Path Difference Research Articles

Meta-Learning Assisted Source Domain Optimization for Transfer Learning Based Optical Fiber Nonlinear Equalization

Transfer learning (TL) has been demonstrated its feasibility on fast remodeling for fiber nonlinearity equalization. It will be very efficient with fine-tuning rather than retraining when the channel parameters have been changed, e.g., the optical launch power and fiber distance. However, the performance is dependent on the highly correlation between the source and target tasks. An improper source domain cannot improve the efficiency or even negative transfer. In this paper, we propose a meta-learning assisted source domain optimization scheme in single- and multi-channel transmissions. We construct the support dataset and query dataset from the information of the inner-task and the across-task to train the neural network so that we can find a unified source domain and efficiently adapt to different tasks with fewer epochs. We conduct an experiment to transmit 120-Gb/s dual polarization (DP) 16-QAM signal over 800-km standard single mode fiber (SSMF). The required epochs for convergence can be significantly reduced and there is slight Q-factor improvement with meta-learning algorithm compared with retraining and conventional TL. We also analyze the TL strategy in multi-channel system. The simulation results show that meta-learning can also help to optimize the source domain with lower complexity.

Fiber Laser Methane Detection Device based on TDLAS and Its Application

Methane is a flammable and explosive gas, which is the main component of natural gas, mashgas, and biogas. Using the technology of tunable diode laser absorption spectroscopy (TDLAS) to detect methane, which does not require oxygen, no poisoning, no calibrating, has gas selectivity, strong environmental adaptability, and is stable and reliable. The laser transmission with the optical fiber has the characteristics of low optical loss, long distance and flexible networking. This manuscript demonstrated an optical fiber laser methane detection device based on TDLAS and its application. The laser was transmitted to the methane detection position by optical fiber, and the laser interacted with methane in the optical fiber gas box. The optical fiber collimator coupled the surplus laser into the optical fiber and sent it back to the monitoring device.The photoelectric detector converted the surplus laser to an electrical signal. The monitoring device operation was based on the principle of TDLAS and analyzed the returned laser strength to obtain the concentration of methane. This manuscript described the principle, composition and monitoring spot network of the fiber laser methane monitoring device and shown a practical application.

The design of an optomechanical microphone using a photonic waveguide interferometer

Most commercial microphones rely on capacitive or piezoelectric readout of a vibrating diaphragm. Optical methods have also been used to read the motion of a vibrating microphone diaphragm. Most optical microphone demonstrations to date use free-space optics. We present the design of an optical interferometric microphone that uses a photonic waveguide embedded within the diaphragm. The deflection of the moving diaphragm causes a strain field within the diaphragm, and this strain in turn changes the optical path length of the waveguide. Light traversing the sensing arm of the interferometer combines with light traversing an on-chip reference arm to yield an output light power that depends on the instantaneous displacement of the diaphragm. The output of the combined beams is coupled from the chip to a photodetector to complete the interferometer. An advantage of such transduction method is the absence of a microphone backplate and its associated thermal-mechanical noise. Electrical noise associated with high input-impedance amplification, as required for small capacitive and piezoelectric sensors, is also circumvented. This embodiment does, however, introduce several implementation challenges including the routing of light onto and off the microphone diaphragm.

Research on determining the position of zero optical path difference with the wavelet transform

Research on determining the position of zero optical path difference with the wavelet transform

Image Enhancement in Lens-Coupled Detectors for industrial CT systems

These days, lens-coupled detectors have been taken into consideration due to their cost effectiveness, but they have some disadvantages relative to other detectors such as flat panel detectors. Meanwhile, white spot noise and light spreading due to optical distance, will result in reduction of the quality of the reconstructed object in computed tomography applications. In this paper, due to limited power of classical methods, to fix these dysfunctions, we have learned a Neural network for improvement of the images.

Effects of Shock Waves on Shack–Hartmann Wavefront Sensor Data

Shock waves will form by turning supersonic or locally supersonic flow and result in an increase in the freestream density downstream of the shock. This increase leads to optical distortions that limit the effectiveness of aircraft-mounted laser systems. In this paper, analytic expressions are developed to describe these optical distortions in terms of the optical-path difference (OPD). Pupil-plane disturbances imposed by the shock are studied for two cases: when the shock is parallel to the propagation direction and when the shock is on an angle relative to the propagation direction. Upon propagation from the pupil plane, the analysis shows that shock-induced phase discontinuities can sometimes cause the irradiance pattern in the image plane to bifurcate. Despite a large amount of tilt in the pupil plane, the bifurcated irradiance pattern does not map to a proportional shift in the image plane. The implications that these findings have on Shack–Hartmann wavefront sensor (SHWFS) data are also explored. The results show that least-squares reconstruction from the SHWFS data yield accurate estimates of the change in OPD across the shock when the magnitude of the phase difference caused by the shock is between 0 and approximately . However, when , the results show that least-squares reconstruction begins to severely underestimate the change in OPD across the shock. Such results will inform future efforts looking to develop aircraft-mounted laser systems.

