Injection current and temperature impact the gain curve, and thereby influence the output wavelength of the laser. The injection current changes the carrier density, modifying both the index of refraction and the bandgap energy of the device. If left unmodified, their usefulness is limited in applications requiring narrowband radiation. Initial measurements of AlGaAs laser linewidths in the early 1980's revealed a Lorentzian shape linewidth, with a full width at half maximum intensity that varied inversely with laser output power. The linewidth broadening was subsequently attributed to variations of the refractive index of the laser cavity with instantaneous carrier density.
Optical feedback reduces the linewidth primarily by increasing the optical cavity length.
The extended cavity decouples the resonant laser frequency from strong dependence on the index of refraction of the semiconductor. As a result, linewidth broadening due to index of refraction fluctuations is reduced. 5,16 Optical feedback is also used to influence the laser's frequency. As a result, the gain of the system only weakly depends upon wavelength and the lasing wavelength is easily perturbed. By using a wavelength-selective cavity to externally impose the desired wavelength, wavelength selection can be achieved.