Though it won't happen anytime soon, physics eventually will halt the exponential growth of per-fiber bandwidth. A report by researchers at Lucent Technologies Inc.'s Bell Labs in Murray Hill, N.J., suggests that the capacity of an optical fiber in dense wavelength division multiplexing is limited by cross-phase modulation to 3 bits per second per hertz. The next-generation 40-Gb/s systems, which will feature 100-GHz spacing, will have a spectral efficiency of 0.4 bits per second per hertz. Cross-phase modulation is a nonlinear effect caused by perturbations in the refractive index of the fiber by the modulated intensity of signals running through it. The use of hollow-core fiber and polarization-resolved detection can reduce this effect. The researchers published their findings in the June 28 issue of Nature.