OCQNAUNIT7

8/2/2019 OCQNAUNIT7

1/10

OpticalCommunications UnitVII

UNIT 7

1. Discuss about the choice of different components in designing an opticalfiber link. Discuss the shortcomings of decency of each component.

Ans:

The system designer has many choices when selecting components for an optical fiber

system. The major components choices are,

(i) Optical Fiber Type and ParametersMultimode or single mode, size, refractive index, attenuation, dispersion,

mode coupling, strength, joints etc.

(ii) Source TypeLaser or LED, optical power launched into the fiber, rise and fall time,

stability etc.,

(iii) Transmitter ConfigurationDesign for digital or analog, input impedance, supply voltage, dynamic

range, feedback etc.

(iv) Detector Type and Characteristics p-n, p-i-n or avalanche photodiode, response time, active diameter, biasvoltage, dark current etc.

(v) Receiver ConfigurationPreamplifier design, BER, SNR, range etc.

(vi) Modulation and CodingSource intensity modulation, pulse frequency modulation, PWM and PPM

transmission.

(vii) Digital transmission or analog transmission such as biphase scheme and FMrespectively. These decisions will be taken depending on the system performance,

ready availability of suitable components and cost.

The shortcomings of the components can be mentioned as follows,

(i) LED may appear ideally suitable for analog transmission most of the LED displaysome degree of non-linearity in their output.

w.jntuworld.com

www. ntuworld.com

8/2/2019 OCQNAUNIT7

2/10


(ii) The thermal behavior of LED and Lasers can limit their operation. Significantincrease in junction temperature may cause loss of lasing and reduction in optical

output power.

(iii) Finite spectral width can cause pulse broadening due to material dispersion on anoptical fiber communication link.

2. Discuss system considerations in point-to-point optical link.

Ans:

Point-to-Point Link

It has transmitter at one end and the receiver at the other end. The components must becarefully chosen to ensure the desired performance level and can be maintained for the expected

system life time.

Figure represents the block diagram of a simplex point-to-point link. The three majoroptical links building blocks are,

(i) Transmitter

(ii) Receiver and

(iii) Optical fiber.The characteristics of transmitter and receiver are given the first priority and then the

characteristic of optical fiber is considered to meet the system performance requirements.

Optical sources (such as LED or LASER) are used based on the following characteristics.

(i) Signal dispersion

(ii) Data rate(iii) Transmission distance and cost.

The characteristics of photo detector such as,(i) Responsivity

(ii) Operating wavelength

(iii) Speed and

(iv) Sensitivity.

The choice of optical fiber in single mode and multimode (step or graded index) depends on the

light source used and the amount of dispersion .

3. Give an account of fiber optic link power budget with an example.

w.jntuworld.com

www. ntuworld.com

8/2/2019 OCQNAUNIT7

3/10


Ans:

The power budget determines, whether the fiber optic link meets the attenuation

requirements or amplifiers are needed to boost the power level. It also determines the powermargin between the optical transmitter output and the minimum receiver sensitivity needed toestablish a specific Bit Error Rate (BER).

Figure shows the power loss model for a point-to-point link. In addition to losses, a link

power margin is provided in the analysis to allow for component aging, temperature fluctuation

and losses arising from components that might be added in future.

The link loss budget considers the total optical power loss PTthat allowed between the

light source and photo detector. This loss is allocated to cable attenuation, connector loss,

splicing loss and system margin. The total power loss in the link is given by,

= 2 System margin

Where,

Ps= Optical power emerging from the end of a fiber fly lead.

PR= Receiver sensitivity

lc= Connector loss

f= Fiber attenuation in dB/km

L = Transmission distance.

A margin of 6 to 8 dB is generally used for systems which do not have additionalcomponents incorporated into the link in future.

w.jntuworld.com

www. ntuworld.com

8/2/2019 OCQNAUNIT7

4/10


4. Write short notes on system considerations in the design of a fiber optic

link.

Ans:

The design of an optical fiber link involves many considerations regarding thefiber, source and photo detector operation and performance characteristics. Expected life timewith performance and cost constrains were considered while designing a fiber link. The systemconsiderations are choosing the transmitting wave length and components that operates in thisregion. The wavelength range 800-900 nm is chosen for shorter distances and 1300-1550 nm forlonger distances with low attenuation and dispersion.

Next, we consider receiver, transmitter and implement them on a fiber to check whetherthe desired performance characteristics are obtainable or not. We first select a photo detector. Indoing so, we should determine the minimum detectable optical power in order to satisfy Bit ErrorRate at specified data rate. Cost and complexity are also considered here. The systemsconsiderations with cheap and less complex one is preferred. P-i-n diodes are used as they aresimple, high temperature tolerance, less bias voltages. APD's are used to detect low power levelswhich are not done by P-i-n.

