Standing Waves Between Mirrors
A laser cavity is a pair of mirrors facing each other with a gain medium in between. Light bouncing between the mirrors forms standing waves — only wavelengths that fit an exact integer number of half-wavelengths into the cavity length are sustained. These are the longitudinal modes of the resonator, and their frequency spacing is simply c/(2L). For a 30 cm cavity, this spacing is 500 MHz — a tiny fraction of the optical frequency itself.
Gain, Loss, and Threshold
The gain medium amplifies light passing through it, while the mirrors let a small fraction escape as the useful laser beam. Lasing begins when the gain per round trip exceeds the total losses — mirror transmission, scattering, and absorption. This threshold condition determines the minimum pumping power needed. The threshold gain coefficient equals −ln(R₁R₂)/(2L), so higher mirror reflectivity lowers the threshold.
Multi-Mode vs Single-Mode Lasers
When the gain bandwidth is wide enough to span several mode spacings, multiple longitudinal modes can oscillate simultaneously. A helium-neon laser at 632.8 nm with a 30 cm cavity might support 5–6 modes. For applications demanding extreme spectral purity — interferometry, spectroscopy, coherent communications — single-mode operation is achieved with shorter cavities, intracavity etalons, or distributed feedback gratings.
Finesse and Stored Energy
Cavity finesse measures how many times a photon bounces between the mirrors before escaping. High-finesse cavities (F > 1000) store enormous amounts of optical energy and produce extremely narrow emission lines. This principle is exploited in ring-down spectroscopy to detect trace gases at parts-per-trillion levels, and in gravitational wave detectors like LIGO where finesse exceeds 400.