Wavelength-switchable watt-level continuous wave near-infrared Pr3+:LiYF4 lasers pumped by an InGaN laser diode.

We report a high-performance wavelength-switchable near-infrared Pr3+:LiYF4 (Pr:YLF) laser by InGaN laser diode (LD) pumping. The 895, 922, and 924 nm lasers with low emission cross sections in the Pr:YLF crystal have been successfully realized using a birefringent filter Lyot as well as designing and optimizing optical thin films and the laser resonant cavity. The maximum output powers of the 895, 922, and 924 nm lasers are 2.01, 1.92, and 1.95 W, respectively. As far as we know, these are the highest power for Pr:YLF lasers at 895, 922, and 924 nm so far. The beam quality M x2 and M y2 factors are measured to be 1.85 and 1.71 at 895 nm, 1.94 and 1.67 at 922 nm, and 1.76 and 1.60 at 924 nm, respectively. The laser output power fluctuates within ±3%. In addition, the transmittance of the Lyot is theoretically calculated to achieve laser wavelength switching. The successful realization of the wavelength-switchable watt-level continuous wave near-infrared Pr:YLF laser can provide many practical applications in biomedicine and other fields.

AI-algorithm-assisted 895-nm praseodymium laser emitting sub-100-fs pulses.

Praseodymium (Pr) lasers have achieved outstanding pico- and sub-picosecond pulsations covering the near-infrared (NIR) and visible spectral range in recent years. However, it has been a stagnant task for more than two decades to leapfrog into the sub-100 femtosecond (fs) regime as the Pr gain bandwidths are too narrow for their major transition lines. Although the wide tunability at the NIR bands in the Pr:YLF crystals has been explored, the spectral tails in these transitions suffer severely from weak gains for mode locking, combined with the intricate dispersion control to achieve transform-limit formation. In this work, we target the Pr:YLF's 895-nm line with a specially designed edge-pass filter to balance the gain bandwidth and transitional strength. By deploying a symmetric dispersion scheme and tuning with the soft actor-critic artificial intelligence (AI) algorithm, we have achieved the pulse duration down to sub-100-fs in a Pr laser for the first time. This work also enriches the AI-assisted methodology for ultrafast solid-state laser realizations.

Efficient UV-visible emission enabled by 532†nm CW excitation in an Ho3+-doped ZBLAN fiber.

Rare-earth-doped ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fibers have evolved to become promising candidates for efficient UV-visible emission because of their low phonon energy and low optical losses, as well as their well-defined absorption bands. We investigate the efficient emission of UV-visible light in a low-concentration (0.1 mol%) Ho3+-doped ZBLAN fiber excited by a 532 nm CW laser. In addition to the direct populating of the thermalized 5F4+5S2 levels by ground-state absorption, the upconversion processes responsible for UV-visible emission from the higher emitting levels, 3P1+3D3, 3K7+5G4, 5G5, and 5F3, of the Ho3+ ions are examined using excited-state absorption. The dependence of UV-visible fluorescence intensity on launched green pump power is experimentally determined, confirming the one-photon and two-photon characters of the observed processes. We theoretically investigate the excitation power dependence of the population density for nine Ho3+ levels based on a rate equation model. This qualitative model has shown a good agreement with the measured power dependence of UV-visible emission. Moreover, the emission cross-sections for blue, green, red, and deep-red light in the visible region are measured using the Füchtbauer-Ladenburg method and corroborated by McCumber theory, and the corresponding gain coefficients are derived. We propose an alternative approach to achieve efficient UV-visible emission in an Ho3+-doped ZBLAN fiber using a cost-effective, high-brightness 532 nm laser.

High-power yellow DSR pulses generated from a mode-locked Dy:ZBLAN fiber laser.

Ultrafast yellow lasers are in high demand in recent biomedical and medical applications; however, direct emission of mode-locked pulses in yellow at the high-power level still presents a huge technical challenge to date. By integrating the nonlinear polarization rotation (NPR) scheme into a Dy:ZBLAN fiber laser, dissipative soliton resonance pulses at ∼575 nm are demonstrated for the first time, to the best of our knowledge. The average output power reaches ∼240 mW at maximum, which is an improvement of almost two orders of magnitude over those reported from the latest mode-locked visible fiber lasers. The laser scheme combines a piece of large-core Dy:ZBLAN gain fiber and free-space NPR components designated at the yellow bandwidth. The maximal pulse energy is 2.4 nJ at the repetition rate of ∼100 MHz and the minimal pulse duration is 83 ps. The achieved wavelength of 575 nm is the shortest ever reached from a fiber-based mode-locked laser to date.

