A fiber Bragg grating is a microstructure typically a few millimeters in length that can be inscribed in the core of an optical fiber. The index of refraction within the core of the fiber changes along its length, from high-index to low-index. The modulation of the refractive index causes the Fiber Bragg Grating to behave like a mirror that reflects certain wavelengths and transmits others. The wavelength that a Fiber Bragg Grating reflects depends on the spacing between the low and the high index regions within the fiber. The distance between two high-index regions is called the “period of the Fiber Bragg Grating”, denoted as Λ. Fiber Bragg Gratings preferred reflect light at the Bragg wavelength „λB“, defined by λB = 2neffΛ, where neff is the average effective refractive index of the fiber. The strength of the reflection depends on how large the index modulation is.

The modulation of the refractive index can be a steady periodic change or a variable “quasi-periodic” change. If a Fiber Bragg Grating contains regions with different periods, a single optical fiber can contain multiple “mirrors,” causing different wavelengths of light to reflect from different positions along the fiber. Fiber Bragg Gratings featuring a period that changes smoothly along the fiber length are called “chirped” Fiber Bragg Gratings and can have elaborate period profiles.

Principle of the fiber bragg grating sensor

A periodic refractive index change DN the fiber core in the order of magnitude of 10-3 to 10-4 with the distance of L leads to a formation of a wavelength selective mirror at l=2*n*L in the fiber core.

Details on measurement principle

Strain and Temperature changes result in a wavelength change Dl, which is within a large interval relatively linear and in the order of ~10pm /°C and ~1pm/µe. From this a wide range of derived variables like pressure, curvature or acceleration can be measured.