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A diffraction-limited bi-aspherical readout lens for the digital audio disc system has been designed and developed based on a high -precision lens replication process using UV-polvmeri_ able coatings. This single element read -out lens consists of a ball-shaped quartz substrate with two aspherical replicated coating surfaces. The design of the bi-aspherical lens is presented and its production described. The optical performance of the lenses proved to be reasonable with an rms -OPD of 0.055 A, the primary lens aberration being spherical aberration. I INTRODUCTIONAspherical lenses made by replication have recently succesfully been used in optical scanning systems like the video and digital audio disc systems (1,2). The replication process and the physical properties of the UV-polymerized coatings have been described in detail (3,4). The main advantages of these high precision aspherical lenses, made in a mass -production process, are a reduction of the number of optical components, of the lens aberrations and of the total weight of the optical system. The described aspherical lenses for the digital audio disc system were single element objective lenses that have to be used in combination with a collimator lens; this collimator is either a two element glass lens , a replicated single element asphere or eventually a single element plastic asphere. Considering the need for further miniaturization of the optical scanning system, it goes without saying that a lens design omitting the collimator lens would be interesting, especially if such a lens can be produced cheaply.At first sight the application of a ball-shaped bi-aspherical read out lens is not particularly interesting; this shape severely limits the degrees of freedom in the design which restricts the ultimate performance. However, from a production point of view, the ball geometry would have several interesting properties that could be more important than a relatively small image field. The knowledge of the replication process and the design of aspherical lenses using replication coatings developed within our company has led us to the the investigation of the biaspheric single element readout lens described in this paper. H LENS DESIGNThe standard optical light path of a Compact Disc player consists of a collimator with a numerical aperture (NA) of 0.10 and a focussing objective with a NA equal to 0.45 . The focal lengths of these components are approximately 20 and 4.5 mm and both have one conjugate located at infinity. The combination has a focal length of approximately 3.7 mm and the magnification is -0.22. The desired field diameter is 150 um. For the focussing action the objective is actuated with the aid of a loudspeaker coil in a magnetic field. The optical correction is not affected by this movement because of the parallel section in the light path. The peak -peak excursion of this movement amounts to 1.2 mm in practice.When we combine the collimator and objective in one lens, the focussing movement changes both conjugate distances and the state of opt...

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