Course 6. Fiber Optics and Communications

This course builds on and expands modules 1-5, 1-6 and 4-7

Module 1. Fiber Properties and Specifications

    1.1 Fiber losses
        1.1.1 Absorption
            1.1.1.1 Material absorption
            1.1.1.2 Impurity absorption
            1.1.1.3 Wavelength dependence
            1.1.1.4 Numerical values
        1.1.2 Scattering
            1.1.2.1 Linear scattering
            1.1.2.2 Nonlinear scattering
        1.1.3 Inhomogeneities
            1.1.3.1 Changes in shape and size of core
            1.1.3.2 Coating imperfections
            1.1.3.3 Impurities
        1.1.4 Microbending and macrobending
            1.1.4.1 Bending losses
            1.1.4.2 Multimode fiber loss
            1.1.4.3 Single mode fiber loss
    1.2 Dispersion
        1.2.1 Material dispersion
            1.2.1.1 Index of refraction varies with wavelength
            1.2.1.2 Cause of pulse broadening
            1.2.1.3 Values of dispersion for typical materials
            1.2.1.4 Variation of dispersion with wavelength
            1.2.1.5 At some wavelength, the dispersion is zero
        1.2.2 Waveguide dispersion
            1.2.2.1 Cause of wavegu9ide dispersion
            1.2.2.2 Only important when material dispersion is near zero
            1.2.2.3 Typical values
        1.2.3 Modal dispersion
            1.2.3.1 Different paths for different modes
            1.2.3.2 Causes pulse spreading
            1.2.3.3 Values for step index fiber
            1.2.3.4 Values for graded index fibers
    1.3 Fiber strength
        1.3.1 Fiber breaking strength
        1.3.2 Time-dependent strength
    1.4 Cables
        1.4.1 Main parts of cables
            1.4.1.1 Fiber
            1.4.1.2 Buffer
            1.4.1.3 Strength member
            1.4.1.4 Jacket
        1.4.2 Indoor cables
            1.4.2.1 Simple
            1.4.2.2 Duplex
            1.4.2.3 Multifiber
            1.4.2.4 Light duty
            1.4.2.5 Heavy duty
            1.4.2.6 Plenum
            1.4.2.7 Breakout
        1.4.3 Outdoor
            1.4.3.1 Overhead
            1.4.3.2 Direct burial
            1.4.3.3 Indirect burial
            1.4.3.4 Submarine
    1.5 Cable specifications
        1.5.1 Duty specifications
            1.5.1.1 Light duty
            1.5.1.2 Heavy duty
            1.5.1.3 Plenum
            1.5.1.4 Riser
        1.5.2 Quantities specified
            1.5.2.1 Attenuation at specified wavelengths
            1.5.2.2 Bandwidth at specified wavelengths
            1.5.2.3 Core and cladding diameters
            1.5.2.4 Operating temperature range
            1.5.2.5 Tensile load limit
            1.5.2.6 Crush resistance
            1.5.2.7 Minimum bend radius
            1.5.2.8 Flame resistance
            1.5.2.9 Weight per unit length


