# What is "wavelength allocation planning"?

**URL:** <https://forum.ofscn.net/t/what-is-wavelength-allocation-planning/2943>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [September 28, 2026, 8:04am UTC](https://forum.ofscn.net/t/what-is-wavelength-allocation-planning/2943 "2026-09-28T08:04:17Z")\
**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 28, 2026, 8:04am UTC](https://forum.ofscn.net/t/what-is-wavelength-allocation-planning/2943/1 "2026-09-28T08:04:17Z")

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How should the initial wavelength of each sensor be designed to ensure they do not “clash”?

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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 28, 2026, 8:04am UTC](https://forum.ofscn.net/t/what-is-wavelength-allocation-planning/2943/2 "2026-09-28T08:04:37Z")

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To avoid spectral overlap (i.e., “wavelength crosstalk” or “collision”) when multiple sensors are connected in series on the same optical channel in a Wavelength Division Multiplexing (WDM)-based Fiber Bragg Grating (FBG) sensing network, systematic **Wavelength Allocation Planning** is essential.

The initial center wavelength (\lambda\_0) for each sensor is designed, primarily following these physical calculation and engineering design steps:

* * *

### 1. Calculate the Maximum Dynamic Wavelength Drift of a Single Sensor (\Delta\lambda\_{\text{dynamic}} )

Based on the maximum variation range of the measured physical quantity (temperature, strain, stress, etc.), calculate the maximum redshift (drift towards longer wavelengths) and blueshift (drift towards shorter wavelengths) that the sensor may experience during operation:

- **Wavelength Drift due to Temperature** :
\Delta\lambda\_T = K\_T \cdot \Delta T
_The temperature sensitivity of a bare fiber grating at 1550\ \text{nm} is approximately K\_T \approx 10\ \text{pm/}^\circ\text{C}; if encapsulated in metal such as seamless steel tubes, the sensitivity typically increases to 20\sim35\ \text{pm/}^\circ\text{C} due to differences in thermal expansion coefficients._
- **Wavelength Drift due to Strain** :
\Delta\lambda\_\varepsilon = K\_\varepsilon \cdot \Delta\varepsilon
_In the 1550\ \text{nm} wavelength band, the strain sensitivity is K\_\varepsilon \approx 1.2\ \text{pm/}\mu\varepsilon ._

Based on engineering limit conditions, the maximum operating range for the sensor is determined as:

[\lambda\_{0,i} - \Delta\lambda\_i^{-}, \lambda\_{0,i} + \Delta\lambda\_i^{+}]

* * *

### 2. Set a Guard Band (\Delta\lambda\_{\text{guard}} )

To prevent interference between the boundaries of adjacent channels, a guard band \Delta\lambda\_{\text{guard}} must be added between the dynamic ranges of two sensors during wavelength planning. The following factors should be considered:

1. **Reflection Spectrum Bandwidth of the FBG Itself** : The 3dB bandwidth of a conventional FBG is typically around 0.2\sim0.4\ \text{nm}.
2. **Manufacturing Tolerances** : The factory wavelength of fiber gratings usually has manufacturing tolerances (e.g., \pm0.2\ \text{nm}\sim\pm0.3\ \text{nm}).
3. **Demodulation Algorithm Resolution Margin** : The minimum signal-to-noise ratio requirement for peak identification algorithms in demodulators to separate dual peaks.

_Engineering practice typically recommends a guard band of \Delta\lambda\_{\text{guard}} \ge 0.5\sim1.0\ \text{nm} ._

* * *

### 3. Calculate the Initial Wavelength Spacing Between Adjacent Sensors (\Delta\lambda\_{\text{spacing}} )

The difference between the initial center wavelengths of two adjacent sensors (sensor i and sensor i+1) must satisfy the non-overlap criterion:

\lambda\_{0,i+1} - \lambda\_{0,i} \ge \Delta\lambda\_i^{+} + \Delta\lambda\_{i+1}^{-} + \Delta\lambda\_{\text{guard}}

**Example Calculation** :  
If sensors i and i+1 are both temperature sensors, with a maximum operating temperature of +100^\circ\text{C} (resulting in approximately +3.0\ \text{nm} redshift) and a minimum operating temperature of -20^\circ\text{C} (resulting in approximately -0.6\ \text{nm} blueshift), and a guard band of 0.8\ \text{nm} is set:

\lambda\_{0,i+1} - \lambda\_{0,i} \ge 3.0\ \text{nm} + 0.6\ \text{nm} + 0.8\ \text{nm} = 4.4\ \text{nm}

This means the initial center wavelength spacing between adjacent sensors at normal temperature must be set to at least 4.4\ \text{nm} .

* * *

### 4. Match the Total Operating Bandwidth of the Demodulator and the Channel Multiplexing Capacity

Map the planned wavelength ranges of all sensors to the operating spectral range of the demodulation equipment:

- **Demodulator Bandwidth Window** : For example, the standard operating wavelength range of the OFSCN® Fiber Bragg Grating Interrogator is 1525\sim1565\ \text{nm} (total bandwidth B = 40\ \text{nm} , but custom wideband ranges like 1510\sim1590\ \text{nm} are also available).
- **Maximum Measurement Points per Fiber (Multiplexing Capacity)**:
N\_{\max} \le \left\lfloor \frac{B}{\Delta\lambda\_{\text{spacing}}} \right\rfloor

If the number of measurement points required within a single channel exceeds the bandwidth capacity limit, spatial division multiplexing (SDM) can be used to expand capacity by increasing the number of physical channels of the interrogator (e.g., using 4-channel, 8-channel, 16-channel, or 32-channel interrogators).

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