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This paper presents the results of a detailed theoretical study of the bending properties of original all-glass leakage channel microstructured optical fibers (LC MOFs) over a bending radius range from 3 cm to 11 cm. These LC MOFs contain two layers of fluorine-doped silica glass elements with reduced refractive index, different diameters, and different distances between them. We determined the spatial distributions of the electric field components of different modes in addition to the usual parameters such as effective refractive indices, bending losses, and spatial intensity distributions. A detailed analysis showed that three modes for each polarization have to be considered to correctly calculate the bending losses. Two pairs of these three modes couple in two distinct bending radius ranges, specifically near 3.68 cm and near 5.95 cm, and the mode coupling in these pairs is resonant. The resulting bending losses of the LC MOF for two polarizations are very close to each other and have two maxima at bending radii of 3.68 cm and 5.95 cm. However, the nature of these maxima is not resonant; they are caused by the combined influence of all three modes, each of which has specific dependencies of losses and other parameters on the bending radius that exhibit quasi-resonant behavior near the corresponding bending radii. [ABSTRACT FROM AUTHOR]

Multimode polymer optical fibers (POFs) are considered to be the best choice for short-distance communication lines due to their advantageous properties. One must pay close attention to POFs' most crucial transmission property, bandwidth (BW), which affects the fibers' transmission capacity. The improvement of POF transmission qualities, particularly BW, which is its most crucial property, has been the subject of many researches during the past few decades. Both experimental and theoretical approaches have been used to address this issue. Up to now, there were no commercial simulation tools available for studying the transmission characteristics of multimode optical fibers. To overcome this problem, the modal continuum approach is employed to characterize light transmission in various kinds of multimode optical fibers. This modal continuum approach, which uses the time-dependent power flow equation (TD PFE), plays the most important role in the theoretical approaches. The PFE is the most comprehensive because it takes into account attenuation, mode coupling and modal dispersion, which significantly affects the BW. It can also be modified to accommodate various refractive index profiles, including step-index (SI), W-type index and graded index (GI) profiles. This review analyzes the pertinent literature related to the use of modal continuum-based models for BW modeling in conventional and microstructured multimode POFs. An overview of methodologies for optical fiber analysis with a focus on modal continuum approximation, and solutions to the TD PFE for various multimode fiber types, is provided. [ABSTRACT FROM AUTHOR]

This paper presents the results of a detailed theoretical study of the bending properties of original all-glass leakage channel microstructured optical fibers (LC MOFs) over a bending radius range from 3 cm to 11 cm. These LC MOFs contain two layers of fluorine-doped silica glass elements with reduced refractive index, different diameters, and different distances between them. We determined the spatial distributions of the electric field components of different modes in addition to the usual parameters such as effective refractive indices, bending losses, and spatial intensity distributions. A detailed analysis showed that three modes for each polarization have to be considered to correctly calculate the bending losses. Two pairs of these three modes couple in two distinct bending radius ranges, specifically near 3.68 cm and near 5.95 cm, and the mode coupling in these pairs is resonant. The resulting bending losses of the LC MOF for two polarizations are very close to each other and have two maxima at bending radii of 3.68 cm and 5.95 cm. However, the nature of these maxima is not resonant; they are caused by the combined influence of all three modes, each of which has specific dependencies of losses and other parameters on the bending radius that exhibit quasi-resonant behavior near the corresponding bending radii.

Up to now, there have been no commercial simulation tools accessible for researching the transmission properties of multimode microstructured optical fibers (MOFs). In order to avoid this problem, this study uses the time-independent power flow equation (TI PFE) numerical solution to examine the wavelength dependency of the equilibrium mode distribution (EMD) and steady state distribution (SSD) in multimode graded-index microstructured polymer optical fibers (GI mPOF) with a solid core. We showed that the lengths zs at which an SSD is obtained in GI mPOF and the coupling length Lc necessary to create an EMD are shorter at X = 568 nm than they are found to be at X = 633 nm. The lengths Lc and zs stay constant when the wavelength decreases further from X = 568 to 522 and then to 476 nm. As a result, it is anticipated that a faster bandwidth enhancement in the tested GI mPOF will take place at wavelengths around X = 568 nm as opposed to X = 633 nm. Such a bandwidth improvement is not brought about by additional wavelength reduction. The study's findings can be used in communication and sensory systems that use multimode GI mPOFs at different wavelengths. [ABSTRACT FROM AUTHOR]

With the development of laser technology, microstructured optical fibers (MOFs) have become an important part of ultrafast optics, providing excellent platforms for ultrafast laser pulse generation, amplification, and compression, promoting the development of fiber laser systems to generate high power, high pulse energy, and few-cycle duration pulses. MOFs extend the ultrafast laser spectrum to the vacuum ultraviolet (VUV) and even extreme ultraviolet (EUV) regions based on dispersive wave emission and high harmonic generation, as well as to the mid-infrared region based on soliton self-frequency shift (SSFS), contributing compact and low-cost light sources for precision microscopy and spectroscopy. In this paper, first several common types of MOFs are introduced, then the various applications of MOFs in ultrafast optics are discussed, mainly focusing on the aspects of ultrafast laser pulse scaling in pulse energy and spectral bandwidth, and finally the possible prospects of MOFs are given. [ABSTRACT FROM AUTHOR]

