2f Signal Research Articles

Research on the NO concentration of marine diesel emission by tunable diode laser absorption spectroscopy technique

This work investigates the feasibility of TDLAS technique in the NO concentration measurement for the marine diesel engine. A system in laboratory is devised for the NO steady state measurement. An ICL laser whose center wavelength is 5263nm is chosen for the detection. With the direct detection method, a resolution about 2ppm is realized, and the relative error is less than 5%. In contrast to the direct detection method, the harmonic detection method realizes a higher resolution, and the error is better less 3%. Considering with the temperature fluctuation of the diesel emission, the absorption line strength is calculated from 300K to 500K. At the same time, the intensity of the 2f signal is measured, which is proportional to the change of the line strength. So a modification can be imported easily to avoid the error produced by the temperature fluctuation. The TDLAS proposed in this work can realize a high precision, high resolution, and transient measurement for the marine diesel emission.

Calibration-free 2f WMS with in situ real-time laser characterization and 2f RAM nulling.

This Letter demonstrates a new calibration-free 2f wavelength modulation spectroscopy (WMS) technique to measure gas concentration and pressure without the need for laser precharacterization. A 1650-nm laser diode is used for methane concentration and pressure measurements for pressures up to 4 bar and for a modulation index (m) of 2.2. All laser parameters such as the intensity, linear and nonlinear intensity modulation (IM), frequency modulation (FM) characteristics, the phase difference ψ1 between the FM and the linear IM, and the phase difference ψ2 between the FM and the nonlinear IM are accurately estimated in situ and in real time. This technique accounts for variations in these parameters that arise due to scanning of the laser's center wavelength, laser temperature variations, and aging of the laser. The laser is modulated at its phase quadrature frequency at which the linear IM and the FM are orthogonal to each other (ψ1=90°). This ensures that the two linear IM-dependent distorting Fourier components are orthogonal to the detection axis, and the undistorted 2f signal is recovered. This simplifies the simulation and gas parameter-extraction process. Finally, 2f RAM nulling is implemented to remove the significant absorption-independent 2f residual amplitude-modulation (RAM) signal that is seen to cause significant distortion of the 2f signal and detector saturation.

Immunity to Laser Power Variation in a DFB Diode Laser Based Optical Gas Sensor Using a Division Process.

The division process used in a DFB diode laser-based optical gas sensor was studied to improve the immunity to laser power variation. Residual amplitude modulation (RAM) in wavelength modulation spectroscopy (WMS) detection was eliminated by intensity normalization using a division process. As a result the detected harmonic signals showed a significant improvement in line shape. For the first harmonic (1f) signal, Bias was improved from 38.7% to 1.2%; Baseline Difference was improved from 2.7% to 0.69% and Asymmetry was improved from 15.4% to 0.22%. For the second harmonic (2f) signal, the Asymmetry Coefficient was improved from 103% to 5.1%. Moreover the division process can further suppress the influence of unstable laser power. As a result, for the 1f signal, stable detection with a variation coefficient of 0.59% was obtained over a wide dynamic range (0.38–8.1 mW). For the 2f signal, stable detection with a variation coefficient of 0.53% was obtained from 0.64 mW to 8.27 mW. The test results showed a good agreement with the theoretical analysis and the proposed method has considerable potential application in gas sensing.

Analysis of inaccuracy induced by intensity variation of a DFB laser in fibre optic multipoint 2f-WMS measurements of methane near 1666 nm

The analysis of methane concentration measurement inaccuracy induced by the intensity variation of a DFB laser at 1666nm in a multipoint fibre optic sensor has been reported. The measurement is based on wavelength modulation spectroscopy (WMS) with second harmonic (2f) detection technique. The 2f signals are normalised to the average laser intensity and the detector gain to eliminate the effect of transmission loss and laser intensity variation on the measurements. The measurements are conducted at two laser intensities, defined as nominated intensity and 73% of the nominated intensity, to quantify the measurements errors due to the intensity change. The experimental results show that a significant change in the laser intensity yields differences in the 2f signals; as a result, errors were induced. A model is developed to quantify the errors in the concentration measurements. The maximum deviations for single cell, 2-Cell, and 3-Cell measurements to the given concentration of 10%, 7.5%, and 5.667% are calculated to be 9.764%, 7.2235%, and 5.368%, respectively. The concentration error increases with the cell number because of the accumulated background transmission loss of the gas cells in 2-Cell and 3-Cell in comparison to a single cell measurement.

