JP4375107B2 - Optical pickup device - Google Patents

Description

  The present invention relates to an optical pickup device for writing information on an optical disc or reading information from an optical disc (hereinafter, writing information on an optical disc or reading information from an optical disc is referred to as “recording / reproducing of an optical disc”). The present invention relates to an optical pickup device for recording / reproducing an optical disc using light beams having at least two different working wavelengths.

  2. Description of the Related Art Conventionally, optical disks known as CDs and DVDs are widely used for storing music information, video information, etc., recording data output from a computer, and the like. With the advancement of advanced information technology in recent years, demands for larger capacity and higher density of optical discs are increasing.

  In order to achieve high recording density on the optical disc, it is necessary to reduce the spot diameter of the light beam collected through the objective lens (hereinafter referred to as spot diameter). In general, this spot diameter is known to be proportional to the wavelength of the light beam used (hereinafter referred to as the used wavelength) and inversely proportional to the numerical aperture (NA) of the optical system. In this case, the wavelength used may be shortened or the numerical aperture of the optical system may be increased.

  Densification from CD to DVD is realized by shortening the operating wavelength from about 780 nm to about 650 nm and increasing the numerical aperture of the optical system from 0.45 to 0.60 or 0.65. Yes. In order to further increase the density in the future, an optical apparatus using a light beam having a wavelength shorter than 440 nm as a light source has been studied.

These optical devices are required to be able to record / reproduce conventional optical disks. In order to meet the above requirement, that is, to record / reproduce an optical disk using light beams having a plurality of use wavelengths, aberrations are minimized with respect to one of the two use wavelengths. In addition to the objective lens, there has been a method of providing another element (hereinafter referred to as an aberration correction element) that corrects aberrations and the like by diffraction with respect to the remaining light beam having the used wavelength (see Patent Document 1).
JP 2003-177226 A

  However, such a conventional technique for recording / reproducing an optical disc has a problem that new aberration is generated due to decentration generated between the objective lens and the aberration correction element.

  The present invention has been made to solve such a problem, and an optical pickup capable of reducing the amount of aberration caused by decentering of an optical element such as an aberration correction element and an objective lens as compared with a conventional apparatus. A device is provided.

In view of the above, the invention according to claim 1 can first recording and reproducing for the first optical disk with a light beam using a wavelength _{λ 1 (380nm ≦ λ 1 ≦} 440nm) is , an optical pickup apparatus capable of recording and reproducing the second optical disk by using the light flux of the second wavelength used _{λ 2 (630nm ≦ λ 2 ≦} 680nm), the first used wavelength lambda ₁ of a first light source for emitting a light beam, the second light source for emitting a second light flux used wavelength lambda _2, keeping the wave front of the first working wavelength lambda ₁ of the light beam constant, the second a specific wavelength wavefront adjustment element for changing a wavefront of the light beam used wavelength lambda _2, an objective lens for converging a light flux passing through the specific wavelength wavefront adjusting element to the optical disc determined in advance in accordance with the used wavelength, the Receives the light beam reflected from the optical disk Comprising a light receiving element, the that, the objective lens, the two surfaces intersecting with the optical axis, the first used wavelength lambda ₁ of the numerical aperture of the recording and reproducing for the first optical disc using a light beam 0 .6 or higher, the third-order off-axis coma aberration at an image height of 0.05 mm is an RMS value of 0.6 λ ₁ or more, and the wavefront aberration on the optical axis is an RMS value of 0.03λ. _It has a configuration having an aspherical shape of ₁ or less .

With this configuration, the specific wavelength wavefront adjusting element changes the wavefront of the light beam having the second use wavelength without changing the wavefront of the light beam having the first use wavelength, and the shape of the two surfaces intersecting with the optical axis of the objective lens is changed. Since the off-axis coma aberration at the position where the image height is 0.05 mm is made to have an RMS value of 0.6 wavelength or more when the optical disc is recorded / reproduced, the influence of decentration is reduced. Thus, an optical pickup device capable of reducing the amount of aberration caused by the decentering of the optical element and the objective lens can be realized as compared with the conventional device.
The objective lens has two surfaces intersecting the optical axis on the optical axis when the first optical disk is recorded / reproduced with a numerical aperture of 0.6 or more using a light beam having the first used wavelength. Since the wavefront aberration has an aspherical shape with an RMS value of 0.03 wavelength or less, the axial aberration when the first optical disk is recorded / reproduced using the light beam having the first use wavelength. Can be achieved, that is, an optical pickup device having diffraction-limited performance can be realized.

