JP2008130190A - Coupling lens and optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a chromatic aberration correction coupling lens available at two or more wavelengths and having high light use efficiency. <P>SOLUTION: This coupling lens for chromatic aberration correction arranged on the light source side of an objective lens for converging luminous fluxes having a plurality of wavelengths on the information recording surface of an optical recording medium, and provided with a plurality of rings constituted of concentric steps around an optical axis in the surface of the objective lens side, has a wavelength of λ<SB>1</SB>=380 to 430 nm, a size d of the step satisfying λ<SB>1</SB>d=m<SB>1</SB>λ<SB>1</SB>/(n<SB>1</SB>-1) (m<SB>1</SB>is a real number, and n<SB>1</SB>is a refractive index of the coupling lens at wavelength λ1), and 9.9≤m<SB>1</SB>≤10.1 are established. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention is a multi-wavelength optical system that uses a plurality of types of monochromatic light, such as CD (Compact Disc), DVD (Digital Versatile Disc), HD-DVD (High Density Digital Versatile Disc), and Blu-ray. The present invention relates to a coupling lens that can be used in a compatible optical disc apparatus that can handle different optical recording media, and an optical pickup device using the same.

  With the recent increase in capacity and density of optical discs, new standard high-density optical discs such as HD-DVD and Blu-ray discs have been proposed and put into practical use. Accordingly, there is a demand for a compatible optical disk apparatus capable of recording / reproducing both the new standard high-density optical disk and the conventional optical disks of different types such as CD and DVD.

  In order to enable recording / reproduction with respect to a high-density optical disc having an improved recording capacity, it is necessary to reduce the diameter of the light spot obtained from the optical system of the optical pickup device used in the optical disc apparatus. Since the spot diameter is proportional to λ / NA (where λ is the wavelength of the light source and NA is the numerical aperture of the objective lens), the spot diameter can be reduced by shortening the wavelength and increasing the numerical aperture. Regarding the shortening of the wavelength, as described above, research on a blue-violet semiconductor laser having a wavelength of about 400 nm has been advanced and put into practical use.

  By the way, in general, in the optical pickup device, the laser power at the time of recording is larger than the laser power at the time of reproducing the optical disk. Therefore, when switching from reproduction to recording, a phenomenon in which the wavelength of light increases by several nanometers, a so-called mode hopping phenomenon occurs. With this mode hopping phenomenon, the focus position shifts. This focus shift can be eliminated by auto-focusing the objective lens. However, when the wavelength of light is as short as about 400 nm, the focus shift becomes larger than the conventional one. For this reason, a recording failure occurs due to a focus shift for several nsec until autofocusing is performed. Therefore, it is necessary to make correction to reduce the amount of defocus due to the mode hopping phenomenon, that is, chromatic aberration.

In response to this problem, Patent Document 1 discloses a lens that corrects chromatic aberration by the diffraction action of a diffraction pattern. Further, Patent Document 2 discloses a coupling lens that corrects chromatic aberration by a method that does not use a diffractive action without reducing light utilization efficiency.
JP 2002-303788 A JP 2004-185746 A

  However, the method using the diffraction action described in Patent Document 1 increases the total number of annular zones, leading to a decrease in diffraction efficiency, that is, light utilization efficiency. The coupling lens for correcting chromatic aberration described in Patent Document 2 is dedicated to a single wavelength of a blue-violet semiconductor laser having a wavelength of about 400 nm, and has two wavelengths together with CD (wavelength of about 780 nm), DVD (wavelength of about 650 nm), and the like. Thus, it could not be used as a compatible objective lens that can be used. Therefore, it is possible to reduce the size of the optical pickup device by using a three-wavelength compatible objective lens corresponding to all of the CD, DVD, and the high-density optical disc of the above-mentioned new standard and using a common three-wavelength optical system. There wasn't.

  The present invention has been made to solve such problems, and an object of the present invention is to provide a chromatic aberration correcting coupling lens that can be used at two or more wavelengths and has high light utilization efficiency.

