Search

Silica microspheres have been demonstrated as the optical micro-cavity for laser generation based on whispering-gallery modes (WGM). They can achieve a high-quality factor (Q) within a tiny volume. Traditionally, complex processes are needed to process a coating containing photoluminescent elements, with enough thickness and uniformity, for laser generation. We developed a novel sol-gel fabrication method of Er3+/Yb3+doped silica microspheres with a homogeneous doping concentration over the entire microsphere volume. The sol-gel precursors were doped with 1–2 mol% Er3+ and/or Yb3+. The viscosity of the precursor was controlled at around 4000 cP, and gel fibers can be conveniently drawn from the precursor. After firing at 1000 °C for 1 h, transparent fibers with diameters of 40–180 µm were obtained. The fiber tips were quickly melted into microspheres using a CO2 laser. The diameters of microspheres were determined by the fiber diameter and laser parameters, such as laser power and irradiation time duration. Typically, the microspheres had diameters between 90 to 160 µm. The emission spectrum under 357 nm and 527 nm excitation showed characteristic Er3+ emission peaks, that match the literature well. The UV-VIS spectra confirm the photoluminescence results and showed both Er3+ and Yb3+ characteristic absorptions. The optical behaviors of the microspheres indicate that the Er3+ and Yb3+ were well dispersed in the silica matrix and the microspheres had typical optical activities of Er3+/Yb3+ glasses. Highlights: Silica microspheres, fabricated using laser melting, with homogeneously doped Er3+/Yb3+. No need for multiple layers of Er3+/Yb3+ sol-gel coatings. Microspheres demonstrated strong optical activities with Er3+/Yb3+ doping. [ABSTRACT FROM AUTHOR]

The outbreak of the COVID-19 Omicron variant in Shanghai in 2022 elicited complex emotions among Shanghainese during the two-month quarantine period. This paper aims to identify prevailing public themes and sentiments by analyzing social media posts from Weibo. Initially, we conducted research based on a dataset of 90,000 Weibo posts during the 2022 COVID-19 outbreak in Shanghai. By examining social media data that mirrors residents' emotional shifts and areas of focus during unforeseen circumstances, we have developed an analytical framework combining hotspot analysis and public sentiment assessment. Subsequently, we employed the Latent Dirichlet Allocation (LDA) method to conduct topic modeling on the Weibo text data. The SnowNLP sentiment classification method was then utilized to quantify sentiment values. Ultimately, we performed spatial visualization of sentiment and concern data, categorizing them into distinct time periods based on Shanghai's infection curve. This approach allowed us to investigate concern focal points, sentiment trends, and their spatiotemporal evolution characteristics. Our findings indicate that variations in public sentiment primarily hinge on the severity of the epidemic's spread, emerging events, the availability of essential resources, and the government's ability to respond promptly and accurately. It is evident that, while residents' concerns shift over time, their primary objective on social media remains expressing demands and releasing emotions. This research offers an avenue for leveraging public opinion analysis to enhance governance capacity during crises, fortify urban resilience, and promote public involvement in governmental decision-making processes. [ABSTRACT FROM AUTHOR]

This paper empirically investigates the effect of product life‐cycle on trade patterns. We measure the product life‐cycle length with patent citation data at the industry level. Using bilateral trade data from 2002 to 2006, we find that countries with more knowledge capital endowment export more in industries with shorter product life‐cycles. We show that this pattern is largely driven by the risk of imitation. The pattern is reversed when imitation is limited by stronger intellectual property rights. [ABSTRACT FROM AUTHOR]

