Page 269 - Electrical Properties of Materials

Optical fibres 251
14
Taking an operational frequency of f =3 × 10 Hz, a typical dielectric
–7
constant = 2.25, and the best material available at the time with tan δ ≈ 10 ,

we get
3
–1
A ≈ 4 × 10 db km , (10.76)
a far cry from 20. No doubt materials can be improved, but an improvement
in tan δ of more than two orders of magnitude looked at the time somewhat
beyond the realm of practical possibilities. Nevertheless, the work began, and
Fig. 10.25 shows the improvement achieved. The critical 20 db was reached at
–1
the end of 1969, and by 1983 the figure was down to 0.2 db km , an amazing
improvement on a difficult enough initial target.
The most usual material used for these successful fibres has been purified
silica (SiO 2 ) with various dopants to produce the refractive index profile across
the diameter to contain the light ray in a small tube along the axis, with total
internal reflection from the lower refractive index cladding. To get very low
attenuations, the wavelength of the light has to be carefully chosen. Even with
highly purified silica, there are some hydroxyl (OH) impurity radicals, which
are stimulated into vibrations and hence absorb bands of frequencies. There are
‘windows’ in this absorption spectrum, one of which between 1.5 and 1.7 μm
wavelength was used to obtain the 0.2 db km –1 result. A new impetus to the
in any case fast-growing optical fibre communications has been given by
the invention of a fibre amplifier that makes orthodox repeaters superfluous.
The principles of operation will be explained in Section 12.10 after some
acquaintance with lasers.
The lowest attenuation available in 2003 was 0.15 db/km –1 at a wavelength
of 1568 nm, not much less than that achieved in 1983. The latest fibres, free
of OH absorption, can cover the wavelength range from about 1275–1625 nm.
Since the attenuation is now around 1% per km, there is not much scope for
improvement. The amount of information one can get through these fibres,
using Wavelength Division Multiplex, is enormous, much above present de-
mand. Demultiplexing is usually done by Bragg reflection filters (an example

10 000 –7
10 –8
Optical fibre attenuation (db km –1 ) 100 10 –9 Approximate value for tan δ of fibre
1000
10

10
–10
10
1.0

0.1 10 –11 Fig. 10.25
1967 1969 1971 1973 1975 1977 1979 1981 1983 Improvement in optical fibre
Year attenuation.