Enrichment of high‐purity nervonic acid ethyl ester from Acer truncatumBunge seed oil by combination method of urea inclusion and molecular distillation

AbstractAcer truncatum Bunge is a type of maple that is unique to China. Its fruit setting rate and seed oil content are both high. The oil is mainly composed of C16–C24 fatty acids and contains 5%–7% nervonic acid (NA). NA and its derivatives can delay ageing and prevent and treat disorders such as senile dementia and Alzheimer's disease. The content of NA ethyl ester prepared by the seed oil of A. truncatum Bunge is 5.84%. We report a new process for the enrichment of NA ethyl ester by urea inclusion (UI) and molecular distillation (MD), with the aim of obtaining highly pure NA ethyl ester. First, based on the difference in fatty acid ethyl ester saturation and carbon chain length, unwanted compounds such as oleic acid ethyl ester, linoleic acid ethyl ester, and sterol were removed by one‐stage UI, the content of NA ethyl ester was increased to 18.69%. The oil in the UI compound was used as a feed, and differences in the mean molecular free paths between the components were exploited to separate the C16–C20 fatty acid ethyl esters (FAEEs) by two‐stage MD. The total content of C16–C20 FAEEs decreased to 3.69% and the purity of NA ethyl ester increased to 47.47%. A new purification process of UI‐MD‐UI was established and NA ethyl ester could be purified to 91.8%. The combination of MD and UI has an important reference value for the industrialization of producing high‐purity NA ethyl ester.

Tuning of optical fiber laser based on super-mode interference in a seven-core fiber

Tuning of optical fiber laser emissions based on super-mode interference in a seven core fiber (SCF) is presented. This super-mode interference is generated by a super-mode Mach–Zehnder interferometer constructed with a few millimeters of SCF spliced between two single mode fibers (SMFs). In the SCF, two super-modes are exited via the fundamental mode of the SMF. Then, an interference pattern caused to the optical path difference between super-modes is generated at the final of the SCF, which is collected with the spliced SMF. This interference pattern induces and tunes laser line emissions by adjusting the laser cavity losses in an optical fiber laser. By bending the Mach–Zehnder interferometer, a wavelength shift of the interference pattern is induced, which in turn causes a wavelength shift of the laser cavity losses and, therefore, a displacement of the laser wavelength emission. Laser line emissions were tuned by linear factors of around 0.91 nm mm−1, and the switching of simultaneous emissions was also obtained. The stability and reliability of the laser emission on the proposed method are also discussed.

Exact formula to relate optical path differences and transversal aberration components.

An exact equation to relate the optical path differences (OPD) with its transversal aberration components (TAC) is determined. The OPD-TAC equation reproduces the Rayces formula and introduces the coefficient for the longitudinal aberration. The defocus orthonormal Zernike polynomial (Z DF) is not a solution for the OPD-TAC equation since the obtained longitudinal defocus depends on the ray height on the exit pupil, meaning that it cannot be interpreted as a defocus. In order to find an exact expression for OPD defocus, first, a general relationship between the wavefront shape and its OPD is established. Second, an exact formula for the defocus OPD is established. Finally, it is proved that only the exact defocus OPD is an exact solution of the exact OPD-TAC equation.

Investigation of the interference phenomenon between ultrashort pulses based on an all-fiber laser

We experimentally report on the interference phenomenon between ultrashort pulses in fiber media. When the pulse interval goes further to a certain small scale, the interference effects can arise. As a result, the spectrum appears to be a modulation phenomenon. Different from the phenomenon of bound-state pulses in a fiber laser cavity, the relationship between the modulation period of the spectrum and the pulse interval in the time domain seems to go against the interference principle. At the same time, the time interval between the two pulses is far greater than the time difference caused by the optical path difference. We are very confused. It seems that when two pulses overlap by a certain proportion in time, they will push each other apart to increase the time interval. Moreover, when the pulse interval becomes smaller and the two pulses are overlapped in time, the corresponding autocorrelation trace exhibits only one peak with regular modulation instead of the conventional three peaks with equal spacing, which looks very strange and is hard to explain. Limited by our technical tools, we could not illustrate the physical causes, but there is no doubt that it will be very meaningful to research what happens. Our observations have found some novel optical phenomena in experiment, which may provide some reference for further exploration of interference phenomena between ultrashort pulses.