Rapid expansion of China's exports and inward foreign direct investment (FDI) are characteristic of China's outward-oriented economy. Based on a unique micro-level survey on the co-development between domestic enterprises and foreign-invested enterprises in Kunshan County of Jiangsu Province, this paper finds that domestic enterprises can improve their productivity by matching (in terms of supply, processing, or original equipment manufacturer [OEM] relationship) with foreign-invested enterprises. A self-selection mechanism of matching is at work for domestic enterprises involved in exports: when foreign-invested enterprises operate in China, domestic enterprises compete to match with them, which eventually helps them improve. Empirical analysis supports the emergence of this mechanism, together with a learning-by-exporting effect and a peer effect. In addition, with respect to the peer effect, the improvement in productivity at domestic enterprises whose main product is intermediate inputs depends more on skilled or high-quality labor, while that of domestic enterprises whose main products are capital and consumable products depend more on management staff. [ABSTRACT FROM AUTHOR]

A novel photonic method for low power radio-frequency (RF) signal detection by a tunable optoelectronic oscillator (OEO) with high gain based on equivalent phase modulation (EPM) is proposed and experimentally demonstrated. A dual-parallel Mach-Zehnder modulator is utilized to generate the EPM signal with a controllable optical carrier to sideband ratio, by which the gain to the RF signal by the OEO loop can be enhanced. A phase-shifted fiber Bragg grating (PS-FBG) with a narrow-band notch in its reflection band implements the phase modulation to intensity modulation conversion by removing one sideband of the EPM signal. The gain and noise performance of the detection system are theoretically analyzed and simulated. In the experiment, the maximum gain of 10.4 dB is obtained at 8 GHz, and the detection sensitivity for RF signals from 2 to 18 GHz are within the range of −87.7 to −84.3 dBm by using the developed detection system. Compared to normal phase modulation based scheme, the EPM-based system offers 9.1 dB higher gain. Utility of the proposed approach for detecting modulated RF signal is also investigated and an optimization scheme is discussed. [ABSTRACT FROM AUTHOR]

Using a panel of Chinese domestic enterprises over the 2002–2008 period, we assess the effect of the state-owned commercial bank shareholding reform on the domestic enterprises' outward foreign direct investment (OFDI) decisions. We find that a modern market-oriented bank system significantly increases the likelihood of OFDI, especially for the purpose of facilitating trade and services. [ABSTRACT FROM AUTHOR]

A novel photonic method for microwave downconversion by using a tunable optoelectronic oscillator (OEO) based on a phase-shifted fiber-Bragg grating (PS-FBG) and the polarization-multiplexed dual loop is proposed and investigated. By the virtue of the narrow notch bandwidth of the PS-FBG and the side-mode suppression of the optimized dual loop, the frequency tunable local oscillator (LO) signal with high spectral purity and low phase noise is generated by changing the frequency of the optical carrier from the laser source. The microwave downconversion is conducted directly by inputting the radio frequency (RF) signal into the OEO without any additional components. The experiment of the LO signal generation and the microwave downconversion is carried out. The frequency tunable LO signals from 2.5 to 7 GHz with the side-mode suppression ratio as high as 56 dB and the phase noise of −109 dBc/Hz at 10 kHz are generated successfully. By tuning the LO frequency generated by the proposed OEO, the RF signals with different frequencies from 3.5 to 8 GHz are converted to the same intermediate frequency band of 1 GHz. The spurious-free dynamic range of the downconversion system is measured to be 102.2 dB $\cdot $ Hz $^{2/3}$. The downconversion performance for RF carrying 16 quadrature amplitude modulation signal is also analyzed. [ABSTRACT FROM AUTHOR]

Based on the MIKE 21 numerical model combined with measured data, a numerical model for the coupling of water and salinity in the Changjiang (Yangtze) River estuary was established, and based on good verification, the influence of the tidal current intensity on the transport and variations of salinity concentrations in saline water from the North Branch to the South Branch was numerically evaluated. The time and space mean root mean square of the tidal current velocity can be expressed as a parabolic function of an adjustment coefficient for the amplitude of the M2 tidal constituent along the open boundaries of the model. Under the advection of runoff and tidal currents, the saline pool is transported downstream in an oscillatory pattern. With the enhancement of tidal current intensity, the oscillatory motion of the saltwater pool is increasingly significant in a tidal cycle forced by rising and falling tides. Along four set paths, the daily average concentrations of the saline core were generally similar, and in the process of transportation downstream, the concentrations of saltwater declined. The decay of the tidal-period-averaged salinity of the saltwater was linearly related to the square of the tidal current. Thus, the tidal current has a significant and direct impact on saltwater transport and diffusion in the Changjiang River estuary. [ABSTRACT FROM AUTHOR]

