JPS6214109A - Objective lens for recording/reproducing optical information recording medium - Google Patents

Abstract

PURPOSE:To correct the spherical aberration of an objective lens by constituting the objective lens by joining a transparent material having aspherical surfaces with the light source side surface of a glass lens having projected spherical surfaces on its both sides and satisfying the objective lens with specific conditions. CONSTITUTION:The objective lens is constituted by joining the transparent material having the aspherical surfaces with the light source side of the glass lens having the projected spherical surfaces on both sides and satisfied with the conditions of (1) -5.0<r3/f<-1.0... and (2) 1.6<n2...; provided that (f) is the composite focal distance of the whole system, r3 is the radius of curvature on the opposite side surface of the spherical glass lens to the light source and n2 is the refractive index of the spherical glass lens and it is preferable that the lens is satisfied with the condition of the shown formulas; provided that NA is an numerical aperture on the image side, DELTA1 is an optical direction difference between the aspherical surface on the most peripheral side of the effective diameter on the surface closest to the optical source side and a reference spherical surface having r1 radius of curvature of a vortex and becomes a positive value when said aspherical surface is displaced to the optical source side in accordance with the separating distance from the optical axis and d2 is the thickness of the spherical lens.

Description

[Detailed Description of the Invention] Purpose of the Invention (Field of Industrial Application) The present invention is an objective lens for optical disks, which is composed of a single lens suitable for use in particular when the distance between the light source and the information recording surface is relatively small. This invention relates to an objective lens.

(Prior art) An optical system used in recording/reproducing devices for information recording media such as optical disks, which has become common in recent years, is one that converts light emitted from a light source 4 through a collimator lens 3, as shown in FIG. The parallel light is made into parallel light and focused on the information recording surface 1 by the objective lens 2. In this optical system, focusing is performed by moving the objective lens 2 in the optical axis direction in response to surface wobbling of an optical disc or the like. This method has the advantage that the performance of the optical system remains unchanged even if the objective lens 2 is moved, but on the other hand, the optical system becomes expensive because it requires two lenses: the objective lens 2 and the collimator lens 3. There's a problem.

On the other hand, as shown in FIG. 10, a method is also known in which the light from the light source 4 is directly focused onto the information recording surface 1 using the objective lens 2 without using a collimator lens. With this method, focusing is performed by moving only the objective lens, but since the numerical aperture and performance of the objective lens change due to movement, it is not possible to increase the imaging magnification very much, and the standard imaging magnification is - It was about 1/40 to -1/8.

In recent years, in optical systems for compact disc playback, (1) The optical system must be made more compact.

(2) Due to the improvement in the quality of compact discs, there is no longer a practical problem even if the focusable range is narrow.

For these reasons, after examining the optical system, it has become clear that the optical system shown in FIG. 10 can be used at a standard imaging magnification of about -178 to -1/4.

In this optical system, if the objective lens 2 is composed of two lenses, it takes a lot of man-hours to assemble and adjust the lenses.

On the contrary, it would be cheaper to construct the optical system shown in FIG. 9 by constructing each of the objective lens 2 and collimator lens 3 by a single lens, so they must be constructed by a single lens.

Various types of objective lenses for recording and reproducing optical information recording media having a single lens structure have been proposed.

Among them, the most common ones are those that utilize aspherical surfaces, and plastic lenses with aspherical surfaces on both sides and single lenses in which a transparent material having an aspherical surface is bonded onto a spherical glass lens have been put into practical use. (MICRo 0PTIC3
NEWS Vol 3. No, 1. p, 20 and p,
15) These lenses were developed as objective lenses for the optical system shown in Figure 9, but it is practically advantageous to manufacture the objective lenses for the optical system shown in Figure 10 using a similar manufacturing method. be. Among these, plastic lenses with one surface aspherical have the advantage of being inexpensive, but have drawbacks such as low heat resistance, high birefringence, and performance changes depending on temperature and humidity. On the other hand, a single lens in which a transparent material having an aspherical surface is bonded onto a spherical glass lens has a manufacturing cost of 1~ higher than that of a plastic single lens, but the above-mentioned drawbacks can be overcome if the transparent material is appropriately selected.

JP-A-59-12412 describes a single lens in which a transparent material having an aspherical surface is bonded onto a spherical glass lens, but this is for the optical system shown in Fig. 9.
This type of objective lens for use in the optical system shown in FIG. 10 cannot be found.

(Problems to be solved by this invention) This invention is the first
This objective lens has a large imaging magnification and is suitable for use in the optical system shown in FIG.

Structure of the Invention (Means for Solving Problems) In this invention, as shown in FIG. 1, the objective lens is constructed by bonding a transparent material having an aspherical surface to the light source side of a biconvex spherical glass lens. Single lens: 5,0<ra/f<-1,0...(1)1.6<
n, ・ ・ ・ It is characterized by satisfying the condition (2).

