Scanned-wavelength-modulation spectroscopy near 2.5 μm for H2O and temperature in a hydrocarbon-fueled scramjet combustor

The design and demonstration of a two-color tunable diode laser sensor for measurements of temperature and H₂O in an ethylene-fueled model scramjet combustor are presented. This sensor probes multiple H₂O transitions in the fundamental vibration bands near 2.5 μm that are up to 20 times stronger than those used by previous near-infrared H₂O sensors. In addition, two design measures enabled high-fidelity measurements in the nonuniform flow field. (1) A recently developed calibration-free scanned-wavelength-modulation spectroscopy spectral-fitting strategy was used to infer the integrated absorbance of each transition without a priori knowledge of the absorption lineshape and (2) transitions with strengths that scale near-linearly with temperature were used to accurately determine the H₂O column density and the H₂O-weighted path-averaged temperature from the integrated absorbance of two transitions.     

利用可调谐激光吸收光谱技术对光路上气体温度分布的测量

利用可调谐激光吸收光谱技术(TDLAS),扫描多条吸收谱线以实现气体温度分布的测量。文章给出了温度分布测量的原理和方程离散化的方法,在气体浓度和压力均匀时,利用带约束最小二乘法计算得到温度分布。根据HITRAN中6 330 cm-1附近的4条CO谱线的参数,建立了温度在300和600 K时,路径长度均为55 cm的两段温度分布模型,模拟了测量误差与温度区间长度约束条件的影响。结果表明随着测量误差的增大和约束条件的减弱,计算结果误差相应增大。在5％的测量误差下,计算结果的最大误差为11％,平均误差为2.2％。以管式炉中的高温段和室温下的低温段作为两段温度分布模型进行试验。利用6 330 cm-1处的垂直腔面发射激光二极管(VCSEL)扫描得到的4条CO谱线,通过背景信号的三次多项式拟合得到基线,求出温度分布计算所需的光谱吸收率积分值。在四种情况下, 计算温度分布结果与模型误差分别为7.3％,6.5％,4.7％和2.7％。     

腔增强吸收光谱技术中的腔镜反射率标定方法研究

腔增强吸收光谱技术具有实验装置相对简单、灵敏度高、环境适应性强等特点,是高灵敏吸收光谱技术的重要分支之一,在其应用过程中,腔镜反射率是影响其测量准确性的重要因素。利用2.0 μm可调谐二极管激光器作为光源搭建了一套腔增强吸收光谱测量系统,使用两片反射率为99.9%的高反镜作为腔镜,以CO₂气体在5 001.49 cm-1处的吸收谱线作为研究目标,对两种简单实用的腔镜反射率标定方法进行了对比研究。第一种标定方法利用已知程长多通池作为参考池,通过测量谐振腔和多通参考池的输出吸收信号,比较二者的吸收率推导出腔增强系统中的有效吸收路径,然后通过镜片反射率和有效吸收路径的关系对腔镜反射率进行标定;第二种标定方法根据理想气体状态方程得到气体分子数密度,并结合数据库中的谱线线强值,实现了对腔镜反射率进行标定。结果表明,方法一中积分腔与参考池测得信号的积分吸收面积之比为10.5,经过多次测量并计算得到积分腔的有效吸收路径与镜片的反射率分别为302.65 m和99.85%,得到大气中CO₂气体的浓度为0.037 3%,与实际大气CO₂的含量相符合,验证了此方法的准确性;该方法的优点是不受样品浓度影响,但因引入新的参考池,需要两池中气体的压强和温度都保持一致,此方法适用于开放式的腔体结构。方法二中测得大气中CO₂分子位于5 001.49 cm-1处吸收光谱,并结合大气中CO₂气体的分子数密度N为9.099×1015 molecule·cm-3,Hitran数据库中该条谱线线强为3.902×10-22 cm·molecule-1,计算得到镜片反射率约为99.84%;此方法优点是结构相较前一种方法更简单,但需要已知被测气体的分子数密度,因此在配置气体的过程中浓度、压力的误差会影响腔镜反射率的标定。由此可见两种镜片标定方法均可精确实现对腔镜反射率的标定,根据两种方法的特点,在实际应用中可选取相应适合的方法作为参考。     

