Wavelength Modulation Technique Research Articles

Standoff detection of backscattered light based on tunable diode laser absorption spectroscopy

Standoff detection is of great significance in greenhouse gas measurement monitoring, early warning security and leak detection. Variations in topographic characteristics and distances will undoubtedly increase background noise. However, the effect of real-time changes in the angle between the sensor and the target on dynamic detection is rarely studied. In this article, a portable laser sensor is developed to retrieve gas concentration of CH4 by collecting the backscattered light from a distant non-cooperative target. The experimental results show that “2f/1f” based on wavelength-locked wavelength modulation technique can effectively suppress the background interference caused by a change in the detection angle. Increasing the sampling frequency does not achieve denoising for static detection but can lead to significant noise reduction for dynamic detection. For a modulation frequency of 4 k Hz, the mean standard deviation of the gas concentration is improved to 42.8 % and 7.04 % of that at 1 k Hz and 250 Hz, respectively. Kalman filtering can significantly improve the denoising effect for dynamic detection. The background concentration perturbation is reduced by a factor of 4.19 ∼ 4.71 after applying Kalman filtering. Combined with Kalman filtering, the minimum detection limit for Allan variance prediction can reach 20.5 ppb@ (0.15 sec) at a modulation frequency of 4 k Hz.

Light-induced thermoelastic spectroscopy by employing the first harmonic phase angle method

A highly sensitive gas sensor based on light-induced thermoelastic spectroscopy (LITES) with the first harmonic phase angle(1f-PA) wavelength modulation technique is reported. In this research, methane (CH4) is selected as the target gas, and the detection target wavelength is close to 1653.72 nm. Modulated laser beam passes through a 35 cm absorption cell and is focused on the root of the quartz tuning crystal fork (QCTF). The retrieved in-phase and out-of-phase first harmonic (1f) components are processed with 1f-PA method. With this detection method, the amplitudes of the 1f-PA have demonstrated immunity to light intensity dithering as well as demodulation phase shift. The fluctuation of 1f-PA amplitudes is less than 0.5% while the light intensity varies between 2.11 mW and 10.20 mW. Also, the 1f-PA amplitudes have ranged within 0.4% while the demodulated phase shifted from 0° to 360°. In addition, the system also demonstrates an excellent linear response. While the concentration varies from 200 ppm to 10,000 ppm, the linearity coefficient (R2) is 0.999. According to Allan deviation analysis, the lowest detection limit is 2.85 ppm, and the corresponding normalized noise equivalent absorption coefficient (NNEA) is 3.24 × 10−8 cm−1WHz. With the 1f-PA method, the precise gas concentration which is immune to both light intensity dithering and phase shift can be obtained conveniently for LITES. It has shown great advantages and application prospects in long-term field applications.

Wavelength modulation‐based active laser heterodyne spectroscopy for standoff gas detection

AbstractThe wavelength modulation‐active laser heterodyne spectroscopy (WM‐ALHS), which introduces wavelength modulation technique into active laser heterodyne spectroscopy, is reported to enhance the gas standoff detection capability of weak return optical power. The ALHS gas standoff detection system is developed using a 1.65 μm butterfly packaged distributed feedback laser with tail fiber and an all‐fiber structure, which successfully detects methane concentrations at a distance of 35 cm using an aluminum plate as the reflective surface. Allan variance analysis shows that the system has good stability, and the detection limit at 29.2 s integration time is 0.84 ppm m. Standoff detection experiments with direct absorption‐active laser heterodyne spectroscopy (DA‐ALHS) and wavelength modulation spectroscopy (WMS) are carried out on the basis of the WM‐ALHS system and the detection limits of 4.49 and 1.64 ppm m were achieved within integration times of 18.6 and 29 s for DA‐ALHS and WMS, respectively. The superiority of the WM‐ALHS method compared to these two spectral detection methods is demonstrated. Furthermore, the feasibility of applying WM‐ALHS to gas standoff detection has been verified, laying the foundation for the development of this spectroscopic technology.

