A research group at the Electronics and Telecommunications Research Institute in Taejon, South Korea, has shown that it is mathematically possible to obtain relatively flat gain across the C- and L-bands by combining the gain from erbium ions' stimulated emission and from stimulated Raman scattering in a single, long optical fiber. That's important because, as demand grows for bandwidth in fiber optical telecommunications systems, engineers are seeking efficient ways to expand existing systems from the C-band into the neighboring L-band.The group modeled its calculations on several kilometers of erbium-doped germano-silica fiber, pumped bidirectionally by high-power diode lasers at 1.495 µm (Figure 1). The optical signal to be amplified is an 80-channel wavelength division multiplexed signal spanning the spectral space from 1530 to 1610 nm, with a channel separation of 1 nm and a signal strength of –25 dBm in each channel. Stimulated emission from the erbium ions primarily amplifies the C-band (1530 to 1565 nm) channels, and stimulated Raman scattering within the fiber core primarily amplifies the L-band (1565 to 1610 nm) channels.Figure 1. The model used for the calculations incorporates several kilometers of erbium-doped germano-silica fiber, pumped bidirectionally by a pair of semiconductor lasers. The researchers calculated the gain across this spectral range and showed how varying the erbium-ion concentration, the fiber length and the pump power could optimize it. Obtaining the proper erbium-ion concentration must be performed correctly for the amplifier to provide flat gain across the two spectral bands. The concentration must be low enough so that the limited number of pump photons can create a population inversion over most of the fiber's 5-km length. (Sections of the fiber in which an inversion is not achieved will absorb the C-band channels.) But if the concentration is too low, the stimulated-emission gain in the C-band will be insufficient to match the Raman gain in the L-band. The scientists repeated their calculations for different values of erbium concentration, fiber length and pump power and showed how these parameters could be manipulated to optimize the gain in a number of situations.Figure 2. The calculated gain across 80 wavelength division multiplexed channels spaced at 1 nm is relatively flat and can be readily flattened with a gain-flattening filter. In this example, the bidirectional pump power is 600 mW, the fiber length, 5 km, and the erbium concentration, 3 X 1016 cm.The term "flat" may seem optimistic when applied to the calculated gain, for which the output varied by at least 5 dB across the spectrum (Figure 2). Bear in mind, however, that the gain of an ordinary erbium-doped fiber amplifier can vary by 15 dB or more across the C-band, so that the gain in the figure is relatively flat and can be flattened to any desired degree with a gain-flattening filter without causing unacceptable system loss.