We examine the validity of Eq. (5) in [A. Sabatyan and M. T. Tavassoly, “Application of Fresnel diffraction to nondestructive measurement of the refractive index of optical fibers,” Opt. Eng., 2007, Vol. 46(12), pp. 128001-1–128001-7] to describe the diffraction pattern of an optical fiber. We show that Eq. (5) must be changed to account for the phase introduced by the fiber correctly. In particular, we show that, to agree with the implicit criterion for representing traveling waves, given when using the Fresnel–Kirchoff integral [Eq. (4)], the phase introduced by the object must be carried by positive exponentials. The results obtained correcting Eq. (5) are compared with those obtained by the rigorous solution of Maxwell equations in the optical fiber. We demonstrate the importance of adequately taking into account the criterion implicitly assumed in the Fresnel–Kirchoff integral to represent traveling waves.

A methodology based on convolutional neural network (CNN) is proposed for joint classification of transmitting user number and modulation format in a multiuser free-space optical communication (FSOC) link. The proposed methodology relies on amplitude information of received mixed signal. In-phase and quadrature components of users that are sharing time and bandwidth resources transmitting into the same optical wireless access point and interfering within each other are analyzed. The proposed approach utilizes the constellation diagrams of the received mixed symbols to generate image data sets that are fed into CNN input. The designed CNN model with three convolutional layers was tested for: varying image resolutions, image-data set size, varying number of received symbols, and atmospheric turbulence to identify optimal parameters and processing time for system design and implementation. The results indicate that the CNN model can blindly and accurately identify the communicating device number and their optical modulation format with classification accuracy up to 100% for various SNRs. Moreover, the CNN demonstrated robustness against atmospheric turbulence and suggested immunity to additive noise. Therefore, the proposed methodology proved to be a promising and feasible solution for practical implementation of an intelligent optical wireless receiver for aerial and terrestrial FSOC links.

Fiber Bragg grating has embraced the area of fiber optics since the early days of its discovery, and most fiber optic sensor systems today make use of fiber Bragg grating technology. Researchers have gained enormous attention in the field of fiber Bragg grating (FBG)-based sensing due to its inherent advantages, such as small size, fast response, distributed sensing, and immunity to the electromagnetic field. Fiber Bragg grating technology is popularly used in measurements of various physical parameters, such as pressure, temperature, and strain for civil engineering, industrial engineering, military, maritime, and aerospace applications. Nowadays, strong emphasis is given to structure health monitoring of various engineering and civil structures, which can be easily achieved with FBG-based sensors. Depending on the type of grating, FBG can be uniform, long, chirped, tilted or phase shifted having periodic perturbation of refractive index inside core of the optical fiber. Basic fundamentals of FBG and recent progress of fiber Bragg grating-based sensors used in various applications for temperature, pressure, liquid level, strain, and refractive index sensing have been reviewed. A major problem of temperature cross sensitivity that occurs in FBG-based sensing requires temperature compensation technique that has also been discussed in this paper.

This guest editorial introduces the Special Section on Terahertz and Infrared Optics: Towards Biophotonics.

Functional brain network analysis is important for understanding the causes of neurological disorders and relevant brain mechanisms. Lately, functional near-infrared spectroscopy (fNIRS) yields outputs similar to the blood-oxygen-level-dependent signals of functional magnetic resonance imaging (fMRI), and numerous studies have been conducted on functional connectivity and causality using fMRI and fNIRS. Despite the existence of numerous analysis toolboxes for fNIRS, most of them are difficult to use because they involve numerous steps, coefficients, and related files. In this study, we developed a MATLAB toolbox called OptoNet, to analyze cortical networks in the brain for fNIRS. Given that OptoNet consists of a simple and intuitive graphical user interface, users can readily analyze the cortical networks of the brain for fNIRS signals. To evaluate the efficacy of the developed toolbox, the finger tapping task experiment—extensively used in brain functional activities and causal connectivity studies—was employed. The experiment was performed using the right and left hands, and both hands simultaneously, and the consequently elicited brain cortical network activity was analyzed using developed OptoNet.

We developed an optical cryostat with a sample-rotation unit for polarization-sensitive measurement in terahertz (THz) and infrared (IR) ranges. The cryostat, in combination with two metal-grid polarizers, provides full control of mutual orientation of the sample’s crystallographic axes and the light polarization plane. Importantly, this control is realized in-situ, i.e., during the sample cooling–heating cycle. To demonstrate the abilities of the developed cryostat, we used it in combination with a laboratory-made THz time-domain spectrometer, for polarization-sensitive measurements of an orthoferrite (YFeO₃) in the range of 5 to 50  cm^−  1. These measurements revealed strong angular dependence of the sample transmission. The developed cryostat is capable for solving numerous demanding problems of THz and IR spectroscopy in condensed matter physics and materials science, biophysics, chemical, and pharmaceutical sciences.

Infrared thermography can be used as a measuring technology that records the thermal reaction of body to an external effect. In this case, the external impact is low-intensity optical radiation (LOR). Temperature is measured using a short-wave (1.5 to 5.1  μm) infrared camera. It is known that LOR produces a therapeutic effect. At the same time, the reason for this action remains unclear. The effect of LOR of the human forearm with λ  =  640  ±  10  nm with dose 5.04, 8.4  J  /  cm² on the temperature of the palm of the irradiated hand is studied. Temperature measurements continue during and after irradiation for 20 min. To interpret the results, a change in the temperature of the palm of volunteers who drank hot water is also investigated. It is found that the temperature after exposure to LOR is definitely established at a new level, depending on the radiation dose. Changes in surface temperature are associated with stimulation of blood flow. Literature data suggest that the observed response of the body to radiation may be caused by the work of the vascular relaxation factor, photoreactivation of superoxide dismutase, and the death of blood cells during irradiation.

