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Tensor models have been used extensively in signal processing applications to design different types of communication systems. This paper proposes, for the first time, the use of tensor models for optical communications. The signals of an optical dual-core coupler network are modeled as a multiway array (tensor), which satisfies a generalized Tucker train decomposition. This tensor model is then used to develop an estimation algorithm that enables the network parameters to be estimated from the input and output signals. The performance of this algorithm was evaluated by means of computer simulations, in terms of NMSE of the estimated parameters and convergence speed. For the tested configurations, good levels of NMSE with fast convergence were obtained, demonstrating the effectiveness of the proposed method as a promising tool for studying and designing optical devices, with a wide range of applications in the context of lightwave systems.

(6+1)×1 pump and signal combiner with high power handing capability, negligible beam quality degradation and bi-directional signal transmission ability has been developed by employing signal-fiber non-taper fabrication approach and in-line active splicing process. The influences of lateral core offset and angular misalignment between the signal input and output fibers in combiner have been studied theoretically, showing that M2 factor rather than signal efficiency is more suitable to be the evaluation criterion. Three splicing methods have been compared experimentally, revealing that the M2-based active splicing is the most effective to preserve beam quality and obtain good efficiency simultaneously. Signal laser transmitted backward through 25-μm fiber core has shown efficiency of 95.5% and M2 factor deterioration no more than 10% at 1 kW of power. Combined pump from laser diodes into 400-μm fiber clad has reached beyond 5 kW of power at efficiency of 98.2%, with the maximum temperature on combiner being 70.5°C, which was tested in the condition without cooling on fiber pigtails and with flat-cleaved fiber termination aggregating thermal load. Both results indicate the effectiveness of the proposed fabrication technique to make bi-directional (6+1)×1 combiner for high power and superior beam quality fiber laser systems.

In recent times, usage of fixed wireless access (FWA) is gaining momentum even though fiber based existing services such as fiber-to-the-home (FTTH) are widely used for home Internet applications. We explore a time-division multiplexing passive optical network (TDM-PON) and long term evolution advanced (LTE-A) based cost-efficient access network planning strategy for combined usage of FWA and FTTH with the same network infrastructure. The proposed fiber-wireless (FiWi) access network satisfies all feasibility constraints and users’ quality of service (QoS) requirements. The proposed planning method judiciously identifies the network architecture and user service mode (FWA/FTTH). [ABSTRACT FROM AUTHOR]

To find out which kind of mode-locked fiber oscillator is superior, six kinds of typical mode-locked Yb:fiber oscillators, including nonlinear polarization evolution (NPE)-based oscillators (i.e. conventional NPE-based oscillator, polarization maintaining (PM) NPE-based oscillator), nonlinear amplifying loop mirror (NALM)-based oscillators (i.e. figure-8 oscillator, all-fiber figure-9 oscillator, free-space figure-9 oscillator), and semiconductor saturable absorption mirror (SESAM)-based oscillator, were presented and crosswise compared. Except for the conventional NPE-based one, all the others were constructed with all-PM fiber components. Experimentally, detailed crosswise comparisons were carried out quantitatively and qualitatively for different oscillators in terms of spectral bandwidth, spectral shape, pulse repetition rate, signal-to-noise ratio (SNR), mode-locking threshold, self-starting mode-locking pump power, robustness, component life, construction simplicity, and so on. In addition, subjective sensations on each oscillator were also provided, which could be helpful for decision-making when one wants to make or purchase an oscillator. Finally, the all-fiber figure-9 oscillator was suggested to be the best choice even it needs higher pumping power, which could be an ideal seed for high power ultrafast laser amplifiers. [ABSTRACT FROM AUTHOR]

In this paper, a new apodized fiber Bragg grating (FBG) structure, the Chebyshev apodization, is proposed. The Chebyshev polynomial distribution has been widely used for the optimal design of antennas and filters, but it has not been used for designing FBGs. Unlike the function of traditional Gaussian-apodized FBGs, the Chebyshev polynomial is a discrete function. We demonstrate a new methodology for designing Chebyshev-apodized FBGs: the grating region is divided by discrete n sections with uniform gratings, while the index change is used to express the Chebyshev polynomial. We analyze the Chebyshev-apodized FBGs by using coupled mode theory and the piecewise-uniform approach. The reflection spectrum and the dispersion of Chebyshev-apodized FBGs are calculated and compared with those of Gaussian FBGs. Moreover, a sidelobe suppression level (SSL), a parameter of the Chebyshev polynomial, along with the maximum ac-index change of FBGs are analyzed. Assume that the grating length is 20mm, SSL is 100 dB, the section number is 40, and the maximum ac-index change is 2 × 10−4. The reflection spectrum of Chebyshev apodized FBGs shows flattened sidelobes with an absolute SSL of −95.9 dB (corresponding to SSL=100 dB). The simulation results reveal that at the same full width at half maximum, the Chebyshev FBGs have lower sidelobe suppression than the Gaussian FBGs, but their dispersion is similar. We demonstrate the potential of using Chebyshev-apodized FBGs in optical filters, dispersion compensators, and sensors; Chebyshev apodization can be applied in the design of periodic dielectric waveguides. [ABSTRACT FROM AUTHOR]

Fiber-reinforced composite parts used in drones, automobiles, and sports equipment are now being manufactured by additive manufacturing (AM), where the material parameters such as fiber direction can be changed within a layer or from one layer to the other. Nondestructive evaluation methods are required to assess the quality of such customized printed parts. In this work, a microcomputed tomography ($\mu $ CT) dataset is analyzed to determine the fiber content in a 3-D printed composite material part using a digital binary image processing method. The existing literature on binary image analysis methods to measure the fiber volume fraction is limited to continuous fiber reinforced composites. Discontinuous fiber reinforced 3-D printing filaments are popular in manufacturing parts with increased strength. The methods developed in this work expands the binary image process to scans that show fibers embedded lengthwise in different directions in the 3-D printed layers. An optimized thresholding method is trained on the filament sample and then applied to 3-D printed samples. The results show fiber volume fraction measurements with standard deviations below 0.15%. The results in this work will be useful for product quality validation. [ABSTRACT FROM AUTHOR]

