Page 199 - Optical Communications Essentials

Optical Amplifiers

Optical Amplifiers 189

trations are required compared to the C-band. Whereas pumps with wave-
lengths of 980 and 1480nm are used for the C-band, high-power 960-nm pump
lasers are more advantageous for the L-band. These lasers are constructed from
AlGaInAs and have optical output powers greater than 200mW in uncooled
devices while dissipating less than 1W of electric power. Typical gains range
from 27dB (a factor of 500) at 25-dBm (3-µW) input powers to 20dB (a factor
of 100) at 6-dBm (250-µW) input powers. Noise figures are nominally less
than 5.5dB.

11.5. Raman Amplification
Whereas an EDFA requires a specially constructed optical fiber for its opera-
tion, a Raman amplifier makes use of the transmission fiber itself as the ampli-
fication medium. A Raman amplifier is based on an effect called stimulated
Raman scattering (SRS). This effect is due to an interaction between an optical
energy field and the vibration modes of the lattice structure in a material.
Basically what happens here is that an atom first absorbs a photon at one
energy and then releases another photon at a lower energy, that is, at a wave-
length longer than that of the absorbed photon. The energy difference between
the absorbed and the released photons is transformed to a phonon, which is a
vibration mode of the material. The power transfer results in an upward wave-
length shift of 80 to 100nm, and the shift to a longer wavelength is referred to
as the Stokes shift. Figure 11.12 illustrates the Stokes shift and the resulting
Raman gain spectrum from a pump laser operating at 1445nm. Here a signal
at 1535nm, which is 90nm away from the pump wavelength, is amplified.
In a Raman amplifier this process transfers optical energy from a strong laser
pump beam to a weaker transmission signal that has a wavelength which is 80
to 100nm higher than the pumping wavelength. For example, pumping at
1450nm will lead to a signal gain at approximately 1530 to 1550nm. Owing to
the molecular structure of glass, a number of vibration modes exist so that the
optical gain region is about 30nm wide. In practice one uses several pump lasers

Figure 11.12. Stokes shift and the resulting Raman gain
spectrum from a pump laser operating at 1445nm.

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