Single and Multiple Phase Shifts Tilted Fiber Bragg Gratings

The spectral behavior of single and multiple phase shifts tilted fiber Bragg gratings has been experimentally investigated. To this aim, a simple and cost-effective postprocessing technique based on local thermal treatment was used to create arbitrary phase shifts along the tilted grating structure. In particular, UV written tilted fiber Bragg gratings were treated by the electric arc discharge to erase the refractive index modulation in well-defined regions. We demonstrate that these defects give rise to interference pattern for all modes, and thus defect states can be achieved within all the attenuation bands, enabling a simple wavelength independent spectral tailoring of this class of devices. 1. Introduction In the last decades, intense research on fiber Bragg gratings (FBGs) as strain and temperature sensing elements has been conducted: however, in order to increase the applicability of this technology, new research paths have been opened. One of them consists in studying structures far more complex than the widely used uniform FBG. Among these, structures such as long-period grating [1], chirped FBG [2], and tilted FBG (TFBG) [3] have been proposed for various sensing applications. In particular, TFBGs have been recently investigated for various sensing applications ranging form external refractive index, bending, and many others [4, 5], providing an exciting technological platform to be employed in many industrial sectors. In order to enlarge the potentialities of this class of devices, here, we investigate the spectral behavior of single and multiple phase shifts TFBGs fabricated by a low cost and simple postprocessing technique based on local heat treatment. Different from common phase shift gratings [6], here the electric arc discharge (EAD) was used as postprocessing technique to locally erase the grating modulation, producing single and multiple phase shifts without phase masks or patterned coatings [7] independently on the operating optical wavelength [8]. For the first time to the best of our knowledge, we report on the spectral characteristics in weakly TFBG with single- and multiphase shifts focusing the attention on the capability of the proposed method to reshape and to force defect states in all attenuation bands. 2. Experiment The investigated TFBG has been manufactured into hydrogen-loaded Corning single-mode fiber by means of frequency-doubled Argon ion laser emitting at 244？nm [9]. A 1060？nm uniform phase mask was mounted on a rotating stage in order to apply the desired tilt. The transmitted spectrum of a 1？cm long 3 ° TFBG