Coherent optical receiver testing

An apparatus for measuring a characteristic of a coherent optical receiver (COR) that comprises one or more optical mixers followed by one or more differential photodetectors, the apparatus comprising:

• one or more coherent light sources configured to provide first and second lights with an optical frequency shift f therebetween;
• first and second output optical ports for coupling one of the first and second lights into a local oscillator (LO) port of the COR and the other of the first and second lights into an optical signal port of the COR;
• an optical modulator disposed in an optical path of the first light and operable to modulate the first light in amplitude at a modulation frequency F>f,
• an electrical signal recorder configured to connect to an output port of the COR and to record an output COR signal received therefrom, said output COR signal comprising a first frequency component and a second frequency component;
• and
• a controller coupled to the electrical signal recorder and configured to determine the characteristic of the COR based at least in part on the first and second frequency components.

A method for measuring a characteristic of a coherent optical receiver (COR) that comprises one or more optical mixers followed by one or more differential photodetectors, the method comprising:

• splitting light from a coherent light source into first and second lights;
• frequency shifting one of the first or second lights by a frequency shift f;
• modulating the first light in amplitude at a modulation frequency F that is greater than f,
• providing one of the first and second lights into a signal port of the COR, and the other of the first and second lights into a local oscillator (LO) port of the COR;
• recording an output signal of the one or more differential photodetectors, said output signal comprising a first frequency component at a shifted modulation frequency (F−f) or (F+f) and a second frequency component;
• and
• determining the characteristic of the COR based at least in part on the first and second frequency components.