JP2009070423A - Objective lens and optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To supply an objective lens which suppresses the change of optical characteristics due to temperature change while suppressing manufacturing cost and is easily handled. <P>SOLUTION: The objective lens 4 is a compound lens comprising a biconvex lens 41 and a meniscus lens 42. The biconvex lens 41 is an injection-molded plastic aspheric lens. The meniscus lens 42 is a resin lens having negative refractive power, and is formed on a surface of a recording medium 8 side of the biconvex lens 41 by molding resin. The surface of the recording medium 8 side of the biconvex lens 41 is bonded to the surface of a half mirror of the meniscus lens 42. In addition, the temperature dependency of the refractive index of the meniscus lens 42 is larger than the temperature dependency of the refractive index of the biconvex lens 41. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an objective lens, and more particularly to an objective lens used in an optical pickup device. The present invention also relates to an optical pickup device using this objective lens.

  At present, as an objective lens used in an optical pickup device used in a reproducing / recording apparatus such as an optical disk, an aspheric lens that is advantageous for suppressing aberrations is often used. If this aspherical lens is formed of a glass mold lens (for example, Patent Document 1), it is difficult to process the glass, and the manufacturing cost of the lens increases. Further, since the lens is glass, its own weight is large, and it is difficult to increase the focusing speed of the optical pickup device. On the other hand, if an aspherical lens is formed using a plastic lens (for example, Patent Document 1), injection molding is possible, so that the manufacturing cost can be reduced and the weight of the lens can be reduced. However, the plastic lens has a large change in optical characteristics due to temperature change, which is a problem.

Therefore, as a technique for suppressing changes in optical characteristics accompanying changes in temperature of the plastic lens, a method using a plastic diffraction grating lens has been proposed (for example, Patent Document 2).
JP 2001-324673 A JP 2001-143301 A

  However, since the diffraction grating lens is difficult to mold, the manufacturing cost increases. In addition, the diffraction grating lens has a problem that the grooves forming the diffraction grating are apt to be crushed and are easily damaged when touched with a hand.

  The present invention was made to solve such problems, and an object thereof is to supply an objective lens that suppresses a change in optical characteristics due to a temperature change and is easy to handle while suppressing a manufacturing cost. To do.

  The objective lens according to the present invention is an objective lens including a biconvex lens and a meniscus lens, and the temperature dependence of the refractive index of the meniscus lens is larger than the temperature dependence of the refractive index of the biconvex lens, and the meniscus A lens is bonded to at least one surface of the biconvex lens.

  According to the above configuration, the objective lens according to the present invention includes the biconvex lens and the meniscus lens, and has a negative refractive power of the meniscus lens because the temperature dependency of the refractive index of the meniscus lens is larger than that of the biconvex lens. The surface can be designed to cancel out changes in the optical characteristics due to temperature changes, and deterioration of the optical characteristics due to temperature changes can be suppressed. Further, since the meniscus lens is bonded to at least one surface of the biconvex lens, it is not necessary to combine a plurality of separately formed lenses. Therefore, the optical axis alignment required when using a plurality of lenses in combination is unnecessary, which contributes to cost reduction.

  In the objective lens according to the present invention, the difference in thickness of one surface of the biconvex lens is smaller than the thickness difference of the other surface, and the meniscus lens is bonded to the one surface where the difference in thickness of the biconvex lens is small. preferable.

  According to the above configuration, the meniscus lens is joined to one surface of the biconvex lens having a small thickness difference, so that joining is easy and contributes to cost reduction. In addition, the magnitude | size of the thickness deviation in the same surface is shown with the distance of each point on the surface in the optical effective diameter, and the center surface of a lens within the same surface. Furthermore, let the largest difference of the magnitude | size of a thickness deviation be a thickness difference.

  In the objective lens according to the present invention, it is preferable that the surface on which the meniscus lens of the biconvex lens is cemented has an aspherical shape.

  According to the above configuration, the surface on which the meniscus lens of the biconvex lens is joined is aspherical, so that the design that cancels the change in the optical characteristic due to the temperature change can be facilitated, and the deterioration of the optical characteristic due to the temperature change can be further suppressed.

  In the objective lens according to the present invention, it is preferable that the meniscus lens has a negative refractive power.

  According to the above configuration, the meniscus lens can be designed to reduce the Petzval sum having a negative refractive power, so that the influence when the lens is tilted is reduced.

