The \"FP\" stands for Fabry-Perot.

Fabry-Perot laser diode has very simple structure: two reflective mirrors and an active section between the mirrors. Most laser systems can be simplified as FP oscillators.

FP laser diode is made by cleaving the facets of a semiconductor PN junction. The facets are naturally reflective because of the difference on refractive index of the semiconductor material and the air. Different high reflective and wavelength selective coatings can also be deposit on the facets to enhance the performance of the laser diode.

In general, the FP laser diode emits multiple longitudinal optical modes. The wavelength spacing between two modes is determined by the cavity optical length. Changing the temperature of the diode can tune the wavelength of the laser, normally in the scale of several nanometers. The temperature changes the optical length of the FP cavity, as well as affecting the band gap of the active material. Varying the driving current also changes the wavelength slightly, because the effective index is changed with the density of electrons and holes.

Heterostructure diode structure is developed to decrease the threshold of the laser diode, by increasing the electrons/holes density.

Coaxial pigtailed FP laser diode usually has a photodiode integrated inside the package. The photodiode is located at one of the ends of the laser cavity, generating current when the diode is lasing. This current output can be used in a negative feedback loop circuit to automatically control the optical power of the laser diode, to achieve very stable optical output.