ATTENUATION AND DISPERSION IN OPTICAL COMMUNICATION
Saleh Ahmed Hakim
The efficiency of any optical transmission system depends largely on how effectively the attenuation and dispersion of the fiber can be reduced, The introduction of optical fibers in transmission systems is one of the latest landmarks in the field of telecommunication. In the early stages of development of optical fibers, the loss was much higher. For example, losses in fiber manufactured in 1968 was as high as about 1000 dB/km. Also the bandwidths, associated with fibers manufactured during that period, was not enough to transmit a large number of channels. As such optical communication was not considered technically and economically viable in those days.
But some scientists were very optimistic about the future of optical communication. They were of the opinion that by improving fiber design, fiber material and manufacturing techniques, it would he possible to manufacture low loss and high capacity optical fibers in the near future. Soon it was evident that their optimism would become a reality as it was possible to bring down the fiber loss to 20dB/km in 1970.
Encouraged by this achievement, various telecom manufacturing companies began their research on reducing loss and improving bandwidth in optical fibers. Their continuous efforts resulted in the production of single mode fiber with very large bandwidth. Also it has been possible to bring down the fiber loss to mere 0.2 dB/km or even lower.
Discussions in this paper will be limited to the causes and impact of losses and dispersion in optical transmission systems.
LOSSES IN FIBERS:
There are many reasons which contribute to losses in optical fibers. The main sources of losses are summarized below:
(i) Absorption loss:
Absorption due to impurities present in the fiber are a major source of fiber loss. Normal glass is relatively impure with copper, iron and manganese being the common contaminants. The absorption loss due to these
impurities varies with the wavelength being used. Also presence of hydroxylions (OH) add to loss. In order to manufacture low loss fibers, the impurity level must be brought down to the barest minimum.
ii) Intrinsic absorption:
Even if all the impurities are eliminated from the optical fiber materials, absorportion loss will still occur. This happens because pure glass has its own absorption loss at some wavelengths like ultra violate and infrared, which is used in optical communication.
iii) Rayleigh scattering:
It is the scattering of light due to micro-irregulalrities iii the dielectric medium through which electromagnetic wave propagates. The Rayleigh scttering varies inversely as the wave length. So higher scattering occurs at lower wavelength and gradually diminishes at larger wavelength. Small defects such as hubbies in the fibre cause localised scattering.
Even slight deformation of the fiber axis cause the optical power to he redistributed among the rays. Such loss, referred to as Microbending loss, may he minimised by either decreasing the core size or increasing the refractive index difference along the fiber axis.
(v) Bend or Curnature loss:
Bending a fiber causes radiation of previously guided rays. The loss resulting from this radiation depends on he angle and radius of bending. Generally bending radius of a few cms or more arc not hazardous.
(vi) Cladding effects:
The loss due to absorption In the cladding material also add to The total loss. In a multimode fiber, almost entire power is cofined to the core and therefore the effect of lossy cladding is not much felt. However, as the core is much smaller dimension in monomode fibers, there is much more power in the cladding. As such the purity of cladding is very important.
The overall loss spectrum of a typical single mode fiber is show below:
It is evident from the above figure that the loss is least around three wave lengths of 850 nm 1300 nm and ism nm. These are commonly referred to as he 1st, 2nd and 3rd window respectively. Obviously such low loss wavelength have been used in optical fiber communication.
DISPERSION IN FIBERS.
Dispersion is the broadening of light pulses as it propagates through the fiber. It increases with length of the fiber. Excessive dispersion causes over-lapping of adjacent pulses or inter symbol interference. So dispersion has a negative effect on the bandwidth of a fiber. The higher the dispersion, lie lower will he bandwidth of the system. Dispersion also decreases the peak optical power of the pulse and therefore increases the effective attenuation of a fiber,
Dispersion may be classified into two categories depending on the cause. These are
(I) Modal and (ii) Material dispersion.
(I) Modal dispersion:
These are dominant in multimode fibers where the optical rays propagate in different modes. Thus the path lengths are different for different modes and consequently propagation time is different for different rays, This results in broadening or dispersion of light pulses.
In order to solve this problem, graded index fibers have been discovered where the refractive index varies accross he cross-section of the fiber. Therefore the speed of optical rays arc different for different modes. The refractive index profile is so designed that the propagation lime is almost the same for different modes.
Another solution to tie problem of modal dispersion is to reduce the core diameter to such an extent that only a single mode of propagation is possible. Such a fiber is called the monomode fiber. With the introduction of this type of fiber it has been possible to virtually eliminate modal dispersion. The fiber bandwidth, therefore, is much greater now a days.
(ii) Material dispersion:
The refraction index of a optical material varies with the wavelength of operation. The variation pattern is shown below:
It can be seen from the above figure that the refractive index decreases with the increasing lengths. Practical optical sources have a non-zero spectral width. So, there is a difference in refractive index and, consequently, speeds of light rays are different for this non-zero spectral width. This results in broadening or dispersion of optical signals. The LED source has a much larger spectral line width and thereby material dispersion is much higher. Later on LASER diode sources have been developed with much less spectral line width and higher optical output power. Thus it has been possible to reduce the material dispersion to a great extent. The unit of material dispersion is ps/nm/km (Pico seconds per manometer of source spectral line width per kilometer of fiber)
Dispersion is the major factor which limits the bandwidth of a fiber, Overall or total dispersion may he obtained from the following relationship:
(Total dispersion)2 = (Modal dispersion) 2 + (material dispersion)2
The bandwidth of system may be calculated from the following empirical relation:
BW= l80/T GHz km
Where T is total dispersion expressed in ps/km
In the early stages of operation of optical communication, the loss and dispersion were much higher. So, the repeater spacing and also channel capacity were much less. Continuous study and research resulted in much lower attention. With the introduction of LASER diode source and single mode fiber, it has been possible to increase the transmission bandwidth to a great extent. The operating wave lengths have also been changed from 1st to 2nd low loss window and then to the 3rd window in order to achieve more efficiency.
As Bangladesh is criss-crossed by numerous rivers the use of optical fibers have so far been limited to junctions between exchanges in multi exchange area and between Microwave stations and exchanges, of course, gradually more and more optical fiber links are established in Bangladesh like other countries. There is no doubt that optical fiber communication will continue to he technically and economically viable for many years to come.Engr. Saleh Ahmad Hakim was born in 1952. Rejoined Bangladesh T&T Board in 1978 and was promoted to the rank of Divisional Engineer in 1982. He worked in various field of Telecommunication and also obtained training on various fields in home and abroad. At present he is working as Divisional Engineer, Digilal. -3, Moghbazar. He contributed several articles in the Teletech journal