The deflection that can reflect the vertical stiffness of a bridge plays an important role in the structural evaluation and health monitoring of bridges. In the past 20 years, the bridge deflection measurement methods based on computer vision and photogrammetry have been gradually applied to the field measurement due to the advantages of noncontact measurement, simple experimental setup, and easy installation. The technical research progress of vision-based bridge deflection measurement is reported from four aspects: basic principles, measurement methods, influencing factors, and applications. Basic principles mainly include camera calibration, three-dimensional (3D) stereo vision, photogrammetry, feature detection, and matching. For measurement methods, the single-camera two-dimensional measurement, the dual-camera 3D measurement, the quasistatic measurement based on photogrammetry, the multipoint dynamic measurement based on the displacement-relay videometrics and the deflection measurement based on UAV platform are introduced, respectively. In the section of influencing factors, this part summarizes the work of many researchers on the effects of camera imaging factors, calibration factors, algorithm factors, and environmental factors on measurement results. The field measurement results at different measurement distances and measurement accuracy based on these are presented in terms of applications. Finally, the future development trends of vision-based bridge deflection measurement are expected.

We review the physics underlying thermal infrared emission, focusing on the processes that generate polarization. We also discuss a number of features of the theory that are not widely discussed in the literature, including the Straubel theorem for absorbing media, the mixed Poynting vector, and the emission of nonisothermal media. Using a model for emission from nonisothermal media, we present a quantitative discussion of the conditions under which Kirchhoff’s law is valid. Using this model, we show how it is possible for many blackbody simulators and infrared camera shutters to produce emission very close to true blackbodies.

Guest editors Vitaly Gruzdev and Jonathan Arenberg introduce the Special Section on Laser Damage VI.

We propose a figure of merit that characterizes the femtosecond laser damage behavior of optical coatings. This figure of merit, based on the complete spatiotemporal evolution of the field in a multilayer system, can be included in optics design. The monochromatic intensity enhancement widely used in “electric field-engineering” is sufficient only in certain structures such as high-reflectivity quarter-wave mirrors. In more complex systems, for example, in group delay dispersion mirrors and frequency tripling mirrors, one should consider the actual (typically smaller) intensity enhancement produced by short pulses and the change (typically increase) of pulse duration within the stack.

Dielectric sesquioxide films (Sc₂O₃, Y₂O₃, and Lu₂O₃) were fabricated by pulsed-laser deposition and tested in terms of their laser damage properties for pulses of 500 fs duration, at a wavelength of 1030 nm and at a 10 Hz repetition rate. Comparable tests were performed with magnetron-sputtered thin films of established optical-coating materials (SiO₂, HfO₂, and Nb₂O₅), whose results served as a benchmark. The laser-induced damage thresholds of the sesquioxides are comparable to each other, and in the multi-pulse test regime show values close to ones of HfO₂ coatings. A lower damage threshold was observed for the polycrystalline Lu₂O₃ film grown on sapphire compared to single-crystal Lu₂O₃ grown on yttrium aluminium garnet (Y₃Al₅O₁₂), attributed to the highly textured morphology and potential for a greater density of defect states in these films. We conclude that pulsed-laser deposition is a potential fabrication method of sesquioxides for use in high-power resistant optical components for ultrashort-pulse lasers.

Deterministic finishing methods of optical components for high-peak-power laser applications that can meet the requirements for high laser damage resistance are not sufficiently developed to meet all needs. This is especially in the case for ultraviolet (UV) laser applications. Fused silica is the material of choice for optics operating at UV wavelengths owing to its intrinsically large bandgap, high transparency, and excellent uniformity. Here, we report on the laser damage behavior of fused silica surfaces finished by fluid jet polishing (FJP) as a function of removal depth. Fused silica test substrates were processed by FJP to depths ranging from 0.7 to 18  μm. Laser damage testing was conducted on these surfaces at 351 nm and 1-ns pulse lengths for both, 1-on-1 and R-on-1 testing protocols. The results for 1-on-1 testing showed no degradation in the laser-induced damage threshold (LIDT) of the substrates. Instead, a gradual improvement starting at a depth of 2.1  μm was observed and continued to the 18  μm surface. At 18  μm of removal, the LIDT was 16% higher than a surface that was not finished by FJP. For R-on-1 testing, all surfaces treated by FJP demonstrated an improvement in laser damage resistance. At depths greater than 5  μm, the improvements were significantly more pronounced and a 30% increase in the LIDT was realized.

