Among the many photonic devices, the Bragg grating of chalcogenide glass fibers is important. As a linear device, it can be applied to infrared sensors. In nonlinear optical applications, it can achieve all-optical signal processing. In addition, it is necessary to apply Bragg grating with high reflectivity to obtain efficient integrated chalcogenide glass fiber lasers, such as the distributed feedback Brillouin lasers. For chalcogenide glass fibers, it is complex to write Bragg gratings, mainly because the cladding and core components of chalcogenide glass fibers are similar; thus, both show photosensitivity, resulting in the weak photosensitivity of the fiber core. In our previous study, near-bandgap light close to the absorption edge of the material was selected to irradiate the chalcogenide glass fiber. The fiber core showed high photosensitivity, which solved the weak photosensitivity caused by sub-bandgap light far from the absorption edge of the material. However, when the fiber is used to write a Bragg grating, the depth of the grating transmission peak is not extended and cannot satisfy the actual usage requirements. This requires methods to further improve the photosensitivity of chalcogenide fibers. Therefore, this study extensively investigated the photosensitivity of chalcogenide optical fibers in tension applied in the axial direction to improve the photosensitivity of chalcogenide fibers and provide a new approach to writing gratings with high reflectivity. The findings provide new ideas and approaches for preparing chalcogenide grating photonic devices and will promote the development of chalcogenide photonics.

The photosensitivity of chalcogenide optical fibers was determined by measuring the change in the refractive index of the fiber core based on the Fabry-Perot (FP) etalon principle. In this experimental device, an optical fiber clamp with a slider was designed to apply tension, and a continuous dual-frequency Nd∶YAG laser with a working wavelength of 532 nm was used as the illumination source. After the laser beam was expanded using the beam-expansion system, it was focused onto the sample surface through a cylindrical lens to form a light spot with a height of approximately 1.1 mm and width of approximately 5 mm. The As₂S₃ optical fiber was cut using an ultrasonic cutter to produce an FP etalon of approximately 15 mm in length.

An improved Sagnac interference system, constructed using the same Nd∶YAG laser and a phase mask with a +1/-1 diffraction order, was used to form interference fringe patterns for writing the fiber Bragg gratings. In this system, the optical path of the Sagnac interference system was optimized to ensure that the two Gaussian light spots have improved coincidence and obtain interference fringes with increased contrast.

The experimental results show a significant difference in photosensitivity between the presence and absence of tension (Figs. 2 and 3). By applying tension, the photorefractive index change in the first fast process presents a decrease in the amplitude of the negative change of the refractive index, and the duration of the first fast process shortens. During the second slow process under tension, the recovery time of the photorefractive index change in the positive direction also shortens, and the change in the photorefractive index slowly recovers with the extension of the illumination time and can reach 10^-3 orders of magnitude. Under the same tension, during the first fast process of photorefractive index change, its duration is significantly shortened with an increase in light power [Figs. 2(b) and 3(b)], which can be shortened to tens of seconds. During the second slow process, the recovery of the photorefractive index change is also accelerated, and the change in the photorefractive index shows a positive increase under a specific tension, which can reach 5.41×10^-3. Under the same light power, the duration of the first fast process is significantly shortened with an increasing axial tension, whereas the negative change amplitude of the photorefractive index change decreases. In the second slow process, the change in the photorefractive index recovers faster under larger tension (Fig. 4).

The dynamic characteristics of Bragg grating written on a small-diameter fiber sample under a tension of 0.196 N were investigated. Without applying axial tension, the maximum depth of the transmission peak of the engraved Bragg grating was approximately 6.65 dB, and the reflectivity is approximately 78% (Fig. 6). However, when a tension of 0.196 N is applied, the transmission peak of the Bragg grating can reach 9.85 dB, the reflectivity increases to 89.7%, and the grating bandwidth is approximately 0.48 nm (Fig. 7). During the exposure time of 70-110 s under tension, the average depth of the transmission peak of the grating reaches 9.46 dB, the average reflectivity reaches 88.6%, and a good-quality grating spectrum can be obtained (Fig. 7).

In this study, the characteristics of the photorefractive index change of an As₂S₃ chalcogenide optical fiber under axial tension were experimentally investigated. The experimental results show that there is a significant difference in the photosensitivity with and without tension. By applying tension, the negative change amplitude of the photorefractive index change in the first fast process can reduce, and the duration of the first fast process shortens significantly, which can be shortened to tens of seconds under a particular tension. In the second slow process, under tension, the recovery time of the photorefractive index change in the positive direction also shortens, and the photorefractive index change shows a positive increase under a certain tension, which can reach 5.41×10^-3. On this basis, the As₂S₃ chalcogenide fiber Bragg grating was written based on the remarkable photosensitivity caused by the application of tension. The grating spectrum shows that within 70-110 s short exposure time, the average depth of transmission peak of the grating reaches 9.46 dB, the maximum depth of peak is 9.85 dB, the maximum reflectivity reaches 89.7%, and the grating spectrum quality is good. These experimental results are crucial in applying chalcogenide glass fibers in the near-middle infrared field as force sensors, achieving all-optical signal processing, and improving photosensitivity for preparing fiber gratings with high reflectivity.