--- title: "Stimulated Emission and How a Laser Works" description: "An atomic laser amplifies light by stimulated emission. Pumping creates an inversion between laser levels. In a two-mirror design, cavity feedback supports oscillation when gain overcomes losses. Cohe" canonical: https://lightmysky.com/learn/science/stimulated-emission-and-how-a-laser-works-mt_xgjD0mzEAV source: https://lightmysky.com/learn/science/stimulated-emission-and-how-a-laser-works-mt_xgjD0mzEAV.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Stimulated Emission and How a Laser Works An atomic laser amplifies light by stimulated emission. Pumping creates an inversion between laser levels. In a two-mirror design, cavity feedback supports oscillation when gain overcomes losses. Coherence describes field phase relationships, not brightness. Subject: Science · Area: Waves, Light & Sound · Ages 21 to 22 Page: https://lightmysky.com/learn/science/stimulated-emission-and-how-a-laser-works-mt_xgjD0mzEAV ## Ready when they can - Distinguishes spontaneous from stimulated emission and identifies matching optical-mode properties, with phase referring to the light field - Explains inversion between laser levels of equal statistical weights and follows the pumping cycle in a three-level scheme - Explains cavity feedback, output coupling and the balance of round-trip gain and loss ## Lesson: How a laser builds its beam An atom can emit a photon when it moves from a higher energy level to a lower one. The photon carries the energy gap. Spontaneous emission needs no incoming beam, and the timing of a particular decay is probabilistic. Pumping supplies excitation energy but does not schedule each spontaneous decay. Resonant light can stimulate an excited atom to emit into its occupied optical mode, defined by frequency, direction and polarization. With a coherent incident field, the added light has a matching field phase. A single photon need not have a definite phase. Real transitions respond over a finite frequency range near resonance, not at one infinitely exact frequency. Here the two laser levels have equal statistical weights: each contains the same number of quantum states. In this model, absorption and stimulated emission have equal transition probabilities per atom for a given field. Lower-level atoms absorb; upper-level atoms can add light by stimulated emission. More atoms in the upper than the lower level gives net stimulated gain. This population imbalance is an inversion. *(drawing: With equal statistical weights, 70 upper-level atoms versus 30 lower-level atoms gives net stimulated gain before other losses.)* In this three-level scheme, ground is the lower laser level. Pumping raises atoms to a higher pump level. Fast decay feeds a longer-lived upper level, where atoms accumulate. Laser emission returns atoms to ground; pumping replenishes losses. With equal statistical weights, inversion requires more upper-level atoms than ground-level atoms. Neglecting pump-level population gives the more-than-half-total rule. For a small nonzero pump population, that threshold is only approximate. In this two-mirror design, light makes repeated passes through the gain medium. This feedback favors resonant modes: field patterns that reproduce after a round trip. One mirror reflects most light; the output coupler transmits part as the beam. Mirrors supply no pump energy and do not give a fixed doubling on each pass. An inverted medium can also amplify light in a single pass without mirrors; that alone does not establish laser oscillation. At startup, a small circulating signal grows only when round-trip gain exceeds losses. Multiply its intensity gain factor by the fraction retained after losses. Gain 1.3 and retention 0.8 give 1.04, so the signal grows. A product below 1 makes it fade; 1 marks the threshold balance. In steady operation, gain saturation reduces gain as intensity grows until gain balances losses. Laser output has a nonzero frequency spread and finite coherence. Coherence means predictable field phase relationships over a range of times or positions. Laser beams often have narrow spectra, spread little and have greater coherence than ordinary lamp light, but may contain several modes. Intensity is energy per unit area per second, not a measure of coherence. As in the Fourier lesson, compressing a fixed pulse shape in time widens its frequency spread. **Recap.** Pumping creates inversion between laser levels. Stimulated emission adds light to a mode. Cavity oscillation needs gain to overcome losses. Laser coherence and frequency spread are finite. *Lesson, questions, worked solutions and flashcards written for this dataset. Factual background includes OpenStax, University Physics Volume 3 and College Physics.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Separating the Schrödinger Equation for Hydrogen](https://lightmysky.com/learn/science/separating-the-schrodinger-equation-for-hydrogen-mt_Yk2qHB0_v2) ## Optional connections - [Fourier Analysis and the Wave Packet](https://lightmysky.com/learn/science/fourier-analysis-and-the-wave-packet-mt_1ZaojWUZEw) ## Opens up - [Ultrafast Spectroscopy and Watching a Reaction Happen](https://lightmysky.com/learn/science/ultrafast-spectroscopy-and-watching-a-reaction-happen-mt_wxQgetzzW5)