Recently, it has been found that power scaling in polarization-maintaining single-frequency Er/Yb co-doped fiber (EYDF) amplifiers is not only limited by the amplified spontaneous emission effect, the excited Brillouin scattering effect, but also by the transverse mode instability (TMI) effect. However, to the best of our knowledge, studies on the TMI threshold of EYDF amplifiers are still lacking. In this manuscript, we experimentally investigate and theoretically discuss the dependence of the pump wavelength and pump direction on the TMI threshold of the polarization-maintaining single-frequency EYDF amplifier. In our counter-pumped amplifier with 4.5 m EYDY, by pumping at 940 nm, the TMI threshold can be enhanced by 55.3% as compared to that pumped at 976 nm. Besides, with pumping at 940 nm, the co-pumping system exhibits a lower TMI threshold, only half that of the counter-pumped amplifier. Furthermore, based on the steady-state rate equation of the EYDF amplifier, we simulate heat load along the active fiber. The simulation results demonstrate that reducing the pump absorption coefficient, as well as employing counter-pumping configurations, could decrease peak heat load and axial thermal gradients along the active fiber, thus suppressing the TMI effect. Our work could provide a valuable reference for optimizing the system to suppress the TMI effect in high-power polarization-maintaining single-frequency EYDF amplifiers.

High-power narrow-linewidth pulsed fiber lasers with near-diffraction-limited beam quality are critical for diverse scientific and industrial applications. However, scaling their output power while preserving narrow linewidth and near-diffraction-limited beam quality remains a significant challenge, primarily due to the severe trade-offs between suppressing nonlinear effects and mitigating transverse mode instability (TMI). To address these constraints, we employ an ultra-low numerical aperture (NA) large-mode-area fiber to synergistically balance the suppression of multiple nonlinear effects and TMI. Additionally, rectangular short-duration pulses are adopted to mitigate stimulated Brillouin scattering (SBS) and self-phase modulation (SPM). We experimentally demonstrate a nanosecond-pulsed fiber amplifier system achieving an average output power at the kilowatt level. The system delivers narrow-linewidth pulses (∼1.5 GHz spectral linewidth, 0.98 ns pulse duration) at a repetition rate of 10 MHz. Near-diffraction-limited beam quality is maintained with measured M2 factors of ∼1.21 and ∼1.23 in the x and y directions, respectively. To the best of our knowledge, this represents the highest average power reported for a pulsed fiber laser that simultaneously maintains near-diffraction-limited beam quality and narrow-linewidth operation.

Stratified flow detection is one of the key technologies for achieving refined injection and regulation in oil and gas fields. This paper proposes a novel stratified flow detection method based on distributed fiber Raman scattering temperature sensing. A multi-physics coupling model of the fiber temperature field and fluid flow state is established to simulate the complex temperature distribution and flow characteristics in horizontal wells, with experimental verification conducted under a controlled downhole temperature gradient environment. The experimental results indicate that, under single water injection port conditions, the temperature difference change rates are 0.343 degrees C/min, 0.325 degrees C/min, and 0.335 degrees C/min, respectively. The flow rate is highest at water injection port 1. Under combined operation of multiple water injection ports, The combination of ports 1 and 3 yields the maximum total flow. Moreover, further investigation indicates that under multi-inlet operating conditions with complex simultaneous injection, the temperature-difference response can still provide a stable linear characterization of the flow rate at each port. This method enables real-time acquisition and inversion of wellbore temperature signals, demonstrating high detection accuracy and engineering practicality. This study provides reliable technical pathways and theoretical support for flow monitoring during stratified injection and production in oilfields, offering broad application prospects.

High-power narrow-linewidth fiber amplifiers at 1150 nm are crucial for producing yellow light via nonlinear frequency conversion, with significant implications in ophthalmology and astronomical detection. This paper presents systematic experimental investigations of 1150 nm high-power narrow-linewidth fiber amplifiers employing single-stage and cascaded hybrid gain structures. A narrow-linewidth Yb-Raman fiber amplifier at 1150 nm delivers 1 kW under the single-stage hybrid gain configuration and 1.22 kW under the cascaded hybrid gain configuration. These results indicate that both configurations are capable of achieving kilowatt-level output power. Crucially, the limitation factors in the single-stage and cascaded hybrid gain architectures are amplified spontaneous emission and spectral broadening, respectively. These findings provide crucial experimental insights and technical guidelines for the development of high-power narrow-linewidth fiber amplifiers at 1100-1200 nm.