Next, we choose the transmitting elements. While choosing LED or laser, we shouldconsider data rate, dispersion, cost, and transmission distances. Laser provides the advantage oflonger repeater less transmission distance and low dispersion. LED's offer low cost and simplercircuitry.

Next we have to choose an optical fiber that may be single mode or multimode with step orgraded index core. The choice is made depending on dispersion tolerance and type of source.Edge emitting LED's along with single mode fiber launches sufficient power and supporttransmission data rates >500 Mbps. LED's can be used with multimode fibers.

While choosing attenuation characteristics of fiber, we should consider cabling processloss, and fiber attenuation, connector loss, splice loss and losses due to atmospheric conditionsalso.After considering a system satisfying above conditions, the "Link power Budget" and "Rise time

budget" techniques are used to ensure the desired performance characteristics are met or not.

5. List the estimates and conclusions possible from transmission distance

versus bit rate plot for the given wavelength LED-PIN diode combination.

Ans:

The plot of transmission distance Vs the bit rate for a short wavelength (800-900 nm)

LED-PIN diode combination is shown in figure 7.5.1.

w.jntuworld.com

www. ntuworld.com

8/2/2019 OCQNAUNIT7

5/10


The above plot shows the limitation of attenuation and dispersion on transmission

distance as a function of data rate. The Bit Error Rate (BER) is considered to be 10-9

for all the

cases. The fiber-coupled LED output power is taken as -13 dBm for all data rates up to 200Mbps. The attenuation limit curve is drawn by taking a fiber loss of 3.5 dB/km. We include 1 dB

connector coupling loss at both ends and a system margin 6 dB. We consider material dispersion

to be 3.5 ns/km with 50 nm spectral width. From figure 7.5.1 it is clear that a data rate of 40

Mbps over which the transmission distance is attenuation limited and below which the

transmission distance is material dispersion limited.

Suppose, take a data rate of 200 Mbps, the attenuation is 3 km x 3.5 dB/km = 10.5 dB and

dispersion is, 2 km x 3.5 ns = 7 ns, the transmission distance is just 2 km. Now, consider a data

rate of 2 Mbps, the attenuation is 10 km x 3.5 dB/km = 35 dB and dispersion is almost '0'. The

transmission distance is 10 kms.

From the above two estimations we conclude that higher data rates can be transmitted to a

smaller distance with less attenuation and high dispersion while, lower data rates (2 Mbps) can

be transmitted up to a larger distance of 10 km with high attenuation and no dispersion. If we

choose an optimum data rate of 20 Mbps, the attenuation and dispersion are high and can be used

for optimum transmission distance.

6. Discuss the difference between dispersion limited and an attenuation

limited fiber optic link.

Ans:

Dispersion Limited Fiber Optic Link Attenuation Limited Fiber Optic Link

1. When the data rates are higher than 40 Mbps,

the transmission distance is limited by dispersion

parameter.

2. In this, there is no effect of attenuation losses.

3. In this, losses are occurred due to increasepulse width, which in turn results in overlapping

with neighboring pulses.

4. This limit is taken at a distance at which

tmat is 70% of a bit duration.

5. Rise time budget analysis is a good method forcalculating the dispersion limitation of anoptical fiber link.

1. When the data rates are lower than 40 Mpbs,

the transmission distance is limited by attenuation

parameter.

2. In this, there is no effect of dispersion.

3. In this, losses are produced due to theabsorption by fiber, material imperfections, and

fiber bents.

4. The attenuation limit curve has a negative slope because the minimum optical power at receiverincreases with increasing data rates at a constantBER.

5. Link power budget is a convenient method tocalculate attenuation limitation of an optical fiberlink.

w.jntuworld.com

www. ntuworld.com

8/2/2019 OCQNAUNIT7

6/10


7. Describe the procedure to determine the maximum allowed NRZ or RZ

data rate on a given fiber optic link.

Ans:

The procedure which is followed to determine the maximum allowed NRZ or RZ data

rate on a given fiber optic link is described below.

1. Initially, calculate the source rise time (rrs), inter modal dispersion rise time (tmod), intra modal

pulse broadening rise time (rmat) and detector rise time (rrd) by using given data.

2. Then, calculate the total system rise time (tsys ) using below formula as,

3. In the rise time calculations involving optical fiber systems, the constant 0.35is often utilized.

Hence,

0.35

Where,

= Pulse duration.

4. For an RZ pulse format, the bit rate B = 1/ and the expression for maximum allowed RZ data

rate of a given fiber optic link is given by,

.

5. For NRZ pulse format, the BT = B/2 = (1/2) and the expression for maximum allowed NRZ

data rate of given fiber optic link is given by,

.

6. In a RZ pulse format, the upper limit on tsys should be less than 35% of the bit interval. In a

NRZ pulse format, the upper limit on tsys should be less than 70% of the bit interval.

w.jntuworld.com

www. ntuworld.com

8/2/2019 OCQNAUNIT7

7/10


8. Discuss the effect of mode mixing factor on modal dispersion for calculating

the maximum allowable transmission data rate in a fiber optic link.

