Optical Fiber Structure Research Articles

Low-cost fabrication and characterization process for development of a sensitive optical fiber structure.

The structure of silica single-mode fiber (SMF) must be modified in order to develop optical fiber-based biosensors. To reduce the diameter of the optical fiber, a low-cost chemical etching method is very popular. The proposed chemical etching method is a simple, rapid, and cost-effective technique for removing the silica cladding up to a desired diameter. In the laboratory, hydrofluoric acid (HF acid, 40% concentration) is used for etching. A variation on etching is also proposed and tested with 40% HF as well as with magnetic stirring at the different speeds. The etching experiments are also carried out at different temperatures. The etching results of silica fiber are presented through a step-by-step procedure using a rapid and resource-efficient method for the fabrication of optical fiber-based biosensors. The etched diameter characterization is done using a calibrated compound microscope. The sensing experiment with unetched and etched optical fiber is also performed for the detection of different concentrations of glucose biomolecules.

Numerical and experimental study on temperature measurement performance of SNS fiber optic sensor with liquid-sealed

Single-mode-no-core-single-mode (SNS) optical fiber structures have valuable potential to encapsulate as high-precision temperature sensors, due to their great sensitivity. This paper is to improve the temperature sensitivity of SNS fibers by studying the parameters of fiber length, diameter, and refractive index of the seal liquid. Transmission spectra of the fibers are given in both aspects of experiment and numerical simulation. We find that using a slim no-core fiber section sealed with liquid of high refractive index can effectively improve the sensitivity, while the choice of fiber length mainly affects the width of transmission peak. The results may contribute to the development of high-sensitivity optic temperature sensor.

Surface plasmon resonance sensor using a polarization-maintaining fiber on a hetero-core optical fiber structure with gold thin film.

This study experimentally demonstrated the effects of two polarization-maintaining fibers (PMFs) as a sensing region instead of conventional single-mode fiber (SMF) using Au-coated hetero-core optical fiber surface plasmon resonance (SPR) sensor. We experimentally observed that the SPR resonant wavelength is shifted toward longer wavelength with refractive index (RI) increasing from 1.332 to 1.396. A PMF sensor exhibits the broad SPR spectra, resulting in a higher sensitivity with a 1.5-fold change in the light intensity at 850 nm relative to RI, compared with the SMF case. Discussions most likely responsible for this effect are given by the lower angular distribution peak in the hetero-core region of PMFs.

A High-FOM surface plasmon resonance sensor based on MMF-TUMMF-MMF structure of optical fiber

The figure of merit (FOM) of conventional surface plasmon resonance (SPR) sensors is low due to the low sensitivity and wide full width at half maxima (FWHM) caused by many low-intensity high-order cladding modes. To address this problem, this paper proposes a SPR sensor based on U-shaped tapered arm cascade of multimode fiber (TUMMF). The U-shaped macro-bend increases the intensity of the light field leaking into the cladding, and the tapered area makes the number of higher-order modes smaller to achieve high sensitivity and narrow FWMH sensing performance. Through simulation and experimental verification, the sensitivity of the sensor is proportional to the fiber curvature and taper ratio, and the FWMH is inversely proportional. Therefore, the parameters of the sensor are optimized by integrating the FOM and the mechanical strength of the sensor. The optimized sensor parameters are 3 mm bending diameter, 0.85 taper ratio, and 152 nm gold film thickness. The sensor was tested in the refractive index (RI) sensing range of 1.3324–1.3731 and showed a sensitivity of 4816.092 nm/RIU and a FWMH of 97.54 nm. The FOM of the proposed sensor is 49.37 /RIU, which is 3.18 times higher than that of the U-shaped multimode fiber SPR sensor. The probe has the advantages of simple fabrication, high environmental stability and easy modification, and has a wide range of applications in bioenvironmental and other fields.

