In this paper, we demonstrate and experimentally verify a tunable, multi-wavelength switchable ring-cavity erbium-doped fiber laser (EDFL). The hollow-core anti-resonant fiber (HC-ARF) filters, which are based on polarization interference, are fabricated using a bent HC-ARF. These filters were incorporated into a ring-cavity EDFL, achieving a tunable laser output ranging from 1547 to 1561 nm with a tuning step of 3.5 nm, and all measured optical signal-to-noise ratios (OSNRs) exceeded 35 dB. Additionally, the laser system supports switching from single-wavelength to three-wavelength operation near the 1560 nm region.

In this paper, to reduce the damage or absorption caused by radiation to optical fibers, we study lightweight and flexible anti-radiation films based on optical precision deposition technology. At first, anti-radiation composite thin films based on Kapton, ITO, and Cu (or Al) are designed and homemade with different structures. Subsequently, polarization-maintaining (PM) Yb-doped fiber (Yb-fiber) samples protected by these different kinds of anti-radiation films are irradiated with a dose of ∼150 kGy. At last, we comparatively investigate (1) the radiation-induced attenuation (RIA) of these PM Yb-fiber samples and (2) the lasing performance (threshold and slope efficiency) and gain performance of a 1064 nm fiber laser and amplifier using these irradiated PM Yb-fibers as the gain medium, respectively. The results show that such a film can reduce the RIA of the irradiated Yb-fiber by up to 2.84 dB/m and increase the output power by up to 75.3% at most. In addition, we also study the optical recovery of the PM Yb-fibers after radiation.

Correcting wavefront distortion caused by atmospheric turbulence is crucial for atmospheric optics. To evaluate correction systems, a real and fast atmospheric turbulence time-evolving model is needed. We proposed a model for a time-evolving turbulence phase screen (PS) based on its fractal nature, which achieves scale transformation under time or space. According to fractional Brownian motion, an interpolation algorithm is proposed to enhance the spatio-temporal resolution of PS efficiently. Additionally, a grid-based time-evolving PS generation method is proposed combining the covariance matrix and temporal spectra. The results demonstrate that our method can efficiently generate time-evolving PS with high spatio-temporal resolution and accuracy, and the interpolation algorithm introduces a slight deviation of less than 2%, which has a minimal impact on the overall results. The fractal nature of atmospheric turbulence has enabled the generation of PS with high accuracy, efficiency, and flexibility. This advancement is meaningful for atmospheric turbulence simulation and related atmospheric optics fields.

In this paper, a novel compensation technique is presented for mitigating dispersion in fiber optic microwave frequency transfer systems. By introducing an additional carrier light into the transmission link, the system effectively eliminates the effect of dispersion on temperature in the fiber link. The experimental results show that the long-term stability of microwave frequency transfer over a 100 km optical fiber link is better than 10-18/d. This remarkable result strongly demonstrates the feasibility of our proposed scheme, while also highlighting its significant practical significance in ensuring the long-term stability of microwave frequency transfer across hundreds of kilometers of optical fibers.

Amplified spontaneous emission (ASE) is the most natural optical carrier for covertly conveying messages in the photonic layer and simultaneously serves as a typical optical carrier in optical sensors. Here, an innovative scheme for integrating covert sensing and communication based on ASE light is proposed and demonstrated through a proof-of-concept experiment. The optical covert sensor, based on a Sagnac structure, detects the location of vibration by searching the null frequency in the spectrum. The experimental results show that the impact of covert sensing on covert communication is negligible, and the bit error rate (BER) performance verifies the feasibility of the integration of optical covert sensing and communication. It may be used in the metropolitan area optical network.

