A technique which displays the fractional order of optical interference by the azimuthal angle of radial arm has been improved by using a digital electronic circuit such as phase‐locked loop and D flip–flop. The phase quadrature reference signals of this system are derived by reforming a reference signal and shifting it by a quarter wavelength referring to its waveform. As the result the orthogonal phase relation of the two signals is not affected by the frequency of the signal. This system has been proven to operate properly over the frequency range of 200–600 kHz without readjusting the electric system.