Design and performance of a plastic optical fiber leakage sensor

In this article design and operation of a plastic optical fiber (POF) sensor based on the unconventional light leakage from one fiber to another one causing intensity modulation are presented. The dominant loss mechanism was found to be the evanescent loss and based on this phenomenon the optical fiber sensor was designed. The penetration depth of evanescent wave as a function of different refractive index of cladding was calculated. Operation of this sensor was tested as a liquid level sensor for different liquids and the experimental results are compared. The dry (air interface layer) and wet output (liquid interface layer) signals for this probe were measured for a series of measurements and important factors concerning sensor operation are described. The precision of measured values, reproducibility of the results (1.35% error), and the stability of sensing operation as a function of time at different launching powers are also reported (0.85% error). The reported results for this design are promising and verify the successful operation of such a device as a liquid level probe and also as an on/off level switch.     

Pressure sensing utilizing linear cavity erbium-doped fiber laser

In this paper, we report a fiber laser pressure sensor based on linear cavity erbium-doped fiber laser. The fiber laser structure comprises of an erbium-doped fiber amplifier, a circulator, an optical coupler and a fiber Bragg grating (FBG) which acts simultaneously as a lasing wavelength selecting components as well as a pressure sensor. The FBG is fitted to the shock tube where the pressure is applied. The fiber laser pressure sensor has a low threshold power of 7 mW, an output power of 2.28 mW and an optical signal to noise ratio over 55 dB. The proposed fiber laser sensor is expected to be an attractive choice for long-distance pressure monitoring.     

Power line monitoring system using fiber optic power supply

We propose a novel power-line-monitoring system using optical fibers for transmitting power as well as signal. The principle is experimentally confirmed with a system composed of a monitoring side with a 1.5-μm laser diode, transmission line of a single mode fiber, and a sensing side having an efficient photovoltaic (PV) cell, electrical junction sensor, and low power liquid crystal optical modulator (LCOM). The PV cell generates the electrical power in the sensing side with a conversion efficiency of 20%. The LCOM is driven with low power of less than 50 μW, modulates the laser light with a signal indicating the power line condition, and transmits the optical signal. The developed sensing unit produces an optical signal having an extinction ratio of 15 dB with low optical power of 1.8 mW. Five systems were in operation for two years, faithfully monitoring the oil pressure in electrical cables every 20 min without incident.     

L波段高掺铒光纤超荧光光源

采用半导体激光二极管泵浦12cm长高掺铒光纤,在双程前向装置中,获得了10.8mW最大超荧光输出功率,斜率效率10.6%,在1553.1~1588.6nm接近36nm的范围内,功率抖动小于0.2dBm。作为比较,在相同实验条件下泵浦10cm长的高掺铒光纤,结果显示输出光谱带宽明显下降,C波段的放大自发辐射远大于L波段的,由此也证明了L波段的放大自发辐射是由C波段的放大自发辐射泵浦产生的。     

双圈同轴光纤传感器在润滑油介质中的输出特性

针对双圈同轴式光纤(TCCOF)位移传感器用于滑动轴承油膜厚度检测时,由于传播介质为润滑油,传感器输出特性会受到影响,本文根据折射定律分析了传播介质对光纤最大入射角的影响,考虑传播过程中的光损并利用光场的准高斯分布规律建立了润滑油环境下的传感器数学模型,同时通过仿真计算得到了传播介质不同时的传感器输出特性。在此基础上,搭建了相应的传感器标定实验平台进行验证。结果表明:空气中TCCOF传感器的灵敏度为0.691/mm,润滑油中TCCOF传感器的灵敏度为0.464/mm;TCCOF传感器在润滑油测量环境中可减小光功率不同对传感器特性的影响;TCCOF传感器灵敏度随着反射面曲率半径的增大而增大;由于具有对称结构,当反射面为圆柱面时,TCCOF传感器可减小探头周向安装角度对其输出特性的影响。因此,在利用TCCOF传感器进行油膜厚度检测时,必须在相应润滑油环境下及相应的反射面情况下进行传感器标定。     

Elimination of polarization-induced signal fading and reduction of phase noise in interferometric optical fiber sensor using polarization diversity receivers

Interferometric optical fiber sensor relies on the coherent mixing of two optical signals, which is strongly polarization dependent. The fluctuation in the input polarization to an interferometric optical fiber sensor can result in not only the variation of the visibility but also the generation of excess phase noise in the output. In this paper, the relationship between the visibility and polarization-induced phase noise coefficient is described and theoretically confirmed. The use of tri-state polarization diversity receivers to eliminate polarization-induced signal fading and reduce phase noise is theoretically and experimentally demonstrated. The result shows that tri-state PDR that improves the visibility can reduce the polarization-induced phase noise and the signal can be correctly demodulated.     