Up to now, there have been no commercial simulation tools accessible for researching the transmission properties of multimode microstructured optical fibers (MOFs). In order to avoid this problem, this study uses the time-independent power flow equation (TI PFE) numerical solution to examine the wavelength dependency of the equilibrium mode distribution (EMD) and steady state distribution (SSD) in multimode graded-index microstructured polymer optical fibers (GI mPOF) with a solid core. We showed that the lengths zs at which an SSD is obtained in GI mPOF and the coupling length Lc necessary to create an EMD are shorter at λ = 568 nm than they are found to be at λ = 633 nm. The lengths Lc and zs stay constant when the wavelength decreases further from λ = 568 to 522 and then to 476 nm. As a result, it is anticipated that a faster bandwidth enhancement in the tested GI mPOF will take place at wavelengths around λ = 568 nm as opposed to λ = 633 nm. Such a bandwidth improvement is not brought about by additional wavelength reduction. The study’s findings can be used in communication and sensory systems that use multimode GI mPOFs at different wavelengths.

We propose an air-silica core microstructured optical fiber-based surface plasmon resonance (SPR) sensor to simultaneously measure temperature and refractive index (RI). The sensing channel is formed by coating the outside of the fiber with a gold film and a polydimethylsiloxane (PDMS) layer as a temperature sensing medium, and then being immersed into the liquid analyte. The plasmon mode can penetrate through the PDMS layer and then into the analyte, therefore both the temperature and the analyte RI changings can lead to the variations of SPR spectra that will be measured. Our numerical results demonstrate that the proposed sensor can support two resonance peaks in the x-polarized core mode and one resonance peak in the y-polarized core mode, therefore providing two detection approaches, peak-based and polarization-based approaches. By measuring the two peaks in the x-polarized core mode, for the peak-based approach, the temperature coefficients are −2.077 nm/°C and −2.723 nm/°C, and the RI coefficients are 1252 nm/RIU and 1931 nm/RIU, respectively. While by measuring the second peak in x-polarized core mode and the peak in y-polarized core mode, for the polarization-based approach, the temperature coefficients are −2.723 nm/°C and −3.401 nm/°C, and the RI coefficients are 1931 nm/RIU and 2973 nm/RIU, respectively.

Microcavity lasers are widely applied in bio-chemical sensing, molecular targeted detection, integrated labeling source, and optofluidic control. Particularly, the microstructured optical-fiber-based laser is expected to be a promising candidate for its high-quality factor, low threshold, high integration, and low energy consumption. Moreover, the latest nano technology improves its lasing performance in spectral range, linewidth, and circling lifetime. Considering the specificity in this paper, the discussion presented herein focuses on several typical cases of the microcavity lasers integrated in microstructured optical fiber over the past decades. These micro- and nano-scaled lasers are expected to become a priority in next-generation integrated optics and biomedical photonics. [ABSTRACT FROM AUTHOR]

The paper presents the results of theoretical and experimental studies of all-glass leakage channel microstructured optical fibers (MOFs) with a large mode area and low bending losses. These MOFs contain two layers of fluorine-doped silica glass elements with a reduced refractive index, different diameters, and different distances between them. A numerical analysis of the properties of these MOFs was performed using the finite element method. The leakage losses for the fundamental and higher-order modes were calculated in the spectral range from 0.65 μm to 1.65 μm. Simulation results show that the proposed MOF design allows for single-mode guidance in the spectral range from 0.92 μm to 1.21 μm with a bending radius of down to 0.08 m. The measured losses of the fabricated MOF with a core diameter of 22.5 μm and a bending radius of 0.1 m were less than 0.1 dB/m in the spectral range from 0.9 μm to 1.5 μm. It is demonstrated that the segments of this MOF longer than 5 m are single-mode. [ABSTRACT FROM AUTHOR]

We investigate mode coupling in a multimode graded-index microstructured polymer optical fiber (GI mPOF) with a solid core by solving the time-independent power flow equation (TI PFE). Using launch beams with various radial offsets, it is possible to calculate for such an optical fiber the transients of the modal power distribution, the length Lc at which an equilibrium mode distribution (EMD) is reached, and the length zs for establishing a steady-state distribution (SSD). In contrast to the conventional GI POF, the GI mPOF explored in this study achieves the EMD at a shorter length Lc. The earlier shift to the phase of slower bandwidth decrease would result from the shorter Lc. These results are helpful for the implementation of multimode GI mPOFs as a part of communications and optical fiber sensory systems. [ABSTRACT FROM AUTHOR]

With the development of laser technology, microstructured optical fibers (MOFs) have become an important part of ultrafast optics, providing excellent platforms for ultrafast laser pulse generation, amplification, and compression, promoting the development of fiber laser systems to generate high power, high pulse energy, and few-cycle duration pulses. MOFs extend the ultrafast laser spectrum to the vacuum ultraviolet (VUV) and even extreme ultraviolet (EUV) regions based on dispersive wave emission and high harmonic generation, as well as to the mid-infrared region based on soliton self-frequency shift (SSFS), contributing compact and low-cost light sources for precision microscopy and spectroscopy. In this paper, first several common types of MOFs are introduced, then the various applications of MOFs in ultrafast optics are discussed, mainly focusing on the aspects of ultrafast laser pulse scaling in pulse energy and spectral bandwidth, and finally the possible prospects of MOFs are given.

