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In this research, a novel fiber-optic multimode interference based multi-channel transmissive sensors integrated into a fiber laser loop (FRL) for the detection of $\text{Cu}^{2+}$ and $\text{Zn}^{2+}$ metal ions in aqueous solution is proposed and experimentally corroborated. The multi-channel configuration includes single mode-claddingless- graded index multimode- single mode (SCGS) fiber structure and single mode- claddingless-few mode-single mode fiber (SCFS) combination in parallel for tracing $\text{Cu}^{2+}$ and $\text{Zn}^{2+}$ metal ions simultaneously and implemented in FRL setup to enhance the optical sensing characteristics. The sensing probe utilises the self-imaging effect to detect and measure the concentrations of two different metal ions. The SCGS structure has achieved a sensitivity of $4.347 \times 10^{-4}$ nm/nM and a coefficient of determination $\mathrm{R}^{2}$ value of 0.9922. The SCFS combination, on the other hand, has attained a sensitivity of $4.358 \times 10^{-4}$ nm/nM and a $\mathrm{R}^{2}$ value of 0.99277. The proposed sensing configuration holds promise for multi-point sensing in bio and chemical applications, particularly in real-time scenarios.

A high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on distributed Rayleigh scattering in an all-few-mode fiber random cavity is proposed and experimentally demonstrated. Compared to traditional BRFLs oscillating in the single-mode-fiber cavity, the few-mode-fiber random cavity allows the high-order spatial mode Stokes beams to be activated and amplified via intra-mode stimulated Brillouin scattering for sufficient gain. Besides random feedback of distributed Rayleigh scattering along few-mode fibers, a few-mode fiber Bragg grating (FM-FBG) is employed to provide the one-end transverse mode-dependent reflection, which facilitates a high-purity oscillating random laser mode switchable between LP01 and LP11 mode by readily tunning the working wavelength of the Brillouin pump. Consequently, the all few-mode fiber-based BRFL emits a 600-Hz narrow-linewidth LP11 mode with a high mode purity of ∼91.3 %, which paves the way for spatial multiplexing coherent communication, high-resolution imaging, highly sensitive sensing, etc.

AbstractAn all fiber Mach–Zehnder interferometric (MZI) sensor for measuring temperature and refractive index (RI) is proposed and experimentally demonstrated. The proposed sensor is fabricated with a coreless–few mode–coreless fiber structure. Variations in ambient temperature and RI cause changes in phase differences between the fundamental mode and the higher-order modes, leading to shifts in interference spectrum. The two resonance dips shifts within the wavelength spectrum are used to investigate the temperature and RI characteristics of the sensor. Experimental results show that the two dips have different responses to temperature and RI, indicating that the sensor can realize simultaneous measurement of temperature and RI; the maximum sensing sensitivities are 0.0739 nm/°C and −25.29 nm/RIU, respectively. The proposed sensor has potential applications in physical, biological, and chemical sensing owing to its low cost, good linearity, and simple fabrication.

Fiber optic communications are inevitable to achieve higher data rates of modern telecom networks. After utilization of Wavelength division multiplexing, higher order modulations and polarization multiplexing, mode division multiplexing is a new dimension to achieve higher transmission capacity for optical fiber communication links. Different spatial distributions of optical energy along cross sectional area of optical fiber allows simultaneous transmission of data by considering each mode as an independent channel. During such simultaneous transmissions, possibility of mixing of signals amongst modes causes signal degradations and acts as limiting factor for bandwidth -- distance product of the link. This effect of mode coupling has been explored in this article by presenting its mathematical formulations. A simulation has been performed to study the impact of fiber constructional parameters on mode coupling using optical wavelengths used for telecommunication systems. The observations help to develop fiber for reduced mode coupling for particular group of modes and operating wavelengths. This article paves the way forward for study of mode coupling in micro and macro bending conditions for forthcoming research endeavours. [ABSTRACT FROM AUTHOR]

An all fiber Mach–Zehnder interferometric (MZI) sensor for measuring temperature and refractive index (RI) is proposed and experimentally demonstrated. The proposed sensor is fabricated with a coreless–few mode–coreless fiber structure. Variations in ambient temperature and RI cause changes in phase differences between the fundamental mode and the higher-order modes, leading to shifts in interference spectrum. The two resonance dips shifts within the wavelength spectrum are used to investigate the temperature and RI characteristics of the sensor. Experimental results show that the two dips have different responses to temperature and RI, indicating that the sensor can realize simultaneous measurement of temperature and RI; the maximum sensing sensitivities are 0.0739 nm/°C and −25.29 nm/RIU, respectively. The proposed sensor has potential applications in physical, biological, and chemical sensing owing to its low cost, good linearity, and simple fabrication. [ABSTRACT FROM AUTHOR]

