Conventional Single-mode Fiber Research Articles

High-Density Weak in-Fiber Micro-Cavity Array for Distributed High-Temperature Sensing With Millimeter Spatial Resolution

High-spatial-resolution distributed sensing at high temperature is crucial in many industrial areas, such as aero-engines, nuclear power, furnaces, and fuel cells. Here, we propose and demonstrate a large-scale multiplexed high-density weak in-fiber micro-cavity (MC) array for distributed high-temperature sensing with millimeter spatial resolution. The proposed in-fiber MC, featured by an intrinsic Fabry-Perot interferometer (IFPI) with a short cavity length of 100 μm and a low peak reflectivity of ∼−55 dB, was formed by two weak reflectors created in a conventional single-mode fiber (SMF) by using femtosecond laser point-by-point inscription. Several high-density MC arrays, consisting of identical weak IFPIs over 1000, were fabricated by using different femtosecond laser pulse energy to investigate the transmission loss (TL) of MC arrays. The experimental result shows that the TL induced by a single MC could be low as 0.0009 dB. Moreover, the high-temperature performance of the MCs was studied via cyclic heating and cooling between room temperature and 1000 °C, showing a temperature sensitivity of −2.29 GHz/°C (i.e., 18.4 pm/°C). Furthermore, distributed high-temperature sensing was demonstrated by employing the fabricated in-fiber MC array with the demodulation of optical frequency domain reflectometry, and a high spatial resolution of 1 mm was achieved at a high temperature of 1000 °C. As such, the proposed high-density weak in-fiber MC arrays are suitable for distributed high-temperature sensing in harsh environment, and hence have wide prospection of application in the fields of aerospace, nuclear power, metallurgy, and electrochemical industry.

Weakly Coupled Multicore Fiber Technology, Deployment, and Systems

Space-division multiplexing (SDM) technology is a promising candidate to achieve massive parallelism in optical fiber cables and overcome the capacity limitation of the conventional single-mode fiber (SMF). Among the several kinds of SDM fiber, a weakly coupled multicore fiber (MCF) is advantageous in that it can be easily upgraded for use in existing equipment and requires no complex signal processing for optical multi-input multi-output (MIMO). In this article, we discuss the progress and standardization activity of MCF technologies from the viewpoint of accelerating the practical application of SDM technology. We first describe the possible application areas of SDM fibers and then show the recent progress of the essential technologies to construct the MCF link: the MCF itself, its splicing and connectorization, and the input/output techniques. We also discuss the availability and deployability of MCF as a large-capacity transmission medium and detail the progress on establishing a new international standard for SDM technology, which includes both historical progress and more recent endeavors on the standardization of optical fiber cables and SDM technology in International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and International Electrotechnical Commission (IEC). We close with a summary of MCF technology and its standardization activity with an eye to next-generation transmission systems and the long-term development of ultralarge capacity SDM transmission technology.

Randomly-Coupled Multi-Core Fiber Technology

Randomly-coupled multi-core fiber (MCF) technology has come to attract lots of attention because of its strong applicability to long-haul transmission systems. Compared with weakly-coupled MCFs with independent cores, it can simultaneously realize higher spatial channel density and ultralow transmission loss using existing ultralow-loss single-mode fiber (SMF) core designs. The strong mode coupling characteristics of randomly-coupled MCFs can provide favorable optical properties, such as suppressed accumulation of modal dispersion (MD), mode-dependent loss (MDL), and nonlinear impairments. This article gives an overview of randomly-coupled MCF technology advancements. First, we describe the classification and design of randomly-coupled MCFs and explain what the randomly-coupled MCFs are and how they are designed. State-of-the-art randomly-coupled MCFs can accommodate four, seven, or 12 cores in a standard 125- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> cladding while achieving ultralow transmission loss and/or small MD, which are very promising for long-haul transmission media. Next, we present the methods to characterize the optical properties of randomly-coupled MCFs and the difference compared to conventional SMF measurements. We also show the low-loss low-MDL connectivity of this type of MCF and the cabling that can suppress MD. A field-deployed randomly-coupled MCF cable testbed is also presented, which confirmed the favorable optical properties of randomly-coupled MCFs after deployment. Then, multi-core amplifier technologies are briefly summarized, and finally, we discuss the performance improvements in the transmissions over randomly-coupled MCFs and suitable application areas.

