查看更多>>摘要：? 2021 Elsevier B.V.Multiplet independent resonance modes can be used to control light–matter interactions beyond the capability of conventional optical resonators. Here we present the design and fabrication of multiplexed gratings in a plasmonic waveguide structure to realize multiplet independent resonance modes. Based on the optimally chosen, low-loss plasmonic TM1 waveguiding mode, the multiplet resonance modes demonstrate relatively high-quality factors, field enhancements, overlapping resonance fields, and identical polarization states, advantageous to resonant control and enhancement of light–matter interactions. By adjusting structure parameters of the constitutive gratings, the respective resonance positions and their resonance intensities can be independently adjusted. Experimentally, the plasmonic structures with multiplexed gratings are fabricated, based on multi-exposure holographic lithography processes, which demonstrate arbitrarily adjustable doublet or triplet resonance modes.
查看更多>>摘要：? 2021 Elsevier B.V.Fiber Bragg grating (FBG) is a commonly used optical fiber sensing structure. FBG bent sensors in multimode fiber have been achieved. However, the structure and manufacturing process are complex. In the present study, we designed a simple structure that composed of an ordinary single-mode fiber (SMF) and a section of multimode fiber (MMF) with a FBG. It can realize the dual-parameter sensing of micro-bending and temperature simultaneously. The experimental results suggested that the structure is sensitive to micro-bending and temperature, and the sensitivities do not interfere with each other. The temperature sensitivity is 13.4 pm/°C, and the maximum bending sensitivity is 23.03 dB/m?1.
查看更多>>摘要：? 2021 Elsevier B.V.Integer-order vector beams maintain their polarization properties under the paraxial diffraction condition. Fractional-order vector beam with irregular phase change that leading to changeable focusing properties has aroused huge interest research. Herein, we proposed and experimentally generated a double-mode concentric annular fractional vector beam. Based on the modal superposition, the intensity and spin angular momentum (SAM) density are characterized by periodic changes from rotational symmetry to axial symmetry as the topological coefficient increases. These results reveal that the fractional topological coefficients have a great potential degree of freedom. The experimental results are exploited in this work to emphasize the important connection between fractional coefficient and SAM density.
查看更多>>摘要：? 2021 Elsevier B.V.The sodium laser guide star (sLGS) adaptive optics (AO) system has become an essential component for large ground-based optical/infrared telescope. The performance of the AO system can be significantly hindered if the on-sky spot profile of its sLGS is degraded by the turbulence along the laser launching uplink path. One effective method to overcome this problem is to perform real-time “pre-correction” on the laser before it is launched onto the sky to counter this turbulence effect especially for the lower altitude where turbulence is stronger. The “by-product” Rayleigh backscatter generated by large molecule and aerosols at low altitude when projecting sLGS is a perfect candidate for real time detection of low altitude turbulence. It is therefore important to evaluate whether this “by-product” of a sodium laser guide star could achieve suitable performance for wavefront sensing. In this paper, we attempt to answer the question regarding to the achievable photon return of such Rayleigh backscattered plume. A comparison between results from our field test and theoretical model of MSISE-90 was presented. We showed that the greatest differences is less than 20% which hints the applicability of using the defocused Rayleigh plume as wavefront sensing signal for different turbulence strengths. The result also showed that in case of strong (Fried parameter (r0) of 5 cm) or moderate (r0 of 10 cm) turbulence, the optimum center for range gating is 9 km, and the corresponding value of range gate depths are 3 km, 1.1 km, 0.56 km and 0.28 km when subaperture sizes are 3 cm, 5 cm, 7 cm and 10 cm, with the maximum pulse repetition frequency at 1500 Hz.
查看更多>>摘要：? 2021 Elsevier B.V.Distributions of actual average photon energy (APE) at several outdoor locations around the world are investigated and can be characterized using Gaussian distribution with average APE (μ) and standard deviation (σ) values. For the comparison among different outdoor sites, their APE distributions must be calculated based on the measured solar spectra in the same wavelength range. The method to convert APE values based on different wavelength ranges into those based on same wavelength range is consequently proposed. It is shown that spectral gain and loss (G&L) of different-type photovoltaic (PV) modules calculated from actual APE distribution and its Gaussian fitted distribution are almost similar. General G&L contour maps of the different-type PV modules as functions of μ and σ values are then calculated. These contour maps readily allow us to quantitatively evaluate various-type test PV modules at several outdoor locations around the world, when the μ and σ values are known.