Summary: A multi‐mode supported elliptical ring core fiber is demonstrated in the suggested work. The reported fiber structure is capable of supporting a large number of modes and influences to be favorable for high capacity, less crosstalk in the area of mode division multiplexing, and Internet of Things (IoT) compatible communications. To ensure the efficiency of the described structure, many characteristics such as V‐parameter, birefringence, sensitivity response, and confinement loss are analyzed. The investigation results are obtained from the offered structure by taking 22 distinct eigenmodes and the wavelength range from 1.54 to 1.7 μm. The evaluation of the numerical analysis shows the confinement loss of less than 2.85 × 10−9 dB/m, the sensitivity response of 80,757.73292 nm/RIU. These are, to the best of our knowledge, the maximum responses received in this trade. IoT‐based technologies allow networked tools to communicate, sense, and interact with each other. Where the wireless networks are harsh to operate on the ecological surroundings, the optical technology as fiber optic cables delivers the best resources of handling data and transferring it from one place to another effectively. [ABSTRACT FROM AUTHOR]

This paper introduces a method for fiber Bragg grating (FBG) multiplexing based on spatial mode diversity. By introducing the dimension of spatial modes in few-mode fibers (FMF), the FBGs are allocated into multiple optical paths carried by different spatial modes. The method involves multiplexing and detecting the reflected spectra of FBGs in various spatial modes, effectively enhancing the system's sensing capacity. The paper establishes a mode-diversity multiplexing sensing system using mode-selective photonic lanterns and FMF circulator. As a proof of concept, the system is experimentally validated for multipoint temperature sensing using four spatial modes LP 01 , LP 11 , LP 21 and LP 02 , and its performance is compared with that of traditional multiplexing methods. The experimental results indicate that the proposed mode division multiplexing (MDM) and hybrid MDM/wavelength division multiplexing (WDM) schemes effectively enhance the sensing capacity. Furthermore, the sensitivity to temperature response for the LP 01 , LP 11 , LP 21 and LP 02 modes in the spatial mode diversity multiplexed sensing surpasses 0.0096 nm/°C, maintaining linearity above 97%, and the maximum error contained within 5%. The proposed method offers a novel approach to expand the capacity of sensor networks. • The method enables mulyiplexing of multiple sensing channels by spatial modes via photonic lantern spatial mode diversity. • The proposed method enhances sensing capacity and reduces costs compared to traditional sensing multiplexing systems. • The proposed method can tolerate single-channel failure and improve system stability and reliability. [ABSTRACT FROM AUTHOR]

We propose a new type of all-fiber mode scrambler for mode division multiplexed data transmission based on few mode fibers. By randomly placing multiple point-loads along the fiber length, low loss and wideband mode coupling is obtained amongst all the guided modes. The effect of mode coupling is simulated from a single point-load to multiple point-loads with a specific focus on the insertion loss and wavelength dependence. In an exemplar experimental demonstration we present a mode scrambler for step-index two mode fibers incorporating 150 randomly located point-loads. Uniform mode coupling is achieved over the C band with an insertion loss of less than 1 dB.

A few mode fiber temperature sensor filled with PDMS based on vernier effect is proposed. It uses single mode fiber (SMF), few mode fiber (FMF), and polydimethylsiloxane (PDMS) to form the SMF-air microcavity-FMF-PDMS-FMF structure. It is to slot the glass capillary tube, place the two FMFs in the slot at a suitable distance, and fill the slot with PDMS multiple times and seal it. The PDMS is used as a sensing microcavity. We insert the SMF from the other end of the glass capillary tube, adjust the distance, fix the SMF and seal the connection. Two end faces of the SMF and the first FMF form an air reference microcavity. Since the refractive index of each medium is different, multiple reflective surfaces are formed to reflect. Because the second FMF is infinitely long, whose right end is beveled, it is considered that the sensor has four reflected beams. When the temperature changes, the length and refractive index of the PDMS microcavity will change, which will cause a change in the interference spectrum of the sensor. And because the vernier effect is formed by the superposition of the two microcavities, the temperature sensitivity can be amplified and the temperature can be measured with high sensitivity. The experimental results show that in the range of 40–56°C, the transmission spectrum of the sensor appears red shift with the increase of temperature, and the temperature sensitivity can reach 3.89nm/°C. The sensor is simple to manufacture, has high sensitivity, and has good application prospects.

