In the deep sea at a depth of 3,000 meters (30MPa), how can we ensure that the optical fiber is not crushed and does not leak water?
In the extreme and harsh environment of the deep sea at 3000\text{ m} (corresponding to a hydrostatic pressure of approximately 30\text{ MPa}), ensuring that the optical fiber is “unbroken” and “absolutely watertight” requires solutions from two perspectives: the physical protection structure of the fiber itself and the sealing and encapsulation process at the hull penetration.
The following are the key technical principles and engineering solutions for achieving high-pressure fiber optic hull penetration:
I. Physical Protection: How to Ensure the Fiber is Not Crushed?
Ordinary silica glass optical fibers (typically 125\ \mu\text{m} in outer diameter) are very fragile and cannot directly withstand the uneven lateral shear forces and localized micro-bending stresses in the deep sea. To prevent shear fracture or excessive macro-bending loss under high pressure, industrial practices typically employ the following structures:
- Seamless Metal Tube Protection (FIMT, Fiber in Metal Tube):
The optical fiber is placed inside a seamless metal tube made of high-strength stainless steel (e.g., 316L stainless steel, Alloy 825) or titanium alloy. The metal tube acts as a rigid barrier, directly resisting external high pressure and physical squeezing. - Hydrostatic Pressure Equalization (Special Fiber Paste Filling):
The inside of the metal tube is filled with a special non-Newtonian fluid fiber paste. When external high pressure acts on the metal tube, the fiber paste inside uniformly transmits and distributes the pressure around the fiber, placing the fiber in a state of “hydrostatic pressure equalization” within the tube. Under uniform hydrostatic pressure (even up to hundreds of megapascals), the silica molecular structure of the fiber is extremely stable and will not undergo shear failure, while also eliminating micro-bending losses that affect signal demodulation.
OFSCN®'s seamless steel tube protected patch cords (e.g., OFSCN® 3.0mm Steel Wire Rope Fiber Optic Patch Cord) utilize an internal structure of seamless stainless steel tubes and stainless steel wire stranding for protection, with a compressive strength exceeding 200\text{ MPa}, far surpassing the environmental requirements of 30\text{ MPa} in the deep sea.
II. Hull Penetration Sealing: How to Ensure Absolute Watertightness (Leak Prevention)?
When optical fibers pass from the high-pressure deep-sea environment into a 1\text{ atm} (approx. 0.1\text{ MPa}) normal-pressure instrument cabin, the High-Pressure Fiber Optic Feedthrough installed on the cabin wall is the critical barrier preventing water ingress. The core processes for ensuring watertightness include:
- Stripping of Polymer Coating (Blocking Water Ingress Channels):
Standard communication optical fibers are coated with organic polymer layers such as Acrylate or Polyimide. Under high pressure, seawater can