Single-mode D-shaped photonic crystal fiber surface plasmon resonance sensor with open microchannel

Accessible microchannels that allow microscale fluid control and microparticle-filtering are promising tools for advanced biological and chemical sensing and analysis. To this end, we proposed a d-shaped photonic crystal fiber surface plasmon resonance sensor with open microchannel. The optical characteristics of the sensor were numerically investigated using the Finite Element Method and the fiber structure was then fabricated using a standard stack-and-draw method. The numerical results demonstrate that the d-shaped photonic crystal fiber can be applied as a sensor with wide refractive index detection range between 1.330 and 1.435, a maximum sensitivity of 28400 nm/RIU, finest resolution of 3.52 × 10−6 RIU, and maximum figure-of-merit of 592 RIU−1. The sensor operates in the single-mode regime, has good linearity range, requires simple fabrication, has high coupling efficiency with conventional single-mode fibers, and has low losses, making it a practical candidate for wide range of applications in chemical and biological testing.

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 cladding-pumping based power-scaled noise-like and dissipative soliton pulse fiber laser

We report a high-average-power noise-like pulse (NLP) and dissipative soliton (DS) pulse fiber laser. Average power as high as 4.8 W could be obtained at the fundamental mode-locked repetition rate. The NLP can also be transformed into a more powerful DS mode-locking state by optimizing the polarization and losses of intra-cavity pulses in the nonlinear polarization evolution regime. The operation mode between the NLP and DS can be switched, and the laser output performance in both modes has been studied. The main advantage of this work is switchable high-power operation between the NLP and DS. In comparison with conventional single-mode NLP fiber lasers, the multi-function high-power optical source will greatly push its application in supercontinuum generation, coherence tomography, and industrial processing.

Parametric optimization of hollow core photonic crystal fiber and its comparison with conventional single mode fiber

Photonic Crystal Fibers (PCF) are flexible in its structural dimensions such as core radius, cladding hole radii and pitch. This flexibility permits the researcher to optimize the fiber to its best performance for desirable applications. Therefore, the objective of this paper is to propose an optimized Hollow Core Photonic Crystal Fiber (HCPCF) by investigating the optical parameters of the fiber. In addition to this, the manuscript intends to provide a brief comparative analysis of conventional Single Mode Fiber (SMF) and proposed HCPCF. Major parameters for study are taken as confinement factor, confinement loss, dispersion and birefringence. Based on these optical parameters, efficiency of the design is judged, where all the structural entities are chosen and investigated sequentially. The optimized value for pitch, radius of air-holes of core and cladding region of HCPCF are calculated as 3.8 µm, 18.01 µm and 1.2 µm, respectively. On comparison of HCPCF and conventional SMF, it is observed that the CF for the proposed HCPCF is higher than conventional SMF. On the other hand, confinement loss is lower for the conventional SMF. Thus, it is inferred that this optimized HCPCF can be used for high power small distance communication or sensing based optical applications.

Simultaneous measurement of torsion and strain at high temperature by using a highly birefringent cladding fiber Bragg grating.

