Optical Fiber Sensors for Biomedical Applications

17 Optical Fiber Sensors for Biomedical Applications 663the tissue. The light source characteristics provide a wide range of properties foroptical interrogation of tissues including intensity, phase, polarization state, andwavelength content.2. Optical fibers deliver light from a light source to a sensing mechanism (seeSection 17.2.2 below) located at the fiber end, which is coupled to the interrogatedmedium (e.g. tissue). Here, the light experiences a modulation by theinterrogated tissue, and is returned via the same (Fig. 17.1a) or different fiber(s)(Fig. 17.1b) to a light-measurement device where it can be detected and analyzed.Often, a portion of the interrogating light can be used as a reference (theratio method), to correct for optical fluctuation and to provide improved noiserejection.Fibers can be bundled to increase the interrogation volume and improve signalto-noiseby capturing a larger signal. This typically requires stripping the outerbuffer (jacket) off the fiber to reduce overall diameter. A hexagonal packingconfiguration provides the optimal arrangement for minimizing the dead space(inactive area between fibers) due to the cladding. The total number of packedfibers, T, in a circular cross section for such an arrangement isT = 1 +k∑6i (17.1)where k is the total number of rings. With the jacket removed, a fiber bundle canachieve a total active area of almost 65%.3. Optical components such as connectors, lenses, mirrors, fiber couplers, circulators,polarizers, phase modulators, and beam-splitters can be configured tooptimize the signal in the detector fiber. Single source-detector geometry resultsin a small sensor diameter with a small interrogation volume and high lightcollection efficiency. However, compared to multi-distance sensors and/or fiberbundle geometries, single-fiber sensors suffer from high unwanted backgroundsignals generated within the fiber. These can stem from fiber autofluorescencegenerated by the excitation light or from backscattered illumination light. Assuch, filters (bandpass, monochromatic) can be used to remove background lightor reduce source intensities to appropriate safety levels for both the tissue and thelight detection system. Filters can also be applied to the source beam to improvespectral purity and/or to enable spectroscopy.4. For single wavelength detection and/or spectrally-unresolved total intensity,a photomultiplier tube or photodiode is used, whereas for spectral detection,a charged coupled device (CCD) or spectrograph/monochrometers is used.Conversion of detected light to electrical signal generally requires standardanalog-to-digital circuitry.i=017.2.2 Classification of Fiber Optic Sensors for BiomedicineOptical fiber sensors can be divided into two main categories based on the sensingmechanism: direct and indirect. Direct sensors (e.g., photometric sensors)