Intrinsic temperature sensitivity of tilted fiber Bragg grating based surface plasmon resonance sensors

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 3 pm. This mean error can be further reduced (less than 1 pm) 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.

Research Advances on Fiber-Optic SPR Sensors with Temperature Self-Compensation

The fiber-optic surface plasmon resonance sensor has very promising applications in environmental monitoring, biochemical sensing, and medical diagnosis, due to the superiority of high sensitivity and novel label-free microstructure. However, the influence of ambient temperature is inevitable in practical sensing applications, and even the higher the sensitivity, the greater the influence. Therefore, how to eliminate temperature interference in the sensing process has become one of the hot issues of this research field in recent years, and some accomplishments have been achieved. This paper mainly reviews the research results on temperature self-compensating fiber-optic surface plasmon sensors. Firstly, it introduces the mechanism of a temperature self-compensating fiber-optic surface plasmon resonance sensor. Then, the latest development of temperature self-compensated sensor is reviewed from the perspective of various fiber-optic sensing structures. Finally, this paper discusses the most recent applications and development prospects of temperature self-compensated fiber-optic surface plasmon resonance sensors.

Numerical simulation of optical refractometric sensing of multiple disease markers based on lab-in-a-fiber

A multi-parameter optical refractometric sensor based on lab-in-a-fiber is proposed and its sensing properties have been investigated. Based on the particular three suspended-core fiber, the sensor has three channels for liquid circulation and three suspended cores for detection. The multiple disease markers can be detected by coating the specific bio-recognition layer on the surface of three channels. The bio-recognition layer thickness, representing the concentration of the disease markers, can then be measured by the wavelength of fiber Bragg grating inscribed in each suspended core. Owing to the triple symmetry of the fiber, the sensitivity of each core is similar. The simulation results show that the grating wavelength linearly changes with the bio-recognition layer thickness variation. Through the sensitivity matrix, the sensitivity of the sensor is 0.362 nm/nm and the sensing accuracy is ± 1 nm.

Optical fiber thermo-refractometer

This work presents the implementation of a thermo-refractometer, which integrates the measurement of both refractive index and temperature in a single optical fiber structure. To this purpose, a lossy mode resonance (LMR)-based refractometer is obtained by means of the deposition of a titanium dioxide (TiO₂) thin film onto a side-polished (D-shaped) single mode fiber. Measurement and subsequent temperature compensation are achieved by means of a fiber Bragg grating (FBG) inscribed in the core of the D-shaped region. The LMR wavelength shift is monitored in transmission while the FBG (FBG peak at 1533 nm) displacement is observed in reflection. The LMR is sensitive to both the surrounding refractive index (SRI), with a sensitivity of 3725.2 nm/RIU in the 1.3324-1.3479 range, and the temperature (- 0.186 nm/°C); while the FBG is only affected by the temperature (32.6 pm/°C in the 25°C - 45°C range). With these values, it is possible to recover the SRI and temperature variations from the wavelength shifts of the LMR and the FBG, constituting a thermo-refractometer, where it is suppressed the effect of the temperature over the refractometer operation, which could cause errors in the fourth or even third decimal of the measured SRI value.

Influence of InGaZnO Films with Different Ratios on Refractive Index Sensing Characteristics of LPFG

Sensitive materials are widely used in the field of optical fiber sensing because of their unique advantages such as rich types, controllable composition ratio and diverse structure distribution. In this paper, the surface of long-period fiber gratings with InGaZnO [(In2O3):(Ga2O3):(ZnO)] nano films with different compositions were coated by pulse laser deposition (PLD) technology. The best sensing ratio and the high sensitivity sensing of the refractive index of long-period fiber grating (LPFG) were achieved through the analysis of the influence of different ratios of InGaZnO nano films on the refractive index sensing characteristics of grating. High sensitivities of 337 nm/RIU (refractive index unit) and 145 dB/RIU of the LPFG are achieved when the best doping ratio of InGaZnO is 7:1:2.