Ans:

Information is carried by light in an optical fiber, which is injected intooptical fiber cable by optical Source in the form of many individual modes. These individualmodes do not travel throughout the fiber instead, they transfer large energy to their adjacentmodes. This type of mode conversion is known as mode coupling or mode mixing. This modemixing factor affects the propagation characteristics in many ways, the major one is related todispersive properties of fibers.

The sensitivity of the receiver reduces due to pulse broadening (by dispersion) caused dueto finite bandwidth. This leads to increase in bit error rate and Inter Symbol Interference (ISI).Equalization phenomenon compensates above loss at receiver. This is known as dispersion-equalization or ISI penalty (DL).

dB, given by Gaussian pulses.

Te = 1/e full width pulse broadening due to dispersion.

T = Bit period.

But, for Gaussian pulses.

Te = 22

222

Where,

BT = Bit rate

= r.m.s pulse width.

Pulse broadening is caused due to intermodal and/or intramodal dispersion. Multimode fibers

exhibit considerable intermodal (modal) dispersion. We know pulse broadening (Intermodal) is

proportional directly to length 'L' of fiber.

The presence of mode coupling (or) mixing along with intermodal dispersion slows the rateat which pulse broadening increases with length 'L'.

w.jntuworld.com

www. ntuworld.com

8/2/2019 OCQNAUNIT7

8/10


At lower bit rates, the effect of mode mixing on the dispersion-equalization penalty is

almost negligible. At higher bit rates, mode mixing factor controls the dispersion-equalization

penalty, in fact it reduces the penalty. It-also increases the repeater spacing. Hence, maximum

allowable bit-rate is obtained by providing desired bandwidth. In order to obtain desired bandwidth, the pulse broadening should be reduced. As the mode coupling factor reduces the

dispersion equalization penalty, pulse broadening (due to modal dispersion) is also reduced.Thus, maximum allowable bit rate is attained using mode mixing effect on modal dispersion.

9. The rise times for various components of intensity modulated fiber optic

link are listed below. Determine if these specifications support a 5 km repeater

less fiber optic link with 6 MHz optical bandwidth.

Rise time of LED transmitter electronics = 10 ns

Inter-modal dispersion induced = 8ns/km

Intra-modal dispersion induced = 2ns/km

Rise time of Detector and receiver electronics = 3ns

Ans:

Given that,

Optical bandwidth = 6 Mhz

Length of the fiber optic link = 5 km

trise = 10 ns

tinter= 8 ns/km

tinter= 5 x 8 ns = 40 ns for 5 km

tintra

= 2 ns/km = 10 ns for 5 km

tr= 3 ns

Now,

1010 4010 1010 310 42.5

w.jntuworld.com

www. ntuworld.com

8/2/2019 OCQNAUNIT7

9/10


We know that maximum rise time of system is,

.

58.6

42.5 58.6

Hence, the given specifications give a system rise time that satisfies the bandwidth

requirement of optical fiber. Hence, these support a repeater less communication.

10. (i) What is essential to initiate a system design?

(ii) Explain briefly about multiplexing.

Ans:

(i)

To initiate a system design certain basic system requirements must be specified.These specifications includes,

1. Type of transmission i.e., digital or analog.2. System fidelity. For a digital system, it is specified in terms of BER and for an analog

system it is specified in terms of received SNR and signal distortion.

3. Bandwidth required for transmission.4. Spacing between the terminal equipment or intermediate repeaters.5. Cost.6. Reliability.(ii)

The transfer of information over an optical fiber communication link can beincreased by multiplexing several signals on to a single fiber. These multichannel signals arecarried by multiplexing in time or frequency domain, prior to intensity modulation of opticalsource.

The technique of separating signals in time is called Time Division Multiplexing (TDM).Narrow pulses from multiple modulators under the control of a common clock (commutator) arefed to a single channel. Pulses from individual channels are interleaved and transmittedsequentially, thus enhancing the bandwidth utilization of a single fiber link.

The technique of separating signals in frequency is called Frequency Division Multiplexing(FDM). In FDM, the optical channel bandwidth is divided into a number of non-overlappingfrequency bands and each signal is assigned one of these bands of frequencies. The individualsignals can be extracted from the combined FDM signalby appropriate electrical filtering at the

receiving terminal.

In contrast to the FDM, it is possible to utilize a number of optical sourceseach operating at a different wavelength on the single fiber link. In this technique, referredto as

Wavelength Division Multiplexing (WDM), the separation and extraction of the multiplexed

signals (i.e., wavelength separation) is performed with optical filters.

w.jntuworld.com

www. ntuworld.com

8/2/2019 OCQNAUNIT7

10/10


w.jntuworld.com

Documents

OCQNAUNIT7