A method of fabricating an optical device includes forming on a semiconductor substrate a first optical cavity, a second optical cavity, a first light guide and a second light guide. The first light guide has an input, and is optically coupled to the first optical cavity by a first coupling strength. In addition, the first light guide is optically coupled to the second optical cavity by a second coupling strength. The second light guide has an output, and is coupled to the second optical cavity by a third coupling strength. The first coupling strength is greater than the second coupling strength, and the third coupling strength is greater than the second coupling strength.