Temperature-compensated balloon-like fiber magnetic field sensor with F-P structure based on PDMS

A highly sensitive balloon-like fiber interferometer based on magnetic fluid (MF) nanomaterial for magnetic field measurement is proposed in this paper. The Mach-Zehnder interferometer (MZI) was formed by bending the single-mode fiber (SMF) nested in Teflon tubes, and then MF was filled into the tube of the balloon-like fiber interferometer by using the capillary effect. When the external magnetic field intensity changes, the refractive index (RI) of MF changes, resulting in the change of the difference in optical path. The change of magnetic field intensity can be detected by the shift of interference spectrum. Besides, a Fabry-Perot interference (FPI) by filling the SMF with polydimethylsiloxane (PDMS), which is only sensitive to temperature, is proposed to compensate the temperature during sensing of magnetic field. The transmission signal of the MZI and the reflection signal of FPI are cascaded together to the circulator, optical switch and other components, so that the magnetic field intensity and temperature can be measured simultaneously. The temperature sensitivity of the MZI and the FPI is –0.919 nm/℃ and1.559 nm/℃, respectively. FPI could be used to offset the effect of temperature. Consequently, the sensitivity of the magnetic field sensor can reach 0.683 nm/mT in the range of 0 to 10 mT, and the linear coefficient is 0.987. The system could be widely employed in lots of applications of magnetic field and temperature monitoring.

Fabrication and characterization of porous tissue-mimicking optical phantoms as a tool for optical sensor validation.

This paper presents porous polydimethylsiloxane (PDMS) optical phantoms with tunable microstructural and optical properties to mimic porous biological tissues (e.g., fruit) during the design and optimization of novel optical setups. A well connected salt network formed using salt particles of various size distributions was used to obtain porous PDMS phantoms of different porous features including porosity, pore size distribution, pore number density and pore connectivity. These microstructural features are strongly related to the light scattering from the phantom where a higher reduced scattering coefficient ( ) was observed from the porous PDMS phantom with a higher number of small pores compared to the optical phantom with a lower number of larger pores. The prepared phantoms were used to validate GASMAS (gas in scattering media absorption spectroscopy) H2 O and O2 sensors by quantifying the optical path length through the pores and the O2 concentration inside the pores.

Prosthetic approach following traumatic evisceration: a case report

A 20-year-old male patient was referred to Prosthodontic Department, Dental Hospital of Hasanuddin University, Makassar with chief complaint of disfiguration of the face. Medical history revealed a significant trauma to the left eye 6 months prior which stayed after evisceration. Patient has never used prosthetic eye afterwards. Examination of the left eye socket revealed a healthy conjunctiva covering the posterior wall of the anophthalmic socket with syn-chronous motions and absence of infection or inflammation signs. Sulcus depth was sufficient enough to retain prosthetic eye. This article presents rehabilitation of the ocular defect with customized acrylic-based ocular prosthe-sis to increase patient’s appearance and to prevent further shrinkage of the eye socket. Preliminary impression was done using customized tray fabricated from modelling wax and hydrocolloid irreversible impression material. An in-traocular custom tray for secondary impression was fabricated with acrylic resin and modified with a syringe that at-tached to the custom tray. Secondary impression of the defect was recorded using polyvinyl siloxane light viscosi-ty material followed by wax pattern fabrication using modelling wax. The wax pattern was tried in patient’s socket and checked for size, comfort, support, fullness, and then packed with tooth colored heat cure acrylic resin. After deter-mining the location and diameter of the iris with an optical vernier pupillary distance ruler, the color of sclera was de-termined by shade guide and confirmed with technician using digital photo. Ocular prosthesis was fabricated after-wards and inserted into the eye socket and evaluated for suitability, aesthetic and also movements with the contra-lateral eye. It is concluded that customized ocular prosthesis was significantly more aesthetic than pre-fabricated one with better contouring, color matching, and coordinated movements with the contralateral eye. This prosthetic ap-proach may restore patient’s appearance, increase their self-esteem, and improve their quality of life.