(Digital Presentation) Excitonic Photoluminescence Properties of Locally Functionalized Single-Walled Carbon Nanotubes Using Molecular Modifiers

Excitons in single-walled carbon nanotubes (SWCNTs) show unique properties due to strong quantum confinement and weak dielectric screening effects which give rise to high binding energies based on strong Coulombic interactions. By this feature, stable and mobile excitons are generated in SWCNTs and its radiative recombination produces photoluminescence (PL) in the near infrared (NIR) region. Recently, local chemical functionalization of SWCNTs has attracted great attention because of enhancement of their NIR PL properties.[1-5] This functionalization dopes local defects such as sp3 carbon to the semiconducting sp2 carbon networks of SWCNTs based on the covalent bond formation between the tube walls and modifier molecules. As a result, emissive doped sites that have narrower band gaps and exciton trapping features are created in the locally functionalized SWCNTs (lf-SWCNTs). For lf-SWCNTs, E 11* PL newly appears with red-shifted wavelengths and increased PL quantum yields compared with original E 11 PL of pristine SWCNTs. Currently, chemical functionalization methods and E 11* PL wavelength modulation techniques are being developed for further functionalization of lf-SWCNTs. [1-5] In this presentation, E 11* PL properties of lf-SWCNTs synthesized using newly designed chemical modifiers are discussed. For example, an azide compound is found to produce the emissive doped sites in lf-SWCNTs via a [2+1] cycloaddition reaction. The E 11* PL peak was observed at 1118 nm and N1s peak in X-ray photoelectron spectroscopy was detected around 400 eV regions based on the chemical bond formation. Moreover, solvatochromic PL energy shifts of the E 11* PL showed a different trend from those observed for typical lf-SWCNTs synthesized using diazonium chemistry. This result indicates that the chemical structures of the doped sites could be a key factor to modulate excitonic properties of localized excitons in lf-SWCNTs. Therefore, the molecular chemistry approach would provide unique exciton engineering tools for lf-SWCNTs and develop advanced excitonic NIR materials applicable to quantum cryptography technologies and high-performance bio/medical imaging and sensing.[1] T. Shiraki, Chem. Lett., 50, 397 (2021); [2] T. Shiraki et al., Acc. Chem. Res., 53, 1846 (2020); [3] S. Tretiak et al., Acc. Chem. Res., 53, 1791 (2020); [4] Y. Wang et al., Nat. Rev. Chem., 3, 375 (2019); [5] J. Zaumseil, Adv. Optical Mater. 2101576 (2021).

VOC Biomarker Monitoring for Diabetes Through Exhaled Breath Using Ag/P-TiO2 Composite Plasmonic Sensor

The detection of diabetes at an early stage is considered to be highly significant in the successful treatment of the disease. The patient has been reported to generate unique volatile organic compounds (VOCs) profiles in their exhaled breath which can be utilized as biomarkers to diagnosis the disease conditions. In this article, we theoretically report a simple breath sensor for the highly sensitive detection of isopropanol (IPA), which is the biomarker for both types of diabetes, in exhaled breath using a D-shaped composite plasmonic sensor (DSCPS). The sensing response of the sensor is characterized by using a full-vectorial finite element method (FEM) for the detailed analysis of DSCPS, where a porous TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (P-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) layer is used as low index sensing material on the top of thin Ag film. The design of the proposed sensor is based on the wavelength modulation technique. A maximum sensitivity of 0.25, 0.35, 0.45, and 1.35 nm/% can be achieved for 20%, 40%, 60%, and 80% volume fraction of IPA, respectively, with a 30% porosity of TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , which is the maximum filled porous voids that provides the highest sensitivity.

