A semiconductor laser, also called a laser diode, is a laser that uses a semiconductor material as a working substance. It has the characteristics of small size and long life, and it can be integrated with the integrated circuit by pumping its working voltage and current with a simple injection current. Due to these advantages, semiconductor diode lasers have been widely used in laser communication, optical storage, optical gyro, laser printing, ranging, and radar.
Principle of laser illumination
First, the population number is reversed;
Second, there must be a resonant cavity, which can play a role of optical feedback, forming a laser oscillation; forming a variety of forms, the simplest is the Fabry-Perot cavity.
Third, the laser must also meet the threshold conditions, that is, the gain is greater than the total loss.
(1) Meet certain threshold conditions.
In order to form a stable oscillation, the laser medium must be able to provide a sufficiently large gain to compensate for the optical loss caused by the cavity and the loss caused by the laser output from the cavity surface, and continuously increase the light field in the cavity. This requires a sufficiently strong current injection, that is, there is enough population inversion, and the higher the degree of particle inversion, the greater the gain obtained, that is, the requirement that a certain current threshold condition must be met. When the laser reaches the threshold, light having a specific wavelength can resonate in the cavity and be amplified, and finally a laser is formed to continuously output.
(2) The resonant cavity can act as an optical feedback to form a laser oscillation.
In order to obtain the coherent stimulated radiation, the stimulated radiation must be feedbacked multiple times in the optical cavity to form a laser oscillation. The cavity of the laser is formed by the natural cleavage plane of the semiconductor crystal as a mirror, usually without light. The end is plated with a high-reflection multilayer dielectric film, and the light-emitting surface is plated with an anti-reflection film.
For FP cavity (Fabry-Croove) semiconductor lasers, it is convenient to use the natural cleavage plane of the crystal perpendicular to the plane of the PN junction to form the FP cavity.
(3) Gain conditions:
The inverse distribution of carriers in the lasing medium (active area) is established. The electron energy in the semiconductor is composed of a series of energy bands close to a continuous energy level. Therefore, in order to achieve particle number inversion in the semiconductor, it is necessary to be in the high energy state conduction band between the two energy band regions. The number of electrons at the bottom is much larger than the number of holes at the top of the low-energy valence band. This is achieved by applying forward bias to the homojunction or heterojunction and injecting the necessary carriers into the active layer. The electrons are excited from a lower energy valence band to a higher energy conduction band. When a large number of electrons in a state in which the number of particles is inverted are combined with holes, a stimulated emission is generated.
Semiconductor laser characteristicsA semiconductor laser is a type of laser device in which a semiconductor material is a working substance. It was born in 1962 and has the following advantages in addition to the common features of lasers:
(1) Small size and light weight;
(2) The driving power and current are low;
(3) High efficiency and long working life;
(4) Direct electrical modulation;
(5) Easy to integrate optoelectronics with various optoelectronic devices;
(6) Compatible with semiconductor manufacturing technology; can be mass-produced.
Due to these characteristics, semiconductor lasers have received extensive attention and research from all over the world since their inception. It has become the world's fastest-growing, most widely used, first-class laser that has gone out of the laboratory for commercialization and has the largest output value.
The working principle of the semiconductor laser is an excitation method. The semiconductor material (that is, using electrons) is used to illuminate between the energy bands, and the cleavage plane of the semiconductor crystal is used to form two parallel mirror surfaces as mirrors to form a resonant cavity to oscillate and feedback the light. The radiation that produces the light is amplified and the laser is output.
Semiconductor lasers rely on injected carriers. There are three basic conditions for emitting lasers:
(1) To generate a sufficient population inversion distribution, that is, the number of high-energy particles is sufficiently larger than the number of particles in the low-energy state;
(2) There is a suitable resonant cavity that can play a role in feedback, so that the excited radiation photons proliferate, thereby generating laser oscillation;
(3) To meet certain threshold conditions, so that the photon gain is equal to or greater than the loss of photons.
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