OE thanks the reviewers who served the journal in 2024.

In 2011, the authors introduced a multispot, real-time M2 measurement technology that fulfilled the missing ability of the existing M2 measurement systems to dynamically measure thermal lensing in high-power fiber laser systems. Over the next years, the technology has evolved making the technology of M2 measurement compact, easier to use, and more accurate. The traditional M2 measurement technology, being a time-averaged measurement, is incapable of seeing transient changes in a laser system’s M2, let alone thermal lensing. The multispot approach provides an M2 measurement in a single laser pulse or at the frame rate of the pixelated sensor being used. This paper covers the evolution of the multispot M2 measurement technology and how it has matured into a state-of-the-art M2 measurement tool.

In infrared imaging, pixel non-uniformity due to manufacturing and technological limitations significantly degrades image quality, posing a critical challenge for high-performance applications. This issue is especially pronounced in the field of infrared astronomical observation, where the scientific integrity of the data must be ensured when correcting infrared images. In addition, the coupling capacitance between pixels leads to nonlinear effects in pixel sensitivity. To address these challenges, a non-uniformity correction (NUC) algorithm was introduced that utilizes classification and regression tree segmentation. This approach enables precise corrections by adapting to varying illuminance levels, a capability not fully explored in existing solutions. In our method, we innovatively segment the pixel response curves into distinct low and high illuminance ranges and apply customized corrections for each segment to enhance the accuracy of correction. Evaluations using real image data demonstrate that our method enhances image quality and consistency.

In response to the demand for real-time non-destructive detection of delamination defects in curved structure carbon fiber–reinforced polymer (CFRP), an anisotropic CFRP model was constructed based on the laser thermal elastic effect, and the generation and propagation of ultrasonic waves inside CFPR were analyzed. On this basis, the orientation information of the delamination defect is obtained by calculating the time it takes for the ultrasonic transverse wave to reach the CFRP surface probe after encountering the delamination defect. When the delamination defect with the size of 1×0.2 mm is located between the first and second layers, the detection errors at different probe positions are 0.48% and 0.64%, respectively. Moreover, when the defect is located between the second and third layers, the detection error can also be controlled within 1%.

We proposed and demonstrated a tunable injection-locked laser, which can be locked in a wide range. It can also be used in cascaded injection locking lasers to decrease the influence of the temperature drift. Based on this technology, a photonic scanning receiver is proposed to implement wide-range microwave frequency measurement. The receiver applies optical frequency scanning to simplify the channelization. In the experiment, the receiver can measure weak microwave signals within the frequency of 16.5 GHz.

Fulvic acid is a major component of humus and has a complex composition structure and characterization properties. Proposing an effective detection method for fulvic acid has certain research significance. We detect the three-dimensional (3D) fluorescence spectrum of fulvic acid and apply the parallel factor (PARAFAC) method for qualitative and quantitative analysis. Nine different concentrations of fulvic acid solutions were prepared, and each sample was scanned four times using a fluorescence spectrophotometer, resulting in 36 sets of 3D fluorescence spectra. It is found that the fluorescence characteristic peak is at 428 nm, and the standard excitation wavelength is 360 and 250 nm. The PARAFAC algorithm is applied to decompose the spectrum, resulting in three component factors corresponding to UVA humic-like, t soil fulvic acid, and UVA marine humic-like. By establishing an exponential fitting relationship between the PARAFAC scores of each component and the concentration of fulvic acid, the optimal content prediction model is obtained, with a coefficient of determination of 0.99888 and an average recovery rate of 99.10% for six test samples. Studies show that fluorescence spectroscopy combined with PARAFAC can realize the effective analysis of fulvic acid concentration and composition factor, which can also be applied to the analysis of more complex dissolved organic matter in actual environmental water.

