JP4788783B2 - Objective optical element and optical pickup device - Google Patents

Description

  The present invention relates to an objective optical element and an optical pickup device for condensing a light beam on an information recording surface of an optical information recording medium.

In recent years, with the practical use of short-wavelength red lasers, DVDs (digital video discs), which are high-density optical information recording media (also referred to as optical discs) that have the same size and capacity as CDs (compact discs), are being developed. It has been commercialized.
In the DVD recording / reproducing apparatus, the numerical aperture NA on the optical disk side of the objective lens when a 650 nm semiconductor laser is used is set to 0.6 to 0.65. DVD is a track pitch 0.74 [mu] m, the minimum peak Tsu preparative length 0.4 .mu.m, the track pitch 1.6μm of CD, are densified to less than half with respect to the shortest pit length 0.83 .mu.m. Further, in the DVD, the protective substrate thickness is 0.6 mm, which is half the protective substrate thickness of CD, in order to suppress the coma aberration generated when the optical disk is tilted with respect to the optical axis.

In addition to the above-described CD and DVD, optical discs of various standards having different light source wavelengths and transparent substrate thicknesses, such as CD-R, RW (recordable compact disc), VD (video disc), MD (mini disc). MO (magneto-optical disk) and the like have been commercialized and are in widespread use.
Furthermore, semiconductor lasers have become shorter in wavelength, and a protective substrate thickness of about 0.1 mm using a blue-violet semiconductor laser light source having a wavelength of about 400 nm and an objective lens having an image-side numerical aperture (NA) increased to about 0.85. Research and development of high-density DVDs with a protective substrate thickness of about 0.6 mm using high-density optical disks (hereinafter referred to as “high-density DVDs”) and objective lenses with an image-side numerical aperture (NA) of about 0.65. Progressing.

Various types of so-called compatible optical pickup devices that can converge two types of light beams having different wavelengths onto the information recording surfaces of two types of optical discs through one objective lens have been proposed (for example, patents). Reference 1 to 3).
Patent Document 1 and Patent Document 2 disclose an optical pickup device including a flat hologram optical element and a refractive objective lens as separate bodies.
These devices, for example, transmit a light beam of one of two types of wavelength through a hologram optical element, and then condense it on a predetermined disk via an objective lens, and a hologram of a light beam of the other wavelength. After diffracting so as to diverge when passing through the optical element, the −1st order diffracted light of the diffracted light is condensed on a predetermined optical disk through the objective lens, so that two types of one objective lens are used. Information is recorded / reproduced on / from an optical disc.

In Patent Document 3, a hologram is formed in a concentric region (center region) centered on the optical axis, and a diffraction grating is formed around the center region (peripheral region) and a refractive objective. An optical pickup device provided with a lens separately is disclosed.
This device diffracts a light beam having a wavelength of 780 nm while transmitting a light beam having a wavelength of 635 nm in the central region. In the peripheral region, the light beam having a wavelength of 635 nm is transmitted, while the light beam having a wavelength of 780 nm is substantially blocked by diffraction.
In this way, all of the light beam having a wavelength of 635 nm is incident on the objective lens, and only the light beam that passes through the central region of the light beam having a wavelength of 780 nm is diffracted so as to diverge and enter the objective lens. Information is recorded / reproduced on two types of optical disks.

JP-A-9-54973 JP-A-9-306018 International Publication No. 98/19303 Pamphlet

However, all of the apparatuses disclosed in Patent Documents 1 to 3 diffract one beam of two types of wavelengths with a hologram optical element and transmit the other beam, and then pass through the objective lens to obtain a predetermined beam. The light is condensed on the optical disc.
Here, the diffraction efficiency is determined by the number of projections and depressions formed on the hologram optical element, and there is a limit to the diffraction efficiency of diffracted light when the diffraction efficiency of transmitted light is almost 100% (for example, 4 (About 81% for the step structure, about 88% for the five-step structure, and about 91% for the six-step structure). When used for information recording, the problem is that the amount of diffracted light is insufficient, There is a problem that it becomes difficult to process and mold the mold by increasing the number of steps.

Moreover, since all the apparatuses disclosed in Patent Documents 1 to 3 have the hologram optical element arranged separately from the objective lens, that is, the hologram optical element and the objective lens are arranged apart from each other, There was a problem of increasing the size.
Further, since the light beam is diffracted so as to diverge by the hologram optical element, there is a problem in that the ratio of the light amount used for the total light amount becomes small and the light amount is reduced.

In addition, the hologram optical element and the objective lens are arranged apart from each other, resulting in decentration and deterioration in image height characteristics. As a result, spherical aberration (hereinafter referred to as “spherical aberration”) caused by axial chromatic aberration or a temperature change exceeding the expected value at the design stage. , “Temperature characteristic aberration”) and the like.
In addition, since a flat hologram optical element is used, the number of stepped irregularities (interference fringe patterns) formed on the surface of the hologram optical element is increased, resulting in a complicated manufacturing process.

