6.3. RAMAN AMPLIFIERS 247
Figure 6.13: Gain–saturation characteristics of Raman amplifiers for several values of the un-
saturated amplifier gainGA.
Nd:YAG laser can still be used if a higher-order Stokes line, generated through cas-
caded SRS, is used as a pump. For instance, the third-order Stokes line at 1.24μm can
act as a pump for amplifying the 1.3-μm signal. Amplifier gains of up to 20 dB were
measured in 1984 with this technique [32]. An early application of Raman amplifiers
was as a preamplifier for improving the receiver sensitivity [33].
The broad bandwidth of Raman amplifiers is useful for amplifying several channels
simultaneously. As early as 1988 [34], signals from three DFB semiconductor lasers
operating in the range 1.57–1.58μm were amplified simultaneously using a 1.47-μm
pump. This experiment used a semiconductor laser as a pump source. An amplifier gain
of 5 dB was realized at a pump power of only 60 mW. In another interesting experi-
ment [35], a Raman amplifier was pumped by a 1.55-μm semiconductor laser whose
output was amplified using an erbium-doped fiber amplifier. The 140-ns pump pulses
had 1.4 W peak power at the 1-kHz repetition rate and were capable of amplifying
1.66-μm signal pulses by more than 23 dB through SRS in a 20-km-long dispersion-
shifted fiber. The 200 mW peak power of 1.66-μm pulses was large enough for their
use for optical time-domain reflection measurements commonly used for supervising
and maintaining fiber-optic networks [36].
The use of Raman amplifiers in the 1.3-μm spectral region has also attracted atten-
tion [37]–[40]. However, a 1.24-μm pump laser is not readily available. Cascaded SRS
can be used to generate the 1.24-μm pump light. In one approach, three pairs of fiber
gratings are inserted within the fiber used for Raman amplification [37]. The Bragg
wavelengths of these gratings are chosen such that they form three cavities for three
Raman lasers operating at wavelengths 1.117, 1.175, and 1.24μm that correspond to
first-, second-, and third-order Stokes lines of the 1.06-μm pump. All three lasers are
pumped by using a diode-pumped Nd-fiber laser through cascaded SRS. The 1.24-μm