How do laser diodes work?
basic recipe for laser diode:
1.
medium gain
cleaved mirrors
3 electrodes
4. DC power source
5. packaging
the gain medium is a direct band gap optoelectronic material and generally pn junction (most basic design). this gain medium is surrounded by electrodes with also semiconductors. these must be network or domain-adapted to maximize electrical conductivity and minimize interfacial defects; they must therefore be made by epitaxial techniques (mocv, molecular beam, etc.). it must be properly cleaved at the edges to reflect the light. it is properly packaged and the source of direct current or perhaps alternating current is connected to the electrodes.
* look in the comment for an image *
the gain means acts like a cavity. GaAs optoelectronic material, for example, emits light when electrons and holes combine again. For this, a voltage must be applied. the DC power source must be programmed with capacitor inductors, etc. so that when the laser is on, the current is maximum, allowing a very high spontaneous emission. at launch, the spontaneous emission must be much higher than the stimulated emission. at this point, the laser is basically a led. as more light is created, it reaches the laser emission threshold, the current must drop. now the voltage must be high with the least current. this guarantees a more stimulated emission. the cleaved mirrors facilitate a cavity for the light to increase in intensity exponentially by stimulated emission. once it reaches dynamic equilibrium, a useful laser light is produced. it can be directed via micro-optical components in a neat package.
now it’s a very basic design. Other designs exist, such as planar emission, vertical cavities, and so on. The thickness of the medium gain film can also be pushed to 5 nm or less. This type of nano-engineering creates a reduction in the density of states that increases the gain coefficient (the laser becomes more efficient).
more advanced engineering: gain doping to modify the wavelength, growth of quantum wells under stress, growth of quantum dots (further increases the gain coefficient), use of cobalt electrodes (with sapphire thin film interfaces) light (also requires manganese doping on the gain medium), fiber coupling with external cavities (with wavelength adjustment by mechanical stress on the fiber) and charges of other materials.
