Traditional Single-mode Fiber Research Articles

A High-Energy, Wide-Spectrum, Spatiotemporal Mode-Locked Fiber Laser.

In this article, we demonstrate a high-energy, wide-spectrum, spatiotemporal mode-locked (STML) fiber laser. Unlike traditional single-mode fiber lasers, STML fiber lasers theoretically enable mode-locking with various combinations of transverse modes. The laser can deliver two different STML pulse sequences with different pulse widths, spectra and beam profiles, due to the different compositions of transverse modes in the output pulses. Moreover, we achieve a wide-spectrum pulsed output with a single-pulse energy of up to 116 nJ, by weakening the spectral filtering and utilizing self-cleaning. Strong spatial and spectral filtering are usually thought to be necessary for achieving STML. Our experiment verifies the necessity of spatial filtering for achieving STML, and we show that weakening unnecessary spectral filtering provides an effective way to increase the pulse energy and spectrum width of mode-locked fiber lasers.

Dead Zone Fault Detection Optimization Method for Few-Mode Fiber Links Based on Unexcited Coupled Higher-Order Modes

Dead Zone Fault Detection Optimization Method for Few-Mode Fiber Links Based on Unexcited Coupled Higher-Order Modes

Ultrasensitive RI and temperature sensor based on cascaded microfiber MZIs with Vernier effect

A sensitivity amplified optical fiber sensor based on Vernier effect by cascaded microfiber Mach-Zehnder interferometers (MZIs) is proposed and demonstrated for the simultaneous measurement of refractive index (RI) and temperature. The traditional single mode fiber is tapered into micrometer near the dispersion turning point to constitute MZI, and then cascaded to generate Vernier effect for the sensitivity enhancement. A novel phenomenon that the envelope of Vernier effect shifted in opposite directions decided by the free spectral ranges of the sensing and reference arms is clarified and demonstrated for the first time. Experimental results show that the RI sensitivity can be improved from 944 nm/RIU of single MZI to 18504 nm/RIU of Vernier effect, while the temperature sensitivity can be improved from −0.53 nm/℃ of single MZI to 9.49 nm/℃ of Vernier effect. Moreover, the proposed sensor with the merits of low hysteresis effect, good repeatability, high stability and good reliability, making it of great potential for biochemical trace detections without temperature-induced cross-sensitivity.

Low crosstalk 125 μm cladding heterogeneous six-core few-mode fiber with air-hole array and trench structure

In order to be compatible with the current optical fiber communication network, we propose a 125 μm diameter heterogeneous six-core two linear polarization modes fiber with air-hole array and trench (HAT) structure. The coupled power theory and finite element method (FEM) are used to investigate inter-core crosstalk (ICXT), bending loss, and other properties. The HAT structure can effectively suppress ICXT; and the maximum ICXT is less than −38 dB/100 km in C + L band. The effective refractive index difference (Δneff) between modes is greater than 3 × 10-3, inter-mode crosstalk (IMXT) can be ignored. The maximum intrinsic loss is 0.187 dB/km; relative core multiplicity factor reaches 32.21. The relative spatial efficiency is 18 times that of traditional single-mode fibers. This fiber can be produced by chemical vapor deposition and existing optical fiber manufacturing technology. The designed fiber can be directly connected to the communication network to reduce the cost of equipment renewal, and the communication capacity can be increased by 18 times.

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

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

The effect of fusion current on thermally diffused expanded core of splicing ultra-low loss fiber and traditional single-mode fiber

Ultra-low loss (ULL) optical fiber is already being used at high altitude in optical communication due to the stable performance in long haul links. Whereas, traditional single-mode optical fiber (SMF) is still used in most general areas since the high price of ULL fiber. Accordingly, there will be an increasing number of junctions between SMF and ULL fibers. The main contents of this work are to study the influence of fusion current on the splice loss caused by thermally diffused expanded core (TEC) of splicing SMF and ULL fibers, and the variations on the diffusion of core dopants and fusion time are also taken into account. Furthermore, the analysis was verified by experiment and simulation. The results illustrate that the changes in fusion current will cause varying degrees of changes in the core and lead to different splice losses due to diffusion, which plays a guidance role in the current calibration of fusion splicer in the future when splicing SMF and ULL fibers. The significance of this work is to reduce the splice loss of SMF and ULL fibers. If the method is applied in a modern fusion splicer, the splice loss of SMF and ULL fibers will be deduced further.

