Two basic concepts of lasers

Two basic concepts of laser, one is stimulated emission, the other is the resonator. In this paper, the basic principle of DBR (Distributed Bragg Reflector), which is a resonator in VCSEL type lasers, is introduced. Two basic physics knowledge: reflection phase transition and thin film interference are introduced respectively.

The position of DBR in the VCSEL laser is shown below:

Reflection phase transition

When light is transmitted from the optically sparse medium n1 to the optically dense medium n2 (refractive index n2>n1), the reflected light will undergo a 180 degree phase transition at the interface. However, no phase transition occurs when a photodense medium is transmitted to a photophobic medium.

From an engineering point of view, light is also an electromagnetic wave, and the reflection of light can be analogous to the reflection of an electrical signal when the impedance changes. When an electrical signal enters a low-impedance transmission line from a high-impedance transmission line, it produces a negative phase reflection (phase transition of 180 degrees), and when it enters a high-impedance transmission line from a low-impedance transmission line, it produces a positive phase reflection (no phase transition). The refractive index of an optical transmission medium is analogous to the impedance of an electrical signal transmission.

Deeper explanations are beyond the scope of this article.

Thin film interference

When light passes through a thin film, it will be reflected twice on the upper and lower surfaces, and the thickness of the thin film will affect the optical path difference of the two reflections. If the thickness of the thin film is controlled to be (1/4+N) times the wavelength, the optical path difference of the two reflections is (1/2+2N), and the optical path difference corresponds to a 180-degree phase transition, and one of the reflections will undergo a 180-degree phase transition. Then the reflected light of the two times is eventually in phase, and the superposition is enhanced, that is, the overall reflection coefficient is increased. In fact, DBR is an alternating layer of two refractive index media. When light passes through DBR, each layer will increase a certain reflection system, and the reflection coefficient of DBR can reach a very high level.

Film interference mechanism diagram:

Note 1: In order to show clearly, the three beams of light are drawn separately, but they are actually stacked together;

Figure 2: The first reflection of blue (180 degree phase transition) and the second reflected light of yellow (180 degree phase difference due to optical path difference) are finally in phase, and the superposition is enhanced.

The DBR structure can amplify the reflectance through multiple layers of reflection. However, DBR works using the interference principle, so DBR will have high reflectivity for some specific wavelength ranges of light, and can achieve very low loss, and other types of reflectors (such as metal surfaces) are different in reflection characteristics

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