However, f: Combined focal length of the entire system r3 Radius of curvature of the surface of the 4-spherical glass lens opposite to the light source n2: Refractive index of the spherical glass lens Furthermore, this lens is (), 15<44-"<0.45-・(3)(NA)
f 0.6 spider... (4) (n2-1)
It is desirable to satisfy the condition f.

However, NA: image-side numerical aperture Δ;: aspherical surface and apex radius of curvature r at the most peripheral effective diameter of the surface closest to the light source (position tiI on the surface on the light source side where the peripheral rays of the above NA enter) The difference in the optical axis direction from the reference spherical surface is defined as positive if the aspherical surface is displaced toward the light source as the distance from the optical axis increases.

d2 Effect of Thickening of Bispherical Lens) In the shell lens of the present invention, it is necessary to convert diverging light with a large numerical aperture into convergent light. At this time, the spherical aberration generated at the articulating surface is small, but a large spherical aberration occurs at the surface of the spherical glass lens opposite to the light source, so it is desirable to reduce this as much as possible. Condition (1) is for this purpose. In the lens of this invention,
Since the surface on the light source side is aspheric, correction of spherical aberration is possible in principle, but if the radius of curvature r of the surface of the spherical glass lens on the opposite side from the light source falls outside the range of condition (1). ,
In order to correct spherical aberration, it is necessary to increase the amount of aspherical surface. For aspherical lenses made by normal molding, if the mold can be processed, the amount of aspherical surface does not matter when molding.
However, in the case of a cemented lens such as the one of this invention, when the amount of aspherical surface is large, and when the linear expansion coefficient of the transparent material forming the aspherical surface and the linear expansion coefficient of the glass material of the spherical lens are significantly different, the use environment of the lens may be affected. As , the aspheric surface deforms and the performance changes, which is a fatal flaw. Therefore, as the transparent material forming the aspherical surface, it is not possible to use a material such as a plastic material whose expansion coefficient is significantly different from that of glass, which is a significant disadvantage in manufacturing.

Condition (2) relates to correction of the sine condition. If the lower limit is exceeded, the swell in the intermediate annular zone under the sine condition increases, and off-axis performance deteriorates. When tracking is performed by moving the objective lens parallel to the information recording surface of the optical information recording medium, the optical system shown in FIG. 9 always uses an axial light beam, so in principle there is no need to correct off-axis aberrations. However, in the optical system shown in FIG. 10, tracking causes the light source to deviate from the optical axis of the objective lens, so off-axis performance must also be corrected. Outside the range of this condition, practical problems occur when the imaging magnification is large.

Condition (3) is a condition for correcting spherical aberration based on condition (1). As is clear from aberration theory, third-order spherical aberration is considered as wavefront aberration.

It is proportional to the fourth power of the aperture. Therefore, the aspherical amount is 4 of the numerical aperture.
It is necessary to normalize by multiplying. The higher the refractive index n2 of the spherical glass lens, the greater the amount of asphericity for correcting spherical aberration. The aspheric amount for the peripheral ray regarding the axial object point on the light source side is normalized by 1/(n, -1)', (NA)', f. The larger the value, the greater the effect of overcoming spherical aberration. When the spherical aberration becomes large beyond the upper limit, the spherical aberration becomes over, and when it becomes small beyond the lower limit, the spherical aberration becomes under.

Condition (4) is a condition regarding astigmatism. As mentioned above, in the optical system shown in FIG.

Spherical aberration because off-axis aberrations need to be corrected.

In addition to the sine condition, it is also desirable to properly correct astigmatism. When the lower limit is exceeded, astigmatism becomes significantly undervalued, and even if the sine condition is well corrected, off-axis wavefront aberration worsens.

(Example) The symbols in Table 0 showing examples of the present invention are as follows: riH Vertex curvature radius di of the first lens surface from the light source side: Distance ni of the first lens surface from the light source side: The first lens surface distance from the light source side Refractive index of the first lens material 1: Indicates the Atsube number of the first lens material from the light source side, and the aspherical shape is a rectangular coordinate with the apex of the surface as the origin and the optical axis direction as the X axis. In the system, when the apex curvature is C1, the conic constant is K, the aspherical coefficient is A + , and the power of the aspherical surface is Pi (Pi>2.0), it is expressed as φ;noy +z.

At this time, the aspherical amount Δ is Δ−XSP−XAS, where the height of the peripheral ray on the surface on the light source side is H, where C, = 1/r.

In addition, the value of face plate G is also shown in the table.