A homemade double-tapered Yb-doped fiber is employed in a high-power narrow-linewidth fiber amplifier to improve the thresholds of nonlinear effects and transverse mode instability (TMI) effect. A bidirectional pumping configuration is employed, and a phase-modulated single-frequency fiber laser serves as the seed laser. As a result, an output power of 3630 W with 0.21 nm linewidth (stimulate Brillouin scattering limited) and an output power of 4180 W with 0.59 nm linewidth (TMI limited) are realized, corresponding to the slope efficiency of ∼77%. The beam quality at the maximum output power is ${\rm M}_{\rm x}^{2}= 1.57, {\rm M}_{\rm y}^{\rm 2} = 1.39$. The results could provide a well reference for the power scaling of narrow-linewidth Yb-doped fiber amplifier. [ABSTRACT FROM AUTHOR]

Multicore fibers (MCFs) have attracted intense attention to overcome the capacity limit of single-core optical fiber communication system. However, as indispensable connection devices between the MCF and the individual single mode fiber (SMF), fan-in and fan-out (FIFO) devices are challenging to achieve overall performance in terms of coupling efficiency, crosstalk, return loss, and device compactness. Here we develop an MCF reverse-tapering technique to increase the core-to-core spacing of the MCF and construct a spacing-tailored interface between the MCF and the normal SMF bundle. The fabricated FIFO device for the small-core MCF with narrow core-to-core spacing of 35 μm features a low average coupling loss (∼0.6 dB), extraordinarily low crosstalk (<−65 dB) and a return loss (<−60 dB) without using any bridge elements, where the remarkably low crosstalk is about 20 dB lower than using special bridge fibers for the same type of MCF. The flexibility of reverse tapering of MCFs can overcome its intrinsic integration challenge and provide a powerful approach to constructing all-fiber FIFO devices with compact construction and excellent overall performance. [ABSTRACT FROM AUTHOR]

An easy-to-fabricate and efficient fiber-to-waveguide adiabatic coupling on a high-index-contrast (HIC) InGaAs/InP quantum well (QW) waveguide platform has been theoretically analyzed and experimentally demonstrated through a 3D spot-size converter (3D-SSC). The 3D-SSC has three monolithically integrated sections: a HIC and a fiber-matched section, coupled using a 3D-adiabatic mode converter (AMC). Vertical tapering of 3D-AMC is achieved by $\text{ZrO}_{2}$ based selective area impurity-free QW intermixing (IFQWI) on the photo-lithographically delineated horizontally tapered region. The 3D-AMC has a compact length of 30 $\mu$ m and adiabatically coupled the expanded mode size to a compressed mode size from input to the output of 3D-SSC. A better than 7 dB improvement in the coupling loss is obtained for the vertical tapered waveguide over a non-tapered waveguide of 4 $\mu$ m rib width. An additional $\sim$ 1 dB improvement in coupling loss is obtained for the vertical and horizontal taper for a bare fiber input. These finding matches very well with the simulated analysis. The fabricated 3D-SSC reduces the insertion loss by 6 dB from that of the non-SSC waveguide for a bare fiber coupling, with a facet coupling loss of 3.33 dB and a large misalignment tolerance. A low coupling loss of 1.85 dB is measured for a lensed fiber. The total propagation loss for a 100 $\mu$ m integrated device is as low as $\sim$ 0.1 dB, making it very suitable for monolithic integration with other optical devices. [ABSTRACT FROM AUTHOR]

The demand for information capacity in data center networks has grown exponentially during past decades. Such growth promotes consideration for adoption of coherent transmission technologies in this application space to achieve current and future high data rate requirements at 400G, 800G, and higher. Coherent systems offer higher capacity than direct detection systems using more degrees of freedom to carry information, but with higher system complexity. To meet the cost and power consumption requirements for data center transceivers, coherent-lite technologies have been proposed. Among them, self-homodyne coherent (SHC) is a promising candidate which allows the use of low cost lasers and simplifies digital signal processing (DSP). We study SHC systems and focus on the use of a multicore fiber (MCF) to obtain small channel skew, which allows for further simplification of DSP. We observe and manage stimulated Brillouin scattering (SBS) effects in the local oscillator path. We analyze the optimal power split ratio between the data path and the local oscillator path to maximize data capacity. Moreover, we compare the channel skew and transmission performance between multicore fiber and ribbon fiber. SHC with MCF delivers high data capacity with significant complexity reduction, which may be a promising solution for data center links. [ABSTRACT FROM AUTHOR]

Plasmonic devices have witnessed their success in optical applications due to the boosted light-matter interactions in the subwavelength scale. Among them, optical fiber-based configurations offer excellent portability and accessibility. Here, we report a novel kind of plasmonic fiber devices by introducing Tamm plasmon polaritons (TPPs) into the fiber-optic community. TPPs are cavity modes confined between a distributed Bragg reflector (DBR) and a metallic layer, resulting in high quality factor ∼100, strong field-intensity enhancement of 92, immunity to ambient perturbations, and flexible tunability. As the fabrication process only requires thin-film deposition, fiber-based TPP system is relatively low-cost and high-repeatability. This scenario extends the scope of plasmonic devices and open up new perspectives for active and passive fiber-optic systems. [ABSTRACT FROM AUTHOR]

There are great challenges in the generation of higher-order modes for long period fiber grating (LPFG). The cross-coupling coefficient between the fundamental mode and the higher-order azimuthal modes is extremely small due to the difficulty of angular modulation with traditional methods. Therefore, how to generate higher-order orbital angular momentum (OAM) modes with a high efficiency and low insertion loss is an urgent problem to be solved. In this paper, an efficient all-fiber approach to generate fourth-order OAM mode by employing a preset twisted LPFG (PT-LPFG) in a few-mode fiber (FMF) is presented. The difficulty of generating fourth-order mode utilizing traditional single-side exposure carbon dioxide laser manufacturing method and the advantages of the PT-LPFG in enhancing the coupling coefficient are theoretically analyzed in detail. In experiment, the conversion from the fundamental mode to the fourth-order mode is achieved, and the fourth-order interference patterns demonstrated the successful generation of the OAM±4 modes. The conversion efficiency of the OAM modes is over 99% and the purity is measured to be more than 90%. To the best of our knowledge, this is the first time to generate the fourth-order OAM mode in an all-fiber system using single long-period fiber grating. [ABSTRACT FROM AUTHOR]