In this letter, we demonstrate a high-power narrow-linewidth bidirectional pumping Yb-Raman fiber laser at 1120 nm. A dual-seed configuration employing temporally stable phase-modulated single-frequency fiber lasers at 1071 nm and 1120 nm simultaneously suppresses spectral broadening and the stimulated Brillouin scattering effect. Besides, to raise the transverse mode instability threshold, the fiber main amplifier uses a bidirectional pumping scheme. An output power of 3.58 kW is attained at 1120 nm, achieving 81% optical-to-optical efficiency. As the output power reaches its upper limit, the 3 dB linewidth at 1120 nm is 0.93 nm, and the 2nd-order Raman ratio is -23 dB. This work confirms that the bidirectional pumping Yb-Raman hybrid gain combined with temporally stable seeds provides a promising solution for power scaling of narrow-linewidth fiber amplifiers in the long-wavelength.

Optical filters are essential components in modern high-speed optical communication systems, playing a pivotal role in signal selectivity, channel isolation, and system capacity limits. As demand for high-density wavelength-division multiplexing (WDM) and high-bandwidth signal processing increases, the precision of wavelength screening by filters becomes increasingly critical. However, traditional microwave photonic filters suffer from inherent limitations, including constrained free spectral range, fixed geometric periods, and rigid tuning mechanisms, all of which hinder their ability to meet the flexibility required for advanced WDM systems. In this paper, we present a tunable topological valley photonic crystal notch filter integrated with barium titanate, leveraging cavity-edge coupling for continuous notch frequency tuning. Our simulation results reveal that while the microcavity size determines the filter wavelength as a deterministic design degree of freedom, the intrinsic scattering resistance is sustained by the topological protection of the edge states. By cascading microcavities of different sizes, filters with arbitrary wavelength intervals can be achieved. Furthermore, by utilizing the microcavity as a functional point-defect, the refractive index of the microcavity can be tuned to achieve a sensitivity of 391.7 nm/RIU. The proposed structure features a synergistic wave manipulation mechanism that integrates the robust transport of topological edge states with the highly sensitive modulation of localized point-defects. With its compact footprint, high tuning linearity, and versatile wavelength allocation, this design represents a promising solution for the next generation of reconfigurable, high-density photonic integrated circuits.

A metasurface composed of four U-shaped nanopillars of silicon and quartz as the substrate boasting excellent refractive index (RI) sensing and optical switching properties is designed and analyzed. By breaking the symmetry, the metasurface excites dual ultra-narrow quasi-bound states in the continuum (QBIC) resonance peaks with the Fano shape. Both peaks are located in the near-infrared region, and multipole analysis shows that the enhancement is caused by the toroidal dipole (TD) and electric quadrupole (EQ). Quality factors (Q-factors) of 41,898.34 and 20,563.17 and figure of merit (FOM) values of 2767 RIU- 1 and 2314 RIU- 1 are achieved, respectively. Additionally, the polarization characteristics of the metasurface are evaluated. The metasurface has excellent properties and practicality, and the results provide insights into the development of high-performance refractive index sensors and optical switches.

A highly sensitive optical fiber strain sensor with simple structure, easy preparation and temperature insensitivity is proposed. It consists of a parallel Fabry-Perot Interferometer (FPI) based on the Vernier Effect (VE). The sensing cavity (FPIS) is formed by two single-mode fibers (SMFs) by fusion splicer arc discharge, and the reference cavity (FPIR) consists of two SMFs inserted into a hollow capillary tube(HCT). The sensor has a high strain sensitivity of -42.04 pm/mu epsilon and an ultra-low temperature cross-sensitivity of 0.022 mu epsilon/degrees C, which reduces the measurement errors caused by temperature variations. At the same time, stable experimental measurements show that the interference fringes change by less than 20 pm. This sensor combines high strain sensitivity, ultralow temperature cross-sensitivity, and stable performance for precise strain measurement applications.

A temperature-insensitive Fabry-Perot interferometer (FPI) based on the Vernier effect is proposed for strain measurement. Two structurally similar FPIs are prepared with a fusion splicer, and the air cavity in the middle forms the Fabry-Perot cavity. The one FPI with a thinner wall serves as the sensing cavity, and the other is the reference cavity. The strain sensitivity of the optical fiber sensor for 0-600 mu s is 131.7 pm/mu s, which is 9 times higher than that of the single structure FPI. It has a temperature sensitivity of 2.7 pm/degrees C and a temperature cross-sensitivity of 0.021 mu s/degrees C in the temperature range between 25 and 200 degrees C. The sensor has excellent repeatability and stability in strain measurements. The results show that the strain sensor is simple, cost-effective, and easy to fabricate. In conjunction with its high sensitivity, it has great commercial potential in strain measurements, especially in complex temperature environments.