The coupling lens according to the first aspect of the present invention is disposed on the light source side of an objective lens that focuses a light beam having a plurality of wavelengths on the information recording surface of the optical recording medium, and light is applied to the surface on the objective lens side. A coupling lens for correcting chromatic aberration having a plurality of annular zones composed of concentric steps centered on an axis, wherein the wavelength λ ₁ = 380 to 430 nm, and the size of the step is d using λ ₁ = M ₁ λ ₁ / (n ₁ -1) (where m ₁ is a real number and n ₁ is the refractive index of the coupling lens at wavelength λ ₁ ), 9.9 ≦ m ₁ ≦ 10.1 It is. As a result, it is possible to provide a chromatic aberration correcting coupling lens that can be used at two or more wavelengths and has high light utilization efficiency.

A coupling lens according to aspect 2 of the present invention is characterized in that, in the aspect of the invention described above, the coupling lens includes three wavelengths of the wavelength λ ₁ , the wavelength λ ₂ = 630 to 690 nm, and the wavelength λ ₃ = 760 to 810 nm. is there. The present invention is particularly suitable when the above three wavelengths are used.

The coupling lens according to aspect 3 of the present invention is the above-described aspect of the invention, wherein the step size d = m ₂ λ ₂ / (n ₂ −1) = m ₃ λ ₃ / (n ₃ −1) (provided that , M ₂ and m ₃ are real numbers, and n ₂ and n ₃ are the refractive indices of the coupling lenses at wavelengths λ ₂ and λ ₃ , respectively, 5.9 ≦ m ₂ ≦ 6.1 and 4.9 ≦ m ₃ ≦ 5.1. Thereby, it is possible to effectively correct chromatic aberration at the three wavelengths.

  A coupling lens according to aspect 4 of the present invention is characterized in that, in the aspect of the invention described above, the annular zones have different aspheric shapes.

The coupling lens according to Aspect 5 of the present invention is the coupling lens according to the aspect of the invention described above, wherein the wavefront aberration in each of the annular zones is 0.035λ or less at the wavelengths λ ₁ , λ ₂ and λ ₃ . And the ring zone is formed.

  An optical pickup device according to Aspect 6 of the present invention includes the aspect coupling lens according to the above-described invention.

  According to the present invention, it is possible to provide a chromatic aberration correcting coupling lens that can be used at two or more wavelengths and has high light utilization efficiency.

  Embodiments of the present invention will be described below. However, the present invention is not limited to the following embodiment. Further, in order to clarify the explanation, the following description and drawings are appropriately omitted and simplified.

  In the present embodiment, the coupling lens according to the present invention is applied to an optical disc apparatus. In this embodiment, the case where a module equipped with three light sources including a light source for CD, a light source for DVD, and a light source for HD-DVD is used, but the number of light sources is limited to three. There is nothing, and two or more is sufficient.

  The coupling lens according to the present invention has an aspherical shape so that the optical path of a light beam passing through an arbitrary light beam height has no or little chromatic aberration when combined with an objective lens. It has become. As a result, the chromatic aberration is sufficiently corrected for each optical disc. In addition, since it is realized by using only the refracted light without using the diffraction action, there is no loss of light amount due to diffraction efficiency.

1A and 1B are diagrams showing a configuration of a coupling lens 30 according to the present invention, in which FIG. 1A is a front view and FIG. 1B is a cross-sectional view. As shown in FIG. 1, the lens surface on the exit side has a unique aspherical shape with an adjacent step amount d concentrically predetermined in the lens radial direction around the optical axis (z-axis in FIG. 1B). At least two or more annular zones are formed. Here, in the aspherical shape in the present invention, the distance from the tangential plane of the incident surface on the optical axis of the aspherical surface at the coordinate point on the aspherical surface where the height from the optical axis is h is Z (h), The curvature (1 / curvature radius) on the optical axis of the spherical surface is C, the conic coefficient is K, the aspherical coefficients from the fourth order to the 16th order are A4, A6, A8, A10, A12, A14, A16, constant B, respectively. Is expressed by the following equation (1).

