Axial strain and temperature sensing characteristics of the single–coreless–single mode fiber structure-based fiber ring laser

This paper experimentally demonstrated a singlemode–coreless–singlemode (SCS) fiber structure-based fiber ring cavity laser for strain and temperature measurement. The basis of the sensing system is the multimodal interference occurs in coreless fiber, and the transmission spectrum is sensitive to the ambient perturbation. In this sensing system, the SCS fiber structure not only acts as the sensing head of the sensor but also the band-pass filter of the ring laser. Blue shift with strain sensitivity of \(\sim\) ?2 pm/με ranging from 0 to 730 με and red shift with temperature sensitivity of \(\sim\) 11 pm/°C ranging from 5 to 75 °C have been achieved. Experimental results also show the proposal has great potential in using long-distance operation. The fiber ring laser sensing system has a optical signal to noise ratio (OSNR) more than 50 and 3 dB bandwidth less than 0.05 nm. The result shows that the coreless fiber has no improvement of the temperature and axial strain sensitivity. However, compared to the common singlemode–multimode–singlemode fiber structure sensors, the laser sensing system has the additional advantages of high OSNR, high intensity and narrow 3 dB bandwidth, and thus improves the accuracy.     

Simultaneous measurement of temperature and strain using a long-period fiber grating with a micro-taper

An all-fiber Mach–Zehnder interferometer (MZI) consisting of a long-period fiber grating with a micro-taper is proposed for simultaneous measurement of temperature and strain. The experimental results demonstrate that the temperature and strain sensitivities of the proposed MZI are 83 pm/°C and ?2.6 pm/με, respectively. The strain sensitivity is 20 times as high as that of a long-period fiber written by CO₂ laser pulses combined with a fiber bitaper. In addition, the interferometer requires only a common single-mode fiber, and it is easy to fabricate and is inexpensive for temperature and strain sensing applications.     

Optical fiber strain and temperature sensor based on an in-line Mach–Zehnder interferometer using thin-core fiber

We propose and demonstrate a fiber in-line Mach–Zehnder interferometer using thin-core fibers. This in-line interferometer is composed of a short section of thin-core fiber inserted between two single mode fibers (SMF), and demonstrated as a strain and temperature sensor in this study. A strain sensitivity of ?1.83 pm/με with a measurement range of 0?2000 με, and the temperature sensitivity of ?72.89 pm/°C with a temperature variation of 50 °C are achieved. We also discussed that the influence of strain and temperature change on the relative power ratios among the excited cladding modes in thin-core fibers.     

All-solid multi-core fiber-based multipath Mach–Zehnder interferometer for temperature sensing

A novel multipath Mach–Zehnder interferometer (m-MZI) is proposed and experimentally demonstrated, which is fabricated by fusion splicing a segment of all-solid multi-core fiber (MCF) between two sections of single mode fiber-28 with a well-controlled lateral offset at the splice points. Beam propagation method-based simulation results demonstrated light passing throw MCF from multiple paths. Experiments with different lengths of MCF were implemented to investigate our proposed m-MZI’s response to temperature and strain. Compared with previously reported optical fiber modal interferometers, higher phase sensitivity can be obtained in our scheme due to the multipath interference configuration embedded in one fiber. A very high temperature sensitivity of 130.6 pm/°C has been achieved, and the maximum strain sensitivity is less than 0.284 pm/με in all experiments. A record low strain-to-temperature cross-sensitivity of 6.2 × 10^?4 °C/με has been realized, and it shows great significance of this in-fiber integrated multipath Mach–Zehnder interferometer in practical temperature sensing applications.     

