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.     

Integration of miniature Fabry-Perot fiber optic sensor with FBG for the measurement of temperature and strain

A sensor has been fabricated by the integration of a fiber Bragg gating sensor (FBGs) with a fiber Fabry-Perot (F-P) sensor fabricated by etching method. In the integrated sensor, the FBG was used to measure temperature, while the fiber Fabry-Perot interferometer sensor (FFPIs) was used for strain measurement. Wavelength decoding for FBG and peak tracking for FFPI was employed for demodulation, respectively. The result showed that the temperature and strain sensitivity for the integrated sensor is ~ 2.7 pm/μεand ~ 9.3 pm/°C, respectively.     

Dynamic strain measurement using two wavelength-matched fiber Bragg grating sensors interrogated by a cascaded long-period fiber grating

In this work, we describe a fiber Bragg grating (FBG) sensing system using two wavelength-matched FBG sensors for static and dynamic strain measurement. A cascaded long-period fiber grating (CLPG)-based demodulation technique has been used to interrogate the two wavelength-matched FBG sensors. Experimental results of static strain measurement show that the proposed system has a strain resolution of 1 με. This system has also been used for dynamic strain measurement. An eddy current displacement meter-based system has been used as a comparison measurement. Experimental results of dynamic strain measurement have proved that the FBG sensing system has a good performance in the measurement of dynamic strain. The results of static and dynamic strain measurement indicate that the sensing system using two wavelength-matched FBG sensors is superior to the single FBG sensor system.     

Remote fiber sensor based on cascaded Fourier domain mode-locked laser

We propose a high-speed remote fiber Bragg grating (FBG) sensor interrogation system based on a 1.3-μm cascaded Fourier-domain mode-locked (FDML) laser. It consists of multiple FBGs connected to an optical circulator in the laser cavity. The cascaded FDML laser with these multiple FBGs is operational when the scanning frequency of the fiber Fabry-Perot tunable filter matches the fundamental frequency of the laser cavity. Each FBG provides a separate laser cavity for the FDML laser. The scanning frequencies of each laser cavity are 30.5314, 31.5393, 32.7108, and 33.8023 kHz. Using the cascaded FDML laser, we measure the performance of the long-distance static strain FBG sensor interrogation system in both the time and spectral domains. The slope coefficients of the measured relative wavelength difference and the relative time delay from the static strain are found to be 0.95 pm/μstrain and 0.15 ns/μstrain, respectively. We also demonstrate the dynamic response of the interrogation system with 80-Hz modulation strain using the cascaded FDML laser. Thus, an FBG sensor interrogation system for high-speed and high-sensitivity long-distance monitoring systems can be realized using a cascaded FDML laser.     

基于掺铒光纤/喇曼混合放大的光纤激光器布喇格光栅传感系统

提出了一种提高长距离光纤布喇格光栅信噪比以进行准分布测量的新方法.该方法基于掺铒光纤/喇曼混合放大的光纤激光器结构,掺铒光纤和滤波器构成的环形结构产生激光作为光源,喇曼光纤放大器对布喇格光栅信号进行低噪音的双向放大,置于远处的掺铒光纤利用剩余的泵浦功率产生自发辐射光和放大传感信号,为远处掺铒光纤之后的布喇格光栅传感器提供信号光以及补偿由于长距离传输造成的光纤损耗.实验显示,与使用宽带光源的传感方式相比,系统的性能得到显著提高,仅使用小功率泵浦,分布在50 km光纤上的FBG均获得了超过58 dB的优良信噪比.     

Strain and temperature discrimination technique by use of A FBG written in erbium doped fiber

We propose and demonstrate strain and temperature discrimination technique using a single fiber Bragg grating (FBG) written in the core of an erbium doped fiber. We observed that amplified spontaneous emission power varying linearly from the erbium doped fiber with temperature which determines temperature changes and strain is estimated by subtracting the wavelength shift due to temperature change, from the measured shift corresponding to the dip in the transmission spectrum of the FBG. A simple and compact FBG sensor is presented with improved rms errors of 21.2 μ? and 1 °C over ranges of 0–800 μ? and 40–95 °C, respectively. The sensor is shown to have strain and temperature sensitivity of 0.8 pm/μ? and 12 pm/°C.     

