Influence of laser intensity in second-harmonic detection with tunable diode laser multi-pass absorption spectroscopy

Tunable diode laser absorption spectroscopy （TDLAS） has been widely employed in atmospheric trace gases detection. The ratio of the second-harmonic signal to the intensity of laser beam incident to the multi-pass cell is proved to be proportional to the product of the path length and the gas concentration under any condition. A new calibration method based on this relation in TDLAS system for the measurement of trace gas concentration is proposed for the first time. The detection limit and the sensitivity of the system are below 110 and 31ppbv （parts-per-billion in volume）, respectively.

A remote sensing system of vehicle emissions based on tunable diode laser technology

As being an effective real-time method of monitoring vehicle emissions on-road, a remote sensing system based on the tunable diode laser （TDL） technology was presented, and the key technologies were discussed. A field test in Guangzhou（Guangdong, China） was performed and was found that the factors, such as slope, instantaneous speed and acceleration, had significant influence on the detectable rate of the system. Based on the results, the proposal choice of testing site was presented.

Measurements of argon metastable density using the tunable diode laser absorption spectroscopy in Ar and Ar/O2

Densities of Ar metastable states 1s5 and 1s3 are measured by using the tunable diode laser absorption spectroscopy（TDLAS） in Ar and Ar/O2 mixture dual-frequency capacitively coupled plasma（DF-CCP）. We investigate the effects of high-frequency（HF, 60 MHz） power, low-frequency（LF, 2 MHz） power, and working pressure on the density of Ar metastable states for three different gas components（0%, 5%, and 10% oxygen mixed in argon）. The dependence of Ar metastable state density on the oxygen content is also studied at different working pressures. It is found that densities of Ar metastable states in discharges with different gas components exhibit different behaviors as HF power increases. With the increase of HF power, the metastable density increases rapidly at the initial stage, and then tends to be saturated at a higher HF power. With a small fraction（5% or 10%） of oxygen added in argon plasma, a similar change of the Ar metastable density with HF power can be observed, but the metastable density is saturated at a higher HF power than in the pure argon discharge. In the DF-CCP, the metastable density is found to be higher than in a single frequency discharge, and has weak dependence on LF power. As working pressure increases, the metastable state density first increases and then decreases,and the pressure value, at which the density maximum occurs, decreases with oxygen content increasing. Besides, adding a small fraction of oxygen into argon plasma will significantly dwindle the metastable state density as a result of quenching loss by oxygen molecules.

Development of 2D Temperature and Concentration Measurement Method Using Tunable Diode Laser Absorption Spectroscopy

Exhaust gas temperature is an important factor in NOx, THC and PM emissions of engines. Especially 2D temperature and concentration distribution plays an important role for the engine efficiency. A thermocouple is intrinsically a point temperature measurement method and noncontact 2D temperature distribution cannot be attained by thermocouples. Recently, as a measurement technique with high sensitivity and high response, laser diagnostics has been developed and applied to the actual engine combustions. With these engineering developments, transient phenomena such as start-ups and load changes in engines have been gradually elucidated in various conditions. In this study, the theoretical and experimental research has been conducted in order to develop the noncontact and fast response 2D temperature and concentration distribution measurement method. The method is based on a Computed Tomography （CT） method using absorption spectra of water vapor at 1388 nm. It has been demonstrated that the method has been successfully applied to engine exhausts to measure 2D temperature distributions.

Tunable Single-Passband Microwave Photonic Filter Based on Sagnac Loop and Fabry-Perot Laser Diode

A tunable single-passband microwave photonic filter is proposed and demonstrated, based on a laser diode (LD) array with multiple optical carriers and a Fabry-Perot (F-P) laser diode. Multiple optical carriers in conjunction with the F-P LD will realize a filter with multiple passbands. By adjusting the wavelengths of the multiple optical carriers, multiple passbands are merged into a single passband with a broadened bandwidth. By varying the number of the optical carrier, the bandwidth can be adjusted. The central frequency can be tuned by adjusting the wavelength of the multiple optical carriers simultaneously. A single-passband filter implemented by two optical carriers is experimentally demonstrated.