In this paper, we propose a linear three hole microstructured optical fiber surface plasmon resonance sensor to realize the detection of temperature and refractive index (RI). The sensing analytes are placed inside the central hole to guide light, and gold layers are coated on the inner surface of the side holes to generate surface plasmons. The sensor characteristics have been analyzed via finite element method-based software COMSOL Multiphysics. According to the numerical results, the proposed sensor exhibits average wavelength sensitivity of 33,160 nm/RIU (RI unit) with linear behavior in the RI range of 1.45–1.47. The minimum measurable variation by the sensor lies in the order of 10−6 RIU, and maximum figure of merit is calculated to be 414.5. Besides, when applied for temperature detection, the sensor shows average temperature sensitivity of 9.08 nm/°C with excellent linearity. So, due to its simple structure and competitive sensing performance, the proposed sensor can be useful for sensing high RI analytes in the petroleum and chemical industry and for magnetic field and temperature measurement. [ABSTRACT FROM AUTHOR]

Helical twist provides an additional degree of freedom for controlling light in optical waveguides, expanding their applications in sensing. In this paper, we propose a helical broken-circular-symmetry core microstructured optical fiber for refractive index sensing. The proposed fiber consists of pure silica and its noncircular helical core is formed by a broken air ring. By using finite element modeling combined with transformation optics, the modal characteristics of the fiber are investigated in detail. The results show that for the core located at the fiber center, the confinement loss of fundamental core modes increases with twist rate, whereas for a sufficiently large core offset the modes can be well confined owing to the twist-induced light guidance mechanism, showing decreases with rising twist rate in the loss spectra. Moreover, we have found that for large twist rates and core offsets, resonant peaks occur at different twist rates due to the couplings between the fundamental core modes and the highly leaky modes created by the helical structure. The refractive index sensing performance is also studied and the obtained results show that the proposed fiber has great potential in fiber sensing. [ABSTRACT FROM AUTHOR]

A novel side-opening grapefruit fiber-based SPR sensor is designed to realize simultaneous measurement of refractive index (RI) and temperature. The open section of the grapefruit fiber is coated with a gold layer, which can support two main resonance peaks. The variations of RI and temperature can influence the coupling efficiency between core-guided mode and plasmon mode, thus lead to the two peaks shifts that can be used to determine the variations of RI and temperature. Simulation results demonstrate that the RI coefficients of Peak I and Peak II are 1400 nm/RIU and 226 nm/RIU, respectively. The temperature coefficients of Peak I and Peak II are 34 pm/°C and 527.2 pm/°C, respectively.

We propose a concept of surface plasmon resonance sensor based on double-sided polished microstructured optical fiber coated with graphene-on-silver layers. The silver films on the polishing planes are covered with multilayer graphene which can prevent the oxidation of silver and enhance the adsorption of biomolecules. The sensing performance of the proposed sensors with polishing planes angle of 0°, 60° and 120°, are investigated by using both wavelength and amplitude interrogations. Our numerical results demonstrate that increasing the size of the air hole in the core area can effectively improve the wavelength and maximum amplitude sensitivities. Moreover, growing number layer of graphene can increase wavelength sensitivity, whereas decrease the maximum amplitude sensitivity.

We demonstrate a sensitivity-tunable temperature SPR sensor based on side-opening grapefruit fiber. The opening sections of the fiber are coated with gold layer and then filled with a liquid mixture of ethanol and chloroform as the sensing medium. The variation of the temperature will lead to different coupling efficiencies between the core-guided mode and the plasmonic mode, and then resulting in the change of loss spectra of the fiber that will be detected. Simulation results indicate that the temperature sensitivity of the sensor is 19.9 nm/°C at 10°C when the volume fraction of ethanol in the mixture is 0.1. Moreover, by adjusting the volume fraction, the sensitivity of proposed sensor can achieve the upper limit for a certain temperature range without the excitation of higher order plasmonic modes.

We propose a concept of orthogonal-side polished microstructured optical fiber (MOF)-based surface plasmon resonance (SPR) sensor to implement the simultaneous sensing for two parameters. Two feasible structures, L-shaped MOF based and rectangular MOF based SPR sensors are investigated theoretically, which can support x- and y-polarized resonance peaks that can be used to measure and separate variations of the temperature and refractive index (RI). Our results show that the temperature coefficients (KTx and KTy) are 8 pm/°C and 6 pm/°C, and the RI coefficients (Knx and Kny) are 1470 nm/RIU and 1570 nm/RIU, for the L-shaped MOF-SPR sensor, while the KTx and the KTy are 10 pm/°C and 8 pm/°C, the Knx and Kny are 1460 nm/RIU and 1500 nm/RIU, for the rectangular MOF-based SPR sensor. Moreover, these coefficients can be further improved by choosing the appropriate structure parameters. The proposed SPR sensors with the advantages of simultaneously measuring two parameters and no need to be filled with the sensing media are expected to be more competitive in the MOF-based SPR sensor field.

