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We proposed an original type of multipass cell named symmetric optical multipass matrix system (SMMS), in which the same matrix patterns of various sizes can be formed on both sides. According to its special symmetric configurations, the SMMS design problem is modeled as a variant of the classical traveling salesman problem, which can be rapidly solved by evolutionary optimization algorithms. Two sets of 3-mirror SMMSs are designed, analyzed and built, which show superior characteristics of high stability, desirable beam quality and adjustable optical path lengths. Additionally, they can support simultaneous detection of multiple species with multi-laser channels. The proposed method is further extended to design a 4-mirror SMMS, which verifies the universality and robustness of the design methodology. The experimental observations are in consistent with the theoretical calculations. The newly proposed SMMSs have a broad application prospect in trace gas measurement.

Surfaces with light-trapping structures are widely used in solar cells to enhance light capturing and to transform efficiency. The study of light-trapping character is important for light-trapping structures in solar cells. In the present study, the light-trapping character for the regular hemisphere pit arrays (RHPAs) in solar cells was intensively investigated in terms of reducing light reflection, suppressing light escape, and increasing the length of the optical path. Results show that the RHPAs can decrease surface reflectivity by ~54% compared with the plane structure, and can reflect ~33% of the light that has not been absorbed back into the absorption layer of the solar cell. The total optical path of the cell with the RHPAs structure remarkably increased from 2ω to 4ω. To verify the theoretical research conclusions, we produced the glass structure samples with different aspect ratios by using micro/nanometer-processing technology. The reflection ratios for silicon wafers covered by plane and RHPAs glass samples were tested. The test results were compared with the theoretical calculation results, which showed consistency. [ABSTRACT FROM AUTHOR]

Surfaces with light-trapping structures are widely used in solar cells to enhance light capturing and to transform efficiency. The study of light-trapping character is important for light-trapping structures in solar cells. In the present study, the light-trapping character for the regular hemisphere pit arrays (RHPAs) in solar cells was intensively investigated in terms of reducing light reflection, suppressing light escape, and increasing the length of the optical path. Results show that the RHPAs can decrease surface reflectivity by ~54% compared with the plane structure, and can reflect ~33% of the light that has not been absorbed back into the absorption layer of the solar cell. The total optical path of the cell with the RHPAs structure remarkably increased from 2ω to 4ω. To verify the theoretical research conclusions, we produced the glass structure samples with different aspect ratios by using micro/nanometer-processing technology. The reflection ratios for silicon wafers covered by plane and RHPAs glass samples were tested. The test results were compared with the theoretical calculation results, which showed consistency.

In tunable diode laser absorption spectroscopy (TDLAS), the multipass cell (MPC) that comprises two identical and coaxial spherical mirrors can provide long optical path length (OPL) and reduce the limit of system detection. We proposed an intelligent method to design MPC with long optical path length (OPL) and small size using parallel multi-population genetic algorithm (PMPGA). The developed MPCs presented the maximum mirror utilization in the same size of spherical mirrors with the diameter of 25.4 mm and 40 mm. The performance of the developed MPCs were compared with other teams. We verified the results of PMPGA and obtained a micro MPC design with an OPL of 6.3 m. The diameter of the mirror was 25.4 mm, and the distance between the two mirrors was only 30.5 mm. And the developed MPC was verified by using TDLAS, which can improve the development of portable gas sensors. [ABSTRACT FROM AUTHOR]

We proposed an original type of multipass cell named symmetric optical multipass matrix system (SMMS), in which the same matrix patterns of various sizes can be formed on both sides. According to its special symmetric configurations, the SMMS design problem is modeled as a variant of the classical traveling salesman problem, which can be rapidly solved by evolutionary optimization algorithms. Two sets of 3-mirror SMMSs are designed, analyzed and built, which show superior characteristics of high stability, desirable beam quality and adjustable optical path lengths. Additionally, they can support simultaneous detection of multiple species with multi-laser channels. The proposed method is further extended to design a 4-mirror SMMS, which verifies the universality and robustness of the design methodology. The experimental observations are in consistent with the theoretical calculations. The newly proposed SMMSs have a broad application prospect in trace gas measurement., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Author(s).)

