JPH02271311A - Both-sided aspherical single lens for optical information recording and reproducing device - Google Patents

Abstract

PURPOSE:To obtain the both-sided aspherical single lens which has about 0.53 NA and has excellent image formation performance on and off the axis by employing aspherical surface constitution which satisfies a specific equation among the focal length, thickness on the optical axis, refractive index, etc., of the both-sided aspherical single lens for forming an image through a substrate with thickness (t). CONSTITUTION:The aspherical surfaces of the single lens 1 whose 1st and 2nd surfaces are aspherical are constituted satisfying the equation I, where X is the distance from an optional point on an aspherical surface to the tangential plane at the vertex of the aspherical surface, H the distance from the optional point to the optical axis, Rnu the conic constant of a nuth surface, and Anui the aspherical coefficient of the nuth surface (i=3, 4...). Further, conditional inequalities II - V hold, where F is the focal length of the aspherical single lens 1, D the thickness on optical axis, N the refractive index to in-use wavelength, (t) the thickness of a recording carrier substrate 2. Conse quently, the both-sided aspherical single lens for the optical recording and reproducing device which has the excellent image formation performance both on and off the optical axis can be offered although the lens has brightness of about 0.53 NA.

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention relates to a double aspherical single lens used in an optical head in an optical information recording/reproducing device, and particularly to a double aspherical single lens with an extremely bright NA of about 0.53. It's about lenses.

(Prior Art) In recent years, optical disks such as video disks, compact disks, and magneto-optical disks have been widely used as large storage capacity record carriers.

In order to record information with high density on this type of record carrier or to accurately reproduce recorded information, the objective lens used in optical information recording and reproducing devices has on-axis performance as well as off-axis performance of about 0.5°. As for performance, a resolution of 1 to 2 μm is required.

That is, an objective lens with an NA of about 0.53 is required.

Furthermore, in the objective lens for the above-mentioned use, it is necessary to provide a sufficient distance between the surface of a carrier such as an optical disk and the objective lens to prevent contact between the two to avoid damage to the record carrier and the objective lens.

Furthermore, in the above-mentioned information recording/reproducing apparatus, the mainstream method is to move the objective lens in the optical axis direction or in a direction orthogonal to the optical axis direction in order to perform autofocus or autotracking. Therefore, in order to improve response characteristics, this type of objective lens is required to be smaller and lighter.

Conventionally, as this type of objective lens, Japanese Patent Application Laid-Open No. 58-420
No. 21, Japanese Patent Application Laid-open No. 58-208719, Japanese Patent Application Laid-Open No. 60-122915, etc. disclose lens systems having a configuration of four elements in four groups.

However, the objective lenses disclosed in these publications have a large overall length of the lens system, and cannot be made smaller and lighter as described above.

In order to eliminate the above-mentioned drawbacks, the development of aspherical single lenses has recently been active.
The technology is disclosed in Japanese Patent Publication No. 7, Japanese Patent Application Laid-open No. 11715/1983, and the like.

In the example of the aspheric single lens shown in the above-mentioned publication, if the focal length of the lens is F, then the thickness t of the transparent protective layer of the record carrier to which the lens shown in the example can be applied is
is about 0.26F to 0.28F. However, in order to further improve the autofocus and autotracking operations described above, a lens with a shorter focal length is desired in terms of size and weight reduction. Furthermore, considering the same NA, shortening the focal length of the lens is equivalent to reducing the effective beam diameter, and it is possible to reduce the size of other optical components such as prisms inside the optical head. As a result, the entire optical head can be made lighter, smaller, and thinner. Therefore, it is also desirable to speed up the movement of the optical head in the radial direction of the optical disk, that is, the seek operation.

(Summary of the Invention) An object of the present invention is to eliminate the drawbacks of the conventional example described above, and to reduce the NA to 0.
．． It is an object of the present invention to provide a double aspherical single lens for an optical information recording/reproducing device, which has a brightness of about 53 mm and has good imaging performance both on-axis and off-axis.

The above objects of the present invention are achieved by the double aspherical single lens of the present invention described below.