It is presently well understood that the operational performance limits of optics are determined by three fundamental attributes: the initiation of laser-induced damage, the growth of damage sites, and the transient (nondamaging) modification of optical parameters. The comprehensive characterization of the performance limitations of ultrafast optics requires consideration of all three fundamental attributes. The vast majority of the literature to date, however, has focused primarily on damage-initiation and testing systems that are largely focused on determining the damage-initiation threshold under single- and multipulse excitation. We discuss a testing apparatus that was designed to offer the capability to adequately characterize all three of these performances attributes under femtosecond, near-infrared laser irradiation. Key aspects of the methodology are discussed; these include high-dynamic-range energy control, variable beam size, wavelength tunability, B-integral management, and functional performance characterization to explore the true operational limits of the components. Example results for a metal-dielectric mirror demonstrate the test station’s operation.

The laser damage threshold is a term and figure of merit used ubiquitously in our field. The concept of a damage threshold is deep seated, with origins dating to 1960s. The laser damage threshold has been used as a universal figure of merit for damage resistance, safe operation, and the basis of years of efforts to develop a standard damage test. To address the question, “Is the damage threshold a good and universal figure of merit for characterizing laser damage resistance?” this paper assesses the invariant of the weakest site, the threshold, in a spatial domain of an area A. It is shown that the weakest site in an area is not invariant. The implications of these results for the continued development of damage standards are discussed.

Experimental research is performed on a phase noise compensation (PNC) method to improve the signal-to-noise ratio of a frequency modulated continuous wave (FMCW) ladar under long-distance-detection and atmospheric propagation conditions. In the PNC method, a local self-coherent reference is generated where phase noise of the laser source is recorded. Then the phase noise of long-distance-target echo can be compensated by mean of the recorded source phase noise. The laser source of the ladar is chirped modulated from 1550-nm seed laser, whose bandwidth is 10 GHz. In the experiments, the target for test is an optical corner cube retro-reflector that placed over 10 km away from the ladar. The local self-coherent reference, also refer as auxiliary reference, is the heterodyne signal between the chirped laser source and the delay of itself. The delay path is a 5-km soundproofed fiber optic ring. A 2D data processing technique is proposed for ranging with the concatenately generated phase method applied to implement PNC. To verify the proposed technique under long-distance atmospheric propagation conditions, experiments are performed with the distances of the target as 12 and 19.5 km respectively, and the spectral linewidth of the seed laser as 100 Hz and 3 kHz respectively. By comparing the experimental results, the proposed technique is effective in compensating FMCW laser phase noise, even when the target is further and the phase noise of seed laser is worse.

Digital refocusing is a key feature of Fourier ptychographic microscopy (FPM). It is currently performed by determining and removing the defocus aberration during the iterative phase retrieval process. We examine the feasibility of digitally refocusing an FPM image by numerically propagating the recovered complex FPM image after the phase retrieval process has been completed – in effect, disentangling the defocus correction process from the iterative phase retrieval process. If feasible, this type of postreconstruction digital refocusing can significantly reduce the FPM computational load and provide a quick and efficient way for refocusing microscopy images on the fly. We report that such an approach is infeasible for large defocus distances because the raw FPM dataset associated with a defocused sample is illconditioned for the FPM’s phase-retrieval process, and it will not output a complex-valued image that corresponds to any physically relevant image wavefront. When the defocus distance is small, the FPM can output an approximately correct image wavefront. However, this wavefront does not contain a global defocus phase term and, therefore, cannot be further focused using the digital refocusing application of a reverse global phase term. In totality, this means that postreconstruction digital refocusing does not serve a meaningful function for any defocus distance. To verify our analysis, we performed a series of experiments, and the results showed that the postreconstruction digital refocusing method is not a viable digital refocusing method.

A two-photon excitation fluorescence (TPEF) microscopy system under broadband excitation of femtosecond pulses is built in this study. The TPEF spectrum of an eosin stain is recorded, and the TPEF microscopy system is equipped with a suitable detection window matching the TPEF spectrum of the eosin stain. TPEF microscopy is demonstrated using a solid eosin sample and eosin-stained mouse brain sections through detecting the TPEF signal from eosin staining. Three-dimensional (3D) reconstruction of the mouse brain section across its thickness is realized with several TPEF imaging frames acquired via z-axis scanning with a step size of 1 μm. Two-dimensional TPEF imaging frames and 3D reconstruction of the brain section demonstrated high performance of TPEF microscopy in the observation of eosin-stained biological sections.