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 detection 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 differential 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 (2f) 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.

Absorption spectrum of Quasi-continuous laser modulation demodulation method

A software phase-locked amplifier demodulation method is proposed in order to demodulate the second harmonic (2f) signal of quasi-continuous laser wavelength modulation spectroscopy (WMS) properly, based on the analysis of its signal characteristics. By judging the effectiveness of the measurement data, filter, phase-sensitive detection, digital filtering and other processing, the method can achieve the sensitive detection of quasi-continuous signal The method was verified by using carbon dioxide detection experiments. The WMS-2f signal obtained by the software phase-locked amplifier and the high-performance phase-locked amplifier (SR844) were compared simultaneously. The results show that the Allan variance of WMS-2f signal demodulated by the software phase-locked amplifier is one order of magnitude smaller than that demodulated by SR844, corresponding two order of magnitude lower of detection limit. And it is able to solve the unlocked problem caused by the small duty cycle of quasi-continuous modulation signal, with a small signal waveform distortion.

A new method of background elimination and baseline correction for the first harmonic

A new method of background elimination and baseline correction is proposed, since there are background signal and larger baseline signal in the first harmonic (1f) of the tunable diode laser absorption spectroscopy (TDLAS). The laser-associated intensity modulation signal, electronic noise, and optical interference fringes of the 1f background are analyzed. Harmonic detection in none absorption spectral region (HDINASR) is used to eliminate the background signal. Then the relationship curve between current and intensity is given in different operating temperatures to design a remaining baseline correction method after eliminating the background. The principle of background signal searching and the LabView software flow chart are also given. The TDLAS experimental system is designed to detect hydrogen fluoride (HF) gas. According to spectral line selection principle, the absorption line -1312.59 nm is selected, whose operating temperature is set at 27.0 ℃ and the background temperature is set at 30.2 ℃. After eliminating the background and correcting the baseline, signal distortion is significantly improved and baseline is corrected. Then it is verified that the method is valid at other operating temperature of the laser (26.7-27.2). And the improvement of HF gas concentration is quantitatively analyzed. It is convenient for the subsequent processing of 1f signal.

Apodized 2f/1f wavelength modulation spectroscopy method for calibration-free trace detection of carbon monoxide in the near-infrared region: theory and experiment

We introduce the basics of an apodized 2f/1f wavelength modulation method for the spectroscopy of the R(9) transition line in the first overtone band of carbon monoxide (12C16O) in near-infrared (NIR) region around 2.33 μm. Performance of the method is investigated for high gas concentrations beyond the optically thin limit to generalize common 2f/1f wavelength modulation spectroscopy (WMS) reported by Rieker et al. (Appl Opt 48:5546, [28]). Numerical simulations are performed based on real experimental parameters associated with a NIR spectrometer designed in our laboratory. The results primarily show a more linear response and less error than occurred in the common WMS-2f/1f method for an optically thick sample. It is also theoretically shown that the apodized method enables sharpening the spectrum without peak displacement compared to the common WMS-2f/1f method. The validity of the method is verified experimentally by the trace detection of an air-broadened R(9) CO absorption line centered at 4,294.637 cm−1 at atmospheric pressure and room temperature. The effect of a so-called scaling k-factor on the sharpening of WMS-2f/1f signal is investigated through trace simulation and detection of CO and methane (CH4) lines in the scanning range of a distributed feedback laser. The obtained results show very good agreement between simulation and experiment.

Extending the applicability of the Arndt formula in wavelength modulation spectroscopy for absorbance in the lower percent range

The Arndt formula for Lorentzian signals broadened by modulation is enhanced for the usage on 2f WMS (wavelength modulation spectroscopy) signals produced by spectroscopic lines with high absorption (percent range). Next to the first order approach of the Beer–Lambert law, which is covered by the Arndt formula, a second order term is included for a better approximation of the damped Lorentzian line shape. This second order approximation of the 2f signal can be described by a combination of several components created by the Arndt formula. The error of a pure Arndt evaluation and the improvement of the Arndt extended technique are illustrated in the example of a humidity measurement performed at 100°C and up to 100vol%. The energy transition at ν=10,526.274910cm−1 is used in this setup. With the presented technique, the error is reduced by a factor of 90.

