Semiconductor laser also called laser diode，which is using semiconductor materials as a working substance. The characteristic is small volume, long service life, and it can help its input voltage and operating current by simple injection current, that with compatible with integrated circuit, so it can be integrated with the monolithic chip.
Because of these advantages, semiconductor laser diodes have been widely used in laser communication, optical storage, optical gyro, laser printing, ranging and radar etc...
Laser Principles Of Semiconductor Laser
Laser principles of semiconductor laser, which the laser is produced under the following conditions:
1. population inversion
2. a resonant cavity, in order to play the role of optical feedback, to form laser oscillation; The formation of a variety forms, the simplest of these is Fabry-Perot resonator.
3. The threshold condition is that the gain is greater than the total loss.
(1) Satisfy the threshold condition
In order to form a stable oscillation, the laser medium must be able to provide sufficient gain to compensate for the optical loss caused by the resonator cavity and the loss caused by the laser output from the cavity surface, and the light field in the cavity is constantly increased. This requires a strong enough current injection, and then there`s enough population inversion. The higher degree of population inversion, the greater gain obtained, as well as current threshold condition must be satisfied. When the laser device reaches the threshold value, light with a specific wavelength can resonate in the cavity and be amplified, then finally form a laser and continuous output.
(2) resonant cavity, in order to play the role of optical feedback, to form laser oscillation
In order to obtain the coherent stimulated radiation, it is necessary to make the stimulated radiation in the optical resonator to get multiple feedback to form laser oscillations. The resonant cavity of the laser device is formed by the natural cleavage surface of the semiconducting crystal as a reflector.
Usually plated with the high reflection multilayer dielectric film on the side without light, and the light side is plated the antireflective film.
The semiconductor laser can easily make use of the natural cleavage plane of crystal and the P-N junction plane to form the F-P cavity.
(3) Gain condition
The inversion distribution of the carrier in the maser medium (active region) is established. On behalf of the electron energy is the energy band in the semiconductor, which is composed of a series that is close to continuous energy levels, Therefore, population inversion is required in semiconductors, it must be between two band regions.
The number of electrons at the bottom of the higher conduction bands is much larger than the number of holes at the top of the lower conduction bands. This can be achieved by injecting necessary carriers into the active layer by applying forward bias to the homogeneous or heterojunction to excise electrons from the low-energy valence band to the high-energy conduction band. Excited emission occurs when a large number of electrons in the inversion state recombine with the hole.
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