基于吸收光谱和激光干涉技术的气体压力测量方法研究

气体压力光学非接触测量是目前激光技术重要应用领域之一,其中气压测量过程中温度耦合问题是现在面临的研究难点。故而提出一种光谱测量技术与激光干涉技术组合测量方法,通过积分吸光度和折射率融合的方式实现气体压力、温度解耦的目的。分析可调谐半导体激光光谱技术(TDLAS)的直接吸收法测量原理和基于折射率的激光干涉测量原理,建立基于吸收光谱的气压测量模型和基于折射率的激光干涉气压测量模型,通过利用三次多项式拟合吸收谱线强度函数的方式,建立了基于积分吸光度和折射率的气体压力、温度解耦的数学模型。实验搭建了基于TDLAS技术和激光干涉技术的气体压力检测系统,采用中心波长为2 004 nm的可调谐半导体激光器和波长为632.8 nm的激光干涉仪,气室长度为24.8 cm,将CO₂作为研究对象,并以高精度压力控制器和温度传感器的测量结果分别作为压力温度参考值,以真空为背景信号,在室温环境中测量并计算出气体压力变化后积分吸光度值和折射率值,进而解算得到气体压力和温度值。实验结果显示：压力测量结果最大相对误差为3.61%,最小相对误差为0.5%,测量平均相对误差为1.99%;在以开尔文温度为前提下,温度解算结果最大绝对误差为7.66 K,最小绝对误差为0.78 K,测量平均绝对误差为3.29 K,测量结果与参考结果具有较高的吻合度,该研究可为以后光学法测量气体压力温度影响分析提供参考。     

基于光强与吸收率非线性同步拟合的吸收光谱测量方法

针对高压环境吸收谱线加宽以及波分复用技术合波透射信号分析测试难题,提出利用非线性拟合方法对激光吸收光谱测量中激光强度与吸收光谱进行耦合求解.建立激光强度非线性变化与多谱线吸收拟合函数关系,解决了特殊环境下无法获取光谱基线的难题,实现了波分复用过程合波后光谱信号的分离与诊断.通过仿真验证该方法的可行性,分析计算了激光器特性和特征谱线位置等因素对拟合结果的影响.搭建实验台实现了1—10 atm变压力环境下6330—6337 cm~(-1)波段CO_2吸收光谱叠加信号的诊断分析,对气液两相脉冲爆轰过程中7185.6 cm~(-1)与7444.35 cm~(-1)波段波分复用光谱信号进行测试与拟合,无需分光设备实现了耦合光路分离和温度计算,研究结果对激光吸收光谱技术在高压环境以及燃烧环境下波分复用技术的发展具有重要意义.     

基于便携式柱面镜多通池的二氧化碳高灵敏度探测研究

二氧化碳作为大气中重要的温室气体，与气候变化和人类活动密切相关，因此对其浓度的探测具有重要意义。利用近红外可调谐二极管激光器结合自主设计的便携式小型化柱面镜光学多通吸收池，实现了二氧化碳气体的高灵敏探测。通过Matlab编写光线传输矩阵，优化设计了基于柱面镜的光学多通吸收池，相比于传统Herriott型多通池，具有腔镜利用面积高、在相同体积内可实现有效光程长等特点，在物理基长为15 cm的情况下，实现了14 m的有效光程。实验中使用中心波长为1.57 μm的DFB二极管激光器，采用直接吸收光谱方法对CO₂气体进行了探测研究，并用Allan方差对系统性能进行了分析。结果表明，在平均时间为5 s时，系统的探测灵敏度为33.1 μL·L-1，平均时间为235 s时，系统的探测灵敏度可达到5.3 μL·L-1。此外，利用该系统实现了大气中CO₂的探测，得到大气中的CO₂浓度为383.4 μL·L-1。基于柱面镜多通池搭建的可调谐激光吸收光谱(TDLAS)系统，结合了柱面镜多通池可在小体积内实现长光程和可调谐激光吸收光谱技术高灵敏度、高分辨率、快速响应的优点，大大减小了系统体积，提高了系统探测灵敏度，在气体探测领域有广泛的应用。     

