JPS593724B2 - Inhomogeneous refractive index lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a non-uniform refractive index lens having a non-uniform refractive index sphere (hemisphere) provided with refractive media having different refractive indexes and film thicknesses as a cladding in the optical path thereof.

15 Normally, an optical disk has information recorded on one side of a transparent plate made of plastic or the like with a thickness of about 17 nm, and the information is often read out by passing a laser beam from the other side and condensing the light onto the recording surface.

Therefore, the lens used here requires a certain distance of 20 (total working distance) from the lens surface to the focal point.
On the other hand, in order to reduce aberrations, simplify the servo system that controls position and focus, and improve the performance of the entire device, the lens must be made as small as possible.
However, this is a requirement that conflicts with the above-mentioned setting of the working distance of 25. The present invention aims to provide a non-uniform refractive index lens that satisfies the contradictory demands of miniaturizing the lens and increasing the working distance.

Now, in order to understand the inhomogeneous refractive index lens of the present invention, the functions of the previously proposed ball lens (Japanese Patent Application No. 55-142348 (Japanese Patent Application Laid-open No. 56-138704) will be explained first.

First, Fig. 1a shows a case where a conventional non-uniform refractive index spherical lens 1 is placed in air, for example, and light emitted from one point (left side of the lens) 35Po is imaged by the lens 1 onto the opposite side (right side of the lens) PF. This is to show that aberrations are unavoidable.

Figure 1b shows the magnitude of the aberration that occurs at this time as a function of the angle θ that the focused light at the imaging point PF makes with the optical axis.
It is. The lateral aberration At, which is plotted on the horizontal axis in this figure, is the difference between the rays that converge at each angle deviating from the optical axis in a plane that crosses the point PF where the paraxial rays of θZO converge and is perpendicular to the optical axis. refers to the distance at which Negative lateral aberration refers to the case where paraxial rays are focused on the side closer to the lens than the focal point of the paraxial rays. This negative transverse aberration is therefore shown within the enlarged circle of FIG. 1a. On the other hand, Figure 2a shows that when focused light passes through a spherical shell-shaped medium 2 with a homogeneous refractive index from the left side to the right side, a positive transverse aberration At occurs, contrary to the previous case. It is shown schematically.

The dashed line shows the optical path that the focused light would follow in the absence of this spherical medium 2. FIG. 2b shows the amount of positive transverse aberration produced as a function of the angle .theta. that the focused beam makes with the optical axis, similar to FIG. 1b. In addition, homogeneous refractive index spherical shell medium 2
If has a curved surface opposite to that shown in Figure 2a with respect to the direction of light travel, then the light ray that originates from one point and passes through this medium will be The farther out the object is, the further outward it is refracted.

As a result, when this bundle of light rays is focused, the outer light rays are focused farther away from the medium 2, and a positive transverse aberration as shown in FIG. 2b also occurs. Therefore, if these media 2, 2, which are oriented in opposite directions, are used in combination, the positive transverse aberration can be doubled. In view of the above, the effect that the heterogeneous refractive index spherical lens 1 shown in FIG. 1 has on the bundle of rays to be focused and the effect that the homogeneous refractive index spherical shell 2 shown in FIG. 2 has on this are as follows: It can be seen that there is an inverse relationship between negative and positive aberrations.

Therefore, by combining these two elements, the positive and negative polarities cancel each other out and aberrations are corrected (the ball lens proposed earlier was created based on this technical idea. Figure 3a)
Shown is an example of the configuration of the cladded heterogeneous refractive index spherical lens proposed earlier.

This spherical lens consists of a heterogeneous refractive index sphere 1 (hereinafter also referred to as a core).
The homogeneous refractive index sphere shell 2 is attached as a cladding around the 4b), and the refractive index n(r
) and the distribution type of the function are shown in the graph at the bottom of the same figure, and expressed numerically as follows. ) In the above equation, G2 and G4 are called the second-order coefficient and the fourth-order coefficient, respectively, and the inhomogeneous refractive index is from the center of the sphere r=O to r-R.

This is a quantity that shows how the change is occurring. The refractive index of the cladding 2 is set to Nd=constant, but the magnitude of this and the refractive index n(0) of the center of the sphere does not matter. Further, Nd does not necessarily have to be constant or homogeneous, but can be selected to be non-uniform depending on the design. FIG. 3b shows a numerical example to demonstrate the high performance of this clad heterogeneous refractive index spherical lens.

