Frequency F0 Research Articles

Miniaturized broadband high out-of-band rejection bandpass filter based on spoof surface plasmon polaritons with defected ground structure

In this paper, a novel compact bandpass filter (BPF) with a wide out-of-band rejection is proposed. It can achieve broadband characteristics by combining hollow bowtie-type spoof surface plasmon polaritons (SSPPs) with complementary H-type defected grounded structures (DGSs) through aperture coupling. Compared with the conventional SSPP unit cells, the hollow bowtie-type structure exhibits much better slow-wave characteristics. The introduced slant antenna type port-coupling can produce a very strong high-performance rejection outside the high frequency stopband. Simulation results show that the SSPPs-DGS-based BPF has an excellent band pass characteristics in a broadband range with − 3dB fractional bandwidth of 43.5% at center frequency f0 of 2.04 GHz. The return loss in the passband is better than − 12 dB. Furthermore, because of the multiple transmission zeros generated in upper-stop-band, the designed BPF has an extremely strong out-of-band rejection of -40dB from 1.5 f0 to 4 f0 (f0 is the center frequency). The designed SSPPs-DGS-based BPF is fabricated by conventional printed circuit board (PCB) technology with a compact size of only 0.68λg*0.34λg (λg is the wavelength at the center frequency). The measured results have a good agreement with the simulation ones, which verifies the rationality and feasibility of the design. The miniaturized wideband BPF with broad out-of-band rejection may make it has good application prospect in the new generation microwave communication field.

Seismic site conditions of RESNOM network

The Northwest Seismic Network of Mexico (RESNOM) is operated by personnel from the Center for Scientific Research and Higher Education of Ensenada, Baja California (CICESE), which supervises station installation, improvement, and maintenance. We employed seismic noise and the Horizontal to Vertical Spectral Ratio (HVSR) method to determine, for each station, the following site condition parameters: the depth of the rock layer (Heng_bed), and the geotechnical parameter VS30, obtained from 1D shear wave velocity models. Other parameters as the fundamental frequency (f0) and the average amplitude at the fundamental frequency (A0) were also estimated. Our results show clear differences between the values obtained for the Mexicali Valley and the Peninsular ranges regions. The VS30 obtained for stations of the Mexicali Valley region falls in the range from 173 m/s to 535 m/s, while for the Peninsular Ranges region is between 213 m/s and 958 m/s. Regarding the Heng_bed parameter, the values are similar between both regions, from 23 m to 850 m for the Peninsular and from 42 m to 926 m for the Mexicali Valley. Additionally, from the VS30 values, we propose the site classification according to the U.S. National Earthquake Hazards Reduction Program (NEHRP).

Analytical solution of ground vibration of a half-space induced by sub- and super-critical harmonic moving loads: Theory and application

This paper presents an analytical solution to the ground vibration of a half-space subjected to sub- and super-critical moving loads oscillating with the frequency f0 and distributed in different forms. By applying the contour integral combined with the residue theorem, the approximate values of the integrals for the ground response are determined, with a rigorous rationale provided for the exclusive consideration of the Rayleigh waves on the ground. The validity of the present solutions is verified against existing solutions, including Auersch's. Through the parametric analysis, it is observed that the frequency f0 of dynamic loading of the vehicle exhibits significant influence on the critical speed, the magnitude of ground vibrations and their rate of attenuation. In addition, the effect of rail irregularity on the ground vibration was included in some applications of the present theory for high-speed trains under the sub-and super-critical speeds with respect to the Rayleigh waves speed. It was shown that track irregularity can largely amplify the velocity and acceleration responses of the ground, albeit exerting a comparably little effect on the ground displacement. Besides, resonant vibrations are likely to occur when the train is accelerated to the critical speed of the soil.

Joint phase control in metasurfaces for optical convolution operations

Combining the propagation and geometric phases in a metasurface facilitates the independent control of multiple parameters of the light field. However, the geometric phase often displays a random distribution, making it difficult to observe directly. We introduce a frequency-dependent phase response: at frequency f1, there is a superposition of the geometric and propagation phases, whereas at frequency f2, the propagation phase remains constant, and only the geometric phase is applied. The superposition can be interpreted as a convolution process in far-field Fraunhofer diffraction, enabling convolution metasurface devices to generate complex orbital angular momentum beams array and patterned array. Notably, the geometric phase aligns with the characteristic distribution of orbital angular momentum beams, allowing direct observation of the loaded geometric phase. These findings open what we believe to be new avenues for manipulating and calculating complex vector optical fields, optical information coding, controlling light-matter interactions, and enhancing optical communication.