Further, in the coupling lens according to the present invention, an HD-DVD laser (wavelength λ ₁ = 380 to 430 nm), a DVD laser (wavelength λ ₂ = 630 to 690 nm), a CD laser (wavelength λ ₃ = 760 to 810 nm). In the optical system for the pickup device using the three wavelengths, the adjacent step amount is adjusted so that the chromatic aberration of the objective lens at the three wavelengths can be corrected. The adjacent step amount d is calculated as follows: d = m ₁ λ ₁ / (n ₁ −1) = m ₂ λ ₂ / (n ₂ −1) = m ₃ λ ₃ / (n ₃ −1) (where m ₁ , m ₂ and m ₃ can be expressed as real numbers, and n ₁ , n ₂ , and n ₃ can be expressed as refractive indexes of coupling lenses at wavelengths λ ₁ , λ ₂ , and λ ₃ , respectively. To completely eliminate chromatic aberration at the above three wavelengths, All of m ₁ , m ₂ and m ₃ need to be natural numbers. But this is not possible. In the coupling lens according to the present invention, m ₁ ≈10, m ₂ ≈6, and m ₃ ≈5 were realized. More specifically, 9.9 ≦ m ₁ ≦ 10.1 and 5.9 ≦ m ₂ ≦ 6.1 and 4.9 ≦ m ₃ ≦ 5.1. As a result, chromatic aberration can be effectively corrected at all three wavelengths.

  FIG. 2 is a schematic configuration diagram of the optical pickup device according to the present embodiment. As shown in FIG. 2, the optical pickup device includes a light source 10, a half mirror 20, a chromatic aberration correction coupling lens 30, a diaphragm 40, and an objective lens 50.

  As shown in FIG. 2, the divergent light emitted from the light source 10 passes through the half mirror 20, becomes a substantially parallel light beam by the chromatic aberration correction coupling lens 30, and becomes an infinite system. This parallel light flux enters the objective lens 50 through the diaphragm 40. The light beam transmitted through the objective lens 50 is condensed on the optical disk 60 by the objective lens 50.

  The light reflected from the optical disk 60 passes through the objective lens 50 and the chromatic aberration correction coupling lens 40 and is reflected by the half mirror 20. The light reflected by the half mirror 20 passes through a detection lens and enters a photodetector (not shown), and is photoelectrically converted to generate a focus servo signal, a track servo signal, a reproduction signal, and the like.

(Example)
Specific examples of the present invention will be described below. In the optical pickup device according to the present embodiment, three wavelengths of an HD-DVD laser (wavelength λ ₁ = 407 nm), a DVD laser (wavelength λ ₂ = 658 nm), and a CD laser (wavelength λ ₃ = 785 nm) are used. .

For the chromatic aberration correcting coupling lens 30 according to the present embodiment, the light exit surface shown in FIG. 1 is divided into 20 annular zones in the radial direction from the optical axis, and the surface shape of each section is a blue-violet semiconductor. It was set to reduce the chromatic aberration of the laser. Specifically, the constants B, K, A4, A6, A8, A10, and A12 in Equation 1 were determined as shown in Tables 1 and 2 for the exit surface and the entrance surface shown in FIG. Note that A14 = A16 = 0 in all the annular zones and the entrance surface of the exit surface.

Here, the adjacent step amount d is adjusted so that the chromatic aberration of the objective lens in the HD-DVD laser can be corrected and the three wavelengths are compatible. Specifically, in the chromatic aberration correction coupling lens according to the present invention, d = 10 × λ ₁ / (n ₁ −1) = 5 giving priority to chromatic aberration correction at λ ₁ = 407 nm which is an HD-DVD laser. .98 × λ2 / (n ₂ −1) = 4.98 × λ ₃ / (n ₃ −1) (n ₁ , n ₂ , n ₃ are respectively adjusted to wavelengths λ ₁ , λ ₂ , λ _3. Refractive index of objective lens at

The center thickness of the chromatic aberration correcting coupling lens 30 is 1.5 mm, and the center thickness of the objective lens 50 is 1.28 mm. PMMA (polymethyl methacrylate) was used for the coupling lens 30 for correcting chromatic aberration and the objective lens 50, and PC (polycarbonate) was used for the optical disk. Table 3 shows the refractive index at each wavelength of the glass material used. The glass material is not limited to these, but the refractive index at each wavelength is preferably 1.45 to 1.55.

