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A modulator is reported in which an extensional acoustic wave is launched along a fiber Bragg grating. The acousto-optic superlattice effect causes an enhancement in reflectivity within a narrow spectral region on both sides of the Bragg wavelength. For a fixed acoustic propagation direction, the Doppler shift can be either positive or negative, depending on whether the wavelength of the incident light lies above or below the Bragg condition. The device can function as a Bragg cell and a tunable filter.All-fiber acousto-optic devices have potential uses as frequency shifters, multiplexers, modulators, and tunable filters. The ease with which these devices can be spliced into systems, and the consequent low insertion loss, make them an attractive alternative to pigtailed bulk Bragg cells. Previous designs of intermodal coupler include dual-mode fibers supporting an acoustic flexural wave [1,2], coupling between the polarization normal modes of a high-birefringence fiber by means of a torsional acoustic wave [3], and a high-performance device based on a four-port fused-taper null coupler [4]. All these devices share the requirement that the acoustic wavelength must match the intermodal beat length, L = 2B/)$, where )$ = *$ -$ * and $ and $ are the propagation constants of the modes.In this Letter we describe a modulator (briefly reported for the first time in Ref. 5) in which a fiber Bragg grating is excited by an axially propagating extensional acoustic wave. The underlying principle is acousto-optic superlattice modulation (AOSLM), first proposed in 1986. [6,7] In AOSLM the counterpropagating optical modes (the Bloch waves [8]) of the fine-pitch Bragg grating are coupled by a course-pitch acoustic wave, the superposition of the two forming a superlattice. Coupling is maximum when the inter-Bloch-wave beat period matches the acoustic wavelength.The forward-travelling (group velocity in the +z direction) Bloch wave in a fiber Bragg grating can be closely approximated as a constant superposition of two coupled counterpropagating guided modes with wave vectors k in the form [8]2 ½ where K = 2B/7 is the Bragg grating vector and 7 is its physical pitch. Coupling constant 6 and dephasing parameter h are defined as 6 = Mk /4 and h = 2k -K, where k = Tn /c is the average

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