Page 408 - Fiber Bragg Gratings

8.8 Gain-flattening and clamping in fiber amplifiers 385

Figure 8.25: Schematic of a 1300-nm cascaded fiber grating resonant Raman
amplifier [115].

produces in excess of 6 W CW. The five stages Stokes of conversion is
remarkable, showing ultralow loss of <0.2 dB per grating.
For 1300-nm amplifiers, two configurations can be used. The linear
cavity shown in Fig. 8.25 uses WDM couplers and a set of gratings to
allow gain in the 1300 nm window when pumped by a 1.064 nm source.
The gain is available at the fourth Stokes wavelength. These amplifiers
have gains as high as 40 dB with saturated output powers of 24 dBm.
With higher germania concentration (higher Raman gain) the pump power
can be lowered to 300 mW while providing a gain of 25 dB [114].
The theoretical noise figure for Raman amplifiers is 3 dB [116], while
the achieved figures are around 4.4 dB at 40 dB gain in a ring configuration
[117,118].
The advantage of using bidirectional pumping as in a ring cavity is
reduced cross-talk and polarization sensitivity.
These amplifiers are increasing in importance as the requirement for
the optical bandwidth increases. In regions of the communications window
in which amplification is difficult, e.g., 1350-1500 nm, cascaded fiber
grating resonant amplifiers are likely to provide unique solutions.

8.8 Gain-flattening and clamping in fiber
amplifiers

Rare-earth-doped optical fiber amplifiers are important components in
transmission systems. The transmission bit-rate X distance product is
almost limitless in laboratory-based demonstrations [119]. This is primar-
ily due to the "zero-transmission loss" through periodic amplification as