This paper investigates the Frequency Modulation Continuous Wave (FMCW) radar sensor for multi-target displacement measurement in Structural Health Monitoring (SHM). The principle of three-dimensional (3-D) displacement measurement of civil infrastructures is analyzed. The requirements of high-accuracy displacement and multi-target identification for the measuring sensors are discussed. The fundamental measuring principle of FMCW radar is presented with rigorous mathematical formulas, and further the multiple-target displacement measurement is analyzed and simulated. In addition, a FMCW radar prototype is designed and fabricated based on an off-the-shelf radar frontend and data acquisition (DAQ) card, and the displacement error induced by phase asynchronism is analyzed. The conducted outdoor experiments verify the feasibility of this sensing method applied to multi-target displacement measurement, and experimental results show that three targets located at different distances can be distinguished simultaneously with millimeter level accuracy.

Aiming at the problems of Doppler-induced error and slow measurement rate in frequency-swept interferometry, a novel dynamic clearance measuring method is proposed. By combining frequency-swept and frequency-fixed interferometry, and using fusion processing of interferometric signals, this method not only can eliminate the Doppler-induced error, but also can give the dynamic clearance value at each sampling point. To verify this method, an all fiber-optic measurement system was constructed, and experiments under 500 Hz triangular and 4 kHz sinusoidal vibrations were carried out. Experimental results show that the dynamic clearance can be reconstructed accurately.

Spectral distortions are complex and unpredictable in Fiber Bragg Grating (FBG) sensing applications. An inappropriate choice of FBG demodulation algorithm will lead to unacceptable results in practice. Although many demodulation algorithms have been developed for distorted spectra, they may be limited for some types of spectral distortion. Until now, no model of FBG spectral distortion has been proposed to test different demodulation algorithms. Here, we fulfill this aim by introducing a comprehensive model with asymmetrical distortion, multi-peaks distortion, interferometric noise, background noise and random noise. Using numerical simulations, we evaluate the performance of eight most typical demodulation algorithms with this model. We show that different algorithms should be used within specific ranges of spectral distortions. Altogether, this study provides a guide map as useful reference for selecting an appropriate FBG demodulation algorithm for practical applications. Furthermore, our proposed model paves the way for developing more reliable FBG demodulation algorithms for different spectral distortions.

We present a single longitudinal mode, diode pumped Nd:YVO₄ microchip laser where a pair of quarter-wave plates (QWPs) sandwich Nd:YVO₄ and the principle axes of QWPs are oriented at 45°to the c-axis of Nd:YVO₄. Three pieces of crystals were optically bonded together as a microchip without adhesive. Owing to large birefringence of Nd:YVO₄, two standing waves with orthogonal polarizations compensate their hole-burning effects with each other, which diminish total spatial hole-burning effects in Nd:YVO₄. The maximum pump power of greater than 25 times the threshold for single longitudinal mode operation has been theoretically shown and experimentally demonstrated. The power of output, slope efficiencies and temperature range of single longitudinal mode operation are greater than 730 mw (at 1.25 W pump), 60% and 30°C, respectively.

The Doppler effect of frequency-swept interferometry (FSI) is often seen as an obstacle to the dynamic ranging accuracy. However, the potential of this obstacle is rarely noticed and used. In this paper, by combining the periodical characteristics of the rotational Doppler effect, an FSI-based multi-parameter measurement method for the rotor is proposed. Through the establishment of the rotational Doppler formula of FSI, it is found that the frequency, direct component, and amplitude of the dynamic distance given by FSI can be utilized to estimate the angular velocity, axial clearance, and tilt angle of the rotor, respectively. A rotor platform and a fiber-optic FSI system were constructed, and a series of experiments were carried out to verify the proposed method. The experimental results showed that the relative errors of the measured axial clearance, angular velocity, and tilt angle were less than 3.5%. This work provides a new perspective on the multi-parameter measurement of the rotor and makes it possible to directly perform multi-parameter measurement inside the space-confined rotating machinery as only a single small-size fiber-optic probe is needed.

A near-infrared spectrometer based on novel MOEMS grating light modulators is proposed. The spectrum detection method that combines a grating light modulator array with a single near-infrared detector has been applied. Firstly, optics theory has been used to analyze the essential principles of the proposed spectroscopic sensor. Secondly, the grating light modulators have been designed and fabricated by micro-machining technology. Finally, the principles of this spectroscopic sensor have been validated and its key parameters have been tested by experiments. The result shows that the spectral resolution is better than 10 nm, the wavelength deviation is less than 1 nm, the deviation of the intensity of peak wavelength is no more than 0.5%, the driving voltage of grating light modulators array device is below 25 V and the response frequency of it is about 5 kHz. With low cost, satisfactory precision, portability and other advantages, the spectrometer should find potential applications in food safety and quality monitoring, pharmaceutical identification and agriculture product quality classification.