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TCOM 503 Fiber Optic Networks

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Title: TCOM 503 Fiber Optic Networks


1
TCOM 503 Fiber Optic Networks
Spring, 2007 Thomas B. Fowler, Sc.D. Senior
Principal Engineer Mitretek Systems
2
Topics for TCOM 503
  • Week 1 Overview of fiber optic communications
  • Week 2 Brief discussion of physics behind fiber
    optics
  • Week 3 Light sources for fiber optic networks
  • Week 4 Fiber optic components fabrication and
    use
  • Week 5 Fiber optic components, modulation of
    light
  • Week 6 Noise and detection
  • Week 7 Optical fiber fabrication and testing of
    components

3
New resources
  • Agilent Technologies (old HP test measurement
    division) educators page
  • http//www.educatorscorner.com/tools/lectures
    /slides/
  • Lots of great material on networks, instruments,
    basic electronics, RF networks, and optical
    networking

4
Study topics for final exam
  • Principles of fiber optic cable and devices
    (reflection, refraction, interference,
    diffraction)
  • Types of fiber optic cable
  • Types of distortion and other problems involved
    with optical fiber
  • Operation of LEDs and lasers
  • Operation of detectors
  • Operation of EDFAs
  • Resonant couplers/wavelength selective couplers
    splitters
  • Other optical devices
  • Isolators - Fabry-Perot filters
  • GRINs - Dielectric filters
  • FBGs - Modulators Modulation types

5
Fiber optic components
  • Diffraction gratings
  • Filters
  • Modulators
  • Switches
  • Repeaters

6
Diffraction gratings
  • Change angle of light as a function of its
    wavelength
  • Acts like a prism
  • Effectively does Fourier transform of light
  • Separates waveform in time domain into a number
    of waveforms in frequency domain
  • Used because of control they give over light
  • Can be readily fabricated using technology used
    to make other optical components

7
Types of diffraction gratings
  • Refractive
  • Light passes through material with grating etched
    on its surface
  • Typically glass or plastic
  • Commonly used in optics, but not generally
    employed in communications applications
  • Reflective
  • Light reflects off of surface with closely spaced
    lines
  • Generally fabricated in a medium, often along
    with other components
  • Can be formed in almost any material where
    optical properties can be varied in a regular way
    with period close to wavelength

8
Uses of diffraction gratings
  • Function as wavelength selective filters
  • Block or pass desired wavelengths
  • Enable combining or splitting optical signals
  • Used as reflectors in some devices

9
Principle of operation for diffraction gratings
  • Basic equation
  • ml gs ( sin q sin fm )
  • Where gs grove spacing
  • m order of refracted ray (m 0, 1, 2, 3…)
  • l wavelength of incident ray
  • q angle of incidence
  • fm angle of refraction

10
Operation of diffraction grating (continued)
  • If q 0 we get old diffraction formula sin fm
    ml/gs
  • For m 0 get ordinary reflection (sin q -sin
    fm) or q -fm
  • Solving for sin fm we get
  • sin fm ml/gs sin q
  • If gs gtgt ml, then we can solve for multiple
    values of fm, and as m becomes larger, fm becomes
    smaller

Source Dutton
11
Operation of diffraction grating (continued)
  • By setting grove spacing gs ml, solution only
    exists for m 0 or 1
  • Example l 1550 nm, q 45o, gs 1200 nm
  • Gives sin fm 0.5846 for m 1, fm 35.8o
  • Sin fm 1.87 for m 2 (impossible)
  • Note that if we change l to 1560 nm, sin fm
    0.5929, fm 36.4o
  • Enough that it would be easy to split the two

12
Shapes for diffraction gratings
  • No effect on angles, but does affect strengths of
    diffracted beams or orders
  • Blazed grating transfers large portion of power
    to first order
  • Operates only over restricted range of wavelengths

