ABSTRACT
A continuous-wave mid-infrared radiation from difference frequency generation by mixing a continuous-wave Ti: sapphire laser and a continuous-wave YAG laser in a 15 mm long BaGa4Se7 crystal is demonstrated for the first time. The tunable range from 3.15 to 7.92 µm was achieved by rotating the crystal to fulfill the type I phase-matching condition. A maximum DFG power of 1.41 µW was obtained at 5 µm. Meanwhile the experimental DFG power conversion efficiency was 20.2 µW/W2, with a length-normalized slope efficiency of 15.5 µW/cmW2. The conversion efficiency decreases rapidly from 50 µW/cmW2 at 3.15 µm to 1 µW/cmW2 at 7.92 µm. The wavelength acceptance bandwidth and the angular acceptance bandwidth were measured to be 16.4 cm-1 and 44' for DFG at 5.1 µm, respectively.
ABSTRACT
In this paper, first time as our knowledge, we describe the development and performance evaluation of a 4.4 µm external cavity quantum cascade laser based laser heterodyne radiometer. Laser heterodyne spectroscopy is a high sensitive laser spectroscopy technique which offers the potential to develop a compact ground or satellite based radiometer for Earth observation and astronomy. An external cavity quantum cascade laser operating at 4. 4 µm, with output power up to 180 mW and narrow line width was used as a local oscillation. The external cavity quantum cascade laser offers wide spectral tuning range, it is tunable from 4.38 to 4.52 µm with model hop free and can be used for simultaneous detections of CO2, CO and N2 O. A blackbody was used as a signal radiation source. Development and fundamental theory of Laser heterodyne spectroscopy was described. The performance of the developed Laser heterodyne radiometer was evaluated by measuring of CO2 spectral at different pressures. Analyses results showed that a signal-to-noise ratio of 86 was achieved which was less than the theoretical value of 287. The spectral resolution of the developed Laser heterodyne spectroscopy is about 0.007 8 cm(-1) which could meet the requirement of high resolution spectroscopy measurement in the case of Doppler linewidth. The experiment showed that middle Infrared laser heterodyne spectroscopy system had high signal-to-noise ratio and spectral resolution, and had broad application prospect in high precision measurement of atmospheric greenhouse gas concentration and vertical profile.
ABSTRACT
The accuracy of absorption spectral parameters is very important for the trace gas measurement based on absorption spectroscopy techniques, especially for the isotopic abundance measurement of gas molecules. For some of the applications, spectral parameters listed in HITRAN database were used to retrieve the trace gas concentration. However, these parameters have uncertainty, in order to validate spectroscopic parameters near 2.0 µm of CO2 lines, which are to be used in detecting the CO2 concentration and isotopic abundance, spectra of those lines were recorded at room temperature using a distributed feed-back (DFB) diode laser. The recorded absorption spectra were fitted to Voigt profile. Line position, intensity, self-broadening coefficient and N2-broadening coefficient were deduced from those data. The results show a good consistency in comparison with those listed in HITRAN2012 database. The discrepancy of most line intensities and self-broadening coefficients are less than 2%. The CO2 concentration and Δ(13 CO2 ) in real atmosphere inside laboratory are 440 ppm and -9 per hundred respectively. These results provide a reliable basis for real time and on line detecting the CO2 concentration and Δ(13 CO2) in the wavelength range.
ABSTRACT
In the present paper, the authors focus on the effect of the resonance frequency shift due to the changes in temperature and humidity on the PA signal, present several methods to control the noise derived form gas flow and vibration from the sampling pump. Based on the efforts mentioned above, a detection limit of 1.4 x 10(-8) W x cm(-1) x Hz(-1/2) was achieved for the measurement of atmospheric aerosols absorption coefficient. During the experiments, the PA cell was calibrated with the absorption of standard NO2 gas at 532 nm and the atmospheric aerosols were measured continuously. The measurement results show that the PAS is suitable for the real-time measurement of the absorption coefficient of atmospheric aerosols in their natural suspended state.
ABSTRACT
A new cryogenic cell was developed and could operate at any stabilized temperature ranging from room temperature down to 100 K with temperature fluctuation less than +/- 1 K. The structure and performance of the cryogenic cell was given in detail and the temperature stability of the sample cell was evaluated. The methane low temperature absorption spectra at 1.65 microm were recorded at 296, 248, 198 and 176 K and while the characteristic of methane low temperature absorption spectra at 6 039.70 cm(-1) was given. According to the characteristic of our cryogenic cell, we measured the temperature-dependent exponent n of self-broadening coefficient at 6 039. 657 9 cm(-1).