Module 2. Fiber Design and Fabrication

    2.1 Types of fibers
        2.1.1 Step index
            2.1.1.1 Core/cladding diameter
            2.1.1.2 Single mode step index
            2.1.1.3 Cutoff wavelength for single mode operation
            2.1.1.4 Multimode step index
            2.1.1.5 Number of modes
            2.1.1.6 Numerical apertures for single and multimode
            2.1.1.7 Distribution of power between core and cladding
        2.1.2 Multistep
            2.1.2.1 Depressed cladding profile
            2.1.2.2 The W profile
        2.1.3 Graded index
            2.1.3.1 Index profile
            2.1.3.2 Number of modes
            2.1.3.3 Numerical aperture
            2.1.3.4 Single mode graded index
        2.1.4 Triangular profile
    2.2 Creation of profiles by compositional variation
        2.2.1 Glass
            2.2.1.1 Composition
            2.2.1.2 Index of refraction
        2.2.2 Effect of dopants on index of refraction
            2.2.2.1 Germanium
            2.2.2.2 Boron
            2.2.2.3 Phosphorous
        2.2.3 Design of fiber thru compositional variation
            2.2.3.1 Profile in preform same as in fiber
            2.2.3.2 Dopant profile for step index
            2.2.3.3 Dopant profile for graded index
        2.2.4 Polarization-maintaining
            2.2.4.1 Elliptical core
            2.2.4.2 Bow tie configuration
        2.2.5 Plastic fiber
            2.2.5.1 Lower cost
            2.2.5.2 Lower performance
            2.2.5.3 Applications for plastic fiber
    2.3 Fiber fabrication
        2.3.1 Preform fabrication
            2.3.1.1 Modified chemical vapor deposition process
            2.3.1.2 Outside chemical vapor deposition process
            2.3.1.3 Axial vapor deposition process
        2.3.2 Fiber drawing
            2.3.2.1 The drawing process
            2.3.2.2 Control of diameter
            2.3.2.3 Lengths that can be drawn from a preform
            2.3.2.4 Rolling up the fiber on a drum
        2.3.3 Fiber coating
            2.3.3.1 Hermetic coated fibers
            2.3.3.2 Polymer clad fibers


Module 3. Fiber Connectors and Splices

    3.1 Introduction to joining fibers
        3.1.1 Fiber-to-fiber connectors
            3.1.1.1 Joining fibers is usually done by butt coupling
            3.1.1.2 Connectors can be mated and unmated repeatedly
            3.1.1.3 Need for connectors
            3.1.1.4 Differences in joining fibers and wires
        3.1.2 Splices
            3.1.2.1 Splices are permanent connections
            3.1.2.2 Need for splices
    3.2 Losses when fibers are joined
        3.2.1 Lateral displacement
            3.2.1.1 Cause: cores of fibers are not aligned
            3.2.1.2 Magnitude of loss (losses expressed in dB)
            3.2.1.3 Variation with offset
        3.2.2 Longitudinal gap
            3.2.2.1 Cause: not all light reaches receiving fiber
            3.2.2.2 Magnitude of loss
            3.2.2.3 Variation with gap length
        3.2.3 Angular misalignment
            3.2.3.1 Cause: not all light reaches receiving fiber
            3.2.3.2 Magnitude of loss
            3.2.3.3 Variation with angle
            3.2.3.4 Variation with numerical aperture
        3.2.4 Non-flat fiber ends
            3.2.4.1 Scattering from rough ends is large loss
            3.2.4.2 Use of matching fluids can help
            3.2.4.3 Real remedy is to polish ends
    3.3 Preparation of fiber ends
        3.3.1 Scribe and break method
            3.3.1.1 Bare fiber tip
            3.3.1.2 Nick outer edge of fiber with hard tool
            3.3.1.3 Increase tension till fiber breaks
            3.3.1.4 If done right, yields flat, mirror-like surfaces
        3.3.2 Lap and finish
            3.3.2.1 Bare fiber tip
            3.3.2.2 Insert end into ferrule
            3.3.2.3 Grind end with abrasives to yield polished surface
            3.3.2.4 This method usually used with demountable connectors
    3.4 Connectors
        3.4.1 Butt connectors
            3.4.1.1 Ferrule for each fiber
            3.4.1.2 Precision sleeve into which ferrules fit
            3.4.1.3 Typical loss
        3.4.2 Tapered sleeve (biconic)
            3.4.2.1 Design
            3.4.2.2 Typical loss
        3.4.3 SMA connector
            3.4.3.1 Design
            3.4.3.2 Typical loss
        3.4.4 Overlap Connector
            3.4.4.1 Design
            3.4.4.2 Typical loss
        3.4.5 ST Connector
            3.4.5.1 Design
            3.4.5.2 Typical loss
        3.4.6 Lensed connector
            3.4.6.1 Design
            3.4.6.2 Typical loss
            3.4.6.3 GRIN rod lens connector variation
        3.4.7 Commercial connectors
            3.4.7.1 Types available
            3.4.7.2 Typical losses
    3.5 Splices
        3.5.1 Fusion splices
            3.5.1.1 Method of making the splice
            3.5.1.2 Typical loss
        3.5.2 Adhesive splices
            3.5.2.1 Method of making the splice
            3.5.2.2 Typical loss
        3.5.3 Mechanical splices
            3.5.3.1 V block
            3.5.3.2 Precision sleeve
            3.5.3.3 Three rods
            3.5.3.4 Typical losses


