A near-infrared methane detection system using a 1.654 µm wavelength-modulated diode laser

By adopting a distributed feedback laser (DFBL) centered at 1.654 µm, a near-infrared (NIR) methane (CH4) detection system based on tunable diode laser absorption spectroscopy (TDLAS) is experimentally demonstrated. A laser temperature control as well as wavelength modulation module is developed to control the laser’s operation temperature. The laser’s temperature fluctuation can be limited within the range of ?0.02—0.02 °C, and the laser’s emitting wavelength varies linearly with the temperature and injection current. An open reflective gas sensing probe is realized to double the absorption optical path length from 0.2 m to 0.4 m. Within the detection range of 0—0.01, gas detection experiments were conducted to derive the relation between harmonic amplitude and gas concentration. Based on the Allan deviation at an integral time of 1 s, the limit of detection (LoD) is decided to be 2.952×10-5 with a path length of 0.4 m, indicating a minimum detectable column density of ~1.2×10-5 m. Compared with our previously reported NIR CH4 detection system, this system exhibits some improvement in both optical and electrical structures, including the analogue temperature controller with less software consumption, simple and reliable open reflective sensing probe.     

甲烷检测中电压噪声的二次量化抑制及其实验研究

为了分析并抑制中红外甲烷检测中模数转换器(ADC)参考电压噪声对检测性能的影响,本文理论推导了待测浓度和电压波动的关系式,计算了电压波动对ADC实际分辨率的影响。一方面,为了降低噪声水平,设计了一种三级稳压电源,使参考电压的平均波动由原来的2mV降至0.6mV;另一方面,考虑到噪声无法避免,不同待测电压对应的数字量彼此交叠,致使无法精确测量各性能参数,本文提出了一种新型二次量化抑制方法,以有效消除电压噪声的不利影响。在不同的ADC参考电压噪声水平下,利用提出的二次量化抑制方法,实验测试了系统稳定性、精度、灵敏度和检测下限等性能。结果表明,当ADC参考电压的平均波动由2mV降至0.6mV时,测量精度可由12.9%降至5.0%以下,检测下限可由约26×10-6降至20×10-6。最小检测下限处的检测灵敏度可由约30×10-6降至20×10-6。这些证实,本文给出的基于二次量化思想的噪声抑制方法是可行的,并且该方法也可用于其它具有类似机理的气体检测。     

增益自调整型正交频分复用可见光通信终端

为了改善离散多音(DMT)调制点对点可见光通信(VLC)系统在不同通信距离下的 通信性能,设计并实现了一种基于16-PSK增益自调整型正交频分复用(OFDM )的VLC终端, 研究了自适应增益调整和信道估计技术,分别测试了VLC系统在不同工作条件和不同通信距 离下的误码率(BER)性能。实验结果 显示,在30~120 cm通信范围内,当未采用自适应增益调整以及信道估计技 术时,系统的BER范围为0.1~1．0;使用这两种技术后,系统的BER可 降至10－2 以下,并且在40~100cm 通信距离内,BER更为恒定,其值小于10－3。这表 明,采用自适应 增益调整以及信道估计 技术,可在一定程度上降低BER并改善系统的稳定性。     

基于虚拟信号处理平台的差分式中红外甲烷检测系统

利用上位机及Labview工作平台数据处理能力强 的优势,实现了一种针对甲烷(CH₄)气体的双通道差分式中红外检测系统。采用IRGJ型双 通道传感器(英国E2V公司,3.31μm 和3.90μm)检测气体浓度变化,利用Labview和数据采集(DAQ)卡(PCI6221)实现对探测器输出 信号的采集和处理,采用二阶巴特沃斯数字滤波器滤除噪声以提高信噪比(S NR),对系统硬件和软 件进行了集成。系统具有信号采集、滤波、幅值提取、浓度计算、存储及网络发布等功能 。 配备了不同浓度的CH₄气体样品,开展了标定实验,并测量了系统的精度、稳定性和检 测下限等指标。实验结果显示,系统对CH₄气体的测量范围为0~5%,对9种气体样品的 检测误差均小于6%。受传感器自身光程的限制,系统的检测下限约为60×10－6。对浓度为 2000×10－6(0.2%)的气体进行了 长达4h的浓度检测,除个别突变点外 ,检测的最大误差小于10%。由于数据处理能力强,系统功能不受资 源限制并可任意扩展。     

Wavelet-denoising technique in near-infrared methane detection based on tunable diode laser absorption spectroscopy

