NTT enhances optical fiber communications using space division multiplexing technology − 67% reduction in power consumption achieved

NTT has achieved both increased transmission capacity through simultaneous multi-core amplification and energy efficiency by employing a multi-core structure with 12 cores densely arranged in an amplifying optical fiber designed for optical fiber communication within the major wavelength band (C-band: near 1550 nm). The company has demonstrated for the first time globally that it can reduce power consumption by as much as 67% compared to conventional technologies, creating added value through energy efficiency in transmission capacity expansion technology using Multi-Core Fibers (MCF). Based on the results of this demonstration, the company aims to establish this technology by 2030 as a candidate for space division multiplex transmission lines with more than 10 channels, which is the target of the company's Innovative Optical and Wireless Network (IOWN) concept. These results were adopted at the 49th ECOC, the world's largest international conference on optical communication technology, held in Scotland, and announced on October 4.

To meet the increasing demand for telecommunications, NTT has been promoting further expansion of the capacity of optical fiber communications using space division multiplexing technologies such as MCF. However, as communication capacity expands, existing amplification technologies using a single-core structure struggle. This results in an increase in the number of optical amplifiers required, consequently increasing the overall power consumption of the transmission system.

In contrast, applying optical amplification technology using a multi-core structure allows the shared excitation light for amplification to be distributed among multiple cores. Consequently, it is anticipated that power efficiency comparable to that of existing optical amplification technologies can be achieved.

NTT achieved the world's first and best results using two elemental techniques. The first technique is to maximize the core area ratio (core density). In conventional optical amplifiers (using core excitation method), excitation light is injected at the core level to amplify the signal light propagating within the core. The new optical amplifier adopts the clad excitation method. In this approach, the excitation light injected throughout the cross-section of the optical fiber, enabling simultaneous amplification of all signal lights propagating through multiple cores within the cross-section.

This method was expected to reduce power consumption because one excitation light laser could be shared among multiple cores. However, owing to the wide excitation light propagation across the entire cross-section of the optical fiber, the overlap between the signal light and the excitation light is smaller compared to the core excitation method. This results in a lower energy transfer efficiency from the excitation light to the signal light (with a large proportion of excitation light not being used for optical amplification). Consequently, a significant amount of excitation light power is required, and the anticipated energy efficiency has not been achieved.

In contrast, the new amplifying optical fiber used in this study maximized the core-to-cladding area ratio (core density) by maintaining an equivalent multicore arrangement (number of cores and core spacing) as the transmission optical fiber line while simultaneously reducing/enlarging the outer diameter (cladding diameter) and core diameter of the amplifying optical fiber. This technique achieved the maximum efficiency in excitation light utilization.

However, a new challenge arose at the connection point between the transmission optical fiber line and the amplifying optical fiber owing to the reduction in cladding diameter to increase the core area ratio. The mismatch in cladding diameters caused a partial loss of the excitation light. Similar to conventional techniques, unused excitation light remained after the propagation through the amplifying optical fiber, which was not utilized.

To address this issue, a tapered structure and reflective device were implemented as countermeasures, successfully reducing the excitation light loss and residual excitation light. This approach enhanced the efficiency of optical amplification.