3 to 5 are wavefront aberration diagrams of a light beam emitted from the light source 10 and converted into substantially parallel light by the coupling lens 30. FIG. The horizontal axis is the pupil radius, and the vertical axis is the wavefront aberration. 3 shows the case where the wavelength λ ₁ = 407 nm, FIG. 4 shows the case where the wavelength λ ₂ = 658 nm, and FIG. 5 shows the case where the wavelength λ ₃ = 785 nm.

As shown in FIG. 3, in the case of a blue laser having a wavelength λ ₁ = 407 nm, the wavefront aberration is substantially 0λ. This is because the adjacent step amount d × (n ₁ −1) of the annular zone is exactly 10 times the wavelength λ ₁ .

As shown in FIG. 4, when the wavelength λ ₂ = 658 nm, the wavefront aberration becomes a discontinuous value for each annular zone. This is because the adjacent step amount d × (n ₂ −1) of the annular zone is about 5.98 times the wavelength λ ₂ and not exactly 6 times. However, the magnitude of the wavefront aberration is as small as 0.02λ at the maximum.

As shown in FIG. 5, in the case of the wavelength λ ₃ = 785 nm, the wavefront aberration becomes a discontinuous value for each annular zone as in the case of the wavelength λ ₂ = 658 nm. This is because the adjacent step amount d × (n ₃ −1) of the annular zone is about 4.98 times the wavelength λ ₃ and not exactly 5 times. However, the magnitude of the wavefront aberration is as small as 0.02λ at the maximum.

  As described above, the coupling lens according to the present invention has good wavefront aberration characteristics at all three wavelengths.

For the incident-side surface R1 and the exit-side surface R2 of the objective lens 50, each of the incident surface and the exit surface on the aspherical optical axis at the coordinate point on the aspherical surface where the height from the optical axis is h. The distance Z (h) from the tangential plane (where the unit is mm) can be expressed by Equation 1 (therefore, the constant B = 0 in Equation 1 for both the surface R1 and the surface R2). The constants K, A4, A6, A8, A10, A12, A14, and A16 in Equation 1 were determined as shown in Table 4.

6 to 8 are wavefront aberration diagrams of a light beam emitted from the light source 10 and passed through the coupling lens 30 and the objective lens 50. FIG. The horizontal axis is the pupil radius, and the vertical axis is the wavefront aberration. 6 shows the case where the wavelength λ ₁ = 407 nm, FIG. 7 shows the case where the wavelength λ ₂ = 658 nm, and FIG. 8 shows the case where the wavelength λ ₃ = 785 nm.

As shown in FIG. 6, in the case of a blue laser having a wavelength λ ₁ = 407 nm, the wavefront aberration is substantially 0λ. This is because the adjacent step amount d × (n ₁ −1) of the annular zone is exactly 10 times the wavelength λ ₁ .

As shown in FIG. 7, when the wavelength λ ₂ = 658 nm, the wavefront aberration is a discontinuous value for each annular zone. This is because the adjacent step amount d × (n ₂ −1) of the annular zone is about 5.98 times the wavelength λ ₂ and not exactly 6 times. However, the total wavefront aberration is very small as 0.0070λrms. Here, within the effective diameter, the coupling lens 30 is used from the center to the tenth ring zone.

