What is the most popular Bragg reflector

2022-08-06
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What is a Bragg reflector

Bragg reflector (also known as distributed Bragg reflector) is a kind of reflector structure, which contains an adjustable multilayer structure composed of two optical materials. The most commonly used is the quarter mirror to avoid major accidents. The thickness of each layer corresponds to a quarter of the wavelength. The following conditions are applicable to the case of normal incidence. If the reflector is used for a large angle of incidence, the relative required layer thickness is larger

working principle of Bragg mirror

Fresnel reflection occurs at each interface of the two materials. At the working wavelength, the optical path difference of the reflected light at the two adjacent interfaces is half a wavelength. In addition, the sign of the reflection coefficient at the interface will also change. Therefore, all the reflected light at the interface will have destructive interference and get strong reflection. Reflectivity is determined by the number of layers of the material and the refractive index difference between the materials. The reflection bandwidth is mainly determined by the refractive index difference

Figure 1 shows the electric field penetration curve of Bragg mirrors with 8 layers of TiO2 and SiO2 materials. The blue curve corresponds to the intensity distribution curve of 1000nm light incident from the right side. It should be noted that the oscillation of the intensity curve outside the mirror is due to the interference effect of waves in the opposite direction. The gray curve is the corresponding intensity distribution curve when the wavelength is 800nm. At this time, a large part of the light can pass through the reflector coating

figure 1 Electric field penetration curve of Bragg mirror

Fig. 2 shows the variation curves of reflectivity and group delay dispersion with wavelength. The reflectivity is very high within the range of one story of some optical belt companies, which is related to the refractive index difference and the number of layers of the materials used. The dispersion is calculated by the second derivative of the reflection phase to the optical frequency. The dispersion is small at the central wavelength of the reflection band, but increases rapidly on both sides

figure 2 The reflectance (black line) and dispersion of the same mirror as in Figure 1 will also greatly increase the proportion of emerging countries' exports (blue line)

Figure 3 shows the color scale diagram of the light field penetrating the mirror. It can be seen that only a small part of the light field in the reflection band can penetrate the mirror

figure 3 The variation of field intensity penetrating Bragg reflector with wavelength. The color in the figure represents the light intensity entering the mirror

types of Bragg reflectors

Bragg reflectors can be prepared by the following different technologies:

dielectric reflectors adopt thin-film coating technology, such as electron beam evaporation or ion beam sputtering technology, which can be used as laser reflectors of solid-state lasers. This reflective structure contains amorphous materials

fiber Bragg gratings, including long-period fiber gratings, are commonly used in fiber lasers and other fiber devices. Similarly, volume Bragg gratings can also be made of photosensitive materials

semiconductor Bragg mirrors can be fabricated by photolithography. The mirror can be used in laser diodes, especially in surface emitting lasers

there are also various Bragg reflectors used in waveguide structures, which adopt corrugated waveguide structures and are fabricated by photolithography. This type of grating can be used in some distributed Bragg reflectors or distributed feedback laser diodes

there is also a multi-layer mirror design, which is different from the simple quarter mirror design. With the same number of layers, it usually has a lower refractive index, but can be optimized as a dichroic mirror or chirped mirror for dispersion compensation

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