  An object of the present invention is to take the above-mentioned problems into consideration, and is used for reproducing and / or recording information on two types of optical information recording media having different use reference wavelengths. It is an object to provide an objective optical element and an optical pickup device that can reduce deterioration of characteristics and correct axial chromatic aberration and spherical aberration due to temperature change.

In order to solve the above problems, the invention described in claim 1
Information is reproduced and / or recorded on a first optical information recording medium having a protective substrate thickness t1 using a light beam having a reference wavelength λ1, and the protective substrate is used using a light beam having a reference wavelength λ2 (λ2> λ1). An objective optical element of an optical pickup device that reproduces and / or records information on a second optical information recording medium having a thickness of t2 (t2 ≧ t1),
A predetermined aspherical optical surface is provided with a plurality of diffraction zones centered on the optical axis, and each of the plurality of optical surfaces of the diffraction zone is determined for a light beam passing through the diffraction zone. Provided with an optical path difference providing structure for providing an optical path difference,
In the diffraction ring zone, assuming that there is no optical path difference providing structure, the L-order (L ≠ 0) diffracted light of the light beam having the use reference wavelength λ1 is diffracted to have the maximum diffraction efficiency, and the use reference wavelength λ2 The M-order (M ≠ 0) diffracted light of the light beam is diffracted to have the maximum diffraction efficiency,
The optical path difference providing structure, the L order diffracted light of the light beam used reference wavelength λ1 generated by the diffraction zones, and, with respect to the M th order diffracted light of the light beam used reference wavelength .lambda.2, substantially diffraction orders An optical path difference to be changed is given.

Here, in the present specification, the objective optical element is, in a narrow sense, arranged facing the optical information recording medium at a position closest to the optical information recording medium in a state where the optical information recording medium is loaded in the optical pickup device. The optical element which has a condensing effect | action and is an objective lens, for example.
In a broad sense, the term “optical element” refers to an optical element that can be moved at least in the optical axis direction by an actuator together with the optical element.
Further, the objective optical element is not limited to a single lens, and a lens group configured by combining a plurality of lenses in the optical axis direction may be collectively used as the objective optical element.

The use reference wavelength refers to the wavelength of the light beam emitted from the light source at the use reference temperature.
The use reference temperature is a room temperature (about 10 to 40 ° C.) within a temperature change range of an environment where the objective optical element and the optical pickup device are used.
The optical information recording medium refers to a general optical disc that reproduces and / or records information using a light beam having a predetermined wavelength, such as CD, DVD, CD-R, MD, MO, and high-density DVD.

Information reproduction means reproduction of information recorded on the information recording surface of the optical information recording medium, and information recording means recording information on the information recording surface of the optical information recording medium. . Note that reproduction here includes simply reading information.
In addition, the objective optical element and the optical pickup device in the present invention may be used only for recording or reproducing information, or may be used for performing both recording and reproduction. .

The diffraction ring zone formed on the optical surface is a periodic structure having a function of diffracting an incident light beam by providing a substantially concentric ring zone around the optical axis on the surface of the objective optical element. Say.
Further, the diffractive ring zone does not need to be formed over the entire area of the optical surface, and may be formed only in a predetermined region centered on the optical axis, for example.
The predetermined aspherical optical surface refers to an optical surface having a virtual aspherical shape formed by connecting the starting points of the diffraction zones.
An optical surface having a substantial inclination with respect to a predetermined aspherical optical surface is, for example, a sawtooth or a step along the optical axis direction when the cross section is viewed on a plane including the optical axis (a meridional section). An optical surface having a diffractive ring zone structure such as a shaped one.

In general, optical surfaces with diffractive ring zones produce infinite orders of diffracted light such as 0th order diffracted light, ± 1st order diffracted light, ± 2nd order diffracted light, etc. By doing so, the diffraction efficiency of a specific order may be higher than the diffraction efficiency of other orders, or in some cases, the diffraction efficiency of a specific one order (for example, + 1st order diffracted light) may be almost 100%. it can.
The diffraction efficiency represents the ratio of the amount of diffracted light produced by the diffraction ring zone, and the sum of the diffraction efficiencies of all orders of diffracted light is 1.
Further, the L-order (M-order) diffracted light having the maximum diffraction efficiency means that the diffraction efficiency of the diffracted light is theoretically different from that of the other order when light having the reference wavelength λ1 (λ2) is incident on the objective optical element. The diffracted light at the diffraction order L (M), which is the maximum compared to.

Also, in this specification, the protective substrate refers to an optically transparent parallel plate formed on the light incident surface side of the information recording surface in order to protect the information recording surface of the optical information recording medium. Thickness refers to the thickness of a parallel plate. The light beam emitted from the light source is condensed on the information recording surface of the optical information recording medium through the protective substrate by the objective lens.
Further, in this specification, the numerical aperture on the image side of the objective optical element refers to the numerical aperture of the lens surface located closest to the optical information recording medium among the objective optical elements.
The numerical aperture is a light beam that contributes to the formation of a spot at the best image point by a part or member having a diaphragm function such as a diaphragm or a filter provided in the optical pickup device or a diffraction ring zone provided in the objective optical element. Is the numerical aperture defined as a limited result.