Mode division multiplexing: from photonic integration to optical fiber transmission [Invited

To overcome the capacity crunch of optical communications based on the traditional single-mode fiber (SMF), different modes in a few-mode fiber (FMF) can be employed for mode division multiplexing (MDM). MDM can also be extended to photonic integration for obtaining improved density and efficiency, as well as interconnection capacity. Therefore, MDM becomes the most promising method for maintaining the trend of “Moore’s law” in photonic integration and optical fiber transmission. In this tutorial, we provide a review of MDM works and cutting-edge progresses from photonic integration to optical fiber transmission, including our recent works of MDM low-noise amplification, FMF fiber design, MDM Si photonic devices, and so on. Research and application challenges of MDM for optical communications regarding long-haul transmission and short reach interconnection are discussed as well. The content is expected to be of important value for both academic researchers and industrial engineers during the development of next-generation optical communication systems, from photonic chips to fiber links.

Study of Temporal Thermal Response of Microfiber Bragg Grating

Fiber Bragg grating has been successfully fabricated in the silica microfiber by the use of femtosecond laser point-by-point inscription. Temporal thermal response of the fabricated silica microfiber Bragg grating has been measured by the use of the CO2 laser thermal excitation method, and the result shows that the time constant of the microfiber Bragg grating is reduced by an order of magnitude compared with the traditional single-mode fiber Bragg grating and the measured time constant is ~ 21ms.

Experimental demonstration of multi-parameter sensing based on polarized interference of polarization-maintaining few-mode fibers.

Polarization-maintaining few-mode fibers (PM-FMFs) have found applications in sensors with attractive features that traditional single-mode fibers do not possess. We propose a measurement method for multi-parameter sensing based on PM-FMFs. This method is mainly based on the polarized interference effect of PM-FMF. We experimentally demonstrate the principle of the simultaneous sensing of temperature, strain and transverse force. The sensor has been verified by three groups of multi-parameter measurement experiments showing that the changes of multi-parameter derived from the wavelength shift of the polarized interference spectrum are consistent with the multi-parameter changes set in the experiments. The proposed PM-FMF based sensor has a temperature sensitivity of 0.3 nm/°C, a strain sensitivity of 0.01 nm/µε and a transverse force sensitivity of 0.0065 nm/(N/m).

Refractive Index Sensor Based on Fiber Bragg Grating in Hollow Suspended-Core Fiber

A fiber Bragg grating (FBG) has been fabricated in a hollow suspended-core fiber (HSCF). The HSCF is suitable for refractive index sensor because its core is exposed in the large air hole, which can be used as a microfluidic channel. The refractive index sensitivity of the grating is 8.9 nm/RIU in the range from 1.41 to 1.44. The strain and temperature sensitivities of the HSCF-FBG are 0.8648 pm/με and 9.089 pm/°C, respectively, which are similar to the traditional single-mode fiber FBGs. Remarkably, the bending characteristic of HSCF-FBG has a great dependence on the bending direction and the maximum bending sensitivity is 84.11 pm/m-1. Therefore, the proposed HSCF-FBG can be also utilized as a bending sensor with direction recognition.

Thermally guided Yb-doped fiber-rod amplifier and laser

In this paper, we describe a thermally guided fiber-rod amplifier and laser oscillator, which exploit thermal guiding to achieve mode control. This power scalable approach operates with mode sizes that are typically an order of magnitude larger than traditional single-mode fibers. Operating as an amplifier, we achieve a small-signal gain of 5 dB at 1030 nm whilst maintaining excellent beam quality (M2 < 1.1). In a laser configuration, we obtained 13.1 W of diffraction-limited (M2 < 1.1) output power at 1032.5 nm, with a slope efficiency 53% with respect to absorbed power. A model predicting beam evolution through the thermally guided fiber-rod gain medium is presented and validated.

