Sheila Smith

Distributed stress sensor based on a birefringent fiber Sagnac ring

This paper describes a novel configuration of a distributed optical fiber stress sensor based on a Sagnac ring construction. The principle of operation is based on the FMCW technique. The intensity and location of a stress applied to the fiber can be determined simultaneously by detecting the amplitude and the frequency of the beat signal which is produced by two forward-coupled mode beams in the Sagnac ring. The system was found to have a spatial resolution of 1 meter in a sensing range of 50 meters.

Alteration in the response of fluorescein immobilized in sol-gel thin films as an optical fiber sensing mechanism for pH

Chemical dye immobilization is very commonly used for optical fiber chemical sensors development. The dye may behave differently in aqueous state and in immobilized state. This paper describes the experimental results on absorption, emission and pH response of chemical indicator sodium fluorescein, both as dissolved in water and immobilized in sol-gel thin film, for optical fiber pH sensors development. It was found that in the sol-gel immobilized state, by altering the dye concentration and excitation/emission wavelength, a wide range of measurements from pH 0.5 to pH 10 is possible.

Quasi-distributed pH sensor

A Quasi-distributed pH sensing system based on Optical Time Domain Reflectometry (OTDR) was developed to determine the spatial and intensity information from fluorescence signals coupled back into a single fiber. The evanescent wave due to a 488 nm light pulse from a N2 pumped dye laser was used to excite a pH sensitive fluorophore and the emission around 515 nm was detected. A 3dB Y-coupler was used to convey excitation light to the sensing sites and to deliver emission signals from these sites back to a filtered photomultiplier tube. Data collection was performed using a HP 54540A 500 MHz digital scope and analysis was carried out using a dedicated Pentium 166 MHz PC. A reproducible laboratory fabrication process was developed to produce sensing sites at discrete intervals along the length of the fiber. A polishing process carefully removed the cladding at each sensing site and photo- polymerization was then used to covalently bind the fluorophore fluorescein with a co-polymer directly onto the site. The results show the sensors performance over a range of pH4-pH10 with a pKa value of 6.3. The present system was chosen to have sites 10 m apart, however, based on the propagation rate of 5 ns m-1 for light in the fiber and 10 ns for the fluorescence lifetime, a resolution of approximately 1 m is possible.

Distributed FMCW reflectometric birefringent fiber stress sensor

This paper describes a new distributed optical fiber stress sensor system which consists of a piece of birefringent fiber with a mirror at one end and whose principle of operation is based on the frequency modulation continuous wave (FMCW) technique. The intensity and the location of an applied stress can be determined simultaneously by detecting the amplitude and the frequency of the beat signal which is produced by two forward-coupled mode beams. The system was found to have the advantages of large signal intensity, good signal contrast, 1 meter resolution and up to 100 meters measurement range.

Fast-response sensors for dissolved oxygen

In this work we have been developing fast response optical sensors for dissolved oxygen determination based on measuring the luminescence lifetime of a metalloporphyrin polymer. The sensor is produced by electropolymerization of the metalloporphyrin monomer units of platinum tetraphenylporphyrin (Pt-TPP), platinum octaethylporphyrin (Pt-OEP), palladium tetraphenylporphyrin (Pd-TPP) or palladium octaethylporphyrin (Pd-OEP). The polymerization process results in films which are in the region of micrometers thick. The Stern-Volmer quenching constants as determined from luminescence lifetime measurements for these sensors range from 0.90 (mg 1-1)-1) for Pt-TPP to 1.83 (mg 1-1)-1) for Pd-OEP. The response time of these sensors to a step change from an oxygen free to an oxygen saturated solution is in the millisecond region.

Characterization of sodium fluorescein dye immobilized within sol-gel matrix

This paper details the absorption and fluorescence spectra of sodium fluorescein in aqueous solution and sol-gel thin films as a function of pH. Our results show that the fluorescence spectrum is dependent not only on the microenvironment surrounding the fluorophore but also the concentration the probe in the sol-gel matrix. The pH sensitive range is also shown to be a function of the emission wavelength.

Characterization of a quasi-distributed optical fiber chemical sensor

Techniques for distributed optical fiber chemical sensor development were investigated and a model system for pH measurement was developed and, as a result, discrete, distributed signals were obtained. Fluorescein sodium was chosen as a pH indicator for this work because of its well known properties and high fluorescent intensity. A low temperature sol-gel glass manufacturing process was utilized to immobilize the indicator onto the optical fiber core. Thin (approximately 1 micrometer) porous glass films were deposited on the surface of the optical fiber core with indicator molecules entrapped in the matrix. An OTDR technique was employed to obtain the signal at specific positions along the fiber. A dye laser, pumped by a N2 laser, produced blue light pulses at 440 nm which were launched into a 1 multiplied by 2 optical fiber coupler. A fiber with eight sensitive sections was splice to a 50:50 coupler. The indicator molecules were excited by the blue light via the evanescent wave. Part of the fluorescent light from the indicator molecules was coupled back into the fiber and transmitted back to the coupler. A fast PMT tube was attached to the other arm of the 1 by 2 coupler to detect the fluorescent light. Results were obtained for solutions of various pH value. The system appears to have potential for applications in environmental and safety monitoring.

Applications of electropolymerized porphyrin films to luminescent-lifetime-based fiber optic sensors

Advances in fiber optic sensors and, in particular, the technique of electropolymerization have led to a development which allows for porphyrins, the sensing molecules, to be attached directly onto the tip of an optical fiber. The sensing element as a whole is very robust, since during the process of electropolymerization the molecule forms a matrix on the surface of the fiber-film, thus ensuring no leeching of the film constituents to the surrounding environment. Luminescence lifetime decays were obtained using a novel dedicated instrumentation system. The data was analyzed using high level languages incorporating mathematical modeling capabilities. The results of this work are reported as well as a detailed discussion of the electropolymerization process with respect to the porphyrins.

Practical birefringent fiber Sagnac ring force sensor

A practical birefringent fiber Sagnac ring force sensor has been demonstrated. The sensor is based on the frequency modulation continuous wave technique and simply consists of a birefringent fiber ring. The intensity and the location of a stress applied to the fiber can be determined simultaneously by detecting the amplitude and the frequency of the beat signal which is produced by two forward-coupled mode beams in the Sagnac ring as a result of the applied stress. Similarly, the strain variation of one section of the fiber ring can also be measured by analyzing the phase shift of this beat signal. The experiments demonstrate that resolutions of 1 meter for distributed stress measurements and 4 microstrains for strain measurements can be obtained with this sensor system.

Reflectometric birefringent fiber absolute and relative strain sensor with environment-insensitive lead-in/lead-out fiber

A novel reflectometric birefringent fiber remote strain sensor is reported. The sensor employs the principle of frequency modulation continuous wave interferometry and consists of a single length of single-mode birefringent fiber. The sensor is shown to have several advantages including a resolution of 2 microstrain resolution, a dynamic measurement range of 5000 microstrain, an environment-insensitive lead-in/lead-out signal section and a variable length strain sensing probe. The system is ideally suited to the measurement of absolute and relative strain.