Gamma radiation effects on plastic optical fibers

Highlights

• The effect of gamma-ray irradiation on the Plastic Optical Fibers (POFs).

• Irradiated POFs had optical losses significantly higher at 535 nm than at 650 nm.

• The POFs could be used to connect up to 20 m if gamma-ray does not exceed 17.6 kGy.

• The POFs could be used to connect up to 10 m if gamma-ray does not exceed 28.6 kGy.

• The absorption edge shifts towards the higher wavelengths due to gamma radiation.

Abstract

We report on our study of the effect of gamma-ray irradiation on Plastic Optical Fibers with a 1 mm diameter for short reach communication. For the study, we used gamma-radiation facilities with ⁶⁰Co sources for slow long-term irradiation with a low dose speed of around 70 Gy/hrs and a maximum dose up to 62.90 kGy and a higher speed irradiation dose of around 1 kGy/hrs with a maximum dose up to 51.30 kGy. The optical losses measurement showed that the losses increased linearly during the irradiation and began to decrease immediately after the end of being irradiated. Higher doses of irradiation cause a higher value of the optical loss, and at the dose of 62.9 kGy, the loss was unmeasurably high. A low dose of gamma radiation (1.24 kGy) increased optical losses only slightly and the gamma-ray irradiation at 17.6 kGy for the 20 m long fiber had optical losses of −15.5 dB at 650 nm. The results of the relaxation measurements also showed that optical losses measured at wavelength 535 nm are significantly higher than those at 650 nm. Spectral characteristics measurement done during the irradiation showed that the intensity of the transmitted light decreased and the absorption edge shifted towards the higher wavelengths. Spectral characteristics measured after irradiation during the relaxation period showed that the intensity increased and also that the absorption edge was shifted towards the lower wavelengths, but the same values as before the irradiation were not achieved.

Introduction

Over the past decades, optical fibers have been used for long-distance connections. For this purpose, silica fibers typically operating between the wavelengths of 800 nm and 1625 nm are used, which in practice means 850 nm and 1300 nm for the multimode optical fibers and 1310 nm and 1550 nm for the single-mode ones [1]. Copper-based electrical interconnects dominate short-range communication links, but the increasing demand in recent years for high-speed data communications has led to considering the use of optical fibers for Short Reach (SR) communication links to replace the copper data connections. SR multimode interconnection data transmission is the lowest priced optical interconnects available today for communication lengths up to 80–100 m [2].

Though silica fibers are still the most commonly used ones in optics communication for backbone and metropolitan networks, Plastic Optical Fibers (POFs) as niche applications for short-distance connections are also being studied in today’s industry. The most important advantage of POFs is its large core which makes this type of fiber easy to fit connectors to and facilities a lower installation cost. The most common POFs are made of polymethylmethacrylate (PMMA), a material which is transparent in the visible light spectrum from 400 to 700 nm. The most common operation wavelengths are 650–665 nm (red) and 520 nm (green) since there are low-cost optical sources available. The next key advantage of POFs, when compared to glass optical fibers or copper wires, is their physical resiliency, which means that POFs can easily withstand day-to-day handling as well as industrial environments [3].

Due to the attenuation and the distortion characteristics of the PMMA fibers, they could be commonly used for low-speed, short-distance (up to 100 m) applications in home and industrial networks such as PROFIBUS, PROFINET and car networks MOST [3], [4].

One of the new areas of optical communications is data transmission at nuclear power plants where optical fibers and cables are installed outside of the containment areas and in mild environments. Along with this, researchers are intensively studying also the possibilities of applying such optics communications inside radiation zones in locations with gamma radiation.

In the last decades, several papers describing the properties of optical fibers irradiated with gamma-rays have appeared, as:

Wijnands et al. reported attenuation studies for 12 commercially available single-mode (SM) optical fibers at the light of 1310 nm and 1550 nm. The fibers described there were exposed to gamma-rays from a ⁶⁰Co source. Radiation hard fibers from one manufacturer show an extraordinarily low attenuation for light at 1310 nm that does not exceed 5 dB/km even after a total dose of 1 MGy [5].

