A passively stabilized third-order optical interferometer that spatially separates the local oscillator and signal generation is demonstrated with long-term phase stability. The lack of spatial overlap eliminates unwanted contamination of either field. Fully independent optical control over both fields is exerted after the sample. This independence is taken advantage of with what we believe to be a new approach to scanning the relative phase between the local oscillator and signal that has very high precision and reproducibility. The independence of the fields is also exploited in a flexible balanced heterodyne detection scheme.