Bragg Gratings
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: reflecting structures with a periodic refractive index modulation
More specific terms: fiber Bragg gratings, volume Bragg gratings, holographic bulk gratings
DOI: 10.61835/4ti Cite the article: BibTex plain textHTML Link to this page LinkedIn
An optical Bragg grating is a transparent device with a periodic variation of the refractive index, so that a large reflectance (less precisely: reflectivity) may be reached in some wavelength range (bandwidth) around a certain wavelength which fulfills the Bragg condition
$$\frac{2\pi}\Lambda = 2 \cdot \frac{2\pi n}{\lambda} \cos \theta $$where <$\lambda$> is the vacuum wavelength of light, <$n$> the average refractive index of the medium, <$\theta$> the propagation angle in the medium relative to the direction normal to the grating, and <$\Lambda$> 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).
Around the Bragg wavelength, even a weak refractive 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.
Is the crucial function of a Bragg grating is usually to reflect light, it can also be called a Bragg reflector.
Examples of Optical Bragg Gratings
- Bragg gratings made in a bulk piece (e.g. of some glass or polymer), e.g. by irradiation of a photosensitive material with coherent ultraviolet light which is spatially modulated using an interference pattern, are called volume Bragg gratings (Figure 1). Their reflection bandwidth is much smaller than that of dielectric thin-film mirrors, since they have a low refractive index contrast, which is compensated with a correspondingly longer interaction length. (Peak reflectivities of 99.9% are possible.)
- Volume Bragg gratings can be used e.g. as output couplers for laser diodes; the small reflection bandwidth (e.g. below 0.1 nm) can then lead to a narrow emission bandwidth (linewidth) and a low temperature dependence of the emission wavelength, which can facilitate the pumping of solid-state lasers. One can also insert a volume Bragg grating into the laser resonator of a solid-state laser to stabilize or tune its emission wavelength. For high intracavity powers (e.g. tens of watts), however, detrimental thermal effects can occur, such as a shift of the reflection band and a decrease of reflectance. For not too high power levels, volume Bragg gratings can also be used in spectral beam combining.
- There are fiber Bragg gratings, made in optical fibers. These can reflect light in fibers, or lead to various kinds of mode coupling in multimode fibers. Fiber Bragg gratings are used e.g. for fixing the wavelengths of fiber lasers, for filtering out certain wavelength components, for gain flattening of fiber amplifiers, and in fiber-optic sensors.
- There are also laser diodes with built-in Bragg gratings (based on semiconductor technology) for narrowing and stabilization of the emission wavelength (→ distributed Bragg reflector lasers, distributed feedback lasers).
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Suppliers
The RP Photonics Buyer's Guide contains 26 suppliers for Bragg gratings. Among them:
Technica Optical Components

World leader in premium grade fiber Bragg gratings. High quality components reliably used in hundreds of applications worldwide.
Exail

Exail (formerly iXblue) offers fiber Bragg gratings for a variety of applications: laser cavity mirrors, gain flattening filters, and ultra-narrow bandwidth filters. In conjunction with our extensive doped fiber portfolio, we can offer suitable fibers for single-frequency DFB lasers.
Sylex

FFA-01 is a fiber array sensor with configurable number of FBG elements and their spacing over the fiber length. FFA-01 is typically used for direct gluing to the measured surface to capture strain changes of the monitored structure.
HBK FiberSensing
Based on the Fiber Bragg Grating (FBG) technology and the newLight optical sensor series, the FS70 configurable arrays are optical fibers with multiple FBGs and high-performance coatings. These are optimally suited for multipoint measurement of strain in laboratories and in all kinds of industries.
2023-12-02
Isn't the Bragg condition also fulfilled at integer multiples of the fundamental optical frequency?
The author's answer:
That is an interesting question. If the grating has a sinusoidal modulation of the refractive index, that is not the case. However, if that modulation function has corresponding harmonics, the Bragg condition can be fulfilled for those harmonic frequencies. Basically, we can consider a non-sinusoidal modulation as a superposition of modulations with different spatial frequencies, and the Bragg condition may be fulfilled using any of those spatial frequency components.