Lasers & Fibre Optics
How radiation interacts with matter, how a gain medium is engineered into a laser, and how light is guided, attenuated and dispersed in fibre — each computed as a design decision for marking, sensing, timing and plant networks.
Ruby rod: energy if every Cr³⁺ ion emits
Why optical carriers win
The visible band 400–700 nm spans ≈ 3.2 × 10¹⁴ Hz. At 10 MHz per channel it could carry ≈ 3.2 × 10⁷ channels — millions of times more than a microwave carrier. This is the physical reason behind fibre and free-space optical links.
He-Ne vs ruby vs semiconductor lasers
| Attribute | He-Ne | Ruby | Semiconductor diode |
|---|---|---|---|
| Active medium | He:Ne gas mix (≈10:1) | Al₂O₃ doped with Cr³⁺ | Heavily doped p–n junction (GaAs, InGaAsP) |
| Level scheme | Four-level (He transfers energy to Ne) | Three-level with metastable E₂ | Inversion between conduction & valence bands |
| Pumping | Electrical discharge | Xenon flash lamp (optical) | Forward-bias injection current |
| Wavelength | 632.8 nm (CW) | 694.3 nm (pulsed) | 650 nm – 1.55 µm |
| Efficiency | < 0.1% | ≈ 1% | 30–60% |
| Industrial use | Alignment, interferometry, classic scanners | Holography, pulsed ranging, tattoo/derma | Fibre transmitters, printers, bar-code readers, pump sources |
Bar-code scanner
A diode or He-Ne beam is swept across the code by a rotating mirror. Dark bars absorb and white spaces reflect; a photodiode converts the reflected light into a pulse train that is decoded into the product ID — the backbone of retail and pharma traceability (UC-102).
Laser printer
A modulated laser discharges a photoconductive drum where the page should stay white; toner sticks to the charged regions, transfers to paper and is fused by heat. Resolution is set by the focused spot, i.e. by coherence and directionality.
Laser cooling
Atoms moving toward a slightly red-detuned beam absorb photons and lose momentum h/λ each time; spontaneous re-emission is random, so the net effect is a strong decelerating force. Cold atoms are the basis of atomic clocks and quantum sensors (UC-104).
Type comparison on the current link
2 km · 1310 nm · 10 Gb/s target. Computed by the platform engine.
Absorption, Rayleigh scattering & windows
Rayleigh ∝ 1/λ⁴ dominates short wavelengths; OH⁻ absorption near 1383 nm; IR absorption rises beyond 1600 nm.
Bending losses
Macro-bends tilt rays beyond the critical angle so they escape into the cladding; micro-bends from cabling stress couple modes into radiation. Specify minimum bend radius (typically 10–20× cable diameter) in installation SOPs.
Material & waveguide
Refractive index varies with wavelength, so a source with spectral width Δλ spreads by D·Δλ·L. Narrow-linewidth lasers and operating near 1310 nm (zero dispersion in standard fibre) minimise it.
Intermodal
Different rays travel different path lengths: Δt ≈ Ln₁Δ/c for step-index multimode. Graded-index profiles equalise path delays (Δt ≈ Ln₁Δ²/8c); single-mode fibre eliminates it.