As can be seen from this figure, the radius R of the inhomogeneous refractive index sphere 1.
For example, if the thickness d of the 1 cladding 2 is 0.78 mm, then Sl
Even if nθ, that is, the numerical aperture (NA) of the lens is set to 0.2, the lateral aberration can be corrected to ±0.5 μm, which is below the diffraction limit of light. This is impossible with any conventional ball lens, and the advantage is that it is possible. In addition, below, regarding the absolute value of lateral aberration, θ=
Since the maximum value from O to Sin-1NA is simply referred to as lateral aberration, we have described aberration compensation between the inhomogeneous spherical core and the cladding using the previously proposed ball lens as an example, but the inhomogeneous refraction of the present invention The principle of aberration compensation for the index lens is exactly the same.

However, in the previous ball lens, the cladding had the same refractive index and thickness on both the light source side and the condensing point side. In this configuration, the light source is placed near the lens, so the NAZO. 2 lenses can be considered, but NAZO is the Fourier transform lens that was proposed at the same time when the light source is placed sufficiently far away.
．． I can think of up to 4. NA for optical pickup lenses
ZO. 4 is required, so an optical system with the latter configuration can be considered, but in this case, the distance from the lens surface to the focal point is not very large, and a lens with a core of several diameters has a thickness of 1 mm. The disadvantage is that the light condensing point cannot reach the back side of the disk. Therefore, the lens with the asymmetric cladding of the present invention, that is, the thickness of the cladding on the condensing side (disk side) is made thinner than the cladding on the light source side.
They also proposed a non-uniform refractive index spherical (hemispherical) lens with a high refractive index or both. Now,
The structure of the inhomogeneous refractive index lens of the present invention is shown in FIGS. 4A and 4B.

Figure a is a transmission type, and Figure b is a reflection type, and if the reflective surface 5 is on the plane of symmetry, they are completely equivalent in terms of analysis. Figure b is suitable for picking up commercial laminated disks because it can be configured as a thin head. Hereinafter, the present invention will be explained using the transmission type ball lens shown in FIG. In the optical system shown in FIG. 4a, the refractive index distribution of the core 1 of the lens is assumed to be ^^j-翫'沉-'ノ~9, similar to the previously proposed ball lens.

Here, RO is the core radius, n(0) is the central refractive index G2
, G4 are second-order and fourth-order coefficients, respectively.

When creating a distribution using ion exchange technology or the like, it is difficult to create an exact square distribution, and it is not necessarily a good idea, so a fourth-order distribution was assumed. ? In addition,
In fact, the case of G2KUO becomes a problem. That is, the refractive index decreases from the center toward the periphery. The refractive index of the light source side cladding 2 and the disk side cladding 3 is N.
,,n2, thicknesses are respectively D,,d2, and disk 4
Let the refractive index and thickness of the disc be Ns and S, respectively, and let the working distance between the disk 4 and the cladding 3 be W, and from the center of the core to the light source (the center of the wavefront just before the incidence of the cladding 2 is assumed to be a spherical surface). Let the distance be a, and b=RO+D2+w
+s/Ns and et al. Before presenting the analysis results for the examples, let us explain the asymmetric cladding, D2, Dl,
Let us discuss the increase in working distance due to n2>N.
First, it can be seen from a comparison of FIGS. 4C and 4D that the thinner the thickness D2 of the cladding 3, the easier it is to maintain the working distance. Further, as shown in FIG. 4e, the higher the refractive index of the cladding 3, the farther the light from the core 1 travels due to refraction at the boundary, which likewise lengthens the working distance. Furthermore, the thick cladding on the light source side and the low refractive index serve to lengthen the working distance, but their explanation will be omitted here. Below, we analyzed the following numerical examples. NA(Si
nOmax)=0.45n(0)=N2=1.6,n,
=Ns=1.5, S/RO=0.44 Magnification b/a=0.
02, Table 1 shows these all together. The aberration analysis used the method of ray optics as in the case of the previously proposed ball lens, and the thickness of the cladding was determined so as to minimize the lateral aberration over the O-Si 1 NA with an output angle θ.

FIG. 5 shows the relationship between the thicknesses Dl and d2 of the claddings on the light source side and the disk side when aberration compensation is established, using the quadratic coefficient G2 as a parameter (G4=-0.01).

It also shows isovalue lines of the actuation interval W. From this 1G2
The larger 1 is, the thinner the disk-side cladding can be.However, the light source-side cladding is made thinner by d1/ in order to keep the operating interval W>0.
A thickness of about ROZl or more is required. When manufacturing the cladding, it is advantageous in terms of accuracy to first determine the thickness of the disk side, measure it, and then determine and manufacture the thickness of the light source side cladding. Figure 6 A, b
It is calculated from the curve showing the relationship between cladding thickness accuracy and aberration. Figure 7 shows the thickness D2 of the disc side cladding required for aberration compensation when the refractive index distribution coefficients G2 and G4 of the core are given.
/RO is shown (when D, /RO=1.2)
.

