Stepped chirp fiber Bragg gratings on a pre-stretched fiber with arbitrary group delay responses

We present the design, fabrication and testing of the piece-wise stepped-chirp fiber Bragg gratings (FBGs) with arbitrary group delay responses using a uniform phase mask under pre-stretched condition. The method involves writing a series of sub-gratings on a prestretched fiber whose length is varied during the UV exposure process. Two motorized stages are employed to control the length of the pre-stretched fiber in order to adjust the grating pitch at each writing step to achieve piece-wise stepped chirping of the FBG. Because the fiber is moved relatively to the phase mask, an apodized index modulation profile of the FBG can be obtained by fiber dithering. To demonstrate the effectiveness of the method to produce FBGs with arbitrary group delay responses, a linear-chirp unapodized FBG and a quadraticchirp apodized FBG were fabricated, and their experimental results agree well with the analytical predictions.