Design of fiber Bragg grating-based Fabry–Perot sensor for simultaneous measurement of humidity and temperature

A new sensor for simultaneous measurement of humidity and temperature is proposed. The sensor consists of Fabry–Perot cavity formed by two identical uniform fiber Bragg gratings. To make the cavity serves as humidity sensor, moisture sensitive polymer, which is polyimide, is coated on the FBG and on the cavity with different thickness. When the sensor is exposed to the relative humidity change, the polyimide will expand and stretch the fiber and induces strain on the FBG and on the cavity. The induced strain alters the grating period, cavity length and effective refractive index of fiber. The simulation results show that the humidity sensitivity and thermal sensitivity are 1.92 pm/%RH and 8.87 pm/°C, respectively, for polyimide coating thickness of 10 μm on the FBG and 15 μm on the cavity.     

高灵敏度光纤光栅传感特性测试仪研制

本文报道了一种新颖的光纤光栅传感测试仪,仪器的波长变化分辨率可达0.002nm,应变分辨率达1.7με.本仪器可以用于光纤布喇格光栅传感实用化测量,也可以作为光纤光栅传感技术教学的实验设备.     

A lasing wavelength stabilized simultaneous multipoint acoustic sensing system using pressure-coupled fiber Bragg gratings

A fiber Bragg grating (FBG) sensor head, using a pressure coupling mechanism, was designed for broadband frequency response and structural strain-free characteristic. The pressure-coupled sensor heads were connected to a simultaneous multipoint acoustic sensing system based on a tunable laser. An intelligent lasing wavelength stabilization algorithm capable of identifying the direction of spectrum movement, the wavelength shifting speed, and a fiber bending event was developed so that the simultaneous multipoint acoustic sensing system could be used in environments with rapid temperature variations. The lasing wavelength feedback control algorithm updated the lasing wavelength into the steep slope of the FBG spectrum even under conditions of rapid temperature change. The averaging lasing wavelength updating time was only 21 s because the system can decide a minimal size in scan window by finding the FBG spectrum shifting speed and direction in real time. The system was able to update the lasing wavelength which missed the steep slope of the FBG spectrum under maximum temperature variation rates 0.3014 and −0.3246 °C/s. The proposed system detected simultaneous impact waves at multiple points under conditions of rapid temperature change and change in dynamic strain.     

Discrimination of strain and temperature based on a polarization-maintaining photonic crystal fiber incorporating an erbium-doped fiber

A simple sensing method for simultaneous measurement of temperature and strain is investigated by using a Sagnac fiber loop mirror composed of a polarization-maintaining photonic crystal fiber (PM-PCF) incorporating an erbium-doped fiber (EDF). Amplified spontaneous emission created by a pumped EDF is transmitted to a Sagnac fiber loop mirror. The interference between two counter-propagating signals in a Sagnac fiber loop mirror generates a periodic transmission spectrum with respect to wavelength. When external temperature is increased, the transmission peak power reduces because the amplified spontaneous emission of the EDF is decreased by the applied temperature change (0.04 dB/°C). The peak wavelength is shifted into the shorter wavelength because of the negative temperature dependence of the birefringence of the PM-PCF (0.3 pm/°C). As the applied strain increases, the peak wavelength of the transmission spectrum of the Sagnac loop mirror incorporating the EDF shifts into a longer wavelength (1.3 pm/με) because the phase change of the proposed sensing probe is directly proportional to the applied strain. The transmission peak power, however, is not changed by the applied strain. Since the source and the sensing probe are integrated, the overall system configuration is significantly simplified without requiring any additional broadband light source. Therefore, it is possible to simultaneously measure temperature and strain by monitoring the variation of transmission peak power and peak wavelength, respectively.     

