Page 719 - Introduction to Information Optics

12.3. Design of Optical Network Transport 703
9
15
general requires the BER to vary from 1 x 1CT to 1 x 10~ , typically around
12
12
I x 10~ {a BER of 1 x 10~ corresponds to, on average, one error per
trillion transmitted bits). Since BER is a statistical phenomenon, enough
measuring time has to be allocated to ensure accuracy.
12.3.3.2.2. Optical Signal-to-Noise Ratio
In a contemporary long-haul DWDM transmission system, as shown in Fig.
12.21, high-power optical amplifiers are periodically used after every certain
span of optical fibers, usually 80 or 100 km, to restore the power of the optical
signal. Assume that the optical signal from the transmitter is superclean
without any noise. The signal first encounters some components, such as the
DWDM multiplexer, experiencing insertion loss. The first amplifier not only-
boosts signal power before it enters into optical fiber, but also generates
amplified spontaneous emission (ASE) noise

^noise = 2n sphy(g - 1)B 0, (12.27)

where hy is the photon energy, g is the gain of the amplifier, B 0 is the optical
bandwidth in Hertz, and n sp is related to the noise figure of the amplifier.
The output of the first amplifier is then launched into the optical fiber.
After a certain distance its power is attenuated by the fiber but boosted
again by the second amplifier. Now besides the signal, ASE noise gener-
ated by the first amplifier is also amplified. Moreover, new ASE noise is
generated by the second amplifier and added onto the signal. The process
is repeated at every following amplifier site, and the ASE noise accumulates
over the entire transmission line. The gain of amplifiers in such a system is
usually automatically adjusted to ensure that the output power (signal
power plus noise power) of every channel is fixed. Since more and more
ASE noise is created and accumulated, signal power is sacrificed to keep total
power unchanged. Sometimes the power level of accumulated noise can be very

Fig. 12.21. A multichannel amplified optical transmission system.