Simultaneous measurement of temperature and refractive index based on a hybrid surface plasmon resonance multimode interference fiber sensor

We propose and demonstrate a hybrid fiber-based sensor combining a multimode interference (MMI) structure and a surface plasmon resonance (SPR) structure for simultaneous measurement of temperature and refractive index (RI) of a liquid sample. We configure the MMI structure by connecting a single-mode fiber, a no-core fiber, and a single-mode fiber sequentially. We set up the SPR structure by coating a gold film with a thickness of 50 nm on the surface of the no-core fiber. We measure the sensitivity of RI and the temperature of the MMI and SPR structure, respectively. Then we obtain the coefficient matrix to simultaneously measure the temperature and RI of a liquid sample and obtain the highest RI sensitivity of 2061.6 nm/RIU and temperature sensitivity of 37.9 pm/°C. We verify the feasibility of the sensor in liquid alcohol. The testing results indicate that the proposed sensor and testing method are feasible, accurate, and convenient.

Analysis of a highly sensitive flat fiber plasmonic refractive index sensor

In this paper, we propose an efficient double-layered flat fiber (DLFF) plasmonic refractive index sensor having high resolution and linearity. Thin gold film is used as surface plasmon resonance (SPR) active material protected by a titanium dioxide layer, both deposited on the upper flat surface of DLFF. The sensor consists of an analyte channel in the central core hole as well as on the top of the fiber. Structural parameters of DLFF and thickness of gold and titanium dioxide layer are analyzed based on the finite element method. The optimized structure is studied based on wavelength and amplitude interrogation techniques in the near-infrared region. Numerical results show average wavelength sensitivity of 12172 nm/RIU with a resolution of 8.21×10^-6RIU (refractive index unit) in the highly refractive index (RI) range from 1.445 to 1.490. Further, amplitude sensitivity of this probe is found to be 2910RIU^-1 with a resolution of 3.44×10^-6RIU, which is the highest among all reported PCF SPR sensors, as per the authors' best knowledge. Compared with traditional photonic crystal fiber, the designed DLFF makes the sensor configuration simple to fabricate as well as a potential candidate for developing biochemical sensors and portable devices.

Method for Determining the Plasmon Resonance Wavelength in Fiber Sensors Based on Tilted Fiber Bragg Gratings

Surface plasmon resonance-based fiber-optic sensors are of increasing interest in modern sensory research, especially for chemical and biomedical applications. Special attention deserves to be given to sensors based on tilted fiber Bragg gratings, due to their unique spectral properties and potentially high sensitivity and resolution. However, the principal task is to determine the plasmon resonance wavelength based on the spectral characteristics of the sensor and, most importantly, to measure changes in environmental parameters with high resolution, while the existing indirect methods are only useable in a narrow spectral range. In this paper, we present a new approach to solving this problem, based on the original method of determining the plasmon resonance spectral position in the automatic mode by precisely calculating the constriction location on the transmission spectrum of the sensor. We also present an experimental comparison of various data processing methods in both a narrow and a wide range of the refractive indexes. Application of our method resulted in achieving a resolution of up to 3 × 10⁻⁶ in terms of the refractive index.

A novel immunosensor based on excessively tilted fiber grating coated with gold nanospheres improves the detection limit of Newcastle disease virus

A novel immunosensor for detecting Newcastle disease virus (NDV) was developed using excessively tilted fiber grating (Ex-TFG) coated with gold nanospheres (AuNs). AuNs were coated on the Ex-TFG surface via Au-S bonds using 3-mercaptopropyltrimethoxysilane (MPTMS), and the activated staphylococcal protein A (SPA) was linked to AuNs by covalent bonds via cysteamine. AuNs greatly enhanced the impact of the analyte on the fiber cladding mode through the local surface Plasmon resonance (LSPR) effect, thus improving the detection limit and sensitivity of the immunosensor. Meanwhile, SPA enhanced the bioactivity of anti-NDV monoclonal antibodies (MAbs), thus promoting the effectiveness of specific binding events on the fiber surface. Immunoassays were performed by monitoring the resonance wavelength shift of the proposed sensor under NDV samples containing different particle amounts. Specificity was assessed, and clinical tests for NDV were performed by contrast experiments. Experimental results showed that the detection limit for NDV was about 5~10 times improved compared to that of reference Ex-TFG without AuN treatment. Moreover, the novel biosensor was reusable and could potentially be applied in clinic.

