# Investigation of Crosstalk between Data Signals and Feed Light in Power-over-Fiber
## 4 Evaluation of Signal Degradation in PWoF Using MMFs
### Digital Signal Transmission  
To evaluate the effect of crosstalk between the digital data signal and feed light as well, the BER characteristics of the transmitted digital data signal were measured. Figure 8 shows the BER characteristics and the eye-patterns of the transmitted digital data signal into a 2 km MMF. The power penalty between the back-to-back and the transmitted digital data signals when the laser output power of the feed light was set to 0 W was mainly due to the modal dispersion of the 2 km MMF. However, there was little difference in the power penalty between the 0 W and 1 W laser output powers because there was almost no crosstalk with the digital data signal at a laser output power of 1 W, which was consistent with the analog data signal transmission performance, as shown in Figure 7. However, as the laser output power increased, the power penalty also increased. In the 10 W laser output power, the penalty to the back-to-back signal was approximately 2.9 dB. The insets show the eye-patterns of the transmitted digital data signal when the laser output power was set to 0 W and 10 W, respectively. It can be clearly seen that the intensity fluctuations at the mark and space levels increased in the 10 W laser output power.  
Figure 9 shows the BER characteristics and eye-patterns of the digital signal transmitted into a 4 km MMF. In this case, there was little difference in the power penalty between the 0 W and 1 W laser output powers. However, as the laser output power increased, the power penalty also increased. The insets show the eye-patterns of the transmitted data signals when the laser output power was set to 0 W and 10 W, respectively. Although the waveform distortion was observed even at 0 W laser input power owing to mode dispersion, larger intensity fluctuations at the mark and space levels were observed at 10 W laser input power. This was due to the intensity noise transfer induced by the longitudinal mode beating and mode partition noise of the RFL. The power penalty to the back-to-back signal was approximately 4.7 dB. It was clearly observed that the intensity noise transfer of the feed light not only significantly affects the analog data, but also influences digital data signal transmission performance in PWoF systems using single-core optical fibers.  
The results of the BER measurements are summarized in Figure 10. Here, the power penalty represents the extra received signal power from the back-to-back signal required to obtain a BER = 10\({}^{-9}\). For both the 2 km and 4 km transmissions, it can be seen that the penalty increased as the laser output power increased. In particular, the increasement of the power penalties was higher from 1 to 5 W than from 5 to 10 W, and greater for 2 km than 4 km transmission. This is related to the intensity noise spectra of the RFL, as shown in  
Figure 8: BER characteristics of digital data signal transmitted into 2 km MMF for various laser output powers. Insets show eye-patterns of transmitted data signal when laser output powers were (**a**) 0 W and (**b**) 10 W, respectively.  
Figures 3 and 4. In Figure 3, the spectral broadening from 1 to 5 W was more pronounced than that from 5 to 10 W. This rapid increase in the spectral broadening caused a large mode partition noise, and was a major factor in the degradation of the transmission performance. On the other hand, in Figure 4, the increase in noise floor from 1 to 5 W was also more pronounced. Therefore, the power penalty increase from 1 to 5 W was also larger than from 5 to 10 W in the BER measurement. This effect was more pronounced because the electrical spectrum of digital signals had a much wider bandwidth than that of analog signals.