Accurate Optical Measurements Research Articles

Precision compensation method for large-scale measurements based on full-space refractive index field reconstruction with vibratory mirror background oriented schlieren (VMBOS)

With the development of the large-scale equipment manufacturing industry, such as aircraft component assembly, wind turbine rotor blade measurement and large ship assembly the demand for optical measurement accuracy in large-dimensional spaces within industrial environments has become increasingly stringent. However, due to the presence of internal interference sources, light will no longer propagate in a straight line, thereby introducing measurement errors that cannot be ignored. To compensate for the measurement error, the refractive index field distribution within the large-dimensional measurement space must be reconstructed. Therefore, this paper proposed a vibratory mirror background oriented schlieren (VMBOS) based on vibratory scanning. This method scans the measurement space through the rotation of vibratory mirror to obtain light deflection angles at different directions. Subsequently, the refractive index field of the large-dimensional space is reconstructed by using the tomographic reconstruction algorithm. Then the optical path of the light in the optical measurement can be corrected through the Hamiltonian ray tracing algorithm, thereby achieving compensation for the measurement error. Finally, the VMBOS method has been verified through both simulation and experimental methods. The experimental results demonstrated that the method proposed in this paper can be applied to optical path correction and error compensation in large scale spaces.

Highly accurate absolute optical transfer delay measurement over a long distance assisted by the pulse time signal.

In this paper, an absolute optical transfer delay measurement method based on pulse time signal, pseudo-random code phase, and microwave phase is proposed. By employing two-stage integer ambiguity resolution, not only can a measurement range of several hundred kilometers be achieved, but sub-picosecond level measurement accuracy can also be attained. A test system was built in the laboratory and experimentally verified on a fiber optic link. The experimental results verify an accuracy of ±0.1 ps in measuring an ultrahigh-accuracy optical delay line. In addition, long fiber is also tested, which proves that a measurement range of at least 100 km can be achieved. The refresh rate of the measurement results can reach 100 ms each time.

A Critical Review on Soil Gas Analysis: Modern Technologies and Problems

In this review article, the main techniques for spectroscopic studies of gases in field conditions are considered. The issues related to the study of gas emissions from soils and the determination of their concentrations are analysed. The main types of spectroscopy used in portable devices for soil gas analysis, along with their design features and sampling approaches, are provided. Various studies aimed at optimising the operation of devices for analysing gases emitted from the soil, taking into account agronomic, agrochemical, and ecological specifics, are also presented. The effect of using different types of lasers and reflecting elements on the accuracy of optical measurements and the sensitivity to various substances in the gases is analysed.

Real-time monitoring and control of the layer height in laser metal deposition with coaxial wire feeding using optical coherence tomography

Laser metal deposition (LMD) with coaxial wire feeding is an additive manufacturing technology in which a metal wire is fed into a laser-induced melt pool. The repeated deposition of weld beads allows three-dimensional geometries to be created that can be used for manufacturing, repair, and modification of metal components. However, the process is highly sensitive to disturbances because the fed wire must always be fully melted, and no self-regulating effects as in powder-based LMD exist. The layer height is particularly important for process stability, as even small deviations accumulate over many layers and, ultimately, lead to the termination of the process. Therefore, monitoring and closed-loop control of the layer height during the deposition process are crucial. Due to process emissions, an interruption of the process is usually necessary for the accurate optical measurement of the layer height, which negatively affects the overall productivity. In order to overcome this drawback, an in-axis optical coherence tomography (OCT) sensor was employed in this work, which enabled real-time measurements of the layer height. It was found that positioning the OCT measurement spot as close as possible to the center of the wire provided the highest signal quality. Based on the real-time height data, a closed-loop layer height control was implemented, applying the wire feed rate as the manipulated variable. The experimental results showed that the proposed system was able to compensate for significant disturbances, ensuring dimensional accuracy and process stability.

