An enhanced laser self-mixing Doppler velocimeter by fiber Bragg grating (FBG) is proposed. In fiber sensing, FBG is often used as an optical filter to select the wavelength of the light using the narrow reflection spectrum. The Doppler frequency shift from the rotating target can be parsed in the self-mixing interferometry (SMI) signal. The frequency-fluctuant self-mixing signal is transformed into intensity variations by a FBG when the laser wavelength is set to the edge of the FBG steep transmission profile. An experimental comparison between the enhanced and the traditional SMI approaches is made. The magnitude of obtained FBG-enhanced SMI signal is about 40 times stronger than that from traditional SMI. The result of the experiments indicates that enhanced self-mixing Doppler velocimetry by FBG can be applied to velocity sensing. This method has potential application in the engineering industry because the FBG is easy to fabricate and connect to a SMI system and the wavelength of FBG can be changed to match the different lasers for an enhanced SMI signal.
An advanced sinusoidal phase-shifting technique and a time-domain phase demodulation method were used to improve the measurement accuracy and realize the real-time measurement speed of the laser self-mixing interferometer in a large range of displacement. An electro-optic crystal modulator (EOM) was used to realize the sinusoidal phase-shifting on the laser beam in the external cavity. The interference signal was demodulated using a time-domain phase demodulation method. The speed requirement could be met by combining the two together in a wide range of displacement measurement processes together with the real-time measurement requirement as an interferometer at the same time. It was experimentally verified that the displacement measurement precision of a sinusoidal phase-shifting laser self-mixing interferometer could reach less than 0.5 μm in the hundred mm large-scale displacement measuring process. In addition, the factors affecting the interferometer’s measurement speed in the real-time displacement measurement process is analyzed and the maximum speed of our system was obtained as well. Keywords: self-mixing interference; phase modulation; time-domain phase demodulation