Excessively Tilted Fiber Grating Research Articles

Femtosecond Laser Inscribed Excessively Tilted Fiber Grating for Humidity Sensing.

We propose a humidity sensor using an excessively tilted fiber grating (Ex-TFG) coated with agarose fabricated using femtosecond laser processing. The processed grating showcases remarkable differentiation between TE and TM modes, achieving an exceptionally narrow bandwidth of approximately 1.5 nm and an impressive modulation depth of up to 15 dB for both modes. We exposed the agarose-coated TFG sensor to various relative humidity levels and monitored the resonance wavelength to test its humidity sensing capability. Our findings demonstrated that the sensor exhibited a rapid response time (2-4 s) and showed a high response sensitivity (18.5 pm/%RH) between the humidity changes and the resonant wavelength shifts. The high sensitivity, linearity, repeatability, low hysteresis, and excellent long-term stability of the TFG humidity sensor, as demonstrated in our experimental results, make it an attractive option for environmental monitoring or biomedical diagnosis.

SnO2/polyvinyl alcohol nanofibers wrapped tilted fiber grating for high-sensitive humidity sensing and fast human breath monitoring

A fiber-optic humidity sensor based on excessively tilted fiber grating (Ex-TFG) wrapped with tin dioxide (SnO2)/polyvinyl alcohol (PVA) hybrid nanofibers is presented. Relative humidity (RH) rise causes the changes both in refractive index (RI) and geometry size of the film. The transmitted spectrum of the Ex-TFG changes as a result, owing to the strong interaction of the excited forward propagating cladding modes to the surface coating. Experimental results demonstrate the maximum intensity sensitivity of 0.43 dB/%RH in RH range of 35%−75%. Compared with pure PVA film coated ones, the RH sensitivity of the SnO2/PVA wrapped grating is vastly enhanced by one order of magnitude. The reticular morphology of electrospun SnO2/PVA film around the Ex-TFG promises the unimpeded permeation of water molecules, and then the sensor exhibits an ultrafast response of ∼67 ms and recovery times of ∼83 ms, by monitoring the human breathing with different rhythms. Lastly, the negligible spectral deformation to temperature perturbations verifies the temperature de-correlation of the designed RH sensor. This work indicates that the proposed SnO2/PVA nanofibers wrapped Ex-TFG can provide a simple, robust and high-sensitive optic device for healthcare monitoring, environmental and biochemical sensing applications.

Novel optical fiber Vernier immunosensor based on cascading Sagnac loops embedded with excessively tilted fiber grating for specific detection of canine distemper virus.

A novel optical fiber Vernier effect (VE) biosensor based on cascading Sagnac loops embedded with excessively tilted fiber grating (ExTFG) is proposed for the label free and specific detection of canine distemper virus (CDV). The VE was realized by cascading two different Sagnac loops with similar free spectrum range (FSR), one of which was integrated with panda-type polarization maintaining fiber (PMF) as the reference loop, and the other was embedded with ExTFG as the sensing loop. Owning to the amplified function of the VE, the refractive index (RI) sensitivity of the proposed sensing structure reached -1914.89 nm/RIU, which is approximately 12 times higher than that of the single ExTFG based RI sensor. Furthermore, the ExTFG in sensing loop was modified by graphene oxide (GO) and bio-functionalized by the CDV monoclonal antibodies (anti-CDV MAbs) for the specific detection of the CDV. Experimental results show that the proposed optical fiber Vernier sensor could detect the CDV in buffer solution with concentration as low as 1 pg/mL, and the sensitivity was about -1.18 nm/[log(mg/ml)] in the concentration range of 1 pg/mL ~ 50 ng/mL. The excellent specific and clinical properties of the biosensor were verified by immunoassays for fetal bovine serum, Toxoplasma gondii, rabies virus and CDV serum in sequence. Due to the sensitivity amplification function of VE, dense comb spectrum of the Sagnac loop and the stable interference spectra maintained by the polarized light, the proposed biosensor possesses the combined advantages of high sensitivity, high Q-factor and high stability, which may have potential applications in biosensing fields.

Cladding mode characteristics simulation of an excessively tilted fiber grating coated with gold nanoshells.

