Will the place where the grating is etched break if it is bent 10,000 times?
Whether a Fiber Bragg Grating (FBG) will break after being bent 10,000 times in the area where the grating is written (grating region) depends on three core physical and engineering factors: bending radius, grating fabrication process (writing method), and the intrinsic strength of the fiber used.
Here’s a detailed analysis from the perspectives of optical physics and engineering mechanics:
1. Relationship Between Bending Radius and Bending Strain
When a fiber is bent, the outer side experiences tensile strain, and the inner side experiences compressive strain. The estimated formula for the maximum tensile strain \varepsilon caused by bending is:
\varepsilon = \frac{r_{\text{cladding}}}{R}
Where r_{\text{cladding}} is the radius of the fiber’s silica cladding (for standard cladding diameter fibers of 125\ \mu\text{m} , the cladding radius is r_{\text{cladding}} = 62.5\ \mu\text{m} ) and R is the bending radius.
- Large Bending Radius (e.g., R \ge 25\ \text{mm} ): The corresponding bending strain is extremely small (less than 0.25\% , or about 2500\ \mu\varepsilon ). Under this low stress state, fatigue in silica glass occurs very slowly. Even if bent cyclically 10,000 times (or even millions of times), a high-quality grating will absolutely not break.
- Small Bending Radius (e.g., R \le 5\ \text{mm} ): The corresponding bending strain is very large (greater than 1.25\% , or about 12500\ \mu\varepsilon ). At this level, the internal stress in the fiber is extremely high, accelerating the initiation and propagation of micro-cracks in the material. If subjected to cyclic bending, the fiber is likely to undergo fatigue fracture in far fewer than 10,000 cycles.
2. Grating Fabrication Process: Coating Stripped vs. Through-Coating
The treatment of the fiber surface during grating writing determines the local mechanical lifetime limit of the grating region:
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Traditional UV-Exposed Gratings (Coating Stripped and Re-Coated):
Traditional ultraviolet (UV) grating writing requires the polymer protective coating layer (e.g., polyimide or acrylate) on the fiber surface to be stripped off chemically or mechanically, exposing the bare silica glass. The stripping process inevitably introduces microscopic scratches (Griffith micro-cracks) on the glass surface. Even after re-coating, these micro-cracks will rapidly propagate under cyclic bending stress. Therefore, gratings produced by traditional stripping and re-coating are extremely prone to breakage under frequent cyclic bending, and the probability of fracture after 10,000 bends is very high. -
Femtosecond Laser Through-Coating Gratings (Coating Retained):
Advanced femtosecond laser writing technology allows the beam to be focused directly through the fiber’s coating layer to complete the grating inscription within the fiber core. Because no coating stripping is required, the silica glass surface remains protected by the original factory coating and is undamaged or uncontaminated, preserving the extremely high intrinsic fracture strength of the bare fiber (greater than 5.5\ \text{GPa} ). Such gratings exhibit excellent fatigue resistance.
3. High-Strength Bend-Resistant Grating Product Solutions
For engineering applications requiring frequent bending, high strain, or dynamic fatigue, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) offers high-strength fiber grating products based on femtosecond laser technology:
OFSCN® High-Strength Fiber Bragg Gratings / FBG Strings (Bare)
This product is specifically designed for extreme mechanical environments and can withstand extremely high-frequency and amplitude bending fatigue:
- Non-Damaging Inscription: Utilizes point-by-point femtosecond laser writing, which does not damage the fiber coating during inscription, perfectly preserving the fiber’s original mechanical strength.
- Ultra-High Strength Fiber: Uses strictly selected OFSCN® high-strength single-mode polyimide fiber with an outer diameter of 155\ \mu\text{m} .
- Ultra-Large Strain Range: Usable strain range reaches \ge 25000\ \mu\varepsilon . Under a reasonable bending radius, its fatigue life is exceptionally good, and 10,000 bends will not cause physical damage to its structure.
Official Product Link:
OFSCN® High-Strength Fiber Bragg Gratings / FBG Strings (Bare) Official Link
OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare)
If the bending amplitude or strain requirements are slightly lower, standard-grade femtosecond gratings can also be used:
- Non-Damaging Inscription: Also based on femtosecond laser through-coating technology.
- Usable Strain Range: Usable strain range at room temperature is \le 15000\ \mu\varepsilon .
Official Product Link:
OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare) Official Link
Technical Summary and Recommendation
If you plan to cyclically bend the grating region 10,000 times:
- Strictly avoid ordinary traditional gratings with stripped and re-coated coatings, otherwise, their grating regions are highly prone to brittle fracture under cyclic bending.
- Be sure to choose high-strength gratings written via femtosecond laser through-coating (such as OFSCN® High-Strength FBG).
- Control the working bending radius. It is recommended to design a bending radius of R \ge 20\ \text{mm} . If space is limited, the bending radius should not be less than 15\ \text{mm} to limit the bending strain within the safe fatigue threshold, thereby ensuring a mechanical bending life of 10,000 cycles or more.