Aliasing is an unnecessarily confusing topic in imaging and in general. This commentary’s motivation, in the first half, is to alleviate the confusion and remove the mystery of aliasing as well as provide a basic understanding and appreciation for sampling without the need for Fourier theorems or other higher-level mathematics. The second half discusses examples of where aliasing plays an important role in imaging.

Guest editors Nicholas Madamopoulos, Yizheng Zhu, and Jianzhong Zhang introduce the Special Section on Fiber-Optic Sensors for Environmental Health Monitoring.

Marine sliding bearings re prone to wear in harsh environments. A real-time in-situ measurement method of sliding bearing wear, based on chirped fiber Bragg grating (CFBG), was proposed. The CFBG wear detection sensor and the corresponding demodulation system were designed and fabricated. The CFBG was sealed to ensure that wear synchronously with the sliding bearing. The relationship between the bandwidth and length of the CFBG was established and adopted to calculate the wear length. Meanwhile, a Savitzky–Golay filter was applied to denoising to improve the accuracy of the demodulation system. Experimental results showed that the sensor could achieve high precision real-time bearing detection and fault warning. With a chirp rate of 10 nm / cm and a bandwidth resolution of 20 pm, a wear range >8 mm and an accuracy of 0.3 mm could be obtained.

We propose a white-light interferometric demodulation algorithm for high-finesse fiber-optic F-P sensors, in order to improve the demodulation accuracy and the dynamic range encountered in traditional demodulation techniques. The interferometric spectral signal of the high-finesse F-P cavity was converted to the frequency domain and then a more accurate cavity length was estimated based on full phase on higher-order components. A detailed theoretical analysis was operated. A high-finesse F-P temperature sensor based on a silicon diaphragm was fabricated and tested to verify the proposed method. The demodulation accuracy increases with the increase of order, and the anti-noise performance is improved. For the third-order component, the optical path difference sensitivity obtained by this algorithm is 0.231 ± 0.0188 μm / ° C, and the average error rate of cavity length demodulation value is 0.0152%. The proposed algorithm is applicable to demodulate the high-finesse F-P cavities in the light source bandwidth of 1525 to 1575 nm, providing high accuracy and improved anti-noise performance.

Owing to their advantages of anti-electromagnetic interference, chemical resistance, high sensitivity, and fast response time, optical fiber sensors (OFSs) are widely used in biomedical, environmental monitoring, and food safety fields. We introduce the classification and principles of OFSs and summarize the applications and research progress of OFSs in water quality detection (heavy metals and microorganisms) and air quality monitoring (COx, NOx, and VOCs). Meanwhile, analytical performance, reliability, and environmental adaptability of OFSs are discussed and prospected.

An invisible track bed defect classification method based on distributed optical fiber sensing data acquisition and an attention Transformer model mechanism under the frequency domain is proposed. The vibration sensing data contain structural safety information of vehicles, rails, track beds, etc., covering the entire time period and entire track area of subway operation. To classify the invisible track bed defect rapidly and accurately, the original vibration signals are first reduced by down-sampling and envelope signal extraction. According to the regular characteristics of different types of signals, an fast Fourier transform (FFT) Attention Transformer (FFT-Attn-Transformer) sequence feature extraction architecture with a high recognition accuracy is proposed for model training. The results demonstrate that the accuracy, precision, recall rate, and F1-score are all above 98% using the proposed model, and the recognition accuracy of the defect test area is 99.47%, which has extremely high stability and accuracy, providing an innovative and feasible idea for the lack of effective monitoring scheme for invisible track bed defects.

We primarily address the dynamic humidity sensing requirements within hydrogen fuel cell test stacks. Dynamic humidity testing environments impose stringent demands on sensor stability, detection accuracy, and response speed. However, the dense structure between polyimide layers and the higher film thickness have historically led to extended response time. A fabrication approach for polyimide fiber Bragg grating humidity sensors is introduced, which involves a combination of ethanol and potassium acetate. This development results in a humidity sensor in which the treated coating accelerates the exchange of water molecules within the film’s interior. Furthermore, the moisture-sensitive inorganic salt, potassium acetate, enhances the overall water absorption rate, amplifying the center wavelength humidity sensing drift in the grating. Experimental validation confirms that the sensor exhibits high sensitivity and rapid response capabilities under dynamic humidity conditions, showcasing its potential applications in practical engineering scenarios.

By optically exciting the guided acoustic waves in an optical fiber, forward stimulated Brillouin scattering (FSBS) enables the identification of external substances by measuring the acoustic impedance while introducing no optical power leakage. In most experimental demonstrations, uncoated silica optical fibers are adopted to achieve a better sensing performance. However, fragility makes such bare fibers not ideal for practical applications. In this work, environmental acoustic impedance measurement based on FSBS is demonstrated using an aluminum-coated optical fiber, exploring the stable material properties of aluminum and its role as a low-loss medium for acoustic wave propagation. From the detected FSBS-induced temporal waveforms, acoustic reflections at the silica-aluminum and the aluminum-environment interfaces are identified and processed for acoustic impedance measurement of the surrounding liquid. The impedance values calculated from each set of measurement data maintain a deviation of no more than 5% from the reference values, concurrently achieving a substantial reduction in measurement time.

