AIJobalert.comSarkari Naukri • Behtar Bhavishya

Get Free Job Alerts Request a Job Update

AIJobalert.comSarkari Naukri • Behtar Bhavishya

Engineering Physics: Laser Fundamentals and Systems

This lecture covers the foundational concepts of lasers, including absorption, spontaneous emission, and stimulated emission mechanisms. It details population inversion, metastable states, Einstein coefficients derivation, and the construction and operation of three-level (Ruby) and four-level (He-Ne) lasers. Numerical problems on photon energy, momentum, and population ratios are also solved.

Physics
English · 9 topics
Checked against the video on 25 Oct 2026

Made by AI from the YouTube video Unit-4 LASER I Physics I One Shot I Physics by Gushan Sir I Gateway Classes I AKTU (opens YouTube in a new tab) by GateWay Classes, then checked against it. All credit for the teaching goes to the creator. Tap a topic’s time to confirm a fact in the video.

Engineering Physics: Laser Fundamentals and Systems

English9 topics · 6 MCQs · 12 flashcardsWatch the video (opens YouTube in a new tab)

Checked against the video

In short

This lecture covers the foundational concepts of lasers, including absorption, spontaneous emission, and stimulated emission mechanisms. It details population inversion, metastable states, Einstein coefficients derivation, and the construction and operation of three-level (Ruby) and four-level (He-Ne) lasers. Numerical problems on photon energy, momentum, and population ratios are also solved.

Introduction and Basic Laser Properties

The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation.
Must knowAlbert Einstein introduced the concept of stimulated emission in 1917, which forms the operating principle of all lasers.
In 1960, Dr. T.H. Maiman invented the first working laser, known as the Ruby Laser.
A laser beam is highly monochromatic, meaning it consists of a single frequency and a single wavelength or color.
Laser light is highly intense because all the power or energy is concentrated within a very small spatial area.
Laser light is highly coherent, which signifies that all constituent waves propagate in the exact same phase.
Laser beams are unidirectional and collimated, meaning rays travel parallel over long distances without diverging significantly.

Interaction of Radiation with Matter: Three Basic Mechanisms

In stimulated absorption, an atom residing in lower energy state E1 absorbs an incident photon of energy hν = E2 - E1 and jumps to higher state E2.
The transition rate for stimulated absorption (P12) is directly proportional to the number of atoms in state 1 (N1) and radiation energy density E(ν), given by P12 = N1 × B12 × E(ν), where B12 is Einstein's coefficient of absorption.
In spontaneous emission, an atom in excited state E2 transitions back to E1 on its own to attain stability, emitting a photon of energy hν = E2 - E1.
Must knowThe mean lifetime of an atom in an ordinary excited state is approximately 10⁻⁸ seconds.
The rate of spontaneous emission (P'21) is independent of external energy density and depends solely on the number of atoms in state 2: P'21 = N2 × A21, where A21 is Einstein's coefficient of spontaneous emission.
In stimulated emission, an external incident photon of energy hν triggers an excited atom at E2 to drop to E1, releasing a second identical photon with the same frequency, phase, and direction.
The rate of stimulated emission (P''21) is given by P''21 = N2 × B21 × E(ν), where B21 is Einstein's coefficient of stimulated emission.

Parameter	Spontaneous Emission	Stimulated Emission
Photon direction	Moves in random directions	Moves in one same direction
Coherence	Radiation is non-coherent	Radiation is coherent
Phase relation	No phase relationship between photons	Photons have same phase and frequency
Rate dependence	Proportional only to N2	Proportional to N2 and energy density E(ν)
Resulting light	Ordinary light (e.g., LED, bulb)	Laser light

Population Inversion, Metastable State, and Essential Conditions

Under normal thermal equilibrium, the population of atoms follows N1 > N2, meaning the lower energy state has more atoms than the upper state.
Must knowPopulation inversion is the non-equilibrium condition where the number of atoms in the higher energy state (N2) exceeds that in the lower state (N1), such that N2 > N1.
A metastable state is an intermediate excited energy state having an unusually long lifetime of about 10⁻³ seconds, which is 10⁵ times longer than an ordinary excited state (10⁻⁸ seconds).
Metastable states allow atoms to accumulate in higher numbers than the ground state, making population inversion achievable.
Two essential conditions to achieve laser action are: the rate of emission must exceed the rate of absorption, and the probability of spontaneous emission must be negligible compared to stimulated emission.
An optical resonator (resonant cavity) consisting of two parallel mirrors (one 100% reflective, one partially transmitting) placed around the active medium is required to amplify the coherent beam.

Pumping Methods and Components of a Laser

Pumping is the external process of raising atoms from a lower energy state to a higher energy state to establish population inversion.
Optical pumping supplies energy using short, intense flashes of light from a flash lamp, typically used in solid-state lasers like the Ruby laser.
Electrical pumping applies a high potential difference across a gas medium to produce an electric discharge, where accelerated electrons excite gas atoms through collision (used in He-Ne lasers).
Every laser system consists of three main components: an active medium, a pumping source, and an optical resonant cavity.
The active medium contains active centers responsible for lasing and determines the laser type: solid, liquid, or gas.