Ultrafast true-green Ho:ZBLAN fiber laser inspired by the TD3 AI algorithm.

Ultrafast lasers in the true-green spectrum, which are scarce due to the "green gap" in semiconductor materials, are in high demand for the surging field of biomedical photonics. One ideal candidate for efficient green lasing is Ho:ZBLAN fiber, as ZBLAN-hosted fibers have already reached picosecond dissipative soliton resonance (DSR) in the yellow. When attempting to push the DSR mode locking further into the green, traditional manual cavity tuning is faced with extreme difficulty, as the emission regime for these fiber lasers is so deeply concealed. Breakthroughs in artificial intelligence (AI), however, provide the opportunity to fulfill the task in a fully automated manner. This work, inspired by the emerging twin delayed deep deterministic policy gradient (TD3) algorithm, represents the first application, to the best of our knowledge, of the TD3 AI algorithm to generate picosecond emissions at the unprecedented true-green wavelength of ∼545 nm. The study thus extends the ongoing AI technique further into the ultrafast photonics region.

Wavelength extension and power improvement of red Pr3+:YLF lasers.

High-power red lasers (mainly at 639 and 670 nm) based on Pr3+:YLF crystals have been presented in many works. However, the spectral resources of Pr3+:YLF in the red region have not been fully developed to obtain lasers due to their relatively low emission cross sections and the irrepressible strong emission at ∼639 nm. In this work, we propose a scheme to further develop the spectral resources of Pr3+:YLF in the red region and improve the red laser powers based on this crystal. The laser wavelengths are obtained from 634.5 to 674.7 nm (continuous tunings are achieved at some wavebands). To the best of our knowledge, the output powers obtained at 638.7, 644.6, 670.1, and 674.7 nm (2.88 W, 1.87 W, 3.55 W, and 1.73 W, respectively) are the highest to date. Furthermore, lasing originating from the 3P2 energy level of Pr3+:YLF (∼653 nm) is realized for the first time.

Yellow pumped Pr3+-doped ZBLAN fiber laser at 1015 nm.

To date, lasing in the visible to near-infrared wavelengths has been studied for praseodymium-doped fluoride fibers with the upper energy level of 3P0. In this Letter, a fiber laser operating at 1015 nm has been realized for the first time, to the best of our knowledge, which confirms a new mechanism where 1D2 can be the upper energy level. A maximum output power of 241 mW, with a slope efficiency of 30%, was achieved by using a 150-cm-long active fiber pumped at a maximum pump power of 823 mW. Furthermore, the broad emission spectra of Pr3+-doped fibers in the near-infrared band have been exploited as new, to the best of our knowledge, spectral sources.

High-efficiency broadband tunable green laser operation of direct diode-pumped holmium-doped fiber.

Green laser sources have become increasingly important for the application in scientific research and industry. Although several laser approaches have been investigated, the development of green lasers with the necessary efficiency and spectral characteristics required for practical deployment continues to attract immense interest. In this study, the efficient green laser operation of a Ho3+-doped fluoride fiber directly pumped by a commercial blue laser diode (LD) is experimentally investigated at various active fiber lengths. In the free-running laser, the slope efficiency was optimized up to 59.3% with 543.9 nm lasing, with respect to the launched pump power, using a 20-cm long active fiber. This is the maximum slope efficiency reported to date for a green fiber laser. A maximum output power of 376 mW at 543.5 nm was achieved by using a 17-cm long active fiber pumped at a maximum available launched pump power of 996 mW. Moreover, broadband tuning operation was demonstrated by employing a range of active fiber lengths, together with an intracavity bandpass filter. The operating wavelength was tunable from 536.3 nm to 549.3 nm. A maximum tuning power achieved was 118 mW at 543.4 nm for a 17-cm long active fiber. Moderate Ho3+-doped fiber length is shown to be effective in producing a high performance of a green fiber laser. The short-length of the active fiber considerably extends the green short wavelength operation due to limited reabsorption of the signal below 540 nm.

