Semiconductor laser is a kind of coherent radiation source. To enable it to produce laser light, three basic conditions must be met:
Gain conditions: establish the inversion distribution of carriers in the lasing medium (active region), and excite electrons from the lower energy valence band to the higher energy conduction band.
To actually obtain coherent stimulated radiation, the stimulated radiation must be fed back multiple times in the optical cavity to form laser oscillations.
In order to form a stable oscillation, the laser medium must be able to provide a large enough gain, which must have a strong enough current injection, that is, enough population inversion, the higher the population inversion degree, the greater the gain obtained. That is, a certain current threshold condition must be met. When the laser reaches the threshold, light with a specific wavelength can resonate in the cavity and be amplified, and finally form a laser and output continuously.