In this letter, we propose and experimentally demonstrate a novel measurement method of mode dependent loss (MDL) in few-mode fiber (FMF) based on mode multiplexing power control. The fiber MDL, between mode groups, is computed from output power of FMF under test by controlling the power of each spatial mode of the photonic lantern. With this method, the MDL between LP01 and LP11 modes in about 9.8km 3-mode fiber is successfully measured. The experimental results obtained with our proposed method agree well with those measured by the FMF optical time domain reflectometer (FMF-OTDR) and the conventional cutback method. And it is also demonstrated that the proposed scheme can be scalable to measure long-distance FMFs with a large number of modes, and be independent of photonic lantern mode crosstalk. [ABSTRACT FROM AUTHOR]

Abstract A Mach-Zehnder interferometer (MZI) for high temperature (1000 °C) sensing based on few mode fiber (FMF) was proposed and experimentally demonstrated. The sensor was fabricated by fusing a section of FMF between two single-mode fibers (SMFs). The structure was proven to be an excellent high temperature sensor with good stability, repeatability, and high temperature sensitivity (48.2 pm/C) after annealing process at a high temperature lasting some hours, and a wide working temperature range (from room temperature to 1000 C). In addition, the simple fabrication process and the low cost offered a great potential for sensing in high temperature environments.

A few mode fiber temperature sensor filled with PDMS based on vernier effect is proposed. It uses single mode fiber (SMF), few mode fiber (FMF), and polydimethylsiloxane (PDMS) to form the SMF-air microcavity-FMF-PDMS-FMF structure. It is to slot the glass capillary tube, place the two FMFs in the slot at a suitable distance, and fill the slot with PDMS multiple times and seal it. The PDMS is used as a sensing microcavity. We insert the SMF from the other end of the glass capillary tube, adjust the distance, fix the SMF and seal the connection. Two end faces of the SMF and the first FMF form an air reference microcavity. Since the refractive index of each medium is different, multiple reflective surfaces are formed to reflect. Because the second FMF is infinitely long, whose right end is beveled, it is considered that the sensor has four reflected beams. When the temperature changes, the length and refractive index of the PDMS microcavity will change, which will cause a change in the interference spectrum of the sensor. And because the vernier effect is formed by the superposition of the two microcavities, the temperature sensitivity can be amplified and the temperature can be measured with high sensitivity. The experimental results show that in the range of 40–56°C, the transmission spectrum of the sensor appears red shift with the increase of temperature, and the temperature sensitivity can reach 3.89nm/°C. The sensor is simple to manufacture, has high sensitivity, and has good application prospects. [ABSTRACT FROM AUTHOR]

A Mach-Zehnder interferometer (MZI) for high temperature (1000 °C) sensing based on few mode fiber (FMF) was proposed and experimentally demonstrated. The sensor was fabricated by fusing a section of FMF between two single-mode fibers (SMFs). The structure was proven to be an excellent high temperature sensor with good stability, repeatability, and high temperature sensitivity (48.2 pm/C) after annealing process at a high temperature lasting some hours, and a wide working temperature range (from room temperature to 1000 C). In addition, the simple fabrication process and the low cost offered a great potential for sensing in high temperature environments. [ABSTRACT FROM AUTHOR]

For many years, the number and losses of modes have restricted the large-scale application of hollow core photonic bandgap fibers (HC-PBFs) in data transmission. Researchers have successfully reduced fiber loss to an extremely low level; however, little attention has been given to the specific characteristics of the higher-order modes (HOMs) in HC-PBF. In this paper, the high-order mode characteristics of a 7-cell HC-PBF are investigated in detail, and the non-degenerate phenomenon of HE21 is observed. Through an in-depth theoretical analysis by discussing the influence of the structural deformation of the fiber near the core on the high-order mode characteristics, the cause of the phenomenon is described. Thus, an optimized 7-cell HC-PBF for supporting 6 modes is developed. Additionally, the bending losses of all the HOMs in the fiber are measured. Marked bending loss is not observed even when the bending radius is as small as 0.5 cm, which is different from a solid core four-mode fiber. These findings will provide a reference for HC-PBF as a new type of few-mode fiber for the mode division multiplexing field. [ABSTRACT FROM AUTHOR]

We propose a new type of all-fiber mode scrambler for mode division multiplexed data transmission based on few mode fibers. By randomly placing multiple point-loads along the fiber length, low loss and wideband mode coupling is obtained amongst all the guided modes. The effect of mode coupling is simulated from a single point-load to multiple point-loads with a specific focus on the insertion loss and wavelength dependence. In an exemplar experimental demonstration we present a mode scrambler for step-index two mode fibers incorporating 150 randomly located point-loads. Uniform mode coupling is achieved over the C band with an insertion loss of less than 1 dB. [ABSTRACT FROM AUTHOR]