Terahertz (THz) solid immersion microscopy is a modality of THz imaging, which allows one to overcome the Abbe diffraction limit and provides high energy efficiency due to the absence of subwavelength apertures and probes in an optical scheme. It exploits the effect of a reduction in dimensions of electromagnetic-wave caustic, when it is formed in free space, at a small distance (<λ, where λ is an electromagnetic wavelength) behind a material with high refractive index. In our previous study, we introduced an original arrangement of the THz solid immersion lens (SIL), which provides superior spatial resolution of 0.15λ and is capable of imaging soft biological tissues. We applied the finite-difference time-domain technique for solving Maxwell’s equations in order to estimate the resolution limit and the depth of field for the proposed SIL arrangement as well as to define the confidence intervals for the alignment of optical elements. Next, we described the continuous-wave THz solid immersion microscope, which relies on the proposed SIL and exploits a backward-wave oscillator and a Golay cell as an emitter and a detector, respectively. Finally, we studied experimentally the spatial resolution of this microscope and visualized several representative objects featuring subwavelength structural inhomogeneities. The observed results revealed potential of the THz solid immersion microscopy in nondestructive testing and biophotonics.

We analyze the pumping of the graphene-based laser heterostructures by infrared radiation using the numerical model. To enable the injection of sufficiently cooled carriers into the graphene layer (GL) leading to the interband population inversion, we propose to use the graded-gap black-P_xAs_{1  −  x} absorption-cooling layers. Our calculations are based on the thermodiffusion-drift carrier transport model. We demonstrate that the proposed optical pumping method can provide an efficient injection of the cool electron–hole plasma into the GL and the interband population inversion in the GL. Since the energy gap in b-As layer can be smaller than the energy of optical phonons in the GL, the injected electron–hole plasma can be additionally cooled down to the temperatures lower than the lattice temperature. This promotes a stronger population inversion that is beneficial for realization of the GL-based optically pumped terahertz and far-infrared laser, plasmon emitters, and the superluminescent downconverters. We also compare the efficiency of optical pumping through the graded-gap and uniform absorbing-cooling layers.

Mesenchymal stem cells (MSCs) represent a significant interest for cell therapy applications and, being primary cells, undergo gradual aging in culture. We studied the effects of low-intensity infrared laser irradiation during aging of MSCs in culture. Both young and aged MSCs respond to low irradiation doses (0.17  J  /  cm²) by growth activation and to middle doses (2.1  J  /  cm²) by growth retardation. Aged cells demonstrate a relatively higher growth response to low doses, but they are significantly more susceptible to deleterious effects of middle doses compared to young cells. Studies of MSC aging during long-term culture under hypoxia conditions demonstrate that low-dose irradiation of MSCs every 2 days in culture substantially increases the number of population doublings, compared to the control group. In addition, irradiated cells persisted in culture for two passages (4 days) longer than their control counterparts. However, irradiated cells did not proliferate more rapidly if irradiation was omitted. We conclude that growth responses of young and aged murine MSCs to infrared laser irradiation differ significantly and that regular irradiation affects MSC aging in culture but does not result in a bonafide retardation of aging process.

This review highlights recent and novel trends focused on metallic (plasmonic) and dielectric metasurfaces in photoconductive terahertz (THz) devices. We demonstrate the great potential of its applications in the field of THz science and technology, nevertheless indicating some limitations and technological issues. From the state-of-the-art, the metasurfaces are, by far, able to force out previous approaches like photonic crystals and are capable of significantly increasing the performance of contemporary photoconductive devices operating at THz frequencies.

The excitation power dependence of upconversion luminescence intensity and temperature sensing characteristics are investigated in the NaYF₄  :  Er^3  +, Yb^3  +  @  SiO₂ particles synthesized in-house using a hydrothermal method. The effect of laser-induced heating of the upconversion particles is shown, which introduces distortions in the measured power and temperature dependences of the upconversion luminescence. We propose a technique for calibrating the temperature dependence of upconversion particle luminescence, which should improve the accuracy of temperature measurements. The technique is based on the stabilization of upconversion particles temperature, which provides the suppression of laser-induced heating of upconversion particles.

Application of a fiber Fabry–Perot interferometer for studying the sound response of cellular structures and aqueous solutions is discussed. Aqueous suspensions of baker’s yeast and commercial natural drinking water, treated by electrolysis, were applied as models in order to mimic processes in biological tissues and liquids. The distribution of frequency intensities in the acoustic spectrograms yields evaluation of a biological system response to the external exposure. In a yeast suspension, few minutes after the sound irradiation with the frequency of 3 kHz and the sound pressure of 50 to 60 dB, we observed regular fluctuations in the output acoustic signal, with the maximal period of about 100 s. Furthermore, a sound response in the frequency range of 400 to 600 Hz maintained in signals for few minutes after the exposure. The observed results demonstrate that the proposed interferometric sensor has strong potential in biology and medicine since it is quite simple, portable, and highly sensitive device for analyses the sound response of a living system.

An experimental setup with a laser fiber optic probe has been developed, and the vibration dynamics of the vocal folds (VFs) in the larynx of rabbits have been studied. VF vibrations were excited by a variable pressure air stream. We found that, at an air flow pressure of 50 to 60 mm Hg, VFs generate a white vibration noise in the frequency range of 100 Hz to 10 kHz. The spectrum of excited vibration frequencies becomes narrower when the air flow pressure decreases from 10 to 20 mm Hg, and three discrete lower fundamental frequencies of intrinsic mechanical vibrations of individual VFs are excited at about 360, 750, and 1100 Hz, simultaneously with narrow peaks in the high-frequency region at about 3, 6, and 8 kHz, respectively. The characteristic discrete vibration frequencies of VFs are most efficiently excited near the air flow exhaustion at a pressure of 1 to 5 mm Hg. We detected a difference in the fundamental frequencies of the excited vibrations between intact VF and those treated for a scar defect in one of the VFs. The frequencies of the lowest intrinsic excited modes of the treated VF are slightly higher compared with untreated (intact) VF. The increase in the vibration frequencies may be explained by the growth of VF’s stiffness related to the formation of scar tissue. The mentioned frequency difference was registered with confidence and may serve as a basis for a mildly invasive instrumental diagnostics in the therapy of VF disorders as an aid to a traditional examination and subjective assessment of VF states.

The spectral and amplitude-frequency characteristics of a new pyroelectric detector based on thin tetraaminodiphenyl polycyclic polymer films with a thickness of <1  μm were studied in the electromagnetic radiation ranges of 0.4 to 10 and 300 to 3000  μm and at local wavelengths of 81 and 100  μm, respectively. It is shown that the volt–watt sensitivity of such a detector in the entire range is practically nonselective and is 2 to 10 times higher than the sensitivity of other pyroelectric detectors and the Golay cell. The bandwidth of the proposed pyrodetector was 330 to 500 Hz. The results showed good prospects of these sensors for fast ultrawideband spectroscopy, covering visible, infrared, terahertz, and millimeter wave ranges.