Quartz-Enhanced Photothermal-Acoustic Spectroscopy for Trace Gas Analysis

A crystal quartz tuning fork (QTF) was used as a detector to collect and amplify laser-induced photoacoustic and photothermal waves simultaneously for trace chemical analysis. A wavelength modulation technique was applied to the proposed quartz-enhanced photothermal-acoustic spectroscopy (QEPTAS) to improve the detection signal-to-noise ratio. The QTF detector was exposed to the illumination of a near-infrared distributed feedback laser at distances of 1 m and 2 m to evaluate the QEPTAS sensor performance. The QEPTAS sensor performance was determined by detecting water vapor in ambient air using a near-infrared distributed feedback laser with a power of ~10 mW and a wavelength of 1.39 μm. With an optimized modulation depth of 0.47 cm−1, the normalized noise equivalent absorption (NNEA) coefficients of 8.4 × 10−7 W·cm−1·Hz−1/2 and 3.7 × 10−6 W·cm−1·Hz−1/2 were achieved for a distance of 1 m and 2 m, respectively. The developed QEPTAS technique reduces the requirements for laser beam quality, resulting in a simple but robust sensor structure and demonstrates the ability of remote sensing of gas concentrations.

Highly sensitive acetylene detection based on a compact multi-pass gas cell and optimized wavelength modulation technique

A highly sensitive acetylene (C2H2) sensor system was demonstrated using tunable diode laser absorption spectroscopy (TDLAS) based on a compact multi-pass gas cell with a 10 m optical path. The sensor system can work in two measurement modes: direct measurements and wavelength modulation (WMS). The incident laser beam with a small spot radius (0.5 mm) effectively eliminated interference fringes and improved the signal-to-noise ratio. The minimum detection limit (MDL) of the direct detection mode was 4.9 ppm. As for the wavelength modulation method, after optimization the modulation depth, a MDL of 14.3 ppb was obtained. The corresponding noise-equivalent absorbance (NEA) was calculated and obtained an excellent value of 1.8 × 10-8 cm-1 Hz−1/2. An Allan deviation analysis was used to analyze the system stability. The MDL was improved to 1.3 ppb when the averaging time was 270 s.

Analysis and Detection of a Target Gas System Based on TDLAS &amp; LabVIEW

This paper introduces detection techniques of target gas concentration based on infrared absorption spectrum method and the simulation of the gas detection system using LabVIEW. In this work, the harmonic detection is performed by wavelength modulation technique with a related frequency signal to cover the measured signal and get the gas absorption coefficient that can be analysed to give a gas concentration. A series of simulations are carried out to scan weak absorption lines and extract the second harmonic curve of the absorption spectrum of the target gas. The absorption and second harmonic curves are acquired by changing the parameters of the target gas concentration and line width. This work claims to be a contribution to the study and the comprehension of gas detection system based on tunable diode laser absorption spectroscopy and can serve as a basis for practical data acquisition.

Trace methane gas detection by wavelength modulated off-axis integrated cavity output spectroscopy

An instrument based on wavelength modulation off-axis integrated cavity output spectroscopy (WM-OA-ICOS) was developed for detection of trace methane (CH4) gas. The wavelength modulation technique was employed to obtain the second harmonic of the CH4 absorption signal. The modulation parameters were optimized to obtain a maximum second harmonic signal. A noise-equivalent absorption sensitivity of 9.4 × 10−11 cm−1 Hz−1/2 (corresponding to a detection limit of 28.9 ppbv for CH4 at this wavelength) was achieved using 2f detection. Compared with the traditional off-axis integrated cavity output spectroscopy (OA-ICOS) technique, WM-OA-ICOS provided an 8-fold improvement in detection sensitivity. CH4 concentration measurements were also achieved by normalization of second harmonic signal to first harmonic signal (2f/1f). The dynamic range and linearity of WM-OA-ICOS for both 2f method and 2f/1f method were investigated. The result showed that the 2f/1f method exhibited better linearity than 2f method.