Optimum splicing of high-index core microstructured optical fibers and traditional single-mode fibers using improved field model

Traditional fusion splicing technologies are not usable for the high-index core triangular lattice microstructured optical fibres (MOFs), as the characteristic air-holes pattern often collapse during splicing, significantly increasing splice losses due to distortion of the waveguiding structure. The low-loss splicing of MOF and the traditional single-mode fibre (SMF) can be achieved by enlarging the mode-field diameter (MFD) of MOF, facilitating to an optimum mode-field match at the splicing interface. Using our recently developed improved field model Sharma and Sharma (2016), we aim to investigate the modal characteristics of high-index core MOF as a mode-field expander, and low-loss splicing between an MOF and the standard step-index single-mode fiber by using controlled all air-hole collapse method. For comparison, experimental and the numerical simulation results have been included, as available in the literature. Relative errors are also given.

Demonstration of Single-Mode Multicore Fiber Transport Network With Crosstalk-Aware In-Service Optical Path Control

Multicore fiber (MCF) transmission is considered as one of the promising technologies for breaking the capacity limit of traditional single mode fibers. Managing the crosstalk (XT) and configuring optical paths adaptively based on the XT as well as achieving longer distance and larger capacity transmission are important, because intercore XT could be the main limiting factor for MCF transmission. In a real MCF network, the intercore XT in a particular core is likely to change continuously as the optical paths in the adjacent cores are dynamically assigned to match the dynamic nature of the data traffic. If we configure the optical paths while ignoring the intercore XT value, the Q-factors may become excessive. Therefore, monitoring the intercore XT value continuously and configuring optical path parameters adaptively and flexibly are essential. To address these challenges, we develop an MCF transport network testbed and demonstrate an XT-aware traffic engineering scenario. With the help of a software-defined network controller, the modulation format and optical path route are adaptively changed based on the monitored XT values by using programmable devices such as a real-time transponder and a reconfigurable optical add–drop multiplexer.

High Sensitivity Refractometer Based on Reflective Smf-Small Diameter No Core Fiber Structure.

A high sensitivity refractive index sensor based on a single mode-small diameter no core fiber structure is proposed. In this structure, a small diameter no core fiber (SDNCF) used as a sensor probe, was fusion spliced to the end face of a traditional single mode fiber (SMF) and the end face of the SDNCF was coated with a thin film of gold to provide reflective light. The influence of SDNCF diameter and length on the refractive index sensitivity of the sensor has been investigated by both simulations and experiments, where results show that the diameter of SDNCF has significant influence. However, SDNCF length has limited influence on the sensitivity. Experimental results show that a sensitivity of 327 nm/RIU (refractive index unit) has been achieved for refractive indices ranging from 1.33 to 1.38, which agrees well with the simulated results with a sensitivity of 349.5 nm/RIU at refractive indices ranging from 1.33 to 1.38.

Analysis of phase sensitivity to longitudinal strain in microstructured optical fibers.

We investigate the influence of air holes on phase sensitivity in microstructured optical fibers to longitudinal strain. According to the numerical simulations performed, large air holes in close proximity to a fiber core introduce significant compression stress to the core, which results in an increase in the effective refractive index sensitivity to longitudinal strain. The theoretical investigation is verified by an experiment performed on four fibers drawn from the same preform and differentiated by air hole diameter. We show that introducing properly designed air holes can lead to a considerable increase in normalized effective refractive index sensitivity to axial strain from -0.21 ε-1 (for traditional single mode fiber) to -0.14 ε-1.

Measurement of RI and Temperature Using Composite Interferometer With Hollow-Core Fiber and Photonic Crystal Fiber

A novel, highly sensitive double-parameter sensor based on the composite interference structure has been designed and implemented in this paper. It can be regarded as a cascade of the traditional single-mode fiber (SMF)–hollow-core fiber (HCF)–photonic crystal fiber (PCF)-SMF structure, which forms a reflected Fabry–Perot interferometer and a transmitted Mach–Zehnder interferometer at the same time. The HCF is filled with ethanol, which is sensitive to temperature. Then the simultaneous measurement of the temperature and the refractive index (RI) can be achieved by monitoring the reflected spectrum and transmitted spectrum. The fact that these two spectrums do not interfere with each other makes it easier to obtain a demodulation method for simultaneous temperature and RI measurement that does not involve complex fast Fourier transform and matrix computation. The sensitivities of the temperature and the RI can reach up to 0.454 nm/°C and 178.7 nm/RIU, respectively.

Ultra-stable high-power mid-infrared optical parametric oscillator pumped by a super-fluorescent fiber source.