Terahertz (THz) waves can influence a diverse range of cellular processes. The use of high-power THz sources in biological studies may lead to major advances in understanding biological systems and help to determine safe exposure levels for existing THz technologies. We are devoted to the development of an experimental system for irradiating cells with intense broadband THz pulses. Subpicosecond pulses of THz radiation with intensities of 32  GW  /  cm² and electric field strength up to 3.5 MV/cm are obtained by optical rectification, using an OH1 organic crystal, of near-infrared femtosecond pulses generated by a Cr:forsterite laser. The system has been developed to allow cells to be kept in suitable conditions for long-term exposure and to be irradiated with THz pulses in single-point mode as well as in scanning mode. The transmission in the THz region of various plastic dishes for cell culture is estimated.

Quantum tomography is a widely applicable tool for complete characterization of quantum states and processes. We develop a method for precision-guaranteed quantum process tomography. With the use of the Choi–Jamiołkowski isomorphism, we generalize the recently suggested extended norm minimization estimator for the case of quantum processes. Our estimator is based on the Hilbert–Schmidt distance for quantum processes. Specifically, we discuss the application of our method for characterizing quantum gates of a superconducting quantum processor in the framework of the IBM Q Experience.

The combined use of fluorescence diagnostics (FD) and photodynamic therapy (PDT) is a promising approach to the treatment of cholangiocarcinoma. Information about the probing depth of laser radiation at a therapeutic dose sufficient for the appearance of the photodynamic effect allows planning the PDT process. We aim to assess the probing depth of radiation at a therapeutic dose. The highest fluorescence intensity is observed in the gall bladder and liver tissues. A significant difference was noted in the intensity of backscattered laser radiation, depending on the segments of the wild boar hepatobiliary system. Analysis of the probing depth of radiation (λ  =  660  nm) revealed that when the optical fiber is located outside the drainage, it increases by 2 to 4 mm, which improves the efficacy of treatment. The applicability of video FD of segments of the hepatobiliary system is investigated using the two-channel video fluorescence system. During video FD in the near-infrared range, the contrast of the fluorescence images of the hepatobiliary system is less than in the visible range. The results of the study will improve the quality of diagnostic information and optimize the FD and PDT algorithms for malignant neoplasms of the human hepatobiliary system.

We demonstrate the applicability of near-infrared (NIR) autofluorescence (AF) of skin tissues to differentiating neoplasms based on performing a series of experiments with in vivo and ex vivo skin tumors and analyzing the skin AF spectral shape, the excitation–emission matrices, and the photobleaching properties of malignant and benign neoplasms. The melanin-pigmented lesions showed an increase of the AF in comparison to nonpigmented tissue fluorescent emission using excitation at 785 nm. Autofluorescent spectral differences can be associated with different concentration of melanocytes cells in the investigated skin lesions. The differences in excitation–emission matrices for tested tissues prove that melanin is the dominant NIR fluorophore in human skin. Further, we found that the photobleaching properties of normal skin and neoplasms differ significantly. The highlighted differences in skin tissue AF response can be used in rapid analysis of large tissue areas and can complement other methods of skin tumor detection.

Quantitative analysis of the temporal evolution and spatial distribution of water content in grass and clover leaves has been carried out in vivo, with the aid of the terahertz imaging system comprised of an impact avalanche and transit time-diode source, a condenser lens, and an imaging camera. The leaf samples were exposed to 100-GHz radiation to measure the transmitted power. Progressive variation in the level of the transmitted signal has been detected when the plants were subject to the condition of insufficient water supply, whereas after watering the plants, the transmission was restored to its initial value. The presented experimental results demonstrate that TeraSense imaging instrumentation can be effectively used to monitor the hydration state of plants in their natural environment.

Our paper is devoted to a new type of the precision RF oscillator—the so-called optoelectronic oscillator (OEO). The OEO can be used as the RF oscillator and as the correlator of random variables (signals). The laser modulation modes in OEO are studied, taking into account the problem of the carrier frequency suppression. It is shown that the spectral power density of the phase noise power of OEO depends on the laser phase noise, laser coherence time, the choice of modulation modes, the level of the DC component suppression, the degree of alignment of lateral optical harmonics during modulation, the equality of excitation coefficients of optical channels in the Mach–Zehnder modulator, the geometric length of the optical fiber, and the optical power.

Terahertz (THz) spectroscopy with high sensitivity is essential for biological application considering the strong absorption and scattering effects therein. As the most commonly used THz detector, the photoconductive antenna’s (PCA) response greatly relies on the properties of the substrate’s material. THz detection properties of the PCAs fabricated on low-temperature-grown GaAs (LT-GaAs) and ErAs:GaAs superlattices were compared at the sub-THz band. The detection efficiency of the PCAs with regard to incident laser power was characterized. In addition, using the PCAs as detectors, the signal-to-noise ratio (SNR) and dynamic range (DR) of a terahertz time-domain spectroscopy were quantified. The result indicates that the PCA detector with LT-GaAs has higher efficiency than the one with ErAs:GaAs. Consequently, the corresponding THz spectrometer has better SNR and DR. This result is contrary to the previous report, in which enhanced detection efficiency was observed with ErAs:GaAs-based PCA, which is probably due to the different structures of ErAs:GaAs superlattices used in the experiment.

Photobiomodulation (PBM) using nonionizing light sources, including lasers, light-emitting diodes, and/or broadband light, in the visible (400 to 700 nm) and near-infrared (700 to 1100 nm) electromagnetic spectrum, has been successfully exploited for multiple therapeutic purposes. We analyzed the effects of red and infrared irradiation on neuroblastoma cells in an in vitro rotenone model of Parkinson’s disease. Cell viability was assessed by colorimetric assay for metabolic activity (MTT test), and the oxygen consumption rate was analyzed using a Seahorse analyzer. Low doses of rotenone slightly, but not significantly, suppressed oxygen consumption and did not affect cell viability within 2 hours of treatment. PBM stimulated mitochondrial respiration overcoming rotenone-induced inhibition. At high doses (50  μM), rotenone moderately suppressed cell viability, which was reversed by PBM. Thus, preliminary treatment with red and infrared radiation improves cell viability and enhances mitochondrial oxygen consumption in an in vitro rotenone model of Parkinson’s disease.