• High-order mode random fiber laser in an all-few-mode fiber random cavity. • LP 11 mode random laser emission with high mode purity and narrow linewidth. • Dynamic behaviors and statistics of random laser emission in the spatial domain. A high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on distributed Rayleigh scattering in an all-few-mode fiber random cavity is proposed and experimentally demonstrated. Compared to traditional BRFLs oscillating in the single-mode-fiber cavity, the few-mode-fiber random cavity allows the high-order spatial mode Stokes beams to be activated and amplified via intra-mode stimulated Brillouin scattering for sufficient gain. Besides random feedback of distributed Rayleigh scattering along few-mode fibers, a few-mode fiber Bragg grating (FM-FBG) is employed to provide the one-end transverse mode-dependent reflection, which facilitates a high-purity oscillating random laser mode switchable between LP 01 and LP 11 mode by readily tunning the working wavelength of the Brillouin pump. Consequently, the all few-mode fiber-based BRFL emits a 600-Hz narrow-linewidth LP 11 mode with a high mode purity of ∼91.3 %, which paves the way for spatial multiplexing coherent communication, high-resolution imaging, highly sensitive sensing, etc. [ABSTRACT FROM AUTHOR]

In this paper, we propose a weakly coupled few-mode fiber requiring only 2 × 2 multi-in multi-out equalizer blocks, which makes it compatible with standard coherent receivers with polarization diversity. The fiber has a highly elliptical core, surrounded by a depressed index trench in the cladding, and supports five spatial modes with twofold polarization degeneracy (ten channels). The fiber is designed to mitigate inter-modal crosstalk since the effective index difference between spatial modes is larger than ~1 × 10-3 over the C-band. Through numerical simulations, we report on bending loss and other modal characteristics such as effective area and chromatic dispersion. Finally, we briefly discuss the scalability of the design.

In this paper, we propose and experimentally demonstrate a novel method for simultaneously measuring mode-dependent loss (MDL) and differential modal group delay (DMGD) in few mode fibers (FMFs). This method is based on analyzing the Rayleigh backscattering amplitudes obtained with a special nondestructive measurement system. With this method, the MDL and DMGD in about 9.8 km 3-mode fiber are successfully measured, which are 0.186 dB and 3.581 ps/m between LP01 and LP11a and 0.157 dB and 3.604 ps/m between LP01 and LP11b, respectively. Then, the experimental results of DMGD and MDL with different methods are compared, which show that the values of DMGD and MDL obtained by the proposed method agree well with that using the time-of-flight (TOF) and conventional transmission methods, respectively. In addition, it is also demonstrated that the proposed scheme can be scalable to measure FMFs with a large number of modes.

In this paper, we address the problem of modulation format identification (MFI) for few mode fiber (FMF) transmission in elastic optical networks (EONs). The MFI accuracy is studied under different FMF channel conditions including mode coupling (MC), optical signal-to-noise ratio (OSNR), and chromatic dispersion (CD). Artificial neural network, trained using features extracted from the asynchronous in-phase quadrature histogram (IQH), is proposed to investigate the identification accuracy. Extensive simulation results have been conducted to identify six modulation schemes widely used in polarization division multiplexing coherent optical networks. This includes PDM-BPSK, PDM-QPSK, PDM-8QAM, PDM-16QAM, PDM-32QAM, and PDM-64QAM transmitted at 10 Gbaud network transmission speed. The results show that the proposed MFI achieves a successful average identification accuracy exceeding 98% in the presence of low MC when the incoming signal OSNR is greater than 20 dB. However, the effect of high MC and CD = 1100 ps/nm reduces the average accuracy to 90%. Further, the MFI accuracy is investigated under different symbol rates such as 14 and 20 Gbaud.

This article considers, for the first time, optical performance monitoring (OPM) in few mode fiber (FMF)-based optical networks. 1-D features vector, extracted by projecting a 2-D asynchronous in-phase quadrature histogram (IQH), and the 2D IQH are proposed to achieve OPM in FMF-based network. Three machine learning algorithms are employed for OPM and their performances are compared. These include support vector machine, random forest algorithm, and convolutional neural network. Extensive simulations are conducted to monitor optical to signal ratio (OSNR), chromatic dispersion (CD), and mode coupling (MC) for dual polarization-quadrature phase shift keying (DP-QPSK) at 10, 12, 16, 20, and 28 Gbaud transmission speeds. Besides, M-ary quadrature amplitude modulation (M = 8 and 16) is considered. Also, the OPM accuracy is investigated under different FMF channel conditions including phase noise and polarization mode dispersion. Simulation results show that the proposed 1D projection features vector provides better OPM results than those of the widely used asynchronous amplitude histogram (AAH) features. Furthermore, it has been found that the 2D IQH features outperform the 1D projection features but require larger number of features samples. Additionally, the effect of fiber nonlinearity on the OPM accuracy is investigated. Finally, OPM using the 2D IQH features has been verified experimentally for 10 Gbaud DP-QPSK signal. The obtained results show a good agreement between both simulation and experimental findings. [ABSTRACT FROM AUTHOR]

