The curve shown in above figure is for material dispersion in the absence of modal

   UNIT 8

1. Explain the first window transmission distance?

Ans:

Figure above is a plot of attenuation and repeater less transmission distance as a function of data rate for the short wavelength (800-900 nm) LED pin combination. The fiber coupled LED output power and BER are taken as -13 dBm and 10-19 respectively for all data rates up to 200 Mb/s. The attenuation limit curve is then derived by using a fiber loss of 3.5 dB/km and receiver sensitivities. For a given BER as the minimum optical power required at the receiver becomes higher for increasing data rates, the attenuation curve slopes downwards to right. Here, a 6 dB system operating margin and 1 dB connector coupling losses are included. Dispersion limit is a function of modal and material dispersions. For an LED with 50 nm spectral width the material dispersion at 800 nm is taken as 0.07 ns/(nm.km) or 3.5 ns/km.

dispersion. This limit is the distance at which  is to present of a bit period.

The modal distortion is derived from the equation,

Where, L = length of the link. Q = values between 0.5 to 1. B0= Bandwidth of 1km length of the cable.

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When, q = 0.7 and bandwidth distance product of the fiber is 800 MHz.km the value of tmodal is 70 percent of the bit period.

The achievable distance are shown here by hatched area i.e., below the attenuation limit curve and left to the dispersion line. The attenuation of transmission distance is till 40 Mb/s, later on it becomes material dispersion limited. Transmission distances are more when avalanche photodiode is replaced with laser diode.

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Attenuation and Transmission Distance Versus Data Rate

Example of an NRZ -L Data Format

below figure. For a serial data stream, as on-off

some of which are shown in below figure. In the unipolar RZ data,

   

2. Explain about RZ and NRZ coding and their effects on the bit rate? Ans: 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 this code, a signal level transition occurs during either

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.

NRZ CODES A number of different 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 (or unipolar) signal represents 1 by a pulse of

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current or light filling an entire bit period, whereas for a 0 no pulse transmitted. These codes are simple to generate & decode, but they possess no inherent error-monitoring or correcting capabilities and they have no self-clocking (timing) features.

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Examples of RZ Data Formats

multiplexer and sent over the same fiber as shown in below figure.

A disadvantage of the unipolar RZ format is that long strings of 0 bits can cause loss of timing synchronization. A common data format not having this limitation is the biphase or optical Manchester code. 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 modulo-2 addition of the-baseband (NRZ-L) signal and a clock signal. In this code, there is a transition at the centre of each bit interval. A negative-going transition indicates a 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 an NRZ code. In addition, it has no inherent error-detecting or correcting capability.

3. What are the underlying principles of the WDM technique? What are its various advantages? How is it different from FDM technique? Ans: Wavelength Division Multiplexing (WDM) FDM stands for Frequency Division Multiplexing. In this scheme the numbers of different frequencies are combined together and pass through low pass filling in order to remove unwanted interferences and an adder, which combines frequencies at the output. At the demultiplexer the individual frequency signals can be obtained by means of a decoder. FDM provides high frequency range in order to associate n number of users. This system provides high capacity. This system is almost in all communications. An interesting and powerful aspect of an optical communication link is that many different wavelengths can be sent along a fiber simultaneously in 1300 to 1600 nm spectrum.

This is achieved through WDM (Wavelength Division Multiplexing). Combining a number of wavelengths on a same fiber is known as WDM. 'N' independent optically formatted information streams, each transmitted at a different wavelength are combined with optical

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Optical Communications    Unit VIII 

Each of the streams could be a different data rate. Each information stream maintains its individual data rate after being multiplexed with other streams and operates at its unique wavelength. The basic of WDM has to use multiple sources operating at slightly different wavelengths to transmit and WDM must be properly spaced to avoid inter channel interference.

Features of Wavelength Division Multiplexing 1. Capacity Upgrade

The classical application of WDM is to upgrade the capacity of existing point-to-point fiber optic transmission links. If each wavelength supports an independent network signal of a few gigabits per second, then WDM can increase the capacity of a fiber network dramatically.

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which has been widely adopted for absorption loss measurement is shown in figure 2.

methods, which are the basis for absorption loss measurement. Figure 1 shows the apparatus

in figure below.

2.

Transparency An important aspect of WDM is that each optical channel can carry any transmission format.

3.

Wavelength Routing In addition to the usage of multiple wavelengths to increase link capacity and flexibility, the use of wavelength-sensitive optical routing devices, in designing communication meteorology and switches has a very important role. Wavelength routed networks use the final address.

4.

Wavelength switching

Wavelength routed networks are based on a rigid fiber structure, wavelength switched architectures allow reconfigurations of the optical layer.