红外地球敏感器扫描镜摆角激光动态测试方法

为解决扫描镜摆角实时动态非接触测量问题,基于激光检测技术和CCD探测技术,提出一种红外地球敏感器扫描镜摆角激光动态测试方法,并研制了扫描镜摆角动态测试系统,其可实现扫描镜的摆动频率、零位角、幅值、峰峰值平均等参量的动静态激光非接触测量。介绍了系统的组成和总体结构,着重对扫描镜摆角动态测量理论和大视场、大相对孔径特殊线性扫描光学系统的设计方法进行了分析与探讨,通过建立系统的数学模型,解决了测量数据误差修正与图形处理问题。对测量系统的精度进行了验证,结果表明系统的摆角测量范围为0~±12°,分辨力为0.01°,动静态测量精度优于±0.04°。     

激光器特性在痕量气体检测中的影响

研究了激光器扫描步长和线宽两种特性对可调谐半导体激光吸收光谱检测系统的影响,理论上推导出激光与气体吸收谱线的作用原理,分析出扫描信号(锯齿波)的台阶间隔和高度影响激光器中心波长的扫描原理.设定了仿真参数,仿真出锯齿波台阶数与最大扫描误差关系曲线,得出扫描信号的一个周期内具有4000个台阶时,半高全宽(FWHM)大于0.01 cm-1,误差小于1‰;仿真出激光器线宽与最大幅值、线宽误差关系曲线,给出线宽误差最大为1%,0.5%时激光器线宽对应的最小FWHM.在温度系数n取0.9,大气展宽系数γair取0.005的条件下,给出温度T,压强P与FWHM关系图,推出了适用的压强与温度范围.为指导选取激光器与气体吸收谱线、提高系统检测限提供了相关理论依据.     

Tunable diode laser absorption sensor for the simultaneous measurement of water film thickness,liquid- and vapor-phase temperature

A four-wavelength near-infrared (NIR) tunable diode laser sensor has been developed for the simultaneous measurement of liquid water film thickness, liquid-phase temperature and vapor-phase temperature above the film. This work is an important improvement of a three-wavelength concept previously introduced by Yang et al. (Appl. Phys. B 99:385, 2010), which measured the film thickness in environments with known temperature only. In the new sensor, an optimized combination of four wavelengths is chosen based on a sensitivity analysis with regard to the temperature dependence of the liquid water absorption cross section around 1.4 μm. The temperature of liquid water and the film thickness are calculated from absorbance ratios taken at three wavelength positions assessing the broad-band spectral signature of liquid water. The vapor-phase temperature is determined from the absorbance ratio of two lasers rapidly tuned across two narrow-band gas-phase water absorption transitions. The performance of the sensor was demonstrated in a calibration cell providing liquid layers of variable thickness and temperature with uncertainties smaller than 5% for thickness measurements and 1.5% for liquid-phase temperatures, respectively. Experiments are also presented for time-resolved thickness and temperature measurements of evaporating water films on a quartz plate.     

In situ measurement on nonuniform velocity distributionin external detonation exhaust flow by analysis ofspectrum features using TDLAS

Instantaneous and precise velocity sensing is a critical part of research on detonation mechanism and flow evolution.This paper presents a novel multi-projection tunable diode laser absorption spectroscopy solution,to provide a real-time and reliable measurement of velocity distribution in detonation exhaust flow with obvious nonuniformity.Relations are established between overlapped spectrums along probing beams and Gauss velocity distribution phantom according to the frequency shifts and tiny variations in components of light-of-sight absorbance profiles at low frequencies analyzed by the fast Fourier transform.With simulated optical measurement using H2O feature at 7185.6 cm-1 carried out on a phantom generated using a simulation of two-phase detonation by a two-fluid model,this method demonstrates a satisfying performance on recovery of velocity distribution profiles in supersonic flow even with a noise equivalent absorbance up to 2×10^-3.This method is applied to the analysis of rapidly decreasing velocity during a complete working cycle in the external flow field of an air-gasoline detonation tube operating at 25 Hz,and results show the velocity in the core flow field would be much larger than the arithmetic average from traditional tunable diode laser doppler velocimetry.This proposed velocity distribution sensor would reconstruct nonuniform velocity distribution of high-speed flow in low cost and simple operations,which broadens the possibility for applications in research on the formation and propagation of external flow filed of detonation tube.     

