Abstract:A twist sensor with hybrid few-mode tilted fiber Bragg grating (FM-TFBG) and few-mode long period grating (FM-LPG) in fiber laser cavity is demonstrated. The FM-LPG is utilized to excite LP11 core mode. The FM-TFBG is used for sensing. The transverse modes at 1 553.9 nm and 1 550.5 nm are LP01 and LP21 core modes, respectively, which are coupled from forward-propagating LP11 core mode. These two excitation wavelengths have opposite variation tendencies, which participate in sensing. The twist sensitivity of 0.16 dB/° from −40° to 40° is achieved. The proposed sensor has potentially used for structure monitoring in many areas.

Abstract:Railway turnout contact monitoring is very important in high-speed rail operation systems. In order to measure the distance between the sharp rail and the basic rail in a switch system, a wide-range, high-precision fiber Bragg grating (FBG) displacement sensor was designed. Because the distance between the sharp and basic rails is always greater than 14 cm, the measurement range width and accuracy of the proposed sensor system are ensured through the use of a long spring and a beam of constant strength. A differential compensation method is used to eliminate temperature effects. Test results show that the resolution of the proposed sensor is 0.040 mm and the measuring range is 0—170 mm. A field test was also carried out to evaluate the performance of the sensors.

Abstract:An ultraviolet-infrared dual-color detector is proposed and realized based on the vertical integration of single-layer graphene and a 4H-SiC layer by semiconductor micro-fabrication technology. The spectral response characteristics of the detector are analyzed. The ultraviolet response range is 208—356 nm with a responsivity larger than 0.4 mA/W and the infrared response range is 1.016—1.17 μm with a responsivity larger than 0.4 mA/W at room temperature and 5 V bias voltage. The peak responsivity of the graphene in the ultraviolet-C band at 232 nm is 0.73 mA/W and in the near infrared band at 1.148 μm is 0.64 mA/W. The peak responsivity of SiC layer in the ultraviolet-B band at 312 nm is 2.27 mA/W. Besides, the responsivity increases with the bias voltage.

Abstract:We numerically and experimentally proposed a dual-stop-band terahertz filter based on standard microelectronic fabrication method. The stop bands locate at 0.32 THz and 1.02 THz with 3 dB bandwidths of 0.26 THz and 0.55 THz, respectively. The resonance characteristics of the proposed device were discussed with the help of surface current maps and field density maps extracted from computer simulation software to better understand the working principle of the proposed device. On top of that, a total of seven devices with different dimensions were fabricated to fully discuss the dimension effects on the resonant frequency shift and bandwidth changes. This fabrication process is applicable for related integrated metamaterial devices and provides essential experiment evidences for effective ways of manipulating the transmission spectrum of the proposed filter.

Abstract:To accurately launch laser, a fast-steering mirror (FSM) with flexure hinge is designed. First, actuators, angle sensors and flexure hinge were designed or selected respectively according to requirements of vehicle track-launch system. Then, the servo control system with two closed loops was projected after fine manufacturing and assembling. Finally, the pointing precisions of FSM on the static and vibrancy platforms were tested. The results show that the designed FSM with pointing error range on the static platform is less than 0.9″, and less than 44.5″ on the vibrancy shaker, which can meet the requirements of vehicle track-launch system application.

Abstract:In this paper, a significant enhancement in current efficiency of the green tandem organic light-emitting diodes (TOLEDs) is demonstrated, which is based on a buffer-modified charge generation layer (CGL) of fullerene carbon (C60)/zinc-phthalocyanine (ZnPc). Al and MoO3 were used as the buffer-modified layers on both sides of the bilayer C60/ZnPc, respectively. Experimental results show that the inserted Al and MoO3 layers can effectively increase the electron extraction of the CGL for obtaining the device performance enhancement. Compared with that of the green TOLEDs without buffer-modified layers in CGL (37.3 cd⋅A-1), the current efficiency of the green TOLEDs is increased to 54.1 cd⋅A-1. Further study results find that the performance can also be improved by optimizing the thickness of Al in the CGL. The maximum current efficiency and maximum luminance of the green TOLEDs achieve 63.5 cd⋅A-1 and 17 873 cd⋅m-2, respectively, when the multilayer structure of the CGL is Al (3 nm)/C60 (5 nm)/ZnPc (5 nm)/MoO3 (3 nm).

Abstract:We demonstrate a method for fabricating sapphire rib waveguides with femtosecond laser micromachining technology. Finite difference beam propagation method (FD-BPM) is applied to design sapphire rib waveguides. We explore the process of etching on a sapphire substrate surface via direct laser ablation. Rib waveguides that satisfies the dimensions of our design were fabricated. Actual light propagation performance is tested, and with the cut-back method, the insertion loss of 2.9±0.5 dB/cm is measured.

Abstract:The opal photonic crystals (PCs) were assembled by vertical deposition of polystyrene microspheres (PS), in which Yb3+, Er3+ co-doped ZnO powders were deposited on the surface of PC films. The phase, structure and morphology of the obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The performance of up-conversion photoluminescence (UCPL) was investigated by fluorescence spectrophotometer. The up-conversion (UC) emission of ZnO:Yb3+/Er3+on the PC surface is notably enhanced when the UC emission wavelength is overlapped with the photonic bandgaps of opals, which is attributed to Bragg reflection of photonic bandgap. The results show that PCs may have potential applications in the enhancement of UCPL and optoelectronic devices.