The invention according to claim 2 is the invention according to claim 1, wherein the specific wavelength wavefront adjusting element is surrounded by the innermost stepped portion of a plurality of cylindrical stepped portions having an optical axis as a central axis. A circular portion and an annular portion sandwiched between the adjacent step portions, the step portion and the annular portion have a plane perpendicular to the optical axis, and the first use wavelength λ ₁ the light flux of the first wavelength used lambda ₁ integral multiple causing the phase difference between the value obtained by multiplying the 0.9 to 1.1, the objective lens said second used wavelength lambda ₂ generated by It has a configuration which is a phase correction element you reduce the wavefront aberration.

By this configuration, in addition to the effect of claim 1, a specific wavelength wavefront adjustment element with respect to the first use wavelength lambda ₁ of the light beam, the first integral multiple of the operating wavelength lambda ₁ of 0.9 to 1.1 Therefore, an optical pickup device that can change the wavefront of a light beam having another use wavelength without causing a substantial wavefront change can be realized.

Further, The invention according to claim 3, in claim 1, wherein the specific wavelength wavefront adjusting element, to the second operating wavelength lambda ₂ of the light beam, has a configuration which is a diffraction grating causing a diffraction Yes.

By this configuration, in addition to the effect of claim 1, a specific wavelength wavefront adjustment element, due to a diffraction grating causing a diffraction for the second used wavelength lambda ₂ of the light beam, substantially at the first working wavelength lambda ₁ without causing a change in the wavefront, the optical pickup device can be realized capable of changing a wavefront of the second used wavelength lambda ₂ of the light beam.

The invention according to claim 4, in any one of claims 1 to 3, wherein the two surfaces of the objective lens, the using the first light flux of the wavelength used lambda ₁ of the first light when conducting recording and reproducing of the disc, with a near-Ru configuration between 1.6Ramuda ₁ from 1.0λ ₁ 3-order off-axis coma in the position of the image height 0.05mm is in RMS value.

With this configuration, in addition to the effect of any one of the first to third aspects, an optical pickup device that can further reduce aberrations caused by decentration can be realized.

According to the present invention, the specific wavelength wavefront adjusting element changes the wavefront of the light beam having the second use wavelength without changing the wavefront of the light beam having the first use wavelength, and the shapes of the two surfaces intersecting the optical axis of the objective lens are changed. Since the off-axis coma aberration at the position where the image height is 0.05 mm is made to have an RMS value of 0.6 wavelength or more when the optical disc is recorded / reproduced, the influence of decentration is reduced. It is possible to provide an optical pickup apparatus having an effect that the amount of aberration caused by the decentering of the optical element and the objective lens can be reduced as compared with the conventional apparatus. As a result, when both are used in an integrated manner, the optical axes can be easily aligned, and the assembly cost can be reduced. It is also possible to incorporate both of them separately into the device. In that case, the autofocus or tracking servo only needs to act on the objective lens, compared with the case of acting on both integrated members. In addition, the load is reduced by the weight of the aberration correction element, and the speed and power consumption of the apparatus can be increased.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a block configuration of an optical pickup device according to an embodiment of the present invention. 1, the optical pickup apparatus 100 includes a first light source 1, 2 second light source that emits a light beam of the used wavelength lambda _2, the first beam splitter 3 that emits a light beam using a wavelength lambda _1, the second beam splitter 4, a collimator 5, a reflector 6, a specific wavelength wavefront adjusting element 7, an objective lens 8, and a light receiving element 11.

The optical pickup device 100 performs “recording / reproducing of an optical disc” by writing information on the optical discs 9 and 10 and reading information from the optical discs 9 and 10 using a plurality of types of optical discs using _two light beams having wavelengths λ ₁ and λ _2. (Hereinafter, there are two types of optical discs, which are a first optical disc and a second optical disc, respectively). It is assumed that the wavelengths λ ₁ and λ ₂ of the light beam used for recording / reproducing of the optical disc are predetermined according to the type of the optical discs 9 and 10.

