Ordinary light — from the sun, a lamp, a hot exhaust — is chaos: all colours (wavelengths) mixed together, waves in random phase, radiated in all directions. A laser produces the opposite: light of one wavelength, whose waves all march in step, in one tight beam.
This is achieved through stimulated emission: in an active medium (crystal, gas or semiconductor), atoms are "pumped" to a higher energy level. A passing photon with exactly the right energy stimulates such an atom to release its energy as an identical photon: same wavelength, same direction, same phase. Between two mirrors this light bounces back and forth and builds up like an avalanche — one mirror lets a fraction through: the laser beam.
Specific characteristics of laser
Three properties set laser light apart from all other light, and all three are militarily relevant:
- Monochromatic — one wavelength. Receivers can use a narrowband filter to reject all other light (sun, flares, fires) and let only their own laser wavelength through: enormous immunity to interference. The flip side: laser warning receivers also know exactly which wavelengths to watch for.
- Coherent — all waves in phase (EM waves in phase, in antiphase and out of phase). This keeps the beam tight and allows the energy to stay concentrated over long distances; coherence also makes interferometric precision measurements possible.
- Collimated — extremely small beam divergence, typically 0.1–1 milliradian. A 1 mrad beam is only 1 m wide at 1 km, 10 m at 10 km. Almost all the energy therefore arrives on the target — and almost none beside it, which makes interception and detection from the side very difficult.
A 1 W torch lights up a spot tens of metres wide at 100 m: invisibly faint. A 1 W laser still puts almost its full watt on a spot a few metres wide at 10 km. It is not the power but the collimation that makes the laser special.
Commonly used military wavelengths:
| Wavelength | Type | Use | Remark |
|---|---|---|---|
| 1.06 µm | Nd:YAG | rangefinders, designators | standard; dangerous to the eye |
| 1.55 µm | erbium glass / fibre | modern rangefinders | "eye-safe": the cornea absorbs it, the retina is spared |
| 10.6 µm | CO₂ | older rangefinders, industry | excellent through smoke and haze |
| 0.4 – 0.7 µm | diodes (visible) | pointers, dazzlers | visible; not used for measurement |