According to the invention described in claim 1, the optical path difference providing structure, the L order diffracted light of the light beam used reference wavelength λ1 generated by the diffraction zones and the M th order diffracted light of the light beam used reference wavelength λ2 On the other hand, an optical path difference that substantially changes the diffraction order is given.
Accordingly, the diffracted light corresponding to the orders other than the Lth order of the light beam having the reference wavelength λ1 is emitted to the first optical information recording medium, and the diffracted light corresponding to the orders other than the Mth order of the light flux having the wavelength λ2 The light is emitted to the optical information recording medium.
As described above, the diffraction orders of the light beams of the respective wavelengths can be substantially changed in two steps of the diffraction ring zone formed on the objective optical element and the optical path difference providing structure, so that the diffraction orders of the respective light beams are appropriately changed. Thus, diffracted light having a sufficient amount of light according to the type of optical information recording medium can be obtained. In addition, the degree of freedom in design with respect to diffraction efficiency and diffraction order can be increased.

In addition, the conventional optical pickup device obtains diffracted light with respect to a light beam having a predetermined wavelength by using a diffractive optical element or the like, but the objective optical element in the present invention can be obtained by a diffracting ring zone. Information is reproduced and / or recorded on an optical information recording medium using the diffracted light of the order having the maximum diffraction efficiency among the light flux of wavelength λ1 and the diffracted light of the order having the maximum diffraction efficiency of the light flux of wavelength λ2. Therefore, diffracted light having a sufficient amount of light according to the type of optical information recording medium can be obtained.
In addition, since diffracted light is used for reproducing and / or recording information on an optical information recording medium, various aberrations (axial chromatic aberration, temperature characteristic aberration, etc.) can be accurately corrected.

Further, passing Rutotomoni each of the plurality of optical surfaces of said diffraction zones, said diffraction zones comprises a plurality of diffractive ring-shaped zones around the optical axis on the optical surface formed in a predetermined aspherical shape An optical path difference providing structure for providing a predetermined optical path difference to the luminous flux to be provided is provided.
Therefore, the optical pickup device can be reduced in size as compared with the case where an optical element having a diffraction ring zone and an optical element having an optical path difference providing structure such as a hologram optical element are arranged separately, and the eccentricity or Deterioration of image height characteristics can be prevented.
Further, as compared with the case where the staircase-shaped discontinuous surface is formed on a plane orthogonal to the optical axis as in the case of a flat hologram optical element, the total number of staircase-shaped discontinuous surfaces or the number of diffraction ring zones The number can be reduced and the number of manufacturing steps can be reduced.

Invention of Claim 2 is the objective optical element of Claim 1, Comprising: The said diffraction ring zone is a sawtooth-shaped structure , The said optical path difference providing structure is a step shape along an optical axis direction. It is characterized by comprising discontinuous surfaces .

According to a third aspect of the invention, an objective optical element according to claim 1 or 2, wherein the optical surface is a substantially circular central region around the optical axis, and a peripheral region covering the outer periphery of the central region The diffraction structure and the optical path difference providing structure are provided in a central region .

According to the third aspect of the present invention, the same effect as in the first or second aspect can be obtained, and the incident light beam can be passed through only the central region or only the peripheral region as necessary, or the central region and the peripheral region By allowing both to pass, the degree of design freedom with respect to diffraction efficiency and diffraction order can be further increased.

A fourth aspect of the present invention is the objective optical element according to any one of the first to third aspects, wherein the number of the diffracting ring zones is any one of 3 to 20.

Invention of Claim 5 is an objective optical element as described in any one of Claims 1-4 , Comprising: The said optical path difference providing structure is an integral multiple of use reference wavelength (lambda) 2 with respect to the light beam of use reference wavelength (lambda) 2. The optical path difference is given.

According to the fifth aspect of the present invention, the same effect as in any one of the first to fourth aspects can be obtained, and the optical path difference that is an integral multiple of the wavelength λ2 with respect to the light flux having the wavelength λ2 by the optical path difference providing structure. Is granted. That is, the phase difference is not substantially given to the light flux having the wavelength λ2 by the optical path difference providing structure. Therefore, the M-order diffracted light of the light beam having the wavelength λ2 having the maximum diffraction efficiency, which is diffracted by the diffraction ring zone, can be output as it is to the second optical information recording medium as a light beam corresponding to the M-th order diffraction.

The invention of claim 6 wherein is an objective optical element according to any one of claims 1 to 5 characterized in that it is a L = M.

According to the seventh aspect of the present invention, the light beam having the use reference wavelength λ1 is condensed on the first optical information recording medium having the protective substrate thickness t1, and the light beam having the use reference wavelength λ2 (λ2> λ1) is collected. (t2 ≧ t1) comprises a second objective optical element for condensing light on an optical information recording medium, an optical pickup apparatus for reproducing and / or recording of the information, one of the claims 1 to 6 one It has the optical element of description .

