Biophotonics_Concepts_to_Applications

For a 0.020-mm = 20-μm gap

Lgap¼10 log

25

25 þ20 tan 14

 2

¼ 1 :580 dB

For a 0.025-mm = 25-μm gap

Lgap¼10 log

25

25 þ25 tan 14

 2

¼ 1 :934 dB

Thus the loss variation is 0.354 dB.

Instead of examining transmitted light levels, a sensor can be configured to
measure intensity changes in reflected light, as is shown in Fig.7.16. In this case,
light emerges from a sensing opticalfiber and a mirror or a diaphragm located a
distancedfrom the end of thefiber reflects the light [ 3 ]. Then a percentage of the
reflected light is captured by the sensingfiber core and is transmitted to a pho-
todetector that senses the returning optical power level. For example, if the pressure
on the mirror or diaphragm increases or decreases, the distancedwill decrease or
increase correspondingly. Consequently the measured level of the captured
reflected light will increase or decrease, respectively. Again, the optical power
variation at the photodetector is a measure of the change in an environmental
parameter.
The same analysis as is given in Eq. (7.3) is applicable to the case shown in
Fig.7.16. However, now the parameter s is replaced by 2d, because the light makes
a round trip of 2d through the gap.

Reflected

light Pressure-induced

movement

Fixed fiber

Cladding

Core Reflecting mirror

or diaphragm

Separation d

Captured area of

reflected light

Incident

light

Fig. 7.16 Simple opticalfiber-based pressure sensor based on using a reflecting mirror or
diaphragm (J. Biomed. Opt. 19(8), 080902 (Aug 28, 2014). doi:10.1117/1.JBO.19.8.080902)

7.4 Optical Sensors 213