Design of Optical Digital
Transmission Systems
Xavier Fernando
Ryerson University
Point-to-Point Links
Key system requirements needed to analyze optical fiber links:
1. The desired (or possible) transmission distance
2. The data rate or channel bandwidth
3. The desired bit-error rate (BER)
LED or laser
(a) Emission wavelength
(b) Spectral line width
(c) Output power
(d) Effective radiating area
(e) Emission pattern
MMF or SMF
(a) Core size
(b) Core index profile
(c) BW or dispersion
(d) Attenuation
(e) NA or MFD
pin or APD
(a) Responsivity
(b) Operating λ
(c) Speed
(d) Sensitivity
Selecting the Fiber
Bit rate and distance are the major factors
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
Selecting the Optical Source
• Emission wavelength depends on acceptable
attenuation and dispersion
• Spectral line width depends on acceptable …………
dispersion (LED  wide, LASER  narrow)
• Output power in to the fiber (LED  low, LASER 
high)
• Stability, reliability and cost
• Driving circuit considerations
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)
Typical bit rates at different
wavelengths
Wavelength
LED Systems
LASER Systems.
800-900 nm
150 Mb/s.km
(Typically
Multimode Fiber)
2500 Mb/s.km
1300 nm (Lowest 1500 Mb/s.km
dispersion)
25 Gb/s.km
(InGaAsP Laser)
1550 nm (Lowest 1200 Mb/s.km
Attenuation)
Up to 500
Gb/s.km
(Best demo)
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
Optical power-loss model
PT  Ps  PR  mlc  nlsp   f L  System Margin
PT : Total loss; Ps : Source power; PR : Rx sensitivity
m connectors; n splices
Try Ex: 8.1
Power Budget Example
Specify a 20-Mb/s data rate and a BER = 10–9.
With a Si pin photodiode at 850 nm, the required receiver input signal is –42 dBm.
Select a GaAlAs LED that couples 50 mW into a 50-μm core diameter fiber flylead.
Assume a 1-dB loss occurs at each cable interface and a 6-dB system margin.
The possible transmission distance L = 6 km can be found from
PT = PS – PR = 29 dB = 2lc + αL + system margin = 2(1 dB) + αL + 6 dB
• The link power budget can be represented graphically (see the right-hand figure).
•
•
•
•
•
Example: Spreadsheet Power Budget
Rise-Time Budget (1)
• A rise-time budget analysis determines the dispersion
limitation of an optical fiber link.
• The total rise time tsys is the root sum square of the rise times
from each contributor ti to the pulse rise-time degradation:
–
–
–
–
The transmitter rise time ttx
The group-velocity dispersion (GVD) rise time tGVD of the fiber
The modal dispersion rise time tmod of the fiber
The receiver rise time trx
Here Be and B0
are given in
MHz, so all
times are in ns.
Rise-Time Budget (2)
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Short-Wavelength Band
Attenuation and dispersion limits on the transmission distance vs. data rate for a
770–910-nm LED/pin combination.
• The BER was 10–9 ; the fiber-coupled power was –13 dBm up to 200 Mb/s.
• The attenuation limit curve was derived by using a fiber loss of 3.5 dB/km
• The receiver sensitivities shown in the left figure (8.3)
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Attenuation-Limited Distances for Two Single-Mode Links
1.
2.
3.
4.
5.
6.
A DFB laser that has a fiber-coupled output of 0 dBm at 1550 nm.
At 1550 nm the single-mode fiber has a 0.20-dB/km attenuation.
The receiver has a load resistor RL = 200 Ω and T = 300°K.
At a 10–12 BER a value of Q = 7 is needed.
The InGaAs pin and APD photodiodes have a responsivity of 0.95 A/W.
The gain of the APD is M = 10 and the noise figure F(M ) = 5 dB.
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Power Penalties
•
•
•
•
When any signal impairments are present, a lower optical power level arrives at the
receiver compared to the ideal reception case.
This lower power results in a reduced SNR, which leads to a higher BER.
The ratio of the reduced received signal power to the ideal received power is the
power penalty for that effect and is expressed in decibels.
If Pideal and Pimpair are the received optical powers for the ideal and impaired cases,
respectively, then the power penalty PPx in decibels for impairment x is
• In some cases, increasing the received optical power can reduce the
power penalty. For other cases (some nonlinear effects) increasing the
received power level will have no effect on the power penalty.
• Power penalties may be due to chromatic dispersions and polarizationmode dispersion, modal (speckle) noise, mode-partition noise, the
extinction ratio, chirp, timing jitter, reflection noise, and nonlinear
effects arising from high optical power level in a fiber link.
