Tunable fiber bragg grating filters

Hill et al. reported for the first time in 1978 the formation of refractive-index gratings in germanium doped silica fibers exposed to UV laser radiation. Since then many studies have been performed on fiber gratings and a widespread range of applications has been found. For example, gratings are today used as sensors, for the measurement of strain, temperature and pressure, or in telecommunications, as multiplexers, filters, dispersion compensators, mode converters and fiber laser mirrors. The aim of this work is to conceive and to realize a fast and broadband tunable optical filter device for telecommunications. Up to now, no in-fiber filters combining wide tuning range and high tuning speed have been reported in literature. The main elements of the realized filter are two piezoelectric stack actuators permitting to push and/or pull the optical fiber exploiting the Bragg grating strain sensitivity. Two independent control loops are used to control and to adjust the actuators position. To minimize bending problems during compression, the fiber is inserted in a specially conceived guiding system. Any kind of fiber grating, e.g. Bragg, chirped, ... could be strained. However, the tuning range is inversely proportional to the strained length. A prototype, integrating the piezoelectric stack actuators, the control loops, the guiding system and the fiber grating, was realized. It operates both in reflection and in transmission. Maximum tuning speeds are around 19 nm/ms with setting times less than 1.5 ms. A tuning ranges of ∼45 nm was obtained for a grating of 2.5 mm length. The repeatability in the wavelength selection is better than 0.3 nm. This prototype is tunable over the Erbium Doped Fiber Amplifier range without spectral, reflectivity and bandwidth changes. This device is designed to work with a channel spacing of 100 GHz according to the International Telecommunications Union specifications for Wavelength Division Multiplexing systems. In addition this filter has been successfully used to continuously tune the output mirror of a cw praseodymium doped all fiber laser emitting in the 1.3 μm region. This device is of potential use in telecommunication networks for wavelength multiplexing, switching and routing or as wavelength selective filter.