High sensitivity internal refractive index sensor based on a photonic crystal fiber long period grating

A high sensitivity method is reported for the measurement of the internal refractive index using a photonic crystal fiber long period grating. Long period gratings of different lengths were inscribed in photonic crystal fibers, and the air holes of the fiber had varying refractive indices. A side lobe appeared near the resonant dip through the analysis of the characteristics of transmission spectra showing variation in the refractive index. The resonant dip and its side lobe provided varying sensitivities to the internal refractive index values. The sensitivity of the side lobe was as high as 2343?nm/refractive index unit (RIU), which exceeded the value for the resonant dip (2047?nm/RIU) for refractive indices from 1.3333–1.3792. Due to the high resolution of 8.5?×?10^?6 RIU, this method offers promising applications for biological and chemical analysis in which high-precision refractive index measurements are required.     

Sensitivity-optimized long-period fiber gratings for refractive index and temperature sensing

A long-period fiber grating sensor was fabricated by periodically changing the structure of single-mode fiber with an electric arc discharge technique. After the fabrication, the refractive index and temperature sensitivities were optimized by etching the cladding with hydrofluoric acid solution. The experimental results illustrate that the thinner cladding shows relatively higher refractive index and temperature sensitivities for the same order cladding mode, which are accordant with that of numerical simulation. After the long-period fiber grating was etched for 15?min, average refractive index sensitivities of 214?nm/refractive index unit (RIU) (1.3333?–?1.3931) and 1987?nm/RIU (1.4115–1.4555) were achieved. An extremely higher refractive index sensitivity of 2731?nm/RIU appears near 1.4555. By systematically studying the temperature sensing characteristic of cladding-etched long-period fiber grating in this work for the first time, the temperature sensitivity can reach as high as 144.23?pm/°C when the ambient temperature changes from 30 to 80°C. This work provides a theoretical reference for the fabrication of a high-sensitivity refractive index and temperature sensor based on arc-induced long-period fiber grating.     

Simultaneous measurement of refractive index and temperature based on a long period fiber grating inscribed in a photonic crystal fiber with an electric-arc discharge

A novel fiber sensor composed by two single mode fibers and long period fiber grating based on a photonic crystal fiber prepared by periodic discharge heating has been experimentally investigated to measure refractive index and temperature. A Mach-Zehnder interferometer was formed due to the presence of two fusion spliced collapsed regions in the photonic crystal fiber. The resonance dip and interference pattern were differently influenced by the ambient disturbance, so the dual-parameters were simultaneously measured by analyzing the characteristics of transmission spectrum. After the experimental measurements, refractive index and temperature sensitivities of 117.28?nm/RIU and ?86.29?pm/°C were realized. Therefore, the reported sensor with advantages of easy fabrication, simple structure, and small size has the potential for simultaneous refractive index and temperature measurements involving biochemical sensing applications.     

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.     

纤芯失配的光纤Mach-Zehnder折射率传感器

基于Mach-Zehnder干涉仪原理,利用光纤错位熔接技术设计并制作了一种单模光纤-多模光纤-单模光纤-错位熔接点-单模光纤结构的液体折射率传感器。传感器中的多模光纤和错位连接部分充当光耦合器;多模光纤在后面的单模光纤的纤芯和包层中激发出纤芯模和包层模,不同的模式有不同的模式折射率,经中间单模光纤传输到错位熔接点处时,不同模式光之间将产生光程差,经错位熔接点耦合成为导出光纤的纤芯模从而产生干涉。对该传感器输出的干涉光谱中干涉谷功率随外界溶液折射率变化的规律进行了理论分析和实验研究。结果表明：溶液折射率变化为1.358 9~1.392 2时,干涉谱中1 530 nm附近的干涉谷光功率与溶液折射率呈单调递增关系,可用于折射率的测量;折射率变化为1.372 0~1.392 2时,传感器响应曲线具有很好的线性度,线性拟合系数为0.998,对应的灵敏度为252.06 dB/RIU。该传感器制作简单、结构紧凑、成本低、灵敏度高,可用于生物医学领域液体折射率的实时测量。     

Etched multimode microfiber knot-type loop interferometer refractive index sensor

We propose a novel refractive index sensor based on multimode microfiber knot-type loop (NL) interferometer. The middle portion (~5 cm) of a 15 cm long multimode fiber is etched in 48% hydrofluoric acid to reduce its diameter to ~12 μm. A NL of diameter <1 mm is made from the etched fiber. The ends of etched fiber are spliced with single-mode fibers for launching and detecting light from the NL interferometer. The NL introduces path differences to produce interferometric spectra with free spectral range ~16 nm. The spectrum shifts as the surrounding refractive index of the loop is changed by adding chemicals. We observe the highest sensitivity of the NL interferometer ~172 nm/RIU (refractive index unit) at a refractive index value 1.370 as obtained experimentally using commonly available chemicals. The design could be used as simple, low cost, and highly sensitive biological and chemical sensor.     