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Chromatic Dispersion Penalty
• Chromatic dispersion arises since each wavelength has a slightly different
velocity, and thus they arrive at different times at the fiber end.
• Therefore, the range of arrival times at the fiber end of the spectrum of
wavelengths will lead to pulse spreading.
• The accumulated chromatic dispersion increases with distance.
• A basic estimate of what limitation chromatic dispersion imposes on link
performance can be made by specifying that the accumulated dispersion
should be less than a fraction ε of the bit period Tb = 1/B, where B is the bit
rate:
The ITU-T Rec. G.957 for SDH and the Telcordia Generic Requirement
GR-253 for SONET:
• For a 1-dB power penalty the accumulated dispersion should be less
than 0.306 of a bit period (ε ≤ 0.306).
• For a 2-dB power penalty the requirement is ε ≤ 0.491.
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Polarization-Mode Dispersion Penalty
• Polarization-mode dispersion (PMD) arises since light-signal
energy at a given wavelength in a single-mode fiber occupies
two orthogonal polarization states or modes.
• PMD arises because the two fundamental orthogonal
polarization modes travel at slightly different speeds owing to
fiber birefringence.
• This PMD effect cannot be mitigated easily and can be for
links operating at 10 Gb/s and higher.
• To have a power penalty < 1.0 dB, pulse spreading from PMD
must on the average be less than 10% of a bit period Tb:
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Extinction Ratio Power Penalty
• Let P1-ER and P0-ER are the logic 1 and logic 0 power levels, respectively.
• Then the extinction ratio re in a laser is the ratio of P1-ER to the power level
P0-ER, that is, re = P1-ER / P0-ER.
• With a nonzero extinction ratio, the average power is
• When receiver thermal noise dominates, the 1 and 0 noise powers are equal
and independent of the signal level. In this case, letting P0-ER = 0 and P1-ER
= 2Pave, the extinction ratio power penalty becomes,
• Optical transmitters usually have minimum re of 7 to 10 (8.5 to 10 dB).
• The power penalties range from 1.25 to 0.87 dB.
• A minimum re = 18 is needed for an ER power penalty < 0.5 dB.
Chirping Power Penalty
• A single-mode laser may experience dynamic line broadening when the
injection current is directly modulated called “chirp”.
• Chirping can produce significant dispersion when the laser wavelength is
displaced from the zero-dispersion wavelength of the fiber.
• When the effect of laser chirp is small, the eye closure Δ can be
approximated by
• The power penalty for an APD can be estimated from the SNR degradation
(in dB) due to the signal amplitude decrease as, where x is the excess noise
factor of the APD
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ER and Chirping Power
Penalties
Extinction-ratio, chirping,
and total-system power
penalties at 1550 nm for a
4-Gb/s 100-km single-mode
G.652 link having a
dispersion D = 17 ps/(nmkm) and a DFB laser with
an active layer width of
1.75 mm. (From Corvini
and Koch, Ref. 53.)
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Coherent Fiber Links
• We have been looking at Intensity Modulated / Direct
Detection (IM/DD) links so far where only optical power is
considered (on-off modulation).
• The detector detects only the power (square law detection)
• In coherent fiber optic links both the amplitude and phase
of the light wave is considered.
• Receiver should be phase locked to the transmitter. This is
done by having another ‘phase locked laser’ at the receiver
• In coherent links, phase modulations (FM, QAM, QPSK
etc.) are possible. Superior performance. Very high bit
rates.
Coherent Detection
• Coherent detection provides gain to the incoming optical
signal by combining or mixing it with a locally generated
continuous-wave (CW) optical field.
• The result of the mixing is that the dominant noise in the
receiver is the shot noise coming from the local oscillator.
• Thus the receiver can have a shot noise limited sensitivity.
Differential Quadrature Phase-Shift Keying
• Multilevel modulation formats are of interest for 10 and 40 Gb/s speeds
• This modulation format transmits more than one bit per symbol.
• In the DQPSK method, information is encoded by means of the four phase
shifts {0, + π/2, - π /2, π}.
• The set of bit pairs {00, 10, 01, 11} is assigned to individual phase shifts.
– For example, a phase shift of π means that the bit pair 11 was sent.
• Thus DQPSK transmits at a symbol rate of half the aggregate bit rate.
• A DQPSK transmitter typically uses two nested Mach-Zehnder modulators
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BER Comparisons
Comparison of Number of
Required Photons per Bit
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