6.2 WDM technology
In fiber-optic communications, wavelength-division multiplexing (WDM) is a
technology which multiplexes a number of optical carrier signals onto a single optical
fiber by using different wavelengths (colours) of laser light. This technique enables
bidirectional communications over one strand of fiber, as well as multiplication of
capacity. The term wavelength-division multiplexing is commonly applied to an
optical carrier (which is typically described by its wavelength), whereas frequencydivision multiplexing typically applies to a radio carrier (which is more often described
by frequency). Since wavelength and frequency are tied together through a simple
relationship, the two terms actually describe the same concept.
Fig… WDM is accomplished using a multiplexer to combine wavelengths traveling
on separate fibers into a single fiber. At the receiver end of the link, a demultiplexer
separates the wavelengths and routes them into different fibers
There are different types of WDM.
•
Course wavelength division multiplexing (CWDM, ITU standard G.694.2 [7])
uses a relatively small number of channels, e.g. 4,8 or 16 and a large channel
spacing of 20 nm. The nominal wavelengths range from 1310 nm to 1610 nm.
The wavelength tolerance for the transmitters is quite large, e.g. ±3 nm, so
destabilized DFB lasers can be used. The single-channel bit rate is usually 1
to 3.125 Gbit/s.
•
Dense wavelength division multiplexing (DWDM, ITU standard G.694.1 [6]) is
the extended method for very large data capacities, as required e.g. in the
Internet backbone. It uses a large number of channels (e.g. 40, 80, or 160),
and a correspondingly small channel spacing of 12.5 GHz, 25 GHz, 50 GHz,
or 100 GHz. All optical channel frequencies refer to a reference frequency
which has been fixed at 193.10 THz (1552.5 nm). The transmitters have to
meet tight wavelength tolerances. Typically, they are temperature-stabilized
DFB lasers. The single-channel bit rate can be between 1 and 10 Gbit/s, in
the future also 40 Gbit/s.
Taking into consideration all the technical information, we have chosen CWDM
technology accordingly to the distance of the link, or even with the number of
channels we want to use in the fiber. Considering the 3rd transmission window, Cband, 1550 nm, using CWDM multiplexing technology we are allowed to transmit up
to 8 channels along distances that have less than 80 km. This technology enable us
to adapt perfectly to metropolitan areas (40-80 km) and is cheaper in relation to
DWDM. Considering the theory and the physical medium we established the maximal
distance when using CWDM at 60 km.
We use DWDM technology for links with more than 8 channels and distances larger
than 60 km.
6.2.1 Optical multiplexers / demultiplexers
In that cases where more than one STM frame has to travel through fiber, we will
need optical multiplexers and demultiplexers to implement WDM technology.
For Coarse Wavelength Division Multiplexing we use devices that can multiplex
two,four or eight channels or wavelengths depending on number of STMs to
multiplex. In links where Dense WDM is implemented, we will need MUX capable of
multiplex till 40 wavelengths.
Fig …Type of Mux/ Demux and WDM technology use
Type Mux
Demux
No.
channels
CWDM
JDSU OSP CWDM
ODM
Insertion
loss mux
(dB)
Insertison
loss demux
(dB)
2- channels
4-channels
2,3 dB
3 dB
2,3 dB
3 dB
8- channels
3,5 dB
3,8 dB
JDSU CWDM
Mux/Demux Filter
DWDM
JDSU 100 GHz DWDM
Optical Add/Drop
Module
4- channels 3,7 dB
8- channels 4 dB
JDSU 13 channel
DWDM MUX MDX13 MX2001A
13- channels 3,5 dB
3,7 dB
4 dB
3,5 dB
JDSU 40-channel
MDX-40MD101CB
(PASIVO)
20 -channels
6 dB
6 dB