AN OPTICAL DATA RECORDING DEVICE

Abstract: An optical data recording device for focusing a laser beam (52) on a recording medium (60) for storing and reading out information therefrom. The device is characterized by using a high efficiency holographic grating (30) as a polarization beam splitter. In a forward collimated light path the holographic grating diffracts a light beam from a laser (54) on an objective lens. The beam is reflected from the surface towards the recording medium (60) and transmitted through the grating onto a light detector (64). The grating (30) further provides for correcting a typical elliptical light beam shape from the laser (54) to a circular beam shape as well as for correcting the astigmatism typically found in the wave fronts emitting from a laser diode.

Description: This invention relates to optical data recording device according to the preamble of claim 1. In many optical storage devices a polarized beam splitter is used as an optical switch. The splitter passes all of the light from a laser light source to a recording medium, and at the same time it isolates the reflected beam from the medium to prevent the light from returning to the laser. This phenomenon is based on the principle that a specially coated beam splitter can reflect most of the light which is polarized perpendicular to its plane of incidence and transmit all the light polarized in its plane of incidence. The use of polarized beam splitters with quarter wave retardation plates is expensive. It is the purpose of the present invention to eliminate the need of expensive polarized beam splitters and quarter wave retardation plates, thereby greatly reducing the overall cost of components used in optical data recording. The optical data recording device of the present invention is characterized in that the polarisation-sensitive beam splitter consists of one holographic grating disposed for directing light towards the detector, and in that the polarisation-rotating means is constituted by a reflector arranged in the light path to direct the recording beam towards the recording medium. The invention uses an inexpensive holographic grating as a polarization beam splitter. Further, the holographic grating has the inherent features of creating a circular beam shape from a typical elliptical beam shape transmitted from the laser light source. This greatly reduces the spot size of the focused beam on the recording medium. EP-A-195657 discloses the use of a pair of holograms in combination with a wave plate in the paths between a light source and a recording medium as well as between the recording medium and a detector. The advantages and objects of the invention will become evident from the following detailed description of the drawings when read in connection with the accompanying drawings which illustrate preferred embodiments of the invention and in which: Fig. 1 depicts a prior art optical data recording device. Fig. 1A illustrates a typical quad diode having four zones on a light detector. Fig. 2A and Fig. 2B illustrate a holographic grating for transmission and reflection of a light beam. Figs. 3, 3A, 3B and 3C illustrate an embodiment of the present invention using a holographic grating with an optical recording device. Figs. 4 and 4A illustrate another embodiment of the present invention. Figs. 5A and 5B illustrate an incorporation of a wave front from a cylindrical lens onto a holographic grating. Like numerals and characters designate like elements throughout the figures of the drawings. In Fig. 1 a prior art optical storage device 10 is represented having a collimated beam of light indicated by arrows 12 produced from a laser 14. The light 12 is linearly polarized in the plane of incidence of a beam splitter 16. The light 12 is fully transmitted in this example by the beam splitter 16. The polarization of the light 12 is such that the beam splitter 16 transmits all of its power. The light then passes through a quarter wave retardation plate 18 and through an objective lens 20 where it is focused onto a recording medium such as a rotating optical disk 22. Upon reflection of the light from the rotating disk 22, the return light is polarized at 90 degrees from the original polarization of the laser 14. Accordingly, it is reflected by the beam splitter 16 through a toric lens 24 onto a light detector 26. Little of the return light is transmitted back to the laser source 14. Fig. 1A depicts a typical light detector having quad like zones 1, 2, 3 and 4. Fig. 2A illustrates the general principle of making a high efficiency holographic grating having general reference numeral 30. The grating 30 includes a light sensitive layer 32 and substrate 34. The layer 32 and substrate 34 making up the holographic grating 30 are greatly enlarged in this drawing for illustration only. Two collimated beams of light 36 and 38 interfere onto the light sensitive layer 32 which is supported by the substrate 34. Both beams are polarized perpendicular to the plane of incidence of the holographic grating 30. The holographic grating 30 made in this configuration is a transmission holographic grating