连续和准连续调制激光吸收光谱的软件锁相解调比较研究

根据对激光器施加的调制信号是否连续,波长调制光谱分为连续和准连续调制激光吸收光谱技术。为了深入的比较研究这两种方案,设计准连续调制谱专用的软件锁相放大器,将准连续调制信号因间断产生的无效信号滤除后,再对吸收信号解调得到二次谐波信号。与连续激光调制谱软件解调的二次谐波信号进行了比较,结果表明,参数相同,准连续调制谱比连续调制谱的信噪比提高5%、检测限降低11.3%。在滤除无效信号后,准连续调制谱也可以解调出标准的二次谐波信号,因此有望用于与气体线型相关的研究中。该工作为选取更加适用的激光调制谱技术,提供了准确依据。     

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection

The development of a compact tunable mid-IR laser system at 3.5 μm for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO₃ and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1σ replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5×10^-10 cm^-1. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-1492, E-mail: dr@ucar.edu     

WMS的玻璃药瓶内氧气含量检测及其标定方法改进

应用激光波长调制光谱(WMS)技术,建立了一种开放光路短光程检测玻璃药瓶内氧气含量的方法。选择氧气分子位于760.885(13 142.58 cm-1)的吸收谱线,通过多次调试优化了系统相关参数,给出了实时扣背景及实时谱线校正等数据处理的方法和步骤。采集七种不同氧气含量的玻璃药瓶样本,获取相应的二次谐波信号,分别建立二次谐波峰值、半高谱峰面积与浓度的线性回归方程进行定量预测。实验结果表明,其拟合系数分别为0.996 6和0.997 8,后者相比前者的标定方法提高了0.12%。采用完全交互验证的方法来评价两个模型的预测精度,其预测的均方根误差(RMSEP)分别是0.003 1和0.002 0,后者相比前者降低了37.69%。对浓度是4%的气体样品,比较不同时间的20次测量结果,标准差分别为0.002 2和0.001 6,后者相比前者降低了27.3%,同时其测量灵敏度分别为0.198%和0.097%,后者相比前者的灵敏度提高了约51%。证明了该系统及数据处理方法对玻璃药瓶内氧气含量检测是可行的,且利用半高谱峰面积更丰富的幅值信息来反演气体浓度可以降低波峰失真影响,检测精度更高,稳定性更好。     

Resonant photoacoustic detection of trace gas with DFB diode laser

A resonant photoacoustic detection system based on a low-power distributed feedback diode laser is developed. This sensor has been applied to the detection of acetylene (C₂H₂) using a specifically designed photoacoustic cell operating on its second longitudinal mode. The minimum detectable limit of about 10 parts-per-million volume (SNR=1) is achieved with an average laser power of 3.5 mW at atmospheric pressure, and an integration time constant of 3 ms; thus, the minimum detectable absorption coefficient normalised by power and bandwidth is 4.0×10⁻⁸ W cm⁻¹/√Hz. The optimum operating pressure buffered with N₂ is also investigated. The realisation of our system is described and experimental results are compared with different modulation techniques and other results reported in the literature. A number of issues arising from the conventional use of mechanical chopping of the beam can be effectively suppressed in wavelength modulation PA spectroscopy (WM-PAS) and second harmonic detection.     

基于半导体光放大器交叉偏振调制效应实现正、反相波长变换

利用琼斯矩阵,对基于半导体光放大器中的交叉偏振调制效应实现全光波长变换进行了理论分析.结果表明,利用这种机制可以实现同相与反相波长变换.在实验上同时实现了10Gb/s归零信号的正相与反相波长变换,输出消光比分别达到了6.9 dB和8.1 dB.     

Photoacoustic spectroscopy for trace gas detection with cryogenic and room-temperature continuous-wave quantum cascade lasers