We propose a theoretical analysis method for the denoising capability of Hadamard transform spectrometer (HTS). We establish the equivalent relationship between the HTS’s signal-to-noise ratio (SNR) and the coding matrix generalized Rayleigh quotient. Then, based on noise source during spectral detection, we analyze the changes of SNR with different types of spectrometers when detector noise or photon noise is dominant. The theory shows that when detector noise is dominant, the SNR of HTS with H-matrix coding is about 3.01 dB higher than S-matrix coding, and the SNR of HTS with S-matrix coding is about 10 log₁₀ ( n + 1 / 2 ) dB (n is the coding matrix order) higher than the slit-based spectrometer. When photon noise is dominant, the SNR of slit-based spectrometer and the SNR of HTS with H-matrix coding are better than S-matrix coding by about 1.51 dB. The theory is testified by computer simulations and a self-built HTS system.

In practice, the wrapped phase in interferometry is often affected by noise and discontinuity, and among the various types of phase unwrapping (PU) method, the weighted least-square (WLS) PU algorithm, as a global strategy, is widely utilized. However, the excessive smoothing effect exists within the process of PU. Therefore, it is necessary to conduct a comprehensively analysis of different WLS PU algorithms, which includes noise resistance, discontinuity characteristics, convergence speed, and accuracy. First, different weighting strategies were compared with detail for the WLS approach. Under slight noise condition, the edge detection map (EDM) and the filtering method both obtained relatively accurate and reliable phase information. However, when it comes to global multiplicative noise such as speckle noise, the filtering method showed better anti-noise performance than the EDM method, whereas EDM was more capable of dealing with discontinuous phase. Second, to improve the iterative convergence speed and accuracy, the initial value selection was analyzed in detail, and a new initial value selection method was proposed. Simulation and experiments were carried out and validated the results of the analysis.

In practical vision systems, due to the limitation of manufacturing technology, charge-coupled device pixel cells are difficult to satisfy the standard square, and the lens inevitably has imaging nonlinearity, which leads to geometric distortion of the image. To ensure the accuracy of subsequent visual processing, it is necessary to correct the image distortion. A non-metric correction method for complete lens distortion using dual-characteristics joint measurement is proposed. We not only considered the influence of the distortion center in the correction process but also constructed a mapping matrix to standardize the pixel cells, thus establishing a complete lens distortion model. Simultaneously, the linear residual function of the dual-characteristics joint measurement was defined according to the geometric constraint between the vanishing point and the straightness. In addition, the distortion model coefficients were determined by nonlinear iterative optimization. Extensive experiment results demonstrated that the proposed method enables effective and accurate lens distortion correction, which is significantly superior to the existing single characteristic measurement distortion correction approach.

A practical tapered optical fiber (TOF) biosensing system was developed for label-free detection using antigen-antibody pairs with repeatable results and a very high degree of sensitivity. This was done by attaching molecular recognition agents to a tapered fiber surface for augmenting sensitivity and specificity of analyte. The entire system included three main parts: a tunable laser, a tapered fiber, and an optical detector. Light from an unpolarized tunable fiber laser was introduced into the tapered fiber from one end, and the transmitted intensity was detected by a photodetector. In the tapered fiber area, the evanescent electromagnetic field, which extends outside the fiber, was able to detect minute changes in the refractive index caused by antigen-antibody pairs. Recorded data was analyzed using an innovative Fourier analysis method to find phase changes, which are directly related to the biomolecular concentration coated on fiber, from which antibody-antigen concentrations are obtained. Two experiments were performed to confirm the concept using two very different agents. The first was the protein Interleukin-8 (IL-8). Repeatable results with a sensitivity of 10 pg/mL were achieved. The second was human coronavirus OC43 (HCoV-OC43), a surrogate viral particle for SARS-CoV-2, with a sensitivity of 50 viruses/mL. Critical sources of error were identified and addressed for the purpose of using the device for real clinical diagnosis in various real-life environments, where viruses can reside in water, phosphate-buffer solution, or saliva, the most popular three environments in real clinical diagnosis. Our device was designed according to the principle that only one specific kind of antibody and antigen can be combined together. The device demonstrated good accuracy to chosen analyte(s) tailored to specific applications and offered the potential to develop a point-of-care device used in clinics, as well as for detecting a variety of viruses and biocontaminants. The reproducibility of TOFs was confirmed through multiple fabrications and consistent results.