We demonstrate the fabrication of a new highly birefringent cladding fiber Bragg grating (Hi-Bi CFBG) consisting of a pair of sawtooth stressors near the fiber core by using a femtosecond laser direct writing technology. The unique sawtooth structure serves as in-fiber stressor and also generates Bragg resonance due to its periodicity. After optimization of laser pulse energy, the Hi-Bi CFBG with a high birefringence of 2.2 × 10-4 and a low peak reflectivity of ∼ -24.5 dB (corresponding to ∼ 0.3%) was successfully fabricated in a conventional single-mode fiber (SMF). And then, a wavelength-division-multiplexed Hi-Bi CFBGs array and an identical Hi-Bi CFBGs array were successfully constructed. Moreover, a simultaneous measurement of torsion and strain at high temperature of 700 °C was realized by using the fabricated Hi-Bi CFBG, in which the torsion can be deduced by monitoring the reflection difference between the two polarization peaks and strain can be detected by measuring polarization peak wavelength. A high torsion sensitivity of up to 80.02 dB/(deg/mm) and a strain sensitivity of 1.06 pm/µɛ were achieved. As such, the proposed Hi-Bi CFBG can be used as a mechanical sensor in many areas, especially in structural health monitoring at extreme conditions.

Operando optical fiber monitoring of nanoscale and fast temperature changes during photo-electrocatalytic reactions

In situ and continuous monitoring of thermal effects is essential for understanding photo-induced catalytic processes at catalyst’s surfaces. However, existing techniques are largely unable to capture the rapidly changing temperatures occurring in sub-μm layers at liquid-solid interfaces exposed to light. To address this, a sensing system based on a gold-coated conventional single-mode optical fiber with a tilted fiber Bragg grating inscribed in the fiber core is proposed and demonstrated. The spectral transmission from these devices is made up of a dense comb of narrowband resonances that can differentiate between localized temperatures rapid changes at the catalyst’s surface and those of the environment. By using the gold coating of the fiber as an electrode in an electrochemical reactor and exposing it to light, thermal effects in photo-induced catalysis at the interface can be decoded with a temperature resolution of 0.1 °C and a temporal resolution of 0.1 sec, without perturbing the catalytic operation that is measured simultaneously. As a demonstration, stable and reproducible correlations between the light-to-heat conversion and catalytic activities over time were measured for two different catalysis processes (linear and nonlinear). These kinds of sensing applications are ideally suited to the fundamental qualities of optical fiber sensors, such as their compact size, flexible shape, and remote measurement capability, thereby opening the way for various thermal monitoring in hard-to-reach spaces and rapid catalytic reaction processes.

A strain reflection-based fiber optic sensor using thin core and standard single-mode fibers

We propose and demonstrate a fiber optic strain sensor based on a simple splice between a thin core fiber and a piece of conventional single-mode fiber. Mode dispersion generates an interference reflection spectrum sensitive to strain and temperature. The sensor offers a competitive strain sensitivity of 2.4 pm/μɛ, good linearity (R2=0.9969), and a cursory repeatability analysis yielded variation. The thermal studies indicate minimal cross-sensitivity (2.9 μɛ/°C). The sensor offers a simple implementation without any intricate manufacturing.

Single-Mode Input Fiber Combined with Multimode Sensing Fiber Used in Brillouin Optical Time-Domain Reflectometry

Conventional single-mode fiber (SMF) Brillouin optical time-domain reflectometry (BOTDR) suffers from a low signal-to-noise ratio (SNR) and severe sensing reliability due to the influence of the stimulated Brillouin scattering threshold and bend loss. In this study, a simple and low-cost distributed sensing structure, with a single-mode input fiber alignment fusion and a 50 μm diameter graded index multimode sensing fiber, is designed, and the SNR characteristic is investigated. Through theoretical derivation and experimental verification, a higher SNR and excellent bending resistance are realized in BOTDR. The experimentally measured improvements in the SNR of the proposed sensing structure over the SMF at the beginning and end of a 5 km fiber are 2.5 dB and 1.3 dB, respectively. The minimum bending radius of the sensing structure is 2.25 mm, which is much better than that of the SMFs. The bidirectional optical losses between the SMF and the 50 μm graded index multimode fiber are measured by a simple experiment system and are 0.106 dB and 1.35 dB, respectively. The temperature-sensing characteristics of the sensing structure are measured by the self-built frequency-shift local heterodyne BOTDR sensor, and the measured temperature sensitivity and accuracy are 0.946 MHz/℃ and 1 ℃, respectively. The design provides a reference for BOTDR with a high SNR and has great potential for structural safety and health monitoring of infrastructures.

