Conventional Optical Fiber Research Articles

New method for the investigation of mode coupling in graded-index polymer photonic crystal fibers using the Langevin stochastic differential equation

The mode coupling in a graded-index polymer photonic crystal fiber (GI PPCF) with a solid core has been investigated using the Langevin equation. Based on the computer-simulated Langevin force, the Langevin equation is numerically integrated. The numerical solutions of the Langevin equation align with those of the time-independent power flow equation (TI PFE). We showed that by solving the Langevin equation, which is a stochastic differential equation, one can successfully treat a mode coupling in GI PPCFs, which is an intrinsically stochastic process. We demonstrated that, in terms of effectiveness, the Langevin equation is preferable compared to the TI PFE. The GI PPCF achieves the equilibrium mode distribution (EMD) at a coupling length that is even shorter than the conventional GI plastic optical fiber (POF). The application of multimode GI PCFs in communications and optical fiber sensor systems will benefit from these findings.

Diameter Optimization of No-Core Fiber for High Refractive Index Sensing

This study presents a simulation-based modal analysis to optimize the no-core fiber (NCF) sensor design for high refractive index (HRI) applications. The goal is to improve the sensitivity of the sensor towards HRI by solving problems in conventional optical fiber sensors (OFS), such as sophisticated fabrication methods, high fragility, and potential effects on long-term reliability. Furthermore, in previous work, most OFS applications focused on detecting mediums with a low refractive index (LRI). Hence, the goal of this study is to overcome these constraints. For this research, the Wave Optics module in COMSOL Multiphysics® software was used to optimize the NCF diameter, which ranged from 100 μm to 150 μm. Simulations include changes in the refractive index (RI) of the analyte in the surrounding medium of the sensor to analyse the efficiency of the NCF in HRI sensing completely, ranging from 1.46 RIU to 1.56 RIU. The performance is then assessed by monitoring the intensity change in the power spectrum, which reflects the reaction of leaky modes to various HRI conditions. The findings indicate that decreasing the size of the NCF sensor enhances sensitivity in detecting HRI mediums by amplifying the leakage loss of each order leaky mode, as opposed to a larger NCF diameter. This effect is due to the reduced confinement of modes in a smaller diameter NCF. The 100 μm diameter NCF has exceptional refractive index (RI) sensitivity and linearity, measuring 88.996 dB/RIU and 0.7783, respectively. Accordingly, NCF proves to be a promising candidate for utilization in HRI sensing applications, given its high sensitivity and straightforward fabrication, making it easily implementable and practical for real-world use.

Citrate polymer optical fiber for measuring refractive index based on LSPR sensor

Fiber optic localized surface plasmon resonance (LSPR) sensors have become an effective tool in refractive index (RI) detection for biomedical applications because of their high sensitivity. However, using conventional optical fiber has caused limitations in implanting the sensor in the body. This research presents the design and construction of a new type of polymer-based LSPR sensors to address this issue. Also, finite element method (FEM) is used to design the sensor and test it theoretically. The proposed polymer optical fiber (POF) based on citrate is biocompatible, flexible, and degradable, with a rate of 22% and 27 over 12 days. The step RI structure utilizes two polymers for light transmission: poly (octamethylene maleate citrate) (POMC) as the core and poly (octamethylene citrate) (POC) as the cladding. The POF core and cladding diameters and lengths are 700 µm, 1400 µm, and 7 cm, respectively. The coupling efficiency of light to the POF was enhanced using a microsphere fiber optic tip. The obtained results show that the light coupling efficiency increased to 77.8%. Plasma surface treatment was used to immobilize gold nanoparticles (AuNPs) on the tip of the POF, as a LSPR-POF sensor. Adsorption kinetics was measured based on the pseudo-first-order model to determine the efficiency of immobilizing AuNPs, in which the adsorption rate constant (k) was obtained be 8.6 × 10–3 min−1. The RI sensitivity of the sensor in the range from 1.3332 to 1.3604 RIU was obtained as 7778%/RIU, and the sensitivity was enhanced ~ 5 times to the previous RI POF sensors. These results are in good agreement with theory and computer simulation. It promises a highly sensitive and label-free detection biosensor for point-of-care applications such as neurosciences.