As shown in FIG. 8, in the case of the wavelength λ ₃ = 785 nm, the wavefront aberration becomes a discontinuous value for each annular zone as in the case of the wavelength λ ₂ = 658 nm. This is because the adjacent step amount d × (n ₃ −1) of the annular zone is about 4.98 times the wavelength λ ₃ and not exactly 5 times. However, the total wavefront aberration is as small as 0.0093 λrms. Here, within the effective diameter, the fourth annular zone from the center of the coupling lens 30 is used.

(Comparative example)
In the comparative example, a normal coupling lens without a ring zone was used. Each constant in this equation 1 was determined as shown in Table 5. As shown in Table 5, the surface shape of the exit surface is different from that of the example. Other conditions are the same as those in the example.

FIG. 9 shows a wavefront aberration diagram of a light beam emitted from the light source 10 and converted into substantially parallel light by the coupling lens 30 of the comparative example. The horizontal axis is the pupil radius, and the vertical axis is the wavefront aberration. As a representative example, the case of wavelength λ ₂ = 658 nm is shown.

Since no ring zone is formed in the coupling lens of the comparative example, the wavefront aberration is a continuous value, and the maximum wavefront aberration is about 0.48λ, which is very large compared to the example. Further, in the coupling lens of the example, based on the data of the coupling lens of the comparative example at this wavelength λ ₂ = 658 nm, the maximum wavefront aberration in each annular zone is 0.023734. The width of the band was determined. Specifically, in FIG. 9, the step formation position of the annular zone is indicated by an arrow. The width of the annular zone is preferably determined so that the magnitude of the wavefront aberration in each annular zone is 0.035λ or less for all three wavelengths.

Tables 6 to 8 show wavefront aberrations and chromatic aberrations of light beams emitted from the light source 10 and passed through the coupling lens 30 and the objective lens 50 in Examples, together with comparative examples. Table 6 shows the case of wavelength λ ₁ = 407 nm, Table 7 shows the case of wavelength λ ₂ = 658 nm, and Table 8 shows the case of wavelength λ ₃ = 785 nm. Regarding the wavefront aberration, together with the total wavefront aberration, SA3, which is the third-order component of spherical aberration, and SA5, which is the fifth-order component of spherical aberration, are also shown for reference. As for chromatic aberration, Table 6 shows the positional deviation of the focus, that is, the chromatic aberration and the average value of both when the deviation is ± 1 nm from the reference wavelength of 407 nm. Tables 7 and 8 show the chromatic aberration and the average value per 1 nm of the magnitudes of both when shifted by ± 3 nm from the respective reference wavelengths of 658 nm and 785 nm.

As shown in Table 6, when the wavelength λ ₁ = 407 nm, the wavefront aberration of the comparative example is 0.0000 λrms, whereas the wavefront aberration of the example is 0.0001 λrms, which is a very good numerical value. In addition, the average value of chromatic aberration of the comparative example is as large as 0.498 μm / nm, whereas the average value of chromatic aberration of the example is 0.069 μm / nm, which is an order of magnitude smaller and reaches a very good value. It has improved.

As shown in Table 7, when the wavelength λ ₂ = 658 nm, the wavefront aberration of the comparative example is 0.0017 λrms, whereas the wavefront aberration of the example is 0.0070 λrms, which is slightly inferior, but is a very good numerical value. It is. Further, while the average value of chromatic aberration in the comparative example is 0.116 μm / nm, the average value of chromatic aberration in the example is equivalent to 0.122 μm / nm, which is a favorable numerical value.

As shown in Table 8, when the wavelength λ ₃ = 785 nm, the wavefront aberration of the comparative example is 0.0021 λrms, whereas the wavefront aberration of the example is 0.0093 λrms, which is slightly inferior, but is a very good numerical value. It is. In addition, the average value of chromatic aberration in the comparative example is 0.068 μm / nm, whereas the average value of chromatic aberration in the example is 0.053 μm / nm, which is slightly improved and is a very good numerical value.

  Next, a beam profile of a light spot which is emitted from the light source 10 and converted by the coupling lens 30 and then passed through the objective lens 50 and then collected on the optical disc 60 will be described with reference to FIG.

