Femtosecond laser-based generation of radio frequency signals has produced astonishing improvements in achievable spectral purity, one of the basic features characterizing the performance of an radio frequency oscillator. Kerr frequency combs hold promise for transforming these lab-scale oscillators to chip-scale level. In this work we demonstrate a miniature 10 GHz radio frequency photonic oscillator characterized with phase noise better than −60 dBc Hz−1 at 10 Hz, −90 dBc Hz−1 at 100 Hz and −170 dBc Hz−1 at 10 MHz. The frequency stability of this device, as represented by Allan deviation measurements, is at the level of 10−10 at 1–100 s integration time—orders of magnitude better than existing radio frequency photonic devices of similar size, weight and power consumption.

We combine fiber Bragg grating ͑FBG͒ technology with a wet chemical etch-erosion procedure and demonstrate two types of refractive index sensors using single-mode optical fibers. The first index sensor device is an etch-eroded single FBG with a radius of 3 m, which is used to measure the indices of four different liquids. The second index sensor device is an etch-eroded fiber Fabry-Pérot interferometer ͑FFPI͒ with a radius of ϳ1.5 m and is used to measure the refractive indices of isopropyl alcohol solutions of different concentrations. Due to its narrower resonance spectral feature, the FFPI sensor has a higher sensitivity than the FBG sensor and can detect an index variation of 1.4ϫ 10 −5 . Since we can measure the reflection signal, these two types of sensors can be fabricated at the end of a fiber and used as point sensors. Since the early 1990s, fiber Bragg grating ͑FBG͒ sensors have been intensively developed due to their many desirable advantages such as the small size, absolute measurement capability, immunity to electromagnetic interference, wavelength multiplexing, and distributed sensing possibilities. [1][2][3][4][5] Thus far, the FBG sensors' capability to measure physical quantities such as the temperature, strain, pressure, etc., has been studied extensively. [2][3][4][5][6][7][8] However, the use of FBG sensors for detection of environmental refractive index change has not been fully explored. Refractive index sensing is important for biological and chemical applications since a number of substances can be detected through measurements of the refractive index. [2][3][4][8][9][10][11][12][13] For normal FBGs, removal of the fiber cladding is required to increase the evanescent field interaction with the surrounding environment. This concept has been demonstrated using D-shaped fiber and sidepolished fiber. 9,11,12 In both cases, the strength and durability of the sensor were greatly reduced. Special fiber was also needed, which would raise the costs and limit the possible applications. Long-period fiber gratings have also been demonstrated to have high sensitivity to the refractive index of the ambient media, 2,3,13-15 however, their multiple resonance peaks and broad ͑typically tens of nanometers͒ transmission resonance features limit the measurement accuracy and their multiplexing capabilities. 9 In addition, the relatively long length of the grating limits their application as point sensor devices.In this letter, we first demonstrate a single etch-eroded FBG sensor using standard single-mode telecommunication fiber ͑Corning SMF-28͒. Fiber Fabry-Pérot interferometers ͑FFPIs͒ have also been widely used as sensors. 2,3,16,17 Compared to a single FBG, the FFPI sensors possess narrower resonance peaks and are more desirable for high accuracy wavelength measurement. 2,3,9,18 To that end we propose and demonstrate an etch-eroded FFPI sensor formed by two FBGs. The FFPI sensor is used to measure the refractive index of isopropyl alcohol ͑IPA͒ solutions of different concentrations, exhibiting the capabi...

Abstract-Smart grid is a promising power delivery infrastructure integrated with communication and information technologies. Its bi-directional communication and electricity flow enable both utilities and customers to monitor, predict, and manage energy usage. It also advances energy and environmental sustainability through the integration of vast distributed energy resources. Deploying such a green electric system has enormous and far-reaching economic and social benefits. Nevertheless, increased interconnection and integration also introduce cybervulnerabilities into the grid. Failure to address these problems will hinder the modernization of the existing power system. In order to build a reliable smart grid, an overview of relevant cyber security and privacy issues is presented. Based on current literatures, several potential research fields are discussed at the end of this paper.

Advanced applications in optical metrology demand improved lasers with high spectral purity, in form factors that are small and insensitive to environmental perturbations. While laboratory-scale lasers with extraordinarily high stability and low noise have been reported, all-integrated chip-scale devices with sub-100 Hz linewidth have not been previously demonstrated. Lasers integrated with optical microresonators as external cavities have the potential for substantial reduction of noise. However, stability and spectral purity improvements of these lasers have only been validated with rack-mounted support equipment, assembled with fibre lasers to marginally improve their noise performance. In this work we report on a realization of a heterogeneously integrated, chip-scale semiconductor laser featuring 30-Hz integral linewidth as well as sub-Hz instantaneous linewidth.

We analyze a mode-locked regime in Kerr frequency combs generated in nonlinear microresonators. Using damped driven nonlinear Schrödinger equations we show that the combs can produce subpicosecond optical pulses when the resonators are characterized with a small enough anomalous group velocity dispersion. We provide an analytical solution of the problem for the case of small damping.

No abstract

Abstract-This paper studies a green paradigm for the underlay coexistence of primary users (PUs) and secondary users (SUs) in energy harvesting cognitive radio networks (EH-CRNs), wherein battery-free SUs capture both the spectrum and the energy of PUs to enhance spectrum efficiency and green energy utilization. To lower the transmit powers of SUs, we employ multi-hop transmission with time division multiple access, by which SUs first harvest energy from the RF signals of PUs, and then, transmit data in the allocated time concurrently with PUs, all in the licensed spectrum. In this way, the available transmit energy of each SU mainly depends on the harvested energy before the turn to transmit, namely energy causality. Meanwhile, the transmit powers of SUs must be strictly controlled to protect PUs from harmful interference. Thus, subject to the energy causality constraint and the interference power constraint, we study the end-to-end throughput maximization problem for optimal time and power allocation. To solve this nonconvex problem, we first equivalently transform it into a convex optimization problem and then propose the joint optimal time and power allocation (JOTPA) algorithm that iteratively solves a series of feasibility problems until convergence. Extensive simulations evaluate the performance of EH-CRNs with JOTPA in three typical deployment scenarios and validate the superiority of JOTPA by making comparisons with two other resource allocation algorithms.

Continuous organ initiation and outgrowth in plants relies on the proliferation and differentiation of stem cells maintained by the CLAVATA (CLV)-WUSCHEL (WUS) negative-feedback loop. Leucine-rich repeat receptor-like protein kinases (LRR-RLKs), including CLV1, BARELY ANY MERISTEMS and RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2), a receptor-like protein CLV2 and a pseudokinase CORYNE (CRN) are involved in the perception of the CLV3 signal to repress WUS expression. WUS, a homeodomain transcription factor, in turn directly activates CLV3 expression and promotes stem cell activity in the shoot apical meristem. However, the signalling mechanism immediately following the perception of CLV3 by its receptors is poorly understood. Here, we show that a group of LRR-RLKs, designated as CLAVATA3 INSENSITIVE RECEPTOR KINASES (CIKs), have essential roles in regulating CLV3-mediated stem cell homeostasis. The cik1 2 3 4 quadruple mutant exhibits a significantly enlarged SAM, resembling clv mutants. Genetic analyses and biochemical assays demonstrated that CIKs function as co-receptors of CLV1, CLV2/CRN and RPK2 to mediate CLV3 signalling through phosphorylation. Our findings not only widen the understanding of the underlying mechanism of CLV3 signal transduction in regulating stem cell fate but also reveal a novel group of RLKs that function as co-receptors to possibly mediate multiple extrinsic and intrinsic signals during plant growth and development.