Simple method for self-referenced and lable-free biosensing by using a capillary sensing element

We demonstrated a simple method for self-reference and label free biosensing based on a capillary sensing element and common optoelectronic devices. The capillary sensing element is illuminated by a light-emitting diode (LED) light source and detected by a webcam. Part of gold film that deposited on the tubing wall is functionalized to carry on the biological information in the excited SPR modes. The end face of the capillary was monitored and separate regions of interest (ROIs) were selected as the measurement channel and the reference channel. In the ROIs, the biological information can be accurately extracted from the image by simple image processing. Moreover, temperature fluctuation, bulk RI fluctuation, light source fluctuation and other factors can be effectively compensated during detection. Our biosensing device has a sensitivity of 1145%/RIU and a resolution better than 5.287 × 10^-4 RIU, considering a 0.79% noise level. We apply it for concanavalin A (Con A) biological measurement, which has an approximately linear response to the specific analyte concentration. This simple method provides a new approach for multichannel SPR sensing and reference-compensated calibration of SPR signal for label-free detection.

Micro-capillary-based self-referencing surface plasmon resonance biosensor for determination of transferrin

A novel self-referencing surface plasmon resonance (SPR) biosensor for detection of transferrin is demonstrated using a micro-capillary as the sensing element. The biosensor employs the SPR mode as a measuring signal and the Fabry-Perot (FP) mode as a referencing signal. The SPR mode is generated in the gold film that is coated on the outside of the capillary; instead, the FP mode is excited in the capillary, which is filled with de-ionized water. The FP mode is sensitive to temperature and insensitive to refractive index, which can be used as a referencing signal to compensate the effects caused by the temperature fluctuation. The sensor provides a high sensitivity of 1783.943 nm/RIU (refractive index unit) and a resolution of about 7.287×10^-5  RIU. The self-referencing biosensor was applied to measurement of transferrin protein. It can monitor the interaction of transferrin protein with anti-transferrin in real time (0-5.228 μM). The simple and low-cost SPR sensor can be used for highly sensitive self-referencing biosensing for further investigations.

Theoretical and experimental analysis of excessively tilted fiber gratings

We have theoretically and experimentally investigated the dual-peak feature of tilted fiber gratings with excessively tilted structure (named as Ex-TFGs). We have explained the dual-peak feature by solving eigenvalue equations for TM_0m and TE_0m of a circular waveguide, in which the TE (transverse electric) and TM (transverse magnetic) core modes are coupled into TE and TM cladding modes, respectively. Meanwhile, in the experiment, we have verified that one of the dual peaks at the shorter wavelength is due to the TM mode coupling whereas the other one at the longer wavelength arises from TE mode coupling when a linearly polarized light launched into the Ex-TFG. We have also investigated the peak separation of TE and TM cladding mode for different surrounding medium refractive indexes (SRI), revealed that the dual peaks separation is decreasing as increasing of SRI, which agrees very well with the theoretical analysis results.

Highly sensitive surface plasmon resonance sensor utilizing a long period grating with photosensitive cladding

In this study, we propose and investigate a novel grating-assisted surface plasmon resonance (SPR) platform based on a silver coated long period fiber grating having a photosensitive cladding (C-LPFG). We show that the SPR mode is transited from the higher EH mode with an effective refractive index (ERI) close to that of the surrounding refractive index (SRI) and is highly sensitive to a change in the SRI. Compared with a conventional SPR sensor, a much higher sensitivity is obtained for the novel C-LPFG-based SPR sensor. The sensitivity can be further improved by reducing the fiber diameter. The numerical results show that the highest local sensitivity, which can be as high as ∼4900  nm/RIU, and the corresponding resolution of ∼2.04×10(-6)  RIU are achieved for the reduced silver coated C-LPFG sensor. Moreover, the Q-factor and the figure of merit exhibit better characteristics than those of the conventional LPFG-based SPR sensor. Our findings provide insight into the C-LPFG-based SPR platform being a potentially important area to explore for biochemical sensing.

