We report a generally designed optical modulation method to garner the tunable three-dimensional (3D) polarized optical multi-focal spots arrays in the focusing system of high numerical aperture (NA) objective lens, and it has good compatibility with vector beams of different polarization states. Based on the vector angular spectrum theory and Fourier transform, the vector light fields of Gaussian beam, Bessel-Gaussian (BG) beam and Laguerre-Gaussian (LG) beam are modulated with the generally designed multi-focal spots phase filter (MSPF). Meanwhile, a generally designed sidelobe suppression transmission (SST) function is employed to reduce the interference effect of the adjacent focal spots in the arrays. Thus, the one-dimensional (1D) axial multi-focal spots array and the 3D multi-focal spots arrays with high spatial resolution can be generated. The MSPF is flexible and reliable in modulating various vector beams, and exhibiting high tunability for the number (3, 12, 21), spacing (4λ—11.3λ) and spatial distribution (1D—3D) of focal spots in the array. With the generally designed MSPF, the generation of multi-focus array is promoted to be more universal and flexible and shows potential application prospects in femtosecond laser multi-beam parallel processing, particle capture, as well as high-density data storage.
2022, 18(12): 705
Long-term ultra-precision synchronization between optical frequency combs (OFCs) and microwave oscillators is important for various fields, including scientific observation, smart grid, positioning and navigation, etc. Here, a phase-locked loop system based on fiber loop optical-microwave phase detector (FLOM-PD) is proposed to realize the synchronization of the repetition rate of OFCs and rubidium atomic clocks. Firstly, the scheme and locking process of the system are elaborated, then the mathematical model of the system is established, and the feasibility of the scheme is proved by theoretical analysis and experimental verification. After synchronization, the instability of the system reaches 8.69×10-12 at 1 s and 2.94×10-13 at 1 000 s, indicating that the phase synchronization system can achieve ultra-precision and stability of OFCs repetition rate.
2022, 18(12): 712
We report the experimental observation of evolving phenomenon of a split multi-wavelength bright-dark pulse in the nonlinear amplifying loop mirror (NALM)-based mode-locked thulium-doped fiber laser (TDFL) with a figure-eight configuration. Bright-dark pulse with 10 wavelengths was successfully obtained at the pump power of 3 W. The time interval between the bright and dark pulses was discovered not only increasing linearly with the pump power but also approximately equaling to the reciprocal of modulation frequency of radio frequency (RF) spectrum. Moreover, we also observed that the spectrum of split multi-wavelength bright-dark pulse can further present up to 13 wavelengths.
2022, 18(12): 717
Aiming at the problem of external parameter calibration in the combined rotating three-dimensional (3D) scanning process of laser radar and turntable, a fast 3D data scanning and accurate equipment calibration method suitable for industrial application scenarios is proposed. The coordinates of the center of the circle in the laser radar coordinate system are obtained by fitting the coordinates of the center of the circle with the scanning arc. Through the standard size and rotation angle of the target ball, the coordinates of the ball center in the turntable coordinate system are obtained. The rotation parameter R and the translation parameter T of the minimum value of the data set conversion error function in the two coordinate systems are calculated. The experimental results show that the error between the converted data and the actual data is within 0.1 mm, which meets the actual work requirements. This calibration method has the advantages of high accuracy and strong robustness.
2022, 18(12): 730
A temperature-insensitive sensor for glucose brix measurement based on compact spindle-shaped structure with two-mode fiber (TMF) is proposed. Due to the bending of optical fiber caused by flame baking, some of the light energy transmitted in the core leaks into the cladding area as an evanescent wave, which excites the higher-order cladding mode of the sensor. The experimental results show that when the length of TMF is 3 cm and the bending diameter is 4 mm, the maximum glucose brix sensitivity of the sensor is 0.368 nm/% from 0 to 21%. The sensor is insensitive to temperature from 10 °C to 50 °C, which can avoid the problem of temperature cross-sensitivity. A compact spindle- shaped sensor is a potential effective sensor with a simple structure, easy fabrication and low cost. The sensor can be used to detect glucose content in areas such as crops quality assessment and the research of pharmacy and bioengineering.
2022, 18(12): 737
When measuring the concentration of multi-component gas mixtures based on supercontinuum laser absorption spectroscopy (SCLAS), there are interferences between the absorption spectral lines. For the spectral interference problem of CO2 and CH4 at 1 432 nm, a method based on support vector regression (SVR) is proposed in this paper. The SVR model, the k-nearest neighbor (KNN) model and the least squares (LS) model are used to analyze and predict the absorption spectral data, and the prediction accuracies were 96.29%, 88.89% and 85.19%, respectively, with the highest prediction accuracy of the SVR model. The results show that the method can accurately measure the concentration of gas mixtures, realize the detection of mixed gases using a single waveband, and provide a solution to the overlapping spectral line interference of multi-component gas mixtures.
2022, 18(12): 743
Taking the standard solution and actual water samples as the research object, this paper studies the influence of various environmental factors (temperature, alkalinity (pH), inorganic salt ions and salinity) on the detection of chemical oxygen demand (COD) in water by ultra-violet (UV) spectrometry, analyzes the influence mechanism, and draws relevant conclusions. The results show that the UV absorbance of the COD standard solution and the actual river water samples varies very little with temperature. The value of pH has certain impact on both the standard solution and the actual water samples, but the pH values of the affected experimental samples are in different ranges. The effects of nitrogen salt ions (NO2 - and NO3 -) on the UV absorption spectra can be eliminated by intercepting the UV spectrum interval less affected for modeling. The influence of salinity values below 35‰ on the UV absorption spectrum of water is mainly concentrated in the spectral band of 200—250 nm, and the influence above 250 nm is very little. This paper provides a preliminary analysis and discussion on the influence mechanism and laws of various environmental factors in the detection of water quality parameters by UV spectrometry, which provides an experimental basis for selecting the optimal test conditions when establishing the COD prediction model.
2022, 18(12): 749
On an internet of video things (IoVT), an encoder needs to collect a large number of signal samples to improve the reconstruction quality. It is challenging to some occasions where the resources of an encoder are extremely limited. The distributed video compressive sensing (DVCS) can save a lot of resources for the encoder. For the skip-block coding at such an encoder, this paper proposes a motion-adaptive adjacent-reference skipping (MAS) algorithm for DVCS with general decoders. The proposed algorithm makes full use of the spatial-temporal correlation between consecutive frames, and the reconstruction quality can be improved significantly. What’s more, the skipping ratio of non-keyframes is adaptive to the difference of their motion-speeds. The proposed algorithm does not need to change any decoder, so it can be easily applied to general decoders. The simulation results show that under different skipping ratios, the proposed algorithm can achieve better reconstruction quality than other existing algorithms, and thus improve the energy-efficiency of the encoder.
2022, 18(12): 755
Existing temporal segmentation methods suffer from the problems of high computational complexity and complicated steps. To address this issue, we present a method that combines the binary tree and spatio-temporal tunnel (STT) for temporal segmentation of rough videos. First, we compute initial cumulative spatio-temporal flow to determine flow overflow of sub-video which is divided from a rough video. Second, the decision tree is generated by combining binary tree and balance factor to dynamically adjust the sampling line of the STT. Finally, pixels on the sampling line are extracted to generate an adaptive STT for temporal proposals. Experimental results show that the computational complexity of the proposed method is significantly better than that of the comparison methods while ensuring accuracy.
2022, 18(12): 763