Guest editors Xinbin Cheng, Sven Schroder, Christopher Stolz, and Xu Liu summarize the Special Section on Frontiers of Optical Coatings.

HfO₂ prepared by ion beam sputtering (IBS) is widely used as a high-refractive index material for making low-loss laser films. The challenge of a high-performance HfO₂ film is to simultaneously obtain an amorphous morphology with low scattering and a stoichiometric structure with low absorption. Furthermore, nanometer-sized voids are commonly present in IBS films due to excessive oxygen and argon adsorption during deposition, which is the primary barrier to achieve a smooth surface and low optical loss of HfO₂. Thin amorphous SiO₂ layers were added periodically into HfO₂ coatings using the IBS process to synthesize amorphous HfO₂  /  SiO₂ nanolaminate-based composites. The resulting composites exhibited excellent comprehensive performance with a dense amorphous microstructure and a void-free smooth surface. High-temperature annealing was performed to ensure superior stoichiometry and lower absorption. However, the crystalline states and microstructure of some composites evolved during the gradual annealing. We present a detailed study of the crystallization, surface topography, and absorption evolution in HfO₂  /  SiO₂ nanolaminates as a function of HfO₂ sublayer thickness and thermal annealing temperature. Moreover, the interplay between crystallization, surface topography, and absorption is elucidated. The HfO₂  /  SiO₂ nanolaminate with 19 thin layers maintained a dense amorphous structure with low absorption after annealing. Finally, a 1064-nm HfO₂  /  SiO₂ nanolaminate-SiO₂ high-performance reflector was prepared and achieved lower absorption with a smooth surface after annealing, which demonstrated the great potential of the HfO₂  /  SiO₂ nanolaminates for considerably improving optical loss.

The correlation between the process parameters of auxiliary ion source and the SiO₂ film stress was systematically studied using dual-ion-sputtering deposition. Spectrophotometer and ellipsometer were used to measure the SiO₂ film’s transmittance spectrum and reflection ellipsometry. The refractive index and thickness of SiO₂ film were obtained by total spectrum inversion calculation. The laser interferometer measures the substrate surface shape to obtain film stress. The experimental result shows that film stress is related to sputtering energy during deposition, high-energy oxygen ion assisted deposition can significantly reduce it and the assisted ion beam voltage plays an important role in controlling it by data normalization analysis. Ultra-low stress high reflective film was prepared by dual-ion-beam sputtering deposition, where stress was 70% lower than traditional film. At the same time, this film can ensure sufficiently high optical quality to keep it reliable and stable in high-precision laser systems.

SiO₂ is a very important low refractive index film material that can operate in the UV to mid-IR regions. The SiO₂ films used were deposited on Si substrates by different deposition techniques. We designed and manufactured a high-temperature infrared spectrum measuring equipment connected with a Fourier transform infrared (FTIR) spectrometer; the measuring temperature could range from room-temperature to 600°C. The FTIR transmission spectra of SiO₂ films under different working temperatures were measured in the wavenumber region from 4000 to 400  cm^−  1. From the measured spectra, it can be seen that IBS-SiO₂ and IAD-SiO₂ films are more stable and operating temperature can reach 400°C. EB-SiO₂ film is worse and operating temperature is only 300°C. Complex dielectric functions of SiO₂ films under different temperature conditions were calculated from FTIR transmittance spectra, and the best fitted method was obtained for calculating optical constants of dielectric materials in the high temperature condition. The results show that the structure of the Si-O-Si network in IBS-SiO₂ films was the most stable structure. The experimental results were of great significance to practical application.

The far ultraviolet (FUV) spectrum is critical for the observation of phenomena in space, among various other applications. Hence, the aim of our study was to investigate the effect of the humidity of Al mirrors coated with LiF and enhanced MgF₂ within the FUV spectrum. The samples were first coated with Al and LiF films on a super-polished silicon wafer at room temperature, heated to 220°C, and then coated with an enhanced MgF₂ film. All materials were deposited by thermal evaporation. The samples were stored in environments with 20% relative humidity (RH), 40% RH, 80% RH, and 90% RH. The reflectivity remained stable when the environment was 20% RH, which was the optimal storage environment. The reflectivity of the Al  /  LiF  /  eMgF₂ mirrors stored in high RH decreased over time, the roughness increased over time. Furthermore, the decrease rate of reflectivity and increase rate of roughness decreased over time. The degradation of reflectivity can be attributed to the presence of oxygen in environments with a high RH.