Mid-infrared Raman lasers and Kerr-frequency combs from an all-silica narrow-linewidth microresonator/fiber laser system.

Mid-infrared (mid-IR) lasers have great applications in bio-molecular sensing due to strong vibrational fingerprints in this wavelength range. However, it is a huge challenge to realize mid-IR lasers in conventional silica materials. Here, we demonstrate the generation of mid-IR Raman lasers and Kerr-frequency combs from an all-silica microresonator/fiber laser system. A single wavelength narrow-linewidth laser at ∼2 µm is first realized by using an ultrahigh Q-factor silica whispering-gallery-mode (WGM) microresonator as mode-selection mirror, and thulium-doped silica fiber as gain medium. Due to the strong intensity enhancement in the microresonator itself, multiple third-order nonlinear optical effects are observed, which include stimulated Stokes and anti-Stokes Raman scattering, and (cascaded) four-wave-mixing (FWM). The stimulated Stokes and anti-Stokes Raman scattering shift the initial 2 µm narrow-linewidth laser to as far as ∼2.75 µm and ∼1.56 µm, respectively. While the cascaded FWM helps to form a Kerr-frequency comb with a broad bandwidth of ∼900 nm and a mode spacing of twice of the microresonator free-spectral-range. This work offers a simple and effective route to realize all-silica mid-IR lasers based on enhanced optical nonlinearity in WGM microresonators.

Adaptive Weighted Graph Fusion Incomplete Multi-View Subspace Clustering.

With the enormous amount of multi-source data produced by various sensors and feature extraction approaches, multi-view clustering (MVC) has attracted developing research attention and is widely exploited in data analysis. Most of the existing multi-view clustering methods hold on the assumption that all of the views are complete. However, in many real scenarios, multi-view data are often incomplete for many reasons, e.g., hardware failure or incomplete data collection. In this paper, we propose an adaptive weighted graph fusion incomplete multi-view subspace clustering (AWGF-IMSC) method to solve the incomplete multi-view clustering problem. Firstly, to eliminate the noise existing in the original space, we transform complete original data into latent representations which contribute to better graph construction for each view. Then, we incorporate feature extraction and incomplete graph fusion into a unified framework, whereas two processes can negotiate with each other, serving for graph learning tasks. A sparse regularization is imposed on the complete graph to make it more robust to the view-inconsistency. Besides, the importance of different views is automatically learned, further guiding the construction of the complete graph. An effective iterative algorithm is proposed to solve the resulting optimization problem with convergence. Compared with the existing state-of-the-art methods, the experiment results on several real-world datasets demonstrate the effectiveness and advancement of our proposed method.

1.83-µm high-power and high-energy light source based on 885-nm in-band diode-pumped Nd:YAG bulk laser operating on 4F3/2→4I15/2 transition.

We report on 1.8-µm laser generation based on a 885-nm diode laser in-band pumping of conventional Nd:YAG bulk crystal. The maximum output power reaches 2.72 W at 1834 nm with slope efficiency of about 12.1% with respect to the absorbed power. With a Cr:ZnSe saturable absorber, passively Q-switched operation is also demonstrated with maximum average output power of 1.25 W. The achieved shortest pulse width, maximum pulse energy and peak power are 54 ns, 125.9 µJ and 2.27 kW, respectively. The results are very competitive to many reported Tm3+ lasers at 1.9 µm. However, this 1834-nm wavelength is indeed difficult to generate from Tm3+ solid-state lasers, which bridges the wavelength gap for potential applications.

Compact all-fiber 2.1-2.7 µm tunable Raman soliton source based on germania-core fiber.

Although ultrafast rare-earth-doped fiber lasers mode-locked at near-infrared and ∼3 µm wavelengths have been well developed, it is relatively difficult to achieve ultrafast fiber laser emitting in the 2.1-2.7 µm spectral gap between ∼2 µm (Tm fiber) and ∼2.8 µm (Er or Ho fluoride fiber). In this paper, we report the generation of 2.1-2.7 µm tunable femtosecond Raman solitons from a compact fusion-spliced all-fiber system using a home-made 1.96 µm ultrafast pump source and a MIR-available germania-core fiber. At first, a Tm-doped double-clad fiber amplifier is used to not only boost up the power of 1957 nm femtosecond seed laser, but also to generate the first-order soliton self-frequency shift (SSFS). The first-order Raman solitons can be tuned from 2.036 to 2.152 µm, have a pulse duration of ∼480 fs and can reach a pulse energy of 1.07 nJ. The first-order Raman solitons are further injected into a 94 mol.% germania-core fiber to excite the second-order SSFS. The second-order solitons can be tuned to longer wavelengths, i.e. from 2.157 µm up to 2.690 µm. Our work could provide an effective way to develop compact, all-fiber ultrafast MIR laser sources with the continuous wavelength tuning of 2.1-2.7 µm.

