Abstract:
There is provided an assembling method for expanding a maximum admissible astigmatism of objective lens which includes defining a maximum admissible astigmatism for the entire optical system, installing the collimator lens on the optical system to cause a predetermined amount of astigmatism lying within a range of up to the maximum admissible astigmatism by tilting the collimator lens from a condition where a point source of the semiconductor laser lies on an optical axis of the collimator lens, and installing the objective lens on the body case of the optical system to bring a total amount of astigmatism of the entire optical system to a level smaller than or equal to the maximum admissible astigmatism for the entire optical system by rotating the objective lens from a condition where the point source of the semiconductor laser and the center of the collimator lens lie on the optical axis of the objective lens.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an assembling method for an optical system of an optical pick-up in which a semiconductor laser is employed. 
     In general, an optical system of an optical pick-up is configured to converge a laser beam emitted by a semiconductor laser on an optical disc. Since in general the semiconductor laser causes an astigmatism, the laser beam obtained by converging the laser beam emitted by the semiconductor laser through the optical system has an astigmatism. 
     More specifically, a shape of a beam spot formed on the optical disc by the optical system changes from a circular shape state to an elliptical shape state depending on a defocus state of the laser beam on the optical disc, which raises a problem, such as deterioration of an RF signal or a jitter property. For this reason, the optical system is required to decrease the astigmatism. 
     Japanese Patent Provisional Publication No. 2004-342145A (hereafter, referred to as JP2004-342145A) discloses a technique for canceling an astigmatism caused in components in an optical system excepting a collimator lens with an astigmatism caused by the collimator lens. More specifically, the astigmatism caused in components in the optical system excepting the collimator lens is cancelled with the astigmatism caused by the collimator lens by adjusting the position of the collimator lens by rotating the collimator lens about an optical axis of the collimator lens. In this case, the collimator lens is required to be designed to have a certain astigmatism corresponding to the amount of the astigmatism that the optical system has. It is understood that since the amount of astigmatism varies from an optical system to another, a collimator lens designed for a certain optical system is not always suitable for other optical systems. 
     Recently, new technical standards, such as a BD (Blu-ray Disc) and a HD DVD, have been proposed. Based on the fact that the diameter of a beam spot is inversely proportional to a numerical aperture defined on an image side in an optical system (hereafter, frequently referred to as an image side numerical aperture) and is proportional to the wavelength of a beam, such a new technical standard is designed to decrease the beam spot diameter by increasing the image side numerical aperture of an optical system and decreasing the wavelength of a beam of a light source in the optical system. 
     On the other hand, the amount of astigmatism increases in proportion to the square of the image side numerical aperture. Therefore, if the image side numerical aperture of an objective lens in the optical pick-up is increased, the entire astigmatism caused in the optical system by a semiconductor laser or manufacturing errors increases. 
     In general, aberrations including an astigmatism caused in an optical system are evaluated in the unit of a wavelength. Considering that an evaluation wavelength for the new technical standard is approximately 405 nm which is shorter than that for the DVD (approximately 660 nm) and the CD (approximately 790 nm), the amount of aberration in the optical system according to the new technical standard increases in inverse proportion to the used wavelength even if an error in the shape of the objective lens in the optical system according to the new technical standard is substantially the same as that for the optical system for the DVD or CD. 
     By configuring an objective lens having a small amount of astigmatism, it may be possible to suppress the entire astigmatism in an optical system. However, it is considerably difficult to manufacture an objective lens of which astigmatism is very small even if the designed shape of the objective lens exhibits a small amount of astigmatism. In general, it is difficult to avoid an astigmatism due to a manufacturing error. In addition, the amount of astigmatism due to a manufacturing error varies from an objective lens to an objective lens. 
     In general, a maximum admissible astigmatism is determined for manufacturing of objective lenses. If the number of manufactured objective lenses not satisfying the maximum admissible astigmatism is relatively large (i.e., the yield is relatively low), a manufacturer is required to decrease the manufacturing error of each objective lens. However, to decrease the manufacturing error, the manufacturer needs to improve a production line, which also increases the cost for manufacturing objective lenses. For this reason, it is desirable for the manufacturer that the maximum admissible astigmatism is as large as possible so as to prevent the deterioration of the yield and the increase of the manufacturing cost. 
     SUMMARY OF THE INVENTION 
     The present invention is advantageous in that it provides an assembling method for an optical system of an optical pick-up capable of suppressing the total amount of astigmatism caused in the optical system while achieving a relatively large maximum admissible astigmatism for an objective lens. 