We investigated the X-ray radiation impact on the performances of “backup” erbium-doped fiber amplifiers (EDFAs) and erbium-ytterbium-doped fiber amplifier (EYDFA). These devices are exposed to the space radiation constraints, unpowered in the “OFF” mode, up to their hypothetic activation during the mission. This atypical profile of use must be considered as it could lead to a higher radiation vulnerability. To carry out this study, active optical fibers differing in terms of radiation hardness have been exposed to steady-state X-rays under different profiles of amplifier use: the gain degradation is measured by varying the pumping conditions, that drive the photobleaching (PB) phenomenon, during the radiation exposure: “ON” (100% ON), “OFF” (0% ON), and “OFF–ON–OFF” with powering 2% or 10% of the time. Our results show that the impact of the pump and its related PB efficiency depend on the selected active fiber to build the amplifier. An important result is that the “backup” EDFA designed with radiation-hardened Ce-doped fiber does not suffer from an extra-degradation due to the “OFF” mode, contrary to the one designed with a nonrad-hard fiber. For this latter one, it is indeed possible to reduce the extra-gain degradation by powering the amplifier for short time periods during the “OFF” state; otherwise, the EDFA recovers this extra-gain degradation in a few hours after turning “ON” in our test conditions. For the tested radiation-tolerant EYDFA, an enhanced vulnerability is observed for the “OFF” amplifier, that can be reduced too by briefly powering the amplifier. The origins of these effects are discussed based on the properties of point defects related to the aluminosilicate (EDFA) and phosphosilicate (EYDFA) glasses of the fiber. [ABSTRACT FROM AUTHOR]

The radioluminescence (RL) response under X-rays is investigated for five different types of multimode silica-based optical fibers doped with Ge, P, Al, F, or Ce. The results indicate that all tested fibers show a measurable RL signal at dose rates from 0.1 to 15 Gy(SiO2)/s, using 10-cm-long samples and a photomultiplier-based acquisition chain. Other influences of radiation, such as radiation-induced attenuation, are discussed in order to evaluate the potential of such fiber types for radiation detection or dosimetry applications. [ABSTRACT FROM AUTHOR]

This study presents a highly miniaturized, handheld probe developed for rapid assessment of soft tissue using optical coherencetomography (OCT). OCT is a non-invasive optical technology capable of visualizing the sub-surface structural changes that occur in soft tissue disease such as oral lichen planus. However, usage of OCT in the oral cavity has been limited, as the requirements for high-quality optical scanning have often resulted in probes that are heavy, unwieldy and clinically impractical. In this paper, we present a novel probe that combines an all-fiber optical design with a light-weight magnetic scanning mechanism to provide easy access to the oral cavity. The resulting probe is approximately the size of a pen (10 mm × 140 mm) and weighs only 10 grams. To demonstrate the feasibility and high image quality achieved with the probe, imaging was performed on the buccal mucosa and alveolar mucosa during routine clinical assessment of six patients diagnosed with oral lichen planus. Results show the loss of normal tissue structure within the lesion, and contrast this with the clear delineation of tissue layers in adjacent inconspicuous regions. The results also demonstrate the ability of the probe to acquire a three-dimensional data volume by manually sweeping across the surface of the mucosa. The findings of this study show the feasibility of using a small, lightweight probe to identify pathological features in oral soft tissue. [ABSTRACT FROM AUTHOR]

Orbital angular momentum (OAM) multiplexing technology is expected as one of the prospective candidates for sustaining the further increment of data-transmission capacity in the optical communication systems. Advanced multiplexing technology of combining OAM-based mode division multiplexing (MDM) in the multicore fiber (MCF) with wavelength division multiplexing (WDM) can further improve the transmission rate and spectrum efficiency in the communication systems. In this paper, a multi-ring-air-core fiber (MRACF) supporting thousands of OAM modes is designed and analyzed. The tradeoffs between ring number/density and their corresponding system performance parameters in this fiber are systematically investigated. Based on the analytical results, MRACF with 37 rings can support a record high 4440 radially fundamental OAM modes at 1550 nm, featuring <−53 dB inter-ring crosstalk and >5.67 × 10−4 intra-ring modal effective refractive index difference. Furthermore, a record-high 4144 radially fundamental OAM modes with <−40 dB inter-ring crosstalk can be supported across C and L bands from 1530 nm to 1625 nm, which is one order of magnitude larger than the number of linearly polarized (LP) modes or supermodes ever reported within one single fiber. [ABSTRACT FROM AUTHOR]

We investigatethe potential of space division multiplexing (SDM) for the power-over-fiber (PWoF) technique, in terms of weakly-coupled multicore fiber (WC-MCF) design and experimental verification. Our simulation results indicate that, the SDM-enabled PWoF scheme is superior to the wavelength division multiplexing (WDM) enabled counterpart, due to the scalability of the energy light delivery and the save of wavelength division multiplexers. Then, we experimentally demonstrate the co-transmission of optically carried fifth generation new radio (5G NR) signals at 1550-nm and the 60-W energy light at 1064-nm over the 1-km weakly-coupled seven-core fiber (WC-7CF) fronthaul link. Since the optical power delivery characteristic per the WC-MCF core is similar to that of the standard single-mode fiber (SSMF), the SDM-enabled PWoF scheme can evenly distribute the high-power energy light into multiple spatial channels, to maximize the efficiency of the optical power transition efficiency (OPTE). Consequently, the total collected optical power of the energy light over the 1-km WC-7CF is 4.6 times higher than that of the 1-km SSMF, mainly due to the insertion loss of the fan-in and fan-out (FIFO) devices. Meanwhile, the error-vector magnitude (EVM) of 1.5-Gbit/s 5G NR signal is 0.28%, when the received electrical signal power is −25 dBm. The EVM value of the co-transmitted 5G NR signal with the 60-W energy light only has a fluctuation of 0.01% over six hours. The SDM-enabled PWoF is useful for the optically powered 5G pico-cells, with the capability of high-speed access and centralized management. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a novel method to achieve pulsed operation of thulium-doped fiber lasers at 1.7 μm. In our design, the thulium-doped fiber laser cavity was embedded within the cavity of an erbium/ytterbium-codoped fiber laser operating at 1.5 μm. The thulium-doped fiber in the erbium/ytterbium-codoped fiber laser cavity served as a saturable absorber, forcing the 1.5 μm laser to operate in a passively Q-switched scheme. The 1.7 μm thulium-doped fiber laser was in-turn pumped by the Q-switched 1.5 μm laser and was gain-switched. This made the pulse duration of 1.7 μm emission much shorter than that of 1.5 μm emission. 1.02 W average output power at 1720 nm was obtained for 10 W of pump power at 976 nm. The pulse repetition frequency, pulse energy and pulse duration were 134 kHz, 7.6 μJ and 102 ns, respectively. A rate equation model was developed to examine the power- and temporal-behavior of the laser system. The calculated pulse duration and pulse repetition frequency matched well with the experimental results. Methods of further improving the laser performance are discussed based on insights gleaned from our modeling. [ABSTRACT FROM AUTHOR]

