thanks for the answer. but it can’t makes me clear . isn’t it represent the amplitude of the RF spectrum. If an optical spectrum contains two sidebands at 193.07 and 193.14 THz, then when PIN detects it Why it place the rf spectrum at 60 GHz ? this i want to know , why in 60GHz? not in 193.07 and 193.14 THz ? Thanks in advance .