JPS6267740A - Light converging optical system for recording and reproducing optical system of optical information storage medium - Google Patents

Abstract

PURPOSE:To reduce the image forming magnification by constituting the titled system by the 1st lens being a spherical signal lens having a specific condition and with the 2nd lens being an objective single lens. CONSTITUTION:The light converging optical system is constituted of the 1st lens being a spherical lens the 2nd lens being a single objective lens in order from the light source side, and the 2nd lens itself has the least wave aberration on the axis when the image forming magnification is m0' and the diffraction limit performance is provided. Let's the image forming magnification of the 2nd lens in the optical system comprising the 1st lens and the 2nd lens be m0, the refractive index and arrangement of the 1st lens are selected to satisfy the relation of 0.002<m0-m0'<0.12. Further, a magnification m0' satisfies the relation of -0.1<m0'<0.15 and -0.7<r2<n1f1<-0.4, where r2 is a radius of curvature of the face opposite to the light source of the 1st lens, n1 is a refractive index and f1 is the focus. Thus, the image forming magnification is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial Field of Application) The present invention relates to a condensing optical system for recording and reproducing optical information that has a large numerical aperture on the light source side and has a simple configuration.

(Prior art) Recording/reproducing device for optical information recording media such as optical disks
The most common condensing optical system used is as shown in FIG. Let's make friends. In this optical system, focusing is performed by moving the objective lens 2 in the optical axis direction in response to a focal shift caused by surface wobbling of the optical disc or the like.

A typical objective lens in a compact disc playback optical system has a focal length of 4.5 W.

It has a NA of about 0.45 and has a three-element structure in two groups, as typified by Japanese Patent Laid-Open No. 55-4068. On the other hand, the collimator lens has a focal length of 1.7w% and a NA of 0.1.
A typical example is a 4 in 1 detailed 2-sheet configuration. The imaging magnification of these composite systems is as small as -0.2647.

In recent years, we have succeeded in making an objective lens into an aperture lens using an aspherical surface, and it has begun to be widely used (Optical Technology) Contact VO1, 23, 7 P.

465~), this has significantly reduced the cost of objective lenses, but in order to further reduce costs, there has been a demand for the collimator to be an exposure lens.

(The rough point that this invention attempts to solve) Since the collimator lens is made into a single lens with the above specifications, the refractive index is 1.8.
The 0-axis wavefront aberration is 0.077λnm5 (λ=7
8 On1ll), which is acceptable to Marechal, and the optical performance is insufficient.

It is known that the collimator is made into an exposure lens by making the focal length of the collimator long < LNA small, but in this case, a high output light source is required to compensate for the decrease in the utilization efficiency of the light source light. This invention aims to obtain a condensing optical system for recording and reproducing optical information with a small imaging magnification and a large numerical aperture on the light source side with a simple configuration consisting of a wide-lens objective lens and one spherical lens. It is.

Structure of the Invention (Means for Solving the Problem) The condensing optical system of the present invention includes, from the light source side, a first lens that is a spherical lens, and a second lens that is an objective lens. The imaging magnification is =. When , the axial wavefront aberration is the smallest and diffraction limited li'! ft exists, and when the imaging magnification of the second lens in the composite optical system of the first lens and the second lens is mo, o, oo2<mo-m, <o, x2 -...-(1) The refractive power and arrangement of the first lens are selected so as to satisfy -0,7 (r2/n
, 1. (-g, 4...It is desirable to satisfy (3).

(Function) As is well known, a condensing optical system for a recording/reproducing optical system of an optical information medium needs to have diffraction-limited performance. That is, the spherical aberration of the light beam emitted from the axial object point must be sufficiently corrected. In conventional optical systems, each of the collimator and objective lens has diffraction-limited performance.

On the other hand, in the optical system of the present invention, the spherical aberration is undersized when the first lens is used alone, and excessive spherical aberration is generated in the second lens, so that it is necessary to properly correct the spherical aberration in the entire optical system. be.

The usual design method when considering such a condensing system is that when the second lens is an aspherical lens, l! is automatically designed using the aspherical coefficient of the second lens as a variable. It is possible to perform k11 conversion. In this case, since the second lens alone has aberrations, it is necessary to directly measure the aspherical surface or measure the wavefront aberration and investigate its aberration components. Not suitable.

