Wide-field color imaging through multimode fiber with single wavelength illumination: plug-and-play approach.

Multimode fiber (MMF) is extensively studied for its ability to transmit light modes in parallel, potentially minimizing optical fiber size in imaging. However, current research predominantly focuses on grayscale imaging, with limited attention to color studies. Existing colorization methods often involve costly white light lasers or multiple light sources, increasing optical system expenses and space. To achieve wide-field color images with typical monochromatic illumination MMF imaging system, we proposed a data-driven "colorization" approach and a neural network called SpeckleColorNet, merging U-Net and conditional GAN (cGAN) architectures, trained by a combined loss function. This approach, demonstrated on a 2-meter MMF system with single-wavelength illumination and the Peripheral Blood Cell (PBC) dataset, outperforms grayscale imaging and alternative colorization methods in readability, definition, detail, and accuracy. Our method aims to integrate MMF into clinical medicine and industrial monitoring, offering cost-effective high-fidelity color imaging. It serves as a plug-and-play replacement for conventional grayscale algorithms in MMF systems, eliminating the need for additional hardware.

Efficient polarization-insensitive quasi-BIC modulation by VO2 thin films.

Bound states in the continuum (BIC) offer great design freedom for realizing high-quality factor metasurfaces. By deliberately disrupting the inherent symmetries, BIC can degenerate into quasi-BIC exhibiting sharp spectra with strong light confinement. This transformation has been exploited to develop cutting-edge sensors and modulators. However, most proposed quasi-BICs in metasurfaces are composed of unit cells with Cs symmetry that may experience performance degradation due to polarization deviation, posing challenges in practical applications. Addressing this critical issue, our research introduces an innovative approach by incorporating metasurfaces with C4v unit cell symmetry to eliminate polarization response sensitivity. Vanadium Dioxide (VO2) is a phase-change material with a relatively low transition temperature and reversibility. Here, we theoretically investigate the polarization-insensitive quasi-BIC modulation in Si-VO2 hybrid metasurfaces. By introducing defects into metasurfaces with Cs, C4, and C4v symmetries, we enable the emergence of quasi-BICs characterized by strong Fano resonance in their transmission spectra. Via numerically calculating the multipole decomposition, distinct dominant multipoles for different quasi-BICs are identified. A comprehensive investigation into the polarization responses of these structures under varying directions of linearly polarized light reveals the superior polarization-independent characteristics of metasurfaces with C4 and C4v symmetries, a feature that ensures the maintenance of maximum resonance peaks irrespective of polarization direction. Utilizing the polarization-insensitive quasi-BIC, we thus designed two different Si-VO2 hybrid metasurfaces with C4v symmetry. Each configuration presents complementary benefits, leveraging the VO2 phase transition's loss change to facilitate efficient modulation. Our quantitative calculation indicates notable achievements in modulation depth, with a maximum relative modulation depth reaching up to 342%. For the first time, our research demonstrates efficient modulation using polarization-insensitive quasi-BICs in designed Si-VO2 hybrid metasurfaces, achieving identical polarization responses for quasi-BIC-based applications. Our work paves the way for designing polarization-independent quasi-BICs in metasurfaces and marks a notable advancement in the field of tunable integrated devices.

High-temperature-insensitivity high-power Yb-doped fiber laser directly in-band pumping by 1018 nm laser diodes.

In this Letter, we demonstrate a high-power ytterbium-doped fiber laser (YDFL) based on a directly in-band pumping scheme (DIPS) which employs 1018 nm laser diodes (LDs) as pump sources. The wavelength of the LDs is designed theoretically, considering the distribution of Yb3+ absorption cross section (σa) as well as quantum defect (QD). The flat distribution of σa around 1018 nm ensures excellent temperature insensitivity and flexibility for the YDFL. Besides, lower QD and more compact structure promise higher optical-to-optical (O-O) and electrical-to-optical (E-O) efficiencies. Based on the experimental setup, as the cooling temperature of the 1018 nm LDs ranges from 6 to 23°C, an output power of 2 kW level is achieved that varies by only 2.01% without adjusting the operating current of the LDs subjectively. The output power is then scaled up to 5 kW level. Furthermore, there is a great potential to achieve higher output power and E-O efficiency in YDFLs based on the DIPS.

Comparison of tandem pumping and direct pumping on high-power linearly polarized fiber laser.

In this article, a thorough model of linearly polarized fiber laser considering polarization coupling, mode coupling, SBS, and SRS effects is established. The output results of direct pumping and tandem pumping linearly polarized fiber laser under different SBS and SRS intensity settings are simulated. The results show that direct pumping is a better pumping scheme at present, and if the doping concentration of gain fiber can be further increased and the mode field quality of corresponding passive fiber can be optimized, the disadvantages of tandem pumping can be suppressed. To explore the potential of tandem pumping, a backward tandem pumped linearly polarized fiber amplifier is built and 875 W over 13 dB linearly polarized laser output is obtained.