This paper investigates wavelength dependence of equilibrium mode distribution and steady-state distribution in W-type (double-clad) microstructured polymer optical fibers (mPOFs) with a solid core for parametrically varied refractive index and width of the intermediate layer (IL) (inner cladding) by solving the time-independent power flow equation (TI PFE). In the case of wider IL, independent of wavelength, the lengths for establishing the equilibrium mode distribution and steady-state distribution are larger. We have demonstrated that the wavelength has no effect on these lengths for IL’s width that is larger. These lengths drop in a wavelength-dependent manner as the IL's width decreases. Equilibrium mode distribution and steady-state distribution occur at shorter optical fiber lengths as the depth of the IL diminishes, which is due to the similarly declining number of leaky modes. The smaller the depth of the IL, the shorter the fiber length is required for completion of the coupling process. These programmable characteristics allow double-clad W-type mPOFs to be easier customized for a particular use in optical fiber sensors and communications at various wavelengths.

A surface plasmon resonance (SPR)-based microstructured optical fiber (MOF) is designed and theoretically investigated for dual-channel optofluidic sensing. The design of the MOF consists of a solid silica core surrounded by double air holes, and the hollow section is coated with thin Ag film to excite the surface plasmon waves on the dielectric interface. The optofluidic sensing characteristics of the individual channel have been analyzed based on the mode-coupling theory between core and plasma modes. By filling the dual channel with different liquid analytes, the resonance wavelengths are adjusted away from each other to achieve dual-optofluidic-channel sensing. When the refractive index (RI) of liquid samples varies from 1.33 to 1.39 RIU, the optofluidic sensing sensitivities of channel 1 and channel 2 in MOF are 1439.29 nm/RIU and 4003.57 nm/RIU, respectively. The proposed SPR-based MOF is appropriate for multi-channel biological and chemical sensing fields. [ABSTRACT FROM AUTHOR]

Microstructured optical fiber (MOF) possesses unique capability of long interaction path-length between light and analyte samples, which shows distinct advantages in constructing in-line optofluidic sensing microchannel. In this paper, a side-hole MOF integrative twin-microchannels is proposed and experimentally demonstrated for optofluidic sensing application. The structure of in-line optofluidic microchannel consists of a side-hole MOF sandwiched between two sections of pre-processed microfiber, which is fabricated by utilizing ultrasonic vibration cutting and arc discharge splicing technology. For the optofluidic MOF, two microfluidic accesses allow liquids to flow in/out the cladding hollow holes, and enable optical waveguide signals to transmit through the microchannels simultaneously. Experimental results show that the optofluidic MOF achieves a sensing sensitivity of 1309.994 nm/RIU in the low refractive index (RI) range. Moreover, the proposed microfluidic approach provides an efficient strategy for solid-core-type MOF to load fluid liquids into their cross-sectional air holes for the study of light-matter interaction. Owing to the features of enhanced sensing performance and simplified microfluidic-system design, the side-hole MOF integrative twin-microchannels is well suitable for a wide range of optical sensing, filtering, bioimaging and micro-nano particle manipulation. [ABSTRACT FROM AUTHOR]

We demonstrate a sensitivity-tunable temperature SPR sensor based on side-opening grapefruit fiber. The opening sections of the fiber are coated with gold layer and then filled with a liquid mixture of ethanol and chloroform as the sensing medium. The variation of the temperature will lead to different coupling efficiencies between the core-guided mode and the plasmonic mode, and then resulting in the change of loss spectra of the fiber that will be detected. Simulation results indicate that the temperature sensitivity of the sensor is 19.9 nm/°C at 10°C when the volume fraction of ethanol in the mixture is 0.1. Moreover, by adjusting the volume fraction, the sensitivity of proposed sensor can achieve the upper limit for a certain temperature range without the excitation of higher order plasmonic modes. [ABSTRACT FROM AUTHOR]

We propose a concept of orthogonal-side polished microstructured optical fiber (MOF)-based surface plasmon resonance (SPR) sensor to implement the simultaneous sensing for two parameters. Two feasible structures, L-shaped MOF based and rectangular MOF based SPR sensors are investigated theoretically, which can support x- and y-polarized resonance peaks that can be used to measure and separate variations of the temperature and refractive index (RI). Our results show that the temperature coefficients (KTx and KTy) are 8 pm/°C and 6 pm/°C, and the RI coefficients (Knx and Kny) are 1470 nm/RIU and 1570 nm/RIU, for the L-shaped MOF-SPR sensor, while the KTx and the KTy are 10 pm/°C and 8 pm/°C, the Knx and Kny are 1460 nm/RIU and 1500 nm/RIU, for the rectangular MOF-based SPR sensor. Moreover, these coefficients can be further improved by choosing the appropriate structure parameters. The proposed SPR sensors with the advantages of simultaneously measuring two parameters and no need to be filled with the sensing media are expected to be more competitive in the MOF-based SPR sensor field. [ABSTRACT FROM AUTHOR]

