Design and optimization of slab coupled microfiber sensors

We investigated the sensing performance of a tapered microfiber, which was in evanescent contact with a multimode slab waveguide. The effects of the sensor configuration parameters on its transmission features and sensitivity were found for both the resonant-wavelength-shift scheme and light-intensity-variation scheme through theoretical and numerical calculations. On the basis of these results, we propose rules for optimizing geometrical parameters of the slab coupled microfiber sensors. With this approach, the slope of resonant spectra and resonant wavelength shift were increased. As a result, the sensitivity was substantially improved. It was shown that the potential device sensitivity can be as high as approximately 20,000?nm/RIU by matching optimal geometrical parameters. These results may provide beneficial references in optimally designing and fabricating slab coupled microfiber sensors to satisfy the requirements in applications.     

Simultaneous measurement of humidity and temperature using a polyvinyl alcohol tapered fiber bragg grating

An optical fiber sensor based on a multimode tapered fiber cascading fiber Bragg grating has been proposed and experimentally demonstrated for the simultaneous measurement of humidity and temperature. The sensor was constructed using a tapered fiber that was coated with polyvinyl alcohol and a fiber Bragg grating with high reflectivity. The measurement of humidity and temperature was achieved by monitoring changes in reflective optical power and spectral shift, respectively. Due to the different measurement methods, the effect of temperature on humidity measurement may be ignored. The theoretical analysis and experimental results show that the highest sensitivities of 0.33 µW/%RH and 10.9?pm/°C were achieved when the diameter of the taper waist was 26?µm and the thickness of coating was 3.3?µm. Due to the advantages of good linearity, low cost of fabrication and convenient operation, the proposed sensor is promising for simultaneously measuring humidity and temperature.     

Microfiber knot resonator based electric field sensor

An optical fiber electric field sensor was constructed by coating a microfiber based knot resonator with propylene carbonate, a liquid electrooptic material. The Kerr electrooptic effect of propylene carbonate changed the refractive index of the liquid in the presence of an electric field and shifted the fringe pattern of the resonator. The electric field was demodulated by monitoring the fringe shift. The sensor was used to characterize sinusoidal electric fields with magnitudes from 200–4000?V/cm and a pulsed field with a 200?µs duration time. This work may provide practical applications for the characterization of electric fields.     

High efficiency slotted photonic crystal waveguides for the determination of gases using mid-infrared spectroscopy

We present the design and analysis of a high efficiency slotted photonic crystal waveguide for gas sensing applications in the mid-infrared wavelength range. We designed the slotted photonic crystal waveguide structure by engineering the interfaces between the input and output slot waveguides and a resonant coupler. Coupling and transmission spectra of the sensor have been modeled using the three-dimensional finite-difference time domain method. The sensing principle is based on the shift of the upper band edge of the sensor output transmission spectrum which arises due to changes in the ambient refractive index. Transmission spectrum and sensitivity of the proposed sensor are analyzed by tuning the radii of the air holes localized on each side of the slot. The results show that a change of the refractive index of the gas by 10^?4 leads to a shift of the upper band edge wavelength by 540?pm which corresponding to a sensitivity of 1720?nm per refractive index unit. The proposed design may be an ideal platform for developing mid-infrared gas sensing devices characterized by high coupling efficiencies and high sensitivities.     

高折射率镀膜聚合物波导传感器的制备

为了提高利用倏逝波传感的光纤传感器的灵敏度问题,仿真并验证了一种基于高折射率镀膜的光纤传感器.首先两根光纤之间利用激光诱导波导自行成技术形成聚合物波导,并在波导表面镀上一层高折射率Ta2O5薄膜以增强波导表面倏逝波强度,从而增加传感器灵敏度.根据聚合物波导制备结果,使用COMSOL Multiphysics?软件对Ta...     

