Zhenhui Du

Modulation index optimization for optical fringe suppression in wavelength modulation spectroscopy

Optical fringes constitute one of the major obstacles in the gas detection based on wavelength modulation spectroscopy (WMS). In order to suppress optical fringes, a convenient method of modulation index optimization was presented, using the signal-fringe ratio as a criterion. In addition to suppressing optical fringes, the optimized modulation index enables the detection of gas absorption. This method was demonstrated in a WMS based oxygen sensor. By comparing the sensor performances with and without the use of the optimized modulation index, we showed that the optical fringes are reduced by using the optimized modulation index; furthermore, the system stability and detection limit are improved. More specifically, the long-term fluctuation of the sensor measurement is dramatically reduced by a factor of 8, and a detection limit of as low as 120 ppm (with effective optical path length of 32 cm and integral time of 2.6 s), characterized by the Allan variance, was derived. This method can be applied in other existing WMS systems without the need for additional devices or complex algorithms and has the potential to be used in both laboratory and industrial settings.

Using skin impedance to improve prediction accuracy of continuous glucose monitoring system

The continuous blood glucose monitoring system using interstitial fluid (ISF) extracted by ultrasound and vacuum is proposed in this paper. The skin impedance measurement is introduced into the system to monitor the skin permeability variation. Low-frequency ultrasound is applied on skin surface to enhance the skin permeability by disrupting the lipid bilayers of the stratum corneum (SC), and then ISF is extracted out of skin continuously by vacuum. The extracted ISF is diluted and the concentration of glucose in it is detected by a biosensor and used to predict the blood glucose concentration. The skin permeability is variable during the extraction, and its variation affects the prediction accuracy. The skin impedance is an excellent indicator of skin permeability in that the lipid bilayers of the SC, which offer electrical resistance to the skin, retard transdermal transport of molecules. So the skin impedance measured during the extraction is transformed to skin conductivity to estimate correlation coefficient between skin conductivity and permeability. Skin conductivity correlates well with skin permeability. The method and experiment system mentioned above may be significative for improving the prediction accuracy of continuous blood glucose monitoring system.

Hollow Waveguide-Enhanced Mid-Infrared Sensor for Real-Time Exhaled Methane Detection

A hollow waveguide (HWG)-enhanced tunable laser absorption spectroscopy sensor was demonstrated for the detection of exhaled methane in the mid-infrared range. A 3392-nm interband cascade laser was used to simultaneously target two neighboring strong absorption peaks and wavelength modulation spectroscopy (WMS) with the second harmonic signal normalized by the first-harmonic signal (WMS-2f/1f ) was employed for data processing. With a time resolution of 0.1 s and a detection limit of 48 ppb, the evolution of exhaled methane concentration in a single breath was observed. In addition, the HWG-based optical path is mirror-less, flexible, and light, which makes the proposed sensor suited for clinical instrumentation, point-of-care breath monitoring, and disease diagnostics.

Dynamic spectral characteristics measurement of DFB interband cascade laser under injection current tuning

The dynamic spectral properties of semiconductor lasers during its tuning are very important for frequency modulation-based applications. The spectral properties of a distributed feedback (DFB) interband cascade laser (ICL) under injection current tuning (i.e., slope efficiency, dynamic tuning rate, and instantaneous linewidth) were measured by using short delayed self-heterodyne interferometry combined with time-frequency analysis of the interferometric signal. The relations of these spectral characteristics with the injection current, tuning frequency, and operating temperature of the laser were investigated as well. The dynamic tuning rate of the laser varies from 0.07 nm/mA to 0.16 nm/mA depending on the injection current and tuning frequency, which is considerably below the static tuning rate 0.20 nm/mA. The laser instantaneous linewidth increases within 360 kHz to 760 kHz as the injection current increases or the tuning frequency increases. Unexpectedly, both the dynamic tuning rate and linewidth seem not to be related to the operating temperature of the laser. These results will be very useful for understanding the spectral properties and optimizing the frequency modulation of DFB-ICLs.

Detection of Atmospheric Methyl Mercaptan Using Wavelength Modulation Spectroscopy with Multicomponent Spectral Fitting

Detection of methyl mercaptan (CH3SH) is essential for environmental atmosphere assessment and exhaled-breath analysis. This paper presents a sensitive CH3SH sensor based on wavelength modulation spectroscopy (WMS) with a mid-infrared distributed feedback interband cascade laser (DFB-ICL). Multicomponent spectral fitting was used not only to enhance the sensitivity of the sensor but also to determine the concentration of interferents (atmospheric water and methane). The results showed that the uncertainties in the measurement of CH3SH, H2O, and CH4 were less than 1.2%, 1.7% and 2.0%, respectively, with an integration time of 10 s. The CH3SH detection limit was as low as 7.1 ppb with an integration time of 295 s. Overall, the reported sensor, boasting the merits of high sensitivity, can be used for atmospheric methyl mercaptan detection, as well as multiple components detection of methyl mercaptan, water, and methane, simultaneously.

