An apparatus for interferometrically measuring an optical device under test, characterized by: a laser monitor interferometer coupled to first detectors (61, 62); an optical device under test interferometer coupled to second detectors (70, 71); a splitter for directing light from a laser to the laser monitor interferometer and to the optical device under test interferometer, the laser configured for scanning the optical device under test over a range of different wavelengths of light; and electronic circuitry, coupled to the first and second detectors (61, 62, 70, 71), to: process laser monitor signals received from the first detectors (61, 62) during the scanning to determine an instantaneous laser phase or a derivative of the instantaneous laser phase, the instantaneous laser phase being time-dependent, and for each optical length of one or more optical lengths associated with the optical device under test, compensate for a phase variation of the laser in an interferometric response of the optical device under test to the scanning by: computing, from the instantaneous laser phase or the derivative, a delay-specific phase for the optical length, the delay-specific phase being time-dependent and corresponding to a difference between two delayed versions of the instantaneous laser phase, mixing the delay-specific phase for the optical length with interferometric intensity signals of the optical device under test received from the second detectors (70, 71), to generate mixed data, the mixed data being delay-specific, and processing the mixed data to determine a phase-compensated interferometric response of the optical device under test, the processing comprising low-pass-filtering the mixed data. a laser monitor interferometer coupled to first detectors (61, 62); an optical device under test interferometer coupled to second detectors (70, 71); a splitter for directing light from a laser to the laser monitor interferometer and to the optical device under test interferometer, the laser configured for scanning the optical device under test over a range of different wavelengths of light; and electronic circuitry, coupled to the first and second detectors (61, 62, 70, 71), to: process laser monitor signals received from the first detectors (61, 62) during the scanning to determine an instantaneous laser phase or a derivative of the instantaneous laser phase, the instantaneous laser phase being time-dependent, and for each optical length of one or more optical lengths associated with the optical device under test, compensate for a phase variation of the laser in an interferometric response of the optical device under test to the scanning by: computing, from the instantaneous laser phase or the derivative, a delay-specific phase for the optical length, the delay-specific phase being time-dependent and corresponding to a difference between two delayed versions of the instantaneous laser phase, mixing the delay-specific phase for the optical length with interferometric intensity signals of the optical device under test received from the second detectors (70, 71), to generate mixed data, the mixed data being delay-specific, and processing the mixed data to determine a phase-compensated interferometric response of the optical device under test, the processing comprising low-pass-filtering the mixed data. process laser monitor signals received from the first detectors (61, 62) during the scanning to determine an instantaneous laser phase or a derivative of the instantaneous laser phase, the instantaneous laser phase being time-dependent, and for each optical length of one or more optical lengths associated with the optical device under test, compensate for a phase variation of the laser in an interferometric response of the optical device under test to the scanning by: computing, from the instantaneous laser phase or the derivative, a delay-specific phase for the optical length, the delay-specific phase being time-dependent and corresponding to a difference between two delayed versions of the instantaneous laser phase, mixing the delay-specific phase for the optical length with interferometric intensity signals of the optical device under test received from the second detectors (70, 71), to generate mixed data, the mixed data being delay-specific, and processing the mixed data to determine a phase-compensated interferometric response of the optical device under test, the processing comprising low-pass-filtering the mixed data. computing, from the instantaneous laser phase or the derivative, a delay-specific phase for the optical length, the delay-specific phase being time-dependent and corresponding to a difference between two delayed versions of the instantaneous laser phase, mixing the delay-specific phase for the optical length with interferometric intensity signals of the optical device under test received from the second detectors (70, 71), to generate mixed data, the mixed data being delay-specific, and processing the mixed data to determine a phase-compensated interferometric response of the optical device under test, the processing comprising low-pass-filtering the mixed data.