How Optical Fiber Cables Prevent Longitudinal Moisture Propagation

Optical fiber cables are highly sensitive to moisture. Once water enters a cable—whether from a damaged sheath, rodent bite, mechanical stress, or manufacturing defect—it can migrate longitudinally along the cable and reach the optical fibers. This leads to:

1. Increased attenuation
2. Hydrogen-induced losses
3. Ice expansion damage
4. Long-term degradation of fiber coatings

To prevent these failures, modern optical fiber cables use a combination of water-blocking materials and structural protection mechanisms that stop water from moving along the cable’s length.

Below is a detailed breakdown.

1. Water-Blocking Materials: Mechanisms and Engineering Principles

Water-blocking materials work through two primary mechanisms:

A. Water Swelling (Hydro-Expansion)
These materials absorb water quickly and expand into a gel-like state, blocking water pathways.

B. Physical Isolation (Barrier Formation)
These materials do not absorb water but instead create a mechanical barrier that prevents water from entering fiber-containing areas.
Different cable types use different combinations depending on installation scenario.

2. Detailed Explanation of Water-Blocking Materials

2.1 Water-Blocking Yarn (Swelling Yarn)
Mechanism: Water Swelling

Water-blocking yarn is typically polyester filament coated with super-absorbent polymer (SAP). When water contacts the yarn:

• SAP absorbs water up to 20–50 times its mass.
• SAP turns into a hydrogel.
• The yarn expands rapidly in diameter.
• The swelling fills empty spaces inside the cable core.
• Longitudinal water propagation is stopped within seconds.

Applications

• Direct-buried cables
• ADSS cables
• Loose tube cables
• Submarine cables (inner layers)

Advantages

• Fast swelling response (<10 seconds)
• Lightweight
• Clean, non-messy
• Excellent for dry-type cables

2.2 Water-Blocking Tape (Swelling Tape / SAP Tape)
Mechanism: Water Swelling + Physical Isolation

Water-blocking tape is a laminate of polyester nonwoven fabric + SAP powder.

How it blocks water:

• The tape forms a physical wrap around loose tubes or the cable core.
• When water enters, SAP absorbs and expands laterally.
• It forms a flat, gel-like barrier preventing water from moving along the cable length.

Applications

• Direct-buried cables
• Submarine cables
• High-protection outdoor cables
• Hybrid ADSS/OPGW designs

Advantages

• Very high swelling volume
• Good coverage and uniformity
• Prevents both radial and longitudinal penetration

2.3 Water-Blocking Gel (Thixotropic Gel)
Mechanism: Physical Isolation

This gel fills empty spaces inside loose tubes or central buffer tubes.

How it blocks water:

• Gel fills 100% of the internal tube space.
• Water cannot pass through because the gel is hydrophobic.
• Even if the tube is punctured, water infiltration is localized.

Characteristics

• Thixotropic: flows under stress, stays firm at rest
• Maintains fiber strain-free environment
• Provides cushioning and vibration damping

Applications

• GYTA / GYTS
• Submarine cables (multi-gel designs)
• Power OPGW inner tubes

Advantages

• Excellent isolation
• Long-term stability
• High temperature resilience

2.4 Water-Blocking Grease / Petroleum Jelly
Mechanism: Physical Isolation

This is a thicker hydrophobic gel covering metallic elements and cable cores.

Used in:

• Armored direct-buried cables
• Submarine fiber optic cables

OPGW

• High-moisture, high-pressure environments
• Grease fills all micro-gaps to stop water pathways.

2.5 Super Absorbent Polymer (SAP) Powder
Mechanism: Water Swelling

Used in some low-cost or lightweight designs.

How it works:

• SAP powder is sprinkled in the core during manufacturing.
• Upon water entry, the powder forms swollen gel granules.
• It fills and seals the area.

Limitations

• Easy to lose uniformity
• Used less in high-end cables today

3. Structural Moisture-Blocking Mechanisms

3.1 Loose Tube Structure

Loose tubes isolate fibers from direct water contact.

Protection roles:

• Tube walls slow down water movement
• Combined with gel or yarn, tubes act as enclosed sub-channels
• Even if water penetrates one tube, others remain dry.

3.2 Corrugated Steel Tape Armor (CST)
Mechanism: Physical Barrier

• Provides a strong anti-burial moisture barrier
• Prevents water diffusion through the jacket
• Increases mechanical strength

Used heavily in direct burial cables.

3.3 Aluminum Tape (APL) Moisture Barrier
Mechanism: Hermetic Seal

Aluminum tape blocks both water and vapor molecules.

Used in:

• GYTA
• GYTY53
• Duct cables requiring high-level moisture protection

3.4 Multiple Jackets (Dual / Triple Jackets)

Each layer reduces penetration risk.

Direct-buried and submarine cables often use:

• Inner PE jacket
• Outer HDPE jacket
• UV-resistant layer

3.5 Sealant Rings and Special Closures

For submarine cables:

• Elastomeric seals
• Thermoplastic barriers
• High-pressure-resistant joints

These prevent water from entering at splice points.

4. Moisture Protection in Special Cable Types

4.1 Direct-Buried Optical Cable

Uses the strongest anti-water system:

• Water-blocking yarn
• Water-blocking tape
• Gel-filled loose tubes
• Steel tape armor
• Double jackets

Engineered to survive decades in soil moisture.

4.2 Submarine Optical Cable

Highest protection level:

• Full gel-filling design
• Multiple steel wire armors
• Polypropylene yarn flooding
• Waterproof layers
• Pressure-resistant sheath

Even micro-cracks must not allow water ingress.

4.3 ADSS Cable

Uses dry water-blocking materials to prevent dripping:

• Water-blocking yarn
• Dry water-blocking tape
• Non-gel structure
• UV/heat resistant jacket

4.4 OPGW (Optical Ground Wire)

Moisture control inside metal tubes:

• Stainless steel tube filling compound (gel)
• Helically wrapped steel/aluminum wires block external water
• Sealed stainless-steel welds prevent ingress

5. Summary Table