In a conventional circular cross‐section single‐mode fiber, built‐in and induced birefringences remove the degeneracy and make the fiber a two‐Eigenmode system. We have constructed the first all‐fiber interferometric sensor system and have performed preliminary studies on the effects of birefringence in these interferometers. Theoretical analysis and experimental data supporting this analysis are presented for the 3‐dB coupler and all‐fiber interferometer sensor.