# What is "bandwidth" (FWHM)?

**URL:** <https://forum.ofscn.net/t/fwhm-what-is-bandwidth-fwhm/3082>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [October 19, 2026, 2:55pm UTC](https://forum.ofscn.net/t/fwhm-what-is-bandwidth-fwhm/3082 "2026-10-19T14:55:55Z")\
**Posts on this page:** 2\
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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 19, 2026, 2:55pm UTC](https://forum.ofscn.net/t/fwhm-what-is-bandwidth-fwhm/3082/1 "2026-10-19T14:55:55Z")

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Does a narrower or wider reflection peak perform better? Does it affect the demodulator’s peak finding?

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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 19, 2026, 2:58pm UTC](https://forum.ofscn.net/t/fwhm-what-is-bandwidth-fwhm/3082/2 "2026-10-19T14:58:08Z")

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In Fiber Bragg Grating (FBG) sensing technology, the “thickness” or “thinness” of a reflection peak refers to its **Full Width at Half Maximum (FWHM)**, commonly characterized in fiber optics engineering as the **3\text{dB} bandwidth** (denoted as \Delta \lambda\_{3\text{dB}} ).

The engineering conclusion is: **The reflection peak should be neither excessively thin nor excessively thick; it needs to achieve optimal matching with the spectral sampling characteristics of the fiber grating demodulator.**

* * *

### I. Effects of an “Overly Thin” Reflection Peak (Narrow Bandwidth, e.g., \< 0.1\ \text{nm} )

1. **Negative Impact on Demodulator Peak Finding (Sampling Distortion)**:
  - Whether it’s a swept laser or spectrometer-based demodulator, wavelength axis sampling is discrete (typical sampling interval is 5\ \text{pm} \sim 20\ \text{pm}).
  - If the reflection peak is extremely narrow, the effective sampling points falling within the peak region will be very few (even just 2 \sim 3 points). In this scenario, common methods like Gaussian Fitting or the Centroid Algorithm will suffer from fitting distortion due to insufficient data points, potentially leading to demodulated wavelength jumps or jitter.

2. **Reduced Optical Power and Signal-to-Noise Ratio (SNR)**:
  - At the same peak reflectivity, a narrower bandwidth means lower total integrated optical power reflected back to the demodulator, which can easily lead to a worsening system SNR.

3. **Physical and Process Limitations** :
  - Achieving an extremely narrow bandwidth typically requires a longer physical grating region (e.g., 15\ \text{mm} \sim 20\ \text{mm} or more). Longer gratings are more susceptible to non-uniform strain/temperature distributions when installed under stress or in a temperature gradient, easily causing distortion or splitting (Peak Splitting) of the originally narrow, symmetrical peak.

* * *

### II. Effects of an “Overly Thick” Reflection Peak (Broad Bandwidth, e.g., \> 0.8\ \text{nm} )

1. **Negative Impact on Demodulator Peak Finding (Reduced Positioning Accuracy)**:
  - The curvature at the peak of a wide reflection peak is very small, tending to be flat (a region where the derivative is close to 0 is wide). In the presence of background light noise or detector noise, the uncertainty in positioning the peak center wavelength \lambda\_B by extremum detection or fitting algorithms increases significantly, thus reducing the wavelength measurement resolution of the demodulation system.

2. **Severely Limited Multiplexing Capacity** :
  - Wavelength Division Multiplexing (WDM) fiber sensing networks rely on wavelength division channels. For instance, if the demodulator’s working window is 1525\ \text{nm} \sim 1565\ \text{nm} (a 40\ \text{nm} range), and the bandwidth of a single measurement point is excessively wide, larger channel spacing must be reserved to avoid spectral overlap when the sensor is subjected to strain or temperature changes. This drastically reduces the number of sensors that can be multiplexed on a single fiber.

* * *

### III. Specific Mechanisms of Influence on Common Demodulator Peak Finding Algorithms

Demodulators typically employ the following types of wavelength calculation algorithms, and the reflection peak bandwidth affects each differently:

- **Centroid / Center-of-Gravity Method** :
\lambda\_c = \frac{\sum \lambda\_i \cdot I\_i}{\sum I\_i}
This algorithm is highly dependent on peak symmetry and the number of effective data points. It performs most stably with a moderate bandwidth (typically covering 10 \sim 30 discrete sampling points); discretization errors increase sharply with too narrow a bandwidth, and the centroid shifts significantly with too wide and asymmetric a bandwidth.
- **Gaussian / Polynomial Nonlinear Fitting** :  
This method performs nonlinear least-squares fitting based on a model that approximates the theoretical reflection spectrum as a Gaussian distribution. If the peak is too narrow, lacking edge profile points, the fitting matrix can easily become singular or underfit; if the peak is too wide and accompanied by side lobes, the fitting residual will significantly increase.
- **Threshold Peak Detection** :  
Wide peaks with a flat top region are prone to extremum determination drift due to minor noise fluctuations.

* * *

### IV. Engineering Standards and Recommended Parameters

Considering the demodulator’s sampling interval, algorithm stability, multiplexing capacity, and the sensor’s resistance to strain gradients, the standard FBG reflection peak 3\text{dB} bandwidth for industrial sensing is typically controlled within **0.2\ \text{nm} \sim 0.4\ \text{nm}** (corresponding to a grating physical length of approximately 8\ \text{mm} \sim 10\ \text{mm} and a reflectivity \ge 70\% ). This range allows fiber grating demodulators to achieve excellent SNR and peak fitting accuracy.

For equipment related to fiber grating wavelength acquisition and high-precision peak finding processing, please refer to:  
[OFSCN® Fiber Bragg Grating Interrogator](https://www.ofscn.net/fbg-products/analyzer.html)

 ![OFSCN® Fiber Bragg Grating Interrogator 8CH](https://forum.ofscn.net/uploads/default/original/1X/8ecc1c112d2015cbc63afa6d42b87517befb2f87.jpeg)  
 ![OFSCN® Fiber Bragg Grating Interrogator 32CH](https://forum.ofscn.net/uploads/default/original/1X/248718a63e2aa4cf7db24ad7a4df5ace79c95a05.jpeg)