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6.1 Fundamentals of laser operation: stimulated emission and population inversion

4 min readLast Updated on July 22, 2024

Lasers are fascinating devices that produce powerful, focused light through stimulated emission. This process occurs when excited atoms emit photons with the same properties as an incident photon, amplifying light within the laser cavity.

Population inversion, a crucial concept for laser operation, occurs when more atoms are in an excited state than the ground state. This non-equilibrium condition enables stimulated emission to dominate, leading to the generation of coherent, monochromatic, and highly directional laser light.

Fundamentals of Laser Operation

Process of stimulated emission

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  • Incident photon interacts with excited atom or molecule, causing emission of additional photon
    • Emitted photon has same frequency, phase, polarization, and direction as incident photon
    • Amplifies original light, one photon leads to emission of two photons
  • Key mechanism behind laser operation
    • Allows amplification of light within laser cavity
    • Emitted photons can further stimulate more excited atoms or molecules, leading to cascading effect
  • Requires population inversion for efficient occurrence
    • Condition where more atoms or molecules are in excited state than ground state

Concept of population inversion

  • Non-equilibrium condition with more atoms or molecules in excited state than ground state
    • In thermal equilibrium, population of ground state always higher than excited state (Boltzmann distribution)
  • Crucial for laser operation, enables stimulated emission to dominate over absorption
    • Without inversion, incident photons more likely absorbed by ground state atoms or molecules
  • Achieved through pumping process
    • Can be optical, electrical, or chemical (depending on laser type)
    • Excites atoms or molecules from ground state to higher energy levels
  • Once achieved, stimulated emission occurs efficiently, amplifying light and generating laser beam

Components of laser systems

  1. Active medium (gain medium)
    • Material where population inversion and stimulated emission occur
    • Examples: gases (CO2, He-Ne), liquids (dye lasers), solids (ruby, Nd:YAG)
  2. Pumping mechanism
    • Process used to achieve population inversion in active medium
    • Can be optical (flashlamps, laser diodes), electrical (current injection), or chemical (chemical reactions)
  3. Optical resonator (laser cavity)
    • Two mirrors, one highly reflective and one partially transmissive, arranged parallel
    • Confines light within cavity, allows multiple passes through active medium
    • Provides feedback for amplification and determines laser's wavelength and beam characteristics
  • Additional components:
    • Q-switch: Generates short, high-intensity laser pulses
    • Mode-locking elements: Generates ultrashort laser pulses
    • Frequency conversion crystals: Changes laser's wavelength

Energy level diagrams for lasers

  • Represent electronic structure of atoms or molecules in active medium
  • Illustrate energy levels and transitions, essential for understanding laser operation
  • Three-level laser system (ruby laser):
    • Ground state, pump level, and metastable level (upper laser level)
    • Pumping excites atoms from ground state to pump level
    • Atoms quickly relax from pump level to metastable level, creating population inversion
    • Stimulated emission occurs between metastable level and ground state, producing laser light
  • Four-level laser system (Nd:YAG laser):
    • Ground state, pump level, upper laser level, and lower laser level
    • Pumping excites atoms from ground state to pump level
    • Atoms quickly relax from pump level to upper laser level, creating population inversion
    • Stimulated emission occurs between upper and lower laser levels, producing laser light
    • Atoms in lower laser level quickly relax back to ground state, maintaining population inversion
  • Efficiency depends on energy level structure and transitions involved
    • Four-level lasers generally more efficient than three-level lasers

Laser Operation and Characteristics

Laser light amplification within optical resonator

  • Optical resonator (laser cavity) consists of highly reflective and partially transmissive mirrors
    • Active medium placed between mirrors
  • Light passes through active medium, stimulates emission of additional photons, amplifying light
  • Emitted photons travel back and forth between mirrors, passing through active medium multiple times
    • Each pass further amplifies light
  • Highly reflective mirror reflects most light back into cavity, partially transmissive mirror allows portion to escape as laser output
  • Optical resonator acts as frequency filter
    • Only light with wavelengths satisfying resonance condition (integer multiples of half cavity length) can be sustained and amplified
    • Results in laser output with very narrow frequency bandwidth

Characteristics of laser light

  1. Directionality
    • Highly directional, travels in narrow, collimated beam
    • Due to geometry of optical resonator, only light traveling perpendicular to mirrors is amplified
  2. Monochromaticity
    • Monochromatic, consists of single wavelength or very narrow range of wavelengths
    • Result of resonance condition of optical cavity, selectively amplifies specific wavelengths
  3. Coherence
    • Coherent, photons have fixed phase relationship with each other
    • Two types of coherence:
      • Temporal coherence: Photons emitted at different times maintain fixed phase relationship, resulting in long coherence length
      • Spatial coherence: Photons emitted from different points in laser beam have fixed phase relationship, resulting in uniform wavefront
  • Characteristics make laser light suitable for wide range of applications (precision measurements, material processing, optical communication)

Factors affecting laser beam quality and divergence

  • Laser beam quality measures how close beam is to ideal Gaussian beam
    • Perfect Gaussian beam has lowest divergence and highest focusability
  • Beam quality factor (M²) quantifies deviation from ideal Gaussian beam
    • Ideal Gaussian beam has M² = 1, real laser beams have M² > 1
  • Factors affecting beam quality and divergence:
  1. Active medium inhomogeneities
    • Variations in refractive index, density, or temperature cause distortions in laser beam
  2. Thermal lensing effects
    • Heat generated during pumping causes temperature gradient in active medium, changing refractive index and creating "thermal lens"
    • Can cause laser beam to focus or defocus, affecting quality and divergence
  3. Misalignment of optical resonator
    • Imperfectly aligned mirrors cause laser beam to deviate from ideal path, increasing divergence
  4. Aperture effects
    • Size and shape of aperture through which laser beam passes affect quality and divergence
    • Smaller aperture can lead to diffraction effects, increasing beam divergence
  • Techniques like beam shaping, mode selection, and adaptive optics can improve laser beam quality and reduce divergence


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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.