Biochemical sensor based on a novel all-fibre cavity ring down spectroscopy technique incorporating a tilted fibre Bragg grating

A novel all-fibre cavity ring down spectroscopy technique is proposed where a tilt fibre Bragg grating (TFBG) or long-period grating (LPG) in the cavity provides sensitivity to surrounding medium. Such configuration with an LPG as the representative was theoretically analyzed. Two spectral bands were identified employable for sensing of surrounding refractive index for a weak LPG while only one band existed for a strong LPG. A TFBG, with enhanced sensitivity compared to usual LPGs, was used in a ring down cavity of 1 m constructed with 2 fibre Bragg gratings as the reflectors and the decay time changed from 220 to 450 ns when the TFBG was immersed into water from air.     

Cascaded tilted fiber Bragg grating for enhanced refractive index sensing

We proposed and experimentally demonstrated a cascaded tilted fiber Bragg grating(TFBG) for enhanced refractive index sensing. The TFBG is UV-inscribed in series in ordinary single-mode fiber(SMF) and reduced-diameter SMF with the same tilt angle, and then excites two sets of superposed spectral combs of cladding modes. The cascaded TFBG with total length of 18 mm has a much wider wavelength range over 100 nm and narrower wavelength separation than that of a TFBG only in the SMF, enabling an enlarged range and a higher accuracy of refractive index measurement. The fabricated TFBG with the merits of enhanced sensing capability and temperature self-calibration presents great potentials in the biochemical sensing applications.     

利用倾斜光纤光栅的可开关双波长光纤激光器

报导了一种在四波混频作用下利用倾斜光纤布拉格光栅进行波长选择的可开关双波长掺铒光纤激光器。通过将倾斜光纤光栅与单模光纤进行横向错位焊接,使光栅的反向LP01和LP11两个模式具有相近的有效反射率,从而可以用来进行激光器的双波长选择。接入腔内的一段高非线性光子晶体光纤引入的四波混频效应克服了模式竞争,使得双波长激光在室温下稳定振荡。腔内起偏器和偏振控制器的联合作用可产生依赖于波长的损耗,以补偿光纤光栅两反射峰峰值的大小差异。基于以上原理,通过调节腔内的偏振态,该激光器实现了室温下稳定的双波长输出,也实现了在两波长之间的转换。两波长激光均有超过45 dB的信噪比,最大的功率波动为0.8 dB     

Design of triangular FBG filter for sensor applications using covariance matrix adapted evolution algorithm

The design of triangular fiber Bragg grating (TFBG) filters using covariance matrix adapted evolution strategy (CMAES) algorithm is presented. The TFBG filter can be used as a readout device in FBG-based sensor applications. Two different filter design problems are investigated. The first problem is the design of TFBG filter for a specified bandwidth. The other is maximizing the bandwidth of the TFBG filter for a given grating length and maximum index modulation of the FBG. A more practical asymmetrical triangular FBG filter design is also considered. The results show that for the same grating length and maximum index modulation, a larger bandwidth can be achieved with a chirped grating. The maximized bandwidth design can be used to achieve the maximum possible bandwidth for the specified grating length, maximum index modulation and chirp rate. The results obtained using CMAES algorithm (codes available at http://www.ntu.edu.sg/home/epnsugan) is compared with the results of PSO and other algorithms. Comparisons show that the CMAES algorithm is more consistent in obtaining the best solution with reduced computation time. Hence, the CMAES algorithm is effective for the design of TFBG filters for sensor applications.     

Highly sensitive bend sensor with hybrid long-period and tilted fiber Bragg grating

We demonstrate a new type of fiber optic bend sensor with a hybrid structure made up of a long period grating (LPG) and a tilted fiber Bragg grating (TFBG). The sensing mechanism is based on the spectrum of power transfers between the core and cladding modes from a TFBG located downstream from a LPG. We show that the curvature of a beam can be determined by the reflected power difference between the core mode and the recoupled cladding modes. We further provide design rules for the LPG and TFBG to optimize and linearize the sensor response. In addition, the temperature cross-sensitivities of this configuration are also investigated for two different types of fiber.     

