# What is polyimide coating?

**URL:** <https://forum.ofscn.net/t/what-is-polyimide-coating/1999>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [June 18, 2026, 2:55pm UTC](https://forum.ofscn.net/t/what-is-polyimide-coating/1999 "2026-06-18T14:55:55Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\
**Post date:** [June 18, 2026, 2:55pm UTC](https://forum.ofscn.net/t/what-is-polyimide-coating/1999/1 "2026-06-18T14:55:56Z")

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Why can this yellow casing withstand temperatures of 300 degrees Celsius?

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**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\
**Post date:** [June 18, 2026, 2:56pm UTC](https://forum.ofscn.net/t/what-is-polyimide-coating/1999/2 "2026-06-18T14:56:25Z")

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In the field of optical fibers, what you’re referring to as the  
“yellow coating” is actually the **Polyimide (PI) coating**. Polyimide typically has a semi-transparent amber (dark yellow) color. Its ability to operate stably for extended periods in high-temperature environments of 300\ \text{°C} and above stems mainly from its unique chemical structure and excellent material properties:

### 1. Why Can Polyimide (PI) Withstand High Temperatures of 300\ \text{°C} ?

- **Aromatic Backbone** :  
The main molecular chain of polyimide consists of a large number of alternating aromatic rings (benzene rings) and imide rings highly conjugated. This structure possesses extremely high covalent bond energy, making the chemical bonds very stable and difficult to break upon heating.
- **High Glass Transition Temperature ( T\_g ) and Decomposition Temperature ( T\_d )**:  
Standard acrylate coatings used in optical fibers soften or decompose at temperatures above 85\ \text{°C} to 150\ \text{°C} . In contrast, the glass transition temperature ( T\_g ) of polyimide is typically as high as 350\ \text{°C} to 410\ \text{°C} or more, and its decomposition temperature ( T\_d ) exceeds 500\ \text{°C} . Therefore, at 300\ \text{°C} , its physical properties and mechanical strength remain highly stable, without melting or flowing.
- **Excellent Low Thermal Outgassing and Low Coefficient of Thermal Expansion (CTE)**:  
In a 300\ \text{°C} environment, polyimide exhibits minimal outgassing (low volatility). Additionally, its coefficient of thermal expansion (CTE) is relatively close to that of the silica glass substrate. This effectively prevents excessive micro-bending stress between the coating and the fiber’s quartz cladding due to drastic temperature changes, thus ensuring low-loss transmission in high-temperature conditions.

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### 2. Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) High-Temperature Polyimide Fiber Products

Leveraging high-performance polyimide materials, Beijing Dacheng Yongsheng Technology Co., Ltd. has introduced a range of high-temperature resistant special optical fibers suitable for demanding environments such as aerospace, oil and geothermal exploration, and high-temperature sensing.

#### Single-Mode Polyimide Fiber

[OFSCN® 300℃ SM Polyimide Optical Fiber](https://www.ofscn.org/optical-fibers/polyimide-coated-sm-300.html)

- **Operating Temperature Range** ： -200\ \text{°C} to +350\ \text{°C} (or -270\ \text{°C} to +350\ \text{°C} )
- **Dimensional Specifications** ：Cladding diameter 9\ \mu\text{m} , coating diameter 125\ \mu\text{m} , and jacket diameter 155\ \mu\text{m} .
- **Typical Appearance** ：Due to the extremely thin (approximately 15\ \mu\text{m} per side) polyimide coating, the fiber exhibits its characteristic golden-yellow color.

 ![OFSCN® 300℃ SM Polyimide Optical Fiber](https://forum.ofscn.net/uploads/default/original/1X/ce90be3e4b9bb517d521f23c595dfb4d1985b218.jpeg)

 ![OFSCN® 300℃ SM Polyimide Optical Fiber 2](https://forum.ofscn.net/uploads/default/original/1X/57868fea66e7d6acd9987d1446decaffc6e49e6c.jpeg)

#### Polarization-Maintaining Polyimide Fiber

[OFSCN® 300℃ Polyimide Panda-type PM Optical Fiber](https://www.ofscn.org/optical-fibers/pi-coated-pm1550-125-155-300.html)

- **Product Features** ：High-temperature resistant polarization-maintaining fiber designed with a high-precision panda stress structure, capable of operating at temperatures ranging from -200\ \text{°C} to +350\ \text{°C} .

 ![OFSCN® 300℃ Polyimide Panda-type PM Optical Fiber](https://forum.ofscn.net/uploads/default/original/1X/37f0bcf3b6e1e25f0a7bfba3f020e7cd16c17132.jpeg)

 ![OFSCN® 300℃ Polyimide Panda-type PM Optical Fiber 2](https://forum.ofscn.net/uploads/default/original/1X/87ff2e8aa14697b19e94e3244bd04ae952bce9a9.jpeg)

#### Large-Core Polyimide Fiber

[OFSCN® Polyimide Large-Core Optical Fiber](https://www.ofscn.org/optical-fibers/300c-polyimide-large-core-fiber.html)

- **Product Features** ：A series of high-temperature resistant, large-core products widely used for high-power laser transmission or multimode fiber sensing.

 ![OFSCN® Polyimide Large-Core Optical Fiber](https://forum.ofscn.net/uploads/default/original/1X/596c6107a14110c5451441273f46a1ab31866f10.jpeg)

#### Small Diameter Single-Mode Polyimide Fiber

[OFSCN® 300℃ Small diameter optical fiber](https://www.ofscn.org/optical-fibers/small-diameter-pi-sm.html)

- **Dimensional Specifications** ：Cladding diameter of only 80\ \mu\text{m} and jacket diameter of 100\ \mu\text{m} , suitable for ultra-small bend radii or high-integration sensing packaging.

 ![OFSCN® 300℃ Small diameter optical fiber](https://forum.ofscn.net/uploads/default/original/1X/6ad91989a10bc68960adc770697b07c1c82c851b.jpeg)

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For more technical specifications and other special fiber product categories, please visit:  
[OFSCN® Special Fiber Product Categories Link](https://www.ofscn.org/optical-fibers.html)