The first light source 1 is constituted by, for example, a semiconductor laser, and emits a divergent light beam with linearly polarized light at a wavelength λ ₁ (380 nm ≦ λ ₁ ≦ 440 nm) in the vicinity of 405 nm. Similarly, the second light source 2 is composed of, for example, a semiconductor laser, and emits a divergent light beam with linearly polarized light at a wavelength λ _{2 in the} vicinity of 655 nm (630 nm ≦ λ ₂ ≦ 680 nm). Here, the first light source 1 and the second light source 2 are arranged so that the polarization direction of the light beam from the first light source 1 and the polarization direction of the light beam from the second light source 2 are orthogonal to each other. ing. The value of the second wavelength used is based on compatibility with a conventional DVD or the like.

  The first beam splitter 3 has a role of transmitting the light beam from the first light source 1 and reflecting the light beam from the second light source 2 so as to guide them onto the same optical axis. The second beam splitter 4 transmits the light beams from the first light source 1 and the second light source 2 and reflects the light beams from the information recording surface of the first optical disk 9 and the information recording surface of the second optical disk 10. The light is reflected and guided to the light receiving element 11. A half mirror may be used instead of the beam splitters 3 and 4.

  The collimator 5 has a role of converting light beams from the first light source 1 and the second light source 2 into substantially parallel light, and it is preferable to use a lens made of glass, synthetic resin or the like and having a focal length of about 10 mm to 20 mm. It is. Further, as the reflecting plate 6, a reflecting mirror, a prism, or the like can be used.

Specific wavelength wavefront adjustment element 7, the two used wavelength lambda _1, the particular wavelength used of lambda ₂ (hereinafter referred to. The wavelength used in the first use wavelength lambda ₁₎ maintaining the wavefront of the light beam constant The phase is changed by an integral multiple of the wavelength so that the phase is not substantially changed. That is, the specific wavelength wavefront adjusting element 7 acts on the light beam having the first use wavelength λ ₁ as if it does not exist.

On the other hand, the phase of the light beam having the second use wavelength λ ₂ can be changed, and the wavefront of the light beam having the second use wavelength λ ₂ can be adjusted. For this reason, the specific wavelength wavefront adjusting element 7 can change the wavefront of the light beam having the second use wavelength λ ₂ to correct the aberration of the light beam incident on the optical disc 10, and for the light beam having the first use wavelength λ _1. The specific wavelength wavefront adjusting element 7 acts as if it does not exist. Specific wavelength wavefront adjustment element 7, the phase correcting element, to use a diffraction element or the like which diffracts the light beam of wavelength lambda ₂ without diffracting the light flux with wavelength lambda ₁ is preferred.

The objective lens 8 condenses the light beam that has passed through the specific wavelength wavefront adjusting element 7 onto an optical disc that is predetermined according to the use wavelengths λ ₁ and λ ₂ (in this embodiment, the first use wavelength λ _{The first} light beam is condensed on the first optical disk, and the light beam having the second use wavelength λ ₂ is condensed on the second optical disk. Specifically, the focal length is about 1 mm to 5 mm, the numerical aperture is 0.6 or more, the axial wavefront aberration is 0.03λ ₁ or less in terms of the RMS value with respect to the light beam having the first use wavelength λ ₁ , and off-axis. third-order coma aberration components of wavefront aberration definitive image height 0.05mm has 0.6Ramuda ₁ greater characteristics RMS value, glass, it is preferred to use a lens made of synthetic resin or the like.

  The light beam emitted by the first light source 1 sequentially passes through the first beam splitter 3, the second beam splitter 4, and the collimator 5, is reflected by the reflecting plate 6, and has a specific wavelength wavefront adjusting element 7 and an objective lens 8. And is condensed on the information recording surface of the first optical disc 9. Similarly, the light beam emitted by the second light source 2 is condensed on the information recording surface of the second optical disk 10. The light beams condensed on the information recording surface of the first optical disc 9 or the second optical disc 10 are reflected by the respective information recording surfaces, and sequentially pass through the objective lens 8 and the specific wavelength wavefront adjusting element 7 for reflection. Reflected by the plate 6, transmitted through the collimator 5, reflected by the second beam splitter 4, and enters the light receiving element 11.

  An output signal from the light receiving element 11 is used to obtain a read signal, a focus error signal, and a tracking error signal for information recorded on the information recording surface of the first optical disc 9 or the information recording surface of the second optical disc 10. It is supposed to be. The optical pickup device 100 includes a mechanism (focus servo) for moving the lens in the optical axis direction based on the focus error signal, and the lens in a direction substantially perpendicular to the optical axis based on the tracking error signal. A moving mechanism (tracking servo) is included, but is omitted in the configuration shown in FIG.