Highly birefringent microstructured optical fiber for distributed hydrostatic pressure sensing with sub-bar resolution.

We demonstrate distributed optical fiber-based pressure measurements with sub-bar pressure resolution and 1 m spatial resolution over a ∼100 m distance using a phase-sensitive optical time-domain reflectometry technique. To do so, we have designed a novel highly birefringent microstructured optical fiber that features a high pressure to temperature sensitivity ratio, a high birefringence and a mode field diameter that is comparable to that of conventional step-index single mode fibers. Our experiments with two fibers fabricated according to the design confirm the high polarimetric pressure sensitivities (-62.4 rad×MPa-1×m-1 and -40.1 rad×MPa-1×m-1) and simultaneously low polarimetric temperature sensitivities (0.09 rad×K-1×m-1 and 0.2 rad×K-1×m-1), at a wavelength of 1550 nm. The fiber features a sufficiently uniform birefringence over its entire length (2.17×10-4 ± 7.65×10-6) and low propagation loss (∼3 dB/km), which allows envisaging pressure measurements along distances up to several kilometers.

Impact of Fiber Attenuation and Effective Area on Spectrum Efficiency of Elastic Optical Networks

The ultra-low-loss optical fibers becoming available now offer large effective areas with much better transmission performance than conventional standard single-mode fibers (SSMFs). However, how low the attenuation should be and how large the effective area has to be to benefit the transmission performance of a fiber-optic system have been open questions until now. In the context of an elastic optical network (EON), this paper evaluates for the first time the extent to which both fiber attenuation and effective area impact the network capacity. We first present an optical signal-to-noise ratio (OSNR) model to evaluate the performance of a lightpath considering various optical transmission impairments. We also propose different service-provisioning approaches based on this OSNR model. Simulation studies show that a spectrum-dependent provisioning scheme that considers different levels of cross-channel interference (XCI) requires minimum spectrum resources when provisioning incremental demands, and a margin-reservation scheme that considers different alternate routes shows the lowest bandwidth blocking probability (BBP) when provisioning dynamic lightpath demands. We also estimate optimal launch powers needed by lightpaths for different fiber attenuations and effective areas. Under different launch powers, it is found that lowering fiber attenuation and increasing fiber effective area demonstrate saturation in performance improvement, i.e., initially decreasing fiber attenuation and increasing effective area can significantly improve the network service provisioning performance, but the improvement is much less when the fiber attenuation and effective area are lower than 0.165 dB/km and greater than 110 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. This implies that it is not necessary to fabricate a fiber with extremely low attenuation and large effective area for the best cost-to-performance ratio.

Experimental Evaluation of Hybrid Fibre–Wireless System for 5G Networks

This article describes a novel experimental study considering a multiband fibre–wireless system for constructing the transport network for fifth-generation (5G) networks. This study describes the development and testing of a 5G new radio (NR) multi-input multi-output (MIMO) hybrid fibre–wireless (FiWi) system for enhanced mobile broadband (eMBB) using digital pre-distortion (DPD). Analog radio over fibre (A-RoF) technology was used to create the optical fronthaul (OFH) that includes a 3 GHz supercell in a long-range scenario as well as a femtocell scenario using the 20 GHz band. As a proof of concept, a Mach Zehnder modulator with two independent radio frequency waveforms modifies a 1310 nm optical carrier using a distributed feedback laser across 10 km of conventional standard single-mode fibre. It may be inferred that a hybrid FiWi-based MIMO-enabled 5G NR system based on OFH could be a strong competitor for future mobile haul applications. Moreover, a convolutional neural network (CNN)-based DPD is used to improve the performance of the link. The error vector magnitude (EVM) performance for 5G NR bands is predicted to fulfil the Third Generation Partnership Project’s (3GPP) Release 17 standards.

