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Bragg Gratings

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Definition: reflecting structures with a periodic refractive index modulation

German: Bragg-Gitter

Category: general optics

How to cite the article

An optical Bragg grating is a transparent device with a periodic variation of the refractive index, so that a large reflectivity may be reached in some wavelength range (bandwidth) around a certain wavelength which fulfills the Bragg condition

volume Bragg grating

Figure 1: A volume Bragg grating within a glass block reflects an incident beam, if the Bragg condition is met (left-hand side). For a different angle of incidence, hardly any reflection occurs.

Bragg condition for grating reflection

where λ is the vacuum wavelength of light, n the refractive index, θ the propagation angle in the medium relative to the direction normal to the grating, and Λ the grating period. If this condition is met, the wavenumber of the grating matches the difference of the wavenumbers of the incident and reflected waves.

Other wavelengths are only weakly affected by the Bragg grating, except for some side lobes in the reflection spectrum. Similarly, the reflection can nearly totally disappear when the angle of incidence is modified (see Figure 1).

Calculator for the Bragg Condition

Wavelength:
Refractive index:
Grating period: calc
Angle: calc

After you have modified some values, click a "calc" button to recalculate the field left of it.

Around the Bragg wavelength, even a weak index modulation can be sufficient for achieving nearly total reflection, if the grating is sufficiently long. Due to the wavelength dependence of reflection and transmission, a Bragg grating can serve as an optical filter.

Examples of Optical Bragg Gratings

See also: Bragg mirrors, fiber Bragg gratings, optical filters

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etalon

Color-coded field penetration in an etalon structure as a function of wavelength.

This diagram has been made with the RP Coating software.

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