Tunable Diode Laser Research Articles

Investigations on Pressure Broadening Coefficients of NO Lines in the 1←0 Band for N2, CO2, Ar, H2, O2 and He

A tunable diode laser absorption spectroscopy (TDLAS)-based spectrometer employing a mid-infrared (Mid-IR) interband cascade laser (ICL) was developed and used to determine pressure broadening coefficients of two NO absorption transitions at 1914.98 cm−1 and 1915.76 cm−1 in the fundamental (1←0) band (R11.5 Ω1/2 and Ω3/2) for CO2, N2, Ar, O2, He, and H2. For the first time, a reliable and consistent set of six different pressure-broadening coefficients for the NO line has been measured by a consistent approach covering pressures from 100 to 970 mbar at a temperature of 294 K. Air pressure broadening has been calculated based on N2 and O2 coefficients. The stated pressure-broadening coefficients for N2, CO2, Ar, H2, O2, He, and Air have relative errors in the 0.5–1.5% range. For CO2 and H2, broadening results of NO (1←0) band (R11.5 Ω1/2 and Ω3/2) lines are reported for the first time. The results are also compared to previously available literature data. It was found that the broadening coefficients for O2 and Air are in agreement with literature values, whereas results for Ar and He show larger differences.

1.7 THz tuning range pivot-point-independent mode-hop-free external cavity diode laser.

A novel mode-hop-free (MHF) tunable external cavity diode laser (ECDL) is demonstrated without mechanical pivot-point tuning. By corotating a periscope with an etalon and a narrow band pass (NBP) filter inside an external cavity, the cavity single longitudinal mode selection can be maintained, and continuous tuning can be achieved with optimal synchronization. A MHF continuous tuning range of 1.7 THz has been achieved with a semiconductor gain chip at the wavelength of 652 ± 2 nm experimentally, and the theoretical tuning range can reach over 4.8 THz. The laser linewidth is estimated to be less than 1 MHz (FWHM) by a scanning Fabry-Perot (F-P) and a Michelson interferometer. 1 mW output power with variation of less than 10% in the tuning region of roughly 1.7 THz has been demonstrated.

Absolute wavelength scanning interferometry for measuring the thickness of optical elements.

A technique for measurement of the thickness of optical elements using absolute wavelength scanning interferometry is presented in this paper. To achieve high-grade optical components and systems, the thickness of both planar and non-planar optical components must be measured with an accuracy of a few micrometers. The proposed technique is based on the Fizeau interferometer and interconnects data from three different tunable laser diodes yielding a long effective wavelength range and thus low measurement uncertainty. The uncertainty of the central thickness measurement ranges from hundreds of nanometers to a few microns. The method allows to measure the thickness of both flat optical elements as well as lenses with curved surfaces. Moreover, the areal information provided by the interferometry and its high angle sensitivity help to quickly and precisely align the measured component and reduce misalignment errors. The results of thickness measurements have been validated and cross-tested with other techniques. In addition to the thickness, the technique provides some additional information (wedge, surface form error) in the case of flat samples and can be easily and quickly modified (mounting of a Fizeau transmission sphere) to measure other essential parameters of optical elements. Thus, this one approach can replace many single-purpose measuring devices while maintaining high accuracy.

Determination of methane during anaerobic digestion by tunable diode laser absorption spectroscopy (TDLAS)

This paper presents the design and application of a near-infrared laser absorption-based sensor for methane measurements during anaerobic digestion using an optical probe inserted into an anaerobic tank reactor. The wavelength of the laser was scanned across the absorption lines of methane near 1.65 µm to provide the real-time concentration. A probe was designed for optical measurement. This technique was validated by measuring methane in the laboratory and anaerobic digestion tank reactor. A set of reference values spanning from 30 to 52%, covering the typical concentrations during anaerobic digestion, were measured to validate the reliability of the system. The spectrometrically derived methane concentrations showed high consistency with the reference values, and an R2 of 0.999 revealed that the system was reliable. In addition, the detection limit was estimated by the uncertainty of the intercept to be 0.27% with an accuracy of 0.1% and a temporal resolution of 1 s.

