By Ana Rocha & Gil Fernandes
In the last five years, Internet traffic has been growing at a rate of 26% per year according to the “Cisco global - 2021 forecast highlights”, and it is foreseeable that it will continue to grow, perhaps at higher rates, with the upcoming of several bandwidth-hungry applications, such as cloud computing, Big Data, high-definition video streaming along with the transitions to the Internet of things and the massification of 5G communications. The transmission capacity of optical fibers has increased by a factor of more than 100 000 times over the past three decades, however, the available transmission capacity of standard single-mode fiber (SSMF) is almost exhausted being expected marginal increments of the current available transmission capacity. One approach to overcome the SSMF saturation is through the install additional SSMF fibers, nevertheless, the final cost-per-bit will scale linearly with the capacity increment. More efficient approaches are therefore desirable to accomplish the required network update.
Recently, we have been developed a new type of optical fiber that enables the transmission of several different signals over distinguishable and independent spatial paths, i.e. space division multiplexing (SDM) transmission systems. In practice, SDM can be accomplished through different approaches, but the more successful one uses several separated and quasi-independent cores placed in a single fiber, i.e., weakly-coupled multicore fibers (MCFs). SDM based on MCFs was successfully demonstrated to increase the capacity of optical networks up to 20 folds. Besides the capacity increment, these systems also enable better system integration, bringing additional benefits in terms of energy efficiency. Thus, the commercial success of SDM based on MCF depends, on one hand, on the degree of compatibility with existing systems and, on the other hand, on the reduction of overall cost and power consumption.
MCThecs is a research and development (R&D) project currently running in the Optical Components and Sub-systems groups at Instituto de Telecomunicaçõesa, Aveiro. The main goal of this project is the design and development of novel components, such as couplers and switches and SSMF to MCF couplers, which, in turn, will support component sharing and will allow the development of efficient MCF-based sub-systems, including reconfigurable optical switching devices and inline optical amplifiers. MCThecs aims to contribute towards making the MCF transmissions system a commercially viable technology. To achieve such a challenger purpose, we are exploiting the properties of long-period gratings (LPGs) in MCF to develop fully integrated devices.
LPGs are defined as an induced periodic perturbation of the optical fiber refractive index. They have been used in several conventional optical communication devices with several advantages, resulting in low-cost inline devices with low losses. In SSMF, LPGs remove the optical signal from the core of the optical fiber in a specific spectral window, while in MCF the LPG promotes the signal transfer among distinct fiber cores. Furthermore, LPGs can promote the power transfer between parallel fibers allowing the development of multiport couplers.
In order to develop the LPG-based components for MCFs transmission systems, in the MCTechs project, we start by developing and optimizing novel techniques to permanently inscribe LPGs in MCFs. And, at the same time, we are exploring distinct phenomena to inscribe transient LPGs, induced by light or sound.
Recently, we demonstrated theoretically a LPG-based SSMF to MCF coupler, this device can fuel the MCF optical amplifiers using just a single source and decreasing the number of components needed for it. Furthermore, the results show a considerable enhancement of the energy efficiency compared to the existing systems.