The longterm stability of the laser system is very important in many applications. In this letter, an ultra-stable, broadband, mid-infrared (MIR) optical parametric oscillator (OPO) pumped by a super-fluorescent fiber source is demonstrated. An idler MIR output power of 11.3 W with excellent beam quality was obtained and the corresponding pump-to-idler conversion slope efficiency was 15.9%. Furthermore, during 1h measurement at full power operation, the peak-to-peak fluctuation of idler output power was less than 1.9% and the corresponding standard deviation was less than 0.4% RMS, which was much better than that of a traditional single mode fiber laser pumped OPO system (10.9% for peak-to-peak fluctuation and 1.8% RMS for the standard deviation) in another experiment for comparison. To our knowledge, this is the first demonstration on a high-power, ultra-stable OPO system by using the modefree pump source, which offered an effective approach to achieve an ultra-stable MIR source and broadened the range of the super-fluorescent fiber source applications.

High sensitivity refractive index sensor based on a tapered small core single-mode fiber structure.

A high sensitivity refractive index (RI) sensor based on a tapered small core single-mode fiber (SCSMF) structure sandwiched between two traditional single-mode fibers (SMF28) is reported. The microheater brushing technique was employed to fabricate the tapered fiber structures with different waist diameters of 12.5, 15.0, and 18.8 μm. Experiments demonstrate that the fiber sensor with a waist diameter of 12.5 μm offers the best sensitivity of 19212.5 nm/RIU (RI unit) in the RI range of 1.4304 to 1.4320. All sensors fabricated in this Letter show good linearity in terms of the spectral wavelength shift versus changes in RI. Furthermore, the sensor with the best sensitivity to RI was also used to measure relative humidity (RH) without any coating materials applied to the fiber surface. Experimental results show that the spectral wavelength shift changes exponentially as the RH varies from 60% to 95%. A maximum sensitivity of 18.3 nm per relative humidity unit (RHU) was achieved in the RH range of 90.4% to 94.5% RH.

On-Demand Spectrum and Core Allocation for Reducing Crosstalk in Multicore Fibers in Elastic Optical Networks

In the past few years, many researchers have studied elastic optical networks, which exhibit a dramatically improved transmission capacity compared with conventional optical networks. However, the transmission capacity per fiber will soon reach the physical limit for traditional single-mode fibers. Multicore fiber (MCF) is among the innovative fibers based on space-division multiplexing technology. MCF has multiple cores and achieves a far larger transmission capacity than traditional single-mode fibers. However, signals transmitted in these crowded multiple cores interfere with each other and are degraded. This degradation has a serious impact on the network resource management in elastic optical networks. Previous research indicates that the crosstalk effect of MCF is dependent on an arrangement of signals regarding the spectrum and core. Therefore, we approach the problem of the crosstalk in MCF with regard to the spectrum and core allocation, from the network perspective. First, this paper summarizes the related work regarding elastic optical networks and MCFs. Next, we propose an &#x201C;on-demand&#x201D; spectrum and core allocation method that reduces both the crosstalk and fragmentation in elastic optical networks with MCFs. This proposed method is based on two predefined policies related to the crosstalk and fragmentation. The first, the core prioritization policy, is based on the MCF&#x2019;s structure, and the other is a core classification policy based on the required bandwidth of the connections. The core prioritization policy realizes the core allocation that reduces crosstalk by avoiding filling adjacent cores. The core classification policy reduces the spectrum fragmentation by allocating a uniform bandwidth connection for each core. Finally, we evaluate the proposed method using computer simulations. The results indicate that the proposed method can, under various network conditions, improve both the crosstalk and blocking probability of the total network through our two policies.

Magnetic Field Sensing Based on Magnetic-Fluid-Clad Fiber-Optic Structure With Up-Tapered Joints

A kind of magnetic field sensor based on mode interference effect is proposed. The sensing arm consists of two special up-tapered joints formed on the traditional single-mode fiber by fusion tapering technique. Magnetic fluid is used as the cladding of the structure. The interference valley wavelength or the transmission loss of the sensing structure is sensitive to the external magnetic field, which is utilized for magnetic field sensing. The sensitivity of shift of interference valley wavelength with magnetic field can be up to 325.3 pm/mT in the range of 0-16 mT. The variation of transmission loss at valley wavelength with magnetic field has a sensitivity of -0.2121 dB/mT in the range of 2-30 mT.