Fiber optic temperature sensor using the orbital angular momentum and gaussian beams

A fiber optic temperature sensor is reported based on the interference between the orbital angular momentum and Gaussian beams. A Mach-Zehnder interferometer was employed to convert the temperature signal to a light signal of the phase difference between the reference and measuring paths. The fiber Bragg grating is included in the fiber of the measuring path. Different from the traditional sensing of fiber Bragg gratings by wavelength interrogation, the temperature-dependent phase delay near Bragg wavelength was utilized in this design. The resolution of system was better than 0.01°C. Moreover, the sensitivity of the system was obtained across various ranges of temperature. This system has potential applications in temperature measurements that require high resolution.     

倾斜光纤光栅周期对其透射谱的影响

倾斜光纤光栅作为一种特殊结构的光纤光栅,近年来受到了研究人员的广泛关注。基于模式耦合理论,仿真研究了光栅周期对倾斜光纤光栅透射谱的影响规律。研究结果表明,纤芯导模与包层模的波长差和光栅周期之间存在良好的线性关系。利用这一结论可提高倾斜光纤光栅在应变测量中的灵敏度和检测精度,且能够实现单光纤多测量的目的。     

Temperature and refractive index sensor using a high-birefringence fiber loop mirror and single mode-coreless-single mode fiber structure

A novel fiber optic sensor for the simultaneous measurement of refractive index and temperature is reported. The sensor consists of a high-birefringence fiber loop mirror and a section of single mode-coreless-single mode fiber structure. The single mode-coreless-single mode fiber structure served as a refractometer while the high-birefringence fiber loop mirror was used to measure temperature. The multimode interference valley of the single mode-coreless-single mode fiber structure was sensitive to the surrounding refractive index of liquids (96.42 nm/refractive index unit) and had almost no response to temperature fluctuations. The high-birefringence fiber loop mirror was highly sensitive to temperature (1.98 nm/°C) but was insensitive to changes in refractive index. The theoretical and experimental results demonstrated simultaneous measurement of temperature and refractive index. The optimum resolution was 2.07 × 10^?4 refractive index units and 0.01°C.     

基于光纤布拉格光栅的化学传感器

除掉光纤布拉格光栅的包层,可以使它的布拉格波长对外界环境的折射率变化敏感。采用氢氟酸腐蚀掉光纤布拉格光栅的包层,获得了直径约为6 μm的布拉格光栅。实验研究了布拉格波长对化学溶液的浓度敏感性特征,结果表明:采用10 pm分辨率的光谱仪,丙二醇溶液在低浓度时的浓度分辨率为0.7%,在高浓度时的分辨率为0.32%;糖溶液在低浓度时的分辨率为0.55%,高浓度时为0.1%。采用商用的分辨率为1 pm的高精度波长解调系统,它们的分辨率可以提高一个数量级。     

少模光纤倾斜光栅制备及扭转传感研究

设计了一种基于少模光纤倾斜布拉格光栅的反射型扭转传感器,小角度倾斜光栅中的纤芯基模与二阶模在光栅处发生互耦,通过测量该过程形成的反射峰强度变化实现扭转测量。分析了光栅倾斜角度对模式耦合效率的影响,采用相位掩模板法在线刻写光栅技术,在少模光纤上刻写了不同的小角度倾斜光栅;在此基础上选择倾斜角度为1°的光栅进行了单点、双点扭转传感实验,实验表明其反射光谱中基模与二阶模的互耦合峰(LP₀₁-LP₁₁)对光纤扭转敏感,可以实现对扭转角度大小的测量。在-50°～-150°逆时针扭转过程中,扭转灵敏度为0.52 dB/(rad·m^-1),而在40°～190°顺时针扭转的角度范围内,扭转灵敏度为0.34 dB/(rad·m^-1)。这种传感器具有在单根光纤上实现多点扭转传感的潜力,在多点扭转监测方面具有应用前景。     

Optimization of the frequency-modulated continuous wave technique for referencing and multiplexing intensity-based fiber optic sensors

In this paper, we demonstrate the interrogation of fiber optic intensity sensors by using the combination of the frequency-modulated continuous wave concept with the spectral selective reflectivity of fiber Bragg gratings. Thus, we multiplex these kinds of sensors with this technique having simultaneously a referenced system. The basis of this dual functionality is described and results are presented for the case of interrogation of two multiplexed intensity sensors. Their evaluation permits to establish the conditions to address a sensor network of this type. Also, it is proposed a strategy to implement this sensing approach without the requirement of using optical fiber delay lines in the sensor heads.     