Actual example 1 f = I NA O, 453 Magnification - 1/4.51
r+ di ni
νi1 1.19335 0.0278
1.48595 55.0 Aspheric coefficient/power number 1st surface K Knee 1.460760+OO A, =-1,665930-01P, =4.0000A
2=-1,22055D-01P,=6.0OOOA,
=-1,24128D-01P, =8.0O00A4=
1.29532D-02P4=lO,0000)1□,
0.548 Example 2 f=l NA 0.451 times, rate -1
153-2, 109600, 6042 Aspheric coefficient/power number 1st surface K = -1,475920÷00 A□=-1,58541D-01P1=4.0000A
2=-1,39464D-01P2:6.0000A,
, -9,98946D-02P, =8.0000A, =
-2,23087D-03P4=10.0O0011□
= 0.535 Example 3 f=I NA 0.451 Magnification -175i
rI di nl
V iI L15412 0.014
7 1.48595 55.02 0.9778
0 0.8823 1.74411 27.53
-1.53008 0.6221 Aspheric coefficient/power number 1st surface K = -1,562840+OO A, =-2,020880-01P = 4.0000A
,=-2,23316D-01P,=6.0OOOA3
．． -1.563921) -01P, =8.0000A4
=-3,960880-02P4=10.0OO011
□=0.530 Example 4 f=INA 0.451 Magnification -178i
ri di old V i
l 1.15032 0.0062 1.48
595 55.02 0.86702 0,900
0 1.74411 27.53 -1.6639
8 0.5583 Aspheric coefficient/power number 1st surface K = -1,44436D+OO 80=-1,76685D-01P=4.0000A
, = -2,28214D-01P, = 6.0000
A, =-1,934990-01P, =8.0O00A
4=-7,17142D-02P4=10.0O00H
1=G, 5QO Example 5 f”l NA O, 451 Magnification -178i
rI dint
ν11 1.19030 0.00
62 1.57300 30.02 0.
76849 0,9000 1.74411
27.53 -1.59430 0.5665
Aspheric coefficient/power number 1st surface K = -1,370510+00 Ai=-1,608850-01Pl=4.0000A
,=-2,13539D-01P=6.0000A,
=-1,74260D-01P, =8.0000A4=
-1,77711D-01P4=10.0OOOH,=
0.500 Example 6 f=I NA O, 451 magnification -178i
ri di ni
Ya11 0.99945 0.0062 1
．． 48595 55.0j O,821690,8
2491,6498850,53-1,353290,
5868 Aspherical coefficient/power number 1st surface H, = 0.501 ``Muni Eno/distribution - 0.200 (NA)' f (n, -1)f'' 1°269 Example 7 f=I NA O,452 Magnification -178 Aspheric coefficient/power number 1st surface A, "-4,159120-Of P4"1
0.0000+11=0.504 1 to epi-rainbow・0.325 (NA)' f Effects of the Invention The objective lens of this invention has an imaging magnification as shown in the various aberration diagrams in FIGS. 2 to 8. is larger than the conventional one, spherical aberration including the cover glass G is almost completely corrected, and the sine condition is also good.

This lens made it possible to create an optical system for optical discs in the simplest form, making it possible to achieve a significant improvement.
In optical systems for optical discs, an optical element such as a polarizing beam splinter is often arranged on the light source side of the objective lens, but this can be accommodated by making some design changes to the above embodiments.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the configuration of one embodiment of the objective lens of the present invention, and FIGS. Aberration curve diagrams of Examples 1 to 7, FIG. 9 is a layout diagram of an optical disk optical system using a conventional collimator lens, and FIG. 10 is a layout diagram of an optical system using an objective lens of the present invention. 1: Optical disk (optical information recording medium) 2: Objective lens 3: Collimator lens 4: Light source Patent applicant Konishi Roku Photo Industry Co., Ltd. Applicant agent Patent attorney Fumio Sato and 2 others @1 Figure 2 Spherical aberration Sine condition Astigmatism Figure 3 Spherical Aberration Sine Condition Astigmatism Figure 4 Spherical Aberration Sine Condition Astigmatism Figure 5 Spherical Aberration Sine Condition Astigmatism Figure 6 Spherical Aberration Sine Condition Astigmatism Figure 7 - 0,00200 ,002-0,0200112-0,
00200,002 Spherical aberration sine condition
Astigmatism tube 8 Figure

Claims (1)

[Claims] A single lens constructed by bonding a transparent material having an aspherical surface to the light source side of a biconvex spherical glass lens, with -5.0<r_3/f<-1.0 1.6<n_2. Objective lens for recording and reproducing optical information recording media characterized by satisfying the following conditions: f: composite focal length of the entire system r_5: radius of curvature of the surface of the spherical glass lens opposite to the light source n_2: of the spherical glass lens refractive index