## 1 Introduction  
Integrated-optical circuits based on proton-exchange channel waveguides are the basis of integrated-optical modulators (IOMs) of radiation used in fiber-optic communication lines and are also the heart of navigation systems based on fiber-optic gyroscopes [1]. In addition, such modulators are used in quantum cryptography for the transmission of quantum keys [2] and in single-photon detectors [3]. At the moment, the integrated optical circuits based on silicon [4], InP [5] and thin-film lithium niobate (LN) [6] are most actively developing. However, traditional diffusion technologies for the formation of waveguides in LN are the basis for commercial integrated-optical circuits, especially in the field of fiber-optic gyroscopes and navigation systems [7]. In addition, proton-exchange waveguides in LN are used for the creation of new types of compact gyroscopes [8]. This is due to both the relative simplicity of the process of forming waveguides, and their extremely high polarizing properties [9]. Despite the fact that proton-exchange waveguides are low-contrast and do not fit well with the modern ideology of integral photonics (which requires a high density of elements), low optical losses and a high operating modulation frequency (up to 100 GHz) prompt researchers to look for new ways to form more compact devices based on proton exchange technology in LN [10]. The undoubted importance of these IOMs applications requires their stable operation in a wide temperature range, typically from minus 60 to plus 80 \({}^{\circ}\)C. The study [11] shows that heating of the LN integrated-optical circuit, pre-cooled to low temperatures, can lead to a significant increase in optical losses in the channel or even complete cessation of radiation channeling. This phenomenon is most dangerous when using IOMs as part of a fiber-optic gyroscope since the conditions of \"switching off\" the waveguides may well be reproduced during actual system operation. The essence of the phenomenon under study is that the action of the pyroelectric effect in a lithium niobate crystal results in a redistribution of the charged defects in the crystal lattice, leading to a local decrease \(\Delta\)n--the refractive index of optical waveguides with respect to the surrounding crystal. In some cases, the \(\Delta\)n value for waveguides may become so small that the conditions for channeling of the fundamental mode of radiation stop being fulfilled. This phenomenon, described theoretically in [12], was not observed in the experimental works before [11] due to relatively rare conditions of the experiment --cooling the sample to low temperatures and its rapid heating. Usually, IOMs are not tested under such conditions; for most commercially available IOMs, the operating temperature range declared is from 0 to 80 \({}^{\circ}\)C. Additionally, pyroelectric influence to proton exchanged waveguides was studied in [13] and it was shown that electrodes play huge role in changing waveguide optical properties. The total the optical power at the IOM output only is analyzed in [11]. However, neither longitudinal character of losses (local or distributed) nor their localization within OIM length was determined. The present work aims to answer these questions and investigate in more detail the processes occurring inside the waveguide during a change in the crystal temperature. Several experimental approaches were used to solve the task. In the first stage, the IOM transmittance and their temperature dependence were analyzed. To study the backscattered signal, we used the distributed methods based on coherent frequency domain reflectometry (OFDR), which, in contrast to classical time-domain reflectometry, allows one to obtain micron spatial resolution [14]. Its principle is based on the spectral analysis of the interference signal arising from mixing the probing and scattered radiation. This analysis can be performed by sweeping the optical frequency of probing radiation.