Fiber laser strain sensor based on an optical phase-locked loop.

In this Letter, we present a high-strain resolution fiber laser-based sensor (FLS) by a novel optical phase-locked loop (OPLL) interrogation technique based on a root mean square detector (RMSD). The sensor consists of a distributed feedback (DFB) fiber laser as a master laser for strain sensing and a fiber Fabry-Perot interferometer (FFPI) as a reference. The laser carrier locks to the reference by the PDH technique, and the single sideband laser working as a slave laser locks to the DFB sensing element using the OPLL technique, respectively. A strain resolution of 8.19 pε/√Hz at 1 Hz and 35.5 pε in 10 s is achieved in the demonstrational experiments. Significantly, the noise behaves a 1∕f distribution below 0.2 Hz due to the very low pump power for the DFB sensor and an active thermostat testing environment. The proposed OPLL interrogation brings new thinking for the demodulation of FLS. This strain sensor based on FLS has a great performance in strain measurement and can be a powerful tool for geophysical research.

The Frequency-Variable Rotor-Blade-Based Two-Degree-of-Freedom Actuation Principle for Linear and Rotary Motion.

Piezoelectric accurate actuation plays an important role in industrial applications. The intrinsic frequency of previous actuators is invariable. However, variable frequency can approach the range near the low-intrinsic-frequency and realize a high actuation capability. The frequency-variable linear and rotary motion (FVLRM) principle is proposed for rotor-blade-based two-degree-of-freedom driving. Inertial force is generated by frequency-variable piezoelectric oscillators (FVPO), the base frequency and vibration modes of which are adjustable by the changeable mass and position of the mass block. The variable-frequency principle of FVPO and the FVLRM are recognized and verified by the simulations and experiments, respectively. The experiments show that the FVLRM prototype moves the fastest when the mass block is placed at the farthest position and the prototype is at the second-order intrinsic frequencies of 42 Hz and 43 Hz, achieving a linear motion of 3.52 mm/s and a rotary motion of 286.9 mrad/s. The actuator adopts a lower operating frequency of less than 60 Hz and has the function of adjusting the natural frequency. It can achieve linear and rotational motion with a larger working stroke with 140 mm linear movement and 360° rotation.

Tunable plasmon-induced transparency with a dielectric grating-coupled graphene structure for slowing terahertz waves.

We present a tunable plasmon-induced transparency (PIT) structure that is composed of dielectric grating and a graphene system to manipulate terahertz (THz) waves. The graphene system consists of a graphene sheet and a graphene ribbon layer, with a spacer between them. By exploiting the diffraction coupling of THz wave with dielectric grating, graphene plasmonic resonance is efficiently excited on both graphene sheet and graphene ribbons. This leads to the surface plasmon mode of the graphene sheet and the localized plasmon mode of the graphene ribbons. The coupling between the two-plasmon modes via near-field destructive interference generates a strong PIT effect with slowing the group velocity of THz waves. A group delay over 0.2 ps and group index beyond 170 can be achievable. The group slowing effect is dynamically tunable with varying the Fermi level of graphene. The work suggests a promising scheme for on-chip graphene slow-wave devices at the THz regime.

Broadband single-polarization optical fiber based on surface plasmon resonance.

We present, to the best of our knowledge, a new scheme of broadband single-polarization optical fiber with high extinction ratio based on surface plasmon resonance (SPR). The double-hole optical fiber with a Ge-doped core is modified by integrating the stacks of conductive and dielectric layers to support SPR. The strong couplings between the guided modes and surface plasmon mode can bring about serious polarization loss of TM mode, while supporting the efficient transmission of TE polarization in broadband. The achievable extinction ratio can be more than 25 dB covering a wide telecom band of 1.17-1.42 µm in short fiber lengths=2.5mm. Meanwhile, the insertion loss of the fiber is less than 0.25 dB. The modified SPR fiber shows promising application in high-quality fiber-integrated polarizers.

Optofluidic in-fiber integrated surface-enhanced Raman spectroscopy detection based on a hollow optical fiber with a suspended core.

