Figure 1 shows the optical system. The beam that exits from the semiconductor laser will be focused onto the pinhole (P) with the aid of collecting lens (L1).
Although the laser beam is polarized light beam P horizontal to the paper surface, it spreads after exiting from the pinhole (P) first then penetrates the polarizing beam splitter (PBS), passes through the 1/4 wavelength plate and becomes a circularly polarized light from a linearly polarized light. Then, this beam will be emitted as a parallel beam by means of the collimator lens (L2) via the mirrors (M1 to M4).
This parallel beam will be reflected by the mirror for measurement (M5) and become a circularly polarized light in a reverse rotation direction. The beam will enter the 1/4 wavelength plate via the mirrors (M1 to M4) once again. At this stage, the circularly polarized light in a reverse rotation direction becomes the polarized light beam S which is perpendicular to the paper surface, then it will be reflected by the polarizing beam splitter (PBS) and enter the position detection sensor (CMOS).
What is shown in Figure 2 is the light path excluding the mirrors (M1 to M4) shown in Figure 1.When the mirror for measurement (M5) is inclined only by θ, the beam which is reflected by M5 and directed back toward its source enters the collimator lens (L2) under the condition that it remains inclined only by 2 θ and produces an image of the pinhole (P) at a position which is deviated only by d from the center line on PSD. This has such a relation as d = ftan2θ≒ 2fθ.
For this reason, detecting d enables the tilt angle θ of mirror for measurement (M5) to be found.