Managing an optical probe beam for displacement sensing

An apparatus comprising:

• a transmitter configured to provide an optical beam from a transmitter aperture, the optical beam comprising a plurality input beams each having a different spatial mode, from a set of mutually orthogonal spatial modes, where the set of mutually orthogonal spatial modes include:
• a lowest order spatial mode,
• a highest order spatial mode, and
• one or more intermediate order spatial modes each having a mode order between the lowest order spatial mode and the highest order spatial mode;
• and
• a receiver configured to receive the optical beam at a receiver aperture after displacement of the optical beam within the receiver aperture caused by a path shifting component, and to provide initial displacement information characterizing at least one of:
• an initial estimate of the displacement, or
• an indication that the displacement is below a predetermined threshold;
• wherein the transmitter is further configured to determine a relative amount of each of the input beams to be included in the optical beam based at least in part on:
• corresponding diffraction loss estimates for each of the input beams, and
• the initial displacement information;
• wherein one of the input beams that has a largest relative amount in the optical beam is one of the intermediate order spatial modes.