UNIT-VI
Optical Link Design
Optical Link Design
When designing a fiber optic system, there are many
factors that must be considered – all of which contribute
to the final goal of ensuring that enough light reaches the
Receiver.
Without the right amount of light, the entire system will
not operate properly.
To determine the correct optical transmitter and receiver
combination is based upon the signal to be transmitted
(Analog, Digital, Audio, Video).
Determine the operating power available (AC, DC etc.)
Determine the special modifications (if any) necessary
(Impedances, Bandwidths, Special Connectors, Special
Fiber Size, etc.)
Calculate the total optical loss (in dB) in the system by
adding the cable loss, splice loss, and connector loss.
Compare the loss figure obtained with the allowable
optical loss budget of the receiver.
Point-to-Point Links
The simplest transmission link is a point-to-point line that
has a transmitter on one end and a receiver on the other.
This type of link places the least demand on optical fiber
technology and thus sets the basis for examining more
complex system architectures.
The design of an optical link involves many interrelated
variables among the fiber, source, and photodetector
operating characteristics, so that the actual link design
and analysis may require several iterations before they
are completed satisfactorily.
Performance and cost constraints are very important
factors in fiber optic communication links, the designer
must carefully choose the components to ensure that the
desired performance level can be maintained over the
expected system lifetime without overspecifying the
component characteristics.
The key system requirements are needed in analyzing a link:
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:
4. 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-fi eld 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
(g) Responsivity
(h) Operating wavelength
(i) Speed
(j) Sensitivity
Two analyses usually are carried out to ensure that the
desired system performance can be met:
1. Link power budget analyses
2. System rise-time budget analyses
The link power budget analysis one first determines the
power margin between the optical transmitter output and
the minimum receiver sensitivity needed to establish a
specified BER.
This margin can then be allocated to connector, splice,
and fiber losses, plus any additional margins required for
other components, possible component degradations,
transmission-line impairments, or temperature effects.
If the choice of components did not allow the desired
transmission distance to be achieved, the components
might have to be changed or amplifiers might have to be
incorporated into the link.
Once the link power budget has been established, the
designer can perform a system rise-time analysis to
ensure that the desired overall system performance has
been met.
System Considerations
In carrying out a link power budget, we first decide at
which wavelength to transmit and then choose
components that operate 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 770-to-910
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 in the O-
band through U-band region.
Having decided on a wavelength, we next interrelate the
system performances of the three major optical link
building blocks; that is, the receiver, transmitter, and
optical fiber.
The designer chooses the characteristics of two of these
elements and then computes those of the third to see if
the system performance requirements are met.
If the components have been over- or underspecified, a
design iteration may be needed.
The procedure we follow here is first to select the
photodetector.
Then choose an optical source and see how far data can
be transmitted over a particular fiber before an amplifier
is needed in the line to boost up the power level of the
optical signal.
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.
In making this choice, the designer also needs to take into
account any design cost and complexity constraints.
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 5V, whereas those of avalanche photodiodes
range from 40V to 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.
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 the laser output is much narrower
than that of an LED.
This is of importance in the 770-to-910-nm region,
where the spectral width of an LED and the dispersion
characteristics of multimode silica fibers limit the data-
rate-distance product to around 150 (Mb/s) . km.
For higher values [up to 2500 (Mb/s) · km], a laser must
be used at these wavelengths.
At wavelengths around 1.3 mm, where signal dispersion is
very low, bit-rate-distance products of at least 1500 (Mb/s)
· km are achievable with LEDs in multimode fibers.
For InGaAsP lasers, distances of 150 m can be achieved at
100-Gb/s rates in OM4 multimode fiber at 1.3 mm.
A single-mode fiber can provide significantly higher rates
over longer distances. Since laser diodes typically couple
from 10 to 15 dB more optical power 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.
Link power budget
The optical power received at the photodetector
depends on the amount of light coupled into the fiber
and the losses occurring in the fiber and at the
connectors and splices.
The link loss budget is derived from the sequential loss
contributions of each element in the link.
Each of these loss elements is expressed in decibels
(dB) as
where Pin and Pout are the optical powers entering and
leaving the loss element.
The loss value corresponding to a particular element
generally is called the insertion loss for that element.
link power margin is normally provided in the analysis
to allow for component aging, temperature fluctuations,
and losses arising from components that might be added
at future dates.
Fig. Optical power loss model for a point-to-point link.
A link margin of 3 to 6 dB is generally used for systems
that are not expected to have additional components
incorporated into the link in the future.
The link loss budget simply considers the total optical
power loss PT that is allowed between the light source
and the photodetector, and allocates this loss to cable
attenuation, connector loss, splice loss, and system
margin.
