Direct Side Pumping of Double-Clad Fiber Laser by Laser Diode Array Through the Use of Subwavelength Grating Coupler

An electron-beam-fabricated subwavelength grating coupler for direct side coupling of light emission from a high-power laser diode array (LDA) was studied theoretically and implemented experimentally. A gold-embedded silica-based design for grating coupler was employed to minimize the thermal expansion caused by the accumulated heat from light absorption by metal part of the grating coupler. In addition, with the consideration of the backward diffraction loss and the groove wall nonverticality caused by fabrication distortion, the grating pitch and groove width were optimized for the highest coupling efficiency. According to the experimental results, the grating coupler is capable of coupling light power up to 21 W from a 976-nm continuous-wave-operated LDA into the inner clad of a 400-μm-diameter double-clad fiber with an overall coupling efficiency of 50%. Furthermore, an LDA side-pumped ytterbium-doped DCF laser by using the grating coupler was demonstrated. By fine tuning the slow-axis collimation lens array, the laser-pumping scheme can easily be switched between bidirectional pumping and unidirectional pumping. Compared with the unidirectionally pumped fiber laser of the same gain fiber length, the laser slope efficiency of the bidirectionally pumped fiber laser was increased by 18% because of a better gain distribution over the fiber length. Finally, the signal output power of 10 W with a slope efficiency of 61% was achieved for the bidirectional side-pumped fiber laser.