A machine learning method designing flat broadband erbium-doped fiber amplifier (EDFA) + Raman hybrid amplifier was demonstrated. First, we trained a neural network (NN) using data consisting of Raman amplifier pump parameters and gains arranged in descending, ascending, and random order. This trained NN is utilized to calculate pump parameters based on the target gain of the hybrid amplifier. Then, the gain accuracy is optimized through the application of fitting equations that establish a relationship between the adjustment power of Raman’s pump and the gain ratio of the EDFA/Raman amplifier. The optimized results show that when using descending datasets, the error of output power is minimized, with means of root-mean-square-error and maximum error of 0.24 and 0.54 mW, respectively. Its average flatness is not significantly different from the results obtained using ascending and randomly arranged training data. Second, we divided the training data into five parts according to gain ratio of EDFA and Raman amplifier. The accuracy deteriorates when training the NN with a portion of the data, but the accuracy could be enhanced by repeatedly training the NN with the same data. Our results contribute to the construction of intelligent fiber optic transmission networks.

By fully considering display characteristics, an innovative multi-primary-color light-emitting diode (LED) display scheme is proposed aimed at achieving wider color gamut coverage (CGC) and lower blue light hazard efficiency of radiation (BLHER) while meeting the requirements for melanopic efficacy of luminous radiation (MELR). The multi-verse optimizer algorithm is employed to optimize the peak wavelengths and luminous proportions of each primary color LEDs. Results show that when MELR is below 1.5, the BLHER is as low as 0.08, and the CGC exceeds 0.98 under the Rec.2020 standard for the multi-primary-color LED display, which is significantly outperforming BLHER of 0.16 and CGC of 89% for the three-primary-color LED display. Furthermore, it is demonstrated the four-primary-color LED display has slightly better performance in BLHER at night compared with the five-primary-color LED display scheme. As a result, when the number of primary colors reaches 4, the increase in the number of LEDs cannot significantly improve the display performance. Therefore, if the gamut area is not pursued very much, the four-primary-color LED display is the best display scheme with high-cost performance.

Building on conventional dual-fiber optical trap systems, we have developed a multichannel dual-fiber optical trap chip that is simple and low-cost. This chip features three channels, enabling simultaneous capture of three individual microspheres within a liquid environment. With each microsphere being captured by opposing optical fibers at 45-mW power, the measured axial optical trap stiffnesses of each channel were 0.4626, 0.7184, and 0.6467 pN/μm, respectively, whereas the transverse optical trap stiffnesses were 1.7794, 1.6885, and 1.4560 pN/μm. This chip may have various applications in the field of microfluidics, such as optical stretching, viscosity coefficient measurements, cell analysis, and micromixing.

We propose all-optical NOT, XOR, XNOR, and NOR logic designs for NRZ-formatted signal at 340 Gbps. For the optical NOT gate, it is proposed to utilize two orthogonally polarized laser sources at the same frequency to induce cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA). For XOR and XNOR logic gates, it is proposed to cross-couple two orthogonally polarized data streams to induce XGM in SOAs, and the output of these two is combined using a polarization controller to form XOR logic operations. Subsequently, XNOR logic operations are implemented by adding a NOT gate in cascade with XOR logic operations. For the realization of NOR logic operations, two data streams having the same polarization, combined and co-propagating with an orthogonally polarized continuous wave laser source, are proposed to excite XGM in SOA.

We thoroughly evaluate four different taper profiles for fiber-optic surface plasmon resonance (SPR) sensors. We utilized the transfer matrix method to these profiles to evaluate their effect on key performance parameters, including sensitivity, figure of merit (FOM), detection accuracy, full-width at half maximum, amplitude dip, and half-power point. With six different metals (Au, Ag, Cu, Al, Ni, and Pt), we found that the sinusoidal taper profile with Au has the maximum sensitivity, reaching 11.9 μm/RIU at a taper ratio (TR) of 5, and a corresponding FOM of 23.36 RIU−1. In comparison, at TR=6, the exponential taper profile using Cu was able to attain a sensitivity of 25.1 μm/RIU. To optimize the sensor for visible to near-infrared applications, the results demonstrate that raising the taper ratio considerably moves the SPR dip toward longer wavelengths (e.g., from 600 to 850 nm for Au). For real-time bio-sensing applications, these findings are crucial in determining the design of high-performance, tunable SPR sensors that can detect minute changes in refractive index.