Optically powered FiWi system for B5G hybrid VLC/RF access networks

This paper proposes the concept and reports the demonstrator of an optically powered fiber/wireless (FiWi) system based on a hybrid visible light communication (VLC) and RF access network for beyond 5G (B5G) indoor applications. The power-over-fiber (PoF) technology is properly applied to simultaneously energize two components from the remote site, namely: a radio-over-fiber (RoF) module, which contains a photodetector and an RF amplifier; a red laser from the VLC system. The proposed PoF system is composed of a 500 m conventional multimode optical fiber (MMF) with a 62.5 μm core diameter, which enables the transmission of up to 12 W optical power with a power transmission efficiency (PTE) of 14%. In the proof of concept, 8.1 W optical power is generated to feed the 5G system components. Regarding the communication setup, a 5G new radio (5G NR) signal is initially transported through a RoF fronthaul, composed of 10 km of single-mode fiber (SMF), followed by the hybrid VLC/RF access network. The performance investigation of the B5G communication system is based on an analysis of the root mean square error vector magnitude (EVMRMS) parameter. Experimental results indicate 1.2 Gbps throughput in the VLC link using a 200 MHz bandwidth and 64-quadrature amplitude modulation (64-QAM), resulting in approximately 4.02% of EVMRMS. Moreover, the RF link at 3.5 GHz provides 360 Mbps with an EVMRMS of 2.67%. Finally, a comparison between the PoF system and conventional power supply validates the proposed approach's applicability for B5G wireless communications.

Enhancement of SPR effect and sensing characteristics of no-core fiber with bubble structure

In this work, we propose the SPR fiber sensor based on a bubble structure. Conventional SPR optical fiber sensors have an anomalous broadening of the resonance dip in the near-infrared band. Therefore, we propose to utilize a bubble structure to improve the FWHM of the spectrum. The sensor utilizes no-core fiber as the sensing platform and multimode fiber as the transmission fiber, and fabricates a bubble at the fusion of the two fibers. The effect of bubble size on the SPR effect is investigated experimentally, and the results show that bubbles can improve the sensitivity and reduce the FWHM of SPR sensor. The refractive index sensing test was implemented on the proposed SPR bubble sensor. The experimental results showed that the bubble structure enhanced the maximum sensitivity of the SPR sensor by 927.7 nm/RIU. Finally, the effect of the bubble on the response time of the sensor is discussed. The method of using a bubble structure to improve sensor performance is simple to operate and easy to implement.

Enhanced frequency and time domain feature extraction for communication infrastructure type classification using optical fiber sensing

We present a feature extraction method for a feedforward-type neural network (FNN) designed to realize a distributed acoustic sensing (DAS) technique that suits conventional optical fiber. This is, to the best of our knowledge, the first trial in which an FNN is used to interpret the field communication infrastructure type surrounding optical cables. Three classes are taken to represent the field environment: the cable tunnel (class 1), circular duct (class 2), and overhead area (class 3). We investigate and compare frequency- and time-domain feature extraction. We also show that the frequency-domain features yielded by spectral envelope shape (SES) processing have better performance than simple fast Fourier transform features. Two types of time-domain features are verified: one is the short-time maximum magnitude (STMM), which shows the largest value in the time frame, and the other is the short-time average magnitude (STAM), which indicates the average value in a time frame. Note that all features are optimized for multi-class classification. In this paper, we present the suitable number of both features and the number of training iterations. An accuracy rate of 79.0% is achieved using FNN analysis with the features studied here. Furthermore, by considering the similarity of neighboring classes, classes are refined into higher probability classes. As a result, accuracy is improved to 87.2%.

High-Resolution and Large-Dynamic Range Fiber-Optic Sensors Based on Dual-Mode Direct Spectrum Interrogation Method

Conventional optical fiber temperature/strain sensors often have to make compromises between the resolution and the dynamic range. Here we present a new method that meets the measurement requirements for both high resolution and large dynamic range. A high-quality optical fiber Fabry-Perot Interferometer (FPI) constructed using a pair of chirped fiber Bragg gratings is employed as the sensor and a dual-mode direct spectrum interrogation method is proposed to identify the small drift of external temperature or strain. As a proof-of-concept illustration, a temperature resolution of 0.2 °C within 30–130 °C is demonstrated. For strain sensing, the resolution can be 10 µε within 0–1000 µε. The measurement resolution can be improved further by routinely increasing the reflectivity of the CFBG and the cavity length and the sensor can also be mass-produced. This new sensing schema not only resolves the conflict between the resolution and the dynamic range of fiber-optic temperature/strain sensors but can also be extended to other sensors and measurands.

