There are currently lots of research activities concerning explosive hazards detection, and stand-off detection of explosives is in main focus. The reason for this interest is the occurrence of terrorist attacks on the civilian society involving Improvised Explosives Devices (IED). Laser-based spectroscopies are the only currently viable techniques that can be utilized to detect trace amounts of explosives at stand-off distances. In particular, Raman Spectroscopy (RS) has been shown to be effective for stand-off detection and has the ability to both detect and identify explosive materials. Raman spectroscopy is virtually instantaneous, non-destructive in nature and provides high selectivity. The traditional Raman spectrometer utilizes continuous lasers and CCDs to detection the scattering signal, which greatly limits the application of Raman spectroscopy in the stand-off detection of explosive hazards due to the weak signal, strong background fluorescence, ambient light interference, and long analysis time. Time-gated Raman spectroscopies are based on ultrafast pulsed lasers and time-resolved single-photon detection techniques. Through the time-gated method, the Raman signal intensity can be greatly improved, and the influence of fluorescence and environmental light can be effectively suppressed. In this work, the time-gated Raman system utilizing frequency-doubled Nd:YAG lasers at 532 nm excitation was developed. The Cassegrain telescope was coupled to the Raman spectrometer using a fiber optics cable, and notch filter was used to reject Rayleigh scattering light. The Raman scattered light was collected by a telescope and then transferred via fiber optic to spectrometer and finally directed into Intensified CMOS (ICMOS) detector. The applications of time-gated Raman spectroscopy in stand-off detection of hazardous explosives have been performed. The Raman spectra of DNT, TNT, RDX and NaNO3 at a stand-off distance of 50 m have been identified with a detection limit of 1 mg/cm2.
With the increase of output power, more heat generation and higher operation current have become important issues, which affect the electrical-optical performance and reliability of high power semiconductor lasers. For the past several years, high power semiconductor laser chips utilizing double or triple quantum wells have been developed to achieve higher output power. However, the operation current of diode laser chips with double or triple quantum wells is much higher than that with single quantum well. Diode laser chips with double or triple quantum wells could only operate at a much lower duty cycle. In this paper, a compact quasi-continuous wave (QCW) high power semiconductor laser array based on dual-chip integration techniques has been developed. For this packaging structure, two diode laser bars were welded above and below a micro-channel heat sink, without significant increase in volume. By means of this integration method, the output power of the semiconductor laser could reach kilowatt-level at a lower operation current. The thermal behavior of the semiconductor laser array with different operation parameters was carried out using finite element method. The structure parameters of semiconductor laser array based on dual-chip integration were optimized and characterized. The output power is 1485 W operated at a current of 700 A and the maximum electro-optical efficiency is 75%, which is the record-high level for a high power semiconductor laser array.
Improvised explosive devices (IED) and homemade explosives (HMEs) have become the preferred choice for terrorists and insurgents. It’s a challenge to develop the techniques to detect explosive hazards at standoff distances. In this paper, a standoff UV Raman spectrum detection system for explosive detection was developed, which can realize 2-10m Raman spectrum detection of solid, solution and trace potassium nitrate samples. The relationship between Raman signal intensity (RSI) and pulse energy, detection distance and sample concentration was studied. The experimental results show that the RSI is approximately proportional to the pluse energy and contains nonlinear terms. It has an inverse square relationship with the detection distance and a linear relationship with the sample concentration. The concentration of solution and trace potassium nitrate samples of were successfully predicted at 2m distance, and the root mean square error of prediction (RMSEP) was 11.7 and 6.1, respectively.A simple and effective method for preparing trace potassium nitrate is presented.
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