Anritsu Corporation (“Anritsu”) and KDDI Research, Inc. (“KDDI Research”) have, for the first time in the world, designed a remote monitoring system for next-generation optical submarine cables using multicore optical fiber (Note 1), and have successfully demonstrated the measurement of optical path characteristics (Note 2) in a trial environment (Note 3) (hereinafter referred to as “this trial”).

In this trial, leveraging the expertise of KDDI Research, a trial environment was established for an optical submarine cable system incorporating an optical circuit that enables remote monitoring with multicore optical fiber. Within this environment, Anritsu's optical pulse tester, Coherent OTDR (Coherent Optical Time Domain Reflectometer) MW90010B (Note 4, 5), was used to confirm the ability to measure optical path characteristics such as fault location and fiber loss profile, which are essential for the operation and maintenance of submarine cables.

This achievement enables remote visualization of the quality and faults of multicore optical fiber, which had previously been difficult, greatly enhancing the reliability and operational efficiency of optical submarine cables.

Both companies will unveil these achievements at the 51st European Conference on Optical Communication (ECOC 2025), the world's largest international conference on optical communications, to be held in Denmark from September 28 to October 2, 2025 (Note 6).

About the Trial

1. Background

With the rapid proliferation of AI and IoT technologies, global data traffic is expected to continue increasing dramatically, driving the need for greater capacity in optical submarine cables that support international communications. Currently, optical submarine cable systems utilize single-mode fibers, which contain a single optical path (hereinafter “core”) per fiber. However, due to the limitations of a single core, multicore fibers—which incorporate multiple independent cores within a single fiber—are being explored to significantly increase transmission capacity per fiber.

The introduction of multicore fiber also requires new technological advancements in system operation and maintenance. At present, coherent OTDR technology is used to locate fiber breaks and measure losses in single-mode fibers. However, multicore fibers have a unique degradation factor known as inter-core crosstalk (Note 7), which can affect measurement accuracy (Note 8) using coherent OTDR. Until now, measuring optical path characteristics in optical submarine cable systems employing multicore fiber has been challenging (Note 9).

2. Achievements

This trial investigated whether optical path characteristics of multicore fiber could be measured using coherent OTDR, similar to single-mode fiber. KDDI Research designed a new transmission system enabling measurement with coherent OTDR and constructed a simulated environment for an optical submarine cable system using multicore fiber. Measurement light transmitted from Anritsu’s Coherent OTDR MW90010B propagates through the multicore fiber, and the reflected and scattered light from each core is analyzed, enabling accurate measurement of optical path characteristics.

For the first time worldwide, it was confirmed that coherent OTDR can detect faults and measure fiber loss profiles in optical submarine cable systems using multicore fiber. Additionally, the distribution of inter-core crosstalk along the fiber, including through repeaters, could be measured, marking the first successful visualization of inter-core crosstalk over long distances.

This achievement demonstrates the practical applicability of conventional coherent OTDR for multicore fiber and is expected to accelerate the adoption of multicore fiber in optical submarine cable systems while ensuring high communication quality.

Inter-core crosstalk: Comparison between coherent OTDR measurement and theoretical value

Anritsu and KDDI Research will continue to collaborate on research and development to further increase the capacity of optical submarine cables and ensure stable international communications.

Notes

• (Note 1) A type of optical fiber with multiple optical paths (cores), designed to suppress interference between cores.
• (Note 2) The various properties and characteristics, such as fiber loss, occurring when optical signals are transmitted through fibers and repeaters.
• (Note 3) As of September 2025, this is the world’s first measurement of optical path characteristics in a multicore optical submarine cable system using coherent OTDR (according to both companies).
• (Note 4) An optical pulse testing instrument utilizing coherent detection (optical heterodyne) technology to measure optical fiber characteristics.
• (Note 5) Coherent OTDR MW90010B
• (Note 6) ECOC2025 (Bella Center) Presentation: September 29, 2025, 16:00–16:15 (local time), Session: M.03.08 Fiber-Optic Sensing SC 8, Title: Impact of Inter-Core Crosstalk on Coherent Optical Time-Domain Reflectometry in Repeatered Multicore Fibre Systems
• (Note 7) Crosstalk occurs when an optical signal in one core leaks into an adjacent core, degrading the signal-to-noise ratio and communication quality.
• (Note 8) Fiber loss profile and reflections occurring within optical submarine cables.
• (Note 9) As of September 2025, according to research by both companies.