Single-wavelength-pump bi-directional hybrid fiber amplifier for bi-directional local area network application

This paper proposes a novel bi-directional hybrid fiber amplifier using a single-wavelength pump laser diode (LD) at 1495 nm. The hybrid amplifier is theoretically applied in a 50 km bi-directional local area network (LAN) with 26 ch × 10 Gb/s for bi-directional transmission. Thirteen C-band channels serve as downlink signals while the other 13 L-band channels are employed as uplink signals. Without loss of generality, four channels (two from each band) are experimentally analyzed. Erbium doped fiber (EDF) provides amplification for the C-band channels and Raman amplification amplifies the L-band channels. The pump efficiency is improved by employing a double-pass scheme for both the Erbium doped fiber amplifier (EDFA) and Raman fiber amplifier (RFA). The chromatic dispersion incurred by all the channels is precisely compensated for by inserting a fiber Bragg grating (FBG) array in appropriate locations along the dispersion compensating fiber (DCF) segments. Moreover, gain equalization of all the channels is achieved by adjusting the FBG reflectivity. Both the simulation results and experimental measurements confirm the proposed device feasibility and potential application in a bi-directional LAN.     

基于宽谱光源的光纤传感波长解调系统的研究

研究制作了基于宽谱光源的光纤传感波长解调系统, 以多光纤光栅作为波长参考基准、采用可调谐光纤法布里-珀罗(F-P)滤波器作为波长扫描器件。系统中采用三次多项式拟合的方法对滤波器锯齿波的扫描电压与透射波长关系曲线进行非线性拟合, 解决可调谐光纤F-P滤波器的电压—波长非线性关系对系统测量带来的较大误差问题, 实现波长的高精度解调。采用五光纤光栅做波长参考, 单根光纤光栅传感器的解调实验结果表明：待测光纤光栅布拉格波长短期测量分辨率为3.5 pm, 长期测量稳定性为7 pm。采用该系统对光纤非本征法布里-珀罗干涉型(EFPI)应变传感器的测试结果表明, 测量应变灵敏度为2.41 nm/με, 并且应变和波长之间存在良好的线性关系, 线性相关度达到0.99991。     

Highly reliable strain-tuning of an Erbium-doped fiber laser for the interrogation of multiplexed Bragg grating sensors

The authors implemented a tunable linear cavity Erbium-doped fiber (EDF) laser for the interrogation of multiplexed fiber Bragg grating sensors (BGS). A high-strength single-pulse draw-tower fiber Bragg grating (FBG) was used as the mechanically tunable element enabling the rapid wavelength sweeping of the laser center wavelength. A simple algorithm to process the noisy acquired data and identify the peak of the BGS spectra was employed. For the first time, this type of configuration was used to reliably interrogate at least three fiber Bragg grating sensors with a standard wavelength spacing between the Bragg wavelengths of 3 nm, without risk of breaking the stretched fiber, using an FBG with a simple PZT driven mount as the tuning elements.     

Fiber optic Fabry-Perot pressure sensor with low sensitivity to temperature changes for downhole application

Pressure and temperature are two important parameters in reservoir engineering. The fiber optic sensors can be used for permanent downhole monitoring. In this paper, we propose an extrinsic fiber Fabry-Perot interferometer (EFPI) sensor for pressure measurement with low sensitivity variation. The pressure sensitivity of EFPI sensor and of the fiber Bragg grating (FBG) sensors have been measured. The experimental pressure sensitivity for EFPI and FBG sensors are measured to be 2.75 × 10⁻⁸ 1/kPa and 1.52 × 10⁻⁸ 1/kPa, respectively. The temperature cross-sensitivity problem of the EFPI sensor has been solved by a new technique. The temperature sensitivity of EFPI sensor has been decreased to 1.2 × 10⁻⁶/°C, while the temperature sensitivity of non-compensated EFPI sensor has been measured to be 16.4 × 10⁻⁶/°C. The results show that the EFPI sensor has a higher pressure sensitivity and good capability to decrease temperature sensitivity in comparison to FBG sensor.     

