Biomedical Engineering Reference
In-Depth Information
4
Fiber-Optic
Sensors
4.1 Introduction
Fiber-optic sensing is a relatively new area of technology in which a number
of systems are currently in development. The sensing mechanisms in a fiber
sensor can include one or all of the following modulation techniques: inten-
sity, phase, polarization, or frequency. The high sensitivity of fiber sensors
makes them attractive in many applications where electronic sensors are
inadequate due to limitations in sensitivity, range, and resolution. The inter-
ferometric class of fiber sensors offers the highest sensitivity and dynamic
range. The parameters to be sensed cause a change in the phase of the inter-
ferometer, eliminating the need for a separate modulator [1].
The general advantages of fiber sensors include the following: fiber com-
munication system compatibility, large bandwidth/high data rate, high sen-
sitivity and dynamic range, EMI immunity, and geometric versatility [2].
A wide range of physical parameters can be detected with the fiber sensor,
as evidenced by the following representative fiber sensor types [3]: hydro-
phone (pressure), magnetometer (magnetic field), gyroscope (rotation rate),
position sensor (displacement), pressure sensor, vibration sensor (accelera-
tion), flow sensor, liquid-level sensor, pollution monitor, temperature sensor,
and pH sensor.
4.2 AmplitudeModulationSensors
Fiber sensors are often separated into two broad categories: amplitude mod-
ulation (or intensity sensor) and phase modulation sensor. The intensity
sensor simply monitors the interruption of light propagation through a (nor-
mally multimode) optical fiber due to the perturbing field to be measured. A
transduction device or material transfers the influence of the measured field
into a disruption or interruption of the optical fiber path, modulating the
intensity of light transmitted to a detector. Signal processing at the detector
may be used to measure frequency information related to the perturbing
field or to separate out the effect of interest. This type of sensor is illustrated
in block diagram format in Figure 4.1.
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