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Characteristics of multimode fiber Bragg gratings (MMFBGs) are systematically studied. Dependence of the reflection spectra of MMFBG on excitation conditions, refractive-index change profile, and grating length is investigated experimentally. It is observed that the transmission spectra of the MMFBGs are more sensitive to the polarization state of input light than that of single-mode FBGs (SMFBGs). The influences of the fabrication conditions of MMFBGs (i.e., UV-exposure time) and the rotation of MMF on the output spectra of the MMFBG-based external-cavity semiconductor lasers are then investigated. It is found that the side-mode-suppression ratio and the spatial wavelength-locking region at a specific Bragg wavelength of an MMFBG are dependent on the refractive-index change profile of the grating.