Temperature and Strain 7.5 Polarization Stability of Interrogation
Schemes
3
7.Fiber Bragg Grating Sensors
7.6 Multiplexing Techniques 7.7 Sensors Based on Chirped Bragg
L
E
(7.2a)
n n

n2P 2E
(1
2v)(2
p12

p11 )
(7.2b)
Where E is Young’s modulus
13
7.2.1 Pressure Sensitivity
GivenL / L / ,the normalized pitch pressure is given by
The most fundamental means for interrogating a FBG relies on broad band illumination of the device.
The grating used in sensor applications have bandwidth of 0.05 to 0.3nm.
Bragg Gratings and Interferometric Sensors
17
7.3 Wavelength Demodulation of Bragg Grating Point Sensors
For FBG point sensors 1 pm resolution is required to resolve ~ 0.1o C or 1 By OSA+ Tunable laser.
FBG’s have been used for DMFS thru the Faraday effect.
n

n

VH 2
(7.5)
16
7.3 Wavelength Demodulation of Bragg Grating Point Sensors
7.3.1 Quasi-Static Strain Monitoring 7.3.2 Dynamic Strain Sensing 7.3.3 Simultaneous Interrogation of
2
7.Fiber Bragg Grating Sensors
7.1 Introduction 7.2 Sensing External Fields 7.3 Wavelength Demodulation of Bragg
Grating Point Sensors 7.4 Simultaneous Measurement of
25
7.3.1.1 Passive Broadband Interrogation
This approach provides (1) Very limited sensitivity for broadband filter.
or(2) Limited range for edge filter. (3) Rely on bulk optic components; alignment stability is critical which reduces portability.
Gratings 7.8 Distinguishing Bragg Grating Strain
Effects 7.9 Bragg Grating Fiber Laser Sensors 7.10 Bragg Gratings as Interferometer
Sensors and Reflective Markers
12
7.2.1 Pressure nsitivity
A pressure change of p wavelength shifp :
p (n) ( 1 1 n )P B n P n P
(7.1)
For SMF
L (1.2v)P
23
7.3.1.1 Passive Broadband Interrogation
24
7.3.1.1 Passive Broadband Interrogation
Comparing light Xmitted thru the filter with light passed along a reference path recovers the wavelength shift of the sensor.(ref.Fig.7.2(a))
7
7.1 Introduction
The key detection issue for FBG is - the determination of small measurand-induced
wavelength shifts. Category of FBG sensors - point sensor - array (WDM/TDM/SDM) or distributed - a reflective marker (OTDR) - extremely long grating
p

B[
(1 2v) E

n2 2E
(1
2v)(2 p12

p11 )]P
(7.3a)
For the refractive-pressure coefficients are given by
1 (1 2v)
P
E
(7.3b)
Therefore
1 n
n p
10
7.2 Sensing External Fields
Eq(3.9)&(3.10) & (3.11) works for measuring
- acceleration - ultrasonic waves - forces
11
7.2 Sensing External Fields
7.2.1 Pressure Sensitivity 7.2.2 Dynamic Magnetic Field Sensitivity
WDM Fiber Coupler 7.3.1.3 Interrogation via Scanning
Optical Filter 7.3.1.4 Bragg Grating Interrogation
Using Wavelength Tunable Source
20
7.3.1 Quasi-Static Strain Monitoring
7.3.1.7 Mode-Locked Fiber Laser 7.3.1.8 Other Lasers
21
7.3.1 Quasi-Static Strain Monitoring
A lot of schemes used for recovery the wavelength- shift information is required for smart structure application.
4
1.1 Fiber Bragg Gratings
7.11 Other Bragg Sensor Types 7.12 Applications of Bragg Grating
Sensors
5
7.1 Introduction
FBG is suitable for measuring - Temperature - strain - pressure
22
7.3.1 Passive Broadband Interrogation
Broadband or edge filters provided a wavelength - dependent loss when the cutoff is lose to the signal wavelength.(ref.Fig.7.1(a))
18
7.3 Wavelength Demodulation of Bragg Grating Point Sensors
19
7.3.1 Quasi-Static Strain Monitoring
7.3.1.1 Passive Broadband Interrogation 7.3.1.2 Ratio metric Detection with a
7.3.1.5 Recovery of Bragg Grating Wavelength Shift Using CCD Spectrometer
7.3.1.6 Analysis of Bragg Grating Wavelength Shift Using Fourier Transform Spectroscopy

for the two cp (circular polarization ,CP)
2 Bragg conditions:
B 2n B 2n
Where “+” refer to right cp. “ ” refer to left cp.
15
7.2.2 Dynamic Magnetic Field Sensitivity (DMFS)
光纖布拉格光柵
Fiber Bragg Gratings 國立中山大學 電機工程研究所
林武文教授 2006.11.1
1
Ref: Andreas Othonos & Kyriacos Kalli, Fiber Bragg Gratings, Artech House , Norwood, MA.U.S.A.1999
8
7.2 Sensing External Fields