Multiplex and simultaneous measurement of displacement and temperature using tapered fiber and fiber Bragg grating

A simple method to work out the multiplexing of tapered fiber based sensors is proposed and demonstrated. By cascading a tapered fiber with a fiber Bragg grating (FBG), the sensor head is provided with a wavelength identification, different FBGs provide the sensor heads with different reflective peaks and they can be distinguished in optical spectrum. By compositing several such sensor heads with a multi-channel beam splitter, a star-style topological structure sensor for multipoint sensing is achieved. At the same time, the output intensity at the peak wavelength is sensitive to one external physical parameter applied on the related FBG-cascaded tapered fiber and the central wavelength of the peak is only sensitive to temperature, so that that parameter and temperature can be measured simultaneously. A sensor for dual-point measurement of the displacement and temperature simultaneously is experimentally demonstrated by using a 2 × 2 coupler in this paper. Experiment results show that the sensor works well and the largest sensitivities reach to 0.11 dB/μm for displacement in the range of 0-400 μm, and ～0.0097 nm/°C for temperature between 20 °C and 70 °C.     

Refractive index sensor based on fiber loop ring-down spectroscopy

An optical fiber refractive index sensor based on fiber loop ring-down spectroscopy and a tapered fiber was fabricated using an ordinary single mode fiber with an arc fusion splicer. The performance of the sensor was controlled by the parameters of the tapered fiber. A fiber loop ring-down spectroscopy system was employed to enhance the sensitivity and demodulate the transmission spectrum. The results showed that a sensor with a waist diameter of 14 m and a length of 1.2 mm had good optical performance. By monitoring the ring-down time of the system, relatively high sensitivity of 411.576 s/ RIU was achieved with refractive index values from 1.333 to 1.412. This sensor offers few interferences, high sensitivity, easy fabrication, and low cost.     

Graphene-based optical fiber ammonia gas sensor

To precisely monitor and forecast harmful gases in the air, a high-performance fiber gas sensor based on graphene nanometer-functional materials is proposed and experimentally demonstrated. The sensing area of the proposed sensor was a graphene-wrapped fiber which was cascaded between a down-taper and an up-taper. The graphene wrapping on the subuliform fiber sensing area substantially increased the evanescent field which transmits along the surface of fiber and will have a significant sensitization effect on the gas sensing. At the same time, the gas molecules adsorbed by graphene lead to changes of the effective refractive index of the composite waveguide, which causes corresponding wavelength drift and attenuation. By detecting the change of the output optical signal, the concentration of gas molecules was detected. Based on this principle, the sensor realized a gas sensitivity of 0.015?nm/ppm. The research shows that the sensing structure has the advantages of small volume, good mechanical strength, excellent spectral quality, and high sensitivity.     

Planar optical waveguide temperature sensor based on etched bragg gratings considering nonlinear thermo-optic effect

This paper demonstrates the development of optical temperature sensor based on the etched silica-based planar waveguide Bragg grating. Topics include design and fabrication of the etched planar waveguide Bragg grating optical temperature sensor. The typical bandwidth and reflectivity of the surface etched grating has been ∼0.2 nm and ∼9 %, respectively, at a wavelength of ∼1552 nm. The temperature-induced wavelength change is found to be slightly non-linear over ∼200 °C temperature range. Typically, the temperature-induced fractional Bragg wavelength shift measured in this experiment is 0.0132 nm/°C with linear curve fit. Theoretical models with nonlinear temperature effect for the grating response based on waveguide and plate deformation theories agree with experiments to within acceptable tolerance.     

基于拓扑单向波导的电磁诱导透明效应研究

为了研究电磁诱导透明效应在拓扑单向波导中的表现,设计了一种基于磁性光子晶体的耦合谐振腔波导。通过对谐振腔位置的调控实现了具有单向性质的电磁诱导透明效应,并利用有限时域差分仿真证明了电磁诱导透明效应在单向拓扑波导中的相关特性。该研究可为拓扑波导中实现光延迟、光开关等提供参考。     

AN OPTICAL FIBER TEMPERATURE SENSOR BASED ON AN ETHANOL FILLED FABRY-PEROT CAVITY

Optical fiber Fabry-Perot interferometers have been widely used as sensors. A novel anhydrous ethanol-filled optical fiber Fabry-Perot temperature sensor was reported in this article. Based on the characteristics of the temperature-controlling refractive index, the ethanol may serve as a sensitive medium for a sensor. According to the experimental results, the refractive index of the ethanol was changed by approximately 0.02617 when the temperature increased from 16°C to 74°C. A cavity length of 94 μm was used to demonstrate the feasibility of this sensor. The resonance wavelength of the sensor shifted about ?0.42 nm/°C. The novel optical fiber Fabry-Perot temperature sensor was simple, compact, sensitive, and immune to electromagnetic interferences.     

