Mode-locked Yb-doped fiber laser based on mode coupler

Overview: High-order modes (HOMs), surpassing the capacity barrier in the traditional single-mode fiber (SMF) communication system, have attracted extensive attention and been widely applied in the fiber laser, optical communication, particle trapping, remote sensing technology, and so on. The HOMs include linear polarization LP₁₁, LP₂₁, LP₀₂, LP₃₁, and even higher-order modes, which can be generated by using free-space and fiber-based mode conversion devices. LP₁₁ mode is one of the most important HOMs, which has four vector eigenmodes. These eigenmodes are called as cylindrical vector beams (CVBs) with the axially symmetric polarization and circular intensity distribution. A polarization controller (PC) added on the fiber can effectively eliminate the degeneracy of LP₁₁ mode to excite individual vector modes in different polarization states. Additionally, the orbital angular momentum (OAM) characterized by helical wavefront can be generated by superimposing two orthogonal vector modes. Recently, ultrafast fiber lasers combined by HOMs have been reported owing to their outstanding characteristics, such as compactness, high peak power, narrow pulse width, and low cost. However, the HOMs were converted outside the laser, and the fundamental mode (LP₀₁) was still transmitted in the laser cavity.

In this paper, a HOM directly oscillating in a mode-locked Yb-doped fiber laser (YDFL) is demonstrated. Two PCs and a polarization-dependent isolator are used to achieve the mode-locked mechanism of nonlinear polarization rotation. A pair of home-made mode selective couplers (MSCs) connecting through their few-mode fiber (FMF) ports, acts as an efficient mode convertor to generate and oscillate HOMs in the FMF section of the YDFL. A MSC is composed of a SMF and a FMF, which are fused by using hydrogen oxygen flame technology to keep two fiber cores close to each other. The claddings of two fibers are partly fused to form a coupling region. If the phase matching condition is satisfied, the LP₀₁ mode is transferred to the LP₁₁ mode in the coupling region. The MSC has a central wavelength of 1064 nm, a mode conversion bandwidth of 80 nm, and a HOM purity of 94%. Meanwhile, according to the reversibility of MSCs, the LP₁₁ mode can be lunched in the FMF port and output the LP₀₁ mode in the SMF port. The pulsed laser with a 3 dB spectral width of 7.4 nm, a pulse repetition frequency of 10.9 MHz, and a signal-to-noise ratio of radio frequency of 55 dB is obtained, and the slope efficiency of the pump and output power is 2.3%. The pulse LP₁₁ mode, CVB, and first-order OAM are obtained from the YDFL. These results demonstrate that the HOM can be generated by the MSC and be directly oscillated in the YDFL, and this approach is promising for directly generating pure and efficient HOMs in all-FMF ultrafast Yb-doped fiber lasers.