O-band lasers find widespread use in fiber optic communication, medical treatments, measurements, and sensing. For example, in fiber optic communication, they serve as light sources for short-distance communications, such as those between data centers, and for detecting optical signals in fiber optic communication links. In medical applications, the main components of biological tissues—such as water, oxygenated hemoglobin, and melanin—exhibit minimal transmission losses at a wavelength of 1.3 μm, making O-band lasers suitable for laser surgeries and imaging. The specific wavelength range renders O-band lasers indispensable tools in these applications. However, achieving a narrow linewidth output in semiconductor lasers can result in high costs. Additionally, the challenge of fusion splicing fluoride-based fibers with conventional silica fibers significantly restricts the widespread adoption of praseodymium-doped fiber lasers.

Benefiting from its broadband emission characteristics in the near-infrared spectral region, bismuth (Bi) is considered a highly promising gain medium for manufacturing optical devices. A series of lasers based on bismuth-doped fibers has rapidly developed. Although some O-band bismuth-doped fiber lasers have been reported internationally, these lasers typically require pump powers in the range of several watts to achieve high slope efficiencies, while also exhibiting relatively low signal-to-noise ratios. Therefore, further optimization of O-band bismuth-doped fiber laser performance is necessary.

We initially examine the emission characteristics of the bismuth-doped fiber. Our measurements reveal that the emission spans the entire O-band, ranging from 1260 nm to 1360 nm, with notable amplified spontaneous emission (ASE) power levels between 1300 nm and 1350 nm. Taking into account losses from optical devices and fibers, we designate 1310 nm as the operational wavelength for the bismuth-doped fiber laser. Constructed with a ring cavity structure, the bismuth-doped fiber laser comprises a total cavity length of approximately 125 m. This length includes 120- m-long bismuth-doped fiber (BDF) and a 5-m-long fiber connecting other optical components inside the cavity through all-fiber fusion splicing. A semiconductor laser, boasting a maximum output power of 550 mW at a wavelength of 1240 nm, acts as the pump source. This laser is linked to the pump end of wavelength division multiplexers (WDM1 and WDM2) for 1240 nm and 1310 nm. To safeguard the pump, isolators (isolator 1 and isolator 2) are strategically positioned between the pump source and WDM. For wavelength selection, a circulator and a fiber Bragg grating (FBG) are employed. To ensure unidirectional transmission within the cavity, the isolator 3 and the circulator are employed, with the isolators mitigating optical component reflection effects. Laser output is facilitated through a coupler. Upon constructing the bismuth-doped fiber laser (BDFL), we examine the impact of varying coupling ratios on laser output and determine the optimal ratio. Additionally, we compare the effects of different pumping methods on laser output.

We explore the impact of varying output coupling ratios on laser output power by employing couplers set at 10∶90, 20∶80, and 50∶50, with results depicted in Fig. 3. Across different output coupling ratios, once the input pump power surpasses the laser threshold power, the laser output demonstrates a linear increase with pump power. For a given pump input power, the laser output power steadily rises with the output coupling ratio escalating from 10∶90, through 20∶80, reaching 50∶50 and 80∶20, and marginally decreases as the coupling ratio peaks at 90∶10. At a pump output power of 1000 mW, the maximum output power registers at 127.3 mW (80∶20 coupling ratio), boasting a maximum slope efficiency of 14.44% (80∶20 coupling ratio). Figure 4 illustrates the fluctuation of output power concerning output coupling ratio across various pump input powers. At lower input powers, the optimal output coupling ratio falls between 70∶30 and 80∶20, progressively increasing to approximately 80∶20 as the input power escalates. Figure 5 showcases the output spectrum of the BDFL, revealing a peak wavelength of 1310.22 nm, an OSNR exceeding 60 dB, a 3 dB linewidth of around 0.036 nm, output power stability within ±0.06 dB, and wavelength stability below 0.01 nm.

After identifying the optimal coupling ratio as 80∶20, we proceed to compare the effects of different pumping methods on laser output. Figure 8 displays the laser output spectra under forward and backward pumping configurations, both showcasing a central wavelength of 1310.22 nm and consistently maintained OSNRs around 60 dB. The 3 dB linewidths under forward and backward pumping configurations measure approximately 0.032 nm and 0.04 nm, respectively. In Fig. 9, we depict the variation of output power with input power under three pumping configurations. The laser thresholds for all three pumping configurations hover around 240 mW. Once the input power surpasses the threshold, the output power linearly increases with input power. Considering the pump coupling efficiency, backward pumping maximizes both output power and slope efficiency, peaking at 136.4 mW and 18.92%, respectively, at an input power of 1000 mW.

In summary, we have constructed a bismuth-doped fiber laser utilizing a ring cavity structure and explored its output under varying coupling ratios. Our findings reveal that at the optimal coupling ratio of 80∶20, the laser outputs a central wavelength of 1310.22 nm, an OSNR exceeding 60 dB, a slope efficiency of 14.44%, and achieves a maximum output power of 127.3 mW at an input power of 1000 mW. Furthermore, we conducte a comparison of different pumping methods on laser output. Among the three methods, forward pumping yields the lowest laser output power and slope efficiency, while backward pumping produces the highest, reaching 136.4 mW and 18.92%, respectively. In conclusion, we have realized a bismuth-doped fiber laser with high OSNR and relatively high efficiency, boasting a simple structure. This laser offers valuable insights for the advancement of O-band fiber lasers.