JNTU
JNTU B.Tech IV OPTICAL COMMUNICATIONS Previous Question Paper (July/August - 2022 - R16)
Answer any five questions. All questions carry equal marks.
Main
Explain the transmission of light through fiber using Ray theory concept.
Answer
Ray theory describes light propagation in an optical fiber by treating light as rays traveling in straight lines within the core. The core has a refractive index n1, surrounded by a cladding with a lower refractive index n2. When a light ray enters the fiber core at an angle, it undergoes total internal reflection (TIR) at the core-cladding interface, provided the angle of incidence is greater than the critical angle.
The critical angle is defined as:
For TIR to occur, the ray must strike the interface at an angle greater than θc. This process allows light to be trapped and guided along the fiber axis through successive reflections. The Acceptance Angle is the maximum angle at which light can enter the fiber and be successfully guided, which is related to the Numerical Aperture (NA) of the fiber:
Rays that satisfy these conditions are called meridional rays, which pass through the fiber axis, while skew rays follow a helical path along the fiber.
Draw a block diagram of fiber optic communication system and describe the function of each component.
Answer
A fiber optic communication system consists of the following primary components:
- Transmitter (Optical Source): Converts electrical signals into optical signals. Common sources include LEDs or Laser Diodes.
- Optical Fiber (Channel): The transmission medium that carries the light signal over long distances using total internal reflection.
- Optical Amplifier/Repeater: Used in long-haul systems to boost the attenuated optical signal strength.
- Receiver (Optical Detector): Converts the received optical signal back into an electrical signal using photodiodes like PIN or APD.
- Signal Processing Unit: Includes amplifiers, filters, and decoders to restore the original information from the electrical signal.
The block diagram follows the sequence: Input Data -> Electrical Transmitter -> Optical Source -> Optical Fiber -> Optical Detector -> Electrical Receiver -> Output Data.
Draw and explain the acceptance angle and numerical aperture of an optical fiber and derive expressions for both.
Answer
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A graded index fiber core with a parabolic refractive index profile (a=2) and diameter of 50 µm, the fiber has numerical aperture of 0.2. Estimate the number of the guided modes propagating in the fiber when the transmitted light has a wavelength 1µm.
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With a neat diagram derive an expression for intra model dispersion.
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Discuss about pulse broadening in graded index fibers with necessary equations.
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Describe various types of fiber connectors and couplers.
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With a neat diagram, explain in detail about macro and micro bending losses.
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Explain in detail about various fiber splicing techniques with necessary diagrams.
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Describe the possible lensing schemes used to improve optical source-to-fiber coupling efficiency.
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Describe the operation of injection laser diode in detail.
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A double hetero junction LED emitting at a peak wavelength of 1310nm has radiative and non-radiative recombination time of 45ns and 95ns respectively. The drive current is 35mA. Determine internal quantum efficiency and internal power level. If the refractive index of the light source material is n=3.5, find the power emitted from the device.
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Derive necessary expressions for the SNS of both PIN and APD by incorporating all noise sources.
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Define the term Quantum limit and derive a suitable expression for probability of error with respect to receiver.
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With a neat block diagram, explain about point-to-point fiber optic link system.
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Explain the power budget with the help of power loss model for point to point link. Also give the graphical representation of a link loss budget.
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