Path Difference Research Articles

Aberration analysis of reflective and transmissive type optical spectrometer using Zemax

This study presents a detailed analysis of geometrical aberrations in reflective and transmissive optical spectrometers using Zemax optical design software. Wavefront aberration, a standard metric for assessing geometrical aberration in optical systems, is employed to evaluate performance. Spectrometers are designed in sequential mode and analyzed with the physical optics propagation (POP) algorithm. Key performance metrics, such as optical path difference (OPD), transverse ray plot, spot diagram, modulation transfer function (MTF), and geometric encircled energy, are used to assess the spectrometers. This analysis aims to study aberration effects that cause distorted and blurry spectra, ultimately impacting spectrometer accuracy and efficiency. A comparative analysis of both spectrometers in the visible range for zero and first-order diffraction grating is reported. Line and edge spread functions are simulated to examine the spectrometer’s resolution capability. This study comprehensively compares optical spectrometers, highlighting their applicability across various fields.

Research on Noise Suppression in High-Speed Optical Communication Systems

With the rapid advancement of optical fiber communication technology, the speed and distance of information transmission have increased unprecedentedly. Multi-core fibers (MCF), which contain multiple cores, enable high-capacity and high-density information transmission. However, during signal transmission, four-wave mixing (FWM) and intercore crosstalk (ICXT) noise are generated, significantly impacting signal propagation quality. Based on the characteristics of FWM and ICXT noise, two effective noise suppression schemes are proposed. The first scheme involves reducing noise power by increasing the wavelength interval or using non-equally spaced wavelengths to mitigate phase-matching conditions. The second scheme focuses on altering the structure of the MCF; in weakly coupled MCFs, longitudinal random bending, torsion, and structural fluctuations randomly affect the power coupling coefficient. According to the noise power formula, reducing the effective fiber length, increasing the cores effective area, and using a band with a low nonlinear effect on the MCF can effectively minimize noise generation. Finally, we evaluate the proposed scheme and analyze and discuss the results.

Picometer sensitive prototype of the optical truss interferometer for LISA

Abstract The optical truss interferometer (OTI) is a contingent subsystem proposed for the LISA telescopes to aid in the verification of a 1 pm Hz optical path length stability. Each telescope would be equipped with three pairs of compact fiber-coupled units, each forming an optical cavity with a baseline proportional to the telescope length at different points around the aperture. Employing a Pound–Drever–Hall approach to maintain a modulated laser field on resonance with each cavity, the dimensional stability of the telescope can be measured and verified. We have designed and developed prototype OTI units to demonstrate the capability of measuring stable structures, such as the LISA telescope, with a 1 pm Hz sensitivity using a set of freely mountable fiber-injected cavities. Aside from its initial motivation for the telescope, the OTI can also be readily integrated with other systems to aid in ground testing experiments. In this paper, we outline our experimental setup, measurement results, and analyses of the noise limitations.

A general design method for ultralong optical path length multipass matrix cells

A general design method for ultralong optical path length multipass matrix cells

A Trace C2H2 Detection Based on Near‐Infrared Dual‐Comb Spectroscopy

ABSTRACTTo achieve low‐concentration measurement using near‐infrared dual‐comb spectroscopy (DCS), this study presents a trace C2H2 detection using near‐infrared DCS. A multi‐pass gas cell is employed to extend the optical path length, improving the sensor's sensitivity. Signal post‐processing via interpolation method is applied to reduce noise and achieve a flatter dual‐comb. The sensor's sensitivity and transient characteristics are evaluated by selecting absorption lines at 6526.53, 6529.17, and 6531.78 cm−1, with the experimental profile being well‐fitted by a Voigt function. The stability of the system is confirmed with a C2H2 concentration of 200 ppm over a 2.5 h observation period, showing a stability better than 2.305 × 10−2. Allan variance analysis indicates an optimal integration time of 597 s, yielding a minimum detection limit of 6.15 × 10−4 for absorbance and 350 ppb for concentration. Additionally, the detection of nine absorption lines demonstrates the multi‐peak measurement capability, with deviations ranging from 0.059% to 12.496%.

Interaction of light with gas-liquid interfaces: influence on photon absorption in continuous-flow photoreactors.

Light interacts with gas bubbles in various ways, potentially leading to photon losses in gas-liquid photochemical applications. Given that light is a valuable 'reagent', understanding these losses is crucial for optimizing reactor efficiency. In this study, we address the challenge of quantifying these interactions by implementing a method that separately determines the photon flux and utilizes actinometric experiments to determine the effective optical path length, a key descriptor of photon absorption. The results reveal the unexpected impact of gas phase introduction in continuous-flow photoreactors. Notably, photon absorption, and consequently the throughput of a photoreactor, can be increased by the introduction of a gas phase. This enhancement arises from the reflection and refraction effects of gas bubbles, which can intensify light intensity in the liquid volume and thereby offset any loss in residence time. The photon absorption losses that were observed were associated with large bubbles and were less significant than anticipated. In contrast, the introduction of small bubbles was found to increase photon absorption, suggesting it is a potential strategy to optimize photoreactor performance.

