# Investigation of Crosstalk between Data Signals and Feed Light in Power-over-Fiber
## 1 Introduction  
Power-over-fiber (PWoF) is a unique power supply system that uses optical fibers. Compared to conventional copper wires, optical fibers are lighter and more resistant to corrosion. In addition, because it is a power line that does not conduct electricity, it can avoid the effects of reverse currents caused by lightning damage and leakage [1, 2]. To date, a number of experimental demonstrations have been reported using various kinds of optical fibers such as single-mode fibers (SMFs) [3, 4], multimode fibers (MMFs) [5, 6], multicore fibers (MCFs) [7, 8, 9], and double-clad fibers (DCFs) [10, 11, 12]. In PWoF, simultaneous data signal and power transmission using a single optical fiber is an attractive way to effectively use the fiber and integrate communication and power supply systems. In this case, unlike MCFs and DCFs, SMFs and MMFs have a single core, and the data signal and high-power feed light must be simultaneously transmitted to the same core [3, 4, 5, 6]. Therefore, even when these wavelengths are significantly different, the quality of the feed light, which has a much higher power than the data signal, strongly influences the quality of the transmitted data signal. Indeed, the signal quality degradations due to feed light has been observed in PWoF using SMFs [3, 4], MMFs [5, 6], and multicore fibers (MCFs) [7, 8] in terms of signal transmission performances such as error-vector magnitude (EVM) and bit-error-rate (BER) measurements. For all the demonstrations, the signal degradations were strongly dependent on the feed optical power and transmission distance, although various wavelengths were used for data signals and feed light in these demonstrations. In terms of the effect of high-power pump light on simultaneously transmitted data signals, the analytical expression in co- and counter-pumped Raman amplification hasbeen given, and this phenomenon also affects the relative phase noise of the transmitted data signal through nonlinear effects such as cross-phase modulation [13, 14, 15]. In addition, Refs. [3, 4] have evaluated and simulated the crosstalk in PWoF using SMFs due to nonlinear effects such as self-phase modulation, cross-phase modulation, and stimulated Raman scattering, and the effect of the noise fluctuations in the feed light has been also evaluated. Although our preliminary investigations of crosstalk between the data signal and feed light in PWoF using MMFs have been reported [5, 16, 17], our detailed investigation of the crosstalk has not been reported. In this work, we experimentally evaluate the crosstalk between data signals and high-power feed light in PWoF using a single-core, conventional MMF. To observe the effect of the crosstalk more clearly, a high-power Raman fiber laser (RFL) with a larger intensity noise than high-power laser-diodes (LDs) and a maximum laser power injected into the fiber of up to 10 W is used as a feed light source. In this measurement, we compare and evaluate the intensity noise spectra of the RFL with different laser powers and transmission distances. Furthermore, we experimentally observe the intensity noise of an analog data signal, which is simultaneously transmitted with the feed light into the fiber. We also measure the transmission performance of analog and digital data signals to evaluate and quantitatively compare the signal degradation. The results showed that the intensity noise induced by the feed light source affects the signal degradation of the transmitted data signal, and that it is related to the longitudinal mode beating and mode partition noise determined by the property of the laser. The remainder of this paper is organized as follows. In Section 2, we explain the crosstalk between the data signal and high-power feed light in PWoF using a conventional MMF, and its relation to longitudinal mode beating. In Section 3, we investigate the intensity noise of the feed light source while varying the laser power and transmission distance.