Traceable characterisation of fibre-coupled single-photon detectors

Abstract The detection of single photons plays an essential role in advancing single-photon science and technologies. Yet, within the visible/near-infrared spectral region, accurate fibre-based optical power measurements at the few-photon level are not yet well-established. In this study, we report on a fibre-based setup, enabling traceable optical power measurements at the few-photon level in this spectral region. The setup was used to calibrate the detection efficiency (DE) of four single-photon avalanche diode (SPAD) detectors. The relative standard uncertainties on the mean DE values obtained from repeat fibre-to-detector couplings ranged from 0.67% to 0.81% (k = 2). However, the relative standard deviation of DE values, which ranged from 1.38% to 3.20% (k = 2), poses a challenge for the metrology of these devices and applications that require high accuracy and repeatability. We investigated the source of these variations by spatially mapping the response of a detector’s fibre connector port, using a focused free-space beam, allowing us to estimate the detector’s spatial non-uniformity. In addition, we realise a novel calibration approach for fibre-coupled SPADs in a free-space configuration, enabling a direct comparison between the fibre-based setup and the National Physical Laboratory’s established free-space facility using a single SPAD. Finally, we investigated alternative coupling methods, testing the repeatability of different fibre-to-fibre connectors in addition to direct fibre-to-detector couplings: SPADs from three manufacturers were tested, with both single-mode and multi-mode fibre.

Design and construction of an affordable optical power meter: micro- to milli-Watt in the 400–800 nm range

This study introduces the design, construction, and evaluation of an affordable optical power meter prototype, AYR (Affordable Yet Reliable) version 1.0, which operates effectively within the 400–800 nm range, using a silicon photodiode. Aimed at bridging the gap in accessibility to precise and reliable photonics instrumentation, especially in resource-constrained settings, AYR 1.0 leverages advancements in photodiode technology, additive manufacturing, and do-it-yourself electronics. The device incorporates a custom-built electronic circuit that facilitates accurate optical power measurement by converting light into electrical current. Through rigorous testing against a reliable commercial optical power meter, AYR 1.0 demonstrated exceptional accuracy and reliability. Sensitivity values ranged from ∼13 μA mW−1 at 405 nm to ∼796 μA mW−1 at 805 nm. The operational power range spanned from 0.003 mW to 242.0 mW, with linearity (R 2) values consistently above 0.9981, indicating high fidelity in measurement. Repeatability percentages varied between 99.4% and 99.9%, and response times ranged up to 55 μs, showcasing the prototype’s rapid and reliable response to changes in optical power. The key components include a low-cost silicon photodiode (2DU10), a differential trans-impedance amplifier circuit for signal processing, and a 3D-printed housing for the sensor head and console, contributing to its cost-effectiveness and robustness. The prototype’s total cost was 116 US dollars, highlighting its affordability and potential for widespread adoption.

A novel technology based on Raman spectroscopy for effective differentiation of alanine chiral isomers

In this study, Raman time-domain spectroscopy (R-TDS) was used to record the spectra of β-alanine, D-alanine, L-alanine, and DL-alanine molecules in the Raman shift range of 250–2000 cm−1. Baseline correction and smoothing of the original spectra yielded considerably different scattering peaks for β-alanine than the other three alanine isomers. The positions and intensities of the peaks in the spectra of D-alanine, L-alanine and DL-alanine were very similar, enabling β-alanine to be easily distinguished from the aforementioned isomers based on the Raman spectra. The Raman spectral data of D-alanine, L-alanine and DL-alanine were subjected to variational mode decomposition (VMD), whereby each spectral line was decomposed into four intrinsic mode functions (IMFs). Finally, a grid search-support vector machine (GS-SVM) classification recognition model was established for Raman spectral analysis and classification analysis using the first four IMFs as datasets. VMD-GS-SVM classification had a 96.67 % discriminative accuracy for predicted samples of the three isomers of D-alanine, L-alanine and DL-alanine. The classification recognition accuracy gradually trended downward as the number of mode components increased. The results show that the fast identification model of alanine isomers based on R-TDS and VMD-GS-SVM is effective and feasible, providing an accurate and fast optical measurement solution for the development of novel drugs and quality monitoring of fine chemicals.

A simple and highly accurate method for fuzzy edge-based 3D morphometry

As the level of engineering technology increases, the mechanical problems posed in the area of mechanics become complex and diverse. The traditional measurement methods and content of past measurements no longer meet the needs, so more advanced experimental methods are needed to accurately measure the three-dimensional shape of objects and their deformations. Nitrate ester plasticized polyether (NEPE) propellants, are the highest energy solid propellant that have been applied in public reports in the world. Due to the material properties of the material itself, its mechanical properties cannot be accurately measured by conventional methods. This paper proposes a simple and highly accurate optical measurement method to study the volume change rate, Poisson’s ratio and true stress-strain curves of NEPE at different displacement loading rates of NEPE. In this paper, the strain of solid propellant NEPE in three directions during unidirectional stretching was measured by defocusing method and digital image correlation(DIC) method and the volume change rate, Poisson’s ratio and true stress-strain curves of NEPE under different stretching rates were obtained. The measured damage initiation engineering strain of NEPE at different tensile strain rates is 0.4687.

