TW492001B - Double focused optical read/write head - Google Patents

Abstract

The present invention discloses a double focused optical read/write head, which is applicable in a laser optical read/write device with single lens, for producing two reading spots of different focal length (double focused). Therefore, it is possible to facilitate the conduction of reading/writing data from/to a compact disc (CD) and digital versatile disc (DVD) of different thickness. Whit the optical device disposed in an optical path, as the light beam (laser beam) passes through, difference between the optical path length and the range of radius make it possible for such device to have functions of controlling the numerical aperture of the system and reducing the spherical aberration. After such light beam of different aperture passes through the focus lens, it is then possible to achieve an effect of producing a read/write spot of different focal length.

Description

[Application range of the invention] The present invention relates to a dual-focus optical pickup head, in particular, an optical reading device applied to a single objective lens. It has a compact disc (CD) and a DVD (digital versatile di) with different thicknesses. sc) for data access. [Related technology] The technology of using CD (Compact Disc) and DVD (Digital Versatile Disc) § recorded data, and the use of optical access devices to access data on the data surface of these two types of optical discs has long been known, Data access is performed through an optical pick-up head. The principle of reading is to focus a laser beam on the data storage surface of the optical disc into a light spot, and then use an image detection element. (Phot〇Detector) will reflect back through the surface of the optical disc, and convert the light beam containing the data signal into an identifiable electrical signal, that is, the reading of the optical disc data is completed, and the process of storing the data is reversed. The dimensions and specifications of CD and DVD are shown in [Table 1]: THICKNESS _ TRACK PITCH μ WAVELENGT nHm NA (numerical aperture) CD 1.2 1.6 780 0.45 DVD 0.6 * 2 0774 ~ 635-650 0.6 NA: Numerical Aperture (光 圏 値） [Table 1] From the contents of [Table 1], it can be found that, relative to CD, DVD display

492001 V. Description of the invention (2) Of course, it has a relatively high storage density, which can be called a high-density optical disc. Due to the difference in size (especially the storage density), it also causes the optical reading device to take Conditions have different requirements; in order to be able to access data on the data storage surfaces of Cj) and DVD, the optical pickup is a key component in the optical reader, and the optical system design is in the optical pickup. The most important part. Therefore, in order to read the data of CD and DVD in an optical reading device of a single objective lens (Objective Lens), an optical system must be designed according to the thickness of the optical disc, so that a light beam passes through two different numerical apertures. Causes the spot size on the disc to change in time and to be in the correct focus position. Due to the compatibility requirements of commercial products for CD and DVD, the use of a single optical pickup must be able to access the data in CD and DVD discs, and how to generate a double focal length optical system is the key technology. It is known that the technology of generating double focal length in the optical pickup head is used to separately access the data of the CD and the order. 1) There are roughly the following: 1 · The design of the dual objective lens · The use of two objective lenses (Objective lens) The two objective lenses have different focal lengths respectively. According to the type of the optical disc (CD or DVD), one of the objective lenses is selected using a driving mechanism. The disadvantages are the increase in the weight of the optical pickup and the increase in the price of the product. , 2 · Create a diffraction element in the objective lens to achieve a double focus effect. 3 · Use of Holographic optical element (HOE; Holographic optical element) to generate double focal length; its disadvantages are difficult to manufacture and manufacturing

Page 5 492001 V. Description of the invention (3) · 1 Cost South. 4 Use LCD (Liquid Crysta 1 Disp1 ay) shutter (LCD Shutter) to make a NA controller, make a single beam pass through two mirrors with two M's to achieve the effect of double focal length; its disadvantage is that the polarity of the laser light source must be accurate In order to cooperate with the LCD, in addition to maintaining the operation of the LCD with power.

5 · _ Make a ring structure in the objective lens. The light beam through the center is used to read the lean material of the CD. The light beam for reading DVD data is formed by the inner circle and the outer circle beam a. For example, US Patent No. 5,665, No. 957 (illustrated in FIG. 1, 2) 'A similar technique is disclosed therein, in which a holographic optical element (H0iogran) lens with a pattern on the surface is used to achieve the effect of double focal length ; Its disadvantages are high production cost, low tolerance for installation errors (high accuracy requirements), and reduced beam availability by about 15% ° 6 · Using dual light sources and two lenses, such as US Patent No. 5 ,? Patent 77, gw 'discloses that a laser light source using two different wavelengths is used to match different lenses to provide reading spots with different focal lengths and different diameters. The disadvantages are high production cost and complicated structure.

