All-fibre optical sensor system for chemical sensing based on fibre Bragg grating technology

Sammanfattning: This thesis deals with the realization of an all-fibresensor system for chemical sensing through the evanescentfield, based on fibre Bragg grating technology. The advantagesof all-fibre systems are most apparent for the potential userof the system. These systems are of small size, have highmobility, and require no precision alignment. Optical fibresare higly adapted to remote sensing and are very robust toexposure of hazardous chemicals, radiation, and hightemperatures. They are electrical isolators and chemicallyinert, thus offering for instance an intrinsic medicalsafety.Fibre Bragg grating technology can be used to produceversatile in-fibre components which may be utilized as atransducer elements for many generic sensor types or as afeedback elements in fibre lasers, which are highly suitablelightsources for fibre sensor systems.A new method for realizing very long-non-homogenous fibreBragg grating structures is presented. These structures offerconsiderable performance advantages, both for transducerelements and as feedback structures in fibre lasers.A long non-homogenous grating structure is utilized toaccomplish a Yb3+ DFB fibre laser. This laser exhibits robustsingle-mode operation and has a potential continuous tuningrange over 10 nm. A theoretical model is introduced to describesome of the characteristics of rare-earth doped fibre DFBlasers.A sampled grating structure has been realized to increasethe potential tuning range in linear fibre laserconfigurations. It may' ultimately result in widely tunable,narrow linewidth lightsources.A sensor concept based on the wavelength response of a longnarrow linewidth etched fibre Bragg grating is proposed anddemonstrated. Different system aspects as multiplexing andself-referencing are also addressed. The sensor system iseasily multiplexed in the wavelength domain by the utilizationof a continuously tunable DBR laser and the use of additionalfibre Bragg gratings with slightly different Bragg wavelengths.By including a grating that is not etched, self-referencedmeasurements are demonstrated. This makes the sensor systemimpervious to temperature fluctuations and wavelength drift ofthe lightsource.The sensor system is finally demonstrated in a modelexperiment of an industrial application where some of theadvantageous characteristics of optical fibres are fullyutilized.Keywords:fibre optical sensors, chemical sensing,evanescent field sensing, fibre Bragg gratings, fibrelasers.