This invention relates to a small-sized optical element and its manufacturing method, which is used for a light pickup lens of light pickup device and so on.
Lately, there are manufactured and on sale, various kinds of optical element of various sizes, such as large-sized optical elements used for telescope lenses and microscope lenses and small-sized optical elements used for light pickup lenses of CD (Compact Disc) and DVD (Digital Video Disc). These optical elements are usually manufactured through so called “injection molding” process in which specific resin corresponding to the application is injected into a prescribed die for molding, however on the surface layer of the mold, there occurs birefringence to some extent, created by a residual stress, and therefore, a light scattering phenomenon occurs on the surface layer to make it difficult to maintain the focal distance to be the wanted design value.
In Patent Document 1, it is described that the residual stress generated in a molded product can be relieved and birefringence generated in the mold can be minimized by means of annealing the product at a temperature below the glass transition-temperature after cooling (leaving as it is) at the room temperature.
[Patent Document 1] U.S. Pat. No. 3,055,443
However, the objective of the technique disclosed in Patent Document 1 is to apply annealing process to relatively large-sized optical elements when they are molded to add some preferable performance to the molds, and though the conditions for the annealing process are specified in detail, the conditions are not available to every optical element made of resin manufactured through an injection molding process. For example, regarding a small-sized optical element of a several millimeter scale used as the aforementioned light pickup lens, only if the conditions for the annealing process slightly differ in each manufacturing lot, the difference of the conditions affects performance of forming a spot of condensed light greatly. Therefore, the annealing process of the technique disclosed in the Patent Document 1, can not relieve the residual stress sufficiently and can not maintain the thermal balance condition stably during the annealing process so that sufficient performance can not be obtained for a optical element of light pickup lens.