JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III OPTOELECTRONICS AND LASER INSTRUMENTATION Previous Question Paper (February - 2023 - R18)
Part A is compulsory (25 Marks). In Part B, answer any one question from each unit (50 Marks).
Part A
What are different fibers, and which fiber is having the least dispersion?
Answer
- Optical fibers are primarily classified based on the refractive index profile and the number of modes of propagation:
- Step-index Multimode Fiber (SIMM): Features a core with a uniform refractive index and a sudden change at the cladding interface.
- Graded-index Multimode Fiber (GIMM): The core refractive index decreases parabolically from the center toward the cladding, which helps in reducing intermodal dispersion.
- Single-mode Fiber (SMF): Possesses a very small core diameter (typically 8–10 μm) allowing only the fundamental mode to propagate, effectively eliminating intermodal dispersion.
- The fiber with the least dispersion is the Single-mode Fiber (SMF). Because it supports only a single transverse mode, it eliminates intermodal dispersion (the primary cause of pulse broadening in multimode fibers). Furthermore, SMFs are typically designed to operate at wavelengths where chromatic dispersion is minimized, making them the ideal choice for high-bandwidth, long-distance communication systems.
Generalize any two required properties of light sources used in optical fiber communication.
Answer
- Spectral Purity (Narrow Spectral Width): An ideal light source for optical fiber communication must have a narrow spectral width to minimize chromatic dispersion. Since optical fibers exhibit dispersion where different wavelengths travel at different velocities, a broad emission spectrum would cause pulse broadening, thereby limiting the bandwidth-distance product of the communication system. Lasers are preferred over LEDs for high-speed, long-haul applications due to their extremely narrow linewidth.
- High Coupling Efficiency and Direct Modulation Capability: The light source must be capable of being efficiently coupled into the small core of an optical fiber. This requires a small emitting area and a directional output beam (high radiance). Furthermore, the source must support direct modulation, meaning its optical output power can be varied rapidly in response to an electrical input signal, enabling high data transmission rates without the need for external modulators.
Define Moire fringes.
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How optical beam polarization is achieved in external modulators?
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List the parameters of a typical fiber optic sensor.
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Give the significance of polarization maintaining fibers in optical communication.
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Explain “Q-switching”.
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Explain how population inversion is achieved for Laser Generation?
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Describe the components of Hologram.
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Classify the uses of laser in medicine.
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Part B
Elaborate about the various attenuation losses possible in optical fibers. Also discuss how intramodal and intermodal dispersion can be minimized.
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Draw and explain the acceptance angle and numerical aperture of an optical fiber and derive expressions for both.
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What is the role of optical fibers in the measurement of pressure? Describe in detail.
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Explain the various measures of efficiency in PIN photodiode and briefly explain the working principle of Schottky barrier photodiodes.
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Explain in detail about Moire fringe modulator of displacement measurement.
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What is the role of optical fibers in the measurement of pressure? Describe in detail.
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With the help of an energy diagram, analyze how four level laser system is advantageous to three level laser system. Also explain the construction and working of a four-level laser.
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Examine the construction and operation of solid state lasers with neat diagram.
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Classify the methods of holography and explain each method in detail with suitable diagrams.
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Explain in detail about measuring the voltage and velocity by using laser.
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