Module 4. Basic and Specialty Fiber Devices

    4.1 Couplers
        4.1.1 Coupler fundamentals
            4.1.1.1 Represent a basic device
            4.1.1.2 Enable transfer of optical energy from one
                        waveguide to another
            4.1.1.3 Coupler is a multiport device
            4.1.1.4 Throughput loss
            4.1.1.5 Tap loss
        4.1.2 Tee couplers
            4.1.2.1 Three port device
            4.1.2.2 Structure
            4.1.2.3 Use in bus topologies
            4.1.2.4 Losses
        4.1.3 Star coupler
            4.1.3.1 Distributes power from one input port to two
                        or more output ports
            4.1.3.2 Structure
            4.1.3.3 Star coupler applications
            4.1.3.4 Losses
        4.1.4 Directional coupler
            4.1.4.1 Power from an input port transferred only to specified output ports
            4.1.4.2 Structure
            4.1.4.3 Provides optical isolation
            4.1.4.4 Losses
        4.1.5 Fused couplers
            4.1.5.1 Made by wrapping fibers together and heating
            4.1.5.2 Can be very small
            4.1.5.3 Can be used to form star couplers
    4.2 Fiber amplifiers
        4.2.1 All-optical amplifiers
            4.2.1.1 Fibers doped with certain elements are pumped at one
                        wavelength, provide optical gain at another
            4.2.1.2 Repeaters could use fiber amplifiers
            4.2.1.3 System would be all optical
            4.2.1.4 No need for optoelectronic detection, electronic
                        amplification or retransmission
            4.2.1.5 Pulse retiming and reshaping would not be needed
            4.2.1.6 Developing technology for future application
        4.2.2 Erbium doped fiber amplifiers (EDFA)
            4.2.2.1 Pumped at 980 nm
            4.2.2.2 Amplifies signals at 1550 nm
            4.2.2.3 They provide wide gain bandwidth
        4.2.3 ZBLAN fiber amplifier
            4.2.3.1 Zirconium barium lanthanum aluminum sodium fiber
                        doped with neodymium
            4.2.3.2 Pumped at 795 nm
            4.2.3.3 Provides gain at 1345 nm
    4.3 Solitons
        4.3.1 Nature of solitons
            4.3.1.1 Solitary wave, single peak moving in isolation
            4.3.1.2 Generated in media with both dispersion and optical nonlinearity
            4.3.1.3 Pulse lengthening effects of dispersion balanced by pulse
                        shortening effects of nonlinearity
            4.3.1.4 Solitons preserve pulse shape and duration over large distances
            4.3.1.5 Solitons generated in fibers with rare earth dopants
            4.3.1.6 Pulses can have picosecond duration
        4.3.2 Soliton transmission experiments
            4.3.2.1 Solitons generated in EDFA
            4.3.2.2 Demonstrations at 20 Gbps over 13000 km
            4.3.2.3 Soliton based systems offer promise for very wide bandwidth
                        communications in the future
    4.4 Wavelength division multiplexing (WDM)
        4.4.1 Many signals at different wavelengths transmitted in one fiber
            4.4.1.1 Beams are modulated independently
            4.4.1.2 At the receiver, beams separated by optical filters and detected
                        by different detectors
            4.4.1.3 All beams can be amplified by same wide bandwidth amplifier,
                        like an EDFA
        4.4.2 WDM increases number of channels that can be carried on a fiber
            4.4.2.1 Can increase system capacity without changing cables
            4.4.2.2 Use of all-optical amplifiers with WDM offers future promise