Microbiological total viable count detection based on tunable diode laser wavelength-modulation spectroscopy

Total viable counts (TVC) are vitally critical to health management, and the quantitative testing of TVC is an essential part of the microbiological testing process. This work demonstrates the rapid and accurate measurement of microbiological TVC using a wavelength-modulated tunable diode laser based on absorption spectroscopy (WM-TDLAS). To detect CO2 in sealed culture bottles, a tunable distributed feedback (DFB) laser with a central wavelength of 2004 nm was employed as the light source. Experiments with Salmonella show that the technique can generate effective second harmonic signals, allowing the detection limit to be 71.8 ppm. Simultaneously, the optimal parameters for measuring the growth curve of Salmonella at various concentrations were determined to establish the standard curve used to calculate the TVC. When compared to the conventional method ISO 4833:2013, the new detection method illustrates good correlation (R2 = 0.98), and the results can be obtained within the detection range of 1–8 log10(CFU/mL) in 4–12 h, saving at least 36 h. Thus, it was shown that WM-TDLAS is a user-friendly, non-invasive technique that allows rapid and accurate assessment of TVC.

Temperature Measurements by Wavelength Modulation Diode Laser Absorption Spectroscopy with Logarithmic Conversion and 1f Signal Detection

A new version of a sensor for temperature measurements in the case of strong laser intensity fluctuation was developed. It was based on tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation, logarithmic conversion of the absorption signal, and detection of the first harmonic of the modulation frequency. The efficiency of the technique was demonstrated under experimental conditions with excess multiplicative noise. Temperature was evaluated from the ratio of integrated absorbance of two lines of the water molecule with different lower energy levels. Two algorithms of data processing were tested, simultaneous fitting of two spectral ranges with selected absorption lines and independent fitting of two absorption lines profiles. The correctness of the gas temperature evaluation was verified by simultaneous measurements with a commercial thermocouple. An error in temperature evaluation of less than 40 at 1000 K was achieved even when processing a single scan of the diode lasers.

Optimization of beam arrangement for tunable diode laser absorption tomography reconstruction based on fractional Tikhonov regularization

Beam arrangement with limited projections based on tunable diode laser absorption spectroscopy is the key to achieving a more accurate two-dimensional reconstruction of the combustion. Using fractional calculus theory, a beam optimization method based on fractional Tikhonov regularization is proposed. The beam arrangement function based on fractional Tikhonov regularization is established by extending the standard Tikhonov regularization to fractional modes. The genetic algorithm is used to analyze the calculation results of different orders in a range of (0, 1), and the beam arrangement is obtained. Using 20 laser beams to scan the characteristic absorption spectrum of H<sub>2</sub>O in the near-infrared band 7185.6 cm<sup>–1</sup>, modeling the calculations in a 10×10 element discrete tomography domain, and comparing the reconstruction results of the five beam arrangements for different Gaussian distribution models, the beam arrangement based on fractional Tikhonov regularization shows more obvious advantages. This design method proposed in this work is valuable for the theoretical study of the optimal design of two-dimensional measurement beams based on the tunable diode laser absorption spectroscopy technique, which can promote the application of this technique in the two-dimensional reconstruction of complex engine combustion and combustion efficiency improvement.

Research on highly sensitive detection of oxygen concentrations based on tunable diode laser absorption spectroscopy

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

Noise Immune Absorption Profile Extraction for the TDLAS Thermometry Sensor by Using an FMCW Interferometer

Tunable diode laser absorption spectroscopy techniques usually utilize direct absorption spectroscopy (DAS) to measure absorbance profiles for temperature measurements. The DAS method is sensitive to thermal noises and fails to work in harsh application cases. A noise immune method is proposed for robust temperature measurement by using coherent detection and retroflected optical alignment. The absorption spectrum is shifted to a high-frequency band of MHz level away from ambient noises. The optical alignment suppresses thermal radiations along the laser path. Gas temperatures were measured in cases of both laboratory and porotype setup of aeroengine, i.e., the steady and the harsh states. Transition lines of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O at 7182.94 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , 7185.59 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , and 7444.35 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> were selected for performance evaluations of target molecules, i.e., of water vapor. Performance comparisons were made with the DAS method for temperature detection. In a steady-state case, standard deviations of the proposed method were below 0.9 K at SNR of 25 dB, while those of the DAS method were above 3.9 K. In an onsite harsh application case, the fluctuations of path-averaged temperatures obtained by the proposed method were also smaller. The coherent detection and retroflected alignment enable better performance in hostile cases.

