Tilted Bragg grating in a glycerol-infiltrated specialty optical fiber for temperature and strain measurements.

In this Letter, we propose an in-line tilted fiber Bragg grating sensor for temperature and strain measurements. The grating is inscribed in a specialty optical fiber using tightly focused femtosecond laser pulses and the line-by-line direct writing method. Beside the central core in which the grating is produced, a hollow channel filled with glycerol aqueous solution significantly improves the sensitivity of the fiber cladding modes due to its high thermo-optic coefficient. We show that the temperature sensitivity of the core mode is 9.8 pm/°C, while the one of the cladding modes is strongly altered and can reach -24.3 pm/°C, in the investigated range of 20-40°C. For the strain measurement, sensitivities of the core mode and the cladding modes are similar (∼0.60 pm/µÎµ) between 0 and 2400 µÎµ. The significative difference of temperature sensitivity between the two modes facilitates the discrimination of the dual parameters in simultaneous measurements.

Cascaded plastic optical fiber based SPR sensor for simultaneous measurement of refractive index and temperature.

A cascaded side-polish plastic optical fiber (POF) and FONTEX optical fiber based surface plasmon resonance (SPR) sensor is proposed for simultaneous measurement of refractive index (RI) and temperature. The side-polish POF and FONTEX optical fiber are connected by using the UV glue in a Teflon plastic tube. The SPR phenomenon can be excited at both of the side-polish region and the FONTEX fiber cladding. The polydimethylsiloxane (PDMS) is coated on the side-polish POF to get a temperature sensing channel. Due to the low RI sensitivity of the FONTEX optical fiber, the cascaded fiber sensor can obtain a broader RI measurement range with a low crosstalk. An RI sensitivity of 700 nm/RIU in the RI measurement range of 1.335-1.39 and a temperature sensitivity of -1.02 nm/°C measured in deionized water with a range of 20-60 °C are obtained. In addition, the cascaded POF based SPR sensor has potential application prospects in the field of biochemical sensing.

NY-ESO-1 antigen-antibody interaction process based on an TFBG plasmonic sensor.

Autoantibodies against New York esophageal squamous cell cancer 1 (NY-ESO-1) play a crucial role in the diagnosis of esophageal cancer. In this work, a surface plasmonic tilted fiber Bragg grating (TFBG) biosensor is proposed for the detection of NY-ESO-1 antibody, as well as the investigation of the hook effect (which refers to the false negative result in some immunoassays when the concentration of antibodies in the sample is very high) during biomolecular binding between NY-ESO-1 antigen and antibody. The biosensor is made by an 18° TFBG coated with a 50-nm-thick gold film over the fiber surface together with NY-ESO-1 antigens attached to the metallic surface serving as bio-receptors. This biosensor can provide a limit of detection at a concentration of 2 × 10-7 µg/ml with a good linearity in the range from 2 × 10-7 to 2 × 10-5 µg/ml. For a concentration higher than 2 × 10-3 µg/ml, the performance of the sensor probe is reduced owing to the hook effect. Furthermore, experimental results have also demonstrated the repeatability of the proposed biosensor. This proposed biosensor features label-free, compactness, and fast response, which could be potentially applied in the diagnosis of esophageal cancer.

Influence of Annealing on Polymer Optical Fiber Bragg Grating Inscription, Stability and Sensing: A Review.

This article reviews recent research progress on the annealing effects on polymer optical fibers (POFs), which are of great importance for inscription, stability and sensing applications of fiber Bragg gratings (FBGs) in POFs due to their unique properties related to polymer molecular chains. In this review, the principle of annealing to reduce frozen-in stress in POFs drawing and different annealing timings are firstly summarized. Then, the annealing methods for POFs are introduced under several different conditions (temperature, humidity, strain, stress and solution). Afterwards, the principle of FBGs and several inscription techniques are reported. Subsequently, the annealing effects on the properties of POFs and polymer optical fiber Bragg gratings (POFBGs) quality are discussed. Finally, the influence of annealing on POFBG sensitivity is summarized. Overall, this paper provides a comprehensive overview of annealing techniques and their impact on both POFs and POFBGs. We hope that it will highlight the important progress made in this field.

Recovery of a highly reflective Bragg grating in DPDS-doped polymer optical fiber by thermal annealing.

We report fiber Bragg grating manufacturing in poly(methyl methacrylate) (PMMA)-based polymer optical fibers (POFs) with a diphenyl disulfide (DPDS)-doped core by means of a 266 nm pulsed laser and the phase mask technique. Gratings were inscribed with different pulse energies ranging from 2.2 mJ to 2.7 mJ. For the latter, the grating reflectivity reached 91% upon 18-pulse illumination. Though the as-fabricated gratings decayed, they were recovered by post-annealing at 80°C for 1 day, after which they showed an even higher reflectivity of up to 98%. This methodology for the fabrication of highly reflective gratings could be applied for the production of high-quality tilted fiber Bragg gratings (TFBGs) in POFs for biochemical applications.

Wearable cardiorespiratory monitoring with stretchable elastomer optical fiber.

This work presents a stretchable elastomer optical fiber sensor incorporated into a belt for respiratory rate (RR) and heart rate (HR) monitoring. Different materials and shapes of prototypes designed were tested in terms of performance and the best choice was identified. The optimal sensor was tested by 10 volunteers to evaluate the performance. The proposed elastomer optical fiber sensor can achieve simultaneous measurement of RR and HR in different body positions, and also ballistocardiography (BCG) signal measurement in the lying position. The sensor has good accuracy and stability, with maximum errors of 1 bpm and 3 bpm for RR and HR, respectively, and average weighted mean absolute percentage error (MAPE) of 5.25% and root mean square error (RMSE) of 1.28 bpm. Moreover, the results of the Bland-Altman method showed good agreement of the sensor with manual counting of RR and with electrocardiogram (ECG) measurements of HR.

Femtosecond laser line-by-line tilted Bragg grating inscription in single-mode step-index TOPAS/ZEONEX polymer optical fiber.

In this Letter, we demonstrate 8°-tilted fiber Bragg grating (TFBG) inscription in single-mode step-index TOPAS/ZEONEX polymer optical fibers (POFs) using a 520 nm femtosecond laser and the line-by-line (LbL) writing technique. As a result of the tilt angle and the fiber refractive index, a large spectral range of cladding mode resonances covering 147 nm is obtained. The evolution of the transmitted spectrum is analyzed as a function of the surrounding refractive index (SRI) in a large range from 1.30 to 1.50. The cutoff cladding mode shows a refractive index sensitivity of 507 nm/RIU (refractive index unit). For single-resonance tracking near the cutoff mode, the sensitivity is at least 6 nm/RIU, depending on the exact wavelength position of the cladding modes. The main originality of our work is that it produces, for the first time, to the best of our knowledge, a TFBG in POF that operates in the refractive index range of aqueous solutions. The sensing capability for a large range of refractive index values is also relevant for (bio)chemical sensing in different media.

Simultaneous measurement of refractive index and temperature based on a side-polish and V-groove plastic optical fiber SPR sensor.

A simple plastic optical fiber (POF) based surface plasmon resonance (SPR) sensor is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. The sensor consists of a series of V-grooves along the POF and a side-polish structure at the other side of the fiber. The V-groove structure can alter the SPR excitation angle and act as a mode filter, effectively enhancing the SPR effect and narrowing the SPR wavelength width. After coating a layer of thermosensitive material-polydimethylsiloxane (PDMS) film on half part of the fiber probe, a dual-parameter sensor probe is obtained for RI and temperature measurement. Experimental results show the RI sensitivity of the prepared probe can reach 1546 nm/RIU in the RI range of 1.335-1.37 RIU and the temperature sensitivity is -0.83 nm/°C in the temperature range of 20-80°C. The sensor is simple in structure and low cost, and has potential applications in the biochemical sensing fields.

