1.  Field of the Invention
The present invention relates an objective lens, an optical pickup, an optical information recording/reproducing apparatus, such as those preferably applied to an optical disc device in which information is recorded on an optical disc as an information recording medium and the information is reproduced from the optical disc. 
2.  Description of the Related Art
Optical disc devices have been widely used in the art.  Any of these devices is designed to record information on an optical disc, such as a compact disc (CD) or a digital versatile disc (DV), and read out the information therefrom.  In recent years, Blu-ray Disc (registered trademark, hereinafter also referred to as “BD”), an optical disc with a significant increase in information recording density, is becoming common in addition to these optical discs. 
The optical disc device uses an objective lens to focus a light beam on a track spirally or concentrically formed on the recording layer of an optical disc and follow the focal point of the light beam. 
For example, in order to accommodate the DVD system, an objective lens is designed to have a numeric aperture (NA) of about 0.6 or more to concentrate a light beam of about 660 nm in wavelength on a recording layer under a cover layer of about 0.6 mm in thickness in an optical disc.  In contrast, a very small spot of the light beam is made on the information recording surface of an optical disc in order to accommodate the BD system.  Thus, it is desirable to provide an objective lens having a numeric aperture (NA) of about 0.8 or more to concentrate a light beam of about 405 nm in wavelength on the recording layer of the optical disc.  Such a recording layer is formed under a cover layer of about 0.1 mm in thickness in the optical disc. 
The single object lens that satisfies the functions of an objective optical system a numeric aperture (NA) of 0.8 or more is almost 10 times sensitive to a production tolerance than any of other objective lenses commonly used for CDs and DVDs because the NA of the former is larger than that of the latter. 
Therefore, even if such a kind of the objective lens has a production tolerance in a micrometer order, it may lead to a decrease in production yield because of being provided as a defective product that does not satisfy the desired optical characteristics of the optical lens. 
Here, the single objective lens is formed, for example, by filling a so-called glass material, such as a material made of glass or resin, into a pair of dies in a certain manufacturing device. 
At this time, in the manufacturing device, a positional displacement between dies or molding members that form the opposite sides of an objective lens may occur due to a problem in accuracy or the like.  The degree of the positional displacement corresponds to a level defined as production tolerance of the objective lenses.  Mainly, there are two kinds of production tolerance, a decentering error and a thickness error. 
For example, when a die structure for molding a plurality of parts is used in injection-molding of a glass material to enhance productivity, the processing accuracy of each part and so on are accumulated.  Therefore, critical values for producing objective lenses while ensuring the yielding percentage thereof include a decentering tolerance of almost ±2.5 μm that represents an allowable range of the decentering error and a thickness tolerance of almost ±1.0 μm that represents an allowable range of thickness error. 
For confirming such critical values, objective lenses were actually manufactured using dies built with high precision and the critical values were calculated back from the aberration of the objective lens.  As a result, an estimated decentering tolerance of almost ±2.5 μm and an estimated thickness tolerance of almost ±1.0 μm were obtained. 
These variations in the respective critical values are not only limited in any injection-molding device that performs injection-molding of a glass material.  It is desirable to estimate the same levels of such variations in an injection-compression molding machine, a compression molding machine, or the like. 
In consideration of the production tolerance, the objective lens is allowable as long as it is designed to exert desired optical characteristics even when the decentering error and thickness error thereof are within their acceptable ranges in production.  In other words, if the objective lens is designed in this way, the production of any inferior objective lens can be prevented so that the yield of the production can be improved. 
Here, the term “decentering sensitivity” is referred to as a wavefront aberration level occurred when the wavefront aberration between the lens surface on the light side and the lens surface on the disk side of an objective lens is one micrometer.  If there is a thickness error of +1 μm, then a wavefront aberration level generated when the lens thickness of the objective lens on the optical axis is +1 μm higher than a predetermined thickness.  A k-order coma aberration (k is an uneven number of three or more), which is generated when a decentering error is 1 μm, is referred to as a k-order decentering sensitivity (DCmk).  In addition, k-order spherical aberration level (k is an uneven number of three or more) generated when a thickness error is 1 μm is referred to as a k-order thickness sensitivity (TSAk).  Then, the decentering sensitivity can be represented by square-root of sum of squares of the decentering sensitivities of the respective orders and the thickness sensitivity can be represented by square-root of sum of squares of the thickness sensitivities of the respective orders.  However, the wavefront aberration is positive when progressing to a converging spherical wave. 
A technique for reducing decentering sensitivity has been proposed as a technology that focuses attention on the yield of aberration resulting from a variation in production accuracy.  This technique pays attention to aberration deterioration due to the decentering of the objective lens and sets limitations to the differential value and the second order differential value of each surface shape of the lens, so that the surface shape of the lens is less curved (see, for example, U.S. Pat.  No.  4,130,938 (particularly, FIG.  4 thereof)).