光纤布拉格光栅.ppt

L
E
(7.2a)
n n

n2P 2E
(1
2v)(2
p12

p11 )
(7.2b)
Where E is Young’s modulus
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7.2.1 Pressure Sensitivity
GivenL / L / ,the normalized pitch pressure is given by
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

n2 2E
(1 2v)(2 p12

p11 ) (7.4)
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7.2.2 Dynamic Magnetic Field Sensitivity (DMFS)
Faraday effect
- A longitudinal magnetic field applied to
the grating changes the refractive index
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7.3.1.1 Passive Broadband Interrogation
IS IR

A{B
O

}
(7.6)
This system offer several advantages - low cost - ease of use - low resolution
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7.3.1.1 Passive Broadband Interrogation
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7.2 Sensing External Fields
Eq(3.9)&(3.10) & (3.11) works for measuring
- acceleration - ultrasonic waves - forces
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7.2 Sensing External Fields
7.2.1 Pressure Sensitivity 7.2.2 Dynamic Magnetic Field Sensitivity
7.3.1.7 Mode-Locked Fiber Laser 7.3.1.8 Other Lasers
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7.3.1 Quasi-Static Strain Monitoring
A lot of schemes used for recovery the wavelength- shift information is required for smart structure application.
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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
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7.3.1.1 Passive Broadband Interrogation
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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))
FBG’s have been used for DMFS thru the Faraday effect.
n

n

VH 2
(7.5)
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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
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7.2 Sensing External Fields
In ch.3 we learned (1)Strain response of the grating arises
from the physical elongation of the sensor, leading to a fractional change in the grating pitch with a corresponding change in the fiber index because of the photo elastic effect.
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7.2 Sensing External Fields
(2) Thermal response - inherent thermal expansion of the fiber material. - the temperature dependence of the refractive index.
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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.
WDM Fiber Coupler 7.3.1.3 Interrogation via Scanning
Optical Filter 7.3.1.4 Bragg Grating Interrogation
Using Wavelength Tunable Source
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7.3.1 Quasi-Static Strain Monitoring
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7.3 Wavelength Demodulation of Bragg Grating Point Sensors
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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
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7.1 Introduction
The principal advantage is that the measured information is
- wavelength encoded - flexibility to network - linearity (from ppb~%) - low insertion loss & narrow band wavelength - reflection - easily embedded into materials
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.
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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))
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7.2.1 Pressure Sensitivity
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
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
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1.1 Fiber Bragg Gratings
7.11 Other Bragg Sensor Types 7.12 Applications of Bragg Grating
Sensors
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7.1 Introduction
FBG is suitable for measuring - Temperature - strain - pressure
Bragg Gratings and Interferometric Sensors
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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.
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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
p

B[
(1 2v) E

n2 2E
(1
2v)(2 p12

p11 )]P
wenku.baidu.com(7.3a)
For the refractive-pressure coefficients are given by
1 (1 2v)
P
E
(7.3b)
Therefore
1 n
n p

for the two cp (circular polarization ,CP)
2 Bragg conditions:
B 2n B 2n
Where “+” refer to right cp. “ ” refer to left cp.
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7.2.2 Dynamic Magnetic Field Sensitivity (DMFS)
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
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7.3.1.2 Ratiometric Detection with a WDM Fiber Coupler