Rare-earth doped fibers, owing to their broad gain bandwidth, high efficiency, light weight, and low cost, are widely utilized in fiber lasers and amplifiers^[1–3]. In recent years, with the continuous exploration of the space environment, active optical fiber devices have played a key role in fields such as space-borne laser communications, lidar, fiber optic gyroscopes, and gravitational-wave detection^[4–6]. However, the space environment is filled with radiation sources such as gamma rays, X-rays, and high-energy particles, which can seriously affect the performance of the optical fiber system^[7]. In the nuclear field, nuclear reactors have also begun to use fiber Bragg gratings to detect parameters such as temperature, and the radiation within nuclear facilities can also seriously affect their performance^[8]. In the medical field, embedded structure fiber optics is often used as a radiation dosimeter in radiotherapy, and optical fiber devices can also be affected by some radiation^[9].

At present, the anti-radiation performance of passive optical fibers has basically met the practical application requirements. However, improving the radiation resistance of active optical fibers remains a challenge, and active optical fibers are more susceptible to radiation^[10]. In the space environment, the aircraft usually uses a certain thickness of metal materials to encapsulate and protect internal devices against radiation^[11]. Metal casings such as optical fiber water-cooled disks are commonly used for radiation protection, which belongs to passive anti-radiation technology. In addition, researchers have also investigated some active radiation suppression for active optical fibers. For examples, (1) the radiation resistance of Yb/Al co-doped silica fiber can be improved by adding phosphorus (P)^[12]; (2) preloading with H2 can enhance the radiation resistance of the active fiber^[13]; and (3) subjecting silica fibers to high-dose pre-irradiation treatment improves their radiation resistance in subsequent irradiation scenarios^[14].

Although there are many active irradiation suppression techniques for optical fibers, their respective limitations exist for practical applications. While passive anti-radiation techniques are easy to operate, they usually use metal materials for shielding, which will cause the optical fibers to lose their original lightweight, flexible, and bendable characteristics. Therefore, there are strong motivations to develop new passive anti-radiation technology with the advantages of being lightweight, flexible, and cost-effective, which can be well adhered to the surface of optical fiber.

In this Letter, we propose composite multilayer anti-radiation films based on optically precise deposition technology. Using electron beam evaporation combined with an ion-assisted deposition system, five kinds of composite multilayer anti-radiation films with different materials (Kapton, ITO, and Cu/Al) and structures were prepared. Then, these anti-radiation films were wrapped around polarization-maintaining (PM) Yb fibers as testing samples, respectively. After applying ∼150kGy irradiations twice to these PM Yb-fiber samples, experiments were designed to measure their radiation-induced attenuation (RIA), lasing, and gain performance to verify the effectiveness of the anti-radiation films. Moreover, the recovery characteristics of these irradiated PM Yb fibers were also studied experimentally.

The anti-radiation technology studied in this paper is for the application of active optical fibers in radiation environments. Therefore, the following three factors are mainly considered in the design: (1) having an effective anti-radiation effect; (2) having good flexibility and bending properties to fit the peripheral surface of the optical fiber adequately; (3) maintaining the original small size and light weight of the optical fiber. Based on the above considerations, a composite multilayer symmetrical anti-radiation film based on optical precision deposition technology is proposed in this paper. This film is mainly composed of a metal layer, an ITO (indium tin oxide) layer, and a Kapton (polyimide film) layer. Its structure is shown in Fig. 1(a). The X layer in the figure represents the metal layer, and its specific structure and thickness are shown in the table in Fig. 1(a); that is, five anti-radiation films (Film A–E) with different substrates were developed in this paper. The ITO layer (0.6 µm) and Kapton layer (50 µm) of Film A–E have the same structure. The only difference lies in the thickness and material of the metal layer.

Among these three layers, the core structure is the ITO layer. ITO has been used as an effective anti-static and thermal control coating on various spacecraft^[15,16], and it also has good gamma-ray irradiation performance^[17]. Through the optical precision deposition technology, granular ITO materials can be deposited on a suitable substrate. We designed the thickness of the ITO layer to be 0.6 µm. This can not only have a strong anti-radiation ability but also maintain the lightweight and flexible characteristics of the anti-radiation film. Considering the extensive use of metal materials (such as Al) for physical shielding in the aerospace field, metal foils of appropriate thickness are selected as the substrate for ITO deposition in this paper. Metal substrates of Cu and Al are used, respectively, with a thickness of 10–30 µm. However, the bare ITO coating may crack and be damaged after being irradiated by near-ultraviolet radiation in a vacuum environment^[18]. Therefore, a supplementary protective structure is required. Kapton film has many unique advantages^[19]: First, Kapton film can withstand extreme temperatures from −269 to 400°C, which can cope with many complex radiation environments; second, Kapton film has a certain anti-radiation ability. Therefore, in this paper, Kapton film (50 µm) produced by DuPont is used as the external protective layer. Meanwhile, considering the convenience in practical use, the anti-radiation film designed in this paper adopts a symmetrical structure film system.

