Versatile objective lens with adjustable correction for different wavelengths and substrate thicknesses for testing optical disks

A typical aspheric singlet used as the objective lens in optical-disk data-storage systems will not work at different wavelengths or with different substrate thicknesses because of spherochromatism. Using two microscope objectives with adjustable collars and a pair of relay lenses, we have constructed a system in which a diffraction-limited spot of any wavelength in the range of 0.4-0.7 mm can be moved by as much as ±100 μm in both the focusing and tracking directions. We accomplished this by simply moving an aspheric singlet mounted in an off-the-shelf optical head. The system uses the adjustable collars of the microscope objectives to correct for the spherochromatism of the singlet and to accommodate the various thicknesses of the substrates.