FINAL YEAR

CLASS: IV YEAR / VII SEMESTER ECE

SUBJECT CODE AND NAME: EC2402 – OPTICAL COMMUNICATION AND NETWORKING

UNIT I – INTRODUCTION

1. Define numerical aperture of a step index fiber.                                                                                              (Apr/May 2005 Chennai)

 The numerical aperture gives the measure of light gathering capacity of the fiber. It is given by NA= (n1²-n2²) The relative refractive index difference is given by

                                         n1²-n2²                       n1-n2

                                  ∆=   ——                      ∆= ———

                                           2n1²                            n1

2. Define Mode-field diameter.  (Nov/Dec 2007 Covai)

The fundamental parameter of a single mode fiber is the mode-field diameter.              This can be determined from the mode field distribution of the Fundamental LPo1 mode.

3. What are meridional rays and skew rays (Apr/May 2009 Chennai)

Meridional rays are the rays following Zig Zag path when they travel through    fiber and for every reflection it will cross the fiber axis.                  

Skew rays are the rays following the helical path around the fiber axis when They travel through the fiber and they would not cross the fiber axis at any time.

4. What is fiber birefringence? Nov/Dec 2007 Chennai)

Imperfections in the fiber are common such as asymmetrical lateral stress,Non circular imperfect variations of refractiveindex profile. These imperfectionsBreak the circular symmetry of ideal fiber and mode propagate with different phaseVelocity and the difference between their refractive index is called fiber birefringence.                             B=ko(ny-nx)

5. What is Snell’s law? (Nov/Dec 2010 Covai)

 The relationship at the interface is known as Snell’s law and is given by         n1sin_1=n2 sin_2

6. Define acceptance angle. (Nov/Dec 2009 Covai)

The maximum angle ‘_max’ with which a ray of light can enter through the

Entrance end of the fiber and still be totally internally reflected is called acceptanceAngle of the fiber.

7. Define V-number.  (Apr/May 2010 Chennai)

 V-number is used to find the number of modes that can support by the fiber.It is a dimensionless number and it is given by

                               2πa

                        V=   —   (NA)

                                λ

8. Define fiber beat length.

 When the fiber phase difference is an integar multiple of 2π, the two modes will beat At this point and the input polarization the state will reproduce.Therefore the length over which this beating occurse is the fiber beat length

                     Lp=2π/β

9. Compare single mode fiber with multi mode fiber. (Apr/May 2009 Chennai)

Single mode fiber	Multimode fiber
It supports only a single propagation            Mode. Characterized by small core diameter             Ranging from 4-10micro meter.	It supports more then hundred propagating modes. Relatively large core diametervalue of 50,62.5,80,100 micro meter.

10. Mention the advantages of optical fiber.     (Apr/May 2011 Covai)

·Low transmission loss and wide bandwidth.

·Small size and weight.

·Electrical isolation.

·Immunity to interference and crosstalk.

·Signal security.

11. Write short notes on Total internal reflection.     (Nov/Dec 2010 Covai)

 Total internal reflection is defined as the complete reflection of light into the same medium without any transmission of light.

      It has two conditions:

·Light should travel from high refractive index material lower refractive index material.

·Incident angle should be greater then the critical aqngle.

12. What are the applications of fiber optical communication?

·Enormous potential bandwidth()

·Small size and small weight.

·Electrical isolation.

·Low transmission loss 0.2 db/km.

·Flexibility and raggedness.

13. A Typical relative refractive index difference for on optical fiber designed for long distance transmission is 1%.Estimate the numerical aperture for the fiber when the core index is 1.47.                                                                                                               (Nov/Dec 2008 Chennai)       Given data:

                    ∆=1%=0.01,  N1=1.47

                     NA=n1(2∆)½         =1.47(2(0.01))½      NA=

14. A silica fiber as a core refractive index of 1.5 and a cladding refractive index of 1.47 .Determine the acceptance angle in air for the fiber.          (Nov/Dec 2007 Chennai)

      Given data:   N1=1.5,  N2=1.47           max=sin (NA)

      NA=(n1²-n2²)½  =(1.5²-1.47²) NA =0.30    amax=sin(0.30) amax=17.4°

15. A step index fiber has a normalized frequency V=26.6 at 1300nm wavelength. If the core radius is 25pm.Find the numerical aperture                                                                     (Nov/Dec 2011 Chennai)

Given date:

               V=26.6,  λ=1300nm,a=25pm

                      2πa

               V=   —   (NA)                  

                               Λ

                            Λv           26.6x1300x10ˉ9

                   NA= —      =    ————―—       =220.143x10^3

                           2πa            2πx25x10ˉ12

16. It is desired to make a single-mode fiber at an operating wave length=1300nm with Ncore=1.505 and Ncladd =1.502.Find the numerical aperture and core radius.                                                                                                                      (Nov/Dec 2006 Chennai) Given data:

               Λ=1300nm, n1=1.505, n2=1.502

                 NA=(n1²-n2²)½   =(1.505²-1.502²)½    =0.094

                               Vλ           2.405x1300x10ˉ9

       Core radius=———  =  ————————

                              2πxNA        2xπx0.094

                     A=5.29x10ˉ6 m

17. A single mode fiber has a beat length of 8cm at 1300nm.Find modal bi-refringence.                                           (Nov/Dec 2010 Covai)

                 

          Given data:                     Lp=8cm=0.08m, λ=1300nm

                         2λ          2x1300x10ˉ9    

                   ß= ——    = ——————     =78.5mˉ1

                         Lp               0.08

         Alternatively                   Bf=ny-nx

                         Λ        1300x10ˉ6

                       —— = ————— =1.63x10ˉ5                      Lp        8x10ˉ2

18. Define normalized propagation constant. (Nov/Dec 2006 Chennai)

 The number of modes  that can exist in a waveguide as a function of V may be conveniently represented in terms of a normalized propation constant.It is defined as

                         a²w²       

                   B= ——

                          v²

19. What is tunnel effect in optical fiber?           (Nov/Dec 2009 Chennai)

            The power radiation out of the waveguide result from a quantum mechanical phenomenon called tunnel effect.

20. How light gathering capacity of a fiber is expressed in terms of numericalaperture?                                           (Nov/Dec 2008 Chennai)

            Numerical aperture gives the measure of light gathering capacity of the fiber.It is referred as the figure of merit of the fiber.Numerical aperture is given by

              NA=(n1²-n2²)½  (or) NA=n1(2∆)½

21. Give the refractive index expression of a graded index fiber.

                                                                                  (Nov/Dec 2006 Chennai)

              Refractive index of the graded index  fiber is given by

                N(r)={n1(1-2∆(r/a)ª)½     r<a(core)    {n1(1-2∆)½=n2      r>a(cladding)

22. What is the fundamental parameter of a single mode fiber?        

                                                                                 (May/jun2007. Chennai)

            The single mode fiber has two fundamental parameter followed by

• Horizontal. Vertical.

23. Multimode step index fiber with a core diameter of 80 µm and a relative index of 1.5% is operating at a waveguide of 0.85µm.If the core refractive index is 1.48.Determine (i)Nomalized frequency of the fiber  (ii)The number of guided modes.

    D=80µm, a=40µm=40x10ˉ6, ∆=1.5%=0.015, λ=0.85µm, n1=1.48

(i)                            2π                          2xπ

V=—— an1(2∆)½  =  ———     x40x10ˉ6x1.48(2x0.015)½

       Λ                         0.85x10ˉ6

V=75.8     V²       (75.8)²

(ii)                Ms=——  =———  =2879.5

     2           2

24. What is the structure of an optical fiber?

       An optical fiber is dielectric wave guide that operates  at optical frequencies. This fiber wave guide is normally cylindrical in form.

25. What is need for cladding?

       i) It reduces scattering loss resulting from dielectric discontinuity at the core surface.

       ii) It adds mechanical strength to the fiber.

26. Define the refractive index of a medium.

 The refractive index of a medium is defined as the ratio of the velocity of light in vacuum to the velocity of light in the medium.                N = c / v

27. Define critical angle.

When light the angle of refraction is 90 and the refracted ray emerges parallel to the interface between the angle of incidence must be less than 90.this angle of incidence is known as critical angle.

28. What is external reflection?

When light travelling in a certain medium is reflected off an optically denser material the process is referred to as external reflection.

29. What is internal reflection?

The reflection of light of less optically dense material is called internal reflection.

30. What are the conditions for total internal reflection?

i) The angle of incidence of the ray exceeds the critical angle.

ii) When light is incident on the dielectric of lower index from the dielectric of higher index.