An acousto-optic (AO)-based electric field sensor is presented for time domain measurement under magnetic resonance imaging (MRI). A fully MR-compatible sensor is designed and fabricated using a phase-shifted fiber Bragg grating mechanically coupled to a piezoelectric transducer. Mechanical resonance of the piezoelectric transducer is matched to the operating frequencies of commonly used MRI systems to increase the sensitivity of the sensor. Sensitivity of the sensor is measured as 1.27 mV/V/m, with a minimum detectable electric field of 4.4 mV/m/√Hz. Directivity of the sensor is measured with a 18 dB orthogonal component rejection. The dynamic range of the sensor is calculated as 117 dB/Hz, which allows the measurement of electric fields up to 3.2 kV/m. In MRI studies, the AO sensor was able detect local hot spots around a reference implant accurately with high signal-to-noise ratio. AO sensor exhibited similar or better performance when compared with commercially available MRI compatible electric field sensors. Furthermore, the small size of the sensor with the flexible fiber optic link could allow in situ measurements of electric fields during critical interventional procedures such as pacemaker lead or deep brain stimulator placement as an MRI dosimeter during diagnostic scans.

This study introduces a scanning system that utilizes a motor platform to mount a line laser and a camera. Two crucial parameters are considered to extract three-dimensional information from a single frame: a rotation axis and a specific point along this axis. A calibration approach for rotation axis identification is presented to determine these parameters. The rotation axis corresponds to the normal vector of the camera’s movement plane and defines the rotation matrix during the scanning process. The identified point on the rotation axis serves as the translation matrix’s center and represents the focal point of the camera’s trajectory. A comprehensive point cloud is generated by assembling multiple frames using the scanner’s rotation angle. Experimental findings substantiate the method’s efficacy in enhancing the system’s scanning for plane reconstruction and accuracy, and the scanning quality is much better than that of the previous approach.

As part of developing a sub-terahertz imaging scanner for automatic security screening applications, we investigate the detection and recognition of concealed objects in sub-terahertz images using various image processing techniques capable of addressing the low quality of such images. First, we propose a method that combines locally adaptive and multilevel thresholding with connected components labeling. This method enables the removal of the nonuniform background, facilitating the detection and localization of multiple concealed objects. Subsequently, we analyze the performance of several local feature-based matching methods for selecting the optimal detector–descriptor combination, allowing for efficient detection and recognition of concealed objects. Furthermore, we assess the applicability and performance of the supervised machine learning method based on the support vector machine classifier combined with the bag of visual words model. Finally, we explore the effectiveness of transfer learning by fine-tuning two pretrained convolutional neural networks, namely AlexNet and YOLOv3 models. The two fine-tuned models achieve recognition accuracies of 87% and 92% for AlexNet and YOLOv3, respectively. Benchmarking the study results demonstrates that deep learning methods provide an efficient solution for automatic security screening systems.

We introduce an analytical model for Fourier ptychography (FP). The model represents recorded intensity images as linear equations. Here, the sample’s self-correlating components are the unknown variables, while the optical system’s point-spread function’s corresponding components serve as coefficients. This framework facilitates the direct computation of high-resolution complex images from the sample. A standout feature of our model is its ability to verify the uniqueness of the reconstructed image, a critical attribute for numerous quantitative applications. Our linear model offers clear insights into the impact of various experimental factors on accurate reconstruction, such as scanning step, random scanning mode, spatial resolution, and noise. While these factors have been previously acknowledged, their precise roles have remained nebulous. We have also developed an efficient computational method tailored for our model, adept at managing large matrices, thus enhancing the translation of low to high-resolution images. This research elucidates the foundational mathematics behind FP’s efficacy, underscoring its potential for optical measurements and metrology where mathematical uniqueness is paramount.

Our study examines how laser dazzling affects human performance, specifically accuracy and reaction times, using a laser dazzling shooting simulator at the Royal Military Academy, Belgium. The research assesses the performance degradation under laser dazzling in a simple, baseline scene, including different target contrasts and the use of laser eye protection. Utilizing a 532 nm green laser for a safe yet effective dazzle, trained shooters’ performances were measured and analyzed. The results align strongly with a live shooting trial and correlate with Adrian/CIE visibility levels. Additionally, electrical brain activity data, acquired via electroencephalography (EEG), provided insights into the shooters’ mental states. EEG-derived metrics, particularly frontal alpha asymmetry and frontal alpha power, revealed that participants experienced heightened negative and avoidance emotions, coupled with increased cognitive load prior to shooting. These responses returned to baseline levels postshooting. Moreover, distinct cognitive and emotional states were observed in relation to different types of laser eye protection goggles, potentially correlating with variations in shooting performance. These findings pave the way for future research with more advanced simulation scenes and deepen understanding of the effects of laser dazzle.

Gray code assisted phase shifting technology can achieve robust and noise-tolerated three-dimensional (3D) shape measurements. To solve the issues of unsynchronized brightness changes, local overexposure, and edge coding errors caused by inconsistent reflectivity of the surface in complex industrial scenes, as well as defocusing caused by noncontinuous surfaces and varying distances, we combine the advantages of a large imaging range in passive stereo vision and high precision in active structured light imaging. It uses a consumer-grade projector to project gray code and stripe patterns, whereas two precalibrated color industrial cameras capture raw images and obtain the original channel data. Gray code and reverse gray code images are projected to solve the problems of binarization and boundary blur. In addition, an error point filtering strategy is proposed to retain pixels with decoding errors of less than two bits. The use of softargmin for subpixel matching of absolute phase results in a high precision disparity map. We present a simple and high precision 3D measurement system for industrial objects. Experiments on 3D measurements in complex industrial scenes showed that the proposed method can achieve high precision and robust 3D shape measurements.

