Fibre optic reflection system

Fibre optic-based flow-through optical biosensors The dramatic advances in fibre optic development in die last decade have promoted construction of sensors where radiation, whether emitted, transmitted or reflected, is conducted fi-om the sample to the detection system. The wide variety of available optical waveguide types (solid rods, hollow cylinders, micro-planar geometries) has been used with varying success in sensor development. 

To increase the sensitivity a long cavity length is used, sometimes with multiple passes, achieved by reflection from a corner cube or concave mirrors (25). Mechanical design of a stable cavity is critical for multiple reflections and to refocus the light back into the small core of a fibre. The optical measurement system generally includes an IR LED, interference filter and dual photodetectors (25) and a differential absorption technique for signal and reference channels (24). Further developments need to be made to provide a stable high-resolution optical detection system at low cost. 

Applications of the fibre optics transmittance or ATR probe are in quality control, reaction monitoring, skin analysis, goods-in checking, analysis at high and low temperature, radioactive or sterile conditions, and hazardous environments. Applications of the reflectance probe are for turbid liquids, powders, surface coatings, textiles, etc. By using an on-line remote spectrophotometer, real-time information is gathered about a chemical process stream (liquids, films, polymer melts, etc.), as often as necessary and without the need to collect samples. This determines more reliable process control. Remote spectroscopy costs less to maintain and operate than traditional techniques. Fernando et al. [48] have compared different types of optical fibre sensors to monitor the cure of an epoxy resin system. 

Figure 8.1. In the FODA system, light reflected from a fineparticle moving toward the end of a fibre optic cable is Doppler shifted. The reflected light is heterodyned with original light reflected from the end of the cable and used to measure the speed of the fineparticle.

In practice, very few applications of FEWS sensors can be found outside laboratory applications and demonstration systems. In the near-IR, suitable fibres are readily available but usually there is no real necessity to use them. Possible transmission pathlengths are sufficiently large to allow using standard transmission probes, while turbid samples can be measured using transflection or diffuse reflection probes. In the mid-IR, high intrinsic losses, difficulties in fibres handling and limited chemical and mechanical stability limit the applicability of optical fibres as sensor elements. 

Figure 1. Schematic of the optical fibre system. Excitation light is launched into the fibre. Due to the refractive index differences between the fibre core and cladding materials, the light is internally reflected and travels through the fibre with minimal loss (see inset). The emitted light is carried hack from the fluorescent sensor located on the tip of the fibre to a CCD camera detector. Reprinted with permission from Science, 2000, 287, 451-452. Copyright 2000 AAAS.

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