31. What are the three major performance criteria must be considered, when choosing an optical wave guide fiber?

i) Signal losses

ii) Ease of light coupling and inter connection.

iii) band width.

32. Define numerical aperture.

It is the relationship between the acceptance angle and the refractive indices of the three media involved namely core, cladding and air. The numerical aperture is a dimensionless quantity which is less than unity, with values ranging from 0.14 to 0.50.

33. Discuss the important parameters that characterize optical communication system.

 i) Signal losses.             ii) Ease of light coupling and inter connection.            iii) band width.

34. What type of fiber is used in optical communication?

Passive fibers

35. What are the various types of fibers?

i)Active fiber    ii) Conical fiber        iii)Multiple fiber    iv)passive fiber    v)lasing fiber

36. What are the two types of rays that can propagate in a fiber?

i) Meridional ray                  ii) Skew ray

37. Write down the formula for the NA for skew rays.

       NA= sinθas cos£

38. What are the modes of waveguide?

The propagation of light along a wave guide can be described in terms of a set of guided electromagnetic waves called modes.

39. What is TE,TM and Hybrid Mode?

When Ez = 0 the modes are called TE modes

When Hz = 0 the modes are called TM modes

If both Ez and Hz are non zero then the modes are called hybrid modes.

40. What is caustic surface?

The path followed by the ray lies completely within the boundaries of two coaxial cylindrical surfaces known as caustic surfaces.

41. What are leaky modes in optical fibers?

Leakey modes are the modes that are only partially confined to the core region and attenuate continuously by radiating their power out of the core as they propagate along the waveguide.

42. What is meant by cut off condition?

The cut off condition is the point at which a mode is no longer bound to the core region.

43. What are bounded and unbounded rays?

Bounded rays are the rays that are trapped in the core and propagate along the fiber axis according to the laws of geometrical optics.

Unbounded rays are the rays that refracted out of the fiber core.

44. What are the two types of step index fiber?

i) Monomode step index fiber.    ii) Multimode step index fiber.

45. What are the techniques used in fabrication of optical fiber wave length?

i)Vapour phase oxidation process  ii) Direct melt method

46. What are the methods to reduce the number of modes in a step index fiber?

i) Reduce the fiber diameter

ii) Decrease the difference between the cladding and core indices.

iii) Increases the wave length.

47. What is W- type fiber?

W-type fiber is step index fiber. the refractive index profile for W type fiber is known as doubly clad fiber.

48. What is the advantage of W- type fiber?

It can compensate material dispersion of a normal single clad fiber by introducing a negative guide dispersion.

49. What is the disadvantage of multimode fiber?

Intermodal dispersion

50. What are the materials that satisfy the above requirements?

i) Glasses     ii) plastics

.                                              

Part –B (8-marks)

1)         Discuss the signal distortion in single mode fibers.(Nov/Dec 2005 Chennai)

2)         Discuss pulse broadening in graded index fibers with necessary equations.

                                                                                 (Nov/Dec 2005 Chennai)

3)        Briefly explain the evolution of fiber optic system. (Nov/Dec 2005 Chennai)

4)        Compare the configuration of different types of fibers.(Nov/Dec 2005Chennai)

5)        Derive the wave equations for step-index fiber and explain.(Nov/Dec 2006 Chennai)

6)        What is meant by ‘weakly guiding approximation’? (Nov/Dec 2006 Chennai)

7)        Draw the elements of an optical fiber transmission link and explain.                                                         (May/jun 2006 Chennai)

8)        Explain with a neat block diagram the fundamentals of optical fiber communication.                                           (Nov/Dec 2008 Chennai)

9)        A graded index fiber with a parabolic refractive index profile core has a refractive index at the core axis of 1.5 and the refractive index difference of 1%.Estimate the maximum possible core diameter which allow single mode operation at a wave length of 1.3µm.                                                                 (Nov/Dec 2011 Chennai)

10)     What are the fiber modes? Explain mode theory for optical fibers in detail.                                                          (Nov/Dec 2011 Chennai)

Part –B (16-marks)

1. (i) Discuss the propagation modes in single -mode fiber   (ii) Discuss the structure of graded index fiber.                                                         (Apr/May 2005 Chennai)

2.Briefly explain the various generations of fiber optic communication.                                                       (Nov/Dec 2005 Chennai)

3. Compare single mode fibers and graded index fibers. Explain the requirements for fiber materials.                           (May/jun 2007 Chennai)

4. Draw the structures of single and multimode step index fibers and graded index fiber with their typical dimensions. (Nov/Dec 2008 Chennai)

5. What are the functions of the core and cladding in an optical fiber? Why should their refractive indices the different? Explain with suitable sketches.                                                                  (Apr/May 2008 Chennai)

6. Describe the fiber structure utilized to provide dispersion shifting and dispersion flattering in a single mode fiber.      (Apr/May 2008 Chennai)

7.Explain in detail about the fiber types, modes and its configurations.                                                              (Nov/Dec 2010 Covai)

8. What are fiber modes? Explain mode theory for optical fibers in detail. (Nov/Dec 2011 Covai)

UNIT II – TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS 

1. Calculate the numerical aperture and acceptance angle in air for a fiber with core refractive index of 1.46 and core cladding index difference is 0.01. (Nov/Dec 2008 Chennai)

Given data:   The numerical aperture NA=n1 (2) ^1/2 =1.46(2(0.01)) ^1/2 =0.21

2.      What is the advantage of multimode fiber?    (Apr/May 2005 Chennai)

The larger core radius of multimode fibers makes it easier to launch optical fiber into the   fiber. Connecting together of similar fiber is easy. It can be launched in to a multimode fiber using a LED.

3.      Difference B/W Intramodel &Intermodal? (Apr/May 2007 Chennai)

        Intra model:Pulse spreading that takes place within a  single    mode.  This       spreading across from the  finite spectral emission width an optical source. This phenomenon known as group velocity dispersion.

            Intermodal:

It appears is multimode fibers. The signal distorting mechanism is a   result of each mode having a difference value of the group velocity at a single frequency.

   4. Define Snell’s law. (Apr/May 2007 Chennai)

         Snell’s law of refraction at the core cladding interface

     5. Define signals attenuation of fiber loss. (Nov/Dec 2008 Chennai)

      It is defined as the ratio of the optical output power pout from a fiber of length L to the optical input power pinThe symbol α is commonly used to express the attenuation in decibel per kilometer

            Where L is fiber length

6. Why is signal attenuation one of the most important properties of an optical fiber? (Apr/May 2011 Chennai)

     Signals attenuation is one of the most important property of optical fiber, because it largely determines the maximum repeater less separation between a transmitter and receiver.

7. What is waveguide dispersion? (Apr/May 2010 Chennai)

     The waveguide of the fiber may create intra modal dispersion. This results from the variation in group velocity with wavelength for particular mode. Wavelength dispersion, occurs because a single- mode fiber only confines about 80% of the optical power to the core.

8. Write down the formula for group velocity. (Apr/May 2010 Chennai)

 Where   - propagation constant
             

9. Name three different mechanism caused by absorption.     (Nov/Dec 2010 Chennai)

     1. Absorption by atomic defects in the glass composition.

     2. Extrinsic absorption by impurity atoms in glass material.

     3. Instrinsic absorption by the basis constituent atom of the fiber material.

10. Write Urbach’s rule.                                (Nov/Dec 2008 Chennai)

     Where C, E0 are empirical constants, E-Photon energy

11.What are the different types of linear scattering?                                             (Apr/May 2005 Chennai)

        i) Rayleigh scattering    ii) Mie scattering

12. How will scattering loss occurs?     (Apr/May 2005 Chennai)

        Scattering losses in glass arise from

i) Microscopic variation in the material density

ii)  Compositional fluctuations’

iii) Structural in homogeneities or defects occurring during fiber manufacture

13. Write the formula for Ray Leigh scattering. (Nov/Dec 2009 Chennai)

Where - Rayleigh scattering coefficient, λ- Optical wavelength

            N-refractive index of the medium, p- Average photo elastic coefficient

            - Isothermal compressibility      T_p – Fictive temperature

            K- Boltzmann’s constant

14. What is meant by inter modal dispersion? (Nov/Dec 2010 Chennai)

     Pulse broadening due to intermodal dispersion results from the propagation delay differences between the modes within a multimode fiber. As the different modes which constitute a pulse, the pulse width at the output is dependent upon the transmission times of the slowest and fastest modes.

