In this paper, a refractive index (RI) sensor based on the twin-core photonic crystal fiber (TC-PCF) is presented. Introducing the rectangular array in the core area makes the PCF possible to obtain high birefringence and low confinement loss over the wavelength range from 0.6 μm to 1.7 μm. Therefore, the core region can enhance the interaction between the core mode and the filling material. We studied theoretically the evolution characteristics of the birefringence and operating wavelength corresponding to the strongest polarization point under the condition of filling the rectangular array with RI matching fluid range from 1.33 to 1.41. Simulation results reveal that the proposed TC-PCF has opposite evolutions of change rates between the B and wavelength, and the maximum RI sensing sensitivities of 1.809×10-2 B/RIU and 8 700 nm/RIU at low and high RI infill are obtained respectively, which means that the TC-PCF features of dual-parameter demodulation for the RI sensing can maintain a high refractive index sensing sensitivity within a large scope of RI ranging from 1.33 to 1.41. Compared with the results of single-parameter demodulation, it is an optimized method to improve the sensitivity of low refractive index sensors, which has great application potency in the field of biochemical sensing and detection.

A novel whispering gallery mode (WGM) strain sensor based on microtube has been proposed, where perceiving strain variations are reported via the dynamical regulation of a whispering gallery mode. The WGMs in the microtube resonator were evanescently excited by a micro-nano fiber fabricated by the fusion taper technique. The structural changes of microtubes under axial strain were simulated with finite element software, and the effect of microtube wall thickness on strain sensitivity was systematically studied through experiments. The experimental results show that the strain sensitivity of thin-walled microtube is found to be 1.18 pm/με and the Q-factor in the order of 4.4×104. Due to its simple fabrication and easy manipulation as well as good sensing performance, the microtube strain sensor has potential applications in high-sensitivity optical sensing.

In this work, we design a silicon-on-insulator (SOI) sidewall grating with tunable wavelength, delay, and bandwidth through thermal tuning. Incorporating uniform sidewall corrugations and a linearly chirped rib waveguide, the thermo- optic effect imposed on chirped rib waveguide changes the effective refractive index, due to the resultant of the temperature gradient. Consequently, tunable wavelength, delay, and bandwidth in SOI sidewall grating are achieved. In the numerical simulations, the designed SOI grating is demonstrated with a tunable wavelength from 1 560.7 nm to 1 561.9 nm, a tunable delay from 0 to 38 ps, and a tunable bandwidth from 1 nm to 1.5 nm.

Active constellation expansion (ACE) and iterative clipping and filtering (ICF) are simple and effective techniques for reducing the peak-to-average ratio (PAPR) in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems, but effective PAPR suppression requires a lot of iterations. To overcome this shortcoming, a joint algorithm based on improved active constellation expansion (IACE) and ICF (IACE-ICF) is proposed. The simulation results show that at the complementary cumulative distribution function (CCDF) of 10-4, the PAPR of IACE-ICF (G=4, iter=4) algorithm is optimized by 1.507 dB, 1.13 dB and 0.204 dB compared with that of the IACE, ICF (iter=4) and ICF-IACE (G=4, iter=4) algorithms, respectively. Meanwhile, when the bit error rate (BER) is 10-3, the optical signal to noise ratio (OSNR) of the proposed scheme is optimized by 2.04 dB, 1.75 dB and 1.4 dB compared with that of clipping, ICF (iter=4) and ICF-IACE (G=4, iter=4) algorithms, respectively. On the other hand, the proposed scheme can reduce the number of complex multiplications by 14.29% and complex additions by 28.57% compared with the ICF (iter=14) scheme.

The performances of multi-hop parallel free space optical (FSO) communication system are investigated over double generalized gamma (double GG) distribution for plane and spherical waves considering path loss and pointing errors (PE). Specifically, the closed-form expressions of outage probability and average bit error rate (ABER) are derived with Meijer-G function and further confirmed by Monte Carlo (MC) simulation. Subsequently, the outage performances of this system are analyzed in detail with the influence of PE, turbulence strengths, structure parameters and weather conditions for plane and spherical waves. Moreover, cyclic coding is used in this work to further optimize system performance.