According to invention of Claim 7 , the effect similar to any one of Claims 1-6 can be acquired . .

  According to the present invention, the diffraction orders of the light fluxes of the respective wavelengths can be substantially changed in two stages of the diffraction ring zone formed on the objective optical element and the optical path difference providing structure. It is possible to obtain an objective optical element and an optical pickup device in which diffracted light having a sufficient amount of light according to the type of optical information recording medium can be obtained and the degree of design freedom with respect to diffraction efficiency and diffraction order is increased. .

It is the schematic which shows an example of the optical pick-up apparatus and optical element which concern on this Embodiment. It is a principal part side view which shows the structure of an objective lens. It is the principal part side view (a) of an objective lens, and the enlarged view (b) of the location enclosed with the circle in FIG. 3 (a). It is a principal part side view which shows the structure of an objective lens. It is a principal part side view which shows the structure of an objective lens. It is a principal part side view which shows the structure of an objective lens. It is a principal part side view which shows the structure of an objective lens. It is a principal part enlarged side view which shows the structure of an objective lens. It is the schematic which shows an example of another optical pick-up apparatus and an optical element.

  Embodiments of an objective optical element and an optical pickup device of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, an optical pickup device 1 has a wavelength from a first semiconductor laser 3 (light source) to a first optical information recording medium 2 (DVD in the present embodiment) that is an optical information recording medium. A light beam of λ1 (= 650 nm) is emitted, and a light beam of wavelength λ2 (= 780 nm) is emitted from the second semiconductor laser 5 (light source) to the second optical information recording medium 4 (CD in the present embodiment). By emitting the light, information is recorded on the information recording surface 6 of the first optical information recording medium 2 or the second optical information recording medium 4, and the recorded information is read. The first semiconductor laser 3 and the second semiconductor laser 5 are unitized as light sources.

When information is recorded on or reproduced from the DVD 2, the light beam having the wavelength λ1 emitted from the first semiconductor laser 3 passes through the collimator 7 and becomes a parallel light beam as shown by a solid line in FIG. Further, the light is narrowed down by the diaphragm 9 through the beam splitter 8, and is focused on the information recording surface 6 by the objective lens 10 through the DVD protective substrate.
The effect of the objective lens 10 on the light beam having the wavelength λ1 at this time will be described later.
Then, the light beam modulated and reflected by the information pits on the information recording surface 6 is reflected again by the beam splitter 8 through the objective lens 10 and the aperture 9, is given astigmatism by the cylindrical lens 11, and the concave lens 12 is Then, a read signal of information recorded on the DVD 2 is obtained using the signal incident on the photodetector 13 and output from the photodetector 13 .

When recording or reproducing information on the CD 4, as indicated by a broken line in FIG. 1, the light beam having the wavelength λ 2 emitted from the second semiconductor laser 5 passes through the collimator 7 and becomes a parallel light beam. Further, the light is narrowed down by a diaphragm 9 through a beam splitter 8 and focused on the information recording surface 6 by the objective lens 10 through the protective substrate of the CD4.
The effect of the objective lens 10 on the light beam having the wavelength λ1 at this time will be described later.
Then, the light beam modulated and reflected by the information pits on the information recording surface 6 is reflected again by the beam splitter 8 through the objective lens 10 and the aperture 9, is given astigmatism by the cylindrical lens 11, and the concave lens 12 is Then, using the signal incident on the photodetector 13 and output from the photodetector 13, a read signal for information recorded on the CD 4 is obtained.

  In addition, focus detection and track detection are performed by detecting a change in the amount of light due to a change in the shape of the spot and a change in position on the photodetector. Based on the detection result, the two-dimensional actuator 14 forms an image of the light flux from the first semiconductor laser 3 or the light flux from the second semiconductor laser 5 on the information recording surface 6 of the DVD 2 or CD 4. 10 and the objective lens 10 is moved so as to form an image on a predetermined track.

As shown in FIG. 2, the objective lens 10 as an objective optical element is a double-sided aspherical single lens that constitutes the optical system of the optical pickup device, and a diffraction ring zone is formed on one (light source side) optical surface 10a. 20 and an optical path difference providing structure 30 are provided.
Specifically, the objective lens 10 includes a plurality of diffraction ring zones 20 which are serrated discontinuous surfaces having a substantial inclination with respect to a predetermined aspherical optical surface centered on the optical axis L. Further, on the optical surface of each diffraction ring zone 20, a step-like discontinuity along the optical axis that gives a predetermined optical path difference to the light beam passing through these diffraction ring zones 20. An optical path difference providing structure 30 including a surface 31 (step) is formed.