15. What are the types of non-linear scattering losses?

   i) Brillouin scattering         ii) Raman scattering

16. Give the reasons for the pulse broadening in optical communication system

     i) Dispersion                        ii) Distortion

17. What are the effects of dispersion in optical fiber system?

     i) Pulse broadening              ii) Inter symbol interference

18. Define dispersion in optical fibers.

        Dispersion of the transmitter optical signal causes distortion for both digital and analog transmission along optical fibers. Dispersion mechanisms within the fiber causes broadening of transmitted light pulses as they travel along the channel.

19. Two main causes of intra modal dispersion.

       i) Material Dispersion                     ii) Waveguide Dispersion

20. Give the other name for material dispersion.

       i) Chromatic dispersion                  ii) Spectral dispersion

21. What is meant by material dispersion in fibers?

       Material dispersion arises from the variation of the refractive index of the core material as a function of wavelength. This can=uses a wavelength dependence of the group velocity of any give mode, that is pulse spreading occurs even when different wavelength follows the same path.

22. Draw the characteristics of attenuation versus wavelength of optical fiber communication system

This can=uses a wavelength dependence of the group velocity of any give mode, that is pulse spreading occurs even when different wavelength follows the same path.

23. Under what condition “Material Dispersion” occurs.

        i) When the phase velocity of a plane wave propagating in the dielectric medium varies nonlinearly with wavelength.

        ii) When the second differential of the refractive index with respect to wavelength is not zero.

24. Under what condition “Wavelength dispersion” occurs in single mode fiber?

        20MHz KM - For multimode SI         1 GHz KM    - For multimode GI

        100 GHz KM - For multimode SI

25.  Define Fictive temperature.

      It is defined as the temperature at which the glass can reach a state of thermal   equilibrium

26. What is bandwidth – distance product?

     A measure of the information capacity of an optical wave guide is known as bandwidth – distance produce.

27. What are the different types of material absorption?

     i) Intrinsic Absorption           ii) Extrinsic Absorption

28. Write the formula to find out the overall fiber dispersion.

29. What is meant by mode coupling?

      It is the coupling of energy from one mode to another arises because of structural imperfection fiber diameter and refractive – index variation and cabling induced microbends.

30. What are micro bending losses? How is it reduced?

       Micro bends are repetitive small scale fluctuation in the radius of curvature of the fiber axis. They are caused either by non uniformities in the manufacturing of the fiber or by nonuniform lateral pressures created during the cabling of the fiber. Microbending losses are minimized by extruding a compressible jacket over the fiber.

31. What is Zero dispersion wavelength?

         The material and waveguide dispersion can have opposite signs and the net effect is a complete cancellation. This is zero dispersion wavelengths.

32. Semiconductor junction devices of the type grown on substrates have been used as sources for operation in zero wavelength range.

       Ga In, AsP, InP

33. What meant by intrinsic absorption?

        It means the IR absorption and UV absorption by fiber material (SiO₂). IR absorption is due to Si – O coupling. UV absorption is due to electronic absorption bands.

34. Among the different fibers, which has the highest dispersion?

        Multimode step index fiber. This fiber has enormous multimode dispersion along with material dispersion and waveguide dispersion.

35.  Why do we have smaller dispersion in graded index fibers?

         Due to shaping the refractive index profile in the parabolic manner and by self focusing effect, the dispersion is small.

36.  What is dispersion shifted fibers?

        Normally at 1.3µm, there is minimum dispersion and finite loss. But at 1.55µm, the loss and dispersion are very minimum for SiO₂ fibers. At 1.55µm the material dispersion is positive and wavelength dispersion is negative. When these are equal, there is no dispersion. So the operating wavelength is now shifted to 1.55µm instead of 1.3µm.

37. What is pulse dispersion?

       Spreading or broadening of pulse during transmission of light through the fiber is called pulse dispersion. Due to overlapping of pulse the proper signal cannot be received.

38. What are the other sources to produce dispersion except intermodal and intra modal dispersion?

       The spectral spread of the light source and improper shaping of refractive index profile create dispersion in the fiber.

39. What is meant by modal noise?

      This is the 3^rd effect of the transmitted signals on the optical channel. The speckle patterns observed in multimode fiber as fluctuation which have characteristic times longer than the resolution time of the detector, and is known as modal noise.

40. What are the loss or signal attenuation mechanisms in s fiber?

              i) Absorption ii) Scattering iii) Radiative losses

41. Define polarization.

       It refers to the electric-field orientation of a light signal, which can vary significantly along the length of the fiber.

42. Define Birefringence.

        The polarization modes propagate with the different phase velocities and the difference between their effective refractive indices is called the Birefringence.

43.  What is meant by cross talk?

           It is also referred to as the extinction ratio at the fiber output between the unwanted mode and the launch mode.

44. What are the two types of fiber joints?

        (a) Fiber splices - Semi permanent

        (b) Demountable fiber connectors

45. Define Fresnel reflection?

      When the two joined fiber ends are smooth and perpendicular to the fiber axes, and the two fiber axes are perfectly aligned, a small proportion of the light may be reflected back into the transmitting fiber causing attenuation at joint. This is known as Fresnel reflection.

46. What are the types of misalignment?

      a) Lateral/radial/axial misalignment

      b) Longitudinal misalignment

      c)  Angular misalignment

47. What are the techniques used in splicing?

       a) Fusion Splice

       b) V-groove mechanical splice

       c) Elastic tube splice

48. State the drawbacks of fusion splicing?

      a) Heat necessary to fuse the fiber

      b) With careful handling, the tensile strength of the optical power emitted may be as low as            30% of the uncoated fiber before fusion.

49. Mention the tree types of fiber couplers.

      i) Three and four port coupler   ii) star coupler   iii) wavelength division multiplying devices.

50. List out the variation loss parameters with four port couples?

       a) Excess loss    b) Insertion loss c) cross talk d) split ratio

.Part –B (8-marks)

1)        Sketch the fundamentals mode field in a curved optical wave guide and explain how bending losses occurs?                         (Nov/Dec 2006 Chennai)       (Nov/Dec 2011 Covai)

2)        Find the radius of curvature at which the number of modes (in a bent fiber) decreases by 50% in a graded index fiber α=2, n2=1.5,∆=0.01,a=25µm and γ=1.3µm.                                                                                (Nov/Dec 2006 Chennai)

3)        What is wave guide dispersion? Derive the expression.            (Nov/Dec 2006 Chennai)

4)        Discuss the design optimization of single mode fibers.(Nov/Dec 2006Chennai)

5)        When the mean optical power launched into an 8 Km length of fiber is 120µW, the mean optical power at the fiber output is 3µW.Determine (1) Overall signal attenuation in dB/Km and (2) The overall signal attenuation for a 10 Km optical link using the same fiber with splices at 1 Km intervals, each giving an attenuation of 1 dB.                            (Nov/Dec 2007 Chennai)

6)        Explain with suitable diagrams the different mechanisms that contribute to attenuation in optical fibers.                   (Nov/Dec 2007 Chennai)

7)        Explain the scattering and bending losses that occurs in an optical fiber with relevant diagrams and expressions. (Nov/Dec 2008 Chennai)

8)        Discuss polarization modes dispersion and limitations. (Nov/Dec 2008 Chennai)

9)        Describe the linear and non-linear scattering losses in optical fiber.

                                                                (Nov/Dec 2009 Chennai)

Part –B (16-marks)

1. (i) Derive modal equation(ii) Discuss the modes in step- index fibers.         (Nov/Dec 2005 Chennai)

2. (i) what is meant by material dispersion? (ii) Derive an expression for material dispersion (iii) Derive the expression for wave guide dispersion. (Apr/May 2005 Chennai)     (Nov/Dec 2011 Covai)

3. Discuss the pulse broadening in graded index fibers. (Apr/May 2005 Chennai)

4. Discuss the mode theory of circular wave guide.   (May/Jun2006 Chennai)

5. Discuss various kinds of losses that an optical signal might suffer while propagating through fiber, which is most important one? What is the effect of these losses on light power and pulse shape?          (Nov/Dec 2010  Covai)                                    (May/Jun 2007 Chennai)

6. What is mode coupling? Discuss pulse broadening in Gl fibers.                                                                            (May/Jun 2007 Chennai)

7. Discuss in detail intra-model dispersion with relevant expressions and diagrams (ii) write a brief note on design optimization of single mode fibers.                                                                                             (Nov/Dec 2008 Chennai)

8. Describe the fiber structures utilized to provide dispersion shifting and dispersion flattering in a single mode fiber.                                                                                                         (Apr/May 2008 Chennai)

9. Discuss the theory of GI fiber. Derive the expression for the numerical aperture of GI.                             (Nov/Dec 2011 Covai)

10)  Discuss in detail about wave representation in a dielectric slab wave guides.                      (Apr/May 2011 Covai)

UNIT III – SOURCES AND DETECTORS

11)  1. What are the principle of light sources used for fiber optic  communication? (Apr/May 2011 Covai)

            They are heterojunction structured semiconductor laser diodes (or injection laser, diodes – ILDs ) and light emitting diodes ( LEDs).

