Tellurite And Fluorotellurite Glasses For Active And Passive

1. Introduction; MDO 2
communication systems operate by passing electrons through wires. Microwave (λ ≈ 10 -4
to 10 -1 m) and radio (λ ≈ 10 -1 to 10 3 m) systems function by sending EM waves through
open space, including satellite systems, and fibreoptic systems by the transmission of
light (λ ≈ 10 -8 to 10 -3 m) through transparent materials. The final application probably
determines the choice of medium for a communications system [2]. For example, radio-
frequency appears suited for non-directional transmission, such as radio, but for a
cellular-phone network, local airborne systems are most appropriate. For physical links
between two or more fixed points, such as telephone and cable television, cable systems
are appropriate.
The separate types of systems have very different data carrying characteristics and
physical utility. The capacity a cable can carry (data transfer rate or bandwidth, measured
in Mbit.s -1 ), and the distance the information must go, and hence loss, are the most
important considerations. Simple metal wiring can carry a low speed signal; coaxial cable
can carry high speed signals over short distances, however optical fibre can carry data at
high speeds over long distances. Transatlantic fibreoptic links have around 100 times the
capacity of coaxial cables [2]. Optical fibre systems have the advantage of not being
affected by electromagnetic interference (EMI), and hence the signal cannot be
eavesdropped, important for secure links. Optical fibres are also flexible, light, and small,
compared to coaxial cable [2].
Focussing now on fibreoptic telecommunications systems, their operation wavelengths
are defined by residual impurity hydroxyl (OH) which manifest as extrinsic absorption
bands in the transparent window of the silica glass optical fibre. The first short, low speed
systems used GaAs / GaAlAs sources emitting in the 750 to 900 nm region. Long