External feedback requirements for coherent optical communication systems

Even though it is well known that the external optical feedback requirements for coherent optical systems are very severe, definite data are difficult to obtain. Based on a numerical and an analytical study, values for the maximum tolerable feedback level are derived. In order to fulfil a given linewidth requirement, for example 70 MHz, the external f4eedback level has to be controlled below f_ext=10 ^-7 for typical fiber pigtail lengths, and below f _ext=10^-4 for cavities with an air equivalent of 1 cm as in typical integrated receiver structures