2. What is hetero junction?                               (Nov/Dec 2011 Covai)

            It consists of two adjoining semiconductor materials with different band gap energies. These devices are suitable for wide range of applications in fiber transmission system.

3. What is active or recombination region? (Apr/May 2005 Chennai)

            When pn junction is forward biased, electrons and holes are injected  into the p  and n region respectively. These injected minority carriers can recombine either radiatively or non – radiatively. The recombination every is dissipated in the form of heat. This pn junction is known as the active or recombination region.

4. Distinguish between splice and connector. (May/Jun 2007 Chennai)

Splice is a permanent joint and the bonding formed is permanent. Connector is a temporary joint or demountable joint. Splice loss is very small when it is compared with connector loss.

5. What is radiance or brightness? (May/Jun 2006 Chennai)

            Radiance is a measure in watts of the optical power radiated into a unit solid angle per unit area of the emitting surface.

6. What is lambertian pattern? (May/Jun 2008 Chennai)

            In the surface emitter LED, the emission is essentially isotropic with a 120­­^o half power beam width. The isotropic pattern from a surface emitter is called lambertian pattern. In this pattern, the source is equally bright when viewed from any direction. This radiation pattern decides the coupling efficiency of LED.

7. What is emission response?                  (Nov/Dec 2011 Covai)

            Emission response time is a delay between the application of a current pulse and on set optical emission.

8. Mention the LED configurations used in optical fibre links.

            The two basic LED configurations are used in optical fibre links.

(a)   Surface emitters LEDs

(b)   Edge emitters LEDs

9. Define external quantum efficiency? (May/Jun 2007 Chennai)

It is defined as the ratio of photons emitted from the LED to the number of photons generated internally.

10. Name the factors to determine frequency response of a LED.

            The following three factors are used to determine the frequency response of a  LED.

a)     Doping levels in the active region

b)     The injected carrier life time in the recombination region

c)     Parasitic capacitance of the LED.

11.  Define Internal quantum efficiency? (Apr/May 2005 Chennai)

            The internal quantum efficiency (η_int)is define as the ratio of radiative recombination rate to the total recombination rate,

                        η_int =          R_r

                                  ____________   

                                    R_r+R_nr

Where R_r  is radiative recombination rate

R_nr is non-radiative recombination rate.

12. What is gain-guided laser ?

         The injection of electron and holes into the device alters the refractive index of the active layer.

The profile of these injected carries creates a weak, complex waveguide that confinies the light laterally. The type of device is referred to as gain-guided laser.

13. What is meant by modal noise?

            The four fundamental structure are:

I)                 Buried hetero structure

II)               Selectively-diffused construction.

III)             Varying thickness structure.

IV)             Bent layer configuration.

14.  Define SQW and MQW ? (Apr/May 2008 Chennai)

·        Single Quantum –Will (SQW) laser hs a single active region.

·        Multi Quantum-Will (MQW) laser are having multiple active regions.

15. What do you meant by modified multiquantum well laser?

            When the bandgap energy of the barrier layer differ from the cladding layer in a MQW device, it  is usually referred to as a modified quantum-well laser.

16. Mention the advantages of quantum well laser  over DH laser?

            I)  Allow high gain at low carrier density

            II)  Narrow line widths

            III)  Higher modulation speeds

            IV)Lower  frequency chirp

            V) Less temperature dependence.

12)  17. Define quantum efficiency ? (Apr/May 2011 Covai)

Quantum efficiency (η) is define as the number of electron-hole pairs generated per incident photon of  energy, and is given by

            η=number of electron-hole pairs generated

                ___________________________________

                    Number of incident photons

18. Define responsivity ? (Apr/May 2008 Chennai)

            The  responsivity is a useful parameter as it gives  the transfer characteristic of the detector. It is defined as the ratio of output photocurrent to the incident optical power.

                                                R=  I_p

                                                                                    ____   (AW^-1)

                                                      P_O

19. What is adalance effect ?            (Nov/Dec 2011 Covai)

            Due to impact ionization effect new carrier are generated. The newly created carriers also accelerated by high electric field, thus gaining enough energy to cause further impact ionization, this phenomenon is known as avalanche effect.

20.Define output power signal-to-noise ratio of an optical power ?

21. what  is the transit time of the photo carriers?

            The transit time (t_d) of the photocarriers in the depletion region is the ratio between carrier drift velocities (v_d) and the depletion layer width (W) and is given by

                        t_d=W

                             ___

                               V_d

 22. Write down the differences between LED and Laser diodes. (Apr/May 2011 Covai)

           

SNO	LED	LASER
1	Optical output is in coherent	Optical output is coherent
2	No optical resonant cavity	Optical energy from optical resonant cavity
3	Output radiation has broad spectral width	Highly monochromatic
4	No spatial and temproal coherence	It has spatial and temporal coherence

23. Compare homojunctions and heterojunction laser diode.

           

SNO	HOMOJUNCTION LASER	HETEROJUNCTION LASER
1	Because of high threshold current density output power will be low	High output power can be achieved with low threshold current
2	Stability and coherence are very poor	High coherence and stability are obtained

24. What are the laser light properties? How are they produced?

            1. Highly monochromaticity

            2. High intensity

            3. High coherence

            4. High directionality. The above properties are obtained by stimulated emissions.

25. What laser emission is not obtained in a atomic system under thermal equilibrium?

            Under thermal equilibrium the probability for spontaneous emission is far greater than the probability of stimulated emission. Therefore laser emission is not possible. Laser emission can be obtained when there is nonthermal equilibrium i.e. by getting population inversion through pumping the matter.

26. What is meant by laser action? What are the conditions to achieve it?

            Laser action means the amplification of light by stimulated emission of radiation. To get laser action there should be population inversion and stimulated emission should take place.

27. What is meant by threshold condition for laser oscillation?

            There should be a minimum amount of population inversion from which laser oscillation

Starts. This is called threshold condition for laser oscillation. Therefore to start the laser oscillation, the gain coefficient should exceed the threshold value.

28. Mention  the important semiconductors used in laser and LED’s.

            1. InGaAsP  - to produce wavelengths from 1.3 µm to 1.7µm

            2. GaALAs   -  to produce wavelengths from 0.8 µm to 0.9µm

29. What are the drawbacks of homojunctions laser diodes?

            1. Threshold current is very large.

            2. The output beam has large divergence.

            3. Coherence and stability are poor.

            4.Optical confinement is very poor.

30. How will you increase the modulation bandwidth of laser diode?

            1. The average life time of carriers should be very small.

            2. The injection current should be more than the threshold current.

31. What are factors that decrease the life time of laser diode?

            1. Increase of temperature 2. Aging  3. High current density

32. What are the factors that effect the light source linearity?

            1. When the modulation index is greater than 1 or 100 %.

            2. The harmonic distortion and inter modulation distortion.

33.What are the different noises arise during the laser transmission through optical fibre link ?

            1. Modal or speckle noise due to coherent nature of light.          

            2. Mode partition noise due to fluctuations in the intensity of longitudinal modes               of a laser.

            3.Reflection noise due to back reflection at the connector and joints.

34.What are the different factors that produce the degradation of light sources?

            1. Internal damage due to crystal defects in the active medium.

            2. Ohmic contact degradation due to increase of thermal resistance between the                light source  chip and heat sink.

            3. Damage due to decrease in mirror reflectivity and increase of non radiative               recombination at the laser facets.

35.What are the factors that affect the internal quantum efficiency ?

            1. Non-radiative recombination at the boundaries of hetero structure layers.

            2.Self absorption in the active region.

36.Why do we prefer laser diodes over LED’s for communication application ?

            1. Longer life time

            2. Higher modulation rates

            3. Very narrow spectral width of the source

            4. High optical power output

            5. Efficient waveguide structure

37. What is meant by quantum well laser?

            Quantum well laser cavity has dimensions of 50 -100 Å. They have low threshold current.