6 GHz lamb wave acoustic filters based on A1-mode lithium niobate thin film resonators with checker-shaped electrodes

The first-order antisymmetric (A1) mode lamb wave resonator (LWR) based on Z-cut LiNbO3 thin films has attracted significant attention and is widely believed to be a candidate for next-generation reconfigurable filters with high frequency and large bandwidth (BW). However, it is challenging for traditional interdigitated electrodes (IDTs) based LWR filters to meet the requirement of a clean frequency spectrum response and enough out-of-band (OoB) rejection. To solve the problem, we propose LWRs with checker-shaped IDTs for the design of filters that meet the Wi-Fi 6E standard. By taking advantage of checker-shaped IDTs with unparalleled boundaries, the fabricated 6-GHz resonators successfully suppress higher-order A1 spurious modes, demonstrating a spurious-free impedance response and a high figure-of-merit (FOM) up to 104. Based on the demonstrated checker-shaped electrode design, the filter features a center frequency (f0) of more than 6 GHz, a 3 dB BW exceeding 620 MHz, and an excellent OoB rejection >25 dB, consistent with the acoustic-electric-electromagnetic (EM) multi-physics simulations. Furthermore, through the capacitance-inductance matching network technology, the filter’s voltage standing wave ratio (VSWR) is successfully reduced below 2, showing an excellent 50 Ω impedance matching. This study lays a foundation for ultra-high-frequency and ultra-wideband filters for the Wi-Fi 6/6E application.

Transmission Coefficient Measurements Using a 2.48-4.7 GHz Lateral Multiband Microstrip Patch Antenna (MB-MPA)

We designed a standard triple-wavelength multiband microstrip patch antenna (MB-MPA) by electromagnetic simulation and fabricated it using an FR-4 substrate. The fundamental resonant frequency f1 was 2.48 GHz, the second-order resonant frequency f2 was 3.7 GHz, and the third-order resonant frequency f3 was 4.7 GHz. The f2 signal resonated at 1/2 the length of the patch width W, and the electrical field intensities of Eφ and Eθ showed π/2 rotation polarization relative to the f1 signal. The polarization angle of the f3 signal was about 34 degrees as the f3 signal resonated at a length of 1/2 of the oblique line of the right triangle consisting of the shortened length L and 1/2 of the width W of the patch. The 0.1 - 6 GHz frequency band reflection and transmission coefficients of S-parameters S11 and S21 measured at a spatial propagation distance of 0.1 - 0.3 m with MB-MPAs as the transmitting and receiving antenna were consistent with the transmission system simulation results. The simulated transmission coefficient S21 at three wavelengths with a spatial propagation distance of 10 - 50 m with two-stage LNAs showed practical values. The lateral resonant MB-MPA can be practically designed by using 3D electromagnetic current density distribution, electric field intensity radiation, and evaluating the transmission coefficient S21.

Monaural and binaural masking release with speech-like stimuli.

The relevance of comodulation and interaural phase difference for speech perception is still unclear. We used speech-like stimuli to link spectro-temporal properties of formants with masking release. The stimuli comprised a tone and three masker bands centered at formant frequencies F1, F2, and F3 derived from a consonant-vowel. The target was a diotic or dichotic frequency-modulated tone following F2 trajectories. Results showed a small comodulation masking release, while the binaural masking level difference was comparable to previous findings. The data suggest that factors other than comodulation may play a dominant role in grouping frequency components in speech.

Subsurface Sediment Layer Analysis at the Dendam Tak Sudah Lake Flyover Construction Site in Bengkulu City Using the HV-Inv Method