The main characteristics that a sensor must possess for trace gas detection and pollution monitoring are high sensitivity, high selectivity and the capability to perform in situ measurements. The photacoustic Helmholtz sensor developed in Reims, used in conjunction with powerful Quantum Cascade Lasers (QCLs), fulfils all these requirements. The best cell response is # 1200 V W⁻¹ cm and the corresponding ultimate sensitivity is j 3.3 × 10⁻¹⁰ W cm⁻¹¹ Hz^−11/2. This efficient sensor is used with mid-infrared QCLs from Alpes Lasers to reach the strong fundamental absorption bands of some atmospheric gases. A first cryogenic QCL emitting at 7.9 μm demonstrates the detection of methane in air with a detection limit of 3 ppb. A detection limit of 20 ppb of NO in air is demonstrated using another cryogenic QCL emitting in the 5.4 μm region. Real in-situ measurements can be achieved only with room-temperature QCLs. A room-temperature QCL emitting in the 7.9 μm region demonstrates the simultaneous detection of methane and nitrous oxide in air (17 and 7 ppb detection limit, respectively). All these reliable measurements allow the estimated detection limit for various atmospheric gases using quantum cascade lasers to be obtained. Each gas absorbing in the infrared may be detected at a detection limit in the ppb or low-ppb range.     

考虑非线性效应的路由与波长分配算法

本文研究了在波长路由的智能光网络中选择路由与波长分配(RWA)过程中如何考虑非线性效应的影响.在简要分析了考虑非线性效应影响的必要性后,以交叉相位调制为例推导出了一种简单的计算非线性效应引起的光功率代价计算公式,并讨论了如何将其引入路由与波长分配算法中.仿真结果表明非线性效应确实对RWA的优化结果产生一定的影响,其程度与物理拓扑、业务需求和具体的网络传输参量有关.     

Direct EPR detection of transient and continuous wave signals at 2.5 GHz

Direct detection of free induction decays and electron spin echoes, and the recording of echo-detected EPR spectra and electron spin echo envelope modulation patterns at a microwave frequency of 2.5 GHz is demonstrated. This corresponds to the measurement of the transverse magnetization in the laboratory frame, rather than in the rotating frame as usually done by down-converting the signal (homodyne detection). An oscilloscope with a 6-GHz analog bandwidth, a sampling rate of 20 GigaSamples per second, and a trigger frequency of 5 GHz for the edge trigger and 750 MHz for the advanced trigger, is used in these experiments. For signal averaging a 3-GHz microwave clock divider has been developed to synchronize the oscilloscope with the frequency of the EPR signal. Moreover, direct detection of continuous wave EPR signals at 2.5 GHz is described.     

临界及非临界相位匹配KTP光学参量振荡器

报道了利用Nd:YAG激光器二次谐波532nm及基频波1064nm作抽运源,采用临界及非临界相位匹配方式抽运KTP光学参量振荡器的实验结果.实验中获得了具有实用价值的1.53-1.84um人眼安全激光,KTP光学参量振荡器输出总量最大达130mJ,最高能量转换效率64%.分析比较了调谐范围、离分角、接收角、阈值及转换效率,讨论离分角对参量输出的光束质量、转换效率的影响.     

Broadly tunable difference-frequency spectrometer for trace gas detection with noncollinear critical phase-matching in LiNbO3

3 is reported. Using an external cavity diode laser, tunable from 795 to 825 nm, and a cw diode-pumped Nd:YAG ring laser at 1064 nm, we produced narrowband mid-ir laser light between 3.16 and 3.67 μm. This broad tuning range of 440 cm^-1 can be critically phase-matched by changing the external crystal angle within a range of only 0.4°. Even for a fixed crystal angle broadband phase-matching over 230 cm^-1 was achieved. No realignment was required when tuning the laser over the whole wavelength range, which enabled the use of a compact 36-m multipass cell as gas chamber. The conversion efficiency could be improved by almost a factor of 3 by applying noncollinear instead of collinear phase-matching, resulting in 30 nW of idler power with good beam quality. Spectra of methane were recorded in laboratory air, which demonstrate the rapid and continuous broad tunability at high sensitivity, enabling sub-ppm detection. Finally, wavelength modulation spectroscopy at high resolution was applied as a promising tool for further inproving the performance of this laser spectrometer. Received: 16 March 1998/Revised version: 1 July 1998     

Mid-infrared virtually imaged phased array spectrometer for rapid and broadband trace gas detection

We present and characterize a two-dimensional (2D) imaging spectrometer based on a virtually imaged phased array (VIPA) disperser for rapid, high-resolution molecular detection using mid-infrared (MIR) frequency combs at 3.1 and 3.8?μm. We demonstrate detection of CH₄ at 3.1?μm with >3750 resolution elements spanning >80 nm with ～600 MHz resolution in a <10 μs acquisition time. In addition to broadband detection, we also demonstrate rapid, time-resolved single-image detection by capturing dynamic concentration changes of CH₄ at a rate of ～375 frames per second. Changes in absorption above the noise floor of 5×10^-4 are readily detected on the millisecond time scale, leading to important future applications such as real-time monitoring of trace gas concentrations and detection of reactive intermediates.     