To overcome the invalid phase and phase jump phenomenon generated during the phase unwrapping, a phase error correction method based on the Gaussian filtering algorithm and intensity variance is proposed in this paper. First, a threshold of fringe intensity variance is set to identify and clear the phase in the invalid region. Then, the Gaussian filtering algorithm is employed to correct the phase order at the fringe junction, and then the absolute phase is corrected. Finally, the phase correction experiments of different geometric objects are carried out to verify the feasibility and accuracy of the proposed method. The method proposed in this paper can be extended to the correction of absolute phase error obtained by any coding method.

With the absence of on-situ temperature monitoring of optical fiber composite overhead ground wire (OPGW) for the process of sleet-thawing, early temperature warning and safety control of direct current (DC) in sleet-thawing process is difficult. Here we propose a Brillouin optical time-domain reflectometry (BOTDR) with broadband receiving for fast measurement and with distributed Raman amplification for long distance measurement of about 100 km. A field experiment for on-situ temperature monitoring of sleet-thawing of OPGW is also reported, which shows uneven change of temperature along the OPGW. The difference between the maximum and the minimum temperature change can be greater than 40 °C.

In order to implement 3D point cloud scanning of small hole structure, which could not be contacted or damaged, we propose a noncontact 3D measuring method. The system contains a laser triangulation displacement sensor, a Michelson interferometer system and a coordinate measuring machine, with the advantages of non-invasive scanning, fast measurement speed and high precision. Focusing on reconstructing 3D point cloud data, random sample consensus is used to separate surface data and hole data respectively from the raw dataset. Least square optimization determines the function of the cylinder, as well as hole diameter and inclined angle between the hole and the surface. In the experiment scanning a round hole, the estimated result has diameter error and angle error within 30 μm and 0.2°, respectively. Results manifest the effectiveness and feasibility of this system and express practicality in manufacturing industry.

In order to study the influence of backscattering of indicating laser in laser guidance process and laser guidance countermeasure test, the scattering function and volume extinction coefficient of typical aerosol distribution are calculated, and the backscattering detection model of 1.06 μm horizontally transmitted laser is established, based on Mie scattering theory and scattering function optimization algorithm; the model is used to study the change of backscattering energy detected by the detector at different positions and different detection angles, and the false-alarm area of laser guidance along the indicating laser path under different detection thresholds is obtained. The results can help to deepen the understanding of the influence of atmospheric scattering on the laser guidance process, and provide theoretical reference for the scheme design of the laser guidance countermeasure test.

Siamese tracking methods have recently drawn extensive attention due to their balanced accuracy and efficiency. However, most Siamese-based trackers use shallow backbone network, in which extracting high-level semantic features is difficult. When the appearance of distractors and targets is particularly similar, these methods may lead to tracking drift or even failure. Considering this deficiency, we propose a Siamese network with enriched semantics, named ESDT. First, a semantic enrichment module (SEM) comprising dilated convolution layers is designed to improve the classification capability of the siamese tracker. In addition, the target template is updated adaptively to cope with the target texture information changes caused by illumination and blur and further promote the tracking performance. Finally, exhaustive experimental analysis on the public datasets shows that the proposed algorithm outperforms several state-of-the-art algorithms and could track the target stably despite disturbances.

Deep learning (DL) based semantic segmentation methods can extract object information including category, location and shape. In this paper, the identification of prohibited items is regarded as a task of semantic segmentation, and proposes a universal model with automatic identification of prohibited items. This model has two improvements based on the general semantic segmentation network. Firstly, the N-type encoding structure is applied to enlarge the receptive field of the network aiming at reducing the misclassification. Secondly, consider the lack of surface texture in X-ray security images. Inspired by feature reuse in Densenet, shallow semantic information is reused to improve the segmentation accuracy. With the use of this model, when using input images of size 512×512, we could achieve 0.783 mean intersection over union (mIoU) for a seven-class object recognition problem.

Aiming at the problem of underwater polarized laser scattering caused by underwater suspended particles, the equivalent spherical particle Mie scattering theory simulation method is used to study the polarization characteristics of underwater scattered light. The relationship between underwater suspended particle characteristics and optical characteristics is analyzed, and the effects of particle size, polarization characteristics of incident light, and angle of incidence on the degree of polarization of forward and backward scattering light are studied. The results show that: When the incident light is natural light, the degree of polarization of scattered light is very low at the forward-scattering angle, which increases with the increase of the scattering angle, but changes frequently with the increase of the particle size. When the incident light is linearly polarized, the degree of linear polarization of the scattered light is related to the azimuth Angle. The degree of circular polarization is largely unaffected by particle size.