A real-time automated system for remote substance identification on various surfaces without preliminary sample preparation is presented. In practice, it can be used, for example, as an alerting system to signal the presence of some contaminants. The main components of the system are diffuse reflectance spectra acquisition module, data processing module, and identification module. Development of each module was based on the choice of appropriate devices and algorithms, either existing or newly designed. The experimental setup consists of a quantum cascade laser emitting in the spectral range of 5.3 to 12.8  μm with a HgCdTe photodetector. To achieve better selectivity of substance recognition, identification algorithms were based on the absorption and transmission spectra calculated from the recorded diffuse reflectance spectra. Spectra conversion algorithms employed Kramers–Kronig relations, phase spectra extrapolation, and phase correction. The system was supplied with the recognition database composed of certain commercially available substances. The experiments showed that the usage of transmittance spectra significantly improved the sensitivity of the identification method; the remote identification limit of 30  μg acetylsalicylic acid has been experimentally confirmed. For similar substances, such limit was estimated as 10  μg   /  cm² at a distance of 1 m.

Terahertz reflectometry technique is applied for noninvasive precorneal tear film assessment. The results of the receiver operating characteristic analysis show a good applicability of the proposed method. The direct comparison of the results obtained by reflectometry approach and results of Norn testing shows a good correlation. The in vivo measurement of the dynamic of tear film thinning could be useful for clinical diagnosis of dry eye syndrome.

We studied the formation of a composite from an aqueous dispersed medium with albumin and carbon nanotubes under the action of laser radiation in continuous wave (CW) mode and pulsed mode with a repetition rate of 10 Hz and pulse duration of 16 ns. During the experiments, the temperature was monitored at the site of exposure, as well as its distribution in the liquid. Pulsed solid-state Nd:YAG laser and CW diode laser with an irradiation power of ∼500  mW were used as radiation sources. However, a three-dimensional composite was formed only with constant exposure. The effect of pulsed laser radiation with an intensity corresponding to nonlinear interaction with water dispersion led only to its enlightenment. Thus, it is important not only the energy parameters of radiation but also the frequency of energy portions exposure for the fabrication of tissue-engineered structures (composites). As a result, it was found that the curing of the dispersion and the composite formation occurs under the action of continuous or pulsed (with a high pulse repetition rate) laser radiation at a temperature in the range from 45°C to 50°C; in case the pulse repetition rate is insufficient, composite formation is not observed even under the action of high intensity radiation and heating occurs only to a temperature of ∼40  °  C. This formation process can be generated both in the visible 532 nm and in the infrared 810-nm wavelength ranges. In this case, one of the main conditions is the absence of albumin or cells absorption at these wavelengths so that absorption occurs mainly with single-walled carbon nanotubes. Studies of the surface and internal structure of the composite made it possible to demonstrate the binding of nanotubes to each other. This happened under the influence of laser radiation. This led to high hardness values of the composites. The average value of hardness was 0.26  ±  0.02  GPa.

Infrared optical coherence tomography angiography (OCT-A) is a promising method that can be used for assessing the level of choroid blood supply. The aim of the work is to develop a technique for quantitative assessment of choroidal blood flow for early primary open-angle glaucoma (POAG) diagnostics. The study was performed using OCT-A SPECTRALIS (Heidelberg Engineering). The technique consists of a quantitative assessment of sagittal OCT-A scans in the optic nerve disk area (peripapillary zone). The boundaries of the outer plexiform layer (OPL), the Bruch membrane (BM), and the borders of the suprachoroidal space were identified on each analyzed scan. For each region, blood flow indices proportional to the total number of pixels were determined. The sagittal OCT-A scans of 28 patients (53 eyes), including 12 patients (24 eyes) aged 52 to 79 (62.1  ±  9.5  years) without ophthalmic pathology (control group) and 16 patients (29 eyes) aged 55 to 74 (64.5  ±  6.0  years) with various POAG stages were analyzed. The highest value of blood supply was revealed in the choroid and the lowest above the OPL border and between the BM and upper OPL layers. A significant decrease in choroidal blood supply was revealed in stage I POAG as compared to the control group. Microcirculation of the choroid in the peripapillary area assessed using sagittal OCT-A scans can be considered an informative criterion for early POAG diagnostics.

Decoherence is a fundamental obstacle to the implementation of large-scale and low-noise quantum information-processing devices. We suggest an approach for suppressing errors by employing preprocessing and postprocessing unitary operations, which precede and follow the action of a decoherence channel. In contrast to quantum error correction and measurement-based methods, the suggested approach relies on specifically designed unitary operators for a particular state without the need in ancillary qubits or postselection procedures. We consider the case of decoherence channels acting on a single qubit belonging to a many-qubit state. Preprocessing and postprocessing operators can be either individual, which is acting on the qubit effected by the decoherence channel only, or collective, which is acting on the whole multiqubit state. We give a classification of possible strategies for the protection scheme, analyze them, and derive expressions for the optimal unitary operators providing the maximal value of the fidelity regarding initial and final states. Specifically, we demonstrate the equivalence of the schemes where one of the unitary operations is individual while the other is collective. We then consider the realization of our approach for the basic decoherence models, which include single-qubit depolarizing, dephasing, and amplitude damping channels. We also demonstrate that the decoherence robustness of multiqubit states for these decoherence models is determined by the entropy of the reduced state of the qubit undergoing the decoherence channel.

We present a computational modeling approach for imitation of the time-domain optical coherence tomography (OCT) images of biotissues. The developed modeling technique is based on the implementation of the Leontovich–Fock equation into the wave Monte Carlo (MC) method. We discuss the benefits of the developed computational model in comparison to the conventional MC method based on the modeling of OCT images of a nevus. The developed model takes into account diffraction on bulk-absorbing microstructures and allows consideration of the influence of the amplitude–phase profile of the wave beam on the quality of the OCT images. The selection of optical parameters of modeling medium, used for simulation of optical radiation propagation in biotissues, is based on the results obtained experimentally by OCT. The developed computational model can be used for imitation of the light waves propagation both in time-domain and spectral-domain OCT approaches.

