## 3 Results and Discussion
### Frequency Responses for Red and Green Light  
We obtained the frequency responses with the experimental protocol described above, which was specifically designed to maintain similar launching conditions with both wavelengths by using a pigtail that was alternatively connected to the two laser diodes. In addition, the detection conditions were identical because the connection to the SPD receiver remained in place when changing sources. In these experiments, we tested the three fiber types for both sources using segments of three different lengths: 50, 20, and 15 m. The measurements were repeated twice, but in Figure 6, the results of only one session are shown. Normalized frequency responses for both wavelengths and the three fiber lengths are plotted on the left, middle and right graphs, respectively. In all graphs, a second set of measurements is shown for the GH fiber. These measurements were taken using the other fiber of the duplex segment in order to assess fiber variability. The results show that, for all fiber types and lengths, there are only very small differences between the frequency responses with red and green light. However, with a few exceptions (15 and 20 m MH, and 15 m GH), the measurements with red light are above those obtained with green light. In the three exceptions, the curves show oscillations that can be attributed to small variations in the reference due to the changes at the connection of the RF cable from one mount to the other, which are necessary to perform both measurements. The S\({}_{21}\) transmission parameter measurements were similar for short fibers and for the reference, so that any small alteration was magnified with the calculation of the fiber frequency response as its quotient (see Equation (1)). In addition, the differences between the two measurements for the GH fiber, particularly those obtained for the 20-m segment, are greater than for the measurements with red and green light, suggesting that fiber variability has more impact than the possible differences between wavelengths. At this point, it is important to note that the SPD receiver from Graviton was equipped with a lens to focalize the light onto its active area, so that it has a small aperture of 0.25 (14.5\({}^{\circ}\)). Thus, the SPD lens filtered out a high percentage of the fiber output power. The EAFs in Figure 2 show that this percentage was between 25% and 55% for GH and BH, and even less for MH. We suppose that this filtering effect could have been masking the differences between measurements with red and green light. For this reason, we also performed the control experiment using the SPA receiver, also from Graviton, whose lens has an aperture of 0.5 (30\({}^{\circ}\)), and thus, it is able to capture practically all the power that exits the fibers. Figure 7 shows the results using the SPA in the same way as in Figure 6.  
Figure 6: Normalized frequency responses obtained with a pigtail launch and the two laser diodes using the SPD receiver: (a) fiber length of 15 m; (b) fiber length of 20 m; (c) fiber length of 50 m.  
A comparison of the corresponding graphs in Figures 6 and 7 reveals that the frequency responses obtained with the SPD were flatter than those obtained with the SPA. This effect was not owed to the better response of the SPD, because its effect was taken into account by the reference in Equation (1). The reason was the spatial filtering due to the small aperture of the SPD receiver, which enhanced the frequency response, because it blocked power carried by middle and high order modes that, as they propagated through longer paths inside the fiber, introduced higher delays that degraded the fiber frequency response [23]. Thus, the use of the SPA receiver with a higher aperture revealed larger differences between both wavelengths for the GH and BH fibers. On the other hand, for MH they are similar to those obtained with the SPD, because both receivers were able to capture most of its output power due to its lower NA.