Intensity-based fibre optical sensors

The quickest path to the fabrication of an optical sensor is to make a film that can be used in one of the commercially available film holders such as the one from Ocean Optics. Since absorbance-based probes require a differential measurement, they employ three fibres with source, signal and reference legs. The need for two spectroscopic channels is usually avoided by storing a reference spectrum. For luminescence measurements, the reference leg can be ignored or used to correct the spectra for variations in source intensity. A similar commercial probe was employed by Kriz and Mosbach [33]. 

Those based on the fluorescence lifetime, rather than intensity (e.g. the Ipitek system), allow one to avoid problems of light loss and other factors that could affect calibration, as discussed above. Assessment of tilted Bragg gratings and long-period gratings on optical fibres has shown them to be a probe of cure of the resin as well as being both temperature- and strain-sensitive (Buggy et al, 2007). The complexity of the response of these and fibre-optics based Fabry-Perot interferometers to strain, temperature and refractive index makes it necessary to employ combinations of sensors if measurements of all of these properties are required separately. 

Badcock R A and Fernando G F, Intensity-based optical fibre sensor for fatigue damage detection in advanced fibre-reinforced composites , Smart Mater. Struct., 1995, 4(4), 223-30. 

The first intravascular sensor for simultaneous and continuous monitoring of the pH, pC>2, and pCC>2 was developed by CDI-3M Health Care (Tustin CA)14 based on a system designed and tested by Gehrich et al.15. Three optical fibres (core diameter = 125 pm) are encapsulated in a polymer enclosure, along with a thermocouple embedded for temperature monitoring (Figure 3). pH measurement is carried out by means of a fluorophore, hydroxypyrene trisulfonic acid (HTPS), covalently bonded to a matrix of cellulose, attached to the fibre tip. Both the acidic ( eXc=410 nm) and alkaline ( exc=460 nm) excitation bands of the fluorophore are used, since their emission bands are centred on the same wavelength (/-cm 520 nm). The ratio of the fluorescence intensity for the two excitations is measured, to render the sensor relatively insensitive to fluctuations of optical intensity. 

Rothmaier et al. (2008) describe a textile pressure sensor based on flexible POFs. The approach is based on thermoplastic silicone fibres, which can be integrated into woven textiles. When pressure is applied on the textile, the cross-section of the sensing optical fibre will change and therefore transmit less light. The variation in light intensity is detected and relates to the applied pressure. 

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