Optical fibres losses

The attenuation of an optical fibre results from physical phenomena either occurring within the fibre or coming from the environment. This is the sum of light lost by scattering in the fibre, absorption by the fibre materials, leakage of light out of the core due to environmental factors (e.g. microbends). Scattering and absorption losses dominate in every fibre. 

In connectors light source to optical fibre, power losses result from different space characteristics of the light source and the fibre. Each light source emits a beam in its typical shape whereas a fibre can accept only these rays within the acceptance cone. Some typical losses are presented in Figure 2. 

Figure 2. Different sources of optical power losses in source to fibre connectors.

Optical fibres were originally studied and developed for the transmission of information in telecommunications and have been used for building of long-haul as well as local networks for years. Such kinds of optical fibres have to be of ultra-low optical losses, standard size, resisting to temperature changes in a large interval or to chemical influence of their surroundings, etc. In contrast to these standard widely used fibres, the special ones are... 

So the function of special optical fibres for sensing is to produce a sensitive response to changes in the fibre surroundings. Such requirements on optical hardware as durability to the analyte, transparency (i.e. minimum optical losses) in a wide spectral range and common availability should be pointed out. Related to the these requirements, the choice of the fibre material as well as of the fibre coating and fibre structure belong to fundamental tasks in the design of fibre-optic sensors. 

From the spectral attenuation curve of an optical fibre drawn from Optran-UV (Figure 6) it can be seen that the losses are relatively high, but still acceptable (of the order of dB/m in the UV) for practical sensing. Refractive index of silica is 1.457 at 633 nm and falls down with increasing wavelength. 

Optical fibres composed of plastics are also transparent in the visible spectral region but optical losses reach 102 - 103 dB/km13. Their refractive index varies from 1.35 to 1.6 depending on the kind of polymer used (e.g. polymethymethacrylate PMMA -1.49). The chemical resistance is much worse than that of silica fibres and thermal stability is incomparable. On the other hand, low temperature processes of plastic fibre preparation allow us mix the starting polymer with organic dyes which enables the production of luminescent fibres suitable e.g. for fluorescence-based sensing13. 

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 7.15 Schematic loss spectrum of an optical fibre.

The intrinsic absorption properties of the fibre material at the wavelength, X, of the radiation. This is usually expressed in optical power loss, a, in dB/m, per unit length of fibre, L, which is related to the spectrophotometric absorbance of the material, A (Equation (3.21)), by the relation... 

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. 

A novel research use for this PVDF tube, making use of its pyroelectric response to measure the absorption loss in singlemode optical fibre, has already been reported (18). 

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.

Selection of the coating polymer (Table 8.1) is often dictated by conditions of future application of POF and the core material itself [3]. The first polymer which was used as cover a POF for a PS core was PMMA. Optical fibres from this polymer are widely used. It is easily produced and processed numerical aperture (NA) = 0.56. It also displays rather low light losses at A. = 670 nm (< 110 dB/km) [2]. However, (PMMA-PS) are not optimal due to some significant disadvantages of PS (low light resistance and light... 

Optical power losses are very small, about 0.03 dB/km. This means that over 100 kilometres the optical power is halved this is an important requirement for telecommunications. Optical power loss is minimized near the wavelengths of 1300 nm and 1550 nm, where the minimum Rayleigh scattering or minimal infrared absorption is found. For sensing purposes, the losses in the fibre optics are irrelevant, but still 1550 nm is the preferred window, since the optoelectronic components are more readily available. 

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