Radiation to Photonics Project: Optical Time-Domain Reflectometry (OTDR) Data Analysis

1. Introduction

This is part of my class at University Jean-Monnet (Saint-Étienne, France) during my Erasmus Mundus RADMEP third semester last October 2024.

2. Rayleigh Scattering and OTDR

Rayleigh scattering refers to the scattering of light by a particle whose dimension is smaller than the wavelength of light [1]. This scattering elastic, implying that the scattered light has the same wavelength and intensity as the incident. For optical time-domain reflectometry (OTDR), a pulsed laser is injected into an optical fiber. The interaction of this laser with the material backscatters some of the light, and this backscattered light is measured by a detector. The schematic is shown below:

Figure 1. OTDR measurement setup.

Under irradiation, the backscattered light does not have the same intensity as the incident but becomes attenuated. This phenomenon is called Radiation-Induced Attenuation (RIA). This is measured using the following formula [2]

$$\displaystyle \text{RIA}(t, \lambda) = -\left(\frac{10}{L} \right) \cdot \log_{10} \left(\frac{\text{I}(t, \lambda) - \text{I}_{\text{N}}(\lambda)}{\text{I}_{\text{ref}}(0, \lambda) - \text{I}_{\text{N}}(\lambda)} \right)$$

where:

• $\text{I}(t, \lambda)$ $\rightarrow$ intensity of backscattered light at time $t$ and wavelength $\lambda$
• $\text{I}_{\text{N}}(\lambda)$ $\rightarrow$ background noise
• $\text{I}_{\text{ref}}(0, \lambda)$ $\rightarrow$ reference intensity/intensity of light at the beginning
• $L$ $\rightarrow$ length of the fiber

Below is an example curve of the measured intensity of the reflected light as a function of distance at a certain time:

Figure 2. Image from [3].

3. Goal

The goal of this project was to plot the RIA as a function of dose.

4. Experiment

The optical fiber was illuminated using three telecom wavelengths and the coverage windows of the OTDR curve for each are chosen as follows:

Wavelength [nm]	Coverage Window [km]
1310	0.06-6.39
1625	0.06-6.39
1310	0.06-4.80

Table 1. Telecom wavelengths and the corresponding measurement range.

At each period of time, the OTDR data for each wavelength had been measured. The optical fibers were irradiated at 105.56 µGy/s, allowing conversion from time to dose.

5. Data Analysis and Result

The data analysis used the following scripts:

• ria.py : Responsible for the data extraction and processing from raw values of the OTDR measurement
• radiation_to_photonic_analysis.ipynb : Jupyter notebook containing the analysis to yield the RIA vs. dose plots for the three telecom wavelengths.

The result of these analyses is shown in the plot below:

Figure 3. RIA vs. dose plot for the three telecom wavelengths.

The plots above show increasing RIA with dose for each telecom wavelength. Moreover, the RIA rate with respect to dose increases according to ascending order of the given telecom wavelengths.

6. Applications

One application of this measurement is the possibility of using this OTDR setup as a dosimeter if the RIA vs. Dose plot shows some linearity.

6. References

[1] https://en.wikipedia.org/wiki/Rayleigh_scattering.

[2] A. Hasan, Y. Aguiar, R. García Alía, C. Campanella, A. Morana, A.K. Alem, S. Girard, A. Raj Mandal, M. Ferrari, "Online and offline Radiation-Induced Attenuation measurements on FD-7 radiophotoluminescence dosimeters irradiated at high X-ray doses," Radiation Measurements, vol. 177, 2024. [Online]. Available: https://doi.org/10.1016/j.radmeas.2024.107246.

[3] https://www.thefoa.org/tech/ref/testing/OTDR/OTDR.html.