Bengkulu City is situated within a subduction zone where the Indo-Australian and Eurasian tectonic plates converge, rendering the area highly susceptible to seismic activity. This study employs the microseismic method to assess seismic vulnerability and the subsurface rock structure at the Dendam Tak Sudah Lake Flyover Construction Site in Bengkulu City, which encompasses a swampy region. The microseismic method used was an inversion of the horizontal to vertical (H/V) spectral ratio (HV-Inv) for determining the dominant frequency (f0), amplification factor (A0), seismic sensitivity index (Kg), and shear wave velocity (Vs). The findings reveal that f0 in the study area range from 2.16 to 7.53 Hz, A0 vary from 0.40 to 3.79, and Kg values span from 0.03 to 6.04. The sedimentary layers exhibit an average thickness of 5-10 meters, with some locations showing significantly thicker sedimentary deposits. Notably, the highest seismic susceptibility is recorded at point T8. The Vs values range from 185.19 to 539.49 m/s, which are inversely proportional to the Kg values and indicate soil classifications varying from soft to medium. The overall seismic risk in the study area is moderate. These results offer key insights into geophysical and geological conditions in Bengkulu City, crucial for earthquake mitigation.

“I like the sound of that”: understanding the effectiveness of audio in ads

PurposeSports advertisements such as the Super Bowl showcase products and brands that have invested increasingly large sums financially to gain viewers’ attention. However, how audio features in advertisements impact viewers' behavior remains unexplored.Design/methodology/approachUsing the lens of signaling theory, this research uses advanced data analytics of voice and music audio in Super Bowl advertisements to examine its impacts on viewers.FindingsResults show that advertisement viewers prefer more voiced frames and have a greater liking behavior of voiced frames with a low intensity (less loud) and a higher F1 frequency, which is typically associated with male vocal tracts. A fast music tempo works better for longer advertisements. The model controls for various types of ad appeals. The research underlines the importance of voice and music in signaling successful brand features that are likely to increase the ad-liking behavior of consumers (positive effect).Research limitations/implicationsThe current research implies that brands advertising through sports ads must carefully select voice actors and music in order to provide the most positive signals for a brand to have the most significant effect and, thus, a greater return on the high sums invested in the ads.Originality/valueFirst, this research contributes in terms of a new research process for using audio analytics in advertising. The detailed research process outlined can be used for future research examining audio and music from advertisements. Second, our findings provide additional support to the important role of voice features (e.g. intensity and frequency) as signals in inducing responses from consumers (Biswas et al., 2019; Hagtvedt and Brasel, 2016). Third, the study surfaces a new theoretical association: the effect of tempo in moderating the relationship between duration and propensity to like an ad.

Disturbance-immune, fast response LITES gas sensor based on out-plane vibration mode employing micro Fabry-Perot cavity with heterodyne phase demodulation

The disturbance-immune, fast response light-induced thermoelastic spectroscopy (LITES) gas sensor based on out-plane vibration (OPV) mode employing micro Fabry-Perot cavity with heterodyne phase demodulation was firstly reported in this paper. Compared to the traditional in-plane vibration (IPV) mode, the OPV mode has a lower resonance frequency (f0) to enhance the energy accumulation time of the system. The OPV mode based H-LITES (OPV-H-LITES) sensor and the IPV mode based H-LITES (IPV-H-LITES) sensor were established using a standard quartz tuning fork (QTF) at the same conditions for performance comparison. Acetylene (C2H2) was selected as the target gas to verify the sensor performance. In comparison to the IPV-H-LITES sensor, there has been a 3.5-fold improvement in signal-to-noise ratio (SNR) of OPV-H-LITES sensor. In order to further improve the sensor performance, a novel self-designed low-frequency QTF was adopted to replace the standard QTF, leading to a 2.3-fold increase in the SNR of the OPV-H-LITES sensor. Finally, a minimum detection limit (MDL) of 7.22 ppm for C2H2 detection was obtained. The proposed technique holds the merits of high sensitivity, power disturbance resistance, wavelength independence, excellent linear concentration response and long-term stability.

Frequency-hopping along with resolution-turning for fast and enhanced reconstruction in ultrasound tomography