Single Spectral Line Method for TDLAS Imaging of Temperature and Water Vapor Concentration

A single spectral line method for gas temperature as well as concentration extraction was proposed by using iterations with a fuzzy proportional–integral-derivative (PID) controller in tunable diode laser absorption spectroscopy (TDLAS). Usually, two or more spectral lines are essential to extract temperatures from coupled gas concentration and pressure. The proposed method utilized the line shape of a single absorption spectrum to decouple gas parameters along the laser path simultaneously and simplified the optical implementations. Differential equations of the Voigt profile were introduced at multiple wavenumbers to extract gas parameters from a single spectral line. A fuzzy PID controller was utilized to iteratively achieve the single spectral line from widely existed overlapping spectra. A relative temperature error within 2.5% at reference temperatures ranged from 300 to 2300 K was achieved by using the spectral line of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7444.35\,\,{\mathrm {cm}}^{-1}$ </tex-math></inline-formula> from noise free data. At low temperatures below 350 K, the proposed method using the single spectral line of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7181.14\,\,{\mathrm {cm}}^{-1}$ </tex-math></inline-formula> yielded smaller errors in noisy cases, and the temperature errors were verified within 5 °C on the water-based thermostat below 100 °C using the single spectral line. Experiments of axisymmetric counterflow flames at different heights were also implemented. The proposed method effectively reconstructed distributions of flame temperature and water vapor concentration inside, by only using a single spectral line of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7444.35\,\,{\mathrm {cm}}^{-1}$ </tex-math></inline-formula> .

A near infrared H2S leakage detection system based on WDO-ELM using a digital lock-in amplifier combined with discrete wavelet transform filter

A gas leakage detection system was developed based on tunable diode laser absorption spectroscopy (TDLAS) technique with the combination frequency absorption band of hydrogen sulfide (H2S) gas molecules at 1578 nm for telemetry. For further telemetry distance and signal-to-noise ratio (SNR), a Fresnel lens with a diameter of 250 mm is utilized to focus the laser beam that diverges gradually with the increase of telemetry distance. A digital lock-in amplifier (DLIA) combined with discrete wavelet transform (DWT) filter was devised to process faint photoelectric signal received by InGaAs photodiode. Meanwhile, a global optimization algorithm, combined wind driven optimization (WDO) with extreme learning machine (ELM), named WDO-ELM algorithm, is applied for H2S concentration inversion to make up the limitation of ELM algorithm and seek for the optimal weight and threshold in a more extensive range. The reliability and sensitivity of the H2S leakage detection system have been verified by calibration and stability experiments. The limit of detection (LoD) reaches a minimum of 0.983 parts-per-million·meter (ppm·m) with a 96 s integration time by Allan-Werle deviation analysis. This self-developed H2S leakage detection system with high integration and low cost is more convenient for commercial application in tank farm to realize remote detection of gas leakage.

Infrared Trace Gas Sensing With a Fast Perovskite Nano-Structure Laser Photodetector

In the present work, we firstly demonstrated infrared perovskite nanorods as a state-of-the-art fast photo-sensing core so that they could adapt to the demand of infrared trace gas sensing. Periodic perovskite nanorods can enhance light absorption in the infrared regime, boost the distributed electromagnetic field, and benefit the collection of photo-generated carriers. An as-fabricated photodetector (18 A/W) with trans-impedance amplifier circuits can be compatible with the designed high-precision tunable diode laser absorption spectroscopy (TDLAS) gas sensing system. A digital quadrature phase-locked-loop amplifier is used to demodulate the second harmonic (2f) signal for the TDLAS gas sensing circuit, where the circuit exhibits a high detectivity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.138\times 10^{11}$ </tex-math></inline-formula> Jones) and 2800 signal-to-noise ratio (SNR). A good linear relationship is observed between the amplitude of the 2f signal and the CH4 concentration in the range from 100 ppb to 2000 ppm. Our perovskite-based gas-sensing system is quite promising for future infrared trace gas sensing applications, such as medical expiratory gas sensors and greenhouse gas monitoring.

