- Photodetectors Adapt to Emerging Applications
Dr. Kenneth J. Kaufmann
New materials and configurations will allow photomultipliers to become
more sensitive and rugged or smaller and less expensive.
Technology and economics drive photodetector innovations. Technology determines what can
be developed, while economics governs both what is developed and the rate
of development. However, economics varies from market to market and over time.
Semiconductor and optical communications are two
markets that have been declining over the last year. Nonetheless, these markets
are so large and so much driven by innovation that photodetector technology continues
Until recently, the needs of the military
and law enforcement did not influence detectors very much. With the events of Sept.
11, this has changed, generating a large increase in demand for the detection of
explosives and nuclear material.
The aging population continues to fuel
advances in medical diagnostics, and detector technology has changed to suit the
needs of both existing instruments and of those undergoing development.
Authorization of reimbursement for
positron emission tomography (PET) scans of patients who have a variety of cancers
has doubled sales of PET scanners over the last two years. Because these scanners
use hundreds of photomultiplier tubes, there is a strong motivation to find a way
to reduce their cost. However, because of the high skill level required, workers
in developed countries make these tubes. Therefore, the manufacturers must rely
on technological innovation and not lower labor costs to improve their price point.
A microPET scanner developed at the University of California in Davis
uses 90 photomultiplier tubes. Conventional scanners use many more, encouraging
photomultiplier tube suppliers to produce more inexpensive devices. Courtesy of
Simon Cherry, University of California.
Although the innovations are not public
knowledge, they probably involve the use of electron trajectory simulations to find
dynode configurations that are easier to assemble and cheaper to manufacture. At
Hamamatsu, we are also developing a multielement flat panel photomultiplier that
can be used in a high-throughput PET scanner.
Drug discovery has also influenced
the development of optical detection devices. Animal PET scanners provide an excellent
method to noninvasively test a drug candidate on laboratory animals such as mice.
A scanner optimized for such applications requires an array of detectors placed
close together to provide resolution sufficient to locate the site of activity of
Manufacturers are developing compact
multielement photomultipliers for small-animal scanners, and arrays of avalanche
photodiodes will compete hard for this segment. Because the total detection area
of the PET scanner is small, the higher cost-per-unit area of an avalanche photodiode
detector is not a big concern. The solid-state detector’s lack of sensitivity
to magnetic fields makes it easier to combine animal PET scanning with magnetic
resonance imaging (MRI). PET registers the location of the drug, while MRI measures
For drug discovery in cells and medical
diagnostics, manufacturers are trying to increase throughput with devices that simultaneously
measure more components. This requires the use of more dyes and, in turn, requires
extending the range of the dyes into the blue and red. It also has generated efforts
to improve sensitivity at 300 and 800 nm. Look for new photocathodes based on GaN
and extended red multialkali. Multi-element detectors will also increase throughput
or the amount of information from a single measurement.
Other biomedical applications such
as gene chip scanners and flow cytometers have identical requirements for extended
spectral coverage and multiple elements. These instruments will see accelerated
sales as research proves their capability to detect and analyze the presence of
biological and chemical agents.
Telecommunications will continue to
grow. Most people believe that detector sales will be the greatest for metropolitan
networks and, eventually for applications in the local loop. Thus, emphasis will
be on lower-cost, higher-volume devices. High-gain devices such as avalanche photodiodes
with solid-state amplifiers will soon be available for this market. Initially, they
will be hybrid devices, but monolithic detectors will follow to help lower costs.
For short distances, it is more economical
to transmit data at a low bit rate over multiple fibers because it incurs less cost
for electronics. We can expect to see multielement detector arrays. In silicon,
they will be in the form of optoelectronic integrated circuits in which processing
electronics and the detector will occupy one chip. For InGaAs, a hybrid array will
be available, but suppliers will soon be placing the detector array and the electronics
on one device.
Finally, as photolithography implements
157-nm F2 lasers, we will see photodiodes able to withstand long-term exposure to
this wavelength without damage. One such approach is to use a phototube with a diamond
The next year will yield evolutionary
changes in photodetectors, some driven by the need to reduce costs, others by performance
Meet the author
Kenneth J. Kaufmann is in marketing at Hamamatsu
Corp. in Bridgewater, N.J.
- optical communications
- The transmission and reception of information by optical devices and sensors.
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