Dynamic vision sensors (DVS) represent a promising new technology, offering low power consumption, sparse output, high temporal resolution, and wide dynamic range. These features make DVS attractive for new research areas including scientific and space-based applications; however, more precise understanding of how sensor input maps to output under real-world constraints is needed. Often, metrics used to characterize DVS report baseline performance by measuring observable limits but fail to characterize the physical processes at the root of those limits. To address this limitation, we describe step-by-step procedures to measure three important performance parameters: (1) temporal contrast threshold, (2) cutoff frequency, and (3) refractory period. Each procedure draws inspiration from previous work, but links measurements sequentially to infer physical phenomena at the root of measured behavior. Results are reported over a range of brightness levels and user-defined biases. The threshold measurement technique is validated with test-pixel node voltages, and a first-order low-pass approximation of photoreceptor response is shown to predict event cutoff temporal frequency to within 9% accuracy. The proposed method generates lab-measured parameters compatible with the event camera simulator v2e, allowing more accurate generation of synthetic datasets for innovative applications.

Additive manufacturing (AM) holds great development potential in several applications. In particular, the fabrication of metal parts has been rapidly developing as an industry recently. In such applications, a powder feeder with low material loss is required to ensure effective material usage. Thus, the manufacturing process using laser-induced forward transfer (LIFT) is attractive as a candidate for advances in powder feeder technology. We show that stainless steel particle with diameters of 26 to 53 μm can be transferred via the particle LIFT process; the transfer of particles started at a peak fluence of ∼0.05 J cm ^{− 2} (26- and 36-μm diameters) and ∼0.15 J cm ^{− 2} (53-μm diameter), and the particles were fully transferred (i.e., the probability of transfer reaches unity) at a peak fluence of 0.2 (26 μm), 0.4 (39 μm), and 0.5 J cm ^{− 2} (53 μm). The initial velocity, v₀, at which the particle leaves the substrate increased proportionally with the increase in peak fluence. When the point of irradiation of the laser beam was displaced from the contact point of the particle on the substrate, the particle was transferred with a transfer angle (i.e., not vertically). These results demonstrate the possibility of transferring metal particles via the LIFT process. To use this technology for AM, the laser power should be adjusted to control the transfer angle of the particle.

We present a low-cost online system for monitoring the axial thermal displacement of machine tools. The proposed monitoring system includes an embedded optical sensor derived from a laser mouse; an image acquisition microcontroller; speckle patterns; and an edge computer that hosts software, including an image display module, a displacement calculation module, an image enhancement module, and a data visualization module. The proposed sensing system can measure the displacement in two orthogonal directions simultaneously by employing digital image correlation; thus, the proposed system is a two-dimensional displacement sensor. The sensing system is benefited from image enhancement techniques and customized optimal speckle patterns printed using a standard low-cost monochrome laser printer. Experimental results indicate that the proposed displacement sensing system has an accuracy and a precision of <5 μm in both orthogonal directions. The two-dimensional displacement sensing system was used for the online monitoring of the thermal deformation of a feed drive system for machine tools, and the performance was assessed experimentally. The complete procedure for measuring the displacement due to thermal deformation was conducted automatically and can be completed in 2 min using a program coded in a computer numerical control controller.

For achieving perfect absorption (PA), the absorbing film usually has either high loss or a thickness much greater than its wavelength. We propose a simple bilayer structure composed of a perfect electric conductor substrate and an indium tin oxide (ITO) film to solve the situation. Results show that PA can be realized with a film thickness of several nanometers based on the epsilon-near-zero (ENZ) response and relatively low loss of ITO film. Notably, PA at different thicknesses of the film can be achieved by simply selecting the appropriate incident angle, and it is further demonstrated that the incident angle should be reduced linearly with an increase of the thickness of the film to maintain PA. Furthermore, the ENZ wavelength can be flexibly controlled by changing the carrier density via electrostatic field, resulting in the tunability of PA. These adjustable simple and tiny perfect absorbers may find practical applications in photodetectors and sensors.