We design a new ultra-sensitive optical methane sensor by combining such optical approaches as appropriate use of absorption bands of methane by HITRAN database in the near-infrared (NIR) region, effective absorption path length in circular multi-pass optical cell, a wavelength modulation, and noise suppression with dual beam configuration. The design offers a compact and portable optical sensor for methane gas measurements in real time and high sensitivity. In the design, one can select different laser diode source to expand the selectivity labels for the interest many other trace gases. This is inexpensive and simpler experimental design without complexity of laser frequency locking and phase stabilization. It can find wide applications in many fields such as physical, environmental, and atmospheric research as well as physics, analytical chemistry, biology, medical diagnosis and process technology. [ABSTRACT FROM AUTHOR]

Purpose: Near-infrared spectroscopy (NIRS), performed with a commonly available noninvasive tissue oxygenation monitoring device, is based on the modified Beer–Lambert law (MBLL). Although NIRS based on MBLL (NIRSMBLL) assumes that the optical path length (PL) is constant, the effects of changes in PL during exercise on muscle oxygenation calculated by MBLL are still incompletely understood. Thus, the purposes of this study were to examine the changes in optical properties during ramp incremental exercise and to compare muscle oxygen dynamics measured by time-resolved NIRS with those calculated based on MBLL. Methods: Twenty-two healthy young men performed ramp incremental cycling exercise until exhaustion. Optical properties (reduced scattering coefficient and PL) and absolute oxygenated, deoxygenated, and total hemoglobin and myoglobin concentrations (oxy[Hb + Mb], deoxy[Hb + Mb], and total[Hb + Mb], respectively) at the vastus lateralis were continuously monitored by a three-wavelength (763, 801, and 836) time-resolved NIRS device. The values of oxy-, deoxy-, and total[Hb + Mb] were then recalculated by assuming constant PL. Results: PL at all wavelengths statistically significantly shortened during exercise. In particular, PL at 763 nm was greatly shortened, and the average changes during exercise were a 9.8% ± 3.1% reduction. In addition, significant differences in the kinetics of oxy-, deoxy-, and total[Hb + Mb] between directly measuring PL and assuming constant PL were found. The average changes in measured PL and assuming constant PL-deoxy[Hb + Mb] were increases of 28.8 ± 16.0 μM and increases of 16.4 ± 9.3 μM, respectively. Conclusion: Assuming constant PL in NIRSMBLL significantly underestimated actual muscle oxy/deoxygenation as compared with measurements obtained by real-time PL determination. The percent degree of the underestimated oxy/deoxygenation was greater than the percent degree of the changes in PL. [ABSTRACT FROM AUTHOR]

A mathematical model of a methane concentration sensor has been developed and studied, which takes into account the use of materials with different optical properties in the channel, spectral and spatial adjustment of the sensor components, losses at modulation and signal processing, as well as scattering and absorption of the optical emission flux in the channel. Taking into account results of modelling and experimental studies of the prototype sensor, confirming the adequacy of the proposed model, types of optoelectronic sensor components, as well as its design parameters with allowable deviations in manufacturing, have been substantiated and selected. The emission losses in the optical channel have been minimized when the informative parameter is maximized, which is absorption of optical emission by the measured methane concentration, by providing the length of the sensor measuring channel not more than 30 mm and the use of light and photodiodes with covering of their sensitive elements with chalcogenide glass. It has been established that the main measurement error of methane concentration with the application of these recommendations is not more than 0.04 vol.% with the regulated value of 0.20 vol.% in the range from 0 to 5 vol.%.

Porosity, one of the important quality attributes of pharmaceutical tablets, directly affects the mechanical properties, the mass transport and hence tablet disintegration, dissolution and ultimately the bioavailability of an orally administered drug. The ability to accurately and quickly monitor the porosity of tablets during manufacture or during the manufacturing process will enable a greater assurance of product quality. This tutorial systematically outlines the steps involved in the terahertz-based measurement method that can be used to quantify the porosity of a tablet within seconds in a non-destructive and non-invasive manner. The terahertz-based porosity measurement can be performed using one of the three main methods, which are (i) the zero-porosity approximation (ZPA); (ii) the traditional Bruggeman effective medium approximation (TB-EMA); and (iii) the anisotropic Bruggeman effective medium approximation (AB-EMA). By using a set of batches of flat-faced and biconvex tablets as a case study, the three main methods are compared and contrasted. Overall, frequency-domain signal processing coupled with the AB-EMA method was found to be most suitable approach in terms of accuracy and robustness when predicting the porosity of tablets over a range of complexities and geometries. This tutorial aims to concisely outline all the necessary steps, precautions and unique advantages associated with the terahertz-based porosity measurement method. [ABSTRACT FROM AUTHOR]