Fiber-optic Quantum clock synchronization has been proposed and experimentally demonstrated for its potential enhancements in precision and better guarantee of security. To further prove its advantage in practical applications, it is necessary to test its synchronization performance between clocks apart for at least intracity optical fiber distance. In this paper, we report a fiber-optic two-way quantum clock synchronization experiment with the distance reaching 50 km. With the common reference clock, a synchronization stability of 54.6 fs at 57300 s and an accuracy of 1.3 ps±36.6 ps have been achieved. With independent reference clocks, comparable performance has also been achieved combined with the microwave frequency transfer technology, which shows a stability of 89.5 fs at 57300 s. These results nicely showcase the feasibility of the two-way quantum clock synchronization to provide femtosecond-scale synchronization precision over intracity optical fiber links. [ABSTRACT FROM AUTHOR]

The silicon photonic integrated network based on the thermo-optical or electro-optical effect generally has the disadvantages of volatile, large footprint, and significant energy loss. Here, we propose and demonstrate an all-optical controllable switch device with non-volatile, broadband adjustable, and bistable fast switching performance by integrating the phase-change material (Ge2Sb2Te5) on the optical fiber. By adjusting the energy of the laser pulse train exerting on the optical fiber, the phase state of the phase-change material can be switched, thereby realizing the “ON” and “OFF” function of the optical switch. As a result, a non-volatile optical switch with a wide bandwidth working range (up to 60 nm) and a significant transmission contrast (2 dB) is demonstrated. This device is expected to help realize fast and broadband optical routing in fiber optical communication networks as the optical switch. Furthermore, its logic operation function can impart optical fiber with new storage and computing functionalities except for transmission and sensing. It can be combined with an optical field-programmable gate array (OFPGA) and play a part in optical fiber photonic circuitry in the future. [ABSTRACT FROM AUTHOR]

We developed a passive silica based large mode area fiber with the nanostructured core with a diameter of 30 μm devoted to fiber Bragg grating inscription for application in an all-fiber laser cavity. The fiber is perfectly matched to the commercial active fiber with NA = 0.06 and core/cladding diameters 30/250 μm. Measured modal characteristics and bending loss of the fiber confirm single-mode operation for bending diameter of 8 cm with bending losses as low as 0.15 dB/m. Although the nanostructured fiber core is low germanium doped, we fabricated high reflectivity (98.5%) UV induced fiber Bragg grating (FBG) and verified its performance in fiber laser setup. The laser cavity formed with highly reflective FBG inscribed in the fiber and Fresnel reflection on the free end of active fiber results in lasing efficiency of 65.5%. The flexibility of the nanostructuring approach gives prospects for the development of free-form large mode area fibers for passive fiber components and matched with any type of active fibers. [ABSTRACT FROM AUTHOR]

Interfacial bonding is significant in determining the mechanical properties and dielectric loss of polymer composites. In this study, NH3 plasma treatment is used to reinforce the interfacial bonding between polyisophthaloyl metaphenylene diamine (PMTA) and the cellulose matrix. The mechanism of modification is investigated via x-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Additionally, molecular dynamics (MD) simulations are performed to explore the microscopic mechanism of the performance improvement of the composite insulation paper. The experimental results show that the mechanical property is improved and dielectric loss decreased to a certain extent after treatment. According to the results of XPS and DFT, the treatment introduces a polar group, −NH2, into the C18 position of PMTA. PMTA treated with NH3 plasma (T-PMTA) is conducive to form H-bonds with cellulose. It increases the interaction energy and strengthens the interfacial bonding, causing the T-PMTA molecules to penetrate wider and deeper on the cellulose surface. Thus, the mechanical properties are improved. Additionally, the improved interfacial bonding impedes the rotation and movement of molecules at the interface, which reduces the degree of freedom of the dipole steering and the resulting low dielectric loss. This study reveals the mechanism of NH3 plasma treatment and provides insights into the microscopic mechanism of the improvement of properties of composite insulation paper at the molecular level. [ABSTRACT FROM AUTHOR]

We report the results obtained by using a newly designed 300-μm-core-diameter Yb-doped fiber with a large cladding absorption of 23 dB/m at 976 nm. An average power of 512 W, a maximum pulse energy of 51.2 mJ laser is realized using the 300-μm-core Yb-doped fiber in the main amplifier with a slope efficiency of 72%. The entire laser system adopts an all-fiber construction with pulse control system. These results extend average power and peak power of nanosecond all fiber-based laser, exceeding to the 500 W and 1 MW regime simultaneously for the first time to the best of our knowledge. [ABSTRACT FROM AUTHOR]

A successful migration from current C-band based optical networks to a multiband scenario primarily depends on the development of solutions that can reliably measure physical properties of optical links over broad spectral transmission windows. Additionally, these solutions must be capable of delivering wavelength-dependent and spatially-resolved indicators that can empower network operators to identify faults before they lead to severe service disruptions. Recently, the exploitation of receiver based digital signal processing as a tool for optical performance monitoring has gained tremendous popularity. One successful example is the so-called in-situ power profile estimator, which can reconstruct the per-channel longitudinal power profile along the optical fiber link solely processing the received signal samples. In this work, we propose a novel application for the in-situ power profile estimator by harnessing it on multiple wavelengths to accurately estimate the spectral gain profile of C+L-band in-line Erbium-doped fiber amplifiers deployed in a 280-km single mode fiber link. Furthermore, we show how this scheme can be efficiently used to detect amplification-related anomalies, such as gain tilt and narrowband gain compression. In our measurements, we achieved a sub-dB estimation accuracy by comparing the proposed gain extraction approach with the back-to-back characterization obtained from an optical spectrum analyzer. [ABSTRACT FROM AUTHOR]