If PS is the optical power emerging from the end of a
fiber flylead attached to the light source or from a source-
coupled connector, and if PR is the receiver sensitivity,
then PT = PS – PR
= 2lc + αL + system margin
Where,
Ic is the connector loss,
α is the fiber attenuation (dB/km)
L is the transmission distance, and the system margin
is nominally taken as 6 dB.
Assume that the cable of length L has connectors only on
the ends and none in between.
The splice loss is incorporated into the cable loss for
simplicity.
A convenient procedure for calculating the power budget
is to use a tabular or spreadsheet form.
Link Power Budget Example
• Example: [SONET Component/loss Output/ Power margin
parameter sensitivity/loss (dB)
OC-48 (2.5 Gb/s)
link] Laser output 3 dBm
Transmitter: 3dBm APD Sensitivity -32 dBm
@ 1550 nm; @ 2.5 Gb/s
Receiver: InGaAs Allowed loss 3-(-32) dBm 35
APD with -32 dBm
Source connector 1 dB 34
sensitivity @ 2.5 loss
Gb/s;
Jumper+Connect 3+1 dB 30
Fiber: 60 km long or loss
with o.3 dB/km Cable attenuation 18 dB 12
attenuation; jumper
Jumper+Connect 3+1 dB 8
cable loss 3 dB each, or loss
connector loss of 1
Receiver 1 dB 7(final margin)
dB each. Connector loss
Rise time budget
A rise-time budget analysis is a convenient method for
determining the dispersion limitation of an optical fiber
link.
This is particularly useful for digital systems.
In this approach, the total rise time t sys of the link is the
root sum square of the rise times from each contributor t i
to the pulse rise-time degradation:
The four basic elements that may significantly limit system
speed are
The transmitter rise time ttx,
The group-velocity dispersion (GVD) rise time t GVD of
the fiber,
The modal dispersion rise time tmod of the fiber, and
The receiver rise time trx.
Single-mode fibers do not experience modal dispersion, so
in these fibers the rise time is related only to GVD.
Generally, the total transition-time degradation of a digital
link should not exceed 70 percent of an NRZ (non-return-to-
zero) bit period or 35 percent of a bit period for RZ (return-
to-zero) data, where one bit period is defi ned as the
reciprocal of the data rate.
The rise times of transmitters and receivers are generally
known to the designer. The transmitter rise time is
attributable primarily to the light source and its drive
circuitry.
The receiver rise time results from the photodetector
response and the 3-dB electrical bandwidth of the receiver
front end.
The response of the receiver front end can be modeled by
a first-order lowpass filter having a step response
g(t) = [1 - exp (-2π Bet)]u(t)
Where
Be is the 3-dB electrical bandwidth of the receiver and
u(t) is the unit step function which is 1 for t ≥ 0
The rise time trx of the receiver is usually defined as the time
interval between g(t)=0.1 and g(t) = 0.9.
This is known as the 10- to 90-percent rise time.
If Be is given in megahertz, then the receiver front-end rise
time in nanoseconds is
The fiber rise time tGVD resulting from GVD over a length L
can be
Where σλ is the half-power spectral width of the source, and
the D is dispersion.
The bandwidth BM in a link of length L can be expressed to
a reasonable approximation by the empirical relation
Where the parameter q ranges between 0.5 and 1, and B0 is
the bandwidth of a 1-km length of cable.
A value of q = 0.5 indicates that a steady-state modal
equilibrium has been reached, whereas q = 1 indicates little
mode mixing.
Based on field experience, a reasonable estimate is q = 0.7.
Total system rise time of
Where all the times are given in nanoseconds,
σλ is the half-power spectral width of the source, and the
dispersion D [expressed in ns/(nm · km)]
Transmission Distance for MM-Fiber
NRZ signaling, source/detector: 800-900 nm LED/pin
or AlGaAs laser/APD combinations.
BER=10-9; LED output= -13 dBm; fiber loss=3.5
dB/km; fiber bandwidth 800 MHz.km; q=0.7; 1-dB
connector/coupling loss at each end;
6dB system margin, material dispersion is 0.07
ns/(km.nm); spectral width for LED=50 nm.
Laser at 850 nm spectral width=1 nm; laser ouput=0
dBm, Laser system margin=8 dB;
Transmission Distance for a SM Fiber
Communication at 1550 nm, no modal dispersion, Source: Laser;
Receiver: InGaAs-APD (11.5 log B -71.0 dBm) and PIN (11.5log
B-60.5 dBm); Fiber loss =0.3 dB/km; D=2.5 ps/(km.nm): laser
spectral width 1 and 3.5 nm; laser output 0dBm, laser system
margin=8 dB;
Line Coding
In designing an optical fiber link, an important
consideration is the format of the transmitted optical
signal. This is of importance because, in any practical
digital optical fiber data link, the decision circuitry in the
receiver must be able to extract precise timing
information from the incoming optical signal.
