Fiber Optic Sensor Basics
(PDF)
This guide is intended to help the new user understand the
basics of DavidsonSensors™
fiber optic sensing. See
Configuring a Fiber Optic Sensing System
for more advanced information to help with the planning of a
fiber optic sensing system.
For more detailed technical information about
fiber optic sensing systems, see
Davidson Technical Publications.
1. Optical Fiber
Fiber optic sensors use
optical fiber and light to make useful measurements of
temperature, pressure, and other physical parameters. Optical
fibers are strands of glass that transmit light over long
distances and are the equivalent of wire in electronic systems.
The glass optical fibers are 0.005 inch in diameter and are
protected from damage by acrylate, polyimide, plastic, or metal
coatings. Once properly protected, the delicate glass fibers
operate maintenance free for years.
2. Sensing Theory
Davidson fiber optic sensing
systems are based on white light interferometric sensing
technology using extrinsic Fabry-Pérot sensors. The same
Fabry-Pérot sensing technique is used regardless of whether the
parameter being measured is temperature, pressure, vacuum, flow,
level, density, vibration, or acceleration.
The sensor consists of two
partially reflective and parallel surfaces that are placed in
close proximity to one another and coupled to an optical fiber
that transmits the light. One of the reflective surfaces is
held in a fixed position while the other reflective surface
moves (transduces) in response to an environmental stimulus such
as temperature or pressure. The light reflected from the sensor
experiences a phase modulation that is caused by the
interference of the reflections at these two surfaces. The
characteristics of the modulation are defined by the
reflectivity of the two surfaces in the sensor, the wavelength
of light transmitted to the sensor, and by the length of the
sensor gap. Two scientists, Charles Fabry and Alfred Pérot,
first documented this phase modulation in 1899 and the equation
that defines the modulation is called the Fabry-Pérot equation.
3. Practical Transducers
In practical pressure
measurement applications, the second reflector is attached to a
metal diaphragm and process pressure deflects the metal
diaphragm. At full scale pressure, for example 3000 psi, the
diaphragm deflects approximately 0.0005 inch or 12,700
nanometers (for reference consider that a dollar bill is 100,000
nm thick). Since the signal conditioner can resolve changes of
less than a nanometer, the system is able to make precise
measurements of the pressure. Temperature sensors are made by
using two materials with slightly different coefficients of
thermal expansion. As the temperature changes, the difference
in the expansion rates of the two materials causes the gap to
increase or decrease. As in the pressure sensor, the full range
of displacement is only 0.0005 inch. A temperature sensor may be
inserted into the pressure transducer and used to make
corrections of the pressure output based on the actual
temperature of the transducer. In this manner, the accuracy of
the pressure measurement is improved.
4. Cabling and Connector Considerations
The fiber optic cabling and connectors used for
telecommunications systems may not be appropriate for fiber
optic sensing systems. Unlike telecommunication systems which
use a very narrow band of light and share many signals on a
single fiber, Davidson's discrete fiber optic sensing systems
require a dedicated fiber for each sensor. The characteristics
of the optical transmission fibers in the cable must match the
characteristics of the optical fiber used in the transducer and
signal conditioner or severe degradation will occur. The
standards of acceptable quality of the connectors and
terminations for fiber optic sensing systems exceed the
standards for telecommunication systems.
5. Signal Conditioning
A fiber optic signal
conditioner is the equivalent of a transmitter in conventional
electronic sensing systems. During operation, the signal
conditioner sends a pulse of light in sequence to each of the
interferometric transducers. The light signal received
from each transducer is focused through a lens and transmitted
through a Fizeau interferometer (optical wedge) and onto a CCD
array. The Fizeau interferometer acts as an optical cross-correlator
and instantly converts the modulated light into a
cross-correlated signal that is projected onto a linear CCD with
thousands of pixels. The effect of the cross-correlation is
that the peak of the signal occurs at that location on the CCD
where the optical length of the interferometric gap in the
sensor matches precisely with the optical length of the
interferometric gap in the Fizeau interferometer. Each pixel in
the CCD is calibrated to a precise optical thickness in the
Fizeau interferometer.
The CCD converts the light
signal into an electronic signal that is processed by a
microprocessor. The microprocessor in the signal conditioner
converts the peak signal into a known length of gap. Through
rigorous calibration done at the factory, the calibration
constants are known for each transducer and loaded into the
microprocessor. The microprocessor converts the known gap into
the proper engineering units, (i.e. psig, inches of water, °F),
for the transducer.
Since all of Davidson fiber optic sensors are based on the same
interferometric sensing technology, different measurands can be
multiplexed and processed by a single signal conditioner.
Multiplexing a variety of sensors with a single signal
conditioner allows multivariate signal processing, error
correction, etc. The result is unprecedented measurement
accuracy in harsh industrial environments.
Once the measurement is corrected and converted into the
appropriate engineering units, the signal conditioner then
transmits the measured result to the process control system
based on the specified analog or digital protocol, i.e. 4-20mA
analog or RS-485 Modbus digital protocol. Ideally, a multichannel
signal conditioner is located in a control room environment and
interrogates multiple transducers. The signal conditioner can
be packaged in a 19" rackmount, NEMA enclosure,
explosion-proof container, laboratory bench top unit, or other
custom packaging.
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