Cladding Mode Resonances Research Articles

Unraveling Plasmonic Tilted Fiber Bragg Gratings (TFBG): A Journey From “Anomalous Resonances” to Refined Refractometry

AbstractPlasmonic tilted fiber Bragg gratings (TFBGs) have emerged as versatile tools for refractometric analyses and biochemical sensing. Their applications have significantly blossomed these last years, from proteins and cellular bioassays to operando monitoring in batteries, to cite just a few. They are widely recognized for their cutting‐edge performance and low limits of detection, arising from their dense multimodal spectral nature featuring tens of narrowband cladding mode resonances. Their comb‐like spectrum is so rich that numerous demodulation techniques have been reported, without benchmark of their relative performance while they possess important distinctions. This review highlights developments in detangling techniques from the pioneering works based on single‐peak analysis up to the most recent approaches involving Fourier analysis, the implementation of machine learning, and cascaded spectral decomposition processes. To fairly compare the different techniques of the literature, we implemented each analysis on original experimental refractometric calibrations, revealing the assets of the most updated methods. This paper therefore reviews these demodulation techniques based on the same datasets, obtained under the same conditions. We show and discuss the results obtained from bioassays and pinpoint the importance of advanced analytical methodologies to maximize the reproducibility, reliability and performance of plasmonic‐based TFBGs biosensors.

Application of Gabor, Log-Gabor, and Adaptive Gabor Filters in Determining the Cut-Off Wavelength Shift of TFBG Sensors

Tilted fibre Bragg gratings are optical fibre structures used as sensors of various physical quantities. Their unique measurement capabilities result from the high complexity of the optical spectrum consisting of several dozen cladding mode resonances. TFBG spectra demodulation methods generate signal features that highlight changes in the spectrum due to changes in the interacting quantities. Such methods should enable the distinction between two slightly different values of the measured quantity. The paper presents an effective method of processing the TFBG spectrum for use in measuring the refractive index of liquids. The use of Gabor and log-Gabor filters and their adaptive version eliminates the problem of discontinuity in determining the SRI value related to the existence of the cladding mode comb. The Gabor filters used make visible the shifting and fading of spectral features related to the decrease in the intensity of leaking modes. Subsequent modifications of the proposed algorithm led to an increase in the quality factor of the processed spectrum.

Femtosecond laser direct writing annular small-period long-period fiber gratings for simultaneous refractive index and temperature sensing.

A new type of small-period long-period fiber grating (SP-LPFG) consisting of a series of annuli inscribed by a femtosecond laser in a single-mode fiber is proposed and demonstrated. The effects of the annuli radii and the number of annuli in each period on the transmission spectrum are studied. The transmission spectrum of the annular SP-LPFG exhibits both strong Bragg resonances and cladding mode resonances, which have similar temperature sensitivities of 9.26 and 9.75 pm/°C, respectively. The Bragg resonances are insensitive to the change of environment refractive index (RI), while the cladding mode resonances show a RI sensitivity of 141.89 nm/RIU. The proposed sensor offers a potential solution for biomolecular sensing applications by effectively eliminating the issue of temperature cross-sensitivity. Its compact design and ease of spectral acquisition enhance its practicality and convenience.

A large-dynamic range conductivity-modulated PANI online analysis and sensing platform for ammonia gas detection using coupled optical probing mode

Due to the large-dynamic range and reversible conductivity modulation capability, polyaniline (PANI) shows promising prospects for diverse applications, such as supercapacitors, rechargeable batteries, and electrochromic materials. However, online characterization of the optical properties of the conductive polymer in the infrared band throughout the synthesis and redox state changing processes poses a challenge. Here, we propose and demonstrate an analysis and sensing platform for real-time evaluation of both thickness and the complex refractive index of PANI nanofilm, and employed for ammonia gas concentration sensing. The coupled optical probing mode of tilted fiber Bragg grating (TFBG) as a robust and portable permittivity perception tool is utilized for online perceiving the large-dynamic-range conductivity change of PANI nanofilm. By reproducing the spectral evolution of the probing cladding mode resonance, we introduce a simulation method for analyzing the optical properties of polymer nanofilms within the infrared band. For ammonia gas detection, the sensing platform exhibited continuous and reliable detection within a concentration range of 25–500 ppm, with an optimal film thickness of 156 nm. A good linear response was observed with a sensitivity of up to 5.2 × 10−3 dB/ppm, and a linearity (R2 = 0.9876) with a detection limit of 10 ppm. Our proposed low-cost and large dynamic range conductivity-modulated PANI-TFBG platform for optical properties online analysis and sensing shows potential in environmental monitoring, healthcare, and the biochemical field.