In this Letter, we propose, to the best of our knowledge, the first in-fiber optofluidic Raman surface-enhanced Raman spectroscopy (SERS) sensor based on a microstructured hollow fiber (MHF) with a suspended core. Taking advantage of the unique internal structure, we immobilize silver nanoparticles with an SERS effect in the MHF by chemical bonding. The Raman signal of the microfluidic sample is excited by the excitation light in the suspended core through an evanescent field. Then the online SERS signal can be coupled back into the core and detected. To demonstrate the feasibility of the device, rhodamine 6G is chosen as the analyte, and high-quality SERS spectra are detected with the limit of detection of 1×10-14 M. Furthermore, an online optofluidic test is conducted on ceftriaxone (C18H18N8O7S3) to examine its capabilities in antibiotic sensing. The results show that the LOD of the samples is 10-6 M. Significantly, this Letter provides an integrated optofluidic in-fiber SERS sensor with a microchannel that can be integrated with chip devices without spatial optical coupling, which has a broad application in medicine and food safety, as well as various aspects of biological in-fiber sensing.

Graphene/liquid crystal hybrid tuning terahertz perfect absorber.

We present, by simulations, a metastructured graphene/liquid crystal hybrid tuning terahertz perfect absorber that consists of graphene disk resonator arrays above a metallic layer separated with liquid crystal substrate. The liquid crystal refractive index and the graphene Fermi level are utilized to implement double-tuning operation to push the spectra scanning limit of the terahertz absorber. Our simulations demonstrate high performance of a near-linear broad tuning region and near-unity absorbance with wide incident angle and polarization independence. The range of the resonant frequency scan is notably enlarged at a spectral ratio of as high as Δf/f=50% while ensuring absorbance beyond 90%. Such graphene/liquid crystal hybrid tuning scheme would be preferable to push the working-band limit of terahertz perfect absorbers.

Double D-shaped hole optical fiber coated with graphene as a polarizer.

A double D-shaped hole optical fiber coated with graphene is proposed as a polarizer at the wavelength of 1.55 µm. As the planar surfaces of D-shaped holes are both coated with graphene, the interaction between the core and graphene can be doubled. Moreover, the interaction can be further improved by introducing functional materials into the holes. The proposed fiber provides a high extinction ratio (ER) and low insertion loss, and it operates in the single polarization mode. The ER of 42.5 dB with a 2.5-mm-long optical fiber can be achieved for a transverse-electric-pass polarizer, and the insertion loss is approximately 1.08 dB. Specifically, the proposed fiber can achieve simultaneously dual-band polarization at 1.55 µm and 1.31 µm. The proposed fiber is feasible for seamless integration in existing fiber systems. We hope our work benefits high-efficiency polarizers, and we believe that the proposed fiber has some potential applications in photonic integrated circuits.

Two-core single-polarization optical fiber with a large hollow coated bimetallic layer.

A two-core hollow optical fiber for a polarizer based on surface plasmon resonance (SPR) is proposed and studied by the full-vector finite-element method. The proposed fiber consists of two circular cores, inner cladding, outer cladding, and a large central air hole. The two cores are arranged symmetrically in inner cladding and couple weakly with the air hole. There is no cross talk between cores because they are insulated by the air hole. A nanodimension Ag/Au bimetallic layer can be coated on the inner surface of the central air hole to support SPR. The numerical results show that single polarization of two cores is achieved simultaneously at the wavelength of 1.310 µm, due to strong coupling between the TM mode and the surface plasmon polariton mode. The extinction ratio 40.90 dB with 3 mm length is obtained, and the confinement loss of TE mode is 0.19 dB/cm. Moreover, the resonance wavelength is tunable by varying the refractive index of materials in the central air hole. The scheme is helpful for coupling an all-fiber polarizer with multicore polarization maintaining fibres (PMFs) and makes possible application in in-fiber integrated interferometric sensors while maintaining polarization.

Design and numerical analysis of a THz square porous-core photonic crystal fiber for low flattened dispersion, ultrahigh birefringence.