A two-step phase-shifting hiding technique for a fully phase-based image is proposed. A reference beam was generated with a laser illuminating the host image using our modified Mach–Zehnder interferometer (MZI) design, and an object beam was injected into a fully phase-based image that is to be hidden. A reference beam was used to engage the image-hiding function while providing phase changes in the hologram recording process. After Fresnel diffraction occurred on the two paths of the MZI, the diffracted waves were detected as interference patterns on a charge-coupled device plane, which resembled the host image’s Fresnel diffraction pattern. A theoretical analysis is provided alongside the experimental results.

A line-field scanning Fourier-domain optical coherence tomography (OCT) system (LF-FDOCT) that makes high-resolution and large-dynamic-range imaging possible was demonstrated. Unlike the conventional flying-spot OCT system, the x-axis parallel imaging (one B-scan) has a coherent imaging mode. A theoretical simulation of parallel interference imaging was derived, and the anisotropic resolution along two orthogonal directions was achieved. Validated by experimental results, the spatial resolutions along the x and y axis directions were 2.46 and 2.19  μm, and the theoretical resolutions were 1.8 and 1.34  μm, respectively. The field of view (FOV) in the lateral direction was 900  μm   (  x  )    ×  850  μm   (  y  )  , and the axial resolution and FOV in the experiment were 2.5 and 700  μm, respectively. The maximal axial sensitivity was measured to be 90.5 dB when the sample was a specula. The en face of tomato and the cross-section of multilayer glass were demonstrated based on the LF-FDOCT system. The three-dimensional image of adherend sample including gels and microelectrodes was realized, proving the LF-FDOCT system had the capability of high resolution and high-dynamic-range imaging.

Strain is one of the conventional parameters that evaluate structural performance in the field of bridge health monitoring. Among the various techniques that have been utilized to measure strain, fiber Bragg grating (FBG) sensing system has been broadly applied for long-term strain monitoring in civil engineering, whereas microimage strain sensing (MISS), with its corresponding sensors designed and fabricated, is a new technology for strain measurement. Furthermore, the MISS system is developed to monitor the strain of small and medium span bridges. To verify the performance of the MISS system in strain measurement, a comprehensive and comparative study of the MISS system and FBG strain sensor system is provided. First, the technical features of the two technologies were described. Moreover, more details were expounded in terms of service life, deployment, coverage, cost, and temperature dependence. Then, assisted data comparisons were carried out through component testing. Results show that the proposed MISS system is effective and robust. Therefore, the proposed MISS system will serve as a promising strain measuring alternative in the field of bridge structural health monitoring.

We demonstrate how a simple optical setup based on the incoherent self-imaging phenomenon can be used to capture two-dimensional phase maps of transparent (phase) objects. We prove that the derivative of the object’s phase map can be extracted from the self-image. Simulations of self-images from a semi-spherical transparent object confirm the theoretical results. As proof-of-principle examples, experimental phase profiles obtained for water droplets on glass, and also a hydrophobic substrate are presented and compared. The phase maps are extracted from a single self-image, using the Fourier analysis, that makes this technique favorable for real-time measurements on dynamic samples.

Tunable diode laser absorption spectroscopy is a robust method for simultaneous measurements of temperature and gas species. Different from commercial detectors integrated with photodiode receivers and amplifiers as the detecting system, a custom-made photoelectric detecting system comprised of Ge detectors and separated amplifiers was designed. First, Ge detectors received optical power from laser beams and then converted it into electrical current signals. The trans-impedance amplifiers were used to convert the current signals to voltage signals. The second stage amplifiers further amplified the voltage signals. All amplifiers that integrated the electronic circuits were packed in a portable case. Finally, the amplified voltage signals were received by data acquisition card. In this way, detectors could be arranged more compact and are easier to facilitate expandability. In addition, absorption data were reconstructed using twice adaptive algebraic reconstruction technique (AART) algorithm. During the reconstruction, after obtaining the temperature distribution through AART algorithm, AART algorithm was adopted for the second time for H₂O concentration distribution. This system was validated with measurements of one-dimensional average temperature in a high temperature tube furnace and the system combined with twice adaptive algebraic algorithm demonstrated in a square flow field containing McKenna burners for simultaneous measurements of temperature and H₂O distribution. The results indicated the viability and potential of this system combined with twice adaptive algebraic algorithm for sensitive temperature and H₂O distribution tomography.

As a crucial part of a high-speed polar coordinate laser writing system, the alignment of the rotatory stage axis and the writing laser optical axis has been a key point that influences the accuracy of the whole system and the fabrication patterns. An alignment method of the rotatory stage axis and the writing laser optical axis is proposed. An alignment system is established and the imaging subsystem is calibrated by a high precision calibration object. A circle center lookup algorithm is applied to find the axis of the rotary stage. The accuracy of the alignment could reach 400 nm. The results of the alignment indicate that the proposed alignment method is a good method for the alignment of the rotary stage axis and the writing laser optical axis. The proposed alignment is useful in the fields of high-resolution maskless laser lithography in polar coordinates.

