Tilted Fiber Grating Research Articles

Ultrafast and Reproducible Fiber-Optic Hydrogen Sensor via a Tilted Fiber Grating With Pd/WO3 Nanocoating

Hydrogen, a high-density and clean energy, has been widely used in various critical applications. However, the safety risk caused by hydrogen leakage during storage and transportation is still a non-negligible issue. Therefore, it is necessary to offer hydrogen sensors with fast response and high repeatability, and it will be perfect for achieving in situ monitoring over the lifecycle of hydrogen production and utilization. Here, we propose a compact optical fiber sensor with a short section of the tilted Bragg fiber grating (TFBG) inscribed in the fiber core and a palladium and tungsten trioxide (Pd/WO3) combined film of 40 nm thickness over the fiber surface. The TFBG excites tens of narrow cladding resonances, part of which possess refractive indexes matching that of the Pd/WO3 coating and providing the high sensitivity to the surrounding hydrogen concentration change. The sensor offers improved sensing characteristics, including the fast response time (less than 10 s), high repeatability (over tens of measurement), and excellent linear response (higher than 99.6%) over the 0% to 3% concentration range.

Renewable regeneration optic fiber glucose sensor based on succinylaminobenzenoboronic acid modified excessively tilted fiber grating

BackgroundOptical fiber sensors have been used to detect glucose owing to advantages such as low cost, small size, and ease of operation etc. phenylboronic acid is one of the commonly used receptors for glucose detection, however phenylboronic acid based regenerative optical fiber sensors are commonly cumulative regeneration, renewable regeneration sensor has been missing from the literature. ResultsIn this work, instead of using phenylboronic acid, we synthesized succinylaminobenzenoboronic acid molecule (BPOA) by introducing a short chain containing carboxyl group at the other end of phenylboronic acid then covalently bonded BPOA on the surface of excessively tilted fiber grating (Ex-TFG). This provides a very stable platform for renewable regeneration and the regenerative buffer was also optimized. The proposed renewable regeneration method exhibited higher linearity and sensitivity (R2 = 0.9992, 8 pm/mM) in relative to the conventional cumulative regeneration method (R2 = 0.9718, 4.9 pm/mM). The binding affinity between BPOA and glucose was found to be almost constant over 140 bind/release cycles with a variation of less than 0.3 % relative standard deviation. SignificanceThe regenerative and label-free sensing capacity of the proposed device provides a theoretical foundation for label-free saccharide detection and the development of wearable glucose monitoring devices based on fiber optic sensors.

Trace detection of cadmium (II) ions based on an excessively tilted fiber grating

Cadmium (Cd2+) ion is one of the most crucial industrial pollutants that cause serious harm to the human body. We proposed and experimentally demonstrated a highly sensitive Cd2+ sensor based on hydrogel coated excessively tilted fiber grating. The hydrogel with the functional monomer of the allyl thiourea can specifically bind to Cd2+, and hence forming a complex. The grating excites high order cladding modes, and ensures a sufficient interaction between the light and hydrogel binding to Cd2+, providing highly sensitive monitoring. The results show that the sensor can detect 0-160 pM Cd2+ in aqueous solution. The maximum sensitivity is 10600 nm/µM, and the minimum detection concentration is 20 pM (about 0.004 ppb), which is much less than that of the international standard (3 ppb). The proposed sensor exhibits high sensitivity, ultra-low detection limit, specificity, and a compact structure, offering potential as a tool for Cd2+ detection in aqueous solution.

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.

Fiber Laser Frequency Comb Based on a 45° Tilted Fiber Grating (Invited)

Fiber Laser Frequency Comb Based on a 45° Tilted Fiber Grating (Invited)

Femtosecond Laser-Inscribed Excessively Tilted Fiber Grating Sensor with CFBG for Simultaneous Measurement of Refractive Index and Temperature

Femtosecond Laser-Inscribed Excessively Tilted Fiber Grating Sensor with CFBG for Simultaneous Measurement of Refractive Index and Temperature

Label-Free DNA Detection Using Etched Tilted Bragg Fiber Grating-Based Biosensor.

A label-free-based fiber optic biosensor based on etched tilted Bragg fiber grating (TFBG) is proposed and practically demonstrated. Conventional phase mask technic has been utilized to inscribe tilted fiber Bragg grating with a tilt angle of 10°, while the etching has been accomplished with hydrofluoric acid. A composite of polyethylenimine (PEI)/poly(acrylic acid) (PAA) has been thermally deposited on the etched TFBG, followed by immobilization of probe DNA (pDNA) on this deposited layer. The hybridization of pDNA with the complementary DNA (cDNA) has been monitored using wavelength-dependent interrogation. The reproducibility of the probes has been demonstrated by fabricating three identical probes and their response has been investigated for cDNA concentration ranging from 0 μM to 3 μM. The maximum sensitivity has been found to be 320 pm/μM, with the detection limit being 0.65 μM. Furthermore, the response of the probes towards non-cDNA has also been investigated in order to establish its specificity.