We propose a new concept to generate efficient one-third harmonic light from an unseeded third harmonic process in optical fibers. Our concept is based on the dynamic constant (Hamiltonian) of the nonlinear third harmonic generation in optical fibers and includes a periodic array of nonlinear fibers and phase compensation elements. We test our concept with a simulation of the nonlinear interaction between the fundamental and third harmonic modes of a realistic optical fiber, demonstrating high-efficiency one-third harmonic generation. Our work opens a new approach to achieving the so far elusive one-third harmonic generation in optical fibers. • Develop a concept to achieve unseeded one-third harmonic generation in optical fibers. • Confirm the concept by using a realistic simulation of real microstructured fibers. • A pathway to demonstrate and achieve high down-conversion efficiency in fibers. [ABSTRACT FROM AUTHOR]

A high sensitivity temperature sensor based on spot pattern demodulation of rare doped dual-core fiber (RDCF) is proposed. By utilizing the interference and scattering in RDCF, the related temperature variations can be captured by optical field. Furthermore, In order to improve the sensitivity, the RDCF is given a certain amount of bending. Due to the change of the spot mode caused by bending, the temperature changes more to the refractive index of the fiber core, so that the spot of the fiber changes more to facilitate the subsequent spot identification. In addition, considering the global and subtle nature of spot changes, a deep learning algorithm based on AlexNet transfer model is used to process image recognition at multiple temperatures. The experimental results show that the sensor can perceive different temperature in range of 30–199 °C, and the resolution is higher than 0.05 °C. These outstanding results indicate that the bending assisted temperature sensor based on spot pattern decoding of RDCF has potential applications in temperature quantitative sensing and artificial intelligence perception. • A Rare-doped dual-core fiber temperature detection was realized. • By using asymmetric bending improve the temperature response of spot. • Taking transfer model of Alex for wide range of temperature classification. [ABSTRACT FROM AUTHOR]

Abstract Fiber optofluidic laser (FOFL) integrates optical fiber microcavity and microfluidic channel and provides many unique advantages for sensing applications. FOFLs not only inherit the advantages of lasers such as high sensitivity, high signal-to-noise ratio, and narrow linewidth, but also hold the unique features of optical fiber, including ease of integration, high repeatability, and low cost. With the development of new fiber structures and fabrication technologies, FOFLs become an important branch of optical fiber sensors, especially for application in biochemical detection. In this paper, the recent progress on FOFL is reviewed. We focuse mainly on the optical fiber resonators, gain medium, and the emerging sensing applications. The prospects for FOFL are also discussed. We believe that the FOFL sensor provides a promising technology for biomedical analysis and environmental monitoring.

The paper presents the results of theoretical and experimental studies of all-glass leakage channel microstructured optical fibers (MOFs) with a large mode area and low bending losses. These MOFs contain two layers of fluorine-doped silica glass elements with a reduced refractive index, different diameters, and different distances between them. A numerical analysis of the properties of these MOFs was performed using the finite element method. The leakage losses for the fundamental and higher-order modes were calculated in the spectral range from 0.65 μm to 1.65 μm. Simulation results show that the proposed MOF design allows for single-mode guidance in the spectral range from 0.92 μm to 1.21 μm with a bending radius of down to 0.08 m. The measured losses of the fabricated MOF with a core diameter of 22.5 μm and a bending radius of 0.1 m were less than 0.1 dB/m in the spectral range from 0.9 μm to 1.5 μm. It is demonstrated that the segments of this MOF longer than 5 m are single-mode.

We investigate mode coupling in a multimode graded-index microstructured polymer optical fiber (GI mPOF) with a solid core by solving the time-independent power flow equation (TI PFE). Using launch beams with various radial offsets, it is possible to calculate for such an optical fiber the transients of the modal power distribution, the length Lc at which an equilibrium mode distribution (EMD) is reached, and the length zs for establishing a steady-state distribution (SSD). In contrast to the conventional GI POF, the GI mPOF explored in this study achieves the EMD at a shorter length Lc. The earlier shift to the phase of slower bandwidth decrease would result from the shorter Lc. These results are helpful for the implementation of multimode GI mPOFs as a part of communications and optical fiber sensory systems.

In this work a plasmonic sensor with a D-Shaped microstructured optical fiber (MOF) is proposed to detect a wide range of analyte refractive index (RI   ; n a) by doping the pure silica ( SiO 2) core with distinct concentrations of Germanium Dioxide ( GeO 2) , causing the presentation of high spectral sensitivity. In this case, the fiber is shaped by polishing a coating of SiO 2 , on the region that will be doped with GeO 2 , in the polished area, a thin gold (Au) layer, which constitutes the plasmonic material, is introduced, followed by the analyte, in a way which the gold layer is deposited between the SiO 2 . and the analyte. The numerical results obtained in the study shows that the sensor can determine efficiently a range of 0.13 refractive index units (RIU) , with a limit operation where n a varies from 1.32 to 1.45. Within this application, the sensor has reached an average wavelength sensitivity (WS) of up to 11 , 650.63   nm / RIU . With this level of sensitivity, the D-Shaped format and wide range of n a detection, the proposed fiber has great potential for sensing applications in several areas. [ABSTRACT FROM AUTHOR]

A surface plasmon resonance sensor based on double-sided polished microstructured optical fiber with hollow core is put forward for refractive index sensing. Two gold films parallel to each other attached to the polished surface act as microfluidic sensing channels for the analyte. The artificially introduced air hole can facilitate the phase matching between the core mode and the plasmon mode. The sensitivities of the proposed sensor are investigated by the wavelength, amplitude and phase interrogation methods when the analyte refractive index increases from 1.33 to 1.34. In contrast to the D-shaped design, the double-sided polished structure demonstrates narrower resonance spectral width and greater phase sensitivity. Moreover, the numerical results indicate that the proposed sensor shows a good stability in the fabrication tolerances of ±5% of the thickness of gold film and the depth of polishing, respectively.