S. Girard et al. have published the properties of the radiation effects on silica-based optical fibers with the description of the mechanisms inducing microscopic and macroscopic changes under radiation-induced attenuation. They discussed the influence of various parameters related to the optical fiber in harsh environments [6].

G. Cheymol et al. presented the measurements of standalone rad-hard optical fibers and the fibers embedded in cables, and showed a significant increase of radiation-induced attenuation (RIA) at 1550 nm for fibers in the cable. This increase was assigned to molecular hydrogen generated by the compounds’ radiolysis of the cable [7].

Temperature and radiation effects on radiation-sensitive SM optical fibers were presented by C. Campanella in [8]. The authors presented in 2020 that the aluminium-doped fibers provided the highest radiation sensitivity in the IR. It was also shown that phosphorus-doped fibers demonstrated a good sensitivity to radiations in the IR without saturation tendency up to 100 kGy.

The radiation resistance of six pure-silica-core SM optical fibers from different manufacturers were presented by P.F. Kashaykin et al. [9]. They reported on their comparative study using strong fission-reactor irradiation and they found that polyimide and acrylate-coated SMFs failed mechanically and lost transparency.

Z. Šaršounová et al. presented the properties of two kinds of cables. One cable contained four fibers in one tube while the other cable contained 12 tubes with 12 fibers per tube. The cables containing multimode fibers MM 62.5/125 (J‐fiber, MIL‐PRF‐49291/6C) and also single-mode fibers SM 9/125 μm (ITU‐T G.652, J‐fiber) were exposed to gamma-irradiation, resulting in a darkening and an attenuation increase and was then followed with a recovery process, when the attenuation decreased again [10].

Radiation effects on pure-silica multimode optical fibers in visible and near-infrared domains were also presented by Campanella et al. in 2021 [11].

Few papers dealing with the properties of the polymeric optical fibers irradiated with gamma-rays have also been presented.

The possible use as on-line dosimeters of commercially-available PMMA plastic optical fibers was presented by Fernandez et al. [12]. The PMMA fibers showed low radiation-induced attenuation between 650 m and 800 nm, but at lower wavelengths the radiation-induced attenuation becomes more significant. The fibers were found to have a threshold between 100 Gy and 1000 Gy. Above this threshold, they began to show changes in their transmission spectra. Eight days after the end of the gamma irradiation, a clear recovery was noticed.

A gamma-ray irradiation test on the POFs in order to use the fibers in low-level radiation environments was presented by K. Toh et al. [13]. The optical fibers used in the experiment were commercially available 5 m long PMMA POFs fabricated by Mitsubishi Rayon Co., ltd with the diameter of its core/clad 0.24/0.25 mm. One irradiation was done by the ⁶⁰Co gamma-ray irradiation facility at the Japan Atomic Energy Agency with a maximum absorbed dose of 17.5 kGy and the second irradiation test was performed at Tohoku University at a low dose rate using gamma-rays whose intensity was 5 ∼ 6 orders of magnitude lower than that in the previous test.

The study of gamma radiation effects on a low-loss perfluorinated polymer optical fiber (PF-POF) based on Cytop® polymer was presented in [14]. The radiation-induced attenuation (RIA) in the visible and near-infrared region was measured and its origins were discussed. The optimal operation wavelength region of PF-POF-based systems intended for use in radiation environments was determined to be around 1.1 µm.

In this paper, we are going to present our study of the effect of gamma-rays on the 1 mm POFs. The research was focused on the possibilities of using such types of fibers for Short Reach (SR) communication (up to 20 m). To obtain a thorough amount of information, we measured the optical properties of the POFs irradiated with (i) low dose gamma-rays for a longer amount of time and (ii) higher doses for a short amount of time. We expect that this information would provide general value which could be utilized in nuclear power plants, nuclear pharmacies, cosmic research and etc., where optical fibers are exposed to gamma-rays.