     According to an aspect of the invention, there is provided an assembling method for an optical system of an optical pick-up including a semiconductor laser, a collimator lens and an objective lens. The method includes defining a maximum admissible astigmatism for the entire optical system, installing, on a rear side of the semiconductor laser, the collimator lens on a body case of the optical system to cause a predetermined amount of astigmatism lying within a range of up to the maximum admissible astigmatism for the entire optical system by tilting the collimator lens with respect to a center of the collimator lens from a condition where a point source of the semiconductor laser lies on an optical axis of the collimator lens, and installing, on a rear side of the collimator lens, the objective lens on the body case of the optical system to bring a total amount of astigmatism of the entire optical system to a level smaller than or equal to the maximum admissible astigmatism for the entire optical system by rotating the objective lens about an optical axis of the objective lens under a condition where the point source of the semiconductor laser and the center of the collimator lens lie on the optical axis of the objective lens. 
     Such a configuration makes it possible to reduce the total amount of astigmatism of the entire optical system to a level smaller than or equal to the maximum admissible astigmatism while allowing the objective lens to have a maximum admissible astigmatism of up to a double value of the maximum admissible astigmatism of the entire optical system. 
     Consequently, it becomes possible to suppress the total amount of astigmatism of the entire optical system to a sufficiently low level while achieving a relatively large amount of maximum admissible astigmatism for the objective lens. 
     In at least one aspect, the predetermined amount of astigmatism caused by tilting the collimator lens is equal to the maximum admissible astigmatism for the entire optical system. 
     In at least one aspect, the maximum admissible astigmatism for the entire optical system lies within a range from 0.005 to 0.030 λrms. 
     In at least one aspect, the maximum admissible astigmatism for the entire optical system lies within a range from 0.005 to 0.015 λrms. 
     In at least one aspect, a wavelength of a beam of the semiconductor laser is shorter than or equal to 450 nm, and a numerical aperture defined for the objective lens on a image side is larger than or equal to 0.65. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  is a block diagram of an optical system of an optical pick-up to which an assembling method according to an embodiment of the invention is applied. 
         FIG. 2  is an explanatory illustration explaining the assembling method according to the embodiment. 
         FIG. 3  is a table showing a relationship between the astigmatism of a front side optical system, the astigmatism of an objective lens, and the total astigmatism of the entire optical system. 
         FIG. 4  is a flowchart illustrating the assembling method of the optical system. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, an embodiment according to the invention is described with reference to the accompanying drawings. 
       FIG. 1  is a block diagram of an optical system  10  of an optical pick-up to which an assembling method according to an embodiment of the invention is applied. As shown in  FIG. 1 , the optical system  10  includes a semiconductor laser  11 , a half mirror  12 , a collimator lens  13 , an objective lens  14 , and a photoreceptor  15 . The semiconductor laser  11  is configured to emit a light beam from a point source. The half mirror  12  has a function of passing a beam entering the half mirror  12  from a side and reflecting a beam entering the half mirror  12  from another side. 
     The collimator lens  13  converts the light beam which passed through the half mirror  12  into a substantially collimated beam. The objective lens  14  converges the collimated beam to form a beam spot on a record surface of an optical disc D 1 . The photoreceptor  15  detects light incident thereon after reflecting from the record surface of the optical disc D 1 , passing through the objective lens  14  and the collimator lens  13  in this order and then being reflected by the half mirror  12  toward the photoreceptor  15 . 
     The objective lens  14  is mounted on an actuator in a body case of the optical pick-up (not shown) so that the objective lens  14  can be moved in a radial direction (i.e., a tracking direction) of the optical disc D 1  and in a direction (i.e., a focusing direction) perpendicular to a surface of the optical disc D 1 . With this structure, the position of the objective lens  14  can be adjusted so that a beam spot having a suitable size is formed at a desired position on the record surface of the optical disc D 1 . 
     Hereafter, the assembling method of the optical system  10  is described.  FIG. 2  is an explanatory illustration explaining the assembling method according to the embodiment.  FIG. 4  is a flowchart illustrating the assembling method for the optical system  10 . 
     First, a manufacturer determines a maximum admissible astigmatism to be applied to the entire optical system  10  (step S 101 ). Hereafter, the maximum admissible astigmatism is denoted by a variable “A”. “A” is represented in the unit of λrms (root-mean-square) and is not zero. 
     Next, objective lenses ( 14 ) are produced (step S 102 ). For producing of the objective lenses  14 , the manufacturer assigns 2A (i.e., a two-fold value of the maximum admissible astigmatism for the entire optical system  10 ) to a maximum admissible astigmatism for the objective lens  14 . Therefore, in the production of objective lenses  14 , objective lenses having the amount of astigmatism larger than 2A are rejected as defective products, and objective lenses having the amount of astigmatism smaller than or equal to 2A are regarded as non-defective products. 
     Next, an assembling process for the semiconductor laser  11  and the collimator lens  13  is conducted (step S 103 ). In this process, a worker prepares the semiconductor laser  11  and the collimator lens  13 , and mounts the semiconductor laser  11  and the collimator lens  13  on the body case (not show) of the optical pick-up. More specifically, the worker mounts the semiconductor laser  11  and the collimator lens  13  on the body case of the optical pick-up such that the point source of the semiconductor laser  11  is positioned on an optical axis of the collimator lens  13 . 
