## 4 Experimental Results  
The results of measurement P\({}_{\text{out}}\)(\(t\)) for Y-splitter during temperature cycling of the sample and the action of the pyroelectric effect are presented in Figures 3 and 4, where the red and black curves correspond to the two arms of the Y-splitter, and the gray broken line corresponds to the temperature in the temperature chamber during the experiment. The range of temperature changes was 140 \({}^{\circ}\)C. Because of the long thermocycle we showed only the most important part of signal changing and \(X\)-axis start not from zero.  
Figure 3: The change in the output signal of the Y-splitter under the action of the pyroelectric effect. Black and red lines are P\({}_{\text{out1}}\) and P\({}_{\text{out2}}\) respectively, gray line is temperature. The rate of temperature increase is 0.16 \({}^{\circ}\)C/min, the lowest sample temperature is about \(-\)55 \({}^{\circ}\)C.  
Figure 2: Scheme of the experimental assembly for optical measurements.  
As can be seen from the presented figure, a sharp drop in the output optical power (-40 dBm) is observed in the heating areas on one arm of the Y-splitter. During subsequent heating, the P\({}_{\text{out}}\) value is restored, and this behavior is typical for all heating cycles. The cooling of the sample does not give a similar effect, and the analysis of the graph does not allow us to identify a stable temperature at which P\({}_{\text{out}}\) value decreases.  
The power decrease for such a heating rate is characteristic of only one arm of the Y-splitter. At the same time, a small power output variation is observed in the second arm. The considered power drop can be conditionally classified into three sections: A-B is a sharp increase in optical loss (\"disappearance\" of the waveguide), B-C is the absence of the waveguide and C-D is the waveguide restoration. The length of the section _A-B_ in the presented figure is 24 min, the length of B-C is 5.5 h and the length of C-D is about 2.5 h. The signal power in the section B-C (-49 dBm) corresponds to the value that would be observed in the absence of a waveguide channel, but in the presence of input radiation.  
For higher heating rates, similar phenomena are observed, accompanied by changes in the signal of the second arm of the Y-splitter (Figure 4). The heating rate is 5.3 and 2.5 K/min.  
The condition for the waveguide \"disappearance\" was also investigated from the point of view of the starting temperature used for sample heating. For this purpose, a thermal cycle was proposed in which the sample was heated at different speeds under the influence of temperatures of minus 40 \({}^{\circ}\)C, minus 20 \({}^{\circ}\)C and 0 \({}^{\circ}\)C. The heating rates varied during the thermal cycle, but the temperature drop remained constant and was equal to 60 \({}^{\circ}\)C. The results show that the depth of P\({}_{\text{out}}\) degradation heavily depends on the temperature at which the sample was heated. Short-term, but complete \"disappearance\" of both waveguides is observed at an initial temperature of minus 40 \({}^{\circ}\)C. With an increase in the initial heating temperature, this phenomenon weakens and completely disappears when the sample is heated at the temperature of 0 \({}^{\circ}\)C. For this reason, the phenomena we discovered may not have been observed previously by other researchers, since the used parameters of the thermal cycle (low initial temperature, high heating rate and large temperature differential) are not typical for testing of optical integrated circuits.  
Figure 4: P\({}_{\text{out}}\) degradation upon heating at a rate of 5.3 and 2.5 \({}^{\circ}\)C/min. Black and red lines are P\({}_{\text{out1}}\) and P\({}_{\text{out2}}\) respectively, gray line is temperature.  
The sample covered by conductive paste was measured to make sure that \(P_{\text{out}}\) changes are due to pyroelectric effect. Signal changes are about 1 dBm and optical connections have good stability (Figure 5).