The cladding mode characteristics simulation of an excessively tilted fiber grating (ExTFG) coated with gold nanoshells was conducted in this study. First, the effective refractive indices of the core and cladding mode before coating were obtained by solving the eigenvalue equation of the three-layer waveguide structure, and the coupling characteristics were briefly analyzed. Then HE1,m and EH1,m modes were selected as the research objects, and the spectral characteristics of ExTFG coated with gold nanoshells were simulated by the finite element method. The simulated refractive index sensitivity of HE1,29 and EH1,29 modes is 160.16 and 185.03 nm/RIU, respectively. Compared with the non-localized surface plasmon resonance (LSPR) effect, it increased by 10.76 nm/RIU (7.2%) and 19.53 nm/RIU (11.8%), respectively. Thus, the LSPR effect was verified to be beneficial to improve the refractive index sensitivity of ExTFG.

Ultrasensitive Glucose Biosensor Using Micro-Nano Interface of Tilted Fiber Grating Coupled With Biofunctionalized Au Nanoparticles

Monitoring glucose levels is one of the main diagnostic methods for preventing and treating diabetes. Conventionally monitoring of blood glucose through a finger-stick procedure brings pain. It is essential to develop biosensors for low concentration glucose detection in non-invasive samples. In this paper, a tilted fiber grating based localized surface plasmon resonance (LSPR) biosensor for ultrasensitive glucose detection has been demonstrated. The glucose biosensor was fabricated by immobilization of Au nanoparticles (AuNPs) onto an excessively tilted fiber grating (Ex-TFG) and subsequent biofunctionalization by coating polydopamine and concanavalin A onto the AuNPs/Ex-TFG surface. The biosensor has the response regions covering low, middle and high glucose concentrations. The resonance peak indicates a redshift of 0.24 nm, 0.06 nm and 0.48 nm in the concentration ranges of 1.0 nM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 1.0\,\, \boldsymbol {\mu }\text{M}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.0\,\, \boldsymbol {\mu }\text{M}- 50.0\,\, \boldsymbol {\mu }\text{M}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50.0\,\, \boldsymbol {\mu }\text{M}-5.0$ </tex-math></inline-formula> mM, respectively. The limit of detection (LOD) of the biosensor in pure phosphate buffered saline is 2.5 nM. The high sensitivity and specific selectivity for D-glucose detection are attributed to strong coupling between the evanescent fields from transverse magnetic (TM) cladding modes of the Ex-TFG and the localized electric fields from LSPR of the AuNPs and their biofunctionalization, and resulting in remarkable enhancement of localized effective refractive index with slightly increased D-glucose concentration. Especially, the glucose biosensor based on the Ex-TFG coupled with the biofunctionalized AuNPs, has been demonstrated to be of practical utility to detect the glucose in artificial urine, which has potential applications in non-invasive glucose monitoring for early diagnosis and better management of diabetes.

Sensitivity adjustable biosensor based on graphene oxide coated excessively tilted fiber grating

Biosensors play a significant role in biomedical, clinical and disease diagnosis areas. Here, we proposed Langmuir adsorption model to explain sensing mechanism of biosensor based on excessively tilted fiber grating (Ex-TFG) functionalized with graphene oxide (GO). Due to GO containing plenty of six-membered rings and oxygen-containing groups, the biomolecules can be easily adsorbed through π-π interaction and hydrogen bond. The whole interaction process obeys the Langmuir adsorption model in which there always an equilibrium during detecting process, inducing the sensor with adjustable bio-sensitivity and detection range. Three biosensors based on Ex-TFG coated with three different amount of GO were investigated for hemoglobin (Hb) detection experiment, showing pronounced bio-interaction induced resonance shifts. The experiment results indicate that GO coating could enhance the surface biological activity of Ex-TFG, making the Ex-TFG sensitive to the Hb biomolecule solutions. The three GO coated Ex-TFG sensors have the bio-sensitivity of 3.83 nm/(mg/ml), 4.33 nm/(mg/ml), and 8.21 nm/(mg/ml), and the detection range of 0.8 mg/ml, 0.6 mg/ml and 0.4 mg/ml, respectively, which are in good agreement with the prediction from the Langmuir adsorption model. By controlling the amount of the bio-functionalized materials, the bio-sensitivity and detection range of the Ex-TFG based biosensors can be easily adjusted.

