Bragg grating formation in PMMA fibers doped with trans-4-stilbenemethanol

Abstract

Polymer fiber Bragg gratings (PFBGs) are an emerging sensor technology in smart structures and biomedical applications. However, the formation mechanism of PFBGs has not been fully understood despite some limited successes in laboratory-made sensors. Therefore, this thesis is focused on a systematic study of the PFBG formation in PMMA based fibers doped with trans-4-stilbenemethanol (TS). Step-index single-mode polymer optical fibers (POFs) were fabricated by the preform-drawing method. First, the photosensitivity of the core film was investigated by the refractive index reduction induced by UV irradiation and confirmed by formation of volume film gratings. Secondly, the refractive index profile of the fabricated fibers was directly measured, and step-index profile was achieved by prepolymerizing the monomer for fiber core. Thirdly, the diameter fluctuation of the fabricated POFs was significantly reduced by stopping natural convection in the heating jacket of the drawing tower with a cap. PFBGs have been inscribed by using phase masks that have a wide range of zeroth order diffractions, for example, a phase mask with zeroth order diffraction up to 49%, far exceeding the maximum requirement to write fiber Bragg gratings (FBGs) as previously reported (<5%). In addition, the microstructure of the fabricated PFBGs was found to be dependent on phase masks. Hence, the near-field of phase masks was simulated based on their diffraction ratios. The calculation revealed that the near-field of phase masks was influenced not only by the zeroth order diffraction, but also the second and the third. The simulated near-fields agreed with the observed microstructures of the fabricated PFBGs to a large extent. The formation dynamics of PFBGs during long-time phase masked UV exposure was studied. The Bragg peak increased rapidly immediately after UV exposure and reached the first highest level within ~5 minutes, and then dropped slightly or maintained constant. About 20 min after UV exposure started, the Bragg peak increased again and reached the second highest value. Finally, the Bragg peak decreased slowly and vanished. The observation of grating microstructure revealed that the POF was severely damaged after 20 min of exposure, and hence the optimal inscription time was ~5 min. For PFBGs that were inscribed by 5 min (also for 1, 15, or 30 min) of phase masked UV exposure, the Bragg peak continued to vary after exposure. The peak power decreased and almost disappeared within a few hours, but slowly recovered over one day. This relaxation process was observed for the first time in PFBGs. It has been found that heating the as-fabricated PFBGs can effectively speed up this relaxation process. Several hypotheses, such as the moisture absorption/desorption and back-reactions forming TS, were proposed and tested to explain this relaxation process. After careful investigation, however, the most probable reason for this relaxation process was ascribed to the stress relaxation induced by photoisomerization because of geometrical difference in trans and cis isomers of 4-stilbenemethanol. The fabricated PFBGs were found to be highly sensitive to relative humidity (RH). For example, the measured humidity sensitivity of a PFBG around 1310 nm was -66 pm/%RH. Moreover, the humidity and temperature were found to have a synergetic effect on PFBGs. That is, the temperature sensitivity is humidity-dependent and vice versa. Thermal response of the fabricated PFBGs was measured in ambient conditions and at constant RH. For a 1308 nm PFBG, the measured temperature sensitivity at 0 %RH and 60 %RH were -1 and -8.7 pm/°C, respectively, which are one to two orders of magnitude lower than the value measured in ambient conditions. A sensor integrated with two PFBGs was developed for simultaneous measurement of shear stress and pressure by placing one PFBG horizontally and the second tilted in a silicone cube. Finite element analysis revealed that the induced strain along fiber axis was not uniform if the fiber was bonded to the silicone matrix. The identified better configuration was to have a friction-free interface between the fiber and the silicone matrix, and loading the PFBGs by the deformation of the elastic cube at its boundary. Hence, release agent was applied on the surface of POFs to prevent the bonding, and two gaskets were adhered to two ends of each fiber to stretch the fiber under loading. The measured sensitivity of pressure and shear stress were 0.82 and 1.33 nm/kPa, respectively.