ofc systems design considerations
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OFC SYSTEMS: Design Considerations
BC Choudhary, Professor
NITTTR, Sector 26, Chandigarh.
Receiver
TransmitterElectrical to
Optical
Conversion
Optical to
Electrical
Conversion
Coupler
Coupler
Optical Fiber
OFC point-to-point Link
System Measurements & Design Considerations
Design & installation of an OFC system require measurement
techniques for verifying the operational characteristics of the
constituent components.
Of particular importance are accurate & precise measurements
of optical fiber cannot be readily replaced once it has been
installed.
Two groups of people interested in fiber measurements are:
Manufacturers- concerned with the material composition and
fabrication effects on fiber properties
System Engineers- must have sufficient data on the fiber to
perform meaningful design calculations and to evaluate
systems during installation and operation.
Systems Communication Requirements
Mainly Two Parameters of concern
Link Length
Repeater less distance (50km, 100km, 150km)
Maximum data transmission rate (Mbps, Gbps)
These requirements will decide the type of input
data, transmitter (launch power, modulation), optical
fiber cable, receiver(sensitivity) etc.
Input
Signal
Coder or
Converter
Light
SourceSource-to-Fiber
Interface
Fiber-optic Cable
Transmitter component serves two functions.
Optical Transmitter
Must be a source of the light coupled
into the fiber optic cable.
Must modulate this light so as to
represent the binary data that it is
receiving from the Source.
ILDs or LEDs?
LEDs : LED's have advantages over LD's because they
have
• Higher reliability
• Better linearity
• Lower cost
ILDs : LDs have advantages over LED's in
the following ways.
• Can be modulated at very high speeds.
• They produce greater optical power.
• They have higher coupling efficiency to
the fiber
How much light can be coupled into the core through the
external acceptance angle?
How much attenuation will a light ray experience in
propagating down the core?
How much time dispersion will light rays representing the
same input pulse experience in propagating down the
core?
Fiber Optic Cable
Consideration Parameters
Fiber Optic Cable can be one of two types
Multi-mode or Single-mode.
These provide different performance with respect
to both attenuation and time dispersion.
Glass fiber optic cable has the lowest attenuation and comes at the
highest cost.
Plastic fiber optic cable has the highest attenuation, but comes at
the lowest cost.
Optical Receiver
Receiver component serves two functions.
Detect the light coupled out of the fiber optic
cable then convert the light into an electrical
signal.
Demodulate the light to determine the identity
of the binary data.
Optical to
Electrical
ConversionCoupler
Optical Fiber
Receiver
There are two types of photodiode structures;
• Positive Intrinsic Negative (PIN) and
• Avalanche Photo Diode (APD).
Detectors
In most premises applications the PIN is the preferred element in the
Receiver. This is mainly due to fact that it can be operated from a
standard power supply, typically between 5 and 15 V.
APD devices have much better sensitivity. In fact it has 5 to 10 dB more
sensitivity. They also have twice the bandwidth. However, they cannot
be used on a 5V printed circuit board. They also require a stable power
supply. This makes cost higher.
APD devices are usually used in long haul communications links.
Fiber Connectors
The connector must direct light and collect light. It must also
be easily attached and detached from equipment. This is a
key point. The connector is disconnectable.
FC, FC/PC, SC, SMA, ST, Biconic, D4,
Commonly Used Connectors
Fiber Parameters
Fiber parameters of interest are:
• Multimode fibers: core & cladding diameters,
numerical aperture, refractive index profile/difference,
fiber attenuation and dispersion
• Single mode fibers: Effective cut-off wavelength, mode
field diameter, fiber attenuation and dispersion.
Fiber manufacturers supply the values of these parameters.
Fiber geometry, refractive index profile, NA, cutoff
wavelength, MFD are not expected to change during
cable installation and operation.
Once these parameters are known, there is no need to
remeasure these.
