System and method for measuring group delay based on zero-crossings

A system for measuring a group delay of an optical device under test (DUT) comprising:

• a light source that generates an input optical having a frequency that varies with time;
• a test interferometer optically coupled to said light source to receive said input optical signal, said test interferometer including said optical DUT;
• a DUT zero-crossing detector coupled to said test interferometer to detect DUT zero-crossings of a DUT heterodyne beat signal from said test interferometer;
• and
• a processor configured to calculate a derivative of a DUT zero-crossings function defined by said DUT zero-crossings with respect to one of time and optical frequency of said DUT zero-crossings function, said processor being further configured to compute a group delay of said optical DUT using said derivative of said DUT zero-crossings function.

A method of measuring a group delay of an optical device under test (DUT) comprising:

• transmitting an input optical signal having a frequency that varies with time into a test interferometer and a reference interferometer, said test interferometer including said optical DUT;
• detecting DUT zero-crossings of a DUT heterodyne beat signal produced by said test interferometer from said input optical signal;
• calculating a derivative of a DUT zero-crossing function defined by said DUT zero-crossings with respect to time;
• detecting reference zero-crossings of a reference heterodyne beat signal produced by said reference interferometer from said input optical signals;
• calculating a derivative of a reference zero-crossing function defined by said reference zero-crossings with respect to time;
• and
• computing a group delay of said optical DUT using said derivative of said DUT zero-crossing function and said derivative of said reference zero-crossing function, said computed group delay being indicative of a phase response of said optical DUT.