  In the objective lens according to the present invention, it is preferable that at least one of the biconvex lens and the meniscus lens is formed of a resin material.

  According to the above configuration, since at least one of the biconvex lens and the meniscus lens is formed of the resin material, the lens weight can be reduced as compared with the case of being formed of the glass material. Accordingly, it becomes easy to move the objective lens, and it becomes possible to increase the focusing speed when used in an optical pickup device or the like. Further, by forming the lens from a resin material, it is possible to manufacture an aspheric lens at a low cost.

  The objective lens according to the present invention is suitably used for an optical pickup device.

  According to the present invention, it is possible to supply an objective lens that is easy to handle while suppressing a change in optical characteristics due to a temperature change while suppressing a manufacturing cost.

  Hereinafter, an embodiment of an optical pickup device embodying the present invention will be described with reference to the drawings.

  As shown in FIG. 1, in the optical pickup device according to the present embodiment, the objective lens 4, the half mirror 3, the collimator lens 2, and the laser light generating means 1 are recorded on a CD or the like in order from the recording medium 8 side. In this structure, they are arranged on a straight line perpendicular to the signal recording surface 8 a of the medium 8. The laser light generated by the laser light generating means 1 passes through the collimator lens 2 and becomes parallel light, passes through the half mirror 3, is condensed by the objective lens 4, and is a signal recording surface 8 a inside the recording medium 8. Focused on top.

  The light condensed on the signal recording surface 8a is modulated and reflected by signal pits provided on the signal recording surface 8a. The reflected light again passes through the objective lens 4 to become parallel light, and is reflected by the half mirror 3 in a direction perpendicular to the incident direction. The reflected light is collected by the condenser lens 5, the optical axis is corrected by the optical element 6, and then detected by the photodetector 7, whereby the information recorded on the recording medium 8 is read.

  As shown in FIG. 2, the objective lens 4 is a compound lens composed of a biconvex lens 41 and a meniscus lens 42. The biconvex lens 41 is an injection molded plastic aspheric lens. The meniscus lens 42 is a resin lens having a negative refractive power, and is formed by molding a resin on the surface of the biconvex lens 41 on the recording medium 8 side. Therefore, the surface on the recording medium 8 side of the biconvex lens 41 and the surface on the half mirror 3 side of the meniscus lens 42 are joined. Here, since the objective lens for the optical pickup device needs to keep a certain distance between the lens end and the recording medium while shortening the focal length, the uneven thickness of the surface of the biconvex lens 41 on the half mirror 3 side. The difference is larger than the thickness difference of the surface of the biconvex lens 41 on the recording medium 8 side. Since the meniscus lens 42 is bonded to the surface of the biconvex lens 41 on the side of the recording medium 8 where the difference in thickness deviation is small, manufacturing is easy. In addition, the size of the thickness deviation in the same plane is indicated by the distance between each point on the surface within the same surface and within the optical effective diameter and the center plane of the lens, and the maximum difference in the size of the thickness deviation. Is the uneven thickness difference. For example, since the maximum value of the thickness deviation of the surface on the half mirror 3 side of the biconvex lens 41 is D1, and the minimum value is D2, the thickness deviation difference of the surface on the half mirror 3 side is represented by D1-D2. It is. Similarly, since the maximum value of the thickness deviation on the surface of the biconvex lens 41 on the recording medium 8 side is D3 and the minimum value is D4, the thickness deviation on the surface on the recording medium 8 side is D3-D4. Indicated. Further, the temperature dependence of the refractive index of the meniscus lens 42 is larger than the temperature dependence of the refractive index of the biconvex lens 41. Further, when the Abbe number of the biconvex lens 41 is ν1 and the Abbe number of the meniscus lens 42 is ν2, ν1−ν2> 15.

  Here, the influence of temperature in a general objective lens will be described with reference to FIG. Generally, the pickup has a focusing function for adjusting the distance between the recording medium and the objective lens. For this reason, there is little occurrence of defocusing, so-called defocusing, due to a temperature change, and the influence of the temperature change is a large change in wavefront aberration. Therefore, wavefront aberration will be examined. In the following description, the surfaces of the objective lens are referred to as a first surface and a second surface in order from the half mirror 3 side.

