Suppressing sidelobes in fiber Bragg gratings
By Yvonne Carts–Powell
A method of making Bragg gratings in fiber removes crosstalk–inducing sidelobes. Fiber Bragg gratings (FBGs), which act as wavelength–selective mirrors, are a building block of many WDM devices, used in add/drop filters for DWDM and for stabilizing wavelengths in erbium–doped fiber amplifiers. Sidelobes in the grating's reflection spectrum, however, reflect unwanted wavelengths, resulting in crosstalk between channels and other problems.
Gratings that suppress sidelobes can be made in several ways, most of which require additional equipment or processing that is not ideal for mass production. A method called self–apodization also suppresses sidelobes and can be made in a mass–producible way with a relatively simple writing setup. The method was first proposed by Ho–Jin Jeong and others at Korea Telecom (Taejeon) in 1999, and has been developed by researchers at TuiLaser AG (Germering, Germany) since then.1
Because ultraviolet light alters the refractive index of a fiber core, excimer lasers are used to create FBGs.2 They can either be made by interferometric methods (in which a beam is split and then recombined to produce interference fringes in the fiber core) or by phase–mask methods (in which the laser beam passes through a grating and creates an interference pattern between the +1 and −1 order diffraction). Phase masks that apodize the FBG can be made, but they are complex and expensive.
One of the easiest ways of suppressing sidelobes is to make sure that the change in the grating's refractive index follows a Gaussian curve. However, such gratings tend to have self–induced chirp, due to the changing average index of refraction. Jeong's fabrication method manages to achieve both requirements at the same time: a constant index of refraction across the grating as well as a Gaussian change of refractive index.
This is done by first raising the index of refraction in the fiber where the edges of the grating will be formed, creating an area where the refractive index dips in a bowl–like curve (see figure). Then when the grating is formed in this bowl, the change in index is roughly a Gaussian shape, while the change in the average index is relatively small. (The order of the steps is not important.)
The beauty of the method is that no additional equipment is required to produce these gratings; they can be made simply by changing the laser exposure time, laser intensity, and the position of exposure.
For more information contact Heinz Huber at firstname.lastname@example.org, or Sebastian Spörlein at email@example.com.
1.H.–J. Jeong et al., Proc. ECOC 1999, I–306.