Aiming at the problem of constraints between reconstruction range and splicing accuracy when 3D reconstruction is carried out on large-scale equipment in aerospace and other fields, we investigate the combined 3D scanning measurement method based on the guidance of the all-space laser positioning system (ALPS) on the basis of analyzing the fusion of the ALPS and scanning target. The method establishes a global 3D scanning measurement model based on multiple measurement points by Creo and simulates and analyzes the data with MATLAB, then designs the structure of the circumferential combined photoelectric receiver by combining with the receiving characteristics of photodetectors for laser signals in the full-space field, and then completes the optimal layout design of the receiver based on the accessibility. Finally, a 30 m×30 m experimental platform for the fusion of ALPS and scanning target is constructed, and the measured standard bat and Zeiss standard ball are used as the measured objects to verify the feasibility of the research method. The simulation and experimental results show that the optimal number of receivers needed for the global measurement system is N=6, the measurement accuracy is better than 0.5 mm, and the repeatability measurement accuracy is up to 0.02 mm, which provides a basis for the 3D scanning of large-scale equipment.

Large-scale space measurement is of great significance for the assembly of large parts. In view of the problems of traditional measurement methods, such as difficult expansion of the measurement range, complicated calibration process, and poor portability, this work proposes a multicamera vision measurement network that can freely expand the measurement range based on the auto calibration method. First, cameras were fixed to auxiliary targets and every camera with a target was mounted on a precision turntable. Second, the rotation axis parameters of the camera and the target pose relative to the camera coordinate system were estimated. Finally, the extrinsic parameters between the cameras could be estimated by interactive scanning method. The experimental results show that the method in this paper simplifies the calibration method of the multicamera vision measurement network, and the measurement accuracy basically reaches the accuracy of the Zhang type calibration method under the premise that the measurement range can be expanded freely.

The fabrication method of microlens arrays with diameters from 20 to 90 μm based on thermal reflow and dry etch structure transition process is demonstrated. The key process parameters of microlens design and production are obtained by theoretical calculation. And the research of photoresist (AZ4330) surface topography changes during thermal reflow process is studied in detail, which is helpful for microlens array design and process optimization. The photoresist microlens arrays are transferred into quartz by dry etching process. The measurement data indicate that the photoresist microlens arrays have good surface topography and high surface quality, and the optical property of quartz microlens is in good agreement with the theoretical design. This method can be widely used to prepare high quality microlens arrays in optical applications.

To eliminate motion-induced errors, a multi-step fitting position transformation method is proposed for measuring the three-dimensional (3D) profile of an object. The method employs a neural network to perform position transformation on the objects in the captured motion phase-shifting fringe projection images, aligning them to the same position, and then uses the phase-shifting method to obtain the 3D profile of the objects. Theoretically, the rigid body motion transformation matrix is used to deduce the change of points on the objects in the fringe pattern under different motion conditions, and the 3D profile measurement results of randomly moving objects are simulated and analyzed. Experimentally, a neural network model was trained by using 5000 sets of data obtained from experiments to eliminate motion-induced errors in various motion states. Simulation and experimental results show that this method significantly reduces the motion-induced errors that occur when measuring moving objects using the traditional phase-shifting method. It has broad application prospects in the field of real-time measurement of moving objects’ 3D profiles.

Infrared smoke plays a vital role in battlefield photoelectric countermeasures. Transmittance and effective shielding area are crucial indicators in the evaluation of infrared smokescreen shielding performance. The traditional evaluation technique has drawbacks of lacking precision, time consumption, and low automation level. Therefore, an efficient and accurate technique is urgently desired for the evaluation of infrared smokescreen shielding performance in the battlefield environment. We propose a technique based on the grayscale value for evaluating infrared smokescreen shielding performance. First, a description of the field test system is introduced. Second, the transmittance and effective shielding area mathematical models are deduced and constructed. In particular, we discuss the assignment methods of the background and smokescreen grayscale value in the transmittance mathematical model. Finally, the evaluation software is designed, and a field test sample calculation is included to illustrate the procedure of the proposed technique. The results prove to be of enough precision for engineering applications, which is of great significance for engineering guidance.

On-chip Si3N4 microcavities composed of multi-mode waveguides can achieve high-quality factor and anomalous dispersion simultaneously, which is necessary for solitons generation. It has been proven to be a promising platform for compact optical (Kerr) frequency comb generation. To further reduce the threshold power for generating a single soliton microcomb, a racetrack Si3N4 microcavity with Euler bends is reported. Compared with traditional racetrack microcavities, Euler bending can significantly suppress the sudden change in bending radius at the waveguide connection, which suppresses mode interactions and reduces propagation losses, resulting in an increase in Q factor value. Based on this racetrack microcavity, using the auxiliary laser heating method, a single soliton frequency comb with a repetition rate of 34 GHz and a 3 dB bandwidth exceeding 18 nm (corresponding to a pulse duration of 143 fs) can be generated using only a 19 mW pump laser (estimated on-chip pump power below 10 mW). The optimized racetrack microcavities can be building blocks for integrated photonics systems.

The application of three-dimensional (3D) shape measurement technology based on the phase-shift method on bright and dark surfaces is susceptible to image saturation and low grayscale values. This is due to the need to analyze the absolute grayscale values of pixels. Although multiple exposures and adaptive stripe methods can restore high dynamic scene 3D morphology with high accuracy, the requirement for multiple rounds of projection leads to low computational efficiency. In this work, we proposed a high dynamic surface 3D shape measurement technology that combines the phase-shifting method and line-shifted Gray code. This technology can accurately restore the 3D shape of a highly dynamic scene with a single projection. To avoid the decrease in measurement accuracy caused by low projector calibration accuracy, we developed a binocular structured light 3D measurement system to verify the method proposed in this work. Experimental results demonstrate that the proposed method improves measurement integrity by ∼121.1% and reduces measurement error by 71.1% compared to the phase-shifting method in overexposed scenarios. In low signal-to-noise ratio scenarios, the measurement error is reduced by 62.6%. Furthermore, compared to multi-wheel projection methods, the calculation time is reduced by 88.4%.