Hollow waveguide-enhanced mid-infrared sensor for fast and sensitive ethylene detection

Purpose This paper aims to provide a sensor for fast, sensitive and selective ethylene (C2H4) concentration measurements. Design/methodology/approach The paper developed a sensor platform based on tunable laser absorption spectroscopy with a 3,266-nm interband cascade laser (ICL) as an optical source and a hollow waveguide (HWG) as a gas cell. The ICL wavelength was scanned across a C2H4 strong fundamental absorption band, and an interference-free C2H4 absorption line located at 3,060.76 cm−1 was selected. Wavelength modulation spectroscopy with the second harmonic detection (WMS-2f) technique was used to improve the sensitivity. Furthermore, the HWG gas cell can achieve a long optical path in a very small volume to improve the time response. Findings The results show excellent linearity of the measured 2f signal and the C2H4 concentration with a correlation coefficient of 0.9997. Also, the response time is as short as about 10 s. The Allan variance analysis indicates that the detection limit can achieve 53 ppb with an integration time of 24 s. Practical implications The ethylene sensor has many meaningful applications in environmental monitoring, industrial production, national security and the biomedicine field. Originality/value The paper provides a novel sensor architecture which can be a versatile sensor platform for fast and sensitive trace-gas detection in the mid-infrared region.

Numerical investigation of two-dimensional imaging for temperature and species concentration using tunable diode laser absorption spectroscopy

A technique has been developed to obtain simultaneous tomographic images of temperature and species concentration based on line-of-sight (LOS) diode laser absorption spectroscopy. Multiple LOS measurement paths divide the combustion zone into an M × N grid. With the assumption of uniform distribution around the grid intersection points, spatial distribution of temperature or a particular chemical species can be inferred from the M+N path averages because each measurement path passes through different grid boxes and carries their information of temperature and species concentration. The measurement accuracy and resolution depend on the total number of paths measured. The feasibility of this method is verified by simulated image reconstruction of an engine temperature distribution.

A Laser Doppler Interferometric System for Measuring Motion of Vibrating Combs

Micro-Electro-Mechanical Systems have been growing rapidly over recent years. However, measurement of their motions is a difficult problem. A laser Doppler interferometric system for measuring motion characteristics of vibrating combs in MEMS is presented. The working principle and optical diagram of the system are explained. Some key challenges in the system design are discussed. The velocity, displacement, and frequency can be acquired by demodulating the phase-modulated Doppler signal and processing the data by software.

Preliminary investigation of the capillary adsorption for a hollow waveguide based laser ammonia analyzer

Hollow Waveguide (HWG) is usually used as a gas cell in an infrared gas sensor feathered with low-volume and high-sensitivity. However, the measured concentration is often distorted by the interference of the adsorption of gas molecules on the inner wall surface of the HWG. This adsorption is a type of physical absorption called capillary adsorption. In order to correct this distortion, the characteristics of HWG adsorption of ammonia were investigated by using the laser analyzer itself under HWG heating-cooling process and various ammonia flow rate in the HWG. The results showed that the readout of ammonia concentration increased by 17.8% when heating the HWG for no-flowing ammonia in the HWG, and the readout undergone a process of increase to fast decrease to slow increase when heating the HWG for flowing ammonia in the HWG at various flow rate. These surely come from the adsorption and desorption of ammonia on the inner wall surface of the HWG. The preliminary investigation provides a quantitative readout distortion and a creditable evidence for further study about the adsorption of HWG.

Indoor carbon dioxide monitoring with diode laser absorption at 2 μm

In order to investigate the variation of indoor carbon dioxide concentration and how it changes with human activities, a tunable diode laser absorption spectroscopy (TDLAS) system was used to monitor the indoor CO2 concentration. Based on Wavelength Modulation Spectroscopy double frequency detection (WMS-2f), the 2v1+v3 characteristic line (4991.26 cm-1) of CO2 was measured by a DFB laser. The measured concentration values were calibrated by means of a cell filled with reference gas. The results show that the daily average indoor CO2 concentrations is about 419ppm which is slightly higher than that of the outdoor and the changing range is between 380ppm and 510ppm in a day. The indoor CO2 concentration was influenced by the change of ventilation and indoor staff. The respiration of the indoor staff makes a greater impact on a relatively confined indoor CO2 concentration. The CO2 increasing rate is measured to be 80ppm/hour in the case of occupant density of 0.06 people/m3. Therefore, the staff crowded indoor should ventilate timely to prevent excessive CO2 causing people discomfort.