Integrated Forward-Propagating Add-Drop Filter based on Cladding-Modulated Multimode Long Period Grating

We propose an integrated forward-propagating add-drop filter based on the co-directionally mode conversion by a cladding-modulated long period grating (LPG). The cladding-modulated corrugation is realized by etching periodic silicon cubes along the multimode strip waveguide. Two-mode waveguide grating for TE
0
-TE
1
-TE
0
mode conversion and three-mode waveguide grating for TE
0
-TE
2
-TE
0
mode conversion are simulated, fabricated and tested. Experimental results show that the two-mode waveguide grating has an extinction ratio of 10 dB at the through-port and a 3-dB bandwidth of 4.1 nm at the drop-port, while the three-mode grating has an extinction ratio of 6 dB at the through-port and a 3-dB bandwidth of 3.05 nm at the drop-port. Our devices may play an important role in the field of on-chip optical interconnect, spectrometer and microwave photonics.