7 · In addition, a variable aperture is used in conjunction with the design of the movable objective lens to achieve the effect of double focal length, such as US Patent Nos. 5,6 5 9,5 3 3 and 5,2 8 1,7 9 7 All have similar technologies. [Objective and Summary of the Invention] The main object of the present invention is to provide a new optical read head capable of generating a double focal length, which has a simple structure, low cost, easy assembly, and assembly.

Page 6 492001 V. Description of the invention (4) High tolerance for mounting error According to the optical element of the present invention, it can be used with Laser diode, Optical element; HOE), (Objective Lens), without the need to add a lens or dual objective lens Etc.) advantages. The disclosed double-focus light has an optical pickup head beamsplitter (Beam collimator lens (Col)), and other optical components (such as a laser hologram that can store the optical pickup head separately) Leveling to achieve the optical path length, aperture and passing function in the rooting. Another purpose of the present invention is to provide an image-type optical pickup head reading head, all optical elements, with different light beams. The optical read head can be processed to produce different focal lengths. According to the present invention, the optical principle can be achieved when the spherical aberration-reducing mirror is focused, and the laser on the image plane can be directly in the middle. By using the same reading light technology, the main beam (lightning difference makes the effect, and 'the beam can be reached, the current collimation through the _ objective lens point is to expose the beam of light to the optical reading of this one Head, just use the laser diode (or holographic limator Lens), objective lens element (Photo Detector, such as variable aperture, mobile objects to obtain CD and DVD information. It is suitable for the current traditional or dual-source optical reading to separate a single laser beam, and this one contains a traditional optical pickup head with different optical paths, and a mirror (Collimator Lens (Objective Lens)). (Configured in the optical path) Through this optical element, because the element has a control system, the value of the reading spot of the beam with different numerical apertures and different focal lengths is known. 'A beam of approximately monochromatic light traveling on the object plane will form two. Focus, this invention makes use of existing

492001 V. Description of the invention (5) An optical element, such as a three-beam grating, or the other side of a holographic optical element, is made of this optical element, and a cylinder is made at the center of the laser beam corresponding to the optical element. Or a circular concave hole, the laser beam can be divided into two beams. The cylindrical path or circular concave hole can be used to change the optical path difference of the central beam. On the other hand, the numerical aperture (NA) of the optical element is controlled. Among them, the beam passing through this optical element is more efficient and can be used as a reading beam for a DVD, and the other central beam only passing through a cylinder or a circular concave hole can be used as a reading beam for a CD.

The technical content and embodiments of the present invention are described below with reference to the drawings. [Explanation of the drawings] Fig. 1 shows the structure of the first embodiment of the optical element. Fig. 2 is a construction diagram of a bifocal optical pickup head according to the first embodiment of the present invention. Figure 3 is an enlarged view of the partial structure of Figure 1, showing the focus of the light beam when reading a DVD. Figure 4 is an enlarged view of the partial structure of Figure 1, showing the focus of the light beam when reading a CD. Fig. 5 shows the structure of the second embodiment of the optical element.

Fig. 6 is a schematic diagram of a bifocal optical pickup head according to a second embodiment of the present invention, and discloses the case where the optical element is applied to a conventional optical pickup head. Fig. 7 shows the structure of the third embodiment of the optical element, showing another example in which the optical element is provided in a three-beam grating.

Page 8 492001 V. Description of the invention (6) Fig. 8 is a construction diagram of a double focal length optical pickup head according to the third embodiment of the present invention, which reveals the situation where the optical element is applied to a full-image type optical pickup head. Fig. 9 is a construction diagram of a dual focus optical pickup according to a fourth embodiment of the present invention, in which an example in which a combination of an optical element and a three-beam grating is applied to a dual light source optical pickup is disclosed. FIG. 10 is a structural diagram of a dual focal length optical reading head according to a fifth embodiment of the present invention, in which an optical element, a three-beam grating, and a holographic optical element are combined and applied to a dual light source optical reading head. Another example. [Detailed description of the embodiment] Please refer to "Fig. 1". The optical element 10 is arranged in the learning path. This optical element has a first refractive surface 11 perpendicular to each other. The surfaces 1 1 and 1 2 are along the optical axis of each other. A material, such as flat glass, or columnar body 1 30, which is distributed on different optical axes half-on-a-plate-shaped translucent body 13 becomes the first refractive surface 1 on the transmissive body surface A] is the so-called second refraction The center of the surface 1 2 is a circle centered around the axial end and passing through different refractive surfaces 1 1 and 1 2 According to the technology of the present invention, the light passing through the laser beam in an optical pickup head has two light axes respectively corresponding to the laser light axis. To the second refracting surface 1 2, the optical paths of the light beams in the two refracting directions are different, and the range is; the specific implementation method can be formed on the surface (the light transmitting body 13 must be a quartz plate with high light transmittance) Among the protrusions 13, the axial end face b of the surface columnar body 1 30 on the same side as the columnar body 13 is the surface A of which is the periphery of the surface B of the axial end face b. After the light beam passes through, it will be divided into two lines with different light.