Theoretical Research on Optofluidic Photonic Crystal Waveguide for Broadly Tunable and Ultra-Wideband Slow Light

Broadly tunable and ultra-wideband slow light was theoretically demonstrated in a line defected photonic crystal waveguide (PCW) by infiltrating fluid with suitable refractive index into the first row of air holes adjacent to the waveguide. Simulation results showed that the working wavelength of the flat slow light in the optimized optofluidic PCW could be tuned from 1542.5 nm to 1569 nm with nearly constant group index of 80. Namely, broadly tunable slow light could be realized with the ultra-wide bandwidth over 26.5 nm and the group index up to 80. These achievements could not only open the possibility for achieving many excellent optical devices based on slow light in PCW, but also highlight the versatility, flexibility, and tunability offered by optofluidic infiltration schemes.     

High sensitivity all-fiber Sagnac interferometer temperature sensor using a selective ethanol-filled photonic crystal fiber

An all-fiber Sagnac temperature sensor based on 3?dB tapered coupling and ethanol selective-filled photonic crystal fiber was demonstrated theoretically and experimentally. The ethanol selectivity-filled photonic crystal fiber has noncircular symmetry and thus exhibits birefringence. The ethanol selective-filled photonic crystal fiber, acting as the sensing element, was inserted in a Sagnac loop interferometer to measure temperature. The output spectra of Sagnac interferometer applied different temperatures were measured and analyzed. Experimental results have shown the temperature sensitivity of the Sagnac interferometer is 1.65?nm/°C in the range of 25–33°C.     

Sensitivity-enhanced single-mode fiber-tapered hollow core fiber-single-mode fiber Mach–Zehnder interferometer for refractive index measurements

In this paper, a sensitive-enhanced single-mode fiber—tapered hollow core fiber—single-mode fiber Mach–Zehnder interferometer is demonstrated for refractive index sensing. The sensitivity was improved by forming an up-taper at the two splicing joints and concave cone in hollow core fiber. The up-tapered regions served as a more effective mode splitter/combiner, and the tapered hollow core fiber was used to generate a stronger evanescent field to enhance the interaction of light with the analyte. According to the principles of interference between the cladding and fundamental modes, we performed refractive index measurements. The experiments indicated that the proposed sensor has a high refractive index sensitivity of 214.97?nm/RIU in the refractive index range of 1.333–1.379, with a minimum refractive index measurement resolution of 9.3?×?10^?5. In addition, the sensor had a low temperature response of 2.96?pm/°C in the range from 50 to 85°C and a low cross sensitivity of 1.377?×?10^?5 RIU/°C. The proposed sensor is attractive for its high refractive index sensitivity, easy fabrication, low cross sensitivity, and good mechanical strength, making it of potential value for refractive index measurements for chemical and biological sensing.     

Design and optimization of tapered structure of near-field fibre probe based on finite-difference time-domain simulation

The finite-difference time-domain method was employed to simulate light propagation in tapered near-field fibre probes with small metal aperture. By conducting large-volume simulations, including tapered metal-cladding waveguide and connected optical fibre waveguide, we illustrated the coupling between these guiding modes as well as the electric field distribution in the vicinity of the aperture. The high collection efficiency of a double-tapered probe was reproduced and was ascribed to the shortening of the cut-off region and the efficient coupling to the guiding mode of the optical fibre. The dependence of the efficiency on the tapered structure parameters was also examined.     

