We report the first demonstration of 640-Gb/s return-to-zero ON-OFF keying channel transmission over a 100-km standard single-mode fiber link employing midspan phase conjugation. A frequency-degenerate conjugate field spanning more than 20 nm is created in a low-birefringence parametric mixer for the first time. Physical separation of the conjugate field from the original field is enabled by utilizing pump polarization nondegeneracy. Rigorous link characterization using a high-quality 640-Gb/s transmitter and a high-sensitivity receiver revealed error-free (bit error ratio <;10-9) performance, eliminating the need for impractical fiber length control or electronic signal processing. The compatibility of wavelength-transparent conjugation with spectrally inefficient channel implies that channels with higher bit rates and better spectral efficiency can also be compensated.