The residual lateral aberration and operating distance W/RO at that time are also shown. Distribution coefficients G2 and G4 that can reduce aberrations from this
Understand the range of For example, the innermost (lateral aberration/RO) ×
R when 103≦0.2. 〒2.5m7! A lens with an L core can have a lateral aberration of ≦0.5 μm, which is below the diffraction limit, making it a very good lens. The figure shows a range of G2 of -0.07 to -0.2, but this is a refractive index difference between the core center and the periphery of approximately 3.5 to 10%, which is within the range that can be manufactured by ion exchange etc. be. Regarding G4, (lateral aberration/RO)XlO3<. To fall within the range of 0.2, for example, when G2=-0.15, -0.025≦G4≦-
It is necessary to set it to 0.01. This value may seem severe at first glance, but in reality, this value can be raised or lowered (not shown) by changing the parameters of the optical system, such as the refractive index of the cladding, so that a wide range of cores can equivalently be used. It is expected that the spherical non-uniform refractive index lens of the present invention will not only be comparable in light gathering performance to the assembled lenses used in conventional optical pickups, but will also be superior in terms of size, weight, mass production, etc. be done.

In addition, the hemispherical inhomogeneous refractive index lens has the advantage that it can be made into a thin head, and when reading from the back side of the recording surface of a 1 mm thick disk, it is approximately 8 to 10111cm thick.When reading from the front of the recording surface of a disk with a thickness of 1 mm, it can be made into a head with a thickness of several M7lL. can. Therefore, the lens is suitable for picking up not only consumer optical discs but also commercial laminated optical discs such as computer memories, broadcast station memories, and shakeboxes. In addition, the inhomogeneous refractive index lens of the present invention, like the previously proposed ball lens, can be used as a relay lens for images, a high-density optical energy transmission path, a highly efficient coupling lens from multiple light sources to multiple light guide paths, and a Fourier lens for optical information processing. Conversion lenses, wide-angle measurement lenses, camera lenses,
It is expected to have a wide range of applications such as medical endoscope lenses and laser scalpel heads.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram showing the aberrations of a spherical lens without a cladding.
Figure 2 is a conceptual diagram showing aberrations due to a homogeneous refractive index sphere shell, Figure 3
The figure is an explanatory diagram showing the configuration and aberration correction effect of the basic cladded heterogeneous refractive index spherical lens. Figures 4A and 4b are configuration diagrams of the heterogeneous refractive index lens of the present invention, and Figures C, d, and e is a conceptual diagram showing changes in working distance due to asymmetry of the cladding, Figure 5 is a diagram illustrating the relationship between the thicknesses Dl and d2 of the cladding on the light source side and disk side for aberration compensation, and Figure 6 is the manufacturing accuracy of the cladding. FIG. 7 shows the thickness D2 of the disc-side cladding required for aberration compensation according to the refractive index distribution coefficients G2 and G4 of the core (the thickness d1 of the light source-side disc is constant). , a diagram also showing residual lateral aberration and operating distance. In the figure, 1 is the core, 2 and 3 are the cladding, 4 is the disk, and 5
is a reflective surface (film).

Claims (1)

[Claims] 1. In a heterogeneous refractive index lens in which a homogeneous spherical shell-like medium is concentrically covered with a spherically symmetric refractive index distribution sphere 1 whose refractive index decreases from the center to the periphery in an approximately square law distribution, a lens that corresponds to the optical path 2. One side of the periphery is set as the light source side and the other as the condensing side, and the thickness of the condensing side spherical shell medium is made thinner or the refractive index is made higher than that of the light source side spherical shell medium, or the condensing side spherical shell is A heterogeneous refractive index lens is characterized by having a thin medium, a high refractive index, and a long working distance. 2 In a hemisphere obtained by cutting a sphere with a homogeneous spherical shell-like medium concentrically over a sphere with a spherically symmetric refractive index distribution whose refractive index decreases from the center to the periphery in an approximately square law distribution, this plane The spherical shell-like medium on the light-collecting side is made thinner or has a higher refractive index than the spherical shell-like medium on the light source side, with the part as a total reflection surface and one of the two peripheries corresponding to the optical path as the light source side and the other side as the condensing side. Alternatively, the inhomogeneous refractive index lens is characterized in that the thickness of the condensing side spherical shell medium is reduced, the refractive index is increased, and the working distance is increased. 3. The heterogeneous refractive index lens according to claim 2, comprising a reflective film on the plane of the hemisphere of the heterogeneous refractive index.