1. § 4 Optical Fiber Sensors
Sensor classification
Intensity modulation sensors
Phase modulation sensors
Frequency modulation sensors
Polarization modulation sensors
Wavelength distribution sensors
Mode number sensors

2. Intensity modulation sensors
MModulation mechanisms   
Position dependent coupling between two fibers
Fig. 4.1: Simple sensor based on the position-dependent coupling between two fires which move relative to each other (a) lateral (b) longitudinal.

3. Absorption based sensor
Fig. 4.2

4. Example (a C2H2 sensor)

5. Without sample gas

6. With 9% of acetylene (C2H2)

7. Measured C2H2 absorption

8. Evanescent field sensor
Fig. 4.3(b): Photograph of a section of D fiber.
Fig. 4.3(a)

9. Evanescent field

10. Detection of intensity modulation:
The intensity variation can be converted into an electric signal (current or voltage) by a light detector (e.g., PIN photo-detector).

11. Phase modulation sensors
Modulation mechanisms
The total phase ( = L = 2neff L /) of the light path along an optical fiber depends on three properties of the fiber guide:
Its total physical length L
The refractive index and the index profile (affect neff)
The geometrical transverse dimensions of the guide (affect neff)
The total physical length of an optical fiber may be modulated by:
Application of a longitudinal strain
Thermal expansion
Application of a hydrostatic pressure causing expansion via Poisson’s ratio

12. The refractive index varies with:
Temperature
Pressure and longitudinal strain via the photoelastic effect
The guide dimensions varies with
Radial strain in a pressure field
Longitudinal strain through Poisson’s ratio
Thermal expansion

13. The light intensity at the photo-detector is given by:
Detection of Phase Modulation – Optical Interferometers
1. Mach-Zehnder Interferometer
Fig. 4.4: An all-fibre Mach Zehnder interferometer using homodyne detection incorporating the required quadrature bias via modulation of the feedback voltage to the PZT modulator
The light intensity at the photo-detector is given by:
(4-1)

14. Limitation : I Operating point not stable
-   
90 bias point (operation point)
Fig. 4.5
Limitation :
Operating point not stable
Not suitable for static measurement

15. All fiber Mach Zehnder has approved to be very useful for high accuracy dynamic phase measurement. Assuming there is dynamic phase modulation (t) = Xsint, the output of the interferometer may be written as
(4-2)
If  can be kept to /2, for small phase modulation, i.e., X<<1, the AC part of I(t) may be written as
(4-3)
IAC is directly proportional to Xsint and can thus be used to measure dynamic phase modulation.

16. 2. Interferometric fiber optic gyroscope-Sagnac Interferometer
Fig. 4.6: The optical fibre Sagnac interferometer (a) Interferometer configuration,

17. Fig. 4.6: The optical fibre Sagnac interferometer (b) the principles of the Sagnac effect

18. (4-5)
where L is the length and D is the diameter of the fiber coil,  is the wavelength of light and c is the light velocity in vacuum.
(4-6)
Low sensitivity
Not sensitive to rotation direction
A 90-degree phase bias is introduced between the two counter-propagating waves, the interferometer output then becomes:
(4-7)
Maximum sensitivity at  = 0, and can also tell the direction of rotation.

19. Fig. 4. 7(a): Example of the intermediate grade I_FOG products
Fig. 4.7(a): Example of the intermediate grade I_FOG products. This I-FOG employs the configuration with the I-FOG chip and the single-mode fiber coil, and the closed-loop operation. Bias drift: 0.5deg/h max, scale factor stability: 0.05%, max. range:  200 deg/h. (product of Japan Aviation Electronics Industries Ltd.: by courtesy of Dr. K. Sakuma of JAE).

20. Frequency modulation sensors
Doppler effect
If radiation at a frequency f is incident on a body moving at velocity v as viewed by an observer, then the radiation reflected from the moving body appears to have a frequency f1 where:
(4-8)
In an optical system, Doppler shifts provide a very sensitive detector of target motion. For instance, with a He-Ne laser as the light source, the frequency shift is 1.6MHz per meter per second. A laser Doppler probe should be capable of detecting target velocities in the range from microns per second to perhaps metres per second, depending on the choice of the detection electronics.

21. Example: a fiber optic Doppler anemometer
Fig. 4.8: Schematic diagram of a fibre optic Doppler anemometer.
(4-9)

22. The light beam at the photo-detector reflected at face A is:
(4-10)
The interference of the two beams at the photo-detector gives:
(4-11)
This signal oscillate at a frequency equals to the Doppler shift f=f1-f=fv/c and can there be used as a measure of the particle velocity v.

23. Wavelength distribution (colour) sensors
Fig. 4.9: The principal feature of a colour modulation sensor.

24. The basic principles of the most common spectrometer components.

25. The basic principles of the most common spectrometer components(Con’t).

26. Example: an optical fiber pH probe
Fig. 4.11: An optical fibre pH probe.

27. Example: an optical fiber Bragg grating (FBG) sensor
(4-12)
where  is the grating pitch and n is the fibre refractive index.
(4-13)
Fig. 4.12: Basic Bragg grating-sensing mechanism.

28. Detection of Bragg Wavelength
Fig. 4.13: Basic filter approach to grating wavelength shift detection.

29. Fig. 4.14: Scanning filter approach for grating wavelength shift detection.

30. Question:
Consider the FBG system shown in Fig N=4. The Bragg wavelength the four gratings are 1545nm, 1550nm, 1555nm and 1560nm respectively. When the control voltage of the tunable filter is changed from 0 to 5V, the center wavelength of the transmission band of the tunable filter varies from 1540nm to 1570nm. Sketch the output waveform from the photo-detector when the tunable filter is controlled by an tri-angular wave within its voltage varies from 0 to 5V.

31. Polarization modulation sensors
Faraday effect
Fig. 4.15
(4-14)
where V is the Verdet constant, and L is the length of optical path in the material.

32. Example:
An optical fiber is placed within a long sole-roid of n turns perimeter with current I. If a linearly polarized is launched from the input end of the fiber, the output light will still be a linearly polarized light with its direction rotated by an angle  relative to the input light direction. What are the relation between  and I (solution VnIL)
When N-turns of optical fibers are wound around a current I, the angle of rotation may be written as:
(4-15)
i.e. the angle of polarization rotation is proportional to current I.

33. A fiber optic electric current sensor
Fig. 4.16: An optical fibre electrical current probe using Faraday rotation as the modulation process.
Assume the input light is x-polarized and an analyzer is position along y-axis direction, the light intensity after the analyzer is
(4-16)

34. taking the ratio of I1-I2 to I1+I2 gives
If a Wollaston prism, instead of analyzer, is placed at 45 degree of x-direction, the light intensity of the two linearly polarized lights after the prism may be expressed as:
(4-17)
(4-18)
taking the ratio of I1-I2 to I1+I2 gives
(4-19)
This is much more sensitive than Eq. (6.16)

35. The sensitivity of Faraday rotation current sensor may be calculated from the Verdet constant of silica (3.3x10-4 degrees/ampere turn) and by assuming that a polarimeter with a resolution of 0.1 degrees is available. The resolution is then 300 ampere turns, so that a 10-turn coil of fiber will resolve 30 amperes. The dynamic range is determined by the detection technique. The simple ratio intensity polarimeter is linear to 1% to a total rotation of 7 degree, giving a maximum current of 2.1kA and a resolution of slightly worse than 1% over this range.