基于超窄线宽激光的一氧化碳多参数检测研究

基于超窄线宽激光特性和激光器波长扫描技术,通过对一氧化碳气体近红外吸收光谱的测量及分析,设计了一氧化碳气体多参数实时在线检测系统.系统采用超窄线宽可调谐半导体激光器作为光源,利用气体直接吸收光谱测温法,实现了一氧化碳气体温度实时检测.根据所测温度值并结合气体浓度差分检测原理,实现了一氧化碳气体温度和浓度同时测量.利用超...     

Near-infrared methane sensor based on a distributed feedback laser

In this article, a near-infrared methane detection system using tunable diode laser absorption spectroscopy technology was designed and implemented. The distributed feedback laser was driven by a self-developed temperature and current controller to allow scanning the selected absorption wavelength at 1654?nm. Laser temperature fluctuation was lower than 0.01%, and the output emission wavelength was linear and stable. The emitted beam passed a reflective gas chamber and was received by the Indium Gallium Arsenide photodiode detector. Through a data acquisition card, a digital lock-in amplifier was developed to extract the second harmonics with real-time monitoring and adjustment. Based on Allan deviation analysis, the limit of detection was about 48?ppm with a path length of 30?cm, at an integration time of 6?s. The experimental results revealed a maximum detection error of less than 3% at a gas concentration higher than 100?ppm. The fluctuations rates in long-term (9?hr) stability measurements for 1?×?10³?ppm and 1?×?10⁴?ppm methane samples were 0.8% and 0.48%, respectively, indicating good stability for the sensor. In the control module design, compared with previous reports on methane detection systems, the current system uses a self-developed temperature controller, a current driver and a signal processor, to allow real-time display and adjustments. The potential for adjustable wavelength scanning is available for multi-gas detection based on a single detection system.     

相敏式激光啁啾色散光谱技术在高吸收度情况下的应用

研究了相敏式激光啁啾色散光谱法在高吸收度情况下的应用.用窄频半导体激光器作为光源,利用一工作于载波抑制模式的铌酸锂电光强度调制器调制单频激光,在单频激光两侧产生两个边频分量,并通过两边频分量产生外差干涉信号.利用外差干涉的相位波动来测量甲烷气体位于1653.7 nm附近的折射率波动,通过气体折射率与吸收系数之间的Kramers-Kronig关系计算甲烷气体浓度.传统的波长调制光谱法受限于郎伯-比尔定律,在应用于高吸收度的情况时,存在灵敏度下降的问题,甚至出现随气体浓度上升输出信号反而下降的现象.实验结果显示,相同实验条件下,波长调制光谱法的线性测量范围为38.1—1500 ppm·m,线性测量的动态范围仅为16 d B;而相敏式激光啁啾色散光谱法在很大的吸收度范围内均具有线性输出,检出限低至47.3 ppm·m,线性测量范围上限为174825 ppm·m,具有超过35 d B的动态范围.     

Development of a tunable diode laser sensor for measurements of gas turbine exhaust temperature

A tunable diode laser (TDL) temperature sensor is designed, constructed, tested, and demonstrated in the exhaust of an industrial gas turbine. Temperature is determined from the ratio of the measured absorbance of two water vapor overtone transitions in the near infrared where telecommunication diode lasers are available. Design rules are developed to select the optimal pair of transitions for direct absorption measurements using spectral simulations by systematically examining the absorption strength, spectral isolation, and temperature sensitivity to maximize temperature accuracy in the core flow and minimize sensitivity to water vapor in the cold boundary layer. The contribution to temperature uncertainty from the spectroscopic database is evaluated and precise line-strength data are measured for the selected transitions. Gas-temperature measurements in a heated cell are used to verify the sensor accuracy (over the temperature range of 350 to 1000 K, ΔT∼2 K for the optimal line pair and ΔT∼5 K for an alternative line pair). Field measurements of exhaust-gas temperature in an industrial gas turbine demonstrate the practical utility of TDL sensing in harsh industrial environments. PACS 42.62.Fi; 42.55.Px; 42.62.Cf; 39.30.+w     