Attenuation and Dispersion of a fiber can change during
fiber cabling and cable installation
SMFs : Chromatic and Polarization Mode Dispersions are important
factors that limit the bandwidth-distance product.
MMFs : Modal dispersion effects important to be examined
In addition to optical fiber parameters, system engineers are interested in
knowing the characteristics of passive splitters, connectors, and
couplers and those of electro-optic components such as sources,
photodetectors and optical amplifiers.
Furthermore, when a link is being installed and tested, the operational
parameters of interest include bit-error-rate, timing jitter and signal-to-
noise ratio.
During actual operation, measurements are needed for maintenance and
monitoring functions to determine factors such as fault locations in fibers
and status of remotely located optical amplifiers.
Typical WDM link & Performance Measurement
Parameters
Performance-measurement parameters of users interest
Designing A Fiber Optic Link
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.
Step-by-step procedure to be followed while designing any
system
• Determine the correct optical transmitter and receiver
combination based upon the signal to be transmitted (Analog,
Digital, Audio, Video, RS-232, etc.)
• 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. These parameters
should be available from the manufacturer of the electronics and
fiber.
After performing the above calculations, if it is discovered that
the fiber bandwidth is inadequate for transmitting the required
signal the necessary distance, it will be necessary either select a
different transmitter/ receiver (wavelength) combination, or
consider the use of a lower loss premium fiber.
• Compare the loss figure obtained with the allowable optical loss
budget of the receiver. Be certain to add a safely margin factor of
at least 3 dB to the entire system.
• Check that the fiber bandwidth is adequate to pass the signal
desired.
BUDGET CALCULATIONS
Two analyses are usually carried out to ensure that the
desired system performance can be met:
Link Power Budget
Rise-time budget
Link Power Budget :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 margin required for possible component degradation,
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.
If PS is the optical power emerging from the end of the fiber
attached to the light source, and PR is the receiver sensitivity, then
PT = PS - PR
= 2 lc + f L + system margin
where lc is the connector loss, f is the fiber attenuation in (dB/km) and L is the transmission length. System margin is normally taken 6dB for LED and 8 dB for ILD.
A power budget example
• Data Rate of 50 Mbps. BER of 10-9.
• Link length of 5 km (premises distances).
• Multi-mode, SI, glass fiber optic cable 62.5/125m
• Transmitter LED at 850 nm, 3dBm, 5dB coupling loss.
• Fiber cable-device coupling loss 1dB each
• Receiver PIN with sensitivity of -40 dBm at 50 Mbps.
• Fiber optic cable has 1 splice.
Link Performance Analysis
LINK ELEMENT VALUE COMMENTS
Transmitter LED output power 3 dBm Specified value by vendor
Source coupling loss -5 dB Accounts for reflections, area mismatch etc.
Transmitter to fiber optic cable
connector loss-1 dB
Transmitter to fiber optic cable with ST connector.
Loss accounts for misalignment
Splice loss -0.25 dB Mechanical splice
Fiber Optic Cable Attenuation -20 dB Line 2 of Table 2-1 applied to 5 km
Fiber optic cable to receiver
connector loss-1 dB
Fiber optic cable to Receiver with ST connector. Loss
Accounts for misalignment
Optical Power Delivered at
Receiver-24.25 dB
Receiver Sensitivity -40 dBm Specified in link design. Consistent with Figure 2-14
LOSS MARGIN 15.75 dB
Power Budget for a fiber optic data link
• Clearly, the optical power at the Receiver is greater than
that required by the sensitivity of the PIN to give the
required BER.
• Important to note is the entry termed Loss Margin? This
specifies the amount by which the received optical power
exceeds the required sensitivity.
• Loss margin is 15.75 dB. Good design practice requires it to
be at least 10 dB. Why?
Because no matter how careful the power budget is put
together, entries are always forgotten, are too optimistic
or vendor specifications are not accurate.
