Laser Cavity Lab for a playground swing

A laser fires only when the optical gain from the pumped medium exceeds the losses of the mirror cavity. Below that threshold it merely glows like a lamp; above it, stimulated emission takes over and output rises steeply and linearly with pump power. This sharp threshold is the defining signature of laser action.

A distributed Bragg reflector stacks alternating quarter-wave layers of high and low index. Their reflections add up in phase over a band of wavelengths — the stopband — creating a mirror that can exceed 99.99% reflectivity, far better than metal. More layer pairs and higher index contrast deepen the reflectivity. Used in lasers, fiber gratings, and dielectric mirrors.

A real laser beam is not a perfect ray — it narrows to a minimum waist then spreads. Within one Rayleigh range of the waist it stays roughly collimated; beyond, it diverges at an angle set by wavelength over waist size. Tighter focus means faster spreading — the fundamental diffraction trade-off behind every laser and telescope.