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V. Description of the invention (7) After a light beam is parallelized, it passes through the reading spot with the same focal length. Cheng ’s laser beam, and then using this objective lens, can produce different optimal implementations. The radial cross-sectional shape of a fairy scale, a cylindrical body 1 30 (usually a cylinder) is based on the band stopper μ. The cross-sectional shape of Λ ^ ^,,, and Tian Shexian is made, and its light energy efficiency will be higher. Optical element 1 η manufacturing 1 卞 丄 0 manufacturing method, pattern conversion can be done by photolithography (Photllllography)

Transfer), and then carry out the rest of the time to control the desired depth; of course, it can also be made by injection molding or other similar methods. Please refer to the "structure diagram of the optical pickup head in Fig. 2", which is a dual focal length & component component package of the dual focal length optical pickup head of the first embodiment of the present invention for generating an access laser Laser diode required for laser diode 8 (CD or DVD); an optical element 10 for dividing the laser beam emitted by laser diode 20 into two different optical paths Beam Splltter Cube 30 to separate two different laser beams from the reflected beam from the disc 8 (CD or); a collimator lens 40 for parallel processing of two light beams with different optical path lengths into parallel light beams;, :: Objective Lens 50 for focusing a parallel light beam containing two non-f light beams on the optical disc 8 (CD or DVD) the lean storage surface becomes an access point;

492001 V. Description of the invention (8) An image detection element (Photo Detector) 6 1 for converting the reflected light beam from the optical disc 8 (CD or DVD) into an electrical signal; and a focusing lens 6 0 for The reflected light beam from the beam splitter 30 is focused on the image detection element 6 1. As can be seen from the contents of "Figure 2", the laser beam used to read the optical disc 8 (CD or DVD) is emitted from the laser diode 20, and after passing through the light car element 10, it will be divided into two lights. The laser beams with different distances pass through a collimating lens 4 0 and an objective lens 5 0 in succession, and then focus on the lean material storage surface of the optical disc to become a light spot. The light beam completely passing through the optical element 10 is It is synthesized by the light beams of 11 angstroms and 2 -9 on the first refracting surface, and its light energy efficiency can be used as the reading beam of DVD 8 i (see "Figure 3"), while the other passes only through the cylinder 13 The central beam can be used as the reading beam of CD 8 2 (see Figure 4). The assembly of the optical element 丄 〇 only needs to coincide the center of the cylinder with the center of the laser beam, that is, the main two CM ten miles. Early in the step, because the light beam reflected from the surface of the optical disc, from the beam splitter 30 to the focusing lens 60 to the image detection element 61, and no longer passes through the science element 10, it can have a larger The tolerance value of the assembly of ". 5 pictures" reveals another feasible light Example of the element, the way it is realized is-flat plate-shaped light-transmitting body (3, face shape); Therefore, using the optical element process of the present invention, the laser beam generated: 需 need to cooperate with the collimating lens 40 'As well as the objective lens 50, the data of CD 8 2 and DVD 8 1 can be accessed separately without adding other optical components (such as variable aperture, moving objective lens or dual objective lens).

