A double-pass configured all-fibre Mach-Zehnder interferometric optica
l filter has been developed and used in a distributed fibre optic sens
or for separation of Rayleigh and Brillouin signals. Its superior perf
ormance over a single-pass all-fibre Mach-Zehnder interferometer is hi
ghlighted by its 15dB improved rejection of the Rayleigh signal and co
mparable throughput.