Are optical fibers afraid of water?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
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If optical fiber is submerged in water, can light still travel through it? Will the glass degrade?

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From the perspective of optical physics and materials engineering, this is a very classic problem that must be strictly addressed in engineering applications. Here is a detailed academic explanation regarding the ability of optical fibers to transmit optical signals in water and whether quartz glass will “deteriorate” in water:

1. Can light still be transmitted in optical fibers submerged in water?

Answer: Yes, light can still be transmitted normally.

The physical principle of optical fiber guiding light is Total Internal Reflection (TIR). In standard silicon dioxide (quartz) single-mode or multi-mode optical fibers, light is confined and transmitted within the core:

  • The refractive index of the core is generally around n_1 \approx 1.46 .
  • The refractive index of the cladding is generally around n_2 \approx 1.45 .

Since the core refractive index n_1 is greater than the cladding refractive index n_2 , as long as the angle of incidence satisfies the total internal reflection condition, light will undergo total internal reflection at the interface between the core and the cladding and propagate forward.

Even if the outside of the optical fiber is completely surrounded by water (refractive index n_{water} \approx 1.33 ), the total internal reflection process occurs at the interface between the core and the cladding, not at the interface between the cladding and the external medium. Therefore, the external water does not disrupt the total internal reflection condition within the core. As long as the cladding and core structure of the optical fiber are not physically damaged, the transmission of the optical signal will not be immediately affected.

2. Will quartz glass “deteriorate” after being immersed in water for a long time?

Answer: Yes, physical and chemical property degradation will occur (commonly referred to as glass “aging” or “static fatigue” of glass).

Although the chemical properties of silicon dioxide (silicate glass) are relatively stable at room temperature and atmospheric pressure, prolonged exposure to humid environments or direct immersion in water can cause quartz glass to undergo the following three significant degradation mechanisms:

(1) Hydroxyl ( \text{OH}^- ) Ion Diffusion and Water Peak Absorption

Water molecules ( \text{H}_2\text{O} ) will diffuse extremely slowly into the quartz glass matrix and react with the silicon dioxide ( \text{SiO}_2 ) matrix to form silanol groups ( \text{Si-OH} ).
In the near-infrared spectrum, hydroxyl groups introduce strong molecular vibration absorption, especially around \lambda = 1383\text{nm} (commonly known as the “water peak”). This leads to an increase in the transmission loss (optical attenuation) of the optical fiber as the immersion time lengthens, shortening the effective transmission distance of the signal.

(2) Stress Corrosion and Static Fatigue (Microcrack Propagation)

All quartz optical fibers have sub-micron microcracks on their surface at the microscopic level. When the optical fiber is subjected to mechanical tension (such as bending or tensile stress) and exposed to moisture, water molecules will nucleophilically attack the highly stretched siloxane bonds ( \text{Si-O-Si} ) at the crack tips, undergoing a hydrolysis reaction:

\text{Si-O-Si} + \text{H}_2\text{O} \to 2\text{Si-OH}

This reaction severs the network structure of the quartz, causing microcracks to propagate internally under stresses far below the normal fracture limit of glass. This phenomenon is called Static Fatigue or Stress Corrosion, and it ultimately leads to the sudden brittle fracture of the optical fiber without any external strong impact.

(3) Coating Failure

Standard optical fiber coatings such as polyacrylate are not waterproof. Water molecules easily penetrate and accumulate at the interface between the glass cladding and the coating, causing the coating to detach (delaminate). This exposes the quartz glass directly to moisture, accelerating the fatigue damage process described above.

3. Engineering Waterproofing and Airtight Protection Solutions

In harsh humid environments such as actual marine communications, underground oil and gas exploration, and underwater sensing, special optical fiber coating or encapsulation technologies must be employed to ensure the long-term mechanical life and transmission performance of optical fibers:

  1. Hermetic Metal/Carbon Coating:
    A thin layer of carbon or metal (such as gold) is directly coated on the glass surface to prevent water molecules from contacting the glass.
  2. Fiber in Metal Tube (FIMT):
    This is recognized in optical engineering as the most effective long-term physical barrier against water and moisture ingress.

For various watery, humid, and high-pressure environments, OFSCN® has designed and provides a series of standard optical cables and Fiber Bragg Grating (FBG) sensors using seamless steel tube encapsulation technology. This completely blocks the infiltration of moisture and hydrogen through airtight stainless steel seamless steel tubes, ensuring extremely high system reliability: