Tiêu đề PHY-QB-Solution.pdf ✅

A Ruby laser operates by stimulating the emission of light within a synthetic ruby crystal to produce a coherent and monochromatic beam of laser light. Here's a simplified of how it works: 1. Gain Medium: The laser's core component is a synthetic ruby crystal doped with chromium. 2. Pumping: Intense light, often from a flash lamp, excites the chromium ions in the ruby crystal, raising their electrons to higher energy states. 3. Population Inversion: As the excited electrons return to their ground state, they release photons, creating a population inversion where more electrons are at higher energy levels. 4. Optical Resonator: The ruby crystal is placed between two mirrors, forming an optical resonator. One mirror is highly reflective, and the other is partially transparent. 5. Stimulated Emission: Photons passing through the ruby crystal trigger the release of more photons in a similar direction, wavelength, and phase, a process called stimulated emission. 6. Output Coupling: Some of the laser light escapes through the partially transparent mirror, which serves as the laser's output. The remaining light is reflected back into the cavity to stimulate further emission. 7. Laser Light: The stimulated emission and reflection between the mirrors cause the light to amplify in intensity, resulting in a coherent and monochromatic laser beam, often in the red part of the spectrum. Drawbacks of Ruby Laser are Following:- • Large and Bulky • Limited Pulse Duration • Limited Power • Short Lifespan Applications of Ruby lasers are: laser rangefinders, high-speed photography, pulsed holography, tattoo and hair removal Q3 (a) Give construction and working of ND: YAG laser. (b) What is population inversion? Explain why laser action cannot occur without population inversion between atomic levels? (a)
Nd:YAG is a four level laser system. The pumping of neodynium (Nd3+) ions to upper state (E4) is done using krypton arc lamp. Thus optical pumping is used in this laser. The wavelength of light of wavelength 7200 Å to 8000 Å excites the ground state (E1) Nd3+ ions to E4 states. From E4 states, they make a non- radiative transition and come to E3 state. E3 is the metastable state so population inversion is achieved between the levels E3 and E2. (b) Population inversion is the condition in laser gain medium in which higher-lying energy levels are more strongly populated than a lower-lying level. Laser action cannot occur without population inversion between atomic levels because it is the necessary condition of stimulated emission Q4 Give construction and working of Semiconductor lasers. Principle and working of a semiconductor laser When a p-n junction diode is forward biased, the electrons from n- region and the holes from the p-region cross the junction and recombine with each other. During the recombination process, the light radiation (photons) is released from a certain specified direct band gap semiconductors like Ga-As. This light radiation is known as recombination radiation. The photon emitted during recombination stimulates other electrons and holes to recombine. As a result, stimulated emission takes place which produces laser. Q5 Explain with neat diagram absorption, spontaneous emission and stimulated emission of radiation. What is the reason for the monochromaticity of laser beams?
Q6 Give the principle of propagation of light through an optical fiber. Derive an expression for acceptance angle , acceptance cone and Numerical Aperture. Optical fiber: It is a very thin fiber made of glass and plastic having a radius of the order of a micrometer (10-6 m). It works on the principle of Total Internal Reflection. Q7 What is optical fiber? Explain the types of fiber on the basis of modes and their index profiles with a neat diagram wherever required. It is a very thin hair like structure, Which allows the light to pass through it along the Zig-Zag path and follow Total Internal Reflection.