Measuring Two Dimensional OTF Applying CT Principle

In optical disc system, the precise measurement and evaluation of focused micro-laser-spot (usually less than lum in diameter) is very important. To meet this requirement, a new method using the concept of CT scanner has been developed. A knife-edge scans the spot on the image plane. Differentiating the signal received by the detecter under the knife-edge, we get a one-dimension (1-D) projection of 2-D laser spot. Rotating the knife-edge slightly, we can get different projections at different angles. The projections measured are processed by taking the convolution and back-projecttion, just done as in the CT scanner. In this way, we get the 2-D profiles of the spot, i.e. the Point Spread Function (PSF) of the optical head. The shape and size of the spot can be got directly from the profile. The 2-D Fourier Transform of PSF results in the OTF of the optical head, which discribes the quality of the optical system quantitatively.