Objective lens for optical memory device

An objective lens for an optical memory device comprising a plane surface on one side thereof for emitting a luminous flux oppositely to a disk, and an aspherical surface on the other of the side thereof for receiving a luminous flux reversely to the disk in such a manner that the following conditions (1) and (2) are satisfied: EQU 1.69.ltoreq.n.ltoreq.1.74 (1) EQU n-1.gtoreq.(n.sub.C -n.sub.A')/0.00056 (2) where n is a refractive index of a lens medium for the wavelength of an incident light, n.sub.A' is a refractive index of a lens medium for A' beam (768.2 nm of wavelength) of a reference light beam, and n.sub.C is a refractive index of a lens medium for a C beam (656.3 nm of wavelength). Thus, the lens can sufficiently obtain the thickness of the periphery thereof, has a plenty of utility in light weight, and can follow a variation in a laser wavelength due to the switching of the emitting light power of a semiconductor laser.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an objective lens for use in an optical memory 
device for writing and reading by a laser and, more particularly, to an 
objective lens for an optical memory device having preferable chromatic 
aberration and a plenty of utility. 
2. Description of the Prior Art 
An objective lens having an aspherical surface on one side and a plane 
surface on the other side for use in an optical disk device is disclosed, 
for example, in Japanese Patent Application Laid-open No. 88213/1986. If a 
refractive index is determined in the lens having an aspherical surface on 
one side and a plane surface on the other side of this type, its central 
thickness and a variation in its aspherical surface are automatically 
determined when its sine conditions and spherical aberration are 
corrected. 
In the objective lens disclosed in said Japanese Patent Application 
Laid-open No. 88213/1986, its refraction index n has a condition of 
n&gt;1.68. However, assume, for example, that the lens has a condition of 
n&gt;1.68, the thickness of the periphery of the lens becomes excessively 
thin, and, when it is adhered with an adhesive, there is a possibility 
that a distortion is affected in an effective diameter from the periphery 
of the lens. 
Further, in the conventional objective lenses including the lens disclosed 
in the above-described laid-open application, a countermeasure for its 
chromatic aberration is not considered in case of manufacturing the lens. 
In an optical memory device, an emitting light power in case of writing 
information on a disk and an emitting light power in case of reading 
information from the disk are switched. Normally, the emitting light power 
in case of writing is approx. 30 mW, and the emitting light power in case 
of reading is approx. 3 mW. However, in a semiconductor laser realized at 
present, a variation in the wavelength of approx. .+-.5 nm occurs upon 
switching of the emitting light power. When the wavelength is varied in 
this manner, a variation in its focal distance occurs due to the 
difference of the wavelengths in case of switching from the reading to the 
writing or vice versa, and its focus servo is offset so that there arises 
a problem that the servo becomes impossible in operation. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide an objective lens 
for an optical memory device which can eliminate the above-mentioned 
problems and which can sufficiently obtain the thickness of the periphery 
thereof, has a plenty of utility in light weight, and can follow a 
variation in a laser wavelength due to the switching of the emitting light 
power of a semiconductor laser. 
In order to achieve the above and other objects of the invention, there is 
provided according to the present invention an objective lens for an 
optical memory device comprising a plane surface on one side thereof for 
emitting a luminous flux oppositely to a disk, and an aspherical surface 
on the other side thereof for receiving a luminous flux reversely to the 
disk in such a manner that the following conditions (1) and (2) are 
satisfied: 
EQU 1.69.ltoreq.n.ltoreq.1.74 (1) 
EQU n-1.gtoreq.(n.sub.C -n.sub.A')/0.00056 (2) 
where n is a refractive index of a lens medium for the wavelength of an 
incident light, n.sub.A' is a refractive index of a lens medium for A' 
beam (768.2 nm of wavelength) of a reference light beam, and n.sub.C is a 
refractive index of a lens medium for a C beam (656.3 nm of wavelength). 
The above and other related objects and features of the invention will be 
apparent from a reading of the following description of the disclosure 
found in the accompanying drawings and the novelty thereof pointed out in 
the appended claim.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be described in detail with 
reference to the accompanying drawings. 