The present invention has the third order coma aberration components of wavefront aberration at the image height 0.05mm off-axis uses a lens having a 0.6Ramuda ₁ larger axis characteristics in RMS value, thereby, she will be described later As described above, the magnitude of the aberration caused by the eccentricity between the objective lens 8 and the specific wavelength wavefront adjusting element 7 can be reduced. In the following embodiments, the specific shape of the objective lens having off-axis coma aberration, the element shape of the specific wavelength wavefront adjusting element 7 used according to the objective lens, and the amount of aberration generated according to the decentration of both And. In the following, it is assumed that the specific wavelength wavefront adjusting element 7 is a phase correcting element.

  Further, as a comparison object, a configuration example using an objective lens without off-axis coma aberration and a phase correction element used in accordance with the objective lens (hereinafter, referred to as a comparative example) is taken up. The shape of the correction element and the amount of aberration caused by the decentration of both are shown and compared. A comparison between this example and the comparative example shows that the magnitude of the aberration caused by the eccentricity differs depending on the magnitude of the off-axis coma aberration.

  Hereinafter, specific examples of the specific wavelength wavefront adjusting element 7 and the objective lens 8 and comparative examples thereof will be described.

The objective lens 8 according to the embodiment of the present invention will be described with specific numerical values. Objective lens 8, the third-order coma aberration components of wavefront aberration at off-axis image height 0.05mm is designed to be 0.6Ramuda ₁ in RMS value (see FIG. 2). Table 1 is a table showing the focal length f of the objective lens 8 and the refractive index n of the lens material of the objective lens 8 at the used wavelengths λ ₁ and λ ₂ .

  Here, λ is the center wavelength of the used wavelength (first used wavelength 405 nm, second used wavelength 660 nm), and the focal length f is relative to the first used wavelength and the second used wavelength, respectively. 3.00 mm and 3.09 mm, and the refractive index n is 1.5208 and 1.5057, respectively, for the first use wavelength and the second use wavelength. The objective lens 8 is a double-sided aspheric single lens, and the lens center thickness is 1.84 mm.

  The aspheric shape of the objective lens 8 was determined based on the following formula (1).

Here, the symbol i is an aspherical order and takes one of 2, 4, 6, 8, and 10. The symbol j is the surface number, the surface on the light source side is surface number 1, and the surface on the optical disk side is surface number 2. Further, symbol h is a distance from the optical axis (hereinafter referred to as height), and symbol z _j is a tangent plane of the apex of the j-th surface aspheric surface (generally, in contact with the aspheric surface near the optical axis, Vertical plane.) To the point of height a on the aspheric surface in the optical axis direction, and r _j , k _j , a _{i, j} , and j are coefficients for determining the j-th plane. is there.

  Table 2 shows the values of the coefficients used in the above equation (1).

In Table 2, “Ep (p is an integer)” means “× 10 ^−p ”.

Next, the off-axis characteristics of the objective lens will be described. FIG. 2 is a graph showing off-axis characteristics of the objective lens 8 with respect to the first use wavelength (405 nm). The horizontal axis of the graph shown in FIG. 2 is the image height (in mm) indicating the distance from the axis, and the vertical axis represents the wavefront aberration in units of wavelength λ and RMS value. In FIG. 2, a solid line indicates wavefront aberration including all types of aberrations (hereinafter referred to as total wavefront aberration), and a broken line indicates third-order coma aberration. The same applies to the other drawings unless otherwise specified. In the objective lens 8, the value of the total wavefront aberration is almost 0 at the position where the horizontal axis (image height) shown in FIG. 2 is 0 mm, that is, on the axis, and the third-order coma aberration is off-axis at the image height of 0.05 mm. It is designed to be substantially 0.6Ramuda ₁ in RMS value.

FIG. 3 is a graph showing off-axis characteristics of the objective lens 8 at the second use wavelength λ ₂ (660 nm). Total wavefront aberration on the axis shown in FIG. 3, the total wavefront aberration is located 0.124Ramuda ₂ in RMS value, not in a small aberration to be able to perform recording and reproducing of the optical disc. FIG. 4 is a diagram showing a distribution shape of wavefront aberration generated at the position of the pupil (hereinafter referred to as the pupil position) when the light beam having the second use wavelength is passed through the objective lens 8. The horizontal axis indicates the radial coordinate when the pupil radius is normalized to 1.0, and the vertical axis indicates the value when the wavefront aberration is normalized by the wavelength.