  As shown in FIG. 3, the light that enters the objective lens in parallel from the half mirror 3 side is refracted by the first surface having the positive refractive power of the objective lens. The refracted light is refracted again by the second surface of the objective lens and collected on the recording medium. When the temperature of the objective lens changes, the refractive power of the objective lens changes, and the refraction at the first surface and the refractive index at the second surface of the light incident from the half mirror 3 side change. Therefore, the angles and positions at which the light transmitted through the objective lens reaches the recording medium are different. This is the cause of wavefront aberrations due to temperature changes.

  Next, the influence of temperature in the objective lens 4 according to the present embodiment will be described with reference to FIG. Since both the biconvex lens 41 and the meniscus lens 42 are plastic lenses, the thermal linear expansion coefficient is larger and the temperature dependency of the refractive index is larger than that of glass. Consider the impact of both.

  First, the thermal expansion is examined. The thermal linear expansion described here is defined in ASTM (American Society for Testing and Materials) E831. A material having a large coefficient of thermal expansion has a large shape change due to temperature, and a material having a small coefficient of thermal linear expansion has a small shape change due to temperature. Plastics have a larger coefficient of thermal expansion than glass, and the thickness of plastic lenses varies greatly with temperature compared to glass lenses. For example, in a high temperature state, the lens thickness of both the biconvex lens 41 and the meniscus lens 42 is larger than that at normal temperature. Therefore, since the radius of curvature at the refractive surface is small, the positive refractive power is larger than that at normal temperature on the surface having a positive refractive power (convex surface), and the negative refractive power is negative at normal temperature on the surface having a negative refractive index. Become bigger. As shown in FIG. 2, the light incident from the half mirror 3 side is positively refracted by the first surface having a positive refractive power. This positive refraction is greater than at normal temperature. Next, the light transmitted through the biconvex lens 41 is negatively refracted on the second surface having negative refractive power as well as the interface with the meniscus lens 42. This negative refraction is greater than at normal temperature. Accordingly, a change in refractive power due to a high temperature state is partially offset. Here, when the thermal expansion coefficients of the biconvex lens 41 and the meniscus lens 42 are approximately the same, the objective lens 4 is a convex lens as a whole, and therefore the difference in thickness of the first surface of the biconvex lens 41 is the deviation of the second surface. Greater than meat differences. Therefore, the increase in the positive refractive power is larger than the increase in the negative refractive power, and the positive refractive power increases as the objective lens 4 as a whole. However, by making the thermal linear expansion coefficient of the meniscus lens 42 larger than the thermal linear expansion coefficient of the biconvex lens 41, the above-described canceling effect can be increased. Further, by the same effect, even when the temperature is low, the effect of canceling out the thermal linear expansion can be further increased by making the thermal linear expansion coefficient of the meniscus lens 42 larger than the thermal linear expansion coefficient of the biconvex lens 41.

Δｎ：屈折率の温度変化
ｎ ：２５℃での屈折率
Δｎ：屈折率の温度変化
ΔＴ：温度変化
Ｄ_０：屈折率の温度依存性
従って、屈折率の温度依存性Ｄ_０が高い材料は、温度変化ΔＴに対する屈折率の温度変化Δｎが大きいといえ、屈折率の温度依存性Ｄ_０が低い材料は、温度変化ΔＴに対する屈折率の温度変化Δｎが小さいといえる。 Next, the temperature dependence of the refractive index is examined. The temperature dependence of the refractive index is approximated by the following equation.

Δn: temperature change of refractive index n: refractive index at 25 ° C. Δn: temperature change of refractive index ΔT: temperature change D ₀ : temperature dependency of refractive index Therefore, a material having a high temperature dependency of refractive index D ₀ is It can be said that the temperature change Δn of the refractive index with respect to the temperature change ΔT is large, and the material having a low temperature dependence D ₀ of the refractive index has a small temperature change Δn of the refractive index with respect to the temperature change ΔT.

