Optical information recording/reproducing element

An element of the invention focuses and projects a parallel or substantially parallel light beam onto an optical information recording medium. The element consists of a transparent columnar body having a convex surface as the input surface and a flat surface as the output surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical element which can be suitably 
used for recording/reproducing information using an optical information 
recording medium such as a video disk, a compact disk, an optomagnetic 
recording apparatus or the like. 
2. Description of the Prior Art 
An optical element used for the purpose described above focuses parallel 
light from a light source onto a recording medium. Aberration of the 
element outside the optical axis, especially coma, must be sufficiently 
corrected in consideration of an assembly error introduced by assembly of 
an optical system and application conditions, e.g., a plurality of light 
beams are passed through a single optical element and a tracking beam 
among these incident beam is incident at an element position outside the 
optical axis. In practice, when the wavelength of light used is 
represented by .lambda., the residual wavefront aberration must be within 
.lambda./4. In addition, the element must be compact in size, light in 
weight, and inexpensive. In the past, various types of optical information 
recording/reproducing optical elements have been proposed. However, each 
of such elements is subject to some problems. 
For example, when a focusing lens consisting of a number of normal 
spherical lenses each comprising a transparent material having a uniform 
refractive index is used, since the lens becomes large in size and heavy, 
the overall apparatus cannot be rendered compact in size and light in 
weight. In addition, due to the heavy weight of the lens, tracking and 
servo control of the optical pickup is adversely affected. 
When light beams are passed through a number of lenses, reflection loss 
occurs at each lens surface, and the optical power of light reaching the 
recording medium is reduced. For this reason, beam control reliability is 
impaired. 
In addition, since the optical system becomes complex in structure, 
high-precision processing and assembly cannot be performed. 
In order to solve these problems encountered with an assembled spherical 
lens, a graded index lens having one concave surface has been proposed as 
in Japanese Patent Disclosure No. 58-122512. However, in general, an 
optical information recording/reproducing element has a very small 
diameter below several millimeters, for example. Therefore, it is very 
difficult to process one surface into a concave surface having a 
predetermined radius of curvature. Even if a concave surface with a 
predetermined radius of curvature can be obtained, a sharp edge of the 
lens is subject to chipping. Furthermore, with a lens having a flat 
surface at the light source side and a concave surface at the recording 
medium side, since the concave surface has a negative power, a desired NA 
cannot be obtained unless a very large power is obtained with a refractive 
index distribution within the lens. This requires a large difference in 
refractive index within the lens, and control of terms of high orders of a 
refractive index constant, rendering the manufacture of the lens extremely 
difficult. 
OBJECT OF THE INVENTION 
It is a main object of the present invention to provide a compact and 
lightweight optical information recording/reproducing element which is 
free from the problems of a conventional element, which has a high optical 
performance with small aberration and which is easy to manufacture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the present invention will now be described 
with reference to the accompanying drawings. 
Referring to FIG. 1, reference numeral 1 denotes a light source; and 2, an 
optical element according to an embodiment of the present invention. A 
parallel light beam 3 emitted from the light source 1 is focused onto a 
point O on a recording surface 4A of an optical information recording 
medium 4 by the element 2. Optical recording of information or readout of 
recorded information of the recording medium 4 is performed at the point 
O. In general, the light beam 3 is a completely parallel light beam or a 
light beam having a convergence angle (half angle) within .+-.5.degree.. 
The light beam 3 is incident on the element 2 parallel to or at a 
predetermined inclined angle with respect to a central axis 5. A working 
distance WD from an output end surface 2B of the element 2 to the 
recording medium 4 is generally 0.5 to 2.5 mm. The element 2 comprises a 
columnar body of a transparent material (transparent to light of 
wavelength used) such as glass or a synthetic resin which has a refractive 
index distribution which decreases from the central axis 5 toward the 
outer periphery of the element 2. An input end surface 2A of the element 2 
at which the light beam 3 from the light source 1 is incident is a convex 
surface, and the output end surface 2B is a flat surface perpendicular to 
the central axis 5. 