Real-time optical performance monitoring (OPM) is of the utmost importance in adaptive optical networks to enable awareness of channel conditions and to achieve high quality of service. In single-mode fiber (SMF)-based networks, optical signal-to-noise ratio (OSNR) and chromatic dispersion (CD) monitoring have been extensively studied in the literature. In this work, we consider OPM in few-mode fiber (FMF) networks employing non-coherent detection. OPM in such networks is a challenging task, as FMF has an additional performance-limiting impairment over SMF, namely mode coupling (MC). Here, we propose an OPM scheme to estimate three FMF channel parameters: OSNR within the range of 8 to 20 dB, CD within the range of 160 to 1120 ps/nm, and different levels of MC. The proposed scheme uses a stacked auto-encoder (AE) to extract features with reduced dimensionality compared to the original data. These features are used to train an artificial neural network (ANN) regressor. Simulation results show that the proposed OPM scheme can accurately estimate the OSNR, CD, and MC with root mean square error (RMSE) values of 0.0015 dB, 0.28 ps/nm, and 7.88 × 10−6, respectively. The performance of proposed OPM scheme is also evaluated against different types of features commonly used in literature.

The performance of few-mode (FM) erbium doped fiber amplifiers (EDFAs) with different pumping configurations is investigated and evaluated. An improvement in both gain and noise figure is achieved with a bidirectional core pumping FM-EDFA using unequal pump power with pump wavelengths of 980 nm and 1480 nm for forward and backward pumping, respectively. Moreover, the bidirectional pumping FM-EDFA is studied in case of equal pumping wavelength at 980 nm and different pump powers. Then, the system performance for the case of 6-spatial modes is tested for backward core pumping at 980 nm. The backward core pumping achieved a high modal gain with a low noise figure compared to bidirectional core pumping. Meanwhile, we report a cladding-pumped 6-modes EDFA with higher modal gain ~ 35 dB over the C-band, zero differential modal gain and low noise figure using both backward and bidirectional pumping. [ABSTRACT FROM AUTHOR]

Abstract A scheme of relay-assisted transmission system with few-mode all-optical relay is proposed in this paper to resist turbulence for atmospheric channels. The all-optical relay is composed of few-mode EDFA and photonic lantern which could amplify the distorted light field after atmospheric channels and covert the distorted light field to fundamental Gaussian fields. At the relay, using few-mode all-optical relay leads to the improvement of the turbulence induced fading which brings improved system performance. A dual-hop model for all-optical relaying system with few-mode EDFA and photonic lantern is derived. The communication performance is calculated with the help of turbulence phase screens to model atmospheric turbulence channel. Numerical results show that power budgets are increased by more than 4 d B for fixed gain amplification systems compared with all-optical relaying systems with single-mode all-optical relay and compared with few-mode all-optical relaying systems including few-mode EDFA only. Moreover, the BER performance is the best when mode-dependent gain (MDG) of few-mode EDFA is zero and becomes worse with the increase of MDG. [ABSTRACT FROM AUTHOR]

This paper studied the relationship between the temperature/strain wavelength sensitivity of a fiber optic in-line Mach–Zehnder interferometer (MZI) sensor and the wavelength separation of the measured wavelength to the critical wavelength (CWL) in a CWL-existed interference spectrum formed by interference between LP01 and LP02 modes. The in-line MZI fiber optic sensor has been constructed by splicing a section of specially designed few-mode fiber (FMF), which supports LP01 and LP02 modes propagating in the fiber, between two pieces of single-mode fiber. The propagation constant difference, $\Delta \beta $ , between the LP01 and LP02 modes, changes non-monotonously with wavelength and reaches a maximum at the CWL. As a result, in the sensor operation, peaks on the different sides of the CWL then shift in opposite directions, and the associated temperature/strain sensitivities increase significantly when the measured wavelength points become close to the CWL, from both sides of the CWL. A theoretical analysis carried out has predicted that with this specified FMF sensor approach, the temperature/strain wavelength sensitivities are governed by the wavelength difference between the measured wavelength and the CWL. This conclusion was seen to agree well with the experimental results obtained. Combining the wavelength shifts of the peaks and the CWL in the transmission spectrum of the SFS structure, this paper has shown that this approach forms the basis of effective designs of high sensitivity sensors for multi-parameter detection and offering a large measurement range to satisfy the requirements needed for better industrial measurements. [ABSTRACT FROM AUTHOR]