Plasmon-assisted fiber polarizers with high extinction ratio and low insertion loss

We propose a compact plasmon-assisted in-line fiber polarizer with metal films selectively coated on the lateral surface of the cladding in a conventional single mode fiber. The polarization-dependent loss mechanism intrinsically originates from the polarization-selective excitation of surface plasmon polaritons (SPPs) at both interfaces of metal films. Both the polarization extinction ratio (PER) and insertion loss (IL) of the plasmonic fiber polarizer strongly depend on the strength of the interaction between the polarized core modes and SPP modes. We numerically demonstrate the performances of an Au-coated and an Al-coated fiber polarizers, respectively. Both of them can achieve a PER at least higher than 10 dB/mm over the whole C- and L-bands. Notably, the PERs at 1550 nm in the Au-coated and Al-coated fiber polarizers can be up to 37.3 dB/mm and 29.4 dB/mm, respectively. The IL in the Al-coated fiber polarizer is about an order of magnitude higher than that in the Au-coated one, but it can still be lower than 0.1 dB/mm over the C- and L-bands. The achievable excellent performance shows that our proposed plasmonic fiber polarizer can provide a promising candidate for compact all-fiber polarizers with a high PER and a low IL.

Stabilized Long-Distance Superluminal Propagation Based on Polarization-Matched Low-Noise Brillouin Lasing Resonance

This paper comprehensively investigated noise characteristics of superluminal propagation based on low-noise single-frequency Brillouin lasing oscillation with the aid of a population inversion dynamic grating. Thanks to high-degree polarization alignment between the Brillouin pump and the lased Stokes lightwaves in polarization maintaining fibers, efficient Brillouin lasing resonance with over 10-dB relative intensity noise suppression has been demonstrated to activate Brillouin loss-induced anomalous dispersion in the vicinity of pump signals, benefiting a noise-insensitive superluminal propagation along kilometer-long optical fibers with robust resistance to ambient disturbance. Consequently, sinusoidally modulated pump signals experienced the time advancement of 4634.0 ns at the group velocity of 10.63 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> . Results show that the variance of the fractional advancement with polarization maintaining fibers is 2.54 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> which is two orders of magnitude lower than that of conventional single mode fibers. Furthermore, the dependence of the group velocity on the modulation frequency was experimentally investigated, showing good agreement with the theoretical analysis.

Coupled-core fiber Bragg gratings for low-cost sensing

Sensors based on Bragg gratings inscribed in conventional single mode fibers are expensive due to the need of a sophisticated, but low-speed, interrogation system. As an alternative to overcome this issue, in this work, it is proposed and demonstrated the use of coupled-core optical fiber Bragg gratings. It was found that the relative reflectivity from such gratings changed when the coupled-core fiber was subjected to point or periodic bending. This feature makes the interrogation of such gratings simple, fast, and cost-effective. The reflectivity changes of the gratings are attributed to the properties of the supermodes supported by the coupled-core fiber. As potential applications of the referred gratings, intensity-modulated vector bending and vibration sensing are demonstrated. We believe that the results reported here can pave the way to the development of many inexpensive sensors. Besides, coupled-core fiber Bragg gratings may expand the use of grating technology in other areas.

A kind of single trench 19-core single-mode heterogeneous fiber with low crosstalk and anti-bending performance

