246 CHAPTER 6. OPTICAL AMPLIFIERS
Figure 6.12: Variation of amplifier gainG 0 with pump powerP 0 in a 1.3-km-long Raman am-
plifier for three values of the input power. Solid lines show the theoretical prediction. (After
Ref. [31];©c1981 Elsevier; reprinted with permission.)
Noise in Raman amplifiers stems from spontaneous Raman scattering. It can be
included in Eq. (6.3.2) by replacingPsin the last term withPs+Psp, wherePsp=
2 nsphνs∆νRis the total spontaneous Raman power over the entire Raman-gain band-
width∆νR. The factor of 2 accounts for the two polarization directions. The fac-
tornsp(Ω)equals[ 1 −exp(−h ̄Ωs/kBT)]−^1 , wherekBTis the thermal energy at room
temperature (about 25 meV). In general, the added noise is much smaller for Raman
amplifiers because of the distributed nature of the amplification.
6.3.3 Amplifier Performance.....................
As seen in Fig. 6.12, Raman amplifiers can provide 20-dB gain at a pump power of
about 1 W. For the optimum performance, the frequency difference between the pump
and signal beams should correspond to the peak of the Raman gain in Fig. 6.11 (occur-
ring at about 13 THz). In the near-infrared region, the most practical pump source is a
diode-pumped Nd:YAG laser operating at 1.06μm. For such a pump laser, the max-
imum gain occurs for signal wavelengths near 1.12μm. However, the wavelengths
of most interest for fiber-optic communication systems are near 1.3 and 1.5μm. A