The distorted Born iterative (DBI) method is considered to obtain images with high-contrast and resolution. Besides satisfying the Born approximation condition, the frequency-hopping (FH) technique is necessary to gradually update the sound contrast from the first iteration and progress to the actual sound contrast of the imaged object in subsequent iterations. Inspired by the fact that the higher the frequency, the higher the resolution. Because low-frequency allows for low-resolution object imaging, hence for high-resolution imaging requirements, using low-frequency to possess a high-resolution image from the first iteration will be less efficient. For an effective reconstruction, the object’s resolution at low frequencies should be small. And similarly, with high frequencies, the object resolution should be larger. Therefore, in this paper, the FH, and the resolution-turning (RT) technique are proposed to obtain object images with high-contrast and -resolution. The convergence speed in the initial iterations is rapidly achieved by utilizing low frequency in the frequency-turning technique and low image resolution in the resolution-turning technique. It is crucial to ensure accurate object reconstruction for subsequent iterations. The desired spatial resolution is attained by employing high frequency and large image resolution. The resolution-turning distorted Born iterative (RT-DBI) and frequency-hopping distorted Born iterative (FH-DBI) solutions are thoroughly investigated to exploit their best performance. This makes sense because if it is not good to choose the number of iterations for the frequency f1 in FH-DBI and for the resolution of N1 × N1 in RT-DBI, then these solutions give even worse quality than traditional DBI. After that, the RT-FH-DBI integration was investigated in two sub-solutions. We found that the lower frequency f1 used both before and after the RT would get the best performance. Consequently, compared to the traditional DBI approaches, the normalized error and total runtime for the reconstruction process were dramatically decreased, at 83.6% and 18.6%, respectively. Besides fast and quality imaging, the proposed solution RT-FH-DBI is promised to produce high-contrast and high-resolution object images, aiming at object reconstruction at the biological tissue. The development of 3D imaging and experimental verification will be studied further.

Measuring attentional selection of object categories using hierarchical frequency tagging.

In the present study, we used Hierarchical Frequency Tagging (Gordon etal., 2017) to investigate in electroencephalography how different levels of the neural processing hierarchy interact with category-selective attention during visual object recognition. We constructed stimulus sequences of cyclic wavelet scrambled face and house stimuli at two different frequencies (f1 = 0.8Hz and f2 = 1Hz). For each trial, two stimulus sequences of different frequencies were superimposed and additionally augmented by a sinusoidal contrast modulation with f3 = 12.5Hz. This allowed us to simultaneously assess higher level processing using semantic wavelet-induced frequency-tagging (SWIFT) and processing in earlier visual levels using steady-state visually evoked potentials (SSVEPs), along with their intermodulation (IM) components. To investigate the category specificity of the SWIFT signal, we manipulated the category congruence between target and distractor by superimposing two sequences containing stimuli from the same or different object categories. Participants attended to one stimulus (target) and ignored the other (distractor). Our results showed successful tagging of different levels of the cortical hierarchy. Using linear mixed-effects modeling, we detected different attentional modulation effects on lower versus higher processing levels. SWIFT and IM components were substantially increased for target versus distractor stimuli, reflecting attentional selection of the target stimuli. In addition, distractor stimuli from the same category as targets elicited stronger SWIFT signals than distractor stimuli from a different category indicating category-selective attention. In contrast, for IM components, this category-selective attention effect was largely absent, indicating that IM components probably reflect more stimulus-specific processing.

Site Characterization and Shallow Shear Velocity through HVSR Measurements around the Hyderabad Metropolitan Region, India

ABSTRACT We investigate the site characterization and shallow shear velocity profiles from the analysis of the Horizontal to Vertical Spectral ratio (HVSR) around the Hyderabad metropolitan region (HMR), which falls under the southern Indian shield. This work uses both the ambient noise and microearthquake data to compute the HVSR, and additionally the Random Decrement technique to compute the HVSR of extracted Rayleigh waves. This study indicates comparable HVSR curves at each station with the three different datasets, from which we obtain the average dominant frequency (f0) and amplification value (A0). They are further used to calculate the seismic vulnerability index value (Kg). We observe that the value of f0 around the HMR is not fixed, but is varying in the range of 3.4 to 18 Hz, whereas the value of A0 is in the range of 1.7 to 12 approximately and Kg in the range of 0.16 to 1.68 approximately, with an exception of ∼ 33 at VKB (Vikarabad) station, which may be due to a local unconsolidated sub-surface structure. Based on the Diffused Field Assumption (DFA), we invert the average HVSR curves and average dispersion curves of Rayleigh waves, and provide the shallow shear velocity profiles up to 300 m, along with an approximate estimate of VS30 (in upper 30 m depth). The estimated VS30 values vary between 911 to 3143 m/s, falling under the classifications A and B (mostly Hard Rock and Rock type) of National Earthquake Hazards Reduction Program (NEHRP) (BSSC, 2001). However, our study shows some stations with shear velocity inversions at shallow depths within 300 m, indicating layers of low velocity, needing further study. In the absence of detailed near-surface findings, these findings are valuable inputs for geotechnical engineering studies and urban-city planning around the HMR, and emphasizes the effectiveness the HVSR method to determine sub-surface topography and/or unknown soil structures as an economical investigation viability.