We present an investigation of bias drift caused by the Faraday effect in differential fiber optic gyroscope (DFOG) based on dispersion compensation factor and broadband source (BS) parameters. The theoretical model of bias drift is established and the influence of dispersion compensation factor and BSs on the bias drift is revealed. The DFOG prototype is designed by optimizing the dispersion compensation factor and two BSs parameters. Then, the bias drifts caused by the Faraday effect in the DFOG and the traditional single-source fiber optic gyroscope (SFOG) are tested and compared. The bias drift caused by the axial magnetic field in the DFOG is about 0.0072 deg/h for 1Gs, which is about 1/22 of the SFOG. The bias drift caused by the radial magnetic field is about −0.0059  deg  /  h for 1Gs, which is about 1/16 of the SFOG. A discovery is that the bias drift caused by the Faraday effect in DFOG can be suppressed by optimizing the dispersion compensation factor and the two BSs parameters.

Motorized stages are widely used in the fields of processing and measurement, such as lithography, semiconductor packaging, biochips, and micro/nanostructure characterization. We propose a tightly focused laser marking method for measuring the motion performance of motorized stages. In this method, a test sample is coated with heat-mode resist thin films and is fixed onto motorized stages. A tightly focused laser spot with a size of ∼520  nm irradiates the heat-mode resist thin films, which then absorb the laser energy and are heated. A laser-induced marking of the films’ structural change from an amorphous state to a crystalline state occurs when the temperature exceeds the crystallization threshold temperature. The motion locus and performance of motorized stages can be clearly observed via the marking of the structural change of heat-mode resist thin films, which can be further clarified through the lithography developing process in alkaline or acidic solutions. The measurement results showed that the minimum nonuniformity coefficient of the motorized stages was 0.016, the minimum parallelism error was 80 nm, the minimum positional accuracy was 0.76  μm, and the accuracy of the measurement system was within 80 nm. The test results demonstrate that the motorized stage can achieve high-precision uniaxial motion with a period of 300 nm and biaxial coordinated motion with a period of <3  μm through parameter optimization, thereby reaching its ultimate performance. Compared to the non-interferometric methods, this tightly focused laser marking method has the advantages of strong environmental adaptability and high accuracy. This study provides an effective and convenient approach for the measurement and optimization of the motion performance of motorized stages.

Uneven illumination, vignetting, and stray light lead to the undesirable decrease of response uniformity of high-throughput fluorescence imaging systems, which brings difficulties to subsequent image processing and data analysis. Here, we propose a flat-field correction method using array targets specially designed for fluorescence microscopy. Based on several dedicated correction sources and the response model, the proposed algorithm performs step-by-step correction to eliminate nonuniformities caused by stray light, high-frequency inhomogeneity, and low-frequency response inhomogeneity. For verification, the correction method is applied to a high-throughput microscopic imaging system with several time-delay-integration detectors. Experimental results show that the proposed flat-field correction method is effective and practical.

A photonic approach for radio-frequency (RF) self-interference cancellation (SIC) incorporated into an in-band full-duplex radio-over-fiber system is proposed. A dual-polarization binary phase-shift keying modulator is used for polarization multiplexing at the central office (CO). A local oscillator signal and an intermediate-frequency signal carrying the downlink data are single-sideband modulated on the two polarization directions of the modulator, respectively. The optical signal is then transmitted to the remote unit, where the optical signals in the two polarization directions are split into two parts. One part is detected to generate the up-converted downlink RF signal, and the other part is re-modulated by the uplink RF signal and the self-interference, which is then transmitted back to the CO for the signal down-conversion and SIC via the optical domain signal adjustment and balanced detection. The functions of SIC, frequency up-conversion, down-conversion, and fiber transmission with dispersion immunity are all incorporated into the system. An experiment is performed. Cancellation depths of more than 39 dB for the single-tone signal and more than 20 dB for the 20-Mbaud 16 quadrature amplitude modulation signal are achieved in the back-to-back case. The performance of the system does not have a significant decline when a section of 4.1-km optical fiber is incorporated.

We describe a multichannel camera operating in the visible to near-infrared wavelengths (450 to 900 nm) with an etendue of 0.08  cm² sr that can be used to image extended sources across multiple optical bands, capable of 3σ detection of 20 Rayleigh signal in 120 s. It has an angular resolution of 0.1 deg and a 35  deg  ×  25  deg field of view, employing a charge-coupled device detector in a compact 1U CubeSat compatible form factor (10  ×  10  ×  8  cm³). The design uses commercial off-the-shelf components while offering versatility using a mosaic of bandpass filters to select different spectral channels to address a wide range of remote sensing needs. The specific implementation described here covers the application of the instrument to explore the neutral sodium and potassium atom density distribution in the atmospheres of the Earth or the Moon.