The currently reprinted aspheric objective lens has diffraction-limited performance for parallel incident light]7 (result is magnification richness = 0).
. When such an objective lens is used with diverging or converging light, the amount of change in spherical aberration is determined by the amount of change in spherical aberration, including third-order spherical aberration, Ike's aberration (coma aberration, astigmatism, Perapart sum, distortion aberration, and pupil spherical aberration). It depends on the state of correction of aberrations), and it is not possible to immediately estimate the amount of aberrations. Focal length 4.5fi with aberration correction for parallel incident light, NAo, 45
Figure 5 shows the relationship between changes in imaging magnification m0 and changes in wavefront aberration for the double-sided aspherical lens.This lens has excessive spherical aberration for convergent light.
Marechal tolerance 1m (0.07λ) at imaging magnification 003
It can be seen that excessive spherical aberration is generated.

In order to use the second objective lens for convergent light incidence, it is necessary to convert the light beam from the light source into a convergent light beam using the first lens. The amount of spherical aberration is larger when the diverging light is made into a convergent light than when the diverging light is made into a thousand line lights.

In the optical system of this invention, it is necessary that the first lens has aberrations that do not change depending on the imaging magnification.This relationship is shown in FIG.

The horizontal axis represents the magnification m of the second lens. The root mean square of the wavefront aberration WFErmsf is plotted on the vertical axis. The relationship between mo and the residual wavefront aberration of the second lens is represented by curve 11, and the corresponding residual wavefront aberration of the first lens is represented by curve 12.13. curve 12
When the aberration of the first lens does not change much depending on the imaging magnification as in the case shown in FIG. On the other hand, when the aberration of the first lens changes greatly depending on the imaging magnification as shown in curve 13, the intersection with curve 11 does not occur, or even if it does, its @ ratio is very far away on the design @ vehicle. In this case, the optical system no longer works.

Similarly, regarding the first lens, the slope of the curve 12.13 cannot be estimated stationarily. Therefore, as a result of examining the specific range in which the optical system of the present invention can be realized, the imaging magnification m when the axial wavefront aberration is the smallest among the second lenses was determined. and the result of the two lenses in the optical system consisting of the first lens and the second lens.
It is clear that equation (1) must be satisfied between the rate m0.

When the upper limit of this conditional expression is exceeded, the spherical aberration is good, but
If the sine condition is significantly exceeded, the optical system must be assembled with high precision in order to prevent the image height range in which diffraction-limited performance is achieved from becoming small. If the lower limit is exceeded, it becomes difficult to correct the spherical aberration that occurs in @luns.

If the upper limit of condition (1) is exceeded, the amount of spherical aberration generated by the first lens increases, and if you try to forcefully correct this with the second lens, the upper limit of item # (1) will be exceeded.Second There is no lower limit on mo as long as the lens has diffraction limited performance.

However, if @ is exceeded under condition (2), the design and manufacture of the second lens? rX becomes difficult. In addition, in order to improve off-axis performance, it is desirable that the second lens be a double-sided aspherical lens.It is also desirable that the first lens alone has a shape that causes less spherical aberration. The one that satisfies (3) is good.

(Example) Examples of the present invention will be shown below.

The symbols in the table indicate the following.

r;: Vertex curvature of the atth lens surface from the light source side Color di: Distance between the i-th lens surfaces from the light source side n,: Refractive index of the i-th lens material from the light source side Spot: i-th lens surface from the light source side Atsbe number fi for the d-line of the th lens material: Focal length of the ith lens f: Focal length of the composite system of the 1st lens and the 2nd lens The aspherical shape has all the vertices of the surface as the origin, and the optical axis direction is the X axis. In the orthogonal coordinate system, the vertex curvature raw color is 01, the conic coefficient is 1, the aspherical coefficient is Ai, and the power of the aspherical surface is Pi (Pi).
0) Expressed as when ・In addition, the table also shows the direction regarding disk G.

Example 1 m. =00246 mo=O f,=15.95 f=6.3306
ω Aspheric coefficient/power number n1!1 Example 2 m. =0.0138 mo=Of,=19.33 f=5.893rid
in iv i Aspheric coefficient/power number n1!1 Example 3 mo=00]o2 n'i0;.

f, =21.63 f=5.695 Aspherical coefficient/power number 1=-0562 n1f.

Example 4 m, =0.0872 m0=O f1=15.92 f=21.73 Aspheric coefficient/power number n,,y1 Example 5 mo=00301 m0==Q f, =14.94 f= 3.698 Aspheric coefficient/power number Example 6 m. =0.0089 rno20f, =21.4 Of=3.871 ridiniνl Aspheric coefficient/power number n, j.