Infrared neurostimulation (INS) is a new approach for modulation or control of neuronal pulses. Recently, different studies have been presented to investigate the origin of generation of the action potentials during INS, and it seems that the photothermal mechanism has an important role during INS. So, spatial and temporal temperature changes are important parameters, because the heating of neural tissue can excite or block the activity of neurons and an excess deposit of thermal energy could damage the neural tissues. We aim to explore the effects of heat diffusion during INS. We model the generation of action potential using the photothermal mechanism to study the changes of electrical properties of the membrane of neural cell in the earthworm (as a simple neuronal network) during INS. The variation of electrical properties of the membrane causes the changes in the concentration of ions such as K⁺ and Na⁺ inside the cells, which can originate the action potentials. This study includes three sections: (1) exploring the effect of laser light properties (wavelength of 1450 and 1550 nm, repetition rate and energy per pulse) on the measurement of temperature rise inside a phantom similar to neuronal tissue, (2) theoretical modeling to predict the generation of action potentials induced by the local temperature rise inside the neuronal network of earthworm, and (3) detecting the variation of voltage of peripheral nervous system of the earthworm during INS. This modeling can help us to better understanding the mechanism of the blocking and controlling the action potentials for in-vivo applications in the brain cognitive studies and treatment of some neuron system diseases.

A mathematical model for the calculation of the optical and thermal properties of an ensemble of gold nanostars with different tip lengths L  =  10 to 20 nm in an aqueous environment is proposed. The heating of a nanoparticle ensemble under irradiation by a laser pulse of a nanosecond duration: resonant λ  =  808  nm and nonresonant λ  =  1064  nm is studied. A significant blue shift of surface plasmon resonance during the formation of a vapor bubble around the nanoparticle was detected and described. Based on the analysis of the kinetics of nanoparticle heating under the action of a laser pulse before and after the formation of a vapor bubble, a theoretical description of the experimentally observed threshold nature of particle photomodification is given. It was found that the laser intensity I_m necessary to achieve the melting temperature T_m (photomodification threshold) tends to increase with decreasing nanostar tip length and for an individual nanostar differs by an order of magnitude. However, up to 45% of the nanoparticles are heated to the melting temperature T_m by a single pulse with an intensity only 2.5 times higher than the minimum value of I_m. At a wavelength of 1064 nm, I_m is ∼6 times higher than at a wavelength of 808 nm.

We review the advances of terahertz (THz) science and technology in biophotonics, including related challenges and solutions. The main impediment to THz spectroscopy and imaging in this field is the high absorption of the THz beam in water. Hence, transmission imaging and spectroscopy of thick wet tissue using THz radiation has generally been quite difficult. However, the absorption of THz waves by water molecules is so strong that increasing the power of the THz source can lead to structural and functional changes in tissues, so solutions must go beyond a larger power output. In terms of resolution, THz imaging is superior to ultrasound but inferior to visible light microscopy. Owing to its unique material analysis capabilities, promising diagnosis applications have been demonstrated through THz imaging and spectroscopy. Unfortunately, many applications are limited by beam penetration depth and resolution. Hence, researchers from a wide variety of scientific and technical fields have been actively improving these features through the development of electronic devices and materials. In addition, groundbreaking optical architecture and materials to reduce beam absorption in the optics of a system and generate focused beams with smaller diameters have been proposed. On the software side, image processing techniques to computationally enhance the resolution and quality of THz imaging have been proposed. Data science and machine learning to automate the diagnosis of defects and diseases through processing THz images and spectroscopy data have been proposed. We have reviewed the applications of THz radiation in biophotonics and research achievements toward advancing these applications. A conclusion with a roadmap toward increasing the footprint of the THz technology in biophotonics is also proposed.

A photonic-assisted image rejection mixer is proposed based on Hilbert transform in the optical domain using a phase-shifted fiber Bragg grating (PS-FBG). The image signal and the desired RF signal are converted to the optical domain by carrier-suppressed single-sideband modulation, which is then split into two parts. One part is Hilbert transformed by a specially designed PS-FBG and a 90-deg optical phase shifter and then combined with the other part to directly reject the image signal in the optical domain. The image-free optical signal is downconverted to an intermediate frequency signal by combining it with an optical local oscillator signal and then detecting them in a photodetector. The image rejection capability of the system is analyzed, and an image rejection ratio of 68.0 or 58.6 dB is achieved when the bandwidth of the image signal is 20 or 60 MHz, respectively, while having an acceptable influence on the desired RF signal. Quadrature phase-shift keying (QPSK) signals, 8 phase-shift keying (8PSK) signals, and 16 quadrature amplitude modulation (16-QAM) signals are used to evaluate the image rejection performance. When the bandwidth of the image signal is 18.75 MHz, the error vector magnitudes of the downconverted QPSK, 8PSK, and 16-QAM signals with the same power, modulation format, and bandwidth are 2.5%, 2.5%, and 2.6%, respectively.

Transverse translation-diverse phase retrieval (TTDPR), a ptychographic wavefront-sensing technique, is a viable method for freeform optical surface metrology due to its relatively simple hardware requirements, flexibility, and demonstrated accuracy in other fields. In TTDPR, a subaperture illumination pattern is scanned across an optic under test, and the reflected intensity is gathered on an array detector near focus. A nonlinear optimization algorithm is used to reconstruct the wavefront aberration at the test surface from which we can solve for surface error, using intensity patterns from multiple scan positions. TTDPR is an advantageous method for aspheric and freeform metrology because measurements can be performed without null optics. We report on the development of a concave freeform mirror measurement using this technique. Simulations were performed to test algorithmic performance as a function of various parameters, including detector signal-to-noise ratio and position uncertainty of the illumination, with <λ  /  100 root-mean-square (rms) wavefront-sensing error achieved in most cases (λ  =  632.8  nm). An experimental measurement is then demonstrated, and results of reconstructed surface form and midspatial frequency error are presented. Surface reconstructions from two disjoint datasets agree to 13-nm rms, or λ  /  50 at λ  =  632.8  nm.

Optical metrology is a critical and complex technique for the fabrication of precision optics in which the surface figure is better than peak-to-valley 1  /  10λ or RMS 1  /  30λ. Careful calibration of the intrinsic system errors of the experimental setup, including the alignment error of the metrology tool and the manufacturing error of the reference optics, should be performed. However, any surface deformation caused by the mounting supporter or a gravity effect can result in an incorrect surface figure correction, especially in mid-to-large optics. The system error of the experimental setup and deformation by external conditions of the optics, such as temperature drift, air turbulence, and vibration, affect the measured result. In the proposed method, the magnitude and phase of all nonrotationally symmetrical Zernike coefficients were obtained through multiple measurements by rotating the optics. These coefficients were used to analyze absolute low-spatial frequency figures. To verify the reproducibility of the proposed method, three metrology tools with distinct measurement methods were used to obtain surface figures and the results were then compared.