In the process of mode coupling measurement in FMF, the reflection from the front facet of the fiber generates a dead zone, which seriously affects the accuracy of the measurement. This paper analyzed mode coupling over FMF based on Rayleigh backscattering and built an experimental system using two photonic lanterns as the mode converter and demultiplexer respectively. And then we successfully measured the mode coupling distribution along a 4-km six-mode fiber. The values of the mode coupling coefficients obtained with the experimental system agreed well with those obtained with the conventional transmission method, and the deviations were less than 0.66 dB/km while the deviations were more than 1.31 dB/km using other experimental system which cannot eliminate the Fresnel reflection when a 300 ns optical pulse was coupled into the LP 01 mode. The experimental system based on two photonic lanterns can eliminate the Fresnel reflection and the associated dead zone from the front facet of the fiber. [ABSTRACT FROM AUTHOR]

[Display omitted] Strain and temperature sensing based on mode interference in a few-mode fiber is analyzed theoretically and experimentally. The effect of using different mode combinations is explored. A fiber guiding four LP modes is investigated as an example. Good agreement between modeling and experimental results is found for strain sensor. We demonstrate the sensors feasibility by measuring strain ranging from 0 mm to 16 mm with a sensitivity of 3.43 nm/mm and measuring temperature ranging from 30 °C to 100 °C with a sensitivity of 4.8 pm/°C. [ABSTRACT FROM AUTHOR]

Spatial mode-division multiplexing (MDM) that exploits the emerging few-mode fiber (FMF) technology is one potential candidate for next-generation petabyte optical communication links. In this paper, we focus on the design issues and specifications of the next-generation FMF that will better enable future MDM networks. These systems suffer from two main optical fiber impairments: intermodal coupling and dispersion. In this paper, we focus on the dispersion management issues through an FMF optical link. We first define two potential differential mode group delay (DMD) management strategies, namely, sawtooth and triangular. Moreover, we propose and investigate a novel parametric refractive index profile few-mode fiber, referred to as raised cosine (RC) function, which has been extensively used in digital communication pulse shaping. We investigate the achievable DMD for a wide range of the RC shape parameter and identify the design values of a two-mode FMF fiber. We then improve the RC profile by including additional shaping parameters. This enabled us to develop one four-mode fiber (4-FMF) and one six-mode fiber (6-FMF) having particularly attractive dispersion characteristics for three kinds of FMF applications: nz-, p-, and n-DMD.

A simple sensor for simultaneous measurement of refractive index (RI) and temperature based on Mach–Zehnder interference (MZI) is proposed and studied experimentally and theoretically. It consists of a section of few mode fiber and two spherical structures. The two spherical structures act as couplers that can excite high-order core modes and recouple the high-order core modes with the cladding modes back into the core. Two selected dips of the interference spectrum shift with the variation of RI or temperature, and simultaneous measurement of temperature and RI can be realized by means of a sensitivity coefficient matrix. The temperature sensitivity is 0.054 nm/°C as high as in the range from 25 °C to 80 °C, and the RI sensitivity is −27.77 nm/RIU in the range from 1.335 to 1.398. The proposed MZI is characterized by low cost, easy fabrication, and good robustness. [ABSTRACT FROM AUTHOR]

This paper proposes a novel detection scheme simultaneously to measure strain and temperature, based on a simple to construct device using a section of a specially designed few-mode fiber (FMF). The parameters and index profile of the FMF used as the key sensor element are such that the fiber supports only the LP01 and LP02 modes. The propagation constant difference between LP01 and LP02 modes Δβ has a maximum corresponding to the critical wavelength (CWL) in the fiber transmission spectrum. Because the two peaks located closest to the CWL from both sides, left peak 1 and right peak 1, shift in opposite directions, with different sensitivities under axial strain and temperature variations, the FMF device is capable of measuring the strain and temperature simultaneously. A theoretical analysis has been carried out as part of the design process and the experimental results obtained are found to agree well with the theoretical predictions. The characteristics of this sensor scheme are discussed in light of other competing approaches to simultaneous temperature and strain monitoring and are found to show advantages that suit several practical applications including compactness, ease of fabrication and implementation, relatively high sensitivities, and low cost. [ABSTRACT FROM AUTHOR]

It is abundantly clear that obtaining the channel state information (CSI) is of great importance for the equalization and detection in coherence receivers. However, to the best of the authors’ knowledge, in most of the existing literatures, CSI is assumed to be perfectly known at the receiver. So far, few literature discusses the effects of imperfect CSI on MDM system performance caused by channel estimation. Motivated by that, in this paper, the channel estimation in few mode fiber (FMF) mode division multiplexing (MDM) system is investigated, in which two classical channel estimation methods, i.e., least square (LS) method and minimum mean square error (MMSE) method, are discussed with the assumption of the spatially white noise lumped at the receiver side of MDM system. Both the capacity and BER performance of MDM system affected by mode-dependent gain or loss (MDL) with different channel estimation errors have been studied. Simulation results show that the capacity and BER performance can be further deteriorated in MDM system by the channel estimation, and an 1e-3 variance of channel estimation error is acceptable in MDM system with 0–6 dB MDL values. [ABSTRACT FROM AUTHOR]

