# What is "wavelength drift"?

**URL:** <https://forum.ofscn.net/t/what-is-wavelength-drift/2913>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [September 22, 2026, 11:52am UTC](https://forum.ofscn.net/t/what-is-wavelength-drift/2913 "2026-09-22T11:52:38Z")\
**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:** [September 22, 2026, 11:52am UTC](https://forum.ofscn.net/t/what-is-wavelength-drift/2913/1 "2026-09-22T11:52:38Z")

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Why does the color (wavelength) of light change with temperature?

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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:** [September 22, 2026, 11:58am UTC](https://forum.ofscn.net/t/what-is-wavelength-drift/2913/2 "2026-09-22T11:58:45Z")

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In Fiber Bragg Grating (FBG) sensing technology, the “color change of light” essentially refers to the **center wavelength drift** of the fiber grating’s reflected spectrum.

This physical phenomenon can be fundamentally explained by the Bragg reflection condition.

* * *

### I. Core Physical Principle: Bragg Reflection Condition

A fiber grating is a periodically modulated refractive index structure formed within the fiber core. When broadband continuous light is injected into the fiber, only light waves that satisfy the specific Bragg condition are coherently reflected back. This reflected wavelength (Bragg wavelength, \lambda\_B) satisfies:

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

Where:

- \lambda\_B is the center reflection wavelength of the fiber grating
- n\_{\text{eff}} is the effective refractive index of the fiber core
- \Lambda is the spatial modulation period of the grating

* * *

### II. Two Key Effects of Temperature Change Causing Wavelength Drift

When the temperature changes (\Delta T), the \lambda\_B drift is primarily caused by the combined effects of the following two physical phenomena:

#### 1. Thermo-optic Effect (Dominant Factor)

The refractive index of the silica (\text{SiO}\_2) fiber material changes with temperature.

- The physical quantity describing this characteristic is the **thermo-optic coefficient** (\zeta = \frac{1}{n\_{\text{eff}}} \frac{\partial n\_{\text{eff}}}{\partial T}).
- The thermo-optic coefficient for fused silica fiber is approximately \zeta \approx 8.6 \times 10^{-6}\text{ / }^\circ\text{C}.
- The thermo-optic effect typically accounts for **over 90%** of the total wavelength drift caused by temperature changes.

#### 2. Thermal Expansion Effect

The fiber material undergoes slight thermal expansion or contraction when heated or cooled, directly altering the physical period \Lambda of the grating.

- The physical quantity describing this characteristic is the **thermal expansion coefficient** (\alpha = \frac{1}{\Lambda} \frac{\partial \Lambda}{\partial T}).
- The thermal expansion coefficient of fused silica glass is relatively small, approximately \alpha \approx 0.55 \times 10^{-6}\text{ / }^\circ\text{C}.

* * *

### III. Comprehensive Temperature Response Sensitivity

By taking the total differential of the Bragg equation with respect to temperature T, the relationship between the relative change in wavelength and the change in temperature can be derived:

\frac{\Delta \lambda\_B}{\lambda\_B} = (\alpha + \zeta) \Delta T

This yields the temperature drift formula:

\Delta \lambda\_B = \lambda\_B (\alpha + \zeta) \Delta T

In the communication band (taking \lambda\_B \approx 1550\text{ nm} as an example), the temperature sensitivity of a bare fiber grating is approximately **10 \sim 11\text{ pm/}^\circ\text{C}** (meaning for every 1^\circ\text{C} increase in temperature, the reflection wavelength drifts about 0.01\text{ nm} towards longer wavelengths). By capturing this wavelength drift with a high-precision fiber grating demodulator, the temperature change of the measured environment can be accurately inverted.

* * *

### IV. Related OFSCN® Product References

Based on this principle, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) has developed a series of fiber Bragg grating temperature sensors encapsulated in seamless stainless steel tubes, covering environments from normal temperature to ultra-low temperatures:

- [OFSCN® 100°C Fiber Bragg Grating Temperature Sensor](https://www.ofscn.net/fbg-products/temperature-sensor-100.html)
- [OFSCN® 300°C Fiber Bragg Grating Temperature Sensor](https://www.ofscn.net/fbg-products/temperature-sensor-300.html)
- [OFSCN® 500°C Fiber Bragg Grating Temperature Sensor](https://www.ofscn.net/fbg-products/temperature-sensor-350.html)
- [OFSCN® 800°C Fiber Bragg Grating Temperature Sensor](https://www.ofscn.net/fbg-products/temperature-sensor-800.html)
- Product Aggregation: [OFSCN® FBG Temperature Sensor Products Aggregation Link](https://www.ofscn.net/fbg-topics/temperature.html)

 ![OFSCN 300°C Fiber Bragg Grating Temperature Sensor](https://forum.ofscn.net/uploads/default/original/1X/74932c0f3b71797adb12ca84a053059151051842.jpeg)  
 ![OFSCN 300°C Fiber Bragg Grating Temperature Sensor Detail](https://forum.ofscn.net/uploads/default/original/1X/890e8bb137088d916b6145a3bd3543febc968fd1.jpeg)