Long Period Gratings in New Generation Optical Fibers

Abstract

The development of fiber gratings has had a significant impact on research and development in telecommunications and fiber optic sensing. Fiber gratings are intrinsic devices that allow control over the properties of light propagating within the fiber—they are used as spectral filters, as dispersion compensating components and in wavelength division multiplexing systems (Erdogan, 1997). The sensitivity of their properties to perturbation of the fiber by the surrounding environmental conditions has led to extensive study of their use as fiber sensor elements (Kersey et al., 1997). Fiber gratings consist of a periodic perturbation of the properties of the optical fiber, generally of the refractive index of the core and/or geometry, and fall into two general classifications based upon the period of the grating. Short-period fiber gratings, or fiber Bragg gratings (FBGs), have a sub-micron period and act to couple light from the forward-propagating core mode of the optical fiber to a backward, counter-propagating one (Kashyap, 1999; Canning, 2008; Cusano et al., 2009a). The long-period gratings (LPGs), instead, have period typically in the range 0.1-1 mm (James & Tatam, 2003). The LPG promotes coupling between the propagating core mode and co-propagating cladding modes. The high attenuation of the cladding modes results in the transmission spectrum of the fiber containing a series of attenuation bands centred at discrete wavelengths, each attenuation band corresponding to the coupling to a different cladding mode. The exact form of the spectrum, and the centre wavelengths of the attenuation bands, are sensitive to the period of the LPG, the length of the LPG and to the local environment: temperature, strain, bend radius and the refractive index (RI) of the medium surrounding the fiber. The peculiar spectral features of LPGs made them broadly used in many applications ranging from telecommunications to sensing (Bhatia, 1999). In particular, LPGs represent above all one of the most promising fiber grating technological platforms, to be employed in a number of chemical applications because of their intrinsic sensitivity to surrounding RI (SRI) changes (Shu et al., 2002). Up to now great efforts have been done in order to enhance the performance of LPGs in single mode fibers (SMFs) in terms of tuning capability and/or sensitivity. For instance several approaches have been proposed to achieve remarkable sensitivities such as cladding etching, LPG design for coupling to higher order modes near their dispersion turning points or in-fiber complex