Temperature and strain discrimination using a single tilted fibre Bragg grating

A fibre optic sensor capable of discriminating between temperature and strain, using a single fibre Bragg grating, is presented. The technique exploits the core-cladding mode coupling of a tilted fibre Bragg grating (TFBG). The core and cladding modes exhibit different thermal sensivities, while the strain sensivities are approximately equal. Monitoring the core-core mode coupling resonance and the core-cladding mode coupling resonance of the TFBG spectrum allows the separation of the temperature and strain induced wavelength shifts.     

Analysis of the bandwidth of resonant spectra for tilted fiber grating

Based on the coupled-mode equations, the formula of null-null bandwidth for tilted fiber grating is deduced in this paper. Numerical simulations and theoretical analysis of the effects of the tilt angle on bandwidth and maximum reflectivity are performed. Furthermore, under the maximum reflection of 95%, the relation of bandwidth with the tilt angle, modulation amplitude, and the needed length of grating are demonstrated in two three-dimensional diagrams. It reveals that the grating tilt can obviously narrow the bandwidth, especially in the case of large modulation amplitude and grating length, which is meaningful to its application in optical communications and fiber sensing.     

Lateral force sensor based on a core-offset tilted fiber Bragg grating

A novel lateral force sensor based on a core-offset tilted fiber Bragg grating (TFBG) is proposed and experimentally demonstrated. The lateral force is determined by the differential reflected powers between the cladding mode and Bragg mode in the TFBG. The sensors respond monotonically with the lateral force increasing from 0 to 1.75 N. The sensitivity of such a core-offset TFBG sensor can be tailored by choosing different core-offset values. The simple differential power detection method makes the implementation of the sensor system cost-effective and free of the influence of environmental and system fluctuations.     

A Continuously Tunable Erbium-Doped Fibre Laser Using Tunable Fibre Bragg Gratings and Optical Circulator

A continuously tunable erbium-doped fibre laser （TEDFL） based on tunable fibre Bragger grating （TFBG） and a three-port optical circulator （OC） is proposed and demonstrated. The OC acts as a 100%-reflective mirror. A strain-induced uniform fibre Bragger grating （FBG） which functions as a partial-reflecting mirror is implemented in the linear cavity. By applying axial strain onto the TFBG, a continuously tunable lazing output can be realized. The wavelength tuning range covers approximately 7.00nm in C band （from 1543.6161 to 1550.3307nm）. The side mode suppression ratio （SMSR） is better than 50 dB, and the 3 dB bandwidth of the laser is less than 0.01 nm. Moreover, an array waveguide grating （AWG） is inserted into the cavity for wavelength preselecting, and a 50 km transmission experiment was performed using our TEDFL at a 10 Gb/s modulation rate.     

倾斜光纤光栅溶液折射率传感器的实验研究

根据倾斜光纤光栅(TFBG)和表面镀金的TFBG传感器测量折射率的基本原理,通过OptiGrating软件模拟了不同浓度溶液下TFBG的透射谱和芯层模与某阶包层模耦合引起的谐振峰,初步得出了TFBG各阶包层模随着外界折射率的增大而向右偏移、在一定的传感范围内中心波长与外界折射率呈线性关系的结论。用小型离子溅射仪对TFBG镀45 nm厚度左右的金膜,并用扫描电镜在微观上观察镀膜效果。通过不同浓度下的NaCl溶液、MgCl₂溶液、CaCl₂溶液实验,对比研究了裸TFBG和镀金TFBG传感器对溶液折射率的传感特性。从而验证了模拟仿真得出的结论并定量分析得知：镀金后具有表面等离子体共振的TFBG溶液折射率灵敏度大于500 nm·RIU-1,而裸TFBG为2 nm·RIU-1左右,大约提高了200～300倍,且在一定范围内中心波长与溶液折射率的线性拟合度都在0.99以上。     