Module 5. Transmitters for Fiber Optic Communication Links

    5.1 Sources
        5.1.1 LED sources
            5.1.1.1 Surface emitting LEDs
            5.1.1.2 Edge emitting LEDs
            5.1.1.3 Output power
            5.1.1.4 Beam spread
            5.1.1.5 Spectral width
            5.1.1.6 Modulation bandwidth
            5.1.1.7 Lifetime
        5.1.2 Laser diode
            5.1.2.1 Wavelength choices
            5.1.2.2 Edge and surface emitting diodes
            5.1.2.3 Gain guided and index guided devices
            5.1.2.4 Distributed feedback (DFB) lasers
            5.1.2.5 Output power
            5.1.2.6 Beam spread
            5.1.2.7 Spectral width
            5.1.2.8 Single frequency diode lasers
            5.1.2.9 Heat removal
            5.1.2.10 Lifetime
    5.2 Drive circuits
        5.2.1 Input
            5.2.1.1 Two-wire line
            5.2.1.2 Coaxial cable
            5.2.1.3 Rectangular waveguide
            5.2.1.4 Input signal levels
        5.2.2 LED drive circuits
            5.2.2.1 Output vs drive current
            5.2.2.2 Duty cycle
            5.2.2.3 Analog drive circuits
            5.2.2.4 Digital drive circuits
        5.2.3 Laser drive circuits
            5.2.3.1 Output vs drive current
            5.2.3.2 Duty cycle
            5.2.3.3 Analog drive circuits
            5.2.3.4 Digital drive circuits
    5.3 Coupling of light into fiber
        5.3.1 Lens coupling
            5.3.1.1 Reflection losses
            5.3.1.2 Spatial pattern of source
            5.3.1.3 Fraction of light collected
        5.3.2 Pigtails
            5.3.2.1 Description
            5.3.2.2 Efficiency
        5.3.3 Fiber configurations
            5.3.3.1 Bulb end fiber
            5.3.3.2 Tapered end fiber
            5.3.3.3 Graded index fiber coupling
     5.4 The transmitter package
        5.4.1 Functions of transmitter
            5.4.1.1 Convert electrical signal to optical
            5.4.1.2 Input optical signal to cable
            5.4.1.3 Transmitter package contains source, driver and coupling to fiber
        5.4.2 Standard specifications for transmitters
            5.4.2.1 Data rate
            5.4.2.2 Optical output power
            5.4.2.3 Center wavelength
            5.4.2.4 Spectral width
            5.4.2.5 Duty cycle
            5.4.2.6 Risetime and fall time
            5.4.2.7 Supply voltage and current
            5.4.2.8 Operating temperature


Module 6. Receivers for Fiber Optic Communication Links

    6.1 Receiver basics
        6.1.1.Receiver function
            6.1.1.1 Recover highly attenuated signal
            6.1.1.2 Recover signal in presence of noise
            6.1.1.3 Amplify signal
            6.1.1.4 Output recovered data
        6.1.2 Elements of a receiver
            6.1.2.1 Detector
            6.1.2.1 Amplifier
            6.1.2.3 Output circuit
    6.2 Detectors
        6.2.1 PIN Photodiode
            6.2.1.1 Materials
            6.2.1.2 Spectral response
            6.2.1.3 Noise equivalent power
            6.2.1.4 Frequency response
            6.2.1.5 Circuit for PIN photodiode
        6.2.2 Avalanche photodiode (APD)
            6.2.2.1 Materials
            6.2.2.2 Spectral response
            6.2.2.3 Noise equivalent power
            6.2.2.4 Frequency response
            6.2.2.5 Circuit for avalanche photodiode
            6.2.2.6 Choosing between PIN photodiode and APD
    6.3 Amplifier
        6.3.1 Typical amplifier circuits
            6.3.1.1 Low impedance amplifier
            6.3.1.2 Transimpedance amplifier
        6.3.2 Amplifier noise
            6.3.2.1 Noise in low impedance amplifier
            6.3.2.2 Noise in transimpedance amplifiers
        6.3.3 Amplifier bandwidth
            6.3.3.1 Bandwidth of low impedance amplifier
            6.3.3.2 Bandwidth of transimpedance amplifiers
    6.4 Output circuit
        6.4.1 Functions of output circuit
            6.4.1.1 Pulse reshaping and retiming
            6.4.1.2 Separation of clock and data
            6.4.1.3 Level shifting for computability with external circuit
            6.4.1.4 Gain control for constant output level
        6.4.2 Typical output circuit elements
            6.4.2.1 Equalizer
            6.4.2.2 Postamplifier
            6.4.2.3 Filter
            6.4.2.4 Clock recovery
            6.4.2.5 Decision circuit
    6.5 Receiver performance
        6.5.1 Receiver noise
            6.5.1.1 Includes quantum noise in signal, detector noise and amplifier noise
            6.5.1.2 Threshold level for signal detection
            6.5.1.3 Bit error rates
        6.5.2 Standard specifications for receivers
            6.5.2.1 Data rate
            6.5.2.2 Sensitivity
            6.5.2.3 Wavelength
            6.5.2.4 Duty cycle
            6.5.2.5 Risetime and fall time
            6.5.2.6 Supply voltage and current
            6.5.2.7 Operating temperature


