Optical fiber sensors

1. FIBER OPTIC SENSORS
Dr. R. K. Shukla
Department of Physics
University of Lucknow

2. We have five sense organs i.e.
 Eyes
 Nose
 Skin
 Ears
 Tongue
We can tell the smell, temperature, sweet/or chilly
taste, colour of the object, sound or noise by using
above sense organs.

3. We can not exactly know the numerical value of measurement of
above things
For, Exactly knowing the numerical value of any measurement, we
require Sensors.
Meaning of Sensors:
A sensor is a device which can quantitatively measure a physical
property and records, indicates or responds to it, is known as Sensor.
Meaning of Transducer:
A transducer is a device which converts one physical quantity to
another from of physical quantity, e.g., acceleration to voltage or
voltage to pressure.

4. Difference between Sensor and Transducer
The primary function of the sensor is to sense the physical
changes, A sensor is a measurement device that produces an
output signal in proportion to some kind of input.
The accelerometer, barometer, gyroscope are the examples of
the sensors.
A transducer simply converts one form of energy into another
or transducer converts the physical quantities into an
electrical signal or vice versa.
A microphone is a transducer that converts incoming
mechanical energy (sound waves) into an analogous electrical
signal. The thermistor, thermocouple are the examples of
the transducer.

5. Any Sensor has the following Characteristic features
 Low attenuation
 Smaller diameter
 Long distance signal transmission
 Transmission security
 Robust
 Reliable
 Long life
 Easy replacement
 Low cost
 No technical training is required for operations

6. Fiber-optic technology was primarily developed for
telecommunication applications
Due to the advances in fabrication technology, high quality
and competitive price optoelectronic components and
fibers are available, this has contributed to the expansion
of guided wave technology for sensing applications.
A fiber-optic sensor detects changes in the light guided
through an optical fiber when it is affected by external
physical, chemical, biomedical, or any other parameters.

7. The basic fiber-optic system consists of optical source
 Optical source
 Optical fiber
 Modulating element
 Optical detector
 Signal processor
 Readout
The light output of the source is coupled to the optical
fiber, which is then given to the modulating element, where
light is modulated by the measured (e.g. temperature,
pressure, …).
Modulated light is detected by photo-detector and
processed and calibrated to get the direct readout of
measured quantity.

8. The attractive features of fiber-optic sensing are as follows:
 High sensitivity
 Large bandwidth
 operation at very temperature, pressure or voltage
 Multiplexed or distributed measurement
 Long range operation
 Explosion-proof and immune to radio frequency and
electromagnetic interference due to non-electrical
method of operation
 Small size, light weight and great flexibility, allowing
access to restricted areas
 Resistant to chemically aggressive and ionizing
environments
 Easy interface with optical data communication systems
and secure data transmission

9. Block Diagram of Fiber Optic Sensor
The general block diagram of fiber-optic sensor consists of
optical source (Light Emitting Diode, Laser and laser diode),
optical fiber, sensing element, optical detector and end-
processing devices (optical-spectrum analyzer, oscilloscope) is
shown

10. Classification of the Fiber-Optic Sensors
Fiber-optic sensors are classified as follows:
Based on the sensor location or site: The fiber optic sensors are
classified into two types
(a) Intrinsic Fiber-optic Sensors
(b) Extrinsic Fiber-optic Sensors
Based on the operating principles: Based on the optical parameter
modulated by the measurand, sensors can be divided into five basic
categories
(a) Intensity Modulated: The intensity of light launched into the fiber
is changed intrinsically or extrinsically
(b) Phase Modulation: The phase of monochromatic light propagating
through the fiber is changed intrinsically by the measurand.
(c) Polarization Modulated: The direction of the E-field vector with
respect to the direction of the light’s propagation is changed by
measurand.
(d) Wvelength Modulated: The wavelength of light is changed
extrinsically by the measurand.

11. Based on application: The fiber-optic sensors are classified according to
its applications such as
(a) Physical Sensors: Used to measure physical quantities like
temperature, pressure, displacement, liquid level, flow rate etc.
(b) Chemical Sensors: Used for gas analysis, pH measurement etc.
(c) Biomedical Sensors: Used in measurement of blood flow, glucose
content etc.
Based on the basis of response to the measurement point:
(a)Point to Point Sensors
(b) Multiplex Sensors
(c) Distributed Sensors

12. Intrinsic Sensors: Intrinsic sensors are also
known as waveguide sensors in which the
sensing is the waveguide itself and
sensing takes place within the fiber. The light
need not come outside the fiber.
Extrinsic Sensors: Extrinsic sensors make use of
an optical modulator or optical transducer coupled
to the waveguide, the optical parameters of which
are modulated by the measurand.
Sensing takes place in a region outside the fiber
waveguide. The light propagating through input
fiber comes out, reaches the measurand site, where
the information is impressed upon the light in
response to measurand, and then modulated light
comes back to the optical fiber (output).
The information may be impressed in the form of
intensity, frequency, polarization, phase or spectral content.