JP2511279B2 - Optical system for recording / reproducing optical information media - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical system for recording / reproducing an optical information medium including a light source, a collimator lens and an objective lens, and in particular, chromatic aberration and other aberrations are corrected in the entire optical system. Optical system.

(Prior Art) As is well known, a recording / reproducing optical system of a compact disc or a laser disc uses a monochromatic light such as a semiconductor laser as a light source, and after converting a light flux from the light source into a parallel light by a collimator lens. Then, it is focused as a spot of about 1 μφ on the disc by the objective lens.

In the case of reproduction, the reflected light from the pit surface further advances in the same optical system from the objective lens toward the collimator lens. Then, in general, a beam splitter is arranged in the middle of this optical path, and the optical path of the reflected light from the pit surface is changed and guided to the photo-detecting element to obtain a focus signal and a tracking signal.

Conventionally, as this type of collimator lens, although the lens type is different, correction of chromatic aberration is not taken into consideration as in JP-A-51-18557 and JP-A-58-87521, or JP-A-58- Even when correction of chromatic aberration is taken into consideration, such as 72114, 6032-519, and 62-35311, most of them are designed to correct chromatic aberration of the collimator lens or the objective lens alone. Met.

(Problems to be solved by the present invention) When a semiconductor laser is used as a light source, a mode hop is caused by an external environment such as temperature, and a sharp change in oscillation wavelength occurs. However, there is a limit to the response speed of the focusing operation mechanism described above, and it is not possible to follow the deviation of the focusing position of the light flux due to a sudden wavelength change, etc., and the recording error and the reproduction error are caused by the increase of the spot diameter on the disc. It was the cause.

On the other hand, the objective lens used in combination with the collimator lens needs to be as light and compact as possible in order to facilitate the focusing operation and the tracking operation. In recent years, an aspherical surface has been used as the objective lens. The single lens made of plastic or glass is often used. Single-lens lenses have the advantage of being small and lightweight, but they have the drawback of not being able to correct chromatic aberration.
When used in combination with a conventional collimator lens,
The above problems due to changes in the wavelength of the light source cannot be avoided.

This invention attempts to improve the performance of the entire optical system by performing aberration correction by integrating an objective lens and a collimator lens, and setting the residual aberration of the collimator lens according to the aberration correction status of the objective lens. It is a thing.

(Means for Solving the Problems) An optical system for an optical disc of the present invention comprises a light source, a collimator lens and an objective lens, and the collimator lens cancels this depending on the aberration correction state of the objective lens. Aberration correction with residual aberration is performed to reduce the occurrence of aberrations such as chromatic aberration when a collimator lens and objective lens are used in combination. At this time, a first lens group including a biconvex first lens element, a biconcave second lens element, and a positive third lens element from the front, and a five-element configuration including a biconvex fourth lens element and a biconcave fifth lens element. And is configured to satisfy the following conditions.

However, fc: Composite focal length of the entire collimator lens system Ri: Radius of curvature of the i-th lens surface from the front Ni: Refractive index of the i-th lens from the front νi: Abbe number of the i-th lens from the front It is preferable to satisfy the following conditions.

(3) 1.6 <N ₁ , N ₄ <N ₃ (Function) Condition (1) is a condition for correcting spherical aberration, and if the upper limit is exceeded, the correction function for spherical aberration by the second surface and the sixth surface will occur. Get smaller. In particular, in the case of this lens type, it is only the second surface and the sixth surface that the correction effect is large. On the contrary, when the lower limit is exceeded, the spherical aberration largely generated by the first surface is reduced to the second surface and the sixth surface. Since it is necessary to correct only the six surfaces, not only overcorrection is likely to occur, but also high-order overflare occurs, which makes it impossible to obtain a bright lens.

The condition (2) is a condition for appropriately correcting the chromatic aberration. When the upper limit is exceeded, the correction of the chromatic aberration by the biconcave second lens and the fifth lens is insufficient, and the condition (1) is combined with the single-lens objective lens. At that time, it becomes impossible to correct chromatic aberration as a whole. On the other hand, if the lower limit is exceeded, chromatic aberration will be overcorrected, and at the same time, the Abbe number of the currently available glass materials will be limited, and the curvatures of the second surface, the third surface, and the sixth surface will be too strong. It cannot be a good lens.

The condition (3) makes it possible to reduce the Petzval sum, and is effective for obtaining a flat image surface.

(Examples) Examples 1, 2 and 3 below show only the collimator lens of the present invention, and an optical system in which a general aspherical objective lens is combined with the collimator lens of Example 1 as a reference example,
Including cover glass *.

In the table, fc is the synthetic focal length of the collimator lens NA is the numerical aperture R is the radius of curvature of each lens surface D is the distance between the refracting surfaces N is the refractive index of each lens at a wavelength of 830 nm.

Example 1 Example 2 Example 3 Reference example (Effect of the Invention) The present invention compensates aberrations of the objective lens and the collimator lens by correcting the aberration of the objective lens in consideration of the aberration correction state of the objective lens. While using the objective lens, it is possible to excellently correct aberrations such as chromatic aberration of the entire optical system. Further, as seen in each of the examples, although it is a bright collimator lens having a large NA, it is possible to satisfactorily correct aberrations.
Since it was possible to form a group structure, adjustment of eccentricity and the like during assembly was not complicated, and an optical system that was easy to manufacture could be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of a first embodiment of a collimator lens of the present invention, FIG. 2 is its aberration curve diagram, and FIG. 3 is an optical system in which the collimator lens of the first embodiment and a general aspherical objective lens are combined. 4 is an aberration curve diagram of the general aspherical objective lens shown in FIG. 3 alone, and FIG. 5 is an aberration curve diagram of the entire optical system of FIG.

Claims (1)

(57) [Claims] 1. A light source, a collimator lens, and an objective lens. According to the aberration correction state of the objective lens, the collimator lens performs aberration correction so as to have residual aberration that cancels the aberration, and the collimator lens and the objective lens. In an optical system that reduces the occurrence of aberrations such as chromatic aberration when used in combination with lenses, the collimator lens includes a biconvex first lens and a biconcave second lens from the front when the light source section is behind the lens. And a first lens group consisting of a positive third lens element, a biconvex fourth lens element, and a biconcave fifth lens element, which is configured to satisfy the following conditions. Optical system for recording / reproducing characteristic optical information media However, fc: Composite focal length of the entire collimator lens system Ri: Radius of curvature of the i-th lens surface from the front Ni: Refractive index of the i-th lens from the front νi: Abbe number of the i-th lens from the front