Integrating mobile nodes into wireless light communication networks requires overcoming the challenges of light alignment. Here, we use white and blue lights to establish an all-light communication network with mobile light communication (MLC) links for diverse environments. The integration of visual tracking with a gimbal stabilizer enables tracking and pointing mobile nodes during motion. The MLC achieves a robust transmission control protocol (TCP) connection, maintaining a packet loss of 6.8% and a delay of 48 ms even when the gimbal rotates at speeds exceeding 91.6 deg/s. The network demonstrates full-duplex real-time video communication between mobile and fixed nodes. Furthermore, a minimum requirement for establishing a TCP-based MLC link is presented: the motion time over a given path should exceed the sum of the TCP transmission delay, visual tracking delay, and gimbal rotation time. The mobile all-light communication network holds significant potential for providing various services across diverse environments to different users simultaneously.

In this Letter, we provide a novel maximum a posteriori probability detection-based decision-directed carrier phase estimation (MAP-DDCPE) algorithm. The introduced probability-aware maximum a posteriori probability (MAP) detection avoids the decision errors brought by an ununiform probability distribution, which enhances the phase-tracking ability for the probabilistic shaping (PS) signals. With the proposed MAP-DDCPE, we experimentally demonstrate the 96-channel transmission that delivers 40-GBaud polarization division multiplexing (PDM) PS-64-ary quadrature amplitude modulation (64QAM) signals over the 2-km nested anti-resonant nodeless fiber (NANF). We believe the PS-assisted broadband NANF transmission enabled by the MAP-DDCPE is a promising solution for large-capacity optical communication.

Phase unwrapping is a crucial process in the field of optical measurement, and the effectiveness of unwrapping directly affects the accuracy of final results. This study proposes a multi-level grid method that can efficiently achieve phase unwrapping. First, the phase image of the package to be processed is divided into small grids, and each grid is unwrapped in multiple directions. Then, a level-by-level coarse-graining mesh method is employed to eliminate the new data “faults” generated from the previous level of grid processing. Finally, the true phase results are obtained by iterating to the coarsest grid through the unwrapping process. In order to verify the effectiveness and superiority of the proposed method, a numerical simulation is first applied. Further, three typical flow fields are selected for experiments, and the results are compared with flood-fill and multi-grid methods for accuracy and efficiency. The proposed method obtains true phase information in just 0.5 s; moreover, it offers more flexibility in threshold selection compared to the flood-fill and region-growing methods. In summary, the proposed method can solve the phase unwrapping problems for moiré fringes, which could provide possibilities for the intelligent development of moiré deflection tomography.

Brain imaging techniques provide in vivo insight into structural and functional phenotypes that are physiologically and clinically relevant. However, most existing brain imaging techniques suffer from balancing trade-offs among the temporal and spatial resolutions as well as the field of view (FOV). Here, we proposed a high-resolution photoacoustic microscopy (PAM) system based on a transparent ultrasound transducer (TUT). The system not only retains the advantage of the fast imaging speed of pure optical scanning but also has an imaging FOV of up to 20 mm × 20 mm, which can easily enable rapid imaging of the whole mouse brain in vivo. Based on experimental validation of brain injury, glioma, and cerebral hemorrhage in mice, the system has the capability to visualize the vascular structure and hemodynamic changes in the cerebral cortex. TUT-based PAM provides an important research tool for rapid multi-parametric brain imaging in small animals, providing a solid foundation for the study of brain diseases.

Considering practical unknown magnetic detection, fast magnetic field search and locking scheme is needed in atomic magnetometers. Here, based on the in-phase response of atomic polarization rotation, we provide an iterative search and intensity-modulation feedback locking scheme for a single-beam nonlinear magneto-optical rotation (NMOR) atomic magnetometer. It takes about 0.8 s to find the unknown magnetic field with a search range of 10 to 104 nT. The measurement accuracy is within (0.2% ± 4) nT, and the 3-dB bandwidth is 87 Hz. Our scheme should be useful in cases where variations of the magnetic field cover both the weak and strong field regimes.

