Optical Link Design for Digital
Communication Systems
POINT-TO-POINT LINKS
The simplest transmission link is a point-to point line
having a transmitter at one end and a receiver on the
other, as shown below:
Fig: Simple point-to-point link
The design of an optical link involves many interrelated variables such as
the fiber, source, and photodetector operating characteristics, so that the
link design and analysis may require several iterations before they are
working satisfactorily.
POINT-TO-POINT LINKS
The key system requirements needed in analyzing a link are:
1. The desired (or possible) transmission distance
2. The data rate or channel bandwidth
3. The bit error rate (BER)
To fulfill these requirements the designer has a choice of the following
components and their associated characteristics:
1. Multimode or single-mode optical fiber
(a) Core size
(b) Core refractive-index profile
(c) Bandwidth or dispersion
(d) Attenuation
(e) Numerical aperture or mode-field
diameter
2. LED or laser diode optical source
(a) Emission wavelength
(b) Spectral line width
(c) Output power
(d) Effective radiating area
(e) Emission pattern
(f) Number of emitting modes
3. pin or avalanche photodiode
(a) Responsivity (~quantum efficiency)
(b) Operating wavelength
(c) Speed
(d) Sensitivity
System Considerations:
(a)
Wavelength of operation: In carrying out a link power budget, we first
decide at which wavelength to transmit and then choose components
operating in this region. If the distance over which the data are to be
transmitted is not too far, we may decide to operate in the 800- to 900-nm
region. On the other hand, if the transmission distance is relatively long, we
may want to take advantage of the lower attenuation and dispersion that
occurs at wavelengths around 1300 or 1550 nm.
(b)
Source: The system parameters involved in deciding between the use of
an LED and a laser diode are signal dispersion, data rate, transmission
distance, and cost. The spectral width of laser output is much narrower than
that of an LED. Since laser diodes typically couple from 10 to 15 dB more
optical into a fiber than an LED, greater repeaterless transmission
distances are possible with a laser. This advantage and the lower
dispersion capability of laser diodes may be offset by cost constraints. Not
only is a laser diode itself is expensive than an LED, but also the laser
transmitter circuitry is much complex.
System Considerations:
(c)
Photo Detector: In choosing a particular photodetector, we mainly need to
determine the minimum optical power that must fall on the photodetector to
satisfy the bit error rate (BER) requirement at the specified data rate. A pin
photodiode receiver is simpler, more stable with changes in temperature,
and less expensive than an avalanche photodiode receiver. In addition, pin
photodiode bias voltages are normally less than 50 V, whereas those of
avalanche photodiodes are several hundred volts. However, the
advantages of pin photodiodes may be overruled by the increased
sensitivity of the avalanche photodiode if very low optical power levels are
to be detected.
System Considerations:
(d)
Optical fiber: For the optical fiber we have a choice between single-mode
and multi-mode fiber, either of which could have a step- or a graded-index
core. This choice depends on the type of light source used and on the
amount of dispersion that can be tolerated. Light-emitting diodes (LEDs)
tend to be used with multimode fibers (although special LEDs called edgeemitting LEDs can launch sufficient optical power into a single-mode fiber
for transmission at data rates up to 560 Mb/s over several kilometers. The
optical power that can be coupled into a fiber from an LED depends on the
core-cladding index difference , which, in turn, is related to the numerical
aperture of the fiber (for  = 0.01 the numerical aperture NA = 0.21). As 
increases, the fiber-coupled power increases correspondingly. However,
since dispersion also greater with increasing , a tradeoff must be made
between the optical power that can be launched into the fiber and the
maximum tolerable dispersion.
System Considerations:
(d)
Optical fiber (continued): Either a single-mode or a multimode fiber can be
used with a laser diode A single-mode fiber can provide the ultimate bitrate-distance product, with values of 30 (Gb/s).km being achievable. A
disadvantage of single-mode fibers is that the small core size (5 to 16 um in
diameter) makes fiber splicing more difficult and critical than for multimode
fibers having 50 um core diameters.
• Two types of design and analysis procedures
are normally carried out for digital optical
systems:
(I) Link power budget analysis
(power margin calculations between the transmitter and the receiver
considering attenuation, connectors, splices and other losses)
(II) rise-time budget analysis
(rise time calculations and speed of response of system considering
various dispersive effects)
(i) Link power budget analysis
The optical power received by the receiver depends on the power
transmitted and on the various losses occurring over the fiber. The
power received at the output must be sufficiently higher than the
receiver sensitivity (after accounting for safety margin).