不同包层直径的倾斜光纤光栅折射率传感特性

倾斜光纤光栅的透射谱中有纤芯模和大量的包层模,它们具有与布拉格光栅相同的温度特性.利用HF酸腐蚀的方法得到具有不同包层直径的倾斜光纤光栅,研究了其对外界折射率的传感特性.结果表明,外界环境折射率在1.333～1.4532之间变化时,同一直径倾斜光纤光栅的高阶包层模的敏感性要比低阶包层模强;随着包层直径的减小,包层模的敏感性增强,且在折射率比较高的环境中有更高的敏感性.因此,利用倾斜光纤光栅的温度特性不仅可以解决温度交叉敏感问题,而且通过小同的腐蚀程度能定制所需要的灵敏度,以实现对环境折射率的高灵敏度测量.该办法可应用于对生物和化学等高灵敏度传感领域的各种溶液进行实时监控.     

Spectrum-profile-identification-based wavelength division multiplexing method for a fiber Bragg grating sensor

A new wavelength division multiplexing method for fiber Bragg grating(FBG) sensors based on spectrum profile identification is proposed. In this method, FBGs and tilted FBG(TFBG) sensors are cascaded in a single fiber in one sensing channel. The different spectrum profiles enable the cross-correlation method to demodulate the wavelength. Therefore, the different types of sensors can occupy the same central wavelength band. Using this method, the multiplexing capacity is optimized. Experiment results demonstrate the feasibility of this method and it is useful for applications where large numbers of FBGs are needed.     

A compact graphene Q-switched erbium-doped fiber laser using optical circulator and tunable fiber Bragg grating

We demonstrate a passively Q-switched tunable erbium-doped fiber laser(EDFL)based on graphene as a saturable absorber(SA).A three-port optical circulator(OC)and a strain-induced tunable fiber Bragg grating(TFBG)are used as the two end mirrors in an all-fiber linear cavity.The Q-switched EDFL has a low pump threshold of 23.8 mW.The pulse repetition rate of the fiber laser can be widely changed from 9.3 kHz to 69.7 kHz by increasing the pump power from23.8 mW to 219.9 mW.The minimum pulse duration is 1.7μs and the highest pulse energy is 25.4 nJ.The emission wavelength of the laser can be tuned from 1560.43 nm to 1566.27 nm by changing the central wavelength of the straininduced TFBG.     

Temperature-independent bending sensor with tilted fiber Bragg grating interacting with multimode fiber

A new type fiber bending sensor based on a tilted fiber Bragg grating (TFBG) interacting with a multimode fiber (MMF) is presented. The sensing head is formed by insertion of a small section of MMF between a single-mode fiber (SMF) and the TFBG. The average reflective power in the cladding modes decreases with the increase of curvature. The measurement range of the curvature from 0 to 2.5 m⁻¹ with a measurement sensitivity of −802.4 nW/m⁻¹ is achieved. The proposed sensor is also proved as temperature-independent from the experimental investigation.     

多模光纤中简并模式群的分别探测

用多模倾斜光纤光栅(Tilted fiber Bragg grating,TFBG)实现多模光纤中简并模式群(Degenerated modegroup,DMG)的分别探测。多模倾斜光纤光栅将光纤中的导模耦合到辐射光线,用透镜将辐射光线聚焦在焦平面上,使非简并模式群对应的辐射光线的像在焦平面上分离。在焦平面上用空间滤波器使简并模式群选择透过并进行分别探测。用芯径为62.5μm的标准通信多模光纤进行了简并模式群分别探测实验,用单模光纤激励多模光纤,测量简并模式群功率随激励光纤的横向对准偏差的变化关系。低阶简并模式群的测量结果与仿真计算结果基本一致,高阶简并模式群的测量结果与仿真结果有一些不同。用倾斜光纤光栅进行单简并模式群激励,在单简并模式群条件下测量简并模式群耦合效率的想法还需进一步实验验证。     

Method for measuring liquid phase diffusion based on tilted fiber Bragg grating

We propose and experimentally demonstrate a method for measuring liquid phase diffusion based on tilted fiber Bragg grating (TFBG). By monitoring the transmission spectra of the TFBG placed at different positions of the diffusion zone and calculating the normalized area enclosed by the upper and lower envelope curves of the cladding modes, the distribution curves of the glycerol concentration are obtained, according to the experimental calibration formulas between the glycerol concentration and the normalized area. This method can conveniently achieve remote and distributed measurement of the liquid phase diffusion in hostile environment because of its all-fiber structure.     