Accurately perceiving the multidimensional geometric information of complex equipment is crucial for improving product quality and production efficiency. We propose a multichannel time-domain wavelength division multiplexing frequency modulated continuous wave (FMCW) LiDAR integrated with the optical switch system scheme. This enables the implementation of time-domain wavelength division multiplexing technology for FMCW lasers, achieving the unified transmission of multi-length information through a single optical fiber channel. This system scheme enables parallel measurement of multiple targets and enhances the measurement accuracy of single targets by measuring the mean through multichannels, featuring versatility. In experiment, we achieved an overall absolute distance measurement accuracy better than 14 µm and individual channel accuracy better than 20 µm for non-cooperative targets at a distance of 1.3 m. The overall measurement standard deviation reached 14.73 µm, and the minimum Allan deviation was 189 nm at a 2.84 s averaging time. Additionally, we demonstrated 3D imaging experiments with “TIF” patterned cardboard and corridor stairs, obtained data precision better than 0.8 cm, and achieved high reliability in 3D imaging.

It is meaningful to develop a high-performance optic bending sensor characterized by effective direction judgment, compact length, and high sensitivity. In this Letter, a compact Mach–Zehnder interferometer (MZI) fiber sensor for vector bending measurement is proposed and investigated. This sensor is prepared by off-axis splicing seven-core fibers (SCFs) and multi-mode fibers (MMFs) with different core diameters, which achieves a compact sensing length of 8 mm. The chirped core fiber structure excites the high-order cladding mode in the interference component, which enhances the sensing sensitivity. Experimental results indicate that the maximum bending sensitivity of the sensor is -230 nm/m-1. Moreover, the three bending directions of the sensor can be distinguished by judging the variations of the two interference dips during the measuring process. The proposed method and thought can provide some operating experience and principles for the all-fiber curvature sensor design.

A high-energy and high-efficiency 2 µm nanosecond optical parametric oscillator (OPO) with excellent energy stability is reported. The cavity adopts a plane–plane configuration with two potassium titanyl phosphate (KTP) crystals inserted using a spatial walk-off compensated orientation. The KTP-OPO is pumped by a 1064 nm Nd:YAG Q-switched laser at a repetition rate of 10 Hz and produces a maximum pulse energy of 162.6 mJ at a pump energy of 431 mJ, corresponding to an optical conversion efficiency of 37.7% and a slope efficiency of 45.2%. The energy stability shows a record root mean square (RMS) of 0.4% over 30 min. To our knowledge, this represents the highest 2 µm pulse energy achieved via the 1 µm laser-pumped KTP-OPO scheme, which could be an excellent laser source for driving extreme ultraviolet (EUV) radiations in the subsequent demonstration experiments.

Bound states in the continuum (BICs) have gained significant attention in recent years for enhancing light–matter interaction. Here, we numerically and experimentally demonstrate quasi-BICs in a terahertz photonic crystal (PhC) slab induced by breaking the structural symmetry. The terahertz PhC slab can support four symmetry-protected BICs, exhibiting multipole properties in the near fields and vector vortex characteristics in the far fields. By altering the shape of the holes to break the in-plane inversion symmetry, the quasi-BICs can be excited in the PhC slab under normal incidence. Furthermore, by elaborately adjusting the asymmetry parameter, accidental BICs can also be created in the asymmetric PhC slab. Experimental fabrication of both symmetric and asymmetric terahertz PhC slabs confirms the observation of quasi-BICs in the PhC slabs. The high-Q quasi-BICs in the asymmetric terahertz PhC slab show promise for applications in terahertz sensors, filters, and modulators.

In this paper, a new strategy is proposed based on arbitrary selection of perturbation in a dielectric metasurface to achieve multiple quasi-bound states in the continuum (BICs) with identical modes under dual polarizations. Three distinct symmetry-broken perturbations are discussed. By selecting an arbitrary perturbation, triple quasi-BICs can be induced in transverse magnetic polarization modes at wavelengths of 1071.18, 1098.8, and 1199.6 nm, respectively. Simultaneously, double quasi-BICs at wavelengths of 1375.9 and 1628.5 nm are generated in transverse electric polarization modes. Moreover, the excited quasi-BICs exhibit excellent sensing performance with a maximum sensitivity of 900 nm/RIU, which is better than similar previous studies.

