# Investigation of Crosstalk between Data Signals and Feed Light in Power-over-Fiber
## 3 Investigation of Intensity Noise of Feed Light Source  
Figure 2 shows the experimental setup for the signal noise evaluation. As a feed light source, we used a commercially available RFL (3 SP Group Inc., (Picardie, France) ML10-CW-R-TKS-1550). The output wavelength and maximum output power were 1550 nm and 10 W, respectively. The laser cavity and output fiber of the RFL was a SMF, but it was connected to the MMF with a core diameter of 62.5 \(\upmu\)m in the latter stage by direct fusion splicing. The output feed light was passed through a commercially available wavelength-division-multiplexing (WDM) coupler (Opneti Inc., Hong Kong, HPFWDM-1x2-1310T/1550R-900-3-1-NE-10W) and injected into a 2 km or 4 km MMF with a core diameter of 62.5 \(\upmu\)m. After transmission, the feed light was divided using a WDM coupler. Here, the feed light was intentionally received using a commercially available photodiode (PD) (Thorlabs Inc., Newton, NJ, USA, DET08 CFC) with a 3 dB bandwidth of 5 GHz at the 1310 nm WDM coupler output port to dramatically reduce the received feed light power to the PD. Conversely, the feed light power transmitted into the MMF was measured using an optical power meter (OPM) at the 1550 nm WDM coupler output port. The electrical spectrum of the feed light was measured at the receiver using an electrical spectrum analyzer (ESA) (ADVANTEST Corp., Tokyo, Japan, R3172). Figure 3 shows the electrical spectra of the RFL output without transmission (0 km), when the laser output power was set to (a) 1 W, (b) 5 W, and (c) 10 W, respectively. As the laser output power increased, the higher-frequency spectral component increased. The power suppression of the spectra at the higher frequency side included the bandwidth limitation of the PD with a 3 dB bandwidth of 5 GHz. The insets show the expanded spectra in the frequency range between 0 and 100 MHz. The main 10 MHz comb component, which corresponds to the longitudinal mode spacing of the RFL determined by the laser cavity length, was due to the longitudinal mode beating of the RFL. Although the sub 10 MHz comb component was also due to the longitudinal mode beating, the power ratio of these combs depended on the state of polarization of the laser output [14]. The obtained results show that an increase in the laser output power increases the longitudinal mode beating and generates higher-frequency beat noise components. Figure 1: Schematic view of simultaneous data signal and high-power feed light transmission using a single core, conventional multimode fiber. Figure 2: Experimental setup for intensity noise evaluation of RFL. WDM coupler: Wavelength-division-multiplexing coupler, PD: Photodiode, ATT: Attenuator, ESA: Electrical spectrum analyzer, OPM: Optical power meter. Figure 4 shows the electrical spectra of the RFL output after (a) 0 km, (b) 2 km, and (c) 4 km transmissions, when laser output power was set to 5 W, respectively. As the transmission distance increased, the noise floor level increased. This is because a longer transmission distance increases the time delay between oscillation frequencies owing to chromatic dispersion, resulting in an increase in the mode partition noise. In addition, as shown in the insets of Figure 4, it was confirmed that the sub 10 MHz comb components disappeared as the transmission distance increased. This is because the noise component of the feed light was unpolarized as the transmission distance increased. Figure 5 shows an example of the 20 dB bandwidth and noise floor level of the electrical spectrum of the RFL output after a 2 km transmission while varying the laser output power. As the laser output power increased, the 20 dB bandwidth and noise floor level of the electrical spectrum initially increased. However, when the laser output power exceeded 5 W, the 20 dB bandwidth became smaller, and the noise floor level continued to increase. This indicates that the mode partition noise generated by fiber transmission is dominant.