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 :