Processing and performance evaluation of 23-inch glass spheres for deep-sea optical modules

The High-energy Underwater Neutrino Telescope (HUNT) will deploy approximately 1200 underwater neutrino detector strings at depths of 1000 m to 3000 m. Carrying about 55,000 optical modules (OMs), these strings will form a three-dimensional observation array with an instrumented volume exceeding 30 km 3 . Each OM mainly comprises a 20-inch photomultiplier tube, associated electronics, and a power supply system. The OM system’s long-term stability critically depends on the mechanical reliability and optical performance of its pressure-resistant glass sphere. HUNT therefore requires a sphere with a 23-inch outer diameter that can withstand 30 MPa deep-sea hydrostatic pressure while maintaining optical transmittance of at least 85 % from 380 nm to 780 nm. These combined requirements impose exceptional demands on sphere dimensions, structural strength, material quality, manufacturing precision, and broadband transparency under sustained deep-sea operating conditions. To the best of our knowledge, no commercial product currently satisfies all these specifications. To meet these requirements, stress simulations were first performed to determine the optimal wall thickness. The glass material and manufacturing process were subsequently developed in collaboration with a glass-instrument manufacturer, and prototype 23-inch pressure-resistant spheres were fabricated. The prototypes underwent comprehensive performance tests, including residual-stress measurement, dimensional inspection, optical-transmittance measurement, and pressure-resistance testing. Results show that the developed spheres possess highly stable mechanical and optical properties and satisfy the requirements of the present 2000 m prototype sea-trial campaign. Their successful development provides essential technical support for HUNT and offers potential applications in ocean observation and deep-sea engineering, including spherical housings for precision deep-sea instruments and deep-sea buoyancy modules.