Abstract:In this paper, we present a stable single-photon detection method based on Si-avalanche photodiode (Si-APD) operating in Geiger mode with a large temperature variation range. By accurate temperature sensing and direct current (DC) bias voltage compensation, the single-photon detector can work stably in Geiger mode from −40 °C to 35 °C with an almost constant avalanche gain. It provides a solution for single-photon detection at outdoor operation in all-weather conditions.

Abstract:A switchable dual-wavelength erbium-doped fiber laser (EDFL) with tunable wavelength is demonstrated. The ring cavity consists of two branches with a fiber Bragg grating (FBG) and a spherical-shape structure as fiber filters, respectively. By adjusting the variable optical attenuator (VOA), the laser can be switched between the single-wavelength mode and the dual-wavelength mode. The spherical-shape structure has good sensitivity to the temperature. When the temperature changes from 30 °C to 190 °C, the central wavelength of the EDFL generated by the branch of spherical-shape structure varies from 1 551.6 nm to 1 561.8 nm, which means that the wavelength interval is tunable.

Abstract:Al-doped ZnO (AZO) has been used as an electron transport and hole blocking buffer layer in inverted organic solar cells (IOSCs). In this paper, the AZO morphology, optical and structural properties and IOSCs performance are investigated as a function of precursor solution concentration from 0.1 mol/L to 1.0 mol/L. We demonstrate that the device with 0.1 mol/L precursor concentration of AZO buffer layers enhances the short-circuit current and the fill factor of IOSCs simultaneously. The resulting device shows that the power conversion efficiency is improved by 35.6% relative to that of the 1.0 mol/L device, due to the improved surface morphology and transmittance (300–400 nm) of AZO buffer layer.

Abstract:A transparent 3-mercaptopropyl trimethoxysilane (MPTMS)/Ag/MoO3 composite anode is introduced to fabricate green organic light-emitting diodes (OLEDs). Effects of the composite anode on brightness and operating voltage of OLEDs are researched. By optimizing the thickness of each layer of the MPTMS/Ag/MoO3 structure, the transmittance of MPTMS/Ag (8 nm)/MoO3 (30 nm) reaches over 75% at about 520 nm. The sheet resistance is 3.78 Ω/□, corresponding to this MPTMS/Ag (8 nm)/MoO3 (30 nm) structure. For the OLEDs with the optimized anode, the maximum electroluminescence (EL) current efficiency reaches 4.5 cd/A, and the maximum brightness is 37 036 cd/m2. Moreover, the OLEDs with the optimized anode exhibit a very low operating voltage (2.6 V) for obtaining brightness of 100 cd/m2. We consider that the improved device performance is mainly attributed to the enhanced hole injection resulting from the reduced hole injection barrier height. Our results indicate that employing the MPTMS/Ag/MoO3 as a composite anode can be a simple and promising technique in the fabrication of low-operating voltage and high-brightness OLEDs.

Abstract:Conventional angle-tuned thin-film filters have serious angle sensitivity for their low spacer effective refractive index, and it is difficult to fabricate their angle control system. In this paper, we propose and fabricate a novel 100 GHz angle-tuned thin-film filter stack with low angle sensitivity, which uses the high refractive index material TiO2 as the spacer, and its incident angle can be expanded to 25°. Compared with the traditional Ta2O5-SiO2 thin-film filter stack, the novel stack has fewer layers. Using the polarization beam splitters and the half wave plates, the polarization sensitivity of the angle-tuned filter can also be suppressed. Simulation results and the experiments show that the thin-film filter with low angle sensitivity has an effective tuning range of 33 nm, which can cover the whole C-band, and its angle control system is easy to be fabricated.

Abstract:In this paper, a microring resonator (MRR) system using double-series ring resonators is proposed to generate and investigate the Rabi oscillations. The system is made up of silicon-on-insulator and attached to bus waveguide which is used as propagation and oscillation medium. The scattering matrix method is employed to determine the output signal intensity which acts as the input source between two-level Rabi oscillation states, where the increase of Rabi oscillation frequency with time is obtained at the resonant state. The population probability of the excited state is higher and unstable at the optical resonant state due to the nonlinear spontaneous emission process. The enhanced spontaneous emission can be managed by the atom (photon) excitation, which can be useful for atomic related sensors and single-photon source applications.

Abstract:A major challenge in GaN based solar cell design is the lack of holes compared with electrons in the multiple quantum wells (MQWs). We find that GaN based MQW photovoltaic devices with five different Mg-doping concentrations of 0 cm-3, 5×1017 cm-3, 2×1018 cm-3, 4×1018 cm-3 and 7×1018 cm-3 in GaN barriers can lead to different hole concentrations in quantum wells (QWs). However, when the Mg-doping concentration is over 1×1018 cm-3, the crystal quality degrades, which results in the reduction of the external quantum efficiency (EQE), short circuit current density and open circuit voltage. As a result, the sample with a slight Mg-doping concentration of 5×1017 cm-3 exhibits the highest conversion efficiency.

