Few-mode fiber (FMF)

A few-mode fiber sits between standard single-mode fiber, which guides one spatial mode, and multimode fiber, which guides hundreds. Its core is sized to guide a handful of modes, few enough that each can be launched, amplified and recovered as an individual channel. Sending independent signals on those modes is mode-division multiplexing, one of the families of space-division multiplexing, and each mode can carry a full set of wavelength channels.

The mode count follows from the V-number, $V = 2\pi a \,\mathrm{NA}/\lambda$ for core radius $a$. In a step-index core the linearly polarized (LP) modes appear at fixed values of $V$: LP01 always, LP11 above 2.405, LP21 and LP02 above 3.832, LP31 above 5.136, and LP12 above 5.520. A core of 9 µm radius with a numerical aperture of 0.12 has $V = 4.38$ at 1550 nm and guides LP01, LP11, LP21 and LP02. Counting the two orientations of LP11 and LP21, that is six spatial modes, or twelve with polarization. At 1310 nm the same fiber has $V = 5.18$ and also guides LP31, eight spatial modes, so the mode count belongs to a fiber and a band together. Designs keep $V$ away from a cutoff, because the highest mode close to cutoff is weakly guided and loses power in bends.

The modes travel at different group velocities, the modal dispersion that limits multimode links, and they exchange power at splices, bends and imperfections along the way. Coupled modes cannot be separated with passive optics at the receiver; they are unscrambled with multiple-input multiple-output equalization in a coherent DSP, whose filter must span the spread of arrival times between the fastest and slowest modes. That differential mode delay, which grows with length, is the specification that most shapes the fiber design, and graded-index cores with a depressed-index trench around them are used to minimize it. Mode-dependent loss, the difference in attenuation between modes, is the other: unlike delay, it cannot be equalized away and costs capacity directly.

The fiber needs matching components. Mode multiplexers, among them photonic lanterns, multi-plane light converters and mode-selective couplers, map several single-mode inputs onto the fiber's modes, and few-mode erbium amplifiers must hold their gain nearly equal across modes. These components, more than the fiber, are what keep mode-division multiplexing in laboratory and trial systems. Few-mode fiber has a second life in high-power fiber lasers, where a large-mode-area core that guides a few modes is operated in the fundamental mode, with bending used to strip the higher ones.

References: D. J. Richardson, J. M. Fini, L. E. Nelson, Nat. Photonics 7, 354 (2013); P. Sillard, M. Bigot-Astruc, D. Molin, J. Lightwave Technol. 32, 2824 (2014); A. W. Snyder, J. D. Love, Optical Waveguide Theory (Chapman and Hall, 1983).