Method and system for contactless detection of rotational movement

A method of detecting rotation of a rotatable object over an axis coinciding with an optical axis of a probe beam, the method comprising:

• providing the probe beam comprised of at least a first monochromatic wavelength and a second monochromatic wavelength, wherein the first and second monochromatic wavelengths have a fixed frequency difference and a circular polarization of respective opposite chirality;
• providing a neutral beam splitter, where the probe beam is split in a section that is transmitted and a section that is reflected, the neutral beam splitter directing the reflected beam section via a polarizer filter towards a first photodetector to generate a reference signal;
• directing the transmitted beam section of the probe beam via a quarter wave plate attached to the rotatable object,
• wherein a mirror is arranged to reflect the transmitted beam section of the probe beam, via the quarter wave plate, back to the neutral beam splitter,
• wherein the neutral beam splitter directs the reflected transmitted beam section, via a polarizer filter toward a second photodetector to generate a periodic measurement signal, and
• wherein an object rotation is derived from a relative phase difference between the reference signal and the periodic measurement signal from the first photodetector and the second photodetector, respectively.