General principles for designing mode converters based on air-hole collapse in photonic crystal fibers

The general principles for designing mode converters based on air-hole collapse in photonic crystal fibers (PCFs) are theoretically investigated in this paper. The splice-like point in mode converters is found unnecessary for mode conversion; however, two different and separated cores, the adiabatic transitional part and the final merged core, are essential. Three structures are investigated using logical deduction, and the numerical results agree well with the logical conclusions.