Dispersion management in few mode fiber (FMF) technology is crucial to support the upcoming standard that reaches 400 Gbps and Terabit/s per wavelength. Recently in Chebaane et al. (2016), we defined two potential differential mode delay (DMD) management strategies, namely sawtooth and triangular. Moreover we proposed a novel parametric refractive index profile for FMF, referred as raised cosine (RC) profile. In this article, we improve and optimize the RC profile design by including additional shaping parameters, in order to obtain much more attractive dispersion characteristics. Our improved design enabled to obtain a zero DMD (z-DMD), strong positive DMD (p-DMD) and near-zero DMD (nz-DMD) for six-mode fiber, all appropriate for dispersion management in FMF system. In addition, we propose a positive DMD (p-DMD) fiber designs for both, four-mode fiber (4-FMF) and six-mode fiber (6-FMF), respectively, having particularly attractive dispersion characteristics. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate a switchable narrow linewidth single-longitudinal-mode (SLM) erbium-doped fiber (EDF) ring laser with LP 11 transverse mode output. The laser is based on a mode selective all-fiber fused coupler which is composed of a single-mode fiber (SMF) and a two-mode fiber (TMF). By controlling the polarization state of the output light, the laser can provide narrow linewidth SLM output with LP 11 transverse mode at two specific wavelengths, which correspond to two transmission peaks of the chirped moiré fiber grating (CMFBG). The 20 dB linewidth of the fiber laser for each wavelength is approximately 7.2 and 6.4 kHz. [ABSTRACT FROM AUTHOR]

We propose an all-fiber-optic sensor for simultaneous rotation and displacement measurement based on elastic-optic effect in a dual mode fiber. LP 01 and LP 11 modes are both excited in the fiber and are distinguished using wavelength-scanning Spatially and Spectrally resolved imaging technique (S2 Imaging) to accomplish rotation and displacement detection. Mode rotation is characterized by the LP 11 mode rotation using elastic-optic theory; a displacement area of 120 × 120 μm is detected and measured by recording the trace of LP 01 mode pattern. Balance between the detection range and resolution of the sensor is adjustable by simply changing the fiber length or adjusting the relative position of fiber-end, lens and the CCD camera. Real-time detection at ∼0.5 s per measurement speed is achieved, and twist rate and moving velocity can also be measured. [ABSTRACT FROM AUTHOR]

Novel optical fibers for increasing capacity for transmission system was discussed.For conventional fibers for long haul transmission,because transmission impairments such as chromatic dispersion,polarization-mode dispersion can be perfectly compensated for by digital signal processing,the only fiber parameters that can be optimized further are fiber attenuation and effective area.System figure of merit for the two parameters was discussed and recent results on ultra-low loss and large effective area fibers were presented.For short reach applications,the current efforts in improving multimode fiber bandwidth were reviewed and new trends in multimode fibers to meet future bandwidth demand were discussed.For next generation fibers,multicore and few mode fibers for space division multiplexing would be focused on,which has the potential to increase the capacity by an order of magnitude.Fiber design considerations were presented and recent progress on multicore and few mode fibers was reviewed.Finally,major challenges in space division multiplexing applications using multicore and few mode fibers were discussed.

The paper presents an analysis of the main trends in the development of transport networks in general, and data transmission technologies in particular. The requirements for fiber-optic transmission lines of transport networks of new generation are formulated. Options and choices of the optical fibers of these lines are considered. In particular, we considered the data transmission on super channel of lines with standard single-mode fibers, few-mode fiber, in a few-mode regime multimode fiber and multicore fiber. The comparison of the channel characteristics for these fibers is executed. In particular, the application with multimode fiber and multicore fiber technologies spatial division multiplexing and MIMO was considered. Presents forecasts for choosing optical fibers for the next generation transport networks.

We experimentally demonstrate space division multiplexed data transmission using the LP01, LP11a and LP11b modes over a 1 km length of elliptical-core few mode fiber at 1550 nm using 10 Gb/s ON–OFF keying data per spatial channel. Space division multiplexed transmission without the use of any multiple input multiple output digital signal processing showed no power penalty relative to the single-mode or the back to back cases. [ABSTRACT FROM PUBLISHER]

We experimentally demonstrate a narrow linewidth single-longitudinal-mode erbium-doped fiber ring laser with an output of tunable second-order optical vortex beams (OVB). The laser is based on a mode selective all-fiber fused coupler, which is composed of a single-mode fiber (SMF) and a few-mode fiber (FMF). The fused SMF–FMF coupler inserted in the cavity of the laser acts as a mode converter, which converts LP01 mode to LP21 mode group. The tunable OVB is produced by combing two even LP21 modes with orthogonal polarization directions. The tunability of second order OVB in the fiber laser is experimentally realized by adjusting a polarizer at the output end of the fiber. The 20 dB linewidth of the fiber laser is approximate 7.2 kHz. [ABSTRACT FROM PUBLISHER]

