Table of Contents
Advertisement
Commissioning/application notes
3.3.1
Principles of fiber-optic technology
When using fiber-optic cables for the transmission of data, there are various factors that influence the signal
transmission and have to be observed in order to guarantee reliable transmission. Important principles of
fiber-optic technology are described below.
Attenuation
Less light reaches the end of a connection with fiber-optic cables than is input at the start of the connection.
This loss of light between the start and end of the transmission link is called attenuation. The attenuation
between two points is often stated in decibels (dB). However, the decibel is not a unit, but a ratio – in the
case of a fiber-optic cable it is the ratio of the light energy at the start of the connection to that at the end. It is
one tenth of a Bel (B) (1 B = 10 dB). In general, decibel indicates a power level L
power P
to another power P
1
• L
[dB] = 10*log
(P
P
10
A positive power factor is a signal amplification, a negative power factor conversely a weakening or
attenuation of the signal.
The attenuation of a fiber-optic connection is essentially determined by three influencing factors. These
influencing factors are the attenuation in the fiber-optic, the attenuation in the connector and the attenuations
that result from the splices in the fiber-optic connection. The total attenuation is therefore given by
• Fiber-optic link attenuation [dB] = fiber loss attenuation [dB] + connector insertion attenuation [dB] +
splice insertion attenuation [dB]
Where
• fiber loss attenuation [dB] = fiber attenuation coefficient [dB/km)] x length [km]
• connector insertion attenuation [dB] = number of connectors x connector insertion attenuation [dB]
• splice insertion attenuation [dB] = number of splices x splice insertion attenuation [dB]
Dispersion
A further influence that needs to be observed with the signal transmission is the dispersion. Dispersion
describes the spreading or widening of a light pulse. Due to propagation differences resulting in the fiber-
optic cable from different injection angles of the light waves, the optical pulse widens and is therefore wider
at the output than at the input. The longer the transmission link, the greater the dispersion.
Fig. 26: Dispersion
If higher data rates are to be transmitted by the fiber-optic cable, the pulses must be sent faster at the input.
What may happen then, however, is that pulses at the output run into one another and can no longer be
distinguished from one another. The dispersion thus limits the maximum bandwidth of the fiber-optic
connection.
The maximum bandwidth is specified in the data sheet for a fiber-optic cable as the bandwidth/length ratio in
the unit MHz*km. Therefore, the longer a transmission link, the smaller the available bandwidth. The
bandwidth/length ratio or product is always specified in the data sheet for a fiber-optic cable. The length of
the transmission link can then be calculated with the necessary bandwidth.
s [km] = bandwidth [MHz] / bandwidth/length ratio [MHz/km]
38
.
2
/P
)
1
2
Version: 3.6
from the ratio of one
P
EK112x, EK15xx
Advertisement
Table of Contents