In addition, since errors resulting from channel noise and
distortion mechanisms can occur in the signal-detection
process, it may be desirable for the optical signal to have
an inherent error detecting capability.
These features can be incorporated into the data stream by
restructuring (or encoding) the signal. This is generally
done by introducing extra bits into the raw data stream.
Signal encoding uses a set of rules arranging the signal
symbols in a particular pattern. This process is called
channel or line coding.
One of the principal functions of a line code is to introduce
redundancy into the data stream for the purpose of
minimizing errors that result from channel interference
effects. Depending on the amount of redundancy
introduced, any degree of error-free transmission of digital
data can be achieved, provided that the data rate that
includes this redundancy is less than the channel capacity.
The three basic types of two-level binary line codes that can
be used for optical fibres transmission links are:
The non-return-to-zero (NRZ) format
The return-to-zero (RZ) format
The phase-encoded (PE) format
NRZ Codes:
A number of NRZ codes are widely used, and their
bandwidths serve as references for all other code groups.
The simplest NRZ code is NRZ level (or NRZ-L), shown in
figure,
For a serial data stream, an on-off (or unipolar) signal
represents a 1 by a pulse of current or light filling an
entire bit period, whereas for a 0 no pulse is transmitted.
These codes are simple to generate and decode, but they
posses no inherent error-monitoring or correcting
capabilities and they have no self-clocking (timing)
features.
The minimum bandwidth is needed with NRZ coding,
but the average power input to the receiver is dependent
on the data pattern.
For example, the high level of received power occurring
in a long string of consecutive 1 bits can result in a
baseline wander effect, as shown in figure.
This effect results from the accumulation of pulse tails
that arise from the low-frequency characteristics of the
ac-coupling filter in the receiver. If the receiver recovery
to the original threshold is slow after the long string of 1
bits has ended, an error may occur if the next 1 bit has a
low amplitude.
In addition, a long string of NRZ ones or zeros contains
no timing information, since there are no level
transitions.
Thus, unless the timing clocks in the system are
extremely stable, a long string of N identical bits could
be misinterpreted as either N-1 or N+1 bits.
RZ Codes
If an adequate bandwidth margin exists, each data bit can
be encoded as two optical line code bits. This is the basis
of RZ codes. In these codes, a signal level transition
occurs during either some or all of the bit periods to
provide timing information.
A variety of RZ code type exist. In the unipolar RZ data,
a 1 bit is represented by a half-period optical pulse that
can occur in either the first or second half of the bit
period. A 0 is represented by no signal during the bit
period.
The baseband (NRZ-L) with the RZ data is shown in
figure.
A disadvantage of the unipolar RZ format is that long
strings of 0 bits can cause loss of timing synchronisation.
A common data format not having this limitation is the
biphase or optical Manchester code shown in figure.
Note that this is a unipolar code which is in contrast
to the conventional bipolar Manchester code used in
wire lines. The optical Manchester signal is obtained
by direct modul-2 addition of the baseband (NRZ-L)
signal and clock signal.
In this code, there is a transition at the centre of each bit
interval. A negative-going transition indicates 1-bit,
whereas a positive-going transition means a 0 bit was
sent. Since it is an RZ-type code, it requires twice the
bandwidth of a NRZ code.
Block Codes
An efficient category of redundant binary codes is the
mBnB block code class. In this class of codes, blocks of
m binary bits are converted to longer blocks of n > m
binary bits.
These new blocks are then transmitted in NRZ or RZ
format. As a result of the additional redundant bits, the
increase in bandwidth using this scheme is given by the
ratio n/m.
The mBnB block codes provide adequate timing and
error- monitoring information, and they do not have
baseline wander problems, since long strings of ones and
zeros are limited.
A convenient concept used for block codes is the
accumulated or running disparity, which is the
cumulative difference between the numbers of 1 and 0
bits.
The key factors in selecting a particular block code are
low disparity and a limit in the disparity variation. A low
disparity allows the dc component of the signal to be
cancelled. A bound on the accumulated disparity avoids
the low-frequency spectral content of the signal and
facilitates error monitoring by detecting the disparity
overflow.
Generally, one chooses codes that have an even n value,
since for odd values of n there are no coded words with
zero disparity.
A comparison of several mBnB codes is given in table
above. The following parameterare shown in this table:
• The ratio n/m, which gives the bandwidth increase.
• The longest number Nmax of consecutive identical
symbols
• The bounds on the accumulated disparity D.
• The percentage W of n-bit words that are not used.
Suitable codes for high data rates are the 3B4B, 4B5B,
5B6B, and 8B10B codes
