Singlemode-Multimode-Singlemode Optical Fibre Structures for Optical Sensing

Abstract

This thesis describes theoretical and experimental investigations on all-fibre multimode interference (MMI) devices using a singlemode-multimode-singlemode (SMS) fibre structure for use as a new type of edge filter for a ratiometric wavelength measurement system and as novel stand alone sensors. The use of two edge filters, so called X-type edge filters based on SMS fibre structures in a ratiometric wavelength measurement system is proposed and demonstrated. The use of X-type edge filters can improve the resolution and accuracy of wavelength measurement compared to the use of one edge filter in a conventional ratiometric system. Several aspects of the SMS edge filters have been investigated, including the effect of misalignment the SMS fibre cores due to fabrication tolerances, polarization dependence, and temperature dependence. These aspects can impair the performance of a ratiometric wavelength measurement system. Several approaches have been proposed and demonstrated to achieve high resolution and accuracy of wavelength measurement. Misalignment effects due to the splicing process on the spectral characteristics and PDL of SMS fibre structure-based edge filters are investigated numerically and experimentally. A limit for the tolerable misalignment of the cores of an SMS fibre structure-based edge filter is proposed, beyond which the edge filter’s spectral performance degrades unacceptably. It is found that a low PDL for an SMS fibre structure-based edge filter can be achieved with small lateral core offsets. Furthermore, the rotational core offsets position is proposed to minimize the PDL. Analysis of the temperature dependence of SMS X-type edge filters is presented. The temperature variation in the system can be determined and compensated by using an expanded ratiometric scheme with an additional reference arm. New sensing applications of multimode interference in an SMS fibre structure are proposed and demonstrated as a temperature sensor, a voltage sensor based on the strain effect, and a strain sensor with very low temperature dependence. All the sensors utilize a simple intensity-based interrogation system using a ratiometric power measurement system. It is found that the temperature and strain characteristics of SMS fibre structures are linear in nature and can be used for temperature and strain sensors. Based on the strain effect in an SMS fibre structure, a voltage sensor is also proposed. The SMS fibre structure is attached to a piezoelectric (PZT) stack transducer. The displacement of the PZT due to the voltage induces a strain on the SMS fibre structure and in turn results in a change in the ratio response. Finally, a strain sensor with very low temperature induced strain measurement error is investigated. For this purpose two SMS fibre structures were proposed and demonstrated in a ratiometric power measurement scheme, one SMS structure acts as the strain sensor and the other SMS structure acts as the temperature monitor. The use of this configuration can effectively minimize the temperature induced strain measurement error.