Asymmetric dual-side-grating coupler for high power side-pumping

A novel concept for an asymmetric dual-side-grating is proposed and rigorously analyzed for laser-diode (LD) bars side-pumping double-clad fiber (DCF). It consists of a metal reflection grating and a dielectric transmission grating, which are placed behind the upside and downside of DCF, respectively. The concept provides a robust side-pump coupler requiring neither complicated beam-shaping system for LD bars nor any risk of damaging the DCF, and is very tolerant to potential fabrication errors and the variations of the incident angle and wavelength. Up to 94.2% (for TE-polarized) and 93.94% (for TM-polarized) coupling efficiencies are obtained with practical grating structural parameters, which can be realized easily by a commercial EB-lithography system JBX-9300FS of JEOL Ltd.