A hollow-core antiresonant fiber (HC-ARF) using nested hybrid silica/silicon cladding is proposed for single-polarization single-mode (SPSM) and broadband. The HC-ARF design consists of four cladding tubes with nested tubes, and a cladding tube with an inner surface coated with silicon. Single-polarization guidance is realized by adjusting the thickness of the silicon layer to control the coupling between the y-polarization and cladding mode, and by optimizing the diameter of the nested tube, the high-order modes (HOMs) can be significantly suppressed. The simulation results show that the performance of SPSM can be improved by selecting appropriate structural parameters. The polarization extinction ratio (PER) is up to 139,804 and the y-polarization loss is 133  dB  /  m, the x-polarization loss is as low as 0.00095  dB  /  m at 1550 nm. Moreover, high-order mode extinction ratio (HOMER) between the HOMs and x-polarization, as high as 40,360, is obtained in proposed fiber with HOMs loss of 32.38  dB  /  m. Particularly, the proposed fiber also has a bandwidth of 230 nm (1450 to 1680 nm) with x-polarization mode loss <1 and 0.08  dB  /  m bending loss when the fiber is bent in the y-direction at a bending radius of 5 cm. The proposed fiber has potential applications in short-distance, polarization-sensitive fiber systems.

We present a cavity compensation technique based on a tunable long-wave infrared zinc germanium phosphide ZnGeP₂ (ZGP) optical parametric oscillator (OPO). The slope efficiency increased from 3.6% to 7.0% at 9.15  μm after using the cavity compensation technique in the tuning range of 8.02 to 9.15  μm. We used a Q-switched 2.1  μm Ho:YAG laser pump source with a pulse repetition frequency of 10 kHz. Under the incident pump power of 23.03 W, the maximum average idler output power of 2.16 W at 8.02  μm was achieved with a pulse width of 21.5 ns and spectral bandwidth of 29.5 nm; the slope efficiency was 16.5%. To the best of our knowledge, this is the first application of the cavity compensation technique in tunable long-wave infrared ZGP OPO.

In a free-space optical communication system with an avalanche photodiode (APD) detector, the average bit error rate (ABER) is studied based on pulse position modulation (PPM) considering Gamma–Gamma atmospheric turbulence and fiber coupling efficiency (FCE). By analyzing the shot noise and thermal noise, the approximate analytical expression of the ABER of binary PPM is theoretically derived. Then, the approximate analytical expression of ABER union bound is derived for L-ary PPM. The FCE has a greater impact on the ABER of the APD detection system than it does on the PIN detection system. By adopting the adaptive optics technology, compared with the PIN detection system, the communication performance of the APD detection is greatly improved. Moreover, the optimal average APD gain of the APD detection system is strongly correlated to the detector temperature but weakly dependent on the atmospheric turbulence intensity, FCE, number of wavefront compensation terms, average received photon number, and receiving aperture size. By optimizing the receiving aperture and designing an accurate temperature control system, the communication performance of the APD detection system can be further improved.

A bidirectional high-speed chaotic optical communication scheme with physical layer encryption is proposed and studied theoretically. The external cavity optical feedback method is analyzed. An erbium-doped fiber amplifier is used to represent the all-optical delay feedback loop of the traditional external cavity; it can overcome the shortcomings of the traditional method such as lower precision and high equipment volume requirements. The bidirectional communication scheme is discussed; it sets the encryption device at the transmitter, uses the correlation between the two encryption signals to decrypt, and cancels the decryption device at the receiver, which not only simplifies the experimental equipment but also solves the problem that the receiver cannot decrypt synchronously due to the channel damage in remote communication. Further, it is easily applied in production and life. Finally, the simulation results show that the bit error rate is <10^−  5, and the bidirectional transmission of 10-Gb  /  s information over an 85-km single-mode fiber is successfully realized.

Characterizing far-field high energy laser (HEL) propagation during laser weapon system (LWS) test events in an open-air environment is challenging due to many variables that affect laser transmission. These influencing variables can be divided into two different categories, the first of which is the test architecture and LWS configuration. Missions to be accomplished by the system under test (SUT) have varying degrees of difficulty, which include fast-moving dynamic targets. The SUT may also have various operating modes, wavelengths, and shifting internal optics. The second category is the underlying physics occurring from inside the laser cavity and along the optical path to the target. Phenomena including—but not limited to—cavity stability, optical physics, optical turbulence, atmospheric absorption and scattering, and thermal blooming will affect laser propagation and the characterization methodology for an LWS. It is important to consider all these factors as weapon systems are tested and scored to requirements and performance specifications for assessing lethality and effectiveness. Therefore, a year-long measurement study was conducted on the Potomac River Test Range (PRTR) to assess the optical turbulence conditions of the laser range in support of LWS test and evaluation (T&E) events. Experimental data is analyzed for averaged optical turbulence seasonal trends on a complex nonuniform range. The results are used to coordinate optimal test event timing, predict a laser’s far-field properties at the laser termination site, and generate atmospheric forecasts in conjunction with HEL and atmospheric modeling software. The results will contribute to improving future HEL testing methodology and examining accuracy of models producing optical turbulence calculations.

