When is Carbon Coating a Must? A Deep Dive into the Dual Protection Mechanism of Polyimide + Carbon Coated Fiber

In the field of sensing and communication in extreme environments, standard optical fibers often fall short. Our 300°C Polyimide Multimode Fiber from the OFSCN® brand, under [Beijing Dacheng Yongsheng Technology Co., Ltd. (DCYS)], has become an industry favorite due to its superior temperature and chemical resistance.

However, for some extremely harsh applications, relying solely on the Polyimide (PI) coating is not enough. Engineers often face a critical choice: is the additional Carbon Coating necessary?

This article will deeply analyze the protection mechanism when a carbon coating is added to polyimide fiber and clearly define the scenarios where carbon coating is an indispensable "life support."

I. The Foundation: The Role of Polyimide (PI) Coating

The polyimide coating serves as the primary protection layer for this fiber, mainly addressing two core weaknesses of traditional fibers in high temperatures:

1. Thermal Stability: The PI coating withstands an ultra-wide operating temperature range of -200°C to +350°C, overcoming the fatal flaw of standard acrylate coatings, which soften and decompose above 85°C.
2. Chemical and Mechanical Protection: PI offers excellent resistance to chemical solvents and acid/alkaline corrosion, and provides a tensile strength of 100 kpsi, superior to standard fibers.

Conclusion: Polyimide coating performs excellently in hot, dry environments or where basic mechanical strength is required.

II. The Ultimate Reinforcement: The Protection Mechanism of Carbon Coating

If the polyimide coating is the fiber’s “thermal stability jacket,” the carbon coating is an “hermetic armor.”

The carbon coating is a thin layer, approximately 50 nanometers thick, of Amorphous Carbon, directly applied to the silica cladding surface (sandwiched between the carbon layer and the polyimide coating). Its core mechanism involves two key functions:

1. Blocking Moisture Ingress (Moisture Barrier)

One of the greatest threats to the long-term reliability of optical fiber is static fatigue: water diffuses into micro-cracks on the silica glass surface, causing these cracks to slowly propagate under stress, eventually leading to fiber fracture (also known as hydrolysis).

• Mechanism: The carbon coating has extremely low gas permeability, forming a dense hermetic seal that completely prevents water molecules (H₂O) from the environment from penetrating the silica glass surface, thereby eliminating static fatigue entirely.
• Value: The lifetime and reliability of carbon-coated fiber far exceed those of non-hermetic fibers, even in high-temperature/high-humidity or submerged environments.

2. Resisting Hydrogen Darkening

In downhole oil/gas, geothermal exploration, or certain chemical reaction environments, large amounts of hydrogen gas (H₂) are present. Hydrogen can diffuse into the fiber core and react with defects (like germanium oxygen vacancies), causing a sharp increase in fiber absorption loss, known as hydrogen darkening.

• Mechanism: The carbon coating effectively blocks the diffusion of small, highly permeable hydrogen molecules (H₂) to the fiber core, fundamentally preventing the phenomenon of hydrogen darkening.
• Value: Ensures the long-term attenuation stability of the fiber in hydrogen-rich environments, especially critical for sensing applications requiring precise signal transmission.

III. When is Carbon Coating a "Must"?

Carbon coating represents the highest standard of reliability. If your application meets any of the following conditions, a strong recommendation or even a mandatory requirement is to choose the Polyimide + Carbon Coating dual-protection fiber:

Scenarios Where Carbon Coating is a Must	Pain Point (PI Coating Alone is Insufficient)	Typical Application Fields
High Humidity / Submersion	Presence of moisture leads to silica hydrolysis, causing static fatigue and fracture.	Subsea cables, long-term underwater monitoring, high-humidity tropical environments.
High Hydrogen Environments	Hydrogen molecules diffuse into the core, causing sharp deterioration of fiber attenuation (Hydrogen Darkening).	Downhole Sensing (Oil & Gas), hydrogen production and storage, geothermal exploration.
Prolonged High Stress	The fiber constantly endures sustained high tensile or bending stress during its service life.	Structural Health Monitoring (SHM), long-term high-stress applications in specialized cables.
Highest Reliability Standard	Projects with extremely demanding fiber lifespan requirements, where failure is unacceptable.	Core aerospace systems, critical sensors in nuclear power plants.

Summary: Dual Protection for Extreme Reliability

The OFSCN® Polyimide + Carbon Coated Fiber combines the advantages of both coatings:

1. PI Layer: Provides temperature resistance and basic mechanical/chemical protection.
2. Carbon Layer: Provides hermeticity, completely eliminating the risks of hydrolysis fatigue and hydrogen darkening.

For mission-critical projects demanding extreme longevity and ultra-high reliability in harsh environments, the Polyimide + Carbon Coating is your wise and necessary choice.

For more technical parameters, splicing recommendations, or fiber selection consultation based on your specific application, please feel free to contact us.

[Beijing Dacheng Yongsheng Technology Co., Ltd. (DCYS)]

Official Website: https://www.ofscn.org

Product Link: https://www.ofscn.org/optical-fibers/polyimide-coated-mm-300.html