We propose and demonstrate a self-reconfiguration network of high-precision time-frequency synchronization. The local oscillator (LO) signal and time reference signal generated by the central site are transmitted to the two-fiber unidirectional ring in opposite directions. The all-optical microwave phase conjugation and the time-frequency domain transform measurement are utilized to eliminate phase fluctuation and time delay introduced by optical fiber. The two-fiber unidirectional ring can automatically switch the working loop when the fiber link is broken and avoid manual intervention, which realizes the self-healing of the network. A 20 GHz frequency signal and 1 pulse per second time signal are transferred along a 7 km fiber ring. At 1000 s averaging time, the long-term frequency stability of the order of 10 to 16 and time stability of the order of ps can be achieved at remote sites. When the network is interrupted, the proposed system can automatically recover the time-frequency synchronization from the failure state within 37 ms.

Diffractive optical elements can greatly improve the performance of optical imaging systems by achieving wideband high diffraction efficiency with bilayer diffractive optical elements. The conventional design method for bilayer diffractive optical elements (DOEs) utilizes two specific design wavelengths to optimize the diffraction efficiency. However, this method may not provide enough viable solutions when the required diffraction efficiency cannot be achieved at these two wavelengths. In this paper, we propose a direct search method that can enhance the diffraction efficiency of bilayer DOEs further. We achieve higher diffraction efficiency and lower microstructure heights of double-layer discs with linear optics (DLDOEs) across a wide range of fields of view and working wavebands. A detailed comparison of a proposed method with two wavelength method is presented. When the period width is 100 μm, our method can reduce the microstructural heights of the first and second layers of DLDOEs by 22% and 25%, respectively. This method enables us to produce diffractive optical elements with higher diffraction efficiency and lighter weight. Our direct search method provides a fundamental framework for the parametric fabrication of diffractive optical components and enables their application in advanced optical imaging systems.

A location algorithm and a spectral extraction algorithm for a static broadband snapshot imaging spectrometer (SBSIS) are presented. The high-energy target is dispersed into a V-shaped pseudo-image (VPI) in the focal plane of the SBSIS. The location algorithm accurately calculates the target azimuth based on the one-to-one mapping relationship between the intersection position of the extension lines of the two arms of the VPI and the azimuth of the target. The spectral extraction algorithm is described to extract the characteristic spectrum of the target based on the azimuth angle of the target and the VPI.

We proposed a Raman and erbium-doped fiber amplifiers (EDFA) hybrid bidirectional optical amplifier (HBOA). The hybrid amplifier consists of a fiber Raman amplifier (FRA) and two EDFA, which are ingeniously combined by four WDMs. Gain and noise figure (NF) are the metrics used to evaluate the performance. The effects of Raman and EDFA pump on the gain and NF of the proposed hybrid amplifier are explored, resulting in a maximum gain exceeding 40 dB and an NF below 0 dB. Furthermore, when the input signal power is small (−35 dBm), the proposed hybrid amplifier, exhibits a total gain increased of ∼14 dB, and NF reduction of ∼4 dB across the wavelength span of 1545 to 1565 nm, as compared with EDFA-only. Finally, The Allan deviation can reach 1.41×10−14 when HBOA was used, significantly better than the stability of 2.62×10−14 obtained by Bi-EDFA, which confirmed the feasibility of the proposed amplifier for radio frequency synchronization systems.

Exponentially increasing network traffic requires hundreds of terabits per second data transmission rate, consuming around 9% of the electricity worldwide. Massive parallel optical links demanded to support the data transmission capacity, but not favorable in terms of scalability, energy consumption, complexity, cost, and size. A single laser source followed with semiconductor optical amplifier based optical frequency comb enabling data transmission of 6.78 Tbps over an 80 km single-mode fiber (SMF) is a possible solution to reduce the energy consumption, complexity, cost, and area. The data transmission over 80 km SMF also makes it suitable for inter-data center communication. So, in the proposed work, the evenly spaced and phase-coherent 113 frequency lines individually modulated using differential quadrature phase-shift keying at the bit rate of 30 Gb/s and then multiplex using polarization and wavelength multiplexing technique to transmit signal over 94.4 km (80 km single core and single mode fiber and 14.4 km dispersion compensating fiber). The proposed work has also carried out the detailed analysis of optical frequency comb parameters required for its application in telecommunication. Following the procedure mentioned earlier, a signal at the minimum quality factor of 23 dB with total transmission rate of 6.78 THz for 226 users is achieved at the receiving end, making the work suitable for telecommunication and inter-data-center communication applications.

A museum is a historical and cultural center with important conservation significance. In this work, the minimum light damage spectrum of paper cultural relics and solar spectrum are mixed in 11 different proportions by MATLAB software. In addition, REDOX index and the full-spectrum index are used as evaluation indices to calculate the damage degree and comfort level of the mixed 11 spectra. The monochrome light emitting diodes (LEDs) on the market are investigated, and a simulation database of 109 monochromatic LEDs is established by using the state joint state density function as the spectral radiation model. The nine mixed spectra with solar spectral weights from 0.1 to 0.9 are used as the target spectra. The monochromatic LEDs and 2850 K warm white LED in the database are fitted to calculate the optimal solution, and the spectral combination scheme is obtained by parameter comparison. A hybrid light source is designed by LIGHTTOOLS simulation software, and chromaticity analyses are carried out for the five light sources with solar spectral weights of 0.1, 0.3, 0.5, 0.7, and 0.9. The results prove the feasibility of the theory of mixing the solar spectrum with the minimum light damage spectrum to improve the comfort and color rendering of the light source. This will have a certain guiding significance for the lighting of the museum.

