基于薄芯光纤模态干涉技术的折射率 传感特性实验研究

报道了一种具有微结构缺陷的折射率传感器,并对其折射率特性进行了实验研究.将一部分薄芯光纤熔接于标准单模光纤中,由于插入的薄芯光纤和单模光纤纤芯失配,导致包层的高次模被激发并与纤芯模在单模光纤内形成干涉仪.通过减小薄芯光纤的包层直径,以增强包层中的传输模在环境中的倏逝场,从而提高对环境折射率测量的灵敏度.实验表明,该折射率传感器具有损耗低、成本低、灵敏度高和线性度好等特点.     

基于薄芯光纤模态干涉技术的曲率光纤传感器

提出了一种新的基于薄芯光纤模态干涉技术的光纤曲率传感器。在单模光纤的中间部分插入薄芯光纤用于传感光路,没有插入薄芯光纤的单模光纤用于参考光路,以消除环境对曲率测量的影响。由于插入的薄芯光纤和单模光纤纤芯失配,导致包层的高次模被激发,并与纤芯模在单模光纤内形成干涉。当改变薄芯光纤的曲率时,沿纤芯和包层传播的模态和光纤长度会发生改变,使得干涉谷峰发生平移。将传感光纤的两端固定的平移台上,当调节平移台距离时,薄芯光纤的曲率发生改变,导致干涉谷峰向短波方向平移。通过观察谷峰的平移距离可以实现曲率的传感测量。实验表明,该装置具有低损耗、低成本和高灵敏度的特点。     

A high temperature sensor based on a peanut-shape structure Michelson interferometer

A fiber temperature sensor with high sensitivity based on a Michelson interferometer is realized by fusion-splicing a peanut-shape structure in single-mode fiber (SMF). The theory and experimental results show that the peanut-shape structure can couple the light energy of the core mode into the cladding and re-couple the light in the cladding into the core. A high-quality interference spectrum with a fringe visibility of about 18 dB is observed. Experimental demonstration shows that the device can be heated up to 900 °C with a sensitivity of about ∼0.096 nm/°C. The device has the advantages of low-cost, high sensitivity and easy fabrication, which makes it attractive for sensing applications.     

Hydrostatic pressure sensor based on a gold-coated thin-core fiber modal interferometer and ripple shift measurement

We propose a hydrostatic pressure sensor based on a gold-coated thin-core fiber modal interferometer (TCFMI). A thin-core fiber is spliced to a single mode fiber forming a single-end fiber modal interferometer (FMI) due to the core mismatch and the fiber end reflection. Relative reflection spectra of TCFMIs based on thin-core fibers with different lengths are investigated. The TCFMI is gold-coated to enhance the optical reflectivity, which also results in the ripple of the relative reflection spectra of TCFMI. A high hydrostatic pressure sensor test system is proposed and the performance of the pressure sensor has been experimentally investigated. A pure-ripple-shift measurement method is used to achieve the demodulation of the sensor. The proposed pressure sensor has a sensing range up to 40 MPa and a sensitivity of 44.8 pm/MPa.     

Fabry-Perot interference-based fiber-optic sensor for small displacement measurement

A simple fiber-optic sensor based on Fabry-Perot interference for small displacement measurement is investigated both theoretically and experimentally. A broadband light source is coupled into a Fabry-Perot cavity formed by the surfaces of a sensing fiber end and the measured object. The interference signals from the cavity are reflected back into the same fiber. A small displacement via changes in cavity length can be obtained by measuring the wavenumber spacing of the interference fringes. The experimental data meet with the theoretical values very well. Given the light source bandwidth of 40 nm and the initial distance of 30 μm, the proposed displacement sensing system could achieve high resolution measurement of 16 nm.     