(Example) A double aspherical single lens according to the present invention that forms an image through a substrate with a thickness t is a double aspherical single lens whose first and second surfaces are both aspherical. The distance from any point on the aspherical surface to the tangent plane of the aspherical apex is X1 The distance from the arbitrary point to the optical axis is H1 The reference radius of curvature of the νth surface is R, The conic constant of the νth surface Then, when the aspherical coefficient of the ν-th surface is A,l (f=3.4°...), it is an aspherical surface expressed by the following formula, and the following condition (1) is satisfied.
A double aspherical single lens that satisfies (2), (3), and (4).

+A94H'+... (ν=1.2) O, 36<-< 0.46 However, F is the focal length of the aspherical single lens, t is the thickness of the substrate, and D is the optical axis of the aspherical single lens. The upper wall thickness, N, is the refractive index of the aspherical single lens for the wavelength used.

Next, conditions (1) to (4) will be explained.

Condition (1) of the present invention was discovered by taking into consideration the physical conditions when used in an optical information recording/reproducing device.

That is, if the lower limit of condition (1) is not reached, it is contrary to short focal length, which is the subject matter of the present invention, and is unsuitable for weight reduction and size reduction. On the other hand, if the upper limit of condition (1) is exceeded, the distance between the objective lens and the record carrier (working distance) becomes too short, and the optical information recording/reproducing device is exposed to unexpected external forces and vibrations during operation. When this occurs, there is a risk that the record carrier and the objective lens may come into contact with each other and may be destroyed. Therefore, when aiming for short focal length while ensuring sufficient working distance, it is important to satisfy condition (1) while satisfying conditions (2) to (4) shown below to achieve good imaging performance. It is also necessary to obtain

Conditions (2) and (3) of the present invention are for properly correcting spherical aberration and comatic aberration in the third-order region.

According to "Lens Design Method" by Yoshiya Matsui (Kyoritsu Shuppan), the third-order spherical aberration coefficient r of the first and second surfaces, T2 and the coma aberration coefficient of the first and second surfaces I, II2 is expressed as follows when the entrance pupil is made to coincide with the first surface and the object distance is infinite.

Here, ψ1. ψ2 are third-order aspheric terms of the first surface and the second surface, R1 is the paraxial radius of curvature of the first surface, and R2 is the paraxial radius of curvature of the second surface.

The third-order spherical aberration coefficient ■ and the coma aberration coefficient II of the entire lens are the sum of the aberration coefficients of each surface, I=1.
+l2 TI = II, + II 2 is determined, and the lens shape and each aspherical amount are determined so that I and II have appropriate values.

As is clear from the above equation, when the shape of the lens (focal length 1 working distance, etc.) is determined, R, , R2, and D.

The value of N is almost fixed, and the degree of freedom left to set the aberration coefficients I and ■ to appropriate values is ψ1. There is only ψ2.

Therefore, the lens shape must be determined with some consideration for aberration correction, and the numerical range for this is the condition (
2), (3).

When the numerical value range of condition (2) is exceeded, the spherical aberration of the first surface in particular becomes large, and the aberration cannot be corrected completely by the aspherical term ψ of the first surface.

Condition (3) is a condition for correcting comatic aberration in a well-balanced manner on the second surface. If this value falls outside this numerical range, the aspherical term ψ2 of the second surface will not be able to fully correct the aberration, resulting in off-axis imaging. Performance deteriorates significantly.

Condition (4) is mainly a condition for satisfying the sine condition. In the present invention, not only on-axis aberration but also off-axis aberration within a certain range, especially coma aberration, is corrected well.
If it is outside the range of 4), the isoplanatic condition will be significantly lost, which is undesirable. If the upper limit of the condition is exceeded, the radius of curvature of the first surface becomes too small, and a large amount of negative spherical aberration occurs, making it difficult to correct the aberration.
Another disadvantage is that the working distances W and D become short, which poses a practical problem.

Examples of the double aspherical single lens of the present invention are shown below.

