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Chapter: Physics - Photonics and fibre Optics

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Displacement Fiber Optic Sensor (Extrinsic Sensor): Principle, Description and Working

Light is sent through a transmitting fiber and is made to fall on a moving target. The reflected light from the target is sensed by a detector. With respect to intensity of light reflected from its displacement of the target is measured.

DISPLACEMENT SENSOR (EXTRINSIC SENSOR)

 

Principle:

 

Light is sent through a transmitting fiber and is made to fall on a moving target. The reflected light from the target is sensed by a detector. With respect to intensity of light reflected from its displacement of the target is measured.

 

Description:

 

It consists of a bundle of transmitting fibers coupled to the laser source and a bundle of receiving fibers coupled to the detector as shown in the figure.


The axis of the transmitting fiber and the receiving fiber with respect to the moving target can be adjusted to increase the sensitivity of the sensor.

 

Working:

 

Light from the source is transmitted through the transmitting fiber and is made to fall on the moving target. The light reflected from the target is made to pas through the receiving fiber and the same is detected by the detector.

 

Based on the intensity of the light received, the displacement of the target can be measured, (i.e.) if the received intensity is more than we can say that the target is moving towards the sensor and if the intensity is less, we can say that the target is moving away from the sensor.

 

MEDICAL ENDOSCOPE

 

Optical fibers are very much useful in medical field. Using low quality, large diameter and short length silica fibers we can design a fiber optic endoscope or fibroscope.

 

A medical endoscope is a tubular optical instrument, used to inspect or view the internal parts of human body which are not visible to the naked eye. The photograph of the internal parts can also be taken using this endoscope.

 

Construction

 

Figure shows the structure of endoscope. It has two fibers viz., 1. Outer fiber(f0)

2.  The inner fiber (fi).


Outer fiber:

 

The outer fiber consists of many fibers bundled together without any particular order of arrangement and is called incoherent bundle. These fiber bundles as a whole are enclosed in a thin sleeve for protection. The outer fiber is used to illuminate or focus the light onto the inner parts of the body.

 

 

Inner fiber:

 

The inner fiber also consists of a bundle of fibers, but in perfect order. Therefore this arrangement is called coherent bundle. This fiber is used to collect the reflected light from the object. A tiny lens is fixed to one end of the bundle in order to effectively focus the light, reflected from the object. For a wider field of view and better image quality, a telescope system is added in the internal part of the telescope.

 

 

Working:

 

Light from the source is passed through the outer fiber (f0). The light is illuminated on the internal part of the body. The reflected light from the object is brought to focus using the telescope to the inner fiber (fi).

Here each fiber picks up a part of the picture from the body. Hence the picture will be collected bit by bit and is transmitted in an order by the array of fibers.

 

As a result, the whole picture is reproduced at the other end of the receiving fiber as shown in the figure. The output is properly amplified and can be viewed through the eye piece at the receiving end.

 

The cross sectional view is as shown in the figure.

 

In figure, we can see that along with input and output fibers, we have two more channels namely, (i) Instrumental Channel (C1) and (ii) Irrigation channel (C2) used for the following purposes.

Instrumentation channel (C1):

It is used to insert or take the surgical instruments needed for operation.

 

Irrigation channel (C2):

 

It is used to blow air or this is used to clear the blood in the operation region, so that the affected parts of the body can be clearly viewed.

 


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