Optical Sources: LED and Laser Diode With easy examples.

Optical sources convert current into light for fiber communication, and the two workhorses are the LED and the laser diode. Both are semiconductor p-n junctions, but they emit by different physics — spontaneous versus stimulated emission — and that difference decides cost, speed, distance and reliability of every optical link.

Choosing between them is a core engineering call. LEDs are cheap, reliable and simple but too slow and spectrally broad for long-haul fiber; laser diodes are fast and pure but need threshold control and cost more. Exams and interviews test the why, not just the which.

Key Concepts

• LED: injected carriers recombine spontaneously, each photon independent — incoherent light, broad spectrum (30–50 nm), output linear from zero current.
• Laser diode: above threshold current (Ith), stimulated emission dominates — photons clone themselves inside a mirrored Fabry-Perot cavity, giving coherent, narrow light (~1–5 nm).
• Threshold: lasers barely glow below Ith, then output rises linearly; they are always biased above threshold. LEDs have no threshold.
• Speed: LEDs manage tens–hundreds of MHz (slow spontaneous lifetime); laser diodes modulate at 10–50+ Gbps.
• Coupling: a laser's tight beam puts 30–50% into single-mode fiber; an LED's wide pattern couples only a few percent.
• Wavelength: set by bandgap — GaAs ≈ 850 nm, InGaAsP ≈ 1310/1550 nm. Photon energy E = 1240/λ(nm) eV.

Formulas

Photon energy E(eV) ≈ 1240 / λ(nm): 1310 nm → 0.947 eV, 850 nm → 1.459 eV — the bandgap must roughly match. Above threshold, laser power follows P = ηslope × (I − Ith) with slope efficiency ηslope ≈ 0.2–0.5 W/A per facet. LED power is P ≈ ηext × I × (hc/λq) with external quantum efficiency of a few percent. Spectrum width and modulation bandwidth then decide each source's suitable fiber length and bit rate.

Worked Example

Compare sources at 1310 nm. Step 1: E = 1240/1310 ≈ 0.947 eV — InGaAsP territory, confirming material choice. Step 2: laser with Ith = 15 mA, ηslope = 0.3 mW/mA, biased at 35 mA → P = 0.3 × 20 = 6 mW. Step 3: LED at 50 mA, ηext = 3% → P ≈ 0.03 × 0.05 × 0.947 ≈ 1.42 mW over 40 nm, with ~5% coupling into single-mode fiber ≈ 71 µW launched. Step 4: the laser launches roughly 80× more usable power with a 10× narrower spectrum — the quantitative reason long-haul links never use LEDs.

Common Misconceptions

• "A laser is just a brighter LED." The real gaps are coherence, threshold and spectral purity from stimulated emission and the cavity — not brightness.
• "Lasers are always better." For short cheap links (remotes, TOSLINK, short multimode data), LEDs win on cost, reliability, temperature stability and eye safety.
• "More current always means more light." Heating causes thermal rollover, and overdrive destroys laser facets — respect maximum ratings.

Real-World Applications

• Long-haul: DFB lasers at 1550 nm feed EDFA-amplified DWDM links across continents.
• Data centres: 850 nm VCSEL arrays drive 100–800 Gbps multimode links.
• Consumer: LEDs in remotes, indicators, TOSLINK audio, pulse oximeters.
• Storage/sensing: laser diodes read Blu-ray discs, scan barcodes, power LIDAR.

LEDs give cheap, forgiving, incoherent light; laser diodes give fast, focused, coherent light above threshold. The physics — spontaneous versus stimulated emission — explains everything from the 40 nm LED spectrum to gigabit laser modulation, and tells you which source each link deserves.