Simultaneous detection of multiple-gas species has for the first time (to our knowledge) been demonstrated by using a multimode diode-laser-based correlation spectroscopy (MDL-COSPEC) scheme. Concentration measurements of a mixture of CO2 and CO gas were performed by probing overlapping line spectra around 1.57 using an MDL. Species identification and corresponding quantitive analysis were implemented by correlating the recorded absorption signals of the sample gas mixtures with those of the reference gases of particular interest, attaining accuracies of 2% and 1%, respectively. MDL-COSPEC is a generic technique with potential application for simultaneous detection of multiple gases having resolvable narrow lines.
We demonstrate a method for elemental mercury detection based on correlation spectroscopy employing UV laser radiation generated by sum-frequency mixing of two visible multimode diode lasers. Resonance matching of the multimode UV laser is achieved in a wide wavelength range and with good tolerance for various operating conditions. Large mode-hops provide an off-resonance baseline, eliminating interferences from other gas species with broadband absorption. A sensitivity of 1 mu g/m(3) is obtained for a 1-m path length and 30-s integration time. The performance of the system shows promise for mercury monitoring in industrial applications.
Temperature-corrected oxygen measurements were performed by using multi-mode diode laser correlation spectroscopy at temperatures ranging between 300 and 473 K. The experiments simulate in situ monitoring of oxygen in coal-combustion exhaust gases at the tail of the flue. A linear relationship with a correlation coefficient of -0.999 was found between the evaluated concentration and the gas temperature. Temperature effects were either auto-corrected by keeping the reference gas at the same conditions as the sample gas, or rectified by using a predetermined effective temperature-correction coefficient calibrated for a range of absorption wavelengths. Relative standard deviations of the temperature-correction coefficient calibrated for a range of absorption wavelengths. Relative standard deviations of the temperature-corrected oxygen concentrations obtained by different schemes and at various temperatures were estimated, yielding a measurement precision of 0.6%.
Diode laser absorption spectroscopy was utilized for non-intrusive assessment of gas content in human body cavities, including intestines and lungs of a new-born, the mastoid bone, and sinus cavities for monitoring sinusitis recovery in adults.
The fluorescence of different types of planthopper (Hemiptera) and moth (Lepidoptera), which constitute important Chinese agricultural pests, was investigated both in situ in a laboratory setting and remotely using a fluorescence light detection and ranging (lidar) system operating at a range of about 50 m. The natural autofluorescence of different species, as well as the fluorescence from insects that had been dusted with fluorescent dye powder for identification were studied. Autofluorescence spectra of both moths and planthoppers show a maximum intensity peak around 450 nm. Bleaching upon long-time laser illumination was modest and did not affect the shape of the spectrum. A single dyed rice planthopper, a few mm in size, could be detected at 50 m distance by using the fluorescence lidar system. By employing various marking dyes, different types of agricultural pest could be determined. We suggest that lidar may be used in studies of migration and movement of pest insects, including studies of their behavior in the vicinity of pheromone traps and in pheromone-treated fields.
The frequency-modulated continuous-wave (FM-CW) technique, based on the beat signal of a Mach-Zehnder interferometer employing a frequency-ramped light source, is studied for solid scattering media applications. The method is used to evaluate the mean time-of-flight (MTOF) of light traveling in scattering media, specifically polystyrene foams. We assume that each the time-of-flight (TOF) time corresponds to different light scattering paths resulting in a different phase shift. The phase shift variations produce a speckle pattern, which together with the frequency leakage induced by the discrete Fourier transform (DFT) cause "spikes" in the power spectrum of the beat signal, thus decreasing the accuracy of the measured MTOF values in solid scattering media. For comparison, time-of-flight spectroscopy (TOFS) is also employed to evaluate the MTOF for the same samples, while the geometrical difference between these two techniques is compensated for by using diffusion theory. The MTOFs measured by the FMCW and TOFS techniques agree well, which demonstrates a great potential to develop a robust FMCW setup for simplified MTOF assessment.
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.
