Lasers
Go beyond the A-Level syllabus with stimulated emission, population inversion, optical feedback and laser beam properties, using a helium–neon example and practice.
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Before you begin
A laser is not simply a bright lamp. The lessons start with how a single atom absorbs and emits light, show how pumping and a pair of mirrors turn stimulated emission into a working laser, then explain the properties of laser light, the helium–neon laser and how lasers are used safely. Lasers go beyond the 9478 syllabus.
Be comfortable with: atomic energy levels and line spectra and standing waves.
Learning goals
- Explain excitation, spontaneous emission and stimulated emission.
- Explain metastable states, pumping, population inversion, threshold, optical feedback, output coupling and cavity modes.
- Relate laser operation to coherence, linewidth, divergence, irradiance, photon quantities and a helium-neon example.
Lessons
Work through them in order.
- Atomic transition processes for lasersCompare absorption, spontaneous emission and stimulated emission using energy levels and photon energy.Beyond the syllabus
- How lasers workConnect pumping, metastable levels, net gain, threshold, cavity modes and output coupling.Beyond the syllabus
- Properties of laser lightDistinguish coherence, narrow spectral width, low divergence and high irradiance.Beyond the syllabus
- Helium–neon laserTrace discharge pumping, resonant energy transfer, inversion and the 632.8 nm red transition.Beyond the syllabus
- Laser applications, limits and safetyMatch laser properties to sensing, communication, manufacturing and medical uses, within diffraction and safety limits.Beyond the syllabus
Practise and check
Topic reference
The organising model
Laser output builds only when pumping sustains inversion and the round-trip optical gain exceeds all cavity losses. Stimulated emission provides matched photons; the cavity provides repeated opportunities for amplification and selects the supported modes.
Core knowledge to remember
- Absorption needs a photon with hf = E₂-E₁, and moves a particle to the upper state.
- Spontaneous emission has random timing, phase and direction.
- Stimulated emission produces a photon matching the stimulating photon in frequency, phase, direction and polarisation.
- Net gain needs population inversion on the laser transition.
- Pumping supplies energy and maintains the inversion; a long-lived upper state helps particles accumulate.
- The mirrors send light back through the gain medium and select the modes; lasing starts only when the round-trip gain exceeds the losses.
- The output coupler must let some light out while keeping enough feedback.
Common mistakes
- Saying the mirrors make laser light coherent; stimulated emission produces the matched photons.
- Saying pumping alone guarantees laser output; the gain must also exceed the cavity losses.
- Describing population inversion as all the atoms being excited; it compares the two laser levels only.
- Saying laser light does not spread; every beam diffracts.