A method and apparatus for measuring a propagation delay of an optical signal between first and second devices in an optical transmission network, the optical signal being transmitted from the first device to the second device via a first optical fiber and from the second device to the first device via a second optical fiber. The second device includes a loopback having a first mode enabling the optical signal to be routed between the two devices. The method includes: detecting by the second device a triggering by the first device of a propagation delay measurement; receiving a measurement signal transmitted by the first device via the first optical fiber; and configuring the loopback in a second mode in which the loopback injects a return signal into the first fiber in response to the measurement signal. The first device implements a method for determining the propagation delay.