Machine learning aided inverse design for flattop beam fiber

The flattop (FT) beam, one important laser beam, is often applied to the high-power fiber laser. It is preferred to generate the FT beam by M-type fiber. However, the design of optical fiber structure is complex and time-consuming. In this work, based on the M-type fiber, a machine learning method using artificial neural network (ANN) is proposed to inversely design the FT beam fiber. By using this trained ANN, the inverse design of the FT beam fiber is realized, according to the performances of FT beam, the structural parameters of M-type fiber are determined. In addition, the influence of structural parameters on the performances of FT beam, including the flatness, the power confining factor and the effective area, are discussed in detail. The proposed ANN-based machine learning method provides an efficient, accurate prediction for FT beam fiber with excellent performances.

Damage self-healing method of composite structures based on bionic optical fiber

Aiming at the self-diagnosis and self-healing problems of composite materials, inspired by the self-healing mechanism of organisms, this paper proposes a damage self-healing method for composite structures based on bionic optical fiber. The paper introduces the designed structure and material composition of the bionic optical fiber, and the self-healing principle of the bionic optical fiber-composite structure. Finally, the three-point bending method is used to conduct damage self-healing experimental research on the composite material, and the influence of the embedding of the bionic optical fiber on the structural properties of the composite material is also analyzed. The repaired images of the composite structure obtained by the microscope can be seen that the physical structure repair is well. Moreover, the damage self-healing experimental results show that the self-repair efficiency of the flexural properties of the composite structure is 65.05%. The proposal of bionic optical fiber has important significance and practical application value for the research on the damage self-healing of composite materials.

Small-volume highly-sensitive all-optical gas sensor using non-resonant photoacoustic spectroscopy with dual silicon cantilever optical microphones

A small-volume highly-sensitive photoacoustic spectroscopy (PAS) methane detection system based on differential silicon cantilever optical microphones (SCOMs) is proposed and experimentally demonstrated. The system contains a compact non-resonant photoacoustic cell with a small volume of 1.2 mL and symmetrically-located dual SCOMs, as well as a distributed feedback laser at 1650.96 nm. The two identical SCOMs utilize the Fabry-Perot interferometric fiber-optic structure, with the differential Q-point demodulation algorithm to suppress the external vibration noise. Experimental results show that the SCOM has a high displacement sensitivity about 7.1 µm/Pa at 150 Hz and within 2.5 dB fluctuation between 5 Hz and 250 Hz. In the PAS gas sensing experiment, the normalized noise equivalent absorption coefficient of the PAS system is estimated to be 1.2 × 10−9 cm−1·W·Hz−1/2 and the minimum detection limit for methane is about 111.2 ppb with 1 s integration time. External disturbance is also applied to the dual SCOM system and results show excellent stability and noise resistance. The proposed PAS system exhibits superiorities of low gas consumption, high sensitivity and immunity to vibration and electromagnetic interference, which has an enormous potential in medicine, industry and environment.

Intramode Brillouin Scattering Properties of Single-Crystal Lithium Niobate Optical Fiber

Ordinary step-type fiber usually has only one obvious Brillouin scattering gain peak with a low gain coefficient, resulting in a poor sensing performance. As a promising material for nonlinear photonics, lithium niobate can significantly improve the Brillouin gain due to its higher refractive index when replaced with the core material. Furthermore, the higher-order acoustic modes make the Brillouin gain spectrum exhibit multiple scattering peaks, which could improve the performance of sensors. In this study, we simulated the Brillouin scattering properties of different modes of intramode in step-index lithium niobate core fibers. We analyzed the intramode-stimulated Brillouin scattering properties of different pump–Stokes pairs for nine LP modes (LP01, LP11, LP21, LP02, LP31, LP12, LP41, LP22, and LP03) guided in fiber. The results show that both the effective refractive index and Brillouin scattering frequency shift are decreased with the increase in the nine mode orders, and the values of which are 2.2413 to 2.1963, and 21.17 to 20.73 GHz, respectively. The typical back-stimulated Brillouin scattering gain is obtained at 1.7525 m−1·W−1. These simulation results prove that the Brillouin gain of the LiNbO3 optical fiber structure can be significantly improved, which will pave the way for better distributed Brillouin sensing and for improving the transmission capacity of communication systems.

