Abstract

Abstract
Long Period Fibre Gratings (LPFGs) were fabricated by femtosecond (fs) laser direct writing in a standard single-mode fibre (SMF-28e) to measure variations in the surrounding refractive index (SRI). The sensing sensitivity of these structures was optimized with the deposition of homogeneous thin layers of titanium dioxide (TiO2) by physical vapour deposition (PVD) process. A set of LPFGs were coated with different thickness layers of TiO2, and the spectral features were monitored for different SRI solutions. The wavelength shift and the optical power variation of the LPFG minimum attenuation band were measured achieving sensitivities of similar to 570 nm/RIU at using SRI near to 1.3600 in the case of the LPFG coated with 60 nm of TiO2, a 10-fold increase over the corresponding for a bare LPFG. For SRI values higher than the cladding refractive index, a sensitivity over similar to 3000 nm/RIU was determined for 30 nm of TiO2 thick film, a region where the bare LPFGs are useless. For 30 nm of TiO2, the optical power variation follows a quasi-linear function of the SRI, with a range of similar to 10 dB. Moreover, values as high as 50 and 120 dB/RIU at 1.3200 and 1.4200, respectively, can be obtained by choosing the proper film thickness. Preliminary studies revealed that coating fs-laser direct writing LPFGs with titanium dioxide improves their performance.

Abstract
Long period fiber gratings (LPFGs) were fabricated in a standard single mode fiber (SMF-28e) through femtosecond (fs) laser direct writing. LPFGs with longer and shorter periods were fabricated, which allows coupling from the fundamental core mode to lower and higher order asymmetric cladding modes (LP(1,6)and LP1,12, respectively). For the grating periods of 182.7 and 192.5 mu m, it was verified that the LP(1,12)mode exhibits a TAP at approximately 1380 and 1448 nm in air and water, respectively. Characterization of the LPFGs subjected to high-temperature thermal treatment was accomplished. Fine-tuning of the resonance band's position and thermal stability up to 600 degrees C was shown. The temperature sensitivity was characterized for the gratings with different periods and for different temperature ranges. A maximum sensitivity of -180.73, and 179.29 pm/degrees C was obtained for the two resonances of the 182.7 mu m TAP LPFG, in the range between 250 and 600 degrees C.

Abstract
Near-surface optical waveguides were fabricated in alkaline earth boro-aluminosilicate glass (Eagle2000), by femtosecond laser direct writing, using two distinct approaches. First, the capability of directly inscribing optical waveguides close to the surface was tested, and then, compared to the adoption of post writing wet etching to bring to the surface waveguides inscribed at greater depths. Laser ablation was found to limit the minimum surface to core center distance to 6.5 mu m in the first method, with anisotropic wet etching limiting the latter to 3 mu m without any surface deformation; smaller separations can be achieved at the cost of the planar surface topography. Furthermore, the waveguide's cross-section was seen to vary for laser inscription nearing the surface, observations that were also corroborated by its distinct guiding characteristics when compared to the adoption of post writing wet etching. The spectral analysis (in the 500-1700 nm range) also evidenced an increase in insertion loss for longer wavelengths and smaller surface to core center separations, caused, most likely, by coupling loss due to the interaction between the propagating mode and the surface. Different lengths of waveguide exposed to the surface were also tested, revealing that scattering loss due to surface roughness is not an issue at the centimeter scale.

Abstract
The potential of evanescent Mach-Zehnder interferometers, embedded in Eagle2000 substrates, as refractive index sensors was assessed. For that, femtosecond laser direct writing and wet etching were used to fabricate and expose the sensing arm at the surface of the glass substrate, while keeping the reference arm buried. From the analysis of the structures' spectral response, we found that the wavelength shift of the different order peaks increased greatly for refractive indices nearing that of the glass, indicating a greater overlap between the guided mode's evanescent field and the external medium. Therefore, a maximum sensitivity of 10271 nm/RIU was obtained at a refractive index of 1.491. The sensitivity in the refractive index range of water-based solutions was, on the other hand, limited to 446 +/- 39 nm/RIU. Due to the geometry of the device, applications with films deposited at the surface of the substrate and PDMS based microfluidic channels can be explored.

Abstract
Optical waveguides were fabricated at the surface of Eagle2000 glass substrates, using femtosecond laser direct writing and wet etching, and their potential as intensity-modulated refractometers was assessed. Through the analysis of their broadband spectral response to different refractive index oils, we observed that mode mismatch is present when the guided mode reaches the surface of the substrate and interacts with the external medium, thus enabling the use of such optical waveguides in refractive index sensing. Refractive indices equal to or greater than that of the substrate also induced a coupling mechanism that was shown not to be suitable in these devices. The device's wavelength of operation was found to be tunable by controlling the distance between the surface and the center of the optical waveguide. However, the sensitivity was seen to diminish by increasing the latter, being nonexistent for distances greater than 5.5 mu m. In this study, the maximum sensitivity values were found for a surface to core center distance between 1 and 2 mu m, in the biological range, and 2.5 to 3 mu m, for a refractive index nearing that of the substrate. Accordingly, maximum sensitivities of approximate to 25 dB/RIU and approximate to 1200 dB/RIU were found between 1.300 < n(D)(25)degrees(C) < 1.400 and 1.490 < n(D)(25)degrees(C) < 1.500, respectively.