Digital shearography (DS) is a noncontact, whole-field, and laser-based optical interferometric method. It is a valuable tool in the field of nondestructive testing (NDT) and received considerable acceptance in the automotive and aerospace industries. Conventional DS only detects the flaws in one or two shearing/sensitive directions, which leads to the missed inspection and leaves quite a lot of suspected defects underdefined. A trishearing DS for NDT is proposed to solve the mentioned issue. The presented shearography is designed based on a multiplexed Mach–Zehnder interferometer embedded with the spatial phase shift technique. It measures the defects in three sensitive directions simultaneously and has balanced optical paths with high light efficiency. Moreover, the measurement sensitivity of each direction can be adjusted during the test, without affecting the carrier frequencies, so it meets various needs for NDT applications. The principle and the experimental validation of the proposed system are introduced in detail.

We propose a multihop underwater wireless optical communication (UWOC) convergent with free-space optical (FSO) system for an optical internet of underwater things (O-IoUT) and underwater optical wireless sensor network (UOWSN) applications. A closed-form expression of outage probability was derived for the proposed system using the cumulative distribution functions gamma–gamma and hypertangent log-normal for FSO link and UWOC link, respectively. The outage performance of the proposed multihop UWOC convergent with FSO system was analyzed over various oceanic water types (clear ocean, coastal ocean, and turbid harbor) and different FSO weather conditions (clear air, haze, drizzle, and light fog). The results also depict the end-to-end system performance for different pointing errors (strong, moderate, and weak) and varying the number of hops. The maximum link range of ∼2.5 km, which includes 2 km of FSO link and 450 m of multihop UWOC link (nine hops with each of 50 m clear ocean), is considered.

A simple ring-structured photonic crystal fiber (PCF) is designed to support the stable propagation of multiple orbital angular momentum (OAM) modes. The PCF is numerically analyzed and optimized by the finite element method. The results show that the number of OAM modes supported by the PCF reaches 166 in the wavelength range of 1.4 to 1.7 μm. Meanwhile, the confinement loss of the OAM modes is lower than 10 ^{− 8} dB / m, and the smallest dispersion variation is 12.7 ps / ( km · nm ) . Moreover, the mode quality of all of the eigenmodes is higher than 94.1%, and the nonlinear coefficient is lower than 0.7 w ^{− 1} / km. The simple PCF structure with excellent properties has immense application prospects in high-capacity OAM communication.

We propose a broadband tunable negative dispersion photonic crystal fiber (BT-ND-PCF) for dispersion compensation over the S + C + L wavelength bands. BT-ND-PCF is a dual-core structure in which air holes are arranged with a hexagonal lattice and temperature-sensitive liquid is infiltrated into the air holes of the fourth layer. The performance of BT-ND-PCF is numerically analyzed by the finite element method with an anisotropic perfect matching layer. The results show that BT-ND-PCF has a broadband negative dispersion coefficient of −1533.2 to −2836.4 ps / km / nm over the S + C + L wavelength bands, a dispersion coefficient of −2268.9 ps / km / nm at 1550 nm, and a kappa value of 287.9 nm at 1550 nm, which matches with Corning single-mode fiber 28 (SMF). The residual dispersion (RD) after compensating for the dispersion of 1 km SMF is in the range of −0.22 to +0.16 ps / nm. In addition, the dispersion coefficient is changed by 0.679 ps/km/nm per 1°C by controlling the temperature, which is conducive to the accurate compensation of RD. Due to its unique optical properties, BT-ND-PCF can be used in broadband dispersion compensation for dense wavelength division multiplexing optical fiber communication systems.