Most of the reports on Innoslab master-oscillator power-amplifier (MOPA) amplifiers are about experimental results or models that do not adequately take into account all the factors that affect the performance of Innoslab amplifiers. A modified Frantz–Nodvik (F-N) equation and a simple finite-element slice model based on graphical methods for the Innoslab MOPA laser system are theoretically developed for the first time. The pump absorption saturation effect, the pump transverse spatial distribution, the spatial overlap between the seed and pump, the influence of the tilted optical path and repetition rate, and the overlap effect of the non-collinear beam in the slow axis are fully included. Based on the above model, the performance of a partially end-pumped MOPA system is studied and designed. At the repetition rate of 1 kHz and pulse width of 10 ns, an output pulse energy of 64 mJ is obtained. The consistency of the experimental results and the numerical simulation indicates that this model is a powerful tool to optimize and design a high-power Innoslab MOPA system. The method has been applied to the mass production of lasers with good economic benefits.

Photonic crystals–based cascaded microring resonator structure is proposed to improve the performance using the concept of two-dimensional constrained topological edge states. The topological edge state is created when two microring resonators with distinct band topologies meet an interface and are contained inside a slab of dielectric material which results in a sharp increase in quality factor. The suggested results of the proposed structure were accepted for sensing application, as demonstrated here for temperature sensing. The estimated sensitivity, with a very high-quality factor of 2516, is 3.79 nm/°C.

Recent experiments with optically levitated particles have shown great promise for high-precision sensing of accelerations and gravitational fields as well as exploring mesoscopic physics. One barrier that often stands in the way of improved acceleration sensitivity or quantum state coherence time is high particle temperatures due to the absorption of the light from the trapping laser. In optically levitated acceleration sensing architectures, one limitation on the precision of such sensors is often the upper limit on the size of the particle that can be trapped: larger particles require more laser power to levitate, but too much absorption of the trapping light can overheat and vaporize the particles. We present a detailed analysis of a levitated optomechanical accelerometer to understand what combinations of acceleration sensitivities and maximum-tolerated accelerations can be reasonably achieved, and we analyze the extent to which anti-Stokes optical refrigeration may solve the problem of overheating particles. We also analyze the effect of blackbody radiation (BBR) pressure shot noise on a force and acceleration sensor concept involving free-falling particles that are released and recaptured by an optical trap. We find that, although optical refrigeration is likely insufficient to solve the problem of large particles vaporizing in high-power traps, it would help mitigate BBR pressure shot noise in future accelerometers based on free-falling particles.

We design and intensively investigate a polymer-buried electro-absorption modulator (EAM) utilizing the epsilon-near-zero (ENZ) material. By utilizing the finite element method, the ENZ material’s properties are explored. A strong indium-tin-oxide (ITO)–light interaction is achieved by employing ITO inside the polymer waveguide as the ENZ material. As a result of this interaction, a notable modification of the waveguide’s optical mode and effective mode index (EMI) is observed. The investigated EAM achieves an extinction ratio (ER) of 0.78 dB/μm and a figure of merit (FOM) of 116. In addition, an FOM of 153 and an ER of 3.82 dB/μm were observed by a polymer-buried EAM using a Si waveguide as the core. The investigated modulators could significantly progress the field of polymer-based photonic integrated circuit technology and can solve the issues of passive polymer waveguide-based high-speed optical modulators.

We introduced a innovative approach to fabricating a 1×2 power splitter under the silicon-on-insulator (SOI) platform, which can be scaled to a 1×N system device. The current design strategy is based on the well-known simplified coherent coupling, which allows for the construction of sharp bends. To split the incoming light, a small multimode interferometer (MMI) is added at the entrance of the splitter. Analysis and discussion regarding the sensitivity of the design parameters will be expanded throughout the paper. In addition, the proposed splitter architecture was fabricated and evaluated in 1×2 and 1×4 versions, and these devices were compared with splitters based on standard MMI also performed on the same SOI photonic chip. The splitter scheme demonstrates similar performance to standard MMI; however, a slight spectral flat response was observed in the new devices ensuring reliable operation across various wavelengths. This presented approach enables the design of highly compact devices, potentially opening different avenues to improve and enhance the performance of integrated devices developed under the SOI scheme.

