## 5 Attenuation Method Experiment
### Phase III: Building the Final Route and Tuning  
The final step of this paper is to build a final and functional optical polygon fully utilizing a shared fiber to carry the quantum and both service channels. Unlike the previous step, the functionality of the service channels must now also be considered, as their detectors also have only limited sensitivity. For this reason, several changes are made compared to the previous setup, as seen in diagram Figure 5. While most of them are related to service channels, only one is related to the quantum channel. This is the relocation of the attenuator closer to the detector. Thus, not only Channel 32 but also the noise is attenuated. Such a solution ensures a higher OSNR. As already indicated, the limiting element is no longer just the quantum channel. For proper functioning, it is necessary to attenuate the service channels in such a way that their resulting optical power is far below the desired sensitivity of the detector. For this reason, EDFA and lower attenuation components must be used, as shown in Figure 6.  
The first change is the deployment of new SFPs with a more stable optical power value (\(-3\) dBm). This allows us the removal of part of the added attenuation. Both channels are measured, and their spectra can be found in plot Figure 7A. The quantum channel region is also marked in purple for comparison. Figure 5: Schematic of the final and functional route of the polygon. Compared to the previous design, it contains a number of different components. In particular, these are circulators and EDFAs whose signal is subsequently “trimmed” by using a CWDM filter. Figure 6: Schematic diagram of one of the service channels with power values at various points along the optical route. In the original topology, the service channels are first multiplexed into a single fiber by using multiplexers with an attenuation of approximately 4.5 dB. In the same way, they are separated on the opposite side. However, this arrangement adds high attenuation to the path, which is not problematic in the section before the connection to the shared path where the service channels are intentionally attenuated. However, once the shared section is passed, the power of the service channels must be at least equal to the sensitivity of the detectors. For this reason, further attenuation is not desired. The two channels are opposite to each other, which allows replacement of the multiplexers by circulators with much lower attenuation (approximately 0.5 dB). Figure 7: Graphs showing the spectra of both service channels (CH29 and CH30) in different parts of the optical route. The purple lines show the region of occurrence of the quantum channel (CH32). Removing multiplexers, however, leads to noise in the quantum channel. Although the service channels transmit on adjacent channels, they produce noise in the CH32 region in addition to the useful signal. This must be filtered out before all channels are combined. For this reason, a multiplexer is connected to the start of the route, but it only serves as a filter. By reconfiguring the attenuator, the same input power is obtained as in the previous stage. The spectrum of service channels after multiplexer \"trimming\" is shown in Figure 7B. As shown in Figure 7C, after passing through the shared path and the circulators (total attenuation of approximately 7 dB), the power of both service channels is much lower (\(-35\) dBm) than the sensitivity of the SFP modules used (\(-24\) dBm). For this reason, an EDFA is added on each side. The spectra of both channels after passing through the amplifier are located in Figure 7D. This shows that the EDFA amplifies most in the region at approximately 1530 nm. Since the detector accepts wavelengths in the range of approximately 1528-1569 nm, there would be too much error due to the small signal-to-noise ratio. For this reason, a CWDM filter operating at \(\lambda=1550\) nm is placed between the amplifier and the detector itself. The resulting signal therefore appears in Figure 7E.