We propose a kind of square porous-core photonic crystal fiber (PCF) for polarization-maintaining terahertz (THz) wave guidance. An asymmetry is introduced by implementing rectangular array air holes in the porous core of the PCF, and ultrahigh birefringence and low effective material loss (EML) can be achieved simultaneously. The properties of THz wave propagation are analyzed numerically in detail. The numerical results indicate that the proposed fiber offers a high birefringence of 0.063 and a low EML of 0.081 cm-1 at 1 THz. Moreover, a very low flattened dispersion profile is observed over a wide frequency domain of 0.85-1.9 THz. The zero flattened dispersion can be controlled. It is predicted that this PCF would be used potentially in polarization maintaining and dispersion management of THz waves.

Self-powered optical fiber biosensor integrated with enzymes for non-invasive glucose sensing.

To alleviate the discomfort associated with frequent blood glucose detection in diabetic patients, a novel non-invasive tear glucose biosensor has been developed. This involved the design and preparation of a photoelectrochemical probe based on an optical fiber and biological enzymes. One end of the optical fiber connects to a light source, acting as an energy source and imparting, self-powered capability to the biosensor. The opposite end is loaded with nanomaterials and glucose oxidase, designed for insertion into the sample to realize photoelectrochemical sensing. This innovative configuration not only improves the integration of the biosensor but is also suitable for analyzing minuscule voluminal samples. The results show that the proposed biosensor exhibits a linear range from 10 nM to 100 µM, possesses a low detection limit of 4.1 nM and a short response time of 0.7 s. Benefiting from the high selectivity of the enzyme, the proposed biosensor demonstrates excellent resistance to the interference of common tear components. In summary, this work provides a more effective method for non-invasive glucose detection and affords valuable ideas for the design and fabrication of non-invasive and self-powered biosensors.

ZnO/Cu2O heterojunction integrated fiber-optic biosensor for remote detection of cysteine.

Indium tin oxide, semiconductor nanomaterial ZnO, and Cu2O were first loaded on the surface of the optical fiber to form an optical fiber probe. Large-volume macroscopic spatial light is replaced by an optical fiber path, and remote light injection is implemented. Based on the optical fiber probe, a photoelectrochemical biosensor was constructed and remote detection of cysteine was realized. In this tiny device, the optical fiber probe not only acts as a working electrode to react with the analyte but also directs the light exactly where it is needed. Simultaneously, the electrochemical behavior of cysteine on the surface of the working electrode is dominated by diffusion-control, which provides strong support for quantitative detection. Then, under the bias potential of 0 V, the linear range of the fiber-optic-based cysteine biosensor was 0.01∼1 µM, the regression coefficient (R2) value was 0.9943. In spiked synthetic urine, the detection of cysteine was also realized by the integrated biosensor. Moreover, benefiting from the low optical fiber loss, the new structure also possesses a unique remote detection function. This work confirms that photoelectrochemical biosensors can be integrated via optical fibers and retain comparable sensing performance. Based on this property, different materials can also be loaded on the surface of the optical fiber for remote detection of other analytes. It is expected to facilitate the research on fiber-optic-based integrated biosensors and show application prospects in diverse fields such as biochemical analysis and disease diagnosis.

Temperature and salinity sensing characteristics of embedded core optical fiber based on surface plasmon resonance.

An embedded core fiber sensor based on surface plasmon resonance (SPR) principle is developed. In the structure of optical fiber, the middle of the optical fiber cladding is hollowed out. The hollowed-out part is then filled with a temperature-sensitive layer. For the temperature sensitive layer, polydimethylsiloxane(PDMS) is chosen. A metal layer is placed outside the cladding of the optical fiber to detect changes in the external environment and stimulate the SPR effect.The gold metal(Au) layer is also placed between the cladding and the PDMS to stimulate the SPR effect.The refractive index of seawater varies with salinity and temperature through COMSOL Multiphysics finite element simulation. We can measure the two parameters of salinity and temperature at the same time based on the SPR principle. The sensitivity of salinity and temperature calculated by this sensor is 0.193 nm/%, 0.397 nm/°C. Fiber optic sensors use the SPR principle to detect dynamic, real-time, continuous processes. The measurement range is very wide, and the brightness is also very high.Compared with single-channel measurement of single parameter, this sensor can greatly improve the efficiency of two-parameter measurement. The sensor has the advantages of simple structure, low production cost and high sensitivity, which can realize the simultaneous measurement of two parameters and avoid the crosstalk between parameters. It has great research significance.

