It is based on the principle of interference between the beams emerging out from the reference fiber and the fiber kept in the measuring environment.
It consists of a Laser source to emit light. A beam splitter, made of glass plate is inclined at an angle 450 with respect to the direction of the laser beam.
Two fibers viz,
i. Reference fiber which is isolated from the environment
ii. Test fiber kept in the environment to be sensed, are placed as shown in the figure. Separate lens systems are provided to split and to collect the beam.
1. A monochromatic source of light is emitted from the laser source.
2. The beam splitter kept at an angle 450 inclination divides the beam emerging from the laser source into two beams (i) main beam and (ii) splitted beam, exactly at right angles to each other.
3. The main beam passes through the lens L1 and is focused onto the reference fiber which is isolated from the environment to be sensed.
4. The beam after passing through the reference fiber then falls on the Lens L2.
5. The splitted beam passes through the Lens L3 and is focused onto the test fiber kept in the environment to be sensed.
6. The splitted beam after passing through the test fiber is made to fall on lens L2.
7. The two beams after passing through the fibers, produces a path difference due to change in parameters such as pressure, temperature etc in the environment.
8. Therefore a path difference is produced between two beams causing the interference pattern as shown in the figure.
9. Thus the change in pressure or temperature can be accurately measured with the help of the interference pattern obtained.