Performance degradation of an optical Costas loop due to non-ideal response of 90 degrees hybrid

Effect of non-ideal response of a 90 degrees hybrid in optical Costas loops has been examined. Even a very slight phase deviation psi from the ideal h ybrid phase shift of 90 degrees leads to increased power penalties and rece iver sensitivities. Choice of the optimum power splitting ratio leading to a minimum BER, for a fixed laser linewidth, also increases. The greater the magnitude of the phase deviation, the more the system performance degradat ion. The nature of these Variations does not depend significantly on the nu mber of in-line optical amplifiers.