Glucose sensing based on hydrogel grating incorporating phenylboronic acid groups.

We proposed a hydrogel grating sensor functionalized with phenylboronic acid (PBA) group for glucose concentration detection. A PBA functionalized polyacrylamide hydrogel film was first prepared via ultraviolet polymerization. Then, the diffraction grating was written on the hydrogel film via the femto-second (fs) laser point-by-point direct inscription. Binding between the PBA groups in the hydrogel and glucose molecules would lead to the swelling of hydrogel and the thus grating structure, thus modifying the diffraction properties of the grating. We experimentally characterized the swelling and transmission of the grating with different glucose concentrations. Sensitivity of the sensor was defined as variations in relative diffraction efficiency in response to glucose concentration changes, and was experimentally found to 0.61%/mM. The proposed sensor showed fast response towards the presence of glucose, and its reusability and biocompatibility were also confirmed. The use of fs-laser inscription technique does not require a pre-fabricated template, and would allow to directly modify the fabrication parameters such as scanning speed, pulse energy and frequency. Therefore, one is able to conveniently optimize the grating structure and improve the inscription efficiency. The proposed hydrogel grating could be potentially fabricated into wearable sensors, namely, contact lenses, for continuous monitoring of tear glucose level with rapid response.

Ultrasensitive cascaded in-line Fabry-Perot refractometers based on a C-shaped fiber and the Vernier effect.

We propose and experimentally demonstrate a fiber refractometer based on a C-shaped fiber and the Vernier effect. The sensor is fabricated by cascading a single mode fiber (SMF) pigtail together with a C-shaped fiber segment and another SMF segment. Thus, the C-shaped fiber would constitute an open cavity (sensing cavity) in which test analytes could be filled, while the SMF segment would constitute another reference cavity. Due to the similar optical path length of these two cavities, the Vernier effect would be activated, thus forming spectral envelops in the reflection spectrum of the sensor. Variations in the refractive index (RI) of analytes would result in the shifts of the spectral envelops. Both theoretical calculations and experiments are carried out in the characterization of the sensor measuring liquid and gaseous analytes. The experimental sensitivity of the sensor is found to be ∼37238 nm/RIU for gas RI measurement. The proposed sensor features the advantages such as ease of fabrication, extremely high sensitivity, capability of sensing of both gaseous and liquid analytes, small footprint, and good mechanical strength. Compared to other existing Vernier effect-based fiber refractometers typically fabricated using PCFs, the proposed sensor would allow analytes to have much easier and quicker access to the sensor probe.

Nomogram constructed by immunological and inflammatory indicators for predicting prognosis of patients with esophageal squamous cell carcinoma treated with neoadjuvant chemoradiotherapy plus surgery.

Objectives: At present, esophageal squamous cell carcinoma (ESCC) patients accepting neoadjuvant chemoradiotherapy (nCRT) plus surgery lack corresponding prognostic indicators. This study aimed to construct a prognostic prediction model for ESCC patients undergoing nCRT and surgery based on immune and inflammation-related indicators. Methods: We retrospectively analyzed the levels of serum immune- and inflammation-related indicators of ESCC patients before receiving nCRT plus surgery in the training cohort (99 patients) and validation cohort (67 patients), which were collected from 2007 to 2020. Univariate and multivariate Cox survival analyses were conducted to evaluate the indicators to set up a nomogram associated with the patients' overall survival (OS). The prediction accuracy and discriminative ability of the nomogram were measured by the concordance index (C-index), decision curve, calibration curve, integrated discrimination improvement (IDI), and net reclassification improvement (NRI). Results: Univariate and multivariate Cox analyses demonstrated that immune globin A (IgA) and C-reactive protein (CRP) were independent risk factors. A nomogram based on IgA, CRP, and cTNM stage was established for predicted OS in the training cohort and validated in the validation cohort. The C-index of the nomogram was 0.820 (95% CI: 0.705-0.934), which was higher than that of the cTNM stage (0.655 (95% CI: 0.546-0.764), p < 0.05) in the training cohort, and similar results were observed in the validation cohort (0.832 (95% CI: 0.760-0.903 vs 0.635 (95% CI: 0.509-0.757), p < 0.001). Furthermore, the prediction accuracy and net benefit of the nomogram verified by the calibration curve, decision curve, NRI, and IDI were satisfactory in the training and validation cohorts. Conclusion: The newly constructed nomogram concluding serum IgA, CRP, and cTNM stage might be helpful in the prognosis prediction for ESCC patients receiving nCRT plus surgery.

