光纤传感器的基本知识

Optical Modulator Optical Input (Electrooptic, Acoustooptic, Magnetooptic) Optical Output (Amplitude, Phase Frequency, Polarization)
Electrical Control Signal

4
Photoelastic Material
Polarizer at

4
I0
I0 4 2tC 1 cos x
t
I
2tC 1 cos y
The Transmitted optical intensity yields
The above figure is extrinsic fiber optic sensors model and the following one is the model of Intrinsic fiber optic sensors
2
Optical Fibers(1)

Step-Index Multimode Fiber
I0
2R
I


Retardation Plate with Fast Axis at Angle to the x Axis And Having Retardation
1 I 1 cos2 2 sin 2 2 cos 2
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Multimode Polarizatiosor Construction
Gradient index lens
Mirrors
Multimode fiber
Pressure
Temperature sensor
Spacer
Pressure sensor
Refractive index test liquid Temperature
I Er2 E s2 2 Er E s
r k1k2 s (1 k1 )(1 k2 ) I I0 2 s r k1k2 (1 k1 )(1 k2 ) cos(r s )
Where k1 and k 2 are the power coupling coefficients of the two couplers. s and r are optical loss through two paths.
Light
Wavelength 1 2 3 4 Wavelength w1 w3 w4 Photodiodes
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Wavelength divider w1 w2 w3 w4 Combiner
Light
Intensity-Based and Fabry-Perot Interferometer Sensors(5)
a
I
a sin A1 s sin A2
Where, N is the number of slits m is the diffraction order
s Incoming Light

I 0
Diffraction from multiple slits
t Light Beam Stress or Strain applied along the X Axis
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Multimode Polarization Sensors(3)

Basic photoelastic stress Sensing
Unploarized Monochromatic Light Polarizer at
Index Matching Fluid
Detector
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Optical Modulators

A key component of optical fiber systems, including amplitude, phase, frequency, and polarization modulation. The model
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Multimode Polarization Sensors(1)

Retardation and Special Properties of the Half-Wave plate
Light Polarized in X Direction Polarizer with Transmission Axes along X
I I I0 4
If the two stresses act simultaneously
I I0 2tC 1 cos x y 4
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Sensors Based on the Mach-Zehnder and Michelson Interferometers(1)
L2
L L1 L2
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Sensors Based on the Mach-Zehnder and Michelson Interferometers(2)

Numerical Analysis
Er E0 r k1k2 cos 0t r
E s E0 0 1 k1 1 k2 cos 0 s
s s0 1 T T0
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Multimode Grating Sensors(4)

Based on Relative Movement of Opposed Gratings
Light in
Light out
GRINrod Microlens Opposed Grating Structure
As V number is increased, additional modes are allowed to propagate and high-order modes appears in the fiber
4
Fiber properties as sensors(1)

Bending Directional Coupling and Sensing
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Examples: Bulk Phase modulation

The phase variation
(t )
2
V(t)

Ln(t )
output d Input
n 3r n(t ) E 2 2 n 3 r L (t ) V (t ) 2 d
L
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Intensity-Based and Fabry-Perot Interferometer Sensors(1)
Fiber Optic Sensors

Basic Components of FOS Types of FOS Application of FOS
1
Components of FOS

Optical Fibers Optical Light Sources Optical Detectors Optical Modulators

Mach-Zehnder interferometer nkL
Signal Path Lens Beamsplitter Detector Reference Path coupler Mirror
L1
Sensing Fiber Coil
Detector Detector
Fiber
Reference Fiber Coil
Mirror Laser Beam Splitter Mirrored End Environmental Effect Detector
5
Fiber properties as sensors(2)

Evanescent Coupling and Sensing
Input Light Beam
Fiber Cores
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Multimode Grating Sensors(3)

1.
Optical Encoding Techniques
The total amount of energy passed through a grating is proportional to the ratio a/s.
a ( P) I ( P) I 0 s
2.
Used as wavelength encoding devices. The ratio a/s remains a constant, but s is modulated by some parameter of interest, the angular spectral distribution transmitted will be modulated. Example: A wavelength-encoded temperature sensor.
Air
Liquid
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Intensity-Based and Fabry-Perot Interferometer Sensors(4)

Encoder-Based Position Sensors
Using an encoder plate to provide linear or rotary displacement measurement. Encoder plate
2 1 Numerical Aperture
nco ncl na
2 n2 1 / 2 NA sin n c co cl
3


Optical Fibers(2)

Step-Index Single Mode fiber
V 2an

2
n12 n22 2n12
V is known as the modal volume, and a is the radius of the fiber core. The cutoff condition, where only one mode is allowed to propagate down the fiber, is given by V < 2.405
Displacement Fiber

The sensed parameter (strain, pressure, force, position, acceleration can be mechanically coupled to displacement of a device that deforms the fiber

Intensity Sensors(1)
Light Multimode Fiber Movable Reflector
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Intensity-Based and Fabry-Perot Interferometer Sensors(2)

Intensity sensors(2): Microbend Sensors
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Intensity-Based and Fabry-Perot Interferometer Sensors(3)

Intensity sensors(3): Liquid-level sensor
The light returned in air with total internal reflection from a prism is 15 dB above the value in water.
Temperature sensor
Liquid refractive index
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Multimode Grating Sensors(1)

Theoretical Background
Slit
Screen
s sin m sin 2 A1 sin 2 NA2 I A12 sin 2 A2

Photoelastic Effect

2C t

x
2C t

x
Where C is the stress-optic coefficient, C the strain-optic coefficient, t , the optical path length through the material, , x uniaxial stress, x ,uniaxial strain.