Source Dutton
13
Wavelength selection with diffraction gratings
Source Dutton
14
Wavelength selection (continued)
  • Devices tend to be costly because of very high
    precision required
  • Large number of closely spaced wavelengths can be
    separated

15
Examples of use of diffraction gratings
Source Anritsu
16
In-Fiber Bragg gratings (FBGs)
  • Extremely simple, low cost wavelength selective
    filter
  • Wide range of applications
  • Construction
  • Ordinary single mode fiber a few cm long
  • Grating formed by variation of RI of core
    lengthwise along fiber
  • Resonant wavelengths reflected back, others passed

Source Dutton
17
Construction of FBG
  • Only small variation in RI required 0.0001
  • Center wavelength given by
  • l 2 neff L
  • Where l center wavelength reflected back
  • neff average RI of material
  • L physical period of fiber grating

Fig 168
Source Advanced Optics Solutions Gmbh
18
Application of FBGs
  • Wavelength stable lasers stabilize laser so that
    it produces a narrow band
  • See p. 161 use two FBGs and one Erbium-doped
    fiber
  • Dispersion compensation (requires Chirped
    gratings where spacing varies along length)
  • Allow existing optical networks designed for 1300
    nm band to operate in 1550 nm band
  • Effectively shapes pulses
  • See p. 419
  • Wavelength selection in WDM systems
  • Allow separating out particular wavelengths when
    used with circulators

19
Characteristics of FBGs
  • Center wavelength wavelength at center of
    reflection band
  • Bandwidth range of wavelengths reflected
  • Reflectance peak fraction of incident light
    reflected back at center wavelength
  • Parameters which determine characteristics
  • Grating period
  • Grating length
  • Modulation depth (grating strength, determined by
    RI)
  • RI contrast profile
  • Can vary over length of grating, called
    apodization

20
Reflection spectra
.2 nm
  • Extra peaks caused by change in RI which looks
    like a mirror to wavelengths out of band
  • By tapering strength, can reduce peaks

Low RI contrast
High RI contrast
After Apodization
Source Dutton
21
Spectra (continued)

Source Advanced Optics Solutions Gmbh
22
Spectra (continued)
Source Corning
23
Chirped FBGs
  • Chirp in optical context means some change in
    frequency
  • A chirped FBG is one in which period of grating
    changes
  • Leads to variation in response to wavelength
    (frequency) along length of grating
  • Can be done in 2 ways
  • Vary period of grating
  • Vary average RI of grating
  • Different wavelengths reflected from different
    parts of grating
  • Imposes wavelength dependent delay on signal

Source Dutton
24
Chirped FBGs (continued)
  • Shorter wavelengths must travel farther before
    being reflected
  • Major use equalizing response of older fiber
    networks (1300 nm) so that they can operate in
    1550 nm band
  • Refers to dispersion compensation
  • Works because shorter wavelengths tend to be
    ahead of longer wavelengths in smeared
    (dispersed) pulse

25
Chirped FBGs (continued)
  • Characteristics
  • Need to be long for most applications
  • Require apodization in order to smooth response
  • Apodizationchanging shape to smooth
    discontinuities
  • Ripple effectively adds 3 db to noise of signal
  • Response

Source Dutton
After apodization
No apodization
26
Chirped FBGs (continued)
  • Improved response with higher grating strength

After apodization, 2x grating strength
Source Dutton
After apodization
No apodization
27
(No Transcript)
28
Source http//www.laser2000.co.uk/pdfs/view2.pdf
29
Multiple FBGs
  • Possible to put multiple FBGs on a single fiber
    to achieve better results
  • Either sequentially or on top of one another

30
Use of FBGs to make bandpass filters
  • Use blazed (slanted) FBGs to reflect selected
    wavelength out of the fiber
  • Use of multiple sections allows creation of a
    bandpass filter
  • Each section tuned to different band of
    frequencies

