Question Bank Optical Fiber Communication

CHAPTER: 1

  1. Briefly describe the block diagram of Optical Communication Systems and Mention the applications of optical communication.
  2. Give the advantagesand disadvantages of optical   communication   over   conventional communication. State the optical transmission windows.


CHAPTER: 2

Define:
  1. Acceptance angle& Acceptance cone.
  2. Critical angle.
  3. Numerical Aperture
  4. Refractive index
  5. ∆
  6. Normalized frequency
  7. Cutoff wavelength
  8. MFD
  9. Leaky ,Guided and radiation modes

Compare:

  1. Step and Graded index fibers.
  2. Single mode & Multimode Fiber.
  3. Skew rays and Meridional rays

  1. Using simple ray theory, describe the mechanism for the transmission of light within an optical fiber and show how acceptance angle is related to the fiber numerical aperture.
  2. Explain TIR. And give the conditions for total internal reflection.
  3. Explain Mode of fiber and “A mode remains guided as long as propagation factor p satisfies the condition n2k< p< n1k”.
  4. Types of fiber
  5. Give the requirement for fiber material and explain in brief glass fiber and plastic fiber.
  6. Explain fiber drawing techniques and fiber fabrication techniques:
    1. OVPO
    2. VPAD
    3. MCVD
    4. PCVD
    5. Double crucible method


EXAMPLES:

  1. A step index fiber in air has a numerical aperture of 0.16, a core refractive index of 1.45 and a core diameter of 60 µm. Calculate refractive index of cladding, relative refractive index difference (∆) and acceptance angle. Determine the normalized frequency for the fiber when light at a wavelength of 0.9 µm is transmitted. Further, estimate the number of guided modes propagating in the fiber.
  2. A fiber with n1=1.5 and n2=1.47, find acceptance angle, NA,critical angle and ∆.
  3. A multimode step index fiber with a core diameter of 80µm and a relative index difference of 1.5% is operating at a wavelength of 0.85 µ If the core refractive index is 1.48 calculate the normalized frequency for the fiber and the number of guided modes.
  4. A multimode step index fiber has a refractive index difference of 1% and a core refractive index of 1.5.The number of modes propagating at a wavelength of 1.3µm is 1600.Calculate the acceptance angle, numerical aperture and the diameter of the fiber core
  5. The refractive index of the core of step index fiber is 1.46 and the relative refractive index difference between core and cladding of the fiber is 2%. Estimate (1) Numerical Aperture (2) Critical angle at the core cladding interface within the fiber.
  6. A multimode step index fiber with relative refractive index difference 1.5% and core refractive index 1.48 is to use for single mode operation. If the koperating wavelength is 0.85µm calculate the maximum core diameter.
  7. A step index fiber having M=1.47, a=4.3, ∆=0.20%, find out cut off wave length.

CHAPTER: 3
  1. Describe briefly the losses in optical fibers.
  2. What is dispersion in optical fiber? Classify different dispersion with reasons. Explain its effect in optical communication. How to reduce the effect of dispersion?
  3. Justify :Signal distortion mechanism in optical fiber limits the information rate of the signal
  4. Define signal attenuation and how is it mathematically expressed. Explain the following:
    1. Scattering Losses.
    2. Bending losses.
  5. Explain difference between micro bending and macro bending loss. An optical signal at a specific wavelength has lost 55% of its power after traversing 3.5 Km of fiber. What is the attenuation in dB/km of this fiber?
  6. How does material dispersion occur in an optical fiber? Obtain the expression for group delay Tmat resulting from the material dispersion and from this, deduce the relation for the pulses spread Ϭmat in terms of material dispersion Dmat (λ).
CHAPTER: 4

  1. Explain the principles of operation of the laser using suitable diagrams.
  2. Briefly discuss the advantages and drawbacks of the LED in comparison with the laser for use as a source in optical fiber communication.
  3. Explain different types of LEDs.
  4. Discuss briefly the distributed feedback and Fabry perot LASER with neat sketch.
  5. Explain following with respect to laser diodes:
    1. Modes of the cavity
    2. Threshold conditions for lasing.
  6. Explain the significance of the following terms related to the optical source
    1. Optical confinement
    2. Carrier confinement
    3. Internal Quantum efficiency
    4. External Quantum efficiency.
    5. Population Inversion
  7. Explain the high radiance surface emitting LED. Highlight the drawbacks of same and how it can reduce with the help of edge emitting LED.
  8. Describe with a neat and labeled sketch, the construction of an Edge emitting LED
  9. Explain Light source Material.
  10. Explain Longitudinal Modes and Frequency Separation with equation.
  11. Laser Diode Rate Equation.

Examples:
  1. A continuous 12 km long optical fiber link has a loss of 1.5 dB/km: (a) What is the minimum optical power level that must be launched into the fiber to maintain an optical level of 0.3 µW at the receiving end. (b) What is the required input power if the fiber has a loss of 2.5 dB/km.
  2. A Double heterojuction LED operating at 1310 nm has radiative and non-radiative recombination times of 30 and 100 ns respectively. The current injected is 40 mA. Calculate.
    1. Bulk Recombition
    2. Internal quantum efficiency.
    3. Internal Power Level.
  3. Find optical Gain at Threshold of a Laser diode having following parameter R1=R2=0.32, effective absorption coefficient=10 cm-1 and L=500µm.
  4. A GaAs Laser operating at 850 nm and has length of 500µm, refractive index n=3.7. Calculate frequency and wave length spacing.

