Multi-quantum well (MQW)
An active region consisting of multiple thin quantum-well layers separated by wider-bandgap barrier layers. The dominant gain medium architecture for modern telecom diode lasers.
A multi-quantum-well (MQW) active region stacks several quantum well layers separated by barrier layers of higher-bandgap material. Each well confines carriers in one dimension and contributes its own optical gain to the cavity mode. The barriers prevent quantum-mechanical coupling between adjacent wells while supplying carriers to all wells in parallel via classical drift and diffusion.
Typical MQW configurations:
| Material system | Well thickness | Barrier thickness | Number of wells | Emission wavelength |
|---|---|---|---|---|
| InGaAsP/InP | 5 – 10 nm | 5 – 15 nm | 5 – 10 | 1300 – 1600 nm |
| InGaAlAs/InP | 5 – 10 nm | 5 – 15 nm | 5 – 10 | 1300 – 1600 nm |
| AlGaAs/GaAs | 5 – 10 nm | 5 – 20 nm | 1 – 5 | 750 – 870 nm |
| InGaAs/GaAs | 5 – 8 nm | 10 – 20 nm | 1 – 5 | 900 – 1100 nm |
| InGaAs/InGaAsP strained | 5 – 8 nm | 10 – 20 nm | 4 – 8 | 1100 – 1300 nm |
Advantages over bulk double-heterostructure (DH) active regions:
- Lower threshold current density — quantized density of states concentrates carriers at energies near the gain peak rather than spreading across a broad band
- Higher differential gain — increases modulation bandwidth and reduces frequency chirp
- Tunable emission wavelength via well thickness — independent of material composition, enabling fine wavelength engineering
- Strain engineering — biaxial strain in the wells reshapes the valence band structure, can reduce Auger recombination and the linewidth enhancement factor
Number of wells trades off:
- Few wells (1 – 3): highest differential gain, narrowest linewidth, low confinement factor, requires careful optical design
- Many wells (5 – 10): higher modal gain via larger confinement factor, used in DFB lasers requiring strong gain × length to overcome grating losses, but with diminishing returns due to non-uniform carrier injection
Compressively strained MQW designs (InGaAsP wells with lattice constant larger than the InP substrate) became standard in telecom DFB lasers because the strain-induced valence band restructuring reduces Auger and intervalence band absorption losses, particularly important at 1550 nm.