To meet the growing demand for high efficiency stealth dicing (SD), generating dual-focus in silicon to produce double dicing paths has been widely used. However, the challenges in dual-focus SD are the aberration compensation methods based on spatial light modulators (SLM). Until now, the effect of SLM flyback regions that causes focusing intensity reduction has not been considered. Here, an aberration compensation method that considers the effect of SLM flyback regions is proposed to enhance the focusing intensity. This method takes into account not only the total number of flyback regions but also their distribution corresponding to the laser intensity distribution. In addition, the total area of flyback regions is utilized to quantify the impact and determine the optimal compensating phase pattern. Through simulations and experiments, we investigate the generation and control of aberration compensated dual foci. The simulated results demonstrate excellent agreement with theoretical analysis. Moreover, a back crack propagation of 29.9 μm is achieved in the dual-focus SD experiment.

A rigorous method is presented to compute the vector diffraction pattern of microscope objectives considering on-axis aberrated and radially polarized wavefronts. For that, the diffraction pattern of the microscope objective is computed considering the polarization and the wavefront at the exit pupil of the optical system. These computations do not assume that at the exit pupil of the microscope objective the wavefront is spherical but aberrated. Examples of real microscope objectives’ performance are presented.

The results of full-scale experiments on models of a smart window with a double-grating optical filter are presented. Such a filter has an angular-selective direct light transmission due to two thin-film gratings with transmissive and non-transmissive (absorptive, reflective, scattering, or chromogenic) parallel strips. The gratings are angled to match the solar path relative to the window to optimize the daylight and solar heat gain without the use of shading/redirection devices. Due to the strip widths, relative positions, and slope angles of both gratings, calculated to minimize transmission at a preset time, such a smart window self-adapts to daily and seasonal changes in the position of the Sun. Diffused radiation passes through the transmissive strips of both gratings, increasing the daylight and thermal comfort indoors. Experimental data for the four hottest months confirmed the results of numerical simulation based on a previously developed method for calculating the temporal characteristics of the directional light transmission of filters. The minimum transmittance was achieved at a preset time on June 15, for which all geometric parameters of the filters were calculated; on July 15, August 15, and September 15, the light transmission consistently increased, indicating good seasonal adaptation. Conclusions regarding the selection and optimization of the strip widths of the gratings, as well as the processing of experimental data, were obtained.

In this work, an optical 8-amplitude phase shift keying (8APSK) modulator has been designed and successfully simulated using three silicon microring modulators (MRMs). The first two MRMs jointly generate a QPSK signal, while the third MRM is connected in an add-drop configuration to synthesize the final 8APSK signal. The MRMs have a junction volume of 4.5×10−12 cm−3 with carrier recombination lifetime of 100×10−12 seconds. We have obtained the simulated constellation plots at various signal-to-noise ratios and achieved a bit error rate of 10−3 at around 10.7 dB optical signal to noise ratio.

Stimulated Raman scattering (SRS) must be suppressed owing to its adverse effect on high-power fiber lasers. We propose a bending-sensitive multi-ring fiber composed of three parts: a core, multiple high-refractive-index rings, and a cladding. Appropriate bending of the fiber produces a high loss of the first-order Raman Stokes wavelength at 1114 nm and low loss of the signal wavelength at 1064 nm, thus enabling the signal light to be confined to the core while maintaining single-mode transmission. This principle relies on the resonant coupling between the core and ring modes. Numerical analysis indicated that the loss ratio of the fundamental mode could reach 2650.041 by optimizing the structural parameters. Moreover, when the three-ring fiber was subjected to a bending radius of 6 cm, a bend-induced loss of the Raman wavelength reached 11.315 dB/m, which effectively suppressed the first Stokes SRS generation.

This paper proposes a trace hydrogen sensing system based on distributed feedback fiber laser (DFB-FL). The sensor head was fabricated by inserting the DFB-FL into a glass capillary coated with hydrogen sensitive material. The wavelength of the DFB-FL changed under the exothermic reaction of Pt-WO3 with hydrogen and was demodulated by a Michelson interferometer. The experimental results at hydrogen concentration of 50 to 3800 ppm show that the hydrogen sensitivity of the system is 0.0386 pm/ppm and the linearity index R2 is >0.99. The repeatability of the system is 99.4% and the accuracy is 97.43% at hydrogen concentration of 50 ppm. The figure of merit of the system is 0.0385. The proposed DFB-FL sensor with good repeatability and anti-electronic interference has application prospects in many fields such as the detection of trace hydrogen in transformer oil.

A highly sensitive, reversible, and linear sensor, exhibiting excellent stability in response to temperature and humidity, has been successfully proposed and demonstrated for the first time. This sensor is achieved by wrapping a polyvinyl alcohol/graphene nanofiber film onto a chiral long-period fiber grating (CLPG), which is fabricated by periodically twisting single mode fiber. In the experiment, the CLPG sensor demonstrates a temperature sensitivity of 74 pm/°C, which is approximately twice as high as that of conventional fiber grating sensors. Note that, by wrapping the graphene nanofiber film on CLPG, the temperature sensitivity of the sensor is up to 115.23 pm/°C in the range of 30°C to 75°C. In addition, CLPG using for humidity sensing is first demonstrated. The humidity sensitivity measures −9.92 pm/%RH with linearity of 0.995 during a change from 40%RH to 80%RH. In comparison to other humidity sensors, the sensitivity of the CLPG is comparable, whereas its sensing linearity stands out notably above the rest. The results show that CLPG has the characteristics of simple fabrication, easy combination with materials, stable performance, and high sensitivity and holds significant development potential in optical fiber sensing application fields.

