Performance optimization and exact laser linewidth requirements evaluation for optical PSK homodyne communication systems with Costas loop or DDL

For an optical homodyne receiver comprising a Costas loop a formula for the phase error variance determining is derived more precisely than even before. The requirement laser linewidth to achieve the bit-error rate 10^-10 is obtained for the optimally chosen power splitting ratio. Two type of optical PSK homodyne communication systems are taken into consideration: the Costas-type receivers and the DDL ones. It is shown that the requirement laser linewidth are less stringent when power splitting ratio is optimally chosen than that for the fixed power splitting ratio obtained starting from the permissible power penalty due to the power splitting.