A 1550-nm linearly tunable continuous wave single-mode external cavity diode laser based on a single-cavity all-dielectric thin-film Fabry Pérot filter

A 1550-nm linearly tunable continuous wave （CW） single-mode external cavity diode laser （ECDL） based on a singlecavity all-dielectric thin-film Fabry-Pérot filter （s-AFPF） is proposed and realized in this paper. Its internal optical components as well as their operation mechanisms are introduced first, and then its longitudinal mode output characteristic is theoretically analyzed. Afterwards, we set up the experimental platform for the output characteristic measurement of this tunable ECDL; under different experimental conditions, we execute accurate and real-time measurements for the output central wavelength, output optical power, output longitudinal mode distribution, and the line-width of the tunable ECDL in its tuning process. By summing up the optimal experimental condition from the measured data, we obtain the optimal tunable ECDL relevant parameters： the tunable ECDL has a linear mode-hop-free wavelength tuning region of 1547.203 nm-1552.426 nm, a stable output optical power in the range of 40 μW-50 μW, and a stable output longitudinal mode distribution of a single longitudinal mode with a line-width in the range of 100 MHz-150 MHz. This tunable ECDL can be used in environmental gas monitoring, atomic and molecular laser spectroscopy research, precise measurements, and so on.

Generation of Tunable Amplitude-Squeezed Light by Injection Locking of a Laser Diode

Tunable amplitude squeezing around the D_(2)line of cesium has been experimentally accomplished at room temperature in a quantum-well laser diode with light injection from a single-mode distributed Bragg-Reflector laser diode.While the master laser frequency is tuned,amplitude squeezing of the output light from the slave laser can be maintained at about 0.9dB throughout a tunabIe range of~l.7 GHz around the cesium D_(2)line.

Application of Tunable Diode Laser Absorption Spectroscopy with Optical Hollow Fiber to Engine Exhaust Gas Measurement

In recent years, tighter regulation has been already enforced on harmful substances such as NOx, CO, and particles. Considering the above situation, it is important to monitor controlling factors of engine systems in order to improve efficiencies of their operations. As to car engines, an increasing concern in environmental issues such as air pollution, global warming and petroleum depletion has helped drive researches into various ways. Laser diagnostics has been applied to measure species concentration in the actual industrial fields. However there are several challenges to proceed in applying laser diagnostics to practical application. Especially stability of the measurement system is one of the most difficult issues. The purpose of this research is the development of a prompt measurement technique which can be applicable to various engine conditions. The Tunable Diode Laser Absorption Spectroscopy （TDLAS） using the hollow fiber has been developed to satisfy above requirements. By using a hollow fiber, misalignment of an optical axis and vulnerability of measurement environment such as vibration can be greatly reduced with sensitive and fast response features. It was demonstrated that this method can be applicable to measure gas compositions in engine exhaust with a range of millisecond response time. A sensitive method using tunable UV diode laser absorption spectroscopy was also discussed to detect NOx in exhausts.

Dynamic thermal modeling and parameter identification for a monolithic laser diode module

We improved the thermal equivalent-circuit model of the laser diode module（LDM） to evaluate its thermal dynamic properties and calculate the junction temperature of the laser diode with a high accuracy.The thermal parameters and the transient junction temperature of the LDM are modeled and obtained according to the temperature of the thermistor integrated in the module.Our improved thermal model is verified indirectly by monitoring the emission wavelength of the laser diode against gas absorption lines,and several thermal parameters are obtained with the temperature uncertainty of 0.01 K in the thermal dynamic process.

Diode-pumped Kerr-lens mode-locked Ti: sapphire laser with broad wavelength tunability

We report a direct blue-diode-pumped wavelength tunable Kerr-lens mode-locked Ti: sapphire laser.Central wavelength tunability as broad as 89 nm(736-825 nm) is achieved by adjusting the insertion of the prism.Pulses as short as 17 fs are generated at a central wavelength of 736 nm with an average output power of 31 mW.The maximum output power is 46.8 mW at a central wavelength of 797 nm with a pulse duration of 46 fs.

A high sensitivity spectrometer with tunable diode laser for ambient methane monitoring

Tunable diode laser absorption spectroscopy （TDLAS） has been widely employed in atmospheric trace gases detection. In the measurement of these trace gases, harmonic detection combined with a multi-pass white cell could remarkably enhance the detection sensitivity. In this paper, a portable TDLAS system built specifically for long time monitoring methane in the atmosphere is introduced. The detection limit is below 100 ppb that is enough for the monitoring of ambient methane, and the long time monitoring results obtained in Beijing are given, which is well coincident with that of the Fourier transform infrared （FTIR） spectroscopy.

Fast Tunable Wavelength Sources Based on the Laser Diode Array

We report a demonstration of a fast wavelength tunable source (TWS) based on the laser diode array coupled to the arrayed waveguide grating (AWG) multiplexer. The switching and optical characteristics of TWS make it a candidate for implementing the wavelength-division space switch fabric for an optical packet/burst switching.

