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Fiber Optic Temperature Sensor structure, working, advantages, disadvantages

This page describes structure, working operation, advantages and disadvantages of Fiber Optic Temperature Sensor. It mentions Fiber Optic Temperature Sensor advantages or benefits and Fiber Optic Temperature Sensor disadvantages or drawbacks.

Introduction:
There are various methods of temperature measurement which include following.
• Classic mercury glass thermometer
• Infrared Pyrometer
• Electronic thermometer (made of thermocouple, thermistor, RTD etc.)

Above temperature measurement systems have very little immunity from nearby EM interference and stray radiation. This leads to inccurate measurements. Fiber optic temperature sensors offer excellent performance compare to above mentioned temperature measurement techniques due to its many benefits outlined below. Hence such sensors are employed in various space applications and hazardous environments like high voltage machines (e.g. generators, motors, transformers), nuclear power plants, chemical power plants etc.

Fiber Optic Temperature Sensor structure | Fiber Optic Temperature Sensor working operation

The fiber optic temperature sensors are mainly classified into two types viz. Interferometric and non-interferometric. The figure-1 depicts simple non-interferometric and non-luminescent type fiber optic temperature sensor.

Fiber Optic Temperature Sensor
Figure-1: Non-Interferometric fiber optic temperature sensor

The simple non-interferometric type sensor consists of multi-mode optical fiber and temperature sensitive material. The temperature sensing materials include GaAs, CdTe, Si etc. GaAs is preferred over others due to its better wavelength variation with temperature. These materials exhibit changes in their optical parameters such as absorption, transmission and reflection qualities with variation in the temperature.

The working principle depends on phenomenon of energy bandgap shrinkage with increase in the temperature of such semiconductor materials. The figure-1 depicts schematic of such temperature sensor and its temperature vs wavelength curve.

In this sensor type, thin semiconductor ship is used as active element. This active element is sandwiched between light source (e.g. LED or laser) and photodetector. In this sensor, constant intensity of light signal us modulated by external temperature when it travels through the optical fiber cable. Moreover its wavelength shifts towards higher side due to gradual increase in the temperature. This is due to optical absorption edge.

Interferometric temperature sensor
Figure-2: Interferometric fiber optic temperature sensor (Mach-Zehnder type)

The figure-2 depicts Mach-Zehnder Interferometric temperature sensor. This sensor offers flexible geometry and higher sensitivities. Hence it can be used for measurement of various measuranads such as temperature, pressure, rotation, strain etc. It works based on phase modulation by external measurands. Here phase of the beam through sensing fiber is compared with the reference beam. Beam splitter is used in the design of such Mach-Zehnder Interferometric sensor as shown in the figure. Beam splitter divides the light beam into two parts, one is launched into the sensing part and the other is used as reference.

Benefits or advantages of Fiber Optic Temperature Sensor

Following are the benefits or advantages of Fiber Optic Temperature Sensor:
➨It is immune from nearby EM (electromagnetic) and stray radiation.
➨It can be used in environments where high levels of electrical interference exists or where intrinsic safety is a concern.
➨It (i.e. non-interferometric type) offers greater accuracy (+/- 1o) and faster response time (~2 sec).
➨It is light in weight and compact in size.
➨It is cheaper due to low manufacturing cost.
➨It supports wide temperature range of measurement from -10 oC to 300 oC. The GaAs offers better wavelength variation with temperature.

Drawbacks or disadvantages of Fiber Optic Temperature Sensor

Following are the drawbacks or disadvantages of Fiber Optic Temperature Sensor:
➨The different types of optical temperature sensors will have different temperature range of operation and their accuracy of measurements also vary. These depend on their construction and materials used.
➨It is complex to develop measurement systems using fiber optic sensors.
➨The users require training before they start using such sensor types.
➨Some fiber optic temperature sensors are expensive.

The other fiber optic temperature sensors include Fabry-Perot Interferometric Temperature sensor, fluorescent temperature sensor, optical scattering temperature sensor etc. Also refer advantages and disadvantages of temperature sensor >> for more information.



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