Single-core Single-mode Fiber Research Articles

Coherent Optical Transceivers Scaling and Integration Challenges

The advancement of digital coherent technologies has dramatically increased the system capacity per single-core single-mode fiber to the point that we can now approach the Shannon limit by utilizing high-order modulation formats and high-coding gain forward error correction (FEC) codes. Because the required energy per bit increases exponentially the closer we get to the Shannon limit, extending the available optical bandwidth by using ultrawideband wavelength-division multiplexing (WDM) and/or spatial-division multiplexing (SDM) is indispensable for increasing the system capacity with high energy efficiency. However, simple extensions of wavelength resources and spatial parallelization dramatically increase the number of transceivers (TxRxs) in proportion to the wavelength/spatial multiplicity. The key to achieving cost- and energy-efficient systems is to reduce the system complexity by using high-density integration and broadband optelectronics. In this article, we overview and discuss the recent advances of coherent optical transceivers integrated with an optical front end and digital signal processing (DSP)/application-specific integrated circuit (ASIC). We then present the transponder architectures and the challenges involved in applying them for massive parallelized transmission systems.

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.

Fragmentation-Aware Traffic Grooming with Lane Changes in Spectrally–Spatially Flexible Optical Networks

Traffic in current networks is constantly increasing due to the growing popularity of various network services. The currently deployed backbone optical networks apply wavelength division multiplexing (WDM) techniques in single-core single-mode fibers (SMFs) to transmit the light. However, the capacity of SMFs is limited due to physical constraints, and new technologies are required in the near future. Spectrally–spatially-flexible optical networks (SS-FONs) are proposed to provide a substantial capacity increase by exploring the spatial dimension. However, before this technology will reach maturity, various aspects need to be addressed. In particular, during traffic grooming, multiple small requests are aggregated into large-capacity optical corridors in an optical layer to increase the spectral efficiency. As the summary traffic volume is dynamically changing, it may be required to set up and tear down optical channels, which results in network fragmentation. As a consequence, in a congested network, part of the requests can be blocked due to the lack of spectrum resources. Thus, the grooming of traffic and the creation of lightpaths should be carefully designed to minimize network fragmentation. In this study, we present several fragmentation metrics and develop a fragmentation-aware traffic grooming algorithm that reduces the bandwidth blocking probability.

Regression-based fragmentation metric and fragmentation-aware algorithm in spectrally-spatially flexible optical networks

Spectrally-spatially flexible optical networks (SS-FONs) are seen as a next frontier in optical backbone networks that allow supplying demanded high-capacity transmission. In SS-FONs, signals are co-propagating in spatial modes of suitably designed optical fibers, e.g., in the bundles of single-core single-mode fibers. Despite significant fiber capacity, SS-FONs operate on a flexible (elastic) grid which allows for assigning an adjustable amount of spectrum resources according to the requested bit-rate. The full potential of SS-FONs’ spectral and spatial flexibility can be exploited when nodes are equipped with switching devices enabling lane changes, i.e., the devices that support arbitrary switching between input and output spatial modes connected to the node. However, before the SS-FON will reach maturity and become ready for commercial applications, several crucial issues need to be solved. In this paper, we study the fragmentation problem for dynamic traffic in SS-FONs with lane changes. We propose a novel weighted fragmentation metric that accounts for vertical and horizontal fragmentation in the considered scenario. The machine learning regression model is created and solved to obtain the best weights combination that minimizes the network fragmentation. We run experiments on the representative network topology using our developed fragmentation-aware algorithm showing that the proposed metric and assigned fiber weights result in network fragmentation decrease. As a consequence, the proposed solution allows for bandwidth blocking probability reduction when compared to the reference methods. Finally, we discuss several optimization strategies that decrease the computational complexity of our algorithm.

Simultaneous Long-Distance Transmission of Discrete-Variable Quantum Key Distribution and Classical Optical Communication

We theoretically study the issues for long-distance transmission of quantum key distribution (QKD) coexisting with classical signals. The recently proposed phase-matching QKD protocol can drastically improve the transmission distance of QKD. However, in the coexistence system, the noise generated by classical signals, especially spontaneous Raman scattering noise, is a big challenge. Moreover, the classical optical amplifier, which is necessary for the realistic long-distance classical communication, makes the noise more serious. In view of this, we establish the unified Raman noise model for three discrete-variable QKD protocols (BB84, measurement-device-independent and phase-matching QKD protocols) in the presence of classical optical amplifiers, which can be applied to both single-core single-mode fiber and multicore fiber. Then, we derive the key rate of the three QKD protocols coexisting with classical signals using the proposed unified Raman noise model. Finally, simulation results show that multicore fiber is promising for simultaneous long-distance transmission of QKD and the dense wavelength division multiplexing system.