All-fiber intermodal Mach–Zehnder interferometer based on a long-period fiber grating combined with a fiber bitaper

We report a novel all-fiber narrow-bandwidth intermodal Mach–Zehnder interferometer (MZI) based on a long-period fiber grating (LPFG) combined with a fiber bitaper, and the MZI has no special limit for the resonant wavelength of the LPFG. Its responses to temperature and axial strain are studied theoretically and experimentally. Experimental results indicate that the temperature sensitivity is 0.0585 nm/°C within the temperature range from 30 °C to 90 °C and the axial strain sensitivity of 0.00013 nm/με can be neglected. Furthermore, as only the common single-mode fiber (SMF) is required during the fabrication process, the proposed device is cost effective and has good practicability in the optical sensing systems.     

基于纤芯失配和光纤布拉格光栅实现温度和应变同时测量

基于纤芯失配理论,提出了一种多模单模多模（MSM）结构与光纤布拉格光栅（FBG）级联实现温度和应变同时测量的光纤传感器。利用MSM结构的干涉谱和FBG对温度和应变的不同响应灵敏度,实现了对温度、应变的同时测量。实验结果表明,在20 ℃~80 ℃的温度范围内,MSM结构的干涉谱和FBG的温度灵敏度分别为0.091 nm/℃和0.0102 nm/℃;在0～650 με的应变范围内,应变灵敏度分别为 -0.0013 nm/με和0.0012 nm/με。因此利用敏感矩阵,即可实现对温度和应变的同时测量,且温度和应变的最大测量误差分别为±0.2 ℃和±8.25 με。该结构灵敏度高,结构简单,且不易受电磁等干扰,实验结果具有良好的线性度,在工程领域应用前景良好。     

Simultaneous measurement of refractive index and temperature using thinned fiber based Mach–Zehnder interferometer

A novel method for simultaneous measurement of refractive index and temperature based on a small core and cladding diameters thinned fiber Mach–Zehnder interferometer (MZI) using singlemode-multimode-thinned-multimode-singlemode (SMTMS) fiber structure is proposed. Experiments indicate that the selected two interference orders have sensitivities of ?16.1936 nm/RIU and 0.0534 nm/°C, and ?23.0473 nm/RIU and 0.0575 nm/°C, respectively, among RI range from 1.3325–1.3720 and temperature range from 22 °C–82 °C. We can thus use the coefficient matrix of these two peaks to simultaneously determine the surrounding refractive index and temperature. The fabrication is easy, safe and cost effective, includes only the fusion splicing, making the device properly attractive for practical sensor applications.     

A non-contact FBG vibration sensor with double differential temperature compensation

This paper has presented a non-contact fiber Bragg grating (FBG) vibration sensor with double differential temperature compensation. Two FBGs and two states of the sensor have been employed to achieve double differential temperature compensation. Based on magnetic coupling and FBG sensing principle, it can be used to realize non-contact measurement of vibration of the rotating shaft. Experimental results show that the working band ranges are within 0–150 Hz; the sensitivity is ?0.67 pm/µm, and the linearity is 3.87 % within a range of 2–2.6 mm. The fitting equation of temperature compensation which is caused by structural inflation can be expressed as: Δλ ₁′ ? Δλ ₂′ = 1.51 × T ? 32.97. When used to amend a temperature error, the sensor’s temperature error will be reduced to 1.19 % in the range of 25–60 °C.     

Ultra-long-period fiber grating cascaded to a knob-taper for simultaneous measurement of strain and temperature

This study presents a simple Mach–Zehnder interferometer (MZI) to obtain the bimodal characteristics that realize simultaneous measurement of strain and temperature through cascading an ultra-long-period fiber grating and a knob-shaped taper. We obtain the multi-dip feature from the MZI, and the Dips 2 and 5 are selected from 11 interference dips. Experimental results indicated that the wavelength sensitivities of Dips 2 and 5 are ??0.54 nm mε^?1 and 0.058 nm °C^?1, and ??0.53 nm mε^?1 and 0.055 nm °C^?1 to strain and temperature, respectively. The depth sensitivities are ??3.3 dB mε^??1, ? 0.015 dB °C^?1 and ?5.8 dB mε^?1, and 0.06 dB °C^?1 for Dips 2 and 5, respectively. It is concluded that the proposed structure is suitable for simultaneous strain and temperature measurements.     

