# What is a "spectral distortion alarm"?

**URL:** <https://forum.ofscn.net/t/what-is-a-spectral-distortion-alarm/2999>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [October 14, 2026, 12:33pm UTC](https://forum.ofscn.net/t/what-is-a-spectral-distortion-alarm/2999 "2026-10-14T12:33:28Z")\
**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:** [October 14, 2026, 12:33pm UTC](https://forum.ofscn.net/t/what-is-a-spectral-distortion-alarm/2999/1 "2026-10-14T12:33:28Z")

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If the sensor is squashed and the spectrum becomes distorted, will the software alert me?

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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:** [October 14, 2026, 12:34pm UTC](https://forum.ofscn.net/t/what-is-a-spectral-distortion-alarm/2999/2 "2026-10-14T12:34:48Z")

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### I. Physical Mechanism: Why Does “Squashing” a Sensor Cause Spectral Distortion?

The central reflection wavelength of a Fiber Bragg Grating (FBG) is determined by:

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

where n\_{\text{eff}} is the effective refractive index of the fiber core and \Lambda is the grating period. In an ideal operating state, the reflection spectrum exhibits a symmetrical, narrow-band Gaussian or near-sinc profile.

When a fiber sensor is subjected to non-uniform lateral squeezing (i.e., “squashed”), it disrupts the waveguide’s symmetry at both microscopic and macroscopic levels:

1. **Photo-induced Birefringence and Peak Splitting** : Non-axisymmetric transverse stress, through the elasto-optic effect, breaks the cylindrical symmetry of a single-mode fiber, causing a separation in the effective refractive indices of orthogonal polarization modes (x-axis and y-axis) (n\_x \neq n\_y). This leads to the original single Bragg reflection peak splitting into two distinct polarization reflection peaks.
2. **Strain Gradient and Spectral Peak Broadening** : If the squashing region is unevenly distributed along the grating’s axis, it will cause a gradient variation in the grating period \Lambda along the length. This results in severe broadening of the reflection peak, the emergence of side peaks (multi-peaking), and a drastic drop in peak reflectivity (light intensity).

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### II. Can Software Provide Alerts? Common Judgment and Alarm Mechanisms

In fiber grating demodulation systems, software recognition and alerting for such physical distortions rely on the configured demodulation algorithms and threshold rules:

1. **Light Intensity Threshold and “Point Drop/Signal Loss” Alarm (Threshold / Signal Loss Alarm)**

2. **Spectral Fitting Loss of Lock and Spectral Width Exceeded Alarm (FWHM / Fitting Residual Alarm)**

3. **Wavelength Jump and Rate of Change Alarm (Rate of Change Alarm)**

4. **Real-time Spectrum Diagnosis (Spectrum View)**

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### III. Related Equipment and Engineering Protection Solutions

In on-site engineering implementations, to accurately capture the spectral evolution of the grating, the supporting demodulation system must possess stable full-spectrum acquisition and wavelength extraction capabilities.

#### 1. Demodulator Hardware and System Support

[OFSCN® Fiber Bragg Grating Interrogator](https://www.ofscn.net/fbg-products/analyzer.html) supports multi-channel (4, 8, 16, 32 channels optional), configurable sampling rates ( 1\ \text{Hz} \sim 100\ \text{Hz} ), and wavelength resolution up to 1\ \text{pm} or 0.1\ \text{pm}. Its accompanying software provides channel spectrum analysis and underlying data interfaces, facilitating spectral status monitoring and wavelength out-of-bounds alarm configuration.

 ![OFSCN® Fiber Bragg Grating Interrogator 8 Channel](https://forum.ofscn.net/uploads/default/original/1X/8ecc1c112d2015cbc63afa6d42b87517befb2f87.jpeg)

 ![OFSCN® Fiber Bragg Grating Interrogator 32 Channel](https://forum.ofscn.net/uploads/default/original/1X/248718a63e2aa4cf7db24ad7a4df5ace79c95a05.jpeg)

#### 2. Packaging and Protection to Avoid “Squashing”

Bare fiber Bragg gratings have weak resistance to radial squeezing. If the measurement environment involves unavoidable mechanical extrusion, concrete pouring, or heavy lateral pressure, grating sensors encapsulated with seamless steel pipes or rigid alloy sheaths (such as armored structures with metal capillaries) should be used. This prevents the grating core from experiencing shear forces and radial stress concentration, thereby avoiding spectral distortion from the physical source.