Bragg gratings in multimode and few-mode optical fibers

Bragg gratings in optical fibers in multimode propagation are investigated experimentally and theoretically, Bragg gratings formed in optical fibers i n multimode propagation show multiple reflection peaks or multiple transmis sion dips in the reflection or transmission spectra, respectively. For stan dard graded-index multimode fiber, the number of reflection peaks of a Brag g grating depends on excitation condition of propagating modes. The number of reflection peaks of a Bragg grating at around 1.55 mu m is 19 for highly multimode excitation and 3-4 for lower order mode excitation. We analyze t he phase-matching conditions of the propagating modes and identify half of the reflection peaks as the reflection to the same mode and the rest as the reflection to the neighboring modes. In dispersion-shifted fiber, a Bragg grating at around 0.8 mu m in three-mode propagation shows three reflection peaks in the reflection spectrum, The temperature dependence of each refle ction peak is similar to that of a conventional Bragg grating in single-mod e fiber. Polarization dependence measured on a Bragg grating in multimode g raded-index fiber is negligible. An advantage of Bragg gratings in multimod e fiber (MMF) and the applications are discussed.