OptiBench: An Optical Distance Measurement Test Bench using Adaptive Algorithm with Sensor Fusion and Environmental Shielding

OptiBench: An Optical Distance Measurement Test Bench using Adaptive Algorithm with Sensor Fusion and Environmental Shielding

Report on 3,143.6km Time and Frequency Transfer Fiber Link With Ps-level Stability

Abstract The letter describes the implementation of a high-precision time and frequency transmission system using a two-way and wavelength division multiplexing (WDM) scheme over 3,143.6 km of field fiber links, connecting 21 sites between Xi'an and Beijing, China. The scheme incorporates a link noise clean-up system (LNCUS), wherein the controlled very low-phase noise BVA oscillator acts as an internal frequency source at the relay site to effectively suppress the interference caused by fiber link noise. Additionally, high-stability pulse generation technology in LNCPS provides enough short-term stability to significantly improve the stability of the fiber time transfer. The results demonstrate that a time transfer stability in terms of time deviation (TDEV) of less than 2.07 ps@1s, 5.9 ps@100,000s, and the combined uncertainty of less than 49.29 ps can be obtained for the 3,143.6 km field fiber link. The obtained results represent a significant breakthrough in achieving high accuracy of time transfer and continuous operation over ultra-long span field optical fibers exceeding 3,000 km for the first time. This achievement enables the provision of time transfer and synchronization services over the longest international field optical fiber distance, thereby establishing the fundamental basis for constructing a nationwide high-precision time service through optical fiber networks.

Enhancement of Methane Detection in Tunable Diode Laser Absorption Spectroscopy Using Savitzky–Golay Filtering

In order to enhance gas absorption efficiency and improve the detection sensitivity of methane, a gas absorption cell with an effective optical path length of 29.37 m was developed, employing tunable diode laser absorption spectroscopy (TDLAS) and a distributed feedback (DFB) laser with a center wavelength of 1.654 μm as the light source. However, despite these advancements, the detection accuracy was still limited by potential signal interference and noise. To address these challenges, the Savitzky–Golay (S-G) filtering technique was implemented to optimize the TDLAS detection signal. Experimental results indicated a significant enhancement in detection performance. For a methane concentration of 92 ppm, the application of the S-G filter improved the signal-to-noise ratio by a factor of 1.84, resulting in a final device detection accuracy of 0.53 ppm. This improvement demonstrates the effectiveness of the S-G filter in enhancing detection sensitivity, supporting high-precision methane monitoring for atmospheric analysis and various industrial applications.

A Multi-Optical and Mechanical Compensation Robotic Surgery System Based on Augmented Reality for Endoscopic Neurosurgery

Abstract Endoscopic treatment is a widely employed clinical modality for the rapid reduction of cranial pressure in cases of intracerebral hemorrhage (ICH). We are working to make endoscopic surgery simpler and more effective by providing surgeons with robot and augmented reality (AR). This paper proposes an AR-based multi-optical and mechanical compensation robotic surgical system that can precisely match the virtual hematoma region with the anatomical structures seen under a robot-wielded neuroendoscope. The custom mark unified multi-optical localization system enables optical tracking over a wide range of angles and breadths. The neuroendoscope registration employs a multimodal error compensation approach based on an optical tracking system and the kinematic model of the robotic arm. Utilizing innovative methods, we register patients, medical images, optical localizers, and both neuroendoscopic and robotic coordinate systems, thereby enabling dual-view AR tracking through video-stream fusion. In multimodal error compensation for neuroendoscopic registration, the calibration error is 0.37 mm. The system's performance was evaluated through ten phantom experiments, showing the registration accuracy of 0.39±0.10 mm, optical localizer AR fusion accuracy of 3.30±1.08%, neuroendoscope AR fusion accuracy of 5.22±1.02%, and optical localizer AR average distance and angle accuracies of 1.73±0.21 mm and 0.38±0.13 deg, respectively. The AR-based robotic surgical system (ARSS) overlays virtual lesions onto the current endoscopic view of the real lesions in real-time, which can help surgeons to localize lesions hidden deep in tissue by an endoscopy. It is anticipated that the ARSS will reduce surgeon fatigue and improve the intuitiveness of endoscopic surgery.

