Nonlinear Fiber Optics - 4 ed. Agrawal

8.2. Quasi-Continuous SRS 291
wavelengths near 1.12 and 1.40 μm, respectively. However, the signal wavelengths
of most interest from the standpoint of optical fiber communications are near 1.3 and
1.5 μm. A Nd:YAG laser can still be used if a higher-order Stokes line is used as a
pump. For example, the third-order Stokes line at 1.24 μm from a 1.06-μm laser can
act as a pump to amplify the signals at 1.3 μm. Similarly, the first-order Stokes line at
1.4 μm from a 1.32-μm laser can act as a pump to amplify the signals near 1.5 μm.
As early as 1984, amplification factors of more than 20 dB were realized by using
such schemes [74]. These experiments also indicated the importance of matching the
polarization directions of the pump and probe waves as SRS nearly ceases to occur in
the case of orthogonal polarizations. The use of a polarization-preserving fiber with a
high-germania core has resulted in 20-dB gain at 1.52 μm by using only 3.7 W of input
power from a Q-switched 1.34-μm laser [75].
From a practical standpoint, the quantity of interest is the so-called on–off ratio,
defined as the ratio of the signal power with the pump on to that with the pump off. This
ratio can be measured experimentally. The experimental results for a 1.34-μm pump
show that the on–off ratio is about 24 dB for the first-order Stokes line at 1.42 μm but
degrades to 8 dB when the first-order Stokes line is used to amplify a 1.52-μm signal.
The on–off ratio is also found to be smaller in the backward-pumping configuration
[78]. It can be improved if the output is passed through an optical filter that passes the
amplified signal but reduces the bandwidth of the spontaneous noise.
An attractive feature of Raman amplifiers is related to their broad bandwidth. It
can be used to amplify several channels simultaneously in a WDM lightwave system.
This feature was demonstrated in a 1987 experiment [80] in which signals from three
distributed feedback semiconductor lasers operating in the range 1.57–1.58 μm were
amplified simultaneously using a 1.47-μm pump. The gain of 5 dB was obtained at
a pump power of only 60 mW. A theoretical analysis shows that a trade-off exists
between the on–off ratio and channel gains [81]. During the 1980s, several experiments
used the Raman gain for improving the performance of optical communication systems
[82]–[85]. This scheme is called distributed amplification as fiber losses accumulated
over 100 km or so are compensated in a distributed manner. It was used in 1988 to
demonstrate soliton transmission over 4000 km [85].
The main drawback of Raman amplifiers from the standpoint of lightwave system
applications is that a high-power laser is required for pumping. Early experiments
employed tunable color-center lasers that were too bulky for practical applications. Indeed,
with the advent of erbium-doped fiber amplifiers in 1989, Raman amplifiers were
rarely used for 1.55-μm lightwave systems. The situation changed with the availability
of compact high-power semiconductor lasers in the 1990s. As early as 1992, a Raman
amplifier was pumped using a 1.55-μm semiconductor laser [86]. The 140-ns pump
pulses had a 1.4-W peak-power level at the 1-kHz repetition rate and were capable of
amplifying 1.66-μm signal pulses by more than 23 dB in a 20-km-long dispersionshifted
fiber. The resulting 200-mW peak power of 1.66-μm pulses was large enough
for their use for optical time-domain reflection measurements, a technique commonly
used for supervising and maintaining fiber-optic networks [87].
The use of Raman amplifiers in the 1.3-μm region has attracted considerable attention
[88]–[93]. In one approach, three pairs of fiber gratings are inserted within
the fiber used for Raman amplification [88]. The Bragg wavelengths of these gratings