A fiber-optic interferometer is designed to receive and propagate a first single-mode wave along a first optical path and, respectively, a second single-mode wave along a second optical path, the second optical path being the reverse of the first optical path, and to form a first output wave and, respectively, a second output wave, having a modulated phase difference Δϕm(t). According to the invention, the modulated phase difference Δϕm(t) is equal to sum of a first periodic phase difference Δϕπ(t) having a level equal to ±π, a second periodic phase difference Δϕalpha(t) having a level equal to ±alpha and a third periodic phase difference Δϕbeta(t) having a variable level between −beta and +beta, said modulated phase difference Δϕm(t) comprising per modulation period T at least eight modulation levels among twelve modulation levels and said modulated phase difference between such that: Δϕm(t+T/2)=−Δϕm(t).