ABSTRACT
In the present paper the authors designed a vehicle-based remote sensing system using simulated platform and presented a new method of concentration calibration of natural gas pipeline leakage remote sensing. By investigating the performance of different distance, different material, different angle of topographic back scatter and different scan speed, a good coincidence was achieved between experimental results and theoretical results. The system can realize the remote detection of low-level methane concentration at a velocity of 53.3 km x h(-1), and the detecting distance is about 70 m with the minimum detectable sensitivity being 28.9 ppm x m. The research result shows the feasibility in the application.
ABSTRACT
The high resolution spectrum of methane was obtained around 1.65 microm using a tunable DFB diode laser with a long adjustable optical path white * cell (46.36-1 158.90 m) at room temperature through the direct absorption technique. The typical line width of the DFB diode laser is about 10 MHz and the wavelength of DFB laser was calibrated by an optical wavemeter. A total of 259 new absorption lines were studied from 6 043.00 to 6 053.72 cm(-1) at five different pressures and optical lengths. All the data were fitted by Gaussian profile, the line intensities, positions and the percent of the statistical standard deviation (sigmaS/ S) % of the line intensities were obtained, and the absorption lines which are hard to be distinguished were analyzed in this paper. The weakest absorption line is 4.3 x 10(-27) cm(-1) x (mol x cm(-2))(-1), while the lines stronger than 3.0 x 10(-24) cm(-1) x (mol x cm(-2))(-1) were ignored for their saturated absorption due to the long absorption optical length(788-1 000 m). Meantime, the spectrum shows the abundance of methane weak lines and its extremely complex structure around 1.65 microm. All the lines cannot be found in HITRAN2004 database, and as to our knowledge, they were not reported before by other papers.
ABSTRACT
Stable-isotope ratio analysis of water is an important tool for geology, meteorology, and earth sciences. Measurements of water vapor isotopes are very helpful to explaining stratospheric aridity and related issues in atmospheric sciences. The absorption of water vapor near 2.7 microm is very strong so it is suitable for measuring high sensitivity spectra. Based on difference frequency generation and quasi-phase matching, by mixing an Nd : YAG laser with Ti : Sapphire tunable from 750 to 840 nm in a 50 mm long periodically poled lithium niobate (PPLN) crystal, a widely tunable CW laser source was generated for the mid-infrared spectral range from 2.5 to 4 microm. We chose lambda = 20 microm for PPLN crystal, the generated laser was around 2.7 microm. This laser is widely tunable and of inherent narrow linewidth based on difference-frequency generation. Using this idler laser and 100 m multi-pass cell, and direct absorption the water vapor isotopes were measured in the laboratory air. The authors measured isotopes ratios and delta17O, delta18O and deltaD. The values were found to be in excellent agreement with the standard value for three individual lines.
ABSTRACT
The continuously tunable laser source has been realized in a periodically poled LiNO3 crystal based on difference frequency generation and quasi-phase-matching technique. The pump laser is an 1 W tunable Ti: Sapphire laser with a tunable region from 770 to 870 nm. The signal laser is an 1 W diode-pumped monolithic Nd : YAG laser. When the grating period is 20 microm and the temperature is tuned between room temperature and 200 degrees C, the generated wavelength of idler laser is around 2. 8 microm with the general power of .1-2 microW. The direct absorption spectra of (001 <-- 000) band of water in laboratory air were measured based on the laser source. The concentration of water vapor in the laboratory air was estimated with an absorption optical path of 8. 5 cm in open air.
ABSTRACT
The leak of natural gas is not only an economic loss, but also the fountain of danger. Conventional detection techniques of natural gas pipe leak have low efficiency and slow respond time, therefore, it is difficult for them to suit practice application. Optical sensors based on NIR tunable diode laser absorption spectroscopy were widely used because of high sensitivity, small volume and less maintenance. In the present paper, a portable remote sensor of natural gas pipeline leak was reported. The sensor used a ratio of second to first harmonic signals as calibration method, and the results show a good consistency between the concentrations and the ratios of second to first harmonic signals. The effect of different topographic scattering targets on the ratio detection was measured and analyzed. The results show that the ratio of second to first harmonic signals can be used in practical application.