Determining topological charge based on an improved Fizeau interferometer.

We propose a new method to determine topological charge by using an improved Fizeau interferometer. This interferometer is very easy to realize, as well as interference fringes are very distinct. Phases of vortex, Hermite-Gaussian, and elliptical vortex beams are experimentally verified using this method. It provides a convenient way to determine the sign and magnitude of topological charge. This method may have some potential applications in space optical communication.

Direct generation of orthogonally polarized dual-wavelength continuous-wave and passively Q-switched vortex beam in diode-pumped Pr:YLF lasers.

We report on direct generation of an orthogonally polarized dual-wavelength vortex laser, for the first time to our knowledge, by means of a diode-pumped V-shaped Pr:YLF laser platform. A method of misaligning the folded mirror is proposed to realize the simultaneous orthogonally polarized dual-wavelength laser, while a method of orthogonally rotating the laser gain medium is proposed to generate an intracavity vortex beam (LG01 mode). With the two methods, in continuous-wave (CW) mode, we have achieved simultaneous lasing of an orthogonally polarized dual-wavelength vortex laser at 604 and 607 nm with maximum output power of 237.7 mW. Moreover, based on this operation, a simultaneous orthogonally polarized dual-wavelength passively Q-switched vortex laser is also realized by inserting a Co:ASL crystal into the laser resonator as a saturable absorber. This work provides the simplest way for direct generation of an orthogonally polarized dual-wavelength vortex laser for potential applications.

High-efficiency, yellow-light Dy3+-doped fiber laser with wavelength tuning from 568.7 to 581.9 nm.

We report, to the best of our knowledge, the first demonstration of a wavelength-tunable and highly efficient Dy3+-doped fiber laser operating in the yellow spectral region. A 2-m-long Dy3+:ZBLAN fiber pumped by a 447-nm GaN laser diode provides a strong down-conversion gain around 575 nm. A fiber end-facet mirror and a visible reflective grating in the Littrow configuration construct the resonant cavity and introduce the wavelength tunability. A stable yellow laser with a <0.05-nm narrow linewidth is achieved and continuously tuned from 568.7 nm to 581.9 nm, covering more than half of the yellow spectral range. The slope efficiency is as high as 34.9%, and the maximum output power is 142 mW at 576.44 nm, which is 13 times higher than previously reported. It is, to the best of our knowledge, the highest power and conversion efficiency of a yellow-light Dy3+-doped fiber laser with wavelength tunability.

Adversarial training based lattice LSTM for Chinese clinical named entity recognition.

Clinical named entity recognition (CNER), which intends to automatically detect clinical entities in electronic health record (EHR), is a committed step for further clinical text mining. Recently, more and more deep learning models are used to Chinese CNER. However, these models do not make full use of the information in EHR, for these models are either word-based or character-based. In addition, neural models tend to be locally unstable and even tiny perturbation may mislead them. In this paper, we firstly propose a novel adversarial training based lattice LSTM with a conditional random field layer (AT-lattice LSTM-CRF) for Chinese CNER. Lattice LSTM is used to capture richer information in EHR. As a powerful regularization method, AT can be used to improve the robustness of neural models by adding perturbations to the training data. Then, we conduct experiments on the proposed neural model with dataset of CCKS-2017 Task 2. The results show that the proposed model achieves a highly competitive performance (with an F1 score of 89.64%) compared to other prevalent neural models, which can be a reinforced baseline for further research in this field.

The protective mechanisms underlying Ginsenoside Rg1 effects on rat sciatic nerve injury.