Amplification of random lasing enables a 10-kW-level high-spectral-purity Yb-Raman fiber laser.

By amplifying the cascaded random Raman fiber laser (RRFL) oscillator and ytterbium fiber laser oscillator, we present the first, to the best of our knowledge, demonstration of a 10-kW-level high-spectral-purity all-fiber ytterbium-Raman fiber amplifier (Yb-RFA). With a carefully designed backward-pumped RRFL oscillator structure, the parasitic oscillation between the cascaded seeds is avoided. Leveraging the RRFL with full-open-cavity as the Raman seed, the Yb-RFA realizes 10.7-kW Raman lasing at 1125 nm, which is beyond the operating wavelengths of all the reflection components used in the system. The spectral purity of the Raman lasing reaches 94.7% and the 3-dB bandwidth is 3.9 nm. This work paves a way to combine the temporal stability of the RRFL seed and the power scaling of Yb-RFA, enabling the wavelength extension of high-power fiber lasers with high spectral purity.

High-power and high-brightness Er:Yb codoped fiber MOPA operating at 1535 nm.

In this paper, we study the emerging 1535 nm Er: Yb codoped fiber MOPA with high power and high brightness. To characterize the interstage influence of this ASE-sensitive system, we conduct an interstage numerical model based on steady energy transfer model, where the seed and amplifier converge together. We analyze the amplifier setup, the seed pumping scheme, and feedback from inner reflection based on the model. Afterwards, we experimentally demonstrate a 1535 nm all fiber large mode area Er: Yb codoped fiber MOPA with the output power of 174.5 W, the brightness of 13.97 W/µm2sr, and ASE suppression ratio of 45 dB. To the best of our knowledge, this is the highest power and brightness of 1535 nm fiber lasers to date.

Spectral pedestal during the kilowatt-level amplification of a random fiber laser operating near the lasing threshold.

The amplification of random fiber lasers (RFLs) attracts much attention due to their unique characteristics such as wavelength flexibility and low coherence. We present that, in the kilowatt-level amplification of RFL operating near its lasing threshold, a broad and flat spectral pedestal can co-exist with the narrow spectral peak of RFL. This phenomenon is different from the case in the amplification of fixed-cavity laser seeds. Time-domain measurements show that the broad and flat spectral pedestal, which extends to long wavelengths, is composed of temporal pulses, while few temporal pulses exist in the narrow spectral peak. We attribute the spectral pedestal to intensity fluctuations from the random seed laser and modulation instability in the amplification stage. Control experiments reveal that the working status of the random seed laser and the effective length of the amplifier can influence the spectral bandwidth. By taking advantage of this phenomenon, we propose a novel approach to achieve a high-power broadband light source through the amplification of RFLs operating near the lasing threshold.

Spatiotemporal analysis of an all-fiber multimode interference-based saturable absorber via a mode-resolved nonlinear Schrodinger equation.

This paper presents an approach that combines the generalized multimode nonlinear Schrodinger equation with a transmission model to analyze spatiotemporal characteristics of multimode interference in single mode/large mode area fiber-graded-index multimode fiber-single mode fiber (SMF/LMA-GIMF-SMF) structures for the first time. Approximated self-imaging (ASIM) behavior in GIMF and the study of the latter structure used in spatiotemporal mode-locked fiber lasers are first demonstrated. Simulations show that these structures can work as saturable absorbers enabling high-energy pulse output due to nonlinear intermodal interactions and intensity-dependent multimode interference. Otherwise, underlying ASIM is proven that it can perturb the transmission of SMF/LMA-GIMF-SMF, causing instability of their saturable-absorption characteristics. This paper provides a theoretical guide for many applications, such as beam shaping, mode conversion, and high-energy ultrafast fiber laser.

Broadband nanostructured fiber mode convertors enabled by inverse design.

Nanostructured fiber devices enabling mode conversion between arbitrary fiber modes are proposed and numerically validated. The intra-fiber nanostructures are optimized by the inverse design algorithm. We demonstrate a set of designs of nanophotonic fibers that can facilitate high-purity conversion from the fundamental mode to higher-order modes up to 3 orders for both LP and OAM modes inside the fibers. The purity values of the output modes can reach 98% with an ultra-wide operation band exceeding 400 nm around the telecom wavelengths. These devices can be fabricated by technique of thermal drawing of assembled preforms, making them suitable for mass production.

Guided mode meta-optics: metasurface-dressed waveguides for arbitrary mode couplers and on-chip OAM emitters with a configurable topological charge.

