A laser needs a gain medium (pumped into population inversion), placed between two mirrors forming a resonant cavity. Light bounces back and forth, being amplified each pass, until gain equals loss (threshold) — above that, photon number grows and a fraction escapes through the partially-transmissive output coupler as the laser beam.
g(I) = g0 / (1 + I / I_sat) (gain saturation)
lasing when: g(I)·L ≥ −ln(R1·R2)/2 (threshold condition)
P_out ∝ I_cavity · T_output
- Pump power — sets the small-signal gain g0; must exceed the threshold set by cavity losses before lasing starts.
- Output coupler transmission — how much light escapes each pass as usable beam vs. staying in the cavity to keep amplifying.
- Cavity length — the mirror separation; sets the round-trip time and spacing of the visualized photon bursts.
- Gain medium — switches between three common laser wavelengths/colours (He-Ne red, Nd:YAG green-doubled, diode near-IR shown as deep red) with no other physics change.
This exact threshold behaviour is why laser pointers, barcode scanners, and fiber-optic communication lasers all show a sudden "kink" in output power once pump current crosses a critical value — below it you get incoherent glow, above it a coherent beam.