Optical fibres collecting light

Specific optical fibres could be designed to collect light. One study (Nakamura, 1992) describes a structure based on multiple light guides and one collector. The system described in Fig. 21.5 is composed of satellite optical fibres connected to a single lightguide. Satellite... 

Figure 11.5 Schematics of the active OCT imaging catheter, where an optical fibre is scanned by the polymer actuator and a lens focuses the light at the imaging plane. The reflected light is collected by the same optical fibre and processed by the OCT system.

The light emitted from the excitation source is dispersed and recorded by multiple photodetectors placed behind suitably located exit slits (Fig. 6.65). Fibre-optical techniques can also be used to collect light at a suitable location in the spectrometer focal plane for transmission to a battery of PMT.s. Calibration can be provided using standard solutions or standard electrodes. Using a computer-controlled data collection system the concentrations of the selected elements can be printed out shortly after the introduction of the sample. A complete system frequently incorporates means for automatic sample exchange. 

The primary functions of the detection system are to reject unwanted light, to measure the intensity of the output signal, and to analyse spectrally or temporally the output signal if needed. Collection optics (e.g. lenses, optical fibres), wavelength separation devices, detectors, and associated electronics are common components of the detection system. 

Figure 1 (A) Afibre optic spectroscopy system with separate illumination and collection path is based on an excitation source, which is a laser or a white light source (reflectometry) or a monochromator filtered arc lamp (fluorescence). Optics couple the excitation light into the flexible probe. A probe collects the emitted light. Coupling optics adapt the numerical aperture of the probe to the spectrograph or filter system. An optical detector (charge coupled device (CCD), photodiode array, photomultiplier tube) is read out and digitized. (B) A fibre optic spectroscopy system with a probe that incorporates one optical fibre needs a dichroic beam splitter and well aligned optics to separate excitation and fluorescence light. Reproduced with permission of Optical Society of America Inc. from Greek LS, Schulze HG, Blades MW, Haynes CA, Klein K-F and Turner RFB (1998) Fiber-optic probes with improved excitation and collection efficiency for deep-UV Raman and resonance Raman spectroscopy. Applied Optics Z7 ).

As an alternative approach, optical fibres to send the excitation light and to collect the emission light can be profitably utilized (Fig. 9.13). 

In the cell shown in Fig. 9.13 [20] WE is a Pt disk which is placed at a distance of 25-200 pm from the surface of the optical window to speed up electrolysis (1-100 s). The excitation light, sent by means of the optical fibre, encounters the quartz window with an angle of 45°, while the emission light is collected perpendicular to this window by means of another optical fibre. This kind of cell enables to measure the emission in spectroelectrochemical experiments performed on very diluted solutions (5 pM), one order of magnitude lower than the minimum concentration that can be detected in absorption with the same cell and the same electroactive species. This result clearly evidences the higher sensitivity of the emission measurements compared to the absorption ones. Obviously the sensitivity of the spectroelectrochemical experiments coupled to absorption or emission measurements depends on the molar absorption coefficient and the quantum yield, respectively, of the examined species. 

The primary requirement for making any connection is to minimise the optical power that is lost in it. Intrinsic losses result from technological variations of the fibres to be connected i.e. core area mismatch, numerical aperture mismatch, and profile mismatch. These obvious errors can only be omitted by proper matching of connected fibres or additional optical elements should be used. Contrary to the intrinsic losses, extrinsic ones can be corrected by a mechanical alignment. Extrinsic losses are caused if ends of the fibres are in some distance and the light from the input fibre cannot be collected by the sink fibre. A similar situation with power losses occurs in lateral displacement and angular misalignment. 

The basic layout of Raman sensors is similar to fluorescence probes. The common sensor form is that of a fibre optic probe, with excitation and collection fibres. As the excitation light comes from a monochromatic source no excitation filter is required, but a spectrally matched emission notch filter blocking the excitation wavelength is almost always part of the sensor head. 

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