FIG. 2 is a diagram showing the relationship between the peripheral 
thickness t of a plano-convex lens having an aspherical surface on one 
side and a plane surface on the other side and the refractive index n of a 
lens medium by calculating the peripheral thickness t of the lens when the 
operating distance of the lens is WD, the central thickness is d and the 
diameter of 4.4 mm. FIG. 3 shows the relationship between the weight (mg) 
of a plano-convex lens having 4.4 mm of diameter and the refractive index 
n of the lens medium by obtaining the weight (mg) of the lens with 3.83 of 
the specific weight of the lens medium by calculating a lens volume from 
the central thickness of the lens varying in response to the variation in 
the refractive index of the medium of the lens by assuming that the 
diameter of the lens is 4.4 mm. 
From the graphs in FIGS. 2 and 3, the condition that the peripheral 
thickness of the lens is proper is first obtained. When the periphery of 
the lens is secured with an adhesive, the peripheral thickness of the lens 
must be at lest 0.5 mm or more so as to eliminate the influence of a 
distortion due to the securing of the lens to the effective diameter. In 
the diagram in FIG. 2, when a point (a) that the peripheral thickness of 
the lens becomes 0.5 mm or more is obtained, the refractive index of the 
lens at this time is 1.69. Therefore, the following condition (1) is 
required. 
EQU 1.69.ltoreq.n (1) 
Here, the oblique luminous flux characteristics of the light incident to 
the plano-convex lens will be discussed. FIG. 4 shows the relationship 
between a wavefront aberration (RMS) and a refractive index n when the 
inclinations of a parallel luminous flux incident to an objective lens are 
0.degree. and 0.5.degree.. As understood from FIG. 4, if the refractive 
index n is 1.68 or more, when the inclination of the parallel luminous 
flux is 0.5.degree., the RMS of the wavefront aberration becomes 
substantially constant. Accordingly, the condition of the above formula 
(1) is required for the oblique luminous flux characteristic. 
Then, in order to realize a high speed access in the objective lens for the 
optical memory device, it is necessary to reduce in weight not only an 
objective lens supporting section but an objective lens itself. If the 
weight of the lens can be reduced to approx. 100 mg or less, it is 
sufficient to realize a reduction in the weight of a servo movable section 
in case of composing the optical memory device. According to the graph in 
FIG. 3, the weight of the lens becomes 100 mg when the refractive index n 
is set to 1.74. In the diagram in FIG. 2, if the refractive index n is set 
to 1.74, the operating distance WD can be set to 2.0 mm or more. In the 
optical memory device, the operating distance WD so that the disk may not 
be damaged due to the fluctuation of the surface of the disk and the 
displacement of the servo is approx. 1.8 mm at the minimum. Therefore, if 
the refractive index n is set to 1.74 or less and the operating distance 
WD is set to 2 mm or more, it can prevent the disk from damaging. 
In order to satisfy both the above conditions of the weight and the 
operating distance WD, the following formula is required. 
EQU n.ltoreq.1.74 (2) 
From the unequality formulae (2) and (1), the following condition is 
determined. 
EQU 1.69.ltoreq.n.ltoreq.17.4 (1) 
Then, the condition (2) in the present invention relates to its chromatic 
aberration, which is the condition formula relating to the dispersion 
according to the different wavelengths. In the optical memory device as 
described above, the emitting light powers of the semiconductor laser are 
switched in case of between writing and reading, and the wavelength of the 
laser is varied thereby approx. .+-.5 nm. It is necessary in the objective 
lens for the optical memory device to eliminate the influence of the 
variation in the wavelength of the .+-.5 nm to a focusing servo. When the 
lens of the aspherical surface is used as a sole objective lens, the 
variation in the focal distance of the lens to the variation in the 
wavelength of the laser cannot be absorbed by the correction of the 
aspherical surface. Accordingly, in the condition formula (2), conditions 
are imparted to the properties of the glass material for forming the 
objective lens to suppress the dispersion of the light to the variation in 
the wavelength. The condition formula (2) will be obtained. 
First, when the focal distance of the lens is f, the radius of curvature of 
the aspherical surface 1a is r and the refractive index is n, the focal 
distance f of the plano-convex lens is represented by the following 
formula: 
EQU f=r/(n-1) (3) 
The focal distance of the objective lens used for the optical memory device 
is set to f=4 mm. The refractive index of the lens medium is presumed to 
be slightly wider range than that of the condition formulae (1) ((1) and 
(2)), and it is considered in case of n=1.65-1.75. It is necessary in the 
plano-convex objective lens of the aspherical surface used for the optical 
memory device to suppress the variation of the focal distance f due to the 
chromatic aberration of the lens to .+-.0.001 mm or less. If the focal 
distance varies more than this value, the lens becomes a defocusing state 
due to the relation to the focal depth of the lens, and the offset of the 
focus servo occurs. Then, the allowable variation amount of the refractive 
index of the lens when the variation of the focal distance when the focal 
distance is f=4 mm becomes 0.001 mm as described above will be obtained. 