  Most materials have a low refractive index at high temperatures. The same applies to plastic materials. Accordingly, the refractive indices of the biconvex lens 41 and the meniscus lens 42, both of which are plastic lenses, are smaller than those at normal temperature. As a result, both the positive refractive power of the biconvex lens 41 and the negative refractive power of the meniscus lens 42 become small. Therefore, the positive refractive power is smaller on the surface having the positive refractive power than that at normal temperature, and the negative refractive index is negative on the surface having the negative refractive index. Refracting power is smaller than at normal temperature. The light incident from the half mirror 3 side is positively refracted by the first surface having a positive refractive power. This positive refraction is smaller than that at room temperature. Next, the light transmitted through the biconvex lens 41 is negatively refracted on the second surface having negative refractive power as well as the interface with the meniscus lens 42. This negative refraction is smaller than that at room temperature. Accordingly, a change in refractive power due to a high temperature state is partially offset. Here, when the temperature dependency of the refractive index of the biconvex lens 41 and the meniscus lens 42 is approximately the same, the objective lens 4 is a convex lens as a whole, and therefore the difference in thickness difference between the first surface of the biconvex lens 41 is second. Greater than uneven thickness of surface. Therefore, the decrease in the positive refractive power is larger than the decrease in the negative refractive power, and the positive refractive power of the objective lens 4 as a whole decreases. However, by making the temperature dependence of the refractive index of the meniscus lens 42 greater than the temperature dependence of the refractive index of the biconvex lens 41, the effect of the cancellation can be further increased. Further, the effect of canceling out the thermal linear expansion can be further increased by making the temperature dependence of the refractive index of the meniscus lens 42 greater than the temperature dependence of the refractive index of the biconvex lens 41 even at low temperatures.

  Therefore, by combining the biconvex lens 41 and the meniscus lens 42, the above-described thermal linear expansion and the temperature dependence of the refractive index can be suppressed. In addition, by making the thermal linear expansion coefficient of the meniscus lens 42 larger than the thermal linear expansion coefficient of the biconvex lens 41, the effect of suppressing the thermal linear expansion can be further increased. Furthermore, by making the temperature dependence of the refractive index of the meniscus lens 42 greater than the temperature dependence of the refractive index of the biconvex lens 41, the effect of suppressing the temperature dependence of refractive power can be further increased. Accordingly, it is possible to suppress changes in wavefront aberration due to the thermal linear expansion and the temperature dependence of the refractive index. In general, the effect of the temperature dependence of the refractive index has a greater influence on the optical characteristics of the lens than the effect of thermal linear expansion. Therefore, the temperature dependence of the refractive index of the meniscus lens 42 is changed to the refractive index of the biconvex lens 41. It is possible to suppress the change in wavefront aberration only by making it larger than the temperature dependence.

  According to the above embodiment, the following effects can be obtained.

  (1) In the above embodiment, the biconvex lens 41 and the meniscus lens 42 are combined, and the temperature dependence of the thermal linear expansion coefficient and refractive index of the meniscus lens 42 is greater than the temperature dependence of the thermal linear expansion coefficient and refractive index of the biconvex lens 41. Therefore, since the thermal linear expansion and the temperature dependence of the refractive power can be suppressed, a change in wavefront aberration can be suppressed.

  (2) In the embodiment described above, the meniscus lens 42 is joined to the surface of the biconvex lens 41 on the side of the recording medium 8 where the difference in thickness difference is small, so that the manufacture is easy and contributes to cost reduction.

  (3) In the optical pickup device according to the above-described embodiment, the surface on which the meniscus lens 42 of the biconvex lens 41 constituting the objective lens 4 is joined, that is, the second surface is aspherical. The design that cancels out becomes easy, and the deterioration of the optical characteristics due to the temperature change can be further suppressed. In particular, when the first surface is aspherical in order to suppress spherical aberration, the temperature dependence of the thermal linear expansion and the refractive index is complicated. It is effective to suppress this.

  (4) In the optical pickup device according to the above embodiment, since the meniscus lens 42 constituting the objective lens 4 has a negative refractive power, the Petzval sum is reduced. If the Petzval sum is small, it is easy to suppress curvature of field, and a design that suppresses wavefront aberration that occurs when the lens is tilted becomes possible.

  (5) In the above embodiment, since it is a compound lens in which the biconvex lens 41 and the meniscus lens 42 are cemented, the optical axis alignment required for the coupling lens in which the biconvex lens 41 and the meniscus lens 42 are separate bodies is unnecessary. It is easy to handle. Therefore, the cost can be reduced as compared with the coupling lens.

  (6) In the above embodiment, since the biconvex lens 41 and the meniscus lens 42 are plastic lenses formed of a resin material, the weight of the objective lens 4 is smaller than that of the glass lens. Therefore, it is easy to drive the lens, and the focusing speed can be increased as compared with the glass lens.

  (7) In the above embodiment, since the biconvex lens 41 and the meniscus lens 42 are plastic lenses formed of a resin material, it is possible to easily manufacture an aspheric lens by injection molding or mold molding, thereby reducing the cost. Contribute to

  (8) In the above embodiment, since the biconvex lens 41 is a convex lens and the meniscus lens 42 is a meniscus concave lens, it can be easily manufactured by general-purpose technology. In addition, it is easier to manufacture than the diffraction grating lens, and does not have the weakness that the grooves are easily crushed, and is easy to handle.

