Biophotonics_Concepts_to_Applications

Example 7.8 Consider a step-indexfibers with a 25-μm core radius and an

acceptance angle of 14° in the reflective sensor shown in Fig.7.16. What is

the variation in the insertion loss when the longitudinal separation between

thefiber end and the reflector changes from 0.020 mm to 0.025 mm?

Solution:The insertion loss due to a gap betweenfiber and the reflector can

be found by using Eq. (7.3) with the parameter 2d substituted for the

parameter s.

For a 0.020-mm = 20-μm gap

Lgap¼10 log

25

25 þ40 tan 14

 2

¼ 2 :916 dB

For a 0.025-mm = 25-μm gap

Lgap¼10 log

25

25 þ50 tan 14

 2

¼ 3 :514 dB

Thus the loss variation is 0.598 dB.

7.4.4 Microbending Fiber Sensors

As a multimodefiber is progressively bent into a tighter radius, more of the optical
power from the higher-order modes gets radiated out of thefiber. This effect can be
used to build a sensor based on measuring optical power level variations due to
fiber bending [ 29 – 31 ]. An embodiment of one such device is illustrated in
Fig.7.17. Such a microbending sensor was one of the earliestfiber optic sensors.
Here an opticalfiber is run between two interleaved corrugated plates. As the

External force

Deforming rods

attached to plates

Movable plate

Fixed plate

Light input to

optical fiber

Optical fiber

To power

meter

Deformer

spacing

Fig. 7.17 Concept for sensing viafiber microbending

214 7 Optical Probes and Biosensors