Laser-induced fluorescence (LIF) spectra of a bush and numerous branches of Chinese Longjing tea were investigated remotely with lidar techniques. The intensity ratio between the far red fluorescence (FRF) and red fluorescence (RF) due to the chlorophyll content of the tea branches were analyzed to study the growth conditions in different villages around Hangzhou, China. Dried Longjing tea leaves were also measured by LIF techniques in the laboratory. A chemometric method based on singular value decomposition (SVD) and linear discriminant analysis (LDA) was used to evaluate the tea qualities of the dried tea leaves.
A novel interrogation scheme for multiplexing the sensors based on long-period grating (LPG) Mach-Zehnder interferometers (MZIs) is proposed. Each LPG-MZI is formed by a pair of cascaded identical LPGs and induces an additional optical path difference (OPD) proportional to the centre-to-centre interval between the two LPGs. A sub-reflectogram including sensing information can be achieved when the OPD induced in one LPG-MZI is compensated by scanning one arm of a Michelson interferometer to a certain range. For multiple sensors, the grating intervals of different LPG-MZIs are set to different values to ensure that their sub-reflectograms can be well separated in time domain. The spectral response of the LPG-MZI is reconstructed from the corresponding sub-reflectogram with a fast Fourier transformation. Applications of the bending and temperature monitoring are demonstrated by measuring the signals in time domain and wavelength domain, respectively.
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