• Utilize two distinct spectral structures for detection of hydrogen and temperature. • Optimized sensitivity by modifying the device dimensions via wet etching technique. • Be reusable thanks to the reversable reactions between H 2 and palladium. • Eliminate the spark/discharge that may induce explosion based on electric signals. As decarbonization is urgent in global climate, hydrogen, of which the product is pure water after burning, becomes a potentially clean and sustainable energy source as a replacement for fossil fuels. However, the hydrogen storge is challenging due to the highly flammable and explosive properties, limiting its enormous applications in industries. Real-time and sensitive detection of hydrogen is thus imperative to avoid leakage accidents, while the sensors based on electric signals have the risk of spark or arc discharge which may induce explosion. Here, we propose and experimentally demonstrate an optical hydrogen sensor using a functionalized superstructure of Bragg gratings in a helical-core fiber with a thin palladium film. Due to the cross sensing of the two sets of gratings, simultaneous detection of hydrogen concentration and temperature can be realized. By modifying the device diameter using wet etching technique, the sensitivity of hydrogen concentration is enhanced by 2.7 times. The utilization of the superstructure of Bragg gratings sampled in the helical-core fiber provides a promising platform for reliable and safe sensing of hydrogen with the ability of simultaneous monitoring of temperature. [ABSTRACT FROM AUTHOR]

Fiber optic sensors are applied in industry, remote sensing, environmental monitoring and healthcare. A special place is occupied by tilted fiber Bragg gratings, which can significantly expand the capabilities provided by standard Bragg sensors. But these gratings have complex spectral responses, therefore, data processing becomes a critical task for achieving maximum performance. In this paper, machine learning methods for processing spectral data of a plasmonic fiber sensor based on a tilted fiber Bragg grating were applied for the first time for the measurement of small refractive index changes. The responses of two similar but not identical sensors were measured in two independent experiments. The model trained on the data of the first sensor was used to analyze data obtained with another sensor. The best resolution achieved in our experiments was $9 \times {10^{ - 6}}$ RIU. [ABSTRACT FROM AUTHOR]

Few Mode Fibers (FMF) and Space DivisionMultiplexing (SDM) are an attractive solution to offer high capacity in optical networks. Although transmission along FMF presents several issues mainly due to the cross-talk among modes, the use of Multiple Input Multiple Output (MIMO) coherent receivers permits to limit the impact of such physical impairment. However, the complexity of MIMO is not negligible (especially with a large number of modes) and the spatial modes must cover the same path, thus limiting network flexibility, e.g., routing modes along different paths is not admitted. In this paper we exploit the concept of Mode-Group Division Multiplexing (MGDM) and we investigate the network architecture and provisioning supporting MGDM. Modes are divided in groups: the modes within a group are co-routed and received with a reduced-complexity MIMO receiver, while the different groups can be routed along different paths. Different node architectures supporting MGDM are presented taking in account state-of-the-art components and devices, even commercially available on the market. A quality of transmission (QoT) model is also presented accounting to the inter mode group crosstalk, that cannot be neglected in the reach evaluation. QoT is exploited by a proposed connection provisioning scheme for MGDM. Simulations are carried out in metro/rural network scenarios with different link span lengths. Simulations show high throughput increase while limiting the complexity of receivers. [ABSTRACT FROM AUTHOR]

The rod in the composite insulator is a load-bearing component, and glass-fiber-reinforced plastic (GRP) has always been regarded as the most suitable material. However, the investigation reveals that the performance shortcomings of GRP rods are the main cause of emergency heating defects of 131 new 500-kV ac composite insulators in South China. The finite-element simulation is conducted to understand the potential mechanism of heating. It is demonstrated that the holes with moisture distort the electric field, increasing heating power. A hole with moisture in the GRP rod longer than 500 mm could lead to emergency heating defects with temperature rise exceeding 5 K. For the holes in the GRP rod are formed by adjacent voids, the emergency heating defect is inevitable unless the porosity decreases. The fiber infiltration process of the GRP rods is analyzed on the basis of Darcy’s Law, and an improved process is proposed to reduce porosity. Maintaining the mass fraction of fiber by at least 70% and replacing the damp infiltration fluid in time in the production of GRP rods are necessary to restrain porosity. 300-h water diffusion test with a 50- $\mu \text{A}$ threshold is an effective evaluation method of the GRP rod performance. This method should be used before putting composite insulators into operation. [ABSTRACT FROM AUTHOR]

The parts of working machinery have always suffered from various working conditions in the process of use (e.g., high temperature, high humidity, and high pressure) that may generate flaws, pitting, thermal fatigue crack and other types of defects on the surface or inside. Because these tiny defects carry big safety risks, regular detection is necessary to ensure the reliability of the production equipment. [ABSTRACT FROM AUTHOR]

The increase in communications network traffic has led to a demand for a larger transmission capacity. To meet this demand, the combination of wavelength division multiplexing (WDM) and space-division multiplexing (SDM) is being extensively investigated. Among various SDM systems, that based on multi-core fiber is attractive because network flexibility increases while it maintains the transmission capacity to be enlarged in proportion to the number of SDM channel number. However, problems arise because the number of input/output ports increases dramatically in optical nodes. To solve these problems, there is a novel optical configuration technology, the spatial and planar optical circuit (SPOC), which makes the integration of multiple WSSs and filters possible. The SPOC platform has been used to achieve variety of devices such as optical dispersion compensation and WSSs. One of the great advantages of SPOC platform is that we can integrate multiple optics of WSSs in a module, which contribute to miniaturization of devices. In this paper, we review the devices for SDM networks, which are realized with SPOC platform including WSS arrays, wavelength cross-connects (WXCs), multi-channel optical performance monitors, and its related technology. [ABSTRACT FROM AUTHOR]

We investigate recirculating transmission of a wideband S-, C- and L-band signal in a 4-core fiber. We transmit 552 x 25 GHz spaced, 24.5 GBd channels, covering more than 120 nm bandwidth from 1487.8 nm to 1608.33 nm through each core of the multi-core fiber (MCF), which has the same 125μm cladding dimeter as standard single-mode fiber. We first use PDM-16QAM modulation and measure performance at distances up to 3001 km, corresponding to 43 spans of the 69.8 km MCF. At this distance we measure a decoded throughput of 319 Tb/s or 342 Tb/s when estimating from the generalised mutual information (GMI). Extending the transmission distance to 8027 km, a selection of PDM-QPSK modulated channels across the spectrum were received with an average GMI estimated data-rate of 83.7 Gb/s. These results show that wideband transmission including the S-band has the potential to increase data-rates in long-haul optical fiber transmission. Further, our experiment confirms that 125 μm diameter MCFs can multiply transmission capacity whilst offering benefits of resource sharing and integration, expected in SDM systems. [ABSTRACT FROM AUTHOR]