However, as shown in Figure 1, F is the focal length of the lens, and NA
is the numerical aperture, β is the paraxial lateral magnification, R8 is the paraxial radius of curvature of the aspherical surface of the first surface, R2 is the paraxial radius of curvature of the aspherical surface of the second surface, D
is the center thickness of the lens, W and D are the working distance, t is the thickness of the record carrier substrate, N is the refractive index of the lens at the working wavelength λ = 780 nm, and Nt is the refractive index of the record carrier substrate at the working wavelength λ = 780 nm. Refractive index, Δv U) (ν=1.2) is the optical axis direction of the aspheric surface in 10% of the lens effective diameter determined by NA (numerical aperture) on the νth surface and the spherical surface with paraxial radius of curvature R. (However, Δ, (j) is positive in the direction in which the curvature of the aspherical surface becomes weaker.)

The shape of the aspherical surface is defined as: x1 is the distance from any point on the aspherical surface to the tangent plane of the apex of the aspherical surface; H1 is the distance from the arbitrary point to the optical axis; R is the standard radius of curvature of the νth surface; The conic constant of the νth surface is K, the aspheric coefficient of the ν surface is A, + (+ = 3
．． 4. ), it is an aspherical surface expressed by the following formula.

This is what happened when I did it. In the aberration diagrams shown in FIGS. 2 to 5, the horizontal axis corresponds to the entrance pupil radius, and the vertical axis represents the amount of aberration. Furthermore, (A) is the on-axis imaging characteristic, and (B
) and (C) are off-axis imaging characteristics corresponding to angles of view of 0.4° and 0.8°, respectively. The solid line is the lateral aberration of the meridional surface,
The dashed line represents the transverse aberration in the sagittal plane.

+A, 4H'+... (ν=1.2) Further, the lateral aberrations of the lenses of Examples 1 to 4 according to the present invention are shown in FIGS. 2 to 5, respectively. In each example and aberration diagram, the focal length is normalized to F=1.0. Example 1 F=1.0 R,=0.7195O R,=-1,7166O NA=O.

D=0° WD=O.

t=0° K = -1゜ A,, = 6゜ A,, =-1゜ A,, = l.

A,, =-7゜ A,, = 4゜ A,, = 3゜ A,, =-1゜ A110 = 4゜ A,,,=1゜ A,,, =-1゜ A,,, =-8゜ A,,,=2゜ A,,,=2゜ A,,, =-5゜ 83127X10-” 13521X10-' 75084xlO″1 10047xlO” 75959 X 10-' 82531xlO-' 91327X10-' 23485X10゜ 27468X10' 19932xlO' 78222'xlO' 13893xlO-' 02031xlO' 65724xlO' 21685X10' β=O N = 1.57645 N, = 1° 2 = 1, A2. = 4゜ 44 = 9゜ A,, = 2゜ A,, =-1゜ A,, =-1゜ A,, = 3゜ A2. =-4゜ A2Io = 1゜ Az+, = 6゜ A,,,=3゜ A2. ,=5゜ A, 4=8° A2. , =-1゜ A,,, =-8゜ 59047xlO 75410X10-' 54095xlO-' 99007×10+ 66104xlO' 43306xlO6 61485xlO' 45362xlO6 52907xlO' 43103X10 78247xlO 65608xlO 57628X10' 41495xlO' 63734xlO2 (10) =0.2329 (9)=0.1808 (7) = 0. 1030 (5)=0.0506 (10) =-0,0408 (9) =-0,0330 (7) =-0,0199 (5)=-0,0101 Implementation port 2 F=1.0 R, = 0.69795 R,=-2,04045 NA=0° D=0° WD=O.

t=0° K, =-1゜ A,, = 6゜ A,, =-1゜ A,, = 5゜ A,, =-8゜ A,, = 8゜ A,, = 4゜ A,, =-1゜ Ao. =-6° A,,,=1゜ A,,, =-3゜ A,,,=7゜ A,,,=4゜ A,,,=6゜ A,,, -1゜ 66870xlO-2 00268xlO-' 75527xlO” 74300xlO-” 91824x10 38701xlO” 44347xLO-' 12904xlO6 50903xlO' 72591xlO' 51144x10 77967xlO8 97410xlO' 30945xlO 55214xlO' β=O N = 1.57645 N, = 1. 58 , =-2゜ A,, = 4゜ A2. = 1゜ A=s=2゜ A,, = 2゜ A,, =-4゜ A-g = 2゜ A2* = 6゜ A11o=　3゜ A,,,=1゜ A2. ,=　　2゜ A2.3=-2゜ A,,,=1゜ A,,,=l.