As a well-known noncontact optical sensing technique, laser self-mixing interferometry (SMI) exhibits outstanding merits of low-cost, self-alignment, compactness, and high sensitivity, and it has been applied to typical geometrical quantity measurements, tomography, object imaging, as well as nanoparticle sizing. In SMI nanoparticle detection, as a result of Brownian motion, laser beam stochastically interacts with each particle in the illuminating volume, producing self-mixed signals with Lorentz shape power spectra, whose spectral broadening width is directly related with particle sizes. In general, FFT is always the first choice to obtain signals’ power spectra, but due to the influence of spectrum leakage, the heights of spectral lines may rise or fall and then change original Lorentz shapes and further increase sizing errors. Here, an all phase FFT (apFFT) method has been proposed to greatly suppress spectrum leakage, correct spectral line heights and further improve nanoparticle sizing errors for Rayleigh scattering cases. The apFFT method proposed is advantageous to developing precise SMI particle sensors or instruments, which may be applicable to chemical or medical applications.

A method based on Mach–Zehnder interferometer and stress optical coefficients theory was proposed for measuring the photoelastic coefficients of optical glasses. The corresponding apparatus is built to measure the photoelastic coefficients of fused silica and As₂S₃ glasses, with p₁₂  =  0.27 and 0.26 at 632.8 nm, respectively. The validity and accuracy of the present technique were tested on the samples of fused silica and As₂S₃ glass, with the relative error not exceeding 10% compared to that in previous studies and the causes of errors were analyzed. The photoelastic coefficients of tellurite glasses with component 75TeO₂-20ZnO-5Na₂O were measured, with p₁₁  =  0.23 and p₁₂  =  0.22. The acousto-optic (AO) figure of merit M₂ of the three types of glass was calculated and the role of photoelastic coefficient in the equation was discussed, providing an effective technique for investigating the AO interaction in a glass.

As the visual perception window of the drone system, the lens provides great help for obtaining visual information, detection, and recognition. However, traditional lenses carried on drones cannot have characteristics of a large field of view (FoV), small size, and low weight at the same time. To meet the above requirements, we propose a panoramic annular lens (PAL) system with 4K high resolution, a large FoV of (30 deg to 100 deg)   ×  360 deg, an angular resolution of 12.2 mrad of aerial perspective, and great imaging performance. We equip a drone system with our designed PAL to collect panoramic image data at an altitude of 100 m from the track and field and obtain the first drone-perspective panoramic scene segmentation dataset Aerial-PASS, with annotated labels of track and field. We design an efficient deep architecture for aerial scene segmentation. Trained on Aerial-PASS, the yielded model accurately segments aerial images. Compared with the ERF-PAPNet and SwiftNet semantic segmentation networks, the network we adopted has higher recognition accuracy with the mean IoU greater than 86.30%, which provides an important reference for the drone system to monitor and identify specific targets in real-time in a large FoV.

We describe a two-port reflective beam splitter with the elimination of the zeroth order based on T-type metal grating. Under the condition of a normal incidence wavelength λ  =  1.550  μm, for transverse electric (TE) polarization, the grating efficiencies of the zeroth and ±first orders can reach 1.35% and 48.28%, respectively. And for transverse magnetic (TM) polarization, efficiencies reach 0.49% and 48.30%, respectively, where energies of the zeroth order are suppressed well for both TE and TM polarizations. Furthermore, the numerical analysis indicates that the grating has a wide incident bandwidth (the efficiencies of over 47% at a bandwidth of 118 nm) as well as an excellent process tolerance in period and duty cycle. The grating performance is also stable with a simple structure so that it has a wide application prospect in optical polarization devices, laser systems, polarization imaging, etc.

Age-related cataracts are reported to be responsible for over 50% of world blindness and are considered a priority eye disease by the World Health Organization. Cataracts progress slowly and often have no subjective symptoms, so regular examination is required for early detection and treatment. We report on the design and development of a portable slit-light-based cataract detection device using near-infrared light and a scanning mirror. Imaging results were obtained by scanning cataract model eyes made from dimethylpolysiloxane and starch mixture and enucleated swine eye with induced cataracts. The fabricated system has adequate sensitivity to detect the smallest cataract formation and progress of cataract in the induced cataract eye and such measurements are useful for early detection of cataract. The results indicate that our fabricated slit-light device could be an easy-to-handle and low-cost tool for remote/routine detection and diagnosis of ocular diseases (like cataracts) without a need for an ophthalmologist.

