Bragg gratings
2024-3-3
Definition:
Reflective structures containing periodic refractive index modulation.
An optical Bragg grating is a transparent device with a periodically changing refractive index. The reflectivity is very large in a wavelength region (bandwidth) near a specific wavelength, which satisfies the Bragg condition:
where λ is the vacuum wavelength, n is the refractive index, θ is the propagation angle in the medium relative to normal incidence, and Λ is the grating period. If the above conditions are met, the wave number of the grating matches the wave number difference between the incident and reflected waves.
Light of other wavelengths is almost unaffected by the Bragg grating, but will still produce some side lobes in the reflection spectrum. Similarly, there is almost no reflection for beams at other incident angles.
For beams near the Bragg wavelength, when the grating is long enough, even weak refractive index modulation can achieve almost total reflection. Since reflectivity and refractive index depend on wavelength, Bragg gratings can be used as fiber filters.
Figure 1: A Bragg grating contains glass blocks that reflect incident light beams that satisfy Bragg conditions (left). There is almost no reflection for beams at other incident angles.
Reflective structures containing periodic refractive index modulation.
An optical Bragg grating is a transparent device with a periodically changing refractive index. The reflectivity is very large in a wavelength region (bandwidth) near a specific wavelength, which satisfies the Bragg condition:
where λ is the vacuum wavelength, n is the refractive index, θ is the propagation angle in the medium relative to normal incidence, and Λ is the grating period. If the above conditions are met, the wave number of the grating matches the wave number difference between the incident and reflected waves.
Light of other wavelengths is almost unaffected by the Bragg grating, but will still produce some side lobes in the reflection spectrum. Similarly, there is almost no reflection for beams at other incident angles.
For beams near the Bragg wavelength, when the grating is long enough, even weak refractive index modulation can achieve almost total reflection. Since reflectivity and refractive index depend on wavelength, Bragg gratings can be used as fiber filters.
Figure 1: A Bragg grating contains glass blocks that reflect incident light beams that satisfy Bragg conditions (left). There is almost no reflection for beams at other incident angles.