Silicon-based optical media present several desirable properties for a wide range of applications. Herein, we propose a novel hybrid integration scheme that combines a silicon photonics platform and a 5.5%-Δ-silica planar-lightwave circuit (PLC) platform. By exploiting the performance advantages of each platform, we fabricated a polarization-insensitive low-crosstalk 8 × 8 silicon photonics switch butt-jointed with a compact 5.5%-Δ-silica PLC-based 100-GHz 8-channel arrayed waveguide grating (AWG). The device was driven by a smartphone-sized (9 cm × 13.5 cm) control board. The fabricated device exhibits a fiber-to-fiber insertion loss of 12.6 dB, an average polarization-dependent loss of less than 0.57 dB, and less than −40 dB leakage to non-target output ports. We also demonstrate two uses of the proposed device. The first is the “DEMUX and Switch” operation, in which the spectrally divided light by the AWG is routed to an arbitrary output port by a subsequent switch. The second is the “Switch and MUX” operation, in which arbitrary wavelengths from arbitrary input ports are merged by the AWG. No spectral degradation was observed in either operation. These results demonstrate the potential of the 5.5%-Δ-PLC/silicon photonics hybrid platform for compact, low-power, and fast-switching applications. [ABSTRACT FROM AUTHOR]

We analyze the potential suitability or strength of arguments for the application of multicore optical fibers in high capacity submarine cable systems via transmission and techno-economic modeling. We consider hypothetical multicore fibers (MCFs) with 2–4 weakly coupled cores and compare capacity and cost/bit against conventional single-core fibers (SCFs). The analysis is performed in the context of a trans-Atlantic link length system and we evaluate the relative fiber performance with three different, but related, system design approaches. Two SCF coating diameters are assessed in terms of how this parameter affects the cost/bit through fiber density in submarine cables and resulting cable cost. We find that MCFs may enable higher cable capacity when fiber pair limits are imposed, but likely not at lower cost/bit unless optimistic and best case assumptions are made with respect to MCF relative fiber cost. We also find that reduced diameter SCFs can deliver much of the density and cable cost savings that motivates interest in MCF without the challenges of a new eco-system as required by MCF. However, MCF may enable the design of the largest cable capacities such as 1 Pb/s or more that might not be attainable with SCFs without significant cable changes. [ABSTRACT FROM AUTHOR]

Optical frequency domain reflectometry (OFDR) is a family of optical techniques which can be used to produce distributed temperature measurements from the spectral shift of an interference pattern based on the Rayleigh backscatter signature of an optical fiber. Adaptive signal processing techniques have recently been used with OFDR to record meaningful spectral shift data from commercially available SMF-28 optical fibers heated beyond 950 °C. However, a correlation between the measured spectral shift and temperature has not yet been developed at these high temperatures. To extend the measurable temperature range of OFDR in SMF-28, this work describes the development of such a correlation from room temperature (22 °C) to 1000 °C. The relationship between spectral shift and temperature change over this range was found to be best characterized by the fourth-order polynomial $\Delta {T}=(-4.241\ast 10^{-11})\text {S}^{4} +(-2.017\ast 10^{-7})\text {S}^{3} +(-3.677\ast 10^{-4})\text {S}^{2}+(-0.8057)\text{S}$ , where $\Delta {T}$ represents the temperature difference compared to the reference temperature, and S represents the spectral shift measured by the fibers. The calibration developed in this work assumes that the fiber has been fully annealed by heating the fiber to 1000 °C for a few hours. This paper is the first to demonstrate the calibration and use of SMF-28 distributed optical fiber sensors up to 1000 °C, enabled using adaptive OFDR-based signal processing. [ABSTRACT FROM AUTHOR]

We present a compact polarization splitter on a dual hollow-core antiresonant fiber with broad bandwidth covering two well-known communication wavelengths of $1.31~\mu \text{m}$ and 1.55 $\mu \text{m}$. It can achieve excellent polarization splitting performances in a simpler optimized single-layer geometry compared to the related splitters and for the first time, simultaneously offers a very short splitter length of 21.52 mm and a broad bandwidth of 340 nm by maintaining an extinction ratio higher than 20 dB. It also achieves the highest extinction ratio of 140 dB at $1.55~\mu \text{m}$ and maintains a standard single mode performance. Therefore, the proposed splitter has the potentiality to be a promising candidate for the integrated optical devices in communication systems. [ABSTRACT FROM AUTHOR]

A small-coating-diameter fiber is an attractive solution for realizing ultrahigh-density cables. A 6,912-fiber high-density cable with 200-μm-coating fibers was realized with a diameter that was 16% smaller than that with 250-μm-coating fibers. We demonstrated the applicability of a 160-μm-coating fiber with 80-μm cladding to further reduce the cable diameter. The combination of cladding and coating diameters was chosen in terms of the micro-bending sensitivity and abrasion resistance of an optical fiber. The simulated micro-bending sensitivity of the 160-μm-coating fiber was reduced to a level similar to that of a 200-μm-coating fiber by reducing the Young's modulus to one-third that of the current material. The fabricated 160-μm-coating fibers exhibited the expected micro-bending sensitivity. A 1,728-fiber cable with 12-fiber ribbons comprising the fabricated 160-μm-coating fibers realized a reduction in cable diameter of 26% compared with that using 200-μm-coating fibers, exhibiting good attenuation characteristics over a heat cycle test and satisfying the general requirements for optical fiber cables. We realized a fusion splice between 12-fiber ribbons of 160-μm fiber and a multi-fiber push on (MPO) connector between 80-μm- and 125-μm-cladding fibers. The connection technologies facilitated the application of the proposed 160-μm-coating fiber to existing systems whose interface was a conventional 125-μm-cladding fiber. [ABSTRACT FROM AUTHOR]

New transmission techniques for short-reach optical transmission applications especially datacenter interconnections are highly expected to break current bandwidth bottleneck, among which mode division multiplexing (MDM) based on few-mode fiber (FMF) is promising by exploring different linearly-polarized (LP) modes as spatial transmission channels. In this paper, an FMF design utilizing multiple-ring perturbations in fiber core to suppress modal crosstalk during fiber transmission is firstly proposed and a 6-LP-mode double-ring-core (DRC) FMF is fabricated with a high minimum refractive index difference of 1.49 × 10−3 among all LP modes. Secondly, a novel degenerate-mode-selective coupler (DMSC) converting each degenerate LPlm (l > 0) mode into the LP11 mode in a two-mode fiber is proposed. Then intensity-modulation/direct-detection (IM/DD) MDM transmission scheme without any digital signal processing (DSP) can be realized utilizing mode multiplexer/demultiplexer (MUX/DEMUX) consisting of cascaded all-fiber mode-selective couplers (MSC) and DMSCs, in which each degenerate or non-degenerate LP mode is considered as an independent transmission channel. Based on the proposed DRC-FMF and mode MUX/DEMUX, we demonstrate a 4-LP-mode MDM prototype system using commercial single-mode 10-Gbps SFP + modules and 4K video transceivers without any hardware or software modifications. Finally, the intra-LP-mode dispersion effect in a weakly-coupled FMF is analyzed and a modified fixed-analyzer method is proposed for its measurement. Experimental results show that it may be one of the major impairments for IM/DD MDM transmission. [ABSTRACT FROM AUTHOR]

