Bragg Grating
Introduction
A Bragg grating is a type of distributed optical fiber grating that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber's core, which generates a wavelength-specific dielectric mirror. A Bragg grating can be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.
History and Development
The concept of Bragg grating was first proposed by Sir William Henry Bragg and his son, Sir William Lawrence Bragg, in the early 20th century. They discovered that crystalline structures could diffract x-rays, a principle that is now known as Bragg's law. This principle was later applied to the field of fiber optics, leading to the development of Bragg gratings.
Principle of Operation
The operation of a Bragg grating relies on the principle of wave interference. When light of various wavelengths is directed at the grating, only one specific wavelength will meet the condition of Bragg's law and will be reflected back. The remaining wavelengths will pass through the grating unaffected. The reflected wavelength is known as the Bragg wavelength and can be finely tuned by adjusting the periodicity of the grating.
Fabrication
Fabrication of Bragg gratings typically involves the use of ultraviolet (UV) light to inscribe a periodic variation in the refractive index of the fiber core. This is often achieved through a process known as photosensitivity, where the fiber is exposed to intense UV light through a phase mask, resulting in a permanent change in the refractive index.
Applications
Bragg gratings find extensive use in various fields such as telecommunications, sensing, and laser technology. In telecommunications, they are used as band-rejection filters, allowing for the multiplexing of signals. In sensing applications, Bragg gratings can be used to measure strain, temperature, pressure, and other parameters. In laser technology, they are used as reflective elements in distributed feedback (DFB) lasers.