laser-physics Physics

A standing-wave cavity fixes antinodes and nodes of each longitudinal mode, so inversion in the gain medium is depleted into periodic spatial dips matching that pattern — spatial hole burning. Other longitudinal modes place nodes where holes were burned and can extract net gain from undepleted inversion, driving multimode oscillation. Remedies include traveling-wave ring cavities that eliminate the standing pattern, short gain lengths with strong mode pulling, and intracavity etalons or birefringent filters that raise the loss contrast between adjacent modes.

Ordinary light sources rely on spontaneous emission: countless independent atomic or molecular transitions have random phases, polarizations, and propagation directions, so the light spreads isotropically. Laser light instead arises from stimulated emission inside an optical resonator, where each emitted photon duplicates the frequency, phase, and direction of the photon that induced it. The two mirrors of the resonator only sustain modes along the cavity axis; off-axis components suffer repeated diffraction losses and cannot build up, which is why a laser beam typically diverges by only a few milliradians.

Adjacent longitudinal modes are separated by Δν = c/(2L), where L is the optical length of the cavity set by its geometric length and the refractive index of the intracavity medium. A mode can oscillate only if it lies within the gain bandwidth of the medium and its small-signal gain exceeds the round-trip loss, so the number of oscillating modes is roughly the gain linewidth divided by the mode spacing. A 30 cm helium-neon laser has Δν ≈ 500 MHz against a Doppler width near 1.5 GHz, so two or three longitudinal modes typically oscillate simultaneously.