In this paper, the axis-aligned single mode- four mode- single mode (SFS) fiber structure filter is proposed to be an all-optical temporal differentiator. It's shown that differentiators with different orders can be achieved by adjusting the parameters of the axis-aligned SFS fiber structure filter. All-optical temporal differentiators with orders of 0.375, 0.77 and 1 are obtained by using the axis-aligned SFS fiber structure filter, of which processing errors are 8.08%, 2.81%, 0.74%, respectively. The proposed design provides an easy and competitive way for the realization of the all-optical temporal differentiator.

A two-dimensional tapered fiber core mimicking a commonly used tapered fiber coupler is proposed and experimentally demonstrated in a piece of single-mode fiber (SMF) for whispering gallery mode (WGM) excitation. Tapering the fiber core with femtosecond laser ablation can simultaneously enhance the evanescent field and decrease the effective refractive index of the core mode, which lowers the phase mismatch between the core mode and the WGM in a spherical cavity with a diameter less than the fiber cladding radius. Experimental results show that symmetric Lorentzian dip with a quality factor (Q-factor) of $3.6\times 10 ^{3}$ and asymmetric Fano-like lineshape can be achieved. Such a tapered fiber core provides an effective coupling method for a single WGM based photonic device or WGM array-based lab in-fiber due to its inherent compatibility with fiber optic hardware, high integration, and robustness. [ABSTRACT FROM AUTHOR]

Annealing effects on optical losses in silica optical fiber drawn from 3D-printed preform were investigated. An irreversible reduction in loss was observed through multiple annealing cycles at a temperature $T =$ 800 ° C. Changes in Raman spectroscopy confirm densification of the core as the stresses in glass relax with annealing. The temperature-dependent loss in the 3D printed fiber is consistent with modal field overlap at the core-cladding interface. [ABSTRACT FROM AUTHOR]

We present a detailed statistical study on achievable reach of 100 Gbps data center optical links based on vertical cavity surface emitting lasers (VCSEL) and multimode fibers (MMF). Based on the characterization of the spectral and spatial properties of eight lasers and of the modal and dispersion behavior of a large set of 20233 OM3 and OM4 modeled fibers (obtained by properly extending an initial set of 500 measured fibers), we compute the resulting frequency responses of all of the VCSEL-MMF combinations. Then, we feed them to a numerical tool modeling PAM-4 transmission at 100 Gbps net bit rate per wavelength. Our model analyzes performance at distances up to 400 meters, using three different adaptive equalizers at the receiver and considering two forward error correction overheads. We show that 100 Gbps operation is feasible for 99% of the simulated links reaching up to 120 m over OM4 at 850 nm and using a decision feedback equalizer (DFE). Aggregated data rates of 200 Gbps and 400 Gbps per fiber using Shortwave Wavelength Division Multiplexing (SWDM) are achievable for 99% of the links reaching 80 m over OM4 using two wavelengths and feed-forward equalizer (FFE) and four wavelengths and maximum likelihood sequence estimation (MLSE)-based equalizer, respectively. [ABSTRACT FROM AUTHOR]

Optical frequency domain reflectometry measurements in internally clad single crystal sapphire fiber have received attention in recent years due to their high temperature distributed sensing potential. As work with these fibers has proceeded, there have been some inconsistencies in the results. Deeper investigation and testing has identified two critical considerations for the proper functioning of these fibers. First, users must address the propagation of multimode light along the outer surface of the fiber. By observing the far-field image of an internally clad sapphire fiber when adding index matching fluid to the outer fiber surface, we demonstrate the effects of removing the higher order modes from these fibers. The addition of index matching fluid resulted in nearly single mode performance where multimode performance was previously observed. Second, users must address the effect that coupling the fiber to the interrogator via silica based fiber has on the internally clad sapphire fiber's performance. Direct fusion splicing of silica to sapphire, as has been used in the recent work with these fibers, has a mode filtering effect which can be beneficial towards the modal behavior of the fibers. However, in this paper we demonstrate that the splicing can cause a sensing failure due to little or no low order mode light, that is useful for sensing, returning to the detector. The positive results from recent years have demonstrated that optical frequency domain reflectometry sensing performance will be successful in clad sapphire fiber; but only when the considerations described herein are addressed properly. [ABSTRACT FROM AUTHOR]

We proposed a silver-coated 45° radiated tilted fiber grating (45° RTFG) based interferometer, in which the 45° RTFG is working as a beam splitter and a coupler, and the light transmitting in the fiber core is radiated out and resonant in the cladding radial cavity formed by the fully coated silver film, which forms multiple beam interference. We have theoretically and experimentally investigated the proposed fiber interferometer in terms of polarization-dependent transmission spectra, the free spectral range (FSR), spectral visibility and sensing characteristics. Due to the single polarization radiation feature of 45° RTFG, only S-polarized light could form the multiple beam interference and P-polarized light would pass through the grating with low loss. And the FSR of the proposed fiber interferometer is around 6.8 nm, which is determined by the 125 μm cladding diameter. The theoretical results show that the spectral visibility of interference is proportional to the coupling efficient of 45° RTFG and when the coupling efficient reaches 0.92, the spectral visibility is the maximum. In the experiment, the fiber interferometer formed by the 45° RTFG with 11 dB polarization extinction ratio has the spectral visibility of 26 dB. Moreover, we have also investigated the temperature, strain and twist characteristics of the proposed interferometer. The experimental results show that the transmission peak has a red shift with increasing temperature, with a sensitivity of 10 ± 0.1 pm/°C; The strain causes a blue shift for the transmission peak with −0.42 ± 0.01 pm/μϵ sensitivity, and the twist sensitivity reaches 22.408 ± 0.2 dB/rad. The proposed silver-coated 45° RTFG based interferometer has an in-fiber and compact structure, which can be potentially used for optical fiber sensing. [ABSTRACT FROM AUTHOR]