Source Dutton
31
Phase-shifted FBG
  • By shifting phase in middle of FBG, a narrow
    transmission band can be created
  • Too small by itself, but multiple phase shifts
    can make a useable passband

Source Dutton
32
Long-period FBG
  • Grating period is hundreds or thousands of times
    the resonant wavelength
  • Power coupled forward rather than backward
  • In single mode fiber, no mode available in fiber
    for it to couple to
  • Couples into cladding, eventually dissipates
  • Effect is similar to blazed grating, where
    resonant wavelengths removed from system
  • Application is same equalizing gain or response
    curves of devices such as EDFAs

33
Source Fujikura
34
Waveguide Grating Routers (WGRs)
  • Also called Arrayed Waveguide Gratings (AWGs)
  • Use planar waveguide technology
  • Function is similar to Littrow grating
  • Utilize principle of constructive and destructive
    interference to separate wavelengths

35
Array Waveguide Grating (AWG)
l
l
l
l
l
l
l
l
1a
3a
2a
4a
1a
3c
2d
4b
l
l
l
l
l
l
l
l
1b
3b
2b
4b
1b
3d
2a
4c
l
l
l
l
l
l
l
l
1c
3c
2c
4c
1c
3a
4d
2b
l
l
l
l
l
l
l
l
1d
3d
2d
4d
1d
3b
2c
4a
Rows ..
.. translate into ..
.. columns
If only one input is used wavelength
demultiplexer!
Source Agilent
36
Operation of AWGs
  • Input power diffracts into separate waveguides
  • Waveguides have different lengths
  • Signals interfere in output free space coupler so
    that only one goes out on each output line

Source Dutton
37
Performance characteristics of AWGs

Source Dutton
38

Source Lucent
39
(No Transcript)
40
Filters
  • Definition a filter is a device that selects or
    passes a band of frequencies (wavelengths)
  • May have sharp or gradual cut-off characteristic
  • May exhibit ringing or other effects
  • Gratings act as filters in many applications
  • Filters much more important in WDM applications
    than in single-wavelength applications

41
Practical filters

Bandwidth
Source Dutton
42
Filter characteristics
  • Center wavelength mean wavelength between two
    band edges
  • Peak wavelength wavelength at which attenuation
    is least (or greatest for band rejection filter)
  • Nominal wavelength design wavelength
  • Bandwidth distance between band edges where
    response is down a given amount, usually 3 db

43
Filter Characteristics
  • Passband
  • Insertion loss
  • Ripple
  • Wavelengths (peak, center, edges)
  • Bandwidths (0.5 dB, 3 dB, ..)
  • Polarization dependence
  • Stopband
  • Crosstalk rejection
  • Bandwidths (20 dB, 40 dB, ..)

Source Agilent
44
Optical filters Fabry-Perot (Etalon)
  • Essentially a resonator, like an organ pipe or a
    stringed instrument
  • Consists of cavity bounded on either end by a
    partially silvered mirror
  • If mirrors can be moved, called an
    interferometer
  • If mirrors fixed, called an Etalon

Source Dutton
45
Fabry-Perot filter operation
  • Light of a different frequency than the resonant
    frequency is mostly reflected, so very little
    enters chamber
  • What enters the chamber undergoes destructive
    interference
  • Light of resonant frequency which tries to
    reflect actually undergoes destructive
    interference from light already in chamber going
    out through mirror
  • Nearly all light of resonant frequency therefore
    enters
  • Can only exit through opposite mirror
  • Transmission characteristics depend on percent
    reflection of end mirrors

46
Fabry-Perot filter operation (continued)
  • Reflecting surfaces must be extremely flat
  • 1/100th of a wavelength
  • Very difficult to make
  • Mirrors typically made with 99 reflectivity

47
Fabry-Perot filter measures
  • Quality of filter or goodness of filter is
    called finesse
  • Energy stored in filter / energy passing through
    it
  • Analogous to Q in electrical theory
  • Higher reflectivity of mirrors gt higher finesse
  • Resonant wavelengths given by
  • l 2Dn/m
  • Where n RI, D distance between mirrors, m
    1,2,3…