which means that upon illuminating the hologram grating by a reconstruction beam similar to the beam 36 a beam 40 appears as a continuation of the beam 38. The light sensitive layer 32 can be a photosensitive polymer or dichromated gelatin. The thickness of the layer 32 must be many times the wave length of the light used with the recording device 10. The relationship between thickness and optical efficiency of the grating 30 can be found as described in an article entitled, "Coupled Wave Theory for Thick Holographic Grating", by H. Kogelnik, published in the Bell System Technical Journal, Vol. 48, No. 9, page 2909 through 2946, 1969. In this article the author also discusses that the diffraction efficiency is sensitive to the polarization of the illuminating beam in the reconstruction process. The teaching of the Kogelnik article is incorporated herein by reference insofar as may be required for an understanding of the present invention. In Fig. 2A the width of the beam 36 is L₁ and the beam width of the constructed beam 40 is L₂. The relationship between the two widths is L₂ = L₁/sin ϑ. For example, if the angle ϑ as shown in Fig. 2A is 30 degrees, the width L₂ of the reconstructed beam is equal to 2L₁. Hence the incident beam is expanded along one direction by a factor of 2. Fig. 2B is an illustration of a reflection hologram grating 30 similar to that described for Fig. 2A, comprising a light-sensitive layer 32 on a substrate 34. The hollogram is formed by two collimated beams of light 42 and 44 now coming from opposite directions and interfering on the light-sensitive layer 32. Upon reconstruction of the hologram 30 with the beam 44, a beam 46 is reflected from the hologram grating 30. Figs. 3 and 3B shown an embodiment of the present invention having a general reference numeral 50 in which the transmission hologram grating 30 is used. The hologram grating 30 is constructed in such a way that a collimated beam 52 with elliptical cross-section (in dash lines and also shown in Fig. 3B) from a laser 54 will become a beam 56 with a circular cross-section (in dash lines and also shown in Fig. 3C) after passage through the hoologram grating. The beam 56 from the laser 54 and the grating 30 are rotated 45 degrees so that the polarization of the beam 56 is at 45 degrees with respect to the plane of incidence on a prism 58 which reflects the beam towards an objective lens 59. It is known that coated mirrors or prisms can cause phase retardation on the incident beam. Because of the prism 58, the polarization of the return beam from a recording medium 60 and from the prism 58 is rotated by 90 degrees from that of the original beam 56. As a result, most of the reflected beam is transmitted through the hologram grating 30 and focused by a toric lens 62 onto a light detector 64. The detector 64 is shown in Fig. 3A with four quad zones 1, 2, 3 and 4. Fig. 4 illustrates another embodiment of the subject invention in which a reflection hologram grating 30 is used. The advantage of this embodiment is that the toric lens 62 is bonded to one side of the grating 30 to eliminate the need of a support for the toric lens 62. Fig. 4A again shows zones 1, 2, 3 and 4 of the detector 64. Figs. 5A and 5B show the incorporation of a wave front from a cylindrical lens onto th e hologram grating 30. The cylindrical wave front of a light beam 66 is produced by a negative cylindrical lens 70 on a positive cylindrical lens 68. The cylindrical wave front is used to compensate for astigmatism in many of the semiconductor laser diodes. The so-called astigmatic distance in laser diodes ranges from one micrometer to about 25 micrometers. It is difficult and costly to polish a glass cylindrical lens to correct this type of abberation. However, through the use of the hologram grating 30 the light source wave front can be easily and inexpensively corrected.

Claims: An optical data recording device comprising: a laser light source (54) for producing a recording beam; an objective lens (59) disposed between the light source and a recording medium (60) for focusing the recording beam on the recording medium; a polarisation rotating means arranged between the light source and the recording medium; a polarization-sensitive beam splitter arranged between the polarization-rotating means and the light source, and a light detector (64) for receiving the light reflected by the recording medium and passed through the beam splitter, characterized in that the polarisation-sensitive beam splitter consists of one holographic grating (30) disposed for directing light towards the detector (64), and in that the polarisation-rotating means is constituted by a reflector (58) arranged in the light path to direct the recording beam towards the recording medium (60). The device as described in Claim 1, characterized by a toric lens (62) disposed between the light detector and the holographic grating for focusing the reflected light onto the light detector.

Inventors: LEE, WAI-HON, OUDENHUYSEN, ADRIANUS H.
Applicants: LASER MAGNETIC STORAGE INTERNATIONAL COMPANY
Application year: 1987
Main classification: G11B7
IPC codes: G11B7