High-power single-frequency fiber lasers with diffraction-limited spots are indispensable for a wide range of photonic applications and are particularly in advanced detection and sensing technologies. However, the simultaneous achievement of kilowatt-level output power and diffraction-limited beam quality has remained elusive in all reported single-frequency fiber laser systems to date, primarily due to limitations imposed by the stimulated Brillouin scattering (SBS) effect and transverse mode instability (TMI) effect. In this study, we demonstrate the design and manufacturing of an ultra-low numerical aperture (NA) functional Yb-doped fiber featuring a bat-type refractive index distribution, specifically engineered for single-frequency laser amplification. In the fabrication, we implemented multiple chelate gas filling and particle deposition iterations, leading to an active fiber with a bat-type refractive index distribution. The unique capabilities of this large mode area and high-order modes leakage fiber (HOMLF) were demonstrated by stably amplifying the single-frequency laser with more than one kilowatt output power and near single mode beam quality (Mx2 = 1.10, Mx2 = 1.18) for the first time. This fiber design advances the leap forward in single-frequency fiber lasers, which could contribute as a novel and efficient laser amplification technique for the next generation of gravitational wave detection systems.

A kilowatt-level narrow-linewidth all-fiber amplifier at 1178 nm based on cascaded Yb-Raman hybrid gains is demonstrated for the first time, to the best of our knowledge. Three phase-modulated single-frequency fiber sources operating at 1071 nm, 1120 nm, and 1178 nm are applied in this amplifier. They can extract Yb ion gain and function as a temporally stable 1st-order Raman-pumped laser, a 2nd-order Raman-pumped laser, and a Raman signal laser, respectively. This configuration enables simultaneous cascaded Raman conversion and narrow-linewidth operation. By theoretically selecting the suitable length of the large-mode-area Yb-doped fiber in the main amplifier, efficient power conversion to the laser at 1178 nm is realized. A 1.14-kW narrow-linewidth cascaded Yb-Raman fiber amplifier at 1178 nm is obtained with a slope efficiency of 77%. The 3-dB linewidth of the amplifier at 1178 nm broadens little during amplification. At the maximum output power, the higher-order Raman is not observed in the output spectrum, and the beam quality factors of the amplifier are Mx2 = 1.29 and My2 = 1.31. Our study proves that using cascaded Yb-Raman hybrid gains in cooperation with temporally stable fiber sources is promising for the power scaling of narrow-linewidth fiber amplifiers at the long-wavelength extreme of the Yb gain spectrum.

This study presents a high power 1030 nm narrow linewidth fiber amplifier employing confined-doped fiber. Through comparative analysis of conventional fully-doped and novel confined-doped fibers (both with 30/250 µm core/cladding diameter), we demonstrate the latter's superior performance in high-power short-wavelength operation, exhibiting enhanced threshold of transverse mode instability while maintaining high laser efficiency and amplified spontaneous emission suppression. As a result, a 1430 W signal laser with 3.5 GHz linewidth is obtained. To the best of our knowledge, this is the highest average power for an all-fiber laser reported so far operating at 1030 nm just within several GHz spectral linewidth. Furthermore, by narrowing the linewidth of the seed laser, an 848 W signal laser with 1.71 GHz linewidth is achieved. This work demonstrates the significant advantages of the confined-doped fiber design in high-power, short-wavelength, and narrow linewidth fiber laser applications.

Memristors based on quantum dots (QDs) exhibit significant potential in the fields of digital memory and analog computing. However, challenges remain in the research focused on modifying the electronic properties of QDs to enhance the performance of memristors. In this study, we report a novel donor-acceptor (D-A) structured nanomaterial utilizing zinc porphyrin (ZnTPP) covalently modified graphene quantum dots (GQDs). Due to the synergistic effects of charge transfer between the electron-donating ZnTPP molecules and the electron-accepting GQDs, along with the distinctive redox activity of ZnTPP, the Al/ZnTPP-g-GQDs:PVP/ITO device achieves precise modulation of 50 non-volatile conductive states, characteristic of an analog memristor. When subjected to a wider voltage scan, this device exhibits typical digital memristive behavior. Furthermore, the convolutional neural network (CNN) constructed using this memristor displays robust performance in recognizing and classifying five types of animal images with high accuracy. This research establishes a novel pathway for the application of QDs in digital-analog dual-mode memristors.

A fast dynamic mode analyzer (FDMA), based on the matrix operation mode decomposition technique, holds significant potential for characterizing the comprehensive spatial features of few-mode fiber lasers and elucidating multi-mode nonlinear physical phenomena. In this study, the FDMA was first validated within a system where mode dynamic frequencies reached kilohertz, featuring coexistent gain dynamics and mode dynamic coupling. Utilizing the FDMA, the dynamic evolution process of modes on a sub-millisecond timescale was comprehensively characterized, with a mode decomposition (MD) speed exceeding 1500 Hz, 2.5 times swifter than the previously reported theoretical speed of the neural network MD technique—the fastest MD method besides the matrix operation MD approach—and an MD accuracy surpassing 0.98. Additionally, based on MD reconstruction of the beam field, the calculation speed of beam quality exceeded 600 Hz, which is four orders of magnitude faster than the speed of commercial equipment. As a typical application of the FDMA, the thermal-induced dynamic mode coupling threshold power was first determined through the analysis of mode components and rapid beam quality evaluations, offering a novel perspective and efficient supplement, markedly distinct from traditional evaluation methods.