FIG. 5 is a cross-sectional view showing an example of a cross-sectional structure of the specific wavelength wavefront adjusting element 7 for reducing the wavefront aberration shown in FIG. 4, and a plane including the optical axis 51 of the specific wavelength wavefront adjusting element 7 is taken as a cross section. . The specific wavelength wavefront adjusting element 7 shown in FIG. 5 is a phase correction element, and can reduce wavefront aberration generated with respect to the light beam having the _second use wavelength λ ₂ (660 nm), and the light beam having the first use wavelength (405 nm). Is a phase difference that is an integral multiple of the first wavelength used (405 nm). Specifically, a phase difference having a value obtained by multiplying an integral multiple of the first use wavelength λ ₁ by 0.9 to 1.1 may be generated.

In FIG. 5, symbol δ _i (i = 51, 52, 53) is a ring-shaped step portion having the optical axis 51 as a central axis, and symbol φ _i (i = 51, 52, 53) is This is the diameter of the stepped portion δ _i (i = 51, 52, 53). That is, the step portion δ _i (i = 51, 52, 53) has a cylindrical shape with the optical axis as the central axis. Reference sign S _k (k = 52, 53) is a ring zone portion sandwiched between adjacent step portions δ _k and step portions δ _k-1 . Note that reference numeral S ₅₁ is a circular part surrounded by the step part δ ₅₁ , reference numeral S ₅₄ is an annular part outside the step part δ ₅₃ , and all the surfaces indicated by reference numerals S ₅₁ to S ₅₄ are light A plane perpendicular to the axis.

The specific wavelength wavefront adjusting element 7 as the phase correcting element is formed so that the surfaces indicated by the symbols S ₅₁ to S ₅₄ are in different positions in the optical axis direction, and the second wavelength used λ ₂ (660 nm) Thus, the phase of the first light beam (405 nm) is not substantially changed. In FIG. 5, the surface specified by reference numeral 52 is a surface on the objective lens side and is a flat surface.

Table 3 shows the distance and diameter of the annular zone S _k (k = 52, 53, 54) in the optical axis direction when the position on the optical axis of the circular portion δ ₅₁ is set as the reference position (the origin). The numerical value of φk (k = 51, 52, 53) is shown.

In Table 3, the sign of the distance in the optical axis direction of the annular zone S _k (k = 52, 53, 54) is positive in the direction facing the surface 52 on the objective lens side. Table 4 shows the refractive index of the material used for the specific wavelength wavefront adjusting element 7 as the phase correcting element.

FIG. 6 shows the pupil for the _second used wavelength λ ₂ (660 nm) when the specific wavelength wavefront adjusting element (phase correcting element) 7 specified by the above equation (1) and Tables 3 and 4 is used. It is a figure which shows the distribution shape of the wavefront aberration which generate | occur | produces in a position. The horizontal axis and the vertical axis of the graph shown in FIG. 6 are the same as those of the graph shown in FIG.

By providing the specific wavelength wavefront adjusting element 7 as a phase correction element, the total wavefront aberration is 0.048λ _{2 in} RMS value, which is sufficiently smaller than 0.124λ ₂ when there is no phase correction element. In addition, the aberration can be reduced to such an extent that the optical disk can be recorded and reproduced satisfactorily by the phase correction element even at the second use wavelength (660 nm).

  FIG. 7 shows the magnitude of the wavefront aberration generated by the above decentering with respect to the decentering amount representing the degree of decentering between the objective lens 8 and the specific wavelength wavefront adjusting element (phase correcting element) 7. In FIG. 7, the horizontal axis represents the amount of eccentricity (unit: mm) between the objective lens 8 and the specific wavelength wavefront adjusting element 7, and the vertical axis represents the RMS value (unit: wavelength) of the wavefront aberration. The solid line is the total wavefront aberration, and the broken line is the third-order coma aberration.

Based on FIG. 7, when the decentering of the objective lens 8 and the specific wavelength wavefront adjusting element (phase correcting element) 7 is 0.05 mm in the direction perpendicular to the optical axis, the third-order coma aberration component generated by this decentering is RMS. It has become a 0.024λ ₂ in value. The fact that the value that the coma aberration component is the RMS value of 0.024λ is sufficiently smaller than the conventional configuration will be described below by showing a comparative example.