Derivation of Einstein's Coefficients

Under thermal equilibrium, the rate of absorption equals the total rate of emission: P12 = P'21 + P''21.
Substituting the rate expressions gives N1 × B12 × E(ν) = N2 × A21 + N2 × B21 × E(ν).
Rearranging for radiation density yields E(ν) = (A21 / B21) / [ (B12 / B21) × (N1 / N2) - 1 ].
According to Boltzmann's distribution law, the population ratio is N1 / N2 = e^((E2 - E1) / (kT)) = e^(hν / (kT)).
Substituting Boltzmann's ratio gives E(ν) = (A21 / B21) / [ (B12 / B21) × e^(hν / (kT)) - 1 ].
Must knowComparing this with Planck's radiation law, E(ν) = (8 × π × h × ν³ / c³) × [ 1 / (e^(hν / (kT)) - 1) ], yields B12 = B21 and A21 / B21 = 8 × π × h × ν³ / c³.

Three-Level and Four-Level Laser Systems

In a three-level laser, pumping excites atoms from ground state E1 to short-lived pump level E3 (lifetime ~10⁻⁸ s).
Atoms rapidly undergo a non-radiative transition (releasing heat, no photons) from E3 to metastable state E2 (lifetime ~10⁻³ s).
Laser action occurs between metastable state E2 (upper lasing level) and ground state E1 (lower lasing level).
Three-level lasers require very high pumping power because more than 50% of ground-state atoms must be excited to achieve N2 > N1, resulting in pulsed output and low efficiency.
In a four-level laser, lasing occurs between upper lasing level E3 (metastable) and lower lasing level E2 (above ground level E1).
Because level E2 is rapidly depopulated to E1 by fast non-radiative transitions, E2 remains virtually empty, making population inversion between E3 and E2 much easier to achieve with lower pumping power.

Feature	Three-Level Laser	Four-Level Laser
Lasing transition	Between metastable state and ground state	Between metastable state and an intermediate level above ground
Pumping power	Very high	Low
Efficiency	Low	Much higher
Output nature	Pulsed output	Continuous output
Example	Ruby laser	Helium-Neon (He-Ne) laser

Ruby Laser: Construction and Working

The Ruby laser is a solid-state, three-level laser invented by T.H. Maiman in 1960.
Its active medium is a cylindrical ruby rod made of aluminum oxide (Al2O3) doped with approximately 0.05% chromium oxide (Cr2O3).
Cr3+ ions replace some Al3+ ions, giving the rod a pink color and serving as the active centers for laser action.
Optical pumping is provided by a helical xenon flash tube wrapped around the ruby rod.
The resonant cavity is formed by silvering the two flat parallel ends of the ruby rod: one end is fully silvered (100% reflection), and the other is partially silvered.
Must knowThe laser transition in ruby produces a beam with a wavelength of 6943 Å (red color) in pulsed form, and external cooling (water cooling) is required to remove heat generated during operation.

Helium-Neon (He-Ne) Laser: Construction and Working

Invented by Ali Javan in 1960, the He-Ne laser is a four-level gas laser that operates continuously without needing external cooling.
The active medium is contained in a narrow quartz tube filled with a mixture of Helium and Neon gas in a ratio of approximately 7:1 at low pressure.
Neon atoms act as active centers responsible for lasing, while Helium atoms assist in the excitation process.
Pumping is achieved by electrical discharge via a high-voltage DC source connected to cathode and anode terminals.
Energetic electrons collide with ground-state He atoms, exciting them to metastable states at 20.61 eV.
Excited He atoms collide inelastically with ground-state Ne atoms, transferring energy to excite Ne atoms to a matching metastable level at 20.66 eV.
Must knowThe primary lasing transition occurs in neon atoms, emitting a continuous, highly monochromatic laser beam at 6328 Å (red light).

Applications of Lasers and Solved Numerical Problems

Lasers are widely used in medicine for kidney stone treatment, cancer therapy, and eye surgery (cornea reshaping).
Industrial uses include drilling, welding, cutting of hard materials (diamond, iron, steel), and metallurgy processes like hardening and annealing.
Telecommunications utilize lasers with optical fibers for high-capacity long-distance signal transmission.
Other applications include holography, supermarket barcode scanners (He-Ne and semiconductor lasers), and military targeting systems.
In a worked problem with N = 2.8 × 10¹⁹ Cr3+ ions emitting at λ = 7000 Å, the laser pulse energy is calculated as E = n × h × c / λ = 7.94 J.
The population ratio N2 / N1 in a He-Ne laser at λ = 6000 Å and T = 300 K is determined using Boltzmann's formula to be e⁻⁸⁰.