(i) Link power budget analysis
The optical power received by the receiver depends on the power
transmitted and on the various losses occurring over the fiber. The
power received at the output must be sufficiently higher than the
receiver sensitivity (after accounting for safety margin).
Ptx = Prx + CL + Ms
where Ptx is the transmitter power, Prx is the sensitivity of the receiver, CL is
the total link loss or channel loss (including fiber splice and connector loss),
and Ms is the system’s safety margin.
Channel Loss may be expressed as :
CL = f L + con + splice
where f is the fiber loss (dB/km), L is the link length, con is the sum of the
losses at all the connectors in the link, and splice is the sum of losses at all
splices in the link.
Design Example:
An engineer plans to design a 2.5-Gbps SONET OC-48 (or equivalently, an
SDH STM-16 link) link over a 30-km path length. For the 30-km cable span,
there is a splice with a loss of 0.1 dB every 5 Km (a total of 5 splices). The
engineer selects a laser diode that can launch -2 dBm of optical power into
the fiber and an InGaAs avalanche photodiode (APD) with a -32 dBm
sensitivity at 2.5 Gbps. A short jumper cable is needed at each end, assume
that each cable introduces a loss of 1.5 dB. In addition, there is a 0.6 dB
connector loss at each fiber joint (total of 4 connectors in all including two
for jumper cables and two for fiber ends). The problem of the engineer is to
determine whether he can operate the link at 1310 nm, or to use more costly
1550nm equipment. Find out.
(Take fiber attenuation as 0.6 dB/km and 0.3 dB/km at 1310 nm and 1550 nm
respectively.)
Solution:
Spreadsheet for calculating the 1310-nm SONET link power budget
Component/Loss parameter
Output/sensitivity/loss
Coupled laser diode output
-2 dBm
APD sensitivity at 2.5 Gbps
-32 dBm
Allowed loss (-2 -(-32))
Power margin, dB
30.0
Jumper cable loss (2 x 1.5 dB)
-3 dB
27.0
Splice loss (5 x 0.1 dB)
-0.5 dB
26.5
Connector loss (4 x 0.6 dB)
-2.4 dB
24.1
Cable Attenuation (30 km)
-18 dB
6.1 (final margin)
Since the final margin is adequate (keeping in view the safety
margin which is usually 3 dB), it is feasible to operate this link at
1310 nm from the point of view of power calculations (rise-time
budget needs to be also calculated to ascertain final suitability).
(ii) Rise-time budget analysis
Rise time budget analysis deals with calculating the temporal response of the
various system components.
The system design considerations must take into account the temporal response
of the system components, because the temporal response is directly related to
the pulse dispersion and hence the bit rate of the optical fiber channel.
The finite bandwidth of the optical system may result in overlapping of the received
pulses or ISI, giving a reduction in sensitivity at the optical receiver. Therefore,
either a worse BER must be tolerated, or the ISI must be compensated by
equalization within the receiver.
What is rise time?
It is used to calculate the dispersion limitation of an optical fiber link. This
is particularly useful in digital systems at higher bit rates. The rise time of
a linear system tr is the time in which output changes from 10% to
90% of the maximum output value when the input is a step function.
Transfer function of RC circuit
Therefore the 3 dB bandwidth for the circuit is:
f 
1
2RC
Combining this equation with tr, the 3 dB bandwidth for the circuit is:
2.2 0.35
tr 

2B f
In a fiber optic communication system, the three building blocks (i.e. the
transmitter, the fiber (channel) and the receiver) each has its own
response time (or rise time) associated with it.
The total rise time of the system tsys is given as:
tsys = [ ttx2 + tf2 + trx2 ] 1/2
The rise time of the optical fiber includes contributions from intermodal
dispersion (tintermodal) and intramodal dispersion (tintramodal) through the
relation:
tf = [ tintermodal2 + tintramodal2 ] 1/2
For a low-pass system like an fiber optic link (analogous to an RC circuit),
the total rise time and the bandwidth f are related by the standard
expression:
tsys = 0.35 / f
For an RZ format, f = B and for NRZ format, f = B/2, where B=bit rate.
Therefore, for digital systems, tsys
should be below its maximum value, t sys
i.e.