3 (a) and 3 (b), the line indicated by the alternate long and short dash line is an optical surface (having a predetermined aspherical shape) having a virtual aspherical shape formed by connecting the starting points of the diffraction zones 20 as described above. A line indicated by a two-dot chain line is a concentric sawtooth diffraction that has been formed so as to increase in thickness with increasing distance from the optical axis with the optical axis L as the center. This represents the outer shape of the annular zone 20.
3 (a) and 3 (b), a solid line gives a predetermined optical path difference to the light beam passing through each diffraction zone 20 formed on the optical surface of each diffraction zone 20. This represents an actual lens shape including the outer shape of the stepped discontinuous surface 31 (step).
2 and FIGS. 4 to 7, as in FIG. 3, each shape described above is represented by a one-dot chain line, a two-dot chain line, and a solid line.
The depth d1 (length in the optical axis L direction) of each step 31 is substantially equal to the value represented by λ2 / (n−1), where n is the refractive index of the objective lens 10 with respect to the wavelength λ2. An optical path difference corresponding to approximately one wavelength λ2 occurs between the light beam having the wavelength λ2 that passes through one step 31 and the light beam having the wavelength λ2 that passes through the adjacent step 31, and the wavefront is shifted. The length is set so that it does not occur.
Further, the shape of the surface 31a (optical function surface) of each step is obtained by dividing the shape of the surface of the sawtooth diffracting ring zone 20 indicated by a two-dot chain line in FIG. The shape approximates the shape translated in the optical axis L direction.

As described above, the objective optical element of the present invention has a predetermined amount of light that passes through the objective optical element (objective lens 10) by the optical path difference providing structure 30 in which the step 31 having a predetermined depth is provided on the optical function surface. Further, the surface shape 31a of each step 31 is divided into sections corresponding to the respective steps and is translated in the optical axis L direction. Thus, it has a function of extracting the diffracted light having the maximum diffraction efficiency from the light beams having the wavelengths λ1 and λ2.
For example, when a light beam having a wavelength λ1 (650 nm) is incident on the objective optical element, each light beam passes through the areas A to E in FIG. 3, and each light beam has a wavelength of 780 nm−650 nm = 130 nm, that is, 2 / 5π radians. A phase difference is substantially given, and as a result, the phase of the light beam having the wavelength λ1 is changed and diffracted.
On the other hand, when a light beam having a wavelength λ2 (780 nm) is incident, a phase difference corresponding to one wavelength λ2 is given to each light beam by passing through the regions A to E in FIG. The phase difference generated between the light beams that have passed through -E is almost zero. Accordingly, the light beam having the wavelength λ2 is transmitted without being diffracted depending on the optical path difference providing structure 30 as it is.

  In the present embodiment, as described above, the depth d1 (length in the optical axis direction) of each step 31 is set to a length that causes an optical path difference of one wavelength of wavelength λ2, and the surface 31a of each step 31 The shape of the surface of the serrated diffraction ring zone 20 indicated by a two-dot chain line in FIG. 3 is divided into sections corresponding to the respective steps 31 and translated into the optical axis L direction. Although approximated, the depth d1 of the step 31 and the shape of the surface 31a can be appropriately changed according to the wavelength of the light beam used.

Example 1
Next, a first example of the optical pickup device 1 and the objective optical element 10 shown in the above embodiment will be described.
In the present embodiment, as shown in FIG. 2, the optical surface 10 a on one side (light source side) of the objective lens 10 as the objective optical element which is a single lens with a double-sided aspheric surface, which is a certain height from the optical axis L. The diffraction ring zone 20 and the optical path difference providing structure 30 are provided in a range A1 (hereinafter referred to as “central region A1”) in the following range (1.54 mm or less in this embodiment).
Further, a range A2 (hereinafter referred to as “peripheral region A2”) other than the central region A1 on the light source side optical surface 10a does not include the diffraction zone 20 and the optical path difference providing structure 30, and is usually It functions as a refractive lens.
Each step 31 formed in one diffraction ring zone 20 is formed so as to protrude to the light source side as the distance from the optical axis L increases.
Tables 1 and 2 show the lens data of the objective lens.

As shown in Table 1, the objective lens 10 of this example is set to have a focal length f = 3.05 mm and an image-side numerical aperture NA = 0.60 when the use reference wavelength λ1 = 655 nm. The focal length f = 3.07 mm and the image-side numerical aperture NA = 0.50 when the wavelength λ2 = 785 nm are set.
Surface No. 1 in Table 1 2, 2 ′, and 3, respectively, of the optical surface 10 a on the light source side of the objective lens 10, a central region A1 having a height h from the optical axis L of 1.54 mm or less, and a height from the optical axis L of 1. The peripheral area A2 of 54 mm or more and the optical surface 10b on the optical information recording medium side of the objective lens 10 are shown. Surface numbers 4 and 5 represent the surface of the protective substrate of the optical information recording medium and the recording layer, respectively. Ri represents the radius of curvature, di represents the displacement in the optical axis direction from the i-th surface to the (i + 1) -th surface, and ni represents the refractive index of each surface.

