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