Compact mach-zehnder interferometer and wavelength reference employing same

A MZI comprises a pair of folded waveguides (21,22) disposed between a pair of couplers (24,25). Each folded waveguide comprises a pair of segments (21A,21B) intersecting at a reflector (21C). The respective reflectors (21C,22C) are different distances from the couplers to provide a major portion of the differential path length between the two arms. In a preferred embodiment, the reflectors are polysilicon mirrors and can be formed by etching through the intersecting waveguides and depositing polysilicon. The result is a compact MZI that can be used as a multiwavelength reference for a laser light source on a common silicon optical bench chip.