CHAPTER: 5
  1. Derive the equation for the power launched from LED Source in to a G.I. & S.I fiber.
  2. What is splicing? Explain different techniques of splicing.
  3. Write a short note on fiber connectors.
  4. "The optical power launched into a fiber does not depend on the wavelength of the source but only on it's brightness" Justify.
  5. Explain with neat sketches possible lensing schemes for optical sourceto fiber coupling efficiency and discuss the efficient scheme.
  6. Derive the relation for the optical power launched from a surface emitting LED into a graded index fiber.
  7. What is equilibrium numerical aperture? Give the significance of the same.




Examples:
  1. An LED with a circular emitting area of radius 20µm has a lambertian emission pattern with 100 W/cm2.sr. Axial radiance at 100mA drive current. How much optical power can be coupled in to a step index fiber having a 100µm core diameter and numerical aperture of 0.22? How much optical power can be coupled from this source in to a 50 µm core diameter graded index fiber having a = 2.0 n1=1.48 and ∆=0.01.?
  2. Calculate the optical power coupled into the fiber by an optical source with a bias current of 20mA and a forward voltage of 1.5V.Assume an internal efficiency of the source as 2% and the coupling efficiency of 30%.
  3. A Laser diode has lateral θ=0°and transverse θ =90° half power beam widths of 2θ = 60°and 30° respectively what are transverse and lateral power distribution coefficient for this device?
  4. An optical fiber has n1=1.5, n=1. Calculate Fresnel reflection and optical loss in dB.


CHAPTER: 6

  1. Explain detection process in the p-n Define the quantum efficiency and responsivity of a photo detector.
  2. Explain the construction and working of PIN photodiode and also explain following terms relating to PIN photodiode with proper expressions. (i) Cut-off wavelength (ii) Quantum efficiency (iii) Responsivity. Also state the typical responsitivities of pin photodiodes.
  3. Explain the principle characteristics and operation of avalanche photodiode (or RAPD).
  4. A photo diode has a quantum efficiency of 70% when photons of energy 1.5x 10-19 Joules are incident upon it. Calculate (i) Operating wavelength (ii) Responsivity of the photodiode and (iii) Incident optical power required to maintain a photocurrent of 3 µ
  5. A photodiode has a quantum efficiency of 65 % when photons of energy 1.5 x 10 19 J are incident upon it. Calculate operating wavelength of photodiode and incident optical power required to obtain a photocurrent of 2.5 µA when the photodiode is operating at the same wavelength.
  6. A silicon APD has a quantum efficiency of 75 % at a wavelength of 900nm. If 0.5mw of optical power produces a multiplied photo current of 10mA, then what is avalanche gain for this device

CHAPTER: 7
  1. Explain receiver sensitivity and briefly discuss the possible sources of noise in optical receivers.
  2. Draw block diagram of receiver and explain different types of preamplifier.

CHAPTER: 8
  1. Discuss optical power loss model for a point to point link.
  2. Link containing various components and explain it in brief.
  3. Explain link power &rise time budgeting in digital fiber optic system
  4. Explain the performance of passive linear bus and also prove that optical power available at a particular node decreases with increasing distance from source.
  5. To transmit NRZ data stream of 20Mbps, calculate rise time budget for a multi rate link by assuming LED drive circuit rise time of 15 ns. The spectral width of LED is considered as 40 nm over the 6 km link and receiver has 25 MHz bandwidth. The selected fiber has 400MHz*km bandwidth distance product with q=0.7.
  6. Calculate the link power budget to construct an optical link of 15KM and a bandwidth of 100Mb/s.Components are chosen with following characteristics:Receiver sensitivity is -50dBm,fiberloss is 2dB/KM and a transmitter launch power of 0dBm,source and detector coupling loss of 1dB each. It is anticipated that 10 splices are required with each of loss of 0.4dB.Determine whether the system operates with sufficient power margin or not.
  7. A digital fiber link operating at 850nm require a BER of 10-9 calculate the quantum limit in rearms of quantum efficiency.

CHAPTER: 9
  1. Explain Semiconductor optical Amplifiers (SOAS).
  2. Explain the principle of operation of EDFA
  3. How amplification is achieved in Raman Amplifier.
  4. Write a short note on fiber amplifiers. Explain Wideband optical amplifier.
  5. Explain amplifier noise.

CHAPTER: 10
  1. Write short notes on Synchronous optical fiber networks(SONET)
  2. Show the configurations of SONET/SDH Rings.
  3. Discuss key system features of WDM. Draw diagram of a typical WDM
  4. Explain Generic two fiber unidirectional network path switched ring with a counter rotating protection and architecture of four fiber bidirectional line switched ring. Also enlist advantages of BLSR over UPSR.

CHAPTER: 11
  1. Write a brief note on Optical Couplers? Why we need coupler?Explain the 2x2 Fiber Coupler and its function.
  2. Write a short note (i) star coupler (ii) tunable filter (iii) TFF (iv) Optical circulator.(v) Isolator
  3. For a 2x2 fiber coupler show that the phase of the driven fiber always lags 90° behind the phase of the driving fiber. Also define the following related to optical coupler: (1) splitting ration (2) Excess loss (3) Insertion loss (4) Crosstalk
  4. Explain basic parameters in reflection grating and formation of Bragg grating in a fiber core by means of two intersecting ultraviolet light beams.
  5. Explain Mach-Zehnder Interferometer (MZI) multiplexer in detail. In 2x2 MZIs, the input wavelengths are separated by 10GHz. The silicon waveguide has ἠeff = 1.5. Compute the waveguide length difference.

CHAPTER: 12
  1. Explain the OTDRwith field applications and distinct features of backscattered
  2. Discuss the attenuation measurement techniques (i) Cutback technique (ii) Insertion loss (iii) OTDR trace.
  3. Dispersion measurement techniques.
  4. Explain Eye pattern.


By Minkal. B. Patel (Asst. Prof. EC)

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