Experimental observation of anomalous supralinear response of single-photon detectors

The linearity of single-photon detectors allows accurate optical measurements at low light levels and using non-classical light in spectroscopy, biomedical imaging, optical communication, and sensing. However, in practice, the response of single-photon detectors can exhibit intriguing nonlinear effects that may influence the performed measurements. Here, we demonstrate a direct single-source measurement of the absolute nonlinearity of single-photon detectors with unprecedented accuracy. We discover a surprising supralinear behavior of single-photon avalanche diodes and show that it cannot be explained using known theoretical models. We also fully characterize sub- and supra-linear operation regimes of superconducting nanowire single-photon detectors and uncover the supralinearity under faint continuous illumination. The results identify new detector anomalies that supersede existing knowledge of nonlinear effects at the single-photon level.

Impact of graphene coating created by dipping technique on film-wise condensation

Film-wise condensation takes place in most heat exchangers since this mode is the most stable. Modifying cooled surfaces by applying coatings is trendy method to intensify heat transfer. In this work, a new concept allowing study the influence of surface coating on film-wise condensation is proposed. Reliable thermal data and accurate optical measurements of the condensate thickness distribution were obtained. The correctness of the concept was justified by the demonstration of good agreement between film thickness distributions measured for the uncoated surface and classical Nusselt’s theory. Two types of the coating were created and tested as condensers’ surfaces. The first surface has a roughness of 18 µm filled with graphene, did not provide any condensation enhancement. The second coating consisted of the filled roughness of 10 µm and a several graphene layers, demonstrated two times enhancement of the film-wise condensation. For the first time, the impact of the graphene-based coating on film-wise vapour condensation was found when the condensate thickness is comparable with coating. In other words, the level of intensification depends on the technological parameters used to create the coatings.

The Effect of Non-Measured Points on the Accuracy of the Surface Topography Assessment of Elements 3D Printed Using Selected Additive Technologies.

The paper presents the results of research aimed at evaluating the surface topography including the analysis of the number of unmeasured points of the samples 3D printed using four additive technologies (i.e., PolyJet Matrix, fused deposition modeling, selective laser sintering, and selective laser melting). The samples were made in three variants of location on the printing platform of 3D printers. Measurements of the samples' surface topography were carried out using a Talysurf CCI Lite optical profilometer and a Talysurf PGI 1230 contact profilometer. The percentage of non-measured points for each sample and the parameters of the surface topography were determined. Then, the non-measured points were complemented and the topography parameters for the corrected surface were recalculated. In addition, to perform comparative measurements, each surface was measured using a contact profilometer Talysurf PGI 1230. Preliminary results of the research showed that the measurement of the surface topography of the samples made using selective laser sintering technology with the Taysurf CCI optical measuring system is very unreliable, as the number of non-measured points for the analyzed samples was higher than 98%. The highest accuracy of optical measurement was obtained for PJM technology and three variants of location on the printing platform of the 3D printer.

Effect of temperature on evaporation dynamics of sheep's blood droplets and topographic analysis of induced patterns

To characterize various induced phenomena and the blood of healthy sheep using several parameters, the evaporation dynamics of 72 drops of sheep blood evaporated at several temperatures: 23, 37, 60, and 90 °C on glass hydrophilic substrates were studied. This allows the prediction of the sheep blood pattern, knowing the surface temperature and vice versa. To determine the variation in the Marangoni number between the center and the triple line, an infrared thermography method was used to measure the temperature variation along the surface of the drop. Simultaneously, a high-performance camera was used to measure the variation in the height of the drop during the evaporation using a superior algorithm software for image analysis, drop shape analyzer, under controlled conditions (Humidity = 40%, Tatm = 23 °C). The study of the evaporation dynamics and pattern formation shows the effect of temperature on the flow circulation inside the drop, resulting in the final deposit. The results showed two categories corresponding to two different evaporation phenomena induced by the thermal Marangoni effect. Furthermore, to transform the induced pattern of sheep blood evaporation into a 3D image, a topographic study was performed using a highly accurate, fast, and flexible optical 3D measurement system. The topographic parameters were subsequently extracted from these 3D images. The statistical study showed a good correlation between the topographic parameters and the surface temperature, and a significant difference between each temperature group for each parameter.

The use of digital image correlation for measurement of strain fields in a novel wireless intraspinal microstimulation interface.