Page 11 492001 V. Description of the invention (9) --- A cylindrical recessed hole 1 3 1 is formed, and the axial end face C of this cylindrical recessed hole 1 3 1 is like the axial end face of "Figure 1" B—Likewise, as the so-called second and second refractive surfaces 1 2, the same effect can be achieved. σ month reading "Figure 6" is a structural diagram of a bifocal optical pickup head according to an embodiment of the present invention, which discloses how to apply the optical element 1 Q disclosed in the present invention to a conventional optical pickup head. In this case, the dual-focus optical pickup includes: a laser diode 2 0 for emitting a laser beam for accessing the surface data of the optical disc 8 (CD or DVD); a three-beam Grating (Three Beam Grating) 7 0; An optical element 10; A Beam Splitter 3 0; A Collimator Lens 4 0; An Objective Lens 5 0; A Focusing lens 6 0 ; And an image detection component (Photo Detector) 6 1. In this embodiment, the optical element 10 may be in a form as shown in "Figure 1" or "Figure 5", and is directly set in a general two-beam grating 7 0 (or a similar optical element ) (See "Figure 7") and integrated with the three-beam grating 70. Similarly, the optical element 10 disclosed in the present invention can also be applied to a known holographic optical pickup head (such as “FIG. 8”), which includes: a laser diode 2 0 for emitting a laser beam for accessing the surface data of the optical disc 8 (CD or DVD);

Page 12 492001 V. Description of the invention [10] —— The optical element 10 may be in the form shown in "Figure 1 or Figure 5", which is arranged in the optical path of the laser light and is between the laser Between the photodiode 20 and the holographic optical element 80, the laser beam emitted by the laser diode 20 is divided into two light beams with different optical paths; a holographic optical element (HOE; Holograph ic optical el ement,) 8 0 y is used to separate the two reflected laser beams with different optical paths from the surface of the optical disc 8 (CD or DVD); a collimator lens (Collimator Lens) ^ 4 0, It is used to parallelly process the laser beam passing through the holographic optical element (Η0Ε) 80 into a parallel beam (an objective lens) 50, and to focus the parallel beam containing two laser beams with different optical paths on The data storage surface of the CD 8 2 or DVD 8 1 becomes an access light spot; and an image detection element (Photo Detector) 6 1 for The reflected light beam from the aforementioned holographic optical element (HOE) 80 is converted into an electric signal. In the embodiment of the "picture 9 of the factory", another example of an optical pickup with dual / parametric light sources is disclosed, in which An example is shown in which an optical element 10 and a two-beam grating 70 are combined into one (as shown in "Figure 7"), which is directly applied to an optical pickup with dual light sources. In this example, y is used. — A laser diode 'includes a first laser-pole body 2 〇a and a second laser diode 2 0 b' can generate light beams of different wavelengths 5 for accessing optical discs of different specifications. Sheet 8 ((: D or DVD), the first—iniMi shot | L Ι · 1 »| the laser beam emitted by the pole Pfjjnu body 2 〇a first passes through the optical element

492001

The combination of 10 and the two-beam grating 70 is sequentially focused on the data storage surface of the optical disc 8 through the collimating lens 40 and the objective lens 50 to form a light spot, and the second laser diode 2 is The other laser beam 'b' is first guided and guided by a Beam Splitter Cube 3 〇a, and then sequentially passes through the collimator lens 4 〇 and the objective lens 5 〇 to focus on the optical disc 8 The data storage surface becomes a light spot; the laser beam containing the data reflected from the data storage surface of the optical disc 8 is guided by a second beam splitter (Beam Spl it ter Cube) 30b and passes through a focusing lens 6 0 , And focus on the image detection element 6 1 in order to convert the optical signal containing the data into a corresponding electric signal. ~ In the example shown in "Figure 9", if the first laser diode 20a with a laser beam wavelength of 780nm is selected, the laser beam located in the range of the inner aperture will be used to access the CD series. The optical disc 8 is matched with the brother of the laser beam wavelength 650γιπ "—laser ^ one polar body 2 0 b, and the d ν D light stone leather piece is read, so it can be shaped as a kind of lens 5 0 (Objective Lens) Optical pickup for selective access to CD or DVD. If the first laser diode 20a uses a second harmonic laser (SHG Laser) with a laser beam wavelength of 820nm, then the CD-series optical disc is accessed by the laser beam within the aperture range. The second harmonic laser beam (410nm) including the inner and outer apertures is used to access high-density DVD disc data. Finally, please refer to "Figure 10". In the disclosed feasible embodiment, '疋 combines the optical element 1 0 with two beams of thumb 70 and the holographic optical element 80 0 into one set. The optical element 9 is an example of an optical pickup head directly applied to a dual light source. In this example

