JPH05127078A - Objective for optical pickup - Google Patents

Abstract

PURPOSE:To provide the lightweight, inexpensive objective for the optical pickup which has its chromatic aberration excellently compensated. CONSTITUTION:This objective converges laser luminous flux from a semiconductor laser on the recording surface of an optical disk and is constituted by cementing three lenses from the light source side to the recording surface side. The 1st lens 1 is an extremely thin plastic lens which has an aspherical surface on its recording-surface side, the 2nd lens 2 cemented to the recording-surface side of the 1st lens 1 is a biconvex lens made of plastic, and the 3rd lens 3 cemented to the recording-surface side of the 2nd lens 2 is a glass-made meniscus lens which has a convex surface on the recording surface side; and the 2nd and 3rd lenses are spherical surface lenses. Then inequalities I and II of 0.4<¦f/r3¦<1.6 and nu2-nu3>10 are satisfied, where (f) is the focal length of the whole system, r3 the radius of curvature of the 3rd lens surface counted from the light source side, and nu2 and nu3 the Abbe numbers of the 2nd and 3rd lenses.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens for an optical pickup.

[0002]

2. Description of the Related Art Optical pickups for recording and / or reproducing information on the recording surface of an optical disc have been widely known. The objective lens of such an optical pickup is a lens for condensing the laser light flux from the light source as a spot on the recording surface.

As is well known, the semiconductor laser generally used as the light source of the optical pickup as described above has "wavelength jump" due to mode hopping. If such a wavelength jump occurs when the objective lens is not corrected for chromatic aberration, the control speed of the focusing control cannot follow the defocus due to the wavelength change, and high-speed information recording /
Reproduction becomes difficult. Therefore, it is desirable that the objective lens of the optical pickup has chromatic aberration well corrected.

Conventionally, an objective lens having three or more constituent lenses is known as an objective lens in which chromatic aberration is satisfactorily corrected (JP-A-3-2811). However, this lens has a problem that the weight of the objective lens increases according to the number of lens components, and the seek time for the optical disc tends to be delayed.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and is relatively lightweight for an optical pickup, capable of favorably correcting chromatic aberration, and capable of providing a good spot on a recording surface. It is an object of the present invention to provide a novel objective lens capable of forming a lens.

[0006]

The objective lens of the present invention is an "objective lens for converging a laser beam from a semiconductor laser on an optical disc recording surface in an optical pickup", as shown in FIG. The three lenses are cemented from the light source side (left side in FIG. 1) toward the recording surface side (right side in FIG. 1).

The first lens 1 arranged closest to the light source
Is an ultrathin plastic lens whose lens surface on the light source side is formed as an aspherical surface. The second lens 2 cemented to the recording surface side of the first lens 1 is a biconvex lens made of glass, and the third lens 3 cemented to the recording surface side of the second lens 2 is a meniscus made of glass with its convex surface facing the recording surface side. The second lens 3 and the third lens 3 are spherical lenses.

The focal length of the entire system is f, the radius of curvature of the third lens surface (the cemented surface of the second lens 2 and the third lens 3) counting from the light source side is r ₃ , and the second and third lenses are When the Abbe numbers are ν ₂ and ν ₃ , respectively, these satisfy the condition (1) 0.4 <| f / r ₃ | <1.6 (2) ν ₂ −ν ₃ > 10.

[0009]

The condition (1) is a condition for the workability of the objective lens and the wavefront aberration. When the upper limit of this condition (1) is exceeded, the radius of curvature of the cemented surface between the second lens and the third lens 3 becomes too small, making it difficult to process the second and third lenses. On the other hand, when the value goes below the lower limit, the off-axis wavefront aberration deteriorates.

The condition (2) is a condition for correcting the axial chromatic aberration. If the lower limit of condition (2) is exceeded, the second, third
The difference in Abbe number of the lens becomes too small, and it becomes difficult to favorably correct the axial chromatic aberration.

