An apparatus for providing a differential latency, DL, between an upstream, US, transmission and a downstream, DS, transmission of optical data signals of a bidirectional payload channel at upstream and downstream wavelengths (USW, DSW) within a wavelength band via an optical transmission link, comprising an optical fiber having a group index (n g) proportional to the round trip delay, RTD, depending on a signal wavelength (λ) said apparatus comprising: (a) a measurement unit configured to measure the round trip delays, RTD, of at least two measurement signals(MS1, MS2) having different measurement wavelengths (MW1, MW2) which are different to the payload channel upstream and downstream wavelengths, transported through said optical transmission link, OTL; and (b) a processing unit configured to derive an upstream, US, delay of at least one optical data signal at the upstream wavelength from the at least two measured round trip delays, RTD, of the measurement signals (MS1, MS2) and to derive a downstream, DS, delay of at least one optical data signal at the downstream wavelength from the at least two measured round trip delays, RTD, of the measurement signals (MS1, MS2) by linear or non-linear interpolation of the measured round trip delays, RTDs, of the measurement signals (MS1, MS2) wherein the differential latency, DL, is calculated by said processing unit by subtracting the derived upstream, US, delay from the derived downstream, DS, delay, wherein the measurement wavelengths (MW1, MW2) of the measurement signals (MS1, MS2) are preconfigured or tuned to be in a wavelength region where the amplitude of the measurement signals (MS1, MS2) transported through the fiber of the optical transportation link and reflected back to the measurement unit is sensitive to attenuation caused by water absorption and/or bending of a fiber of the optical transportation link. (a) a measurement unit configured to measure the round trip delays, RTD, of at least two measurement signals(MS1, MS2) having different measurement wavelengths (MW1, MW2) which are different to the payload channel upstream and downstream wavelengths, transported through said optical transmission link, OTL; and (b) a processing unit configured to derive an upstream, US, delay of at least one optical data signal at the upstream wavelength from the at least two measured round trip delays, RTD, of the measurement signals (MS1, MS2) and to derive a downstream, DS, delay of at least one optical data signal at the downstream wavelength from the at least two measured round trip delays, RTD, of the measurement signals (MS1, MS2) by linear or non-linear interpolation of the measured round trip delays, RTDs, of the measurement signals (MS1, MS2) wherein the differential latency, DL, is calculated by said processing unit by subtracting the derived upstream, US, delay from the derived downstream, DS, delay, wherein the measurement wavelengths (MW1, MW2) of the measurement signals (MS1, MS2) are preconfigured or tuned to be in a wavelength region where the amplitude of the measurement signals (MS1, MS2) transported through the fiber of the optical transportation link and reflected back to the measurement unit is sensitive to attenuation caused by water absorption and/or bending of a fiber of the optical transportation link.