Abstract:Uncooled InAsSb photoconductors were fabricated. The photoconductors were based on InAs0.05Sb0.95 and InAs0.09Sb0.91 thick epilayers grown on InAs substrates by melt epitaxy (ME). Ge immersion lenses were set on the photoconductors. The cutoff wavelength of InAs0.09Sb0.91 detectors is obviously extended to 11.5 mm, and that of InAs0.05Sb0.95 detectors is 8.3 mm. At room temperature, the peak detectivity of Dλp at wavelength of 6.8 mm and modulation frequency of 1 200 Hz is 1.08×109 cm.Hz1/2.W-1 for InAs0.09Sb0.91 photoconductors, the detectivity D at wavelength of 9 mm is 7.56×108 cm.Hz1/2.W-1, and that at 11 mm is 3.92×108 cm.Hz1/2.W-1. The detectivity of InAs0.09Sb0.91 detectors at the wavelengths longer than 9 mm is about one order of magnitude higher than that of InAs0.05Sb0.95 detectors, which rises from the increase of arsenic (As) composition in InAs0.09Sb0.91 materials.

Abstract:Dy3+/Eu3+ co-doped cubic lattice NaYF4 single crystal with high quality in the size of ~Φ1.0 cm×10.0 cm was grown by an improved Bridgman method using potassium fluoride (KF) as assistant flux. X-ray diffraction (XRD), absorption spectra, excitation spectra and emission spectra are measured to investigate the phase and luminescent properties of the crystal. The effects of excitation wavelength and concentrations of Dy3+ and Eu3+ ions on the luminescent characteristics are analyzed. The NaYF4 single crystal with the doping molar concentrations of 1.205% Dy3+ and 0.366% Eu3+ exhibits an excellent white light emission with chromaticity coordinates of x=0.321, y=0.332. It indicates that the Dy3+/Eu3+ co-doped cubic lattice NaYF4 single crystal can be a potential luminescent material for the ultraviolet (UV) light excited white light emitting diode (w-LED).

Abstract:Er3+/Ce3+ co-doped tellurite glasses with composition of TeO2-GeO2-Li2O-Nb2O5 were prepared using conventional melt-quenching technique for potential applications in Er3+-doped fiber amplifier (EDFA). The absorption spectra, up-conversion spectra and 1.53 µm band fluorescence spectra of glass samples were measured. It is shown that the 1.53 µm band fluorescence emission intensity of Er3+-doped tellurite glass fiber is improved obviously with the introduction of an appropriate amount of Ce3+, which is attributed to the energy transfer (ET) from Er3+ to Ce3+. Meanwhile, the 1.53 µm band optical signal amplification is simulated based on the rate and power propagation equations, and an increment in signal gain of about 2.4 dB at 1 532 nm in the Er3+/Ce3+ co-doped tellurite glass fiber is found. The maximum signal gain reaches 29.3 dB on a 50 cm-long fiber pumped at 980 nm with power of 100 mW. The results indicate that the prepared Er3+/Ce3+ co-doped tellurite glass is a good gain medium applied for 1.53 µm broadband and high-gain EDFA.

Abstract:SrZn2(PO4)2:Sm3+ phosphor was synthesized by a high temperature solid-state reaction in atmosphere. SrZn2(PO4)2:Sm3+ phosphor is efficiently excited by ultraviolet (UV) and blue light, and the emission peaks are assigned to the transitions of 4G5/2-6H5/2 (563 nm), 4G5/2-6H7/2 (597 nm and 605 nm) and 4G5/2-6H9/2 (644 nm and 653 nm). The emission intensities of SrZn2(PO4)2:Sm3+ are influenced by Sm3+ concentration, and the concentration quenching effect of SrZn2(PO4)2:Sm3+ is also observed. When doping A+ (A=Li, Na and K) ions, the emission intensity of SrZn2(PO4)2:Sm3+ can be obviously enhanced. The Commission Internationale de l''Eclairage (CIE) color coordinates of SrZn2(PO4)2:Sm3+ locate in the orange-red region. The results indicate that the phosphor has a potential application in white light emitting diodes (LEDs).