  (9) In the optical pickup device according to the above embodiment, when the Abbe number of the biconvex lens 41 constituting the objective lens 4 is ν1 and the Abbe number of the meniscus lens 42 is ν2, the relationship of ν1−ν2> 15 is established. Since the Abbe number ν2 of the meniscus lens 42 is smaller than the Abbe number ν1 of the biconvex lens 41, it becomes easy to follow the change due to focusing by the mode hopping of the laser. Further, since the difference between the Abbe number ν1 of the biconvex lens 41 and the Abbe number ν2 of the meniscus lens 42 exceeds 15, correction of color misregistration is facilitated.

  In addition, you may change this embodiment as follows.

  In the above embodiment, a CD is used as a recording medium, but other recording media such as a DVD may be used. In this case, the wavelength of the laser beam and the focus distance are also changed. Further, an optical pickup device capable of reproducing and recording various types of recording media may be configured by using the laser beam generating means 1 capable of changing the wavelength of the laser beam.

In the above embodiment, the meniscus lens 42 constituting the objective lens 4 has a negative refractive power, but may have a positive refractive power. If the surface in contact with the biconvex lens 41 has a concave shape, the thermal linear expansion and the temperature dependence of the refractive index can be suppressed, so that the change in wavefront aberration can be suppressed.

  In the above embodiment, the objective lens 4 is used for the optical pickup device, but may be used for other devices. For example, you may use for imaging devices, such as a camera. Occurrence of aberration due to temperature change can be suppressed.

  In the above embodiment, the meniscus lens 42 is bonded to the side surface of the recording medium 8 of the biconvex lens 41, but the meniscus lens 42 may be bonded to the side surface of the half mirror 3 of the biconvex lens 41. In other words, any configuration may be used as long as the change in the optical characteristics due to the temperature change of the meniscus lens 42 is offset by the change in the optical characteristics due to the temperature change of the biconvex lens 41.

  In the above embodiment, the biconvex lens 41 and the meniscus lens 42 are aspherical lenses, but at least one surface may be a spherical lens. In particular, if it is not necessary to use an aspheric lens, the manufacturing cost can be reduced by using a spherical lens.

In the above embodiment, the temperature dependence of the thermal linear expansion coefficient and the refractive index of the meniscus lens 42 is made larger than the temperature dependence of the thermal linear expansion coefficient and the refractive index of the biconvex lens 41, but the temperature of the refractive index of the meniscus lens 42 is increased. The dependence may be made larger than the temperature dependence of the refractive index of the biconvex lens 41. The effect of the temperature dependence of the refractive index on the optical characteristics of the lens is greater than the effect of thermal linear expansion, so the temperature dependence of the refractive index of the meniscus lens 42 is greater than the temperature dependence of the refractive index of the biconvex lens 41. It is possible to suppress changes in wavefront aberrations simply by doing.

The data of one Example which comprises the objective lens shown in FIG. 2 are shown below.
Biconvex lens 41
Material: Zeonex 340R manufactured by Nippon Zeon
Thickness: 1.66mm
Refractive index: 1.5094
Abbe number: 56.47
Linear thermal expansion coefficient: 9 × 10 ⁻⁵ cm / cm ° C.
Do: -0.000288

Meniscus lens 42
Material: Mitsubishi Rayon MP202
Thickness: 0.12mm
Refractive index: 1.5340
Abbe number: 41.15
Linear thermal expansion coefficient: 7 × 10 ⁻⁵ cm / cm ° C.
Do: -0.000501

Working distance: 0.319mm
Recording medium: Teijin AD5503 0.1 mm thickness Light source wavelength: 405 nm
Numerical aperture: 0.85nm
Focal length: about 1.3mm

  Table 1 shows the lens data. However, in Table 1, the number i of each surface is set in order toward the recording medium side with the surface on the half mirror side of the objective lens as the first surface. Ri represents the radius of curvature of each surface, and Di represents the surface spacing between the i-th surface and the i + 1-th surface.

The aspherical shape is represented by the following formula.