Rovibrational fine structure and transition dipole moment of CF3H by frequency-comb-assisted saturated spectroscopy at 8.6 µm

We report on rovibrational fine structure spectroscopy of room-temperature trifluoromethane with a saturated absorption method, using a quantum cascade laser at around 8.6 µm phase-locked to a self-referenced mid-infrared frequency comb synthesizer. The absolute line-center frequencies of 36 lines have been measured by means of the wavelength modulation method with a best fractional uncertainty of ∼3×10−9 in a single acquisition scan from 34.7426 to 34.7443 THz. In addition, an extensive investigation of the measured Lamb-dip profiles using both direct pump-probe spectroscopy method and wavelength modulation technique has been performed for two selected lines, rR40(64) and rR36(38), belonging to the υ5 vibrational band, as a function of either the gas pressure or the pump beam power. This approach allowed the measurement of the main spectroscopic parameters, such as saturation intensity, transition electric dipole moment, as well as self-pressure broadening coefficient.

Wavelength modulation technique-based photoacoustic spectroscopy for multipoint gas sensing.

A multipoint gas sensing scheme based on photoacoustic spectroscopy was proposed. Multiple photoacoustic spectroscopy (PAS) gas cells (resonant frequency f0=5.0 kHz) were connected in series for the multipoint gas sensing with wavelength modulation technique. The PAS signal was excited by modulating the tunable distributed feedback laser diode wavelength at f0/2 using a changing driving current. The gas concentration of each gas cell was obtained by the PAS signal, which was demodulated by the lock-in amplifier. A multipoint PAS experiment to detect the water vapor at 1368.597nm was implemented to verify the scheme we presented. With the three PAS gas cells, the linear response to the water vapor concentration of our sensors achieved 0.9978, 0.99591, and 0.99617, and their minimum detection limits were 479, 662, and 630ppb, respectively.

Wavelength Modulating Self-Mixing Interferometer

Self-mixing interference (SMI) was used to measure the displacement with a resolution of λ/2 by counting the interference signal peaks. In order to increase the measurement accuracy beyond λ/2, sinusoidal wavelength modulation (SWM) technique is introduced in SMI in this paper. Wavelength modulation of the laser beam is obtained by sinusoidal modulating the injection current of the laser diode. Fourier analysis method is proposed to demodulate the phase. Experimentally, the micro-movements of a high precision commercial PZT have been reconstructed, which can obtain a displacement measurement resolution of a few nanometers. It provides a potential displacement sensor with high accuracy and quite compact configuration.

Fiber Bragg grating interrogation using wavelength modulated tunable distributed feedback lasers and a fiber-optic Mach-Zehnder interferometer.

This paper demonstrates a technique of high-resolution interrogation of two fiber Bragg gratings (FBGs) with flat-topped reflection spectra centered on 1649.55nm and 1530.182nm with narrow line width tunable semiconductor lasers emitting at 1651.93nm and 1531.52nm, respectively. The spectral shift of the reflection spectrum in response to temperature and strain is accurately measured with a fiber-optic Mach-Zehnder interferometer that has a free spectral range of 0.0523GHz and a broadband photodetector. Laser wavelength modulation and harmonic detection techniques are used to transform the gentle edges of the flat-topped FBG into prominent leading and trailing peaks that are up to five times narrower than the FBG spectrum. Either of these peaks can be used to accurately measure spectral shifts of the FBG reflection spectrum with a resolution down to a value of 0.47pm. A digital signal processing board is used to measure the temperature-induced spectral shifts over the range of 30°C-80°C and strain-induced spectral shifts from 0 μϵ to 12,000 μϵ. The shift is linear in both cases with a temperature sensitivity of 12.8pm/°C and strain sensitivity of 0.12 pm/μϵ. The distinctive feature of this technique is that it does not use an optical spectrum analyzer at any stage of its design or operation. It can be readily extended to all types of tunable diode lasers and is ideally suited for compact field instruments and for biomedical applications in stroke rehabilitation monitoring.

