Optical device utilizing superresolution effect

Abstract: An optical device which is miniaturized by allowing light to operate in a confined space. Such optical devices are particularly suitable for optical pick-up devices, laser printers, and sensors, which require little space and light weight, wherein the optical device includes an integrated type and a substrate type. The device comprises a system of optical components including a light source, lens, beamsplitter, mirror and optical detector and is such that one of the components blocks or otherwise diminisher the intensity of such a central portion of the beam in order to reduce the spot size via the superresolution effect.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to an optical device of a small size. More particularly, the present invention relates to an optical device miniaturized by allowing light to operate in a confined small space. Such optical devices are particularly suitable for optical pick-up devices, laser printers, and sensors, which require little space and light weight. In general, the optical device has two types; an integrated type and a substrate type. 2. Description of the Prior Art: In recent years, various kinds of optical information processors have been developed. Typical examples are compact disks that are widely used instead of gramophone records, electronic filing systems for storing documents and photographs in magneto-optical disks, and laser printers for writing information in a recording section of a copying machine with a laser beam. An optical information processor is also used in automated manufacturing equipment as an optical encoder and an optical range finder. For example, a compact disk system is provided with pits having a depth of about 1/2 wave depth which are intended to indicate digital signals "1" and "0". By reading data in terms of the pits the data is reproduced on a phonograph record. The pit data is optically read so that large spaces are not required for one bit data but only a tiny area of about 1 µm 2 is sufficient. This leads to a highly dense recording system. Because of this advantage, compact disks are becoming more popular than magnetic tapes or magnetic disks. The compact disk requires an optical component such as a pick-up device for reading pit data. The optical components include a lens, a beam splitter and other components, and the assemblage and adjustment of these are time- and labor-consuming. In addition, the size is likely to become large and heavy, and the cost is likely to be high. In order to explain the background in more detail, reference will be made to Figure 3 which shows a known optical pick-up device intended to overcome the problems mentioned above. This optical device is a self-contained system 59 which includes a hologram 55 formed on a glass plate 52 so as to enable it to function as a beam splitter and a diffracting grating, a semiconductor laser 51 as an optical source and an optical detector 56 for detecting signals. The pick-up device is operated as follows: A laser beam D emitted upward from the semiconductor laser 51 is focused on an optical disk 57 through a collimator lens 53 and a focusing lens 54 both disposed above the hologram 55. The light reflecting upon the disk 57 reversely follows the above-mentioned path but is deflected by the hologram 55, the light being detected by the detector 56. The focusing lens 54 is adjusted as to its position and focal point by means of an actuator (not shown) using an electromagnet. This known pick-up device is comparatively lightweight because of a reduced number of components as well as a reduced size of each component, but ideally, the desired weight should be much lighter. Among the components, it is only the focusing lens 54 that can be moved by the actuator because of its light weight. Figure 4 shows another pick-up device designed to solve the problems pointed out above. A typical example of this device is disclosed in Japanese Laid-Open Patent Publication No. 62-117150. A three beam generating grating 62, a collimator diffracting grating 63, astigmatism generating lenses 76 and 77, a semiconductor laser 71 and an optical detector 78 are all mounted on a glass plate, except for a focusing lens 75 for focusing a source light on the disk 57. This arrangement can dispense with any optical adjustment between one component and other. There is little possibility of causing displacement or deflection due to mechanical vibration or shock. This type of pick-up device will be referred to as "substrate type pick-up". A substrate type pick-up device can be smaller in size than an ordinary pick-up device because the source light is propagated through the substrate 61 even though the distance between the light source and optical disk 57 is the same. However, the small pick-up device requires the laser beam to be as small as possible in diameter so as to pass through the focusing lens without divergence. As a result, the advantage of the small size is traded off against the necessity for requiring a highly precision focusing lens 75. The focusing lens 75 is not mounted on the substrate 61 which prevents the size of the pick-up device from being reduced beyond a certain limit. In order to make most use of the characteristics of light, the beam must be sufficiently focused on the optical disk 57. The minimum radius W 0 of the beam focused by an ordinary lens, the numerical aperture NA of the lens and the wavelength λ of the beam are in the following relationship: W 0 = λ /NA The numerical aperture NA is nearly equal to d/2f where d is the smaller of the diameter of the lens or the diameter of the laser beam (i.e. the effective diameter of the lens), and f is the focal length of the lens. The minimum diameter 2W 0 of the beam is called "diffraction limit". If a lens has a larger numerical aperture NA, the beam can be focused smaller but this results in more abberation and difficulty in fabrication. If the device is to be made compact, the diameter of the beam must be reduced, but in order to maintain the numerical aperture NA constant, the focal length must be reduced. This requires the lens face to have a small curvature, but it is likely to cause aberration. Since a substrate type pick-up device has a relatively short distance from the light source to the focusing lens 75, the effective diameter of the focusing lens 75 can be relatively small. As a result, it is difficult to produce focusing lenses having a large numerical aperture NA. Any other optical systems such as hologram laser printers cannot be miniaturized