Hitherto, a plastic single lens has been generally used as an objective lens used in an optical pickup device or optical information recording/reproducing apparatus for recording or reproducing an optical information recording medium such as a CD, MD and DVD.
Because of lower specific density than a glass lens, a plastic lens has an advantage that it is possible to reduce the burden of an actuator driving the objective lens for focusing and tracking, and to perform tracking of the objective lens in this regard at high speed.
Also, a plastic lens produced by injection molding in a mold can be mass-produced by manufacturing a desired mold with high accuracy. Thereby, although it is made possible to exert high performance of the lens stably, it is made possible to plan to reduce the cost.
By the way, in recent years, study/development of new high-density optical disk system in which a blue-violet laser diode light source having a wavelength of approximately 400 nm and an objective lens having a numerical aperture (NA) enhanced up to approximately 0.85 are used has been progressed. By way of example, as for an optical disk performing information recording/reproducing with descriptions of an NA of 0.85 and a light source wavelength of 405 nm (hereinafter, such an optical disk is referred to as “high-density DVD”), it is possible to record information of 20 to 30 GB per side on an optical disk having a diameter of 12 cm that is the same size as a DVD (an NA of 0.6, a light source wavelength of 650 nm and a storage capacity of 4.7 GB).
Here, in an optical pickup device for such a high-density DVD, spherical aberration generated by refractive index change accompanying temperature change (hereinafter, such spherical aberration is referred to as “thermal aberration”) becomes a problem in case that an objective lens having a high NA is a plastic lens. Such a problem occurs owing to a plastic lens two orders of magnitude larger than a glass lens in terms of change of the refractive index. Usable temperature range becomes very narrow in case that the objective lens having an NA of 0.85 used for a high-density DVD is a plastic lens because the thermal aberration is proportional to 4th power of the NA, and accordingly it becomes a problem in practical use.
In JP Tokukaihei-11-337818A, an art of correcting such thermal aberration of a plastic single lens by using the diffraction effect of a ring-shaped phase structure formed on its optical surface is described.
For correcting thermal aberration of a plastic lens having an NA of 0.85 by this art, it is necessary to set a tilt of a spherical aberration curve in change of wavelength (hereinafter, such tilt of a spherical aberration curve is referred to as “chromatic spherical aberration”) large. Therefore, it is impossible to use a laser diode having an emission wavelength that deviates from a standard wavelength by a manufacturing error, and selection of laser diodes becomes necessary, which causes a high cost.
A specific example with numerical values is shown below. An objective lens whose lens data is shown in Table 1 is a plastic single lens having an incident light beam diameter of 3 mm, a focal length of 2.5 mm, an NA of 0.6, a design wavelength of 650 nm and a design temperature of 25° C., and corrects thermal aberration by the diffraction effect of a ring-shaped phase structure formed on the first surface (optical surface of a light source example). On the other hand, an objective lens whose lens data is shown in Table 2 is a plastic single lens having an incident light beam diameter of 3 mm, a focal length of 1.76 mm, an NA of 0.85, a design wavelength of 405 nm and a design temperature of 25° C., and corrects thermal aberration by the diffraction effect of a ring-shaped phase structure formed on the first surface in the same way as the objective lens of Table 1. Note that a power-of-ten number (e.g. 2.5×10−3) is expressed by using E (e.g. 2.5×E−3) hereinafter (including lens data in Tables).
TABLE 1Surface No.r(mm)d(mm)N650νdRemarks0∞Light source1  1.66032.05001.5409056.7Objective2−4.52371.0105lens3∞0.60001.5775630.0Protective4∞layerAspherical surface coefficients1st surface2nd surfaceκ−6.8755E−01  −7.9005E+00A43.0995E−03  4.3885E−02A62.6042E−04−3.2001E−02A84.5653E−05  1.1954E−02A10−1.2223E−04  −1.9590E−03Diffraction surface coefficients1st surfaceb2−2.3969E−03b4−7.8946E−04
TABLE 2Surface No.r(mm)d(mm)N405νdRemarks0∞Light source1  1.20992.45001.5601356.7Objective2−1.57830.3771lens3∞0.10001.6195030.0Protective4∞layerAspherical surface coefficients1st surface2nd surfaceκ−7.1214E−01−4.3724E+01A4  5.4718E−03  5.2395E−01A6  5.1672E−03−1.1813E+00A8  1.5578E−03  1.2111E+00A10  1.0499E−03−5.0156E−01A12−7.7777E−04  6.2662E−04A14−1.4455E−05A16  1.7285E−04A18−2.2142E−05A20−1.2407E−05Diffraction surface coefficients1st surfaceb2−7.6944E−03b4−8.9900E−03b6  1.1465E−03b8  2.2677E−04b10−3.3067E−04
An aspherical surface in such an objective lens is expressed by the following Formula 1 when the optical axis direction is x-axis, the height of the direction perpendicular to the optical axis is h and the curvature radius of the optical surface is r. Note that κ is a constant of the cone and A2i is an aspherical surface coefficient.
