Wearable sensors are increasingly finding their way into applications of kinesthetic sensing, personal health monitoring, and smart prosthetics/robotics. A graphene-based composite fiber sensor with a "compression spring" structure is fabricated, featuring the ability of detecting multiple kinds of deformation. This fiber sensor is integrated into wearable sensors for monitoring human activities and intricate movements of robotics successfully.

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A Stretchable and Highly Sensitive Graphene-Based Fiber for Sensing Tensile Strain, Bending, and Torsion

DOI: 10.1002/adom.201801433 Scientific American selects plasmonic sensing as the top 10 emerging technologies of 2018.[15] Almost every single new plasmonic or photonic structure would be explored to test its sensing ability.[16–29] These works tend to report the sensing performance of their own structure. Some declare that their sensitivity breaks the world record. However, there is still a missing literature on what the world record really is, the gap between the experiments and the theoretical limit, as well as the differences between metal-based plasmonic sensors and dielectric-based photonic sensors. To push plasmonic and photonic sensors into industrial applications, an optical sensing technology map is absolutely necessary. This review aims to cover a wide range of most representative plasmonic and photonic sensors, and place them into a single map. The sensor performances of different structures will be distinctly illustrated. Future researchers could plot the sensing ability of their new sensors into this technology map and gauge their performances in this field. In this review, we focus on optical refractive index (RI) sensors with no fluorescent labeling required. We will utilize two parameters to characterize and compare the performance of optical RI sensors: sensitivity to RI change (denoted by symbol SRI) and figure of merit (in short, FoM). For simplicity, we restrict our discussions to bulk RI change, where the change in RI occurs within the whole sample. There is another case where the RI variation occurs only within a very small volume close to the sensor surface. This surface RI sensitivity is proportional to the bulk RI sensitivity, the ratio of the thickness of the layer within which the surface RI variation occurs, and the penetration depth of the optical mode.[6] The bulk RI sensitivity defines the ratio of the change in sensor output (e.g., resonance angle, intensity, or resonant wavelength) to the bulk RI variations. Here, we limit our discussions to the spectral interrogations and the bulk RI sensitivity SRI is given by[3,5–7,30]

Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth

Carbon dots (CDs) display tunable photoluminescence and excitation-wavelength dependent emission. The color of fluorescence is affected by electronic bandgap transitions of conjugated π-domains, surface defect states, local fluorophores and element doping. In this review (with 145 refs.), the studies performed in the past 5 years on the relationship between the fluorescence mechanism and modes for modulating the emission color of CDs are summarized. The applications of such CDs in sensors and assays are then outlined. A concluding section then gives an outlook and describes current challenges in the design of CDs with different emission colors. Graphical abstract Schematic representation of the relationship between the color-emitting (blue, green, yellow, red and multicolor) modulation of carbon dots and fluorescence mechanism including bandgap transitions of conjugated π-domains and surface defect states.

The fluorescence mechanism of carbon dots, and methods for tuning their emission color: a review

This paper presents a systematic review of long period fiber gratings (LPFGs) written by the CO2 laser irradiation technique. First, various fabrication techniques based on CO2 laser irradiations are demonstrated to write LPFGs in different types of optical fibers such as conventional glass fibers, solid-core photonic crystal fibers, and air-core photonic bandgap fibers. Second, possible mechanisms, e.g., residual stress relaxation, glass structure changes, and physical deformation, of refractive index modulations in the CO2-laser-induced LPFGs are analyzed. Third, asymmetrical mode coupling, resulting from single-side laser irradiation, is discussed to understand unique optical properties of the CO2-laser-induced LPFGs. Fourthly, several pretreament and post-treatment techniques are proposed to enhance the efficiency of grating fabrications. Fifthly, sensing applications of the CO2-laser-induced LPFGs are investigated to develop various LPFG-based temperature, strain, bend, torsion, pressure, and biochemical sensors. Finally, communication applications of the CO2-laser-induced LPFGs are investigated to develop various LPFG-based band-rejection filters, gain equalizers, polarizers, and couplers.