A high Q-factor evanescent field fiber sensor coated with C-MWCNT for label-free HPV determination

A high Q-factor evanescent field fiber sensor is designed based on a fiber Bragg grating (FBG) coated with carboxylic multi-walled carbon nanotubes (C-MWCNT). In a high-reflectivity FBG, multiple cladding modes are stimulated as the forward-moving guided mode within the core of the multimode fiber is coupled to the FBG’s reverse-moving guided mode within the cladding. Our evanescent field fiber sensor is highly sensitive to the refractive index without altering the architecture of fiber or deviating from the conventional process of fabricating fiber gratings. The proposed sensor exhibits a full-width at half maximum (FWHM) of 78 pm and a Q-factor of 1.9 × 104 for its standard cladding-mode resonance, outperforming the majority of reported evanescent field fiber sensors in these parameters. Furthermore, by coating C-MWCNT on the fiber sensing surface, our sensor has been demonstrated to combine multiple functions into one, such as label-free determination of human papilloma virus (HPV) DNA sequences, the measurement of different concentrations, and mispairing discrimination. The sensor design put forward offers a compelling technique within the realm of optical fiber sensing for refractive index and label-free biomolecule detection.

Machine learning approach for automated data analysis in tilted FBGs

This paper presents the development of an approach for automatic estimation of the regression coefficients as a function of the refractive index (RI) in Tilted Fiber Bragg Gratings (TFBGs) based on their optical spectral characteristics. The methodology is based on machine learning approach, where different samples are analyzed in various test conditions to obtain a dataset for the training of a kNN regression algorithm. In this case, the spectral characteristics related to the wavelength spacing between adjacent cladding mode resonances, number of valleys and amplitude of the normalized spectra are correlated with the linear regression algorithm for RI estimation using the machine learning approach. The analysis are performed as a function of the wavelength shift and optical power, considering two regions on the transmitted spectrum and the envelope of the curve. Furthermore, the number of valleys and the area of the envelope curve are also analyzed as a function of the refractive index, which result in six analyzed features. The Principal Components Analysis (PCA) is used to reduce the number of features to two principal components. The results of the proposed algorithm are obtained with different trained and untrained samples, which indicates a root mean squared error (RMSE) of around 0.003RIU for the trained samples and 0.008 RIU for the untrained ones. Therefore, the results indicate the usefulness of the proposed approach, which can be used to enhance the reproducibility of large set of TFBG sensors in practical applications.

Femtosecond-laser-inscribed Bragg grating in hollow-core fiber for highly sensitive optofluidic sensing.

An optofluidic sensor based on a Bragg grating in hollow-core fiber (HCF) is experimentally demonstrated. The grating is inscribed into the HCF by femtosecond laser illumination through a phase mask. Periodic index modulation is introduced into the silica material surrounding the hollow core, causing cladding mode resonance, and multiple reflection peaks are observed in the grating spectrum. These reflection peaks later shift to longer wavelengths when high-index liquid is infiltrated into the HCF. The new reflection peak results from the backward coupling of the liquid core mode of the waveguide, the mode field of which overlaps with the grating modulation surrounding the liquid core. The resonant wavelength of the liquid-core fiber grating increases with the index value of the infiltrating liquid, and optofluidic refractive index sensing is realized with the device. The highest refractive index sensitivity, 1117 nm/RIU, is obtained experimentally in the index range of 1.476-1.54. The infiltrated hollow-core fiber Bragg grating also exhibits high temperature sensitivity due to the high thermal-optic coefficient of the liquid, and a sensitivity of -301 pm/°C is achieved in the temperature range of 25°C to 60°C.

Spectral responses of tilted fiber Bragg grating coated with a thin Ge2Sb2Se4Te1 layer

This article reports the spectral properties of a Ge2Sb2Se4Te1 (GSST) -coated tilted fiber Bragg grating to the changes of surrounding refractive index (SRI). The variations of the cladding mode resonance amplitudes and wavelengths under two orthogonal polarization modes are analyzed and validated with the simulation results. The introduction of the high index coating has induced non-degeneracy to the cladding modes, but the effect gradually diminishes with increasing SRI. The SRI sensitivities of the s-polarized modes have been substantially suppressed and became 20 times lower than that of p-polarized modes. The findings also reveal the association between the cladding resonance amplitudes, the imaginary part of the effective refractive indices and the evanescent field intensity distributions.

