Fused bitapered single-mode fiber directional coupler for core and cladding mode coupling

We demonstrate a new functional fiber component providing coupling between the LP_1m cladding (LP_1m^cl) mode in one single-mode fiber (SMF) and the LP₀₁ core mode in another SMF. The phase-matching condition in the coupling region is achieved by prepulling the fiber sections. The experimentally demonstrated coupling efficiency between the LP₀₁ core and the LP₁₃^cl modes is more than 60% over 70-nm wavelength range with less than 3% variation over 90°C temperature range.