# POL-MUX System for Noncoherent Optical Networks
## 2 Temperature Influence on the POL-MUX Signal
### Experimental Analysis  
The measurement process was divided into two parts. First, the measured fiber was cooled from \(20\,\mathrm{\SIUnitSymbolCelsius}\) to \(-40\,\mathrm{\SIUnitSymbolCelsius}\) and subsequently warmed back to \(20\,\mathrm{\SIUnitSymbolCelsius}\). Second, the fiber was warmed from \(20\,\mathrm{\SIUnitSymbolCelsius}\) to \(70\,\mathrm{\SIUnitSymbolCelsius}\) and cooled back to \(20\,\mathrm{\SIUnitSymbolCelsius}\). The 3D model in Figure 4 shows how the temperature changed the power in both polarization planes. A wavelength of 1550.1 nm corresponds to the output of the polarization splitter, which was monitored by the OSA. We mark this polarization plane as \(x\).  
During the cooling process, the temperature in the second polarization plane (\(y\)) gradually increased up to \(-40\,\mathrm{\SIUnitSymbolCelsius}\) when the power level increased from the original value \(-21.17\) dBm to \(0.74\) dBm. With further temperature increases, the power level decreased, and for \(20\,\mathrm{\SIUnitSymbolCelsius}\), \(-15.33\) dBm was achieved. After 30 min of tempering, the power level decreased to \(-24\) dBm. At the same time, it can be seen that in the polarization plane \(x\), the power level was minimally decreased by approximately tenths of 1 dB. However, from \(-20\,\mathrm{\SIUnitSymbolCelsius}\) to \(-40\,\mathrm{\SIUnitSymbolCelsius}\), it is possible to see a decrease in the power level with the highest minimum value (\(-4.15\) dBm) at \(-40\,\mathrm{\SIUnitSymbolCelsius}\), which is adequate as the maximal power level in-plane \(y\). At this time, the angle of polarization was rotated almost \(90\,\mathrm{\SIUnitSymbolCelsius}\) due to the changing ambient temperature. After further warming, the power level returned to the original level.  
Figure 3: Spectra of the multiplexed (**a**) and demultiplexed (**b**) signals.  
In the second part of the measurement, the fiber was warmed to \(+70\,^{\circ}\)C. The power level in the polarization plane \(y\) significantly increased as the temperature increased, with a maximum of \(-4.31\,\)dBm at \(70\,^{\circ}\)C. After that, the fiber was cooled to \(20\,^{\circ}\)C, and the power level decreased gradually to \(-21\,\)dBm. In polarization plane \(x\), the power level decreased very slightly. Regarding the maximal power in plane \(y\) the power level in \(y\) decreased by only approximately \(1\,\)dB.  
Figure 5 shows a comparison between the influences of cooling and warming on the change in the signal ratio between the polarization planes (in Figure 5, this is marked as the OSNR). It is obvious that the polarization angle is much more sensitive to temperatures below \(0\,^{\circ}\)C than to higher temperatures. The polarization planes were rotated almost \(90^{\circ}\) during the cooling process because the warming signal ratio between both signals was decreased; nevertheless, the polarization planes did not rotate like they did during cooling. Based on the analysis, we can say that if the temperature of the optical distribution network is kept within \(10\,^{\circ}\)C, the functionality of the POL-MUX system will be preserved even without the use of dynamic polarization control or DSP. A temperature change of \(10\,^{\circ}\)C caused the signal ratio between the polarization planes to be lower than \(5\,\)dB. Practical measurement confirmed that this value does not cause bit error rate (BER) degradation [25].