Synthesis and optical–electrochemical gas sensing applications of Ni-doped LiFePO4 nano-particles

A semiconductor material (LiFeNiPO₄) was prepared using a one-step hydrothermal method and then its gas sensing performances at room temperature were investigated.

Noninvasive and real-time plasmon waveguide resonance thermometry

In this paper, the noninvasive and real-time plasmon waveguide resonance (PWR) thermometry is reported theoretically and demonstrated experimentally. Owing to the enhanced evanescent field and thermal shield effect of its dielectric layer, a PWR thermometer permits accurate temperature sensing and has a wide dynamic range. A temperature measurement sensitivity of 9.4 × 10⁻³ °C is achieved and the thermo optic coefficient nonlinearity is measured in the experiment. The measurement of water cooling processes distributed in one dimension reveals that a PWR thermometer allows real-time temperature sensing and has potential to be applied for thermal gradient analysis. Apart from this, the PWR thermometer has the advantages of low cost and simple structure, since our transduction scheme can be constructed with conventional optical components and commercial coating techniques.

Polarization-resolved evanescent wave scattering from gold-coated tilted fiber gratings

The scatterings of TE- and TM-polarized evanescent wave on the surface of a tilted fiber Bragg grating (TFBG) with a 50 nm thick gold coating were investigated experimentally by observing radiation patterns from discontinuities in the coating. The scattering intensity for TM-polarized light is larger than for TE light when the evanescent wave propagates from the coating towards the discontinuity. The opposite occurs for light propagating from an uncoated section towards the coating edge. However in the latter case the scattering is much weaker. These results confirm that cladding modes with TE and TM polarization can be excited selectively with a TFBG, and that they scatter light differentially at discontinuities. These results are used to propose a simple polarimeter design based on total scattered light intensity monitoring.

Simple and compact reflective refractometer based on tilted fiber Bragg grating inscribed in thin-core fiber

A simple and compact reflective refractometer based on a tilted fiber Bragg grating (TFBG) inscribed in an ultra-high photon-sensitive thin-core fiber is proposed and experimentally demonstrated. The reflective refractometer utilizes a short piece of thin-core fiber containing one TFBG to ensure the recoupling of cladding modes. The reflection spectra occur in two well-defined wavelength bands that correspond to the Bragg core mode and cladding modes, respectively. It is found that the power of the collected cladding modes changes with the external refractive index (RI), while that of the Bragg core mode remains unaffected and can be used as the temperature reference. High RI sensitivity and temperature immunity of the proposed reflective refractometer are experimentally achieved.

Novel D-shape LSPR fiber sensor based on nano-metal strips

In this study, a novel D-shaped localized surface plasmon resonance (LSPR) fiber sensor was introduced. The construction of this sensor involved etching of a single-mode fiber on the cladding layer and core layer, followed by plating using nano-metal strips. The design and calculations of the entire sensor were based on a numerical simulation method combining the finite element method (FEM) and the eigenmode expansion method (EEM). By using graphical representations of the algorithm results, the excitation of the LSPR was clearly observed. The finished D-shaped LSPR fiber sensor possesses several excellent properties, including a short length (2494.4301 μm), high resolution (approximately 35 dB), and high sensitivity (approximately 20183.333 nm/RIU). In addition, compared with LPG-SPR fiber sensor, the framework provides three advantages, namely, a fabrication process that is compatible with semiconductor fabrication, as well as the low-temperature cross-talk and high-temperature stability of surface grating.

Plasmon-enhanced refractometry using silver nanowire coatings on tilted fibre Bragg gratings

A novel technique for increasing the sensitivity of tilted fibre Bragg grating (TFBG) based refractometers is presented. The TFBG sensor was coated with chemically synthesized silver nanowires ~100 nm in diameter and several micrometres in length. A 3.5-fold increase in sensor sensitivity was obtained relative to the uncoated TFBG sensor. This increase is associated with the excitation of surface plasmons by orthogonally polarized fibre cladding modes at wavelengths near 1.5 μm. Refractometric information is extracted from the sensor via the strong polarization dependence of the grating resonances using a Jones matrix analysis of the transmission spectrum of the fibre.

