Liqun Sun

Measuring broadband absorbers remains a challenging task for tunable diode laser absorption spectroscopy (TDLAS), and the simultaneous quantification of multi-component mixtures further complicates such measurements due to overlapping absorption spectra and inter-species interference. In this paper, we propose a new WMS approach based on first harmonic (1f) signal analysis and processing for simultaneous detection of multiple broadband absorbers. By applying a logarithmic transformation to the transmitted intensity, the demodulated 1f component becomes linear in absorbance and thus does not rely on the absorption line shape, which allows for the retrieval of broadband absorbers concentration by measuring 1f signal. A reference gas featuring a narrow absorption line within the absorption band of the broadband absorbers is introduced for wavelength localization, using the peak of its second harmonic (2f) signal as the wavelength marker. Furthermore, we develop a four-point elimination scheme to suppress error amplification in multi-gas measurements. In a proof-of-concept experiment, we selected ethanol (C2H6O) and isobutane (C4H10) as broadband absorbers and ethane (C2H6) as reference gas. Accurate retrieval is achieved under varying C2H6 backgrounds (15, 25, 35, 45, and 55 ppm), with the measured C2H6O and C4H10 concentrations remaining close to their nominal values and a maximum relative error below 2.5%. Continuous measurements yield 1σ precisions of 2.71 ppm (C2H6O) and 2.81 ppm (C4H10), while Allan-deviation analysis provides detection limits of 116 ppb at 6.4 s and 158 ppb at 6.3 s, respectively. This method enables simple, effective, and simultaneous high-precision sensing of multiple broadband absorbers without detailed line-shape modeling, and can be readily extendable to other broadband multi-gas applications.

As the scaling of modern nanoelectronics and integrated photonics pushes the physical limits of miniaturization, highly precise and noncontact optical metrology has become the cornerstone of advanced manufacturing. However, traditional techniques face a severe hardware bottleneck: acquiring richer physical data to ensure measurement precision relies on complex instruments that fundamentally limit throughput. Although broadband Mueller matrix polarimetry captures the highest possible information dimension for extracting complex topography, conventional setups are hindered by bulky spectrometers and slow mechanical polarization modulation. To circumvent this trade-off, we demonstrate a motionless critical dimension metrology framework powered by nonlocal flat optics. By fusing the angular-dependent phase delay of static quartz plates with the spatial dispersion of multilayer thin films, our architecture maps high-dimensional Mueller matrix polarization spectra directly into a single intensity snapshot—eliminating all moving parts. A physics-informed neural network then decodes this pattern to reconstruct the comprehensive optical spectra alongside critical 3D nanoscale topography, including film thickness, grating heights, and sidewall angles. Validated on SiO2 thin films and Si gratings, our reconstructions match commercial metrology and electron microscopy benchmarks. This radically simplified, motionless hardware framework offers a highly scalable pathway for in-line process monitoring in next-generation nanomanufacturing.

We proposed a method for measuring the concentration of broadband absorbers based on wavelength modulation spectroscopy (WMS), logarithmic transformation of the absorption signal, and analysis of the first harmonic (1f) signal. We found that the second harmonic (2f) signal of broadband absorbers is weak, while the 1f signal is much stronger. However, conventional WMS relies on an idealized Voigt absorption line profile for approximation, while the demodulated 1f signal intensity remains concentration-independent. Thus, we proposed a modified WMS method in which, through logarithmic transformation of the absorption signal, the intensity of the 1f signal is linear to the absorbance regardless of the gas absorption line shape. This allowed for the retrieval of broadband absorbers' concentration by measuring the 1f signal. To locate the detection wavelength, we introduced a reference gas that exhibits a narrow absorption peak within the absorption band of the broadband absorbers and utilized the peak of its 2f signal for wavelength calibration. We validated the effectiveness of the method by experiments, where isobutane was measured while ethane was selected as the reference gas. The 1f signal intensity demonstrated a strong linear correlation with isobutane concentration across different ethane background concentrations (R² =0.9989). Based on Allan deviation analysis, the system achieved a minimum detectable concentration of 97 ppb at the optimal averaging time of 19.7 s. Compared to the 2f-based approach, the 1f-based method improved the detection limit by more than 40 times.

Optical coherence tomography (OCT) employs light to acquire high-resolution 3D images and is widely applied in fields such as ophthalmology and forensic science. A popular technique for visualizing the top view (en face) is to slice it with flat horizontal plane or apply statistical functions along the depth axis. However, when the target appears as a thin layer, strong reflections from other layers can interfere with the target, rendering the flat-plane approach ineffective. We apply Otsu-based thresholding to extract the object’s foreground, then use least squares (with Tikhonov regularization) to fit a polynomial curve that describes the sample’s structural morphology. The surface is then used to obtain the latent fingerprint image and its residues at different depths from a translucent tape, which cannot be analyzed using conventional en face OCT due to strong reflection from the diffusive surface, achieving FSIM of 0.7020 compared to traditional en face of 0.6445. The method is also compatible with other signal processing techniques, as demonstrated by a thermal-printed label ink thickness measurement confirmed by a microscopic image. Our approach empowers OCT to observe targets embedded in samples with arbitrary postures and morphology, and can be easily adapted to various optical imaging technologies.

The advancement of biomedical imaging technology has significantly enhanced our understanding of complex physiological processes and the diagnosis of various health issues.^{1 – 3} Understanding the cerebral hemodynamics in rodents is a crucial question of interest in the fields of neuroscience, physiology, psychology, and imaging.^4,5 The vast majority of traditional imaging techniques require the subject animals to remain still or be anesthetized to ensure image quality and stability, classic optical imaging setups for mouse brains typically involve fixing the mouse’s head beneath the lens, and throughout the imaging process, gas anesthetics are delivered to the mouse through a tube and a mask. These setups are widely used in two-photon microscopy,⁶ photoacoustic imaging,⁷ and optical coherence tomography (OCT).⁸ However, under anesthesia, multiple physiological indicators can be affected, leading to research results that may not fully represent the true physiological state.⁹ Therefore, there is a growing awareness of the necessity to image the brain under fully awake conditions.