Budget Outcome
Rise-time budget : Once the link power budget has been
established, the designer can perform a system rise time analysis
(dispersion limitations) to ensure that the desired overall system
performance has been met.
Four basic elements that may significantly limit system speed are
• Transmitter rise time,
• Group velocity dispersion (GVD) rise time of the fiber,
• Modal dispersion rise time of fiber and
• Receiver rise time.
Generally, the total transition-time degradation of a digital link should not
exceed 70% of an NRZ bit period or 35% of a bit period for RZ data.
2
1
N
1i
2
isys tt
The total rise time „tsys‟ of the link is the root mean square of the rise
times from each contributor ti to the pulse rise-time degradation
Testing System Performance
z=0 z=L
Dispersion
z=0 z=L
Attenuation
Attenuation & Dispersion degradation as a function of distance
Attenuation (or Transmission loss): determines the maximum
repeater less separation between a transmitter and receiver.
Measured through : Loss in dB or Signal-to-noise ratio
(SNR)
Dispersion : limit the information – carrying capacity of a fiber
i.e. Bandwidth
Measured in terms of Q-factor or Bit-error-rate (BER); ITU
recommended BER 10-12
Characteristics of Primary Importance :
Optical Test Equipments
Basic test equipment for carrying measurements on optical
fiber components and system include
Optical power meters, Continuity testers, Visual fault locators,
Talk sets, Spectrum analysers, OTDRs and BER-Testers.
• These comes with variety of capabilities, with sizes ranging from
portable, handheld units for field use to sophisticated briefcase
sized instruments for laboratory applications.
• Most of these units has reached a high degree of sophistication
with automated microprocessor-controlled test features and
computer-interface capabilities
Power Meters & Talk Sets
Continuity Testers & Visual Fault Locators
Optical Spectrum Analyzers (OSA)
BER TESTERS
Bit-Error-Rate (BER) Measurements
Performance of any communication system can be
evaluated by one of the following methods:
Eye Diagrams / Patterns.
Histogram Generation
Bit Error Rate Measurements.
Most significant performance parameter in any digital
communications system.
Indeed, it is often accepted as the primary performance figure of merit for a
communication system.
For many applications the maximum specified BER is 10-9 implying that
only one error in 109 received bits is tolerated.
For telecommunication applications the specified maximum BER falls in
the range 10-9 to 10-12 .
Bit Error Rate (BER)
t
e
N
NBER It is simply the probability that an error
will occur in a given bit period.
• Defined as the ratio of the number of errors in a given time
interval (Ne) to the number of bits in that time interval (Nt).
BER Estimates
Many other factors besides SNR degrade the BER and in
their presence the received SNR must be increased to yield
the desired BER.
The increase necessary to completely offset the degradation caused by
a given mechanism is referred to as the power penalty for that
mechanism.
Bit error rate (BER) : Predict the statistical likelihood of
encountering an error during communications.
Can be measured empirically by counting the number of errors over an
adequately long span of transmission
BER depends primarily on the S/N ratio of the received signal, which in
turn determined by transmitted signal power, attenuation of the link and
receiver noise.
Main factors leading to significant penalties are
Intersymbol interference (ISI)
Non zero extinction ratio and
Pulse position jitter
BER estimation is one of the valuable ways of viewing
parametric performance of digital communication systems at
high speeds.
• Requires sophisticated and expensive equipment to achieve
accuracy, particularly at high bit rates.
• Can be investigated qualitatively and perhaps even in a pseudo
quantitative manner by generating the „Eye diagram‟ for the
system.
An intuitive way of viewing parametric performance
Threshold detection and BER
To allow the system designers to determine SNR and threshold
level required to achieve the specified bit error rate.
Useful to calculate the probability of error (BER)
Fig.1: PDFs for levels of 0 and 1 in the presence
of random (Gaussian) noise.