                              X          =                                                                      h                  2                                /                r                                            1                +                                                      1                    -                                                                  (                                                  1                          +                          κ                                                )                                            ⁢                                                                        h                          2                                                /                                                  r                          2                                                                                                                                          +                                          ∑                                  i                  =                  2                                            ⁢                                                          ⁢                                                A                                      2                    ⁢                    i                                                  ⁢                                  h                                      2                    ⁢                    i                                                                                      ⁢                                                      Formula        ⁢                                  ⁢        1            
Additionally, in such an objective lens, the ring-shaped phase structure as a diffractive structure formed on the optical surface is expressed by an optical path difference added to a transmitted wave front by the diffractive structure. As for the optical path difference is expressed by the optical path difference function Φb (mm) defined by the following Formula 2, when the height of the direction perpendicular to the optical axis is h and b2i are the diffractive surface coefficients (also referred to as optical path difference function coefficients).
                              Φ          b                =                              ∑                          i              =              1                                ⁢                                          ⁢                                    b                              2                ⁢                i                                      ⁢                          h                              2                ⁢                i                                                                        Formula        ⁢                                  ⁢        2            
When a diffraction structure is formed on the optical surface of an objective lens based on a value of the optical path difference function Φb (mm), a ring surface is formed each time a value of the optical path difference function Φb (mm) is changed by n-times a predetermined wavelength λB (n is only a natural number). In the present description, “the diffraction structure is optimized at a wavelength λB and a diffraction order n” indicates that a diffraction structure is determined in this way, and the wavelength is referred to as an optimized wavelength or production wavelength.
Table 3 shows RMS values of thermal aberration when an ambient temperature of the two objective lens has risen by 30° C., and RMS values of chromatic spherical aberration when incident wavelength becomes 5 nm longer than the design wavelength.
TABLE 3Thermal aberrationChromatic spherical(+30° C.)aberration(+5 nm)NA 0.60.010 λrms0.003 λrmsNA 0.850.014 λrms0.057 λrms
As found from Table 3, as for an objective lens having an NA of 0.6 has a chromatic spherical aberration suppressed at 0.003 λrms even when the thermal aberration is corrected to 0.010 λrms, and accordingly a laser diode having a wavelength deviating by 5 nm may be used. At the same time, as for an objective lens having an NA of 0.85, the chromatic spherical aberration becomes 0.057 λrms when the thermal aberration is corrected to 0.014 λrms as much as the objective lens having an NA of 0.6, and accordingly a laser diode having a wavelength deviating by 5 nm cannot be used. Laser diodes used as a light source in an optical pickup device have variation of about ±5 nm in its emission wavelength, and accordingly, selection of laser diodes becomes necessary and the production cost of the optical pickup device rises in case of the objective lens having an NA of 0.85.
Note that, in the objective lenses of Tables 1 and 2, both of the change rates of the refractive indexes accompanying the temperature rise are made −9.0×10−5 and the change rates of the wavelength of incident light accompanying the temperature rise are respectively made +0.2 nm/° C. and +0.05 nm/° C.
Also, in the lens data of Table 1, r (mm) denotes a curvature radius, d (mm) denotes a surface distance, N650 denotes a refractive index at a wavelength of 650 nm and νd denotes an Abbe number at the d-line, and in the lens data of Table 2, r (mm) denotes a curvature radius, d (mm) denotes a surface distance, N405 denotes a refractive index at a wavelength of 405 nm and νd denotes an Abbe number at the d-line.
Furthermore, longitudinal chromatic aberration generated in an objective lens becomes a problem in case of using a blue-violet laser diode generating light with a short wavelength of about 400 nm as a light source like such an optical pickup device for a high-density DVD. In an optical pickup device, chromatic aberration of the objective lens is considered not to be a problem because laser light emitted from a laser diode has a single wavelength (single mode). However, actually a phenomenon referred to as mode hopping that a center wavelength is instantly changed by several nm owing to temperature change, output change or the like, is caused. Because the mode hopping is a wavelength change caused instantly which a focusing mechanism cannot track, there is caused a problem that a defocus component corresponding to movement of the image formation position is added and the converging ability of the objective lens is degraded when longitudinal chromatic aberration of the objective lens is corrected.
Because dispersion of general lens materials used for an objective lens is not so large within a range of 600 nm to 800 nm, which is the wavelength region of infrared laser diodes and red laser diodes, the degradation of the converging ability of an objective lens due to mod hopping did not become a problem in CDs and DVDs.
However, because dispersion of lens materials becomes very large in the region of 400 nm, which is the wavelength region of blue-velvet laser diodes, a wavelength change of even slightly several nm causes the image formation position of the objective lens deviate largely. Therefore in a high-density DVD, the converging ability of an objective lens is degraded largely and stable recording and reproducing might be impossible when a laser diode light source causes mode hopping.
The present invention, which has been made in consideration of circumstances as described above, aims at providing a plastic single lens that is applicable as an objective lens of an optical pickup device using an objective lens having a high NA and has an available temperature range being sufficiently wide and slight degradation of converging ability owing to mode hopping of a light source.
Furthermore, the present invention aims at providing a plastic single lens that is applicable as an objective lens of an optical pickup device using an objective lens having a high NA, wherein it is possible to make selection of laser diode light source unnecessary in the production step of an optical pickup device without excessive increase of chromatic spherical aberration even when thermal aberration has been corrected in order to extend the available temperature range.
Furthermore, the present invention aims at providing an optical pickup device where a plastic single lens of these is mounted, and an optical information recording/reproducing apparatus where the optical pickup device is mounted.