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Two-stage all-optical 2R (reamplification and reshaping) signal regeneration based on spectrum broadening due to self-phase modulation in a nonlinear fiber and subsequent off-centered filtering is demonstrated by the use of only one fiber spool in which the signal is transmitted twice in opposite directions. The two-stage configuration allows wavelength shift-free operation of the regenerator. Recirculating-loop signal transmission and regeneration experiment shows that the bidirectional 2R regenerator extends transmission distance by a factor more than two for an unequally-spaced 40 Gb/s short-pulse train with minimum pulse separation of 12.5 ps. Numerical simulation for assessing the influence of Rayleigh backscattering in the bidirectional configuration is performed, which shows that although some influence of Rayleigh backscattering can appear at higher operation speeds such as 80 Gb/s, strong noise reduction is still achievable by the bidirectional two-stage regeneration.