  The light spot has a beam profile as shown in FIG. The horizontal axis indicates the position, and the vertical axis is the light intensity. The light intensity on the vertical axis is normalized so that the peak intensity is 1. As shown in FIG. 10, the light intensity becomes weaker as the distance from the peak becomes longer, and the light intensity becomes minimum at a certain position and becomes substantially zero. As the distance from the peak further increases from this minimum point, there is a high-order term light intensity called a side lobe. A space between the local minimum point including this peak is called a zeroth order spot. A side lobe region formed around the zero-order spot is called a primary ring.

The light spot characteristic is characterized by three parameters. The first parameter is 1 / e ² spot diameter (e: base of natural logarithm (≈2.771828)) indicating the spread of the light spot. The distance between two points AB having a light intensity of 1 / e ² (≈13.5%) of the light peak intensity of the spot is defined as the spot diameter of this light spot. This 1 / e ² spot diameter D = 0.82 × λ / NA (NA is the numerical aperture, and λ is the wavelength of light). Therefore, the spot diameter is proportional to the wavelength and inversely proportional to NA. The smaller the 1 / e ² spot diameter, the smaller the area irradiated with light on the information recording surface of the optical disc, so that a good resolution can be obtained.

  The second parameter is a sidelobe characteristic that is a ratio of the light intensity at the peak in the zero-order spot to the light intensity at the peak in the primary ring. It can be said that a light spot having a smaller side lobe is a better light spot. This is because when light is collected on the optical disk, the light reflected from the optical disk at the position irradiated on the primary ring enters as noise in the signal obtained by reflecting the information recording surface of the optical disk. . Therefore, in a light spot with a large side lobe, a noisy signal is generated. Therefore, a light spot having a smaller sidelobe characteristic is considered to be a better light spot.

  The third parameter is the 0th-order light amount that is the total light amount in the 0th-order spot. This 0th-order light amount corresponds to the light intensity of this light spot. In a light spot with a large 0th-order light amount, the signal intensity generated when the optical disk is irradiated can be increased. Therefore, the S / N ratio (signal / noise ratio) is better when a light spot with a larger 0th-order light amount is used. A high and good signal can be obtained.

Tables 9 to 11 show the above three spot characteristics of the examples together with comparative examples. Table 9 shows the case of wavelength λ ₁ = 407 nm, Table 10 shows the case of wavelength λ ₂ = 658 nm, and Table 11 shows the case of wavelength λ ₃ = 785 nm. The 1 / e ² spot diameter indicates a spot diameter Dx in the x direction and a spot diameter Dy in the y direction of the light spot formed on the xy plane. Regarding the side lobe characteristics, the side lobe SLx in the x direction, the side lobe SLy in the y direction, and the maximum value SLmax of the light spot formed on the xy plane are shown.

As shown in Table 9, when the wavelength λ ₁ = 407 nm, the 1 / e ² spot diameter of the example is 0.513 μm for both Dx and Dy, whereas the 1 / e ² spot diameter of the comparative example is also Dx. And Dy are both 0.513 μm, which is the same value. Further, while the maximum value SLmax of the side lobe of the comparative example is 1.8%, the SLmax of the example is 1.7%, which is an improvement of 0.1%. Further, the 0th-order light amount of the comparative example is 83.8%, whereas the 0th-order light amount of the example is 83.9%, which is slightly improved. That is, the light spot characteristics of the example at the wavelength λ ₁ = 407 nm are equivalent to those of the comparative example.

As shown in Table 10, when the wavelength λ ₂ = 658 nm, the 1 / e ² spot diameter of the example is 0.830 μm for both Dx and Dy, whereas the 1 / e ² spot diameter of the comparative example is also Dx. And Dy are both 0.830 μm and are exactly the same value. Further, while the maximum value SLmax of the side lobe of the comparative example is 1.8%, the SLmax of the example is 1.7%, which is an improvement of 0.1%. Further, the 0th-order light amount of the comparative example is 83.8%, whereas the 0th-order light amount of the example is 83.7%, which is equivalent. That is, the light spot characteristics of the example at the wavelength λ ₂ = 658 nm are equivalent to those of the comparative example.