Optical three-dimensional (3-D) geometry measurements are state of the art when it comes to contactless quality control and maintenance of the shape of technical components that exclude tactile measurements due to filigree or internal structures. Optical inspection methods are also characterized by a fast and high-resolution 3-D inspection of complex geometries. And due to their noncontact principle, they can carry out measurements in places that would otherwise not be accessible due to harsh environmental conditions or specimens such as hot forged parts. However, there are currently no methods to estimate the reconstruction quality for the optical 3-D geometry measurements of hot objects. The mainly used geometric measurement standards cannot be used for the characterization of hot measurements since the calibrated geometrical values are not transferable to high temperatures. For the development of such a metric, we present the fundamentals of the concepts and algorithms for an estimation of the reconstruction quality are presented and evaluated using a two-dimensional simulation model. The generated findings were applied to the 3-D geometry measurement of a hot object in a laboratory environment. The results are compared with general state-of-the-art reconstruction quality metrics.

We fabricated a high-density, large-scale magneto-optical (MO) light modulator array to investigate its performance for holographic display applications. The modulator comprised a magnetic nanowire for light modulation using MO Kerr effect and two hard magnets (HMs) to control the switching property of the nanowire. The magnetization direction of the designated pixels in the array was controlled by the external magnetic field, unlike a spatial light modulator, which drives arbitrary pixels with cell selection backplane transistors. Magnetization of the light modulators with HMs can be reversed using a smaller magnetic field compared with those without HMs; this enables the formation of magnetic patterns by switching only the magnetization direction of the nanowire with smaller switching field; the pattern thus obtained is predetermined and not arbitrary. A diffracted beam in a magnetic stripe pattern displayed on the array was observed as spot patterns, and their spot position was consistent with a diffraction angle of the stripe period. We fabricated a magnetic hologram using a 10  k  ×  10  k pixel array calculated by computer-generated holography and successfully reproduced a holographic three-dimensional image with a wide viewing angle.

An extrinsic Fabry–Perot interferometer (EFPI) acoustic sensor based on a gold diaphragm with an about 100 nm thickness has been proposed and demonstrated. The Fabry–Perot (F-P) cavity consists of a gold diaphragm and a fiber endface. Experimental results illustrate a static pressure sensitivity of about 19.5  nm  /  kPa in the range from 0 to 100 kPa with linearity of about 0.99. Meanwhile, the flat acoustic frequency response of about 35.0  mV  /  Pa between 0.2 and 2.0 kHz is achieved, and the largest acoustic pressure sensitivity is about 80.6  mV  /  Pa at 2.6 kHz. The noise-limited minimum detectable pressure is 1.3  mPa  /  Hz^1/2 @ 2.6 kHz. The proposed EFPI acoustic sensor may have potential application in security and surveillance systems.

Lasers can be identified by their relatively long coherence lengths using interferometry. A Mach–Zehnder interferometer incorporating liquid crystal polarization modulators is demonstrated as a means of low-cost, robust laser detection. Temporal modulations, as a signature of coherence, can be induced by modulating polarization changes in liquid crystal modulators using low voltages. Sensitivities of <10  nW can be achieved. The suitability as a means of laser detection is discussed.

Holo-shear lens-based interferometer is demonstrated to study the influence of gradient magnetic fields (i.e., upward decreasing and upward increasing) and uniform magnetic field on the temperature and temperature profile of a wick stabilized micro diffusion flame created from the candle. Sheared interferograms in the absence and presence of microflame are captured using CCD camera. Fourier fringe analysis method is used to extract the phase-gradient information of ambient air without flame and heated air of microflame separately. The phase difference map of microflame and ambient air is used for the extraction of refractive index difference and temperature distribution inside the microflame. The experimental investigations reveal that temperature and temperature stability of the microflame increase in the upward decreasing and uniform magnetic field, while the temperature and temperature stability inside the microflame decrease in upward increasing magnetic field in comparison to temperature inside the microflame in the absence of magnetic field. Increment in the temperature of the microflame in uniform magnetic field is contrary to macro diffusion flame, where there is a negligible influence on the temperature in uniform magnetic field. Holo-shear lens-based interferometer is simple, lightweight, easy to implement, less vibration-sensitive, and can cover microflame to macroflame under investigation.

A fast and precise spatial-carrier phase-shifting algorithm based on the matrix VU factorization strategy that can realize dynamic real-time phase detection is proposed. First, the proposed algorithm divides the spatial-carrier interferogram into four phase-shifting subinterferograms. Second, the matrix VU factorization strategy, an excellent fast iterative algorithm, is used to accurately obtain the measured phase from these subinterferograms. Numerical simulation and experimental comparison verify that this method is an efficient and accurate single-frame phase demodulation algorithm. Meanwhile, the performance of the proposed method is analyzed and discussed for the influencing factors, such as random noise level, carrier-frequency value, and carrier-frequency direction. The results show that the method proposed is a fast and precise phase detection method that provides another effective solution for dynamic real-time phase measurement.

We present an approach that combines binary structured patterns generated by tripolar pulse width modulation technique with a two-wavelength phase-shifting unwrapping method to realize absolute phase recovery at the same defocusing level. Optimized binary fringe patterns projected by a small defocusing degree projector can significantly alleviate the phase error based on fringe projection profilometry, providing the binary patterns with a similar performance to standard sinusoidal fringe patterns. With this method, a set of images using only two-wavelength fringe patterns is captured by the camera, and it is not obliged to obtain the equivalent wavelengths and their corresponding phase maps. In consequence, the error propagation is avoided, which leads to improved accuracy in the continuous phase map retrieval. Furthermore, The effectiveness of the presented phase recovery technique that can be utilized to measure discontinuous and multiple objects was also verified by experiments. And it can reach high-precision and fast three-dimensional shape measurement.