Greenhouse Gases Monitoring Instrument on GaoFen-5 Satellite-II: Optical Design and Evaluation

The Greenhouse gases Monitoring Instrument on GaoFen-5 satellite-II (GMI-II) uses spatial heterodyne spectroscopy (SHS) for quantitative monitoring of atmospheric greenhouse gases (GHG). Unlike the traditional SHS, the interferometer component of the GMI-II was designed with zero optical path difference offset, effectively improving spectral resolution while maintaining the same detector specifications. The secondary imaging system with non-isometric scaling of spatial and spectral dimensions was designed to decrease the integration time of a frame image or improve the spectral signal-to-noise ratio (SNR) under the same integration time. This paper introduces the design, manufacture, adjustment methods, and test results of the main performance indexes of the GMI-II that indicate that the spectral resolution of the O2 A-band detection channel is better than 0.6 cm−1 and other channels are better than 0.27 cm−1. Under the typical radiance of other carbon monitors’ on-orbit statistics, the spectral SNR of the GMI-II is more than 300. These test results demonstrate that the GMI-II can be well adapted to quantitative remote sensing monitoring of atmospheric GHG.

Effects of Cascading Optical Processes: Part II: Impacts on Experimental Quantification of Sample Absorption and Scattering Properties.

In Part I of the three companion articles, we reported the effects of light scattering on experimental quantification of scattering extinction, intensity, and depolarization in solutions that contain only scatterers with no significant absorption and photoluminescence activities. The present work (Part II) studies the effects of light scattering and absorption on a series of optical spectroscopic measurements done on samples that contain both absorbers and scatterers, but not emitters. The experimental UV-vis spectrum is the sum of the sample absorption and scattering extinction spectra. However, the upper limit of the experimental Beer's-law-abiding extinction can be limited prematurely by the interference of forward scattered light. Light absorption reduces not only the sample scattering intensity but also the scattering depolarization. The impact of scattering on sample light absorption is complicated, depending on whether the absorption of scattered light is taken into consideration. Scattering reduces light absorption along the optical path length from the excitation source to the UV-vis detector. However, the absorption of the scattered light can be adequate to compensate the reduced light absorption along such optical path, making the impacts of light scattering on the sample total light absorption negligibly small (<10%). The latter finding constitutes a critical validation of the integrating-sphere-assisted resonance synchronous spectroscopic method for experimental quantification of absorption and scattering contribution to the sample UV-vis extinction spectra. The techniques and general guidelines provided in this work should help improve the reliability of optical spectroscopic characterization of nanoscale or larger materials, many of which are simultaneous absorbers and scatterers. The insights from this work are foundational for Part III of this series of work, which is on the cascading optical processes on spectroscopic measurements of fluorescent samples.

Reconfigurable highly-sensitive interferometric sensor with Vernier effect and a virtually tunable reference cell

The optical Vernier effect is a powerful tool for increasing the sensitivity of an interferometer greatly. However, when the measurement range is large, the optical path difference between the sensing and reference cells becomes large too, which will cause a significant decrease in the sensitivity. To overcome this problem, we propose a novel method, which introduces a virtual reference cell instead of a real one in the measurement system, with a variable cavity length adapting to the measurement range. Both theoretical analysis and experiment measurement verified that by tuning the cavity length of the reference interferometer, LR, the Vernier effect based sensitivity can be maintained at a high level, although the cavity length of the sensing interferometer, LS, changes in a large range. A new formula presented in this paper also reveals that the Vernier effect induced sensitivity magnification factor M depends only on the ratio between the cavity lengths LS/LR in a more general way (denoted as factor g), instead of the absolute or difference between the two cavity lengths described in the previous literature. The experimental results of a large displacement measurement show that when the displacement changes from 0 to 100μm, g changes from 1.01 to 1.35, which results in M dropping from 110 to 4 as LR is fixed. However, M can be recovered back if LR is adjusted to make g equals 1.01 again when LS is changed. The proposed method provides a new approach to establish a highly sensitive interferometric sensor based on the Vernier effect, consisting of a single real sensing cell and a virtually variable reference cell, which can maintain the sensitivity at a high level even though the measurement range is large.

Differential laser Doppler vibrometry for displacement measurements down to 1 mHz with 1 nm amplitude resolution in harsh environments

Environmental disturbances increasingly limit laser Doppler vibration measurements with decreasing frequency of the moving surface. Particularly limiting for the detection of small displacements of a specimen in a furnace are refractive index fluctuations in the air from heat haze, optical path length fluctuations from thermal expansions in the measurement setup, and mechanical vibrations from the building and other sources. In this paper, we demonstrate how these disturbances can be largely compensated for. In our application, we require a displacement resolution of less than 10 nm to measure the chemical expansion of volume crystals and thin films at an ambient temperature of up to 800 °C. The required sensitivity cannot be achieved with state-of-the-art commercially available laser Doppler vibrometers since they are not designed to operate at frequencies below 0.1 Hz. In order to reduce the dominating low-frequency environmental noise, we employ a differential laser Doppler vibrometer in a setup optimized to minimize low-frequency displacement noise. We prove that the concept of differential measurement can eliminate disturbances by optical turbulence and parasitic mechanical vibrations with a newly constructed laser Doppler vibrometer. The elimination of these noise contributions allows for the first time a displacement sensitivity of about 1 nm for frequencies in the mHz range in a 600 °C environment.