Simultaneous assessment of the spectroscopic absorption signal of gas enclosed in a scattering medium and the corresponding optical path length of the probing light is demonstrated using a single setup. Sensitive gas absorption measurements are performed by a tunable diode laser using wavelength-modulation spectroscopy, while the path length is evaluated by the frequency-modulated cw technique commonly used in the field of telecommunication. Proof-of-principle measurements are demonstrated with water vapor as the absorbing gas and using polystyrene foam as an inhomogeneously scattering medium. The combination of these techniques opens up new possibilities for straightforward evaluation of gas presence and exchange in scattering media.
A method to assess the relative optical porosity in wood by a combination of gas in scattering media absorption spectroscopy (GASMAS) and frequency domain photon migration (FDPM) is presented. Samples of balsa and pine wood are studied.
Laser-induced fluorescence was used to evaluate the classification and quality of Chinese oolong teas and jasmine teas. The fluorescence of four different types of Chinese oolong teas-Guangdong oolong, North Fujian oolong, South Fujian oolong, and Taiwan oolong was recorded and singular value decomposition was used to describe the autofluoresence of the tea samples. Linear discriminant analysis was used to train a predictive chemometric model and a leave-one-out methodology was used to classify the types and evaluate the quality of the tea samples. The predicted classification of the oolong teas and the grade of the jasmine teas were estimated using this method. The agreement between the grades evaluated by the tea experts and by the chemometric model shows the potential of this technique to be used for practical assessment of tea grades.
A combination method of frequency domain photon migration (FDPM) and gas in scattering media absorption spectroscopy (GASMAS) is used for assessment of the mean optical path length (MOPL) and the gas absorption in gas-filled porous media, respectively. Polystyrene (PS) foams, with extremely high physical porosity, are utilized as sample materials for proof-of-principle demonstration. The optical porosity, defined as the ratio between the path length through the pores and the path length through the medium, is evaluated in PS foam and found consistent with the measured physical porosity. The method was also utilized for the study of balsa and spruce wood samples.
Frequency-modulated light field fluctuations due to moving particles in colloidal suspensions are examined using heterodyne interferometry. The power spectrum is the combined result of a time-of-flight-related frequency distribution due to light scattering and frequency shifts due to the Doppler effect. An approximation model is developed based on diffusion theory and verified experimentally. The potential for application towards comprehensive diagnosis of both particle dynamics and optical properties of the examined media is discussed.
A correlation spectroscopy (COSPEC) based on a multi-wavelength fiber laser is first proposed for the detection of gas concentration. The lasing wavelengths are selected to match several characteristic absorption peaks of the gas under test, and the gas concentration is easily measured by correlating it with the reference gas. The present method is immune from the instability of the light source and the influence of other gases. The concentration measurement of C2H2 is demonstrated in the experiment in its near-infrared dominant absorption region. The technique has prospects for simultaneous detection of multiple gases, and the measurement of mixed gases of C2H2 and CO2 is also analyzed.
A novel hybrid plasmonic waveguide with nano-scale confinement, consisting of a metal layer separated from a SOI nano-rib by a thin silica layer has been realized. The loss of 0.01dB/μm allows for ultra-high density photonic integration.
A novel hybrid plasmonic waveguide with nano-scale confinement, consisting of a metal layer separated from a SOI nano-rib by a thin silica layer has been realized. The loss of 0.01dB/mu m allows for ultra-high density photonic integration.
An optical sensor based on differential absorption spectroscopy for real-time monitoring of industrial nitric oxide (NO) gas emission is described. The influence of gas absorption interference from sulfur dioxide (SO2) in the environment was considered and a spectral separation technique was developed in order to eliminate this interference effect. The absorption spectrum of SO2 around 226 nm was evaluated by the SO2 concentration obtained using the experimentally recorded absorption spectrum around 300 nm. The absorption spectrum of NO around 226 nm was obtained by subtracting the absorption of SO2 from the integral absorption spectrum of SO2 and NO. The concentration measurements were performed at atmospheric pressure. The technique was found to have a lower detection limit of 0.8 ppm for NO per meter path length (SNR=2) and be immune from the influence from SO2 on the NO measurement. The sensor based on this technique was successfully employed for in situ measurement of SO2 and NO concentrations in the flue gas emitted from an industrial coal-fired boiler.