Fibre Optic Sensor Based on the Linear and Nonlinear Interaction

Abstract

In recent years the implementation of Erbium-doped fibre amplifiers has allowed the extension of high-bit rate transmission over long-haul distances. But the demand for an increase in transmission capacity is unprecedented and grows continuously. Despite the intrinsically small values of the nonlinear coefficient for silica, the nonlinear birefringence effects in optical fibres can be observed even at low powers considering that the light is confined in a relatively small area over long (i.e. transoceanic) interaction lengths due to the extremely low attenuation coefficient and the event of optical amplifiers. Due to this, nonlinear birefringence effects cannot be ignored when considering light propagation in optical fibres. In this research, both theoretical and experimental investigation of the interaction between linear and nonlinear induced birefringence in a fibre and the possibility of using this interaction to design fibre optic sensors (FOS) was done. Investigations show that the linear birefringence effect leads to a distortion of the signal but when it is frequency independent, its overall effect is just a rotation of the signal state of polarization (SOP) on the Poincaré sphere. Further investigation show that the effect of nonlinear birefringence alone depolarizes the signal, while in high polarization mode dispersion (PMD) links where polarization mode coupling is high, the linear and nonlinear birefringence vectors couple together such that it may reduce the penalty and improve the signal degree of polarization (DOP). As the channel spacing increases, the interaction between the probe and the pump signal is reduced, thus the DOP improvement for 100 GHz (0.8 nm) come later, after that of 50 GHz (0.4 nm) channel spacing. In optical sensing, the study show that the DOP of the probe increases linearly with the applied physical measurand (stress, strain, temperature). Also, the study shows that for temperature sensing with polarization maintaining fibre (PMF) used as a sensing fibre, the rate or frequency at which the DOP varies is faster at high temperatures than at lower temperatures. The design of a stress sensor gave the best sensitivity of 0.051 kg -1 over a range of 0-27.5 kg with PMF as the sensing fibre and 0.049 kg -1 over a range of 0-15 kg with LEAF as the sensing fibre. In strain sensing, a sensitivity of 0.0103 m -1 was obtained with single mode fibre (SMF) as the sensing element. Lastly, the design of a temperature sensor gave the best sensitivity of 0.181 0 C -1 with PMF fibre and 0.0009 0 C -1 with SMF, thus, PMF fibre would be the best choice as a sensing element. The designed sensors offer a unique possibility in high accuracy and sensitivity at low powers than other polarimetric sensors.