38. What is a fiber optical amplifier?

            An amplifier with erbium doped or without doped fiber laser working in the optical domain is called fiber optical amplifier.

39. Why gain guided lasers are not in practise?

            Due to their unstability, higly spectral width and highly astigmatic, they are not used.

40. What are the merits of index guided laser?

            1. Higly stable

            2. Optical confinement is very high

            3. Very narrow spectral width

            4. High directionality.

41. How does the LED work ?

            Due to higher population of injected minority carriers there is enormous radiative recombination when there is forward bias across the active layer of LED and light emission occurs.

42. What is the effect of high rise time of drive current in LEDs?

            If the rise time increases, carrier life time is increased and hence radiative reambination rate is decreased. These lead to decrease of bandwidth and output power of LED.

43. Mention any four factors which affect the power launched into the fiber.

            1. Core radius of the fiber

            2. Radiation pattern of the source

            3. Alignment between source and fiber.

            4. Numerical aperture of the fiber.

44. Define source to fiber coupling efficiency.

            η_c =    Power in to the fiber

         ______________________

            Source power

45. What is the effect of N.A.of fiber on coupled power?

            Coupled power is proportional to the square of the numerical asperture of the fiber.

46. Among the different misalignment losses which misalignment loss is the dominant one?

            Axial offset or lateral offset has the maximum loss when it is compared with the angular off set loss and longitudinal displacement loss.

47. How will you improve fiber to fiber coupling efficiency?

            1. By Proper alignment of two fibers such that axes are in the same straight line.

            2. by introducing index matching liquid between the two fibers.

            3. The two fibers should have same core radius, same refractive index profile and same N.A.

48. How will you improve the laser diode coupling with fiber?

            The coupling efficiency is improved by taper ended fiber so that taper end width is equal to the width of laser beam.Further using spherical or cylindrical lenses one can improve the coupling when the source has smaller emitting area.

49.  What is phonon?

            Phonon is a quantum of energy in the lattice vibrations of crystal.

50. What are the advantages of elastic tube splice?

            1. It performs automatically lateral, longitudinal and angular alignments.

            2.  Fibers with different diameters can be spliced.

Part –B (8-marks)

1)        Draw a typical double- hetero structure light emitter and explain?

                                                                           (Nov/Dec 2006 Chennai)   

2)        Derive the threshold condition for LASER.       (Nov/Dec  2011 Covai)       (Nov/Dec 2006 Chennai)

3)        Explain the laser action with neat diagrams.    (Nov/Dec 2006 Chennai)

4)        A GaAs laser operating at 850 nm has a 500µm length and a refractive index n=3.7.What are the frequency and wavelength spacing’s?                                                                                               (Nov/Dec 2006Chennai)

5)        Draw and explain the different structures used to achieve carrier and optical confinement in laser diodes.                                                                                                             (Nov/Dec 2008 Chennai)

6)        Discuss the effects of temperature on the performance of a laser diode.                                                               (Nov/Dec 2008 Chennai)

7)        Derive expressions for the power coupled from a surface emitting LED into step index and graded index fibers.                                                                                      (Nov/Dec 2008 Chennai)

8)        Explain the mechanical misalignments that can occur between two joined fibers with necessary diagrams.                                                                                                              (Nov/Dec 2008 Chennai)

9)        Compare the threshold current densities at 32^oC and 100^oC for a AlGaAs injection Laser with T_o=160K and the similer ratio for an InGaAlP device with T_o=55K.             (Nov/Dec 2009 Chennai)

Part –B (16-marks)

1.  Draw the structures of edge-emitting LED and surface emitting LED and explain the operation.          (Nov/Dec 2006 Chennai)   (Nov/Dec 2005 Chennai) (Apr/May 2005 Chennai)

2. Discuss the LASER diode principle, modes and threshold conditions.        (Nov/Dec 2005Chennai)

3. Discuss the laser diode structures and radiation patterns.(Apr/May 2005 Chennai)

4. (i) what is meant by hetero junction? Give example. (ii) Derive the internal quantum  

         Efficiency of an LED.                           (May/Jun 2006 Chennai)

5. (i) Explain optical feedback and laser oscillation.(ii) Derive the threshold condition for lasing .          (May/Jun 2006 Chennai)   (Apr/May 2008Chennai)

6. With neat diagram explain the construction and working of high radiance surface emitting LED.                                              (May/Jun 2007 Chennai)

7. Discuss about modulation of Laser diodes. Why thermoelectric cooler are used in Laser diodes?                                         (May/Jun 2007 Chennai)

8. (i) Draw and explain the LED structures based double Hetero structure configuration.(ii) Discuss the principle of operation of laser diodes .What are the effects of temperature on the performance of a laser diode?.  (Nov/Dec 2010Covai)   (Nov/Dec 2007Chennai)

9. (i) Explain the different  lensing schemes available to improve the power coupling efficiency.(ii)Explain  the fiber splicing techniques with necessary diagrams.

                                                                                      (Nov/Dec 2007Chennai)

10)  Explain the different lensing scheme available to improve the power coupling efficiency.(Apr/May 2011 Covai)      (Nov/Dec 2010Covai)

11)   Explain various fiber splicing techniques.           (Nov/Dec 2011Covai)  

UNIT IV – FIBER OPTIC RECEIVER AND MEASUREMENTS

1. What is meant by quantum limit?               (May/ Jun 2007. Chennai)

       To find the minimum received optical power required for a specific bit error performance

In a digital system. This minimum received power level is known as the quantum limit.

2. What is inter symbol interference (ISI)? (Nov/Dec 2005 Chennai)

       ISI occurs from pulse spreading in the optical fiber when a pulse is transmitted in a given

Time slot, most of the pulse energy will arrive in the corresponding time slot at receiver.

3. What is bit-error rate?                     (Apr/May 2011 Covai)\

       To divide the number of error (Ne) occufing over a certain time interval t by the number of pulse (Nt) transmitted during this interval.This is called either the error rate or bit error rate

(BER).

                          Ne   Ne

                    BER= — = ―

                                Nt    Bt

4. Define avalanche effect.                              (Nov/Dec 2011 Covai)

        Due to ionization effect new carriers are generated the newly created carriers also accelerated by high electric field and gain enough to cause a further impact ionization this phenomena is known as avalanche effect.

5. Define impact ionization.                            (Nov/Dec 2007 Chennai)

        A photo generated electron (or) hole gain enough energy  in this high electric field and excte a new electron hole pairs. This process is known as impact ionization.

6. What are the types of photodetectors?       (May/Jun 2006 Chennai)

        There are five types of photodetectors followed by

§  Photo multipliers

§  Pyro electric detectors.

§  Semiconductor based photo conductors.

§  Photo transistor.

§  Photo diodes.

7. How model noise can be minimized?          (May/Jun 2009 Chennai)

·        Use single mode fibers,because it supports only one mode.

·        Use LEDs.This totally avoids model noise.

·        Use a laser which has a large number of longitudinal modes.

·        Use low loss connectors.

·        Use a fiber with a lorge numerical aperture.

8. What is meant by pseudorandom?                   (Nov/Dec 2011Covai)

         The word pseudorandom means that the generated combination or sequence of ones and zeros will euentually repeat but that it is sufficienly random for test purposes.A pseudorandom

Bit sequence comprises four different 2-bit long combinations,eight different 3-bit long combinations,sixteen different 4-bit long combinations and so on up to a limit set by the instrument.

9. Define responsivity of photodiode.                     (Nov/Dec 2005 Chennai)

         It is defined as the ratio of output photocurrent to incident optical power

                         Ip     output photocurrent

                  R= — =———————————

                         Po    incident optical power

10. Define photo diode dark current. (Nov/Dec 2009 Covai)

         It is the current that continues to flow through the bias circuit of the device when no light

Is incident on the photo diode. This is a combination of bulk and surface current.

11. What is flicker noise?

         Flicker noise arises from surface and interface defects and traps in the bulk of the

Semiconductor.