3D cloud masking across a broad swath using multi-angle polarimetry and deep learning

Abstract. Understanding the 3D structure of clouds is of crucial importance to modeling our changing climate. Both active and passive sensors are restricted to two dimensions: as a cross-section in the active case and an image in the passive case. However, multi-angle sensor configurations contain implicit information about 3D structure, due to parallax and atmospheric path differences. Extracting that implicit information requires computationally expensive radiative transfer techniques. Machine learning, as an alternative, may be able to capture some of the complexity of a full 3D radiative transfer solution with significantly less computational expense. In this work, we develop a machine-learning model that predicts radar-based vertical cloud profiles from multi-angle polarimetric imagery. Notably, these models are trained only on center swath labels but can predict cloud profiles over the entire passive imagery swath. We compare with strong baselines and leverage the information–theoretic nature of machine learning to draw conclusions about the relative utility of various sensor configurations, including spectral channels, viewing angles, and polarimetry. Our experiments show that multi-angle sensors can recover surprisingly accurate vertical cloud profiles, with the skill strongly related to the number of viewing angles and spectral channels, with more angles yielding high performance, and with the oxygen A band strongly influencing skill. A relatively simple convolutional neural network shows nearly identical performance to the much more complicated U-Net architecture. The model also demonstrates relatively lower skill for multilayer clouds, horizontally small clouds, and low-altitude clouds over land, while being surprisingly accurate for tall cloud systems. These findings have promising implications for the utility of multi-angle sensors on Earth-observing systems, such as NASA's Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) and Atmosphere Observing System (AOS), and encourage future applications of computer vision to atmospheric remote sensing.

Symmetric Non-Monochromatic Light as Reference in Fourier Transform Spectrometers.

Fourier transform spectrometers typically use a presumed monochromatic reference source to track and correct errors in optical path difference changes. This paper will conduct a theoretical analysis to show that non-monochromatic light sources with symmetric spectral profiles can also be used as reference sources without adding errors. An experiment was carried out using a symmetric broadband superluminescent diode (SLED) as reference light to measure the spectrum of some other SLED light sources to experimentally demonstrate this finding.

Path differences between quasistatic and fatigue cracks in anisotropic media

Path differences between quasistatic and fatigue cracks in anisotropic media

Studying the Effect of Transport Layers on ZrS2/MEH-PPV Solar Cells: Using SCAPS -1D Software

This study investigates the effect of charge transport layers on the efficiency of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH_PPV) and Zirconium Disulfide (ZrS₂) solar cells using Scaps-1D software. It was found that by increasing the MEH-PPV thickness and decreasing its acceptor doping concentration, the efficiency (μ%), fill factor (FF), and short-circuit current density (Jsc ) decreased. Conversely, increasing the thickness of the ZrS₂ electron transport layer and decreasing its donor doping density enhanced the efficiency (μ%) and short-circuit current density (Jsc) while maintaining a constant open-circuit voltage (Voc). These results can be attributed to decreased charge separation and collection in MEH-PPV and reduced optical path length in ZrS2. On the other hand, the back contact with work function is below 4.65 eV, the MEH-PPV/ZrS2 solar cells produced the lowest efficiency compared to different types of back contact. Under optimal conditions, MEH-PPV/ZrS2 solar cell shows a high efficiency of 21% when the dopant concentration of MEH-PPV and the value of the neutral defect density at the ZrS2/ MEH-PPV interface are 1022 cm-3 and 109 cm-3 respectively.

The performance of optic disc-macula distance to disc-diameter ratio in qualitative and quantitative assessment of the optic disc size.

We aimed to evaluate the performance of the optic disc-macula distance to disc-diameter ratio (DM/DD) in qualitative and quantitative assessment of optic disc size. In 300 apparently normal eyes, we determined the correlation between DM/DD and the planimetric disc area (DA) and evaluated the performance of DM/DD in discriminating between small and large discs. The ability of DM/DD to predict the actual DA was validated in a separate cohort of 200 eyes. The disc size in eyes with glaucoma or related diseases was compared with DA- or DM/DD-based disc size. The DM/DD and its square of inverse (DD2/DM2) showed excellent non-linear and linear correlations, respectively, with DA (p < 0.001 and p < 0.001, respectively), demonstrating their performance in discriminating between small (<2.04 mm2) and large (>2.94 mm2) disc sizes with an area under the receiver operating characteristic curve of 0.901 and 0.959, respectively. The difference between the predicted DA and planimetric DA was within ±10% in more than 80% of the eyes. Eyes with suspected and normal-tension glaucoma exhibited larger discs (p < 0.01) when evaluated using both DM/DD and DA. The variation in disc-macula distance seemed to be a confounding factor for DM/DD in inferring the precise disc size. DM/DD, which can be easily calculated from various kinds of fundus photographs or images, was an excellent and interchangeable tool for qualitative and quantitative assessment of disc size, which did not require correction for ocular magnification.