First, the case of n=1.65 will be calculated. When n=1.65 and f=4 mm are 
substituted in the formula (3), the radius of curvature of the aspherical 
surface lens becomes r=2.6 mm. In this case, assume that the focal 
distance becomes f.sub.1 =4.001 mm of the allowable limit, the refractive 
index n.sub.1 at this time becomes as below. 
EQU n.sub.1 =(r/f.sub.1)+1=(2.6/4.001)+1=1.64983754 
That is, the variation amount .DELTA.n of the refractive index allowed to 
suppress the variation in the focal distance to 0.001 mm of the allowable 
limit becomes as below. 
EQU .DELTA.n=n.sub.1 -n=0.0001625 
In the aspherical surface plano-convex lens having n=1.65 and f=4 mm, in 
order to suppress the variation in the focal distance to 0.001 nm of 
allowable limit, it is necessary in the variation in the refractive index 
to fall within n=0.0001625. Similar calculations are conducted in a range 
of the refractive index n=1.65-1.75, and listed in the following table. 
TABLE 
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n r n.sub.1 .DELTA.n 
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1.65 2.6 1.64983754 
0.0001625 
1.66 2.64 1.65983504 
0.0001650 
1.67 2.68 1.66983254 
0.0001675 
1.68 2.72 1.67983004 
0.0001700 
1.69 2.76 1.68982754 
0.0001725 
1.70 2.80 1.69982504 
0.0001750 
1.71 2.84 1.70982254 
0.0001775 
1.72 2.88 1.71982005 
0.0001800 
1.73 2.92 1.72981755 
0.0001825 
1.74 2.96 1.73981505 
0.0001850 
1.75 3.0 1.74981255 
0.0001875 
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In the above table, the variation of the .DELTA.n when the refractive index 
n is varied by 0.01 (e.g., 1.66-1.65) is 0.0000025. This variation is the 
same with respect to the variations in the refractive indices. 
Accordingly, the variation of the .DELTA.n when the refractive index is 
any of those in the above table is as below. 
EQU (n-1.65).times.0.0000025.times.100 (4) 
Accordingly, the value of the .DELTA.n in case of the arbitrary refractive 
index in the table becomes as below from the formula (4) with the 
.DELTA.n=0.0001625 in case of n=1.65. 
EQU .DELTA.n=0.0001625+(n-1.64).times.0.0000025.times.100=0.00025(n-1)(5) 
This formula (5) is the condition formula so that the variation of the 
focal distance f becomes 0.001 mm when the wavelength of the laser is 
varied 5 nm by the switching of the emitting light power of the 
semiconductor laser. In order to further generalize this formula, the 
formula (5) is transformed to correspond to the difference of the 
wavelength (.lambda..sub.A' -.lambda..sub.C)=111.0 nm of the wavelengths 
.lambda..sub.A' =768.2 nm of the A' beam of reference light beam and 
.lambda..sub.C =656.3 nm of the C beam. Since the formula (5) designates 
the varied allowable value .DELTA.n of the n when the variation in the 
wavelength is 5 nm, when the ratio of the difference (n.sub.C -n.sub.A') 
of the refractive index n.sub.A' to the A' beam and the refractive index 
n.sub.C to the C beam to the .DELTA.n is obtained in response to the ratio 
of the wavelengths, the following formula is obtained. 
EQU (n.sub.C -n.sub.A')/.DELTA.n=111.9/5 
From this, the following formula is obtained. 
EQU (n.sub.C -n.sub.A')=.DELTA.n.times.111.9/5 
Here, since the .DELTA.n=0.00025(n-1) is satisfied, the following formula 
is obtained. 
EQU (n.sub.C -n.sub.A')=0.00056(n-1) 
Therefore, in order to suppress the variation in the focal distance to the 
variation in the laser wavelength within the allowable value, the 
condition of the following formula is required. 