Nanomaterial-Functionalized Tilted Fiber Gratings for Optical Modulation and Sensing

The tilted fiber grating (TFG) is a special grating whose grating planes are slanted or blazed relative to the fiber axis. These tilts in the planes break the cylindrical symmetry of the fiber, strongly enhance the light coupling from the fiber core to cladding, and resonantly excite a large number of cladding modes. Then, the TFGs with rich spectral information and strong evanescent field leaking out open up a multitude of opportunities for the enabled light-matter interaction by functionalizing their surface with nanomaterials. In this review, we briefly analyze the architectures, coupling mechanisms, spectral properties, and implementations of different TFGs, including weakly tilted Bragg grating with small tilt angle, radiated TFG with 45° tilt angle, excessively TFG with large tilt angle, etc. Further, the sensing enhancement and spectral modulation by integrating layered nanomaterials on TFGs are discussed, which could be considered as a promising strategy to break the limitation of fiber's one-dimensional structure and promote the light-matter interaction for enhanced and extended applications in optical parameter modulation and sensing.

Tunable Sensitivity in Long Period Fiber Gratings During Mode Transition With Low Refractive Index Intermediate Layer

Double-clad fibers where the second cladding has a lower refractive index than the first cladding, prove to be ideal structures for potentiating and tuning the sensitivity in long-period fiber gratings (LPFGs) operating in mode transition. When a thin film is deposited on the optical fiber, the second cladding performs acts as a barrier that initially prevents the transition to guidance in the thin film of one of the modes guided in the first cladding. Finally, the transition to guidance occurs with a sensitivity increase, in analogy to the tunnel effect observed in semiconductors. This improvement has been demonstrated both as a function of the thin film thickness and the surrounding medium refractive index, with enhancement factors of 4 and 2, respectively. This idea reinforces the performance of LPFGs, adding a new degree of freedom to the mode transition and the dispersion turning point phenomena. Moreover, the control of the variation of the effective index of cladding modes could be applied in other structures, such as tilted-fiber gratings or evanescent wave sensors.

Real-Time Monitoring of Human Breathing Using Wearable Tilted Fiber Grating Curvature Sensors

Respiration rate and volume are important physiological indicators of human health. They can indicate cardiovascular health status and cardiopulmonary function. Here, we demonstrate a compact breath monitoring system based on a wearable tilted fiber Bragg grating (TFBG) curvature sensor for human respiration monitoring. With the assistance of a custom designed breathing tube, TFBGs provide a very reproducible highly sensitive, real time response to curvature changes during respiration. The miniaturized and cost effective sensing system which we propose consists of a tunable laser module, a field programmable gate array acquisition control circuit module, and a sensor module. Different breathing patterns can be successfully identified from the optical amplitude and frequency output of the sensor. The robust instrument displays high repeatability and real-time tracking.

Polarization-independent tilted fiber Bragg grating surface plasmon resonance sensor based on spectrum optimization.

We experimentally demonstrated polarization multiplexing schemes in a tilted fiber grating (TFBG) to achieve polarization-independent fiber-optic surface plasmon resonance (SPR) sensors. The first used two orthogonal polarized lights separated by a polarization beam splitter (PBS) that are p-polarized in polarization-maintaining fiber (PMF) and precisely aligned with the tilted grating plane, so as to achieve the transmission of p-polarized light in two opposite directions of the Au-coated TFBG to excite SPR. Alternatively, polarization multiplexing was also achieved by exploring two polarization components to achieve the SPR effect through a Faraday rotator mirror (FRM). The SPR reflection spectra are polarization-independent of the light source and any perturbations to fibers, which are explained by the superposition of p- and s-polarized transmission spectra in equal proportions. The spectrum optimization is presented to reduce the proportion of the s-polarization component. A polarization-independent TFBG-based SPR refractive index (RI) sensor with a wavelength sensitivity of 555.14 nm/RIU and an amplitude sensitivity of 1724.92 dB/RIU for small changes is obtained, exhibiting unique advantages of minimizing the polarization alterations by mechanical perturbations.

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.

High-Power Chirped and Tilted Fiber Gratings Written by Femtosecond Lasers

High-Power Chirped and Tilted Fiber Gratings Written by Femtosecond Lasers

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.

Reflective-Type High Sensitivity Optical Fiber Hydrogen Sensor Based On Enlarged Taper Cascaded With Tilted Fiber Grating

Highly sensitive reflective-type fiber hydrogen sensor based on an enlarged taper cascaded with a tilted fiber grating (TFBG) was proposed. Hydrogen-sensitive Au-Pd nanofilm was coated on the TFBG surface by magnetron sputtering means. By cascading an enlarged taper upstream to the TFBG, cladding modes and Ghost mode of the TFBG could be coupled into the core to form a reflective-type TFBG-based hydrogen sensor. As the hydrogen was absorbed by the Au-Pd nanofilm, the effective refractive index of the cladding of the TFBG will change accordingly, so as to the reflective spectra of the TFBG. The hydrogen concentration was obtained through the intensity changing of the ghost mode in the reflective spectra of TFBG. The proposed sensor showed a high sensitivity of 4.83 dB/% with a calculated LOD of 0.07% for the hydrogen concentration range from 0% to 0.7% at room temperature with the rapid response time of 26 s and recovery time of 40 s, respectively.

