Characteristics of laser light

Characteristics of laser light

1. What LASER Means

LASER = Light Amplification by Stimulated Emission of Radiation — not just a light source but an amplifier/oscillator giving unique light properties via stimulated emission (Einstein). Ordinary light (bulb, sun) is spontaneous emission, random.

2. Four Hallmark Characteristics (exam table — memorize with contrast to ordinary light)

Property	Laser Light	Ordinary Light (bulb)
Monochromaticity	Very narrow linewidth $Δλ∼10^{-3}$–$10^{-6}$ nm (single $λ$)	Broad $Δλ∼100$ nm (white)
Coherence	High temporal ($L_c∼$ m–km) and spatial (phase locked across beam)	Low $L_c∼µm$, incoherent
Directionality / Collimation	Highly collimated, divergence $θ∼$ mrad (beam stays narrow)	Diverges spherically
Intensity / Brightness	Very high power density (focused to $∼10^{12}$ W/cm² possible) due to concentration in narrow beam/line	Low brightness

Additional: polarization (often linear, well-defined).

3. Why These Properties Occur (preview)

• Monochromatic + coherent ⇒ stimulated emission clones the stimulating photon's $λ, phase, polarization$.
• Directionality ⇒ optical resonator selects one axial mode.
• High intensity ⇒ many photons in same mode, concentrated.

4. Quantitative Note

• Temporal coherence length

— laser $Δλ$ tiny ⇒ $L_c$ huge vs white light $∼1$ µm.

• Beam divergence $θ≈1.22λ/D$ for aperture $D$ — laser's effective $D$ large and $λ$ small ⇒ small $θ$.

5. Worked Contrast

LED (spontaneous): Δλ≈20 nm, L_c≈ λ²/Δλ≈ 12 µm, diverges widely
He-Ne laser: Δλ≈0.001 nm, L_c≈0.3 m? Actually 632.8 nm, Δν≈1 GHz ⇒ Lc≈30 cm, divergence ~1 mrad
No mode selection vs single longitudinal mode gives further narrowing.

6. Exam Points

• Expand LASER — 1 mark, frequent.
• List four characteristics with contrast to ordinary light — 5 marks, very frequent (almost certain table).
• Explain each briefly (why monochromatic, coherent etc.) — 5-7 marks, frequent.
• Relation to stimulated emission — 3 marks.