In situ biosensing with a surface plasmon resonance fiber grating aptasensor

Surface plasmon resonance (SPR) biosensors prepared using optical fibers can be used as a cost-effective and relatively simple-to-implement alternative to well established biosensor platforms for monitoring biomolecular interactions in situ or possibly in vivo. The fiber biosensor presented in this study utilizes an in-fiber tilted Bragg grating to excite the SPR on the surface of the sensor over a large range of external medium refractive indices, with minimal cross-sensitivity to temperature and without compromising the structural integrity of the fiber. The label-free biorecognition scheme used demonstrates that the sensor relies on the functionalization of the gold-coated fiber with aptamers, synthetic DNA sequences that bind with high specificity to a given target. In addition to monitoring the functionalization of the fiber by the aptamers in real-time, the results also show how the fiber biosensor can detect the presence of the aptamer's target, in various concentrations of thrombin in buffer and serum solutions. The findings also show how the SPR biosensor can be used to evaluate the dissociation constant (K(d)), as the binding constant agrees with values already reported in the literature.

Interrogation technique for TFBG-SPR refractometers based on differential orthogonal light states

The generation of near-IR surface plasmon resonance in gold-coated tilted fiber Bragg gratings is strongly dependent on both the polarization state of the transmission light and the property of confining materials (including the coating materials and surrounding media). These dependencies can be advantageously used to demodulate the amplitude spectrum and retrieve the surrounding refractive index. In this paper, we present an automated demodulation technique that measures the surrounding refractive index by comparing the differential amplitude of resonance peaks near the plasmon attenuation for two orthogonal amplitude spectra recorded in the same operating conditions. A mean sensitivity of more than 500 nm per refractive index unit is reported. This new refractive index measurement method is shown to be accurate to 5×10(-5) over a full range of 0.01 in water solutions.

Compact on-chip temperature sensors based on dielectric-loaded plasmonic waveguide-ring resonators

The application of a waveguide-ring resonator based on dielectric-loaded surface plasmon-polariton waveguides as a temperature sensor is demonstrated in this paper and the influence of temperature change to the transmission through the waveguide-ring resonator system is comprehensively analyzed. The results show that the roundtrip phase change in the ring resonator due to the temperature change is the major reason for the transmission variation. The performance of the temperature sensor is also discussed and it is shown that for a waveguide-ring resonator with the resonator radius around 5 μm and waveguide-ring gap of 500 nm which gives a footprint around 140 μm², the temperature sensitivity at the order of 10⁻²K can be achieved with the input power of 100 μW within the measurement sensitivity limit of a practical optical detector.

Fiber optic pressure sensing with conforming elastomers

A novel pressure sensing scheme based on the effect of a conforming elastomer material on the transmission spectrum of tilted fiber Bragg gratings is presented. Lateral pressure on the elastomer increases its contact angle around the circumference of the fiber and strongly perturbs the optical transmission of the grating. Using an elastomer with a Young's modulus of 20 MPa, a Poisson ratio of 0.48, and a refractive index of 1.42, the sensor reacts monotonically to pressures from 0 to 50 kPa (and linearly from 0 to 15 kPa), with a standard deviation of 0.25 kPa and maximum error of 0.5 kPa. The data are extracted from the optical transmission spectrum using Fourier analysis and we show that this technique makes the response of the sensor independent of temperature, with a maximum error of 2% between 25°C and 75°C. Finally, other pressure ranges can be reached by using conforming materials with different modulii or applying the pressure at different orientations.

In situ monitoring of the formation of nanoscale polyelectrolyte coatings on optical fibers using Surface Plasmon Resonances

Deposition of a conformal nanoscale polymer coating was characterized using a fiber SPR sensor. The sensor platform consisted of an unmodified gold-coated single mode fiber where SPR was excited through the coupling of the core mode into the cladding modes using a Tilted Fiber Bragg Grating. The results from this study show how the sensor can monitor in real time the formation of polyelectrolyte coatings during a process consisting of several stages of immersion. The experimental data was further calibrated by simulations and Atomic Force Microscope imaging allowing us to determine the thickness and refractive index of the adsorbed polyelectrolyte.