DL-CSPF: deep-learning-based cell segmentation with a physical framework for digital holographic microscopy

Digital holographic microscopy (DHM) offers label-free, full-field imaging of live-cell samples by capturing optical path differences to produce quantitative phase images. Accurate cell segmentation from phase images is crucial for long-term quantitative analysis. However, complicated cellular states (e.g., cell adhesion, proliferation, and apoptosis) and imaging conditions (e.g., noise and magnification) pose significant challenge to the accuracy of cell segmentation. Here, we introduce DL-CSPF, a deep-learning-based cell segmentation method with a physical framework designed for high-precision live-cell analysis. DL-CSPF utilizes two neural networks for foreground-background segmentation and cell detection, generating foreground edges and “seed points.” These features serve as input for a marker-controlled watershed algorithm to segment cells. By focusing on foreground edges and “seed points”, which have lower information entropy than complete cell contours, DL-CSPF achieves accurate segmentation with a reduced dataset and without manual parameter tuning. We validated the feasibility and generalization of DL-CSPF using various open-source and DHM-collected datasets, including HeLa, pollen, and COS-7 cells. Long-term live-cell imaging results further demonstrate that DL-CSPF reliably characterized and quantitatively analyzed the morphological metrics across the cellular lifecycle, rendering it a promising tool for biomedical research.

Fabry–Perot enhancement of liquid crystals birefringence effects in terahertz range

Abstract THz technology is a rapidly advancing field in both fundamental and applied research, integrating novel approaches from optical and microwave ranges. Despite the wide applications of liquid crystals (LCs) in the visible range, their efficiency in the THz range is limited due to the low optical path length of existing anisotropic LC materials. In this work, we propose and explore the advantages of incorporating a LC core layer into the Fabry-Perot resonator as a simple strategy to enhance the observed birefringence effects. This enhancement is achieved through multiple wave passes through the LC layer, effectively increasing the effective optical path length.&amp;#xD;In particular, we demonstrate a more than tenfold improvement in the efficiency of wave transformation within a plane-parallel LC layer.&amp;#xD;Our findings are well correlated with the THz response of an LC cell composed of two parallel thick quartz substrates infiltrated with commercially available E48 liquid crystal, as observed in our experiments. The low switching voltage for cells with pyrocarbon-based electrodes is also demonstrated.

Application of the Hilbert-Huang transform to evaluate signals in chromatic confocal spectral interferometry

Chromatic confocal spectral interferometry (CCSI) is a hybrid measurement technique that integrates the principles of spectral interferometry and chromatic confocal microscopy. This innovative approach enables scanning-free acquisition of axial dimensions while leveraging interferometric methods to enhance depth accuracy. This feature allows the CCSI signal to be processed using both peak extraction and evaluation of the interferometric optical path length difference. The phase information offers a decreased measurement uncertainty, making it a commonly used approach in existing studies. Methods such as Fourier transform (FT) and wavelet transform (WT) are frequently employed for this purpose. In this paper, we present a signal processing approach based on the Hilbert-Huang transform (HHT). Through simulations and experiments, we compare HHT with Fourier transform (FT) and wavelet transform (WT), demonstrating its stability, noise resistance, and effectiveness in phase extraction for CCSI measurement signals. Additionally, we leverage the characteristics of CCSI signals and the time-frequency analysis capabilities of HHT to address the direction ambiguity problem in white light interferometry.

Miniaturized laser interferometer using dynamic current-wavelength modulation for high precision displacement measurement

Abstract This study proposes a compact displacement measurement interferometer facilitated by a novel dynamic current-wavelength modulation method for high-precision displacement measurement. This technique compensates for the modulation depth by precisely predicting the optical path difference with frequency-scanning interferometry to achieve consistent modulation depth over an extensive measurement range. Additionally, a wavelength-locking method was developed for the 1550 nm band, using the P7 absorption peak of hydrogen cyanide gas to lock the central wavelength of the laser. The experimental results showed that the wavelength stability can be kept within 128 fm for 8 h with the proposed method. Furthermore, the deviations are less than 40 nm compared to a calibrated laser interferometer within the 300 mm measurement range. This research offers precise positioning feedback for precision engineering, paving the way for advanced semiconductor manufacturing processes.

Low temporal radiation coherence of noise-like pulses

Abstract It is quite well-known that coherence is an intrinsic property of laser radiation. However, after the advent of noise-like pulses, it turns out that the temporal coherence of their radiation is low and the electromagnetic field oscillations in adjacent noise-like pulses are not correlated. This work experimentally studies for the first time the degree of coherence of electromagnetic field oscillations inside a noise-like pulse. We measure the visibility of interference fringes at the exit from a Michelson interferometer when its optical path difference is scanned around zero up to and exceeding the length of the noise-like pulse. The experimental data indicate close to zero temporal coherence of radiation within noise-like pulses.