Note: Optical fiber milled by focused ion beam and its application for Fabry-Pérot refractive index sensor

We introduce a highly compact fiber-optic Fabry-Pérot refractive index sensor integrated with a fluid channel that is fabricated directly near the tip of a 32 μm in diameter single-mode fiber taper. The focused ion beam technique is used to efficiently mill the microcavity from the fiber side and finely polish the end facets of the cavity with a high spatial resolution. It is found that a fringe visibility of over 15 dB can be achieved and that the sensor has a sensitivity of ~1731 nm/RIU (refractive index units) and a detection limit of ~5.78 × 10(-6) RIU. This miniature integrated all-in-fiber optofludic sensor may find use in minimal-invasive biomedical applications.     

A two-dimensional polarization interferometry based parallel scan angular surface plasmon resonance biosensor

We describe a two-dimensional polarization interferometry based parallel scan angular surface plasmon resonance (SPR) biosensing technique. The method of line-shaped light illumination and parallel scan offers a high throughput. The simultaneous record of SPR angular spectrum enables the system to be unaffected by the time-dependent variation of the light source. The polarization interferometry technique lowers the minimum of the SPR dip and thereby reduces the noise related to the light intensity. Refractive index resolutions of 1.4 × 10(-6) refractive index unit (RIU) under normal condition and 4.6 × 10(-7) RIU under a more time-consuming condition are achieved in our angle interrogation based sensor. Meanwhile, a manually prepared DNA microarray has been detected, showing the potential applications of this technique in microarray analysis.     

光谱学方法测量多光纤光栅色散研究

用光谱学方法测量多个光纤光栅的反射色散.利用部分相干理论对方法原理进行分析,所建立的系统适用于多个光纤光栅反射带宽不重叠的情况,对系统略作改造也可用于测量单个光纤光栅透射色散.对整幅干涉图进行一次性数据处理或对每个干涉信号分开处理都可以得到待测光纤光栅的群时延.实际测量结果证明了系统的可行性.     

Determination of refractive index by fiber-loop cavity ring-down spectroscopy and a long-period fiber grating

The combination of fiber-loop ring-down spectroscopy and a long-period fiber grating were employed in an intensity-interrogated refractive index sensing method using a broad band light source and the specifically designed long-period fiber grating. Compared with the traditional long-period-fiber-grating based refractive index sensors, this approach monitored the decay of the optical signal passing through the loop of the long fiber grating. The decay time changed regularly when the grating was immersed in surrounding media with varying refractive indices. This instrumentation offered reduced cost and eliminated the influence of source fluctuation. The sensitivity and resolution were 2421 ns/RIU and 3 × 10^?4 for refractive index values from 1.3330–1.3737.     

开环光子晶体光纤化学传感器的设计

为了提高低折射率化学物质监测的灵敏度,采用光子晶体光纤设计了一种在开环内镀有金薄膜的表面等离子体共振传感器。利用仿真软件COMSOL Multiphysics 5.6系统地研究了开环半径、内部气孔大小、金属膜层厚度对该传感器灵敏度的影响。最终在2800～4700 nm的工作波段内设计出折射率检测范围为1.26～1.31的低折射率传感器。该传感器平均灵敏度高达22 500 nm/RIU,最高灵敏度达33 000 nm/RIU。在七氟醚、卤代醚、含氟有机物等低折射率物质检测方向具有较好的应用前景。     

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.     

Near-Field Nanofluid Concentration Measurement by Rayleigh Particle Scattering Bragg Grating Evanescent Wave

We report an approach to detect near-field nanofluid concentration by scattering Bragg grating evanescent wave (EW). Since the suspended nanoparticles can enhance the scattering intensity of the EW from the thinned and tapered fiber with Bragg grating, the reflectance ratio of Bragg grating is dependent on the corresponding refractive index (RI) of the nanofluid at different nanoparticle volume fraction. A critical reflectance ratio measurement identifies the nanofluid concentration. Theory and simulation of scattering Bragg grating EW was analyzed. The scattering Bragg grating EW fiber sensing probe was designed and fabricated by the wet chemical etching method, and calibration was made by several chemical solutions without suspended nanoparticles. The example application of the nanofluid containing dispersed 40 nm SiO₂ nanoparticles demonstrates the feasibility. The reflectance ratio decreases by over 3.2% with the nanofluid concentration increasing from 0.25 wt.% to 4 wt.%, while the temperature disturbance can be negligible.     

Suppression of side lobes in the fiber Bragg grating reflection spectrum

A revision of works on the methods of writing and apodization of fiber Bragg gratings (FBG) is presented. The possibilities of apodizing FBGs recorded by holographic methods with phase masks and Lloyd interferometer using laser radiation with a Gaussian intensity profile are considered. FBG reflection spectra are numerically simulated for different parameters of the writing beam and configurations of the experimental setup. Side-lobe suppression and FBG spectrum smoothing are experimentally demonstrated. It is shown that this effect is due to equalization of the average value of the induced refractive index in the FBGs. FBG applications, in particular, as frequency selectors in fiber lasers (tunable, narrow-band, and multiwavelength) are described.