Shaded region - For a 0 signal
Hatched region - For a 1 signal
• Need to establish the noise statistics
and compute the probability that the
noise level at any given sampling point
pushes the signal to the wrong side of
the threshold for a 1 or 0 transmitted.
thv
11 dvpP
thv
00 dvpP &
Signal Probability distribution
functions for 0 & 1 levels.
BER = Pe = a P0 + b P1
Threshold Detection ….
Bit period 1 2 3 4
Tx Bit 1 0 1 0
Rx Bit 1 0 0 1
VTH
V1
V0Sampling Instants
Total Probability of Error (Pe) : BER = a P1 + b P0
2
Qerf1
2
1PBER e
N
th1
1
th1
0
0th
v
)vv()vv()vv(Qwhere
In term of Error Functions :
Small variations in the Q-factor lead to fairly dramatic changes in the BER.
Cannot afford to let the received SNRR drop below specification.
Q-factor can be estimated from the
measured noise voltages and hence
BER can be determined
Fig.2: Error probability Pe versus error
probability factor Q
For BER in the range of 10-9 to 10-12;
Q should falls between 6 and 7.
“Waterfall” curves
Eye-diagram Test Setup
Basic Equipment for Eye-diagram Measurements
Eye-pattern technique - a simple but powerful measurement method for
assessing the data-handling ability of a digital transmission system.
Has been used extensively for evaluating performance of wire
systems and can also be applied to OFC data links.
Eye-pattern measurements are made in the time-domain and allow the
effects of waveform distortion to be shown immediately on a DSO.
Experimental Set Up & Measurements
Eye Patterns
A visual method to assess the quality of the output of a
transmitter or the input / output of a receiver.
Although the technique is largely qualitative it can provide
some useful quantitative information in terms of trends and
whether or not a system is performing to specification.
Distance 2km from transmitter Distance 6 km from transmitter
Eye Pattern Interpretation
• MN is a measure of noise
margin.
• ST is measure of sensitivity-to-
timing error.
Full width noise
Jitter
20-80% rise time
V1
V0
RMS Noise and Jitter
„DA‟ provides the following information:
RMS noise can be estimated by a rule of the thumb that
total noise on oscilloscope is 5 times the rms noise
The mean 1 and 0 levels can also be calculated and hence
Q can be estimated
Q can be used now to find the BER.
Jitter
„JT’ the range of amplitude differences of the zero crossing, is
a measure of the timing jitter.
Jitter introduces an uncertainty on the sampling position
relative to the centre of the bit period and leads to an increase
in error rate.
Noise Vs Distortion
Eye Diagram Analysis
Eye diagram showing sample measurements of 20-80%
rise time, jitter, full width noise and the mean 0 & 1 levels.
Often used for assessing the quality of received signal and
indeed the quality and integrity of system transmitting it.
Although qualitative; provides
useful data in terms of trends
and system operation as per
specifications.
Semi-quantitative information
about the transmission quality
Determination of “Q”-value
and hence BER.
Q-factor Analysis Software
Softwares enable a DSO to sample the received signals in
the centre of the bit period, transfer the sample to a PC and
then to analyse them.
The analyses algorithms enable the construction of signal
level histogram (i.e. plot of the number of samples occurring
in a narrow voltage range Vs voltage) which is essentially
the probability distribution of the signal levels around 0 and
1 levels.
Theoretical Gaussian distributions are curve fitted within the
software to the measure distribution, signal level (noise)
variance are extracted and Q-factor & BER are determined.
Histogram
A histogram is a function which corresponds to the
number of samples having a particular value
(a) : Good reception.(b) : Poor reception
Sampling for Q-factor & BER Estimation
Factors affecting BER
The main factors affecting BER are:
• Input Power.
• Signal to Noise Ratio (SNR).
Pe
SNR
Dispersion and Power maps
Signal maintenance using Optical Devices
Path Degradation/Engineering
Amplified
& Corrected
Signals/Noise &
Nonlinear gain
Original
Signals
Degraded
& Dispersed
Signals
Unusable
Signal from
Noise
Fiber Fiber
Generally amplifiers (Repeaters) are used to achieve the
required SNR or depending on signal health, regenerators are
used for amplification as well as shaping the signal to desired
level.