In the present invention, the orientation of the end surfaces of the 
element 2 is an important factor; the convex end surface 2A must face the 
light source 1 and the flat end surface 2B must face the recording medium 
4. If the end face of the element 2 facing the light source 1 is a flat 
surface and the other end face facing the recording medium 4 is a convex 
surface, an output beam from the element 2 emerges at a relatively large 
angle with respect to the normal to the refracting surface of the 
recording medium 4. This results in aberration of high order at the 
refracting surface, and the spherical aberration at the periphery of the 
element 2 cannot be fully corrected. In addition, due to an increase in 
the deficiency amount of the sine condition, the characteristics at points 
of the element outside the axis are impaired. On the other hand, if the 
end surface 2A of the element 2 facing the light source 1 is a convex 
surface and the end face 2B facing the recording medium 4 is a flat 
surface, the light beam 3 incident on the element 2 is refracted twice at 
the two surfaces 2A and 2B. Then, the spherical aberration is reduced to 
the minimum and the sine condition can be satisfied. 
The refractive index distribution of the element 2 according to the present 
invention can generally be given by: 
EQU n.sup.2 (r)=n.sub.0.sup.2 [1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 
(gr).sup.6 +h.sub.8 (gr).sup.8 +. . . ] (1) 
where n.sub.0 is the refractive index on the central axis 5, n(r) is the 
refractive index at a point of a distance r from the central axis 5, and 
g, h.sub.4, h.sub.6 and h.sub.8 are distribution constants. When an 
element 2 having the refractive index distribution given by the equation 
(1) above is used, the element can exhibit a particularly high performance 
with low aberration when n.sub.0, g and the effective radius r.sub.0 are 
set to fall within a hatched region shown in FIG. 2. 
In the graph shown in FIG. 2, the product G of the refractive index 
n.sub.0, the distribution constant g, and the effective radius r.sub.0 is 
plotted along the axis of abscissa, and the effective radius r.sub.0 is 
plotted along the axis of ordinate. The hatched region in the graph shown 
in FIG. 2 is bounded by lines given by r.sub.0 =2.5 (mm), G=0.45, r.sub.0 
=-150G+52 (mm), r.sub.0 =-3.87G+2.85 (mm), r.sub.0 =-100G+40.5 (mm), and 
r.sub.0 =-4G+3.41 (mm). 
In a region satisfying the conditions r.sub.0 &gt;-100G+40.5 and r.sub.0 
&gt;-4G+3.41 and outside the hatched region, the correction of the deficiency 
amount of the sine condition is insufficient. In a region satisfying the 
condition r.sub.0 &lt;-150G+52 and outside the hatched region, this 
correction amount becomes excessive. In a region satisfying the condition 
r.sub.0 &lt;-3.87G+2.85 and outside the hatched region, correction of the 
spherical aberration is difficult. When the values of r.sub.0, g and 
n.sub.0 are in the hatched region, the absolute value of the deficiency 
amount of the sine condition can be reduced to a very small value of 0.2% 
or less. 
Examples of specifications of the element according to the present 
invention are given below: 
Z=1.41 mm 
R.sub.1 =2.5 mm 
n.sub.0 =1.6000 
g=0.162 mm.sup.-1 
h.sub.4 =-0.68 
h.sub.6 =-2.5, h.sub.8 =-10 
f=3.42 mm 
WD=1.8 mm 
where Z is the element length, R.sub.1 is the radius of curvature of the 
refracting surface facing the light source, n.sub.0 is the refractive 
index on the central axis, g, h.sub.4, h.sub.6 and h.sub.8 are 
distribution constants, f is the focal length at the side of the image, 
and WD is the working distance from the element output end face to the 
recording medium. The aberration curve of the element having these 
specifications is shown in FIG. 3. 
Referring to FIG. 3, the solid curve represents the spherical aberration 
and the dotted curve represents the deficiency amount of the sine 
condition.