Twin-Spot Launch for Enhancement of Multimode-Fiber Communication Links

Multimode Graded-Index Optical Fibers for Next-Generation Broadband Access

“…In addition to, it is well known than an m-ary digital modulation scheme with m>2 (multi-level coding) can enhance transmission capacity by overcoming the bandwidth limitations of a transmitter or a transmission medium and, therefore, multilevel modulation schemes that are used in radio-frequency communications have also been demonstrated in fiber-optic links [65]. Other attempts to overcome the bandwidth limit includes selective excitation of a limited number of modes, socalled Restricted Mode Launching (RML), in different ways: offset launch [66], conventional center launch [67] or even by means of a twin-spot technique [68]. Since the propagating modes are fewer under RML launch conditions, the difference in propagating times between the fastest and slowest modes is smaller, thus decreasing modal dispersion and increasing the corresponding bandwidth.…”