Distributed acoustic sensing (DAS) systems based on coherent optical time-domain reflectometry (COTDR) can obtain vibration information along the fiber length by demodulating the phase information from the backscattered Rayleigh signal in optical fibers. The phase demodulation algorithm is a key technology for phase extraction in DAS systems, and different algorithms exert varying effects on the demodulation results.This study investigates the impacts of two in-phase quadrature (IQ) demodulation algorithms, i.e.,differential cross-multiplying (DCM) and arctangent (Arctan), on the demodulation results. First, a theoretical analysis and numerical simulations based on DCM and Arctan are conducted. Subsequently, a COTDR-based DAS system is constructed to collect data, which are processed and analyzed using both algorithms. Experimental results indicate that the peak values of the demodulated signals obtained using the two algorithms differ by 0.04 rad, with an observed phase shift of 0.01 rad. In addition, based on the different characteristics of the two algorithms, recommendations for their application scenarios are provided.

Accurate measurement of temperature variations during the pouring and curing of concrete is of great importance for investigating concrete cracking during the construction of the Yellow River Diversion Project in the Yellow River Basin. Notably, the current construction projects involved in the Yellow River Diversion Project in the Yellow River Basin primarily use thermal imaging methods for surface temperature measurement, making it difficult to obtain real-time overall temperature characteristics of concrete. Therefore, to improve the ecological system of the Yellow River Basin and ensure high-quality development in the region, this research proposes a concrete-temperature monitoring method for the Yellow River Diversion Project based on implantable distributed fiber optic sensing;this method has been successfully used in the Yellow River Diversion Project in Shandong Province. Before concrete is poured, the sensing fiber opthic cable is prelaid on both sides of the involved structural-steel bars, enabling real-time and effective monitoring of the internal temperatures during the pouring and curing of concrete. This method is of great value for ensuring real-time analysis of the temperature of concrete throughout its lifecycle. Experimental results show that the use of implantable distributed fiber optic sensing allows real-time monitoring of temperature changes at various points in concrete and temperature anomalies during curing. Moreover, this method can be used to effectively analyze the overall temperature variation during curing. Using implantable distributed fiber optic sensing to monitor the concrete temperature for the Yellow River Diversion Project is of considerable reference value for similar engineering projects in the Yellow River Basin.