Cascaded Bragg gratings in photonic crystal fiber for plasmonic cladding mode-based biosensing of HER2 protein

Spectral multiplexing of biosensors in a single optical fiber has been a long-standing challenge, which we address here for the first time by combining photonic crystal fibers (PCF) with fiber Bragg grating technology. We exploit the features of the optical transmission spectrum of a straight fiber Bragg grating written in a PCF that allows exciting cladding mode resonances within a spectral span of about 60 nm, which is significantly narrower than the width of the transmission spectra of tilted gratings in standard single-mode step-index fibers. More specifically, we consider the cladding mode resonances that feature effective index values close to the refractive index of phosphate buffered saline, and we demonstrate plasmonic label-free biodetection of HER2 (human epidermal growth factor receptor 2) protein. We report on the simultaneous monitoring of the wavelength shifts of said cladding mode resonances from two spatially separated biofunctionalized Bragg gratings and we find that the PCF sensor is able to detect the protein concentration of 8.62 nM with high reproducibility.

Plasmonic biosensing with tilted fiber Bragg gratings interrogated using a 512-pixel spectrometer.

Plasmonic tilted fiber Bragg gratings (TFBGs) are very efficient for fast, accurate, and minimally invasive biosensing. Their transmitted amplitude spectrum is a dense comb of narrowband cladding mode resonances (full width at half maximum < 1 nm) that is usually demodulated using highly resolved (wavelength resolution < 10 pm) devices. This work demonstrates the possibility of using a coarsely resolved spectrometer (166 pm) to read out the amplitude spectrum of a gold-coated TFBG. A refined analysis of the spectral content has allowed us to develop signal processing that provides a refractometric sensitivity of 2656 nm/RIU. This is a fivefold improvement compared to previously reported read-out techniques. Biosensing has then been successfully implemented with gold-coated TFBGs implemented in reflection mode for the detection of insulin, with specific antibodies grafted on the gold surface. Our experimental work is a first step toward the industrialization of the FBG technology, as it opens the door to fast parallel biosensing, profiting from the multiple sensing channels (up to 64) of the interrogator and its high processing speed (repetition rate up to 3 kHz).

Optimized femtosecond laser direct-written fiber Bragg gratings with high reflectivity and low loss.

We propose and experimentally demonstrate a femtosecond laser plane-by-plane (Pl-b-Pl) technology for inscription of high-quality fiber Bragg gratings (FBGs). The spherical aberration (SA) was introduced to elongate the focal volume, and then combined with the scanning process, an expanded rectangular refractive index modification (RIM) region can be achieved. Such RIM regions exhibit a length of 15 µm and a width of 14 µm. Note that it consists of a negative region and a positive region. We have systematically studied the influence of the overlap between the RIM region and fiber core on the spectrum of FBG. After optimizing, the core of a conventional single-mode fiber (SMF) is covered completely by using the positive RIM region, resulting in a significant enhancement of the coupling strength coefficient (i.e., 3177.6 m-1). A 500 µm long FBG assembled by using these RIM regions can achieve a high reflectivity of 95.83%. Moreover, the cladding mode resonances in transmission spectrum are suppressed thoroughly, since the localized effect in RIM region was avoided. In addition, this FBG exhibits a high birefringence of 2.13 × 10-4. Therefore, the proposed fabrication method can be used to inscribe high-quality FBGs that could be used in many fields such as communication, fiber laser, polarization-selective filtering and multi-parameter sensing.

Accurate compensation and prediction of the temperature cross-sensitivity of tilted FBG cladding mode resonances.

The temperature dependence of core mode resonance has been thoroughly studied in fiber Bragg gratings (FBGs), but it is not the case for cladding mode resonances in multi-resonance gratings such as tilted FBGs (TFBGs). In this work, the temperature sensitivity of ultraviolet written TFBGs in SMF-28 fibers is assessed, demonstrating in the first, to the best of our knowledge, place that a single gauge factor K T=6.25⋅10-6±0.02⋅10-6 ∘ C -1 can be employed to characterize the response to temperature of the resonances over the full spectrum in the 10°-50°C range. Then, a simulation model is obtained, enabling to predict TFBG spectra in the 10°-50°C range with high accuracy. This requires a calibration of the core index and dispersion of the TFBG measured in air at 25°C, and determination of the glass refractive index thermo-optic coefficient (d n/d T=8.46⋅10-6±0.1⋅10-6 ∘ C -1, common to both core and cladding glasses), leading to a mean error on the wavelength position of resonances between 1 and 3pm. This mean error can be further reduced (less than 1pm) by considering a linear dependence with temperature of d n/d T. Therefore, this model will enable to completely remove the temperature-induced shifts of all resonances in TFBG sensing applications and measure with great accuracy the variables of interest by using the scaled averages of groups of resonances instead of (less accurate) individual shifts.