Correction: Hu et al. Direct Bragg Grating Inscription in Single Mode Step-Index TOPAS/ZEONEX Polymer Optical Fiber Using 520 nm Femtosecond Pulses. Polymers 2022, 14, 1350.

The authors wish to make a change to the published paper [...].

Femtosecond laser point-by-point Bragg grating inscription in BDK-doped step-index PMMA optical fibers: erratum.

An erratum is presented to correct the laser pulse energy applied on the fiber during grating fabrication in Opt. Lett.47(2), 249 (2022)10.1364/OL.450047.

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.

Cladding Mode Fitting-Assisted Automatic Refractive Index Demodulation Optical Fiber Sensor Probe Based on Tilted Fiber Bragg Grating and SPR.

In the paper based on surface plasmon resonance (SPR) in a tilted fiber Bragg grating (TFBG), a novel algorithm is proposed, which facilitates demodulation of surrounding refractive index (SRI) via cladding mode interrogation and accelerates calibration and measurement of SRI. Refractive indices with a tiny index step of 2.2 × 10-5 are prepared by the dilution of glucose aqueous solution for the test and the calibration of this fiber sensor probe. To accelerate the calibration process, automatic selection of the most sensitive cladding mode is demonstrated. First, peaks of transmitted spectrum are identified and numbered. Then, sensitivities of several potentially sensitive cladding modes in amplitude adjacent to the left of the SPR area are calculated and compared. After that, we focus on the amplitudes of the cladding modes as a function of a SRI, and the highest sensitivity of -6887 dB/RIU (refractive index unit) is obtained with a scanning time of 15.77 s in the range from 1520 nm to 1620 nm. To accelerate the scanning speed of the optical spectrum analyzer (OSA), the wavelength resolution is reduced from 0.028 nm to 0.07 nm, 0.14 nm, and 0.28 nm, and consequently the scanning time is shortened to 6.31 s, 3.15 s, and 1.58 s, respectively. However, compared to 0.028 nm, the SRI sensitivity for 0.07 nm, 0.14 nm, and 0.28 nm is reduced to -5685 dB/RIU (17.5% less), -5415 dB/RIU (21.4% less), and -4359 dB/RIU (36.7% less), respectively. Thanks to the calculation of parabolic equation and weighted Gauss fitting based on the original data, the sensitivity is improved to -6332 dB/RIU and -6721 dB/RIU, respectively, for 0.07 nm, and the sensitivity is increased to -5850 dB/RIU and -6228 dB/RIU, respectively, for 0.14 nm.

Direct Bragg Grating Inscription in Single Mode Step-Index TOPAS/ZEONEX Polymer Optical Fiber Using 520 nm Femtosecond Pulses.

We experimentally report fiber Bragg gratings (FBGs) in a single mode step-index polymer optical fiber (POF) with a core made of TOPAS and cladding made of ZEONEX using 520 nm femtosecond pulses and a point-by-point (PbP) inscription method. With different pulse energies between 9.7 nJ and 11.2 nJ, 12 FBGs are distributed along the cores of two pieces of POFs with negative averaged effective index change up to ~6 × 10−4 in the TOPAS. For POF 1 with FBGs 1­6, the highest reflectivity 45.1% is obtained with a pulse energy of 10.6 nJ. After inscription, good grating stability is reported. Thanks to the post-annealing at 125 °C for 24 h, after cooling the grating reflectivity increases by ~10%. For POF 2 with FBGs 7­12, similar FBG data are obtained showing good reproducibility. Then, the FBGs are annealed at 125 °C for 78 h, and the average reflectivity of the FBGs during the annealing process increases by ~50% compared to that before the annealing, which could be potentially applied to humidity insensitive high temperature measurement.

