An optical feed system using double-clad fibers has been demonstrated to supply 60-W power over a 300-m optical fiber system. The setup could enable future small-cell mobile communications. Delivered optical feed power and power transmission efficiency (PTE) as a function of total optical feed power launched from the two high-power laser diodes. The solid circles show the results of a previous scheme with a tapered fiber bundle divider consisting of two multimode fiber (MMF) outputs. The open circles show results obtained with a tapered fiber bundle divider consisting of six MMF outputs. The insets show how using a greater number of narrower MMFs leaves less empty space, increasing their total cross-section. Courtesy of the University of Electro-Communications. Power supply over fiber is limited by power transmission efficiency, which is impeded by the large fraction of power fed into the optical link that is lost as heat during transmission. As such, restrictions on power feed levels are needed to prevent waste heat from damaging optical components in the link. Researchers from the University of Electro-Communications had previously demonstrated that they could bundle together two multimode fibers for transmitting power with a double-clad fiber for transmitting the data. The bundle was tapered and fused to a double-clad fiber output. However, power was lost in the tapered fiber bundle divider due to the lower cross-sectional area occupied by fiber in the cluster bundle, the researchers said. As a result, the overall power transmission efficiency was only 20 percent, limiting the power that could be fed into the link to just 40 W. Now they say increasing the number of multimode power-carrying fiber to six optimized the cross-sectional area of fiber in the bundle cluster without introducing other limitations, thereby maximizing the power transmission efficiency. The researchers tested the cluster of power and data fibers in a bidirectional system consisting of a central station and a radio antenna unit linked by a double-clad fiber. A laser diode with direct electrical modulation from a signal generator produced test signals at 1550 nm to the standard specifications of the IEEE for the wireless local access network used in Wi-Fi. Commercial laser diodes also fed the optical power. An erbium-doped fiber amplifier boosted the signal and increased the power level of the data signal for the transmission. The system also included elements to reduce the noise, including bandpass filters and cladding mode strippers. The multimode and double-clad fibers in the fibers bundle cluster input were tapered and fused to a 300-m double-clad fibers transmission output. The double-cladding prevents crosstalk, the researchers said, though cost prohibited use of longer-output double-clad fibers in the test system. The researchers identified the combined cross-sectional area of the two multimode fibers as the limiting factor for power transmission efficiency. The two multimode fibers left empty space in the bundle cluster that was unoccupied by fiber, so that the total fiber cross-section was smaller than that of the double-clad fiber it fused to, making power transfer inefficient. Using a greater number of narrower multimode fibers increased the filling factor within the bundle and hence their combined cross-sectional area, the researchers said. However, this also meant each fiber was narrower, reducing its power-handling capability. Ultimately the researchers determined that a bundle of six multimode power-handling fibers gave the optimum compromise between the two limiting factors. The research was published in Optics Letters (doi: 10.1364/OL.40.005598).