This paper discusses the trade policies and practices of Singapore based on the latest WTO trade policy review. Though Singapore's trade policies have been effective and contributing to its remarkable economic performance, it also faces problems and new challenges in the modern globalisation era. Reviewing Singapore's policies also provides guidance for other small open economies. [ABSTRACT FROM AUTHOR]

A novel optical approach to implement RF self-interference cancellation for full-duplex communication using phase modulation and optical sideband filtering is proposed and demonstrated experimentally. Based on the inherent out-of-phase property between the left and right sidebands of phase-modulated signal and optical sideband filtering, the RF self-interference cancellation is achieved by tuning the delay time and amplitude in the optical domain. RF self-interference cancellation for single frequency and microwave with various bandwidths of 1, 5, and 10 MHz is experimentally demonstrated to verify the proposed technique. [ABSTRACT FROM PUBLISHER]

The polymer waveguide optical biosensor based on the Mach-Zehnder interferometer (MZI) by using spectral splitting effect is investigated. The MZI based biosensor has two unequal width sensing arms. With the different mode dispersion responses of the two-arm waveguides to the cladding refractive index change, the spectral splitting effect of the output interference spectrum is obtained, inducing a very high sensitivity. The influence of the different mode dispersions between the two-arm waveguides on the spectral splitting characteristic is analyzed. By choosing different lengths of the two unequal width sensing arms, the initial dip wavelength of the interference spectrum and the spectral splitting range can be controlled flexibly. The polymer waveguide optical biosensor is designed, and its sensing property is analyzed. The results show that the sensitivity of the polymer waveguide optical biosensor by using spectral splitting effect is as high as 10 nm/RIU, with an improvement of 2-3 orders of magnitude compared with the slot waveguide based microring biosensor. [ABSTRACT FROM AUTHOR]

An optical approach to implement radio-frequency (RF) self-interference cancellation using an integrated dual-parallel Mach–Zehnder modulator (DP-MZM) is proposed and demonstrated experimentally. The received signals and the reference signal are converted into optical domain via the sub-MZMs (MZM1 and MZM2) in the DP-MZM, which are biased at the quadrature point and the minimum point with the output of double sideband with carrier signal and double sideband with carrier-suppressed signal, respectively. It avoids the optical interference at the combining Y branch waveguide. The time and the amplitude matching conditions for cancellation between the reference signal and the interference signal are completed by the electrical tunable time delay line and the phase shifter in the MZM2 path. A cancellation depth larger than 80 dB is obtained for the single-frequency signal of 2.4, 5, 8, and 10 GHz. For the bandwidth of 100 MHz, cancellation depths as high as 35.8, 33.1, and 38.3 dB are realized at frequencies of 2.4, 5, 8, and 10 GHz, respectively. [ABSTRACT FROM PUBLISHER]

A 2×2 optical waveguide coupler at 850 nm based on the multimode interference (MMI) structure with the polysilsesquioxanes liquid series (PSQ-Ls) polymer material and the imprint technique is presented. The influence of the structural parameters, such as the single mode condition, the waveguide spacing of input/output ports, and the width and length of the multimode waveguide, on the optical splitting performance including the excess loss and the uniformity is simulated by the beam propagation method. By inserting a taper section of isosceles trapezoid between the single mode and multimode waveguides, the optimized structural parameters for low excess loss and high uniformity are obtained with the excess loss of-0.040 dB and the uniformity of-0.007 dB. The effect of the structure deviations induced during the imprint process on the optical splitting performance at different residual layer thicknesses is also investigated. The analysis results provide useful instructions for the waveguide device fabrication. [ABSTRACT FROM AUTHOR]