Visible light communication (VLC) with non-orthogonal multiple access (NOMA) is one of the most promising technologies in wireless optical communication. To improve the system’s performance, we first proposed a NOMA-VLC system based on polar codes. Second, to evaluate the effect of power allocation algorithm on the transmission quality of the NOMA-VLC system based on polar codes, the performance of the NOMA-VLC system under a fixed power allocation algorithm was studied. Finally, considering the time-varying characteristics of wireless channels and users’ service quality, a quality of service (QoS)-based dynamic power allocation algorithm was proposed. In the dynamic power allocation algorithm, we prioritized the QoS of user1 (the user with a lower channel gain) by ensuring that the transmission rate of user1 was always greater than or equal to the minimum transmission rate, and the remaining power was allocated to users with higher channel gains to maximize the total transmission rate. Compared with the NOMA-VLC system without polar codes, simulation results show that user1 obtained 4.1 dB gain for the polar-coded NOMA-VLC system, when the bit error rate was 10 ^{− 4} and α = 1 / 9. And the total transmission rate of the proposed dynamic power allocation algorithm improved by 38.2% compared with the fixed power allocation algorithm with α = 1 / 9 and improved by 48.1% compared with the gain ration power allocation (GRPA) algorithm, when the signal-to-noise ratio was 10 dB. In addition, compared with the fixed power allocation algorithm with α = 1 / 9 and the GRPA algorithm, the outage probability performance of user2 (the user with a higher channel gain) was significantly improved, whereas the outage probability of user1 was basically unchanged under the dynamic power allocation algorithm.

As a new degree of freedom in optical fiber communication, the transmission of orbital angular momentum (OAM) modes has attracted extensive attention in recent years. Herein, a photonic crystal fiber with a dual-guided mode field was designed for the transmission of OAM modes in the wavelength range between 1.50 and 1.60 μm. The effective index difference, dispersion, nonlinear coefficient, and isolation parameters were analyzed systematically by the finite element method. The results showed that the effective index difference of all OAM modes was greater than 1 × ^{10 − 4}, the nonlinear coefficient was as low as 0.492 km ^{− 1} · W ^{− 1}, and dispersion was very flat with a minimum variation of 4.033 ps / ( km · nm ) . The fiber with outstanding properties has great potential and provides insights into how to improve the efficiency of optical communication.

Lightning strokes can cause an ultrafast rotation of the state of polarization (RSOP) in an optical ground wire cable, which leads to communication interruptions. To address this, a simplified radius-directed linear Kalman filter (S-RD-LKF) for blind polarization demultiplexing was developed. The ultrafast RSOP under thunderstorm weather conditions was simulated based on the Heidler model, and the simulation results indicated that the proposed S-RD-LKF algorithm effectively compensated for the RSOP impairment caused by the first and subsequent lightning strokes. Moreover, the RD-LKF algorithm and the proposed S-RD-LKF algorithm exhibited a similar performance; however, the computation complexity of the proposed algorithm was lower.

We propose two composition-graded quantum barriers (QBs) to improve the performance of AlGaN-based deep ultraviolet laser diodes (DUV-LDs). The optical and electrical properties of three LDs containing conventional QBs, graded increased QBs, and graded decreased QBs were numerically investigated. It was found that the LDs with graded decreased QBs significantly improved the carrier injection efficiency, raised the carrier concentration in the active region, reduced the carrier leakage, and enhanced the stimulated recombination rate of the LDs. Simulation results showed that, at an 80-mA injection current, the threshold current and threshold voltage were reduced to 29.5 mA and 4.78 V, respectively. The slope efficiency was improved to 2.52 W/A, and the electro-optical conversion efficiency was increased to 52.2%. Compared with the conventional structure of LDs, grading the QBs with decreasing Al-content improved the performance of the LD remarkably, which is essential for the development of DUV-LD.

We propose a real-time measurement system that prevents overlapping reflection signals in fiber Bragg gratings (FBGs). The system combines a wavelength-swept laser using Fourier domain mode-locking (FDML) and a buffer stage with three optical paths to obtain a high-speed measurement rate of 304.2 kHz. Systems using high-speed wavelength-swept lasers are affected by the propagation time (delay) of the optical fiber. The delay causes problems such as inaccurate measurements, difficulty in distinguishing FBG reflection signals, and overlapping FBG reflection signals. To prevent these problems, we introduce a method for pulse modulation of a wavelength-swept laser by a fiber switch. This method allows for correction of delays by extracting and identifying only the reflection signal of the FBG at a specific wavelength. By identifying the reflection signals of all FBGs, the overlapping signals can be determined. Accordingly, we devise a measurement method that uses a fiber switch to shade light in the wavelength region of one FBG with overlapping signals from the wavelength-swept light of the laser. This method suppresses overlapping and allows only the reflection signal of the other FBGs to be extracted and demodulated. The proposed system uses bidirectional forward and backward scans with the wavelength-swept laser, such that the shaded FBG signals can be demodulated with either scanning direction. We demonstrate that the system using a three-buffered FDML laser can simultaneously measure the strain or vibration of FBGs installed at arbitrary distances.

