Lasers and Optics

Flashtubes

• A flashtube is an electric arc lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations.
• Made of a length of glass tubing with electrodes at either end.

Drawbacks of Ruby Laser

• Requires high pumping power.
• Efficiency of ruby laser is small.
• Laser output is not continuous, occurs in form of pulses of microsecond duration.

Gas Lasers

• Used for continuous laser beam.
• Vapor metal ions are employed as active media.
• Main advantage: exceptionally high monochromaticity, most pure spectrum, and high stability of frequency.
• Output of gas lasers is moderate but inferior to that of crystal lasers.

He-Ne Laser

• First gaseous laser to be developed.
• Developed in 1961 by Ali Javan and associates.
• A four-level laser.
• Emits a wavelength of 6328A.

Comparison of Ruby and He-Ne Lasers

• Ruby laser: solid-state laser, three-level laser, pulsed laser, optical pumping, coolant required, output power of 10kW, operation duration of a few hours.
• He-Ne laser: gaseous laser, four-level laser, CW laser, electrical pumping, no coolant required, output power of 0.5 to 5mW, operation duration of ~10,000 hours.

Applications of Lasers

• Welding, cutting of materials.
• Eye surgery, treatment of dental decay, and skin diseases.
• Barcode scanners in libraries and supermarkets.
• Laser printers.
• Nuclear Fusion.
• CD/DVD players.
• Holography.

LASERS

• Light Amplification by Stimulated Emission of Radiations.
• Concept of Stimulated Emission given by Albert Einstein in 1917.
• Charles H. Townes utilized the concept to build MASER in 1953.
• Extension to optical region: Theodore H. Maiman built the first Laser in 1960.

Properties of Laser Light

• Monochromaticity: high degree of monochromaticity.
• Coherent: all emitted photons bear a constant phase relationship with each other in both time and phase.
• Directional: a very well-collimated beam that travels long distances with very little spread.
• Intense: a very highly intense beam.

Interaction of Radiation with Matter

• Absorption: probability of absorption, B12 is the Einstein coefficient of absorption.
• Spontaneous Emission: probability of spontaneous emission, A21 is the Einstein coefficient of spontaneous emission.
• Stimulated Emission: probability of stimulated emission, B21 is the Einstein coefficient of stimulated emission.

Relation Between Einstein Coefficients

• In thermal equilibrium, total upward transition equals total downward transitions.
• Substituting for probabilities, we get: A21 = B21 * B12 * N1 / N2.

Principle of Laser Action

• The main principle for amplifying light in a laser system is the stimulated emission.

Concept of Population Inversion and Metastable States

• State of the medium in which there are larger numbers of atoms in higher energy state as compared to the lower energy state.
• The existence of metastable state enhances the probability of population inversion.

Condition for Laser Operation

• If N1 > N2, radiation is mostly absorbed, spontaneous radiation dominates.
• If N2 >> N1, population inversion occurs, most electrons occupy level E2, weak absorption, stimulated emission prevails, and light is amplified.

Population Inversion

• Situation in which the number of electrons in the higher state exceeds that in the lower state (N2 > N1).
• Methods to obtain population inversion: supply energy from outside, optical pumping, electrical pumping, chemical pumping, and laser pumping.

Types of Lasers

• Solid-state lasers: active medium is solid crystal (e.g., Ruby, Nd:YAG).
• Gaseous lasers: active medium is gaseous (e.g., He-Ne, Co2).
• Dye lasers: macromolecules used for different laser light from a single medium.
• Semiconductor lasers: PN junction diode used for producing LASER.

This quiz covers various types of lasers, including flashtubes, ruby lasers, and gas lasers, and their characteristics.