Laser Read-Out System

The information on an optical disc can be read-out with a light,pen. One possible design is shown in Fig. 8a. The light emitted by the solid-state laser is collimated and focused on the information layer, reflected there and on its way back split into two beams which fall on a detector array. This means that when the laser spot strikes a pit, interference will reduce the amount of reflected light. 

There are several possible designs for the lenses used in such a light-pen. In the case of Fig. 8 a where only spherical elements are used, the requirement of only 1 pm spot size can only be fulfilled with an objective consisting of at least three elements. 

a and b. Schematic drawing of Compact Disc light pens with conventional optics (a) and with aspherical lenses (b). The laser beam passes through a beam-splitting prism, is focused on the disc and the reflected beam is directed towards a set of 4 photodiodes. (From Ref. with permission) 

However, with the introduction of one or two aspherical surfaces, depending on the index of refraction chosen, the same optical performance can be achieved with only one element (Fig. 8 b). This has the advantage of smaller size and lower weight and, if a suitable technology is used, lower price For such lenses, the asphericity, that is the deviation from the nearest sphere, needs only to be of the order of +10 pm. 

Aspherical shapes of a precision within +0.2 pm as is required by this application can be made economically in at least three ways, which all depend on copying the aspherical surface from a mold  

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