The rapid growth of the demand for optical communication capacity promotes optical fiber communication technology. As a method to break through the capacity limitation of conventional single-mode fiber, multi-core fiber based on space division multiplexing technology has attracted extensive attention. In order to respond to the capacity of traditional single-mode fiber positively, we design a scheme of single-mode multi-core fiber combining the arrangement of heterogeneous fiber cores with secondary structure of low refractive index trench. The scheme consists of nineteen fiber cores arranged in a hexagonal closed-packed structure. Heterogeneous trench-assisted multi-core fiber (Hetero-TA-MCF) has low inter-core crosstalk and excellent anti-bending performance. Compared with conventional single-mode fiber, the Hetero-TA-MCF has the large transmission capacity and average effective area of each core of about 80 μm&lt;sup&gt;2&lt;/sup&gt;. The transmission capacity of 19 cores is equivalent to the sum of the transmission capacities of 19 single-core single-mode fibers. We use COMSOL Multiphysics to simulate the fiber structure, finding the parameters that affect the properties of the fiber, selecting parameters and structures for optimal performance. Then we calculate the transmission characteristics by the finite element method,and the results of substantive simulating compute are as follows. The Hetero-TA-MCF achieves a low inter-core crosstalk (XT) of about –39 dB/100 km so that each core can be transmitted as a separate channe. It meets the standard of multi-core fiber long distance transmission. The XT of the heterogeneous 19-core single-mode fiber is suitable for multi-core fiber long distance transmission standards. The bending loss of the outermost fiber core is –7.7×10&lt;sup&gt;–5&lt;/sup&gt; dB/m when the bending radius is 10 cm, which reflects the low loss characteristics of the structure. The nonlinear coefficients of three kinds of core are 1.28 W&lt;sup&gt;–1&lt;/sup&gt;·km&lt;sup&gt;–1&lt;/sup&gt;, 1.31 W&lt;sup&gt;–1&lt;/sup&gt;·km&lt;sup&gt;–1&lt;/sup&gt;, and 1.30 W&lt;sup&gt;–1&lt;/sup&gt;·km&lt;sup&gt;–1&lt;/sup&gt; respectively, reducing the nonlinear effect of optical fiber effectively; the dispersions of three kinds of cores are less than 24 ps/(nm·km). In addition, the steady single-mode transmission is achieved in &lt;i&gt;C&lt;/i&gt;+&lt;i&gt;L&lt;/i&gt; band. Compared with traditional single-mode fiber and single-trench homogeneous fiber, the proposed fiber in this work has low crosstalk, good bending resistance and large mode field area, which is suitable for long distance and large capacity transmission in space division multiplexing system.

Fiber strain sensor based on compact in‐line air cavity fabricated by conventional single mode fiber

AbstractA Fabry‐Perot micro‐interferometer has been designed based on the conventional single‐mode fiber end fusion method for measuring strain along the fiber. During the splicing process of single‐mode optical fiber, the uniformly coated glycerine on the end face of the fiber is vaporized due to the high temperature of arc discharge to form a spherical air microcavity (Fabry‐Perot cavity). The experimental results show that the proposed sensor can determine the strain during 0–1.2 N with a linear response and the corresponding sensitivity of ~3.25 nm/N. The Fabry‐Perot micro‐interferometer strain probe proposed in this paper is simple to manufacture, compact in structure and easy to integrate. It has the promising potential in exploring the intelligent wearable devices for real‐time monitoring of micro‐strain changes such as pulse, blood pressure, and respiration.

Highly efficient two-dimensional beam steering using all-dielectric fiber metatip

Optical dielectric metasurfaces have produced a breakthrough in the field of high transmission wavefront engineering. However, its integration with the optical fiber platform is relatively new and has huge potential applications in imaging, sensing, and communication. In this work, we discuss the design and simulation of an all-dielectric metasurface that has been mounted on a conventional single-mode fiber (SMF-28) facet. It has been shown that it is possible to steer the optical beam in a two-dimensional space through appropriate design of the requisite metasurface, supported by both theoretical and simulation results. Numerical simulations reveal that the designed fiber-metatip can work efficiently over the standard telecom C-band with a bandwidth of 60 nm. In addition, the capability of meta-lensing with a focal length of 10 μm, FWHM of ∼1.439 μm and focusing efficiency of 82.2% has been demonstrated. The proposed Fiber meta-tip outperforms in compare to commercially available lensed fibers.