Controlling of symmetric and asymmetric mode coupling in long-period fiber gratings singe-side induced by long-pulse CO2 laser

► A simplified and low-cost setup for writing LPFGs with a cheap CO2 laser. >Writing-spot size correlates with mode coupling in LPFGs. ► Large spot enhances asymmetry of transverse refractive-index distribution. ► Asymmetric mode coupling with multi-dip resonance as a result. >Mode-profile simulation and cladding etching experiment proves the explanation.