Page 705 - Introduction to Information Optics

12.2. Optical Network Elements 689

locking. The second generation of wavelength lockers operate on the same
principle, but the etalon and PDs are put inside an LD hermetic package, and
in some designs, LD back-facet light is used for locking. The advantages of this
technique over the discrete wavelength locker are increased space efficiency and
cost reduction.
Most transponders offer the "3R function"; regeneration, reshaping, and
retiming. Besides wavelength conversion, transponders also process some
SONET overhead bytes, such as Bl and Jl. Forward error correction (FEC),
which helps to combat noise in long-haul transmission, is also done by
transponder. Since more and more data traffic tries to take on DWDM express
directly, without going through a SONET vehicle, transponders are also
designed to interface with signals other than SONET, such as 100 Mb/s
Ethernet, gigabit Ethernet, etc. Transponders only offer 2R without retiming
when the incoming signals are not SONET.
Two reverse trends are currently taking place in transponder development.
One is to add multiplexing functions to an existing transponder. The so-called
transmux or muxponder takes in four channels of OC-48 traffic and multi-
plexes them into one OC-192 DWDM-complaint signal for a DWDM
transport system. The other trend is to build a DWDM-complaint transmitter
into the optical interfaces of data switches and routers, thus eliminating the
need for a transponder altogether.

12.2.5, OPTICAL ADD/DROP MULTIPLEXER

Optical add-drop multiplexer technology substantially reduces the cost of
DWDM optical networks. One of the OADM configurations is shown in Fig.
12.16. A multichannel optical signal appears on the input port. A WDM filter
is used to drop one of the incoming multiple channels and pass through the
rest (the express channels). Another WDM filter is used to add one channel,

WDM filter WDM filter
Input Output
f ^Express channels\\ 1 fc
L 1 r\\ . i J ^
\>
\. } f^ A 3 A 4 2 ^3 ^4 ' \\

i i 1 — . . — 1
>
Drop Add

Fig. 12.16. Schematic of a one-channel OADM.