The increasing use of data is placing greater demands on radio antenna units that transmit data to mobile devices. Powering these antenna units through the same fiber optic systems that transmit data signals could improve and simplify these infrastructures if the power supplied in this way could be increased. Now, Motoharu Matsuura, Hidehito Furugori, and Jun Sato at the Tokyo University of Electro-Communications have demonstrated the ability to supply 60 W in a 300 m test fiber system, exceeding the power supplied in previous work and highlighting the potential of this approach.
Fiber-powered systems are limited by their power transmission efficiency, which is hampered by the large fraction of power introduced into the optical link that is lost as heat during transmission. Consequently, restrictions on power levels are needed to prevent waste heat from damaging the optical components in the link.
Matsuura and his colleagues had demonstrated that they could bundle two multimode fibers for power transmission with a double-coated fiber for data transmission. This bundle was then tapered and spliced to a single double-coated fiber output. However, power was lost at the fiber bundle splitter due to the smaller cross-sectional area occupied by the fiber within the cluster bundle. As a result, the overall power transmission efficiency was only 20%, limiting the power that could be delivered over the link to just 40 W.
By increasing the number of multimode power-carrying fibers to six, they optimized the fiber cross-sectional area within the cluster bundle without introducing any further limitations, thus maximizing power transmission efficiency. In their report, the researchers conclude: "These results demonstrate that optical power systems using double-coated fibers have high potential for practical use in future small-cell mobile communications."
Mobile Data Networks:
Wireless radio antenna units transmit data from a central station to mobile devices on radio frequencies. The central station transmits the data to the antenna units using radio over fibers.
Power to the radio antenna unit is normally supplied by the network, but the proposed power over fiber could simplify the power supply. Furthermore, power over fiber would make the antenna units invulnerable to network power outages, such as those caused by lightning. While power over fiber options have attracted the attention of researchers, the power available over fiber has been limited to date.
Test System Details:
The researchers tested their data fiber and power fiber cluster in a bidirectional system consisting of a central station and a radio antenna unit connected by a double-jacketed fiber.
A laser diode with direct electrical modulation from a signal generator produced test signals at 1550 nm, meeting the IEEE standard specifications for wireless local area networks (WLANs) used in Wi-Fi (IEEE 802.11g). Commercial laser diodes also supplied the optical power.
An erbium-doped fiber amplifier boosted the signal and increased the power level of the data signal for transmission. The system also included noise reduction elements, including bandpass filters and cladding mode strippers.
The multimode fibers and double-coated fiber at the fiber bundle input of the cluster are tapered and spliced to a 300m double-coated fiber transmission output. The double coating prevents crosstalk. Excessive cost precludes the use of a double-coated fiber output for a longer period in the test system. Double-coated fibers are generally produced commercially for use as gain media, for which they do not need to be much longer than 100 meters.
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 assembly, so the total fiber cross-section was smaller than that of the bonded double-clapped fiber, resulting in inefficient power transfer.
Using a greater number of narrower multimode fibers increases their fill factor within the bundle and therefore their combined cross-sectional area. However, this also means that each fiber is narrower, reducing the power handling capacity of each individual fiber. The researchers determined that a bundle of six power-managed multimode fibers provided the optimal compromise between the two limiting factors.
Reference:
Motoharu Matsuura*, Hidehito Furugori, and Jun Sato, 60 W power-over-fiber feed using double-clad fibers for radio-over-fiber systems with optically powered remote antenna units, Optics Letters 40 5598 (2015). Higher School of Informatics and Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
