An optical memory technology which uses an optical disk having pit-like pattern as a storage medium of high density and high capacity have been expanding its applications as a digital audio disk, a video disk, a document file disk, a data file, and the like.  In the optical memory technology, information is recorded and reproduced to and from an optical disk with a light beam narrowed to a very small size with high precision and high reliability.  The recording and reproducing operation mainly depends on the optical system thereof. 
An optical head is a main component in the optical system, and its basic functions are divided into convergence for forming a very small light spot of diffraction limit, focus and tracking control of the optical system, and detection of pit signals.  These functions are realized by combining a various optical system and a various detection technique with photoelectric conversion, according to an object and a use thereof. 
An object lens used in an optical head is designed by considering plate thickness of optical disk.  Its convergence performance is deteriorated due to spherical aberration for an optical disk having thickness different from the design value, and this makes recording and reproduction difficult.  Previously, a compact disc (hereinafter referred to as CD), a video disk, a magneto-optical disk and the like all has plate thickness of 1.2 mm, and one optical head can be used for recording and reproduction for these various optical disks. 
Recently, an optical disk of high density and high capacity, called as DVD (digital video disk), have been used practically, and it is high-lighted as an information medium which can handle a large amount of information such as a dynamic image.  The DVD has smaller pit size in an information recording plane in order to increase recording density, relative to the prior art optical disk, CD.  Therefore, for an optical head used for recording and reproduction of DVD, wavelength of light source and numerical aperture (hereinafter referred to also as NA) of converging lens which determine the spot size are different from the counterparts for CD.  In order to increase recording density, DVD adopts large numerical aperture of object lens.  When numerical aperture is increased, optical resolution is improved and recording density is increased.  On the other hand, the converged light spot has coma aberration caused by inclination of optical disk.  Then, in order to decrease the influence of coma aberration even when the numerical aperture of the object lens is increased, the thickness of the plate of the optical disk, DVD, is decreased to 0.6 mm.  However, when the thickness of the plate of the optical disk is decreased, an object lens used for the optical disk cannot be used for a prior art disk, and the compatibility between DVD and the prior art disk cannot be realized. 
It is to be noted that for CD the wavelength of light source is about 0.78 μm and NA is about 0.45, while for DVD the wavelength of light source is about 0.63 to 0.65 μm and NA is about 0.6.  Therefore, when two types of optical disks, CD and DVD, are recorded or reproduced by a single optical disk drive, an optical head needs two optical systems.  On the other hand, there is a tendency to use a common optical system for CD and for DVD in order to make the drive compact, small and less expensive.  For example, a single light source for DVD is used while two converging lenses for CD and for DVD are used, or even for the converging lens, only one converging lens is used commonly while the numerical aperture thereof is changed between for CD and for DVD mechanically or optically. 
In an example of an optical system of an optical head in a drive compatible with CD and DVD, an object lens of numerical aperture 0.6 is used as the converging lens.  In the object lens, a central portion of numerical aperture equal to or smaller than 0.37 is designed to make aberration minimum when light is converged through a transparent flat plate of thickness 0.9 mm, while an outer portion of numerical aperture equal to or larger than 0.37 is designed to make aberration minimum when light is converged through a transparent flat plate of thickness 0.6 mm.  A light beam of wavelength 650 nm emitted by a laser diode is collimated by a condenser lens to become a collimated light beam, and it is incident on the object lens.  When a DVD is reproduced, the light beam narrowed by the object lens forms an light spot on an information plane in a DVD of thickness 0.6 mm, while it forms an light spot on an information plane of CD in a plate of thickness 1.2 mm.  Next, the light reflected from the optical disk is condensed again by the object lens and detected by a photodetector.  The photodetector is constructed such that a focus control signal is detected by astigmatism technique and that a tracking control signal is detected by a phase difference or push-pull technique. 
By using the optical head, when a CD is reproduced, the light beam transmitting the central portion of the object lens is reflected by the medium plane and enters the photodetector, while the light beam transmitting the central portion is diverged due to large spherical aberration and does not enter substantially onto the photo-receiving plane of the photodetector.  Thus, the numerical aperture is limited substantially to 0.37.  On the other hand, when a DVD is reproduced, the light beam transmitting the central portion is synthesized with that transmitting the outer portion to form an light spot, due to small spherical aberration.  All of the reflected light thereof enters substantially the photodetector, and reproduction is performed with numerical aperture 0.6. 
However, because the prior art optical head obtains the compatibility between CD and DVD by using a light source of wavelength 650 nm, it has a problem that sufficient signals cannot be obtained from an optical disk having wavelength dependence due to difference in reflectivity.  This is evident, for example, for a CD-R standardized as a rewritable CD.  In the standard of CD-R, the reflectivity is defined to be 65% or higher in wavelength range of 775 to 820 nm, but it decreases at wavelengths outside the above range and the absorptivity increases.  For example, the reflectivity decreases to ⅛ times and the absorptivity increases to 8 times, so that reproduction is impossible and even the data recorded by optical absorption are erased. 
In order to solve this problem on the compatibility between CD and DVD, it is proposed to use two light sources of wavelengths 780 and 650 nm and to divide the object lens into a central portion and an outer portion surrounding the central portion, wherein the optimum design plate thickness of plate in the central portion is set to 0.9 mm and that in the outer portion is set to 0.6 mm.  However, this technique cannot be used practically because the spherical aberration becomes too large when a DVD is reproduced.  In a CD drive, the numerical aperture of the object lens is 0.45 for wavelength 650 nm, whereas in the above proposal, the numerical aperture is decreased to 0.37 because the light of lower wavelength 650 nm is used for reproduction of CD.  If the numerical aperture at the central portion of the object lens is about 0.37 in the above example, aberration for CD reproduction is about 40 mλ (rms) and that for DVD reproduction is about 30 mλ (rms), so that reproduction performance is normal.  However, when a CD is reproduced with the light source of wavelength 780 nm, the numerical aperture has to be about the same as in a conventional CD drive, and the numerical aperture of the central portion of the object lens is 0.45.  However, when the central portion of the object lens having the optimum design plate thickness 0.9 mm is enlarged, aberration becomes larger when a DVD is reproduced.  If the central portion is enlarged to numerical aperture 0.45, aberration increases to 80 mλ (rms) or higher though it depends on design conditions, and sufficient reproduction performance cannot be provided.  An optical head which uses light beams of wavelengths 780 and 650 nm and an object lens having double optimum design plate thicknesses, as explained above, has not yet be provided for reproducing both CD and DVD.