In this work, the parametric gain along with bandwidth is investigated for all the four zero dispersion wavelengths (ZDWs) for a highly non-linear tellurite hybrid microstructured optical fiber (HMOF) incorporating single pumping scheme. We have considered a near-zero dispersion profile with D = 0 ± 1.5\;ps/nm/km for communication and near-IR window considering upto 4th higher order dispersion (HOD) parameter which led us to explore the four wave mixing (FWM) effect towards parametric amplification. At 1550 nm, the effect of pump wavelength was investigated for a pump power of 1 W and fiber length of 25 cm with the fiber non-linearity as high as 6642 W−1 km−1 It has been observed that wider gain spectrum and bandwidth for fiber optic parametric amplifiers (FOPAs) can be achieved with the flattened dispersion curve having a proper tuning of the pumping condition. Moreover, a total bandwidth of 950 nm could be achieved with the flat dispersion nature of the proposed HMOF. [ABSTRACT FROM AUTHOR]

Helical twist provides an additional degree of freedom for controlling light in optical waveguides, expanding their applications in sensing. In this paper, we propose a helical broken-circular-symmetry core microstructured optical fiber for refractive index sensing. The proposed fiber consists of pure silica and its noncircular helical core is formed by a broken air ring. By using finite element modeling combined with transformation optics, the modal characteristics of the fiber are investigated in detail. The results show that for the core located at the fiber center, the confinement loss of fundamental core modes increases with twist rate, whereas for a sufficiently large core offset the modes can be well confined owing to the twist-induced light guidance mechanism, showing decreases with rising twist rate in the loss spectra. Moreover, we have found that for large twist rates and core offsets, resonant peaks occur at different twist rates due to the couplings between the fundamental core modes and the highly leaky modes created by the helical structure. The refractive index sensing performance is also studied and the obtained results show that the proposed fiber has great potential in fiber sensing.

Fiber optofluidic laser (FOFL) integrates optical fiber microcavity and microfluidic channel and provides many unique advantages for sensing applications. FOFLs not only inherit the advantages of lasers such as high sensitivity, high signal-to-noise ratio, and narrow linewidth, but also hold the unique features of optical fiber, including ease of integration, high repeatability, and low cost. With the development of new fiber structures and fabrication technologies, FOFLs become an important branch of optical fiber sensors, especially for application in biochemical detection. In this paper, the recent progress on FOFL is reviewed. We focuse mainly on the optical fiber resonators, gain medium, and the emerging sensing applications. The prospects for FOFL are also discussed. We believe that the FOFL sensor provides a promising technology for biomedical analysis and environmental monitoring. [ABSTRACT FROM AUTHOR]

A whispering gallery mode (WGM) microre- sonator based on grapefruit microstructured optical fibers (MOFs) is proposed for in-situ DNA biosensing. The MOF microresonator has a porous structure that offers natural microfluidic channels to load microfluidic samples of volume down to $100~\mu \text{L}$. Moreover, the WGM mechanism greatly enhances the interaction between WGM light and liquid sample kept inside the microcavity. The inner surface of the microcavity is functionalized with poly-L-lysine monolayer to achieve label-free binding of the single-strand probe DNA (ssDNA) through electrostatic force, and the probe immobilization as well as DNA hybridization processes are monitored in real time. The linear fitting of resonance wavelength shift versus DNA concentration in logarithmic scale indicates that DNA detection is available from 0.01 fmol/ $\mu \text{L}$ to 100 fmol/ $\mu \text{L}$. The proposed WGM microresonator biosensor possesses such desirable merits as ease of operation, fast response, label free method, and high specificity, making it a promising candidate for in-situ DNA detection and related biosensing applications. [ABSTRACT FROM AUTHOR]

In this work a plasmonic sensor with a D-Shaped microstructured optical fiber (MOF) is proposed to detect a wide range of analyte refractive index (RI ;na) by doping the pure silica (SiO2) core with distinct concentrations of Germanium Dioxide (GeO2), causing the presentation of high spectral sensitivity. In this case, the fiber is shaped by polishing a coating of SiO2, on the region that will be doped with GeO2, in the polished area, a thin gold (Au) layer, which constitutes the plasmonic material, is introduced, followed by the analyte, in a way which the gold layer is deposited between the SiO2. and the analyte. The numerical results obtained in the study shows that the sensor can determine efficiently a range of 0.13 refractive index units (RIU), with a limit operation where na varies from 1.32 to 1.45. Within this application, the sensor has reached an average wavelength sensitivity (WS) of up to 11,650.63 nm/RIU. With this level of sensitivity, the D-Shaped format and wide range of na detection, the proposed fiber has great potential for sensing applications in several areas.