A Tunable Dual-Wavelength Fiber Ring Laser with a Fabry-Perot Laser Diode in an External Injection Seeding Scheme

A tunable dual-wavelength fiber ring laser with a Fabry-Perot laser diode is proposed and demonstrated. The dual-wavelength outputs have the optical side-mode-suppression-ratio (SMSR) over 31 dB. The wavelength tuning range can be up to 9 run.

An Innovative Gas Sensor with On-Chip Reference Using Monolithic Twin Laser

An innovative gas sensor with on-chip reference using a monolithic twin laser is proposed. In this sensor a monolithic twin laser generates two closer laser beams with slight different wavelengths alternatively, one photodiode is used to catch both absorption and reference signals by time division multiplexing. The detection of nitrous oxide adopting this scheme using a 2.1 I^m antimonide laser and an InGaAs photodiode has been demonstrated experimentally with detection limit below i ppm. Using this on chip reference scheme the fluctuations from the optical path and devices can be compensated effectively; the sensor system is simplified distinctly.

Laser absorption spectroscopy for high temperature H_2O time-history measurement at 2.55 μm during oxidation of hydrogen

Concentration time-histories of H20 were measured behind reflected shock waves during hydrogen combustion. Experiments were conducted at temperatures of 1117-1282 K, the equivalence ratios of 0.5 and 0.25, and a pressure at 2 atm using a mixture of H2/O2 highly diluted with argon. H2O was monitored using tunable mid-infrared diode laser absorption at 2.55 μm （3920.09 cm-1）. These time-histories provide kinetic targets to test and refine reaction mechanisms for hydrogen. Comparisons were made with the predictions of four detailed kinetic mechanisms published in the last four years. Such comparisons of H2O concentration profiles indicate that the AramcoMech 2.0 mechanism yields the best agreement with the experimental data, while CRECK, San Diego, and HP-Mech mechanisms show significantly poor predictions. Reaction pathway analysis for hydrogen oxidation indicates that the reaction H ＋ OH ＋ M = H20 ＋ M is the key reaction for controlling the H2O formation by hydrogen oxidation. It is inferred that the discrepancy of the conversion percentage from H to H20 among these four mechanisms induces the difference of performance on H2O time-history predictions. This work demonstrates the potential of time-history measurement for validation of large reaction mechanisms.

宽光谱吸收应用中光谱扫描区间的选择

与传统的窄带扫描吸收光谱技术相比,宽光谱吸收技术可以获取更宽光谱范围内的丰富吸收数据,具有很大的技术优势。宽光谱吸收技术测量系统的硬件与传统的可调谐二极管吸收光谱技术测量系统基本一致,唯一不同的是宽光谱吸收技术则使用宽带调谐光源。实际应用中,宽光谱吸收光源一般都具有较宽的光谱覆盖范围(>20 nm),而使用全谱段扫描会对数据采集、存储和处理提出较高的要求,从而增加系统成本和测量工作量。因此,选择合适宽度的光谱扫描区间就成为宽光谱吸收应用中的一个关键问题。利用等效下能级概念,提出了针对宽光谱吸收应用的光谱扫描区间选择的数值方法,并针对1.3μm波段和2μm波段开展了扫描区间选择研究。在1.3μm波段,计算给出的优选扫描谱段与文献报道中使用的谱段相重合,验证了选择方法的可行性和正确性。在2μm波段,在给定工况条件下,通过实验数据验证了所选择扫描区间在温度反演精度方面的优越性。针对实际应用中面临的宽工况环境,通过计算给出了2μm波段优选的扫描光谱区间。计算结果表明,在1.8μm的短波段和1.9μm的长波段均存在适用的光谱扫描区间,其中,1.86μm附近测温精度最高;更宽的扫描带宽有助于降低系统在全光谱范围内的整体测温不确定度。相关研究可为宽光谱吸收技术在燃烧流场诊断的现场应用提供关键技术指导。

基于VisuShrink小波阈值变换的甲烷检测降噪技术

在利用可调谐半导体激光吸收光谱技术探测甲烷气体泄漏时,复杂的环境光干扰会降低浓度检测信号的信噪比使波形失真。为了抑制噪声,增强电信号与浓度值的线性相关度,提出一种以VisuShrink方法改善小波阈值函数的小波阈值去噪算法。该算法针对小波阈值去噪算法的软硬度不能平衡信号,存在失真的问题,利用统计方法对阈值进行计算,弱化间断点阈值跳变影响。通过仿真计算对比未加噪声前的信号与滤波后的信号,提出算法的信噪比为28.405 30,优于四阶巴特沃斯滤波器。VisuShrink方法在SNR和NCC参数上性能远优于巴特沃斯滤波器,与软阈值函数相近,在MSE均方误差分析上远小于软阈值精度,与巴特沃斯相近,具有较好的平滑特性。实际搭建检测平台对多个浓度样本采样进行多次采样,采用VisuShrink方法设计的滤波器将系统误差精度从4.14%提升至2.472%,算法可有效用于甲烷检测系统提高光谱数据的平滑性和准确性。

