This paper presents a novel ground surface seismic source location method based on time difference of arrival (TDOA) and local mean decomposition (LMD). The wavelet packet filter is used for environmental noise reduction of seismic signal. Because of the non-stationary and randomness of seismic signal, LMD is applied to analyze seismic signal. The production function (PF) components can be obtained after the local mean decomposition to the seismic signal. Then, the principal PF component is selected based on the cross correlation coefficients. The instantaneous frequency distribution of principal PF component can be acquired by taking a derivative of pure frequency modulated signal with respect to time. In frequency spectrum graph, the frequency corresponding to the maximum of amplitude is selected as characteristic frequency. Finally, the time difference of arrival can be got according to the moment of characteristic frequency first appearance in instantaneous frequency distribution. The results of experiment show that the proposed method is effective.
Rare earth ions doped glasses producing visible upconversion emissions are of great interest due to their potential applications in the photonics filed. In fact, practical application of upconversion emissions has been used to obtain color image displays and white light sources. However, there are few reports on the thermal effect on tuning the emission color of the RE doped materials. In this work, the Er<sup>3+</sup>/Tm<sup>3+</sup>/Yb<sup>3+</sup> tridoped oxyfluoride glasses were prepared through high temperature solid-state method. Under a 980 nm diode laser excitation, the upconversion emissions from the samples were studied. At room-temperature, bright white luminescence, whose CIE chromaticity coordinate was about (0.28, 0.31), can be obtained when the excitation power was 120 mW. The emission color was changed by varying the intensity ratios between RGB bands, which are strongly dependent on the rare earth ions concentration. The temperature dependent color emissions were also investigated. As temperature increased, the intensities for the emission bands presented different decay rates, finally resulting in the changing of the CIE coordinate. When the temperature was 573 K, white light with color coordinate of (0.31, 0.33) was achieved, which matches well with the white reference (0.33, 0.33). The color tunability, high quality of white light and intense emission intensity make the transparent oxyfluoride glasses excellent candidates for applications in solid-state lighting.
A novel fibre-optic methane detection system was proposed, which involved sampled fibre grating and improved
differential absorption detection technique. By this method, near-infrared equal-spaced multi absorption line of methane
was detected simultaneously, and that gas weak absorption detection theory was developed. Using the comb shaped filter
characteristic of sampled fibre grating, R2, R3 and R4 line of molecule absorption spectrum in 2ν 3 overtone band
around 1.66μm was measured at one time. Two sampled fibre gratings of same type were used to fulfill the task of
difference absorption detection. One sampled grating worked as measure grating with its reflection spectrum
corresponding to the absorption line. The other grating worked as reference grating with its reflection spectrum deviate
from that of measure grating to some extent. Chirped fibre grating with its central wavelength around R3 was adopted as
optical band-pass filter. The light power of the three absorption line and the three reference wavelength was detected
alternately by PIN PD at the same time. So that difference absorption detection was achieved. The effect of light source
instability was avoided by ratio treatment. The validity of the system was verified by experiments.