A Fiber Bragg grating (FBG) can be compared with a mirror that reflects a certain wavelength and transmits all others.
By inscribing many of these mirrors at well defined distances, the reflected wavelengths can be controlled with very high accuracy. These mirrors can be inscribed equidistantly for a uniform FBG, or can be inscribed with varying distances along the grating length, which then results to a chirped FBG where the reflected wavelength changes along the grating.
FBGs are generated by exposing the core of a specially prepared optical fiber to a fringe pattern of ultraviolet light. (The core is typically no more than 5 μm in diameter i.e. a tenth of the diameter of a normal human hair). The ultraviolet light will locally create changes in the refractive index of the core. A change in refractive index will function as a tiny mirror, reflecting wavelengths that fulfill the Bragg criteria.
Our versatile and proprietary grating writing technology utilizes a two‐beam interferometer to create the fringe pattern. A highly accurate motion controller can sequentially add up these fringe patterns with sub nanometer precision over more than 10 meters.
By actively controlling the period of the fringe pattern basically any type of FBG can be generated. Grating characteristics such as wavelength range, reflection and dispersion compensation attributes are easily controlled via our software.
The basic principle of measuring temperature with FBGs is that the reflected wavelength depends on the grating characteristics (period, modulation) and is influenced by the ambient conditions such as strain and temperature. This allows the utilisations of FBGs as sensor for strain and temperature.
In the case of temperature monitoring, the Bragg wavelength is a function of the temperature. This temperature dependence results from changes of the refractive index of the fiber as well as from thermal expansion of the glass material.
Dispersion (pulse broadening) is compensated for by precisely chirped FBGs allowing faster wavelengths to travel longer distances than slower, thereby reshaping the pulse. Without FBGs the pulses will eventually start to overlap and bit errors will occur.
Proximion is the only company that offers continuous FBGs, hence providing future proof solutions that are independent of channel plan and modulation format.
“When producing our continuous dispersion products, we induce fringes of the Fiber Bragg Gratings every 500 nm with an accuracy of 2 nm, while moving the fiber at a speed of 4 mm per second.
This can be resembled by placing poles between New York and Washington D.C. (360 km) at every 0.5 meters with an accuracy of 2 millimeters, while flying at a speed of 4 Mach at 10 kilometers’ height.
In fact, it is actually even tougher; the accuracy of two arbitrary poles placed 20 kilometers apart must not be more than 2 millimeters.”
– Krister Fröjdh, Vice President R&D, Proximion
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