Yatao Yang

A distributed optical fibre fluorosensor for pH measurement

Abstract A novel distributed optical fibre pH sensor is reported. The principle of operation of the system is based on the detection of time-resolved fluorescence originating at spatially separated sensor sites. Sodium Fluorescein is used as the pH indicator. The fluorophor is immobilised in porous sol-gel films laid over the fibre core. The distributed system has a spatial range of 100 m and a resolution of the order of 1 m. The pH range of the system is observed to be pH 7–11.

A Multipoint Quasi-Distributed Optical Fiber pH Sensor

This paper reports the development and characterisation of a multipoint quasi-distributed optical fiber sensor for pH measurement. The system is based on a 170 m length of 200 μm core diameter plastic cladding silica fiber where sections of cladding have been removed and replaced with dye immobilised sol-gel glass to form sensing points. Evanescent wave excitation of a dye, immobilised within 2 mm long sections of cladding, enables the pH value of any spillage material to be determined by optical time domain reflectometry along the length of the fiber. The results suggest a spatial resolution of better than 2.5 meters for this fiber system and indicate that this arrangement could form the basis of a practical sensor/actuator system for chemical spillage, provided that suitable dye/analyte combinations are available.

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 distribution signals were obtained. pH indicator fluorescein sodium was chosen 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. Robust porous glass thin film (approximately 1 micrometers ) was grown on the surface of the optical fiber core with indicator molecules entrapped in the matrix. OTDR was employed to obtain the signal at specific positions along the fiber. A dye laser, pumped by a short pulse N2 laser, produced blue light pulses at 440 nm which were launched into a 1 X 2 optical fiber coupler. A fiber with four sensitive sections was spliced 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 X 2 coupler to detect the fluorescent light. Results were obtained for solutions of various pH value. The system appears to have great potential due to the wide applications in chemical, biochemical, environmental and safety monitoring.

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.

Toward a distributed optical fiber chemical sensor

Progress towards a distributed optical fiber fluorosensor for pH is reported. The operation of the sensor is based on the pH dependent quenching of fluorescein dye immobilized in the porous cladding of a PCS optical fiber which has been stripped and reclad in a sol-gel coating. The analyte distribution is recovered from the OTDR response of the system to a short excitation pulse.

Application of sol-gel processes for fiber optic chemical sensor development

Fibre optic chemical sensors represent a relatively new but very rapidly growing research area. This kind of sensor possesses a number of potential advantages: immunity to electromagnetic interference, low transmission loss, low cost, small size, geometric flexibility and all dielectric construction being the most obvious [1].

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.

Optimization of hi-birefringence-fiber-based distributed force sensors

In this paper, we present two new approaches of distributed optical fiber force sensors. The first approach relies on the optical Kerr effect which utilizes two lasers launched into a Hi-Bi fiber from the same end. A He-Ne laser provides a CW probe beam which is launched into fiber to excite two polarization modes with equal intensities. A Nd:YAG laser is used to provide a pulsed pump beam which excites one polarization mode. The polarization state of the CW beam changes when the pump beam propagates through the fiber, or when a force is applied to the fiber. Information regarding the intensity of the force and its position can be obtained using a data acquisition and analysis system. The second approach is based on the FMCW technique. In this case, a frequency modulated laser beam is launched into a Hi-Bi fiber with one polarization mode and reflected from the far end of the fiber by a mirror. When a force is applied to the fiber at any position, mode coupling will occur. By detecting the beat frequency produced by the two coupled mode beams, the intensity and the position of the force can be found. In this paper, the experimental results obtained from the two sensor systems are presented.