Optical fiber modulator derivates from hollow optical fiber with suspended core.

A fiber optic integrated modulation-depth-tunable modulator based on a type of hollow optical fiber with suspended core is proposed and investigated. We synthesized magnetic fluid containing superparamagnetic Fe(3)O(4) nanoparticles and encapsulated it in the hollow optical fiber as the cladding layer of the suspended core by fusing the hollow optical fiber with the multimode optical fibers. The light with a wavelength of 632.8 nm is coupled in and out of the modulating element by a tapering technique. Experimental results show that the light attenuation in the system can be greatly influenced by only 2.0×10(-2) µL of the magnetic fluid under different magnetic field strengths. The saturated modulation depth is 43% when the magnetic field strength is 489 Oe. The response time of the system is <120 ms. Significantly, this work presents information for the development of all-fiber modulators, including other integrated electro-optic devices, such as optical switch, optical fiber filter, and magnetic sensors utilizing the special structure of this hollow optical fiber with suspended core and superparamagnetic magnetic fluid.

Characteristics of embedded-core hollow optical fiber.

We propose a novel embedded-core hollow optical fiber composed of a central air hole, a semi-elliptical core, and an annular cladding. The fiber characteristics are investigated based on the finite element method (FEM), including mode properties, birefringence, confinement loss, evanescent field and bending loss. The results reveal that the embedded-core hollow optical fiber has a non-zero cut-off frequency for the fundamental mode. The birefringence of the hollow optical fiber is insensitive to the size of the central air hole and ultra-sensitive to the thickness of the cladding between the core and the air hole. Both thin cladding between the core and the air hole and small core ellipticity lead to high birefringence. An ultra-low birefringence fiber can be achieved by selecting a proper ellipticity of the core. The embedded-core hollow optical fiber holds a strong evanescent field due to special structure of thin cladding and therefore it is of importance for potential applications such as gas and biochemical sensors. The bending losses are measured experimentally. The bending loss strongly depends on bending orientations of the fiber. The proposed fiber can be used as polarization interference devices if the orientation angle of the fiber core is neither 0° nor 90°.

Integrated fiber Michelson interferometer based on poled hollow twin-core fiber.

We propose an integrated fiber Michelson interferometer based on a poled hollow twin-core fiber. The Michelson interferometer can be used as an electro-optic modulator by thermal poling one core of the twin-core fiber and introducing second-order nonlinearity in the fiber. The proposed fiber Michelson interferometer is experimentally demonstrated under driving voltages at the frequency range of 149 to 1000 Hz. The half-wave voltage of the poled fiber is 135 V, and the effective second-order nonlinear coefficient χ² is 1.23 pm/V.

Optical refractometer based on an asymmetrical twin-core fiber Michelson interferometer.

We report and demonstrate an optical refractometer based on a compact fiber Michelson interferometer. The Michelson interferometer is composed of an asymmetrical twin-core fiber containing a central core and a side core. By chemically etching a segment of the twin-core fiber until the side core is exposed, the effective index of the side core in the etched region is sensitive to the environmental refractive index, which leads to a shift of the transmission spectrum of the Michelson interferometer. The experimental results show that such a device has a refractive index resolution of more than 800 nm/refractive index unit in the range of 1.34-1.37.

Embedded multicore hollow fiber with high birefringence.

We fabricate and demonstrate a hollow fiber with multiple embedded cores (MCHF) based on a modified "suspended core-in-tube" preform technique. Its birefringence properties are controlled by the MCHF core's ovality, which could be controlled by the applied pressure and drawing temperature in the MCHF preform. An in-fiber Mach-Zehnder interferometer with 90.38% visibility is built by fuse-tapering a single-mode fiber to the MCHF, and the splice loss is less than 2 dB. We expect that the proposed MCHF has some potential applications in in-fiber interferometers without the polarization-induced fading problem and in the biosensing area.

Theoretical and Experimental Investigations into a Crawling Robot Propelled by Piezoelectric Material.

