An optical device comprising an optical device comprising a Mach-Zehnder interferometer having a first 2×2 optical coupler, a second 2×2 optical coupler, a first optical arm, and a second optical arm. The first and second arms connecting corresponding pairs of optical ports of the first and second 2×2 optical couplers. The second optical arm has a longer optical path than the first arm. The device also comprises one or more optical resonators optically coupled to the first optical arm and an optical splitter. The optical splitter is coupled to deliver a portion of an input optical signal to one port of the first 2×2 optical coupler and to deliver a remaining portion of the input optical signal to one port of the second optical coupler.