Broadband Vibration Sensor Using Reflected Excessively Tilted Fiber Grating With Clamped Beam

A broadband vibration sensor using reflected excessively tilted fiber grating (ExTFG) with clamped beam was proposed. The two ends of reflected ExTFG were fixed, and the fiber grating part was suspended, thus forming a clamped beam structure. The resonance frequency and vibration mode of the proposed sensor were calculated and analyzed by numerical calculation and software simulation. Axial strain, acceleration response and vibration frequency detection experiments were designed and conducted. Experimental results showed that the axial strain sensitivity of TE/TM mode was −2.57 pm/ $\mu \varepsilon $ and −1.6 pm/ $\mu \varepsilon $ , respectively. The first-order resonant frequency of the sensor was 1900Hz with a flat region ranging from 100 Hz to 800 Hz, and the maximum acceleration sensitivity of TE/TM mode in the flat region was 38.79 mV/g and 31.43 mV/g at 800Hz, respectively. In addition, it could detect the vibration frequency range of 100–10000 Hz, and provide a highest signal-to-noise ratios (SNR) of 55.1 dB at 1900 Hz. The proposed sensor possessed a wide response frequency range, and was able to realize dual-parameter detection for acceleration and vibration frequency, which would make its good application prospects in the mechanical vibration detection for large-scale projects.

Excessively Tilted Fiber Grating Sensors

The development of excessively tilted fiber gratings (Ex-TFGs) provides a new type of sensing device with high refractive index (RI) sensitivity, low thermal crosstalk and vector sensing property. Due to the asymmetric grating structure of Ex-TFG, the light in the core is coupled into high order forward-propagating cladding modes and split into two orthogonal polarization states, resulting in dual-peak resonances in the transmission spectrum. The Ex-TFG also exhibits a non-circularly symmetrical near field distribution, which endows an exceptional orientation sensing capability. Benefitting from the unique mode coupling behavior, Ex-TFGs have been studied and developed for many different sensing applications, such as polarization dependent torsion and loading sensors, vector accelerometer and magnetometer, and a variety of low thermal crosstalk bio/chemical sensors. This paper will review the recent study and development of Ex-TFGs in terms of mode coupling mechanism, fabrication method, transmission and sensor characteristics and the novel applications in sensing areas.

Theoretical and Experimental Analysis of the Directional RI Sensing Property of Tilted Fiber Grating

In this article, we have theoretically and experimentally investigated the unique vector refractive index (RI) sensing property of tilted fiber grating (TFG). Due to the orthogonal symmetric grating structure, TFGs would mainly achieve the coupling between the fiber core mode and the two orthogonal polarization LP1m of cladding mode. And the numerical simulation results showed that the coupling coefficient between fundamental core mode to the LP1m cladding mode is higher than the others. In the experiment, we have furthermore observed the cladding mode field distribution of excessively TFG (Ex-TFG) and long period fiber grating (LPFG), which indicated that the evanescent field distribution of cladding mode of TFG shows an asymmetric near field distribution with two lobes oriented along the fast axis of TFG, and the one of LPFG has a circularly symmetric cladding mode field distribution. In addition, by employing side-immersion method, we have measured the azimuth RI sensitivities of Ex-TFG, tilted fiber Bragg grating (TFBG) and LPFG, which exhibited that both Ex-TFG and TFBG have shown a direction-dependency RI sensitivity, and the RI sensitivity with side-immersion along fast axis is almost half of the one along slow axis, and the RI sensitivity of LPFG is azimuth independent. Overall, the experiment results show that the TFGs inherently show unique directional RI sensing property, which could be potentially applied in vector sensing area.

Polarization dependent cladding modes coupling and spectral analyses of excessively tilted fiber grating.

We report on the detailed analyses of mode coupling from fiber core to cladding in excessively tilted fiber gratings (ETFGs). Cladding modes responsible for the typical dual peak pairs in the transmission spectrum of ETFGs are identified with phase matching condition, which suggests two set of dual peak pairs generated from coupling to cladding modes with even and odd azimuthal order. The polarization dependence of those dual peak pairs are also investigated by calculating the coupling coefficients of cladding modes for two orthogonal polarizations. With the calculated coupling coefficients, the measured polarization dependent spectra can be reproduced numerically.