Simultaneous measurement of salinity and temperature based on Fabry-Perot interference and anti-resonance effect

In this work, we demonstrate a novel cascaded fiber-optic structure depending on Fabry-Perot interference and anti-resonance (AR) effect for seawater salinity and temperature simultaneous measurement. An in-fiber microchannel Fabry-Perot interferometer (FPI) is obtained in a single-mode fiber (SMF) by using femtosecond laser radiation and hydrofluoric acid corrosion, which is filled with thermosensitive polymer and able to measure the temperature. A section of hollow-core fiber (HCF) is fused between the SMFs with the square micro-slots ablated on the ends, and the liquid can easily flow into the HCF. To our knowledge, this is the first time that the capillary-type HCF was used as a microfluidic device to measure seawater salinity. Through model analysis and experimental tests, it is confirmed that the proposed sensor can simultaneously measure the salinity and temperature, the corresponding minimum detectable resolutions reach 0.0008‰ and 0.001 ℃, respectively. The performance tests show that the sensor has accurate measurement results, good stability, and repeatability. Besides, the proposed HCF-based internal liquid analysis structure is a promising approach for highly reliable and ultrasensitive biochemical sensing.

2 μm Laser generation and amplification based on dual Tm3+-doped high-Q silica microsphere using an ASE light source for pumping

A dual Tm3+-doped silica microsphere-tapered optical fiber (TDSM-TOF) structure was proposed to excite 2 μm whispering gallery mode (WGM) laser generation and amplification. A C-band amplified spontaneous emission (ASE) light source was employed as the pumping source of TDSMs. The absorption spectra and the laser spectra of TDSM1 and TDSM2 and their concatenations were measured respectively. The ASE source has polarized light in any direction to satisfy the resonance of the microsphere cavity, the laser longitudinal mode distributions can counteract the influence of polarization variation because of environmental vibration. Meanwhile, six photons up-conversion fluorescence could also be observed.

The effect of immersion time on singlemode-tapered multimode-singlemode (STMS) fabrication using a chemical etching method

Singlemode-Multimode-Singlemode (SMS) is one of the fiber optic sensor structures that is widely applied for many sensing applications. High sensitivity is the preferred characteristic of SMS fiber structure. In this paper, a tapered structure is introduced to the SMS fiber structure to increase its performance. The tapered structure is made by the etching method by immersing the SMS multimode fiber in hydrofluoric acid (HF) solution. The immersion time was varied 5, 10, 15 and 20 minutes. The diameter of the multimode fiber decreases with the longer immersion time, while the length does not change. The STMS transmission spectrum was measured to investigate its performance. The chemical etching method using HF solution for tapering the SMS fibers has great potential as a simple method to achieve a smaller radius without lengthening multimode fibers.

Spectral element modeling of ultrasonic guided wave propagation in optical fibers

Recent advancements in fiber optic methods have enabled their use for guided wave sensing. It opens up new possibilities for Structural Health Monitoring. The aim of this paper is to provide insight for the physics related to guided wave propagation and coupling between the optical fiber and solid structure. For this purpose, a new approach for non-matching interface based on Lagrange multipliers and the time domain spectral element method was developed. A parallelized code has been implemented in order to simulate the guided wave propagation in the structure, its coupling into the optical fiber and the propagation in the fiber in a computationally efficient way. The paper presents four studies showing the efficacy of the modeling approach. The paper first shows the improvement in the computation speed through the use of parallelization and a more efficient implementation. Then the results of the simulation of wave propagation in the fiber are compared with results from previous simulation studies using commercially available software. The third study shows that the spectral element method is able to capture the directional sensitivity of optical fiber based sensors. Lastly, the simulation is used for detection of simulated damage using the spectral element method based simulation. The results indicate that indeed the spectral element implementation is able to recreate the wave coupling phenomena, capture the physics of the system including directional sensitivity and reflections from damage.