     Furthermore, the worker tilts the collimator lens  13  with respect to the center of the collimator lens  13  by a required angle (see an arrow (a) in  FIG. 2 ) so that a front side optical system defined from the semiconductor laser  13  to the collimator lens  13  in the optical system  10  has the amount of astigmatism of “A”. 
     Next, an assembling process of the objective lens  14  for mounting the objective lens  14  on the body case of the optical system  10  is conducted (step S 104 ). In this process, the worker mounts the objective lens  14  on the body case such that the point source of the semiconductor laser  11  and the center of the collimator lens  13  are positioned on an optical axis of the objective lens  14 . 
     Furthermore, the worker rotates the objective lens  14  about the optical axis of the objective lens  14  (see an arrow (b) in  FIG. 2 ) so that the total amount of astigmatism of the optical system  10  including the semiconductor laser  12 , the collimator lens  13  and the objective lens  14  becomes smaller than or equal to “A”. 
     Finally, the worker completes the assembling of the optical system  10  by mounting the half mirror  12  and the photoreceptor  15  on the body case of the optical pick-up (step S 105 ). 
     The reason why the total amount of astigmatism of the entire optical system  10  can be suppressed to a level lower than or equal to “A” regardless of the fact that the objective lens  14  of which maximum admissible astigmatism is “2A” is employed in the optical system  10  will now be explained with reference to  FIG. 3 .  FIG. 3  is a table showing a relationship between the astigmatism of the front side optical system (defined from the semiconductor laser  12  to the collimator lens  13 ), the astigmatism of the objective lens  14 , and the total astigmatism of the entire optical system  10 . 
     As shown in  FIG. 3 , the total amount of astigmatism of the entire optical system  10  is equal to a value obtained by maximally canceling the amount of astigmatism of the front side optical system with the amount of astigmatism of the objective lens  14 . The reason is explained as follows. 
     An astigmatism has directionality that an image formation point by power in a certain direction (e.g., a meridional direction) is different, along the optical axis, from an image formation point by power in a direction (e.g., a sagittal direction) perpendicular to the certain direction. This means that, in an optical system having two lenses, an astigmatism of a certain lens can be cancelled with an astigmatism of another lens. In particular, when two lenses are rotated with respect to each other by 90 degrees from the condition where a direction of astigmatism of one lens coincides with a direction of astigmatism of another lens, the astigmatisms of the two lenses can be maximally cancelled each other. By thus canceling the astigmatism of the two lenses, the astigmatism of the entire optical system including the two lenses decreases, and thereby the wavefront aberration (λrms) of an actual wavefront with respect an ideal wavefront also decreases. 
     As shown in  FIG. 3 , regarding the case where the amount of astigmatism of the front side optical system is zero, the total amount of astigmatism of the optical system  10  is also zero when the amount of astigmatism of the objective lens  14  is zero, and the total amount of astigmatism of the optical system  10  is “A” when the amount of astigmatism of the objective lens  14  is “A”. 
     Regarding the case where the amount of astigmatism of the front side optical system is “A”, the total amount of astigmatism of the optical system  10  can be suppressed within a range from zero through “A” when the amount of astigmatism of the objective lens  14  is from zero through “A”. 
     Furthermore, regarding the case where the amount of astigmatism of the front side optical system is “A”, the total amount of astigmatism of the optical system  10  can be suppressed to a range from zero through “A” even when the amount of astigmatism of the objective lens  14  lies in a range from “A” through “2A”. That is because the amount of astigmatism of the front side optical system can be cancelled with the amount of astigmatism of the objective lens  14 . This means that the total amount of astigmatism of the entire optical system  10  can be suppressed to a level smaller than or equal to “A” as long as the amount of astigmatism of the objective lens  14  is smaller than or equal to “2A”. 
     As described above, the manufacturer of the optical system  10  is allowed to employ the maximum admissible astigmatism of up to “2A” for the objective lens  14  by designing the front side optical system in the optical system to be adjustable to have the amount of astigmatism equal to the maximum admissible astigmatism defined for the entire optical system. 
     In this embodiment, the wavelength of the light beam emitted by the semiconductor laser  11  is, for example, shorter than or equal to 450 nm. The image side numerical aperture of the objective lens  14  is, for example, larger than or equal to 0.65. 
     In an example, the maximum admissible astigmatism for the entire optical system  10  lies within a range from 0.005 to 0.030 λrms. The maximum admissible astigmatism for the entire optical system  10  may lie within a range from 0.005 to 0.015 λrms. 
     Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. 
     This application claims priority of Japanese Patent Application No. P2007-125895, filed on May 10, 2007. The entire subject matter of the application is incorporated herein by reference.