# What is the photoelastic effect?

**URL:** <https://forum.ofscn.net/t/what-is-the-photoelastic-effect/1841>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [June 5, 2026, 12:23am UTC](https://forum.ofscn.net/t/what-is-the-photoelastic-effect/1841 "2026-06-05T00:23:23Z")\
**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 5, 2026, 12:23am UTC](https://forum.ofscn.net/t/what-is-the-photoelastic-effect/1841/1 "2026-06-05T00:23:23Z")

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Why does stretching optical fiber change its refractive index? Does this counteract the effect of the length change?

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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 5, 2026, 12:24am UTC](https://forum.ofscn.net/t/what-is-the-photoelastic-effect/1841/2 "2026-06-05T00:24:13Z")

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In the field of fiber optics and fiber optic sensing, the phenomenon you mentioned is precisely dominated by the **Photoelastic Effect** (also known as the photo-elastic effect or photo-elastic effect).

Regarding your two questions, here are rigorous explanations of the physical mechanisms and quantitative analyses:

### 1. Why does stretching an optical fiber change its refractive index?

When an optical fiber is subjected to axial stretching and experiences strain, the microscopic and macroscopic mechanisms for the change in its refractive index are as follows:

1. **Microscopic Mechanism (Changes in Density and Polarizability)**:  
Optical fibers are typically made of fused silica glass (Silicon Dioxide, \text{SiO}\_2 ). When unstressed, its internal silica tetrahedral network structure is macroscopically isotropic. When the fiber is stretched, the molecular bonds along the axial direction are elongated, and according to the Poisson Effect, there is radial contraction. This physical deformation alters the local mass density and the electronic polarizability of the ions within the fiber material, thereby directly changing the material’s response to the optical electromagnetic field passing through it.

2. **Macroscopic Quantification (Pockels Photoelastic Coefficients)**:  
From the perspective of elasticity and crystal physics, mechanical strain changes the inverse dielectric tensor of a medium. For an isotropic optical fiber, when subjected to axial strain \epsilon , the effective refractive index n\_{\text{eff}} of the fiber core changes. The relationship can be described by the following formula:

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### 2. Will this counteract the effect of length change?

**It will partially counteract it, but not completely.**

We can intuitively understand the dynamic competition between these two effects using the classic Bragg wavelength formula for Fiber Bragg Gratings (FBGs). The reflection center wavelength \lambda\_B of an FBG is defined as:

\lambda\_B = 2 n\_{\text{eff}} \Lambda

Where \Lambda is the physical period (characteristic spatial length) of the grating. When the fiber is stretched and experiences an axial strain \epsilon , the relative change in the Bragg wavelength can be expressed as:

\frac{\Delta \lambda\_B}{\lambda\_B} = \frac{\Delta \Lambda}{\Lambda} + \frac{\Delta n\_{\text{eff}}}{n\_{\text{eff}}}

Let’s compare the two terms on the right side of the equation:

1. **Length (Geometric) Effect**:  
The relative change in the grating period is directly equal to the axial strain:

2. **Photoelastic (Physical) Effect**:  
As derived earlier, the change in refractive index is:

#### Combined Superposition Effect:

Substituting these two terms into the total formula:

\frac{\Delta \lambda\_B}{\lambda\_B} = \epsilon - P\_e \epsilon = (1 - P\_e) \epsilon

Substituting the physical constants for fused silica ( P\_e \approx 0.22 ):

\frac{\Delta \lambda\_B}{\lambda\_B} = (1 - 0.22) \epsilon = 0.78 \epsilon

- **Cancellation Ratio** : The “decrease in refractive index” caused by the photoelastic effect **cancels out approximately 22%** of the wavelength drift caused by the “physical elongation”.
- **Final Result** : Since 1 - P\_e \gt 0 (i.e., 0.78 is greater than 0 ), **the overall effect is still positive (redshift)**. Therefore, when stretching an optical fiber, its reflection wavelength will ultimately become longer, but the sensitivity is reduced by the photoelastic effect. For standard FBGs around 1550\ \text{nm} , their strain sensitivity typically manifests as approximately 1.2\ \text{pm}/\mu\epsilon (microstrain).

* * *

### 3. Physical Calibration in Industrial Sensing Applications

In practical engineering applications, the high-precision fiber optic sensors produced by Dacheng Yongsheng (OFSCN®) are based on this physical phenomenon for their underlying design and factory calibration:

1. **[OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor](https://www.ofscn.net/fbg-products/strain-sensor.html)**  
This sensor utilizes an elastic alloy tube packaging process to ensure that external strain is accurately and losslessly transmitted to the grating in the fiber core. Its wavelength drift calculation formula directly uses the sensitivity constant after the cancellation of geometric deformation and photoelastic effects.

2. **[OFSCN® Fiber Bragg Grating Strain Gauge](https://www.ofscn.net/fbg-products/strain-gauge.html)**  
Suitable for surface bonding or welding strain gauges, it covers short-length surface strain measurements that cannot be achieved with tube packaging. The strain-wavelength calibration performed at the factory is also strictly based on the aforementioned photoelastic theory.

If you need more information on the technical specifications and selection of strain and stress sensors, you can visit the official **[OFSCN® FBG Strain Sensor Products Aggregation Link](https://www.ofscn.net/fbg-topics/strain.html)** for a search.