High refractive index polymers (HRIPs) are characterized with high refractive index, low loss, impact resistance and high transparency, and play an important role in information recording, data storage and integrated circuits. However, their similar molecular structures and small refractive index differences often pose challenges for accurate identification during processing, especially for related polymers under the same chemical group. In this paper, we propose an optical sensor using Rabi splitting to detect HRIPs. The coupled oscillator model reveals the underlying physics, exhibiting an anticrossing gap of 62.04 meV in transmission spectra. Furthermore, the number of periods (N) of the 1D TPC and the position of WS2 can regulate the coupling strength. Importantly, we have successfully achieved effective optical detection of the high refractive index polymers Polythiol-yne (P1-P5) by exploiting the peak changes of the two transmission peaks generated with Rabi splitting. This work provides an effective optical detection method for the accurate identification of HRIPs with the same chemical group, and will also provide a new way to enhance the development of sensing devices for exciton absorption and visible band sensing.

Complete spatial characterization of few-mode fiber lasers is important in photonics. In this letter, based on fast image processing, a noise-resistant complete spatial characterization technique of few-mode fiber lasers was first validated in the experiment. Specifically, an anti-noise fast mode decomposition technique recently proposed by us is used to obtain the mode power shares and relative phases of orthogonal polarization direction. Stokes vector is utilized to reveal the polarization properties. Furthermore, these measurements for both mode and polarization traits rely solely on the near-field intensity profiles of the fiber laser. The experimental comparison shows that the traditional fast mode decomposition approach is almost ineffective in decomposing the noisy near-field beam intensity images, while the anti-noise mode decomposition technique performs with high accuracy. This technique has the potential to serve as a fast and complete spatial characterization technique for few-mode fiber lasers in real-time.

In this work, we experimentally investigate the performance of homemade polarization-maintaining large-modearea erbium and ytterbium co-doped fibers (EYDFs) in power scaling for 1.5 mu m single-frequency fiber amplifiers. By controlling the doping concentrations of erbium and ytterbium ions, two EYDFs exhibiting disparate absorption capabilities have been fabricated. The amplifier employing low-absorption EYDF attains a maximum output power of 104 W and further power scaling is limited to severe backward propagating amplified spontaneous emission (ASE) in the Yb3+ emission band. Notably, in the amplifier utilizing high-absorption EYDF, the output power is amplified to 170 W with excellent Yb-ASE suppression. Nevertheless, further power scaling is limited to the transverse mode instability effect, first observed in the Er/Yb co-doped laser systems pumped by laser diodes. To the best of our knowledge, the result is the highest power record in linearly-polarized allfiberized single-frequency amplifiers at 1.5 mu m. This work could provide a promising strategy in power scaling for 1.5 mu m high-power single-frequency amplifiers.

A high-power narrow-linewidth fiber laser with single-mode beam quality is experimentally demonstrated. By employing a cascaded phase modulation strategy, the stimulated Brillouin scattering (SBS) threshold of the laser is effectively increased from 973 W to 1970 W. High-order modes are well suppressed during power scaling, benefiting from the significant bending loss of a low numerical aperture (NA) ytterbium-doped fiber. A maximum output power of 1970 W is achieved, with a linewidth of 3.2 GHz and a beam quality factor M2 of 1.14. To the best of our knowledge, this represents the highest reported output power for narrow-linewidth fiber lasers (linewidth < 10 GHz) operating at wavelengths below 1040 nm.

We model and demonstrate a self-matching photonic lantern (SMPL) device, which is designed to address the constraint of limited transverse modes generated by fiber lasers. The SMPL incorporates a FMF into the array at the input end of a traditional photonic lantern. The few-mode fiber at the output end is specifically configured to align with the few-mode fiber at the input, therefore named as SMPL. This paper details the design and fabrication of the SMPL device, validated by both simulation and experiment. The 980nm fundamental mode, injected via 980nm single-mode fibers, selectively excites corresponding higher-order modes at the few-mode port of the SMPL. Additionally, 1550nm fundamental and higher-order modes injected at the input end into the SMPL device demonstrates mode preservation and low-loss transmission characteristics. The SMPL is well-suited for developing a ring laser system, enabling selective excitation of 980nm pump light modes and facilitating closed-loop oscillation and transmission of 1550nm laser.