Temperature characteristics of a core-mode interferometer based on a dual-core photonic crystal fiber

A core-mode Fabry–Perot (FP) interferometer is constructed by using a dual-core photonic crystal fiber (DCPCF). The FP cavity is formed by a single piece of DCPCF, which can also serve as a direct sensing probe without any additional components. We theoretically and experimentally studied its temperature responses in the range of 40–480 °C. The temperature sensitivity is 13 pm/°C which matches the theoretical results. Since the temperature sensitivity of the proposed sensor is independent on cavity length, precise control of the length of FP cavity or photonic crystal fiber is not required. The sensor size can be as short as 100–200 μm, and its fabrication only involves splicing and cleaving, which make the sensor production very cost-effective. The proposed FP interferometric sensor based on a DCPCF can find applications in high-temperature measurement especially those that need accurate point measurement with high sensitivity.     

Optical fiber strain sensor using fiber resonator based on frequency comb Vernier spectroscopy

A novel (to our best knowledge) optical fiber strain sensor using a fiber ring resonator based on frequency comb Vernier spectroscopy is proposed and demonstrated. A passively mode-locked optical fiber laser is employed to generate a phased-locked frequency comb. Strain applied to the optical fiber of the fiber ring resonator can be measured with the transmission spectrum. A good linearity is obtained between displacement and the inverse of wavelength spacing with an R(2) of 0.9989, and high sensitivities better than 40 pm/με within the range of 0 to 10 με are achieved. The sensitivity can be proportionally improved by increasing the length of the optical fiber ring resonator.     

Mach-Zehnder all-fiber interferometer using two in-series fattened fiber gratings

We present an all-fiber Mach-Zehnder interferometer consisting of two consecutive fattened sections of dispersion-shifted fiber that act as in-series long period fiber gratings. The proposed Mach-Zehnder interferometer shows a broad fringe pattern ranging from 1000 to 1500 nm and is stable against changes in temperature and strain. By tapering a section of 5mm in length to 50% diameter between the fattened sections we observe an increased sensitivity to temperature changes. The measured temperature and strain sensitivities were in the range of 9–17 pm/°C and 1.44–2.9 pm/μɛ, respectively.     

基于掺铒光纤的微型光纤法布里-珀罗干涉传感器

提出了一种化学腐蚀掺铒光纤制作微型光纤法布里-珀罗干涉传感器的方法。通过对掺铒光纤进行化学腐蚀,形成凹槽,再与单模光纤直接熔接制作而成。实验制作的微型法布里-珀罗干涉传感器干涉条纹光滑,对比度达到15dB。对该微型光纤法布里珀罗干涉腔进行了应变和温度传感实验。实验结果表明,在0-600με￡内,波谷移动随应变改变的灵敏度达到1．7pm／με,线性度为0．9998,从73～23℃,波谷移动随温度改变的灵敏度3．9pm／℃,线性度为0．9982。该方法制作的微型光纤法布里-珀罗传感器具有操作简单,一次成型,制作成本低的优点。     

Fabry–Pérot cavities based on chemical etching for high temperature and strain measurement

In this paper, two hybrid multimode/single mode fiber Fabry–Pérot (FP) cavities were compared. The cavities fabricated by chemical etching are presented as high temperature and strain sensors. In order to produce this FP cavity a single mode fiber was spliced to a graded index multimode fiber with 62.5 μm core diameter. The Fabry–Pérot cavities were tested as a high temperature sensor in the range between room temperature and 700 °C and as strain sensors. A reversible shift of the interferometric peaks with temperature allowed to estimate a sensitivity of 0.75 ± 0.03 pm/°C and 0.98 ± 0.04 pm/°C for the sensor A and B respectively. For strain measurement sensor A demonstrated a sensitivity of 1.85 ± 0.07 pm/μ? and sensor B showed a sensitivity of 3.14 ± 0.05 pm/μ?. The sensors demonstrated the feasibility of low cost fiber optic sensors for high temperature and strain.     