12. List out the values of operating wavelength and responsivity of Si, Ge and GaAs photo diodes.                                    (Nov/Dec 2009 Covai)

Photo diode	Wavelength	Responsivity
silicon	0.3 – 1.1 µm	0.5 A/W
GaAs	1 - 16µm	0.8A/W
Germanium	0.5 – 1.8µm	0.6A/W

    

13. Define quantum efficiency of a photo detector.  (Nov/Dec 2005 Chennai)

         Quantum efficiency (η) is defined as the number of electron-hole pairs generated per incident photon of energy,and is given by

                        Number of electron-hole pairs generated

                   η= ——————————————————

                               Number of incident photons

14. A silicon avalanche photodiode has a quantum efficiency of 65% at a wavelength of 900nm.If 0.5µW optical power produces a multiplied photocurrent of 10µA. Determine its primary photon current and multiplication factor. (Nov/Dec 2010 Covai)

  Given data:

          Δ=65%=0.65,      λ=900nm,      Po=0.5µm,   Im=10µm

                           Im                      ηq          ηqλ

                   M= ——,   Ip=RPo=—— Po =——Po

                           Ip                       hv           hc

                       (0.65)(1.602x10-19)(900x10-9)

                Ip=       ——————————         5x10^-7

                              (6.625x10-34)(3x10^8)

                  Ip =0.2358µA

                         Im       10µA

                  M=——=    ——                  =42.4

                         Ip         0.2358µA

15. List the important requirements of an optical receiver.                                                                       (Nov/Dec2006Chennai)

·        It should respond the signals in a fast manner so that there is no band width limitation

From receiver side.

·        It should have high sensitivity to the signals at the operation wavelength.

·        It should have low noise level so that it has tolerable SNR.         

·        It should reproduce the original signal without any distortion.

16. Explain splice loss?                                   (Apr/May 2008 Chennai)

            Splice losses may be divided into intrinsic splice losses and extrinsic splice losses

Intrinsic splice loss is due to change of refractive index of cores of fiber during fusion and misalignment of fiber.Extrinsic splice los is due to thermalstresses at the joints.

                                           Pf2

   Spliceloss in dB=-10log——

                                           Pf1

Pf2=Coupled power in the receiving fiber, Pf1=power from the emitting fiber.

17. On what factors the response time of a photo detector depends? (Nov/Dec 2010 Covai)

     It mainly depends on

·        Transit time of photo carriers.

·        Diffusion time of photo carrier

RC time constant of photodiode

18. What are the advantages of photodiodes? (Apr/May 2005 Chennai)

         a. Small size

         b. Suitable material

         c. High sensitivity

         d. Fast response time

19. What is meant by excess noise factor?

          The ratio of the actual noise generated in an avalanche photodiode to the Noise that would exist if all carrier pairs were multiplied by exactly m is called the  Excess noise factor (F).

F = (m2) = (m2)

      (m) 2         m2

20. Give the advantages of Pin photodiodes. (Nov/Dec 2007 Chennai)

·         Very low reverse bias is necessary

·        High quantum efficiency

·        Large bandwidth

·        Low noise level

21. Advantaades and Disadvantages of  APD. (Nov/Dec 2009 Covai)

        Advatages    

·        Excellent linearity

·        Detection of very low light level.

·        Increase in sensitivity(5 to 15 dB).

        Disadvatages

·        Cost is high.

·        High bias voltage is required (50 to 400V)which are wavelength dependent.

·        The random nature of photodiode gives an additional noise.

22. Compare the direct bandgap and indirect bandgap.

       Direct bandgap:

·        Conduction band gap electron can recombine with directly with hole in the valance band.

·        Emission of light is possible.

·        Minimum energy of conduction band and maximum energy of valance band having same value of wave vector.

 Indirect bandgap:

·        Conduction band gap electron can recombine with indirectly with hole in the valance band.

·        Emission of light is not possible.

·        Minimum energy of conduction band and maximum energy of valance band having different value of wave vector.

23. . Define long wavelength cut off related to photodiode.

            The upper wavelength cutoff (_c) is determined by the band gap energy Eg of the material. If Eg is expressed in units of electron volts (eV), then _c is given in Units of micrometers (μm) by _c (μm) = hc = 1.24

Eg Eg(eV)

24.What is avalanche multiplication noise?

         Avalanche multiplication noise is due to variation of avalanche gain in a statistical manner.Further it is also due to dependence of gain with temperature.

25. What are the required properties of photodetector?

·        High quantum efficiency.

·        Low rise time or fast response.

·        Low dark current.

26. What are the different noises present in the avalanche photodiode?

·        Quantum noise  

·        Dark current

·        Thermal noise and

·        Avalanche multiplication noise.

27. What are the types of photodiodes?

·        PIN photo detector

·        Avalanche photodiode(APD)

28. Define photocurrent.

         The high electric field present in the depletion region causes the carriers

to separate and be collected across the reverse-biased junction. This gives to a current flow in the external circuit, with one electron flowing for every carrier

pair generated. This current flow is known as photocurrent.

29. Define ionization rate.

       The avg. no. of electron hole pairs created by a carrier per unit distance

traveled is called ionization rate.

                         ß

                   K=——

                         

ß-Hole ionization rate,  -Electron ionization rate

30. What are the conditions to be met for a high signal- to- noise ratio in a photodetector?

·        The photodetector must have a high quantum efficiency to

Generate a large sign al power

·        The p and amplifier noises should be kept as low as possible.

31. Give the classifications of preamplifiers.

·         Low impedence(LZ) preamplifier

·         High impedence(HZ) preamplifier

·         Transimpedence preamplifier.

32. Define Avalanche multiplication.

         In the high electric field the charge carriers’ multiplication take place by a impact ionization (or) avalanche ionization is known as avalanche region or multiplication region.

                      Im

               M= ——

                       Ip

Im-Avarage value of total multiply output current, Ip-primary and multiply photocurrent.

33. Give the 2 analysis that are used to ensure system performance?

The 2 analysis that are used to ensure system performance are:

·        Link power budget analysis

·        Rise time budget analysis.

34. Explain briefly about link power budget analysis?

   In the optical power loss model for a pt-to-pt link, the optical power rxed at the photo detector depends on the amount of light coupled into the fiber & losses occurring in the fiber at the connectors & splices. The link loss budget is derived from the sequential loss contribution of each element in the link.

Loss=10 log (Pout)

(Pin)

The total optical power loss is, PT = PS – P

35. Explain briefly about rise time budget?

           Rise time budget analysis is a method for determining dipersion limitation of an optical power fiber link. The system rise time can be determined from rise time of each component such as the transmitter. the fiber cable and the receiver in the system.

36. What is current mode of operation of photodiode?

           In photo conducting mode, the photocurrent is slightly dependent on the

reverse bias. For a constant reverse bias, the current is linear. This is called current mode of operation of the photodiode.

37. What are the system requirements?

The following are the key system requirements.

·        The desired or possible transmission distance

·        The data rate or channel bandwidth

·        Bit error rate (BER).

·         

38. A Photodiode is constructed of GaAS, which has a band-gap at 1.43ev at 300k.what is meant by long wavelength cutoff.

Sol: band gap energy,Eg =1.43ev

       Cut off wave length,λc =1.24 / Eg(ev)

                                             = 1.24 / 1.43   =08671µm or 869 nm.

39. What are the benefits of a trans-impedance amplifier?

       1) It has a wide dynamic range and layer bandwidth compared with high impedance   network.

       2) Trans-impedance amplifier’s are very useful due to wide dynamic range.

       3) Smaller receiver sensitivity degradation.

40. Mention the important photo detector material. Why it is preferred?

      InGaAsP.since at long wavelengths, it has very low dark current,low rise time and high quantum efficiency.

41. What is dark current noise?

      Dark current noise is due to the flow of current through the bias circuit even though there is no incident light.

42. What are the different components of an optical receiver?

1. Optical detector 2.pre-amplifier 3.variable gain voltage amplifier 4.Fixed gain amplifier 5.Equalizer 6.Decoder.

43. What are the requirements for a preamplifier?

  It should have low noise level, high bandwidth, high dynamic range, high sensitivity to avoid non-linearities and high gain.

44. What are the main advantages of InGaAS photo diodes?

       It has high sensitivity in the wavelength range from 1.0 to 1.8 um. It has high absorption, fast response, low dark current and high quantum- efficiency.

45. What is the drawback of diode capacitance?

The frequency response of the diode is reduced by the diode capacitance. At high frequencies, the diode capacitance acts as e shunt across the output resistance. Further diode capacitance, CjαV^-1/3. That is, it is dependent on bias voltage in a nonlinear manner.