Optimization of GRIN lenses coupling system for twin-core fiber interconnection with single core fibers

We aim at a more compact, flexible, and simpler core-to-fiber coupling approach, optimal combinations of two graded refractive index (GRIN) lenses have been demonstrated for the interconnection between a twin-core single-mode fiber and two single-core single-mode fibers. The optimal two-lens combinations achieve an efficient core-to-fiber separating coupling and allow the fibers and lenses to coaxially assemble. Finally, axial deviations and transverse displacements of the components are discussed, and the latter increases the coupling loss more significantly. The gap length between the two lenses is designed to be fine-tuned to compensate for the transverse displacement, and the good linear compensation relationship contributes to the device manufacturing. This approach has potential applications in low coupling loss and low crosstalk devices without sophisticated alignment and adjustment, and enables the channel separating for multicore fibers.

Free-Space Coupling Optics for Multicore Fibers

In this letter, we report on coupling optics for connecting single-core single-mode fibers with multicore single-mode fibers. After a brief discussion of the options for such coupling systems, we describe our approach of using bulk optics to fabricate low-loss and low-crosstalk devices for both 7- and 19-core multicore fibers with the free-space design approach. This enables the use of the same coupling device with a variety of multicore fibers whose structural parameters differ from one sample to another. We present the results of experimental evaluations of these devices, discuss various causes of insertion loss, and analyze the coupling loss in detail.

Coupling Theoretical Model Between Single-Core Fiber and Twin-Core Fiber

In this paper, a simple theoretical model for describing the coupling between a single-core single-mode fiber and a twin-core single-mode fiber is established. The coupling processing of the two light beams is given and the theoretical predictions are confirmed by the experimental results.

Coupling characteristics between single-core fiber and multicore fiber

A simple method for coupling between a single-core single-mode fiber and a multicore single-mode fiber is proposed and demonstrated. An effective coupling approach is based on a process of standard splicing and tapering between the single-core single-mode fiber and the multicore single-mode fiber.

Twin-core fiber design and preparation for easy splicing

Several methods of preparing twin-core fibers (TCFs) that can be easily spliced to standard single-core single-mode fiber are proposed. Unlike the conventional TCF preparation methods that are used to fabricate a TCF with both cores placed symmetrically with respect to the fiber axis, these methods result in twin-core fibers that have one core in the fiber center. Experimental results obtained with the TCF fabricated by using one of the designs are presented.

Focusing light into a multiple-core fiber. I. Theory

We develop a variational, scalar-field treatment of the guided modes and their propagation constants for a multiple-core optical fiber consisting of many single-mode single fiber cores closely spaced inside a common cladding material. The variational procedure, when supplemented with symmetry considerations, produces without great effort the modes and their wavevectors for a highly symmetrical arrangement of an arbitrary number of cores. By employing these modes, we compute an expression for the coupling efficiency of an arbitrary wavefront focused into such a fiber. We argue that such a fiber should be more strongly coupled than the standard single-core single-mode fiber to an arbitrary wavefront due to a piecewise coupling of the wavefront to the multiple-core fiber. The improved coupling efficiency is illustrated with the example of a focused plane wave, for which a periodic spatial redistribution of energy among the cores, as a function of the propagation distance, is also verified. In the following paper, we demonstrate dramatic improvements in coupling efficiency for an atmospherically aberrated wavefront focused into a multiple-core fiber, a result with considerable implications for ground-based fiber-coupled interferometry.

Focusing light into a multiple-core fiber. II. Application to ground-based interferometry

In the preceding paper, we developed a variational approach to address the coupling of an optical field and the propagation of the coupled field in a multiple-core optical fiber consisting of many single-mode single fiber cores closely spaced inside a common cladding material. Here, we apply the results of that paper to discuss quantitatively the matter of coupling into a multiple-core fiber wavefronts that have been aberrated by propagation through a turbulent atmosphere, a situation characteristic of ground-based imaging systems. Specifically, we compare the coupling efficiencies for a multiple-core fiber with those for a standard single-core single-mode fiber, and show that the multiple-core fiber can out-perform the standard fiber many-fold. We also consider the propagational evolution of the relative core excitation and coherences in the guided light. Under changing conditions of atmospheric turbulence, the optimal design parameters of the MC fiber are remarkably robust, a fact that makes MC-fiber couplers very attractive for ground-based interferometry.