Effects of the Invention Embodiments 3 to 3 have a first lens and a second lens as shown in the first figure.
The distance between the lenses is 8. In the example when it is Off, the result @ magnification mT of the whole system is -0,2647 and -0,2222, respectively.
, -〇, 2.

In Example 4, the distance between the first lens and the second lens is 16 m, which is larger than in Examples 1 to 3, and the mT knee is 0.
It is 2647.

Further, in Example 5.6, as shown in the second figure, the distance between the photo lens and the second lens is 0.2 tm, and each of the four companies -
0,2647, -02.

Examples 1 to 4 are ideal for an optical system in which focusing is performed by moving the second lens ft in the optical axis direction.
In Examples 5 and 6, the first lens and the second lens are assembled into a lens barrel as one body, and the optical system is F&I for focusing by moving them in the entire optical axis direction. This is shown in FIGS. 7 to 12. In the same figure, WFErms is the total light source wavelength λ-780nm.
It is expressed in wavelength units as . As is clear from these figures, the axial spherical aberration has a wavefront aberration of 0.01λrm.
s or less, which is good.

In addition, the off-axis aberration of Example 4 is good at a, which is considered to be at the practical limit.

Furthermore, FIG. 13 shows changes in wavefront aberration when the two-structure lenses of Examples 1 to 4 are moved in a direction perpendicular to the optical axis. In the figure, t represents the amount of movement of the objective lens. In Examples 1 to 3, the wavefront aberration ti is 0.07λ or less and the diffraction limit performance is achieved even when all the tracking is performed from 10, 31 DI to +0.5 m, and the objective lens can be driven in the direction of the disk. It can be seen that even when used in an optical system that performs all tracking, there is very little deterioration in light collection performance due to tracking.

In all of the above embodiments, the same objective lens designed on the basis of incident light is used as the second lens. can be easily obtained, so even if various requirements are met, the second lens can be made common and only the first lens needs to be designed, so there is no need to make multiple types of aspherical lenses, which are difficult to manufacture. The second lens as the mouth objective lens that also produces the effect is not only an ordinary aspherical lens as in the above embodiment, but also a transparent plastic layer having an aspherical outer profile as described in JP-A-60-126616. It is also possible to use a glass fist lens coated with a glass lens, a heterogeneous medium lens, a hololens, or a conventionally used lens having, for example, a three-element structure in two groups.

[Brief explanation of drawings]

Figures 1 and 2 are cross-sectional views showing the configuration of the condensing optical system of the present invention, Figure 3 is a configuration diagram of a conventional condensing optical system, Figure 4 is a diagram of various aberrations of the ball collimator, and Figure 5 is a diagram showing the relationship between the imaging magnification of an aspherical lens and the axial wavefront aberration, FIG. 6 is an explanatory diagram of the condensing optical system of the present invention, FIGS. 7, 8, 9, 10,
11 and 12 are aberration diagrams of Examples 1 to 6, respectively, and FIG. 13 is a wavefront aberration diagram when the second lens of Examples 1 to 4 is moved in a direction perpendicular to the optical axis. 1: Optical disk (optical information recording medium) 2: Objective lens 3: Coupling lens 4: Light source 5: Aperture 6: Collimator lens Patent applicant Roku Konishi Photo Industry Co., Ltd. Applicant agent
Patent attorney Fumi Sato (and 1 other person) Fig. 1 Fig. 2 Fig. 4 Spherical aberration Sine condition Astigmatism Fig. 5 Shoulder Fig. 6 Fig. 7 Spherical aberration Sine condition Astigmatism Fig. 8 Spherical aberration Sine condition Astigmatism W
t9 diagram Spherical aberration Sine condition Astigmatism Figure 10 Spherical aberration Sine condition Astigmatism Figure 11 Spherical aberration Sine condition Astigmatism Figure 12 Spherical aberration Sine condition Astigmatism Figure 13

Claims (1)

[Claims] From the light source side, it consists of the first lens, which is a single spherical lens, and the second lens, which is a single objective lens.The second lens alone has the smallest axial wavefront aberration when the imaging magnification is @m@_0, and has diffraction-limited performance. When the imaging magnification of the second lens in the composite optical system of the first lens and the second lens is m_0, it satisfies 0.002<m_0−@m@_0<0.12. Features: Focusing optical system for optical information recording/reproducing optical system