46. What are the various error sources in the optical receiver?

1. Quantum noise (shot noise) source from the fiber optic link.

2.  Bulk dark current noise source.

3. Surface leakage current noise source.

4. Avalanche multiplication fluctuation noise source.

5. Thermal noise source from the load resister and amplifier components.

6. Amplifier noise source.

           7. Inter symbol interference noise source.

47. Define probability of error. Write an expression for it.

Probability of error means that a transmitted ‘1’in misinterpreted as a ‘0’ (or) transmitted’0’ is misinterpreted as a’1’ by receiver. Probability of error,

Pe=1/2[1-erf [V/2√ 2 σ]]

           Where V/σ is the peak signal to rms noise ratio.

48. Express quantum limit in terms of minimum power and S/N ratio at the receiver.

(Po) min=[S/N] 2hvB/η

Where η= quantum efficiency and B= bandwidth.

49. At 1.3µm, the quantum efficiency of InGaAs photodetector is 0.6.Calculate its responsivity.

     

      R=ηq / hv  = η q λ / h c = 0.6*1.6*10^-19*1.3*10^-6 / 6.625*10^-34 *3*10⁸   = 0.628 A/w.

50. Give the classification of pre amplifiers.

      1. Low impedance (LZ) preamplifier

      2. High impedance (HZ) preamplifier

      3. Transimpedance preamplifier .

Part –B (8-marks)

1)     Explain the error sources in the optical receiver.(Nov/Dec 2006 Chennai)   

2)        A silicon p-i-n photodiode incorporated into an optical receiver has a quantum efficiency of 60 % at a wavelength of 0.9µm.The dark current is 3 nA and load resitence is 4 K ohms. he incident optical power is 200 nw and the receiver bandwidth is 5 MHz .Determine(1)Mean square quantum noise current,(2)Mean square dark current and(3)Mean square thermal noise current at a temperature of 20 degree.                       (Nov/Dec 2007 Chennai)

3)        Discuss the different noise sources and disturbances in the optical pulse detection mechanism.                       (Nov/Dec 2007 Chennai) (Nov/Dec 2008 Chennai)

4)        Derive an expression for the bit error rate of an optical digital receiver.

(Nov/Dec 2007Chennai)                            (Nov/Dec 2011Covai)

5)        When 3* 10 ¹¹ photons each with a wavelength of 085µm are incident on a photodiode on average 1.2*10 ^11. Electronics are collected at the terminals of the device. Determine the quantum efficiency and responsivity of the photodiode at 085µm.                                                                                     (Nov/Dec 2008 Chennai)

6)        Draw and explain the high impedance preamplifier designs based on BJT and FET.                                 (Nov/Dec 2008 Chennai)

7)        Write a brief note on transimpedance amplifier. (Nov/Dec 2008 Chennai)

8)        Describe the various sources of noise that affect the performance of optical receivers. Derive expressions for the S/N of APD receiver.(Nov/Dec 2009 Chennai)

9)     Discuss in brief about noises in photo detector? (May/Jun 2011 Chennai)

10)     Write short notes on Detector response time?   (May/Jun 2011 Chennai)

11)     Briefly describe about Avalanche multiplication noise?(May/Jun 2011 Chennai)

Part –B (16-marks)

1.Draw the schematics of pin photodiode and APD and explain.         (Nov/Dec2005 Chennai)   

2.  Explain the fundamental receiver operation in optical communication.

                                    (Nov/Dec 2005Chennai)     (Nov/Dec 2011Covai)

3. An InGaAs pin photodiode has the following parameters at a wavelength of1300 nm: I D = 4 nA? = 0.9, RL=1000ohms and the surface leakage current is negligible. The incident optical power is 300 nw (- 35dBm), andthereceiverbandwidthis20 MHz Find the various noise terms of the receiver. (Apr/May 2005 Chennai)

4. Discuss the performance of digital receiver by defining the probability of error.                                                                           (Apr/May2005Chennai)

5. Draw the structure of avalanche photodiode and electric fields in the depletion and multiplication regions. Also explain the principle of operation of APD.           (Nov/Dec 2006 Chennai)  

6. Discuss the principle of operation of APD with neat circuit Diagram. Also discuss the requirements of photo detector. (Nov/Dec 2011Covai)   (May/Jun 2006 Chennai)

7. What is known as quantum limit? A digital fiber optic link operating at850 nm requires a maximum BER of 10*9. Find the minimum incidental optical power Po to achieve this BER at a data rate of 10 Mb/s for as implebinary level signaling scheme. (ry: 1), [1/r : B/2].                                                (May/Jun 2007 Chennai)

8. Discuss in detail digital receiver performance calculation and sensitivity calculation in detail?                                   (May/Jun 2007 Chennai)

9. Distinguish between PN diode, PIN diode and APD, with respect to structure and recombination of charges. Is it possible to make these three types of photodiodes using the same semiconductor?  Justify.                                                                        (Apr/May2008Chennai)                                                                       

10. Explain the construction & working of PIN photo detector in detail.                              

                                                                  (Nov/Dec 2010Covai)       

UNIT V – OPTICAL NETWORKS

1. Define network?                              (Nov/Dec 2010Covai)

           Network is defined is to establish connections between these stations , one interconnects them by Transmission  paths to form a network.

 2. Write short notes on rise-time budget. (Nov/Dec 2010 Chennai)  

            A rise time budget analysis is a convenient method for determining the dispersion limitation of an optical link. In this approach the total rise time t_sy of the link is the root-sum-square of the rise times from each contributor ‘t_i’ to the pulse rise-time degradation.

                                                                  t_sys = ( _i²)^1/2

3.      What are the types of logical topology? (Nov/Dec 2006 Chennai)  

           Three common topologies are:

(a)   bus topology.

(b)   ring topology.

(c)   star topology.

4.      State tap loss?                                         (May/Jun 2007 Chennai)

             The power extracted from the bus is called a tap loss and is given by

             Ltap=(-10log Ct)                   where  Ct is the symmetric coupler.

5. Define the term simplex  and full duplex ?  (Nov/Dec 2006 Chennai)  

      The term simplex means that in this configuration, information flows only from left to right. In full duplex communications, in which stations can communicate in either direction, but Simultaneous on both directions.

6.      What is SONET?                                                    (Nov/Dec 2011Covai)

        Synchronous Optical Network is a standard developed by ANSI for fiber optic networks. SONET defines a synchronous frame structure for transmitting TDM signals in the optical fiber networks. It encodes bit streams into optical signals that are propagated over optical fiber.

7. What is SDH?                                              (Apr/May2007Chennai)

         SDH is Synchronous Digital Hierarchy.it is standard developed by ITU-T for fiber optic networks.It is used in North America  and the SDH is the international version of  SONET.

8. What is DWDM?                                          (Apr/May2005Chennai)

           Dense Wavelength Division Multiplexing is an optical technology used to increased bandwidth over existing fiber-optic backbones. It works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fibers.

9.What is meant by wave length division multiplexing?                                                               (Apr/May2005Chennai)

           Wave length division multiplexing is one in which each source sat a different peak wavelength, the integrity of the independent messages from each source is maintained for subsequent conversion to electric signals at the receiving end

10. What is meant by power penalty?             (Nov/Dec 2010Covai)

              When nonlinear effects contribute to signal impairment, an additional amount of power will be needed at the receiver to maintain the same BER. This additional power (dB) is known as the power penalty.

11. What is meant by dispersion length (L _disp)? (Apr/May2005Chennai)

             The normalized distance parameters are called as dispersion length, Ldisp .it is a characteristic Length for the effects of the dispersion term

                                  L=

                      Where

                        C is the speed of light

                        D is the dispersion of the fiber

                        L disp is measured in km.

12. What are the uses of WDM? (Nov/Dec 2009 Chennai)  

            It’s used in angularly dispersive device such as prisms or gratings.

            Filter- based devices.

13. What is LAN? (Nov/Dec 2006 Chennai)  

The local area network is an interconnection topology which is usually confined to either a single building or group of building contained entirely within a confined site or establishment. LAN is typically operated at a transmission rate between 100 k bits^-1 and 100 M bits^-1 over distance of 500m to 10km.LAN is an intermediate between a short range, multiprocessor or network

14. Define crosstalk? (May/Jun 2007 Chennai)

      Crosstalk is defined as the feed through of one channels signal into another channel.

15. Mention the type of crosstalk?

          There are two types of crosstalk.

(a)   Intra channel crosstalk

(b)   Inter channel crosstalk.

16. How intra channel crosstalk aries?

           It aries when interfacing signal is at the same wavelength as the desired signal.this effect is more severe than interchannel crosstalk.

