AbstractA multifunctional all‐fiber device that can simultaneously serve as a multimode‐to‐single mode converter and a wavelength filter is presented in this article. The device has a concatenated structure consisting of a multimode fiber, a hollow core fiber, a single‐mode core mode blocker (made of magnetic nanoparticles), and a long period fiber grating fabricated in a single mode fiber. A quantitative theoretical analysis and experiments are carried out. A device with an extinction ratio of 25.7 dB and an insertion loss of −6.8 dB is obtained, and its performance should be able to be further improved by optimizing the fabrication process in future. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:740–744, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25834