Abstract:To improve the time and frequency reference standards used for radio astronomy and high precision atomic physics applications, we describe a novel stable frequency transmission technique. The approach uses a vertical-cavity surface-emitting laser (VCSEL) based phase correction actuator. Using a frequency mixing process for feedback control, the phase noise induced along the 26 km G.655 optical fibre link was actively reduced. This was achieved by using a 1 310 nm dither controlled VCSEL phase error correction actuator together with the inherent chromatic dispersion properties of the fibre. The technique corrects phase noise and optical signal drift along the fibre at the transmission end. Fractional frequency instabilities across the G.655 fibre link of 4.44×10-12 at 1 s and 4.86×10-14 at 104 s are successfully reported.

Abstract:Based on the theory of dispersion characteristics of double-ring photoelectric oscillator and chirped grating, a broadband ultra-low phase noise spectral frequency shift system is constructed. Firstly, the mathematical model is discussed. Secondly, according to the frequency response of the photonic filter formed by the chirped grating interfering with the optoelectronic oscillation circuit, the simulation of the effect of grating dispersion on oscillation frequency is performed. The simulation results show that the oscillation frequency decreases with the increase of grating dispersion value. In the range of 350—4 000 ps/nm, the oscillation frequency varies from 5 GHz to 16.9 GHz, and the phase noise can reach −140.5 dBc/Hz at 10 kHz, which realizes the generation of ultra-low phase noise spectral shift with broadband tuning.

Abstract:Accurate time transfer and synchronization between different network nodes is a key functional requirement in digital communication. Developments in optical fiber-based frequency dissemination techniques have improved optical frequency stability over time to much lower levels. In this work, we experimentally present the reference frequency transfer employing 850 nm vertical cavity surface emitting laser (VCSEL) over 100.3 m OM3 multimode fiber for synchronization of clocks on networked devices such as servers and racks/pod at different data center network nodes. A low-cost power-efficient multimode VCSEL with central wavelength at 844.26 nm is directly modulated with a 2 GHz reference frequency (RF) clock signal, and transferred over 100.3 m of OM3 multimode fiber. The single side band (SSB) phase noise of −104.62 dBc/Hz and −100.70 dBc/Hz is experimentally measured at back-to-back (B2B) and 100.3 m OM3 multimode fiber transmission respectively at a 1 kHz frequency offset. The jitter stability of 0.14 ps and 0.15 ps is experimentally achieved at B2B and 100.3 m fiber transmission, respectively. This work provides an alternative viable approach for the development of time keeping devices in high-speed short-reach optical communication systems.

Abstract:In a typical infrared optical system, the “Narcissus” effect exists extensively. This concept can be extended into a heterodyne detection. In a heterodyne detection, besides the probe and local oscillator beams, a third coherent beam or even more beams caused by spurious reflection might interfere with each other on the surface of the photodetector. Generation of heterodyne signal is dependent on the interference effects between regular and unexpected waves. Based on the theory of PM demodulation, we analyze the effect of multi-beam interference competition in a laser Doppler vibrometer (LDV). A mathematical analysis demonstrates that the signal competition depends on the modulation index, the amplitude ratio of competing signals and their relative phases. The distortion of demodulated signal due to the appearance of ripples or spikes is also predicted in this study. These effects are verified by setting up an all-fiber LDV system for measuring vibration.

Abstract:This paper develops a variational model for image noise removal using total curvature (TC), which is a high-order regularizer. The TC has the advantage of preserving image feature. Unfortunately, it also has the characteristics of nonlinear, non-convex and non-smooth. Consequently, the numerical computation with the curvature regularization is difficult. In order to conquer the computation problem, the proposed model is transformed into an alternating optimization problem by importing auxiliary variables. Furthermore, based on alternating direction method of multipliers, we design a fast numerical approximation iterative scheme for proposed model. Finally, numerous experiments are implemented to indicate the advantages of the proposed model in image edge preserving, image contrast and corners preserving. Meanwhile, the high computational efficiency of the designed model is verified by comparing with traditional models, including the total variation (TV) and total Laplace (TL) model.

Abstract:The information of expression texture extracted by the completed local ternary patterns (CLTP) method is not accurate enough, which may cause low recognition rate. Therefore, an improved completed local ternary patterns (ICLTP) is proposed here. Firstly, the Scharr operator is used to calculate gradient magnitudes of images to enhance the detail of texture, which is beneficial to obtaining more accurate expression features. Secondly, two different neighborhoods of CLTP features are combined to obtain much information of facial expression. Finally, K nearest neighbor (KNN) and sparse representation classifier (SRC) are combined for classification and a 10-fold cross-validation method is tested in the JAFFE and CK+ databases. The results show that the ICLTP method can improve the recognition rate of facial expression and reduce the confusion between various expressions. Especially, the misrecognition rate of other six expressions recognized as neutral is reduced in the 7-class expression recognition.

Abstract:This study presents a novel and highly efficient superpixel algorithm, namely, depth-fused adaptive superpixel (DFASP), which can generate accurate superpixels in a degraded image. In many applications, particularly in actual scenes, vision degradation, such as motion blur, overexposure, and underexposure, often occurs. Well-known color-based superpixel algorithms are incapable of producing accurate superpixels in degraded images because of the ambiguity of color information caused by vision degradation. To eliminate this ambiguity, we use depth and color information to generate superpixels. We map the depth and color information to a high-dimensional feature space. Then, we develop a fast multilevel clustering algorithm to produce superpixels. Furthermore, we design an adaptive mechanism to adjust the color and depth information automatically during pixel clustering. Experimental results demonstrate that regardless of boundary recall, under segmentation error, run time, or achievable segmentation accuracy, DFASP is better than state-of-the-art superpixel methods.

Abstract:The regeneration rates of rhodopsin under weak and bright light are stimulated, and a model of three energy levels is founded to simulate the photocycle and regeneration rhodopsin when rhodopsin is illuminated by weak and bright light, respectively. Characteristics of the interaction between rhodopsin and light are analyzed theoretically, and this model is used to describe the contribution of rhodopsin to afterimage.