Modulation instability and highly sensitive optical fiber biosensor

The modulation instability (MI) is one of the interesting nonlinear phenomena which is used as a sensing mechanism. The performance of this mechanism is based on the shift of generated sidebands when the effective refractive index of the sensor is changed. The use of the MI process as a biosensing mechanism has capabilities such as high sensitivity, label-free and selective operation. In this paper, the novel optical fiber structures are proposed as lab-on-a-chip biosensors. In the proposed biosensors, capturing target biomolecules is achieved by immobilizing a capture layer. Consequently, after attaching the target layer the effective refractive index and the total dispersion of the structure are changed. As a result, the MI gain profile is changed and the generated sidebands (Stokes and anti-Stokes) around pump frequency are shifted. We demonstrate that the maximum sensitivity of these proposed sensors is 34.88 nm/nm. Moreover, the nonlinear material which is used in these biosensors is semiconductor doped glass (CS 3-68). This material due to the high nonlinear refractive index is an appropriate candidate to realize the MI process at low pump power. The used laser is a conventional laser 1550 nm with a power of 100 mW. The maximum length of the proposed biosensors is 0.199 cm which can realize the concept of lab-on-a-chip biosensors.

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.

Feasibility analysis of an SMS-/MSM-/SMSMS-based optical fiber sensor structure.

The paper discusses the application of single-mode fiber (SMF) and multimode fiber (MMF) to the fabrication of a sensor structure based on the hetero-core optical fiber structure. The proposed structures are SMF-MMF-SMF (SMS), MMF-SMF-MMF (MSM), and SMF-MMF-SMF-MMF-SMF (SMSMS). The transmitted intensity of the probe is used to estimate the strength of the evanescent field. The results indicate that the SMSMS structure generates more evanescent waves that penetrate deeper into the sensing probe, increasing its sensitivity. As a result, the SMSMS structure has enormous development potential in the field of sensing.

Performance analysis of an SMF-/MMF-based single/double/quadruple tapered optical fiber structure.

This paper primarily discusses the structural performance analysis of a single/double/quadruple tapered optical fiber (TOF) structure based on single-mode fiber (SMF) and multi-mode fiber (MMF). Furthermore, the TOF's performance, including its diameter distribution, transmitted intensity, and reproducibility, is also evaluated. According to the experimental results, it can be concluded that the quadruple TOF structure based on SMF has a higher density of evanescent waves (EWs) on the surface of the tapered area, which is essential for the fabrication of high-sensitivity optical fiber sensors. The structure proposed in this article is feasible, and it can be used for optical fiber sensing while offering significant practical and promising applications as well.

Investigation of a spring-shaped fiber modulation based on bending loss for detecting linear displacement

In this paper an innovative spring-shaped fiber modulation (SSFM) based on bending loss characteristic of the optical fiber and spatial helical structure with simple construction and cheap cost was developed for linear displacement monitoring. The structure design and sensing principle of the light intensity with displacement variation were reported. The structural analysis, parameter selection and monitoring performance of the SSFM constructed with two kinds of host materials (nylon carbon fiber spring and spring steel) were investigated through theoretical analysis and laboratory tests. The results showed that the SSFM can be used to modulate the optical fiber loss, and the three equal-pitch coils SSFM made of steel spring material with an initial diameter of 15 mm, an initial pitch of 15 mm would be a good choice. It was characterized by a high linearity, low hysteresis of 4.47%, suitable sensitivity of 0.0202 dB/mm and measurement range of 30 mm, respectively. Additionally, the SSFM can be connected in series with other structures to fabricate a sensor for detection of subsurface properties with a larger measurement range and it has a good multiplexing capacity. The capability supports the promising prospect of the SSFM in civil engineering for deformation monitoring of fill slopes and foundation pits.