High precision measurement of spectroscopic parameters of H&lt;sub&gt;2&lt;/sub&gt;S in 6320—6350 cm&lt;sup&gt;–1&lt;/sup&gt; band

As a highly corrosive and highly toxic gas, hydrogen sulfide (H&lt;sub&gt;2&lt;/sub&gt;S) is an important intermediate product or pollutant in many fields such as chemical industry, energy and environment. Accurate online measurement of its concentration is of great significance for process control and production safety. Tunable diode laser absorption spectroscopy (TDLAS), as a quantitative absorption spectroscopy technique, is suitable for high-precision on-line measurement of H&lt;sub&gt;2&lt;/sub&gt;S concentration in atmospheric environmental monitoring and industrial processes control. Considering that most of the spectroscopic parameters of H&lt;sub&gt;2&lt;/sub&gt;S in the HITRAN2020 database are mainly calculated based on semi-empirical theoretical model and the experimental data to support them are lacking. In this work, direct absorption spectroscopy (DAS) method is firstly used to measure the absorption spectra of H&lt;sub&gt;2&lt;/sub&gt;S in the band of 6320–6350 cm&lt;sup&gt;–1&lt;/sup&gt;. Six groups of characteristic lines with strong absorption and relative independence are selected as the target transitions for experimental measurement. Then, the wavelength modulation-direct absorption (WM-DAS) method with no calibration and high signal-to-noise ratio is used to measure the absorbances of the six groups of transitions under different pressures. Voigt, Raution and quadratic speed-dependent Voigt profiles fit the measured absorbances by least squares method in order to obtain the spectroscopic parameters such as the collision broadening coefficient, line strength and Dicke narrowing coefficient. And the minimum standard deviation of residual error of absorbances is 7×10&lt;sup&gt;–5&lt;/sup&gt;. The measurement uncertainty of each line strength is less than 2%, and the uncertainty of collision broadening coefficients, Dicke narrowing coefficients and the speed-dependent coefficients are all less than 10%. This work is helpful in improving the H&lt;sub&gt;2&lt;/sub&gt;S spectral database and providing the spectral data basis for the high-precision measurement of H&lt;sub&gt;2&lt;/sub&gt;S concentration.

Nanosecond Fiber Laser Step-Tunable From 3.87 to 4.5 μm in HBr-Filled Hollow-Core Silica Fibers

We report here a wavelength widely step-tunable nanosecond 4 μm fiber laser based on HBr-filled hollow-core fibers (HCFs), which is pumped by an acousto-optic modulated narrow linewidth 2 μm thulium-doped fiber amplifier (TDFA) seeded by several thermally tunable diode lasers. The modulated pump pulses excite the first overtone transition of the HBr molecules in HCFs with single-pass configuration, resulting in a number of mid-infrared laser emissions containing both P and R branch fundamental transition lines from 3.87 μm to 4.5 μm. To the best of our knowledge, this is the first tunable pulsed mid-infrared fiber laser beyond 3.8 μm, and it is also the largest tuning range achieved to date for any pulsed fiber laser not by nonlinear effects. The maximum average power of 550 mW associated with a single pulse energy of 0.37 μJ (pulse duration 20 ns, pulse repetition rate 1.5 MHz) is achieved at 4.17 μm, and the laser slope efficiency is about 15.6%. The mid-infrared laser shows a good fundamental mode with a measured beam quality factor M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of about 1.15. The time and frequency domain characteristics are also experimentally investigated in detail. The versatility of such gas-filled HCF laser offers much flexibility in selected molecule gain media and HCF species for power scaling and wavelength extending of pulsed mid-infrared fiber lasers.

Advances in multipass cell for absorption spectroscopy-based trace gas sensing technology [Invited

In the field of absorption spectroscopy, the multipass cell (MPC) is one of the key elements. It has the advantages of simple structure, easy adjustment, and high spectral coverage, which is an effective way to improve the detection sensitivity of gas sensing systems such as tunable diode laser absorption spectroscopy. This invited paper summarizes the design theory and the research results of some mainstream types of MPCs based on two mirrors and more than two mirrors in recent years, and briefly introduces the application of some processed products. The design theory of modified ABCD matrix and vector reflection principle are explained in detail. Finally, trends in its development are predicted.

In situ detection of neodymium isotopes using tunable diode laser absorption spectroscopy for nuclear forensic analysis

As evidence for tracing the history of nuclear materials, in situ detection of neodymium isotopes using tunable diode laser absorption spectroscopy (TDLAS) was investigated.

Quantitative Tomographic Laser Absorption Imaging of Atomic Potassium during Combustion of Potassium Chloride Salt and Biomass.