For comparison with the above embodiment, a case where the third-order coma aberration components of wavefront aberration at off-axis image height 0.05mm was used objective lens is 0.0Ramuda ₁ in RMS value below. Here, the center wavelength of the light sources 1 and 2 used, the focal length f of the objective lens, and the refractive index n of the lens material of the objective lens are as shown in Table 1, and are the same as those used in the examples. . The objective lens is a double-sided aspherical single lens, and the lens center thickness is 1.84 mm.

  It is assumed that the aspherical shape of the lens is specified based on Equation (1) and the coefficients shown in Table 5 below.

In Table 5, “Ep (p is an integer)” means “× 10 ^−p ”.

FIG. 8 is a graph showing off-axis characteristics with respect to the first use wavelength (405 nm) of the objective lens according to the comparative example. In the objective lens, the value of the total wavefront aberration is almost 0 at the position where the horizontal axis (image height) shown in FIG. 8 is 0 mm, that is, on the axis, and the third-order coma aberration is off-axis at the image height of 0.05 mm. It is designed to be substantially 0.0Ramuda ₁ in value.

FIG. 9 is a graph illustrating off-axis characteristics of the objective lens according to the comparative example with respect to the second use wavelength λ ₂ (660 nm). Total wavefront aberration on the axis shown in FIG. 9, RMS value of the total wavefront aberration There 0.272λ _2, not in a small aberration to be able to perform recording and reproducing of the optical disc. FIG. 10 is a diagram illustrating a distribution shape of wavefront aberration generated at the pupil position when a light beam having the second use wavelength is passed through the objective lens according to the comparative example.

In order to enable recording / reproduction of an optical disc at the second use wavelength λ ₂ (660 nm), the above-described diagram is used by using a phase correction element that generates a phase difference that is an integral multiple of the first use wavelength (405 nm). The wavefront aberration indicated by 10 is reduced. FIG. 11 is a cross-sectional view of a phase correction element according to a comparative example, and a plane including the optical axis 111 is a cross section.

In FIG. 11, symbols δ _i (i = 111, 112,..., 118) are ring-shaped step portions having the optical axis 111 as a central axis, respectively, and symbols φ _i (i = 111, 112,...). , 118) is the diameter of the stepped portion δ _i (i = 111, 112,..., 118). That is, the step portion δ _i (i = 111, 112,..., 118) has a cylindrical shape with the optical axis as the central axis. Reference sign S _k (k = 112, 113,..., 118) denotes a ring zone that is sandwiched between adjacent step portions δ _k and step portions δ _k−1 . Note that reference numeral S ₁₁₁ is a circular part surrounded by the step part δ ₁₁₁ , reference numeral S ₁₁₉ is an annular part outside the step part δ ₁₁₈ , and all surfaces indicated by reference numerals from S ₁₁₁ to S ₁₁₉ are light A plane perpendicular to the axis.

A phase correction element is formed so that surfaces indicated by reference numerals S ₁₁₁ to S ₁₁₉ are in different positions in the optical axis direction, phase correction is performed with respect to the _second use wavelength λ ₂ (660 nm), The phase does not change substantially for a wavelength of 1 used (405 nm). In FIG. 11, the surface specified by reference numeral 112 is a surface on the objective lens side, and is a flat surface.

Table 6 shows the distance in the optical axis direction of the annular zone S _k (k = 112, 113,..., 119) when the position on the optical axis of the circular portion S ₁₁₁ is used as a reference (the origin). And the numerical value of the diameter φk (k = 111, 112,..., 118).

In Table 6, the sign of the distance in the optical axis direction of the annular zone S _k (k = 112, 113,..., 119) is positive in the direction facing the surface 112 on the objective lens side. In addition, the refractive index of the material used for the phase correction element is shown in Table 4, and was the same as in the case of the above example.

FIG. 12 shows the pupil position with respect to the _second use wavelength λ ₂ (660 nm) when the phase correction element (phase correction element according to the comparative example) specified by the above equation (1) and Tables 4 and 6 is used. It is a figure which shows the distribution shape of the wavefront aberration in FIG. The horizontal axis and the vertical axis of the graph shown in FIG. 12 are the same as those of the graph shown in FIG. By providing the phase correction element, the total wavefront aberration can be sufficiently reduced to 0.051λ ₂ in terms of RMS value compared to 0.272λ ₂ when there is no phase correction element. The aberration can be reduced to the extent that recording / reproducing of the optical disk can be satisfactorily performed even at a working wavelength of 2 (660 nm).