Dual-polarized reconfigurable intelligent surfaces (RISs) increasingly play significant roles in reshaping wireless transmission environments. In this Letter, we propose a design method for dual-polarized RIS elements. This proposed method develops an equivalent multiport model to quickly calculate reflection electromagnetic (EM) responses of the elements containing multiple structural parameters. Moreover, the genetic algorithm (GA) is utilized to optimize the structural parameters to meet design specifications. A 1-bit dual-polarized RIS is implemented for verification. The simulated and experimental results show good consistency with the calculated results. The proposed method significantly conserves design resources, promoting the development of dual-polarized RISs.

We design a terahertz (THz) biosensor supported by quasi-bound states in the continuum (QBICs) for lung cancer cell sensing. By destroying the in-plane symmetry of the bound state in the continuum (BIC), a QBIC with a high Q-factor is obtained. The designed biosensor exhibits excellent refractive index performance with a sensitivity of 354 GHZ/RIU. Unlike traditional detection schemes that require sample drying, a microfluidic liquid sample pool is utilized to detect different concentrations of lung cancer cells. As the cell concentration increases, the resonance frequency and intensity of the measured spectrum show significant changes. The designed sensor allows non-invasive real-time detection of living lung cancer cells, providing a potentially effective tool for early diagnosis and treatment of lung cancer.

Lenses with desired depth of focus have crucial applications in imaging systems. However, there is little theoretical guidance to extend the depth of focus beyond numerical optimization. The on-demand construction of the Jones matrix using the composite metasurface brings a powerful tool for polarization-multiplexed functionalities. Here, based on polarization-multiplexed focusing in four linear polarization channels, we propose a straightforward method to extend the depth of focus based on the coherent superposition of each linear polarization channel. The metalens shows long and uniform needle beam focusing with a depth of focus of 46λ in circularly polarized excitation in the experiment, which offers a promising tool to tailor the terahertz focal spot for imaging applications.

Comparing the coupling strength with both the mean and the product of the square roots of the respective damping rates for the bright and dark modes is a crucial metric in the study of plasmon-induced transparency (PIT). The flip in the ratio determines whether the coupling state between the structural units is strong or weak and also applies to the group delay. Our study explores two primary coupling channels within PIT structures: the inter-resonator distance (d) between the split-ring resonators (SRRs) and the cut wire (CW) and the spacing (g) between the SRRs. In the simulations, photosensitive silicon is embedded in the openings of the dark mode SRR resonator, actively modulating the dispersion characteristics and the coupling strength. Furthermore, we methodically examine the influence of these coupling channels on the transition between the coupling states, as well as on the maximal group delay in the PIT effect. Theoretically, leveraging the parameter fitting via the Lorentz coupling resonator model identifies the dominant parameters governing coupling state flips and differential regulation mechanisms. Our findings contribute to a deeper understanding of PIT phenomena and offer insights into optimizing PIT structures for diverse applications.

Recently, in the field of nonlinear optics of the terahertz frequency range, numerous unique features have been discovered that distinguish it advantageously from nonlinear optics of the optical frequency range. This study demonstrates that the interference of radiations generated at triple frequencies and those due to self-phase modulation in a cubic nonlinear medium can be either constructive or destructive, depending on the parameters of the pulse at the input of the medium. As a result, for a single-cycle pulse, mutual attenuation of these effects is observed by a factor of 20, while for a single and a half-cycle pulse, mutual enhancement occurs by a factor of 1.7. The obtained features are in good agreement with existing experimental data. Thus, by varying the parameters of few-cycle terahertz waves, it is possible to control the nonlinear processes observed in optical media. This will allow for the future development of light-to-light control devices based on these principles.

