JP2863185B2 - Large aperture aspheric lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a large-diameter aspherical lens suitable for optical information reading, optical communication transmission systems, and the like.

<Prior Art> Large-diameter aspheric lenses have been conventionally used for laser pickups such as CDs (compact discs) and LDs (laser discs). Recently, it is widely used in magneto-optical memories of computers and fine optical systems in optical communication transmission systems.

The use of such a large-diameter aspherical lens is wide-ranging, but generally the numerical aperture NA is 0.45 to 0.55, and the focal length f is about 3.5 mm to 10.0 mm.

As for the material, plastic is widely used because it is inexpensive, has good workability and is lightweight.

A plastic lens having both aspheric surfaces is widely used in CD players and the like.

However, plastic has problems in heat resistance, hygroscopicity, and birefringence (homogeneity).

Since the heat resistance is not sufficient, there is a possibility that the material may be deteriorated by high-temperature treatment for joining to an optical lens barrel or the like. When sealing glass is used when fixing the lens, the outer periphery of the lens is exposed to a temperature of 400 ° to 500 ° during gold plating.

Therefore, a plastic lens cannot be used under such conditions.

As a lens used in an optical fiber connector, there is a gradient index lens conventionally. Such a lens has an unstable refractive index distribution characteristic as seen in the ion exchange method, and has a large variation in quality among lots. This has a bad influence on spherical aberration and the like.

It is also difficult to cope with a high NA, and it cannot be stably used in a high-precision optical system using a semiconductor laser light source or the like.

<Problems to be Solved by the Invention> Recently, a processing method using an ultra-precision mold press technique has been established in lens manufacturing, and it has become possible to manufacture an aspherical lens with extremely high precision. For example, in a connector lens for optical communication, when transmitting a semiconductor laser beam to an end face of a fiber to obtain a high coupling efficiency, it is necessary to use two or more lenses in an infinite optical system.

An object of the present invention is to produce an aspheric lens using an ultra-precision mold press technique using a high-refractive index glass, and a large-diameter aspheric lens having a large aperture despite having a very small focal length. Is to provide.

<Means for Solving the Problems> In order to achieve the above object, in a large-diameter aspheric lens according to the present invention, the first surface and the second surface are aspherical surfaces represented by equations, and It is configured to meet.

Z = [CiY ² / [1+ (1- (1 + Ki) Ci 2 Y 2) 1/2 ]]
+ EiY ⁴ + FiY ⁶ + GiY ⁸ + HiY ¹⁰ 1.0 <(f / NA) <3.0 1.7 <r ₁ /[(n-1)f]<2.3 n> 1.70, where Z: Aspheric surface from a plane perpendicular to the optical axis passing through the vertex Distance Y: Height in the radial direction from the optical axis Ci: Curvature (1 / r _i ) of the aspherical vertex of the i-th surface Ki: Conic constant of the i-th surface Ei to Hi: 4th order of the i-th surface To the tenth order aspherical coefficient NA: numerical aperture f: focal length d: center thickness of the lens r _i : radius of curvature of the aspherical vertex of the i-th surface n: refractive index of glass ν: Abbe number of glass <action In the large-diameter aspherical lens having the above-described configuration, f represents the focal length of the lens, and NA represents the numerical aperture.
Shows the range that f can take when is determined.

In order to satisfy the expression, the absolute focal length value cannot be increased with a high NA.

When f / NA is less than 1.0, it means widening of the angle. In order to satisfy the power, the radius of curvature becomes extremely small on both surfaces or one surface, and it becomes difficult to correct off-axis wavefront aberration.

That is, in terms of the third-order aberration amount, the spherical aberration indicates insufficient correction near the zonal light beam, and the correction is excessive in the annular light beam, the spherical aberration amount increases, and the astigmatism also becomes tangential (Tangen-tial). The direction has a large difference from the sagittal direction, and does not meet the purpose.

Moreover, if the NA is increased under these conditions, the focus becomes shorter,
It is very difficult to obtain an angle of view that satisfies NA, and it is difficult to obtain a curvature radius value that can be processed.

Conversely, if f / NA is larger than 3.0, the design and manufacture are relatively easy because the absolute focal length is large and the absolute radius of curvature is large even at high NA.

In addition, although spherical aberration indicates insufficient correction from the optical axis to the annular light flux, aberration can be improved, but high NA
The purpose of shortening and shortening the focal length cannot be fulfilled.