To compensate the dispersion (pulse broadening)- DCFs/FBGs
are used either in pre- or post-compensation scheme.
Optical Signal Amplification
Conversion of the information
signal from the O-E-O a often
provides a bottleneck within OFC
links.
Restrict both the operating BW
and the quality of the transmitted
signal.
O-E and E-O conversion devices for the realization of
Optical Fiber Communications.
A limiting factor within the implementation of optical fiber systems.
Conventional Method; By Electronic means
Optical Amplifiers
Optical Amplifiers : operate solely in the optical domain
with no inter conversion of photons to electrons.
Require optoelectronic devices for source and detector,
together with substantial electronic circuitry for pulse
slicing, retiming and shaping
Optical amplifiers can be placed at intervals along a fiber
link to provide linear amplification of the transmitted optical
signal.
In principle, provides a much simpler solution
Have emerged as promising network elements not just for
use as linear repeaters but as optical gain blocks, optical
receiver preamplifiers etc.
Basic Operation of optical amplifiers
Principle of Operation
Two main approaches :
Semiconductor Laser Amplifiers (SLA) : Utilize stimulated emission
from injected carriers
Fiber Amplifiers : Gain is provided by either rare earth dopants
(EDFA), stimulated Raman or Brillouin scattering
EDFA
Erbium Doped Fiber Amplifier
Direct amplification of optical signal
Flat gain around 1550nm low loss window
BW 12,500 GHz ; Enormous potential
All channels roughly equal power
INITIAL DWDM SIGNAL
After a series of amplifiers
Signal to noise reduced
Some channels stronger than
others
FINAL DWDM SIGNAL
Typical Long-haul Telecom System
Amplifier spans: 30 to 120 km
Regenerator spans: 50 to 600 km
Terminal spans: up to 600 km (without regenerators)
up to 9000 km (with regenerators)
Terminal
EquipmentAmplifier
Unit
Regenerator
Unit
Terminal
EquipmentAmplifier
Unit
Amplifier
Unit
Two pairs of single-mode fiber
Attenuation limited link
Dispersion Limited Link
&
Dispersion Compensation
Dispersion Compensating Fibers (DCF)
Designed for specific purpose, now used in high data networks
SMFs with Negative Dispersion Characteristic
Total dispersion of the link to be ~Zero : D1L1+D2L2 =0
Pulse Spread compensation with a DCF
Design of DCFs
Manufacturers to modify
• Refractive –index profile ()
• Relative Index value ()
• Decreasing Core radius (a), MFD 4.5 m
Dispersion Compensator- FBG Based
Dispersion Compensation in standard fiber using
chirped grating and optical circulator
High Capacity DWDM OFC Link
Capacity of carrying enormous rates of information in THz
• 1.1 Tb/s over 150 km ; 55 wavelengths WDM
• 2.6 Tb/s over 120 km ; 132 wavelengths WDM
Transmitters
DE
MU
X
Receivers
EDFAEDFA EDFA
Metropolitan-Access Network
Network Management Layer
Add/Drop
li
li
Digital Client Layer Optical Channel (OC-N) Layer
Optical Layers Optical Multiplexing Section Layer (OMS-L)
Optical Transmission Section Layer (OTS-L)
Digital Client Layer SONET/SDH/PDH/ATM/ IP/etc. Digital Hierarchy Layer
MU
X
ln
ln
l
l
l
l
Ro
ute
rEDFA = Erbium-doped fiber amplifier
MUX = Multiplexer
DEMUX = Demultiplexer
The Optical Network System
THANK YOU FOR
YOUR PATIENCE
BC Choudhary Mobile: 9417521382
Email: bakhshish@yahoo.com,
If you have any query, feel free to contact :
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