics.
Broadly speaking, photodetector applications fall into two general categories—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.
Communications photodetectors
Perhaps the most important communications market is fiberoptic communications systems, in which photodetectors, generally operating in the near-IR region of the spectrum, pick up high-speed signals. Detectors for this application do not need high sensitivity, because laser drivers provide plenty of radiation to the fiber, but they have to have extremely fast response and high reliability as well as low cost. Photodiodes, especially those based on indium gallium arsenide (InGaAs) are the workhorses of optical communications, currently achieving data-communication rates as high as 2.5 Gbit/s—more than 200,000 times the capacity of single copper telephone wire.
Although fiberoptic detectors are invisible to the telephone user, detectors are more obvious in TV remote-control devices found in nearly every American home. Again, photodiodes sensitive to near-IR wavelengths are the standard device, but requirements are less stringent than for fiberoptic communications because data-transmission rates are low and only a small amount of information is generally conveyed—simple commands to change channels or turn off the VCR or TV. Even remote-control devices, however, are becoming increasingly sophisticated; for example, cordless mouse devices for PCs use detectors that measure the relative strength of signals to determine mouse orientation and position.
Remote sensing
The simplest types of remote-sensing applications involve the detection of the presence or absence of an object or a condition for safety or security monitoring. Such applications typically use photoconductors, the cheapest and most rugged of detector types. The most common use is in infrared-sensitive motion detectors for home-security systems. More-recently developed applications include automotive uses such as collision detectors that monitor objects in a driver’s “blind spot” and an infrared passenger detector that determines when to activate an air bag. In factories, safety detectors sensitive to optical or UV radiation perform such tasks as arc detection, automatically cutting off currents where arcing occurs.
The next step up in sophistication comes in process control, another large-volume application of photodetectors. In many cases, these devices are as simple as position sensors that check that workpieces are in the proper place or provide feedback for robotics systems. But other systems need accurate comparison of radiation intensity at different wavelengths, generally requiring accurately repeatable detectors such as photodiodes or phototransistors. For example, in recycling plants, the natural fluorescence of certain plastics can be used with filtered detectors to sort different materials, while other spectroscopic tests can differentiate among types of glass.
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 and to detect hot spots for preventive maintenance. In some of these applications, where 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 instruments. Examples of these are thermal cameras that evaluate materials for physical defects or for surface composition and finish (see Fig. 2). Increasingly, imaging systems, especially CCD cameras, are used in machine-vision systems that are incorporated into more flexible robots, not only in factories, but, to a limited extent, in mobile devices used in service institutions such as hospitals.
Process control in factories often re quires detector systems that can endure extreme environmental conditions. In the food industry, for example, sanitizing caustic solutions are sprayed out at high temperature and pressure up to 1200 psi, necessitating extremely durable photodetector packing. In metalworking plants, detectors may require Teflon housing to allow removal of molten-metal splatter from spot welding. Such environments also put a premium on detector materials able to withstand wide ranges in temperature to reduce insulation needs.
A third high-volume detector application is found in all forms of video cameras, from the large cameras used by broadcast and cable-TV studios to a palm-sized camcorder for the consumer market. While video-camera technology 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. The latest development in the field involves the extension of mass-production video-camera technology to IR wavelengths—these “night-vision” applications were previously limited primarily to the military and defense market.
Cutting-edge detector technologies
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 body—has long been a routine diagnostic test requiring IR detectors. But recent developments have allowed research biologists to observe rapidly moving biochemical reactions with ultrahigh-speed CCD cameras. 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 frames per second. In spectroscopic applications, the high sensitivity of PMTs makes them the workhorse of laboratory detectors.
The biggest drivers of high-technology detectors, especially those sensitive to IR radiation, have been defense applications. For decades, the Armed Forces have used IR detectors on heat-seeking missiles, setting off a long-term arms race because decoys, such as magnesium flares, confused basic detectors. More-sophisticated detectors were later developed to distinguish decoys from real targets. This process has led to the development of tunable IR detectors to distinguish jet-exhaust spectra and satellite-borne staring array detectors to isolate warm targets against a warm background; it also has led to continuous improvements in IR video cameras for night vision. Infrared scanning from aircraft can reveal the location of land mines, because they show up as regions of warmer soil.
While it never arrived at a practical way to shoot down ICBMs, the ultimately futile US missile defense program, the Strategic Defense Initiative (SDI), did lead to the development of sensitive far-IR detectors for wavelengths longer than 5 µm, which subsequently found use in astronomy and spectroscopy, among other applications. With the scaling back of SDI and the end of the Cold War, many technologies developed by the military are finding civilian applications. Work is ongoing on adapting military night-vision IR detector systems to enhance driver vision beyond the reach of headlights. Devices that are now commonly used for low-light detection, such as avalanche photodiodes, first emerged from Pentagon-funded research.
Today, environmental monitoring uses a broad range of photodetectors sensitive from the UV to the IR region. Most signals are typically low intensity, so the primary detectors are PMTs and avalanche photodiodes (which can be thought of as solid-state PMTs).
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 is increasingly used. In this method, a laser sends out light pulses at about 50 Hz that 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, a map of a given pollutant can be built up.
Detectors in space
Space-based environmental monitoring, both for tracking pollutants and for weather and climate analysis, places great 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.
Without doubt the most challenging uses for photodetectors are 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 the far-infrared and submillimeter-wavelength space telescope (FIRST), which will open up the one spectral band not yet surveyed--wavelengths from 100 µm to 1 mm. The design will use arrays of germanium gallium photoconductors for 100-200 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 antimonide, mercury cadmium telluride, and platinum silicide, all initially developed for the military, are becoming routine accessories for both ground- and space-based telescopes. Miniaturized IR and CCD cameras on the spacecraft Clementine enabled it to survey the moon at wavelengths from UV to 9.5 µm in the far-IR region.
For detecting UV wavelengths, de vices must necessarily be space-based because of the opaqueness of the atmosphere. Advances in microchannel-plate photomultipliers have led to major improvements in sensitivity, evident in instruments such 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 up new windows into the most energetic processes in the universe. X-ray detectors generally use different approaches than 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 in x-ray detectors as well.
For soft x-rays, below 10 keV, specially designed CCDs are finding greater application. Higher-energy detectors often use 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 the earth’s atmosphere. At installations such as the University of Utah Fly’s Eye Detector, a thousand sensors spread over 0.25 sq km watch for the synchronized flashes of light that signal the cascade produced by cosmic rays colliding with atmospheric atoms (see Fig. 3). Possibly even more exotic are the arrays of visible-light photodetectors emersed in millions of gallons of ocean water, intended to detect the extremely rare interactions of ultrapenetrating neutrinos with hydrogen nuclei in water.
From the lowly photoconductor in a door opener to the most advanced x-ray or far-IR cell on an astronomical satellite, photodetectors span a vast range of technology. In future articles we will survey those technologies in detail.