基于细直径极大倾角光纤光栅的悬臂梁低频振动传感器性能研究

基于细直径极大倾角光纤光栅的悬臂梁低频振动传感器性能研究

基于细直径极大倾角光纤光栅的悬臂梁低频振动传感器性能研究

基于细直径极大倾角光纤光栅的悬臂梁低频振动传感器性能研究

Orientation-dependent optic-fiber accelerometer based on excessively tilted fiber grating

An orientation-dependent optic-fiber accelerometer based on the excessively tilted fiber grating (ExTFG) inscribed in SM28 fiber is demonstrated, which is based on the optical power demodulation scheme. Without any complicated processing, the cladding mode resonances of the bare ExTFG show high sensitivity to slight perturbation of bending. Due to its excellent azimuth-related bending properties, such a bare ExTFG fixed on a simple cantilever beam has exhibited strong orientation-dependent vibration properties. The experimental results show that a TE mode of the sensor can provide a maximum acceleration sensitivity of 74.14 mV/g at 72 Hz and maximum orientation sensitivity of 9.1 mV/deg while, for a TM mode, a maximum acceleration sensitivity of 57.85 mV/g at 72 Hz and maximum orientation sensitivity of 7.4 mV/deg could be achieved. These unique properties enable the sensor to act as a vector accelerometer for applications in many vibration measurement fields.

Label-free and specific detection of soluble programmed death ligand-1 using a localized surface plasmon resonance biosensor based on excessively tilted fiber gratings.

Programmed death ligand-1 (PD-L1) plays an important role in tumor evasion from the host immune system. The level of soluble PD-L1 (sPD-L1) in serum is closely related to tumor aggressiveness and outcomes. This study aimed to propose a localized surface Plasmon resonance (LSPR) biosensor based on excessively tilted fiber grating (ExTFG) coated with large-sized (∼160 nm) gold nanoshells for label-free and specific detection of sPD-L1. The experimental results showed that the limit of detection (LOD) of the immunosensor for sPD-L1 in buffer solutions was ∼1 pg/mL due to the enhanced LSPR effect resulting from the interaction between sPD-L1 molecules and anti-sPD-L1 monoclonal antibodies. The detection of sPD-L1 in complex serum media, such as fetal bovine serum, confirmed that the label-free immunosensor was extremely specific to sPD-L1 and could identify it at a concentration as low as 5 pg/mL. Therefore, it can be potentially applied in clinic for the fast and early diagnosis of cancer.

Excessively tilted fiber grating based Fe3O4 saturable absorber for passively mode-locked fiber laser.

A novel approach to saturable absorber (SA) formation is presented by taking advantage of the mode coupling property of excessively tilted fiber grating (Ex-TFG). Stable mode-locked operation can be conveniently achieved based on the interaction between Ex-TFG coupled light and deposited ferroferric-oxide (Fe3O4) nanoparticles. The central wavelength, bandwidth and single pulse duration of the output are 1595 nm, 4.05 nm, and 912 fs, respectively. The fiber laser exhibits good long-term stability with signal-to-noise ratio (SNR) of 67 dB. For the first time, to the best of our knowledge, Ex-TFG based Fe3O4 SA for mode-locked fiber laser is demonstrated.

Excessively tilted fiber grating-based vector magnetometer.

A compact optic-fiber vector magnetometer is proposed and experimentally demonstrated, which is based on an excessively tilted fiber grating (Ex-TFG) assistant with the magnetic fluid (MF). Without any complicated processing, the cladding mode resonances of the bare Ex-TFG packaged by the MF show high sensitivity to slight perturbations by the magnetic field. Due to the excellent magneto-optical properties of the MF and the azimuth-dependent refractive index sensitivity of the Ex-TFG, such a magnetometer can achieve the magnetic field intensity sensitivity of 2.45nm/mT and the orientation sensitivity of 0.41nm/deg. In addition, based on the spectral interrogation, the detection limit of the magnetic field intensity could reach around 8.1μT at the minimum wavelength measurement accuracy of 0.02nm.

Hybrid Grating in Reduced-Diameter Fiber for Temperature-Calibrated High-Sensitivity Refractive Index Sensing

We propose and experimentally demonstrate a hybrid grating, in which an excessively tilted fiber grating (Ex-TFG) and a fiber Bragg grating (FBG) were co-inscribed in a reduced-diameter fiber (RDF). The hybrid grating showed strong resonances due to coupling among core mode and a set of polarization-dependent cladding modes. This coupling showed enhanced evanescent fields by the reduced cladding size, thus allowing stronger interaction with the surrounding medium. Moreover, the FBG’s Bragg resonance confined by the thick cladding was exempt from the change of the surrounding medium’s refractive index (RI), and then the FBG can work as a temperature compensator. As a result, the Ex-TFG in RDF promised a highly sensitive RI measurement, with a sensitivity up to ~1224 nm/RIU near the RI of 1.38. Through simultaneous measurement of temperature and RI, the temperature dependence of water’s RI is then determined. Therefore, the proposed hybrid grating with a spectrum of multi-peaks embedded with a sharp Bragg resonance is a promising alternative for the simultaneous measurement of multi-parameters for many RI-based sensing applications.