Theoretical analysis of surface plasmon resonance based fiber optic sensor using indium nitride

An extremely sensitive surface plasmon resonance based fiber optic sensor with indium nitride (InN) layer coated on the core of the optical fiber is theoretically analyzed. The proposed sensor exhibits high sensitivity in the near infrared region of spectrum. The optimized value of thickness of InN layer is found to be 70 nm. Possessing high sensitivity of 4493 nm/RIU, the 70 nm thick InN layer based fiber optic SPR sensor illustrates good sensing behavior.     

Hybrid configuration of Sagnac and modal interferometer with high extinction ratio

A hybrid configuration of Sagnac and modal interferometer is proposed to obtain high extinction ratio (ER). Firstly, a modal interferometer based on core-offset splicing of three sections of single mode fiber is obtained through manually controlling the fusion parameters. Then the modal interferometer is inscribed into a Sagnac loop, which can obtain a higher ER by carefully adjusting the polarization controller (PC). There is about 10 dB extinction ratio higher than the modal interferometer. The higher the extinction ratio is, the sharper the bottom of waveform will be, so the wavelength shifts can be measured more accurately when sensing temperature, pressure or other parameters. The measured sensitivity of temperature is 48.67 pm/°C in the band of 1550 nm. It also can be applied in strain measurement, vibration measurement and so on.     

Optimization of photonic bandgap fiber long period grating refractive-index sensors

Long period gratings in low-index contrast solid-core photonic bandgap fibers are a promising platform for fiber-based fluid refractive index sensing with very low detection limits. We provide a comprehensive investigation of the possibilities for refractive index sensing using that principle in a commercial photonic crystal fiber filled with a fluid: using an acoustic grating, we map out the cladding bands, and use this data to optimize a long period grating’s sensitivity. We then implement the optimized long period grating, again using an acoustic grating, and directly measure its sensitivity to refractive index. We demonstrate a sensitivity of 17,900 nm/RIU (6.94 nm/°C) which corresponds to a smallest detectable index change of the fluid of 8.4 × 10⁻⁶.     

All-optical clock extraction from 10-Gbit/s NRZ-DPSK data using modal interference in a two-mode fiber

All-optical clock extraction from a 10-Gbit/s NRZ-DPSK input signal is demonstrated using modal interference in a two-mode fiber (TMF) and a mode-locked fiber ring laser. The TMF has a Mach-Zehnder configuration with two arms along the core and cladding regions. Using the difference in propagation delay between two arms, the non-return-to-zero differential phase shift keying (NRZ-DPSK) signal is converted to the return-to-zero on-off keying (RZ-OOK) signal. To obtain repetitive pulses as a clock signal from the RZ-OOK signal, a ring laser with a semiconductor optical amplifier (SOA) is used. Subsequently, the carrier-to-noise ratio (CNR) of the RZ-OOK and clock signals are enhanced up to 30 dB and 40 dB, respectively, compared to that of the original NRZ-DPSK signal. Also, the clock signal centered at 10 GHz has a low timing jitter of <1.6 ps. It is expected that this method can be applied to high speed fiber-optic systems of >10 Gbit/s due to its small time delay between the core and cladding regions.     

长周期光纤光栅折射率传感器的结构优化

为了提高长周期光纤光栅对环境介质折射率的传感灵敏度,提出一种长周期光纤光栅的周期和包层半径的结构优化.基于长周期光纤光栅的耦合模理论,分析了长周期光纤光栅的周期和包层半径的大小分别与环境介质折射率传感灵敏度的关系,讨论了长周期光纤光栅的周期和包层半径对折射率传感的影响以及控制光栅周期与包层半径对折射率传感的重要性.为使优化的长周期光纤光栅具有实用性,谐振波长设计在1.55 μm的常规波长范围,经过多次摸拟实验,提出最佳优化参量为：Λ=380 μm,rcl=17 μm,对环境介质折射率从1.26~1.38不同值的实验测试,折射率传感灵敏度达到0.000 12,长周期光纤光栅的结构优化获得理想的预期效果.     