48
Fabry-Perot filter measures (continued)
  • Distance between peaks in response curve called
    Free Spectral Range (FSR)
  • FSR l2/(2nD)
  • where n RI, D distance between mirrors
  • Finesse FSR/FWHM pR½ / (1-R)
  • where FWHM Full width half maximum, R
    reflectance

49
Cascading of filters
  • Typically Fabry-Perot filters with different FSR
    are cascaded
  • To get rid of unneeded lobes
  • To sharpen response

Source Dutton
50
Tuning of Fabry-Perot filters
  • Can be made tunable by changing mirror spacing
  • Typically done with piezoelectric crystal
    attached to a mirror
  • High voltage (300-500 v)
  • Slow (1 ms)
  • Also can be made by putting liquid crystal
    material in gap
  • RI changes if current passed through
  • 30-40 nm
  • 10 microseconds

51
(No Transcript)
52
Source TecOptics, www.tecOptics.com/etalons/types
.htm
53
FBGs used as filters
  • Because FBGs block light, must be used together
    with circulator to obtain filtering action

Source Dutton
54
Fiber Ring Resonators
  • Use long rings instead of cavity
  • Non-resonant wavelengths passed through
  • Resonant wavelengths caught in cavity and
    dissipate
  • Resonant if ring length integral multiple of
    wavelength
  • Typically very long 1 km
  • Limits usefulness for optical purposes
  • Some type of tuning device required as precise
    lengths down to nm not feasible

55
Fiber Ring Resonators (continued)
Source Dutton
56
Fiber Ring Resonators (continued)
  • Use when extremely small spacing between
    resonances is desired
  • Spectrum analyzers
  • Sensors
  • Narrowband filters
  • Ring lasers
  • Interferometers
  • Gyroscopes

57
Dielectric filters
  • Same principle as Fabry-Perot filters
  • Use layer of dielectric material to form mirrors

58
Dielectric Filters
  • Thin-film cavities
  • Alternating dielectric thin-film layers with
    different refractive index
  • Multiple reflections cause constructive
    destructive interference
  • Variety of filter shapes and bandwidths (0.1 to
    10 nm)
  • Insertion loss 0.2 to 2 dB, stopband rejection 30
    to 50 dB

0 dB
30 dB
1535 nm
1555 nm
Source Agilent
59
Dielectric filter used as Multiplexor/Demultiplexo
r
  • Deposit a series of dielectric filters of
    different thicknesses instead of just one
  • As light bounces around inside, different
    wavelengths picked off

Source Dutton
60
Switches
  • Device to direct input signal to one of two or
    more output paths
  • In general, the components of switches are called
    optical switching components, and assemblages
    of them to perform real-world functions are
    called switches

61
Simple switching element
  • DOS Digital Optical Switch
  • Y coupler with electrodes added
  • Electrodes can modify RI of waveguide material
    when voltage applied
  • Light follows path of higher RI
  • Low insertion loss of 1 or 2 db

Source Dutton
62
Crossconnect switch element
  • More versatile than previous switch element
  • Two states direct connect (bar) and cross
    connect (cross)
  • Equivalent to 4 simple switches
  • Large space division switches, known as
    crossbar switches in telephony are built using
    them

Source Dutton
63
Realization of optical cross connect switch
element
  • Resonant couplers with electrodes added
  • Normal operation coupling length set so that
    signals cross
  • Voltage applied RI of waveguides changes,
    effectively changing coupling length, so that
    crossover does not occur
  • Main problem difficult to fabricate with
    necessary precision