The characteristics of ultraviolet absorption and electronic excitation of sulfate at high concentrations

The variation of spectra and the characteristics of electronic excitation are critical for establishing a model for quantifying sulfate at high concentrations. The absorption characteristics of sulfate are affected by the optical pathlength and sulfate concentration. The absorption coefficient declines by approximately 86.09–96.20% with an increasing concentration (0–130 g/L) at different optical pathlengths (1–100 mm). Moreover, a high sensitivity and accuracy can be achieved at weak absorption wavelengths or at lower optical pathlengths when high concentrations of sulfate are detected. In addition, the maximum absorption wavelength of sulfate redshifts by approximately 0–10 nm with an increasing concentration and optical pathlength, which is significantly affected by the optical pathlength. The (H2SO4)n‧(H2O)4-n models were established at the PBEPBE/6–311++G(d, p) level of theory. There absorption spectra were calculated by the time-dependent density functional theory (TD-DFT) method. As a result, the maximum absorption wavelength redshifted from 180.16 nm to 192.71 nm with an increasing sulfate concentration, and the corresponding absorption coefficient demonstrated a declining trend. Furthermore, the electron-hole and natural bond orbital (NBO) analysis indicate that the type of electronic excitation changes from a n(O) → σ*(S-O) localized excitation to n → σ* charge-transfer excitation as the sulfate concentration increases. This study provides a theoretical foundation for understanding the spectral behavior of sulfates and constructing the quantification models or methods that can also be applied to analyze the spectroscopy of other chemicals.

Anatomic Peculiarities Associated with Axial Elongation of the Myopic Eye.

To describe anatomical peculiarities associated with axial elongation in the human myopic eye. Reviewing the results of previous histomorphometrical investigations of enucleated human globes, as well as reviewing findings obtained in population-based studies and hospital-based clinical investigations of myopic patients and non-myopic individuals. Myopic axial elongation is associated with a change from a mostly spherical eye shape to a prolate ellipsoid form. It is combined with choroidal and scleral thinning, most pronounced at the posterior pole and less pronounced in the fundus midperiphery. In the fundus midperiphery, the retina and density of the retinal pigment epithelium (RPE) and photoreceptors decrease with a longer axial length, while in the macular region, retinal thickness, RPE cell density, and choriocapillaris thickness are not related to axial length. With axial elongation, a parapapillary gamma zone develops, leading to an enlargement of the optic disc-fovea distance and a decrease in angle kappa. Axial elongation is also correlated with an increase in the surface and volume of Bruch's membrane (BM), while BM thickness remains unchanged. Axial elongation causes moderately myopic eyes to show a shift of BM opening to the foveal direction so that the horizontal disc diameter becomes shorter (with a consequent vertical ovalization of the optic disc shape), a temporal gamma zone develops, and the optic nerve exit takes an oblique course. Features of high myopia are an enlargement of the RPE opening (myopic parapapillary beta zone) and BM opening (secondary macrodisc), elongation and thinning of the lamina cribrosa, peripapillary scleral flange (parapapillary delta zone) and peripapillary choroidal border tissue, secondary BM defects in the macular region, myopic maculoschisis, macular neovascularization, and cobblestones in the fundus periphery. These features combined may be explained by a growth in BM in the fundus midperiphery leading to axial elongation.

Fast-cadence High-contrast Imaging with Information Field Theory

Although many exoplanets have been indirectly detected in recent years, direct imaging of them with ground-based telescopes remains challenging. In the presence of atmospheric fluctuations, it is ambitious to resolve the high brightness contrasts at the small angular separation between the star and its potential partners. Post-processing of telescope images has become an essential tool to improve the resolvable contrast ratios. This paper contributes a post-processing algorithm for fast-cadence imaging, which deconvolves sequences of telescope images. The algorithm infers a Bayesian estimate of the astronomical object, as well as the atmospheric optical path length, including its spatial and temporal structures. For this, we utilize physics-inspired models for the object, the atmosphere, and the telescope. The algorithm is computationally expensive but allows us to resolve high contrast ratios despite short observation times and no field rotation. We test the performance of the algorithm with pointlike companions synthetically injected into a real data set acquired with the SHARK-VIS pathfinder instrument at the LBT telescope. Sources with brightness ratios down to 6 × 10−4 to the star are detected at 185 mas separation with a short observation time of 0.6 s.