In-fiber chirped Fabry-Perot cavity for temperature sensing.

Measurement resolution and dynamic range of conventional optical fiber sensors are often mutually restricted. In this work, an in-fiber chirped Fabry-Perot cavity (interferometer) is proposed, for the first time to our knowledge, to resolve the conflict between the resolution and dynamic range. The chirped Fabry-Perot interferometer is constructed by two chirped fiber Bragg gratings inscribed in the opposite directions, resulting in a gradually varied (i.e., chirp) cavity length for different reflection wavelengths. As such, the interference spectrum exhibits high figure of merit (FOM) and large free spectrum range (FSR) at long and short wavelength regions, respectively, enabling high-resolution and large-dynamic-range measurement simultaneously. Temperature tests are then carried out to confirm the validity of the solution. The proposed sensing schema may be developed further and find vital applications in biomedicine fields such as endosomatic temperature monitoring of living bodies. The proposed concept of chirped Fabry-Perot interferometer can provide breakout ideas for other sensing scenarios where high-resolution and large-dynamic range are demanded and can be further generalized to other measurands or even free-space interference metrologies.

Broadband dispersion compensation and high birefringence photonic crystal fiber for CWDM/DWDM networks

In this study, we propose a new design based on photonic crystal fibers (PCFs) for broadband dispersion compensation in telecommunication networks. The proposed design has a hexagonal structure arrangement of air-holes rings of different diameters between the silica core and the cladding. The PCF properties like effective area, nonlinearity, dispersion slope, confinement loss, and birefringence are reported and discussed. For the best performance we present three designs A, B and C. Simulation results show that the three designs cover the six-telecommunication optical bands O-, E-, S-, C-, L- and U- bands (wavelengths ranging from 1260 to 1675 nm). Design A achieves a large negative dispersion value of about − 1716 ps/(nm.km) with relative dispersion slope equals to that of conventional single-mode optical fibers (SMFs) of about 0.0036 nm−1, which makes it very suitable for long-haul DWDM transmission systems. With a little modification in the core, designs B and C achieve much higher confinement ability and achieve a very large birefringence value for polarization mode dispersion and sensing applications. Design C is engineered to have exact opposite dispersion of SMF with zero dispersion at the wavelength 1310 nm, which makes it a promising design in CWDM transmission system. The numerical values have been investigated using the full vector finite element method.

Opto-hydrodynamic tweezers.

Optical fiber tweezers offer a simple, low-cost and portable solution for non-invasive trapping and manipulation of particles. However, single-fiber tweezers require fiber tip modification (tapering, lensing, etc.) and the dual-fiber approach demands strict alignment and positioning of fibers for robust trapping of particles. In addition, both tweezing techniques offer a limited range of particle manipulation and operate in low flow velocity regimes (a few 100 μm s-1) when integrated with microfluidic devices. In this paper, we report a novel opto-hydrodynamic fiber tweezers (OHT) platform that exploits the balance between the hydrodynamic drag force and optical scattering forces to trap and manipulate single or multiple particles of various shapes, sizes, and material compositions in a microfluidic channel. 3D hydrodynamic flow focusing offers an easy and dynamic alignment of the particle trajectories with the optical axis of the fiber, which enables robust trapping of particles with high efficiency of >70% and throughput of 14 particles per minute (operating flow velocity: 1000 μm s-1) without the need for precision stages or complex fabrication. By regulating the optical power and flow rates, we were able to trap single particles at desired positions in the channel with a precision of ±10 μm as well as manipulate them over a long range upstream or downstream with a maximum distance of 500 μm. Our opto-hydrodynamic tweezers offer an alternative to conventional optical fiber tweezers for several applications in physics, biology, medicine, etc.

Submicrometer optical frequency combs based on SPPs metallic multi-ring resonators

Optical frequency combs (OFCs) have great potential in communications, especially in dense wavelength-division multiplexing. However, the size of traditional OFCs based on conventional optical microcavities or dispersion fibers is at least tens of micrometers, far larger than that of nanoscale electronic chips. Therefore, reducing the size of OFCs to match electronic chips is of necessity. Here, for the first time to our knowledge, we introduce surface plasmon polaritons (SPPs) to the construction of OFCs to realize a miniature device. The thickness of our device is reduced below 1 μm. Though the presence of SPPs may induce ohmic and scattering loss, the threshold of the device is obtained as 9 μW, comparable to the conventional device. Interestingly, the response time is 13.2 ps, much faster than the optical counterparts. This work provides a feasible strategy for the miniaturization of OFCs.