Underwater visible light communication (UVLC) has the advantages of high speed, low latency, and high confidentiality. However, the signal transmission is susceptible to light-emitting diode (LED) modulation bandwidth, non-linear effects of LEDs, and underwater channels. Neural networks, capable of addressing complex nonlinear problems, are increasingly applied to signal equalization in visible light communication. It is found that multilayer perceptron (MLP) is capable of extracting spectral features and nonlinear relationships, and gated recurrent unit (GRU) is capable of handling timing correlation and channel fading problems. Therefore, we propose a GRU-MLP model as a post-equalizer for the UVLC system and experiment using orthogonal frequency division multiplexing modulated signals on a 60-cm underwater experimental platform. The results show that the GRU-MLP equalizer can extend the system transmission bandwidth by 47% higher than the bidirectional gated recurrent unit (BIGRU) and long short-term memory (LSTM) equalizer when only limited by the LED bandwidth; under the influence of the underwater optical channel, the performance of GRU-MLP is similar to that of BIGRU and LSTM. The bit error rate of the GRU-MLP algorithm is significantly lower than other algorithms under the combined effect of two factors. In summary, GRU-MLP demonstrates superior equalization performance in bandwidth-constrained complex channel environments.

With the continuous development of artificial intelligence technology, visible light positioning (VLP) based on deep learning has become a research hotspot for indoor positioning technology to solve the problems of high complexity, low positioning accuracy, and poor real-time performance of the current positioning system. We propose an indoor visible light localization system based on quick response (QR) code recognition technology and transformer–long short-term memory (LSTM) fusion neural network. The system effectively reduces the system complexity by attaching QR code tags to rectangular light-emitting diode (LED) lamp housings for LED-ID data transmission. During the localization process, the vertex coordinates of the LED projection quadrilateral in the image, the diagonal angles, the distance between the centroid and the image center, and the demodulation results are input into a transformer–LSTM fusion neural network for training. The trained model is then used to predict the coordinates of the test set. The experimental results show that in the experimental area of the 9 m×2.2 m×3 m hospital corridor, the average localization error is 8.75 cm, and the localization time of a single sample point is 4.38 ms, which satisfied the localization accuracy and real-time demand of the indoor environment.

We propose and demonstrate a compact ultra-wideband pulse signal generator based on incoherent summation of phase modulation to intensity modulation of two lasers, which are located at the positive and negative linear slope of a chirped fiber Bragg grating (CFBG) reflection spectrum. The relative time delay is derived from the dispersion effect of CFBG. By controlling lasers’ on-off state, negative monocycle pulse, positive monocycle pulse, and negative doublet pulse can be obtained with the central frequency of 5.4, 6.2, and 7 GHz and the fractional bandwidths of 170.37%, 174.19%, and 142.86%, respectively.

In underwater blue-green optical communication, blue-green light is attenuated by seawater absorption, scattering, and ocean turbulence, and the scattering coefficients of blue-green light are different for different depths of seawater; that is, the transmission attenuation of blue-green light varies in different depths of seawater. We calculated the scattering coefficients and albedo of seawater at different depths and analyzed the received optical power of the detector and bit error rate when the blue-green light was line-of-sight (LOS) transmitted in different ways (horizontal, vertical, and oblique range) with the same distance in the same seawater depth range under the effect of absorption and scattering of seawater. Numerical results show that under the effect of absorption and scattering of seawater, the transmission attenuation of blue-green light is large in seawater with large albedo. Therefore, blue-green light in different depths of seawater in the same way LOS transmission of the same distance, the power of light required in the albedo of large seawater is large.

To record the key information in a joint transform correlator cryptosystem, an approach based on ptychography is proposed. This method does not require a reference beam, phase shifting methods, or an additional window in the input plane to record the key information. Instead, the key is illuminated with different probes, and its phase information is recovered employing the probes’ information and their respective cryptosystem’s outputs. Two probes are required for the retrieval of a key capable of successful decryption. Further improvements in the key accuracy and decryption quality are achieved with additional probes. This method is validated with both numerical and experimental results. For an experimental cryptosystem, we obtain similar quality after decryption with a ptychographic key compared with the key registered using the reference arm method.