A novel scheme for the detection of refractive index change based on the mode interference in a special few mode fiber (FMF) is proposed in this paper. The parameters and index profile of the FMF used in the measurement are specifically designed so that only LP01 and LP02 modes are transmitted within the operation wavelength, and the propagation constant difference Δβ between these two modes reaches maximum at critical wavelength (CWL). An inline Mach–Zehnder interferometric sensor for the detection of surrounding refractive index (SRI) is constructed by splicing FMF with two sections of conventional single mode fiber. We demonstrated at the first time that the CWL will change monotonically with the change of SRI in the measurement range of 1.316–1.439. The experimental sensitivity of the CWL increases with the increase of SRI. A sensitiviy of 140.626 ± 12.560 nm/RIU in the range of 1.316–1.383 and a maximum sensitivity of 2489.796 ± 190.179 nm/RIU in the range of 1.433–1.439 were obtained, respectively, in the experiment. At present parameters of the etched FMF, the limit of detection of the refractive index is 3.413 × 10–4 RIU in the SRI range of 1.316–1.383, and 1.928 × 10–5 RIU in the SRI range of 1.433–1.439, respectively. The detection of SRI via monitoring the shift of CWL is superior to the traditional detection method via monitoring merely the shift of peak or dip wavelength of the transmission spectrum, since the latter usually suffers from limited measurement range and multivalue problem because it is difficult to identify and follow one specific peak or dip from the transmission spectrum especially if the system needs to reset in practical application. The novel detection scheme based on monitoring the shift of the exclusive CWL of FMF is promising in practical applications. [ABSTRACT FROM PUBLISHER]

We present a theoretical study on the mode coupling between core modes and cladding modes of the tilted long period fiber grating (TLPFG) in dual-mode fiber. The TLPFG can be applied as mode filters at several specific wavelengths. Using the full vector complex coupled mode theory, numerical simulation shows the maximum coupling coefficients between core modes (LP 01 and LP 11 ) and cladding modes can be found near the tilted angle of 86°. The transmission peaks of modes coupling are sensitive to the titled angle. Furthermore, several peak amplitude between the LP 11 mode and same azimuthal order modes increase with the growth of the titled angle. With the band-stop characteristic of specific mode, the TLPFG written in dual-mode fiber could be exploited as mode filters. [ABSTRACT FROM AUTHOR]

We propose a new method to generate orbital angular momentum (OAM) mode by superposing multiple LP11 modes with microphase difference distributed along fiber propagation. This way, OAM mode can be generated by superposing a series of LP11 mode with phase difference much less than π/2, which is necessary for schemes using two degenerated LP11 mode superposition but is difficult to generate and hard to keep stable. We demonstrate this principle experimentally by utilizing few-mode fiber long period grating (FMF-LPG), in which a series of LP11 modes with microphase difference distribution can be generated by twisting FMF-LPG. This scheme provides a potential all-fiber, compact, and low-attenuation scheme to generate stable OAM modes. [ABSTRACT FROM PUBLISHER]

Many technical capabilities of space division multiplexed systems like low loss and crosstalk have been established in numerous long-haul system experiments. Recently more attention has been given to applications of space division multiplexing for short reach systems, including data center transmission and sensing applications. Short reach systems may be the first high volume application, as space division multiplexing for short reach systems has fewer remaining technical challenges. In addition to higher bandwidth density, space division multiplexed systems offer potential advantages in lower power consumption and cost. In this paper we present penalty-free bit error ratio results down to 10-12 from system tests on 200 meters of 8-core multicore fiber with fan-outs and up to 2 km in dual-core multicore fiber with connectors and fan-outs. [ABSTRACT FROM PUBLISHER]

This article presents a scheme for few-mode EDFA, which allows to choose independent amplification lengths for different mode groups. The EDF is a dual concentric core fiber, where the central core is connected to the line FMF and the ring core is doped with erbium to provide amplification. The modes of FMF are launched into the central core of the EDF, are converted into ring modes using LPG for amplification and then converted back into central core modes using another LPG. The distance between the LPGs determines the amplification length. The amplification length, can thus, be chosen for a given mode group. We demonstrate the working of this concept by choosing LP 11 and LP 21 mode groups of the FMF and show that a suitable choice of amplification lengths for the two mode groups can tailor the differential modal gain (DMG) to any desired value. We demonstrate achieving zero DMG among all the mode of LP 11 and LP 21 mode groups using this concept while having gain in excess of 20 dB. The study should be useful for optical fiber communication system employing space-division multiplexing (SDM). [ABSTRACT FROM AUTHOR]