Module 7. Distribution Systems and Network Architecture

    7.1 Introduction
        7.1.1 System specifications
            7.1.1.1 Type of fiber
            7.1.1.2 Wavelength
            7.1.1.3 Transmitter power
            7.1.1.4 Source type
            7.1.1.5 Detector type
            7.1.1.6 Receiver sensitivity
            7.1.1.7 Modulation code
            7.1.1.8 Bandwidth
            7.1.1.9 Bit error rate
            7.1.1.10 Distance between repeaters
        7.1.2 Power budget
            7.1.2.1 Fiber loss
            7.1.2.2 Transmitter power
            7.1.2.3 Length of link
            7.1.2.4 Connector loss
            7.1.2.5 Receiver sensitivity
        7.1.3 Bandwidth budget
            7.1.3.1 Transmitter rise time
            7.1.3.2 Receiver rise time
            7.1.3.3 Fiber dispersion
    7.2 Networks
        7.2.1 Point-to-point
            7.2.1.1 Simplest network
            7.2.1.2 Connects only two points
            7.2.1.3 Vulnerable to fiber breakage
        7.2.2 Local area networks (LAN)
            7.2.2.1 Limited geographical area, like large building
            7.2.2.2 Less than one kilometer distance
            7.2.2.3 Can be designed not to be susceptible to fiber breakage
            7.2.2.4 Nodes in a LAN
            7.2.2.5 Topologies
                - Star
                - Ring
                - Bus
            7.2.2.6 Dedicated systems and contention systems
        7.2.3 Metropolitan Area Networks (MAN)
            7.2.3.1 Within a city
            7.2.3.2 Telephone local loops, cable TV, etc.
            7.2.3.3 Topologies
                - Star
                - Ring
                - Bus
                - Tree
            7.2.3.4 Connections between clusters and LANs
        7.2.4 Wide Area Networks (WAN)
            7.2.4.1 Long haul trunk communications
            7.2.4.2 High channel capacity, up to 2.4 Gbps
            7.2.4.3 WANs connect to MANs
            7.2.4.4 Sample topology for a WAN
    7.3 Standards
        7.3.1 Development of standards
            7.3.1.1 Need for standards
            7.3.1.2 De facto standards
            7.3.1.3 Development of standards by committees, etc.
        7.3.2 Ethernet
            7.3.2.1 Data rate of 3 Mbps
            7.3.2.2 Data transmitted in 4000 bit packets
            7.3.2.3 Ethernet topology
        7.3.3 Integrated Services Digital Network (ISDN)
            7.3.3.1 International standard
            7.3.3.2 Integrates new services with existing services
        7.3.4 Synchronous Optical Network (SONET)
            7.3.4.1 American National Standards Institute standard
            7.3.4.2 WAN standard
            7.3.4.3 Standard synchronous interface between optical and
                        electrical components
            7.3.4.4 Sets optical transmission rates and formats
        7.3.5 Fiber Distributed Data Interface (FDDI)
            7.3.5.1 American National Standards Institute standard
            7.3.5.2 LAN standard
            7.3.5.3 Uses 1300 nm LEDs
            7.3.5.4 Data rate 100 Mbps
        7.3.6 Other standards
            7.3.6.1 IEEE standards
            7.3.6.2 International Telegraph and Telephone Consultative
                        Committee (CCITT)


