Patent ID: 8861902

Claim:
A planar waveguide circuit comprising: a first optical splitter arranged to receive an input optical signal; a second optical splitter arranged to receive a reference optical signal; a first optical signal combiner; a second optical signal combiner; first and second optical waveguides arranged to couple a first output and a second output of the first optical splitter to respective inputs of the first and second optical signal combiners, and third and fourth optical waveguides arranged to couple a first output and a second output of the second optical splitter to respective inputs of the first and second optical signal combiners; and a phase-shifter located to affect the phase of an optical signal propagating in one of the third and fourth optical waveguides, wherein the first and second optical splitters and the first and second optical signal combiners are arranged such that the first, second, third and fourth optical waveguides do not intersect one another, wherein the phase-shifter comprises a resistive heater arranged to affect a portion of said one of the third and fourth optical waveguides to change a refractive index of the portion of said one of the third and fourth optical waveguides.