Fabrication of Glass Diaphragm Based Fiber-Optic Microphone for Sensitive Detection of Airborne and Waterborne Sounds.

A glass-diaphragm microphone was developed based on fiber-optic Fabry-Perot (FP) interferometry. The glass diaphragm was shaped into a wheel-like structure on a 150-μm-thick glass sheet by laser cutting, which consists of a glass disc connected to an outer glass ring by four identical glass beams. Such a structural diaphragm offers the microphone an open air chamber that reduces air damping and increases sensitivity and results in a cardioid direction pattern for the microphone response. The prepared microphone operates at 1550 nm wavelength, showing high stability in a range of temperature from 10 to 40 °C. The microphone has a resonance peak at 1152 Hz with a quality factor of 21, and its 3-dB cut-off frequency is 32 Hz. At normal incidence of 500 Hz sound, the pressure sensitivity of the microphone is 755 mV/Pa and the corresponding minimum detectable pressure is 251 μPa/Hz1/2. In addition to the above characteristics of the microphone in air, a preliminary investigation reveals that the microphone can also work stably under water for a long time due to the combination of the open-chamber and fiber-optic structures, and it has a large signal-to-noise ratio in response to waterborne sounds. The microphone prepared in this work is simple, inexpensive, and electromagnetically robust, showing great potential for low-frequency acoustic detection in air and under water.

Free-space optical channel performance under atmospheric losses using orthogonal frequency division multiplexing

<span>Free space optics (FSO) is a fast-growing technology that outperforms fiber optics in communication network infrastructure without spectrum licensing. The goal of this work is to evaluate the performance of the FSO communication system using direct detection 4-quadrature amplitude modulation (4-QAM) and 4-phase shift keying (4-PSK) with orthogonal frequency division multiplexing (OFDM) scheme. Simulating a direct detection OFDM-FSO system and comparing constellation diagrams, electrical power, and optical power have been used to conduct the analysis. The model is validated using a constellation diagram of received signals for various FSO channel ranges and weather conditions. According to simulation results, the OFDM-FSO architecture combined with the 4-QAM modulation technique produces the most efficient output with the least amount of power consumption for data rates up to 10 Gb/s and FSO channel ranges of up to <br /> 3 km for clear air, 1.7 km, and 1.3 km for thin and thick fog, and 0.7 km for heavy fog. This investigation can enhance the current fiber optic network's structure connected with the FSO system to provide last-mile connectivity in 5G modules.</span>

Distribution Network Communication Base on Reflective Fiber Optic Current Sensor

Fiber optic current sensor has been brought into the practical application. The main research trend is to use special fiber or improve the system structure, so as to deal with the unstable operation of fiber optic current sensor, thereby improving the measurement accuracy of current. The working principle of the reflective fiber optic current sensor is analyzed, and the Jones matrix is applied to characterize the optical components of the system. With the analysis of the error source of the sensor, a mathematical model of optical path error is constructed, including the fabrication error, temperature error, and strain error. Under these error analyses, the design scheme of twisted fiber optic current sensor is further proposed. In view of the structure of the traditional reflective fiber optic current sensor, twisted optical fiber is used for the 1/4 wave plate and sensing fiber part, and the constant-rate twisted optical fiber is used to replace the ordinary sensing fiber. The error caused by the linear birefringence in the optical fiber is reduced, the temperature sensitivity is lowered, the anti-stress effect is improved, and the adverse interference by temperature and strain on the sensing optical fiber is eliminated. The developed fiber optic current sensor is utilized for long-term pilot application in low-frequency distribution network communication. In the simulation test, the reflective fiber optic current sensor designed with a twisted fiber optic structure has a measurement accuracy of 0.5, which meets the system requirements. As error factors is added and corresponding compensation modules are introduced, the measurement accuracy is increased to 0.2. In the pilot applications of distribution network, it can provide comprehensive and real-time information support, thus laying the foundation for the communication management of the distribution network.