Intracavity multigas detection based on multiband fiber ring laser

We present a new method for multigas detection based on multiband fiber ring laser. Wavelength division multiplexing is used to realize multiband laser operation. Laser intracavity absorption spectroscopy combined with wavelength modulation technique is used to improve the measurement sensitivity. The second harmonic signal is extracted by the software-based lock-in algorithm for gas concentration retrieval. The relationship between the harmonic amplitudes and gas concentrations is investigated under both weak absorption and strong absorption to improve the accuracy of concentration retrieval. When the system is applied to detect the mixed gases of acetylene, carbon monoxide and carbon dioxide in C+L band, the minimum detectable concentrations are 0.6ppm, 17.4ppm and 19.2ppm respectively.

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.

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.

A novel frequency control scheme for comb-referenced sensitive difference-frequency-generation spectroscopy

We present a novel scheme of frequency scan and wavelength modulation of a difference-frequency-generation source for comb-referenced sensitive spectroscopy. While the pump and signal frequencies are phase-locked to an optical frequency comb (OFC), the offset frequency between the signal wave and the nearest comb tooth is modulated to apply a wavelength-modulation technique, and the idler wave frequency is repeatedly swept for signal accumulation by changing the repetition frequency of the OFC. The spectrometer is applied to absolute frequency measurement of weak hyperfine-resolved rovibration transitions of the ν(1) band of CH(3)I, and the uncertainty in frequency determination is reduced by one order of magnitude in compared with that of the previous work published in Optics Express 20, 9178-9186 (2012).

Multi-component gas sensing based on slotted photonic crystal waveguide with liquid infiltration

A high sensitive multi-component gas sensing system was proposed, in which the slotted photonic crystal waveguide (SPCW) was used as the gas cell and the fiber Fabry–Perot (FFP) was adopted as the wavelength filter. By choosing the refractive index that infiltrated in the first and second rows of air holes adjacent to the slot respectively, the working wavelength of SPCW could be tuned to matches with one absorption line of the measured gas, thus, the system could detect different kinds of gases selectively. Meanwhile, a very high group index of 150 with flat bandwidth could be achieved and the electric field enhancement factor would reach to 12. Combined with the wavelength modulation techniques (WMS) for signal processing, the minimum detection limit of 1.56ppm was achieved.

A system for gas sensing employing correlation spectroscopy and wavelength modulation techniques with a multimode diode laser

A tunable multi-mode diode laser system based on correlation spectroscopy and wavelength modulation spectroscopy (TMDL–COSPEC–WMS) is designed and demonstrated for the concentration measurements of oxygen using A-band absorption lines of oxygen around 760nm. The O2 concentrations are conversed from the relation between the normalized WMS-2f signal peak heights of the measurement and reference signals which selected based on high signal to noise ratio and correlation coefficient. The correlation and the fitted slope between the measured and actual O2 concentration are 0.9987 and of 1.025±0.012 respectively over the tested range, which indicate the high linearity and accuracy of the system. A sensitivity of 350ppmm is approved using 30 successive measurements with each measurement time taking ∼20s during 30min. A continuous measurement for oxygen in ambient air during approximately 200min confirms the stability and the capability of the system.

Wavelength modulated surface enhanced (resonance) Raman scattering for background-free detection

Spectra in surface-enhanced Raman scattering (SERS) are always accompanied by a continuum emission called the 'background' which complicates analysis and is especially problematic for quantification and automation. Here, we implement a wavelength modulation technique to eliminate the background in SERS and its resonant version, surface-enhanced resonance Raman scattering (SERRS). This is demonstrated on various nanostructured substrates used for SER(R)S. An enhancement in the signal to noise ratio for the Raman bands of the probe molecules is also observed. This technique helps to improve the analytical ability of SERS by alleviating the problem due to the accompanying background and thus making observations substrate independent.