The term type in this context refers to the underlying mechanism by which grating fringes are produced in the fiber. The different methods of creating these fringes have a significant effect on physical attributes of the produce. The term type in this context refers to the underlying mechanism by which grating fringes are produced in the fiber. The different methods of creating these fringes have a significant effect on physical attributes of the produced grating, particularly the temperature response and ability to withstand elevated temperatures. Thus far, five (or six) types of FBG have been reported with different underlying photosensitivity mechanisms. These are summarized below: Written in both hydrogenated and non-hydrogenated fiber of all types, type I gratings are usually known as standard gratings and are manufactured in fibers of all types under all hydrogenation conditions. Typically, the reflection spectra of a type I grating is equal to 1-T where T is the transmission spectra. This means tha. A fiber Bragg grating (FBG) is a type of constructed in a short segment of that reflects particular of light and transmits all others. This is achieved by creating a periodic variation in the of the fiber core, which generates a wavelength-specific. Hence a fiber Bragg grating can be used as an inline to block certain wavelengths, can be used for sensing applications, or it can be used as wavelength-specific reflector. The first in-fiber Bragg grating was demonstrated by in 1978. Initially, the gratings were fabricated using a visible laser propagating along the fiber core. In 1989, Gerald Meltz and colleagues demonstrated the much more flexible transverse holographic inscription technique where the laser illumination came from the side of the fiber. This technique uses the interference pattern of ultraviolet laser light to create the periodic structure of the fiber Bragg grating. The fundamental principle behind the operation of an FBG is, where light traveling between media of different refractive indices may both and at the interface. The refractive index will typically alternate over a defined length. The reflected wavelength (), called the Bragg wavelength, is defined by the relationship, where is the effective refractive index of the fiber core and is the grating period. The effective refractive index quantifies the velocity of propagating light as compared to its velocity in vacuum. depends not only on the wavelength but also (for multimode waveguides) on the in which the light propagates. For this reason, it is also called modal index. The wavelength spacing between the first minima (nulls, see Fig. 2), or the bandwidth (), is (in the strong grating limit) given by, where is the variation in the refractive index (), and is the fraction of power in the core. Note that this approximation does not apply to weak gratings where the grating length,, is not large compared to. The peak reflection () is approximately given by, where is the number of periodic variations. The full equation for the reflected power (), is given by, where, The structure of the FBG can vary via the refractive index, or the grating period. The grating period can be uniform or graded, and either localised or distributed in a superstructure. The refractive index has two primary characteristics, the refractive index profile, and the offset. Typically, the refractive index profile can be uniform or apodized, and the refractive index offset is positive or zero. There are six common structures for FBGs; 199 uniform positive-only index change,299 ,399 apodized,499 chirped,599 discrete phase shift, and699 superstructure.The first complex grating was made by J. Canning in 1994. This supported the development of the first distributed feedback (DFB), and also laid the groundwork for most complex gratings that followed, including the sampled gratings first made by Peter Hill and colleagues in Australia. There are basically two quantities that control the properties of the FBG. These are the grating length,, given as and the grating strength,. There are, however, three properties that need to be controlled in a FBG. These are the reflectivity, the bandwidth, and the side-lobe strength. As shown above, in the strong grating limit (i.e., for large ) the bandwidth depends on the grating strength, and not the grating length. This means the grating strength can be used to set the bandwidth. The grating length, effectively, can then be used to set the peak reflectivity, which depends on both the grating strength and the grating length. The result of this is that the side-lobe strength cannot be controlled, and this simple optimisation results in significant side-lobes. A third quantity can be varied to he.