We propose and experimentally demonstrate a high-power eight-wavelength distributed feedback (DFB) laser array with 100 GHz spacing using the grating reflector (GR). The GR, which is a uniform sampled grating, can provide light reflection instead of high reflection (HR) facet coating. Thus, the front and rear facets are both coated with anti-reflection coatings (AR/AR) to avoid the influence of a random grating phase at the HR facet. Therefore, more output power, better single-longitudinal-mode operation, and higher wavelength precision can be simultaneously achieved, compared with traditional DFB lasers with HR/AR facet coatings. The maximum output power and side mode suppression ratio of the fabricated eight-channel GR-DFB laser array as high as 70 mW and 55 dB can be achieved, which may benefit wavelength division multiplexing transmission systems, gas sensors, optical computing systems, and microwave photonic signal processing.

Magnetic fluid is a good magneto-optical material. In this paper, a kind of side-etch photonic crystal long-period grating based on magnetic fluid filling is proposed. Simultaneous detection of magnetic field strength and temperature through integrated surface plasma resonance (SPR) and long period grating (LPG) sensing. The refractive index of magnetic fluid changes with the influence of magnetic field intensity and temperature. Based on the above sensing mechanism, the test results show that the magnetic field intensity sensitivity of the LPG sensor is −35.91 pm/GS, and the temperature sensitivity is 51.4 pm/°C. The temperature sensitivity of the SPR sensor is −568 pm/°C, which accurately compensates for temperature crosstalk. The calculated error rate is E(ΔB)=0.124%; E(ΔT)=0.308%. The sensor structure provides a good application prospect for precise magnetic field intensity measurements.

A method for reducing the coherent Rayleigh noise (CRN) in a Rayleigh and Brillouin (RB) self-heterodyne detection Brillouin optical time domain reflectometer (BOTDR) system using phase shift keying (PSK) pulse encoding is proposed and verified experimentally. A phase modulator and an electro-optic modulator driven by the same arbitrary function generator perform PSK pulse encoding modulation on the signal, and the traces obtained by PSK encoded pulses with different encoding patterns are superposed and averaged to reduce the CRN. The experimental results show that, in the case of 10,000 times average to eliminate other random noise effects, after using PSK pulse encoding, the amplitude fluctuation and signal-to-noise ratio at the fiber end of the RB self-heterodyne detection BOTDR sensing system are reduced by 1.12 dB and improved by 3.01 dB, respectively. In addition, the measurement accuracy and measurement stability of the Brillouin frequency shift and temperature are effectively improved. The proposed method offers a robust solution for reducing the CRN in the RB self-heterodyne detection BOTDR sensing system, facilitating high-precision and long-distance sensing capabilities.

A dual-wavelength switchable perfect absorber, comprised of a continuous Au film, an alumina (Al2O3) spacer, an indium tin oxide (ITO) layer, double-layer Dysprosium-doped cadmium oxide (CdO:Dy) films, and a gold ring array from bottom to top, is numerically designed in this paper. The epsilon-near-zero (ENZ) properties are determined by the carrier concentration of these ENZ materials. As for ITO material, the carrier (electron) concentration can be electrically modified by applying a biasing voltage V. And different growth conditions afford significant variation of carrier concentration in CdO:Dy layers. Via changing the biasing voltage V, we can achieve broadband and multifrequency absorption in our infrared absorber. Especially, the proposed infrared absorber demonstrates excellent electrical regulation performance, enabling bidirectional switching of “ON” and “OFF” states at dual-wavelength. We also further reveal the absorption mechanism by establishing quasi-Fabry–Pérot cavity resonance model. In addition, it is shown that the infrared absorber can tolerate a wide range of incident angles as well as has polarization insensitive features by verification. This device has great potential in numerous optoelectronic applications, such as invisibility cloaking, sub-diffraction imaging, and thermal emission.

We explore the characteristics and laser-damage behavior of gallium-based liquid metal alloy mirrors under exposure to ns laser pulses. One of the key advantages of using liquid metal mirrors is the self-healing potential following perturbations arising from exposure to high-power laser pulses. In this work, key performance metrics, such as reflectivity and the laser-damage initiation mechanism and initiation threshold, were investigated using fused-silica cells filled with three different liquid metal Ga alloys. The results suggest that irreversible modification (damage) under 355-nm, 6-ns pulses are associated with the formation of gallium oxide, taking place at a fluence significantly higher than that for damage initiation in conventional metal mirrors. This behavior is believed to arise from the different damage initiation mechanism in liquid metal mirrors requiring increased laser absorbed energy for an irreversible modification of the material surface to occur. This exploratory work is the first of its kind and highlights some favorable performance characteristics of gallium-alloy metal mirrors.

We propose a hybrid plasmonic zeolitic imidazolate framework-8 (ZIF-8) nanostructure to achieve near-infrared dual-band sensing of trace amounts of volatile organic compound (VOC) gases via metal-organic framework patch array integrated on metal (Au) film. To achieve dual-band VOC gas sensing, two absorption peaks related to the surface lattice resonance (SLR) and surface plasmon polaritons (SPP) resonance are constructed in this plasmonic structure, to match the near-infrared absorption bands of methane and ethanol gas molecules respectively. For an 80 nm thick Au layer, the height and width of ZIF-8 patch and the gap between ZIF-8 patches are optimized to be 0.86, 0.925, and 0.43 μm. Then, this structure shows an SLR at 1392.64 and a SPP resonance at 1653.72 nm, which have peak intensities of 0.735 and 0.855, and the Q-values of 1435 and 253, respectively. Furthermore, the third resonance peak related to the Fano resonance is investigated and can be considered as a feasible alternative to calibrate the measurement without external references.

