High sensitivity long-period grating-based temperature monitoring using a wide wavelength range to 2.2 μm

Temperature effects on the various cladding modes of a long-period grating (LPG) fabricated in B-Ge co-doped fibre have been investigated to create a high sensitivity measurement device. The temperature sensitivities of the attenuation bands of the LPG over the wavelength region 1.2-2.2 μm, for a grating with a 330 μm period, were obtained by monitoring the wavelength shift of each attenuation band, with a temperature increment of 20 °C, over the range from 23 °C to 140 °C. The attenuation band appearing over the 1.8-2.0 μm wavelength range has shown a nearly five times higher temperature sensitivity than that of lower order modes, and thus it shows significant promise for fibre optic temperature sensor applications.     

Comparison of single and double cladding long period fiber grating sensor using an intensity modulation interrogation system

The performance of long period fiber grating (LPFG) sensors written in single cladding and double cladding fibers have been compared by using a fast responding interrogation system based on intensity modulation. Temperature and dynamic strain monitoring using this system have been demonstrated. This system is capable of resolving strain to 0.2 and 0.4 με at a loading frequency of 20 Hz, and temperature resolution to 0.02 and 0.19 °C by using LPFG in the single cladding (SC-LPFG) and double cladding (DC-LPFG), respectively.     

Optimal design of temperature-insensitive long period fiber gratings for athermal refractive-index sensor

Based on coupled-mode theory of long-period fiber grating (LPFG), a theoretical analysis and simulations for the optimal design of a temperature-insensitive LPFG is presented in order to achieve an athermal condition for sensing the refractive index of the external medium. Effects of the variation of the cladding radius and grating period on the temperature sensitivity of the LPFG are discussed. Both of these parameters are found to be important to control the temperature sensitivity when the thermo-optic coefficients of core and cladding materials are of the same order. Other grating parameters are also optimized in order to achieve a good contrast of the grating period with resonance wavelength in the 1.5 μm region and to sense the external medium refractive index over a wide range. Variation of external medium refractive index from 1.0 to 1.45 results a red-shift in the LPFG resonance wavelength by 86 nm with its temperature sensitivity as low as 0.008 nm/°C over a temperature range of 0–80 °C for this optimally designed LPFG.     

低成本长周期光纤光栅传感系统

研制了一种低成本长周期光纤光栅传感系统.利用高频CO2激光脉冲写入长周期光纤光栅边缘滤波效应原理,设计参考光路以补偿进入光电探测器前光路中各种扰动引起的光功率起伏.当输入为单波长信号光时,可以通过测量探测器输出的电信号来获得光纤光栅所承受的温度和应变信息.在温度实验里,温度和探测到的电信号成较好的二次关系,温度分辨率为0.014℃；在应变实验里,施加的应变和探测到的电信号成线性关系,应变分辨率为2.5 με.该传感系统结构简单,成本较低,响应快.     

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.     

Simultaneous measurement of refractive index and temperature using dual-period grapefruit microstructured fiber grating

By cascading the long period fiber grating (LPFG) and fiber Bragg grating (FBG) in grapefruit microstructured fiber, a novel dual-period fiber grating sensor is proposed. The refractive index and temperature are measured simultaneously by using the different sensitivity of FBG and LPFG. The relationship between dual-period fiber grating transmission spectrum and refractive index, resonant wavelengths and temperature are analyzed theoretically, respectively. The simulation results show that the accuracy of the sensor in measuring refractive index and temperature is estimated to be 2319.6 nm/RIU in a range from 1.33 to 1.36 and 0.017 nm/°C from 0 °C to 100 °C, respectively. Thus, the sensor has high refractive index sensitivity, and can provide the theoretical foundation for the optical fiber biosensor.     

对弯曲不敏感的长周期光纤光栅传感器

发现了用高频CO₂激光写入的长周期光纤光栅的谐振波长对弯曲灵敏度在圆周的不同方向上呈现周期性,且在两个特定的对称方向上对弯曲不敏感(即弯曲曲率达到1.1m^-1,谐振波长仅漂移-0.018nm).据此提出新型对弯曲不敏感的传感器和可调灵敏度弯曲传感器的设计方案,可望从根本上解决长周期光纤光栅在测量中存在的温度、应变或折射率与弯曲之间的交叉敏感问题.     

Widely tunable long-period fiber grating with nm-thick higher refractive index film overlay

By introducing a four-layer step-index waveguide modeling, the characteristics of long-period fiber grating (LPFG) with an nm-thick film overlay, which has higher refractive index than that of fiber cladding are investigated in detail. The influence of both the overlay thickness and refractive index on the tuning ability of LPFG is analyzed. The numerical results demonstrate that spectral response of LPFG is divided into three distinct regions as the overlay deposition increases and the shift of resonant wavelength is drastic in some special thickness range. In conjunction with higher-order cladding mode coupling and fiber cladding etching, the sensitivity of LPFG to the overlay refractive index is enhanced significantly and over 120 nm resonant wavelength tunable range is achieved.     

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.     

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.     

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.     