A novel wavelet denoising （WD） assisted wavelength modulation technique is proposed for improving near-infrared de- tection performance on methane concentration based on tunable diode laser absorption spectroscopy （TDLAS）. Due to the ability of multi-level analytical resolutions both in time- and frequency-domains, the noise contained in the differen- tial signal is greatly suppressed. Sensor mechanical part, optical part and electrical part are integrated, and a portable detection device is finally developed. Theory and formulations of the WD-assisted wavelength modulation technique are presented, and experiments are carried out to prove the normal function on the extraction of the second harmonic （2j） signal from severely polluted differential signal by using the technique. By virtue of WD＇s suppression on noises, the sensing characteristics on CH4 concentration are improved, and the limit of detection （LOD） is decreased from 4× 10-6 （without WD processing） to 10-6. The proposed technique can also be used for the measurement on the concentration of other gases with corresponding near-infrared distributed feedback lasers.     

Numerical analysis on an OOK-NRZ visible light communication system based on a single white LED

An OOK-NRZ visible light communication (VLC) system is designed by using a single white LED and a 550 nm visible photodetector. The emitting model of the single LED is established, and the general expression of the detector’s output signals under OOK modulation is deduced. With the selected LED, detector and other related parameters, the designed communication system is optimized and its performance is analyzed. The optimized communication distance between the LED and the detector is 0.54 m at the communication bit rate of 1 Mbit/s. With the best communication distance, when the signal-to-noise ratio (SNR) is larger than 6.5 dB, the bit error rate (BER) can drop to 10-4. The analytical model and theory presented in this paper can be of certain practical meanings in the design of similar communication systems.     

一种红外检测气室结构的设计和优化

鉴于传统气室结构检测灵敏度较低的问题,设计了一种椭球体反射镜气室结构,建立了椭球反射镜的数学模型,给出了其结构参数与探测器接收光强之间的关系表达式。为了获得最大的接收光强,优化了椭球镜结构尺寸。当探测头直径为2cm时,所优化的椭球镜长轴为16cm,焦距14.96cm。实验结果表明,该光路结构可使探测器输出信号幅度提高约1.7倍,这将有利于后级电路的信号放大、数模转换等处理过程,从而提高气体浓度检测的灵敏度和精度。     

红外甲烷检测仪的响应时间建模与实验研究

针对研制的基于非对称椭球聚光镜/气室的红外 甲烷(CH₄)检测系统,利用菲克第一定律,建立了用于分 析仪器响应时间的数学模型,在CH₄气体由椭球气室内向气室外扩散和由气室外向气室 内扩散两种情况下, 推导得到了气室中实时CH₄浓度与响应时间的关系表达式,计算并分析了响应时间与气 室结构参数、初始 浓度以及目标浓度的关系。计算结果显示,仪器的10～90%的响应时 间与气室的结构参数有关,而与气室内 的初始浓度和目标浓度均无关。当长轴取为7.6cm、开孔直径分别为 0.50cm和2.24cm时,计算得到的仪 器响应时间约为9.42s。利用所制作的非对称椭球聚光镜/气室和CH ₄检测仪,分别测量了仪器的响应时间, 仪器所处容器的浓度在0～10－4间变化时,其上升和下降时间 分别为7.25s和9.00s;仪器所处容器的浓度 在10－3～10－2间变化时,其上升和下降时间均约8.25s。由于实验条件未能较好满足菲克定律要求,实 验结果和理论计算结果存在一定的差异。尽管如此,本文给出的分析理论和相关模型,在估 算具有类似气室结构的检测仪响应时间方面具有应用价值。     

Performance study of an OFDM visible light communication system based on white LED array

By introducing orthogonal frequency division multiplexing (OFDM) technology, a visible light communication (VLC) system using a 5x5 white LED array is studied in this paper. The OFDM transmitter and receiver are modeled through MATLAB/Simulink tool. The electrical-optical-electrical (EOE) response of the VLC channel, which is also the response of the detector, is derived based on Lambert’s lighting model. Then the modeling on the overall OFDM/VLC system is established by combining the above three models together. The effects of the factors which include the digital modulation, Reed-Solomon (RS) coding, pilot form, pilot ratio (PR) and communication distance on the bit error rate (BER) of the system are discussed. The results show that through the use of RS coding, block pilot, quadrate phase shift keying (QPSK) modulation and a suitable pilot ratio about 1/3, under the communication rate about 550 kbit/s, the BER can be dropped to below 10-5, and the communication distance can reach 0.9 m.