Intraspinal microstimulation (ISMS) has emerged as a promising neuromodulation technique for restoring standing and overground walking in individuals with spinal cord injury. Current and emerging ISMS implant designs connect the electrodes to the stimulator through lead wires that cross the dura mater. To reduce possible complications associated with transdural leads such as tethering and leakage of cerebrospinal fluid, we aim to develop a wireless, fully intradural ISMS implant based on our prior work in the cortex with the Wireless Floating Microelectrode Array (WFMA). Although we have extensive data about WFMA cortical stability, its mechanical and electrical stability in the spinal cord remains unknown. One of the quantifiable metrics to assess long-term implant stability is mechanical strain. The aim of the current work is to develop a method to assess implant stability by measuring strain fields in a surrogate of the human spinal cord. A physical model of the spinal cord was studied using an electromechanical testing apparatus, simulating typical spinal cord motion. Strain fields were digitally analyzed using an optical method known as digital image correlation (DIC). Displacement and strain were visualized using contour plots. The strain values in the vicinity of each WFMA device were significantly different from the strain values in the same locations in the control surrogate spinal cord. The results demonstrate that DIC can be used for in-vitro screening of intraspinal implants. Accurate optical strain measurements will enable researchers to optimize implant design over a wide range of motion conditions.

Accurate Measurement of the Optical Properties of Single Aerosol Particles Using Cavity Ring-Down Spectroscopy.

New approaches for the sensitive and accurate quantification of aerosol optical properties are needed to improve the current understanding of the unique physical chemistry of airborne particles and to explore their roles in fields as diverse as chemical manufacturing, healthcare, and atmospheric science. We have pioneered the use of cavity ring-down spectroscopy (CRDS), with concurrent angularly resolved elastic light scattering measurements, to interrogate the optical properties of single aerosol particles levitated in optical and electrodynamic traps. This approach enables the robust quantification of optical properties such as extinction cross sections for individual particles of known size. Our measurements can now distinguish the scattering and absorption contributions to the overall light extinction, from which the real and imaginary components of the complex refractive indices can be retrieved and linked to chemical composition. In this Feature Article, we show that this innovative measurement platform enables accurate and precise optical measurements for spherical and nonspherical particles, whether nonabsorbing or absorbing at the CRDS probe wavelength. We discuss the current limitations of our approach and the key challenges in physical and atmospheric chemistry that can now be addressed by CRDS measurements for single aerosol particles levitated in controlled environments.

Precise determination of characteristic laser frequencies by an Er-doped fiber optical frequency comb

Femtosecond optical frequency combs correlate the microwave and optical frequencies accurately and coherently. Therefore, any optical frequency in visible to near-infrared region can be directly traced to a microwave frequency. As a result, the length unit “meter” is directly related to the time unit “second”. This paper validates the capability of the national wavelength standards based on a home-made Er-doped fiber femtosecond optical frequency comb to measure the laser frequencies ranging from visible to near-infrared region. Optical frequency conversion in the femtosecond optical frequency comb is achieved by combining spectral broadening in a highly nonlinear fiber with a single-point frequency-doubling scheme. The signal-to-noise ratio of the beat notes between the femtosecond optical frequency comb and the lasers at 633, 698, 729, 780, 1064, and 1542 nm is better than 30 dB. The frequency instability of the above lasers is evaluated by using a hydrogen clock signal with a instability of better than 1 × 10−13 at 1-s averaging time. The measurement is further validated by measuring the absolute optical frequency of an iodine-stabilized 532-nm laser and an acetylene-stabilized 1542-nm laser. The results are within the uncertainty range of the international recommended values. Our results demonstrate the accurate optical frequency measurement of lasers at different frequencies using the femtosecond optical frequency comb, which is not only important for the precise and accurate traceability and calibration of the laser frequencies, but also provides technical support for establishing the national wavelength standards based on the femtosecond optical frequency comb.