Single-sideband modulated radio-over-fiber system based on long period fiber grating

We present a prototype for optical single-sideband (SSB) modulated radio-over-fiber (RoF) system by employing a long period fiber grating (LPFG). A LPFG with 13.78 nm base width of transmission spectrum and 0?23.2 dB of transmission depth was designed by using commercial software. Then it is used in RoF SSB modulation scheme. In the scheme, a Mach?Zehnder modulator modulates the light wave with millimeter-wave driving signals to realize optical double-sideband (ODSB) modulation, the generated ODSB modulation signals pass through a LPFG. Due to the negative slope in transmission spectrum, the lower sideband experiences higher attenuation than the upper sideband. Thus the conversion from ODSB to optical single sideband with carrier (OSSB + C) can be easily achieved by using only one LPFG. Also, the carrier to sideband ratio (CSR) can be reduced by using a LPFG, results show the CSR can be decreased from 12.49 dB to 1.1 dB.     

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.     

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.     

Temperature impact on mechanically induced long-period fiber gratings

The spectral response of mechanically induced long-period fiber gratings (MLPFG) to the ambient temperature variation was experimentally study. In the MLPFG setting, a pressure rig with aluminum grooved plates on standard telecommunication fiber was used. We found that a relatively low change in temperature can produce a severe impact on the characteristic parameters of the attenuation bands, such as a critical decrease in the contrast and fast shift saturation. When the temperature increases from −10 to 70 °C, all the bands are shifted toward longer wavelengths with a mean sensitivity of approximately 180 pm/°C from 0 to 40 °C in the near-linear region, then they present a flattened zone beyond this temperature. Meanwhile, the contrast of the attenuation bands rapidly decreases from the maximum value to 0 dB with a quasi-cosinoidal behavior. These results are important and have to be considered when MLPFG are applied in a medium with ambient temperature variation. Furthermore, we show that MLPFG can be used as low cost ambient temperature sensors through intensity based measurements.     

Bend-insensitive long-period fiber grating sensors

A novel bend-insensitive long-period fiber grating (LPFG) sensor written using focused CO₂ laser pulses is demonstrated, for the first time, to our knowledge. It is found that the central wavelength shift of such a LPFG is only −0.018 nm even for a curvature of 1.1 m⁻¹ at the most bend-insensitive position of the LPFG. Experimental results show that the bend sensitivity of the central wavelength of the LPFG has a periodic distribution along its circular directions. Such a bend-insensitive sensor could be used to solve the problem of cross-sensitivity between bend and other measurands, such as temperature, strain or refractive index, which is an unsolved problem for LPFG sensors in practice. In addition, the bend sensitivity of the LPFG can be adjusted by selecting its circular positions.     

Simultaneous measurement of strain and temperature based on a fiber Bragg grating combined with a high-birefringence fiber loop mirror

We present an all-fiber sensor for simultaneous measurement of temperature and strain. The sensing head is formed by introducing a fiber Bragg grating into a high-birefringence fiber loop mirror that acts as a Mach-Zehnder interferometer for temperature and strain discrimination. A sensing resolution of ±1 °C in temperature and ±21 με in strain has been experimentally achieved over a temperature range of 60 °C and strain range of 600 με.     

Optimization of reversible LPFG for sensing application

We report here for the first time to our knowledge the characterization of mechanically induced long period fiber gratings in novel MSM fiber structure. Reversible grating of same period and length was induced in single mode fiber, multimode fiber and novel multimode-singlemode-multimode (MSM) fiber structure. The spectral response of reversible LPFG in SMF is verified experimentally as well as from simulation results and then compared with the experimental spectral response of reversible LPFG in multimode fiber and MSM fiber structure. Reversible LPFG in novel MSM fiber structure is the most optimized and suitable grating for sensing application. For this grating we have obtained single resonant wavelength over a wide wavelength range and maximum transmission loss peak of around 20 dB.     

Electrically tunable long-period fiber gratings with low-birefringence liquid crystal near the turn-around point

In this work, an electrically tunable long-period fiber grating (LPFG) coated with liquid crystal layer (LC) is presented. As a LC layer, a prototype low-birefringence 1550A LC mixture was chosen. As a LPFG host, two types of gratings were studied: the LPFGs based on a standard telecommunication fiber, produced by an electric arc technique with a period of 222 μm, and the LPFGs based on a boron co-doped fiber written by a UV technique with a period of 226.8 μm. The relatively short period of these gratings allowed exploiting unique sensing properties of the attenuation bands associated with modes close to the turn-around point. Experiments carried out showed that for the UV-induced LPFG with a LC layer, on the powered state the attenuation band could be offset from the attenuation band measured in the unpowered state by almost 130 nm. When the arc-induced LPFG was coated with the LC, the depth of the attenuation band could be efficiently controlled by applying an external E-field. Additionally, all experimental results obtained in this work were supported by the theoretical analysis based on a model developed with Optigrating v.4.2 software.     

基于单长周期光纤光栅光谱特性的温度和应变同时区分测量

根据长周期光纤光栅具有多个不同损耗峰的光谱特性提出了一种使用单个LPG对温度和应变两参数进行同时区分测量的新方案。实验选用了具有不同温度和应变传感灵敏度的第一和第四损耗峰,通过观测其相应的光谱图,得到因测量参数变化而导致的谐振波长的偏移。根据相应的参数求解矩阵方程,当被测量变化较小时,通过计算可知,交叉敏感对参数的测量基本上不产生影响;而当被测量变化较大时,可通过适当补偿消除交叉敏感而带来的偏差。实验测得的温度和应变误差分别是±0.92℃和±22με,该方案能较好地解决测量中存在的应变和温度之间的交叉敏感问题,有效地提高了系统的测量精度。实验结果表明,利用长周期光纤光栅的不同损耗峰同时测量温度和应变的方法是切实可行的,且实验系统体积小,成本较低,简单实用,具有较好的应用前景。