NTT demonstrates optical networking for tunnel inspection

Hazama Ando, NTT and 1FINITY have disclosed results from a communications trial combining optical networking and wireless links inside a mountain tunnel construction site in Japan. The experiment transferred large measurement datasets alongside live monitoring video and supported remote inspection of cracks measuring 0.2mm under conditions simulating a communications distance of 1,000km.

The partners announced the results on 19 October following a proof of concept conducted in March 2026. The experiment took place at the Tsubakihara Tunnel project, associated with the four-lane expansion of the Tokai-Hokuriku Expressway. It examined whether high-capacity communications could be maintained in a construction environment affected by dust, water, vibration and the changing physical conditions of tunnel excavation.

The system incorporated technologies associated with NTT’s Innovative Optical and Wireless Network, or IOWN, including an All-Photonics Network architecture. The partners installed approximately 1,100m of optical fibre inside the tunnel and used six optical transport nodes positioned at intervals of approximately 100m from the working face.

The configuration connected optical equipment inside the tunnel with a simulated long-distance network environment and local wireless links. It was intended to support the movement of monitoring information, video and three-dimensional measurement data from the construction area to locations where engineers could assess conditions remotely.

Tunnel construction creates several communications challenges. Excavation continuously changes the working environment, while large machinery and structural supports can obstruct radio signals. Equipment must also tolerate dust from construction activity, vibration associated with blasting and water entering through the surrounding ground.

Installing communications infrastructure requires consideration of these conditions alongside available power, space and access. Equipment may need to be repositioned as work advances, and connections must remain available without interfering with construction activity or introducing additional hazards.

Optical fibre carries information through modulated light travelling within a glass waveguide. It can support high-capacity links over distances that would create different electrical and signal-integrity requirements for copper connections. The performance of the installed system depends on the fibre, optical equipment, network architecture and the way traffic is handled across the complete communications path.

The partners used 1FINITY’s Ultra Optical System to establish the transport network. During the trial, no packet loss was detected in the optical network section, while jitter was generally maintained at 100 nanoseconds or less. Continuous operation for more than 24 hours was also recorded under the test conditions.

Those results apply to the measured optical network section rather than the complete application path. A remote inspection system also includes cameras, processing equipment, wireless connections, data encoding and receiving devices, each of which can introduce additional delay or affect overall reliability.

Jitter describes variation in the timing of communications events. Excessive variation can create problems for applications requiring consistent delivery of information, particularly where monitoring and control processes depend on measurements arriving within a defined interval. The reported jitter figure describes the experimental network’s behaviour under the specified conditions, not a guaranteed response time for every connected application.

The communications equipment was housed in IP65-rated enclosures mounted along the tunnel. Hazama Ando, NTT and 1FINITY report stable operation despite exposure to dust, vibration and spring water during the experiment. The enclosure rating addresses defined ingress conditions, but does not mean the equipment is qualified for every impact, flooding or long-duration construction environment.

Local wireless connectivity was provided using WiGig, a wireless networking technology operating in the 60GHz frequency band. These links can provide substantial data capacity over relatively short distances, although propagation is sensitive to the arrangement of obstacles, equipment positioning and the physical environment.

The trial examined communication performance across different distances and angles, including an arrangement with an obstruction between wireless devices. The partners reported sufficient capacity to support the visual confirmation of 0.2mm cracks at a viewing distance of 1m, even when wireless equipment was separated by approximately 100m under the tested conditions.

This does not establish that a 60GHz connection can maintain identical performance through arbitrary tunnel structures or at every installation distance. Wireless propagation depends on reflections, attenuation, antenna placement and the characteristics of the surrounding surfaces, making site-specific assessment important.

A separate test examined the ability to transfer measurement data while ongoing monitoring continued. Approximately 20GB of three-dimensional information acquired using a laser scanner was transmitted from inside the tunnel to the site office in around one minute, with video monitoring equivalent to 15 cameras operating at the same time.

Laser scanning creates point-cloud datasets containing measured positions in three-dimensional space. Engineers can use these measurements to assess geometry, compare construction progress with design information and identify changes requiring closer inspection. The amount of data generated depends on measurement density, scanning area and the processing applied to the observations.

Transferring these datasets promptly can reduce the delay between acquiring a measurement and making it available for review. The trial showed that bulk measurement transfers and monitoring traffic could operate concurrently within the experimental communications arrangement, avoiding the need to stop video monitoring while large files were transmitted.

The partners also conducted a remote inspection exercise using 4K video and audio over a network configured to simulate a separation of 1,000km. Personnel were able to inspect images of the tunnel and identify cracks 0.2mm wide. AI image analysis separately detected cracks in images captured under tunnel lighting after training on relevant examples of the wall surfaces.

The 1,000km figure describes the simulated communications distance, not the physical separation of the construction workers and inspectors during the experiment. The results establish performance for the particular imaging and network conditions tested; they do not demonstrate that every 0.2mm defect can be identified reliably across all tunnel materials, lighting environments and camera arrangements.

Image-based inspection depends on the optical resolution of the camera, the distance to the inspected surface, illumination and the clarity of the recorded image. Vibration, motion and compression can reduce visible detail, while moisture and irregular surface texture may make genuine defects difficult to distinguish from harmless markings.

Automated analysis introduces another layer of uncertainty. An AI model trained to identify cracks must be tested against representative construction materials and operating conditions. Differences in lighting, surface appearance or crack orientation can affect detection, while false positives and missed defects must be considered when results contribute to construction decisions.

Hazama Ando provided the construction environment and coordinated the proof of concept. NTT developed the wireless communications arrangements and inspection applications, including AI-based crack detection, while 1FINITY supplied optical transport technology and the environment used to simulate the All-Photonics Network connection.

The work forms part of a broader programme examining remote construction management, where monitoring information and technical measurements can be shared with engineers outside the immediate working area. Such arrangements require reliable communications, but their successful use also depends on inspection procedures, equipment maintenance and appropriate decisions when network information becomes unavailable.

The three organisations intend to conduct additional operational trials connecting multiple tunnel sites, remote offices and data centres. From Japan’s 2027 fiscal year, they plan to examine phased application at actual construction sites and assess performance under operational conditions.

The October announcement therefore reports the results of a completed March experiment, not the commissioning of a permanent remote inspection network. Further trials will establish whether the communication performance, equipment resilience and inspection accuracy observed during the proof of concept can be maintained as construction sites change and the network is extended into routine operations.