A Fiber Bragg Grating Sensor Based on Cladding Mode Resonance for Label-Free Biosensing

A fiber-optic biosensing platform based on ultra-narrowband cladding mode resonances was developed on a high-reflectivity fiber Bragg grating (FBG) for targeting biomolecular detection. The multiple cladding modes with a high sensitivity to the refractive index (RI) were excited in the FBG by coupling between the forward-propagating guided core mode of the multimode fiber and the backward-propagating guided cladding mode of the FBG without any damage to the fiber structure or any change to the standard FBG manufacturing process. The full width at half maximum and the Q-factor of the typical cladding mode resonance operation of the proposed sensor are 80 pm and 19,270, respectively, which are better than those of most fiber-optic biosensors reported to date. In addition, the FBG sensor demonstrated a high sensitivity in protein detection and a high selectivity in serum sample assays. The sensitivity of this sensor was further increased simply by coating it with graphene oxide (GO) sheets on the sensing surface without using a signal amplification strategy. Furthermore, an ultra-low limit of detection (LOD) of 32 pM was obtained by the GO-coated FBG sensor for IgG detection. The proposed FBG sensor provides a competitive fiber-optic platform for biomolecular detection. It has a great potential for applications in label-free biosensing.

Slit Beam Shaping for Femtosecond Laser Point-by-Point Inscription of Highly Localized Fiber Bragg Grating

We propose and experimentally demonstrate a slit beam shaping method for femtosecond laser Point-by-Point (PbP) inscription of highly localized Fiber Bragg Gratings (FBGs). The influence of slit width on the shape and area of Refractive Index Modulation Region (RIMR) induced by a single femtosecond laser pulse was investigated. The shape of RIMR can be changed from a spot to a line with an enlarged RIMR, and hence enhances the coupling strength of core mode and cladding modes. The RIMRs were precisely assembled along the fiber core, producing highly localized FBGs with a spectral comb of pronounced Cladding Mode Resonances (CMRs) intensity of more than 30 dB, a wide wavelength span of 240 nm and a low insertion loss of 0.3 dB. Note that the total processing time for fabricating such a highly localized FBG only requires ∼3.7 s. Subsequently, by including tilted angle on the slit, highly localized tilted FBGs were fabricated and these FBGs show adjustable envelope on the CMRs. Moreover, we investigated the surrounding Refractive Index (RI) response and thermal characteristics of the fabricated highly localized FBGs, which exhibit a sensitivity of 510.87 nm/RIU in a wide RI measurement range and excellent high temperature resistance at 1000°C. Therefore, such highly localized FBGs could potentially be used for multi-parameter sensing in many extreme environments.

Tilted Fiber Bragg Grating Measurements During Laser Ablation of Hepatic Tissues: Quasi-Distributed Temperature Reconstruction and Cladding Mode Resonances Analysis

In this work, we investigate the application of tilted fiber Bragg grating (TFBG) sensors during <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex vivo</i> laser ablation of porcine hepatic tissues. Initially, TFBG’s ability to measure the surrounding refractive index (RI) for different sucrose concentrations and the possibility to measure the RI of the targeted tissue during laser ablation (LA) is analyzed. After, the temperature sensing modality of TFBG is investigated in detail. We have implemented an algorithm for quasi-distributed spatial temperature profile reconstruction along TFBG. The algorithm models the TFBG core mode spectrum as a chain of Bragg gratings (each Bragg grating is modeled <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">via</i> coupled mode theory), where each grating is sensitive to local temperature changes. After, the Gaussian-shape temperature profile along the TFBG is reconstructed using the iterative optimization technique. Temperature measurements have been compared with highly-dense FBG array measurements and with conventional TFBG point temperature measurements based on the core mode tracking techniques (maximum tracking, X-dB Bandwidth, centroid methods). Overall, the proposed reconstruction algorithm is able to provide a quasi-distributed temperature profile along TFBG, which is not possible to obtain using conventional point temperature measurements based on the TFBG’s core mode tracking. The resulted root-mean-square error in comparison to FBG array reference measurements is 7.8±1.7 °C. In general, the results show that the main reliable sensing modality of TFBG during LA is temperature monitoring, which can be significantly improved by the proposed algorithm.