  The second surface, the second 'surface, and the third surface of the objective lens are axially symmetric around the optical axis L, which is defined by a mathematical formula in which the coefficients shown in Tables 1 and 2 are substituted into the following formula (Equation 1). It is formed in a non-spherical surface.

Here, X (h) is an axis in the optical axis direction (the light traveling direction is positive), κ is a conical coefficient, and A2i is an aspheric coefficient.

  Further, the pitch of the diffraction zone is defined by a mathematical formula in which the coefficients shown in Table 2 are substituted into the optical path difference function of Formula 2.

ここで、Ｂ２iは光路差関数の係数である。

Here, B2i is a coefficient of the optical path difference function.

  Equation 3 is an equation representing the optical path difference of the light flux having the wavelength λ1 or the wavelength λ2 at an arbitrary height from the optical axis.

また、λｉ、ｐ、Ｎ、Ｍの値については、表３に示す。

The values of λi, p, N, and M are shown in Table 3.

(Example 2)
Next, a second example of the optical pickup device 1 and the objective optical element 10 described in the above embodiment will be described.
Also in the present embodiment, similarly to the first embodiment, on the optical surface 10a on one side (light source side) of the objective lens 10 as the objective optical element which is a single lens having a double-sided aspheric surface as shown in FIG. The diffraction zone 20 and the optical path difference providing structure 30 are provided in the central region A1 of 1.60 mm or less from the optical axis.
The peripheral region A2 does not include the diffraction ring zone 20 and the optical path difference providing structure 30, and has a function as a normal refractive lens.
Each step 31 formed in one diffraction ring zone 20 is formed so as to protrude to the light source side as the distance from the optical axis L increases.
Tables 4 and 5 show the lens data of the objective lens.

  As shown in Table 4, the objective lens of this example is set to have a focal length f = 2.40 mm and an image-side numerical aperture NA = 0.85 when the use reference wavelength λ1 = 405 nm. When λ2 = 655 nm, the focal length f = 2.46 mm and the image-side numerical aperture NA = 0.65.

  The second surface, the second 'surface, and the third surface of the objective lens are aspherical surfaces that are axisymmetric about the optical axis L and are defined by mathematical formulas obtained by substituting the coefficients shown in Table 4 and Table 5 into Equation 1 above. Is formed.

  Further, the pitch of the diffraction zone is defined by a mathematical formula in which the coefficients shown in Table 5 are substituted into the optical path difference function of Equation 2 above.

  Further, the phase difference of the light beam having the wavelength λ1 or the wavelength λ2 at an arbitrary height from the optical axis is expressed by an equation obtained by substituting the coefficient shown in Table 3 into the above formula 3.

(Example 3)
Next, a third example of the optical pickup device 1 and the objective optical element 10 described in the above embodiment will be described.
In the present embodiment, as shown in FIG. 4, the diffraction zone 20 and the optical path difference are formed on almost the entire optical surface 10a on one side (light source side) of the objective lens 10 as the objective optical element which is a single lens having a double-sided aspheric surface. The provision structure 30 is provided.
Each step 31 formed in one diffraction ring zone 20 is formed so as to protrude to the light source side as the distance from the optical axis L increases.
Tables 6 and 7 show the lens data of the objective lens.

  As shown in Table 6, the objective lens of this example is set to have a focal length f = 2.40 mm and an image-side numerical aperture NA = 0.65 when the use reference wavelength λ1 = 405 nm. When λ2 = 655 nm, the focal length f = 2.48 mm and the image-side numerical aperture NA = 0.65.

  The second surface and the third surface of the objective lens are formed as axisymmetric aspherical surfaces around the optical axis L, which are defined by mathematical formulas obtained by substituting the coefficients shown in Tables 6 and 7 into Equation 1 above. .

  Further, the pitch of the diffraction zone is defined by an equation in which the coefficients shown in Table 7 are substituted into the optical path difference function of the above formula 2.

  Further, the phase difference of the light beam having the wavelength λ1 or the wavelength λ2 at an arbitrary height from the optical axis is expressed by an equation obtained by substituting the coefficient shown in Table 3 into the above formula 3.

Example 4
Next, a fourth embodiment of the optical element and the optical pickup device of the present invention will be described with reference to FIG.
The optical pickup device 70 emits a light beam having a wavelength λ1 (= 655 nm) from the first semiconductor laser 71 (light source) to the first optical information recording medium 80 (DVD in this embodiment), A light beam having a wavelength λ2 (= 785 nm) is emitted from the second semiconductor laser 72 (light source) to the optical information recording medium 81 (CD in this embodiment). These light beams are incident on the objective lens 10 (objective optical element) as divergent light and condensed on the information recording surfaces 80a and 81a of a predetermined optical information recording medium, thereby recording various information and reading the recorded information. Is to do.
Since the first semiconductor laser 71 and the second semiconductor laser 72 are unitized as a light source, FIG. 9 shows a solid line that combines the light flux having the wavelength λ1 and the light flux having the wavelength λ2 emitted from each semiconductor laser. It shall be expressed as

When recording or reproducing information on the DVD 80, the light beam having the wavelength λ 1 emitted from the first semiconductor laser 71 passes through the diffraction grating 73 and is reflected by the half mirror 74. Further, it is stopped by the stop 75 and is focused on the information recording surface 80a by the objective lens 10 via the DVD protective substrate 80b.
Then, the light beam modulated and reflected by the information pits on the information recording surface 80a passes through the objective lens 10, the diaphragm 75, and the half mirror 74 again, passes through the diffraction grating (not shown), and onto the light detector 76. A signal for reading information recorded on the DVD 80 is obtained using a signal incident and output from the photodetector 76.