 0.35
 B for the RZ format

 0.70 for the NRZ format
 B
Thus the upper limit on tsys should be less than 35% of the bit interval for an RZ
pulse format and less than 70% of the bit interval for an NRZ pulse format.
t sys
 0.35
 B for the RZ format

 0.70 for the NRZ format
 B
Design Example:
The
we get
Let us calculate the total rise time tsys.
Power Penalties
(Advanced issue in Power Budget Analysis)
•
•
More sophisticated power budget calculations will include power
penalties.
A power penalty is defined as the increase in receiver power needed to
eliminate the effect of some undesirable system noise or distortion
Typically, power penalties may result from:
•Dispersion.
•Dependent on bit rate and fibre dispersion,
•Typical dispersion penalty is 1.5 dB
•Reflection from passive components, such as connectors.
•Crosstalk in couplers.
•Modal noise in the fibre.
•Polarization sensitivity.
•Signal distortion at the transmitter (analog systems only).
Dispersion Penalty
Defined as:
The increase in the receiver input power needed to
eliminate the degradation in the BER caused by fibre
dispersion
• Typical value is about 1.5 dB.
• Several analytic rules exist:
• Low pass filter approximation rule
• Allowable pulse broadening (Bellcore) rule
Dispersion Penalty
• Defined as the increase in the receiver
input power needed to eliminate the
degradation caused by dispersion
• Defined at agreed Bit Error Probability,
typically 1 x 10-9
• In the sample shown the receiver
power levels required at 1 x 10-9 with &
without dispersion are -35.2 dBm &
-33.1 dBm respectively
• The dispersion penalty is thus 2.1 dB
Low pass filter approximation rule for the Dispersion Penalty
Simple analytic rule of thumb for calculating the dispersion penalty Pd
• Based on two assumptions:
• that dispersion can be approximated by a low pass filter response.
• the data is the dotting 10101010 pattern.
B is the bit rate in bits/sec and Dt is the total r.m.s impulse spread caused by
dispersion over the fibre.
To keep Pd < 1.5 dB, the B.Dt product must be less than 0.25 approximately.
Calculating the Dispersion Penalty
(Low pass filter approx rule)
Total Chromatic Dispersion, Dt = Dc x S x L
where:
Dc is the dispersion coefficent for the fibre (ps/nm.km)
S is transmitter source spectral width (nm)
L is the total fibre span (km)
􀁹 Assuming single-mode fibre so there is no modal dispersion
􀁹 Does not include polarization mode dispersion
􀁹 Typically the dispersion coefficent will be known
􀁹 For Eg., ITU-T Rec.G.652 for single-mode fibres at 1550 nm (typical):
􀁹 Attenuation < 0.25 dB/km
􀁹 Dispersion coefficent is 18 ps/(nm.km)
􀁹 Let’s take 50 km of single-mode fibre meeting ITU G.652
􀁹 Let’s take 1550 nm DFB laser with a spectral width of 0.1 nm
Calculating the Dispersion Penalty
(Low pass filter approx rule)
Total Dispersion, Dt = Dc x S x L
= 18 ps/nm.km x 0.1 nm x 50 km
= 90 ps total dispersion
System operating at 2.5 Gbits/sec
Total Dispersion, Dt = 90 ps
Dispersion Penalty:
The Penalty is thus = 1.2 dB
Calculating the Dispersion Penalty graphically
(Low pass filter approx rule)
Dispersion Penalty for STM-1 (155.52 Mbps)
Dispersion Penalty for STM-4 (622.08 Mbps)
Dispersion Penalty for STM-16 (2488.32 Mbps)
Calculating the Dispersion Penalty
(Low pass filter approx rule):
EXAMPLE:
􀀹 An optical fibre system operates at 1550 nm at a bit rate of 622 Mbits/sec over a
distance of 71 km
􀀹 Fibre with a worst case loss of 0.25 dB/km is available.
􀀹 The average distance between splices is approximately 1 km.
􀀹 There are two connectors and the worst case loss per connector is 0.4 dB.
􀀹 The worst case fusion splice loss is 0.07 dB
􀀹 The receiver sensitivity is -28 dBm and the transmitter output power is +1 dBm
􀀹 The source spectral width is 0.12 nm and the fibre dispersion meets ITU
recommendations at 1550 nm
􀀹Determine worst case power margin, taking account of a power penalty
(Use the Low Pass Filter Approximation rule)
Calculating the Dispersion Penalty
(Low pass filter approx rule):
EXAMPLE:
Step 1: Find the Dispersion Penalty
􀀹 71 km of singlemode fibre meeting ITU G.652
􀀹 1550 nm DFB laser with a spectral width of 0.12 nm
􀀹 System operating at 622 Mbits/sec
Total Dispersion = 153.6 ps
Dispersion Penalty = 0.2 dB
Calculating the Dispersion Penalty
(Low pass filter approx rule):
EXAMPLE:
Step 2: Develop the Power Budget and find the power margin
System Performance with
varying bit rate
Receiver sensitivities vs bit rate
The figure shows receiver sensitivities as a function of bit rate. The Si pin, Si
APD, and InGaAs pin curves are for a 10-9 BER. The InGaAs APD curve is for
a 10-11 BER.
