For the justification of the above explanation i havw given a link of the paper .Go through this paper for better information
Design of coherent receiver optical front end for … – OSA Publishing
Equations given in this paper will gelp you to find the correct LO power
Design of coherent receiver optical front end for … – OSA Publishing – I am browsing [Design of coherent receiver optical front end for … – OSA Publishing]. Have a look at it! https://www.google.co.in/url?q=https://www.osapublishing.org/viewmedia.cfm%3Furi%3Doe-20-3-3225%26seq%3D0&sa=U&ved=0CAcQFjAAahUKEwi0qs_XvfHGAhXFm5QKHSS7BQc&usg=AFQjCNF91DLO_783KISlHe1fTigpfNyPJg