The light-emitting principle and working principle of semiconductor laser

Semiconductor lasers, also known as laser diodes, are lasers that use semiconductor materials as working materials. It has the characteristics of small size and long lifespan, and can use simple injection current to pump its working voltage and current compatible with integrated circuits, so it can be monolithically integrated with it.

Because of these advantages, semiconductor diode lasers have been widely used in laser communications, optical storage, optical gyroscopes, laser printing, ranging and radar.

The light-emitting principle of lasers

The laser must meet the following conditions:

First, the number of particles Reversal;

Second, there must be a resonant cavity, which can play the role of optical feedback and form laser oscillation; there are various forms of formation, the simplest is Fabry-Parrot Resonant cavity.

Third, the laser must also meet the threshold condition, that is, the gain must be greater than the total loss.

(1) Meet certain threshold conditions.

In order to form a stable oscillation, the laser medium must provide a large enough 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 optical field in the cavity. This 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.

(2) The resonant cavity can play the role of optical feedback and form laser oscillation.

To actually obtain coherent stimulated radiation, the stimulated radiation must be fed back multiple times in the optical resonator to form laser oscillation. The resonant cavity of the laser is made of the natural cleavage surface of the semiconductor crystal as a mirror. It is usually formed by coating a high-reflective multilayer dielectric film on the side that does not emit light, and the light-emitting surface is coated with an anti-reflection film.

For the F-P cavity (Fabry-Perot cavity) semiconductor laser, the natural cleavage surface of the crystal perpendicular to the P-N junction plane can be used to form the F-P cavity.

(3) Gain conditions:

Establish the inversion distribution of carriers in the lasing medium (active region). In semiconductors, the energy of electrons is an energy band composed of a series of energy levels that are close to continuous. Therefore, in order to achieve population inversion in semiconductors, it must be in the high-energy state conduction band between the two energy bands. The number of electrons at the bottom is much larger than the number of holes at the top of the low-energy state valence band. This is achieved by applying a forward bias to the homojunction or heterojunction and injecting necessary carriers into the active layer. Electrons are excited from the lower energy valence band to the higher energy conduction band. When a large number of electrons and holes in the state of population inversion recombine, stimulated emission occurs.

Characteristics of semiconductor lasers

Semiconductor lasers are a type of laser device with semiconductor materials as the working material. It was born in 1962. In addition to the common characteristics of lasers, it also has the following advantages:

(1) Small size and light weight;

(2) Compared with the driving power and current Low;

(3) High efficiency and long working life;

(4) Direct electrical modulation;

(5) Easy to interact with various optoelectronic devices Realize optoelectronic integration;

(6) Compatible with semiconductor manufacturing technology; mass production is possible.

Because of these characteristics, semiconductor lasers have received extensive attention and research from all over the world since they came out. It has become the world's fastest growing, most widely used, first out of the laboratory to achieve commercialization and the largest output value of a class of lasers.

The working principle of semiconductor lasers

The working principle of semiconductor lasers is the excitation method, which uses semiconductor materials (that is, the use of electrons) to transition between energy bands to emit light, and the cleavage planes of semiconductor crystals form two A parallel reflecting mirror surface is used as a reflecting mirror to form a resonant cavity, which makes light oscillate and feedback, generate light radiation amplification, and output laser light.

Semiconductor lasers work by injecting carriers. Laser emission must meet three basic conditions:

(1) To produce sufficient population inversion distribution, that is, high-energy state The number of particles is sufficiently larger than the number of particles in a low-energy state;

(2) There is a suitable resonant cavity that can play a feedback role, so that stimulated emission photons are proliferated, thereby generating laser oscillations;

(3) Certain threshold conditions must be met to make the photon gain equal to or greater than the photon loss.