Developing a 3D clumping index model to improve optical measurement accuracy of crop leaf area index

The clumping index reflects the state of leaf aggregation, which is an essential structural parameter for calculating the leaf area index (LAI). Most of the previous clumping index models are one-dimensional (1D) models, in which the input parameters are measured in a long truncated manner (i.e., one-dimensional measurement path, line data), so that is not practical in the measurement of a dense crop canopy with no branch height. To address this issue, it is necessary to develop an area-based clumping index model that is suitable for measuring the LAI of crops. Taking the digital cover photography of crops captured by digital cameras as a case study, we derived equations with the gap fraction to describe the gap distribution which indirectly reflected the anisotropy in the horizontal distribution of leaves. Using the gap fraction as a variable, we established a three-dimensional (3D) clumping index model. Validation on the proposed 3D model using computer simulations shows that the system deviation of the clumping index derived from the model is less than − 4.64%. In the validation using in-situ measurements, compared with the 1D clumping index model, the 3D clumping index model improved the prediction accuracy of LAI by 20.9%. This study demonstrated that the proposed 3D clumping index model successfully addresses the issue about underestimation of LAI by previous models, facilitates the optical measurement, and improves the calculation accuracy in the quadrat. This study provides theoretical assistance for the development of optical equipment in agriculture. • Dealing with underestimation of the leaf area index in the measurement; • Developing a measurement method suitable for the quadrat of crops; • Proposing a 3D clumping index model base on digital cover photography.

Mean Differential Continuous Pulse Method for Accurate Optical Measurements of Light-Emitting Diodes and Laser Diodes.

Limited sources exist for the application of germicidal ultraviolet (GUV) radiation. Ultraviolet light-emitting diodes (UV-LEDs) have significantly improved in efficiency and are becoming another viable source for GUV. We have developed a mean differential continuous pulse method (M-DCP method) for optical measurements of light-emitting diodes (LEDs) and laser diodes (LDs). The new M-DCP method provides an improvement on measurement uncertainty by one order of magnitude compared to the unpublished differential continuous pulse method (DCP method). The DCP method was already a significant improvement of the continuous pulse method (CP method) commonly used in the LED industry. The new M-DCP method also makes it possible to measure UV-LEDs with high accuracy. Here, we present the DCP method, discuss the potential systematic error sources in it, and present the M-DCP method along with its reduced systematic errors. This paper also presents the results of validation measurement of LEDs using the M-DCP method and common test instruments.

Deep learning for tilted-wave interferometry

Abstract The tilted-wave interferometer is an interferometrical measurement system for the accurate optical form measurement of optical aspheres and freeform surfaces. Its evaluation procedure comprises a high-dimensional inverse problem to reconstruct the form of the surface under test from measured data. Recent work has used a deep learning hybrid approach to solve the inverse problem successfully in a simulation environment. A quantification of the model uncertainty was incorporated using ensemble techniques. In this paper, we expand the application of the deep learning approach from simulations to measured data and show that it produces results similar to those of a state-of-the-art method in a real-world environment.

Characterization of predictable quantum efficient detector in terms of optical non-linearity in the visible to near-infrared range

The characteristics of a predictable quantum efficient detector (PQED) in terms of optical non-linearity in the visible to near-infrared range were investigated under zero-bias and reverse-bias voltage conditions. In the zero-bias condition, linear behavior was observed in the wavelength from 405 nm to 1060 nm in the photocurrent range of 1 nA to 10 μA, and saturation occurred for photocurrents over 10 μA for all wavelengths. In the reverse-bias voltage of 10 V, the linear behavior was observed in the photocurrent range of 64 nA to 1 mA except for the wavelength of 1060 nm, and the saturation photocurrent increased up to 1 mA. The supralinearity value at 1060 nm increased sharply from the photocurrent of 100 μA, and its maximum value reached up to 0.34% at the photocurrent of 1 mA because of the back surface recombination and the longer absorption length. The spectral linearity results of the PQED help us to understand the charge-carrier loss mechanism in the PQED, and would lead to more accurate optical measurement with it.

Fabrication and characterization of large-area suspended MoSe2 crystals down to the monolayer

Many layered materials, such as graphene and transition metal dichalcogenides, can be exfoliated down to atomic or molecular monolayers. These materials exhibit exciting material properties that can be exploited for several promising device concepts. Thinner materials lead to an increased surface-to-volume ratio, with mono- and bi-layers being basically pure surfaces. Thin crystals containing more than two layers also often behave as an all-surface material, depending on the physical property of interest. As a result, flakes of layered materials are typically highly sensitive to their environment, which is undesirable for a broad range of studies and potential devices. Material systems based on suspended flakes overcome this issue, yet often require complex fabrication procedures. Here, we demonstrate the relatively straightforward fabrication of exfoliated MoSe2 flakes down to the monolayer, suspended over unprecedentedly large holes with a diameter of 15 µm. We describe our fabrication methods in detail, present characterization measurements of the fabricated structures, and, finally, exploit these suspended flakes for accurate optical absorption measurements.