Performance improvement of a near-infrared CH4 detection device using wavelet-denoising-assisted wavelength modulation technique

By introducing wavelet-denoising (WD) in the traditional tunable diode laser absorption spectroscopy (TDLAS), a novel CH4 detection device based on WD-assisted wavelength modulation technique was experimentally demonstrated. Formulation and detection procedure were presented, and experiments were carried out to prove the normal function on the extraction of 2f signal from severely polluted differential signal with WD algorithm. With the fabricated device, sensing characteristics were measured on CH4 concentration. The response time is less than 8s within the whole detection range of 0–50kppm. By virtue of the suppression on noises with WD, the minimum detection limit (MDL) is dropped from 4 to 1ppm, the maximum detection error is minimized from 6.2% to 3.8% over the concentration range of 4–50kppm, and the decreased Allan deviation (per an average observing time of 9min for 24h) from 0.13 to 0.08ppm suggests enhanced stability. The proposed device also exhibits improved performances compared with our previously reported sensors based on simple direct spectroscopy absorption (DSA) technique and wide-band incandescence wire-source.

Design of a fibre optic sequential multipoint sensor for methane detection using a single tunable diode laser near 1666 nm

The design of a fibre optic sequential multipoint sensor using a single DFB laser source for measurement of average concentration of methane has been reported. A theoretical model for sequential multipoint sensor is developed. The validation of the sensor is conducted using methane gas cells connected in series via fibre optics to form 2-Cell and 3-Cell networks. The second harmonic detection of wavelength modulation spectroscopy (2f-WMS) is employed. Normalisation of the 2f signals with respect to the laser average intensity and the detector gain is used to correct for transmission variation. The measurements are performed at four laser modulation depths to compare the 2f-WMS signals for the target test conditions. The Q(6) transition of the 2v3 band of methane near 1666nm has been selected because the transition is relatively free of interference from water vapour and absorption by other major gases in that region. The sequential multipoint sensor has some unique advantages over the single cell and multiplexed multipoint sensors. These include the average concentration measurements, the use of only a single laser source, a detector, and a fibre optic to connect series of gas cells, no need for multiplexing techniques to distribute the laser intensity among multi gas cells, and relatively simpler data analysis.

Simultaneous detection of CO and CO2 at elevated temperatures using tunable diode laser absorption spectroscopy near 1570 nm

Simultaneous measurements of CO and CO2 at elevated temperatures are demonstrated using a single semiconductor distributed-feedback (DFB) laser near 1570 nm. Wavelength modulation spectroscopy with second-harmonic detection is used to improve the detection sensitivity and accuracy. A proper line pair near 6368.086 and 6368.330 cm−1 is selected using some line-selection criterions for the target temperature range of 300–1000 K. Normalization of the 2f signal with the 1f signal magnitude is used to remove the need for calibration and correct for transmission variation due to beam steering, mechanical misalignments, soot, and windows fouling. The CO and CO2 concentrations measurements are within 2.21% and 2.55% of the expected values over the tested temperature range of 300–1000 K. The minimum detectable concentrations of CO and CO2 at 1000 K are 80 ppm m and 153 ppm m, respectively.

Analysis of Optical Fiber Methane Gas Detection System

With the advances in clean energy, natural gas (methane) takes a greater proportion in total energy consumption. Methane gas detection technology has become an important research direction for fire alarm area. Using near-infrared absorption spectroscopy method, this gas detection technology has many unique advantages, such as intrinsic safe, good stability, good selectivity, long working life, etc. In this paper, a gas detection system based on wavelength modulation and second harmonic (2f) signal detection technology is proposed; the process of the first and second harmonic demodulation is simulated; the feasibility of using first harmonic as light intensity reference and the linear relationship between the concentration and second harmonic is analyzed. Finally, a theoretical guidance for product developing with above technology is provided as well.

