Study of the production of laser-induced graphene coated optical fibers towards sensing applications

• Autor(a) principal: Martins, Luís Carlos Almeida
• Data de Publicação: 2022
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/34102
• topic: Optical fibers; Polyimide; Laser-Induced Graphene; FBG; TFBG; Sensors

In this work the main goal was the development and characterization of optical fiber sensors (OFS) based on graphene. Two types of silica fibers coated with polyimide (PI) with different thickness, one with 16 μm (F1) and the other with 10 μm (F2) were used. Laser-induced graphene (LIG) was produced by direct laser scribing technology using a CO₂ laser, which transforms the PI coating in LIG. In a first stage of the work, only the fiber F1 was used. Different laser powers (4 and 9 W) and scan speeds (200, 250 and 300 mm/s) were used to produce graphene onto the PI coated fiber. After a careful analysis of Raman spectroscopy and scanning electron microscopy (SEM), it was concluded that the best parameters that fit the desired interest were 4 W and 250 mm/s. Strain, temperature and humidity tests were performed on LIG scribed onto PI coated Fiber Bragg Gratings (FBGs). Similarly, Tilted FBGs (TFBGs) were tested to evaluate the response to the refractive index (RI). However the proposed sensor did not respond to the RI variation. Additional FTIR measurements allowed to conclude that there was still PI in the fiber under the produced graphene. Therefore a new fiber, F2, with a thinner coating was explored. After the same initial analysis with Raman spectroscopy, SEM and FTIR, it was decided that the parameters 4 W and 150 mm/s produced the desired characteristics and more importantly did not leave any traces of PI in the inscribed area. The strain, temperature, and humidity tests of the new sensor were performed, yielding values of 1.35-1.36 pm με ˉ¹, 9.90-10.36 pm ˚Cˉ¹ and 1.10-1.18 pm %RHˉ¹, respectively. The RI sensitivity was also calculated for two different fibers containing TFBGs, yielding values of 38.56 nm RIˉ¹ and 37.53 nm RIˉ¹ which are about 1.39 and 1.57 times higher than the same fibers after removal of the LIG. The proposed technique of direct laser scribing stands out as an easier and faster way to produce graphene based optical fiber sensors with promising sensitivities, that could be further increased by the optimization of the scribing process.