Apparatus for measuring the differential group delay .tau..sub.1 in an optical fiber connection. The apparatus comprises at the inlet to said connection, a generator (10) for generating a binary signal sequence at a data rate D and a first polarization controller (30) suitable for subjecting the binary signal of an incoming sequence to a first scan through polarization states; and at the outlet from the connection, a second polarization controller (60) suitable for subjecting the signal resulting from the outgoing sequence to a second scan through polarization states, independently of said first polarization scan, a differential group delay emulator (70) suitable for introducing a variable additional group delay .tau..sub.2, and an analyzer device (90) suitable for detecting the equality .tau..sub.1+.tau..sub.2=1/D in the resulting signal sequence.