We proposed a model to estimate surface topography and transition edge width in diamond-turned non-circular compound freeform optics featuring right angle transitions. The method serves as comparison basis between both full and split radius tools and takes into consideration basic cutting and tooling parameters relevant to a raster tool path, along with multiple variables, such as material response and defects, tool wear, and spindle vibrations. Principles are applied to a set of adjacent rectangular surfaces of different tilts. Fabrication tests validating the model show good agreement between the proposed calculations and the experimental results and offer insight on how and when both tip geometries should be used. We represent a useful asset for optical engineers who want to determine which diamond tips choose for their freeform applications. It can also be employed to assess the attainable surface quality and width of edge transitions in compound freeform designs before manufacturing them.

The optical network-on-chip (ONoC) is a promising alternative to the traditional electrical network. However, in manycore systems, the crosstalk noise and insertion loss significantly limit the scalability of ONoCs. We propose a 2  ×  2 optical switch composed of two microring resonator-based switching elements with no waveguide crossings. Also, the effects of reducing power loss, due to the waveguide crossing elimination, on the transmission spectra are investigated. The crosstalk noise is measured −15  dB for the through port and larger than −20  dB for the drop port. An insertion loss values on through and drop states are equal to 0.8 and 1.3 dB for λ  =  1462.2  nm and λ  =  1501  nm, respectively. The simulation results show that the 2  ×  2 HDMS can be an appropriate candidate in silicon photonic integrated circuits.

We compared different power distributions to design freeform progressive addition lenses (PALs) based on a minimization error function model. We employed straight line, trigonometric, eighth-order polynomial, and directly assigned function curvature laws to process the power distributions over an entire surface and assessed their effects on the PAL design. Four power distribution techniques were constructed to connect far vision and near vision areas. Correspondingly, four PALs were designed, simulated, and machined. The results showed that power distribution from the far vision point to the near vision point, according to the straight line curvature law, caused errors of 0.35 and 0.6 D in the distance and addition power, respectively. The error was reduced by the trigonometric curvature law and further reduced under the eighth-order polynomial curvature law. The assigned curvature law had the lowest error among the four lenses. These results indicate that the outcome of the PAL design is sensitive to different power distributions. The proposed method is expected to help advance the design procedure optimization of PALs in optometry.

Partial-filling hot embossing (PFHE) is a flexible and rapid method to fabricate the glass microlens array (GMLA) with high efficiency and mass production. However, the sidewall of the microhole mold strongly affects the deformation modes and optical performance of the GMLA during PFHE. The microholes of a mold fabricated by electrical discharging machining (EDM) and focused ion beam (FIB) were employed for hot glass embossing to reveal the boundary effect of the GMLA in PEHE. The detailed characterizations were implemented for 3  ×  3 microhole arrays by white-light interfering profilometer and scanning electron microscope to highlight the anomaly in the sidewall topography of different mold cavities. Also, the three-dimensional topography and optical quality of the GMLA were measured by a white-light interfering profilometer, parallel light system, and ultraviolet spectrophotometer. Experimental results indicated that the sidewall of the mold cavity significantly affected the filling modes, appearance, and optical quality of the GMLA. The phenomenon of sidewall slip occurs at the edge of the GMLA in the FIB-mold. The sidewall quality of the EDM-mold strongly affects deformation modes and light transmittance of the GMLA. The work provides a better understanding of fabricating the GMLA in PFHE.

There will be introduced packaging stress and thermal resistance during the packaging process of semiconductor lasers, and if the values of both are too large not only adversely affect the output characteristics of the device but also reduce its lifetime. To reduce the risk, we propose a process method based on bilayer metal theory to improve this situation. The optimal temperature field is determined by a theoretical formulation, and the effectiveness of this method is demonstrated using finite elements and experiments. The experimental results show that different initial temperature of the chips does change the magnitude of packaging stress, the reasonable control of bonding temperature and bonding height can effectively change the amount of heat radiation, and then control the magnitude of packaging stress and thermal resistance, and ultimately improve the packaging quality of the product. The reason for the discrepancy between the experimental results and the theoretical values is attributed to the effect of thermal radiation, too.

Due to the uneven distribution of the underwater laser transmission channels, the transmission beam will be affected by water absorption and scattering effects, which induce inter-symbol interference and the degradation of link performance. Based on the binomial Henyey–Greenstein scattering phase function and considering the forward and backward effects, the pulse response expression of the non-line-of-sight underwater laser channel is derived. The effectiveness of the underwater photon transmission channel model established is simulated and verified by the Monte Carlo method. The fitting effects in the coastal waters between the theoretical calculation value of channel impulse response and simulation are better. Under the same simulation parameters, the pulse-broadening factor in coastal waters is higher than that in harbor waters, and the time-domain broadening effect of laser pulse transmission in harbor waters is large. When the asymmetry factor is large, the influence of front scattering on pulse broadening is greater.