Source Dutton
64
Modified design for easier fabrication
  • Basic idea Use interference together with
    resonant coupling
  • Mach-Zehnder interferometer (MZI)
  • No voltage applied signal passes though MZI
    section with no effect, second resonant coupler
    just extends first
  • Coupling length set to give crossover
  • Voltage applied destructive interference at port
    3, so light exits at port 2

Source Dutton
65
Nonlinear Optical Loop Mirror (NOLM)
  • Very fast optical switch
  • Clean up pulses
  • Demultiplex a time-division multiplexed stream

Source Dutton
66
Operation of NOLM
  • Utilizes nonlinear Kerr effect
  • Light at high intensity changes RI slightly
  • Group velocity of pulses increases
  • Device set for destructive interference at low
    intensity (no Kerr effect)
  • At high intensity, constructive interference
    occurs
  • Still higher intensity shifts phase more, so
    destructive interference occurs again

Source Dutton
67
Operation of NOLM (continued)
Pulse shaper
Source Dutton
Logic gate
68
Optical Add/Drop Mux (ADM) using Mach-Zehnder
Interferometer
  • Function of ADM
  • Device

Source Dutton
69
Operation of ADM
  • Semiconductor Optical Amplifiers (SOAs) offset by
    precise amount
  • One saturates before other
  • When SOA saturates, carrier density depleted,
    causing drop in RI of cavity
  • Phase shift of signal
  • Bar mode both SOAs off
  • Input on 1 exits on 2, input on 4 exits on 3
  • Pulse on 2 saturates SOAs in turn, causes change
    in coupling length
  • When 1 is saturated, causes cross state to occur
  • Pulse on 1 switched to 3 (dropped)
  • Pulse on 4 switched to 2 (added)

70
Modulatorsfunction
  • Replicate variations in electronic signal to be
    transmitted onto an optical carrier
  • Digital on-off
  • Analog continuous variation

71
Modulatorstypes
  • Electrooptic and Magnetooptic
  • Materials change optical properties in presence
    of E or M field
  • Usually RI
  • Depends on polarization
  • Change phase, so require additional device to
    change amplitude (e.g., MZI)
  • Electro-Absorption effects
  • Variable absorption of light as function of E
    field
  • Accoustic modulators
  • Very high frequency sound within a crystal or
    planar waveguide to refract light

72
Electro-absorption modulators
  • Use reverse-biased p-n junction
  • Light absorbed if it has higher energy than
    bandgap
  • Absorption controlled by electric field across
    junction
  • Only on-off modulation possible
  • Only practical when integrated with laser
  • Rates up to 10 GHz, 1550 nm band

Source Dutton
73
Accoustic modulators
  • Basic idea sound waves propagate as compression
    and rarefaction in a material
  • Changes RI
  • Periodicity of sound wave gt periodicity of RI
  • Generates a diffraction grating
  • Very high frequency 100 MHz
  • Light travels 105 times faster than sound in
    materials, e.g. quartz
  • Sound waves appear to be standing still
  • May be standing still if standing waves used
  • Strength of acoustic wave determines intensity of
    diffracted light
  • Modulation done by varying this strength, not
    wavelength
  • Periodicity of acoustic wave determines angle

74
Acoustic modulators (continued)
  • Periodicity of grating 20 to 200 times
    wavelength of light
  • Possible because speed of light wave far less
    than that of light
  • 6,000 m/s in quartz
  • At 100 MHz, wavelength is 6,000 nm or about 60
    times that of light wave of 1550 nm (free space)
  • Basic relationship
  • sin q nl/2L
  • where l light wavelength,
  • L acoustic wavelength

75
Acoustic modulators (continued)
Wide width of sound wave (Bragg)
Source Dutton
Narrow width of sound wave (Debye-Sears)
76
Acoustic modulatorsadvantages and disadvantages
  • Can handle high power
  • Linear response means analog modulation possible
  • Can modulate different optical wavelengths at
    same time
  • Can also operate as switches
  • Relatively high insertion loss
  • Require high drive current
  • Maximum modulation low relative to other methods
    lt 100 MHz
  • Frequency of optical signal changed by frequency
    of sound waves (Doppler effect)

