# POL-MUX System for Noncoherent Optical Networks
## 3 POL-MUX on a Real Optical Network  
After successfully demonstrating the POL-MUX in a laboratory environment, a bidirectional POL-MUX setup was tested on an optical network based on real academic data. In this case, the POL-MUX was used for the separation of \(10\,\)Gbps of downstream and upstream data traffic. The optical link consisted of a \(12.8\,\)km optical fiber between Brno University of Technology (BUT) and Masaryk University (MU), as shown in Figure 6. The  
Figure 4: Power dependence on temperature in both polarization planes. Figure 5: Influences of fiber (**a**) cooling and (**b**) warming. optical link was formed by five segments between six academy buildings and goes through a city with almost 400,000 residents and dense traffic. In the laboratory of transmission media, the Department of Telecommunication at BUT was placed on the first transmission and receiver side. As a transmitter, the EXFO FTB-1 platform with an EXFO NetBlazer 880G module and a 10G SFP+ module (Optoway SPS-2385 MW-D440G) was used. The 880G module can generate high-speed 10 Gbps data traffic and measure the BER and quality of services (QoS) parameters. The wavelength was tuned to a 50 GHz DWDM grid with 1542.14 nm. The output signal from the SFP+ module went through an optical insulator \(ISOL_{1}\) (to avoid damage caused by back reflection) to a polarization controller \(PBS_{1}\), where the angle of polarization was optimally rotated so that the optical signal of the output of PBS1 was at the maximum level (as measured by the OSA), which was then disconnected, and PBS1 was connected to a real optical link. The opposite transmission and receiver side was realized by the Faculty of Informatics at Masaryk University, Brno. The output signal from the optical link was controlled by PC3 to obtain the maximum power level of the output signal. This signal was monitored by an optical spectrum analyzer \(OSA_{2}\). As a transmitter on the opposite side, an EXFO FTB-1 platform with an EXFO NetBlazer 860G module and a 10 Gbps SFP+ module (Optoway SPS-2385 MW-D440G) was also used. The 860G module has similar functionalities to those of 880G. The transmitted 10 Gbps signal went through \(ISOL_{2}\) to polarization controller PC2, which was used to set the maximum optical level of an output of \(PBS_{2}\) (monitored by the OSA). The optical links consisted of many connector connections in each building. It is obvious that optical links are very prone to changes in polarization due to temperature changes. The proposed POL-MUX network was measured for the bit error rate, which is one of the most important parameters with respect to QoS. The reason for this measurement was to determine how the time, temperature and possible mechanical stress changed the state and angle of polarization, which had a major impact on the power level in both polarization planes and of course on the bit error rate. Because the proposed system did not contain a dynamic polarization controller, we can say that the system was'static' and was not able to react to changes in ambient conditions. The BER was tested for five days, and at the beginning of the test, the value of the BER was not measurable (too low) (\(>\)10\({}^{-12}\)). In the following days, the values were:  
* 1 st day-BER \(\approx\) 0
* 2 nd day-BER \(\approx\) 10\({}^{-12}\)
* 3 rd day-BER \(\approx\) 10\({}^{-11}\)
* 4 th day-BER \(\approx\) 10\({}^{-10}\)
* 5 th day-BER \(\approx\) 10\({}^{-8}\)  
Figure 6: Measurement on a real optical link  
Time changes of BER and data throughput are shown on Figure 7. As we can see, BER change in time was almost linear, very slowly reduced about one order per day. BER was in five days measurement reduced from 0 to \(\approx 10^{-8}\). Data throughput is more resistant to polarization changes and was slightly reduced from 10 Gbps to 9.816 Gbps in five days. Table 1 also shows the change of power level measured by the optical spectral analyzer (10% of optical signal). As we can see, signal strength was reduced by about 9 dBm in five days.