As shown in Table 11, when the wavelength λ ₃ = 785 nm, the 1 / e ² spot diameter of the example is 1.287 μm for both Dx and Dy, whereas the 1 / e ² spot diameter of the comparative example is Dx. And Dy are both 1.289 μm and are equivalent. Further, while the maximum value SLmax of the side lobe of the comparative example is 1.8%, the SLmax of the example is 1.8%, which is exactly the same value. Further, the 0th-order light amount of the comparative example is 83.8%, whereas the 0th-order light amount of the example is 83.5%, which is equivalent. That is, the light spot characteristics of the example at the wavelength λ ₃ = 785 nm are equivalent to those of the comparative example.

As described above, by using the coupling lens according to the present invention, the chromatic aberration at the wavelength λ ₁ = 407 nm is drastically maintained while maintaining good wavefront aberration characteristics and light spot characteristics at the three wavelengths. Can be improved. As a result, it is possible to provide a chromatic aberration correcting coupling lens that can be used at two or more wavelengths and has high light utilization efficiency.

It is a schematic diagram which shows the coupling lens which concerns on this invention. It is a schematic diagram which shows the optical system figure of the optical pick-up apparatus which concerns on this invention. It is a wave aberration diagram of the light beam with a wavelength of 407 nm that has passed through the coupling lens according to the present invention. It is a wave aberration diagram of a light beam with a wavelength of 658 nm that has passed through the coupling lens according to the present invention. It is a wave aberration diagram of a light beam with a wavelength of 785 nm that has passed through the coupling lens according to the present invention. It is a wave aberration diagram of the light beam of wavelength 407nm which passed the coupling lens and objective lens shift which concern on an Example. It is a wave aberration diagram of the light beam of wavelength 658nm which passed the coupling lens and objective lens shift which concern on an Example. It is a wavefront aberration figure of the light beam of wavelength 785nm which passed the coupling lens and objective lens shift which concern on an Example. It is a wavefront aberration figure of the light beam of wavelength 658nm which passed the coupling lens of the comparative example. It is a figure which shows a beam profile typically.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Light source 20 Half mirror 30 Coupling lens 40 for chromatic aberration correction Aperture 50 Objective lens 60 Optical disk

Claims (6)

Arranged on the light source side of the objective lens for condensing light beams having a plurality of wavelengths on the information recording surface of the optical recording medium, and having a plurality of annular zones composed of concentric steps centered on the optical axis on at least one surface A chromatic aberration correction coupling lens provided,
Wavelength λ ₁ = 380 to 430 nm, and using λ ₁ , the step size is d = m ₁ λ ₁ / (n ₁ −1) (where m ₁ is a real number and n ₁ is a coupling at wavelength λ ₁ The refractive index of the lens)
Coupling lens in which 9.9 ≦ m ₁ ≦ 10.1. _{2. The} coupling lens according to claim 1, wherein the plurality of wavelengths include three wavelengths of the wavelength λ ₁ , a wavelength λ ₂ = 630 to 690 nm, and a wavelength λ ₃ = 760 to 810 nm. Step size d = m ₂ λ ₂ / (n ₂ −1) = m ₃ λ ₃ / (n ₃ −1) (where m ₂ and m ₃ are real numbers, and n ₂ and n ₃ each have a wavelength λ ₂ and the refractive index of the coupling lens at λ ₃ )
The coupling lens according to claim 2, wherein 5.9 ≦ m ₂ ≦ 6.1 and 4.9 ≦ m ₃ ≦ 5.1.   The coupling lens according to claim 1, wherein the annular zones have different aspheric shapes. _{2. The} annular zone is formed with a width such that the magnitude of wavefront aberration in each of the annular zones is 0.035λ or less at the wavelengths λ ₁ , λ ₂ and λ ₃ . 5. The coupling lens according to any one of 4.   An optical pickup device comprising the coupling lens according to claim 1.

Images