MQW stands for Multi-Quantum Well.

Quantum well laser diode is developed from heterostructure laser diode. When the active layer of the heterostructure diode is extremely thin (around tens of nanometers), the electron levels in the active layer are split into discrete levels. This structure is called quantum well.

The density of the electrons at the discrete energy levels is increased, which results in a lower laser threshold.

Multiple quantum wells are designed in the laser diode in order to achieve higher optical power, to compensate the thin active layer for a single quantum well.

An additional feature for quantum well laser diode is the thickness of the active layer actually affects the energy gap of the electrons and holes. This enables altering the laser wavelength by changing the thickness of the active layer, without the need to change the material of the diode. New wavelengths that are not achievable with the traditional heterostructure laser diode are now available with quantum well lasers.