That is, the conditions of weight reduction and compactness are contradictory to each other, and are outside the limits of the present invention.

If _ri / [n-1) f] is smaller than 1.4 in the formula, the radius of curvature of the first surface becomes relatively small with respect to the focal length, so that astigmatism increases and off-axis wavefront aberration worsens.

On the other hand, if r _i / [(n−1) f] is greater than 2.3, spherical aberration cannot be completely corrected.

When an aspheric lens having an extremely small focal length of about 1.0 mm (0.5 to 1.5 mm) under the conditions of n> 1.70 and v> 30.0, the first lens is used.
The magnitude of the radius of curvature of the surface and the second surface greatly contributes to lens aberrations and manufacturing problems. In general, as the focal length decreases, the radii of curvature of the first surface and the second surface also decrease accordingly, making design and manufacturing difficult. Material plastic with small refractive index (n = 1.49) or FCD1 (n = 1.49)
If 9) is used, the radius of curvature must be further reduced. As a result, conditions for correcting lens aberrations such as astigmatism, coma, and spherical aberration are further deteriorated. Further, when the lens is actually manufactured after the design, whether the lens is made by a mold press or directly ground, the magnitude of the radius of curvature greatly affects the difficulty of processing accuracy. This significantly affects not only the lens performance but also the production cost. When used in an environment of 1300 to 1550 nm for optical communication, when used, the relative value of n becomes even smaller than n, so a lens material with a high refractive index of n> 1.70 is absolutely necessary. Become.

<Examples> Hereinafter, examples of the large-diameter aspherical lens according to the present invention will be described in detail.

FIG. 1 is an optical path diagram of an embodiment of a large-diameter aspheric lens according to the present invention.

The high refractive index glass which is the material of the lens according to the present invention,
Glass transition point (Tg) of 58 to obtain appropriate molding conditions
Use a material that exceeds 0 °.

This improves the heat resistance versus moisture absorption and temperature characteristics of the lens.

 Hereinafter, data of two examples are shown.

Example n 1.782999 f 0.7 d 1.2 WD 0.4 r ₁ 1.22961 K ₁ -4.047151 E ₁ -0.152374x10 ⁰ F ₁ -0.100630x10 ¹ r ₂ -0.56509 K ₂ -3.914536 E ₂ -0.241193x10 ⁰ F ₂ -0.496479x10 ^-1 Wavefront aberration (RNS) On-axis 0.001λ Off-axis 0.002λ In the above embodiment, wavefront aberration and on-axis are numerical values on the optical axis,
The wavefront aberration and off-axis indicate numerical values at 5 μm from the optical axis.

 WD indicates the working distance.

FIG. 2 is a graph showing aberration characteristics of the lens of the example.

<Effects of the Invention> As described above in detail, the lens according to the present invention can be suitably used for an optical fiber transmission line or the like because the above-described favorable aberration characteristics can be obtained with a large diameter by aspherical processing.

 Also, the assembly process is simple.

[Brief description of the drawings]

FIG. 1 is an optical path diagram of an embodiment of a large-diameter aspherical lens according to the present invention. FIG. 2 is a graph showing the aberration characteristics of the embodiment. d: center thickness of the lens r _i : curvature of a vertex of the aspheric surface of the i-th surface n: refractive index of glass

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁶ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (1)

(57) [Claims] 1. A large-diameter aspherical lens wherein the first surface and the second surface are aspherical surfaces represented by an expression, or are configured to satisfy the conditions of the expression. Serial Z = [ciy ² / [1+ (1- (1 + Ki) Ci 2 Y 2) 1/2 ]]
+ EiY ⁴ + FiY ⁶ + GiY ⁸ + HiY ¹⁰ 1.0 <(f / NA) <3.0 1.7 <r ₁ /[(n-1)f]<2.3 n> 1.70, where Z: Aspheric surface from a plane perpendicular to the optical axis passing through the vertex Distance Y: Height in the radial direction from the optical axis Ci: Curvature (1 / r _i ) of the aspherical vertex of the i-th surface Ki: Conic constant of the i-th surface Ei to Hi: 4th order of the i-th surface To the tenth order aspherical coefficient NA: numerical aperture f: focal length d: center thickness of the lens r _i : radius of curvature of the aspherical vertex of the i-th surface n: refractive index of glass