without sacrificing the focusability of the lens. The reduction in the focusability restricts the range of application. In order to focus a light beam to below the minimum diameter decided by the numerical aperture NA of the lens, another method is proposed which consists of blocking light from passing through a central portion of the focusing lens 75. According to this method, the light beams passing through the peripheral portion of the lens are focused, thereby intensifying the secondary peak appearing on both sides of the primary peak on the disk 57. In contrast, the primary peak is narrower than when the light passing through the central portion is not blocked. This means that the light beam is focused beyond the minimum diameter of the beam determined by the numerical aperture NA of the lens. This phenomenon is called the "super-resolution effect", and such lenses are called "super-resolution lens". Under a known system a super-resolution lens is disposed separately from the optical pick-up system. This necessitates the movement of the lens in accordance with that of the pits in the optical disk 57. Because of this movement, the light beam is likely to deviate from the central portion of the lens, thereby failing to achieve the desired focusing characteristics. As is evident from the foregoing description, a light beam cannot be focused beyond a certain limit, thereby making it difficult to achieve a miniaturized optical pick-up device. "Applied Optics", Vol 29, No 20, 10 July 1990, on which the preamble of claim 1 is based, discloses an optical pick-up device for reading an optical disc, the device comprising: an optical system of optical components, the optical system comprising a light source, a reflective element, an objective lens, a beam splitter and an optical detector, wherein the lens, the beam splitter and the optical detector are disposed, in use, in a path of light from the light source; and wherein the optical system is configured so that a light beam is prevented from focusing through a central portion of the lens, whereby the lens focuses light to below the diffraction limit. The lens is not mounted on a substrate, so it is not possible to minaturise the optical device, for the reasons given above. Furthermore, a high precision lens is required, again as described above. Examples of super-resolution optical systems are disclosed in EP-A-0 411 525, US-A-5 025 438 and JP-A-2 177 206. According to the present invention there is provided an optical system of the above type characterised in that the light source, the beam splitter and the optical detector are disposed on a light transmissive substrate ; and in that the objective lens has an aspheric face and is integral with the light transmissive substrate for receiving the light from the light source which passes through the light transmissive substrate. In a preferred embodiment the objective lens comprises a central portion which differentiates the optical characteristics of a light beam incident on the central portion from the properties of a light beam incident on a peripheral portion thereof. In a preferred embodiment the optical system is configured so that a light beam is prevented from passing through a central portion of the objective lens. In a preferred embodiment a central portion of said objective lens is flattened to negate focusibility thereof. In a preferred embodiment a central portion of said objective lens prevents a light beam from passing therethrough. In a preferred embodiment the light beam from the light source passes the objective lens so as to irradiate an optical disc on the outside of the optical system with the light beam. In a preferred embodiment the light beam from the outside of the optical system passes the objective lens so as to make the light beam incident upon another one of the optical components in the optical system. Thus, the invention described herein makes possible the advantages of (1) providing an optical device miniaturized without sacrificing optical characteristics, and (2) providing an optical device allowing the integration of optical components in a relatively small space. These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view showing an optical pick-up device to which a substrate type optical device according to the present invention is applied; Figure 2 is a plan view of an alternative lens for the optical device of Figure 1; Figure 3 is a perspective view showing an optical pick-up device using a conventional hologram; and Figure 4 is a perspective view showing a conventional substrate type optical pick-up device. DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Referring to Figure 1, the exemplary embodiment is a substrate type optical pick-up device which has a multi-sided substrate 110 of transparent glass. The substrate 110 includes an optical system including optical components. The substrate 110 has an inclined face 110a at one end on which a semiconductor laser 111 is disposed as a light source to which a submount 111a made of a silicon substrate having an intergrated semiconductor laser driving circuit (not shown) is connected. The semiconductor laser 111 and submount 111a are integrated with the inclined face 110a . A light beam A from the semiconductor laser 111 enters the substrate 110 , and is injected into a diffracting grating 119. The light beam A is split into three beams A' having different angles of reflection . The three beams A' are injected into a diffracting grating 115 formed on the top surface 110c of the substrate 110 , the diffraction grating 115 including two kinds of hologram beam splitters having different grating period. The diffraction grating 115 splits the light beams A' into a light beam reflected thereon and diffracted light beams which are not used in Example 1. The light beams reflected on the diffraction grating 115 are diffracted vertically upwards by a diffraction grating 123 formed on the bottom 110b and converted into parallel light beams B . The parallel beams B are focused by a super-resolution lens 114 integral with a portion of the substrate 110 corresponding to the diffraction grating 123 whereby they are focused on an optical disk 107 . The super-resolution lens 114 is formed by depositing an aluminum film 114a around the center of an aspheric lens. In addition, it is preferable that the aluminum depositing portion 114a is roughened by a chemical etching so as to prevent the reflecting light beam from returning to the semiconductor laser 111 . Circled in Figure 1 is an enlarged view which shows the distribution of