Gaseous potassium (K) species play an important role in biomass combustion processes, and imaging techniques are powerful tools to investigate the related gas-phase chemistry. Here, laser absorption imaging of gaseous atomic K in flames is implemented using tunable diode laser absorption spectroscopy at 769.9 nm and a high-speed complementary metal oxide semiconductor (CMOS) camera recording at 30 kfps. Atomic K absorption spectra are acquired for each camera pixel in a field of view of 28 × 28 mm at a rate of 100 Hz. The technique is used to determine the spatial distribution of atomic K concentration during the conversion of potassium chloride (KCl) salt and wheat straw particles in a laminar premixed CH4/air flame with an image pixel resolution of up to 120 μm. Due to axisymmetry in setup geometry and, consequently, atomic K distributions, the radial atomic K concentration fields could be reconstructed by one-dimensional tomography. For the KCl sample, the K concentration field was in excellent agreement with previous point measurements. In the case of wheat straw, atomic K concentrations of around 3 ppm were observed in a cylindrical flame during devolatilization. In the char conversion phase, a spherical layer of atomic K, with concentrations reaching 25 ppm, was found within 5 mm of the particle surface, while the concentration rapidly decreased to sub-ppm levels along the vertical axis. In both cases, a thin (∼1 mm) layer without any atomic K was observed in close vicinity to the particle, suggesting that the potassium was initially not released in its atomic form.

A miniaturized multipass cell for measurement of O2 concentration in vials based on TDLAS

Tunable diode laser absorption spectroscopy (TDLAS) is an effective method to inspect possible leakage during filling. However, the sensitivity of TDLAS is confined by the optical path length of the vial headspace. In this letter, the development of a miniaturized multipass cell (MMPC) for improving the sensitivity of laser spectrometer for detecting the O2 content in vial headspaces is presented. Based on analytical vector calculation, a few types of MMPC have been designed and their corresponding beam distributions are derived. The beam spots on the mirrors are compressed in a line-shape parallel with the vial, given the limitation of vial headspace. The simulated configuration with the minimum open path-length is then used for an experimental validating of the MMPC. The horizontal position of the vial has only little influence on the output laser power in the range of -0.5–0.5 mm relative to the optimum one, which is acceptable for practical applications. Wavelength modulation spectroscopy of calibration-free TDLAS was employed to measure the O2 concentration. It is demonstrated that the signal-to-noise ratio improves by 6 times compared to single pass configuration. The R2 of linear fitting the O2 concentration against signal amplitude could reach 0.998 in the range of 0–21% O2. According to the Allan variance analysis, the minimum detection sensitivity of the system is estimated at 0.05% at 141 s.

Real-time, in situ measurement of H2O generated during in situ combustion tests using 1f-normalized wavelength modulation spectroscopy with second harmonic detection.

The development and deployment of a real-time, in situ, non-invasive sensor to monitor the concentration of H 2 O during in situ combustion (ISC) experiments with a heavy-crude oil is described. A real-time sensor to monitor the gas-phase products from ISC can support the study of the kinetics of the complex chemical reactive system in ISC. The mole fraction of H 2 O was measured using tunable diode laser (TDL) absorption spectroscopy coupled with 1f-normalized wavelength modulation spectroscopy (WMS) and 2f detection. The WMS 2f/1f strategy was used to enhance sensitivity with effective noise rejection, particularly suitable when characterizing the water vapor evolved from oil-water emulsions. H 2 O was measured at 3934.10c m -1 from the fundamental band v 3. That transition was selected using the HITRAN database to increase the line strength and minimize interference from neighbor compounds. Measurements of H 2 O concentration were conducted at ambient temperature and pressure using a reference cell (H 2 O=2% at 98.6kPa) to validate the sensor architecture under controlled laboratory environments. The TDL sensor was also successfully validated during real ISC experiments involving heavy-crude oil. Validation and combustion experiments showed the potential of the TDL-based sensor for non-invasive, real-time, in situ measurements of gas-phase species in conditions similar to those of laboratory-scale experimental ISC tests.

TDLAS second harmonic demodulation based on Hilbert transform.

A demodulation method for tunable diode laser absorption spectroscopy (TDLAS) second harmonic based on the Hilbert transform is proposed in this paper. The second harmonic of the TDLAS signal can be easily obtained without a reference signal. The TDLAS signal is firstly processed by band-pass filtering, then the envelope of the processed signal is obtained with Hilbert transform. And finally, the second harmonic is extracted from the 1f component of the envelope. The validity of the proposed method is confirmed by simulation and verified by experiment, and the result shows that the error is acceptable under the cases of weak absorbance, which indicates that the proposed method is applicable to practical trace gas detection.