基于离轴积分腔输出光谱的燃烧场CO浓度测量研究

CO是碳氢燃料不完全燃烧的重要产物,常常被作为反应燃烧效率的标志物,燃烧场CO组分浓度的精确测量对提高燃烧效率、减少污染物排放具有重要意义。离轴积分腔输出光谱(OA-ICOS)是一种利用物质对激光的特异性吸收,实现对该物质分析和测量的技术,具有非接触、稳定和高灵敏度等优点。针对燃烧场CO浓度低,背景信号干扰强等特点,采用分布反馈式(DFB)激光器搭建基于离轴积分腔输出光谱的CO浓度测量系统,通过直接吸收光谱的测量方法实现对高温燃烧场CO浓度测量。利用仿真模拟的方法,在所用激光器中心波长的附近选出了常温下谱线强度较为突出,高温下不受其他燃烧产物干扰的第一泛频带R(10)吸收谱线。通过固定光程池对比吸光度的方法标定了OA-ICOS系统的有效光程;通过比较不同扫描频率下吸收谱线的信噪比和线型拟合残差标准差,得到最佳波长扫描频率;通过测量不同浓度CO混合气体的吸收信号分析了系统误差。探究了不同燃烧情况下CH₄/Air预混平焰炉上CO的产生情况,根据燃烧场测量区域温度分布情况描述了温度分布不确定度对CO测量结果的影响。当量比为1.0时,在10 ms的测量时间分辨率下,噪声等...     

Compact 10-GHz Nd:GdVO4 laser with 0.5-W average output power and low timing jitter

We demonstrate a compact, diode-pumped Nd:GdVO4 laser with a repetition rate of 9.66 GHz and 0.5-W average output power. The laser is passively mode locked with a semiconductor saturable absorber mirror (SESAM), yielding 12-ps-long sech2-shaped pulses. For synchronization of the pulse train to an external reference clock, the SESAM is mounted on a piezoelectric transducer. With an electronic feedback loop of only a few kilohertz loop bandwidth we achieved a rms timing jitter of 146 fs (integrated from 10 Hz to 10 MHz). This is an upper limit because it is mostly limited by the measurement system. The laser setup with a simple linear cavity has a footprint of only 130 mm x 30 mm.     

An approach of open-path gas sensor based on tunable diode laser absorption spectroscopy

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.     

Continuously tunable external-cavity diode laser with a double-grating arrangement

A new external-cavity diode laser system based on an extended version of the Littman configuration is investigated. In our laser system the tuning mirror is replaced with a second grating mounted at the Littrow angle. This double-grating design provides a smaller passive bandwidth than the grating-mirror configuration, which is used to compensate for the large angle-of-incidence-dependent losses of the grazing-incidence grating. This results in a broader continuous tuning range with an improved mode stability. Because two laser diodes, emitting at 820 and 775 nm, respectively, are used, the external-cavity laser is continuously tunable without mode hops across 35 nm at 820 nm and 27 nm at 775 nm. Such a laser was used to measure the absorption lines of the oxygen A band around 762 nm. The complete R - and P -rotational branches of the b summation operator(g)(1) (+)(nu(??)=0)?X (3) summation operator(g)(-)(nu (?)=0) transition were recorded in a single-wavelength scan.     

温度调谐技术测量CO2在6320～6336cm-1波段的吸收光谱

针对当前可调谐半导体激光器吸收光谱(TDLAS)技术中调谐范围、调谐时间以及系统复杂性方面存在的不足,提出了利用激光器模块中的热电制冷器(TEC)和负温度系数(NTC)热敏电阻等元件对激光二极管(LD)进行温度宽谱调谐的方法,并在快速温度调谐过程中精确计算激光器的辐射波长。利用温度调谐二极管吸收光谱技术在3s的时间内测得了CO2气体在6320～6336cm-1波段的高分辨率吸收光谱。在此波段共测得8个较强吸收线。将得到的光谱参数与HITRAN 2008中的数据比较,吸收线位置、线强以及半峰全宽(FWHM)的偏差分别小于1%,3%以及6%。另外,测得的14条较弱的吸收谱线也与谱库中的谱线参数吻合。     

Angle extended linear MEMS scanning system for 3D laser vision sensor

Scanning system is often considered as the most important part for 3D laser vision sensor. In this paper, we propose a method for the optical system design of angle extended linear MEMS scanning system, which has features of huge scanning degree, small beam divergence angle and small spot size for 3D laser vision sensor. The principle of design and theoretical formulas are derived strictly. With the help of software ZEMAX, a linear scanning optical system based on MEMS has been designed. Results show that the designed system can extend scanning angle from ±8° to ±26.5° with a divergence angle small than 3.5 mr, and the spot size is reduced for 4.545 times.