The formation of laser

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The emergence of lasers marked the first time that humans had achieved the control of photons. Lasers are special light sources that amplify light through stimulated radiation, and their formation process involves multiple physical mechanisms such as quantum mechanics, optical resonance, and energy excitation. The following are the main factors contributing to the formation of lasers:
I. Stimulated Emission and Particle Number Inversion
The generation of lasers began with quantum transitions between atomic energy levels. When an electron absorbs energy and transitions to a higher energy state, the system is in an unstable state. At this time, the electron releases energy in two ways:
Spontaneous emission: Electrons randomly jump back to a lower energy state and emit photons with different directions and phases.
Stimulated emission: When the energy of an external photon matches the energy level difference, it will induce electrons in an excited state to simultaneously release exactly the same photon, which forms the basis of optical amplification.
Optical amplification: Particles at high energy levels undergo stimulated emission triggered by incident photons, generating photons of the same phase and frequency. These photons oscillate repeatedly in the resonant cavity, triggering a chain reaction and forming high-intensity coherent light.
Particle number inversion: The pumping system breaks the thermal equilibrium state, enabling the working substance to form metastable energy levels.
Ii. Optical Resonant Cavity
Resonant cavity feedback: An optical resonant cavity composed of two mirrors allows photons in a specific direction to travel back and forth multiple times. When the gain exceeds the loss, a positive feedback loop will be formed, and eventually a coherent beam will be output.
Mode selection: By means of short cavity design or grating feedback, the distribution of longitudinal and transverse modes is controlled to achieve single-frequency and single-mode output.
Energy concentration: Shorten the effective length of the working substance and enhance the laser output efficiency.
Iii. Bose Statistics
Identical photons: Photons produced by stimulated radiation have exactly the same frequency, phase and polarization state.
Wave function superposition: A large number of identical photons form macroscopic quantum states, endowing light with perfect coherence.
Characteristics of laser:
Directionality: The divergence Angle of a common light source is relatively large, while that of a laser light source is relatively small and its direction is fixed.
Monochromaticity: The spectral line width of a laser is narrower than that of an ordinary light source, thus it has better monochromaticity.
High brightness: The laser emits highly parallel beams and can be emitted with a higher concentration
The formation of lasers is a perfect combination of quantum mechanics and optical engineering. Its core lies in achieving controllable amplification of light through particle number inversion and stimulated radiation. The development of lasers is promoting innovations in industries such as manufacturing, healthcare, and information technology.