# What is "optical fiber annealing"? **URL:** \ **Category:** OFSCN Technical Knowledge Base\ **Tags:** fbg-technology\ **Created:** [October 10, 2026, 7:49am UTC](https://forum.ofscn.net/t/what-is-optical-fiber-annealing/2984 "2026-10-10T07:49:59Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\ **Post date:** [October 10, 2026, 7:49am UTC](https://forum.ofscn.net/t/what-is-optical-fiber-annealing/2984/1 "2026-10-10T07:49:59Z")
Does the glass need to be slowly cooled after being drawn out? Will it become brittle if not annealed? ---
**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:** [October 10, 2026, 7:50am UTC](https://forum.ofscn.net/t/what-is-optical-fiber-annealing/2984/2 "2026-10-10T07:50:44Z")
In optical physics and fiber manufacturing engineering, “Optical Fiber Annealing” typically encompasses two technical meanings: first, **controlled cooling (slow cooling/annealing) during the fiber drawing process**; and second, **high-temperature thermal annealing and aging treatment after the preparation of specialty optical fibers and Fiber Bragg Gratings (FBGs)**. Regarding your question, “Does glass need to be cooled slowly after being drawn? Will it become brittle if not annealed?”, the specific mechanisms and engineering principles are as follows: * * * ### I. Slow Cooling and Annealing Mechanism During Fiber Drawing Optical fibers are drawn from a preform made of quartz glass, melted and thinned in a high-temperature drawing furnace (typically at 1900\ \text{℃} \sim 2100\ \text{℃} ), and then drawn downwards at extremely high speeds (several to tens of meters per second). #### 1. Why Can’t It Be Quenched (Why Is Controlled Slow Cooling Necessary)? If the molten glass is rapidly cooled (quenched) by direct exposure to ambient air after being drawn from the high-temperature furnace: - **Freezing of Internal Thermal Stress** : Silica glass has low thermal conductivity. The surface of the optical fiber hardens and cools extremely quickly, while the core cools more slowly. This creates a severe temperature gradient across the radial cross-section. Due to the inconsistent rates of thermal expansion and contraction, the outer layer is compressed, and the inner layer is under tension, resulting in immense residual thermal stress being frozen into the glass network. - **Significant Increase in Material Mechanical Brittleness** : High residual internal stress makes the optical fiber extremely sensitive to microscopic surface defects. When subjected to minor bending or external tensile forces, surface Griffith microcracks can rapidly expand unstably under the drive of residual stress, drastically increasing the probability of **brittle fracture and significantly reducing the mechanical tensile strength**. - **Higher Fictive Temperature and Increased Rayleigh Scattering** : Rapid cooling traps the silica in a non-equilibrium, high-temperature disordered network structure, effectively freezing in a higher fictive temperature (T\_f). This leads to increased microscopic density fluctuations and refractive index fluctuations within the glass, causing a **significant increase in Rayleigh Scattering Loss** at communication wavelengths. #### 2. Engineering Implementation and Slow Cooling/Annealing Apparatus Modern fiber drawing towers are equipped with **slow cooling channels or annealing furnaces (Annealing Furnace / Slow Cooling Tube)** located below the main heating furnace. By precisely controlling the cooling curve, the bare fiber is slowly cooled at a set rate through the glass transition temperature region (around 1000\ \text{℃} \ \sim 1200\ \text{℃} ). This promotes sufficient relaxation of the microscopic network structure of silica, thereby releasing residual stress, reducing optical loss, and ensuring mechanical flexibility and tensile strength. * * * ### II. “Thermal Annealing Treatment” in Optical Fiber Devices and Sensor Fields In addition to the physical slow cooling during the drawing stage, in the field of specialty optical fiber sensing (such as Fiber Bragg Gratings, FBGs), “annealing” typically refers to controlled constant-temperature treatment of the formed fiber or the device after grating inscription, conducted in a high-temperature furnace: 1. **Eliminating Residual Stress and Cold Working Defects** : To remove localized mechanical stresses introduced by stripping, splicing, or mechanical gripping. 2. **Stabilizing Refractive Index Modulation Structure** : After inscription of gratings using UV or femtosecond lasers, metastable color centers or stress defects exist within the core. Annealing in an environment at a temperature higher than the expected operating temperature (e.g., 300\ \text{℃} \sim 800\ \text{℃}) for several hours to tens of hours can pre-emptively eliminate shallow-level unstable defects, preventing Bragg wavelength drift or reflectivity decay when the sensor operates at high temperatures for extended periods. * * * ### III. Industrial Product Line Description - **Drawing and Annealing Equipment** : Drawing towers and glass melting/annealing furnaces are part of the preform drawing equipment for optical fiber raw materials and **are not part of Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®)'s product line**. - **High-Temperature Resistant Specialty Optical Fibers and Annealing Treatment** : OFSCN® focuses on extreme environment optical fibers and fiber optic sensing technologies. For applications requiring long-term stable operation at high temperatures (involving stress release and high-temperature aging treatment), we primarily utilize optical fibers with high-temperature polyimide coating or metallization coating: - [OFSCN® Gold-coated Optical Fiber](https://www.ofscn.org/optical-fibers/gold-coated.html) (Operating temperature range: single-mode gold-coated fiber -270\ \text{℃} \ \sim 700\ \text{℃}, multi-mode gold-coated fiber -270\ \text{℃} \ \sim 650\ \text{℃}) - [OFSCN® 300℃ SM Polyimide Optical Fiber](https://www.ofscn.org/optical-fibers/polyimide-coated-sm-300.html) (Operating temperature range: -200\ \text{℃} \ \sim 350\ \text{℃}) ![OFSCN® Gold-coated Optical Fiber](https://forum.ofscn.net/uploads/default/original/1X/a0527f8c12cba5c5a4c64951106d0779161ec84a.jpeg) ![OFSCN® 300℃ SM Polyimide Optical Fiber](https://forum.ofscn.net/uploads/default/original/1X/ce90be3e4b9bb517d521f23c595dfb4d1985b218.jpeg)