Numerical analysis of plasmon polarition refractive index fiber sensors with hollow core and a long period grating

The main principle of this design is based on the efficient energy transfer between the waveguide mode (WM) and the co-directional SPP provided by a properly designed fiber long period grating (LPG). This LPG is imprinted into a waveguide fiber layer of a specially designed hollow core optical fiber. The simulations are based on the finite element method (FEM) algorithm in electromagnetics and coupled mode theory for gratings. Compared to the previous proposed structure using a fiber Bragg grating (FBG), this novel kind of sensor can greatly enhance the refractive index sensitivity, e.g., from 5.93 nm/RIU (with FBG) to 817 nm/RIU (with LPG) at the sensing refractive index of 1.40. The other advantage is that the working conditions can be performed for the well-developed telecom wavelength windows 1500-1600 nm.     

Intensity curvature sensor based on photonic crystal fiber with three coupled cores

An intensity curvature sensor using a Photonic Crystal Fiber (PCF) with three coupled cores is proposed. The three cores were aligned and there was an air hole between each two consecutive cores. The fiber had a low air filling fraction, which means that the cores remain coupled in the wavelength region studied. Due to this coupling, interference is obtained in the fiber output even if just a single core is illuminated. A configuration using reflection interrogation, which used a section fiber with 0.13 m as the sensing head, was characterized for curvature sensing. When the fiber is bended along the plane of the cores, one of the lateral cores will be stretched and the other compressed. This changes the coupling coefficient between the three cores, changing the output optical power intensity. The sensitivity of the sensing head was strongly dependent on the direction of bending, having its maximum when the bending direction was along the plane of the cores. A maximum curvature sensitivity of 2.0 dB/m⁻¹ was demonstrated between 0 m and 2.8 m.     

Wavelength-encoded fiber-optic temperature sensor with ultra-high sensitivity

We present in this paper a wavelength-encoded fiber-optic temperature sensor with ultra-high sensitivity. The sensor consists of a segment of multimode fiber (MMF) with a polymer cladding spliced between two single mode fibers, forming a multimode fiber interferometer. For a temperature sensor with a 55 mm long MMF and a 45 mm long polymer cladding, a temperature sensitivity of −3.195 nm/°C has been achieved over a temperature range of 10 °C which is mainly limited by the spectral range of the light source used in the experiments. It has been found that the high temperature sensitivity is mainly attributed to the high thermo-optic coefficient of the polymer cladding. Other advantages of the temperature sensor reported here include its extremely simple structure and fabrication process, and hence a very low cost.     

High sensitivity cascaded helical-core fiber SPR sensors

The distributed optical fiber surface plasmon resonance(SPR) sensors have attracted wide attention in biosensing and chemical sensing applications. However, due to the limitation of their sensing structure, it is difficult to adjust their resonant wavelength and sensitivity. Here, novel and flexible cascaded helical-core fiber(HCF) SPR sensors are proposed theoretically and experimentally for distributed sensing applications. It is shown that the resonant wavelength and sensitivity of the sensors can be conveniently controlled by adjusting the twist pitch of the helical core. A high sensitivity of11,180 nm/RIU for refractive-index measurement ranging from 1.355 to 1.365 is realized experimentally when the twist pitch of the helical core is 1.5 mm. It is worth noting that the sensitivity can be further improved by reducing the twist pitch.For example, the sensitivity of the sensor with a twist pitch of 1.4 mm can theoretically exceed 20,000 nm/RIU. This work opens up a new way to implement multi-parameter or distributed measurement, especially to establish sensing networks integrated in a single-core fiber or a multi-core fiber.     

A fiber-optic voltage sensor based on macrobending structure

We propose and demonstrate an optical voltage sensing scheme based on a macrobending optical fiber in a ratiometric power measurement system. This novel approach to sensing has not been utilized before and has the advantage that the sensor involves simple fabrication compared to existing fiber-optic voltage sensors. To prove the feasibility of such a fiber-optic sensor, a sensor for a voltage range from 0∼100 V is demonstrated, with a resolution of 0.5 V. The sensor is robust, linear, and shows a competitive measurement resolution. The sensor can be easily scaled to suit other voltage levels and be effectively combined with optical current sensors.     

