Page 694 - Introduction to Information Optics

678 12. Networking with Optics

nately, because the spontaneous decay of the erbium-excited state has a long
lifetime, and gain saturation is very slow. Typically, it takes 0.1 to 1 ms for gain
compression to occur after a saturation signal is launched into the EDFA.
These times are very long compared to the pulse period of the 6 jus (155 Mb/s,
OC-3) or shorter in DWDM digital systems. EDFAs add little signal distortion
or cross talk for point-to-point transmission systems. However, as DWDM
networking becomes more and more attractive, the gain dynamics of EDFA
invoke interest. In DWDM networks, optical channels are added and/or
dropped along the system. When this happens, the number of channels into
EDFA changes. Since EDFAs used in high-channel count systems are oper-
ated under saturation conditions already, any change of input channels will
change the saturation condition, thus changing the gain. The most serious
difficulty is the relatively big over- or undershooting spike in gain transient
responses. These spikes may result in optical saturation or loss of signal (LOS)
at the receiver. EDFA transient response depends on the level of EDF
saturation, the percentage of input power change, and number of EDFAs in
the chain. Research is still underway to overcome this problem in network
design. Most solutions rely on fast electronics to suppress or alleviate at least
the transient.

12.2,3. WAVELENGTH DIVISION MULTIPLEXER/DEMULTIPLEXER
Dense wavelength division multiplexing (DWDM) is a fiber-optic trans-
mission technology which combines multiple optical channels at different light
wavelengths and then transmits them over the same optical fiber. Using
DWDM, theoretically hundreds and even thousands of separate wavelengths
can be multiplexed into a single optical fiber. In a 100-channel DWDM system
with each channel (wavelength) carrying 10 gigabits per second (billion bits per
second), up to 1 terabits (trillion bits) can be delivered in a second by the
optical fiber. This kind of capacity can impressively transmit 20 million
1
simultaneous two-way phone calls or transmit the text from 300 years worth
of daily newspapers per second. Nevertheless, terabits can be easily consumed
by one million families watching video on Web sites at the same time.
The implementation of a DWDM system requires some critical optical
components. The DWDM multiplexer and demultiplexer are among the most
important ones. Figure 12.5 demonstrates a point-to-point DWDM optical
transmission system where the multiplexer combines N channels into a single
optical fiber; the demultiplexer, on the other hand, separates the N channel
light into individual channels detected by different receivers. Exponentially
growing network traffic demands that many more channels be combined into
limited usable bandwidth of the same single fiber. As a result, the channel
spacing of DWDM systems becomes narrower and narrower so that more and