In this paper, electron beam evaporation-ion-assisted deposition technology is used, and the model of the coating machine is DJ-800. The ITO material used in this paper is 99.9% and is granular, and its melting point is usually between 1526 and 1926°C. The evaporation point is generally above 2000°C. The preparation process is shown in Fig. 1(b). First, the metal foil of a fixed thickness is cleaned and placed in the rotating substrate holder as the coating substrate (target). The high-energy electron beam emitted by the electron gun acts directly on the ITO target placed in the water cooling system. The melted ITO film material molecules evaporate upward. Meanwhile, the high-energy ion beam emitted by the ion source bombards the film material molecules to enable them to obtain sufficient kinetic energy for the uniform deposition of high-quality ITO thin films on the metal Al foil, and the thickness of the ITO thin film is controlled at 0.6 µm. During the coating process, the screen voltage of the ion source is 300 V, the ion beam current is 120 mA, the electron beam current is 140 mA, the electron gun speed current is controlled below 70 mA, the deposition rate is 0.2 nm/s, the working vacuum degree is 3×10−3Pa, and the single coating time is about 1 h. Under this coating process, the film layer does not easily fall off and can meet the flexible conditions.

After the single-sided coating is completed, the metal foil needs to be turned over for secondary coating to achieve a symmetrical film system [Fig. 1(c) shows the metal foil after the ITO thin film is coated]. According to the actual needs, it is cut into the appropriate size and shape, and a 50 µm thick Kapton film layer produced by DuPont is continuously covered outside the ITO film layer. Since Kapton is a tape-like material with stickiness on one side, it can be directly pasted on the surface of the ITO layer. Thus, all the preparation steps of the anti-radiation film are completed.

In the space environment, nuclear environment, and other environments, optical fiber materials will be impacted by a large number of high-energy rays and high-energy particles^[20,21]. In order to better test the radiation resistance ability of the anti-radiation film in these environments, this experiment uses the GJ-2 electron accelerator instead of the traditional Co-60 gamma source as the radiation source. Because the radiation directionality of the electron accelerator is good and the dose rate is high, it can scatter γ rays, α rays, and electron beams. The high-energy electrons it emits have an energy of 2 MeV and a flow rate range of 2–6 mA.

According to the literature^[22], the radiation dose value endured within the spherical structure spacecraft with a 3–4 mm equivalent aluminum plate over 15 years is above 40 kGy, and the radiation dose endured in the field of nuclear electrothermal chambers will be even higher. Therefore, in order to more accurately verify the anti-radiation performance of the anti-radiation film designed in this paper when exposed to a large dose of radiation for an extended period, two completely identical and independent irradiation experiments will be conducted. The total irradiation dose of a single irradiation experiment is set at 150 kGy, and the interval between the two irradiations is three months. After each irradiation experiment, the performance of the optical fiber will be tested to assess the performance of the anti-radiation film.

In order to conduct a comprehensive study on the factors influencing the anti-radiation performance of the anti-radiation films, in this paper, via the five types of anti-radiation films designed previously, eight groups of controlled experiments were established. The optical fibers in each group were all polarization-maintaining ytterbium-doped fibers (PM-YSF-HI-HP) produced by Coherent. The diameters of the fiber core and cladding were 6.0 and 125.0 µm, respectively, the numerical aperture (NA) was 0.11, and the lengths were all 1.1 m. The experimental groups are depicted in Fig. 2(b), and the physical object of the YSF fiber wrapped with the anti-radiation film is presented in Fig. 2(a). The wrapping method of all anti-radiation films is single-layer wrapping. Among them, Fiber A was a pristine YSF fiber, and Fiber H was a YSF fiber directly exposed to the irradiation environment without any protective measures. These two groups served as the control groups. Fiber B, Fiber D, Fiber F, and Fiber G were directly equipped with the four distinct anti-radiation films designed, and the main difference was in the material and thickness of the film substrate. Fiber C was adorned with two different anti-radiation films (Film A + Film D), but the total substrate thickness remained at 30 µm. The difference between the Film E worn by Fiber E and that of Fiber D was that the Kapton protective layer was removed. The purpose of this group was to analyze the role of the Kapton film in enhancing the performance of the entire anti-radiation film.