17. What is solitons?                    (Nov/Dec 2006 Chennai)  

             A solitons are pulses that travel along the fiber without change in shape or amplitude or velocity

18. How the speckle pattern can form?

             The speckle patterns are formed by the interference of the modes from a coherent source when the coherence time of the source is greater than the intermodel dispersion time within the fiber.

19. Define fundamental solitons?  (May/Jun 2007 Chennai)

             The families of pulses that do not change in shape are called fundamental solitons.

20. What are the two different types of WDM? (May/Jun 2009 Chennai)

                 (a) unidirectional WDM.

                 (b) bidirectional WDM

21. Define full-width half-maximum (FWHM)?

             The FWHM is a pulse is defined as the full width of the pulse at its half-maximum power level.

22.  Define Kerr effect?

         Nonlinearity produces a carrier-induced phase modulation of the propagating signal , which is called Kerr effect/

23. Give the important features of time-slotted optical TDM network?

Ø  To  provide backbone to interconnect highspeed neteorks;

Ø  To transfer quickly very large data blocks;

Ø  To switch large aggregations of traffic ;

Ø  To provide both high-rate.

24. What is called higher-order solitons?

             The family of pulses that undergo periodic shape changes are higher-order solitons

25. What are the basic performance criteria of WDM technique?

                 (a) Insertion loss.

                 (b) channel bandwidth.

                 (c) crosstalk.

26. Define the amplifier gain of an optical fiber?

             The signal gain (or) amplifier gain of an optical amplifier is defined as :

                                G=

         Where   Ps,in is the input of optical signal is amplified

                         Ps,out is the output of optical signal is amplified.

27. What are the requirements in analyzing a link?

            The desired transmission distance,       The data rate or channel bandwidth,The bit error rate

28. What is line coding?

            Single encoding uses a set of rules for arranging the signal symbols in a particular pattern. This process is called channel or line coding.

29. What is the function of line coding?

            The function is to introduce redundancy into the data stream for the purpose of minimizing errors resulting from channel interference effects.

30. What are the basic types of two level binary codes used for optical fiber transmission link?

            Non-return-to-zero (NRZ)

            Return-to-zero (RZ)

            Phase-enclosed(PE)

31. What are NRZ, RZ and PE?

            In NRZ codes a transmitted data bit occupies a full bit period.

            For RZ formats the pulse width is less than a full bit period.

            In the PE format both full width and half-width data bits are present.

32. What is meant by topology?

          The topology is the logical manner in which nodes are linked together by information       Transmission channels

33. What are the basic elements that limit system speed?

            Transmitter time t_tx

            The material dispersion rise time t_mat of the fiber

            The model dispersion rise time t_mod of the fiber

            The receiver rise time t_rz

34 .In multimode fibers the rise time depends on

            Modal, material dispersion.

35. Define noise margin.

It is the percentage ratio of the peak signal voltage V₁ for an alternation bit sequence to the maximum signal voltage V₂ as measured from the threshold level.

36. State Link Bandwidth?

            If the N transmitters in a WDM link operate at bit rates  of B1 through BN .the total bandwidth           Is

37. What are the applications of optical amplifier?

            Used to reduce transmission loss and increase the distance between repeaters.

            Used as a front end amplifier for an optical receiver.

            Used as LAN booster amplifier.

38. What are the types of broadcast and select network?

          (i) Single-hop networks,          (ii)Multi-hop networks.          

39. Give the advantages of star system.

            Low optical loss

            Freedom from echopulses.

            Easily designed to give loss and delay which is same for all apair of terminals.

40. What are the various applications of LAN system?

Sl.No	System	Service contents
1	Home use subscriber (LAN) system	TV broad cast service, telephones, home facsimile, video phones, VRS, captain
2	Business use subscriber (LAN) system	Telephone ,PBX ,high speed , data, still picture ,ITV, TV conference, VRS
3.	On premises system	Telephone, data, PBX, tv conference ,ITV
4	CATV dixtribution system	Tv broad casting, interactive tv, high definition tv.

41. What are the basic performance criteria of WDM technique?

            Insertion looss

            Channel width

            Cross talk

42. Define insertion loss.

Insertion loss defines the amount of power loss that arises in the fiber optic line from the addition of a WDM Coupling device.

43. Define channel width.

            Channel width is the wavelength range that is allocated to a particular optical source

44.  What is cross talk?

            That is refers to the amount of signal coupling from one channel to another.

45. What is the tolerable level of inter channel crosstalk?

            Between -20 &-30db level

46. What is the typical insertion loss of a WDM coupling devices?

            1-to3-db range

47. What is meant by cross-phase modulation (XPM)?

            Cross-phase modulation, which converts power fluctuations in a particular wavelength channel to phase fluctuations in the co-propagating channels.

48. What is meant by topology in LAN?

            The manner in which modes are geometrically arranged and connected is known as the by topology of the LAN

49. What are the three basic LAN topology?

            1. Bus, 2. Ring 3. Star

50. What is meant by Ring architecture?

In the ring network, any information fed into the network by a subscribe passes through all the network nodes and hence a number of transmission channel must be provided between all nodes. In this case only information addressed to  a particular subscribe is taken from the subscribe nodes.

51. What is meant by star architecture?

            In a star architecture all nodes are joined a single point called the central node or hob.

Part –B (8-marks)

1)        Explain the operational principles of WDM.      (Apr/May2005Chennai)                                                                        (Nov/Dec 2005 Chennai)

2)        Explain the rise-time budget.                                              (Nov/Dec 2005 Chennai)

3)        Describe the key features of WDM.                (Nov/Dec 2006 Chennai)                  (Apr/May2005Chennai)

4)        Discuss the noise effects on fiber optic system performance.                                                                                                    (Nov/Dec 2006Chennai)  

5)        Explain fiber optic rise time budget with an example.                                                                                (Nov/Dec 2006 Chennai)

6)        Explain the principle of operation of Erbium doped fiber amplifier. Describe the approaches to achieve flatness in its gain. (Nov/Dec 2007 Chennai) (Nov/Dec 2009 Chennai)

7)     Give a brief account of the principles of SONET.              (Nov/Dec 2007 Chennai)

8)        Give a brief account of the different types of losses to be considered in the design of an optical link.                      (Nov/Dec 2008 Chennai)

9)        Draw and explain the basic format of an STS-N SONET frame.                                                                  (Nov/Dec 2008 Chennai)

10)  Why are non-linear effects observed in optical fibers? Why do they become pronounced at high power levels?(Apr/May2008Chennai)

11)     What are the unique properties of a soliton? Explain.                                                                                (Apr/May2008Chennai)

12)     Draw the Soliton pulse it’s enveloped and describe its fundamentals and its propagation in optical fibers.  (Nov/Dec 2009 Chennai)

13)     Explain the Layers of SONET with neat sketches.                                                                                      (Nov/Dec 2009 Chennai)

14)     Describe the operations of Unidirectional and Bidirectional WDM.                                                            (Nov/Dec 2009 Chennai)

15)     What are the key system requirements and characteristics required in analyzing a point- Point link?                        (Apr/May2010Chennai)

Part –B (16-marks)

1.  Explain various types of fiber splicing techniques and fiber connectors.                                                                                               (Nov/Dec2005 Chennai)   

2.  Explain the rise-time budget of a fiber –optic point – to – point link.                                                                                               (Apr/May2005Chennai)

3. Explain fiber optic link power budget with an example.                                                                             (Nov/Dec2006Chennai)

4. Draw the point-to -point fiber optic link and discuss the system considerations.   (May/Jun2006Chennai) (May/Jun2007Chennai) (Nov/Dec2011Covai)

5. Discuss the principle, requirement and applications of WDM.                                  (Apr/May2010Chennai)  (May/Jun2006Chennai) (Nov/Dec2011Covai)

6. Discuss in detail Fiber splicing and connectors. Explain the operation principles of WDM.                                                                (May/Jun2007Chennai)

7. Explain the significance of link power budget and rise time budget with one illustration for each.                            (Nov/Dec 2007Chennai) (Apr/May2010Chennai)

8. Explain the salient features of solitons using relevant expressions and diagrams.                                               (Nov/Dec 2008Chennai)

9. With necessary equations, explain in detail about Rise-time Budget analysis.                                                       (Nov/Dec 2010Covai)

10. With the help of suitable diagram explain about SONET/SDH.     (Nov/Dec2010Covai) (Apr/May2010Chennai) (Apr/May2011Covai)

11. Explain in detail about explain in detail about Erbium-doped fiber amplifiers with neat diagram? Also derive the necessary equation for power conversion efficiency and gain?           (Apr/May2011Covai)