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Review of long period fiber gratings written by CO2 laser

Soybean production is greatly influenced by abiotic stresses imposed by environmental factors such as drought, water submergence, salt, and heavy metals. A thorough understanding of plant response to abiotic stress at the molecular level is a prerequisite for its effective management. The molecular mechanism of stress tolerance is complex and requires information at the omic level to understand it effectively. In this regard, enormous progress has been made in the omics field in the areas of genomics, transcriptomics, and proteomics. The emerging field of ionomics is also being employed for investigating abiotic stress tolerance in soybean. Omic approaches generate a huge amount of data, and adequate advancements in computational tools have been achieved for effective analysis. However, the integration of omic-scale information to address complex genetics and physiological questions is still a challenge. In this review, we have described advances in omic tools in the view of conventional and modern approaches being used to dissect abiotic stress tolerance in soybean. Emphasis was given to approaches such as quantitative trait loci (QTL) mapping, genome-wide association studies (GWAS), and genomic selection (GS). Comparative genomics and candidate gene approaches are also discussed considering identification of potential genomic loci, genes, and biochemical pathways involved in stress tolerance mechanism in soybean. This review also provides a comprehensive catalog of available online omic resources for soybean and its effective utilization. We have also addressed the significance of phenomics in the integrated approaches and recognized high-throughput multi-dimensional phenotyping as a major limiting factor for the improvement of abiotic stress tolerance in soybean.

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Integrating omic approaches for abiotic stress tolerance in soybean.

We experimentally demonstrate a narrow linewidth semiconductor multi-wavelength distributed feedback (DFB) laser array by simultaneously injection locking each laser to the corresponding resonance of a single on-chip microring resonator. The white frequency noises of all the DFB lasers is reduced by more than 40 dB once they are simultaneously injection locked to a single microring resonator with a quality factor (Q-factor) of 2.38 million. Correspondingly, the instantaneous linewidths of all the DFB lasers are narrowed by a factor of 104. In addition, frequency combs originating from non-degenerate four-wave mixing (FWM) between the locked DFB lasers are also observed. Simultaneously injection locking multi-wavelength lasers to a single on-chip resonator may enable the possibilities of integrating a narrow-linewidth semiconductor laser array on a single chip and having multiple microcombs in a single resonator, which are in high demand in wavelength division multiplexing coherent optical communication systems and metrological applications.

Narrow linewidth semiconductor multi-wavelength DFB laser array simultaneously self-injection locked to a single microring resonator.

A new type of polydimethylsiloxane (PDMS) optical microring resonator on MgF2 substrate is proposed and fabricated by nano-imprint lithography. The measured quality factors of the resonators were in the order of 104 in the C band. Our work provides a new method for fabrication of on-chip whispering gallery mode resonators, which can benefit the applications in communication and sensing fields.

Polydimethylsiloxane optical microring resonator by nano-imprint lithography on MgF2 substrate

Tunable Ultra-Narrow Linewidth Linear-Cavity Fiber Lasers Assisted by Distributed External Feedback

We propose and demonstrate a novel concept of Brillouin fiber swept laser with narrow linewidth based on mode following effect. Utilizing the synergy of the Brillouin gain spectrum and cavity mode, the real-time mode following effect is achieved. In the experiment, a tunable laser source was employed to realize synchronous tuning of Brillouin gain spectrum for matching the shift of cavity mode induced by an intra-cavity piezoelectric transducer wound with gain fiber. We theoretically analyzed the influence of the driving parameters and wound fiber length on the frequency sweep range. Experimental results show that the obtained maximum continuous frequency sweep range is ∼445.76 MHz while the driving frequency is 12 kHz and the wound fiber length is 12 m, which is consistent with the theoretical analysis. This new type of fiber swept laser has huge potential application value in optical sensing and communication system.

Real-Time Mode Following Based Narrow Linewidth Brillouin Fiber Swept Laser

An interferometric microfluidic sensor is proposed by perpendicularly coupling light into/out from a glass capillary with a pair of collimators. Light trapped in the wall of a capillary with inner and outer radius of 245μm and 413.5μm travels along different paths and interferes at the output, resulting in a multiple modes interferometer. By successively injecting salt solutions with different concentrations into the capillary air hole, red shift happens in the interferometric spectra with a sensitivity of 42.75nm/RIU. Such a sensing head offers potentials in identifying solutions in real time, especially for those not transparent for light.

Leilei Shi

Papers

Narrow linewidth semiconductor multi-wavelength DFB laser array simultaneously self-injection locked to a single microring resonator.

Polydimethylsiloxane optical microring resonator by nano-imprint lithography on MgF2 substrate

Tunable Ultra-Narrow Linewidth Linear-Cavity Fiber Lasers Assisted by Distributed External Feedback

Real-Time Mode Following Based Narrow Linewidth Brillouin Fiber Swept Laser

Perpendicular coupling glass capillary based interferometric real-time microfluidic sensor