All-fiber zero-insertion-loss add–drop filter for wavelength-division multiplexing

We developed and fabricated an all-fiber add–drop filter by recording a Bragg grating in the waist of an asymmetric mode converter–coupler formed by adiabatic tapering and fusing of two locally dissimilar, single-mode optical fibers. The insertion loss of the device was $\sim 0.1 \mathrm{dB}$. A narrow spectral bandwidth $\left(<1 \mathrm{nm}\right)$ and a large add–drop efficiency $\left(>90\%\right)$ were also demonstrated. In addition, the filter was polarization independent.