Long-distance optical transmission

Optical fibres are flexible, transparent filaments composed of a core made of polymer (polymethylmethacrylate, polycarbonate), silica or quartz and a sheath made of fluo-ropolymer. An additional transparent protective layer is sometimes added. Optical fibres are mostly used for long-distance data transmission. An optical fibre can transport a light ray from one end to the other thanks to the total internal reflection of light. As a consequence, an optical fibre can transmit light without any loss for thousands of kilometres at 200,000 km/h. 

Most present-day fiber-optic sensors use linear diode arrays combined with optical gratings and measure the absorption, transmission, fluorescence, and reflection in UV, visible, and NIR regions (see Table 3.1). Light travels to the sampling probe via one fiber-optic cable and returns to the instrument via a second. Laser excitation permits long-distance transmission of excitation radiation to get a useful signal from the sample. 

Fiber-optic cable has not become a standard in networks, however, because of its high cost of installation. Networks that need extremely fast transmission rates, transmissions over long distances, or have had problems with electrical interference in the past often use fiber-optic cabling. 

Some ionization techniques (El, FAB, and SIMS) are compatible with all mass analyzers. PD, LD, and MALDI are most suited to TOF analyses. Atmospheric pressure ionization methods (TSP, ESI, APCI) are best coupled with quadrupole and ion trap instruments. Sector and FTICR instruments can also operate with chromatographic interfaces however, a significant reduction of pressure in the system is required. Consequently, in FTICR, the ion source and the ICR cells must be separated by a distance of about 1 m. Powerful ion optics is required for the transmission of ions for these long distances. This inconvenience, however, is offset by the advantages of FTICR, such as extremely high resolution and the ability to store the ions of interest for long periods. 

Optical fibres with high transparency allow signal transmission over long distances (up to 1000 m) without loss of quality. This characteristic is valuable in particular for long-distance measurements where the instrument cannot be placed near the sensor. 

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