Further, in the objective lens of the present invention, the first lens 1 is an extremely thin plastic lens, and an aspherical surface is adopted on the side surface of the light source in order to satisfactorily correct the wavefront aberration.

In producing the objective lens of the present invention, first, the second lens 2 and the third lens 3 are produced, these are cemented, and a predetermined light source side lens surface of the second lens 2 is formed. The first lens may be formed by forming a plastic layer having a thickness of 5 mm and forming an aspherical surface on the lens surface of the light source side of the plastic layer by embossing. Alternatively, after the first lens is bonded and formed on the light source side lens surface of the second lens 2 by the above method,
The second lens and the third lens may be cemented.

[0013]

[Examples] Four specific examples will be given below. As shown in FIG. 1, the radius of curvature of the i-th lens surface counted from the light source side is r _i (i = 1 to 4), and the optical axis of the i-th lens surface and the i + 1-th lens surface Let the upper surface spacing be d _i (i
= 1 to 3), and the refractive index and the Abbe number of the material of the j-th lens are respectively expressed by n _j and ν _j (j = 1 to 3). f is the composite focal length of the entire system, N.F. A is the numerical aperture, W. D
Represents the working distance.

As is well known, the "aspherical surface" has a radius of curvature on the optical axis of r, a height from the optical axis of H, and a deviation amount from a reference spherical surface (a spherical surface having a radius of curvature r) of X and K. Cone constant, fourth order,
The 6th, 8th, and 10th aspherical coefficients are A, B, and
When C and D, X = [(1 / r) H ² / {1 + √ [Y]}] + A · H ⁴ + B · H ⁶ + C · H ⁸ + D · H ¹⁰ , Y = 1− (1 + K ) (1 / r) ² H ² is a curved surface. Symbols in this formula: √
[Y] represents the square root of Y.

For the aspherical surface of the first lens surface, the conical constant: K and the higher-order aspherical surface coefficients: A, B, C, D are given,
Identify the shape. In addition, [E in the display of high-order aspherical coefficients
-Number] represents a power. For example, if the [E-10], which means the 1/10 ^10], is that this number is multiplied to the number in front thereof.

Further, in each of the embodiments, the cover (transparent layer for protecting the recording surface) of the optical disk, which is designated by reference numeral 4 in FIG. 1, is considered. The thickness of the cover 4 is dc in each of the examples.
= 1.2100 mm, Refractive index: nc = 1.51633
(Value at d line), Abbe number: νc = 64.1. The material of the first lens is PMM in each of the examples.
It is A. Further, in each of the embodiments, the object point is infinity.

Example 1 f = 3.0, N.V. A = 0.53, W.A. D = 1.150 i r _i d _i j n _j ν _j 1 2.5685 0.1000 1 1.5860 30.3 2 2.5685 1.5171 2 1.7550 52.3 3 -2.4417 0. 6514 3 1.9229 20.9 4 -6.5124.

Aspherical first lens surface K = -0.79108 A = -0.11770E-3, B = -0.16472E
-3 C = -0.96833E-4, D = 0.54783E
-4 condition parameter values _{| f / r 3 | = 1.23} , ν 2 -ν 3 = 31.4 wavefront aberration 0.0044Ramuda (angle: 0 °), 0.038λ (angle:
Once).

Example 2 f = 3.0, N.V. A = 0.53, W.A. D = 1.167 i r _i d _i j n _j ν _j 1 2.4656 0.1000 1 1.5860 30.3 2 2.4656 1.7065 2 1.6425 57.9 3-2.0000 0. 7133 3 1.9229 20.9 4 -3.4582.

Aspherical first lens surface K = -0.89260 A = -0.14919E-2, B = 0.10602E
-2 C = -0.10489E-2, D = 0.24254E
-3 condition parameter values _{| f / r 3 | = 1.50} , ν 2 -ν 3 = 37.0 wavefront aberration 0.019Ramuda (angle: 0 °), 0.043λ (angle: 1
Every time) .

