Perfectly vertical and fully etched SOI grating couplers for TM polarization

We propose and experimentally demonstrate fully etched CMOS compatible silicon-on-insulator (SOI) based input grating couplers (GCs) for perfectly vertical (0°) incidence and transverse magnetic (TM) polarization state. A peak coupling loss of 7.5 dB at 1555 nm was measured for the uniform GC, while 6.5 dB was achieved at 1560 nm for the chirped GC. Compared to the uniform GC, the chirped GC maintained the coupling efficiency while it displayed 8 dB reduction of the back- reflected power to the input fiber reaching −17 dB reflection.