The pursuit of low noise in optical instruments for areal surface topography measurement is relevant to many surface types, ranging from super-polished optical surfaces to weakly reflecting or scattering textures that require enhanced signal sensitivity. We clarify the definition and experimental methods for quantifying random noise in areal surface topography measurements. We also propose a parameter, the topographical noise density, that concisely summarizes the effects of measurement bandwidth. To illustrate these ideas, we present results from a commercial phase-shifting interference microscope showing an RMS measurement noise of 0.03 nm for a 1-s data acquisition of 1 million surface topography image points, after application of a 3  ×  3-pixel convolution filter. The results follow the expected inverse square root dependence on the data acquisition time for fast averaging of topography maps, resulting in a measurement noise of <0.01  nm for a 10-s data acquisition.

Periodic plasmonic nanostructures on a thin homogeneous metal layer are used to excite surface plasmons (SPs) for normal incident light in the optical communication band. The structures are engineered using rigorous coupled-wave analysis by considering sensitivity, linewidth, and reflection amplitude as the evaluation parameters. The presence of SP mode at the thin metal–substrate interface in the proposed plasmonic device adds a self-reference capability while capturing the minute refractive index and thickness variations. The wavelength shift in SP mode at the nanostructure–analyte interface is used to measure the changes in the refractive index of the analyte, and the number of waveguide modes is used to capture the changes in the thickness of the analyte. The proposed engineered plasmonic nanostructures offer a sensitivity of 1100 nm/refractive index unit and a resonance line width of 18 nm while taking into account the fabrication constraints. The proposed structures are further simulated for the detection of hemoglobin concentration (using its refractive index measurement) in human blood in the optical communication band (1450 to 1520 nm). The normal incident action eases the integration of engineered plasmonic substrate with optical fibers that can be used both to excite SP and to interrogate the spectral reflectance.

In data center networks (DCNs), secure multicast provisioning services are obtained from data centers and transported to multiple users in a multicast style, which requires a high demand for security. Moreover, these services are usually stored and maintained in multiple geographically dispersed data centers for more reliable and efficient access. Quantum key distribution (QKD) is a state-of-the-art technology employed to unconditionally distribute security keys, based on the principles of quantum physics. Therefore, one should pay more attention to how to efficiently distribute global quantum keys in quantum DCNs in the near future. We investigate the problem of global QKD while leveraging multicast service backups among multiple geographically distributed datacenters for secure service provisioning. A distributed subkey-relay-tree-based, secure multicast scheme is proposed and used as a basis to develop a distributed subkey-relay-tree-based secure multicast-routing and key assignment algorithm. Numerical results show that the latter can achieve higher security probability and lower consumed keys compared with the traditional routing and key assignment scheme and single-key-relay-tree-based secure multicast scheme.

A randomly distributed freeform cylindrical microlens (RFCML) is proposed for laser beam reshaping and homogenization. By introducing a freeform surface in the microlens, the optical field can be controlled with the expected distribution with a single interface. With the help of randomly distributed subapertures of the microlens, the diffraction orders of the microstructure are smoothed and the optical field is homogenized in the far field. The theoretical design of the RFCML is carried out, and the influence of the field smoothing with the random coefficient is analyzed. With the optimized results, the device is fabricated with the laser beam direct writing method and demonstrates the excellent property of laser beam homogenization in the device. The experimentally reshaped homogenized line with a full angle of 40 deg is in good accordance with the theoretical results.

Higher flexibility and high peak-to-average power ratio (PAPR) are the main challenges for orthogonal frequency division multiplexing passive optical network (OFDM-PON) systems. For this purpose, we propose a low PAPR OFDM-PON system with a dynamic regulatory factor U, which can dynamically adjust the transmission capacity of the system according to the needs of users at optical network units and reduce PAPR at the same time. Dynamic regulatory factor U can assign different probabilistic shaping schemes to different subcarriers, thus improving the system flexibility and adjustability in both the time and frequency domains. Meanwhile, U can flexibly change the number of phase sequence groups in selecting mapping, which is a well-known effective PAPR reduction technique. Simulations of a back-to-back OFDM-PON transmission system with different bit rates are setup to verify the performance of the proposed scheme. It is demonstrated that the proposed scheme achieves effective PAPR reduction performance with considerably lower computational complexity and outstanding bit error performance. More importantly, our proposed OFDM-PON system can be flexible and adjustable in acquiring different and dynamic access bit rates, which suggests a prospective solution for the next-generation PON.

Aiming at the problem of the low sensitivity of conventional optical fiber-based gas refractive index sensors, an ultrasensitive tapered optical fiber coupler-based gas refractive index sensor enhanced by the Vernier effect is proposed and demonstrated. The birefringence property of the tapered optical fiber coupler allows it to support two passes of interferences in two orthogonal polarized states, and the superposition of these two interferences forms the Vernier effect. Theoretical analysis and numerical calculations indicate that, for the fiber couplers working in the gas medium when the waist width is within the range of 1.2 to 2.0  μm, the group birefringence difference between the even mode and odd mode equals zero. Thus the sensitivity toward the ambient gas refractive index can be enhanced significantly. To demonstrate these theoretical results, a tapered fiber coupler with a width of 1.6  μm and a length of 16.5 mm was fabricated, and ultrahigh sensitivities up to 22015.4 and −22690.0  nm  /  RIU were experimentally achieved. The proposed sensor has the merits of being easy to fabricate, having compact structure, and being cost effective. It has significant application prospects in the petrochemical and biomedical detection fields.

Visible light communication (VLC) systems provide illumination and power, and the distributed, specifically made light-emitting diodes (LEDs) are their fundamental sources of light and signal. We propose an arbitrarily distributed LED lamp layout based on a multipopulation genetic algorithm (MPGA) to solve the unevenness of distributions of optical illuminance and power. Twenty LED lamps were taken as an example, and the position coordinates and half-power angles were optimized under the fitness function related to the variance of received power through the coevolution of multipopulations. The simulation results on MATLAB R2016a showed that the distributions of illuminance and power were more uniform, with a variance of power of 0.3978 dBm and a uniformity ratio of illuminance of 87.42%. Moreover, SNR distribution was excellent, with a Q_SNR of 0.2007. An average root mean square delay spread of 2.22 ns was obtained, with a variance of 5.6250  ×  10^−  20. The proposed method is better than the optimization of lamp arrangement, configuration, and genetic algorithm (GA). Furthermore, from iteration curves, the MPGA acts better than the GA in finding the solution. The fitness function value is better than the GA because the latter easily falls into the locally optimal solution. We provide references to design the LED layout and contribute to making a more comfortable communication environment.