Multiresonant analysis improves the limit of detection of tilted fiber Bragg grating refractometers.

A multiresonant approach based on tracking 27 cladding mode resonances of tilted fiber Bragg grating refractometers is shown to improve the limit of detection by a factor of 3 to 4 relative to the conventional approach of tracking the single-most sensitive resonance. Limits of detection below 2 × 10-5 in index change are achieved for dilutions of ethanol in water in repeated experiments. In all cases, wavelengths are referenced to the core mode resonance which eliminates the effect of small temperature changes during and between experiments.

Strong cladding mode excitation in ultrathin fiber inscribed Bragg grating with ultraviolet photosensitivity.

Strong UV-written Bragg gratings written in 50 µm-diameter cladding single mode fibers compatible with conventional fiber couple core guided light to dozens of cladding modes distributed across 140 nm in the 1400-1600 nm region, without the need for complex symmetry breaking mechanisms such as tilted, laterally offset, or localized gratings. The extent of the coupling to high order modes and the smaller cladding diameter both contribute to increasing the sensitivity to surrounding refractive index changes by more than one order of magnitude, and to an increased spacing between mode resonances to facilitate unambiguous measurements of larger index changes between 1.3 and 1.44. These improvements are confirmed by theoretical and experimental studies that also cover the temperature and strain differential sensitivities of the cladding mode resonances for complete multiparameter sensing capability.

Plasmon-Enhanced Refractometry Through Cladding Mode Excitation by a Fiber Bragg Grating in Photonic Crystal Fiber

We report on an extrinsic surface plasmon-enhanced refractometer based on cladding mode resonance excitation in a photonic crystal fiber (PCF) equipped with a straight fiber Bragg grating (FBG). First, we show that the lattice pitch and the air hole diameter of the PCF microstructure define the spectral location of the excited cladding mode resonances. Second, we demonstrate that if the PCF parameters are properly selected, those resonances are sensitive to increases in steps of 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> refractive index units (RIU) of the refractive index value close to that of water. To the best of our knowledge, this is the first time that the sensitivity of PCF cladding mode resonances to refractive index changes in water-based solutions is reported. We achieved experimental values of 40.3 nm/RIU in terms of wavelength sensitivity and −801 dB/RIU in terms of amplitude sensitivity. The performance of our sensor is therefore comparable to that of tilted FBGs in step-index fibers used for water refractometry, which indicates the potential of our PCF sensor for biosensing. In addition, the sensor fabrication does not require any post-processing such as etching or polishing, which allows preserving the integrity of the fiber probe. Finally, the narrow spectrum within which the PCF operates, allows envisaging multi-target detection with a single fiber probe by using cascaded wavelength-multiplexed gratings.

Line by line inscribed small period long period grating for wide range refractive index sensing

Long period grating (LPG) with a small grating period (25 μm) is inscribed with the femtosecond laser line-by-line technique. The fabricated LPG consists of 500 vertical refractive index (RI) modification lines (15μm in length), corresponding to a compact total length of 12.5 mm. With the fabricated LPG, coupling between core modes is significantly enhanced (from previously reported <0.6 dB to ∼11 dB). Moreover, coupling from core mode to forward- and backward-propagating​ cladding modes is also enabled. Thanks to the co-existence of forward- and backward-propagating cladding mode resonances, wide range and sensitive RI sensing is realized by the LPG, with a RI sensitivity more than 500 nm/RIU. In addition, the grating shows a low temperature cross-sensitivity, and the enhanced core mode resonance can serve as a monitor for temperature variation, allowing simultaneous measurement of RI and temperature. Note that these advantages are achieved with only one uniform grating. With a simple structure, the proposed grating will be a good choice for applications where wide RI sensing range and simultaneous temperature monitoring are required.

Optical fiber refractive index sensor based on the double cladding fiber

An optical fiber sensing structure based on double-cladding fiber (DCF) is described. The core and outer cladding of the DCF both supports more than one guided mode and the cladding mode resonance as well as modal interference in the fiber. Due to the W-type refractive index (RI) distribution, the outer cladding generates a stronger evanescent field to obtain higher sensitivity. In the transmitted spectrum, all three dips shift to the longer wavelength with the external RI changing from 1.33 to 1.43, and the high sensitivities are 595, 741, and 686 nm/RIU, respectively.