IFFPI Structures Based on FBG Pairs
Fundamentals of FBG
FBG was first demonstrated by Kawasaki et al.  in 1978 and has been widely used for optical fiber communications, fiber sensors, and fiber lasers. Typical applications of the FBGs include filters, temperature and strain sensors, and laser cavity mirrors. FBG sensors have been developed to be one of the largest markets of all types of optical fiber sensors, especially for structure health monitoring [2,3].
By using an ultraviolet (UV) laser, FBGs can be inscribed in the fiber core by introducing periodic refractive index changes. Usually, photosensitive fibers or hydrogen-loaded fibers are used in order to improve the performance of FBGs. There are several techniques often used for the fabrication of FBGs. One is by using a pulsed excimer laser, at a wavelength of 248 nm, and a phase mask. The coherence length of the excimer laser is often at the 1 mm level or even below. The interference occurs right behind the phase mask so that there is no requirement of a long coherence length on the laser and the optical fiber is put close to the phase mask . This method is most often used for commercial production of FBGs due to its high efficiency for fabrication and high stability of the fabricated FBGs.
The other is by using the two-beam interference method based on the 244 nm continuous wave (CW) Ar+ laser , whose coherent length is much longer than the excimer laser. This method is flexible and can be easily designed for generating FBGs with a desired wavelength by finely tuning the interference between the two beams.
There is also the point-by-point inscription method for the fabrication of FBGs [6,7]. It needs motorized moving parts to scan the laser spot and can be used for the fabrication of chirped or apodized FBGs with a specially designed refractive index profile.
The basic principles of FBGs can be found in the literature. The Bragg wavelength can be expressed as X = 2пЛ, where Л is the grating pitch and n is the effective index of the core . The grating pitch is determined by the half period of the phase mask or the period of the two-beam interference. FBGs are a type of reflective element and often used as a narrow bandpass filter, thanks to their low insertion loss. Usually, the FBG with uniform index changes has a reflective bandwidth of about 0.2 ~ 0.4 nm and the bandwidth can be extended into several or more than 10 nm for a chirped FBG, whose period of the index changes is not uniform .