基于TDLAS的近红外甲烷高灵敏检测技术

为增强甲烷气体检测技术的气体吸收率,提高检测灵敏度,利用可调谐二极管激光吸收光谱技术,采用中心波长为1 653.7 nm的分布反馈激光器作为光源,研制了有效光程为14.5 m的Herriott型气体吸收池,并采用波长调制光谱法进行甲烷气体浓度检测。结果表明,二次谐波峰值信号与甲烷气体浓度成较强的线性关系,线性度为0.998 52,检测下限为4.82 ppm;初始积分时间为1 s时的Allan方差为6.37 ppm;积分时间到112 s时,Allan方差为427 ppb,检测灵敏度为4.27×10^(-7)。

小型化中红外直接吸收CO_(2)光谱仪

二氧化碳(Carbon Dioxide,CO_(2))是一种关键的温室气体,对地球气候和生态系统调节有重要作用,准确检测痕量CO_(2)浓度对评价温室效应意义重大。一般情况下,气体分子在中红外光谱范围存在强吸收,可以实现高灵敏探测,因此,以4.219μm中红外波段的带间级联激光器为光源结合可调谐半导体激光吸收光谱技术(Tunable Diode Laser Absorption Spectroscopy,TDLAS)设计了小型化CO_(2)监测仪器。在不同标气浓度下对仪器进行了标定,并通过分析Allan方差,得出积分时间在8.7 s时对应的最低检测限为16.6×10^(-9),满足温室效应研究对CO_(2)气体的高灵敏测量需求。采用仪器对大气CO_(2)连续监测8天,并与商用仪器LGR进行对比,充分验证了小型化CO_(2)监测仪器的可靠性,以及中红外光源与TDLAS技术相结合的气体浓度检测方法在大气CO_(2)准确测量方面的可行性。该系统具有结构简单、体积小、功耗低和质量轻的特点,为建设空基无人机平台的CO_(2)监测能力提供了技术支持。

基于透射曲线拟合的高精度激光吸收光谱温度检测方法

可调谐激光吸收光谱技术在流场检测领域应用越来越广泛,然而恶劣的现场应用环境对测试系统的环境适应性、抗噪声干扰能力提出了更高的要求。直接吸收光谱技术可以获得吸收峰的全部信息,对多类测试环境均适用,如风洞,发动机喷射流场等,然而受现场测试环境影响,获得的测试信号信噪比通常较差,难以实现高精度检测。针对这一问题,对直接吸收光谱技术的数据处理分析算法进行了分析研究,提出了一种基于透射曲线拟合的高精度温度反演分析方法。利用水汽在7185.6和7444.3 cm-1附近的吸收谱线,研制了基于直接吸收光谱检测技术的温度检测系统,开展了573K-1173K温度范围内测试信号的处理分析,对比分析了基线-吸收线型拟合算法与透射曲线拟合算法的反演结果。当检测温度为773 K时,相比基线-吸收线型拟合算法,透射曲线拟合算法在7444.3 cm-1波段附近的逐点拟合误差峰峰值可以减小约30%,7185.6 cm-1波段附近的逐点拟合误差峰峰值可以减小约16%。573~1173 K温度范围内,透射曲线拟合算法反演温度最大误差为13 K,相比基线-吸收线型拟合算法减小了23 K。给测试信号加入不同幅度的随机噪声信号,两种算法反演温度的标准差均随噪声幅度的增加而增加,同时温度越高,温度反演标准差越大。当噪声的峰峰值分别为20、60和100 mV时,不同温度下基线-吸收线型拟合算法温度反演结果的标准差最小值为18 K,最大值为313 K,透射曲线拟合算法温度反演结果的标准差最小值为4 K,最大值为44 K。实验分析结果表明,相比基线-吸收线型拟合算法,透射曲线拟合算法可以修正激光器功率基线拟合误差,提高透射信号拟合精度,减小温度反演误差。对比不同噪声水平下测试信号的温度反演结果表明,透射曲线拟合算法在噪声干扰下可以实现更高的检测精度,具有更强的抗噪声干扰能力。