The invention discloses an optical interferometer which can be used to provide simultaneous measurements over multiple path lengths and methods to employ such an interferometer as to achieve a variety of functions covering simultaneous measurements at different depths separated by an increment of a multiple differential delay matched in the interferometer as well as imaging. Optical sensors, optical coherence tomography (OCT) set-ups, optical sensing methods and OCT methods are disclosed which can provide: (i) multiple en-face images at several depths with dynamic dispersion compensation, (ii) fast acquisition of cross sections, (iii) fast acquisition of 3D volumes of a scattering object while maintaining dynamic focus; (iv) fast acquisition of long axial measurement profiles, non mechanical, with dynamic focus, range scalable, with applications in tracking and OTDR. Methods are disclosed on the combination of scanning regimes and modes of operation to achieve versatile functionality in measurements, in the 3D imaging of moving tissue such as the eye, heart, or moving embryos or functional/low noise imaging by making use of angular compounding or polarisation. A method for elimination of axial movement effects in measuring the flow profile is also disclosed.