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κ uses f two laser diodes 髀 — L ^ 0 π, let, including the first laser diode 20a (with a wavelength of 780 nm (for accessing the data of ⑶) and the second laser diode Body 2 Ob (wavelength is 650nm, used to access DVD data), can generate laser beams with different wavelengths for accessing different specifications of optical disc 8 (CD or DVD) 〇 its superset 5 〇 and optical discs Then according to the phase hologram 6 1 the second laser spectroscope radiates through the quasi-storage table and returns the first optical element of the full two image inspection focused on the same optical optical element of 8, so that the light-pole (Beam's straight lens surface becomes the signal containing the image-type light output. The laser beam emitted by the laser diode 20a first passes through 90, and then passes through the collimator lens * 0 and the product disk 8 The data storage surface becomes a light spot, and the laser light reflected from the material storage surface returns to the laser light containing the data. The path is focused on the image detection element by the guidance of the collection optical element (90) included in the 80. ^ The optical signals with data are converted into corresponding telecommunications signals. The other laser beam emitted by the body 2 Ob is guided by the bow of a SPlltter Cube) 3 0a, and then focused on the coming-light spot according to 40 and the objective lens 50, and then from the optical disc 8 == Zibu The laser beam of the material is based on the same '丄: subsurface and the element _ 80a; the optical path, box 6 1 a, α will be the light containing the data and: focus on-chip machine number conversion Phase

[Effects of the invention] 1 · Adding other optical components (such as variable aperture lens or dual objective lens, etc.) can produce double focal length to access CD and DVD data. Mobile objects ’are used for

Page 15 492001 V. Description of the invention (13) and 2 • Fine structure ‘easy to manufacture’, low cost, easy assembly, high tolerance for assembly error. 3 · It can be applied to the current traditional optical read head, holographic optical read head, or dual-light source optical read head. [Illustration of Symbols in the Drawings] 0 · 0 a 13 0 13 1 2 0 · 2 0a 2 Ob 3 0 · 3 0a 3 Ob 4 0 · 5 0 · 6 0 · 6 1 · 6 1a 7 0 · Optical Element * Optical Element First Refraction Surface Second Refraction Surface Translucent Cylinder (Cylinder) Cylindrical concave laser diode (first laser diode) • First laser diode • Second laser diode beamsplitter (Beam Splitter Cube) • Beam Splitter Cube • Beam Splitter Cube Collimator Lens Objective Lens Focus Detector Photo Detector Photo Detector) Three Beam Grating

Page 16 492001 V. Description of the invention (14) 8 ....... Optical disc 80 ···· Holographic optical element 8 0 · · · · Holographic optical element

8 1 · · · · DVD

8 2 · · · CD 90 · · · · Collection optics page 17

Claims (1)