Measurements of ultra-low DC fields by high-Tc superconducting cores: The effect of calcination temperature

In this study, the effects of weak links between grains of the ceramic superconductors on the sensitivity of a DC magnetic field sensor were examined. The evolution of 2nd harmonic signal strength, which determines the magnetometer sensitivity, with critical current density Jc was especially analyzed. The Jc values were adjusted by synthesis of Y-123 superconductors at different calcination temperatures. It was observed that the strength of 2f signal increases as the calcination temperature and so, the Jc values increases. This is contrary to the common expectations. Our experiments have shown that the 2f signal changes quite linearly with DC field in a wide dynamic range (10−4 to 10−9T) on the sample calcined at 950°C. The ways of measuring DC fields as low as 1nT are described in the paper.

Novel method for correcting light intensity fluctuation in the TDLAS system

A novel method for online correction of light intensity fluctuation in a practical tunable diode laser absorption spectroscopy (TDLAS) system with wavelength modulation is presented. The proposed method is developed according to the linear relation between peaks at multiple frequencies of sine modulation in the power spectral density of the demodulated second-harmonic (2f) signal and the incident light intensity. Those peaks are demonstrated experimentally and explained as residual power at the first-harmonic and third-harmonic frequencies after 2f demodulation of the residual amplitude modulation signal due to the limited integrating time constant of the lock-in-amplifier. This method can achieve real-time correction of light intensity fluctuations with only little calculation. It can work well in a very large range of light intensity and has great potential applications in the wavelength modulation spectroscopy system.

Research of quantitative remote sensing of natural gas pipeline leakage based on laser absorption spectroscopy

Quantitative remote sensing of natural gas pipeline leakage based on laser absorption spectroscopy is reported. Key technical issues of mobile laser remote sensing are discussed through simulating leak detection. For quantitative remote sensing of trace leakage, the signals with counter-phase and isovalue of residual amplitude modulation are introduced. Then the offset is compensated, the influence of harmonic signal is reduced and sensitivity is improved. The improved soft-threshold denoising method is proposed by analyzing the characteristics of echo absorption spectrum. The results show that the signal-to-noise ratio is improved two times as great as that by the traditional soft-threshold denoising method and the shape of second harmonic (2f) signal is well preserved. The minimum remote sensitivity will be 80 ppm/m under the mobile remote sensing.

Supersonic Mass-Flux Measurements via Tunable Diode Laser Absorption and Nonuniform Flow Modeling

Measurements of mass flux are obtained in a vitiated supersonic ground test facility using a sensor based on line-of-sight (LOS) diode laser absorption of water vapor. Mass flux is determined from the product of measured velocity and density. The relative Doppler shift of an absorption transition for beams directed upstream and downstream in the flow is used to measure velocity. Temperature is determined from the ratio of absorption signals of two transitions (lambda(sub 1)=1349 nm and lambda(sub 2)=1341.5 nm) and is coupled with a facility pressure measurement to obtain density. The sensor exploits wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f) for large signal-to-noise ratios and normalization with the 1f signal for rejection of non-absorption related transmission fluctuations. The sensor line-of-sight is translated both vertically and horizontally across the test section for spatially-resolved measurements. Time-resolved measurements of mass flux are used to assess the stability of flow conditions produced by the facility. Measurements of mass flux are within 1.5% of the value obtained using a facility predictive code. The distortion of the WMS lineshape caused by boundary layers along the laser line-of-sight is examined and the subsequent effect on the measured velocity is discussed. A method for correcting measured velocities for flow non-uniformities is introduced and application of this correction brings measured velocities within 4 m/s of the predicted value in a 1630 m/s flow.

