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Fundamental of Fiber Optics
Optical Fiber
Total Internal Reflection
V-Number and Fiber Modes
Cut-off Wavelength
Definition: the wavelength below which multiple modes of light can be propagated along a particular fiber, i.e., l>=lc, single mode, l<lc, multi-mode
Multi-Mode vs. Single-mode
Wavelength of Transmitted Light
Wavelength-dependant Attenuation
Typical Optical Fiber Sensing Mechanism
Light Parameters
Power/Intensity
Unit:
Watts
dbm: 10log(P*1000)
Attenuation/loss
db: 10log(P1/P0)
Wavelength (spectral distribution)
Polarization
Phase (Optical path)
Optical Transmitter
Definition: a device that converts electrical signal into optical signal
Lasers
Fabry-Perot Lasers (FP)
Distributed Feedback Lasers (DFB)
Vertical Cavity Surface Emitting Lasers (VCSEL)
ASE fiber laser
Light Emitting Diodes (LED)
Surface-Emitting LED (SLED)
Edge-emitting LED (EELED)
Transmitter
Fabry-Perot Laser (FB Laser)
Radiation pattern: 60o angle cone-shape
Output power: several mW
Modulation: at high rates
Spectral width: relatively big
Transmitter
Distributed Feedback Laser (DFB laser)
Single longitudinal mode
Can be internally or externally modulated
Transmitter
Vertical Cavity Surface Emitting Laser (VCSEL)
Single longitudinal mode
Low cost
Low power
Mostly used for MM communication
Transmitter
Light Emitting Diode (LED)
Low cost
Low power
Broad spectral width
Can be modulated to several hundred MHz
Two types
Surface-emitting LED (SLED): <1mW power
Edge-emitting LED (EELED): several mW power
Transmitter Comparison
Optical Receiver
Definition: convert optical signal into electrical signal
Types:
p-i-n photodetector: photon-electron converter
Avalance photodetector (APD): more sensitive for high speed systems
Photodetector parameters:
Responsivity: the amount of current produced per unit of input optical power
Wavelength bandwidth: the bandwidth the PD is sensitive to.
Damage threshold: the maximum optical power the PD can take before damage
Wavelength-dependent Sensitivity of Detectors
Typical Optical Fiber Measurement System
Coupling Light into Fiber
Light Distribution in a SMF
Coupling Light into SM Fiber
Optical Fiber Sensor
Optical Fiber Sensor Types
Intrinsic: the effect of the measurand on the light being transmitted take place in the fiber
Extrinsic: the fiber carries the light from the source and to the detector, but the modulation occurs outside the fiber
Optical Fiber Sensor Types
Optical Fiber Sensor Types
Intensity-based Optical Fiber Sensor
Intensity-based Optical Fiber Sensor
Optical Fiber Components
Fiber connector
Broadband light source (BBS)
Fiber coupler/circulator
Mode scrambler
Index matching fluid
Wavelength division multiplexer
Fiber Connector
Fiber Connector Type
Broadband Light Source
Fiber Coupler
Circulator
Definition: a passive three-port device that couple light from Port 1 to 2 and Port 2 to 3 and have high isolation in other directions.
Mode scrambler
Other Mode Scrambler
Index matching fluid
Definition: A liquid with refractive index similar to glass that is used to match the materials at the ends of two fibers to reduce loss and back reflection.
Applications:
Reduce back reflection
increase coupling between two fibers
Wavelength division multiplexer
Definition: a device that combines and split lights with different wavelengths