optical intensity possessed by each spot on the optical disk 107, and according to the super-resolution lens 114, the diameter 2W 0 of the beam can be reduced by 70% than when an ordinary lens is used. As shown by the enlarged view, each optical spot includes two secondary peaks which appear on each side of a primary peak. The light beam reflecting upon the optical disk 107 passed through the super-resolution lens 114 and is converted into parallel light beams. Then, they are split into a diffracted light beam C and a reflecting light beam by the diffracting grating 115 . In the illustrated example, this reflecting light is not used. The diffracted light beam C is injected into a sextant detector 116 formed on the inclined face 110a of the substrate 110 which is formed adjacent to the semiconductor laser 111 so as to detect signals, a focal point, and the positions of pits. Part of the secondary-peaked reflecting light beams results in noise but the noise is not so strong as a signal which would be caused by the primary peak, thereby providing no problem in detection. The super-resolution lens 114 is not limited to the structure described above, but other structures can be selected; for example, a central portion of the aspheric lens is flattened to negate the focusability of this central portion. Alternatively, it is possible to deposit a highly reflective film such as an aluminum film on the flat portion so as to produce a flat mirror. Light beams which are not focused on the optical disk 107 return to the semiconductor laser 111 , and part of it enters the sextant detector 116 . The reflecting light beams are constant irrespective of the movement of the optical disk 107 , and it results in the overlapping of d.c. bias components in the sextant detector 116 so that no unfavorable influence is inflicted on the detection of signals. These reflecting light beams constitute an external resonator whereby the optical intensity of the semiconductor laser is made constant regardless of any variation in the reflecting light. A super-resolution effect can be achieved by methods other than by using the super-resolution lens 114 ; for example, by not forming the diffraction grating 123 around the central portion of the substrate 110 , a different optical property is imparted to this portion from that of the other portion. In this case, an incident light to this portion cannot be used as a signal. Because of the absence of the diffraction grating 123 , the incident light to this portion passes from the bottom 110b through the substrate 110. Alternatively, it reflects and passes through the substrate 110 from the opposite side 110d . No unfavorable influence is inflicted on the characteristics of this optical pick-up device. The substrate type pick-up device is usable not only for compact disk systems but also for magneto-optical disk systems. In the latter application, it is required that a polarizer such as a Glan-Thompson prism, a polarizing beam splitter, a deep-groove diffracting grating, and a birefringence crystal is disposed individually or integrated in the substrate 110 . The pattern 114a deposited on the hyper-resolution lens 114 is not necessarily required to be circular but may be rectangular as shown in Figure 2 as reference numerals 314 and 314a . This is because restraint is loose upon resolution in a direction normal to the pit pattern on the optical disk. The longitudinal direction of the rectangular pattern depends upon relative directions of the pits and substrate 110 , and therefore the relationship therebetween is not limited to the illustrated example. The application of the present invention is not limited to an optical pick-up device (Example 1 but it can be applied to other optical devices such as optical range finders and optical sensors. When it is applied to an optical sensor, it is not necessary to provide a light source in the substrate by using a detecting system in which the light is taken from outside. Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein.

Claims: An optical pick-up device for reading an optical disc, the device comprising: an optical system of optical components, the optical system comprising a light source (111), a reflective element (119), an objective lens (114), a beam splitter (115) and an optical detector (116); wherein the lens (114), the beam splitter (115) and the optical detector (116) are disposed, in use, in a path of light from the light source (110); and wherein the optical system is configured so that a light beam is prevented from focusing through a central portion of the lens (114) whereby the lens focuses light to below the diffraction limit; characterised in that the light source (111), the beam splitter (115) and the optical detector (116) are disposed on a light transmissive substrate (110); and in that the objective lens (114) has an aspheric face and is integral with the light transmissive substrate (110) for receiving the light from the light source which passes through the light transmissive substrate. An optical device according to claim 1, wherein the objective lens (114) comprises a central portion which differentiates the optical characteristics of a light beam incident on the central portion from the properties of a light beam incident on a peripheral portion thereof. An optical device according to claim 1, wherein the optical system is configured so that a light beam is prevented from passing through a central portion of the objective lens (114). An optical device according to claim 2, wherein a central portion of said objective lens (114) is flattened to negate focusibility thereof. An optical device according to claim 2, wherein a central portion of said objective lens (114) prevents a light beam from passing therethrough. An optical device according to any one of claims 1-5, wherein the light beam from the light source (111) passes the objective lens (114) so as to irradiate an optical disc (107) on the outside of the optical system with the light beam. An optical device according to claim 6, wherein the light beam from the outside of the optical system passes the objective lens (114) so as to make the light beam incident upon another one of the optical components in the optical system.

Inventors: HAMADA TOSHIMASA, KATO SHOHICHI, MATSUI SADAYOSHI, NAKAMICHI MASUMI, YAMAMOTO OSAMU, YAMAMOTO SABURO, YOSHIDA TOSHIHIKO, HAMADA, TOSHIMASA, KATO, SHOHICHI, MATSUI, SADAYOSHI, NAKAMICHI, MASUMI, YAMAMOTO, OSAMU, YAMAMOTO, SABURO, YOSHIDA, TOSHIHIKO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: G02B27
IPC codes: G11B7, G02B27