Transmission distance vs data rate
First Window Transmission Distance
Transmission-distance limits as a function of data rate for an 800-MHz.Km
fiber, a combination of an 800-nm LED source with a Si pin photodiode, and
an 800-nm laser diode with a Si APD.
Transmission distance vs data rate
First Window Transmission Distance
Figure shows the attenuation and dispersion limitation on the repeater-less
transmission distance as a function of data rate for the short-wavelength
(770 to 900-nm) LED/pin combination. The BER was taken as 10-9 for all
data rates. The fiber-coupled LED output power was assumed to be a
constant -13dBm for all data rates up to 200 Mb/s. The attenuation limit
curve was then derived by using a fiber loss of 3.5 dB/km and the receiver
sensitivities shown in earlier Fig. Since the minimum optical power required
at the receiver for a given BER becomes higher for increasing data rates,
the attenuation limit slopes downward to the right.
Transmission distance vs data rate
First Window Transmission Distance
The dispersion limit depends on material and modal dispersion. Material dispersion
at 800 nm is taken as 0.07 ns/(nm.km) or 3.5 ns/km for an LED with a 50-nm
spectral width. The curve shown is the material dispersion limit in the absence of
modal dispersion. This limit was taken to be the distance at which tmat is 70 percent
of a bit period. The modal dispersion limit was taken to be the distance at which tmod
is 70 percent of a bit period. The achievable repeaterless transmission distances
are those that fall below the attenuation limit curve and to the left of the dispersion
line, as indicated by the hatched area. The transmission distance is attenuationlimited up to about 40 Mb/s, after which it becomes material-dispersion-limited.
Transmission distance vs data rate
First Window Transmission Distance
Greater transmission distances are possible when a laser diode is used in
conjunction with an avalanche photodiode. Let us consider an AlGaAs laser
emitting at 850 nm with a spectral width of 1 nm which couples 0 dBm (1 mW)
into a fiber flylead. The receiver uses an APD with a sensitivity depicted in Fig.
depicted earlier. In this case the material-dispersion-limited curve lies off the
graph to the right of the modal-dispersion-limit curve, and the attenuation limit
(with an 8-dB system margin) is as shown in Fig. above. The achievable
transmission distances now include those indicated by the shaded area.
Transmission distance vs data rate
Third Window Transmission Distance
NRZ
NRZ
Transmission-distance limits as a function of data rate for 1550-nm laser
diode, an InGaAs APD, and a single-mode fiber with D = 2.5 ps/(nm . km)
and a 0.3-dB/km attenuation.
Transmission distance vs data rate
Third Window Transmission Distance
NRZ
NRZ
For the third window (1550 nm), let us examine a single-mode link operating
at 1550 nm. In this case the dispersion in the fiber is due only to GVD
effects, since there is no modal dispersion. We take the dispersion to be D =
2.5 ps/(nm . km) and the attenuation to be 0.30 dB/km at 1550 nm. For the
source we first choose a laser which couples 0 dBm of optical power into the
fiber and which has a large spectral width ( = 3.5 nm). Then we select a
laser with  = 1 nm as a second example. The receiver can use either an
InGaAs avalanche photodiode (APD) or a InGaAs pin diode.
Transmission distance vs data rate
Third Window Transmission Distance
The attenuation-limited transmission distances for these two photodiodes
are shown in Fig. with the inclusion of an 8-dB system margin.
For the dispersion limit we examine two cases. First, for the ( = 3.5 nm)
case, Fig. shows limit for NRZ data where the product DL is equal to 70
percent of the bit period. Second, for RZ data the product DL is equal to
35 percent of the bit period. These curves are for the ideal case. In reality
various noise effects leading to laser instabilities coupled with chromatic
dispersion in the fiber can decrease the dispersion-limited distance.
• Assignments
• Reference Books (pl see annexure)
• Quiz (expected next week)