A, 6=-1° 04779xlO' 48359xlO-” 17807xlO-' 73550xlO-' 33495X10~3 14235X10- 14899X10゜ 16208xlO9 41262xlO' 04277X10' 84796X10' 84760xlO' 90557xlO” 39967xlO' 14889xlO” (N-1) F” N.R.-〇.

One = -0° (10) = 0゜ (9) = 0゜ (7) = 0゜ (5) =O.

(N-1) D.

N.R.

D+WD””37 (10) =-0゜ (9) =-0゜ (7) =-0゜ (5) =-0゜ Example 3 F=1.0 R, = 0.7021O R2=-1,92752 NA=O.

D=0° WD=0° 1=0° K, −−1゜ ,A,, = 4゜ Hmm, =-i.

A,, = 7゜ A,, =-9゜ A,, = 8゜ Hmm, -6, A1* =-1゜ A, 1° knee 7° A,,,=1゜ A,,, =-3゜ A,,,=7゜ A,,,=5゜ A,,,=6゜ A,,, =-1゜ 33380X10-2 99151X10'-' 36020 x 10-' 66103xlO-' 98758xfO- 47942xlO 53046X10-' 26724xlO' 04004X10゜ 81251X10 67438X10' 21812X10゜ 41030X10 80399X10' 58810X10' β=O N = 1.57645 N, = 1° 2 = -5° A□, = 2゜ A24 = 2゜ A25=-4゜ A2. =-2゜ A,, = 3゜ A2. = 2゜ A,, =-2゜ A21゜=4゜ A,,,=7, A2. ,=2゜ A□, =-6゜ A,,,=2゜ A,,, =-6゜ A2. , =-2゜ ooooooxio’ 98063X10-2 81447xlO-' 24791xlO-' 09939X10-' 13074X10-' 21370X10゜ 02453X10゜ 48568X10' 40823X10゜ 97764X10 96801X10゜ 21248X10” 51546xlO' 16974X10” (10) = 0゜ (9) = 0゜ (7) =O.

(5) =0°(10) =-0°(9) =-0°(7) =-0°(5) =-0° Example 4 F=1.0 R, = 0.67962 R, =-2,54902 NA=0.53 D=0.61097 WD=0.43340 t=0.37584 β =O N =1.57645 N, =1. 58, =-1,37686xlO-'n, = 2.
49235xlO-" A,, = -2,46348xlO-' A,, = 1.65190xlO"=-1,11976xlO' A, t = 1.47487xlO', =-3
,50309xlO-'A,,=-3.79524xlO'A1,. =-5,90336xlOroAm=3.
17133xlO'A1□=-7,08127xlO' A++s= 1.27092xlO'AI□,=
t, 15587X10″A++s=1.50075
xlO"A++a = -3,74926xlO',
=-4,56598xlO' A2. = 6.18193xlO-” A, I =
9.45675X10-2A,,=-1,03845X
10°A,, = 2.51153X10-'A,, = 1
．． 01118xlO'A,, =-2,00478xlO'A2. =-1,
74017xlO' A2. , = 3.71507xlO Ao, I = 6.98481X10'A21x =
1.12915X10"Ail = 4.843
07xlO"A214 = 3.47351X10'
A21B=-4,12185X10"A,,,=5.32755X10'(10) =O
.

(9) = 0゜ (7) = 0゜ (5) =O.

(10) =-0°(9) =-0°(7) =-0°(5) =-0° or more Examples 1 to 4 according to the present invention are shown, and their imaging performance is shown in FIG. It is shown in Figure 5. In Examples 1 to 4 above, when the thickness t of the record carrier substrate is 1.2 mm,
The focal length corresponds to approximately 2.8 mm to 3.2 mm, and falls within the range of condition (1).

This makes it possible to achieve the objective of the present invention, which is to make the objective lens smaller and lighter, as well as the optical head smaller and lighter.

As seen in the above-mentioned Examples 1 to 4, in the double aspherical objective lens of the present invention, the above-mentioned (1) to (4)
), it is preferable that the following condition (5) is satisfied.