All-optical wavelength converters (AOWCs) based on parametric amplification in highly nonlinear fibers are investigated and evaluated, aiming at application flex-grid optical networks. A multichannel transmission system with a multilevel modulation format at 56-Gb  /  s bit rate is adopted. Parameters such as symbol error rate and signal bandwidth allocation are evaluated considering the optical signal-to-noise ratio, the power reception level, the channel spacing, and the impact of several cascade conversions—series of multiple AOWCs—on the signal quality. This work was carried out with an analytical modeling and computer simulations. Results reveal consistent modeling and adequate operation of the proposed AOWCs for flex-grid wavelength-division multiplexing networks and indicate that the number of conversions is limited by the size of the converter operating band and the allowed flex-grid slot size.

We report on the dissipative soliton operation of a diode-pumped single-crystal bulk Yb:KGW laser oscillator in the all-positive-dispersion regime. Stable passively mode-locked pulses with strong positive chirp and steep spectral edges are obtained. The spectral centering at 1038.6 nm has a bandwidth of about 6.9 nm, and the chirped pulses have a pulse duration of 4.317 ps. The maximum average power can be up to 2.07 W when pumped by absorbed pump power of 5.3 W. The mode-locked slope efficiency and optical–optical conversion efficiency are shown to be 62% and 39%, respectively. Considering the pulse repetition rate with a value of 52 MHz, the corresponding pulse energy is estimated to be 39.8 nJ.

We present the generation and optimization of square-wave noise-like pulses (NLPs) in a mode-locked Tm-doped fiber laser. Mode-locking operation around the 2-μm band is achieved by a nonlinear amplifying loop mirror. To optimize the output performance, the figure-eight cavity is modified by employing a polarization-dependent isolator in a unidirectional loop, and the cavity length is only 17.2 m. First, by employing a cavity with pure anomalous dispersion, a conventional soliton can evolve into a square-wave NLP by properly setting the pump power and polarization controllers. The pulse energy of the fundamental-frequency operation can be varied from 2.29 to 3.4 nJ. Using an ultrahigh-numerical-aperture fiber to reduce the net dispersion to −1.033  ps², the 3-dB bandwidth of the spectrum is broadened to 14.78 nm, and the duration of the autocorrelation spike is only 421 fs. The maximum single-pulse energy can increase up to 4.97 nJ. Due to dispersion management mechanism, the threshold and output power are also significantly improved.

We propose and demonstrate the use of the cost-effective electric arc writing method for the all-fiber cylindrical vector beam (CVB) and orbital angular momentum (OAM) beam generation in few-mode fibers (FMFs) for the first time to the best of our knowledge. We show that this technique enables the writing of long-period fiber gratings (LPFGs) with pitch values as small as 238  μm, which is required in some high-index contrast specialty fibers tailored for the stable guiding of CVB and OAM modes. Conversion efficiencies around 81% are measured for three different symmetric CVBs. The polarization-dependent properties of the fabricated gratings are elucidated, and we report a polarization-dependent loss of about 2.5 dB across the different CVBs. By means of a fabricated LPFG, we further demonstrated the all-fiber generation of the OAM states with topological charge (±1) at the output of the FMF. The results are relevant to the fields of space-division multiplexing, optical sensors, and optical tweezers that would benefit from a compact source of quality CVB and OAM beams of high average optical power.

Pattern noise and nonlinearity are common problems in many image sensors that limit their performance. We present an algorithm based on neural network to correct pattern noise and nonlinearity of the image sensor when the gray value approaches the saturation point to improve the linear range and image contrast of image sensors. The photon transfer curve (PTC) of each pixel is evaluated through a photographic test with an image sensor at different exposures. Assuming that the PTC of the ideal image sensor is a proportional function, the nonlinear region of the PTC of each pixel is corrected to the targeted curve using a neural network. The experimental results show that the image contrast and dynamic range of the corrected image can be significantly improved while the pattern noise of the corrected image is also effectively removed. In addition, the algorithm corrects the damaged pixels of the image sensor.

Recent developments in growing highly n-doped wide bandgap oxides such as β-gallium oxide (β  −  Ga₂O₃) and more recently zinc gallate (ZnGa₂O₄) have opened avenues toward important applications, such as transparent electrodes and ohmic contacts. Magnetoconductivity measurements provide a unique method to assess the contribution of phonons to mobility over a wide range of temperatures. For β  −  Ga₂O₃ and ZnGa₂O₄, initial attempts to interpret the measured magnetoconductivity raised fundamental questions about the interplay between the large number of phonon modes in these lattices, electron–phonon scattering, and lattice disorder. Here, we use density functional theory modeling of electron–phonon scattering to help rationalize magnetoconductivity measurements for a wide range of electron concentrations n and temperatures in β  −  Ga₂O₃ and ZnGa₂O₄. The results provide a first-principles understanding of dominant low-field mobility features suggested by phenomenological models used traditionally for semiconductors with high lattice symmetry.

As an important light absorbing material, space extinction black paint has been widely used for space stray light suppression structures. This is usually not compatible with the Lambert diffusion law. Therefore, in order to describe the light scattering characteristics of a stray light suppressed structure surface accurately, bidirectional reflection distribution data at different angles and wavelengths are obtained by experimental measurements. Then a multivariate and multiparameter spectral bidirectional reflection distribution function model is established according to the characteristics of experimental data, and the parameters of the model are fitted accurately based on the Newton descent method. The quantitative evaluation of model error is completed by comparatively analyzing the measured data and the model data. The calculated model error is 3.07%, which proves the accuracy of the proposed parametric model. This model can provide important technical support for the design and evaluation of a space stray light suppression system.

A facial method was used to enhance the dynamic response of polyvinyl chloride (PVC) gel tunable lenticular microlens array (LMA). The response time could be reduced using a discontinuous hydrophobic layer-modified electrode substrate due to a decrease in friction between the substrate surface and PVC gel. The application of DC voltage (V  =    −  45  V) to LMA induced a response time <240  ms. The proposed PVC gel LMA looks promising for use in image processing and two/three-dimensional switchable displays due to its fast-dynamic response, good stability, compact structure, and easy fabrication.

This erratum corrects an error related to an equation.