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

The uploaded document is the B.Tech Semester IV Theory Examination Paper (2024–25) for the course BOE412 – Laser System and Applications. It is an official university-level question paper designed to test a student’s understanding of the principles of laser physics, laser construction, laser types, optical resonators, beam properties, and real-world industrial and scientific applications of lasers. The exam is of 3 hours duration, carries 70 marks, and is bilingual, containing every question in both English and Hindi, as seen across Page 1 and Page 2 of the document. This bilingual structure helps students from different linguistic backgrounds attempt the exam with comfort and clarity.
 

BOE412-LASER-SYSTEM-AND-APPLICA…

The paper begins with Section A, which includes seven compulsory 2-mark questions designed to evaluate basic understanding. These questions touch upon key foundational concepts such as the difference between spontaneous and stimulated emission, the two fundamental processes responsible for laser action. Students are also asked to define a metastable state, a crucial requirement for population inversion in laser systems. Another question requires them to explain whether a two-level laser system is feasible, pushing them to understand why most practical lasers rely on three- or four-level energy systems. Further, the section asks about the principle of excimer lasers, widely used for microfabrication and UV applications, and the meaning of photolithography, a key process in semiconductor manufacturing. Students must also describe what an active medium is and define the Q-factor, which indicates the efficiency and energy retention of a laser cavity. These short questions are crafted to test a student’s conceptual clarity without requiring lengthy derivations.

Section B demands deeper explanations, containing five 7-mark questions, of which students must attempt any three. The questions in this section expect both theoretical understanding and the ability to relate concepts to physical laser systems. One of the questions requires establishing the relation between Einstein A and B coefficients, fundamental parameters that describe absorption, spontaneous emission, and stimulated emission in radiative transitions. Another question focuses on the concept of population inversion and explains why it is absolutely essential for achieving laser action. It goes further to ask about the plane-parallel resonator, a common cavity design that helps sustain oscillations. The section also questions why four-level lasers are preferred over two- or three-level systems, prompting students to analyze energy transitions and population efficiencies. A detailed description of the construction and working of the He-Ne laser is also included, along with a question on the applications of lasers in material processing, which may cover cutting, welding, drilling, heat treatment, and surface modification. This section tests the student’s ability to give complete, structured, and technically sound answers.

The final part of the paper, Section C, is the analytical section, where students must choose one question from each of the subsections (Q3 to Q7). These are long, 7-mark questions that demand high-level comprehension, derivations, calculations, and clear conceptual articulation. One option asks students to explain laser beam properties such as directionality and brightness, which differentiate laser light from ordinary incoherent sources. Another option asks for the calculation of photons emitted per second by a laser beam, given its wavelength and output power, requiring formula-based numerical reasoning.

More advanced questions appear in further subsections. For example, students may be asked to evaluate the minimum pump power needed to maintain population inversion in a three-level laser, reflecting their understanding of energy pumping requirements. Another question deals with the maximum cavity length required for single longitudinal-mode operation, based on the given half-width of the gain profile, testing optical resonance concepts and cavity mode spacing principles.

Section C continues with questions that require a strong foundation in laser types and operational methods. Students may need to compare CW (Continuous-Wave) lasers with pulsed lasers, discussing their construction, operation, and industrial relevance. Alternatively, they may have to explain mode-locking techniques, which are essential for generating ultrashort pulses in picosecond or femtosecond lasers. The section also includes classical laser types such as the CO₂ laser and Nd:YAG laser, requiring explanations of their construction, working, and energy level diagrams.

The final subsection of Section C covers modern applications, such as LIDAR, requiring students to discuss its components (transmitter, receiver, scanner, detector) and applications in topography, autonomous vehicles, environmental monitoring, and atmospheric sensing. Another option explains laser applications in holography, requiring students to illustrate how coherent light is used to record and reconstruct three-dimensional images.

Overall, this question paper offers a comprehensive evaluation of the subject Laser System and Applications. It examines basic principles like emission processes and metastable states, intermediate topics such as resonators and laser types, advanced numerical and analytical reasoning, and wide-ranging practical applications. By combining short, descriptive, and long analytical questions, the paper ensures that students demonstrate both theoretical mastery and practical insight into laser physics.