77
Acoustooptic Tunable Filters (AOTFs)
  • Can switch multiple wavelengths at same time
  • Depends on superimposing acoustic frequencies
  • Operates by changing polarization of optical wave
    when resonant with acoustic wave
  • Beamsplitter prisms or other devices can then
    separate light beams
  • Maximum tuning (switching) speed about 10
    microsecond
  • Best wavelength resolution 1 nm
  • Tuning accomplished by changing frequency of
    acoustic wave

78
AOTFs (continued)
  • Switching function
  • Structure

Source Dutton
79
Acoustooptic Tunable Filters (AOTFs)
TM transmitted, TE reflected
TM reflected, TE transmitted
Source Dutton
80
(No Transcript)
81
Phase modulation
  • Change phase of a signal relative to some
    reference
  • Usually done by slowing it down for a brief time,
    e.g., while it transits a section of waveguide
  • Generally relies on electrooptic effect, in which
    RI changes in response to applied electric field
  • Not used directly for modulation because it
    requires expensive coherent detection systems
  • Used as part of other systems by converting phase
    changes to intensity changes
  • E.g., by interference

82
Modulation using Mach-Zehnder interferometer (MZI)
  • Basic architecture
  • Use as interferometer

Source NMRC/Ireland
Source Dutton
83
(No Transcript)
84
Pockels Cell Modulators
  • For some crystals, change in RI with applied
    electric field depends on orientation of crystal
    (Pockels effect)
  • Result is that light of one polarization
    experiences different RI from light of another
    polarization

Source Dutton
85
Pockels Cell Modulators (continued)
  • Can act as modulator by varying strength of
    applied electric field
  • More or less of beam will pass through second
    polarizing filter
  • Require high voltage (1000 V)
  • Can handle high signal power
  • Are expensive because of precision required in
    construction
  • Can work up to 1 GHz

86
Faraday Effect Modulators
  • Similar to Pockels effect modulators

Source Dutton
87
Repeaters
  • Seek to perform the 3Rs
  • Reamplification
  • Reshaping
  • Reclocking
  • Layer 1 device
  • Have to be designed for wavelength and speed of
    pulses
  • Contrary to book, dont have to know about
    protocol

88
(No Transcript)
89
Modulation methods
  • Modulation making light carry a signal
  • Methods
  • On-off Keying (OOK) most widely used
  • Multi-state coding
  • Analog amplitude modulation (AM)
  • Frequency shift keying (FSK) 2 frequencies (on,
    off)
  • Phase shift keying (PSK) 2 phases (on, off)
  • Polarity shift keying (PolSK) 2 polarizations
    (on, off)

90
OOK
  • Non-return to Zero (NRZ) i.e., signal does not
    return to zero after every pulse
  • Non-return to Zero Inverted (NRZI) transition1,
    no trans0
  • Return to Zero i.e., signal returns to zero after
    every pulse

91
Pulse shape and bandwidth
  • We dont actually use rectangular pulses of light
  • Bandwidth of square or rectangular pulses gt 10x
    frequency
  • Would impose intolerable burden on receiver
  • 1 GHz square pulse would require 10 GHz frequency
    response
  • Would also require wide channel spacing
  • Modulation generally requires bandwidth 2 x
    frequency content
  • Therefore 10 GHz pulse would require 200 GHz
    bandwidth on carrier
  • At 1550 nm, this would require at least 2.4 nm of
    bandwidth
  • Could only cram about 10 wavelengths into
    1520-1560 band

92
Pulse shape and bandwidth (continued)
  • Instead we use Gaussian shaped pulses
  • Time duration of t seconds corresponds to
    bandwidth of about 1/t Hz
  • Much more efficient use of bandwidth

Source John Venables, ASU
93
Comparison of modulation methods
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