The real-time online detection of oxygen concentrations in medicine glass vials has attracted much attention in the field of pharmaceutical manufacturing. Based on wavelength modulation spectroscopy theory, harmonic line shape is an important factor affecting detection accuracy and speed in the process of gas concentration detection. The gas harmonic line shape is determined by the Voigt profile, which is the convolution of the Gauss profile and the Lorentz profile. However, the high computational complexity of the Voigt harmonic line shape makes its computation speed slow. Therefore, in practical applications, the Gauss or Lorentz harmonic line shape is usually used to replace the Voigt harmonic line shape under certain conditions. Based on the HITRAN database, the difference in the positive and negative peak values (namely, the peak-to-peak value) and the error function are introduced to analyze the approximating effect between the Gauss and Lorentz harmonic line shapes and the Voigt harmonic line shape. Taking the second harmonic of oxygen as the example, the peak-to-peak value of the Gauss and Lorentz harmonic line shapes is simulated at different temperatures and pressures. Then, based on the approximate linear relationship between the modulation depth and peak width of the second harmonic line shape, a method of fitting the actual second harmonic using the Lorentz line shape is presented. Experimental results indicate that the proposed strategy improves the accuracy and stability of oxygen concentration detection in medicine glass vials and can meet the actual detection speed.

We comprehensively investigate the radiation-induced active band attenuation (RIABA) characteristic of erbium (Er)-doped fiber. It is experimentally shown that the RIABA gradually alleviated with radiation dose increasing and the absorption/luminescence characteristics of Er^3  + appear to be unaffected by irradiation ray. By employing the intrinsic luminescence spectrum of Er^3  + as the original output spectrum, a radiation-resistant low noise broadband source with specially customized Er-doped photonic crystal fiber and multiple radiation-resistant methods is proposed. Under 50 krad irradiation dose, it demonstrates mean-wavelength stability of 26.4 ppm and the output power attenuation of <0.1  dB. Throughout the irradiation, the 3 dB bandwidth of the output spectrum is >40  nm. The broadband source proposed exhibits excellently comprehensive performances in the radiation environment and is quite feasible for strategic high-accuracy interferometric fiber-optic gyroscopes.

We demonstrated a nanosecond pulsed Cr:ZnSe laser with both wavelength tunability and burst pulse operation in the mid-infrared (IR) region. The burst pulse operation of the Cr:ZnSe laser was achieved using a pulse-on-demand Tm:YAG laser as a pump source. The number of pulse shots and the repetition rate in a packet were controlled. A tuning range of 2.32 to 2.56  μm was also achieved using a birefringent filter. Our results show that the pulse-on-demand Tm:YAG laser is an attractive pump source for the burst-pulse-operated Cr:ZnSe laser, which paves the way for various mid-IR laser applications.

The average bit error rate (ABER) performance of the decode-and-forward-based multiple-input multiple-output (MIMO) multi-hop underwater wireless optical communication (UWOC) systems has been investigated over the composite exponential-generalized gamma (EGG) fading channel with space-time block coding (STBC). The impacts of absorption, scattering, and the misalignment caused by spatial spreading in the sea, which are modeled by the beam spread function, as well as the turbulence-induced fading are all taken into account in this work. With the help of the moment generating function, Meijer’s-G function, and Gauss–Laguerre quadrature numerical methods, the approximate closed-form expressions of ABER for the UWOC system with binary phase shift keying are mathematically derived. Furthermore, numerical analyses are provided to investigate the effects of various hops, temperature gradients, air bubbles levels, STBC schemes, and turbulence severity on the performance of the multi-hop MIMO UWOC system. In addition, Reed–Solomon (RS) codes are adopted to improve the UWOC system performance. Results reveal that the ABER performance of this multi-hop UWOC system would degrade as the temperature gradients, hop numbers, and bubble levels (BLs) increases, which could be improved considerably by employing the STBC scheme. The ABER performance over the composite EGG distribution is mainly limited by the environmental parameters, whereas the fading can be significantly improved by RS codes. The ABERs of EGG distribution are compared with exponential-gamma and exponential-lognormal distributions in oceanic turbulence under different temperature gradients and BLs, respectively. These analytical results are also verified by Monte Carlo simulations. This work is beneficial for the design of UWOC system.