Operation Principle of WDM The key feature of WDM is that the discrete wavelengths form an orthogonal set of carriers that can be separated, routed and switched without interfering with each other as shown

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The optical bandwidth can be expressed in terms of wavelength deviation “∆ ”.

|∆ | ∆

Since the spectral width of a high quality source occupies only a narrow optical bandwidth. The two loss windows provide many additional operating regions. The light source, each emitting at different peak wavelengths is sufficiently spaced to avoid interference. Fixed frequency spacing is selected because the operating mode of laser is locked, which means the frequency of the laser is fixed. At the transmitter end, there are several modulated light sources which emit signals at different wavelength. A multiplexer is used to combine these signals into a spectrum of closed wavelength signals and mix them into a single fiber. At the receiver, the demultiplexer separates the optical signal into appropriate detection channels for signal processing. A variety of active and passive devices are required to implement WDM networks. The passive devices require no external control for their operation. The active devices can be controlled electronically. Hence they provide a degree of network flexibility.

4. Briefly explain the principle behind the calorimetric method used for the measurement of absorption loss in optical fiber? Ans: Measurement of absorption loss in optical fibers

Generally, the level of impurity content within the fiber material to be checked in the manufacturing process can be made available by the material absorption loss measurements. The methods which can be used to determine the temperature rise in the fiber or bulk material due to the absorbed optical energy within the structure are called as calorimetric

used to measure the absorption loss in optical fibers. The temperature measurement technique

Unit - VIII

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The capillary tubes (in which two fiber samples are mounted shown in figure 2 are

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surrounded by a low refractive index liquid with in the same enclosure of the apparatus for good electrical contact. One of the capillary tubes can be used as dummy fiber (i.e.; reference junction), where both of them must be wound by a thermocouple. The main fiber (not the dummy) through which light can be launched from a laser source. The nanovoltmeter can be used to indicate the temperature rise measured by the thermocouple due to absorption. The electrical calibration may achieved by,

1. Replacing the optical fibers with thin resistance wires. 2. Passing known electrical power through one of the capillary tubes.

Then, the colorimetric technique with electrical measurement instruments can be used for independent measurements. The heating and cooling curves for the fiber sample are provided by calorimetric measurements and which can be used to measure the attenuation of the fiber due to absorption. The time constant, tc can be obtained using curve as

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Where,

t1,t2 = Two points in time

T∞ = Maximum temperature rise of the fiber under test.

Tt1 = Temperature rise at a time t1.

Tt2 = Temperature rise at a time t2.

Then, the fiber attenuation due to absorption is given by,

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Where, C = Constant proportional to the thermal capacity per unit length of silica capillary and the low refractive index liquid surrounding the fiber.

Popt = Optical power propagation in the fiber under test.

5. Describe with suitable diagram, for measurement of fiber attenuation using cutback technique?

Ans:

Experimental setup for Cutback Technique

Cutback technique is the common technique used for the measurement of fiber attenuation. In this technique, the white focused light is mechanically chopped [cut into pieces] at a low frequency [say 200Hz] which enables lock-in amplifier to perform a phase-sensitive detection. Then the light is passed through a monochromator which uses a prism to select the required wavelength for the measurement of attenuation.

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photodiode. Hence any variations in the laser output power are rectified by automatic

below figure. By keeping the launched optical power absolutely constant, reproducible readings

After filtering the light, it is focused onto the fiber by means of a microscopic objective lens. A beam splitter is used before the fiber to provide light for vision and a reference signal is used compensate for output power fluctuations (changes). A mode scrambler is also attached to fiber within the first meter. Fiber is passed through S-shaped groove cut in the Teflon in a cladding mode stripper device through which radiation removes light launched into the fiber and then sends to index matched-glycerin. A p-i-n (or) avalanche photodiode is used to detect the optical power at the receiving end. At last the output from photo detector is fed to a lock-in amplifier and this output is recorded. The relationship optical attenuation per unit length

for the fiber is,

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log

Where, L1 = Original fiber length

L2 = Cutback fiber length

Po1 = Output optical power from original fiber

Po2 = Output optical power from cutback fiber.

6. Explain about an optical attenuation meter?

Ans: There are number of portable measurement test sets specifically designed for fiber attenuation measurements which require access to both ends of the optical link. These measurement devices tend to use cut-back measurement technique unless correction is made for any difference in connector losses between the link and a short length of similar cable. The block diagram of an optical attenuation meter consisting of transmitter and receiver is shown in the

may be obtained. A constant optical output power is obtained using an injection laser and a regulating circuit which is driven from a reference output of the source derived from a

adjustment of the modulating voltage and current.

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A large area photodiode is utilized in the receiver to eliminate any effects from differing fiber end faces. It is generally seen that when a measurement is made on multimode fiber a short cut-back reference length of a few meters is insufficient to obtain an equilibrium mode distribution. Hence unless a mode scrambling device together with a mode stripper is used, it is likely that a reference length of around 500 m (or) more will be required if the measurements are made reasonably accurate. When the measurements are made without a steady state mode distribution in the reference fiber, a significantly higher loss value is obtained which may be as much as 1 dB/km above the steady state attenuation.

