Abstract: A method of determining an optical transform imparted by a multimode optical fibre (300) is disclosed. The wherein the multimode fibre (300) comprises a proximal end, a distal end, and at least one modified region (260) between the proximal end and distal end. The modified region (260) is configured to transmit light toward the proximal end in response to light propagating through the multimode optical fibre (300) from the proximal end to the distal end. The method comprises coupling (101) forward propagating light into the proximal end of the multimode optical fibre; detecting (102), at the proximal end, backward propagating light transmitted from the at least one modified region in response to the forward propagating light; and determining (103) an optical transform from the detected backward propagating light.

Abstract: Polarization compensation in optical system having a primary optical element having polarization-altering characteristics, such as a dichroic element, is provided. A compensation optical element having substantially the same polarization-altering characteristics as the primary optical element is provided in the path of the light beam whose polarization state is to be conserved. The compensation optical element is oriented with respect to the light beam such that by transmitting or reflecting the light beam, it alters its polarization state in a manner opposite to the altering of this polarization state by the primary optical element. Advantageously, this is true for any orientation of the polarization state of the light beam.