We propose a pulse response method to directly characterize the phase of the spatial index modulation, including the dc refractive index distribution (distributed phase shift), discrete phase, and chirp of fiber Bragg gratings (FBGs). <br/>This method is based on the observation that the spatial phase of gratings is directly related to the temporal phase of its pulse response. Therefore, the phase of the spatial index modulation can be characterized directly by measuring the temporal phase of the pulse response of FBGs. This method is then used to characterize an FBG with a current-induced phase shift
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