  FIG. 13 shows the magnitude of the wavefront aberration caused by the decentration with respect to the decentration amount between the objective lens and the phase correction element. In FIG. 13, the horizontal axis represents the amount of decentering between the objective lens and the phase correction element (unit: mm), and the vertical axis represents the RMS value of wavefront aberration (unit: wavelength). The solid line is the total wavefront aberration, and the broken line is the third-order coma aberration.

Based on FIG. 13, when the decentering of the objective lens and the phase correcting element according to the comparative example is 0.05 mm in the direction perpendicular to the optical axis, the third-order coma aberration component generated by this decentering is 0.049λ in RMS value. ₂ In the above embodiment, the coma component is 0.024λ ₂ in RMS value, which is about half of the value obtained for the comparative example. This is because the objective lens 8 of the present invention is designed to have off-axis coma.

  FIG. 14 shows the RMS value of the third-order coma aberration of the objective lens at an image height of 0.05 mm when the specific wavelength wavefront adjusting element 7 and the objective lens 8 according to the present invention are used, and 3 generated with an eccentricity of 0.05 mm. It is a figure which shows the relationship with the RMS value of a next coma aberration. Based on the graph shown in FIG. 14, if the third-order coma aberration at an image height of 0.05 mm is 0.6λ or more as an off-axis characteristic of the objective lens 8, the sensitivity to decentration has no off-axis coma aberration. It is shown that it is less than half that of the lens (comparative example). Further, based on FIG. 14, the use of an objective lens in which the third-order coma aberration at an image height of 0.05 mm has an RMS value of 1.0λ to 1.6λ reduces the aberration caused by decentration. Is preferred.

  In the above description, the phase correction element is taken as an example of the specific wavelength wavefront adjusting element 7. However, the diffraction that diffracts the light beam having the second use wavelength without substantially changing the wavefront of the light beam having the first use wavelength. A lattice may be used.

  As described above, in the optical pickup device according to the embodiment of the present invention, the specific wavelength wavefront adjusting element changes the phase without changing the wavefront of the light beam having the first use wavelength, and the optical axis of the objective lens The shape of the two intersecting surfaces reduces the influence of decentration so that the off-axis coma aberration at the position of an image height of 0.05 mm is 0.6 wavelength or more when the optical disk is recorded / reproduced. As a result, the amount of aberration caused by the decentering of the optical element such as the aberration correction element and the objective lens can be reduced as compared with the conventional apparatus.

Moreover, a specific wavelength wavefront adjusting element, causes a phase difference with respect to the first use wavelength lambda ₁ of the light beam, multiplied by 0.9 to 1.1 in the first integral multiple of the operating wavelength lambda ₁ value Since the phase correction element is used, it is possible to change the wavefront of a light beam having another use wavelength without causing a substantial wavefront change.

In addition, since the specific wavelength wavefront adjusting element is a diffraction grating that generates diffraction with respect to the light beam having the first use wavelength λ ₁ , the wavefront of the light beam having another use wavelength can be changed without causing a substantial wavefront change. Can be changed.

  In addition, when two surfaces intersecting the optical axis of the objective lens perform recording / reproduction of the first type of optical disk with a numerical aperture of 0.6 or more using a light beam having the first used wavelength, the optical axis of the objective lens Since the wavefront aberration has an aspherical shape with an RMS value of 0.03 wavelength or less, the optimal aberration at the optical axis when recording / reproducing an optical disk using a light beam having the first wavelength used Can be achieved.

  The optical pickup device according to the present invention is also applicable to uses such as an optical pickup device in which the effect of reducing the amount of aberration caused by the decentering of an optical element such as an aberration correction element and an objective lens can be reduced as compared with a conventional device. it can.