Compared to fiber-based equivalents, CMOS-compatible silicon-based nonlinear optical devices harness the superior features of lower operating powers, precise dispersion control and extensive transparency range. In this article, we propose a scheme of cascaded soliton compression on CMOS-compatible Silicon Nanophotonic Wire Waveguides in the mid-infrared region which includes the wavelength of 2.0 μm and 2.8 μm. Thanks to the cascaded pulse compression mechanism, we demonstrate the large compression factor on an integrated chip-scale waveguide. We can realize a high degree pulse compression that the pulse width decreases from 1 ps to 24.0 fs with a high compression factor of 41.7 and a low operating peak power of 1.56 W at 2.0 μm. In addition, even as the wavelength migrate to 2.8 μm, our scheme is equally effective. We achieve 31.9 compression factor of 1 ps chirp-free higher-order soliton pulse which can be compressed to 31.3 fs with a small initial peak power of 4.87 W. This research indicates the potential of dispersion engineering waveguides for mid-infrared low-power applications, such as ultrafast integrated lasers, chip-scale frequency-comb generation, minimally invasive skin surgeries and molecular spectroscopy. [ABSTRACT FROM AUTHOR]

We present simple, low loss and broadband mode scramblers for mode division multiplexed (MDM) transmission based on few-mode fibers. By simply shortening the length of the long-period fiber grating (LPFG), the optical bandwidth is significantly enhanced and >260 nm bandwidth is predicted in our simulations with a grating length of 0.613 cm. In an experimental demonstration we fabricate a mechanically induced LPFGs using a commercially available 3-dimentional (3D) printing technique and wideband operation is confirmed over the C band with a low loss (0.2 dB) and low mode dependent loss (0.1 dB). [ABSTRACT FROM AUTHOR]

Using the Jones formalism, it is shown that electro-optic modulators used for phase modulation generate a modulation of the output polarization induced by the difference of phase modulation depth along the crystallographic axes of the modulator. We study two consequences of this polarization modulation in the fiber setups, limiting the performance of high sensitivity measurement devices. The first one is its partial conversion into a residual amplitude modulation (RAM) within any component presenting polarization dependent losses (PDL). The second one is a new effect that consists of the distortion of the signal detected at the output of a fiber cavity. The theoretical expressions of the detected signals are computed in each case. [ABSTRACT FROM AUTHOR]

The use of in-fiber core-to-cladding coupling components for thermal heating purposes has been well assessed in the last decades within the development of fiber optic devices for flow measurements and water thermal conductivity calculation. In these devices, light travelling in the fiber core is transferred into the cladding and absorbed by a metallic layer surrounding the fiber, with the consequent resistive heating generation. Here we demonstrate the use of a Turn-Around-Point (TAP) Long Period Grating (LPG) as resonant core-to-cladding light coupling mechanism for the fabrication of a highly efficient heating device based on metallic coated Fiber Bragg Grating (FBG). A properly designed TAP LPG was fabricated by means of point-to-point UV laser and spliced to a 150 nm thick Au-coated FBG. The heating efficiency characterization of the final device was analyzed, in both air and water, by evaluating the temperature increase in the gold layer surrounding the FBG at incremental values of the injected power. Collected results confirm that the use of LPGs involving the excitation of higher order cladding modes provides an excellent transferring mechanism of the fiber core light into the cladding, which in turn guarantees very high thermal heating efficiency to the final device. Moreover, by comparing such results with the performance of other in-fiber core-to-cladding coupling components already presented in literature, it was found that the TAP LPG-based device exhibits an actuation efficiency 2.5 times greater, thus resulting the most effective and highly performing solution for energy transfer to the metallic overcoat. [ABSTRACT FROM AUTHOR]

Quantitative assessment of myocardial stiffness is crucial to understand and evaluate cardiac biomechanics and function. Despite the recent progresses of ultrasonic shear wave elastography, quantitative evaluation of myocardial stiffness still remains a challenge because of strong elastic anisotropy. In this paper we introduce a smart ultrasound approach for non-invasive real-time quantification of shear wave velocity (SWV) and elastic fractional anisotropy (FA) in locally transverse isotropic elastic medium such as the myocardium. The approach relies on a simultaneous multidirectional evaluation of the SWV without a prior knowledge of the fiber orientation. We demonstrated that it can quantify accurately SWV in the range of 1.5 to 6 m/s in transverse isotropic medium (FA < 0.7) using numerical simulations. Experimental validation was performed on calibrated phantoms and anisotropic ex vivo tissues. A mean absolute error of 0.22 m/s was found when compared to gold standard measurements. Finally, in vivo feasibility of myocardial anisotropic stiffness assessment was evaluated in four healthy volunteers on the antero-septo basal segment and on anterior free wall of the right ventricle (RV) in end-diastole. A mean longitudinal SWV of 1.08 ± 0.20 m/s was measured on the RV wall and 1.74 ± 0.51 m/s on the septal wall with a good intra-volunteer reproducibility (±0.18 m/s). This approach has the potential to become a clinical tool for the quantitative evaluation of myocardial stiffness and diastolic function. [ABSTRACT FROM AUTHOR]

Advanced applications in optical frequency metrology demand improved tunable lasers with high coherence. Herein, a tunable single-frequency fiber laser based on the longitude-purification induced by the distributed feedback has been demonstrated. The key device in the proposed laser structure is a distributed self-injection feedback structure (DSIFS) which function as a mode selector to have a robust frequency-domain mode suppression and wavelength adaptability. Our work is distinctly focused on the theoretical analysis of the formation process of single-longitudinal-mode (SLM) laser output and the wavelength adaptive characteristics in the DSIFS. In this experiment, the side mode suppression of the resonant longitudinal modes can be achieved in a fiber ring laser without any accurate control of the main cavity. The fiber laser obtained a compressed laser linewidth of 855 Hz, enhanced side mode suppression ratio (SMSR) of ∼67 dB, and frequency noise suppression of 40 dB. Furthermore, the fiber laser can be tuned over the entire flat gain region. The measured output power, wavelength and SMSR variations of the proposed laser over a long-term observation are less than 0.034 mW, 0.028 nm and 2.53 dB, respectively. In addition, the SLM operation of the laser can also be obtained at different pump power for every wavelength channel. Moreover, this method is applicable to any other gain-type lasers, indicating that the longitude-purification mechanism has a broad application prospect in other highly coherent tunable lasers. [ABSTRACT FROM AUTHOR]