How Lasers Work

LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser produces a narrow, coherent beam of light where all photons travel in the same direction with the same phase and wavelength — using a quantum process Einstein predicted in 1917, decades before the first laser was built.

1. Three Light-Matter Interactions

An atom has discrete energy levels. When an electron jumps between them, it exchanges a photon. Einstein identified three fundamental processes:

Absorption: A photon arrives with exactly the right energy → electron jumps to a higher level, photon vanishes.
Spontaneous emission: An excited electron spontaneously drops to a lower level → emits a photon in a random direction with random phase. This is how ordinary light (lamps, fire, stars) works.
Stimulated emission: A photon arrives when an electron is already in an excited state → the electron drops and emits an identical photon — same wavelength, direction, phase, and polarisation. The result is two coherent photons where there was one. This is the laser mechanism.

Photon energy: E = hν = hc/λ h = 6.626\times10⁻³⁴ J\cdots (Planck's constant), ν = frequency, λ = wavelength

2. Population Inversion

Normally, more atoms sit in low energy states than high ones (Boltzmann distribution). For stimulated emission to dominate over absorption, you need more atoms in the excited state than the ground state — a situation called population inversion.

Population inversion is thermodynamically unusual: at equilibrium, no system has it. You must continuously pump energy in. This "pumping" comes from:

Optical pumping: Flash a bright external light source (flashlamp, diode laser) at the gain medium.
Electrical pumping: Run current through a semiconductor or discharge through a gas (He-Ne laser, CO₂ laser).
Chemical pumping: Chemical reactions directly produce excited molecules (used in military chemical lasers).

Three-level vs four-level systems: Ruby (three-level) needs very intense pumping. Most modern lasers use four-level schemes (like Nd:YAG) where the lower laser level is not the ground state — it quickly empties, making inversion much easier to maintain.

3. The Optical Cavity

A gain medium alone produces amplified spontaneous emission (ASE) — a very bright, directional glow, but not a laser. To get a true laser you need an optical resonator: two mirrors facing each other, with the gain medium between them.

Photons travelling along the cavity axis bounce back and forth, triggering more stimulated emission each pass — an optical chain reaction. One mirror is 100% reflective; the other is partially transparent (~95–99%) to let light out as the beam.

Round-trip gain > loss: Lasing threshold is reached when the round-trip gain (stimulated emission gain × gain length) exceeds all cavity losses (output coupling, scattering, absorption). Above threshold, output power rises steeply with pump power.

The cavity also acts as a frequency selector (etalon effect): only wavelengths for which the cavity length is an integer multiple of half-wavelengths form standing waves (longitudinal modes) and experience the highest gain. This is why lasers emit narrow spectral lines.

4. Coherence & Monochromaticity

Laser light has properties ordinary sources can't match:

Temporal coherence: Very narrow linewidth (Δλ ~ 0.001 nm). A HeNe laser can maintain a constant phase relationship over metres of propagation (coherence length ~300 m for a single-mode laser).
Spatial coherence: All parts of the beam have a fixed phase relationship. Enables tight focusing below the diffraction limit and long-range collimation.
Low divergence: A well-collimated red laser pointer grows only from 1 mm to ~2 mm diameter over 1 km.

These properties arise because stimulated emission produces identical photons — the beam is a phase-locked amplification of a single initial photon.

5. Types of Lasers

HeNe Gas Laser

632.8 nm (red)

Holography, metrology, optics labs

CO₂ Laser

10,600 nm (IR)

Industrial cutting, engraving, surgery

Nd:YAG

1064 nm (near-IR)

Material processing, range-finding, LIDAR

Diode Laser

400–1600 nm

Optical storage, fibre comms, laser printers

Ti:Sapphire

650–1100 nm

Ultrafast pulses (femtosecond), research

Excimer (ArF)

193 nm (deep UV)

Semiconductor lithography (EUV source)

6. Applications

Manufacturing: CO₂ and fibre lasers cut, weld, and mark metals with precision impossible with mechanical tools. A 6 kW fibre laser cuts 1 cm steel plate at 1 m/min.

Medicine: LASIK eye surgery reshapes the cornea with pulses of ArF excimer laser (193 nm). Surgical lasers seal blood vessels during procedures. Photodynamic therapy activates drug molecules using laser light.

Communications: Fibre optic networks carry 1550 nm laser light (InGaAsP diode lasers) through glass fibres. A single fibre strand can carry 100+ Tb/s using wavelength-division multiplexing.

LIDAR: Autonomous vehicles and mapping satellites use pulsed lasers to measure distances. Time-of-flight: $d = c \cdot \Delta t / 2$ where Δt is the round-trip travel time. Resolution down to ~1 cm at 200 m range.

7. Laser Safety Classes

Laser power and beam exposure time determine hazard. The IEC 60825 classification:

Class 1: Safe under all normal use (laser printers, CD players); beam is enclosed.
Class 2: Low-power visible (≤1 mW); blink reflex protects the eye. Red laser pointers.
Class 3R: Slightly hazardous (≤5 mW); avoid direct eye exposure.
Class 3B: Hazardous on direct beam contact; scattered light generally safe. Lab lasers.
Class 4: Hazardous for eyes and skin; scattered light can ignite materials. Industrial and surgical lasers.

Important: The eye's lens focuses a collimated beam to a spot ~10–20 μm across on the retina. At that concentration factor, even a few milliwatts of visible laser light can permanently damage vision in 0.1 seconds — before the blink reflex can protect you.