The figure which shows the notional block structure of the optical pick-up apparatus which concerns on embodiment of this invention. The graph which shows the off-axis characteristic with respect to the 1st use wavelength of the objective lens which concerns on the Example of this invention. The graph which shows the off-axis characteristic with respect to the 2nd use wavelength of the objective lens which concerns on the Example of this invention. The figure which shows the distribution shape of the wave-front aberration which generate | occur | produces in a pupil position when the light beam of the 2nd use wavelength is passed through the objective lens which concerns on the Example of this invention. Sectional drawing which shows the cross-section of the specific wavelength wavefront adjusting element which comprises the optical pick-up apparatus based on the Example of this invention The figure which shows the distribution shape of the wavefront aberration which generate | occur | produces in a pupil position with respect to the 2nd use wavelength when the specific wavelength wavefront adjustment element (phase correction element) which concerns on the Example of this invention is used. The figure which shows the magnitude | size of the wavefront aberration which generate | occur | produces by eccentricity with respect to the eccentricity amount of the objective lens and specific wavelength wavefront adjusting element (phase correction element) which concern on the Example of this invention. The graph which shows the off-axis characteristic with respect to the 1st use wavelength of the objective lens which concerns on a comparative example The graph which shows the off-axis characteristic with respect to the 2nd use wavelength of the objective lens which concerns on a comparative example The figure which shows the distribution shape of the wavefront aberration generate | occur | produced in a pupil position when the light beam of the 2nd use wavelength is passed through the objective lens which concerns on a comparative example. Sectional drawing which shows the cross-section of the phase correction element which concerns on a comparative example The figure which shows the distribution shape of the wavefront aberration which generate | occur | produces in a pupil position with respect to 2nd use wavelength when the phase correction element which concerns on a comparative example is used. The figure which shows the magnitude | size of the wavefront aberration which generate | occur | produces by eccentricity with respect to the eccentric amount of the objective lens and phase correction element which concern on a comparative example The RMS value of the third-order coma aberration of the objective lens at the image height of 0.05 mm and the RMS value of the third-order coma aberration generated at the decentration of 0.05 mm when the specific wavelength wavefront adjusting element and the objective lens according to the present invention are used. Diagram showing the relationship

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2 Light source 3, 4 Beam splitter 5 Collimator 51, 111 Optical axis 52, 112 Surface on the objective lens side 6 Reflecting plate 7 Specific wavelength wavefront adjusting element 8 Objective lens 9, 10 Optical disk 11 Light receiving element 100 Optical pickup device δ _i ( i = 51, 52, 53, 111, 112,..., 118) Stepped portion φ _i (i = 51, 52, 53, 111, 112,..., 118) Diameter of stepped portion S _i (i = 51, 111) Circular portion S _i (i = 52, 53, 54, 112, 113,..., 119) Ring zone portion

Claims (4)

The first working wavelength lambda ₁ is capable of recording and reproducing for the first optical disk with a light beam _{(380nm ≦ λ 1 ≦ 440nm)} , a second used wavelength _{λ 2 (630nm ≦ λ 2 ≦} 680nm) an optical pickup apparatus capable of recording and reproducing the second optical disk by using a light flux,
A first light source that emits a light beam having the first use wavelength λ ₁ ;
A second light source that emits a light beam having the second use wavelength λ ₂ ;
Said first keeping the wave front of the used wavelength lambda ₁ of the light beam constant, the second specific wavelength wavefront adjusting device used to change the wavefront of the wavelength lambda ₂ of the light beam,
An objective lens for converging a light flux passing through the specific wavelength wavefront adjusting element to the optical disc determined in advance in accordance with the used wavelength,
A light receiving element for receiving a light beam reflected from the optical disc ,
The objective lens, when two surfaces intersecting with the optical axis thereof, recording and reproducing for the first optical disc performed by the numerical aperture of 0.6 or more using the first light flux of the wavelength used lambda _1, Aspherical shape in which the third-order off-axis coma aberration at an image height of 0.05 mm is an RMS value of 0.6 λ ₁ or more and the wavefront aberration on the optical axis is an RMS value of 0.03λ ₁ or less. optical pickup device comprising a Turkey which have a. The specific wavelength wavefront adjusting element includes a circular portion surrounded by the innermost stepped portion among a plurality of cylindrical stepped portions having an optical axis as a central axis, and an annular zone sandwiched between the adjacent stepped portions. The step portion and the annular zone portion have a plane perpendicular to the optical axis, and an integer of the first use wavelength λ ₁ with respect to the light beam having the first use wavelength λ ₁ doubles causing the phase difference between the value obtained by multiplying the 0.9 to 1.1, in the claim 1 is a phase correction element you reduce the second wavefront aberration in the used wavelength lambda ₂ that occur with the objective lens The optical pickup device described. _2. The optical pickup device according to claim 1, wherein the specific wavelength wavefront adjusting element is a diffraction grating that generates diffraction with respect to the light beam having the second use wavelength λ ₂ . The two surfaces of the objective lens, when said first performing use with a light beam wavelength lambda ₁ recording and reproducing of the first optical disc, the third-order axes in the position of image height 0.05mm the optical pickup device according to any one of 1.0Ramuda ₁ comatic aberration in RMS value of claims 1 between Ru near the 1.6Ramuda ₁ to 3.

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