Download - Optical Digital Transmission Systems
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Optical Digital Transmission Systems
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Overview• In this section we cover point-to-point digital
transmission link design issues (Ch8): – Link power budget calculations – Link rise time calculations
A link should satisfy both these budgets
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Fig. 8-1: Simple point-to-point link
System Requirements
1. Transmission Distance
2. Data Rate for a given BER
This p-p link forms the basis for examining more complex systems
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Selecting the Fiber
Other factors to consider: attenuation (depends on?) and distance-bandwidth product (depends on?) cost of the connectors, splicing etc.
Then decide• Multimode or single mode• Step or graded index fiber
Bit rate and distance are the major factors
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Selecting the Optical Source
• Emission wavelength• Spectral line width
(FWHM) and number of modes
• Output power• Stability• Emission pattern• Effective radiating area
LEDLASER
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Selecting the detector• Type of detector
– APD: High sensitivity but complex, high bias voltage (40V or more) and expensive
– PIN: Simpler, thermally stable, low bias voltage (5V or less) and less expensive
• Responsivity (that depends on the avalanche gain & quantum efficiency)
• Operating wavelength and spectral selectivity• Speed (capacitance) and photosensitive area • Sensitivity (depends on noise and gain)
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Typical bit rates at different wavelengths
Wavelength LED Systems LASER Systems.
800-900 nm (Typically Multimode Fiber)
150 Mb/s.km 2500 Mb/s.km
1300 nm (Lowest dispersion)
1500 Mb/s.km 25 Gb/s.km
(InGaAsP Laser)
1550 nm (Lowest Attenuation)
1200 Mb/s.km Up to 500 Gb/s.km
(Best demo)
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Design Considerations
• Link Power Budget– There is enough power margin in the system to
meet the given BER
• Rise Time Budget– Each element of the link is fast enough to meet
the given bit rate
These two budgets give necessary conditions for satisfactory operation
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Fig. 8-3: Receiver sensitivities Vs bit rate
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Fig. 8-2: Optical power-loss model
ystem MarginT s R c sp fP P P ml nl L S
: Total loss; : Source power; : Rx sensitivity
connectors; splicesT s RP P P
m n Try Ex: 8.1
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Fig. 8-4: Example link-loss budget
Try Ex. 8.2
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Rise Time Budget
• Total rise time depends on:– Transmitter rise time (ttx)
– Group Velocity Dispersion (tGVD)
– Modal dispersion rise time (tmod)
– Receiver rise time (trx)
Total rise time of a digital link should not exceed
70% for a NRZ bit period, and 35% of a RZ bit period
1/ 2
2
1
n
sys ii
t t
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Rise Time…
MHzin bandwidth receiver is
wherens; 350
rxB
/Bt rxrx
txtx Bt /350 ns
Similarly
Assuming both transmitter and receiver as first order low pass filters
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Modal Dispersion Rise Time
Bandwidth BM(L) due to modal dispersion of a link length L is empirically given by,
B0 is the BW per km (MHz-km product) and q ~0.5-1 is the modal equilibrium factor
qoM LBLB /)(
(ns) /440/44.0 0mod BLBt qM
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Group Velocity Dispersion
Where,
D is the dispersion parameter (ns/km/nm) given by eq. (3.57)
σλ is the half power spectral width of the source (nm)
L is the distance in km
LDtGVD ||
2/1
20
2222222 440
BLLDttt
q
rxtxsys
Try examples 8.3 and 8.4
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Fig. 8-5: 800 MHz-km Multimode Fiber at 800 nm, (BER=10-9)
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Parameters for Fig 8-5
Power coupled from LED : -13 dBm
Fiber loss 3.5 dB/km
System Margin 6 dB, couplers 1dB (LED-PIN)
Dmat = 0.07 ns/(nm.km)
LED 50 nm
LASER 1 nm
Bo=800 MHz-km
q = 0.7 (modal)
Power coupled from LASER = 0 dBm
Material dispersion limit with LASER is off the graph
System Margin 8 dB (Laser-APD)
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Fig. 8-6: Single Mode fiber, 1550 nm, D = 2.5 ps/nm.km, 0.3 dB/km, two lasers
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Analog Communication Links
Analog (RF) links are used inAnalog TV and audio services (Legacy)Cable modem servicesSatellite base stations
(AmplifierSpontaneousEmission)
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Multi Channel Systems
Number of RF carriers can be summed and directly modulate the laser
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Multi Channel Systems
• These have the capability to multiplex several RF channels
• Each RF channel is independent, it may carry different type of data (analog video, digital video, digital audio etc.)
• The data could be modulated onto the RF carrier using different techniques (AM, FM, QAM etc.)
• Nonlinearity is the major concern
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Sub Carrier Multiplexing
• Each modulating RF carrier will look like a sub-carrier
• Unmodulated optical signal is the main carrier • Frequency division multiplexed (FDM) multi channel
systems also called as SCM
Frequency
Unmodulated (main) carrier
Sub-carriers
f1
f2
f1
f2
f0
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Link Noise
Modal Noise: When a laser is coupled to a multi mode fiber (MMF) modal noise exists. To avoid this,
• Use LED with MMF
• Use a laser with large number of modes
• Use a MMF with large NA
• Use single mode fiber with laser
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Modal noise at a connection of a SMF
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Mode Partition Noise
• This is the dominant noise in single mode fiber coupled with multimode laser
• Mode partition noise is associated with intensity fluctuations in the longitudinal modes of a laser diode
• Each longitudinal mode has different λ
• The SNR due to MPN can not be improved by increasing the signal power
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Fig. 8-15: Dynamic spectra of a laser
Laser output spectrum vary
with time giving
mode partition noise
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Fig. 8-16: Mode-Partition-Noise BER depends onReceiver BER and System BER
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Interferometric Noise
due to multiple reflections
• Increases RIN• Laser instability• Increases with signal power
• Can be decreased byhaving angled, low back reflection connectorsand isolators
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Fig. 8-17: Chirping & extinction-ratio penalties