Acute promyelocytic leukemia with a novel variant (15;17) rearrangement

Dear Sir, Acute promyelocytic leukemia (APL) is characterized by the translocation of chromosomal (15;17)(q22;q21) involving the fusion of the promyelocytic leukemia (PML) and RARA genes, resulting in a PML/RARA fusion gene on derivative chromosome 15 and reciprocal RARA/PML fusion gene on derivative chromosome 17 [1]. Variant rearrangements have been reported in 5% to 10% of APL patients [2]. We report a case of APL associated with a novel variant translocation. The patient is a 67-year-old male presented with epistaxis. He was found to have pancytopenia and rare abnormally hypergranular, bilobed, immature cells consistent with leukemic promyelocytes on peripheral blood smear. No definite Auer rods were seen. Additional laboratory tests show a prolonged prothrombin time (PT) and elevated D-dimer. Bone marrow aspirate smear showed replacement by the same type of cells noted on the peripheral blood smear. Flow cytometric analysis of the marrow aspirate indicated the presence of a large-sized, immature, hypergranular myeloid population. This population brightly expressed CD33, CD13, CD117, and MPO, dimly expressed CD64, and was negative for CD34, HLA-DR, and CD11b. Cytogenetic G-banded analysis of 20 metaphase cells from bone marrow revealed a karyotype of 46,XY, del (17) (q12q21.1), ins(15;17)(q22;q21.2q25.3)[18]/46,XY[2] (Figs. 1 and 2). FISH analysis using the dual-color, dual-fusion PML (15q22-red)/RARA (17q21-green) probe showed a variant abnormal pattern (Fig. 2). Typical APL patients have 1R (red):1G (green):2F (fusion) signals, indicating the presence of two fusion genes. In our case, this typical abnormal pattern (Fig. 3A) was seen in only 1.5% of the cells. On the other hand, 85% of the cells had 2R:1G:1F (Fig. 3B). PML gene remained on the der (15), and PML/RARA was inserted into or translocated to the der (15) (Fig. 2). This indicated an absence of the RARA/PML fusion gene on the derivative (der) chromosome 17 and the insertion or translocation of the PML/RARA fusion gene to the der (15). The remainder of the nuclei (3.5%) demonstrated a normal signal pattern (2R:2G) (data not shown). Metaphase FISH on ten cells using the TelVysion 17q probe showed that the subtelomeric region of the long arm of chromosome 17 was present on both 17 homologs (data not shown). Reverse transcription polymerase chain reaction performed on the RNA extracted from peripheral blood is positive for PML–RARA transcripts. This case was presented at the 2011 annual meeting of the College of American Pathology in Grapevine (Dallas/Ft. Worth), Texas. M. Bellone : T. Ahmed :Y. Hu Department of Anatomic Pathology, Stony Brook University Medical Center, Stony Brook, NY 11794, USA

Calibration-free wavelength modulation absorption spectrum of gas

The calibration-free wavelength modulation absorption spectrum is studied based on the second harmonic (2f) signal to remove the calibration procedure for tunable diode laser absorption gas sensing. The simulated 2f signal is obtain from the analysis of peak and trough from measured 2f signal line shape. The gas concentration is calculated by the linear fitting of measured and simulated 2f signals. The experiment on CO2 detection is carried out at transition of 6336.24cm-1 in a 10 cm absorption cell. The results prove that the calibration-free wavelength modulation absorption spectrum is suited for on-site gas sensing at various conditions. The average absolute error of gas concentration is 0.67% at modulation index ranging from 1.8 to 3.2. With gas concentration and pressure varied, the average absolute errors of gas concentration are measured to be about 0.98% and 0.74%.

Real-time equivalence ratio measurements in gas turbine combustors with a near-infrared diode laser sensor

A rapid (2kHz) near-infrared tunable diode laser (TDL) absorption sensor is developed for real-time measurements of methane concentration (and thus equivalence ratio) in gas turbine combustors. The non-intrusive sensor is based on fixed-wavelength laser absorption of methane near 1.65μm using a single fiber-coupled diode laser. This CH4 absorption wavelength is free of interference from the absorption of other species in air. Wavelength modulation spectroscopy (WMS) is combined with second-harmonic detection to improve the sensor sensitivity and accuracy. WMS-1f signal is used to normalize the 2f signal to remove the need for calibration and account for the laser transmission variations in gas turbine combustors. The near-IR TDL sensor is first calibrated in a static cell with CH4 and N2 mixture to determine the line-strength and find the laser setpoint. Test with a gas turbine combustor operating at atmospheric pressure is then conducted to demonstrate the sensor accuracy and response time. The TDL sensor is used to characterize equivalence ratio fluctuations in the combustor before the fuel/air mixtures enter the combustion chamber. The gain between heat release and equivalence ratio fluctuation is measured when the fuel flow rate is modulated at different levels. The combustion test results demonstrate the utility of the TDL sensor as a useful diagnostic tool to study flame characteristics and evaluate different combustor designs.