Semiconductor laser

1. Overview — Diode Laser (p-n Junction)

Semiconductor laser / laser diode = p-n junction (usually heterojunction GaAs/AlGaAs, InGaAsP) that lases via electron-hole recombination with injection pumping (forward bias). Wavelength determined by band gap:

hν≈E_g

Typical $λ$ : 850 nm (GaAs), 1300/1550 nm (InP for fiber), 405 nm (GaN blue). Compact, efficient (~50%), direct modulation.

2. Construction (heterojunction)

Double heterostructure: n-AlGaAs / p-GaAs (active layer thin ~0.1 µm) / p-AlGaAs
Cleaved facets act as Fabry-Perot cavity mirrors (R≈30% Fresnel, no coating needed; or coated for higher R)
Contacts, stripe geometry for current confinement, heat sink.
Active region sandwiched between larger-Eg cladding layers ⇒ carrier + optical confinement (waveguide).

Homojunction (just p-n GaAs) has high threshold; heterojunction reduces threshold ~10× via confinement.

3. Pumping and Inversion (injection + band structure)

Forward bias injects electrons into conduction band (n-side) and holes into valence band (p-side) into active layer → population inversion when quasi-Fermi levels separate by > $E_g$ :

E_{Fc}-E_{Fv}>hν≈E_g

This is a four-level-like band picture: conduction quasi-level upper, valence lower, lower quickly emptied by recombination. No discrete levels but band.

Gain guided by stripe; resonator is cleaved facets.

4. Characteristics

Threshold current $I_{th}∼10$ – $100$ mA (heterojunction); above that light ∝ $(I-I_{th})$ .
Wavelength set by $E_g$ (e.g., GaAs $E_g=1.42$ eV ⇒ $λ≈870$ nm).
Linewidth ~nm (multimode) without DFB grating; DFB/DBR gives MHz.
Efficiency 30–60%, tiny size mm, direct current modulation to GHz.

5. Comparison to Ruby/He-Ne

Ruby/He-Ne	Diode
Bulk crystal/gas, optical/discharge pump	p-n junction, injection pump
Low efficiency (<1%)	High (~50%)
Large	Chip-scale
Fixed $λ$ (694,632)	Tunable by $E_g$ (all $λ$ )

6. Applications

Fiber communication (1300/1550 nm), CD/DVD/Blu-ray (780/650/405 nm), laser pointers, pumps for solid-state lasers (Nd:YAG diode-pumped), LIDAR, medicine.

7. Exam Points

Structure: double heterojunction diagram — 5-7 marks, very frequent.
Injection pumping and $hν≈E_g$ , quasi-Fermi condition — 5 marks, frequent.
Why heterojunction lowers threshold — 5 marks, frequent.
Wavelength examples and applications — 3-5 marks.

Semiconductor laser