Black arsenic–phosphorus (b-AsP), an alloy containing black phosphorus and arsenic in the form of b-As_xP_1–x, has a broadly tunable band gap changing with the chemical ratios of As and P. Although mid-infrared photodetectors and mode-locked or Q-switched pulse lasers based on b-AsP (mostly b-As_0.83P_0.17) are investigated, the potential of this family of materials for near-infrared photonic and optoelectronic applications at telecommunication bands is not fully explored. Here, we have verified a multifunctional fiber device based on b-As_0.4P_0.6 nanosheets for highly responsive photodetection and dual-wavelength ultrafast pulse generation at around 1550 nm. The fiber laser with a saturable absorber (SA) based on b-As_0.4P_0.6 nanosheets can output dual-wavelength mode-locking pulses with a larger bandwidth and spectral separation than those based on other two-dimensional (2D) materials. Remarkably, it is found that the b-As_0.4P_0.6-based photodetector can achieve a high responsivity of 10,200 A/W at 1550 nm and a peak responsivity of 2.29 × 10⁵ A/W at 980 nm. Our work suggests that b-As_0.4P_0.6 shows great potential in ultrafast photonics, dual-comb spectroscopy, and infrared signal detection.