Key terms

LASER: Light Amplification by Stimulated Emission of Radiation; a device generating coherent, monochromatic, intense, and directional light.
Stimulated Emission: The process wherein an incident photon triggers an excited atom to transition to a lower state, releasing a second identical photon.
Population Inversion: The condition in which a higher energy state contains more atoms than a lower energy state.
Metastable State: An excited atomic state with a comparatively long lifetime (~10⁻³ s) allowing accumulation of excited atoms.
Optical Pumping: Excitation of active medium atoms to higher energy levels using intense flashes of light.
Resonant Cavity: A pair of parallel reflecting mirrors placed around an active medium to amplify light through multiple reflections.
Active Medium: The substance containing active centers that undergo population inversion and emit laser radiation.

Formulas

E = h × ν = h × c / λ: Calculates the energy of a single photon.
P12 = N1 × B12 × E(ν): Rate of stimulated absorption of radiation.
P'21 = N2 × A21: Rate of spontaneous emission of radiation.
P''21 = N2 × B21 × E(ν): Rate of stimulated emission of radiation.
B12 = B21: Einstein relation showing equivalence of absorption and stimulated emission transition probabilities.
A21 / B21 = 8 × π × h × ν³ / c³: Ratio of Einstein coefficient of spontaneous emission to stimulated emission.
N1 / N2 = e^((E2 - E1) / (kT)) = e^(hν / (kT)): Boltzmann distribution law relating population ratio to temperature and energy gap.
p = h / λ: Momentum of a photon derived from de Broglie relation.

Exam tips and traps

In population ratio problems, remember to convert temperatures given in Celsius to Kelvin by adding 273, and convert Boltzmann's constant k to Joules per Kelvin before evaluating the exponent.

Quick revision

Read these the night before the exam.

T.H. Maiman developed the first working ruby laser in 1960.
Stimulated emission was theoretically predicted by Albert Einstein in 1917.
The lifetime of an atom in an ordinary excited state is 10⁻⁸ seconds, whereas in a metastable state it is 10⁻³ seconds.
Laser action requires a minimum of three energy levels because population inversion cannot be maintained in a two-level system.
Einstein's B-coefficients satisfy the relation B12 = B21.
The active medium in a Ruby laser is Al2O3 doped with 0.05% Cr2O3.
Ruby laser emits red light pulses at a wavelength of 6943 Å.
The gas ratio of Helium to Neon in a He-Ne laser is approximately 7:1.
He-Ne laser produces a continuous laser beam at a wavelength of 6328 Å.
Four-level lasers require significantly lower pumping power than three-level lasers because the lower lasing level is rapidly depopulated.

These notes are made by AI from the video. Use the timestamps to check any important fact against the video before you rely on it.

Have a lecture of your own? Paste the YouTube link and get notes, a quiz and flashcards.

Make notes from a YouTube video

Browse every checked set of notes, filed by exam and subject.

Open the notes library

Made this video? Remove these notes

Open the Physics folder →

Motion: Key Concepts, Formulas, and Practice Questions
This lecture covers the physics topic of motion, explaining relativity of rest and motion, distance, displacement, speed, velocity, acceleration, retardation, equations of motion, projectile motion, circular motion, and Newton's laws. It also solves numerical examples and discusses multiple-choice practice questions aligned with teaching exam syllabi.
Physics
English
7 topics
27 Oct 2026
By Balaji Study Adda
Teaching
Physics Revision Notes: Force, Momentum, and Newton's Laws
This lecture covers the fundamental concepts of force, including its definition, SI and CGS units, and conversion factors. It explains the classification of contact and non-contact forces, types of fundamental forces, levers, centripetal and centrifugal forces, torque, and moment of inertia. Finally, it explores Newton's three laws of motion and friction through conceptual questions and practical examples.
Physics
English
6 topics
27 Oct 2026
By Balaji Study Adda
Teaching
Physics Practice Questions on Force, Pressure, and Gravitation
This lecture covers key multiple-choice and numerical questions on force, work, pressure, gravitation, and levers for the UP Super TET teaching exam. Concepts explained include work done by vector dot products, pressure formula calculations, variation of gravitational acceleration, mass versus weight, and Archimedes' principle.
Physics
English
7 topics
27 Oct 2026
By Balaji Study Adda
Teaching
Work, Power and Energy Revision Notes
Comprehensive exam revision covering work, power, and energy definitions, conditions, formulas, units, types, the law of conservation of energy, renewable versus non-renewable sources, and key device transformations.
Physics
English
6 topics
27 Oct 2026
By Balaji Study Adda
Teaching
Super TET Science: Work, Power and Energy Practice Questions
Revision session covering the concepts, units, and formulas of Work, Power, and Energy for teaching exams. Explains the differences between kinetic, potential, and mechanical energy with real-life examples and calculations. Reviews dimensional formulas, energy transformations, and conservation theorems through practice questions.
Physics
English
8 topics
27 Oct 2026
By Balaji Study Adda
Teaching
Optics and Light: Complete Exam Revision Notes
Comprehensive handwritten notes on light based on NCERT Class 7 and Class 10 science syllabi. Covers reflection, refraction, mirrors, lenses, dispersion, scattering, rainbows, optical instruments, and modern optical devices like LASER and LED for teaching exams.
Physics
English
11 topics
27 Oct 2026
By Balaji Study Adda
Teaching
Class 7
Class 10