Fibres, optical

Optical fibres are convenient for sending light from one place to another. Once the light is coupled into one end of a fibre it arrives at the other, regardless of how the fibre is bent or moved. Optical fibres come in three versions - multimode fibres, gradient-index fibres, and single-mode fibres (Fig. 7.21). 

Multimode fibres consist of a core with a high index of refraction and a cladding with a lower index of refraction. The light is kept inside the fibre by total internal reflection. The fibres accept relatively large beam angles. Therefore multimode fibres can be used at an NA of up to 0.4. Multimode fibres made of glass and fused silica fibres eome in diameters up to 1.5 mm, plastic fibres up to 3 mm. To increase the cross section further, a large number of fibres can be combined into a fibre bundle. 

The light from the source, in this case a laser diode, is transferred to the fibre input cross section by a transfer lens system. The first lens is the laser collimator, with a focal length, fl, which is normally a few mm. If the collimated beam is focused into a fibre by a lens of a longer focal length, 12, all aberrations in the laser beam profile are magnified by a factor M = 12 / fl. This requires a fibre of a eorrespondingly large diameter. However, the NA of the beam coupled into the fibre, and eonsequently the pulse dispersion in the fibre, is reduced by the same ratio. 

If a lens of short foeal length is used, e.g. a seeond laser diode collimator, magnification of the aberrations is avoided. Now the laser ean be coupled into a thin fibre. However, the NA is large, and so is the pulse dispersion. An example is shown in Fig. 7.23. Pulses from a 650 nm, 45 ps diode laser were sent through a 1 mm fibre of 2 m length. The pulse shape shown left is for an NA of 0.3, the right pulse shape is for an NA of 0.1. 

In all cases when multimode fibres or fibre bundles are used, eare must be taken that the effective NA of the light eones remains eonstant at the input and the output. The most objeetionable design is an iris diaphragm for intensity regulation in front of a fibre. However, the transmission of neutral-density filters, eolour 

Uhlmann and N.J. Kriedel. Glass Science and Technology, Vol. I (Glass forming systems). Academic Press (1983) TP848 G56. 

La Course, HJ. Stephens. The Physics of Non-crystalline Solid. Taylor and Francis, London (1992). 

Cable and J.M. Parker. High Performance Glasses. Blackie, Glasgow (1992). TP862H54. J.G. Simmons. Contemp. Phys., 11, 21 (1970). 

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Mollenauer L F, Stolen R H and Gordon J P 1980 Experimental observation of picosecond pulse narrowing and solitons in optical fibres Phys. Rev. Lett. 45 1095-7... 

B. S. Aronson, D. R. Powers and R. G. Sommer, Proceedings of Topical Meeting on Optical Fibre Communication, Washiagton, D.C., 1979. 

The running of telephone lines through power lines is long discontinued. They are now run on separate structures within a city and nearby areas at audio frequency (— 0.3-3.4 kHz), and maintain enough distance from HT power distribution lines. They are therefore almost unaffected from such disturbances. Nevertheless, interferences must be kept in mind when installing these lines so that they are out of the inductive interference zone of the power lines. The latest method in the field of communications to avoid disturbances is to use underground optical fibre cables, where possible, as discussed later. Optical fibre cables are totally immune to such disturbances. 

All ctfecis caused by electrostatic or electromagnetic inductions are termed Inductive Interferences. With the use t)f glass optical fibre cables in new installaiioiis. this effect is overcome automatically. Optical fibre cables, as discussed later, have no metal content and cany no electrical signals. Therefore the above discussion is more appropriate for existing installations and also to provide a theoretical aspect and moi c clarity on the phenomena of inductive interferences. These can also be applied to other fields rather than communications alone. 

PMMA has not been able to compete in the field of compact discs, the market having gone to the polycarbonates (see Chapter 20). It is, however, suitable for optical data storage using large video discs. Large-scale acceptance in the field of optical fibres has been held back by problems of obtaining material of an acceptable level of purity. 

Figure 7.11. Optical fibres, showing light trajectories for dilTerent refractive index profiles (after...

Figure 7.12. Chronology of message capacity showing exponential increase with time. The number of voice channels transmitted per fibre increases rapidly with frequency of the signalling medium. The three right-hand side points refer to optical-fibre transmission (after MacChesney and DiGiovamti 1991, with added point).

Telephone cable Pneumatic Optical fibre Optical - infrared Radio... 

Optical fibres are not yet very much in use, but there is no interference between them and electrical signals of any sort. For this reason their use will probably become more widespread. Line-of-sight optical signals require that no obstruction is inserted at any time. Such points are easily noticed when installing and commissioning, but are not so obvious if a malfunction occurs at a later date. 

Figure 1. Optical fibre general scheme and refractive index profile.

Silica is the more conventional material used for fibre manufacturing. The optical fibre telecommunications concentrated the main efforts for waveguide... 

The coherent propagation of star light through optical fibres over long distance has been mainly investigated at IRCOM and kilometric linkages are now possible. The two main points to be managed are ... 

The differential dispersion and polarization properties of optical fibres. 

Figure 10. Left Monomode optical fibre acts as a spatial filter. The coupling efficiency is 1/N where N is the input beam number of modes. Right Using a wavefront corrector the coupling efficiency is significant and quite stable (K band CFHT/ GHANA) (Perrin et al., 2000).

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