Use of acoustic signals in Cape fur seal mother-pup reunions: individual signature, signal propagation and pup home range.

The Cape fur seal (Arctocephalus pusillus pusillus) is one of the most colonial mammals, with colonies of up to hundreds of thousands of individuals during the breeding season. During the lactation period, mothers and pups are regularly separated as females undertake multi-day foraging trips at sea. Mothers and pups use a mutual vocal recognition system to reunite after separation. Such communication is highly constrained by both high background noise and risk of individual confusion owing to the density of seals. This study aimed to experimentally assess the acoustic features relevant for mother-pup vocal identification and the propagation properties of their calls. Playback experiments revealed that mother and pup individual vocal signatures rely on both temporal and frequency parameters: amplitude and frequency modulations, timbre and fundamental frequency (f0). This is more parameters than in any colonial species studied so far. The combinational use of acoustic features reinforces the concept that both environmental and social constraints may have acted as selective pressures on the individual vocal recognition systems. Theoretical propagation distances of mother and pup vocalisations were estimated to be below the range of distances at which mother-pup reunions can occur. This suggests that Cape fur seals may have strong abilities to extract vocal signals from the background noise, as previously demonstrated in the highly colonial king penguin. Investigating the transmission of information throughout the propagation of the signal as well as the ability of the receiving individual to decipher vocal signatures is crucial to understanding vocal recognition systems in the wild.

High‐performance balanced dual‐band bandpass filter with controllable bandwidth and frequency ratio

AbstractIn this study, a novel high‐performance balanced dual‐band bandpass filter (BPF) with controllable bandwidth and frequency ratio is proposed. The symmetrical dual‐band bandpass characteristics on both sides of the center frequency f0 can be achieved by cascading two one‐wavelength edge‐coupled ring resonators through coupling. To suppress the common‐mode (CM) noises, a high‐performance balanced dual‐band BPF is designed and analyzed. The elements affecting the controllable filtering bandwidth and frequency ratio are discussed. For practical verification, a microstrip prototype for GPS applications is manufactured and measured with great out‐of‐band rejection and excellent CM suppression.

Ultrafast Modulations in Stellar, Solar and Galactic Spectra: Dark Matter and Numerical Ghosts, Stellar Flares and SETI

Background: - From new results presented in the literature we discuss the hypothesis, presented in an our previous work, that the ultrafast periodic spectral modulations at fS=0.607±0.08 THz found in the spectra of 236 stars of the Sloan Digital Sky Survey (SDSS) were due to oscillations induced by dark matter (DM) cores in their centers that behave as oscillating boson stars. Two other frequencies were found by Borra in the redshift-corrected SDSS galactic spectra, f1,G=9.71−0.19+0.20 THz and f2,G=9.17−0.16+0.18 THz; the latter was then shown by Hippke to be a spurious frequency introduced by the data analysis procedure. Results: - Within the experimental errors, the frequency f1,G is the beating of the two frequencies, the spurious one, f2,G and fS that was also independently detected in a real solar spectrum, but not in the Kurucz’s artificial solar spectrum by Hippke, suggesting that fS could actually be a real frequency. Independent SETI observations by Isaacson et al., taken at different epochs, of four of these 236 stars could not confirm with high confidence - without completely excluding - the presence of fS in their power spectra and with the same power initially observed. Instead, the radio SETI deep-learning analysis with artificial intelligence (AI) gave an indirect confirmation of the presence of fS through the detection of a narrowband Doppler drifting of the observed radio signals in two stars, over a sample of 7 with a high S/N. These two stars belong to the set of the 236 SDSS stars. Numerical simulations confirm that this drifting can be due to frequency and phase modulation in time of the observed frequencies (1.3–1.7 GHz) with fS. Conclusions: - Assuming the DM hypothesis, the upper mass limit of the axion-like DM particle is ma≃2.4×103μeV, in agreement with the results from the gamma ray burst GRB221009A, laser interferometry experiments, suggesting new physics with additional axion-like particle fields for the muon g-2 anomaly.

