Previous studies on the use of photoacoustic spectroscopy (PAS) techniques in the measurement of CH4 concentrations have been mainly limited to a single point measurement using only one PAS cell. A single point measurement will not satisfy the detection need, for instance in a natural gas transmission pipeline or an underground coal mine, gas explosion may occur at any point of the pipe or tunnel. Therefore, simultaneous multi-point measurements are required to obtain the gas distribution profile along the tunnel. Here we proposed and demonstrated a multi-point photoacoustic spectroscopy gas detection system based on the power compensation technique and fixed wavelength technique for CH4 detection. In the multi-point photoacoustic spectroscopy gas detection system, the gas absorption of the first PAS cell would make the measurement of the second PAS cell inaccurate. Therefore, we proposed a power compensation technique to compensate the absorbed laser power. In addition, a simple fixed wavelength technique was used in multi-point photoacoustic spectroscopy gas detection system to improve the signal to noise ratio (SNR). Finally, the experiment results showed that the two PAS cells achieved the minimum detection limit of 7.72 ppmv and 5.14 ppmv, respectively.
KEYWORDS: Absorption, Modulation, Temperature metrology, Signal to noise ratio, Wavelength tuning, Dielectrics, Absorption filters, Directed energy weapons, Absorption spectroscopy, Data processing
Two measuring methods of wide absorption spectrum by DFB-LDs are presented in detecting water vapor absorption line. One is subsection scanning method, it takes advantage of wide spectrum tuning range by temperature modulation and fast spectrum tuning speed by current modulation, specifically, this method is realized by dividing a target spectral region into several sections which corresponding to specific temperature of DFB-LD, and scanning every section by current modulation for hundreds times and average the data to raise SNR, combining all sections to get the whole spectrum. An accuracy of 10 ppmv had been obtained in the measurement of water vapor with a 10-cm path length by this method. Another is data fitting method, based on absorption line-shape function, the absorption line can be described by fitting with partial measured data. The fitting absorption line is fitted well with the measured data, and the square of correlation coefficient (R-square) is no less than 0.99.
A novel temperature demodulation method which eliminates the impact of Rayleigh scattering on Raman distributed temperature sensors (RDTS) using anti-Stokes light only is presented. This method utilizes two sections of reference fiber which are placed into temperature control chambers with different temperatures, such that the impact caused by the variation of laser’ power and the Rayleigh scattering is eliminated by the two reference temperatures. In the experiment, the temperature error caused by the Rayleigh scattering was decreased by 0.6℃ and 1.7℃ at 30℃ and 50℃compared with conventional method respectively.
KEYWORDS: Signal to noise ratio, Modulation, Absorption, Signal processing, Collimators, Interference (communication), Fiber optics sensors, Gas sensors, Linear filtering, Fiber optics
Noise from optical and electronic components of a fiber optic gas sensor system using wavelength modulation of the
DFB laser diode in either transmission or reflection mode were investigated. Our experimental results indicate that
reflective type cells give poorer performance due to interference effects from connectors and joints within the fiber
system compare to transmissive type cell. Intensity noise from optical coupler, collimator, was measured and its
affection to sensing system was discussed. In order to raise the signal to noise ratio (SNR), signal processing methods,
such as data average, low pass filter were used and compared. The results would be a useful engineering tool to design high SNR optical gas sensing system.
KTP planar optical waveguide was fabricated by combining He+ ion implantation with Rb-K ion exchange in
pure RbNO3. The lattice damage induced by He+ ion implantation and influence of ion implantation on ion exchange
waveguide were studied. TRIM'2003 code was used to simulate the process of 2.0 MeV He+ ion implantation, the
profile of He+ ions concentration as well as the defective lattice versus depth were estimated. The dark mode spectra
of the waveguide were measured by the prism-coupling method at wavelength 633 and 1539 nm. The refractive
index profiles in the novel waveguide were reconstructed by analyzing dark mode spectra and an increased index in
the waveguide region together with a decreased index barrier in damaged layer was obtained. The influence of
irradiation damage on the Rb distribution was investigated and analyzed by means of RBS technique.
Based on the analysis method of the light propagation in isotropic absorption media, the vector propagation constant is introduced and the light propagation in the biaxial absorption crystal is analyzed. The representations of some important physical parameters are derived, which was used to describe the crystal property and light propagation property, such as angle of refraction, refractive index, absorption coefficient. The corresponding results of transparent crystal can be deduced from these representations. When the crystal is absorptive, the reflection and transmission coefficients derived from the vector propagation constant method are in concordance with the results of complex refractive index method. So these two methods are uniform in some aspects, but the method of vector propagation constant is more convenient and available.
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