Suppression of Nonlinear Patterning Effect in Wavelength Conversion Based on Transient Cross-Phase Modulation in Semiconductor Optical Amplifier Assisted with a Detuning Filter

Nonlinear patterning （NLP） effect in wavelength conversion based on transient cross-phase modulation （XPM） in semiconductor optical amplifier （SOA） assisted with a detuning filter is theoretically investigated. A non- adiabatic model is used to estimate the ultrafast dynamics of gain, phase and electron temperature in the SOA. Simulation results show that the NLP can be greatly suppressed by introducing an assist light, especially for the probe wavelength distant from gain peak. Furthermore, the results also indicate that the improvement is more evident for long wavelength probe light and assist light in counter-propagating configuration.     

光传送网中结合传输损伤的路由和波长分配

在波长路由光网优化设计的过程中考虑了物理传输损伤造成的限制,提出了一种联合优化思路.将光信号在传输过程中的物理损伤作为问题公式的另一维约束,给出了相应的启发式算法,并按照由粗到细的机制对所提出的思路做了相应的评估,同时对负荷平衡问题作了初步的分析.数值结果表明这种算法可以获得与优化值相近的波长需求解,并且能够保证网络的传输性能,其业务分布随着备选路径的增加而趋向均衡.     

波长可调谐超短光脉冲的简便高质的生成技术

提出了一种以处于增益开关调制状态下的法布里珀罗(Fabry-Pérot)半导体激光器作为光源,采用简单的自激注入锁定方式,以生成波长可调谐的超短光脉冲的实验系统。该实验装置中,激光器的外腔包括两个串联在一起的布拉格光纤光栅,一个掺铒光纤放大器(EDFA),一个光耦合器和两个环行器,其作用是选模,以及增强并控制反馈回激光器内腔的光强。该实验系统简单而高效,在24nm的波长调谐范围内获得了边模抑制比高于40dB的单模光脉冲输出;而在1521.8nm和1550.0nm之间28.2nm的波长调谐范围内,边模抑制比高于35dB。所得到的各个单波长激光脉冲的时域半峰全宽为140~260ps,各个脉冲的光谱半峰全宽皆为0.1nm。     

The use of diode array detectors in conjunction with continuous wave gas lasers for anti-Stokes Raman spectroscopy

The Raman spectrum of any molecule consists of two mirror-image signals, th e Stokes and anti-Stokes Raman spectra. In most cases, unless highly specific sampling conditions are used, the anti-Stokes signal is much weaker than that of the Stokes. The recent application of intensified diode array detectors to Raman spectroscopy has produced a marked increase in the sensitivity of the technique which makes a study of the anti-Stokes spectrum potentially more rewarding than it has been to date. The present study has shown that, although of limited use for general purposes, there are some specific instances where the anti-Stokes spectrum can be of considerable practical use. Such applications are to extend the operating range of the Raman spectrometer, to study photodegradable samples and for the analysis of samples which exhibit strong fluorescence.     

连续和准连续激光调制谱分析技术的比对研究

为了在面对不同检测需求时,能够选取适合的调谐激光吸收光谱技术方案,对直接吸收光谱、连续调制谱和准连续调制谱三种方法进行了理论分析和实验比较。在相同实验条件下,通过同一激光器测量不同浓度CO2气体,比较了这三种方法的技术特点、信号特征、检测灵敏度。结果表明,准连续调制谱技术具有与连续调制谱相当的检测灵敏度,但是受激光能量间断和较大的寄生幅度调制影响,检测信号相对于气体吸收谱的线形失真较大,因此不太适合依赖光谱线形和线宽的压力、流速测量。为选取更加适用的激光调制谱技术,提供了参考依据。     

Optical Multistability and Wavelength Switching of a Fabry-Perot Semiconductor Laser with Multi-Detuned Light Injections

Optical multistability of a Fabry-Perot semiconductor laser is analyzed when multi-optical inputs detuned from the cavity-resonant wavelength of a semiconductor laser are injected. Many split branches are shown to be produced in the optical output versus optical input characteristics for different detuned optical inputs. It is also shown that optical outputs can be switched in any one of the optical input wavelengths by optical input pulses, based on the optical output versus input characteristics with multi-split branches.