Method of measuring a minute displacement

A method of detecting a minute displacement of a probe which comprises:

• emitting from a laser source two laser beams having different polarizing directions;
• passing the laser beams through a first lens so as to cause the laser beams to diverge;
• passing the diverged laser beams through a beam splitter and then through a second lens and then through a first prism having an angle of separation which varies depending on the direction of polarization of light, the beams passing through the first prism forming first and second measuring beams;
• wherein the first and second measuring beams are focused by the second lens so as to be focused on and subsequently reflected from a first reflecting surface provided in the vicinity of a tip of the probe and a second reflecting surface provided in the vicinity of a probe holder;
• passing the reflected first and second measuring beams through the first prism and the second lens towards the beam splitter;
• deflecting by means of the beam splitter the reflected first and second measuring beams in a direction diverging from a direction towards the laser source so as to enter a polarizing beam splitter operable to pass or reflect the rays of light depending on the direction of polarization of the light, said reflected first measuring beam being deflected by said polarizing beam splitter so as to enter a first photodetector and said reflected second measuring beam being passed through said polarizing beam splitter so as to enter a second photodetector;
• causing the first photodetector to detect a position of focus of the reflected first measuring beam and causing the second photodetector to detect a position of focus of the reflected second measuring beam;
• and
• processing respective output signals from the first and second photodetectors by the use of a signal processing circuit to measure the displacement of the probe.