Optical: systems and elements – Lens – Spherical
Reexamination Certificate
2000-07-25
2001-05-22
Mack, Ricky (Department: 2873)
Optical: systems and elements
Lens
Spherical
C369S112040
Reexamination Certificate
active
06236514
ABSTRACT:
This application is based on application No. 11-213101 filed in Japan, the content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head device and a near-field light emitting device, and more particularly to an optical head device which is suited to be used for a high-density optical memory (record/reading).
2. Description of Prior Art
In the art of optical memories which optically record and read information, in recent years, with speed-up of computer processing and development of multimedia, high-density devices which are capable of recording an extremely large volume of information are demanded, and in order to comply with the demand, a near-field optical recording technique is suggested. In a conventional optical memory using a laser beam, the recording density is limited depending on the diffraction limit of light, and such an optical memory can record and read marks of sizes of at least light wavelength (around 300 nm).
A recently proposed optical memory which uses near-field optics radiates light to a recording medium for recording/reading with the optical head and the recording medium arranged at an interval of only some dozen nanometers. At this time, the optical memory uses a fiber probe with a minuscule aperture smaller than light wavelength and/or an SIL (solid immersion lens) so that a light spot which is smaller than light wavelength can be formed on the recording medium. Thereby, in spite of the diffraction limit, it becomes possible to record and read minuscule marks less than 200 nm.
Incidentally, in the near-field optical technique, in order to obtain a minuscule light spot, it is preferred to converge light to the diffraction limit at a numerical aperture which is as high as possible. For this purpose, U.S. Pat. No. 5,125,750 suggested a method in which a convergent bundle of rays is directed to an SIL made of a material with a high refractive index by use of an objective lens.
In a conventional optical head device using an SIL, specifically, a laser beam is collimated into a parallel bundle of rays and is reflected by a mirror. Then, the laser beam is converged by an objective lens and is incident to the SIL. In this structure, it is necessary to use various optical elements such as a mirror, an objective lens, etc., which indispensably increases the size and weight of the optical head.
Lately, it is suggested to use an SIM (solid immersion mirror) instead of an SIL (see pages 59-62 of Reports of ODF'98, Tokyo, Jun. 16, 1998). The SIM converges light by performing refraction once and reflection twice in a material with a high refractive index, and this eliminates the necessity of using a convergent lens. However, a mirror to reflect a collimated laser beam is still necessary, and it is not perfect as a small and light optical head. Also, since the optical elements such as an SIM, a light source, a mirror, etc. must be assembled, troublesome works, namely optical axis adjustment and focusing must be carried out. Further, there are problems that chromatic aberration may occur because of refraction and that the focal length may shift with a change in temperature.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a small and light optical head device and a near-field light emitting device which are easy to be assembled, and more specifically, do not necessitate an objective lens, a reflection mirror, a focus adjustment mechanism and a chromatic aberration correction mechanism.
In order to attain the object, an optical head device according to the present invention comprises an optical element and a light source. The optical element comprises: a first reflective surface which is planar and which has a light transmitting portion in its center; a second reflective surface which is spheroidal and which converges light on the light transmitting portion; and a concave portion which is located in a center of the second reflective surface and which transmits light. The light source emits a divergent bundle of rays from a point which is a center of the curvature of the concave portion.
In the structure, the divergent bundle of rays emitted from the light source passes through the concave portion and is incident to the optical element. In the optical element, the light is reflected by the first reflective surface and further reflected by the second reflective surface, whereby the light is converged on the center of the first reflective surface. Then, the light is emergent from the optical element through the light transmitting portion.
As the light source, a point light source which emits a divergent bundle of rays from a point, such as an optical fiber, a light guide, an optical waveguide, a laser diode or the like is used. Thereby, neither an objective lens nor a reflective mirror is necessary, and a small and light optical head device can be obtained. Further, because the optical path is formed in one optical element, complicated works, namely, optical axis adjustment and focusing are not necessary. Light emitted from the light source is incident to the optical element through the concave portion, and at this time, refraction by a lens does not occur. Accordingly, chromatic aberration does not occur, and light within a large wavelength range can be used. Moreover, the optical element changes its shape similarly with a temperature change, and the focal length does not change; therefore, a mechanism for adjusting the focus is not necessary.
If the optical element is made of a material with a high refractive index and if the aperture diameter of the light transmitting portion made in the first reflective surface is not more than light wavelength, near field light effuses from the light transmitting portion. Thereby, an optical head device which is suited to be used as a high-density optical memory can be obtained.
REFERENCES:
patent: 5125750 (1992-06-01), Corle et al.
patent: 5615203 (1997-03-01), Fukakusa
patent: 5729393 (1998-03-01), Lee et al.
patent: 5793407 (1998-08-01), Park et al.
Mack Ricky
Minolta Co. , Ltd.
Morrison & Foerster / LLP
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