APS

Figure 1: An optical isolator can be built on a chip using a waveguide that supports two optical modes, I and II, and a wave traveling in one direction with velocity $v$. Downstream light, traveling from left to right in mode I (top), is unaffected by the traveling wave, but upstream light, traveling from right to left (bottom), is resonantly coupled into mode II and is therefore not injected back into the laser.