Recognizing objects directly from optical fiber output images is useful in endoscopic applications when forming a clear image of the object is unnecessary or rather difficult. Conventional fiber-optic systems, such as multicore-fiber-based and multimode-fiber-based systems, suffer from the sensitivity of the fiber to external perturbations. For example, a slight movement of the fiber (a-few-millimeters translation of the tip for meter-long multicore fibers or multimode fibers) can greatly change the output images of the system. In this work, we utilize the light guidance stability of recently proposed glass-air Anderson localizing optical fiber (GALOF) to achieve robust imaging-free objection recognition. We transport five classes of cell images through an 80-cm straight GALOF. A deep convolutional neural network is trained to classify the output images and tested on images never seen, namely, images collected when the fiber is bent or when the fiber facet is placed several millimeters away from the object without any distal optics. Bending-invariant high classification accuracy (86.8% on average) is observed all the way to the maximum bending offset distance of 45 cm (∼74thinsp;° bending angle). High classification accuracy (91.2%) is also preserved when the fiber facet is 0.5 mm away from the object. [ABSTRACT FROM AUTHOR]

In this article we present an all-fiber vector bend sensor by means of a self-fabricated micro-structured multicore optical fiber. The reported solution is based on differential intensity variations of the light transmitted along the cores whose changes are influenced by the bending angle and orientation. The unique asymmetric structure of the air-holes in the optical fiber provides each core with different confinement losses of the fundamental mode depending on the bending radius and orientation, making each of the cores bend-sensitive in a range of at least 80°. It has been experimentally demonstrated that the reported sensor enables the bending angle and orientation to be detected in a full range of 360° without any dead-zones, and the possibility of end point detection with millimeter precision. Additionally, a reconstruction of the bending vector has been carried out theoretically, and a good match can be observed between the experimental and theoretical data. [ABSTRACT FROM AUTHOR]

A compact optical fiber light intensity detector based on an ionic-liquid-adorned microstructured optical fiber (MOF) is proposed. The light intensity detector is implemented by splicing an index-guiding MOF with two air holes in the innermost layer infused with ionic liquid (IL) between two segments of single-mode fibers. The transmission spectral characteristics of the proposed intensity detector under different power densities have been investigated from experimental as well as theoretical perspectives. Owing to modal interference effect and intrinsic absorption of the IL, the power density sensitivity reaches 1.5291 dB/(mW.mm-2) within a relatively linear range. The high sensitivity and feasibility of our proposed intensity detector is anticipated to serve as the key element, such as optical switch, in future all-optical networks.

Ultrasensitive mid-infrared absorption spectroscopy is demonstrated by the use of a novel silica-based hollow-core negative curvature fiber (HC-NCF). The HC-NCF used in this article consists of a single ring of six nontouching cladding capillaries around the hollow core, thus forming a unique core boundary with a negative curvature. Such a silica HC-NCF enables the broadband single-mode transmission in the mid-infrared. By using the HC-NCF as a compact gas cell, a proof-of-principle experiment is conducted to detect the N2O line at 2778.37 cm−1 with a distributed-feedback interband cascade laser emitting at 3.6 μm. A minimum detectable absorbance of 3 × 10−5 is achieved for a fiber length of 120 cm, corresponding to a noise equivalent absorption (NEA) coefficient of 2.5 × 10−7 cm−1. Silica HC-NCFs offer a new opportunity of developing sensitive and compact gas sensors using mid-infrared absorption spectroscopy. [ABSTRACT FROM AUTHOR]

A highly sensitive magnetic field sensor based on mode coupling effect in a microstructured optical fiber (MOF) was theoretically investigated by using the finite element method. Magnetic fluid with refractive index higher than silica was designed to be filled into one cladding air hole in MOF to form a defect core. The coupling wavelength between fiber core modes and defect core modes can be modulated by external magnetic field. Simulation results show that the measuring sensitivity of magnetic field and figure of merit reached 1620 pm/Oe, 0.081/Oe in x-polarized mode and 1790 pm/Oe, 0.045/Oe in y-polarized mode, respectively. Meanwhile, the intensity of magnetic field depicts a good linear relationship with the coupling wavelength for both orthogonal polarized directions in the range of 25–175.9 Oe. The performance of the designed magnetic field sensor can be improved by optimizing the structural parameters in MOF. [ABSTRACT FROM AUTHOR]

• Formed a F-P interferometer on a 45° polished side hole eccentric core fiber. • Can be used as an RI sensing probe for solutions, which is easy to operate. • Can be used as sensors for other parameters, such as temperature by filling suitable materials. A Fabry-Pérot interferometer on the tip of a microstructured fiber and its sensing application was demonstrated. The microstructured side hole eccentric core fiber was fabricated using the drilling hole method. The fiber end face was polished to 45° which could reflect the light transverse to the fiber, utilizing the side wall of the air hole as the reflection surface. Sensing abilities of the interferometer were investigated in the experiment. It can work as a probe for liquid RI sensing by simply plunging the fiber tip into the solution being tested, in addition, it can become sensors for different parameters by filling functional materials into the air hole. An example of filling UV curable adhesive into the fiber to be a temperature sensor was demonstrated in the experiment. The sensing structure on the fiber tip shows the advantages of high linearity, easy fabrication and good reproducibility, as a sensor it is also convenient to operate. [ABSTRACT FROM AUTHOR]

