Improved Efficiency Narrow-Band Acoustooptic Tunable Reflector using Fibre Bragg Grating

Abstract

All-fibre versions of bulk devices are very attractive in fibre systems where low insertion loss is always desirable. We have greatly improved the performance of a recently reported [1] Bragg-grating-based acousto-optic superiattice modulator (AOSLM) by reducing the fibre diameter by HF etching. This improves the overlap of acoustic and optical power, resulting in modulator efficiencies as high as 90%. The modulator produces narrow-band amplitude-adjustable reflections that can be tuned over several nm, which is attractive for WDM and laser applications. In the device, an extensional acoustic wave is launched along a fibre Bragg grating. The ensuing superiattice modulation causes additional bands of reflection to appear on both sides of the Bragg condition. The device works by coupling together the forward and backward propagating Bloch modes in the grating (a principle first proposed in 1986 [2,3]) and operates in reflection mode. This differentiates the AOSLM from all previously reported acousto-optic fibre devices, which all rely on intermodal coupling, examples being LP01 to LP11 coupling in dual-mode [4] and twin-core [5] fibres, four-port null-couplers [6], polarisation mode couplers in Hi-Bi fibre [7] and guided-to-cladding mode coupling in standard telecom fibre [8]. In all these devices the intermodal beat length L B = λ /(n1 – n2) at optical wavelength λ between two modes with phase indices n1, and n2 is matched by the acoustic wavelength v s / f s where v s and f s are the acoustic phase velocity and frequency.