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What is Bragg Grating?

By Mal Baxter
Updated May 16, 2024
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A Bragg grating is a short piece of optical fiber designed to filter light wavelengths. Like speed bumps in a tunnel, small gratings occupy the glass core of a fiber, placed at intervals up to hundreds at a time. These are designed to incrementally reflect back certain portions of a light wave. The gratings disperse portions of the wave as it travels, which permits fine tune control over properties of wave transmissions for numerous purposes.

Collectively, these gratings stabilize laser beam outputs and allow wave division multiplexers to function. These devices separate light waves to increase wave transmissions traveling simultaneously through fiber. Other Bragg gratings work in fiber optic sensors that measure temperature and strain.

Bragg wavelength relates to calculation of a light beam's interference period and incident angle, which allows gratings to be spaced effectively. It is named after British physicist Sir William Lawrence Bragg. A Bragg grating is created by using an ultraviolet (UV) laser to inscribe refractive indexes along a fiber core.

Two methods of achieving periodic or aperiodic variations of refraction include interference and masking. Essentially, the photosensitivity of a fiber is altered by exposure, interference, or masking of UV light. These processes can be automated for the mass production of fiber with refractive grating periods.

Another application of Bragg grating in optical fiber is in the use of sensor technology. One type of fiber optic sensor detects the properties of materials passing through a gap in the optical path. Sensors may also use fiber to conduct information from other types of sensors. Such properties include light intensity, phase, and polarization. Fiber with Bragg grating harmlessly reflects some frequencies of broad spectrum light and clears a path for only the desirable wavelengths being analyzed.

In sensor technology, Bragg grating principles are also employed in other ways. Sensors equipped with fiber Bragg grating may measure temperature and strain. Temperature changes can alter a fiber's refractive index, which alters reflected wavelengths. Degree of alteration corresponds to temperature values, barring other conditions such as tension or compression.

Strain may be caused by similar factors that cause temperature changes; to measure strain requires the use of a strain and temperature sensor. Qualities of reflected wavelengths indicate any changes in refracted index. The temperature reading is simply subtracted from total change, and the difference is attributed to strain. This is referred to as a temperature compensated strain value.

Optical sensors using Bragg grating replace conventional electrical sensors with similar installation characteristics; gauges mount in a similar fashion via bolts, welds, epoxy, and embedded placements. Optical channels, however, can accommodate dozens of sensors and provide safe, clear transmissions over great distances. As such, these sensors can go where conventional sensors fail.

The use of Bragg grating permits fiber with custom wavelengths and bandwidths. It provides reflectivities necessary to accommodate a multitude of applications and field conditions. Fiber innovations mark many improvements on more complex and costly conventional systems.

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