Bidirectional-output fiber lasers have broad application prospects in industrial, medical, communication and other fields due to their unique features of achieving two laser outputs through a resonant cavity. However, the reported high-power bidirectional-output fiber lasers are mainly concentrated in the conventional wavelength band of 1080 nm. Compared with fiber lasers with conventional wavelengths, short-wavelength fiber lasers with wavelengths less than 1060 nm have a wide demand in fields such as biological imaging, nonlinear frequency conversion, and spectral beam combination. However, they are susceptible to nonlinear effects such as amplified spontaneous emission (ASE) and transverse mode instability (TMI), due to the wider absorption cross-section of short wavelength Yb-ions.

In this study, a novel short-wavelength composite cavity bidirectional-output all-fiber laser is constructed and shown in Fig. 1. The laser resonant cavity consists of two output-coupler fiber Bragg gratings (OCFBG-A and OCFBG-B) with reflectivity of approximately 10%, one fiber Bragg grating (FBG) with a reflectivity of approximately 50%, and two pieces of ytterbium doped fibers (YDF#1 and YDF#2) with a core/cladding diameter of 20/400 μm. FBG is placed inside the resonant cavity and fused with YDF#1 and YDF#2 at both ends. The central wavelengths of the OCFBG-A, OCFBG-B and FBG are approximately 1050 nm. Wavelength-locked fiber-pigtailed 976 nm laser diodes (LDs) are utilized as the pump sources, which are injected into the resonant through two (6+1)×1 pump/signal combiners. The signal laser is output through fiber endcaps (QBH-A and QBH-B) with a core diameter of 25 μm.

When the maximum pump power of 6407 W is injected, the total output power reaches 5489 W, with 2278 W of end A and 3211 W of end B, and the corresponding optical-to-optical conversion efficiency of 85.7%, as shown in Fig. 2(a). Figure 2(b) shows the output spectra at the maximum output powers,indicating the center wavelength is 1050 nm, and the Raman suppression ratios at both ends are ~21.9 dB and ~20.1 dB, respectively. Figure 2(c) shows the temporal signals and the corresponding Fourier transform spectra recorded from A and B ends, which confirms that there is no sign of TMI during the power scaling. The measured beam quality factors (M²) of both ends at the maximum power are 1.52 and 1.34, respectively [Fig. 2(d)].

We propose and demonstrate a novel short-wavelength composite cavity bidirectional-output all-fiber laser, and the near single-mode bidirectional output with a central wavelength of 1050 nm and a total output power of 5.5 kW is achieved. The research results are of great significance for suppressing nonlinear effects and transverse mode instability in short-wavelength fiber lasers and bidirectional-output fiber lasers.