Engineers from Sumitomo Electric Industries (Yokohama, Japan) have shown how Raman amplification can be used maintain signal power throughout the length of a hybrid line. Not only does this lead to transmission of a signal with almost the same power out as in—and so longer transmission distances without amplification—but it reduces the amount of noise that the line adds to the signal through nonlinear phenomena. Because of this, said researchers, the configuration should be particularly good for the transmission of solitons.
Based at the Yokohama Research Laboratories, Toshiaki Okuno and his colleagues first demonstrated theoretically how the hybrid line they were using—the recently developed PureCouple, which consists of a pure silica core fiber with an enlarged effective area (Z Plus) and a dispersion-compensation fiber (DCF)—could be Raman-pumped to best effect. Based on their theoretical model, they showed that the output would drop by more than 10 dB over 51 km of hybrid fiber, regardless of direction of travel. Using a conventional configuration, with the Z Plus fiber first and Raman-pumping backwards, the output power was maintained, but the power along the line varied by almost 6 dB. In the reverse configuration, though a slightly higher-power pump beam was required, the variation across the transmission line was on the order of 1 dB.
The team verified their model through experiment (see figure), using an optical time-domain reflectometer to measure the variation in the signal across the transmission line. Due to equipment constraints, they worked with a 20-km rather than a 51-km fiber. The pump power was 23 dBm, going back first through the single-mode fiber, then the DCF, and the resulting power deviation during transmission was less than 1 dB. Soliton transmission is expected to be improved in this circumstance, because the nonlinear phase change should be kept constant by the even signal power. In addition, the noise characteristics for this configuration were shown to be up to 1 dB better than its more conventional DCF-last counterpart.
For more information contact Toshiaki Okuno at firstname.lastname@example.org.