If the sensing application involves
high temperatures or a very tight, almost inaccessible location, fiber
optic sensors could provide the answer. These sensors provide small,
lightweight units that can be quite rugged and reliable as well as very
sensitive with a passive (non-electrical) sensing element.
According to Jeffrey Muhs at Oak Ridge National Laboratory,
fiber optic sensors take advantage of the characteristics of a
propagating light wave such as physical intensity, phase, frequency
(color), polarization state time of flight and modal cross talk that
are altered by an externally induced physical or chemical parameter.
Fiber optic sensors are classified as intrinsic or extrinsic. In
Intrinsic sensors, the fiber acts as the sensing medium and the light
never leaves the fiber but is altered by an external phenomenon. In an
extrinsic design, the fiber acts as light delivery and collection
system - light leaves the fiber and is collected by the same or another
fiber.
Because of their ability to withstand high temperatures and
pressures, fiber optic sensors are a natural for down hole oil and gas
measurements. For example, Opsens Solutions OPP-W,
a Fabry-Perot interferometer based, fiber optic pressure and
temperature sensor has an optical sensing element made of
monocrystalline sapphire. Encased in inconel-718 housing, the sensor
can withstand a maximum temperature of 300C and measure pressures up to
5000 kPa (750 psi) with a pressure accuracy of ±0.2 percent full scale.
The inconel-718 housing of Opsens Solutions fiber optic sensor protects the all-sapphire sensing element.
For industrial measurements, Keyence FS-V30 MEGA POWER
photoelectric sensor is based on fiber optic technology. The sensor
boasts a beam strength that is 64 times greater than conventional
models with response speed of only 33 µs. Fiber-optic photoelectric
sensors from Omron and Pepperl+Fuchs also target industrial application and allow detection in space-restricted and harsh environments.
Keyence MegaFS-V30 MEGA POWER fiber optic photoelectric sensor has a digital output.
For the most demanding industrial and R&D applications, Chiral Photonics HelicaT fiber-optic,
ultra-high temperature sensors operate up to 1000C. The design uses a
patented chiral grating fabricated by twisting, or microforming the
fiber as it passes through a miniature heat zone. This produces a
distinct dip in the transmission spectrum, where the spectral position
of the dip changes with temperature.
Chiral Photonics monolithic glass solution does not rely on
photo-induced gratings or coatings or moving parts, providing
reliability inherent to glass fiber.
Other Special Applications
Optrand's PSIglow
is pressure measuring glow plug that uses fiber optic technology. A
1.7mm-diameter fiber optic based pressure sensor mounted in a housing
with a ceramic heater-based glow plug monitors cylinder pressure in
diesel engines without head modification or loss of glow plug
functionality. While the glow plug reaches a maximum temperature of
1250C in less than 3 seconds, the fiber optic sensor is limited to a
maximum temperature of 350C.
The sensing diaphragm that reflects light in proportion to pressure
is exposed to combustion gasses through a radial orifice in the ceramic
heater and a short connecting passage filled with an Inconel wire mesh.
Operating in pressures up to 300 bar (4350 psi), the sensor has a ±1.5
percent full scale output total pressure reading accuracy.
The small size of fiber optic pressure sensor makes them ideal for medical applications such as in vivo measurement of blood
pressure or intracranial pressure monitoring. In addition, their
ability to function without producing noise in radio frequency,
microwave, magnetic resonance imaging (MRI) and electro-surgery environments, makes them good candidates for temperature, strain and other pressure measurements.
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