Fabrication and characterization of solid-core photonic crystal fiber with steering-wheel air-cladding for strong evanescent field overlap

We report the fabrication and characterization of a photonic crystal fiber with solid-core and steering-wheel pattern air-cladding (SW-PCF). Specifically, SW-PCF is fabricated using sol-gel casting technique. Attenuation spectrum shows the fiber losses of 0.14 dB/m, 0.087 dB/m, and 0.0 32 dB/m at wavelengths of 850 nm, 1000 nm, and 1500 nm, respectively, while the cut-off wavelength for single-mode operation is 1360 nm. Near-field imaging is used for the determination of mode-field diameter. The fiber presents the anomalous dispersion expending to short wavelength range with high non-linearity. Tapered SW-PCFs in the transverse geometries are numerically calculated, which suggests that the tapering of fiber holds a significant promise for the enhancement of power overlap in air holes. Properly designed and fabricated SW-PCF can thus be utilized as attractive platform for evanescent field sensing and detection.     

Torsion sensing with a fiber ring laser incorporating a pair of rotary long-period fiber gratings

We experimentally demonstrate a fiber ring laser for high-resolution torsion measurement, where the laser cavity consists of a Mach-Zehnder interferometer formed with a pair of long-period fiber gratings written in a twisted single-mode fiber by a CO₂ laser. The emitting wavelength of the laser provides a measure of the rate of the torsion applied to the grating pair, while the direction of the wavelength shift indicates the sense of the applied torsion. The narrow linewidth and the large side-mode suppression ratio of the laser can provide a much more precise measurement of torsion, compared with passive fiber-optic torsion sensors. The torsion sensitivity achieved is 0.084 nm/(rad/m) in the torsion range ± 100 rad/m, which corresponds to a torsion resolution of 0.12 rad/m, assuming a wavelength resolution of 10 pm for a typical optical spectrum analyzer. The ultimate resolution of the sensor is limited by the linewidth of the laser and could be an order of magnitude higher.     

General coupled-mode analysis for gain-guided, index-antiguided fibers, and index-guided fibers

Coupling properties of two gain-guided, index-antiguided (GG + IAG) fibers, or GG + IAG fibers and index-guided (IG) fibers, were investigated by the further general coupled-mode theory. The theory can explain any situation of two fiber coupling. The interesting results were obtained by the numerical computation method. For two GG + IAG fiber coupling, total power was decreased as rising real-valued refractive-index difference (real part of refractive-index difference between core and cladding, RVRID), while the power of one GG + IAG fiber was damped oscillations with increased core separation. In the GG + IAG fiber and IG fiber coupler, the variation of the RVRID could change coupling characteristics. These results are different from two IG fibers coupling, and the general coupled-mode theory may provide meaningful references for new concepts of directional coupler and pumping technology of GG + IAG fiber.     

Addressing fiber Bragg grating sensors with wavelength-swept pulse fiber laser and analog electrical switch

A pulse train with a wavelength dependent time sequence is generated in a fiber laser configuration, which contains a cascaded wavelength-division-multiplexing (WDM) fiber Bragg grating (FBG) array and a tunable F-P filter. By distributing pulses to corresponding channels with a 1 × N analog electrical switch, a novel FBG sensors interrogation technique with advantages of high signal-to-noise ratio (SNR) and high interrogation speed is experimentally demonstrated. Then, a FBG sensing system based on this interrogation technique and the mature unbalanced scanning Michelson interferometer (USMI) demodulation technique is realized. The system has shown a sensitivity of 1.610°/με, for the 1555 nm FBG, which agrees well with the theoretical value of 1.674°/με.