The laser diode (LD) is modulated by the injection current of triangular waveform and a photo diode (PD) is packaged in the rear of the LD. The laser reflected by the target re-enters the cavity of LD and contains the target’s displacement information. The information is carried within the laser intensity and can be pickup by the PD. We call this laser intensity as self-mixing interferometry (SMI) signal. While processing the sensing SMI signals, we should carefully determine the windowing function and reduce the effect of windowing in the FFT and IFFT process while applying the mathematical model. Simulation results show that the proposed design is able to accomplish the measurement of micro-displacement with high resolution and great accuracy.
When a part of light emitted by a laser is back-reflected or back-scattered from an external target and re-enters the laser cavity, both the laser intensity and its wavelength can be modulated. This is so-called self-mixing effect (SME), the optical feedback interferometry (OFI) utilizes such effect in an LD developed various applications. In this paper, we use a dualcavity OFI system that operating in period one state, the laser intensity from this system exhibits an oscillation with its amplitude modulated by a traditional single cavity OFI signal. The dual-cavity OFI system has the same measurement resolution as the single cavity which is half laser wavelength. This paper developed a method to improve the resolution by using fringe subdivision. Our simulation result shows that this method can achieve subnanometer resolution.
When a fraction of external optical feedback re-enters inside cavity of a laser diode (LD), the laser intensity and its wavelength will thus be altered. The LD in this case is often called as a self-mixing laser diode (SMLD). This paper presents an SMLD for profile measurement. The LD is modulated by the injection current in triangular waveform and a target to be measured is installed on a mechanic scanning device. The reflection light by the target contains its surface profile. The profile information is then carried in the laser intensity and can be pickup by a photodiode packaged in the rear of the LD. We call this modulated laser intensity as self-mixing interferometric (SMI) signal. In this paper, a new algorithm is developed to retrieve the profile from the SMI signal. Results show that the proposed design is able to achieve the measurement of profile with high resolution.
Self-mixing interferometry (SMI) is a well-developed sensing technology. An SMI system can be described using a model derived from the well-known Lang and Kobayashi equations by setting the system operating in stable region. The features of an SMI signal are determined by the external optical feedback factor (denoted by C). Our recent work shows that when the factor C increases to a certain value, e.g. in moderate feedback regime with 1<C<4.6, the SMI system might enter unstable region and the existing SMI model is invalid. In this case, the SMI signals exhibit some novel features and contain higher-frequency components. To detect an SMI signal without distortion or take suitable correction on the SMI signal, it is must to make an analysis on the system bandwidth and its influence on SMI signals. The results in this paper provide useful guidance for developing an SMI sensing system.