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, the gain medium of most telecom diode lasers, 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 | 1500 – 1650 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
- Emission wavelength set by the well thickness as well as by the composition, which allows 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): the lowest transparency current, suited to long, low-loss cavities; but the small confinement factor makes each well supply a high material gain, where its differential gain is low
- Many wells (5 – 10): a larger confinement factor and higher modal gain, so each well works at a lower carrier density and a higher differential gain, which raises the modulation bandwidth and lowers the chirp; used in short or high-loss cavities and in directly modulated lasers. The returns diminish because carriers, holes especially, do not spread evenly across many wells, and the transparency current grows with the number of wells
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.