基于M-Z干涉仪原理的光纤超声检测

提出了一种基于M_Z(Mach_Zehnder)干涉仪原理的光纤超声检测技术和U形结构的光纤传感器。讨论了光纤传感器与超声波之间的相互作用关系。对传感器的灵敏度与传感光纤的有效长度、U形结构的个数之间的关系进行了理论分析,实验结果和理论分析结果是一致的。通过比较超声波在三种不同入射方式下探测器的输出电压可知,该结构的光纤传感器在声信号垂直入射时由探测器探测到的信号最强,即传感器有很强的方向性,适合于对声表面波的定向检测。     

基于连通器原理的小探头光纤液位传感器研究

介绍了以汞包层光波导为传感探头的光纤液位传感器的原理及信号处理,其工作原理是利用连通器将液位的变化转变为汞包层光波导包层长度的变化。理论分析和实验表明,当光波导直径和光波长一定时,光功率衰减随汞包层光波导长度增加作线性增加。通过对汞包层光波导引起光功率衰减的检测,探头结构的优化设计以及探测器件的选型,可以获得实时性很强的高精度液位测量。实验测得10m液位的最大测量误差为5．2mm,理论分析的相对测量精度可达0．02％。该液位传感器的研制对于油库等易燃、易爆环境中的液位测量具有重要意义。     

光纤参量放大饱和增益特性研究

通过数值分析的方法推导出光纤参量放大饱和信号增益和信号输出功率的数学表达式.计算分三步, 首先数值求解描述参量放大过程的非线性耦合方程得出一系列数值, 然后用控制变量法找到饱和信号增益的函数形式, 最后用最小二乘法拟合出系数（与数字积分结果比较, 最大相对误差不超过0.46‰）.同样也得出了饱和信号输出功率的表达式.计算结果与已有实验结果相吻合.     

Polarization maintaining optical fiber multi-intruder sensor

In this paper, an optical fiber multi-intruder sensor based on polarization maintaining optical fiber (PMF), without any interferometric fiber loop, is introduced. To map the local coordinates of intruders on the beating spectrum of the output modes, radiation from a ramp frequency modulated laser is injected at the input of PMF optical fiber sensor. It is shown that the local coordinates and some characteristics of intruders can be obtained by the measurement of the frequencies and amplitudes of the output mode beating spectrum. Generally the number of beating frequencies is more than the number of intruders. Among the beating frequencies, a group with maximum signal to noise ratio is chosen. The short Fourier denoising method is employed to increase the sensor resolution. Because the output signal is the superposition of finite numbers of discrete frequencies this method is a powerful tool for denoising even for negative signal to noise ratio.     

Underwater Acoustic Sensor with Fiber Bragg Grating

A new type of underwater acoustic sensor is proposed with an optical fiber Bragg grating (FBG). Because of the photoelasticity with respect to the refractive index and the elasticity of the fiber, the sound pressure in water modulates the Bragg reflection wavelength and, in turn, the intensity of the laser light transmitted through the FBG fiber. Good linearity between the detected signal and the sound pressure is obtained in the range from 81 dB to 140 dB re 1 μPa. Since the upper and lower limits of the acoustic signal level for operation of the sensor are limited by the driving circuit and the transduction of the sound in water, the sensor is expected to operate with much larger dynamic range both at higher and lower pressure levels. Operation of the sensor is very stable with the insertion of optical isolators into the system, although without the isolators the output signal fluctuates at low frequency due to the Fabry-Perot interference effect between the FBG and the various facets.     

全光纤低相干光纤位移传感技术

提出并演示了一种由两光纤构成的杨氏干涉解调系统的全光纤低相干光纤位移传感器系统。该技术从根本上避免了光源频率、光强的波动，及光在传输过程中由系统带来的损耗、光的偏振方向的改变引起的信号衰落问题。其结构简单，易于调整，测量分辨率可达０．０５４ｍｍ。     

Preparation and characterization of a liquid level sensor based on plastic fibers

Preparation and characterization of a liquid level sensor based on macro-bending coupling of fibers are demonstrated in this Letter. The sensitive component can be obtained through a twisting and twining structure of transparent cladding plastic fibers. The difference in light power originating from the surrounding media in the fibers is tested. The light power loss for different tested media and the fiber bending magnitude are investigated. The sensing measurements show that the coupling light power in the passive fiber decreases in accordance with increasing liquid level, whereas it exhibits a steady tendency in the case of the active fiber.     