Microcavity Mach-Zehnder refractive index sensing in LiTaO3 crystal waveguides by a combination of femtosecond laser inscription and diamond blade dicing

Based on planar waveguides, refractive index sensors have certain advantages over conventional optical fiber sensors, including better mechanical stability and lower cross-responses to factors such as vibrations. In this study, we report for the first time a microcavity Mach-Zehnder interferometric refractive index sensor fabricated within a planar waveguide. The sensor consists of double-line waveguides made of lithium tantalate crystals written by femtosecond laser and microcavities formed by diamond blade dicing. The refractive index sensitivity of the sensor reaches approximately 1856.1 nm/RIU, with extremely low temperature cross-responses. This work provides a new alternative for developing biosensors based on femtosecond laser-written waveguides. In the future, by selecting appropriate materials and optimizing the structure, the sensing performance of this sensor is expected to be further enhanced.

Fabry-Pérot Cavities with Suspended Palladium Membranes on Optical Fibers for Highly Sensitive Hydrogen Sensing.

Hydrogen (H2) sensors are critical to various applications such as the situation where H2 is used as the clean energy for industry or the indicator for human disease diagnosis. Palladium (Pd) is widely used as the hydrogen sensing material in different types of sensors. Optical fiber H2 sensors are particularly promising due to their compactness and spark-free operation. Here, we report a Fabry-Pérot (FP)-cavity-based H2 sensor that is formed with a freestanding Pd membrane and integrated on a conventional single-mode optical fiber end. The freestanding Pd membrane acts both as the active hydrogen sensing material and as one of the reflective mirrors of the cavity. When the Pd film absorbs H2 to form PdHx, it will be stretched, resulting in a change of the cavity length and thus a shift of the interference spectrum. The H2 concentration can be derived from the amplitude of the wavelength shift. Experimental results showed that H2 sensors based on suspended Pd membranes can achieve a detection sensitivity of about 3.6 pm/ppm and a detection limit of about 3.3 ppm. This highly sensitive detection scheme is expected to find applications for sensing low-concentration H2.

Refractive index SPR sensor with high sensitivity and wide detection range using tapered silica fiber and photopolymer coating.

This paper introduces a surface plasmon resonance (SPR) sensor using tapered silica fiber and photopolymer coating for enhanced refractive index (RI) detection. Tapering the silica fiber to a diameter of 10 µm ensures the evanescent wave leaks into a 1.8-µm thick photopolymer film, which increases the average waveguide RI and broadens the RI detection range accordingly. A 50-nm thick single-side gold film is coated on the photopolymer film, exciting SPR and causing less light transmission loss than a double-side gold film. The method avoids the complex microfabrication processes of conventional polymer optical fiber SPR sensors, while the waveguide RI can be controlled by altering the curing time of the photopolymer during fabrication. The sensor has an overall sensitivity of 3686.25 nm/RIU, enabling RI detection of 1.333 - 1.493. Moreover, the sensor has an ultrahigh sensitivity of 6422.9 nm/RIU in the RI range of 1.423 - 1.493. The temperature response is about 1.43 nm/°C at 20 - 50 °C, which has little impact on RI detection. Finally, we demonstrate that the sensor can grade the severity of hepatic steatosis by measuring the RIs of cytoplasm/triglyceride emulsions with superior sensing performance.

Lipid-coated hetero-core optical fiber sensor for wide-range chemical detection

We developed a chemical sensor based on a hetero-core optical fiber structure where lipid films coat a cylindrical cladding. Two lipids were applied to the fiber surface: oleic acid and phosphoric acid di(2-ethylhexyl) ester. Experiments were performed to observe the differential optical loss spectra over the measured spectral region in response to each chemical substance. The oleic acid- and PAEE-coated sensors induced optical losses of approximately 0.4 and 0.2 dB, respectively, for a refractive index difference of 0.0008 between water and quinine. Compared with a conventional hetero-core optical fiber sensor based on surface plasmon resonance, which uses double-layer films of gold and lipid, the proposed sensors scheme showed the different sensitivity to chemical substances and could detect a wide range of chemical substances such as quinine. The response of the proposed sensor was reproducible at concentrations of >0.01 mM. Changing the type of lipid in the film controls the selectivity of chemical substances. The proposed sensor works based on hydrocarbon groups of fatty acids or carboxyl groups that form self-assembled monolayers on the fiber. The exposure of carboxyl groups at the surface of the fatty acids may be involved in the sensor response.

