Signal waveforms of low frequency vibration of external cavity reflective surface were demodulated from self-mixing interference signals that external cavity length was modulated by a high frequency signal. In order to realize high frequency modulation of external cavity length against the low frequency vibration, a scheme of laser source sinusoidal vibration was proposed, in which a laser diode module was mounted on an electromagnetic vibrator with the length and diameter of 23 mm and 13 mm, respectively. The vibrator excited by a sinusoidal signal drives laser source vibration which amplitude and frequency was about a quarter wavelength and 7 kHz respectively. Using Fourier transform and phase unwrapped means, the low frequency vibration signal waveforms were demodulated from the self-mixing interference signals by digital signal process. The amplitude of the smallest waveform ably resolved was about 0.02 wavelengths. Light density or polarization direction of the laser source unchanged with modulation signal, which was superior to a phase modulator plugged in external cavity. From experimental measurement, it was sure that laser source of sinusoidal vibration can realize modulation of external cavity in self-mixing interference.