EQU (n.sub.C -n.sub.A').ltoreq.0.00056(n-1) 
Accordingly, the following condition (2) is obtained. 
EQU n-1.gtoreq.(n.sub.C -n.sub.A')/0.00056 (2) 
If this condition is satisfied, when the variation in the wavelength is 5 
nm, the variation of the focal distance can be suppressed to 0.001 mm or 
less. 
In the above calculations, the focal distance is set to f=4 mm. However, 
even if the focal distance is out of the 4 mm, if the condition formula 
(2) is satisfied, the variation in the focal distance due to the switching 
of the emitting light power of the semiconductor laser can be suppressed. 
EXAMPLE 
The examples of the present invention will be described. 
The following examples are described in the case that, in a plano-convex 
lens having an aspherical surface 1a of one side and a plan 1b of the 
other side shown in FIG. 1, the aspherical surface shape is given by the 
following formula. The following formula is given where its optical axis 
is an x axis, the radial direction of the lens is a y axis, on the 
meridional surface of an orthogonal coordinates system having a vertex of 
the aspherical surface as an origin. 
EQU x=(y.sup.2 /r)/(1+1-(1+K) (y.sup.2 /r.sup.2))+Ay.sup.4 +By.sup.6 +Cy.sup.8 
+Dy.sup.10 
where r is radius of curvature of reference inscribed sphere at the vertex 
of the aspherical surface, K is conical constant, A, B, C, D are 
quarternary, sexanary, octonary and denary expansion coefficients. 
In the following examples, n is refractive index, and d is the central 
thickness mm of the lens. 
Example 1 
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n = 1.69, d = 1.4296, r = 2.76, 
K = -0.300068 
A = -0.144111 .times. 10.sup.-2 B = -0.146685 .times. 10.sup.-3 
C = -0.810956 .times. 10.sup.-5 D = -0.187406 .times. 10.sup.-5 
______________________________________ 
Example 2 
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n = 1.70, d = 1.5860, r = 2.80, 
K = -0.285694 
A = -0.143985 .times. 10.sup.-2 B = -0.143215 .times. 10.sup.-3 
C = -0.798888 .times. 10.sup.-5 D = -0.176560 .times. 10.sup.-5 
______________________________________ 
Example 3 
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n = 1.71, d = 1.7370, r = 2.84, 
K = -0.270571 
A = -0.144145 .times. 10.sup.-2 B = -0.140229 .times. 10.sup.-3 
C = -0.785993 .times. 10.sup.-5 D = -0.167297 .times. 10.sup.-5 
______________________________________ 
Example 4 
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n = 1.72, d = 1.8830, r = 2.88, 
K = -0.254381 
A = -0.144719 .times. 10.sup.-2 B = -0.137841 .times. 10.sup.-3 
C = -0.772499 .times. 10.sup.-5 D = -0.159751 .times. 10.sup.-5 
______________________________________ 
Example 5 
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n = 1.73, d = 2.0243, r = 2.92, 
K = -0.238529 
A = -0.144946 .times. 10.sup.-2 B = -0.135254 .times. 10.sup.-3 
C = -0.758721 .times. 10.sup.-5 D = -0.152156 .times. 10.sup.-5 
______________________________________ 
Example 6 
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n = 1.74, d = 2.1613, r = 2.96, 
K = -0.223311 
A = -0.144722 .times. 10.sup.-2 B = -0.132356 .times. 10.sup.-3 
C = -0.744761 .times. 10.sup.-5 D = -0.144277 .times. 10.sup.-5 
______________________________________ 
Example 7 
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n = 1.75, d = 2.200, r = 3.00, 
K = -0.208312 
A = -0.146773 .times. 10.sup.-2 B = -0.130142 .times. 10.sup.-3 
C = -0.769152 .times. 10.sup.31 5 D = -0.133531 .times. 10.sup.-5 
______________________________________ 
According to the present invention as described above, the peripheral 
thickness of the lens can be sufficiently obtained, and a light weight 
lens is provided. Further, the oblique luminous flux characteristics of 
the lens are improved, and the operating distance of the lens can be 
sufficient as for the optical memory device. 
The objective lens of the invention can allow for its chromatic aberration, 
and the offset of the focusing servo due to the variation in the 
wavelength upon switching of the emitting light power of the semiconductor 
laser at the time of writing and reading is eliminated.