# What is "wavelength overlap"?

**URL:** <https://forum.ofscn.net/t/what-is-wavelength-overlap/2829>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [September 15, 2026, 11:15am UTC](https://forum.ofscn.net/t/what-is-wavelength-overlap/2829 "2026-09-15T11:15:37Z")\
**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 15, 2026, 11:15am UTC](https://forum.ofscn.net/t/what-is-wavelength-overlap/2829/1 "2026-09-15T11:15:37Z")

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If the wavelengths of the two sensors collide, will the demodulator “fight”?

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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 15, 2026, 11:16am UTC](https://forum.ofscn.net/t/what-is-wavelength-overlap/2829/2 "2026-09-15T11:16:47Z")

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In Fiber Bragg Grating (FBG) sensing systems, when the reflected wavelengths of two gratings mounted on the same fiber channel overlap or cross, the interrogator hardware itself does not sustain physical damage. However, severe data confusion and distortion occur at the **spectral analysis and signal tracking algorithm** level.

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### I. Physical and Spectral Superposition Mechanism

The reflection spectrum of a Fiber Bragg Grating (FBG) is not an ideal single, infinitely narrow linewidth but rather a spectral peak with a certain width (e.g., a typical 3\text{ dB} bandwidth is usually between 0.2\text{ nm}\sim 0.4\text{ nm}).

When the center wavelengths of two sensors approach and overlap due to changes in external physical quantities (temperature, strain, etc.):

1. **Spectral Interference and Superposition** : The reflection spectrum received by the interrogator’s photodetector is an algebraic sum of the two grating reflection spectra.
2. **Spectral Peak Deformation** : Two originally distinct Gaussian reflection peaks, when their spacing is less than the resolution limit or peak width, will merge into a broadened, asymmetric, or flattened-top distorted spectral peak.

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### II. Specific Manifestations in Interrogator Data Processing

The interrogator relies on peak-finding algorithms (such as centroid method, Gaussian fitting, or threshold method) and measurement point tracking algorithms to identify spectra. Wavelength overlap leads to the following phenomena:

1. **Measurement Point Loss (Peak Merging)**  
When two reflection peaks completely overlap into a single peak, the interrogator’s peak-finding algorithm can only detect one peak. The number of valid sensors recorded by the system instantly drops from 2 to 1, resulting in a