The laser-induced damage threshold (LIDT) of plate laser beam splitter (PLBS) coatings is closely related to the subsurface absorption defects of the subs
The laser-induced damage threshold (LIDT) of plate laser beam splitter (PLBS) coatings is closely related to the subsurface absorption defects of the substrate. Herein, a two-step deposition temperature method is proposed to understand the effect of substrate subsurface impurity defects on the LIDT of PLBS coatings. Firstly, BK7 substrates are heat-treated at three different temperatures. The surface morphology and subsurface impurity defect distribution of the substrate before and after the heat treatment are compared. Then, a PLBS coating consisting of alternating HfO2–Al2O3 mixture and SiO2 layers is designed to achieve a beam-splitting ratio (transmittance to reflectance, s-polarized light) of approximately 50:50 at 1053 nm and an angle of incidence of 45°, and it is prepared under four different deposition processes. The experimental and simulation results show that the subsurface impurity defects of the substrate migrate to the surface and accumulate on the surface during the heat treatment, and become absorption defect sources or nodule defect seeds in the coating, reducing the LIDT of the coating. The higher the heat treatment temperature, the more evident the migration and accumulation of impurity defects. A lower deposition temperature (at which the coating can be fully oxidized) helps to improve the LIDT of the PLBS coating. When the deposition temperature is 140°C, the LIDT (s-polarized light, wavelength: 1064 nm, pulse width: 9 ns, incident angle: 45°) of the PLBS coating is 26.2 J/cm2, which is approximately 6.7 times that of the PLBS coating deposited at 200°C. We believe that the investigation into the laser damage mechanism of PLBS coatings will help to improve the LIDT of coatings with partial or high transmittance at laser wavelengths.show less
Imaging three-dimensional, subcellular structures with high axial resolution has always been the core purpose of fluorescence microscopy. However, trade-o
Imaging three-dimensional, subcellular structures with high axial resolution has always been the core purpose of fluorescence microscopy. However, trade-offs exist between axial resolution and other important technical indicators, such as temporal resolution, optical power density, and imaging process complexity. We report a new imaging modality, fluorescence interference structured illumination microscopy (FI-SIM), which is based on three-dimensional structured illumination microscopy for wide-field lateral imaging and fluorescence interference for axial reconstruction. FI-SIM can acquire images quickly within the order of hundreds of milliseconds and exhibit even 30 nm axial resolution in half the wavelength depth range without z-axis scanning. Moreover, the relatively low laser power density relaxes the requirements for dyes and enables a wide range of applications for observing fixed and live subcellular structures.show less
Laser processing with high-power ultrashort pulses, which promises high precision and efficiency, is an emerging new tool for material structuring. High r
Laser processing with high-power ultrashort pulses, which promises high precision and efficiency, is an emerging new tool for material structuring. High repetition rate ultrafast laser highlighting with a higher degree of freedom in its burst mode is believed to be able to create micro/nanostructures with even more variety, which is promising for electrochemical applications. We employ a homemade high repetition rate ultrafast fiber laser for structuring metal nickel (Ni) and thus preparing electrocatalysts for hydrogen evolution reaction (HER) for the first time, we believe. Different processing parameters are designed to create three groups of samples with different micro/nanostructures. The various micro/nanostructures not only increase the surface area of the Ni electrode but also regulate local electric field and help discharge hydrogen bubbles, which offer more favorable conditions for HER. All groups of the laser-structured Ni exhibit enhanced electrocatalytic activity for HER in the alkaline solution. Electrochemical measurements demonstrate that the overpotential at 10 mA cm - 2 can be decreased as much as 182 mV compared with the overpotential of the untreated Ni (-457 mV versus RHE).show less
The Shack–Hartmann wavefront sensor (SHWFS) is commonly used for its high speed and precision in adaptive optics. However, its performance is limited in l
The Shack–Hartmann wavefront sensor (SHWFS) is commonly used for its high speed and precision in adaptive optics. However, its performance is limited in low light conditions, particularly when observing faint objects in astronomical applications. Instead of a pixelated detector, we present a new approach for wavefront sensing using a single-pixel detector, which is able to code the spatial position of a light spot array into the polarization dimension and decode the polarization state in the polar coordinate. We propose validation experiments with simple and complex wavefront distortions to demonstrate our approach as a promising alternative to traditional SHWFS systems, with potential applications in a wide range of fields.show less