Long-length 3D printed hollow-core polymer optical fiber for wideband light guidance

A hollow-core microstructured polymer optical fiber containing a six-pointed star cladding design is directly drawn from a 3D printed preform for wideband light guidance. Fabrication of a complex geometry-based preform is enabled using commercially available translucent polymer filaments in a 3D printer. Light guidance from visible to infrared regions through the core of 57.8 cm long fiber is achieved with a minimum loss of 0.11 dB/cm in the near-infrared spectrum. The bending loss of the proposed design is also investigated to determine the light guidance under a mechanical effect. The modified fabrication process of the polymer optical fiber with novel design could be an alternative to conventional optical fibers for visible and infrared light guidance.

Synthesis of graphene oxide-coated silver nanoparticles modified plastic of fibre optic sensor configuration

This paper presents the synthesis and fabrication of a sensor configuration using graphene oxide (GO) loaded with silver nanoparticles (AgNPs) on a conventional plastic optical fibre (POF) probe. A successful synthesis of GO and AgNPs was achieved using modified Hummers and seed-mediated methods, respectively. The POF was unclad using mechanical slicing and chemical etching, followed by the coating of GO-AgNPs via the dipping method. Scanning electron microscopy (SEM) analysis confirmed the successful coating, with 60% v/v acetone identified as the optimal etching solution. The clear evidence of embedded GO-AgNPs on the POF surface demonstrates the successful fabrication of a coating layer, establishing it as a promising sensing platform for optical fibre sensors.

Hollow core fiber cable technologies

Hollow core fibers (HCF) are innovative optical fibers having the potential to break the limits of conventional optical fibers. Examples of innovation are ultra-low loss potential, ultra-low nonlinearity, resistance against harsh conditions, and long interaction lengths with gases confined to the core. In addition, ultra-low latency characteristics have attracted much attention in recent years, particularly amongst financial trading communication systems. For the first time in the world, we have succeeded in developing a high-performance HCF with practical single mode (SM) properties realized by a resonant couple phenomenon, to apply the HCF in the low latency transmission. Furthermore, we have successfully developed HCF connectors and HCF cables to realize the actual low latency transmissions. These development results will be reported.

Distance controlled resonant radiation from modulated Airy pulses

We investigate the dynamic evolution of resonant radiation (RR) emitted from modulated Airy pulses in an optical fiber with emphasis on the third-order dispersion. We show that the process of RRs emission strongly depends on the parameters of the predesigned spectral phase imposed on the Airy pulses, which is attributed to its linear focusing behaviors. A localized pulse formed at the distance-controlled focusing can effectively generate a large amount of RR after a prescribed propagation distance. At variance with the case of fundamental soliton, the minimum value of third-order dispersion required for the onset of RRs emission becomes much smaller for the modulated Airy pulse. The conversion efficiency of RRs increases with an increasing linear focusing point, but decreases with an increasing truncated coefficient. There is an optimal value of the truncated coefficient for the RRs having highest peak intensity and largest frequency shift. The impact of Raman effects on the RRs is also revealed. Our results not only provide a simply route to actively manipulate the efficient emission of RRs in conventional optical fiber, also could have significant implications of pulse shaping technology in novel highly efficient light sources based on the RRs emission for a variety of application.

Active Fiber Tips With Optoelectronic Integration: State-of-the-Art, Future Trends, and Challenges

The integration of conventional silica optical fibers with optoelectronic materials and electrical structures opens the door to miniaturized and multifunctional all-fiber optoelectronic systems with actively controllable capabilities. The flat endface of an optical fiber tip offers a naturally light-coupling microscopic platform for optoelectronic integration using optimized micro- and nano-fabrication techniques. In recent decades, the increasing demand for active fiber devices has created opportunities to develop various optoelectronic structures on fiber tips, from integrating high-performance single modules for specific applications to integrating multi-function multiple modules for more complex applications. In this paper, we review the rapid development of fiber-compatible fabrication technologies for building optoelectrical circuits on fiber endfaces. Then, we discuss the fast-progressing field of active fiber tips along with its wide applications, including photodetection, modulation, lasing, sensing, and light-emitting. Finally, we discuss challenges and opportunities for future development in this field. We believe that this scheme of active fiber tips will pave the way for next-generation ultracompact optical fiber systems with diverse optical and optoelectronic applications.