Terahertz (THz) radiation generation by two-color femtosecond laser filamentation is a promising path for high-intensity THz source development. The intrinsic characteristics of the filament, especially its length, play a crucial role in determining the THz radiation strength. However, a detailed analysis of the underlying physical mechanism and the quantitative correlation between the laser filament length and the THz radiation intensity under a high-peak-power driving laser is still lacking. In this paper, the effect of filament length on the THz radiation is investigated by modulating the basic characteristics of the two-color laser field and changing the focal length. Experimental results show that the long filament length is advantageous for improving THz radiation intensity. The theoretical simulation indicates that enhancement of THz radiation arises from coherent accumulation of THz wave produced at each cross-section along the filament. These insights suggest that extending the filament length is an effective scheme to enhance the intensity of THz radiation generated by the two-color femtosecond laser filament.

Y-junction photonic power splitters are essential in photonic integrated circuits. In this paper, a tunable Y-junction splitter is introduced using a standard silicon-on-insulator platform. It features a single-point control mechanism of both the turnability of power splitting ratios and the non-volatility with optical phase change materials (O-PCMs). This nonvolatile Y-junction splitter has a broadband of 350 nm (from 1300 to 1650 nm) with an about 0.7 dB low insertion loss. Using the direct binary search (DBS) inverse design algorithm, a circular point was identified to fill the phase change material Sb2S3 within the coupling area of the Y-junction photonic power splitter. Six example power splitting ratios of 1.86, 1.70, 1.50, 1.34, 1.21, and 1.14 were realized under single-point control using phase changes at 1550 nm with a 0.35 dB low insertion loss. Furthermore, we also implemented a five-stage cascaded array, with the final stage consisting of 16 Y-junction splitters. These results are useful for significantly simplifying the control of photonic circuits.

Modulating photoluminescent (PL) materials is crucial for applications such as super-resolution microscopy. The combination of PL materials and photoswitches can achieve this aim by utilizing isomerization of the photoswitches. Here we report an optically PL switchable system by mixing carbon quantum dots (CQDs) and diarylethene (DAE) molecular photoswitches. The PL on/off states of CQDs, switched with alternating visible and UV light, achieve a PL on/off ratio of ∼500 and stable reversibility over 20 cycles. The mechanism of our design is revealed by PL lifetime measurements, temperature-dependent PL spectroscopy, and density functional theory (DFT) calculations, confirming that efficient static quenching and the inner filter effect between CQDs and closed DAEs are the keys to achieving such outstanding performance.

The exceptional temporal and spatial photon confinement properties of whispering gallery mode (WGM) microcavities render them ideally suitable for nonlinear frequency conversion. Here, we present a reliable packaged microcavity device with vibration isolation, air tightness, temperature adaptability, and quality factors greater than 2 billion that can serve as a compact and stable platform for soliton optical comb generation. Low-noise soliton combs can be initiated with a repetition rate of 24.98 GHz at wavelengths near 1550 nm with 4 mW threshold power. Our work provides innovative solutions for investigating and manufacturing miniature, economical, and robust microcomb devices.

A thulium-doped fiber amplifier-enhanced photoacoustic spectroscopy (TDFE-PAS) sensor was developed for carbon dioxide (CO2) detection utilizing a 2004 nm distributed feedback (DFB) laser. The thulium-doped fiber was in-band pumped by a 1567 nm source to amplify the optical power of 2004 nm to enhance the photoacoustic excitation. As a result, the photoacoustic signal was enhanced over 101 times. Based on a 7.9 mL differential PA cell, the sensor achieved a linearity of R2 = 0.9997 on CO2 detection in a wide concentration range of 0–10,000 ppm (part per million). The noise equivalent detection limit was evaluated to be 190 ppb (part per billion) at a response time of 10 s.