Cramming an ever-increasing amount of information through existing optical fibers enhances the value of those fibers and eliminates the need to install expensive new cables. Engineers and scientists around the world have explored several ways to boost the information-carrying capacity of fibers, such as by increasing the modulation frequency, decreasing the channel spacing and utilizing efficient modulation schemes. All of these eventually reach their limits, however, and the only option is to increase the spectral region utilized within the fibers. Figure 1. By configuring the C-band erbium-doped fiber amplifier as a preamplifier for the L-band amplifier, the conversion efficiency of the latter is boosted by more than 20 percent.For this reason, much attention has been focused recently on the possibility of expanding from the conventional C-band (1530 to 1565 nm) into the adjacent S-band (1460 to 1530 nm) and/or L-band (1565 to 1625 nm). A drawback is that erbium-doped fiber amplifiers (EDFAs) are much less efficient in these bands than in the C-band. Researchers originally attempted to develop combined C- and L-band systems by using separate fiber amplifiers for each band. But several laboratories recently have shown that a serial arrangement, in which the C-band device acts as a preamplifier for the L-band one, results in a significantly greater efficiency for the latter (Figure 1).A research group at Tsinghua University in Beijing has achieved dynamic control of a combined C- and L-band EDFA. Dynamic control is necessary to eliminate the variation in gain that normally occurs when the signal level into an EDFA changes. If the input signal increases, for example, it will saturate more of the gain and decrease the gain available for all channels amplified in the EDFA. For a telecommunications system to be stable, the gain in its amplifiers must be independent of the signals they amplify.Figure 2. Gain in the C-band (1530 to 1565 nm) is unaffected by varying the input signal at the L-band wavelengths.Key to the accomplishment was the researchers' observation that gain at the L-band wavelengths is independent of the signal level in the C-band wavelengths, and vice versa. Using the experimental configuration in Figure 1, they measured the gain spectrum in the C-band as they varied the L-band signal from –21 to –6 dBm. The C-band gain was virtually unchanged (Figure 2), and they saw a similar stability in the L-band gain when the C-band signal varied from –21 to –6 dBm. Figure 3. By controlling the pump power to each erbium-doped fiber amplifier independently, a steady 25-dB gain was maintained across the C-band and most of the L-band as the amplifier's input varied between 25 and 483 µW.Because the gain in each band was independent of the signal in the other band, the researchers were able to control two EDFAs as if they were separate devices, even though they were, in fact, strongly coupled by the serial arrangement. They applied pump power to each device that was proportional to the input signal strength in that amplifier's spectral region. The result was a very stable gain, as the total input power in both bands varied from 25 to 483 µW (Figure 3). The 25-dB gain was flat across the entire 77-nm range from 1528 to 1605 nm.