抛光对光纤连接器回波损耗的影响

分别选用氧化铝和氧化硅材料的抛光砂纸,在施加不同抛光压力、抛光时间,以及抛光助剂等工艺条件下,实验研究了抛光对连接器回波损耗的影响规律。通过实验发现:氧化铝砂纸干式抛光使光纤连接器的回波损耗仅保持在32～38dB之间;氧化硅砂纸干式抛光会造成光纤端面污损,使得连接器的回波损耗降低到20dB以下;氧化铝与氧化硅砂纸湿式抛光均可使光纤连接器的回波损耗提高到45～50dB,但氧化铝砂纸湿式抛光会造成80nm以上的光纤凹陷。因此,制作高回波损耗的光纤连接器应优先选用氧化硅砂纸湿式抛光工艺,抛光时间应控制在20～30s。     

Design and operation of a fiber optic sensor for liquid level detection

Design and construction of an optical fiber sensor for liquid level detection are reported. This sensor operates based on light intensity modulation, and such modulation results from alteration of total internal reflection into partial reflection at the interface. The modulated intensity has been measured by using a pair of fibers, one transmitting source light, another acting as receiving fiber, and a glass prism providing the total and partial reflections. During the level measurements, when a liquid in a vessel touches the 45° faces of the 45-90-45° prism, the total internal reflection is disturbed, and the reflected light is modulated. The performance of this sensor is tested with different source lights including a light emitting diode (LED), a diode laser, and a He–Ne laser. Extinction ratio has been measured for different liquids, and compared. This ratio for water using LED source is about 0.03, for diode laser is 0.006 and for He–Ne laser is 0.003. Although this device was tested as a liquid level sensor, but the distinct results obtained for samples with different index of refractions demonstrate that the reported sensor can also be used as a liquid refractometer.     

基于反射法的光纤位移 振动传感器优化设计

采用非相干光作为光源,以平面镜为反射面,对光纤位移 振动传感器的性能进行研究。推导出光纤输出功率与测量距离的解析表达式,并详细探讨了不同光纤半径、光纤数值孔径、发射光纤和接收光纤间距、发射光纤和接收光纤轴线夹角下输出功率比与测量距离之间的内禀关系。仿真结果表明:光纤半径越小,光纤间距越大,可以测试的距离范围越小;光纤数值孔径、光纤轴线夹角越小,可以测试的距离范围越小。通过分析不同情况下,输出功率比随测量距离变化的关系,可以在实际工程应用中根据测量需要选择设计合适的传感器。此外,还对测量误差进行了分析,得到光纤轴线夹角越小,测量距离的范围越大,误差率越小,当光纤轴线夹角为0.25时,误差率约为1%。     

低成本光纤陀螺零相交信号处理

介绍光纤陀螺的低成本信号处理方案。此方案利用光纤陀螺输出信号的过零特性,把位相检测过渡为时域信号检测。     

Conversion wavelength and power dependence of an optical delay system utilizing HNLFs and DCF

Conversion wavelength and power dependence of the time delay and output signal quality for an optical delay system consisting of dispersion compensation fiber (DCF) and highly nonlinear fibers (HNLFs) is investigated in this paper. The numerical results show that the time delay generally varies linearly with the conversion wavelength propagating through DCF and changes slightly with the power launched into HNLFs. But it has abrupt change at some values of conversion wavelength or input power level. Output signal quality of the overall system varies significantly with the conversion wavelength and input power level. The reasons behind these results are analyzed. Finally, the input power level is optimized to achieve linearly varying time delay and desirable output signal quality for an optical delay system.     

光纤光栅与受激布里渊信号的耦合特性

近年来,光纤布拉格光栅传感器与全分布式光纤传感器的融合技术受到了广泛关注,然而光纤布拉格光栅与布里渊信号之间的耦合特性鲜有报道。本文研究了光栅类型、波长、反射率及光纤的光致折射率对受激布里渊信号的影响规律,并探讨了空间分辨率对光纤布拉格光栅定位功能的影响。实验结果表明,在布里渊光时域分析系统中,光纤布拉格光栅处有尖锐的反射峰,而啁啾光栅、长周期光栅及光致折射率变化的光纤处均未出现尖锐的反射峰;光纤布拉格光栅反射率与受激布里渊散射功率谱无关;当光纤布拉格光栅的波长接近1 550 nm时,对受激布里渊频移测量的影响最大;在8 m的长度范围内,光纤布拉格光栅的定位误差约为4 cm,并且与空间分辨率无关。     

Detection of critical micelle concentration (CMC) using uniform and U-shaped optical fiber in sensing region

In this paper the critical micelle concentration (CMC) in surfactant solutions was detected by using an optical fiber. CMC detection is based on an adsorption effect in sample solution. The incident beam passing through the sensing region of the fiber reflected at the interface between the fiber core and the solution with repeating reflections. The change in adsorption condition leads to an effective change in the refractive index at the surrounding surface of the core. The output signal suddenly increases at the CMC due to the reflectivity change caused by adsorption of surfactant molecules onto the surface of the fiber. Plastic clad silica (PCS) fiber with a uniform sensing region was used to measure the CMC point in surfactant solution. It has been found that sensitivity of the sensor depends on the length of the sensing region, the launching condition of the light source and the optical fiber material. The higher the sensing length, the higher the sensitivity of the sensor. The U-shaped sensing region also tried to find the CMC point, and the experimental results indicate that the U-shaped sensing region is more sensitive than the uniform sensing region. Using this method we are able to detect the CMC of the surfactant solution very accurately and in real time. PACS 42.81.Bm; 42.81.Pa