Properties of Fiber Bragg Grating in CYTOP Fiber Response to Temperature, Humidity, and Strain Using Factorial Design.

The characteristics of fiber Bragg grating (FBG) in cyclic transparent fluoropolymer (CYTOP) optical fiber have attracted more and more attention in recent years. However, different results of the FBG response to environmental parameters are reported. This work presents a three-variable two-level factorial experimental method to investigate the FBG response to temperature, humidity, and strain in CYTOP fiber. Two uniform FBGs are inscribed separately in CYTOP fiber with and without over-clad. With only eight measuring points, the interactions among three variable parameters are computed and the parameter sensitivities and cross-sensitivities are estimated. Similar temperature and strain sensitivities were found for both gratings, whereas significant cross-sensitivity between humidity and temperature was present only in FBG inscribed in CYTOP fiber with over-clad.

Recent Achievements on Grating Fabrications in Polymer Optical Fibers with Photosensitive Dopants: A Review.

This review discusses recent achievements on grating fabrications in polymer optical fibers doped with photosensitive materials. First, different photosensitive dopants in polymer optical fibers (POFs) are summarized, and their refractive index change mechanisms are discussed. Then, several different doping methods to fabricate the photosensitive POFs are presented. Following that, the principles of gratings, including standard fiber Bragg gratings (FBGs), tilted fiber Bragg gratings (TFBGs), chirped fiber Bragg gratings (CFBGs), phase-shifted fiber Bragg gratings (PSFBGs), and long period fiber gratings (LPFGs), are reported. Finally, fabrications of different gratings based on photosensitive POFs in the last 20 years are reported. We present our article clearly and logically, so that it will be helpful for researchers to explore a broad perspective on this proposed topic. Overall, the content provides a comprehensive overview of photosensitive POF fabrications and grating inscriptions in photosensitive POFs, including previous breakthroughs and recent advancements.

Femtosecond laser point-by-point Bragg grating inscription in BDK-doped step-index PMMA optical fibers.

In this paper, the inscription of 2-mm-long fiber Bragg gratings (FBGs) on benzyl dimethyl ketal (BDK)-doped poly(methyl methacrylate) (PMMA) optical fibers by means of a femtosecond laser and a point-by-point FBG inscription technique is reported. The highest reflectivity of approximately 99% is obtained with a pulse energy of 68.5 nJ, showing a large refractive index modulation amplitude of 7.2 × 10-4. Afterwards, grating stabilities at room and higher temperatures of up to 80°C are investigated.

Biocompatible and Biodegradable Polymer Optical Fiber for Biomedical Application: A Review.

This article discusses recent advances in biocompatible and biodegradable polymer optical fiber (POF) for medical applications. First, the POF material and its optical properties are summarized. Then, several common optical fiber fabrication methods are thoroughly discussed. Following that, clinical applications of biocompatible and biodegradable POFs are discussed, including optogenetics, biosensing, drug delivery, and neural recording. Following that, biomedical applications expanded the specific functionalization of the material or fiber design. Different research or clinical applications necessitate the use of different equipment to achieve the desired results. Finally, the difficulty of implanting flexible fiber varies with its flexibility. We present our article in a clear and logical manner that will be useful to researchers seeking a broad perspective on the proposed topic. Overall, the content provides a comprehensive overview of biocompatible and biodegradable POFs, including previous breakthroughs, as well as recent advancements. Biodegradable optical fibers have numerous applications, opening up new avenues in biomedicine.