Anodic titanium oxide photonic crystals (ATO PhCs) are promising for optical sensing and photocatalysis due to their photonic band gaps (PBGs). The ability to control the spectral position and width of the PBG are essential for application of ATO PhCs. Anodizing charge density has recently been reported to be a main factor affecting the porosity and hence the effective refractive index (n eff) and PBG position of ATO PhCs. In this paper, we propose a method for accurate determining the dependence of n eff of ATO PhCs on the etching charge density varying in a wide range. The method is based on an analysis of the spectral positions of PBGs in photonic heterostructures. The obtained dependence was used for the synthesis of the ATO PhCs with desired PBG positions in a wide wavelength range (400–1500 nm) by the periodic voltage modulation on optical path length. The deviation of the experimental PBG position from the specified one does not exceed 10 %. The prepared PhCs possess ∼100 % reflectance in the PBG region. • Anodic titania heterostructures consisting of several photonic crystals (PhCs) are prepared. • Dependence of the effective refractive index of PhCs on the etching charge density is revealed. • Parameters for Ti anodizing with voltage versus optical path length modulation are determined. • PhCs with a given photonic band gap (PBG) position and ∼100 % reflectance are obtained. • The deviation of the experimental PBG position from the given one does not exceed 10 %. [ABSTRACT FROM AUTHOR]

The optical Vernier effect is a powerful tool for improving the sensitivity of an optical sensor, which relies on the use of two sensor units with slightly detuned frequencies. However, an improper amount of detuning can easily cause the Vernier effect to be unusable. In this work, the effective generation range of the Vernier effect and the corresponding interferometer configuration are suggested and experimentally demonstrated through a tunable cascaded Fabry–Perot interferometer structure. We further demonstrate a practical method to increase the magnification factor of the Vernier effect based on the device bandwidth. Only the optical path length of an interferometer probe and the sensitivity of the measurement parameters are needed to design this practical interferometer based on the Vernier effect. Our results provide potential insights for the sensing applications of the Vernier effect.

Photometric detection plays a significant role in microfluidics technology. However, the mismatch between the solution concentration and the optical path length will increase detection error. In this study, we proposed a round microfluidic chip for concentration detection to obtain the continuous gradient distribution of concentration. The optimum absorbance can be found by dynamic accurately searching. The solution concentration will be accurately calculated finally according to the relationship between arc length and solution concentration. The overall detection process runs automatically. Under the optimization of injection velocity and concentration, the experimental result shows that the compensation ratio increases as the solution concentration increases. The compensation ratio in the detection of pesticide residue has already reached 14.22% and the reproducibility is acceptable. Therefore, this novel method lays the theoretical foundation for the research of high precision microfluidic photometric detection equipment. [ABSTRACT FROM AUTHOR]

Gasoline engine oil (SAE 5W-20) was subjected to thermal oxidization (TO) for four periods of time (0 h, 48 h, 96 h and 144 h) and exposed to THz-time domain spectroscopy (TDS) measurement. Error contributions from various error sources, such as repeatability errors, assembly errors of the probe volume and errors caused by the TDS system were evaluated with respect to discernibility and significance of measurement results. The most significant error source was due to modifications of the TDS setup, causing errors in the range of 0.13% of the refractive index for samples with a refractive index around 1.467 and a probe volume length between 5 and 15 mm at 1 THz. The absorption coefficient error was in the range of 8.49% for an absorption around 0.6 cm–1. While the average of measurements taken with different setup configurations did not yield significant differences for different TO times, a single, fixed setup would be able to discern all investigated oil species across the entire frequency range of 0.5–2.5 THz. The absorption coefficient measurement showed greater discernibility than the measurement of the refractive index. [ABSTRACT FROM AUTHOR]

Hemisphere cone arrays (HCAs) is a popular light trapping structure in silicon solar cells. However, the light trapping mechanism research of that in solar cells is lacking. In this paper, the light trapping mechanism of the HCAs in silicon solar cells is studied by combination of numerical analysis and geometrical optics simulation. The results show that the HCAs structure can reduce surface reflectance by ∼60% compared with flat structure by multiple injections of the incident light. At the same time, the HCAs can make ∼43% unabsorbed light return to the absorbing layer of solar cells again. To verify the theoretical calculation results, a flat and a square hemispherical cone arrays PDMS thin films are prepared by a micro/nano-processing method, and the reflectivities of a silicon wafer covered the flat and the HCAs PDMS thin films were measured. The experimental measured results are basically consistent with the theoretical calculation results. The numerical calculation results also show that the total optical path length of the thin film Si solar cell covered with the HCAs structure increases from 2 ω to 4 ω . [ABSTRACT FROM AUTHOR]