We introduce an all-optical approach-optical parametric amplification (OPA) processer to suppress the impact of sun outage in laser satellite communication systems, which is implemented by only
We introduce an all-optical approach-optical parametric amplification (OPA) processer to suppress the impact of sun outage in laser satellite communication systems, which is implemented by only one nonlinear semiconductor optical amplifier driven by both the electrical and optical pumps. The optimized OPA processer with the current of 539mA and the pump-to-signal ratio of 16dB could significantly improve the signal quality by 3.5dB in experiment for the elevation angle of sun radiation of 0 rad. The signal-quality improvement is observed at the whole range of elevation angle, confirming the effectiveness of the proposed OPA processer in the field of sun-radiation mitigation.show less
In order to alleviate the impact of turbulence on the performance of underwater wireless optical communication (UWOC) in real time, and achieve high-speed real-time transmission and low cost and
In order to alleviate the impact of turbulence on the performance of underwater wireless optical communication (UWOC) in real time, and achieve high-speed real-time transmission and low cost and miniaturization of equipment, a 2×2 real-time MIMO high-speed miniaturized underwater wireless optical communication system based on FPGA and high-power LED array is designed in this letter. In terms of multiplexing gain, the imaging MIMO spatial multiplexing and high order modulation for the first time is combined and the real-time high-speed transmission of PAM-4 signal based on the LED array light source in 12m underwater channel at 100Mbps rate is implemented, which effectively improves the throughput of UWOC system with high-power commercial LED light source. In light of diversity gain, the system employs the diversity of repeated coding scheme to receive two identical NRZ-OOK signals, which can compensate the fading or flickering sub-links in real time under the bubble-like simulated turbulence condition, and has high robustness. To our knowledge, this is the first instance of a high rate and long distance implementation of a turbulent-resistant real-time MIMO miniaturized UWOC system based on FPGA and high-power LED arrays. With spatial diversity or spatial multiplexing capabilities, its low cost, integrity, and high robustness make the system have important practical prospects.show less
The lens-free on-chip microscopy with RGB LED (LFOCM-RGB) provides a portable, cost-effective, and high throughput imaging tool for resource-limited environments. However, the weak coherence of
The lens-free on-chip microscopy with RGB LED (LFOCM-RGB) provides a portable, cost-effective, and high throughput imaging tool for resource-limited environments. However, the weak coherence of LEDs limits the high-resolution imaging, and the luminous surfaces of the LED chips on the RGB LED do not overlap, making the coherence-enhanced executions tends to undermine the portable and cost-effective implementation. Here, we propose a specially designed pinhole array to enhance coherence in a portable and cost-effective implementation. It modulates the three-color beams from the RGB LED separately so that the three-color beams effectively overlap on the sample plane while reducing the effective light-emitting area for better spatial coherence. And the separate modulation of the spatial coherence allows the temporal coherence to be modulated separately by single spectral filters rather than by expensive triple spectral filters. Based on the pinhole array, the LFOCM-RGB simply and effectively realizes the high-resolution imaging in a portable and cost-effective implementation, offering much flexibility for various applications in resource-limited environments.show less
A flexible-grid 1×(2×3) mode- and wavelength-selective switch which comprises counter-tapered couplers and silicon microring resonators has been proposed, optimized, and demonstrated e
A flexible-grid 1×(2×3) mode- and wavelength-selective switch which comprises counter-tapered couplers and silicon microring resonators has been proposed, optimized, and demonstrated experimentally in this paper. By carefully thermally tuning phase shifters and silicon microring resonators, mode and wavelength signals can be independently and flexibly conveyed to each any one of the output ports and different bandwidths can be generated as desired. The particle swarm optimization algorithm and finite difference time domain method are employed to optimize structural parameters of the two-mode (de)multiplexer and crossing waveguide. The bandwidth-tunable wavelength-selective optical router composed of twelve microring resonators is studied by taking advantage of the transfer matrix method. Measurement results show that, for the fabricated module, the crosstalk less than −10.18 dB, extinction ratio larger than 17.41 dB, the in-band ripple lower than 0.79 dB, and 3-dB bandwidth changing from 0.38 to 1.05 nm are obtained, as the wavelength-channel spacing is 0.40 nm. The corresponding response time is measured to be 13.64 μs.show less