Theoretical analysis of tuning property of the graphene integrated excessively tilted fiber grating for sensitivity enhancement

The graphene integrated optical fiber devices show many excellent characteristics, especially their tunable optical properties. Here we investigate thoroughly the influence of the tuning property of the graphene on the resonance and sensing performance of the excessively tilted fiber grating (Ex-TFG) operating around the dispersion tuning point. An improved piecewise discretization method for the finite-difference mode solver combined with coupling mode theory is presented to explore the general variation rule of mode characteristics and polarization-dependent resonances corresponding to polarized TE/TM0,m and HE/EHv,m modes of the graphene integrated Ex-TFG. The results reveal that both p-polarized and s-polarized modes and their resonances are greatly influenced by the tunable graphene. On this basis, the sensitivity enhancement of the graphene coated Ex-TFG is explored in detail. It is shown that the sensing performance of both polarized cases can be greatly improved by tuning the chemical potential. In particular, the graphene induces a greater influence on both resonance wavelength and resonance strength of the s-polarized mode, but the p-polarized one has a higher sensitivity. We believe that these unique tuning properties make the graphene integrated Ex-TFG devices ideal for the telecommunication and sensing applications, such as tunable fiber modulators and biochemical sensors.

Immunosensing platform with large detection range using an excessively tilted fiber grating coated with graphene oxide.

We report an immunosensing platform with a large detection range using an excessively tilted fiber grating (ExTFG) coated with graphene oxide (GO). ExTFG was inscribed in standard single-mode fiber; GO film was coated on the fiber surface through hydrogen bond. The effectiveness and uniformity of GO deposited on the ExTFG surface were investigated by field emission scanning electron microscopy and energy spectrum method. Bovine serum albumin (BSA) monoclonal antibodies (MAbs) were used as biometric units to link the GO film through a covalent bond for the specific detection of BSA, so as to evaluate the performances of the proposed biosensor. The whole dynamic immobilization process of BSA MAbs and BSA detection were observed by the spectral evolution of the sensor. Experimental results show that the fabricated GO-coated ExTFG biosensor has a large detection range from 1.5nM-75nM and fast response for BSA antigen; the limit of detection is ∼0.88 nM by using an optical spectrum analyzer with a resolution of 0.03nm, and the dissociation constant KD and the affinity constant KA are calculated to be ∼6.66×10-9 M and ∼1.5×108 M-1, respectively. The proposed GO-coated ExTFG immunosensing platform could lay a foundation for the specific detection of other biomolecules.

Titanium Dioxide Film Coated Excessively Tilted Fiber Grating for Ultra-Sensitive Refractive Index Sensor

In this paper, we report a sensitive refractive index sensor based on the titanium dioxide (TiO $ _2$ ) film coated excessively tilted fiber grating (Ex-TFG) operating around the dispersion tuning point. The effects of the TiO $ _2$ film on the cladding mode field, transmission, and sensing characteristics of the Ex-TFG are outlined. It is found that the TiO $ _2$ film induces a two-step mode transition process for both TE/TM $ _{0,l}$ and HE/EH $ _{v,l}$ ( $ v=1,2,\ldots$ ) cladding modes. During the first transition, the same optimized film thickness can be obtained for TE $ _{0,l}$ , HE $ _{1,l}$ , and EH $ _{2,l-1}$ cladding modes. The results show that the sensitivity can be optimized based on the combined effects of the intermodal separation, initial wavelength separation, and the variation in the phase matching curve (i.e., $ \partial \Lambda / \partial \lambda$ ). The improved sensitivities are up to $ 32261.2\;\text{nm}/\text{RIU}$ and $ 23183.3\;\text{nm}/\text{RIU}$ by tracking dual resonance band corresponding to the s-polarized TE $ _{0,10}$ and p-polarized EH $ _{2,9}$ cladding modes, respectively, in the low refractive index region. For the single resonance corresponding to the s-polarized HE $ _{1,9}$ cladding mode, the sensitivity as high as $ 25860.0\;\text{nm}/\text{RIU}$ is achieved. The sensitivity is significantly enhanced by a factor of $ 13.2 \sim 18.1$ and $ 3.7 \sim 5.2$ times as compared with the uncoated Ex-TFG and classical TiO $ _2$ film coated long period fiber grating with reduced fiber diameter, respectively. The sensitivity can be further improved, which makes the film coated Ex-TFG very attractive for the refractive index sensing.