This work proposes a two-dimensional porous lithium niobate grating-coupled birefringent Bloch surface wave (BSW) sensor. Based on the nonlinear characteristics and birefringence of lithium niobate, the theoretical study investigates the impact of two refractive index excitation schemes, ne and no, on the tunability of the BSW. First, we construct a four-period TiO2/MgF2 stacked distributed Bragg reflector (DBR) and deposit a layer of two-dimensional porous lithium niobate grating on the top of the DBR to achieve wave vector coupling. This successfully excites the BSW at the interface of the sensor and the sensing medium. Second, by rotating the incident light, we switch between the ne and no excitation schemes of lithium niobate, introducing the concept of azimuthal detection to explore the tunability of lithium niobate BSW sensors. The results show that the azimuthal angle variation of BSW resonance peaks for both refractive index excitation schemes is approximately 5 deg. Finally, we perform numerical analysis on the sensor performance for both excitation schemes. The results demonstrate that the sensitivity and quality factor of BSW sensors excited along ne and no refractive indices reach 3782 °/RIU (12400 /RIU) and 2967 °/RIU (6682.5 /RIU), respectively. This confirms the great potential of lithium niobate as a nonlinear tunable material in the field of BSW sensors.

With the excitation of a 978 nm laser, the upconversion (UC) emissions of the YVO4: Er3+, Yb3+, Cr3+ nanoparticles include three emission bands located at 524, 552 and 658 nm, respectively. The UC emission characterization confirms that the UC emission of the YVO4: Er3+, Yb3+ nanoparticles can be modulated by the doping of the Cr3+ ions, the increase of the incident laser power and the change of the laser pulse-width. With the doping of the Cr3+ ions, the two green emissions decrease because of the energy transfer from the Er3+ states H211/2 and S3/24 to the Cr3+ ions. With the increase of the incident laser power, the two green emissions increase due to the decrease of the transfer rate of the energy transfer path to the state F9/24 of the Er3+ ions. With the increase of the incident laser pulse-width from 0.2 to 6 ms the intensity ratio of the emission at 524 nm to the emission at 658 nm increases from 0.063 to 0.19, which is due to the number accumulation at the state H211/2 due to the small transfer rate from the Er3+ state H211/2 to the Cr3+ ions. The experimental results confirm that the concentration of the Cr3+ ions has less effect on the average life-time of the UC emissions at 524, 552, and 658 nm.

The pressure sensor is an essential device for pressure measurement and safety evaluation in modern industrial production and engineering applications. A fiber Bragg grating (FBG) pressure sensor is proposed. The FBG pressure sensor is designed based on a Bourdon tube and cantilever beam. Two FBGs are fixed on both sides of the elastomer with a sensitization effect, and the elastomer and the Bourdon tube are connected by spring. The sensing principle of the FBG pressure sensor is introduced in detail, the mathematical model of the sensor is established, and the performance of the sensor is studied. The experimental results show that the average sensitivity is 5.578 nm/MPa in the range of 0 to 0.4 MPa. The maximum error is 1.3% in the temperature range of −10°C to 70°C, and the temperature compensation performance is good. The remote transmission performance of the sensor is tested. The transmission loss of the sensor is about 0.433 dBm/km when the fiber cable length is 8.45 km. The field application is carried out in a pipeline experimental site, which verifies the practicability of the sensor.

We designed a CFRP-FBG sensor with a “sandwich” structure consisting of carbon fiber reinforced polymer (CFRP) encapsulation layers and a fiber Bragg grating (FBG) for strain monitoring of wind turbine blades. This design aims to protect the FBG sensor while maximizing the strain transfer from the CFRP to the FBG sensor. Strain and temperature experiments showed that the linear correlation coefficient of the experimental CFRP-FBG sensor’s central wavelength change-strain evolution curve was above 0.9999, and the repeatability error of the strain sensitivity coefficient was 0.1665%, demonstrating high accuracy and repeatability. Meanwhile, CFRP-FBG was tested in full-size turbine blades in operation, during which time they were subjected to millions of dynamic strain cycles and various loading environments. The results of 11 months of monitoring showed that the CFRP-FBG provides real-time, reliable, and highly accurate strain measurements and can meet the needs of long-term blade strain monitoring.

In a serial wavelength division multiplexing (WDM) fiber Bragg grating (FBG) sensor network, it is well known that there are challenges in separating overlapping signals, which require high precision and low delays. And using an optical spectrum analyzer as a data source result in demodulation models that are impractical for use in engineering applications. Therefore, an overlapping spectral demodulation model based on transfer learning using a charge-coupled device (CCD) interrogator and light gated recurrent unit (Li-GRU) neural networks is proposed. This model can achieve a low signal demodulation error, even when applied to data collected using a CCD interrogator with low spectral resolution and a high signal-to-noise ratio. We describe the operation principle of the Li-GRU neural network and discuss the impact of transfer learning and CNN feature extraction layers on demodulation performance. The experimental results show that lowest root mean square error of our proposed model is 1.93 pm, and the single inference time of the model on the CPU is <45 ms. This serial WDM fiber grating demodulation method can be effectively applied in temperature and strain measurement demodulation.