The performance of the underwater wireless optical communication (UWOC) point-to-point link is strictly affected by the turbulent nature of the ocean. The underwater turbulence mainly arises due to the random variations in the refractive index of the propagating medium, which limits the transmission range of the UWOC links. We study experimentally the effect of salinity-induced turbulence on the propagating optical beam. Three different channel scenarios have been considered, namely, uniform salinity concentrations, salinity gradients, and the different sized air bubbles. In the uniform channel, the received optical power follows an exponential decay with the increase in the salinity concentration in the water. The different gradients and air bubbles create random variations in the refractive index of the water, which results in a random fluctuation in the received intensities. These intensity fluctuations are modeled using the Gaussian mixture model (GMM), which is the sum of the Gaussian functions. The feasibility of both the proposed models is verified by conducting a goodness of fit test in terms of R² and root means square error coefficients. The values of these coefficients indicate that the GMM acceptably matches the acquired experimental observations. Furthermore, the performance of the UWOC point-to-point link is also evaluated using the proposed model. The bit error rate of approximately 10^−  6 that corresponds to a signal-to-noise ratio of 43 dB has been estimated for both uniform and nonuniform channels at an air flow rate of 24  L  /  min. Hence, the proposed model can efficiently describe the effect of salinity variations, gradient conditions, and air bubbles in the turbulent UWOC point-to-point links.

The doping of nanoporous glass (PG) is a prospective technique for producing bismuth (Bi)-doped optical fibers. The refractive index of sintered PG is lower than pure silica glass because of PG high residual content of boron (2 to 3 at.%). This fact impedes the choice of fiber cladding for building a light-guiding structure. We have chosen the microstructured optical fiber with the core from Bi-doped nanoporous glass to overcome this problem. We have studied the glass composition and controlled the luminescent properties of bismuth active centers at each step of the fiber fabrication. The PG is suitable for the production of microstructured fibers.

We present a unique design of weakly coupled few-mode photonic crystal fiber (FM-PCF), which can support six vector modes (HE_11a,HE_11b,TM₀₁,HE_21a,HE_21b, and TE₀₁) with ultra-flattened chromatic dispersion over the C wavelength band. We investigate the impact of fiber parameters on chromatic dispersion, minimum effective refractive index difference (minΔn_eff), confinement loss, bending loss, and differential mode delay using the finite element method. To achieve ultra-flattened chromatic dispersion and large minΔn_eff, an extra small elliptical defected hole is introduced in the fiber core. The circular holes in the first ring are replaced by elliptical holes to further obtain flattened-chromatic dispersion. The simulated results show that the designed weakly coupled FM-PCF can obtain an ultra-flattened average dispersion of 0.011 ns/nm/km with a small dispersion slope (<1.5  ×  10^−  5  ns  /  km  /  nm²) in the wavelength range of 1520 to 1570 nm. And the proposed weakly coupled FM-PCF has large minΔn_eff (>1  ×  ^{10  −  3}), low confinement loss (<10^−  4  dB  /  km), and good bending resistance. In summary, the proposed weakly coupled FM-PCF has potential applications for large-capacity MDM communication systems.

Closed-form statistics are highly desirable in wireless communication while analyzing system performance. Nevertheless, at a few instances, the mathematical intricacy introduced by the probability density function (PDF) limits the analysis of the system. In this context, asymptotic analysis has been entertained in wireless communication to obtain the approximate closed-form statistics of the system performance metrics under high-power regime. With this motivation, the asymptotic framework for exponentiated Weibull (EW) distribution has been presented. First, the origin PDF for EW distribution is proposed. Later, the earlier derived result is utilized in obtaining asymptotic results for the average bit error rate (BER). In addition, we also provide a mathematical framework with maximal ratio combining and selection combining diversity over the average BER. In addition, aperture averaging technique is also implemented over the aforesaid asymptotic results for further improvements in system performance. The results of the asymptotic analysis have been validated using the Monte–Carlo simulation.

We have experimentally demonstrated the efficacy of k-means algorithm-based detection scheme in a two-user-based wavelength division multiplexed (WDM) on-off keying (OOK) power-division non-orthogonal multiple access (PD-NOMA) system over turbulent free-space optical (FSO) channels. The proposed system utilizes both power and wavelength division multiplexing to enhance the capacity of the transmission system. The proposed system also incorporates successive interference cancellation technique that utilizes unsupervised machine learning technique, which is K-means clustering, to detect the signal. Two different weak turbulence conditions in the free-space optical channels have been realized in the laboratory environment having link lengths of 2.2 m (near user) and 3.2 m (far user). The results show that the measured probability density function of intensity fluctuation closely matches the log-normal distribution. The experimental results also show that the K-means-based detection provides the BER, which is well below the Pre-FEC limit. The maximum power penalty for the near (far) user is found to be 3.1 dB (4.2 dB) at a BER of ^{10  −  3} for weak turbulence conditions. The proposed and implemented k-means-based detection scheme for the WDM-OOK PD-NOMA system can also be used for FSO communication systems.

