Abstract: The principle of phase modulating optic fiber sensor and some recent demodulation methods are introduced in this paper at first.Then a phase demodulation method designed by Laview and based on the passive and symmetrical demodulation principle used 3×3 coupler are discussed particularly.The result of experiments shows that this method can implement the accurate demodulation from the phase modulated signal.Compared with the traditional demodulation method using hardware,it has the remarkable characteristics ...