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Chapter: Fiber optics and Laser instruments : Industrial Application of Fiber Optical Sensor

Measurement using fiber optics sensor

1. Measurement of pressure 2. Measurement of temperature 3. Phase modulated temperature sensor 6. Measurement of current 7. Measurement of voltage 8. Measurement of strain 9. Measurement of liquid level

Measurement using fiber optics sensor:


1. Measurement of pressure:


All the displacement sensors can be used to measure pressure. Here the pressure is first converted in to displacement and the change in intensity is reflected or transmitted light is measured in terms of displacement. The pressure sensor based on reflective concept. Depending upon the value of pressure, the radius of curvature of the diaphragm is changed. Hence, the intensity of the reflected light is changed. The response curve shows that as the pressure increases, output voltage decreases. With increase of pressure, the intensity of reflected light is decreased and hence the output voltage decreases.


2. Measurement of temperature:


The bimetallic strip acts as a sensing element. It consist of steel and brass which are welded together to form a strip. The brass has higher linear expansively compared to steel. The strip is attached to a bifurcated reflective fiber optic probe. The strip is designed to move continuously and its movement is directionally proportional to temperature. The amount of reflected light is converted in to voltage by a photodiode.



The amount of light reflected decreases with increase of temperature. So that output of photodiode decreases with increase of temperature.


3. Phase modulated temperature sensor:


Here , the phase shift produced in the sensing relative to reference fiber is a function of temperature. This is given by the equation,


The arrangement is called mach-zender.


The Semiconductor laser acts as a light source.


A 3 db splitter acts as the beam splitter which sense the light through the sensing and reference fiber.


Another 3 dB coupler acts as a combiner of these two beams.


A series of light and dark fringes are formed when light form two fiber interface on the display screen.


A phase changes of 2φ radians causes a displacement of 1 fringe.


By counting the fringe displacement, The magnitude of temperature is determined.


If is negligible. By placing a photodetector to measure the intensity of the fringes, we can get sensitivity as



By taking the difference between the two output signals from the sensing fiber and reference fiber, Sensitivity of the sensor is doubled.


4. Measurement of current:


The linearly polarized laser light from the negative laser is launched in to fiber. Cladding mode stripper removes cladding modes. The direction of polarization of the light in the fiber rotated by the longitudinal magnetic field around the current carrying conductor.

The returning light from the fiber loop is passed through the Wallaston prism which is used to sense the resulting rotation and it resolves the emerging light in to two orthogonal components I1 and I2 . these components are separately detected by photodiode detectors and the difference and sum of these signals are found out

5. Measurement of voltage


The variation of refractive index with respect to electric field E is written as


In this crystal, when we apply electric field/ voltage along Z axis, the light which is linearly polarized at an angle 45° with respect to X axis under goes a phase shift or phase retardation.

If I0be the incident light intensity , then the intensity of the transmitted light through crystal is I=I0sin2 . Thus , phase produced in the linearly polarized wave is directly proportional to applied electric field/ voltage. The polarizer converts the incident ordinary light in to a linearly polarized light. When there is applied voltage across the pocket cell, phase shift is produced for the transmitted polarized beam. Quarter wave plate produces a phase shift of θ/2. The transmitted light is then analysed through a analyzers. The transmitted light intensity can be written as,

Measurement of strain:


Micro bending losses are produced in the fiber when the top block presses the fiber by the applied external force. The micro bend losses are found to increase in force applied to the top block. The intensity changes produced by the applied force are measured with reference to a direct unmodulated signal from the light source. The comparator compares these two values and gives the value of strain produced.



6. Measurement of liquid level


Liquid level sensor consists of two fibers which are connected at the base of a glass micro prism. When the tip of the prism is immersed in the liquid, there is no output at the detector. When the tip of the prism is just above the liquid level, due to contact with air, there is total internal reflection and output is got in the detector.




Not useful for sensing multi liquid level since it operates in digital mode.

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Fiber optics and Laser instruments : Industrial Application of Fiber Optical Sensor : Measurement using fiber optics sensor |

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