Optical burst switching (OBS) is a technique involving a switched mechanism for upcoming all-optical backhauls. However, data bursting congestion may occur as a considerable problem due to the increasing amount of light passing through interconnections and the overall system range. Moreover, the lack of restricted buffers for the infrastructure and normal one-way resource signaling exacerbates the conflict, resulting in the deletion of some contesting bursts as a response. In these types of networks, concerns related to contention can be handled by reducing congestion. In response, the quality of service (QoS) and internet connections provided must be precise and consistent. We propose a scheduling approach for satellite excitation and OBS nodes. The proposed method can satisfy the present program’s QoS, requiring a combination of duration, width hybridization limits, and prioritization scheduling. We use a dynamic barrier scheduling approach that incorporates several levels and enhances the changing efficiency compared with a particular threshold approach. To execute this design, a Xilinx xc7vx690tffg1927-2 field-programmable gate array comprises the bulk of a satellite OBS edge module. The scheduler is implemented in Verilog hardware description language and evaluated with ModelSim SE 10.6d using simulated data.

The performance of an underwater wireless optical communication (UWOC) system is affected by many factors, such as channel conditions, modulation and detection techniques, type of optical source, etc. The UWOC system performance was investigated in the presence of weak and strong turbulence. Pure sea was the water type used for the performance analysis. Binary phase shift keying, quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation, and non-return to zero-on–off keying were used for comparison. Further, a continuous wave laser was used as the optical source, and photodetectors are at the receiver. The performance of the UWOC system was analyzed in terms of the bit error rate (BER). The BER performance was evaluated using the log-normal probability density function (PDF) in weak turbulence and the gamma-gamma PDF in strong turbulence. In all modulation techniques, QPSK had the best BER performance. The BER was reduced further using avalanche photodiode (APD) as the photodetector instead of positive-intrinsic-negative diode for the same transmitter power and channel conditions. In weak turbulence, a BER of ^{10  −  8} was achieved at a transmitter power of 12.5 dBm using the QPSK modulation technique and APD as the photodetector. For the same conditions, a BER of 9  ×  ^{10  −  7} was achieved in strong turbulence. The multiplication gain, dark current, and carrier ionisation ratio of the photodetector were optimized for better BER performance in the UWOC system.

Machine learning has emerged as a powerful tool for physicists for building empirical models from the data. We exploit two convolutional networks, namely Alexnet and wavelet scattering network for the classification of orbital angular momentum (OAM) beams. We present a comparative study of these two methods for the classification of 16 OAM modes having radial and azimuthal phase profiles and eight OAM superposition modes with and without atmospheric turbulence effects. Instead of direct OAM intensity images, we have used the corresponding speckle intensities as an input to the model. Our study demonstrates a noise and alignment-free OAM mode classifier having maximum accuracy of >94  %   and >99  %   for with and without turbulence, respectively. The main advantage of this method is that the mode classification can be done by capturing a small region of the speckle intensity having a sufficient number of speckle grains. We also discuss this smallest region that needs to be captured and the optimal resolution of the detector required for mode classification.

An innovative vibration sensor based on a Fabry–Perot interferometer (FPI) using fiber Bragg grating (FBG) reflectors was demonstrated in this work. The sensor was designed to be compact and easy to fabricate, independent of temperature, to overcome limitations seen in previous designs, providing an effective correction for temperature effects in FBG-based FPI (FBG-FPI) sensors. A laser source with a peak wavelength of 1547.42 nm obtained from the FBG reflective peak was used to illuminate the FBG-FPI so that the light source was always within the FBG-FPI optimum wavelength operating range of 1547.15 to 1547.80 nm. The sensor was shown to capture a 3-kHz burst signal from a signal generator in 1-, 2-, and 3-Hz intervals. In addition, the work carried out has revealed that the sensor could be used to capture sinusoidal signals at frequencies up to 9 kHz, creating a performance comparable with many existing conventional piezoelectric sensors. Furthermore, the ability to operate regardless of any ambient temperature changes [from 26.5°C (room temperature) up to 80°C] opens the way to use such a sensor system over a wide range of engineering applications taking advantage of the next generation of FBG-based FPIs.

A gas pressure sensor based on an in-fiber interferometer with a closed microcavity is proposed. Its temperature and gas pressure characteristics are theoretically and experimentally studied by investigating the dip wavelength of its interference spectrum. Based on the theoretical analysis, Fabry–Perot (F-P) and Mach–Zehnder interferences occur in the sensing process of this sensor. From our experiments, the wavelength sensitivity of this gas sensor is 83.6 pm/KPa at gas pressures from 101 to 120 KPa. And the hybrid interferometer has higher sensitivity than that of the single interferometer. This in-fiber sensor with a closed cavity has advantages of high reusability and simple structure, and it can not only reduce the optical power loss in transmission but also eliminate the influence of the gas refractive index. It can be used stably in a low-pressure monitoring system for flammable and explosive gas.

This erratum corrects errors in the published paper.