  In Equation (2), the optical axis direction is the z-axis, R is the radius of curvature, H is the height in the direction orthogonal to the optical axis, K is the conic constant, and A4, A6, A8, A10, A12, and A14 are respectively The fourth-order, sixth-order, eighth-order, tenth-order, twelfth-order, and fourteenth-order aspheric coefficients.

  The aspheric coefficients on each surface are as shown in Table 2 below.

  [Comparative Example 1]

As shown in FIG. 3, the objective lens according to Comparative Example 1 has a single plastic lens configuration. Numerical data of this objective lens is shown below.
Objective lens (biconvex lens)
Material: Zeonex 340R manufactured by Nippon Zeon
Thickness: 1.769mm
Refractive index: 1.5094
Abbe number: 56.47
Linear thermal expansion coefficient: 9 × 10 ⁻⁵ cm / cm ° C.
Do: -0.000288

Working distance: 0.312mm
Recording medium: Teijin AD5503 0.1 mm thickness Light source wavelength: 405 nm
Numerical aperture: 0.85nm
Focal length: about 1.3mm

Table 3 shows the lens data. However, in Table 3, the number i of each surface is set in order toward the recording medium side with the surface on the half mirror side of the objective lens as the first surface. Ri represents the radius of curvature of each surface, and Di represents the surface spacing between the i-th surface and the i + 1-th surface.

The aspherical shape is represented by the following formula.

  In Equation (2), the optical axis direction is the z-axis, R is the radius of curvature, H is the height in the direction orthogonal to the optical axis, K is the conic constant, and A4, A6, A8, A10, A12, and A14 are respectively The fourth-order, sixth-order, eighth-order, tenth-order, twelfth-order, and fourteenth-order aspheric coefficients.

The aspheric coefficients on each surface are as follows.

  [Comparison between Example 1 and Comparative Example 1]

  Changes in the optical characteristics of the objective lens according to Example 1 and the objective lens according to Comparative Example 1 due to temperature were compared by optical simulation based on the relationship between temperature and wavefront aberration. As shown in FIG. 4, in the objective lens according to Example 1, it can be seen that the wavefront aberration is 0.01λ (λ is the wavelength) or less over the region from 0 ° C to 60 ° C. On the other hand, in the objective lens according to Comparative Example 1, the wavefront aberration is 0.03λ or less over the region from about 20 ° C. to about 30 ° C., but in the other temperature regions, the wavefront aberration is larger than 0.03λ. It can also be seen that the wavefront aberration change accompanying the temperature change is large even in the region of about 20 ° C to about 30 ° C, and the stability of the wavefront aberration with respect to the temperature change is low. Therefore, the objective lens according to Example 1 has suppressed wavefront aberration due to temperature change as compared with the objective lens according to Comparative Example 1.

  Since the objective lens according to the present invention is an objective lens suitable for mounting on an optical pickup device, it can be widely used in an optical pickup device used in a CD / DVD reproducing / recording device.

It is a schematic diagram of the optical pick-up apparatus concerning embodiment. It is sectional drawing of the optical axis direction of the objective lens with which the optical pick-up apparatus concerning embodiment was equipped. It is sectional drawing of the optical axis direction of the objective lens concerning the comparative example 1. FIG. It is a figure which compares the change of the optical characteristic with the temperature of the objective lens concerning Example 1, and the objective lens concerning the comparative example 1, and is a graph which compares the wavefront aberration in each temperature.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Laser light production | generation means, 2 ... Collimator lens, 3 ... Half mirror, 4 ... Objective lens, 5 ... Condensing lens, 6 ... Optical element, 7 ... Light Detector: 8... Recording medium, 8a... Signal recording surface, 41... Biconvex lens, 42.

Claims (6)

In an objective lens composed of a biconvex lens and a meniscus lens,
An objective lens, wherein the temperature dependence of the refractive index of the meniscus lens is larger than the temperature dependence of the refractive index of the biconvex lens, and the meniscus lens is cemented to at least one surface of the biconvex lens. The thickness difference on one side of the biconvex lens is smaller than the thickness difference on the other side,
The objective lens according to claim 1, wherein the meniscus lens is joined to the one surface having a small thickness difference between the biconvex lenses.   The objective lens according to claim 1, wherein a surface of the biconvex lens on which the meniscus lens is cemented has an aspherical shape.   The objective lens according to claim 1, wherein the meniscus lens has a negative refractive power.   5. The objective lens according to claim 1, wherein at least one of the biconvex lens and the meniscus lens is made of a resin material.   An optical pickup device comprising the objective lens according to claim 1.

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