An interferometer suitable for Distributed Acoustic Sensing (DAS) is has two interferometer arms of different length, from which light is fed to an N-way optical coupler (N at least 2). In DAS, the light is backscattered light received from within an optical fiber. Incoming light is amplified by an adjustable factor, which may vary as a function of time in DAS. The N way optical coupler supplies combinations of light from the arms with different relative phase offsets to each other to N outputs. Phase shift between interfering components is computed from the light intensities detected by detectors at the N outputs. Correction of the computed phase shift is applied dependent on the value of the adjustable amplification factor. For this, one interferometer arm comprises a phase modulator, which is used obtaining measured detector output signals from the detectors with different phase shifts by the phase modulator when the value of the adjustable factor is used. Data is determined that defines average of output signals of the different detectors under variation of different phase shift by the phase modulator based on the measured signals. For DAS, data defining time dependent averages according the variation of the factor are determined. A phase shift between interfering light is computed with a correction to compensate for differences between the average detector output signals for the individual optical detectors.