We develop a self-adaptive algebraic tomography algorithm (SAATA) to investigate the simultaneous reconstruction of concentration and temperature distributions in larger temperature range from two views. The simplified optical arrangement with fewer projections is realized by extension of spectral information at multiple wavelengths, resulting in great potential in applications of practical combustion diagnosis. The results show SAATA can perform much better reconstructions in 300–3000 K temperature range than genetic simulated annealing algorithm and least-square orthogonal-triangular decomposition method with two-wavelength scheme. More phantoms are created to demonstrate the capability of SAATA to capture the peaks and adapt for both flat and sharp temperature distributions. Meanwhile, the advantage of high stability ensures better reconstruction performance at noise levels from 0.1% to 10% in projections.

We experimentally demonstrate a simple modulation-free scheme for offset locking the frequency of a laser using buffer gas-induced resonance. Our scheme excludes the limitation of low diffraction efficiency and laser input intensity when an acousto-optic modulator is applied to shift the laser frequency from the resonance. We show the stabilization of a strong 795-nm laser detuned up to 550 MHz from the 87Rb 5S1/2F=2->5P1/2 F'=2 transition. The locking range can be modified by controlling the buffer gas pressure. A laser line width of 2 MHz is achieved over 10 min.

An adaptive filter for cancelling noise contained in the direct absorption spectra is reported. This technique takes advantage of the periodical nature of the repetitively scanned spectral signal, and requires no prior knowledge of the detailed properties of noises. An experimental system devised for measuring CH4 is used to test the performance of the filter. The measurement results show that the signal-to-noise (S/N) value is improved by a factor of 2. A higher enhancement factor of the S/N value of 5.4 is obtained through open-air measurement owing to higher distortions of the raw data. In addition, the response time of this filter, which characterizes the real-time detection ability of the system, is nine times shorter than that of a conventional signal averaging solution, under the condition that the filter order is 100.

The continuous monitoring of H2S gas concentration is a common problem in natural gas desulfurization process technology. Tunable diode laser absorption spectroscopy (TDLAS) is a preferred technology for continuous monitoring of gas in industrial sites, because of its high selectivity, high sensitivity, and fast response. We discuss the technical solutions of on-line monitoring of H2S in natural gas desulfurization process technology based on TDLAS, and study the security design of monitoring system in inflammable and explosive areas. We also design a weak photocurrent signal converting circuit and perform experiments on transmission characteristics of different distances. The signal-to-noise ratio (SNR) of laser absorption spectrum does not decrease after the 1 500-m transmission. The detection limit is 300 ppb. The system can be operated stably and reliably, and satisfies the need for continuous monitoring of the H2S in natural gas desulfurization process.

A modified wavelength modulation spectroscopy (WMS) based on the self-heating effect of the tunable diode laser when driven in quasi-continuous-wave (QCW) mode is investigated. A near-infrared distributed feedback (DFB) diode laser working at the QCW mode is employed as the QCW light source, and CO2 is selected as the target gas. The characteristic of the QCW second harmonic (2f) line profile is analyzed through a comparison with that of the traditional CW WMS with the same system. A noise-equivalent absorbance of 3.2×10 5 Hz 1/2 for CO2 at 1.58 \mu m is obtained with 18-m optical path. The QCW WMS lowers the dependence on lasers and expands selectivity, thus verifying the feasibility of the method.

We present a 657-nm external cavity diode laser (ECDL) system, where the output frequency is stabilized by a narrow-band high transmission interference filter. This novel diode laser system emits laser with an instantaneous linewidth of 7 kHz and a broadened linewidth of 432 kHz.

We establish a single diode laser sensor system to obtain temperature and water concentration in CH4/air premixed flame. Line-of-sight properties are analyzed, but line-of-sight results are not path average values for temperature measurements. The measurements are performed on a flat burner based on scannedwavelength direct absorption spectroscopy using two adjacent water lines at 7153.75 and 7154.35 cm?1. Real-time results are acquired using a data acquisition card with a Labview data processing program. The standard uncertainties of the temperature and water concentration measurements are 2.3% and 5.1%, respectively.

Tunable diode laser absorption spectroscopy (TDLAS) is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region. A time-sharing scanning open-path TDLAS system using two near infrared distributed feedback (DFB) tunable diode lasers is designed to detect CH4 and H2S in leakage of natural gas. A low-cost Fresnel lens is used in this system as receiving optics which receives the laser beam reflected by a solid corner cube reflector with a distance of up to about 60 m. High sensitivity is achieved by means of wavelength-modulation spectroscopy with second-harmonic detection. The minimum detection limits of 1.1 ppm.m for CH4 and 15 ppm.m for H2S are demonstrated with a total optical path of 120 m. The simulation monitoring experiment of nature gas leakage was carried out with this system. According to the receiving light efficiency of optical system and detectable minimum light intensity of detection, the detectable optical path of the system can achieve 1-2 km. The sensor is suitable for natural gas leakage monitoring application.