To overcome the light power fluctuation and frequency mismatch in photoacoustic spectroscopy (PAS), we proposed a self-corrected 1f-only resonant cell-based PAS (1f-RCPAS) gas analyzer. Based on the theoretical analysis of the 1f signal, a signal processing algorithm considering laser power-current nonlinearity is proposed. The 1f-only algorithm is well-tailored for the resonant systems, requiring no time-division multiplexing. The algorithm is further improved to extend the dynamic range. The T-type resonant cell incorporating a graphene sticker is utilized for effectively amplifying the acoustic signals from both the gas and solid to achieve normalization. No optical path alignment is needed. For the low resonance frequency, a digital orthogonal-vector lock-in amplifier is used, further simplifying the system setup. The gas analyzer is used to measure methane (CH₄) with the near-infrared absorption peak at 1651 nm. The experiments demonstrated immunity to fiber coupling loss, laser power drift over time, and frequency mismatch caused by property differences between air and standard gases. The R² value in the concentration calibration reaches 0.99995, and the minimum detection limit given by the Allan variance reaches 3.5 ppb at an average time of 105 s.

Optical coherence tomography (OCT) is an emerging optical imaging technology that holds great potential in medical and biological applications. Apart from its conventional ophthalmic uses, it has found extensive applications in studying various brain activities and disorders in anesthetized/restricted rodents, with a particular focus on visualizing brain blood vessel morphology and function. However, developing a compact wearable OCT probe for studying the brain activity/disorders in freely moving rodents is challenging due to the requirements for stability and lightweight design. Here, we report a robust wearable OCT probe, which, to the best of our knowledge, is the first wearable OCT angiography probe capable of long-term monitoring of mouse brain blood flow. This wearable imaging probe has a maximum scanning speed of 76 kHz, with a 12 µm axial resolution, 5.5 µm lateral resolution, and a large field of view (FOV) of 4 mm × 4 mm. It offers easy assembly and stable imaging, enabling it to capture brain vessels in freely moving rodents. We tested this probe to monitor cerebral hemodynamics for up to 4 hours during the acute ischemic phase after photothrombotic stroke in mice, highlighting the reliability and long-term stability of our probe. This work contributes to the advancement of wearable biomedical imaging.

In this paper, the wavelength modulation spectroscopy (WMS) technique is modified and used for measuring methane with large absorbance. The WMS has been frequently used for gas measurement and relies on the linear relationship between the second harmonic amplitude and the gas volume concentration. However, the conventional WMS technique is only applicable for the gas whose absorbance is much smaller than 1, which is because the first-order approximation of Lambert-Beer's law is required in the derivation of the traditional WMS theory, and the first-order approximation holds only at low absorbance, hence the linear relationship between the second harmonic and the gas concentration does not hold at large absorbance. In the modified WMS in this work, it is not necessary to make any approximation to Lambert-Beer's law. The measured light is absorbed by the gas to be measured and then collected by the photodetector. The reference light is directly detected by another photodetector without being absorbed. The output signals of the two photodetectors are transmitted to the computer after implementing analog-to-digital conversion. In this way, the demodulated second harmonic signal remains linear with the gas concentration even at large absorbance. In this work, the traditional WMS theory and the modified WMS theory are both introduced, and a series of methane gas with concentration gradients are measured separately. The experimental results of the traditional WMS and the modified WMS are compared with each other. It is confirmed that the linearity in the traditional WMS theory no longer holds under large absorbance, but the improved WMS can still guarantee the linear relationship between the second harmonic and the methane concentration, which verifies the advantages of the modified scheme. Finally, through Allan's standard deviation analysis, the stability of this methane measurement system reaches the optimal value at the average time of 103.6 s, and the corresponding Allan's standard deviation is 1/26.62×10–9 volume.

We proposed a six-gas simultaneous detection system for methane (CH ₄ ), ethane (C ₂ H ₆ ), acetylene (C ₂ H ₂ ), ethylene (C ₂ H ₄ ), carbon monoxide (CO) and carbon dioxide (CO ₂ ) based on wavelength modulation spectroscopy (WMS). Five different near infrared distributed feedback (DFB) lasers combined with a custom $5\times1$ wavelength division multiplexer were used to detect the six gases. The lasers were modulated at five different sinusoidal signals and the detection light intensity signals were demodulated with corresponding sinusoidal signals, which can be named frequency division multiplexing (FDM). Among them, CH ₄ and C ₂ H ₆ were measured with one laser based on time division multiplexing (TDM). The principle of four multi-gas detection based on FDM was also derived. To miniaturize the device, a Raspberry Pi was used for data processing based on python, and the detected gas concentrations can be displayed directly on the screen. The six-gas sensing system exhibits detection limits of 247 ppb, 1.28 ppm, 126 ppb, 982 ppb, 2.55 ppm and 4.87 ppm with average time of 1 s for CH ₄ , C ₂ H ₆ , C ₂ H ₂ , C ₂ H ₄ , CO and CO ₂ , respectively. With an average time extended to 100 s, the measurement precision achieves 24 ppb, 135 ppb, 14 ppb, 141 ppb, 495 ppb and 608 ppb, respectively. With a fast response time, compact size and high detection sensitivity, the detection system is promising for dissolved gas analysis (DGA) in transformer oil and other material aging process.