ICTON 2010 Mo.C1.4 978-1-4244-779 - /10/$26.00 ©2010 IEEE Novel Applications of the Acousto-Optic Effect in the Control of Fibre Bragg Grating Parameters Alexandre de Almeida Prado Pohl a , Roberson Assis de Oliveira a,b , Carlos A. F. Marques c , Kevin Cook b , Rogério Nogueira c,d and John Canning b a Federal University of Technology- Paraná, Av. Sete de Setembro, 3165, 80.230-901 Curitiba, Brazil b Interdisciplinary Photonics Laboratories, School of Chemistry, The University of Sydney, NSW 2006, Australia c Instituto de Telecomunicações, Pólo de Aveiro, 3810-193, Aveiro, Portugal d Nokia Siemens Network Portugal SA, 2720-093 Portugal Tel: +55 (41) 3310 4695, Fax: +55 (41)3310 4683, e-mail: pohl@utfpr.edu.br ABSTRACT Recent results on the application of the acousto-optic effect to fibre Bragg gratings are presented, which include the control of phase shifts, the permanent creation of multiple sidebands during the grating writing process and the control of dispersion parameters by acoustical waves. Keywords: fiber Bragg gratings, acousto-optic modulation, acoustical waves. 1. INTRODUCTION Over the past decades the acousto-optic effect has been used in the construction of specialty all-fibre devices due to the fact that it is a fast and accurate mechanism used to modify the properties of fibres. Applied with Bragg gratings inscribed in fibres, it gives rise to a series of useful devices, such as add-drops [1], filters [2], modulators [3] and Q-switched fiber lasers [4]. The excitation and control of flexural or longitudinal acoustic waves allows specific applications. In the case of a longitudinal excitation, the wave causes the formation of a standing mechanical wave, creating compression and rarefaction zones in the grating. When flexural waves are excited, the wave causes bending in the fiber and, consequently, in the grating. Both forms of acoustic excitation change the grating optical spectrum in several ways, resulting in the modulation of the spectrum [5], in the change of the grating reflectivity [6] or in the switching of the Bragg wavelength [7]. A few arrangements can be used to provide the excitation and coupling of acoustic waves in fibres in order to excite flexural or longitudinal waves, both requiring the use of a piezoelectric element (PZT) and a horn. However, the arrangement, where the fibre is placed longitudinally along the silica horn axis [8, 9] is more flexible and compact as it allows the control of both flexural and longitudinal waves to exist, depending on the vibration frequencies of the mechanical system. This fact turns such a device into a flexible mechanism that controls the desired effect on the grating by means of the excitation frequency and the load applied to the piezo. This paper reports on recent achievements using the acousto-optic effect to control phase-shifts and dispersion and to create permanent sidebands in the optical spectrum in Bragg gratings. Particularly, if large acoustic frequency tunability is implemented, the acoustic-optic effect turns out to be an important mechanism for the dynamic control of the grating properties. 2. CONTROL OF PHASE SHIFTS A phase shifted fiber Bragg grating (PS-FBG) is characterized by the introduction of a phase shift across the reflection spectrum, whose location and magnitude can be adjusted according to the desired application. By making this shift tunable, it is possible to construct a dynamic notch filter for specific wavelengths. One way of achieving this selectivity is through the application of the acousto-optic effect. The PS-FBG used in the experiments was inscribed in a standard single mode photosensitive fiber by direct writing through an optical phase mask technique, using a 248 nm KrF laser. The total length of the grating was L g = 25 mm with a phase shift of = in the middle (defined by the cavity round trip). The experimental set-up is based on the silica horn–piezo system composed by a piezoelectric transducer (PZT), a 50.6 mm long silica horn and an optical fiber containing the PS-FBG [8]. The set-up allows the reflection and transmission spectra to be recorded. The interaction length of the acoustic wave in the horn-fiber system is L b = 100 mm. Simulations were performed using a combination of the finite element (FE) and the transfer matrix (TM) methods [10]. The FE method gives the displacement field u(z) along the z-axis due to the acoustic wave and the TM method the resultant spectrum of the acoustically excited fibre Bragg grating. Figure 1a shows the simulation for the case when f = 621 kHz acoustic wave excites the PS-FBG. In this case, side bands appear in the spectrum, which indicates that a longitudinal acoustic mode is excited within the fibre [9]. The result shows very good agreement between simulated and experimental curves. On the other hand, if flexural waves are generated, grating parameters such as the peak transmissivity and its corresponding wavelength can be controlled. For instance, Fig. 1b presents the PS-FBG transmission spectrum behavior under different PZT loads (voltages) keeping the excitation frequency fixed at f = 117 kHz. As the voltage increases, the corresponding transmission peaks decrease (3.2 dB decay is observed at 10 V). Extrapolation of experimental data indicates that the transmissivity