(SEM III) THEORY EXAMINATION 2024-25 LASER SYSTEM AND APPLICATIONS

LASER SYSTEM AND APPLICATIONS – Full Question Paper Description

TIME: 3 Hours | MAX MARKS: 70

This examination evaluates a student’s understanding of laser physics, photon interactions, resonators, population inversion, different types of lasers, coherence concepts, pulse generation techniques, and laser applications in communication, medicine, and metrology.

The paper is divided into three structured sections, testing knowledge from basic principles to advanced analytical and mathematical understanding.

SECTION A – Short Answer Questions (14 Marks)

This section consists of seven compulsory questions, each requiring brief yet conceptually accurate answers.

Key Concepts Covered:

• Ordinary Light vs Laser Light

Understanding differences in coherence, monochromaticity, directionality, and intensity.

• Quality Factor of Resonator (Q-factor)

Definition and significance in determining sharpness of resonance and efficiency of oscillation.

• Role of Metastable State

Why population inversion requires long-lived energy states and how metastability enables stimulated emission.

• Three-Level vs Two-Level Lasers

Why three-level lasers are more efficient, easier to achieve population inversion, and less energy-demanding.

• Measurement of Short Laser Pulses

Interferometry, autocorrelation, streak cameras—methods to detect femtosecond to picosecond laser pulses.

• Role of Helium in He-Ne Laser

Energy transfer to neon atoms through collisions for population inversion.

• Advantages of Laser Eye Surgery

High precision, minimal bleeding, short healing time, and greater accuracy compared to conventional surgery.

This section checks the student’s clarity of laser fundamentals and essential definitions.

SECTION B – Application-Based & Conceptual Questions (21 Marks)

Students must attempt any three questions. These involve explanation-based answers, diagrams, and derivations.

Topics Covered:

• Absorption, Spontaneous & Stimulated Emission + Einstein Coefficients

Photon-atom interactions, detailed labelled diagrams, and deriving the relation among A and B coefficients.

• Gain in Lasers & Threshold Condition

Understanding amplification, population inversion requirement, and mathematical threshold gain formula.

• Three-Level & Four-Level Lasers

Energy-level diagrams, working mechanism, and differences in efficiency.

• CO₂ Laser Construction & Working

Gas mixture, pumping mechanism, resonator structure, and high-power applications.

• Applications in Optical Communication & Medical Field

Fiber communication, dermatology, ophthalmology, surgeries, and diagnostics.

This section tests explanatory depth, derivations, and application knowledge.

SECTION C – Long Descriptive / Analytical Questions (35 Marks)

Each sub-part contains two choices, from which the student attempts one.

C1 – Coherence Concepts and Numerical Analysis

Option A: Coherence Length Numerical Problem

Compute:

Number of oscillations

Coherence time

Spectral width
using given wavelength and coherence length.

Option B: Spatial & Temporal Coherence

Detailed explanation of coherence types and relation of spatial coherence with laser beam directionality.

C2 – Resonators & Photon Calculations

Option A: Fabry–Perot Resonator

Principle, construction, high finesse, multiple reflections, and working.

Option B: Photon Count from Laser Power

Using energy per photon to calculate total emitted photons for given laser power and wavelength.

C3 – Mode Locking & Laser Classification

Option A: Methods of Mode Locking

Active and passive mode locking, Q-switching, Kerr lens technique, plus advantages/disadvantages.

Option B: Types of Lasers Based on Medium

Solid-state, gas, liquid dye, semiconductor lasers—merits and limitations.

C4 – Laser Construction & Operation

Option A: Excimer Laser

Construction, rare-gas halide molecules, ultraviolet output, and working mechanism.

Option B: Ruby Laser

Energy-level diagram, optical pumping, and labelled construction and working.

C5 – Metrology & Holography

Option A: Laser in Metrology

Applications such as interferometry, distance measurement, surface profiling.

Option B: Principle of Holography

Interference and diffraction principles, hologram recording & reconstruction mechanics.

Purpose of This Examination

This exam is designed to evaluate whether a student can:

Understand fundamental laser principles (emission, coherence, population inversion)

Analyze different pumping and laser generation mechanisms

Interpret and construct laser resonator systems

Distinguish among three-level, four-level, and other laser systems

Use lasers in communication, medicine, and measurement technologies

Solve numerical problems related to coherence and photon energy

Explain advanced concepts such as mode locking, holography, and laser fabrication

It tests both theoretical clarity and practical application skills expected from B.Tech (Physics, ECE, EE, or related disciplines).