基于激光器激射特性的新型应变传感系统

报道了一种基于掺铒光纤激光器瞬态特性的新型应变传感系统.用光纤环反射镜和光纤Bragg光栅(FBG)构成Fabry-Perot线型腔.腔内插入一个长周期光纤光栅(LPG)，其透射谱的中心波长为1574.4nm.FBG的带宽为0.23nm，不受力时其反射波长为1557.98nm，位于LPG的透射谱左沿；当FBG受力时激射波长向长波方向移动，激光通过LPG时透射损耗增大，腔损耗的增加将使激光激射延迟时间增加.因此，应变的大小可以通过激光产生的延迟时间来测量.这种新型应变传感器的分辨率和灵敏度由抽运光脉冲的高、 关键词： 应变传感 光纤激光器 时域测量 光纤Bragg光栅     

Radio-frequency interrogation of a fiber Bragg grating sensor in the configuration of a fiber laser with external cavities

A fiber laser sensor, which consists of two coupled cavities based on three fiber Bragg gratings (two of them acting as sensing elements) and is interrogated via the longitudinal mode beating frequency, is presented. The two resonant cavities have lengths of 4250 m and 4297 m, respectively. Their beating frequency is of the order of 24 kHz, and its shift as a function of the variation of the period of one (or both) of the sensing gratings, induced by strain or temperature changes, can be measured by a radio-frequency analyzer. The system is suitable for long-distance sensing with high spatial resolution and high sensitivity.     

基于喇曼/掺铒光纤混合放大的长距离布喇格光栅传感器系统

提出了一种提高长距离光纤布喇格光栅信噪比以进行准分布测量的新方法.本方法基于双向喇曼放大和双掺铒光纤结构,喇曼光纤放大器对布喇格光栅信号进行低噪音的双向放大,置于远处的掺铒光纤利用剩余的泵浦功率产生自发辐射光和放大传感信号,为第二段掺铒光纤之后的远处布喇格光栅传感器提供光以及补偿由于长距离传输造成的光纤损耗.实验显示传感器系统的性能得到显著提高.使用功率为240 mW的单个普通泵浦,分布在50 km光纤上的FBG均可获得15 dB的良好信噪比,比混合放大前提高了10 dB.     

A novel fiber Bragg grating high-temperature sensor

A novel fiber Bragg grating (FBG) sensor for the measurement of high temperature is proposed and experimentally demonstrated. The interrogation system of the sensor system is simple, low cost but effective. The sensor head is comprised of one FBG and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured basis of the wavelength shifts of the FBG induced by strain. A dynamic range of 0–800 °C and a resolution of 1 °C have been obtained by the sensor system. The experiment results agree with theoretical analyses.     

Stable single longitudinal mode fiber ring laser based on polarization maintaining erbium doped fiber

A stable single longitudinal mode (SLM) fiber ring laser that incorporates polarization maintaining erbium doped fiber (PM EDF) acting as gain medium and saturable absorber is proposed and demonstrated. Both theoretical deviation and experimental result prove that optical intensity is time invariant when the laser operates in single longitudinal mode, the SLM operation can be approximately verified by optical intensity analysis, which is practical to analyze longitudinal mode in the application of engineering. The linewidth of the fiber ring laser is measured by the delayed self-heterodyne interferometery, and an acousto-optic frequency shifter is employed to shift the lasing frequency and eliminate zero frequency interfere. The SLM operation is verified by the scanning Fabry–Perot interferometer, whereas the lasing frequency drift slowly during a period of 2 h, PM EDF is proved to be effective on suppressing mode hopping and selecting longitudinal mode.     

合金钢封装光纤Bragg光栅传感器传感特性的研究

提出了一种光纤光栅新型合金钢封装结构。利用等强度悬臂梁和温度控制箱对合金钢封装光纤布拉格光栅的应力和温度传感特性进行了测量。实验表明,采用该种封装的光纤光栅传感器保持了裸光纤光栅的响应灵敏度,其拉应变灵敏度系数为1.17pm/με,压应变灵敏度系数为1.2pm/με,温度灵敏度系数为11.3pm/℃,线性响应度在0.9995以上,可满足实际应用的要求。