Module 8. Installation and Testing of Fiber Communication Systems

    8.1 Types of installation
        8.1.1 Indoor
            8.1.1.1 Tray and duct installation
            8.1.1.2 Conduit installation
        8.1.2 Outdoor installation
            8.1.2.1 Direct burial
            8.1.2.2 Indirect burial
        8.1.3 Aerial
        8.1.4 Submarine
    8.2 Pulling fiber optic cables
        8.2.1 Special requirements
            8.2.1.1 Minimum bend radius
            8.2.1.2 Tensile stress on fiber
        8.2.2 Tools
            8.2.2.1 Pulling grips
            8.2.2.2 Pulling tapes
        8.2.3 Procedures
            8.2.3.1 Process similar to pulling wire cables thru conduits
            8.2.3.2 Monitor pulling force with mechanical gage
            8.2.3.3 Stop if pulling force increases
            8.2.3.4 Use pulley at entrance to conduit to avoid sharp bends
            8.2.3.5 Coil emerging cable in the form of a figure-8 with large radius
    8.3 Splice closures and organizers
        8.3.1 Splices made by methods described in Module 6.3
        8.3.2 Organizer panel
            8.3.2.1 Holds fibers and splices in an orderly manner
            8.3.2.2 Allows fiber splice identification
            8.3.2.3 Allows fiber rearrangement and storage of excess fibers
        8.3.3 Splice closure
            8.3.3.1 Holds one or more organizers
            8.3.3.2 Covers cable splice housings
            8.3.3.3 Provides environmental protection
            8.3.3.4 Provides mechanical strength
    8.4 Distribution hardware
        8.4.1 Routing of signals to final destination
            8.4.1.1 Distribution of signals to final points
            8.4.1.2 Wiring centers
        8.4.2 Types of distribution hardware
            8.4.2.1 Rack boxes
            8.4.2.2 Patch panels
            8.4.2.3 Wall outlets
    8.5 Installation kits
        8.5.1 Functions of tools
            8.5.1.1 Split open multifiber cables
            8.5.1.2 Prepare fibers
            8.5.1.3 Polish fiber ends
            8.5.1.4 Inspect the finish
            8.5.1.5 Make splices
        8.5.2 Common tools
            8.5.2.1 Screw driver
            8.5.2.2 Side cutting pliers
            8.5.2.3 Tape measure
            8.5.2.4 Scissors
            8.5.2.5 Epoxy and relevant applicators
        8.5.3 Specialized tools
            8.5.3.1 Cable stripper
            8.5.3.2 Fiber stripper
            8.5.3.3 Scribe tool for trimming fibers
            8.5.3.4 Crimp tool for attaching connectors to fibers
            8.5.3.5 Fiber polishing compounds
            8.5.3.6 Index matching fluid
            8.5.3.7 Microscope for inspecting fiber ends
    8.6 Test equipment and techniques
        8.6.1 Optical power meter
            8.6.1.1 Range to -80 dBm
            8.6.1.2 Needed for insertion loss measurements and fiber attenuation
        8.6.2 Optical time domain reflectometer (OTDR)
            8.6.2.1 Measures backscattering of light
            8.6.2.2 Determines fiber loss
            8.6.2.3 Evaluates splices and connectors
            8.6.2.4 Determines positions of faults
        8.6.3 Focusing method for determining index profile of fiber
            8.6.3.1 Illuminate fiber with collimated light perpendicular to fiber
            8.6.3.2 Form focused image on observation plane
            8.6.3.3 Use paraxial ray equation to derive fiber profile
    8.7 Testing
        8.7.1 Important parameters
            8.7.1.1 Fiber loss
            8.7.1.2 Insertion losses
        8.7.2 Specifications to be checked
            8.7.2.1 Core and cladding diameters
            8.7.2.2 Numerical aperture of fiber
            8.7.2.3 Attenuation of fiber
            8.7.2.4 Profile of refractive index of fiber
            8.7.2.5 Tensile strength

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