Widely tunable mode-locked all-fiberized Yb-doped fiber laser with near-transform-limited spectrum linewidth

A widely tunable mode-locked all-fiberized Yb-doped fiber laser with near-transform-limited spectrum linewidth is used. It consists of a tunable fiber Bragg grating (TFBG) and a fiber-coupled LiNbO₃ phase modulator (PM) in a linear cavity. The TFBG is used to achieve tunable emission wavelength, and the intracavity PM is used to achieve actively mode-locking operation. We have experimentally demonstrated that the laser-emitting wavelength can be tuned between 1,041 and 1,091 nm with power fluctuation less than 3 dB. The temporal width of the laser pulse is about 1 ns, and the pulses are near transform-limited with a spectral linewidth of 1.3 GHz. The results may find useful application in optical communication and optical measurement system.     

Fiber-optic vector vibroscope

A directional vibration sensor based on polarization-controlled cladding-to-core recoupling is demonstrated. A compact structure in which a short section of multi-mode fiber (MMF) stub containing a weakly tilted fiber Bragg grating (TFBG) is spliced to another single-mode fiber without any lateral offset. Multiple core modes of the MMF are coupled at the junction and appear as well defined resonances in reflection from the TFBG. Some of those resonances exhibit a strong polarization and bending dependence. Both the orientation and the amplitude of the vibrations can be determined unambiguously via dual-path power detection of the orthogonal-polarimetric lowest order LP(1n) modes. Meanwhile, the unwanted power fluctuations and temperature perturbations can be referenced out by monitoring the fundamental LP(01) mode resonance.     

涡旋光束与相位全息光栅不对准时的衍射特性研究

研究了涡旋光束与相位全息光栅不对准时衍射光束的解析特性.利用理论推导的方法得出涡旋光束经相位全息光栅接收后一阶衍射光束的解析表达式.然后通过仿真分析分别得出在发生正常对准、横向偏移、角向倾斜及横向偏移和角向倾斜两者同时出现时衍射光束的质心偏移特性和中心强度变化特性.研究表明: 拉盖尔-高斯光束经相位全息光栅衍射后得到的光场表达式为合流超几何函数形式.光束与相位全息光栅间的不对准会引起衍射光束质心的偏移,而且光束质心的偏移量随入射光束偏移距离和偏离角的增加而增加,与入射光束的偏移方向和方位角无关.角向倾斜时 关键词： 拉盖尔-高斯光束 相位全息光栅 横向偏移 角向倾斜     

基于亚波长光栅的VCSEL偏振控制研究

为了确定亚波长光栅在微电机械系统(MEMS)波长可调谐VCSEL不同位置(上DBR上表面、上DBR下表面以及内腔)中实现TE和TM偏振控制的光栅参数范围以及光栅在哪个位置时实现偏振控制最稳定,通过MATLAB建立MEMS波长可调VCSEL的模型,然后计算光栅在3种位置时上反射镜(包括空气隙和光栅)随光栅参数变化的反射率,以此来确定它们实现TE/TM稳定偏振的光栅参数范围(即高反射范围内的参数)。将各自的高反射所对应反射率减去相同光栅参数范围内TM/TE低反射对应的反射率,通过反射率差值确定光栅在哪种位置时MEMS波长可调谐VCSEL实现偏振是最稳定的。最后得出的结论是光栅在上DBR下表面几乎无法控制TM偏振,而将光栅放置于内腔中,无论是在TE偏振控制上还是TM偏振上都是最稳定的。在实现TE偏振稳定的参数范围内,TE的阈值增益比TM最小少10 cm~(-1);而在实现TM偏振稳定时,在TE偏振稳定的参数范围内,TE的阈值增益比TM最小少5 cm~(-1)。