Example 3 f = 3.0, N.V. A = 0.53, W.A. _{D = 1.150 i r i d i} j n j ν j 1 2.1627 0.1000 1 1.5860 30.3 2 2.1627 1.3163 2 1.6425 57.9 3 -7.2000 0. 6937 3 1.9229 20.9 4 -8.6482.

Aspherical first lens surface K = -0.79062 A = 0.78728E-3, B = -0.27036E
-3 C = -0.89789E-4, D = -0.10486E
-4 Parameter value of conditional expression | f / r ₃ | = 0.42, ν ₂ −ν ₃ = 37.0 Wavefront aberration 0.004λ (angle of view: 0 degree), 0.045λ (angle of view: 1)
Every time) .

Example 4 f = 3.0, N.V. A = 0.53, W.A. D = 1.150 i r _i d _i j j n _j v _j 1 2.7077 0.1000 1 1.5860 30.3 2 2.7077 1.4987 2 1.9225 35.9 3 -2.2428 0. 4700 3 2.0029 23.5 4 -27.3819.

Aspherical first lens surface K = -0.79577 A = 0.96076E-3, B = 0.22033E
-4 C = -0.89625E-4, D = 0.77097E
-4 Parameter value of conditional expression | f / r ₃ | = 1.34, ν ₂ −ν ₃ = 12.4 Wavefront aberration 0.010λ (angle of view: 0 degree), 0.051λ (angle of view: 1
Every time) .

Aberration diagrams relating to Examples 1 to 4 are sequentially shown in FIGS. The chromatic aberration is well corrected in each of the examples.

[0026]

As described above, according to the present invention, a novel objective lens for an optical pickup can be provided. In this objective lens, since the first lens is made of a light plastic material and is extremely thin, the substantial mass is the mass of two lenses by the second and third lenses, and the weight is light as a whole, so the optical pickup It doesn't slow down the seek time. Also, since chromatic aberration is corrected well,
It is possible to effectively avoid defocusing due to mode hopping of the semiconductor laser that is the light source. Further, since the wavefront aberration can be effectively corrected by using the aspherical surface for the first lens surface, it is possible to realize a good spot by making the light intensity distribution of the spot into a simple and clean shape.

Further, since the second and third lenses are spherical lenses, they can be easily manufactured at low cost, and the aspherical surface of the first lens surface of the first lens can also be easily formed by embossing, so that it can be realized at low cost. ..

Although the plastic lens is susceptible to deformation under the influence of temperature and humidity, the plastic lens which is the first lens of the present invention is extremely thin, so the above deformation does not substantially affect the lens performance.

[Brief description of drawings]

FIG. 1 is a diagram showing a lens configuration of an objective lens of the present invention.

FIG. 2 is an aberration diagram for Example 1.

FIG. 3 is an aberration diagram for Example 2.

FIG. 4 is an aberration diagram for Example 3.

FIG. 5 is an aberration diagram for Example 4.

[Explanation of symbols]

 1 1st lens 2 2nd lens 3 3rd lens 4 Optical disk cover

Claims (1)

[Claims] 1. An objective lens for converging a laser beam of a semiconductor laser as a spot on a recording surface of an optical disc in an optical pickup, wherein three lenses are cemented from a light source side toward a recording surface side. The first lens is a very thin plastic lens whose light source side surface is aspherical, and the second lens joined to the recording surface side of this first lens is a biconvex lens made of glass. The recording surface side of this second lens. The third lens cemented to is a meniscus lens made of glass with the convex surface facing the recording surface side.
The lens is a spherical lens, the focal length of the entire system is f, the radius of curvature of the third lens surface counted from the light source side is r ₃ , and the Abbe numbers of the second and third lenses are ν ₂ and ν ₃ , respectively. When these are satisfied, these satisfy the condition (1) 0.4 <| f / r ₃ | <1.6 (2) ν ₂ −ν ₃ > 10.