Similarly, when information is recorded on or reproduced from the CD 81, the light beam having the wavelength λ 2 emitted from the second semiconductor laser 72 passes through the diffraction grating 73 and is reflected by the half mirror 74. Further, the aperture is stopped by the aperture 75 and is focused on the information recording surface 81a by the objective lens 10 via the CD protective substrate 81b. In FIG. 9, for convenience, the CD protective substrate 81b and the DVD protective substrate 80b are shown in the same diagram.
Then, the light beam modulated and reflected by the information pits on the information recording surface 81a again passes through the objective lens 10, the diaphragm 75, and the half mirror 74, passes through the diffraction grating (not shown), and onto the light detector 76. A signal for reading information recorded on the CD 81 is obtained by using a signal incident and output from the photodetector 76.

  In addition, focus detection and track detection are performed by detecting a change in the amount of light due to a change in the shape and position of the spot on the photodetector 76. Based on this detection result, the two-dimensional actuator (not shown) forms an image of the light beam from the first semiconductor laser 71 or the light beam from the second semiconductor laser 72 on the information recording surfaces 80a and 81a of the DVD 80 or CD 81. The objective lens 10 is moved, and the objective lens 10 is moved so as to form an image on a predetermined track.

As shown in FIG. 7, the objective optical element in the present embodiment is on the optical surface 10a on one side (light source side) of the objective lens 10 as the objective optical element which is a double-sided aspherical single lens, and from the optical axis. A diffraction zone 20 and an optical path difference providing structure 30 are provided in a central region A1 of 1.53 mm or less.
Each step 31 formed in one diffraction ring zone 20 is formed so as to fall into the lens as the distance from the optical axis L increases.
The depth d1 (length in the direction of the optical axis L) of each step 31 is a depth at which an optical path difference corresponding to one wavelength of the wavelength λ2 occurs. That is, there is an optical path difference that is almost twice the wavelength λ2 between the light beam having the wavelength λ2 that passes through one splitting surface 31 and the light beam having the wavelength λ2 that passes through the adjacent splitting surface 31, and there is a deviation of the wavefront. The length is set so that it does not occur.
Further, the peripheral region A2 is provided with a sawtooth-shaped diffraction ring zone 40, and the diffraction efficiency of the first-order diffracted light of the light beam having the wavelength λ1 passing through the peripheral region A2 is set to be almost 100%. Yes.
Tables 8 and 9 show the lens data of the objective lens.

  As shown in Table 8, the objective lens of this example has a focal length f = 2.85 mm and an image-side numerical aperture NA = 0.60 when the first wavelength λ1 = 655 nm emitted from the first light source. The imaging magnification m is set to −1 / 8.0, the focal length f = 2.87 mm at the second wavelength λ2 = 785 nm emitted from the second light source, and the image-side numerical aperture NA = 0.47 and imaging magnification m = −1 / 8.1 are set.

  In Table 8, surface numbers 1 and 2 are the surface of the diffraction grating 73 on the light source side, the surface of the diffraction grating 73 on the optical information recording medium side, and surface numbers 3, 3 ', and 4 are the light source side optics of the objective lens 10 Among the surfaces, a central region A1 having a height h from the optical axis L of 1.53 mm or less, a peripheral region A2 having a height from the optical axis L of 1.53 mm or more, and an optical surface on the optical information recording medium side of the objective lens 10 Surface numbers 5 and 6 represent the surfaces of the protective substrates 80b and 81b of the optical information recording medium and the information recording surfaces 80a and 81a, respectively. Ri represents the radius of curvature, di represents the amount of displacement in the optical axis L direction from the i-th surface to the (i + 1) -th surface, and ni represents the refractive index of each surface.

  The third surface, the third 'surface, and the fourth surface of the objective lens 10 are aspherical surfaces that are axisymmetric about the optical axis L and are defined by mathematical formulas obtained by substituting the coefficients shown in Table 8 and Table 9 into Equation 1, respectively. Is formed.

  Further, the pitch of the diffraction zone is defined by an equation in which the coefficient shown in Table 9 is substituted into the optical path difference function of the above formula 2.

Further, the optical path difference of the light beam having the wavelength λ1 or the wavelength λ2 at an arbitrary height from the optical axis is expressed by an equation obtained by substituting the coefficient shown in Table 10 into the above formula 3.