基于光纤布拉格光栅与长周期光栅并联的集成光学传感器

为了提高光纤传感器的性能和进一步缩小传感器的尺寸,通过实验制备出一种基于光纤布拉格光栅(FBG)与长周期光栅(LPG)并联的新型集成光学传感器。该传感器中的FBG和LPG是利用飞秒激光直写技术直接在普通单模光纤中刻写的。FBG和LPG是并联关系,因此很大程度地缩小了传感器的长度。外界的温度和折射率的变化会引起FBG和LPG的谐振峰波长位置发生变化,据此对该集成传感器进行温度和折射率测量。实验结果表明:FBG谐振峰对折射率和温度的灵敏度分别为0 nm/RIU和12.98 pm/℃,而LPG在1 555 nm附近谐振峰对折射率和温度的灵敏度为196.46 nm/RIU和10.93 pm/℃。因此,根据双参数传感矩阵,该传感器可以对温度和外界折射率进行同时传感。     

Analysis of multimode BDK doped POF gratings for temperature sensing

We report a temperature sensor based on a Bragg grating written in a benzil dimethyl ketal (BDK) doped multimode (MM) polymer optical fiber (POF) for the first time to our knowledge. The thermal response was further analyzed in view of theory and experiment. In theory, with the order of the reflected mode increasing from 1st to 60th order, for MM silica fiber Bragg grating (FBG) the temperature sensitivity will increase linearly from 16.2 pm/°C to 17.5 pm/°C, while for MM polymer FBG the temperature sensitivity (absolute value) will increase linearly from ?79.5 pm/°C to ?104.4 pm/°C. In addition, temperature sensitivity of MM polymer FBG exhibits almost 1 order larger mode order dependence than that of MM silica FBG. In experiment, the Bragg wavelength shift will decline linearly as the temperature rises, contrary to that of MM silica FBG. The temperature sensitivity of MM polymer FBG is ranged from ?0.097 nm/°C to ?0.111 nm/°C, more than 8 times that of MM silica FBG, showing great potential used as a temperature sensor.     

Highly birefringent photonic bandgap Bragg fiber loop mirror for simultaneous measurement of strain and temperature

A highly birefringent photonic bandgap Bragg fiber loop mirror configuration for simultaneous measurement of strain and temperature is proposed. The group birefringence and the sharp loss peaks are observable in the spectral response. Because the sensing head presents different sensitivities for strain and temperature measurands, these physical parameters can be discriminated by using the matrix method. It should be noted that this Bragg fiber presents high sensitivity to temperature, of ～5.75?nm/°C, due to the group birefringence variation. The rms deviations obtained are ±19.32?με and ±0.5?°C, for strain and temperature measurements, respectively.     

Temperature self-compensation fiber-optic pressure sensor based on fiber Bragg grating and Fabry–Perot interference multiplexing

A high performance multiplexed fiber-optic sensor consisted of diaphragm-based extrinsic Fabry–Perot interferometer (DEFPI) and fiber Bragg grating (FBG) is proposed. The novel structure DEFPI fabricated with laser heating fusion technique possesses high sensitivity with 5.35 nm/kPa (36.89 nm/psi) and exhibits ultra-low temperature dependence with 0.015 nm/°C. But the ultra-low temperature dependence still results in small pressure measurement error of the DEFPI (0.0028 kPa/°C). The designed stainless epoxy-free packaging structure guarantees the FBG to be only sensitive to temperature. The temperature information is created to calibrate the DEFPI's pressure measurement error induced by the temperature dependence, realizing effectively temperature self-compensation of the multiplexed sensor. The sensitivity of the FBG is 10.5 pm/°C. In addition, the multiplexed sensor is also very easy to realize the pressure and the temperature high-precise high-sensitive simultaneous measurement at single point in many harsh environmental areas.