Polyelectrolyte/Graphene Oxide Nano-Film Integrated Fiber-Optic Sensors for High-Sensitive and Rapid-Response Humidity Measurement.

Optical fiber humidity sensors have sparked enormous interests in many fields because of their excellent features. However, it remains a great challenge to balance sensitivity, humidity response, temperature crosstalk, and wet hysteresis for real-world application. To overcome this trade-off, an optical fiber humidity sensor is developed here by coating functional graphene oxide (GO)/polyelectrolyte nanocomposite film on the excessively tilted fiber grating (ex-TFG), in which GO/polyelectrolyte nanocomposite film is employed for enhancing the hydrophilicity and accelerating the adsorption/desorption of water molecule, while the ex-TFG is utilized for improving the sensitivity of refractive index and eliminating the crosstalk of temperature. By this design, optical fiber humidity sensors achieve high sensitivity, rapid response and recovery, low hysteresis, and temperature crosstalk as well as excellent repeatability and stability in large relative humidity (RH) range. Our work provides a promising platform for effective RH monitoring systems that can be widely applied in rapid diagnostics, pharmacy, precision medicine, and so forth.

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.

Wavelength-tunable noise-like fiber laser based on PbS quantum dot saturable absorber film.

A wavelength-tunable noise-like pulse (NLP) erbium-doped fiber laser incorporating PbS quantum dot (QD) polystyrene (PS) composite film as a saturable absorber (SA) is experimentally demonstrated. The wavelength tuning is implemented via a Lyot filter consisting of a segment of polarization-maintaining fiber (PMF) and a 45° tilted fiber grating. By adjusting the polarization state of the ring cavity, the laser can deliver NLP with a continuous wavelength-tunable range from 1550.21 to 1560.64 nm. During continuous wavelength tuning, the output power varies between a range of 30.88-48.8 mW. Worthwhile noting is that the output power of 48.8 mW is the reported highest output power for wavelength-tunable NLP operation in an erbium-doped fiber laser using composite film as a saturable absorber.

Diffraction characteristics of radiated tilted fiber grating and its spectrometer application.

We have numerically and experimentally presented the diffraction characteristics of radiated tilted fiber grating (RTFG) in terms of the spectrum, bandwidth, degree of polarization, angular dispersion, and temperature crosstalk. The theoretical and experimental results have shown that the polarization property, bandwidth, and dispersion of RTFG highly depended on the tilt angle of RTFG, and the RTFG has ultra-low temperature crosstalk. We have simulated the transmission spectrum of the RTFG with different tilt angles (25°, 31°, 38°, 45°, and 54°), in which the results show that the larger tilt angle has the wider bandwidth. The RTFGs with the tilt angle of 25°, 31°, 38°, 45°, and 54° have the 3dB bandwidth of 110 nm, 144 nm, 182 nm, 242 nm, and 301 nm, respectively. Besides, the degree of polarization (DOP) of the radiated light from RTFG with the different tilt angles are 0.876, 0.944, 0.967, 0.998, and 0.970, respectively, and the RTFG has the maximum DOP at the tilt angle of 45°, which could be used as single-polarization diffraction device. The experimental results show that with further increase or decrease of the tilt angle, the DOP of radiated light of RTFG would decrease. Both the theoretical and experimental results show that the smaller tilt angle could greatly improve the diffraction angular dispersion of RTFG, in which the 25°, 31°, 38°, and 45° RTFG have the angular dispersion of 0.2288 °/nm, 0.1026 °/nm, 0.0714 °/nm, and 0.0528 °/nm, respectively. Due to the broad working bandwidth, the diffraction angles of RTFG have ultra-low temperature crosstalk, where -0.00042, -0.00054, -0.00064, and -0.00099 degree / °C at the tilt angle of 25°, 31°, 38°, and 45°. Finally, we have demonstrated a miniaturized spectrometer integrated by a 25° RTFG, which has a high spectral resolution of 0.08 nm. The proposed RTFG would be an ideal in-fiber diffraction device and widely applied in spectral analysis, space optical communication, and Lidar areas.

Label-free detection of breast cancer cells using a functionalized tilted fiber grating.

The detection of circulating tumor cells (CTCs) still faces a huge challenge partially because of low abundance of CTCs (1-10 cells/mL). In this work, a plasmonic titled fiber Bragg grating biosensor is proposed for detection of breast cancer cells. The biosensor is made by an 18° TFBG with a 50 nm-thick gold nanofilm coating over the surface of the fiber, further immobilized with a specific antibody against GPR30, which is a membrane receptor expressed in many breast cancers, serving as bait. In vitro tests have confirmed that the proposed biosensor can detect breast cancer cells in concentration of 5 cells/mL within 20 minutes and has good linearity in the range of 5-1000 cells/mL, which has met the requirement of CTC detection in real conditions. Furthermore, theoretical analysis based on the experimental results shows that the limit of detection can even reach single-cell level. Our proposed biosensor has a simple structure, is easy to manufacture, is of small size, and has a good performance, making it a good choice for real-time, label-free, and milliliter-volume detection of cancer cells in future.