الفهرس 
chapter 1Only 14 pages are availabe for public view
 |  | from | 122 |  |  |
| | | from | 122 | | |
Abstract
which data are being sent
Another advantage is that signal ’power transmitted at multiple wavelengths can be simullaneously boosted by only a single amplifier, a task that would otherwise require a
separate electronic repeater for each propagated wavelength. This latter feature contributed to
the development of dense wavelength-division multiplexing (DWDM) systems, in which
terabit/sec data rates have been achieved [8].
The reliable gain in an EDFA for example, occupies a wavelength range spanning 1.53 to
1.56 urn. In DWDM systems this allows, for example, the use of some 40 channels having
IOO-GHz spacing.
A fundamental disadvantage of the rare-earth doped fiber amplifiers used as a repeater is that
dispersion is not reset. This requires additional network design efforts in dispersion
compensation and management [9] which may include extra techniques with optical methods
or variation of the optical properties of material of glass of fiber by doping methods [10].
The wide usage of rare-earth doped optical fiber amplifiers in practical or commercial
networks demonstrates the move toward transparent fiber systems, in which signals are
maintained in optical form rather than electronic conversion, and in which multiple
wavelengths, data rates, and modulation formats are supported during transmission.
Beside rare-earth-doped glass fibers, which provide gain through stimulated emission
process, there is a great interest in fiber Raman amplifiers, in which gain at the signal
wavelength occurs as a result of glass-mediated coupling to a shorter-wavelength optical
pump [11]. Raman amplifiers have recently been demonstrated in DWDM systems that
operate in the vicinity of 1.3 Ilm[12].
•. ’4.
This work emphasizes the rare-earth systems particularly thulium doped fiber amplifiers,
since it is important to enlarge the transmission band in practical use to send more data and
information.