492001 VI. Scope of patent application1. A dual-focus optical pickup head that can access data on CDs and DVDs, including: A laser diode used to generate the laser required to access the CD or DVD An optical element for dividing the laser beam emitted by the aforementioned laser diode into two laser beams having different optical paths; a beam splitter cube (Beam Splitter Cube) for dividing the two laser beams having different optical paths The radiation beam is separated from the reflected beam from the data contained in the optical disc (CD or DVD); a collimating lens for parallel processing the laser beam passing through the optical element into a parallel beam; an objective lens for Focus the aforementioned flat beam containing two laser beams with different optical paths on the data storage surface of the CD or DVD as an access light spot; do not use it to collect the beam from the aforementioned CD or DVD, ... Into signals; and 2 The focal lens of the striker is used to focus the inverse residual beam from the aforementioned spectroscope on the image detection element. ::: The dual-focal-length optical pickup head described in item 1 of Liganwei, which has at least: two refracting surfaces and -second refracting surfaces, respectively, with the laser beam, and the first -refraction In the plane: the optical paths K of the light beams coincident with the laser light axis are identical along the laser light axis, and are distributed on the laser light axis # Page 18 492001 VI. Patent application scope Same radius. 5 6 As described in item 2 of Shenjing's patent scope, the optical element is a plate-shaped mask with a protruding columnar body, and the surface on the same side is the first fold, then the second refractive surface, where the The center is the center of the circle. The optical element is a flat mask with a cylindrical concave hole as described in item 2 of the scope of the patent application. The surface on the same side of the hole is the first end face and the second refractive surface. The center of the birefringent surface is the center of the circle. As described in item 1 of the scope of the patent application, the back surface of a three-beam grating is also included, and this type of bifocal optics includes at least a laser diode. The laser beam path between the straight lenses in the double-focus optical pickup head is divided into two optical optical elements having two: a refractive surface and a second fold. The center of the first refracting surface coincides with the bifocal optical pickup head described above, and its light transmitting body is on the surface of the light transmitting body among the light transmitting body and the columnar body. The first refractive surface on the axial end face folds around the second refractive surface. The bifocal-length optical pickup head includes a light transmitting body, a cylindrical concave refractive surface in the light transmitting body, a cylindrical concave refractive surface, and an axis of the cylindrical concave hole. It should be around it again. According to the dual-focus optical pickup head, the optical element is provided in the three-light three-beam grating and integrated. The optical element of the read head is a set body, a collimating lens and an objective lens, which are interposed between the laser diode and the collimator, and are used to shoot the laser diode with laser beams different from each other; The projection surface perpendicular to the axial direction of the laser beam, the aforementioned first refractive surface and the optical axis of the laser, the first refraction 492001 VI. Scope of patent application The optical paths of the light beams of the surface and the second refractive surface along the direction of the laser light axis are different, and they are distributed in different radius ranges of the laser light axis. 7 · The optical element of the optical pickup head with double focal length as described in item 6 of the patent application scope, wherein the optical element is a flat plate-shaped light-transmitting body, and has a protruding column shape on the surface of the light-transmitting body Body, the surface on the same side of the translucent body as the columnar body is the first refractive surface, and the axial end face of the columnar body is the second refractive surface, wherein the first refractive surface is in turn The center of the second refractive surface surrounds the center of the circle. 8 · The optical element of the dual-focus optical pickup head as described in item 7 of the patent application scope, wherein the radial cross-sectional shape of the columnar body is made according to the cross-sectional shape of the laser beam. … 9 · The optical element of the optical pickup head with double focal length as described in item 7 of the scope of patent application, wherein the columnar system is a round material with a cross-section of 1 0 · as in the sixth scope of the patent application scope An optical element, wherein the body 'is on the same surface as the cylindrical recessed hole in the surface of the translucent body, and the first refractive surface in the axial direction of the cylindrical recessed hole surrounds the first refractive surface. 1 1 · A bifocal optical pickup head described in the bifocal optical reading optical element is a plate-shaped light transmission with a circular laurel recess, the surface on the light transmission side is the first refractive end face, The center of the second refracting surface is surrounded by the center of the circle, which is a kind of holographic optical reading 492001 6. The scope of patent application can be used to access CD and DVD, including: a laser diode to generate the laser beam required to access the CD or DVD; an optical element, It is used to divide the laser beam emitted by the aforementioned laser diode into two beams with different optical paths; a holographic optical element is used to separate the aforementioned two laser beams with different optical paths from the disc (CD or CD) DVD) separates the reflected beams containing data on the surface; / / a collimator lens (Collimator Lens) for parallel processing of the laser beam passing through the holographic optical element into a parallel light and an objective lens for A parallel beam containing two thunder beams with different optical paths is focused on the data storage surface of the CD or DVD, and a light-access light spot; and becomes an image detection element for reflecting the reflection from the aforementioned full-image piece. The light beam is converted into a signal. The dual-focal light wind green head according to item 11 of the scope of patent application, wherein the optical element has at least: two first refracting surfaces perpendicular to the axial direction of the two beams, and a second refracting surface light The center of the first refraction surface and the second refraction surface coincide with the + W axis, and the first refraction surface and the second refraction surface have different optical paths in the light beam direction along the light emission direction and are distributed to each other. Yu: Different radius ranges of the two optical axes. ^ Giving light. Bifocal π as described in item 12 of the scope of patent application Page 21 492001 6. The scope of the patent application, wherein the optical element is a flat plate-shaped light-transmitting body, and a protruding columnar body is provided on the surface of the light-transmitting body. The surface on the same side of the body is the first refractive surface, and the axial end surface of the columnar body is the second refractive surface, wherein the first refractive surface surrounds the periphery with the center of the second refractive surface as a circle center. . 1 4 · The dual-focus optical pickup head as described in item 12 of the scope of patent application, wherein the optical element is a flat plate-shaped light-transmitting body, and a cylindrical concave hole is formed on the surface of the light-transmitting body. The surface of the translucent body on the same side as the cylindrical recessed hole is the first refractive surface, and the axial end face of the cylindrical recessed hole is the second refractive surface. The first refractive surface is in turn the first refractive surface. The center of the birefringent surface is surrounded by a circle center. Page 22