Ginsenoside Rg1 (GsRg1), derived from the herb Ginseng, was found to exert protective effects in nerve injury; however, the mechanisms underlying these effects remain to be determined. Oxidant stress and apoptosis are known to be involved in sciatic nerve injury. Thus, the aim of this study was to examine whether GsRg1 was able to modify sciatic nerve injury in a rat model. The following parameters were measured: (1) number of spinal cord motoneurons by Nissl staining, (2) oxidation parameters including spinal cord malondialdehyde (MDA) levels and activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) as well as (3) involvement of apoptosis by determining caspase-3 and X-linked inhibitor of apoptosis protein (XIAP) by immunohistochemistry and Western blot. The number of spinal cord motoneurons was significantly reduced after sciatic nerve injury, while treatment with GsRg1 markedly elevated cell number. Sciatic nerve injury markedly increased spinal cord MDA content concomitant with reduced activities of SOD and GSH-Px. GsRg1 significantly decreased MDA content accompanied by elevated activities of SOD and GSH-Px. Further nerve injury significantly diminished protein expression levels of XIAP accompanied by elevated protein expression levels of caspase-3 in the spinal cord. GsRg1 markedly increased protein expression levels of XIAP, but significantly reduced protein expression levels of caspase-3. Data suggest that the protective effects of GsRg1 in sciatic nerve injury may be associated with reduced oxidative stress involving anti-apoptotic pathways.

Inhibition of oxidative stress by testosterone improves synaptic plasticity in senescence accelerated mice.

It is well known that synaptic plasticity is associated with cognitive performance in Alzheimer's disease (AD). Testosterone (T) is known to exert protective effects on cognitive deficits in AD, but the underlying mechanisms of androgenic action on synaptic plasticity remain unclear. Thus, the aim of this study was to examine the protective mechanism attributed to T on synaptic plasticity in an AD senescence accelerated mouse prone 8 (SAMP8) model. The following parameters were measured: (1) number of intact pyramidal cells in hippocampal CA1 region (2) phosphorylated N-methyl-D-aspartate receptor-1 (p-NMDAR1) and (3) phosphorylated calmodulin-dependent protein kinase II (p-CaMKII). In addition, the content of whole brain malondialdehyde (MDA) as well as activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) were determined. Treatment with T significantly elevated the number of intact pyramidal cells in hippocampal CA1 region and markedly increased hippocampal protein and mRNA expression levels of p-NMDAR1 and p-CaMK II. Further, T significantly decreased whole brain MDA levels accompanied by elevated activities of SOD and GSH-Px. Data suggest that the protective effects of T on synaptic plasticity in a mouse AD model may be associated with reduction of oxidant stress.

Diode-pumped 915-nm Pr:YLF laser passively mode-locked with a SESAM.

A diode-pumped, passively mode-locked laser emitting at 915 nm with a praseodymium-doped yttrium lithium fluoride (Pr:YLF) crystal was demonstrated for the first time, to the best of our knowledge. Utilizing two polarization-combined blue pumping laser diodes (LDs) and a semiconductor saturable absorber mirror (SESAM), stable continuous-wave (CW) mode-locking operations were achieved with a maximum average output power of 408 mW and a slope efficiency of 10.8%. Laser pulse durations of 15 ps were obtained with a spectral full width at half maximum (FWHM) of 0.15 nm and a repetition rate of 1.53 GHz.

Compact self-Q-switched, tunable mid-infrared all-fiber pulsed laser.

A compact self-Q-switched wavelength-tunable Ho3+:ZBLAN fiber laser around 3 µm is experimentally demonstrated for the first time. The mid-IR all-fiber cavity is formed by a pair of fiber end-facet mirrors. A 2 m-long heavily-doped Ho3+:ZBLAN fiber pumped by a homemade 1.15 µm fiber laser not only provides mid-IR optical gain, but also functions as an equivalent saturable absorber. Stable self-Q-switched pulses around 2.9 µm are generated at a low threshold of 36.6 mW, and the maximum average power obtained is 3.17 mW, corresponding to a pulse width of 1.54 µs and repetition rate of 67.8 kHz, respectively. By simply increasing the incident pump power, the mid-IR laser wavelength can be continuously tunable from 2927 nm to 2960 nm. Furthermore, when the pump power is fixed at 207.7 mW, a 42 nm wavelength tuning (2923 ∼ 2965 nm) from the self-Q-switched all-fiber laser is also achieved by applying the novel loss-adjusting technique.