First, spectral tests of RIA were conducted on all the optical fibers after irradiation, and its calculation formula is presented as RIAi(λ)=αi(λ)−αp(λ).(1)

In the formula, αi(λ) is the absorption intensity of the optical fiber before radiation and αp(λ) is the absorption intensity of the optical fiber after radiation. The value of RIA can be obtained through the difference in absorption intensity before and after radiation. In this experiment, we used a tungsten lamp as a continuous light source. To avoid large errors caused by the absorption peaks of ytterbium-doped optical fibers at 915 and 975 nm, the band of 1100 to 1350 nm was selected for measurement. The measurement results after the first irradiation are shown in Fig. 3(a), and the measurement results after the second irradiation are shown in Fig. 3(b).

After the first irradiation experiment, it can be found that the optical fiber loss of those using anti-radiation films is significantly lower than that of Fiber H without any protective measures. By comparing the optical fiber losses of Fibers B–G, it is found that the radiation-induced losses of the anti-radiation films using Cu as the substrate are lower. Moreover, the optical fiber loss of the anti-radiation films with a substrate thickness of 30 µm is lower than that of the anti-radiation films with a substrate thickness of 10 µm. Therefore, the basic rules of the performance of the anti-radiation film can be initially obtained: Under the same thickness, the film with Cu as the substrate has a better anti-radiation effect than the film with Al as the substrate. The greater the thickness of the substrate, the better the anti-radiation performance, but the metallicity will be stronger and a part of flexibility will be lost. The difference between Fiber D and Fiber E is very small, and it is determined that the improvement of the overall performance of the anti-radiation film by the Kapton layer is relatively limited.

After the second irradiation experiment, the RIA of Fibers H–G all increased. However, it can still be found that the optical loss of Fiber B was the lowest, which was more than 2 dB/m lower than that of Fiber H. The overall pattern was largely consistent with that summarized in the previous text, indicating that the anti-radiation film designed in this paper still has the ability to protect the optical fibers when facing the challenge of high-dose radiation.

To further verify the rules obtained in the loss experiment, in this paper, a 1064 nm all PM fiber laser was built to test the laser performance of the anti-radiation optical fibers. The structure of the laser is shown in Fig. 4(a). The resonant cavity is composed of a high-reflection chirped fiber Bragg grating (FBG) with a central wavelength of 1064.05 nm, a linewidth of 6.62 nm, a reflectivity of 99%, and a low-reflection FBG with a central wavelength of 1064.32 nm, a linewidth of 0.38 nm, and a reflectivity of 66%. The gain fiber is the anti-radiation YSF fiber sample, and the pump is 976 nm LD.

Figure 4(b) shows the measured laser experimental powers of YSF optical fibers in the first irradiation experiment, and Fig. 4(c) shows the measured laser experimental powers of YSF optical fibers in the second irradiation experiment. From the first measurement results, it can be found that, compared to Fiber H without protective measures, the laser powers of Fibers B–G using the anti-radiation films were all enhanced, and the slope will also be relatively higher. Compared with Fiber B and Fiber H, the output power increased by 75.3% under a pump of 160 mW. By comparing the measured laser powers after the two irradiation experiments, it can be found that the performance of each group of YSF optical fibers has somewhat declined. However, the performance of Fibers B–G was better than that of Fiber H in both experiments. Therefore, the rule of the anti-radiation performance of the anti-radiation films obtained from the laser experiment is the same as that obtained from the loss experiment.

Finally, in order to verify the protective effect of the anti-radiation film on active optical fiber devices, a 1064 nm band all PM fiber amplifier was built in this paper to detect the change in its output power. The structure of the fiber amplifier is shown in Fig. 5(a). The signal light is generated by a 1064 nm distributed feedback (DFB) laser, the signal light power is 1 mW, and the gain fiber is composed of 8 groups of YSF fibers. The 1064 nm signal light is amplified by pumping the gain fiber with a 976 nm LD.

The measurement results after the first irradiation experiment are shown in Fig. 5(b), and the measurement results after the second irradiation experiment are shown in Fig. 5(c). It can be found that the anti-radiation effect of Fiber B was the best in both irradiation experiments. Under a 160 mW pump, Fiber B had a power increase of 39.3% compared to Fiber H. Except for Fiber B, the other fibers using the anti-radiation film also had better amplification power than Fiber G.

Based on the above three experiments, the fiber performance after the first irradiation experiment and the fiber performance after the second irradiation experiment are summarized in Tables 1 and 2, respectively. It can be seen that the anti-radiation film can effectively protect active optical fiber devices in a high-radiation environment and significantly improve the performance of active optical fibers in terms of loss, gain, etc.

