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Encyclopedia of Physical Science and Technology EN005F-954 June 15, 2001 20:48

808 Fiber-Optic Chemical Sensors

junctions. When photons strike the p junction, electrons
move into the conducting band and act as charge carri-
ers. This process generates a current that is proportional
to the incident light intensity. A series of photodiodes can
be arranged in a linear array and used to detect the entire
spectrum simultaneously. These devices offer advantages
such as small size, wide spectral range, and low power
requirement; however, photodiode sensitivity is relatively
low. Photomultiplier tubes (PMT) are more sensitive than
photodiodes and are used for low-light-level detection.
Photons that strike the photocathode cause the ejection
of photoelectrons that are attracted to the photomulti-
plier electrodes (also called dynodes). The dynodes are
arranged such that each dynode is held at a higher potential
than the previous one. When a high-energy photoelectron
strikes the first dynode, it causes the ejection of several
photoelectrons from the dynode surface. These cascade
processescontinueasmorephotoelectronsstrikeeachsub-
sequentdynode,resultinginahighamplificationoftheini-
tial photon. In addition to their high sensitivity, PMTs have
high signal-to-noise ratios and respond very fast. PMT
detectors, however, are temperature sensitive and can be
irreversibly damaged if exposed to high light levels.
FIGURE 5 Design principle of fiber-optic chemical sensors. (a)
In some fiber-optic chemical sensors, the optical sig-
Two single fibers. (b) Two fiber bundles. (c) Bifurcated fiber.
nal measurement involves image acquisition and anal-
ysis. Charge-coupled device (CCD) chips are typically
used to capture the output image. The CCD is a two-
wavelength operating range. In addition, the light output dimensional integrated-circuit chip array made up of sev-
should be stable over long time periods since light fluc- eral layers of silicon and an array of gate electrodes. These
tuations may add noise to the measurement and reduce electrodes, often referred as metal-oxide semiconductors
the sensor sensitivity. Tungsten–halogen lamps are often (MOS), are individually connected to an external voltage
used as a light source to provide high-intensity light and a source. When photons strike the chip’s light-sensitive sil-
continuous emission spectrum over wavelengths ranging icon layer, they create electrical charges that are detected
from 300 to 2700 nm. This lamp is very useful because of by the gate electrodes. This detection process is called
its small size and high stability, and the potential to use charge coupling and involves the transfer of voltage be-
it in portable devices. Light-emitting diodes (LEDs) are tween the two gate electrodes. Since each pair of gate
also used as light sources for fiber-optic chemical sensors electrodes can detect the localized photon strikes, intensity
because they offer spectral consistency and small size; and spatial position of the image are simultaneously mea-
however, they are limited in their light intensity. Lasers sured. CCD chips offer additional advantages including
are an important light source for fiber-optic chemical sen- their small size (a few square millimeters to a few square
sors since they supply very intense monochromatic light. centimeters),highsensitivity,andstability.Thelimitations
Lasers are used for fluorescence-based fiber-optic chemi- of CCD chips are the requirement for low-temperature op-
cal sensors in which fluorophores with low quantum yields eration and their relatively high cost.
are used.

2. Optical Signal Detectors C. Optical Phenomena Employed for Sensing
in Fiber-Optic Chemical Sensors
In most fiber-optic chemical sensor systems, the light
transmitted from the sensing element (output light) is mea- Chemical sensing via optical fibers is performed by mon-
sured by using photon detection devices, which absorb itoring the changes in selected optical properties. Inter-
photons and convert them into electrical signals. Several actions of light with the analytes are measured either di-
photon detectors are available. The simplest and cheap- rectly or by using indicators. The light is launched from a
est detector is the silicon photodiode. In this solid-state light source into the fiber and guided to the measurement
device, photon detection is based on p–n semiconductor site. The absorbed, reflected, scattered, or emitted light