A typical noiseless Pockels E field sensor

A new practical Pockels E field sensor has been demonstrated. By combining a ferroelectric liquid crystal and a polarization-maintaining optical fiber with the sensor, it is theoretically found that almost all of the optical and electronic noise produced in the system can be completely eliminated. This fact has been experimentally proven not only under the conditions of a uniform electric field but also those of a nonuniform electric field and with ac voltages applied to a stabilized corona discharge. It is shown that if with the new Pockels E field sensor, the smallest detectable phase change was set to be 10(-5) rad, the minimum measurable E field in the electrical discharge could be reduced from 10(2) to 10(-1) kV/m. This technique should be useful not only for the field of electrical discharge studies but also for other optical measurement applications such as length, temperature, and pressure using appropriately configured transducers.     

A fiber loop mirror temperature sensor demodulation technique using a long-period grating in a photonic crystal fiber and a band-pass filter

A fiber loop mirror (FLM) temperature sensor using a long-period grating (LPG) written in a photonic crystal fiber (PCF) and a band-pass filter as a demodulator is proposed. By utilizing the stable filtering function of the LPG in the PCF, the resonant wavelength variation of the FLM with temperature is transferred effectively to the intensity variation of the output light. By monitoring the light intensity of the band-pass of the filter, temperature applied on the FLM is deduced by an optical power meter. Experiment results show that the temperature sensitivity is high as ~1.742 dB/ °C when a filter with a full width at half maximum 3 nm and the center at 1545 nm is used.     

马赫-曾德尔干涉集成化的全光纤磁场与温度传感器

为了简化光纤磁场与温度传感器的结构并提高传感器灵敏度,设计并制作了马赫-曾德尔干涉集成化的全光纤磁场与温度传感器。将单根光纤的马赫-曾德尔模间干涉结构和双臂马赫-曾德尔干涉结构结合:将总长度为1.2m的单模光纤部分制备成长度为2.7cm、锥腰直径为30.1μm的锥形微纳光纤,并得到了拉锥时间与锥腰直径的关系。将锥形微纳光纤放置尼龙槽内并包覆磁凝胶构成传感头,实现模间干涉的马赫-曾德尔磁场传感器;将磁场传感器通过两耦合比为50%∶50%的耦合器并联带有可调谐光衰减器的单模光纤形成马赫-曾德尔干涉的温度传感器。从理论上分析了光谱漂移对磁场和温度传感的特性关系,实验测得室温下磁场强度在25~50mT时,磁场传感的灵敏度为0.301 14nm/mT;在磁场强度为0,温度由25℃升高到30℃时,温度传感的灵敏度为0.518 86nm/℃。该传感器可广泛应用于电力系统放电检测、材料加工、安全监控等领域。     

光纤宏弯损耗与温度传感的理论分析

为了从理论上分析和推导光纤宏弯曲损耗与温度的关系,采用了一个平板波导近似的弯曲损耗模型,对单模光纤的宏弯曲损耗和弯曲半径、波长以及温度的关系进行了分析和仿真,得到了弯曲损耗随弯曲半径的减小而增大,波长的增大而增大,温度的升高而减小的结论。该结论对光纤弯曲损耗在温度传感器方面的应用研究起着重要的参考作用。     

An acoustooptical frequency shifter of speckle-containing optical radiation

The fundamental possibility of using the acoustooptic (AO) Bragg diffraction in problems of a frequency shift of a speckle-containing optical field both with and without field filtering for removing fine-grain background has been demonstrated. Two regimes of anisotropic diffraction observed in a TeO₂ uniaxial crystal, when optical rays propagate close to the crystal optical axis (I) and far from it (II), have been studied. Experiments performed on the basis of an acoustooptical cell made of TeO₂, in which speckle-containing optical radiation at a wavelength of 0.63 μm and an average speckle size of 50–70 μm diffracted at a slow traveling acoustic wave at a frequency of 44 MHz, demonstrated the absence of radiation filtering in diffraction regime I and enlargement of the average speckle size to 220–280 μm in regime II resulting from diffraction.