After a period of time following the irradiation of optical fibers, the fiber loss caused by radiation will gradually recover^[23]. Inspired by this, the loss recovery and gain recovery characteristics of active optical fibers protected by anti-radiation films will continue to be studied in this paper.

Four groups of YSF fibers are selected in this paper, namely Fiber D/Fiber F/Fiber G/Fiber H, which have relatively large losses in the irradiation experiment. They were exposed to the air at room temperature and left to stand for 5 weeks. Subsequently, the loss and gain performance of these fibers was measured. The RIA spectra of the four groups of fibers are presented as follows. Figure 6(a) shows the RIA spectrum of Fiber D, Fig. 6(b) shows that of Fiber G, Fig. 6(c) shows that of Fiber F, and Fig. 6(d) shows that of Fiber H.

It can be found from the loss situation that the RIA of Fiber D has decreased by 79.5% to as low as 1.5 dB/m. The RIA of Fiber G has decreased by 60.0% to 3.2 dB/m. The RIA of Fiber F has decreased by 60.5% and also dropped to 3.2 dB/m. The RIA of Fiber H has decreased by 49.9% to 4.07 dB/m. From the above phenomena, it can be analyzed that the active optical fiber protected by the anti-radiation film has better loss recovery ability after the irradiation ends than the fiber without protection. Moreover, the better the anti-radiation effect of the anti-radiation film, the lower the RIA level that the optical fiber can recover to.

Subsequently, the gain performance recovery of these four groups of fibers was tested in this paper. The method used was still to measure the power using the 1064 nm all PM laser and amplifier built in Sect. 3 of this paper. The laser power curves are shown as follows. Figure 7(a) is the laser power curve of Fiber D, Fig. 7(b) is the laser power curve of Fiber G, Fig. 7(c) is the laser power curve of Fiber F, and Fig. 7(d) is the laser power curve of Fiber H.

Under a 160 mW pump, the output power of Fiber D recovered to 15.2 mW, restoring 25.7% of the power compared to that after the second irradiation. The laser output powers of Fiber G and Fiber F recovered to 12.1 and 14.4 mW, respectively, restoring 45.8% and 23.6% of the power, respectively. However, the laser output power of Fiber H only recovered to 10 mW, restoring 26.6% of the power. From the above results, it can be seen that the lasers using optical fibers with anti-radiation films as gain fibers can recover to a higher output power after the same period of time following irradiation. Moreover, the better the anti-radiation effect of the anti-radiation film used, the higher the recovered laser power will be.

To further verify the gain recovery performance of the above four groups of fibers, the output power was measured using the 1064 nm all PM fiber amplifier built in Sect. 3. The power curves are shown as follows. Figure 8(a) is the amplification power curve of Fiber D, Fig. 8(b) is the amplification power curve of Fiber G, Fig. 8(c) is the amplification power curve of Fiber F, and Fig. 8(d) is the amplification power curve of Fiber H.

Under a 160 mW pump, the amplified output power of Fiber D has recovered to 11.5 mW, which is a recovery of 35.9% compared to that after the second irradiation. Fiber G and Fiber F have recovered to 10.9 and 11.1 mW, respectively, with recoveries of 78.7% and 61.0%, respectively. However, Fiber H has merely recovered to 7.5 mW, with a recovery of 30.8%. Thus, it can be observed that the anti-radiation film is highly beneficial for the recovery of the gain performance of the active optical fiber. It can assist the fiber gain to restore to a higher level within a short period.

Therefore, based on the test results of laser power and amplification power and the loss (RIA) test results of these four groups of YSF optical fibers, it can be considered that the anti-radiation film can not only reduce the irradiation received by the optical fibers during radiation but also improve the gain and loss recovery performance of the optical fibers after irradiation. Moreover, a better anti-radiation film with better anti-radiation performance can enable the irradiated optical fibers to recover to a better state within the same time.

The results of the anti-radiation film developed in this paper, including loss and gain, have been experimentally verified to have an effective anti-radiation effect. Among different anti-radiation films, the anti-radiation film with Cu as the substrate has better performance than that with Al as the substrate. The film with a larger substrate thickness is superior to the one with a smaller substrate thickness. It was also found that the effect of Kapton film on improving the overall anti-radiation performance is relatively limited. In the subsequent performance recovery experiments of active optical fibers, it was found that the performance of irradiated optical fibers recovered to some extent after standing for a period of time. The optical fibers protected by anti-radiation films during irradiation recovered faster than those without protection. Moreover, the better the anti-radiation performance of the anti-radiation films used, the better the performance that could be recovered within the same time.

In conclusion, the anti-radiation film proposed in this paper is not only lightweight and flexible but also has an effective anti-radiation ability. It can also help the optical fiber recover to a better performance more quickly after radiation.