The objective optical element according to the present invention is not limited to those shown in the first to third embodiments, and may be as shown in FIGS.
The objective optical element shown in FIG. 5 is an optical surface on one side (light source side) of the objective lens 10 as an objective optical element which is a single lens having a double-sided aspheric surface, compared with the objective optical elements shown in the first and second embodiments. The central region A1 on 10a includes the diffractive ring zone 20 and the optical path difference providing structure 30, and the peripheral region A2 is the same in functioning as a normal refractive lens, but each step formed in one diffractive ring zone 20 is the same. 31 differs in that it protrudes toward the optical information recording medium as it moves away from the optical axis L, that is, falls into the lens as it moves away from the optical axis L.

  The objective optical element shown in FIG. 6 is an optical surface on one side (light source side) of the objective lens 10 as an objective optical element which is a single lens having a double-sided aspheric surface, compared with the objective optical elements shown in the first and second embodiments. The point that the diffraction zone 20 and the optical path difference providing structure 30 are provided in the central region A1 on 10a matches, but the point that the peripheral region A2 has the sawtooth diffraction zone 40 is different.

  Further, in the objective lens 10 shown in FIG. 3, the shape of the surface of the sawtooth diffraction ring zone 20 is divided into sections corresponding to the respective steps 31, and the shape translated in the direction of the optical axis L is formed at each step. Although the shape of the surface 31a is not limited to this, as shown in FIG. 8, the surface shape of the diffracting ring zone having a step-like discontinuous surface 50 along the optical axis direction having an optical path difference providing function is provided. The shape of the surface 31a of each step may be a shape that is divided in a section corresponding to each step 31 and translated in the optical axis L direction.

  As described above, the objective optical element according to the present invention has a plurality of diffraction ring zones centered on the optical axis in at least a part of the optical function surface formed in an aspherical shape, and the optical surface of the diffraction ring zone What is necessary is just to provide the optical path difference providing structure which consists of the step-shaped discontinuous surface which gives a predetermined | prescribed optical path difference with respect to the light beam which passes through this diffraction ring zone formed above.

L Optical axis 1 Optical pickup device 2 First optical information recording medium 4 Second optical information recording medium 10 Objective optical element 20 Diffraction ring zone 30 Optical path difference providing structure 31 Step-like discontinuous surface (step)
70 Optical Pickup Device 71 First Light Source 72 Second Light Source 80 First Optical Information Recording Medium 81 Second Optical Information Recording Medium

Claims (7)

Information is reproduced and / or recorded on a first optical information recording medium having a protective substrate thickness t1 using a light beam having a reference wavelength λ1, and the protective substrate is used using a light beam having a reference wavelength λ2 (λ2> λ1). An objective optical element of an optical pickup device that reproduces and / or records information on a second optical information recording medium having a thickness of t2 (t2 ≧ t1),
A predetermined aspherical optical surface is provided with a plurality of diffraction zones centered on the optical axis, and each of the plurality of optical surfaces of the diffraction zone is determined for a light beam passing through the diffraction zone. Provided with an optical path difference providing structure for providing an optical path difference,
In the diffraction ring zone, assuming that there is no optical path difference providing structure, the L-order (L ≠ 0) diffracted light of the light beam having the use reference wavelength λ1 is diffracted to have the maximum diffraction efficiency, and the use reference wavelength λ2 The M-order (M ≠ 0) diffracted light of the light beam is diffracted to have the maximum diffraction efficiency,
The optical path difference providing structure, the L order diffracted light of the light beam used reference wavelength λ1 generated by the diffraction zones, and, with respect to the M th order diffracted light of the light beam used reference wavelength .lambda.2, substantially diffraction orders An objective optical element characterized by providing an optical path difference to be changed. The objective optical element according to claim 1,
The diffraction zone has a serrated structure;
2. The objective optical element according to claim 1, wherein the optical path difference providing structure comprises a step-like discontinuous surface along the optical axis direction. The objective optical element according to claim 1 or 2,
The optical surface is divided into a substantially circular central region centered on the optical axis and a peripheral region covering the outer periphery of the central region,
An objective optical element comprising the diffractive structure and the optical path difference providing structure in a central region. The objective optical element according to any one of claims 1 to 3,
2. The objective optical element according to claim 1, wherein the number of diffraction ring zones is any one of 3 to 20. The objective optical element according to any one of claims 1 to 4,
The objective optical element according to claim 1, wherein the optical path difference providing structure provides an optical path difference that is an integral multiple of the use reference wavelength λ2 to the light beam having the use reference wavelength λ2. The objective optical element according to any one of claims 1 to 5,
An objective optical element, wherein L = M. A light beam having a reference wavelength λ1 is condensed on a first optical information recording medium having a protective substrate thickness t1,
An objective optical element for condensing a light beam having a use reference wavelength λ2 (λ2> λ1) onto a second optical information recording medium having a protective substrate thickness t2 (t2 ≧ t1) is provided to reproduce and / or record information. An optical pickup device,
An optical pickup device comprising the optical element according to claim 1.

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