## 5 Attenuation Method Experiment
### Phase I: Design and Safety Verification of the Initial Route  
Within the first phase, a simple polygon was designed, as shown in Figure 2. In the case of neglecting the connections between the individual optical components, the total length was 20 km. This distance was chosen as an educated guess, as it is long enough to be affected by nonlinear effects. However, in terms of attenuation, it respects the manufacturer's specified dynamic range of 10-14 dB for the quantum channel.  
The presented scheme combines the quantum channel in one fiber with both of the service channels. The CH30 works in the same direction as the quantum channel, i.e., is transmitted from Alice toward Bob, and the direction of the second service channel is the opposite. To properly filter out noise from the quantum channel, it is necessary not only to properly filter out extraneous wavelengths, but also to ensure that noise in the area of CH32 does not reach the shared fiber. Such noise would subsequently no longer be distinguishable from the quantum channel.  
Both service channels are first combined into a single fiber by using a conventional multiplexer. However, in addition to its main role, the MUX serves as a filter that removes most of the noise on CH32. To keep the quantum channel interference to a minimum, the service channel is attenuated by 20 dB at the source.  
The basis of the topology is three DWDM filters transmitting CH32, for which qualitative measuring of parameters was conducted. Based on their measured characteristics, they were placed in Positions I, II and III from left to right (marked with underscores).  
* **I. Position--**The main function of the filter is to bind CH30 to a common fiber with a quantum channel and simultaneously extract CH29 from the fiber. Its secondary function is similar to that of the multiplexers described above, that is, to filter out the noise produced by CH30 in the CH32 region. Most of the noise that passes through the MUX is passed to the PASS port, where it is then absorbed by the isolator along with the remnants of CH29.
* **II. Position--**As in the previous case, the main function of the filter is to bind and unbind the service channels to a common fiber. In this case, however, it is no longer appropriate for the filter to pass the noise of service channel B. Unlike Filter I, most of the noise in the CH32 region must be reflected toward Alice, where it again passes to the isolator and is absorbed.  
Figure 2: The original route combining two opposite service channels (CH29 and CH30) with one quantum channel (CH32) is connected. The layout also includes optical power values at important locations in the route, which were obtained by measurement or calculation. The values CH29-r and CH30-r represent the possible remaining power that was passed through the DWDM filters toward the source or detector of the quantum signal.  
* **III. Position--**The filter is mainly used to increase the OSNR (optical signal-to-noise ratio) of the quantum channel. Therefore, its isolation in the COM-PASS direction is important. Service channel residues that are not suppressed by the previous filter are removed from the fiber by the REF port.  
There are two more optical elements on the quantum channel: an isolator, which prevents any light from passing through the filter from reaching the source, and a 1 dB attenuator, which increases the attenuation on the quantum channel to a total dynamic range of 10 dB.