Long-range surface plasmon resonance (LRSPR), generated from a coupled plasmon polariton in a thin metal slab sandwiched by two dielectrics, has attracted more and more attention due to its merits, such as longer propagation and deeper penetration than conventional single-interface surface plasmon resonance. Many useful applications related to light–medium interaction have been demonstrated based on the LRSPR effect, especially in the sensing area. Here, we propose and demonstrate an LRSPR-based refractive index sensor by using a SiO2-Au-TiO2 heterostructure, in which a D-shaped honeycomb-microstructure optical fiber (MOF) is designed as the silica substrate and then deposited with a gold film and thin-layer titanium dioxide (TiO2). By using the full-vector finite-element method (FEM), this heterostructure is numerically investigated and demonstrated to excite LRSPR without a buffer layer, which is usually necessary in previous LRSPR devices. Through comprehensive discussion about the influence of structural parameters on the resonant wavelength, the excitation of the LRSPR in the proposed heterostructure is revealed to be highly related to the effective refractive index of MOF’s fundamental core mode, which is mainly determined by the MOF’s pitch, the thicknesses of the silica web and the planar-layer silica. Moreover, the thin-layer TiO2 plays an important role in significantly enhancing the resonance and the sensitivity to analyte’s refractive index as well, when it is coated on the top of the Au film rather than between the metal and waveguide. Finally, the proposed LRSPR sensor based on SiO2-Au-TiO2 heterostructure shows an ultra-high wavelength sensitivity of 20,100 nm/RIU and the corresponding minimum resolution is as low as 4.98×10−7 RIU. Thus, the proposed LRSPR device offers considerable potential for sensing applications in biomedical and biochemical areas.

Wearable sweat sensors provide real-time monitoring of biomarkers, enabling individuals to gain real-time insight into their health status. Current sensors primarily rely on electrochemical mechanisms, limiting their capacity for the concurrent detection of multiple analytes. Surface-enhanced Raman scattering spectroscopy offers an alternative approach by providing molecular fingerprint information to facilitate the identification of intricate analytes. In this study, we combine a wearable Janus fabric for efficient sweat collection and a grapefruit optical fiber embedded with Ag nanoparticles as a sensitive SERS probe. The Janus fabric features a superhydrophobic side in contact with the skin and patterned superhydrophilic regions on the opposite surface, facilitating the unidirectional flow of sweat toward these hydrophilic zones. Grapefruit optical fibers feature sharp tips with the ability to penetrate transparent dressings. Its microchannels extract sweat through capillary force, and nanoliter-scale volumes of sweat are sufficient to completely fill them. The Raman signal of sweat components is greatly enhanced by the plasmonic hot spots and accumulates along the fiber length. We demonstrate sensitive detection of sodium lactate and urea in sweat with a detection limit much lower than the physiological concentration levels. Moreover, the platform shows its capability for multicomponent detection and extends to the analysis of real human sweat.

Anti-resonant hollow-core fiber is featured by the broadband and low-loss transmission of light in the hollow core thanks to the optical property of anti-resonance cladding design. In this paper, we numerically explore the antiresonance of cladding from a new angle and confirm unique and important roles of refractive index of cladding material played in the confinement, bend and material losses of modes in anti-resonant hollow-core fibers. The numerical simulations demonstrate the dependence of confinement loss in anti-resonant hollow-core fiber on the cladding material refractive index which differs from the capillary waveguide and 1.5 refractive index is found favorable to achieve the lowest confinement loss ever in anti-resonant hollow-core fiber. Meanwhile, higher material refractive index can more effectively mitigate the material absorption and consequent contribution to the overall fiber loss in the anti-resonant hollow-core fiber than in the tube waveguide, by reducing the modal overlap factor. Besides, to the best of our knowledge, it is for the first time to discover that the modal dispersion of anti-resonant hollow-core fiber can be effectively altered by the cladding material refractive index, which was usually regarded to be determined by the core dimension and filled-in gas only. The refractive index around 1.8 is preferred to achieve a flattened profile of group dispersion over the transmission spectral window in anti-resonant hollow-core fibers. Our results bring in a new perspective to investigate the guidance mechanism of anti-resonant hollow-core fibers and provides guidance for material selection in the fiber design for optimization of waveguide properties. [ABSTRACT FROM AUTHOR]

In this paper, the influence of the thickness of metallic-oxide coatings, ITO, and SnO2 on the sensitivity of a microstructured optical fiber Fabry–Pérot (FP) has been studied with the aim of developing ammonia gas fiber optic sensors. Also, the distribution of the optical power that can be coupled to the metallic-oxide sensing films is investigated in order to understand how the sensor’s sensitivity can be improved; the thickness of the coatings plays a relevant role on the sensitivity and response time. Films with thicknesses between 200 and 850 nm were experimentally examined resulting in an optimal thickness of 625 nm for a SnO2 film. The behavior of the sensors toward different concentrations of ammonia gas from 10 to 130 ppm was analyzed by measuring the phase shifts of the reflected signal using the fast Fourier transform of its optical spectrum. The registered response/recovery times of this sensor are below 90 s. [ABSTRACT FROM AUTHOR]