Abstract:In this work, the synthesis and photoluminescence characteristics of two new phosphors Pr3+:CaYAlO4 (CYA) and Pr3+/Tb3+:CYA for light emitting diodes (LEDs) are investigated.0.5% (atom percentage) Pr3+:CYA exhibits the largest bright yellow emission by varying the Pr3+ concentration, owing to the cross-relaxation process of 3P0 +3H4→ 1G4+1G4. The energy level diagram in Pr3+:CYA, especially the positions of 4f5d level and 1S0 level, is discussed. By co-doping Tb3+ ions, the color coordinates of Pr3+/Tb3+:CYA phosphor can be tuned from yellow to white region. Finally, the strongest luminescence emission with color coordinates of (0.339, 0.364) located in the white region can be obtained in 0.3%Tb3+/0.5%Pr3+:CYA phosphor.

Abstract:Because the partial transmit sequence (PTS) peak-to-average power ratio (PAPR) reduction technology for optical orthogonal frequency division multiplexing (O-OFDM) systems has higher computational complexity, a novel two-stage enhanced-iterative-algorithm PTS (TS-EIA-PTS) PAPR reduction algorithm with lower computational complexity is proposed in this paper. The simulation results show that the proposed TS-EIA-PTS PAPR reduction algorithm can reduce the computational complexity by 18.47% in the condition of the original signal sequence partitioned into 4 sub-blocks at the remaining stage of n−d=5. Furthermore, it has almost the same PAPR reduction performance and the same bit error rate (BER) performance as the EIA-PTS algorithm, and with the increase of the subcarrier number, the computational complexity can be further reduced. As a result, the proposed TS-EIA-PTS PAPR reduction algorithm is more suitable for the practical O-OFDM systems.

Abstract:A novel magnetic field sensor based on optical fiber Mach-Zehnder interferometer (MZI) coated by magnetic fluid (MF) is proposed. The MZI consists of two spherical structures formed on standard single mode fiber (SMF). The interference wavelength and the power of the sensing structure are sensitive to the external refractive index (RI). Since RI of the MF is sensitive to the magnetic field, the magnetic field measurement can be realized by detecting the variation of the interference spectrum. Experimental results show that the wavelength and the power of interference dip both increase with the increase of magnetic field intensity.

Abstract:A novel miniature Fabry-Perot interferometric (FPI) temperature sensor is proposed and demonstrated experimentally. The modal interferometer is fabricated by just splicing a section of photonic crystal fiber (PCF) with a single-mode fiber (SMF). The air holes of the PCF are fully collapsed by the discharge arc during the splicing procedure to enhance the reflection coefficient of the splicing point. The transmission spectra with different temperatures are measured, and the experimental results show that the linear response of 11.12 pm/°C in the range of 30–80 °C is obtained. This sensor has potential applications in temperature measurement field.

Abstract:Based on Lee-Low-Pines (LLP) unitary transformation, this article adopts the variational method of the Pekar type and gets the energy and wave functions of the ground state and the first excited state of strong-coupling bipolaron in two-dimensional quantum dot in electric field, thus constructs a bipolaron qubit. The numerical results represent that the time oscillation period T0 of probability density of the two electrons in qubit decreases with the increasing electric field intensity F and dielectric constant ratio of the medium ; the probability density Q of the two electrons in qubit oscillates periodically with the increasing time t; the probability of electron appearing near the center of the quantum dot is larger, while that appearing away from the center of the quantum dot is much smaller.

Abstract:Aiming at the low speed of traditional scale-invariant feature transform (SIFT) matching algorithm, an improved matching algorithm is proposed in this paper. Firstly, feature points are detected and the speed of feature points matching is improved by adding epipolar constraint; then according to the matching feature points, the homography matrix is obtained by the least square method; finally, according to the homography matrix, the points in the left image can be mapped into the right image, and if the distance between the mapping point and the matching point in the right image is smaller than the threshold value, the pair of matching points is retained, otherwise discarded. Experimental results show that with the improved matching algorithm, the matching time is reduced by 73.3% and the matching points are entirely correct. In addition, the improved method is robust to rotation and translation.

Abstract:In lung CT images, the edge of a tumor is frequently fuzzy because of the complex relationship between tumors and tissues, especially in cases that the tumor adheres to the chest and lung in the pathology area. This makes the tumor segmentation more difficult. In order to segment tumors in lung CT images accurately, a method based on support vector machine (SVM) and improved level set model is proposed. Firstly, the image is divided into several block units; then the texture, gray and shape features of each block are extracted to construct eigenvector and then the SVM classifier is trained to detect suspicious lung lesion areas; finally, the suspicious edge is extracted as the initial contour after optimizing lesion areas, and the complete tumor segmentation can be obtained by level set model modified with morphological gradient. Experimental results show that this method can efficiently and fast segment the tumors from complex lung CT images with higher accuracy.