Fibre Bragg Grating Minoli Arumugam Photonics and Optical Communications Instructor: Prof. Dietmar Knipp Jacobs University Bremen Spring 2007
What is a Fibre Bragg Grating? It is a type of distributed Bragg reflector (high quality reflector used in waveguides) constructed in a short segment of optical fibre that reflects particular wavelengths of light and transmits all others. Simply put, it is a wavelength selective filter. The grating is constructed by varying the refractive index of the core lengthwise along the fibre. The resonant wavelengths are reflected backwards towards the source and the non-resonant wavelengths are transmitted through the device without a loss. 2
Principle of Operation The grating consists of regular variations in the refractive index of the core longitudinally along the fibre. As light moves along the fibre and encounters the changes in refractive index, a small amount of light is reflected at each boundary. The centre wavelength of the reflection band is given by: 3
Apodisation Apodisation is a process of tapering the strength of the grating at either end so that the apparent RI change is gradual rather than abrupt. The simplest form is where the grating is tapered in strength linearly. Tapering can occur however following almost any geometric function 4
Chirped FBGs A chirp is where you get a variation in the period of the grating (and hence a variation in its response to different wavelengths) along the length of the grating. When a signal enters a chirped FBG different wavelengths are reflected from different parts of the grating. Thus the grating imposes a wavelength dependent delay on the signal. 5
Phase-Shifted FBGs This causes a transmission fringe to be created in the centre of the reflection band where light is transmitted through the filter rather than being reflected. 6
Temperature Stability in FBGs The wavelength of an FBG varies with the ambient temperature of the device. This is only partially caused by expansion and contraction of the fibre with temperature and consequent change in the spacing of the RI variations in the core. The dominant effect is a variation in the RI of the fibre itself with temperature. Mechanical strain (stretching) in the fibre also changes the wavelength of the FBG. So if a device is constructed that will put tension onto the FBG at low temperatures and relax that tension progressively as the temperature is increased we can arrange for the two temperature dependent effects to compensate one another. This is done by packaging the fibre with another material that has different thermal expansion characteristics to that of fibre. 7
Manufacturing or Writing FBGs The grating is written by exposing the fibre to UV light. There are many ways to write practical gratings. Two of the most important of these are: 1. Interference Pattern Using a beam of light from a single laser, the beam is split and then recombined over the fibres being treated. An interference pattern is generated and this can be arranged such that the period of the grating can be controlled. 2. Phase Mask The phase mask diffracts a single beam of incident light. The diffracted beam has interference fringes which can be controlled to produce periodic variations of the type we are seeking. 8
Applications In-Fibre Laser Dispersion compensation Demultiplexor using FBGs Fibre Bragg Grating Filter 9
References Dutton, Harry J. R. Understanding Optical Communications. IBM. International Technical Support Organization, September 1998 10