Active Mode-Locking All-Fiber Laser With Switchable Multi-Transverse-Mode Operation

An active mode-locking all-fiber laser capable of transverse-mode switching is experimentally demonstrated using a dual few-mode fiber Bragg grating (FM-FBG) configuration. The laser system achieves stable mode-locking operation across five linearly polarized modes and eight orbital angular momentum modes with topological charges of l = 0, ±1, ±2, and ±3, enabling arbitrary switching among these transverse modes. Transverse-mode switching is achieved by cooperatively tuning a tilted few-mode fiber Bragg grating (TFM-FBG) and a non-tilted few-mode fiber Bragg grating (NFM-FBG) with a LiNbO3 Mach-Zehnder intensity modulator providing active mode-locked pulse generation. The TFM-FBG achieves highly efficient coupling from the fundamental mode to different transverse modes by breaking the cylindrical symmetry of the refractive index modulation in the fiber core, while the NFM-FBG acts as a mode-selective cavity mirror. Mode-locked pulses are generated with repetition rates of 7.71 MHz (fundamental mode) and 7.02 MHz (high-order modes), exhibiting signal-to-noise ratios exceeding 67 dB for all modes. This all-fiber mode-locked laser with switchable transverse-mode has potential for applications in material processing and nonlinear optics, and related fields.