We propose and experimentally demonstrate an in-line microfluidic sensor based on a dual S-taper multimode fiber interferometer (MFI) for glucose sensing. The dual S-taper MFI was fabricated by splicing two S-shaped fiber tapers with a commercially available fusion splicer. To realize in-line microfluidic sensing, the sensor was encapsulated into a capillary with the inlet and outlet to pump in and out the glucose sample solution using a syringe. Fourier frequency spectra of the transmission spectra under air and deionized water environments showed that multiple high-order modes simultaneously participated in the modal interference process. Experimental results indicated that the interference dip wavelength sensitivity reached 0.276  nm  /    (  g  /  dL  )   for the glucose concentration ranging from 0 to 25.0  g  /  dL. Our proposed glucose sensor has several advantages such as a compact structure, ease of fabrication, and low cost, which make it a promising candidate for in-line glucose sensing and other microfluidic sensing applications.

We propose an optically switchable ultra-broadband terahertz (THz) perfect absorber based on doped superlattice photonic-crystal silicon. The structure consists of a superlattice photonic crystal silicon slab capped by a SU-8 layer with no metal reflector. It achieves ultra-broadband perfect absorption due to the coupled cavity modes of superposed lattices with two different cavity radii and the enhanced diffraction assisted by the top SU-8 layer. Switchable absorption is realized by changing the carrier concentration of doped silicon controlled by pump beam. Simulation shows that proposed structure exhibits polarization-insensitive and wide-angle absorption with efficiency larger than 90% within 1.03 to 5 THz ultra-broadband, with modulation depth larger than 60% within bandwidth more than 1 THz. The all-dielectric structure uncommonly integrates optical switching, ultra-broadband absorption, polarization, and incident angle insensitivity, which may find potential applications in dynamic THz devices.

A highly sensitive long-period fiber gratings (LPFGs) temperature sensor based on dual-peak resonance operating near the phase-matching turning point in chalcogenide fiber is proposed. The Ge-As-Se chalcogenide glasses with a high linear refractive index, large thermo-optic coefficient, and wide transmission window was prepared and characterized for temperature sensors. Combined with PMTP and cladding etching, the temperature sensing characteristics of different cladding modes were examined systematically. The results of theoretical simulation indicated that different cladding modes show relatively high-temperature sensitivities, and the temperature sensitivity of the seventh cladding mode was as high as 9.296 nm/°C operating near PMTP, which is about two orders of magnitude higher than that of the traditional silica LPFGs. Therefore, this temperature sensor with high sensitivity and simplified design can be applied to high-precision temperature sensing.

Multispectral imaging (MSI) is a valuable sensing modality for applications that require detecting a scene’s chemical characteristics. Existing MSI techniques utilize a filter wheel or color filter arrays, which are subject to reduced temporal or spatial resolution. In this work, we present a cephalopod-inspired multispectral organic sensor (CiMOS) based on geometric phase lenses (GPLs) and organic photovoltaics (OPVs) to enable aberration-based color sensing. We mimic the approach by which animals with single-type photoreceptors perceive colors via chromatic aberration. The intrinsic chromatic aberration of GPLs allows for multispectral sensing by stacking precisely patterned OPVs within specific spectrally dependent focal lengths. We provide simulations and a proof of concept of the CiMOS and highlight its advantages, including its simple design and snapshot multi-color detection using only a single axial position. Experimental results demonstrate the sensor’s ability to detect four colors with full width at half maximum spectral resolution as low as 35 nm.

The influence of a multi-mesa structure on a pin photodiode (pin-PD) was carefully studied. It was found that PDs with multi-mesa structures form an uneven electric field in the device under the working voltage, and the electron obtains the overshoot velocity in the uneven electric field. The device that adopts the multi-mesa structure can also reduce the capacitance. The introduction of multi-mesa structures provides another way to improve the performance of the pin-PD. Compared with the one-mesa structure, the multi-mesa structure improved the bandwidth of 5.5 GHz and reduced the capacitance from 26 to 19 fF. The modified pin-PD with a multi-mesa structure can be used in 100-Gbits optical communication systems.

Erratum corrects the paper’s citation identification number.