Metasurface has achieved fruitful results in tailoring optical fields in free space. However, a systematic investigation on applying meta-optics to completely control waveguide modes is still elusive. Here we present a comprehensive catalog to selectively and exclusively couple free space light into arbitrary high-order waveguide modes of interest, leveraging silicon metasurface-patterned silicon nitride waveguides. By simultaneously engineering the matched phase gradient of the nanoantennas and the vectorial spatial modal overlap between the antenna near-field and target waveguide mode profile, either single or multiple high-order modes are successfully launched with high purity reaching 98%. Moreover, on-chip twisted light generators are theoretically proposed with configurable OAM topological charge ℓ from -3 to +2. This work may serve as a comprehensive framework for guided mode meta-optics and motivates further applications such as versatile integrated couplers, multiplexers, and mode-division multiplexing-based communication systems.

219.6 W large-mode-area Er:Yb codoped fiber amplifier operating at 1600 nm pumped by 1018 nm fiber lasers.

We demonstrate, to the best of our knowledge, the first high-power large-mode-area Er:Yb codoped fiber amplifier pumped by 1018 nm fiber lasers. The output power reaches 219.6 W, which is the highest power operating at 1600 nm with near-diffraction-limitation beam quality. The 1018 nm pumping scheme contributes to the mitigation of Er,Yb fiber bottlenecking, improvement in signal gain, and reduction of heat generation. Also, we inject co-propagating C-band amplified spontaneous emission (ASE) into the master amplifier to avoid unwanted backward-propagating ASE.

Power scalability of a continuous-wave high-power Er-Yb co-doped fiber amplifier pumped by Yb-doped fiber lasers.

The power scaling of Er-Yb co-doped fiber lasers and amplifiers has been limited by the bottleneck effect of energy-transfer saturation between Yb ions and Er ions. The emerging method of Er-Yb co-doped fiber amplifiers pumped by Yb-doped fiber lasers is considered as an approach to enhance the threshold of the bottleneck effect. In this paper, we quantitatively characterize the threshold of the bottleneck effect via the method of extreme value analysis of the second-order derivative. The method facilitates the optimization of the amplifier configuration. Afterward, we numerically investigate the bottleneck effect of various Er-Yb co-doped fiber amplifiers off-peak cladding-pumped by 10××nm Yb-doped fiber lasers for what we believe, to the best of our knowledge, is the first time. The result shows that the most optimal configuration is long gain fiber over 20 m pumped by a 1020-1025 nm fiber laser, with more than two times the output limit of a conventional laser diode pumping scheme. The essential factors of an amplifier are discussed afterward, including the pump-launching direction, the optimization of large-mode-area fiber, the core-cladding ratio, the concentration of doping ions, the nonlinearity limit, and the distribution of the heat load.

YouTube™ as a source of information for Candida auris infection: a systematic review.

BACKGROUND: Candida auris is a novel Candida species, and has emerged globally as a multidrug-resistant health care-associated fungal pathogen. YouTube™ (http://www.youtube.com) as the largest free video-sharing website is increasingly used to search health information. Thus, the aim of this study was to evaluate the content, reliability and quality of YouTube™ videos regarding Candida auris infection, and to identify whether it is a useful resource for people. METHODS: The YouTube™ was used to search systematically for videos using the keywords: "Candida auris infection" and "Candida auris". Strict inclusion and exclusion criteria were used to select the videos. The videos were reviewed and scored by two independent reviewers and recorded the "title", "length", "views", "comments", "dislike", "like", "posted days" and "category of videos". The videos were categorized as "poor", "good" and "excellent" by the score. The DISCERN tool was used to assess the reliability of the YouTube™ videos. RESULTS: Seventy-six videos were included in final analysis in our study. Most videos (59.2%, 55/76) had better quality. There were no statistically significant differences between groups in respect of the number of likes, dislikes, views, comments, percentage positivity, likebility, view rate and viewers' interaction. Length and posted days were significantly associated with the classification. The videos were categorized as "educational video", "new report", "personal experience and blog entertainment" and "interview". Significant differences were found in the source of videos and the characteristics of the individuals appearing in a video between the groups. CONCLUSION: YouTube™ has striking potential to be an effective user-friendly learning interface for people to obtain information of Candida auris infection.

2.19 kW narrow linewidth FBG-based MOPA configuration fiber laser.