Using M-ary quadrature amplitude modulation (M-QAM), the bit error rate (BER) performance of an underwater optical wireless communication (UOWC) system is examined in this research for a variety of water conditions, including a pure sea, clear ocean, coastal water, and a turbid harbor. Attenuation loss, geometrical loss in the case of a semi-collimated laser source, and turbulence, which affect underwater communication, have been taken into consideration for the first time. Taking into account geometrical and attenuation losses, the average BER for the newly proposed underwater channel was derived and expressed in closed form using the lognormal turbulence model. The effect of this underwater channel on coverage and reliability has been illustrated using simulations in MATLAB. With receiver diversity, efforts to enhance both range and reliability have been formulated and evaluated. Moreover, our results show that receiver diversity can improve performance defined in terms of reliability and range especially during weak turbulence, a 1×2 single-input multiple-output transmission on a 30 m pure sea water link with a log-amplitude variance of 0.9, for instance, can result in a 5 m improvement at a BER of 10−4. The proposed system will aid the defence industry especially inter-submarines communication.

A high-resolution optical vector analysis (OVA) based on optical single sideband (OSSB) modulation using a dual port Mach–Zehnder modulator (DP-MZM) is proposed and demonstrated. By using the DP-MZM, an OSSB modulation signal is generated. The generated OSSB signal is propagating through the device under test (DUT). After the optical-to-electrical conversion in photodetector (PD), a photocurrent carrying the frequency responses of the DUT is obtained. After signal processing, the photocurrent corresponding to the frequency component is extracted, and the frequency responses of DUT are measured with high-resolution in simulation. To eliminate the frequency response of the system, a calibration procedure is performed by removing the DUT. Compared to the traditional methods, the proposed OVA can be used to measure the frequency responses of high-quality optical devices with MHz-resolution over a large measurement range. By using the DP-MZM, the stability is improved, and the complexity is reduced. The proposed method is theoretically analyzed and verified by simulation. A simulation for measuring the frequency responses of the ideal phase-shifted fiber Bragg grating and the ideal Fabry–Perot cavity is carried out, the frequency responses in a range of 50 GHz are measured with a resolution of 1 MHz. The proposed method can be potentially applied in the field of characterization of integrated optoelectronic devices and future photonic systems.

A simple photonic approach to generating and anti-dispersion transmitting the quaternary phase-coded microwave waveforms is proposed and experimentally demonstrated. In this approach, an integrated dual-polarization binary phase shift keying (DP-BPSK) modulator is adopted and driven by two parallel pre-encoding signals as well as two RF inputs with 90-deg phase shift. By simply presetting the bias voltages on the DP-BPSK modulator, the phase-coded microwave signals with desired quadrature phase shifts and the immunity to dispersion-induced power fading can be obtained. A complete theoretical analysis on operation principle is presented. Experiments on the generation and anti-dispersion transmission of the 1 Gb/s quaternary phase-coded waveforms with the center frequencies of 6 or 7 GHz over 10 km SMF are successfully carried out. The proposed system can achieve the waveforms with multiple phase coded formats and transmit them over long-distance optical fiber without dispersion-induced power fading. Besides, it also features simple architecture, excellent reconfigurability, better Doppler tolerance and sensitivity of objective recognition, which is highly desirable in modern radar networks based on optical fiber transmission.

We propose and demonstrate an intrachannel fiber nonlinear impairments compensation scheme based on the transformer and simplified triplets. In comprehensive numerical experiments with a transmission distance of 1200 km for 30-GBaud dual-polarization 16-ary quadrature amplitude modulation signals, the Q factor gains of the proposed scheme are 2.48, 1.73, 0.98, and 0.31 dB compared to chromatic dispersion compensation, digital back propagation using 1-steps-per-span split-step Fourier method (1-StSp DBP), deep neural network (DNN), and 10-StSp DBP, respectively. The implementation complexity of the proposed scheme is reduced by 65% and 43% compared to 10-StSp DBP and DNN, respectively.

Our work is devoted to the analysis of a power spectral density (PSD) of amplitude and phase fluctuations of the laser and the laser optoelectronic oscillator (LOEO) with direct modulation. Using the control operator equation of the laser, the amplitude and phase fluctuations of the laser covered by positive optoelectronic feedback were analyzed for the two-level model. Systems of differential stochastic equations of laser and LOEO are compiled, and analytical dependencies of spectral power density of amplitude and phase fluctuations in approximation of small phase noise are determined. The obtained design relationships were compared with experimental PSD of amplitude and phase fluctuations of the laser and LOEO. Shown to achieve in LOEO, the PSD of phase fluctuations less than −140 dB/Hz at the F=10 kHz offset 10 GHz radio frequency (RF) oscillation, it is necessary to use highly coherent laser PSD phase fluctuations less than −100 dB/Hz at the F=10 kHz offset and achieve noise reduction by synchronizing the longitudinal laser modes by directly modulating the laser with a harmonic RF signal of LOEO generation. Our results make it possible to construct the theory of LOEO and the coupled LOEO based on a system of operator differential equations of a laser with fluctuations covered by a positive feedback circuit on the variable component of the pumping current. The content used in this document is only for preview purposes.

The error performance of relay-aided underwater wireless optical communication (UWOC) with wavelength diversity is investigated by considering the compound effects of absorption, scattering, and ocean turbulence. The outage probability and average bit error rate (ABER) of the serial-relay UWOC system based on wavelength diversity are derived. The effects of different relay hops, wavelength diversity orders, and anisotropy factors on the system performance of the signal-combining methods are analyzed in detail. It is observed that relay-aided technology can increase the distance of the communication link and effectively alleviate the limitations of underwater wireless optical fading. The results also show that the wavelength diversity system can help to reduce the scintillation effect of ocean turbulence on the received light, thereby reducing the ABER and outage probability of the system. This study is expected to contribute to the development of more robust UWOC systems operating in turbulent oceans.