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What is a Prague mirror?
Aug 03, 2018
Bragg reflector (also known as distributed Bragg reflector) is a reflector structure consisting of two optical materials composed of adjustable multilayer structures.The most common is a quarter mirror, with each layer of thickness corresponding to a quarter of the wavelength.
Bragg reflector (also known as distributed Bragg reflector) is a reflector structure consisting of two optical materials composed of adjustable multilayer structures.The most common is a quarter mirror, with each layer of thickness corresponding to a quarter of the wavelength.The latter condition applies to the case of direct incidence. If the reflector is used for the case of a large Angle of incidence, the relative thickness of the required layer is larger.

How the Prague mirror works
Fresnel reflections occur at each interface of the two materials.At the working wavelength, the distance difference between the reflected light at the two adjacent interfaces is half a wavelength. In addition, the symbol of the reflection coefficient at the interface will also change.Therefore, all the reflected light at the interface will interfere with each other and get strong reflection.Reflectivity is determined by the number of layers and the refractive index difference between materials.Reflection bandwidth is mainly determined by refractive index difference.

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

The electric field penetration curve of the Bragg mirror.

Figure 2 shows the change curve of reflectivity and group time-delay dispersion with wavelength.Reflectivity is very high in some optical bandwidth, which is related to the refractive index difference and layer number of materials used.Dispersion is calculated by the second derivative of reflected phase with respect to optical frequency.Dispersion is small at the center wavelength of the reflector band, but increases rapidly on both sides. Figure 3 shows the color scale of the optical field penetrating reflector.As you can see, only a small fraction of the light field can penetrate the reflector.
The type of the Bragg mirror

Bragg reflectors can be prepared by the following techniques:
Dielectric reflectors employ thin-film coating techniques, such as electron beam evaporation or ion beam sputtering, which can be used as laser reflectors for solid-state lasers.
This reflection consists of amorphous materials.
Fiber Bragg gratings, including long-period fiber gratings, are commonly used in fiber lasers and other fiber optic devices.
Similarly, the body Bragg grating can also be made from photosensitive materials.
The semiconductor Bragg mirror can be prepared by photolithography.
The reflector can be used in laser diodes, especially in surface-emitting lasers.
There are also various kinds of Bragg reflectors used in waveguide structure, which adopt corrugated waveguide structure and are prepared 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 reflector design, which is different from the simple one-fourth reflector design. With the same number of layers, it usually has a lower refractive index, but can be optimized as a dichroic reflector or chirped reflector for dispersion compensation.
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