In this paper, we demonstrated a monolithic fiber-Bragg-grating-based (FBG-based) master oscillator power amplification configuration fiber laser with a narrow linewidth at high-power level. Several approaches were implemented to reduce the seed laser linewidth and the magnification of spectrum broadening in order to achieve a narrow output linewidth. The narrow seed laser linewidth was obtained by restricting the reflection bandwidth of the FBG. To reduce the magnification of spectrum broadening, a backward pumping scheme was employed in the amplifier stage after its capacity to suppress laser spectrum broadening was preliminarily investigated experimentally. Further, by intentionally shortening the length of the active fiber in the amplifier and sharing the backward pumping power with the oscillator, the spectrum broadening was further inhibited without sacrificing optical efficiency. A maximum output power of 2.19 kW was achieved with a 3 dB spectrum bandwidth of only 86.5 pm. The beam quality at the maximum power was measured to be M2~1.46. No sign of transverse mode instability was shown during the experiments.

Chip-integrated metasurface for versatile and multi-wavelength control of light couplings with independent phase and arbitrary polarization.

While metasurfaces are now widely considered in free-space optics, their potential for coupling and tailoring guided waves is not fully explored. Here we transfer the Jones matrix method to target versatile on-chip coupling using metasurface-patterned photonic waveguides around the telecommunication wavelength of 1.55 µm, which can accommodate both propagation and Pancharatnam-Berry phase metasurfaces for guided waves. One can either encode two arbitrary and independent phase profiles to any pair of orthogonal polarizations or deploy complete control over both the phase and polarization of coupled modes. A set of design scenarios synergizing silicon nanoantennas and low-loss silicon-nitride waveguides are proposed, including directional couplers with mode-selectivity and polarization splitters with directionality ranging from 10 to 20 dB. Furthermore, our optimization method can be further extended to cover multiple working wavelengths. Exemplary on-chip color routers are also numerically demonstrated. This chip-integrated metasurface platform further translates the concept of a metasurface into photonic integrated circuits, serving as a positive paradigm for versatile and complete control over waveguide optical signals and motivating chip-scale applications such as polarization/wavelength demultiplexers, optical switches, and multifunctional mode converters.

High-power 1018 nm ytterbium-doped fiber laser with output of 805 W.

This Letter presents a high-power 1018 nm ytterbium-doped fiber laser (YDFL) with optimized parameters. A record output power reaching 805 W was achieved, along with a light-to-light efficiency of 64.9% and a beam quality factor of M2=1.80. Further, a higher efficiency of 82.8% of 1018 nm YDFL pumped by wavelength-stabilized laser diodes for the first time, to the best of our knowledge, was shown. At last some attempts were made in bidirectional pumping structure. In this Letter, all the measured spectra showed strong amplified spontaneous emission suppression, and the power curves indicated the potential for further power scaling.

Kilowatt-level cladding light stripper for high-power fiber laser.

We designed and fabricated a high-power cladding light stripper (CLS) by combining a fiber-etched CLS with a cascaded polymer-recoated CLS. The etched fiber reorganizes the numerical aperture (NA) distribution of the cladding light, leading to an increase in the leakage power and a flatter distribution of the leakage proportion in the cascaded polymer-recoated fiber. The index distribution of the cascaded polymer-recoated fiber is carefully designed to ensure an even leakage of cladding light. More stages near the index of 1.451 are included to disperse the heat. The CLS is capable of working consistently under 1187 W of cladding light with an attenuation of 26.59 dB, and the highest local temperature is less than 35°C.

933 W Yb-doped fiber ASE amplifier with 50.4 nm bandwidth.

In this paper, a 933 W amplified spontaneous emission (ASE) source with a full width at half maximum (FWHM) of 50.4 nm is presented. The broadband source is generated by a 1271 W ASE amplifier together with 140m ordinary passive fiber for nonlinear transformation. With multi-wavelength rate equations and a nonlinear Schrödinger equation (NLSE), spectral evolution of the ASE source is analyzed theoretically.

Evaluating the beam quality of double-cladding fiber lasers in applications.

We put forward a new ßFL factor, which is used exclusively in fiber lasers and is suitable to assess beam quality and choose the LP01 mode as the new suitable ideal beam. We present a new simple measurement method and verify the reasonability of the ßFL factor in experiment in a 20/400 µm fiber laser. Furthermore, we use the ßFL factor to evaluate the beam quality of a 3-kW-level fiber laser. It can be concluded that ßFL is a key factor not only for assessing the performance of the high-power fiber laser that is our main focus, but also for the simple measurement.

Fiber coupler for mode selection and high-efficiency pump coupling.

A pump/signal fiber coupler with functions of pump light coupling and high-order-mode suppression is reported. This coupler is composed of pump fibers and a double-clad fiber (DCF) with high pump-coupling efficiency. The core of DCF is microdeformed in the coupling region, which serves mode selection. Theoretical analysis and experimental results verify these functions. In the experiment, the pump-coupling and signal-transmitting efficiency of the coupler were ~98% and ~96%, respectively. This coupler can prevent mode distortion with fundamental mode launching, and improve the fiber laser beam quality with high-order-mode launching.