(5) 0.35 << 0.45D+WD Condition (5) relates to the structure and size of the optical head, and being less than the lower limit is undesirable because it leads to an increase in the size of the optical head. On the other hand, if it exceeds the upper limit, the objective lens and the record carrier will come too close to each other, which is unfavorable in terms of safety in the event of unexpected vibrations or the like.

From the viewpoint of aberration correction, it is preferable to satisfy the following conditions (6) and (7) in addition to the above conditions (1) to (4).

Conditions (6) and (7) are the ratio of the aspheric amount of the first surface to the aspheric amount of the second surface at 100% and 70% of the effective diameter, respectively. The above two conditional expressions were found by converting the share of the aberration correction amount between the first surface and the second surface into the ratio of the aspheric surface amount. By satisfying the ranges shown in both equations in addition to the above conditions (1) to (4), it is possible to improve the imaging performance both on-axis and off-axis.

Alternatively, from the viewpoint of aberration correction, it is preferable that in addition to the above conditions (1) to (4), the following conditions (8) to (]l) are satisfied, and more preferably, in addition It is desirable that conditions (12) to (15) below be satisfied in combination.

(8) 0.20F <Δ, (10) < 0
．． 28F (9) 0.09F <Δt (7) <
0.12F(10) -0,045F< A 2
(10)<-0,03F (11)-0,022F<Δ
2 (7)<-0,ot5F'(12) 0.1
6F < Δ h (9) < 0
．． 22F(13) 0.045F<Δ,
(5) < 0.06F(14)-0,
035F<A, (9)<-0,025F(15)
0.012 <Δz (5) < 0,005F
The above conditions (8) to (11) are conditions for determining the amount of asphericity at 100% and 70% of the effective diameter of the first and second surfaces of the lens. Also, conditions (12) and (15) are similarly 90%, 5
These are the conditions for determining the aspherical amount in terms of ratio, and together with conditions (8) to (11), conditions suitable for further aberration correction are disclosed.

Conditions (8), (9), (I2), (13)
If the upper limit of is exceeded, the spherical aberration will be excessive, and if it is less than the lower limit, the spherical aberration will be under, and the axial performance will deteriorate.

Conditions (10), (11), (14), (1
5) mainly relates to correction of off-axis aberrations. Outside the range shown in the conditions, the amount of comatic aberration generated is particularly large, which is not preferable.

(Effects of the Invention) As described above, according to the present invention, it is possible to provide a bi-aspherical objective lens with a short focal length suitable for use as an optical information recording/reproducing device. That is, by employing the dual aspherical objective lens according to the present invention, it is possible to ensure good imaging performance both on-axis and off-axis, and to greatly contribute to miniaturization and weight reduction.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a lens cross section of a double aspherical single lens according to the present invention, and FIGS. 2 to 5 are diagrams showing transverse aberrations of the embodiments according to the present invention.

Claims (1)

[Claims] In a double aspherical single lens that forms an image through a substrate having a thickness t, the first surface and the second surface of which are both aspherical, the aspherical surface is is the distance from an arbitrary point on the aspherical surface to the tangent plane of the aspherical apex, H is the distance from the arbitrary point to the optical axis, and R_ν is the reference radius of curvature of the νth surface.
, the conic constant of the νth surface is K_ν, and the aspheric coefficient of the νth surface is A
When _ν_i (i=3, 4,...), it is an aspherical surface expressed by the following formula, and also satisfies the following conditions (1) and (2).
), (3), and (4), a double aspherical single lens for an optical information recording/reproducing device. ▲There are mathematical formulas, chemical formulas, tables, etc.▼…(ν=1, 2) (1) 0.36<(t/F)<0.46 (2) 0.60<[(N-1)F^2 ]/[N^3R_
1^2<0.77] (3) 0.30<[(N-1)D]/[NR^1]<0
．． 45 (4)-0.46<(R_1)/(R_2)<-0.2
6 Here, F is the focal length of the aspherical single lens, D is the thickness of the aspherical single lens on the optical axis, and N is the refractive index of the aspherical single lens with respect to the wavelength used.