Conventional motors with complicated electromagnetic structures are difficult to miniaturise for millimetre- and centimetre-sized robots. Instead, small-scale robots are actuated using a variety of functional materials. We proposed a novel robot propelled by a piezoelectric ceramic in this work. The robot advances due to the asymmetric friction created by the spikes on the surface. The structural modelling was completed, static and dynamic models were established to predict the moving characteristics, the prototype was built using three dimensional (3D) printing technology, and the models were evaluated via experiments. Compared with conventional inchworm-type robots, the proposed robot is superior in simple structure because the clamping components are replaced by spikes with asymmetric friction. Compared with SMA (shape memory alloy) actuating inchworm-type robots, it has a faster velocity with higher resolution. Meanwhile, the components are printed through an additive manufacturing process that is convenient and avoids assembly errors. This design could make contributions to many areas, such as pipe inspection, earthquake rescue, and medicine delivery.

[Role of hydrogen sulfide/cystathionine-gamma-lyase system in acute lung injury induced by lipopolysaccharide in rats].

OBJECTIVE: To explore the role of hydrogen sulfide/cystathionine-gamma-lyase (H(2)S/CSE) system in lipopolysaccharide (LPS)- induced acute lung injury (ALI) in rats and the underlying mechanisms. METHODS: Sixty-four Sprague-Dawley (SD) rats were randomly divided into four groups: control, LPS (instilled intratracheally to induce ALI), sodium hydrosulfide (NaHS), propargylglycine (PPG). Animals were sacrificed at 4 and 8 hours (n=8) after administration of the above agents. Morphological changes in lung tissues were determined, H(2)S, nitrogen monoxide (NO) and carbon monoxide (CO) concentration in plasma were determined. Malondialdehyde (MDA) content, and myeloperoxidase (MPO), CSE, inducible nitric oxide synthase (iNOS), heme oxygenase (HO) activity of the lung were also determined. The level of P-selectin of lung tissue was measured by radioimmunoassay. Immunohistochemistry technique was performed to examine the expression of iNOS and HO-1 protein in lung tissues. RESULTS: Severe injuries of lung tissues and raised MDA content, MPO activity and P-selectin level were observed in rats treated with LPS. LPS also led to a drop in plasma H(2)S concentration and lung CSE activity. The enzyme activity of iNOS and HO, and their protein expression, plasma NO, and CO levels increased after LPS instillation (P<0.05 or P<0.01). Pre-administration of NaHS before LPS could attenuate the changes induced by LPS. Pre-administration of PPG exacerbated the injuries induced by LPS, with increased MDA content, MPO activity, P-selectin level, the plasma NO level, lung iNOS activity and its protein expression, but there was no prominent variation in CO level, HO activity and HO-1 protein expression compared with those of LPS group. CONCLUSION: Downregulation of H(2)S/CSE is involved in the pathogenesis of ALI induced by LPS. Endogenous and exogenous H(2)S provide protection against ALI, which may be explained by its anti-oxidative effects, attenuation of inflammatory over-reaction in lung induced by polymorphonuclear neutrophils, downregulation of NO/iNOS system and the upregulation of CO/HO-1 system.

Effects of CCK-8 and Cystathionine γ-Lyase/Hydrogen Sulfide System on Acute Lung Injury in Rats.

Acute lung injury (ALI) is mainly characterized by diffusive injuries to lung epithelium and increased permeability of alveolar-capillary membranes caused by various factors, which leads to pulmonary edema and pulmonary closure. Lipopolysaccharide (LPS), which is the main component of the cell wall of gram-negative bacteria, is one of the most important factors causing pulmonary infection and ALI. More and more reports have indicated that hydrogen sulfide (H2S) is closely correlated with ALI and has anti-inflammation function, while the specific mechanism needs further investigation. Cholecystokinin-octapeptide (CCK-8), which is an important endogenous functional fragment belonging to CCK family, participates in anti-inflammatory and anti-endotoxic shock (ES). Whether CCK-8 plays important roles in curing ALI also needs further investigation. Herein, we concluded that CCK-8 alleviated the ALI induced by LPS via regulating the catalytic activity of cystathionine γ-lyase (CSE) and the formation of H2S. This work provides new medicine-designed target for clinical doctor to prevent and cure ALI.