Phase distortion measurement

A test system for measuring phase distortion, the test system comprising:

• a first laser tuned to a first frequency f1 and generating a first optical signal;
• a second laser tuned to a second frequency f2 and generating a second optical signal and phase modulating the second optical signal with a periodic signal with a repetition frequency fM;
• a photo diode that receives and mixes the first optical signal and the second optical signal, and produces a first tone at a third frequency f3, which is a carrier frequency equal to an absolute value of a difference between the second frequency f2 and the first frequency f1, a second tone at a fourth frequency f4 at a difference between the third frequency f3 and the repetition frequency fM, and a third tone at a fifth frequency f5 at a sum of the third frequency f3 and the repetition frequency fM;
• and
• a receiver-under-test (RUT) that measures a first phasor M0 of the first tone, a second phasor M_ of the second tone, and a third phasor M+ at the third tone, and that calculates phase deviations between measurements of the first phasor M0, the second phasor M_ and the third phasor M+, wherein the calculated phase deviations are compared with known phase relationships between the first tone, the second tone and the third tone to measure phase distortion of the RUT.

A test method for measuring phase distortion, the test method comprising:

• generating a first optical signal from a first laser tuned to a first frequency f1;
• generating a second optical signal from a second laser tuned to a second frequency f2 and phase modulating the second optical signal with a periodic signal with a repetition frequency fM;
• mixing, by a photo diode, the first optical signal and the second optical signal;
• and
• producing, by the photo diode, a first tone at a third frequency f3, which is a carrier frequency equal to an absolute value of a difference between the second frequency f2 and the first frequency f1, a second tone at a fourth frequency f4 at a difference between the third frequency f3 and the repetition frequency fM a third tone at a fifth frequency f5 at a sum of the third frequency f3 and the repetition frequency fM, wherein a receiver-under-test (RUT) is configured to measure a first phasor of the first tone, a second phasor of the second tone, and a third phasor at the third tone, and calculate phase deviations between measurements of the first phasor, the second phasor and the third phasor, and wherein the calculated phase deviations are compared with known phase relationships between the first tone, the second tone and the third tone to measure phase distortion of the RUT.