With the rapid development of science and technology, the demand for data traffic has increased greatly in recent decades. The current optical fiber communication system has been unable to meet the growing demand for network data transmission, and there is a notable gap in the operating band of 1150?1500 nm. Bismuth-doped quartz fiber can achieve ultra-broadband luminescence in the ranges of 1150?1500 nm and 1600?1800 nm, compensating for the spectral gap of existing rare-earth-doped fibers. However, the practical application of bismuth-doped fibers still faces some challenges. To enhance the broadband spectral performance of bismuth-doped fibers (BDFs) in the near-infrared region, various post-processing methods have been proposed. Among these, heat treatment is considered one of the key factors affecting the spectral performance of BDFs, with the extent of the effect depending on the specific heat treatment parameters. To date, there have been no sufficient reports investigating the effects of specific heat treatment processes and parameters on the spectral performance of bismuth-doped alumino-silicate optical fibers at near-infrared short wavelengths (1100?1300 nm). Therefore, we systematically investigate the effects of different heat treatment parameters on the spectral properties of BACs in bismuth erbium co-doped fibers (BEDFs) and determine the optimal heat treatment process and parameters for bismuth-doped alumino-silicate fibers, which is of great significance for their practical applications in fiber optic communications, sensing, and detection.

Firstly, the broadband small-signal absorption spectra of BEDFs are measured using the cut-back method to determine the absorption bands of different active centers, and the pump absorption of BEDFs at 830 nm is also measured. Then, a backward luminescence measurement for BEDFs with various thermal treatments is built, and the effects of thermal quenching and annealing, heating temperature and quenching times, and heating duration on the luminescence spectral characteristics of BACs are sequentially investigated. This process aims to derive the optimal heat treatment process and parameters for bismuth-doped alumino-silicate optical fibers step by step. At the same time, to evaluate the actual small-signal amplification capability of the BEDFs used in this paper, a counter-propagating on-off gain measurement is built to explore the effects of the optimal heat-treatment parameters derived from the above experiments on the gain performance of the BEDFs in the near-infrared band. Finally, the mechanism of the effects of heat-treatment parameters on the spectral properties of the BACs in the BEDFs is discussed in terms of thermally induced and unsaturated absorption variations.

First, the broadband small-signal absorption spectrum of the BEDF is measured, which contains three groups of absorption peaks (BAC-Al, BAC-Si, and Er³⁺). The unsaturated absorption of the fiber accounts for 67% of the total small-signal absorption at 830 nm (Fig. 1), indicating that the BEDFs to be tested have a high background loss. Then the effects of different heat treatment processes (thermal quenching and annealing, heating temperature and quenching times, and heating duration) on the luminescence spectral properties of BACs are analyzed and compared. It is found that after thermal quenching, the peak emission intensity of BAC-Al, with its emission peak at 1140 nm, increases by nearly 1.4 times (Fig. 3). Moreover, an excessively slow cooling rate could lead to elevated background loss levels. At heating temperatures of 400 and 500 ℃, successive and repeated quenching further promotes the enhancement of peak luminescence intensity of BAC-Al and gradually saturates, with a maximum increase of up to 1.4 times (Fig. 4). The effect of heating time on the peak luminescence intensity of BAC-Al is further investigated. The luminescence enhancement of BAC-Al at 1140 nm is maximized at a heating time of 2 min with a maximum increase of up to 1.8 times (Fig. 5), However, the background loss level of BEDF continues to accumulate after prolonged heating, reducing the overall near-infrared luminescence emission level. Meanwhile, the luminescence emission spectra of BACs are significantly enhanced between 1000?1350 nm when the heating time exceeded more than 5 min (Fig. 6). The small-signal gain spectra of BEDF at 900?1600 nm are tested, showing a broadband gain of 1250?1600 nm (amplification of stimulated radiation of BAC-Si at 1400 nm and Er³⁺ at 1536 nm) and significant excited state absorption at 900?1250 nm (ESA of BAC-Al at 1050 nm) (Fig. 8). The luminescence decay curves of BAC-Al under different heating duration are measured, and the luminescence lifetime decreases sharply with increased heating duration (Fig. 9). Finally, it is found that when the heating duration reaches 2 min, the background loss increases insignificantly in the heat induced loss spectra; when the heating duration reaches 20 min, the heat induced loss coefficients have a significant elevation and increase with the decrease of wavelength (Fig. 10). Meanwhile, when the heating duration is less than 2 min, the saturable absorption level increases significantly, representing a significant increase in BACs; when the heating duration is more than 2 min, the increase in the unsaturable absorption (the background loss) becomes predominant as the bismuth clusters form.

Rapid cooling of bismuth-doped fibers after heating helps to avoid the accumulation of background loss and increase the concentration of BACs. That is, the thermal quenching process improves the working environment of BEDF. The activation temperature of BAC-Al is about 500 ℃, and the optimal heating duration at 500 ℃ is 2 min, which can enhance the peak near-infrared luminescence intensity at 1140 nm by up to 2 times. A prolonged heating duration not only leads to an increase in the bismuth clustering effect but also changes the spectral shape of BAC-Al, and increases the background loss.