7. Explain intermodal and intramodel dispersion? Ans: Intermodal Dispersion It is defined as the signal distortion that occurs as a result of different values of group delays for different modes of same wavelengths. This results in pulse broadening because group delay leads to difference in time of travel for the zero order modes and the higher order modes. It can be given as,

∆

Where, n1 = Refractive index of the core

c = Velocity of the light

∆ = Change in the path traced by different modes.

Intramodel Dispersion

Intramodel Dispersion also known as Group Velocity dispersion or Chromatic dispersion. It is

defined as the

pulse spreading that occurs because of the changes in group velocity as a function of wavelength occurring within an individual

mode (i.e., single mode). Since this distortion depends on wavelength, its effects on the signal distortion by increasing with the spectral width of the optical source. It is generally calculated as an r.m.s spectral width of a central wavelength. It has two main causes

1.

Material dispersion and 2.

Waveguide dispersion.

1. Material Dispersion

It is

the pulse spreading that occurs when the different wavelengths follow the same path.

It causes

a wavelength

impendence of the group velocity of any given mode. The

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main cause of material dispersion is the variations in the refractive index of the core material as a function of wavelength.

2. Waveguide Dispersion It occurs because of the inability of the single mode fiber to confine the total optical power in its core. Practically it is formed that the single mode fiber can confine up to a maximum of 80% of optical power in its core. So the remaining 20% of light travelling in the cladding travels faster than the light in the core and giving rise to a dispersion called waveguide dispersion. It depends on the fiber design.

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8. A spot measurement of fiber attenuation is performed on a 1.5 km length of optical fiber at a wavelength of 1.1 µm. The measured optical output power from the 1.5 km length of fiber is 50.1 µW. When the fiber is cutback to 2m length, the measured optical power is 385.4 µW. Determine the attenuation per kilometer for the fiber at a wavelength of 1.1 µm. Ans:

Given that, λ = 1.1 µm L1 = 1.5 km L2 = 2 m P1 = 50.1 µW P2 = 385.4 µW Fiber attenuation per unit length is,

    log

= .

log ..

= log ..

= 5.91 x 10-3

Fiber attenuation per kilometer is,

5.91

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9. (i) Convert the optical signal power of 5 mW and 20 µW to dBm. (ii) Convert optical signal power of 0.3 mW and 80 nW to dBµ. Ans: The optical signal power can be expressed in decibels by using,

10  log

Where,

Po = Received optical signal power

Pr = Reference power level

(i) For 1 mW reference power level

10 log 

Hence the given optical signal power is 5 mW

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Optical signal power = 10  log   

 

= 10  log 5

= 6.98 dBm

For 20 µW

Optical signal power = 10  log   

= 10  log 0.02

= -16.98 dBm. (ii)

For a 1 µW reference power level

10  log 

For 0.3 mW is equivalent to

Optical signal power = 10  log . 

= 10  log 300

= 24.77 dBµ For 800 nW is equivalent is

Optical signal power = 10  log   

= 10  log 0.08

= -10.97 dBµ

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10. Describe eye pattern analysis for assessing the performance of digital fiber optic link. Is it possible to estimate BER also from eye patterns? Ans: Eye pattern is a simple and powerful technique in measuring the capacity and performance of a digital transmission system. The measurements are in time domain and the waveform distortion can be seen on the CRO immediately. These eye patterns are formed by superimposing the 2N possible combinations of N-bit long NRZ patterns.

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shown in following figure.

An eye pattern is obtained by superimposing the above 8 patterns as shown in below figure.

              As said, we need a variety of word patterns to measure the performance of a system using eye-pattern technique, these word patterns are provided by pseudo random bit generator. This pseudo random data (bit) pattern generator produces a random data signal that contains 1's and 0's in a random fashion providing uniform data rate. The random data from pseudo random data pattern generator is applied to the vertical input of CRO and the data rate triggers the horizontal sweep. This generates an eye-pattern. For example, consider 8 possible 3-bit NRZ patterns as

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1. The eye-width opening defines the Inter Symbol Interference (ISI) error free sampling rate of signal.

2. The best sampling rate is obtained when the height of eye is maximum.

3. One cannot recognize 1's and 0's if height of eye is reduced. 4. The eye height at a particular sampling period gives noise margin. It is given by,

Noise margin = X 100%

5. The system's sensitivity to time is determined by the closing rate of eye for a variation in sampling period.

6. One can also get rise and fall times of the system from the pattern.

7. Bit Error Rate (BER) is also estimated from the patterns and it can be reduced by inserting a small amount of redundancy into the transmitted pulse train. 

 

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