Harbin Engineering University tests 770 m fibre-optic towed array that measures its own shape at sea
A 770 m towed cable combining 128 interferometric hydrophones and 32 distributed-sensing elements measured its own depth and curvature in an open-sea trial, and the data improved beamforming.
Researchers at Harbin Engineering University built and tested a 770 m towed hydrophone cable that measures its own depth and curvature through the same optical fibre that carries the sound, according to a paper in The Innovation sciencedirect.com
The Innovationsciencedirect.comOpen source ↗. The cable was towed about 500 m astern of a vessel in open-sea trials in December 2025 and January 2026. A cooperative acoustic source 5.7 km away provided 1 kHz test pulses.
Array design and signal bands
Beamforming converts arrival delays across elements into a direction and needs the true position of each element. Conventional arrays carry only a few depth sensors, so the shape between them is estimated, and a curved array blurs the focus and biases the bearing. The new cable has an interferometric section of 128 elements over 104 m, each wound with 22 m of fibre, and a distributed acoustic sensing (DAS) section of 32 elements at 5 m spacing over 160 m. The DAS section reads optical phase along the fibre and doubles as a depth gauge, because hydrostatic pressure shifts that phase.
Both sections use identical air-backed mandrels, 42 mm long and 11.6 mm in outer diameter. The data are split by frequency: below 0.02 Hz for depth and shape, 0.02–0.3 Hz for waves, and 5 Hz–2 kHz for sound.
Trial results
Against a pressure sensor, DAS-inferred depth gave a coefficient of determination of 0.99 and an average deviation of 0.80%. The reconstructed profile showed the cable bowing rather than lying flat sciencedirect.com
The Innovationsciencedirect.comOpen source ↗. Measured wave spectral peaks were 0.102 and 0.098 Hz, against 0.105 and 0.107 Hz expected, a difference of 2.86% and 8.41%. Significant wave height ranged from 1.41 to 2.66 m, within 4.31% and 11.03% of the Copernicus wave model. At 50 m tow depth the authors estimate a detection floor of 0.15 m and a dynamic range of 180 dB.
For sound, the DAS section ran 11.57 dB above its own land-quiet baseline across 5–2,000 Hz, while the interferometric elements were quieter above about 2 kHz. Beamforming tracked a GPS reference bearing in two bands: vessel noise at 120–150 Hz and the 1 kHz pulses from the distant source. Feeding the measured depth profile into the array model raised peak beamforming power by up to 4.10 dB.
The trials covered one vessel, one speed and one sea state, and coastal and shallow water were not tested. Depth and wave retrieval depend on pressure measurements.