Acoustic measurements of the Indonesian oral monophthongs produced by Acehnese speakers

This study explores the characteristics of Indonesian oral monophthong vowels produced by male and female language consultants in Banda Aceh, Indonesia, as their first language. Using purposive sampling, ten Acehnese males and ten Acehnese females were selected to articulate eight target vowels: /i/, /e/, /ɛ/, /ә/, /a/, /u/, /o/, and /ɔ/. Vowel measurements were obtained through recorded word elicitation, using a word list containing the target vowels. F1 and F2 frequencies in Hertz were determined and analyzed using Praat software to assess vowel qualities and subsequently converted to the Bark scale. Vowels were plotted on the F1/F2 formant space. The findings illustrate the distinct measurements of each Indonesian monophthong vowel by male and female consultants, represented in the vowel space. Male vowels generally exhibit higher and more centralized positioning, while female vowels appear more dispersed and lower. This research contributes valuable insights for comparing vowel systems across various languages and dialects spoken in multilingual Indonesia.

Radial Oscillations of Strange Quark Stars Admixed with Dark Matter

We investigate the equilibrium structure and radial oscillations of strange quark stars admixed with fermionic dark matter. For strange quark matter, we employ a stiff equation of state from a color-superconductivity improved bag model. For dark matter, we adopt the cold free Fermi gas model. We rederive and numerically solve the radial oscillation equations of two-fluid stars based on general relativity, in which the dark matter and strange quark matter couple through gravity and oscillate with the same frequency. Our results show that the stellar maximum mass and radius are reduced by inclusion of dark matter. As to the fundamental mode of the radial oscillations, the frequency f0 is also reduced comparing to pure strange stars, and f02 reaches the zero point at the maximum stellar mass with dM/dϵq,c=0. Therefore, the stability criteria f02>0 and dM/dϵq,c>0 are consistent in our dark matter-mixed strange quark stars with a fixed fraction of dark matter. We also find a discontinuity of f0 as functions of the stellar mass, in contrast to the continuous function in pure strange stars. And it is also accompanied with discontinuity of the oscillation amplitudes as well as a discontinuous in-phase-to-out-phase transition between oscillations of dark matter and strange quark matter.

Seismic vulnerability distribution of the earthquake prone area in Central Aceh

Abstract Central Aceh is known as an earthquake-prone area due to the presence of the Sumatra Fault and secondary fault crossing the area resulting the M 6.1 earthquake in 2013. The objective of the research is to investigate seismic vulnerability of Central Aceh. Microtremor observations were conducted at 25 locations in Central Aceh analysed using the Horizontal to Vertical Spectral Ratio (HVSR). Seismic amplification is represented by the maximum values of H/V. The dominant frequency f0 representing the sediment thickness was obtained when the H/V curve is maximum. The frequency values ranged from 14.9 to 25.9 Hz, and the seismic vulnerability values varied from 0.02 to 3.1. The top soil consists of thin layers of sedimentary material dominated by tertiary rocks. The rocks are classified as Type IV and Class I based on the dominant frequency values. This result is consistent with the geology, consisting of sedimentary and volcanic rocks. Areas with a seismic vulnerability value > 1.0 is considered to have a high risk of earthquake damage because they are likely to experience earthquake amplification. Areas with seismic vulnerability value > 1.0 are found in West Aceh and the Geureudong volcano area.

Highly sensitive detection of methane based on LITES and H-LITES techniques

In this paper, a highly sensitive methane (CH4) sensor utilizing light-induced thermoelastic spectroscopy (LITES) and heterodyne light-induced thermoelastic spectroscopy (H-LITES) techniques was developed for the first time. The sensor employed a quartz tuning fork (QTF) with a resonance frequency (f0) of 32762.80 Hz and a quality factor (Q) of 12009.824 as the detector. A continuous wavelength distributed feedback (CW-DFB) diode laser emitting at 1650.96 nm with a maximum output power of 30 mW was used for strong exciting of the target gas. Detailed testing was conducted to assess the sensor’s performance. The results revealed that both CH4-LITES and CH4-H-LITES sensors had excellent linearity concentration response. The minimum detection limit (MDL) for these two sensors was found to be 5.402 ppm and 9.742 ppm, respectively. Compared to CH4-LITES sensor, CH4-H-LITES had the ability to acquire the heterodyne signal and identify the f0 of QTF in just 2 s, showing a fast response capability.