Figure 1 (IMAGE) Compuscript Ltd Caption The setup of DAS-Φ-OTDR systems with different demodulation methods. (a) heterodyne detection and I/Q phase demodulation; (b) heterodyne detection and Hilbert transform phase demodulation; (c) direct detection and phase demodulation based on 3×3 coupler; (d) direct detection and phase demodulation based on phase generated carrier algorithm Credit OEA Usage Restrictions Credit must be given to the creator. License CC BY Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.