查看更多>>摘要：? 2021 Elsevier B.V.A compact high energy co-doped Tm:Ho:YAG Q-switched laser side-pumped by the 785 nm quasi-continuous wave (QCW) laser diodes (LDs) at room temperature is demonstrated. Up to 130 mJ output energy at 2.1 μm has been generated, operating at 5 Hz repetition rate with a pulse duration of 73.7 ns. The average beam quality factor of M2 is measured to be ～3.5. This laser has a simple design and compact structure, which is very suitable for many applications. To the best of our knowledge, this is the highest output energy for LDs pumped single rod Tm:Ho:YAG laser.
查看更多>>摘要：? 2021A novel graphene-enabled electrically tunable optical filter (ETOF) was reported in this work with high voltage response, low power consumption, and wide working bandwidth. A fiber Bragg grating (FBG) was etched in a microfiber using a femtosecond laser, and porous graphene film was coated on the surface of a tapered fiber. Metal electrodes were fabricated using which a voltage was applied to the graphene film to heat the fiber. The wavelength shift of the ETOF has a linear relationship with the applied voltage. The devices illustrated good repeatability and the voltage sensitivity was calculated as 50.36 pm/V. A maximum voltage of 30 V was applied to the devices and a 1.62-nm wavelength shift was observed. The 3-dB bandwidth of the FBG was approximately 0.38 nm, which corresponds to a power consumption of 5.76 μW in the device. The rising and falling times of the device were measured as 44.2 and 26.7 ms, respectively.
查看更多>>摘要：? 2021 Elsevier B.V.A saturable absorber based on graphene film covered single-mode fiber with inner short waveguides is proposed and demonstrated for passively mode-locked laser operation. The output laser pulses produced have the center wavelength of 1556.9 nm and pulse width of 830.5 fs at the fundamental repetition rate of 18.7 MHz. Such a saturable absorber is highly robust as the graphene film is covered on the surface of single-mode fiber, instead on tapered fiber or side-polished fiber. Moreover, compared with the saturable absorber based on clamping the graphene film between the two fiber connectors in a fiber adaptor, our device provides a large interaction surface area, thus effectively avoiding the thermal damage and ensuring a long time and stable system operation. The device is attractive for high power ultra-short laser pulse generation.
查看更多>>摘要：? 2021 Elsevier B.V.A two-color sinusoidal phase modulating (SPM) interferometer based on electro-optic modulator (EOM) is devised for self-correction of air refractive index in distance measurement. Two laser sources with high frequency stability and close wavelengths (633 nm and 782 nm) are used in this scheme. High-accuracy phase demodulation was realized using phase generated carrier (PGC) demodulation techniques with an FPGA-based signal processing board. Experimental results of nanometer displacement measurements show that the resolution and accuracy of PGC demodulation is better than 1°, and the nonlinear error is less than 0.2°. Performance tests of two-color SPM interferometer compared with one-color interferometers under dynamic environment were implemented with an arm length difference of 4 m. The results show that, the measured distance of one-color interferometer drifts 4.8 μm in 600 s, which is caused by the deviation between the measured value of air refractive index and the actual value along the laser propagation path, and this drift reduces to ±1.5 μm using two-color interferometer without environmental sensing.
查看更多>>摘要：? 2021 Elsevier B.V.We proposed a promising method to achieve phase modulation utilizing photonic crystal (PC). The broken parity and time-reversal symmetries induce intrinsic nonreciprocity. The supported one-way phase velocity which has a close relationship with biased magnetic field (BMF). Different from the most metasurfaces, only one structure is required to realize electromagnetic wave spatial distributions by adjusting each cell's BMF. As an application, a deflector with only one column shows obviously nonreciprocity accompanied with magnetically tunable deflection angle. We further designed a flat lens and an axicon under different applied BMF distributions. Such phase modulation by PC provides a new flexible way to realize phase modulation, which can reduce difficulties in design and fabrication.
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