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We propose and demonstrate a technique for phase change measurement of a continuous terahertz (THz) wave, based on 1.55- μm photonic techniques. The generation of the THz wave is based on photomixing using a uni-traveling-carrier photodiode. The detection of the THz wave is based on heterodyne mixing of the THz wave, using a low-temperature-grown GaAs photoconductor with frequency-shifted photonic local oscillator signal. The measurement of a phase shift induced by a single sheet of paper with 85-μm thickness has been demonstrated at 300 GHz.