OPTICAL RECORDING AND REPRODUCING DEVICE

Abstract: A laser light flux is divided and synthesized so that its light intensity distribution is changed from a Gaussian distribution to a non-Gaussian distribution. When a laser light flux (L₀) hav­ ing a Gaussian distribution is irradiated onto a dividing prism (1), it is divided into a left part (l₁) and a right part (l₂) which are refracted right and left invensely from each other. When the right and left parts (l₁, l₂) are incident on a synthesizing prism, they are refracted again and travel to be overlapped with each other on a recording medium. The light intensity distribution on a segment (dd′) at the apex of the synthesizing prism is weaker at the central part and stronger at the peripheral part. This is because the light fluxes resulting from the division are inverted in their position so that the central part of the Gaus­ sian distribution is separated left and right while the peripheral part with a weaker light intensity is centered.

Description: BACKGROUND OF THE INVENTION This invention relates to an optical recording and reproducing device which permits signals to be recorded and reproduced on an optical medium with a high density by irradiating a laser light or the like with its flux converged into a minute spot onto the optical medium and permits the signals once recorded to be erased. More particularly it relates to an optical recording and reproducing device suited to erase information at a high speed. In such a kind of conventional devices, the light flux of the laser light or the like irradiated onto an optical recording medium provided, as disclosed in Japanese Patent Unexamined Publication No. 59-71140, a substantially circular or elliptic shape and a "Gaussian" light intensity distribution where the light intensity is highest at its center and exponentially attenuates towards its periphery. The Gaussian light intensity distribution on the optical recording medium resulted in a Gaussian temperature distribution there, and this gave rise to several difficulties. Fig. 1 shows a temperature distribution 28 on an optical recording medium in the conventional devices. The abscissa indicates a position of an optical recording medium, and the ordinate indicates the temperature at the position irradiated with the Gaussian light flux. In this figure, it is necessary to maintain the temperature between T₁ and T₂ in order to erase the signals recorded on the medium assuming that T₁ is an erasable minimum temperature and T₂ is an erasable maximum temperature which means a writable minimum temperature. Moreover, although the temperature for writing must be higher than T₂, it should be also controlled within an implicit upper limit T₃ for the protection of recording mediums. Thus, it is troublesome and difficult to control the temperature for writing and reproduction. For example, when the information recording in a position range of X₁ - X₄ is intended, the temperature distribution 28 does not permit the recording in ranges of X₁ - X₂ and X₃ - X₄ whereas a possible temperature distribution 28-1 may destroy the medium because of its high temperature. On the other hand, when the information erasing is intended in the range of X₁ - X₄, the temperature distribution 28 does not permit the erasing in a range of X₂ - X₃ but leaves the information there. For these reasons, it has been proposed to individually provide laser light sources for recording and reproducing but it is still difficult to solve the above problem. FR-A-2 133 588 discloses an optical recording and reproducing device with the features included in the first part of claim 1. There, the direction of polarised light is altered by a photo-electric converter element so that the light beam travels along different paths depending on the polarisation. A spatial filter is disposed in the path taken by the writing/erasing beam to render the intensity distribution of that beam more uniform. SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical recording and reproducing device which achieves a temperature distribution suitable for both recording and reproducing information on and from the recording medium. This object is attained by the device characterised in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing a temperature distribution on a recording medium in conventional recording and reproducing devices; Fig. 2 is a view for explaining the basic idea of this invention of providing a non Gaussian distribution light flux from a Gaussian distribution light flux; Fig. 3 is a view showing one embodiment of this invention wherein to realize the basic idea of this invention shown in Fig. 2, a quadrangular pyramid prism is used as a light flux dividing prism while another quadrangular pyramid prism is used as a light flux synthesizing prism; Fig. 4 is a view showing another embodiment of this invention wherein to realize the basic idea of this invention shown in Fig. 2, cone prisms are used as a light flux dividing prism and a light flux synthesizing prism, respectively; Figs. 5a and 5b, and 6 are views for explaining advantages when using an even distribution light flux; Figs. 7 to 11 are views showing embodiments of this invention wherein a light flux dividing prism and a light flux synthesizing prism are incorporated in an optical system of an optical recording and reproducing device, respectively; Fig. 12 is a view for explaining how a minute spot light which is suitable to heat and abruptly cool a part of an optical recording medium and to heat and gradually cool it; Fig. 13 is a view showing a further embodiment of this invention; and Fig. 14 is a graph showing temperature distributions on a recording medium, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Fig. 2, explanation will be given for the basic idea of this invention wherein a laser light flux is divided and synthesized so that its light intensity distribution is changed from a Gaussian distribution to a non-Gaussian distribution. When a laser light flux L₀ having a Gaussian distribution indicated by 3 is irradiated onto a triangle prism 1, it is divided into a left light flux portion ℓ₁ and a right light flux portion ℓ₂ which are refracted right and left inversely from each other. When the light fluxes ℓ₁ and ℓ₂ divided left and right are incident on another triangle prism 2, they are refracted again and travel to be overlapped with each other on a segment dd'. The light intensity distribution on a segment bb' at the apex of the prism 2 is weaker at the central part and stronger at the peripheral part as illustrated by 4. This is because the light fluxes ℓ₁ and ℓ₂ are inverted in their position so that the central part of the Gaussian distribution 3 is separated left and right while the peripheral part with a weaker light intensity is centered. The light intensity distribution on a segment cc' at a position apart from the apex of the prism 2 results as indicated by 5. At this position, the light fluxes ℓ₁ and ℓ₂ are overlapped at the central part of the distribution to provide a stronger light intensity there than that of the distribution 4. Then, the entire light flux width thus formed is narrower than that of the distribution 4. The light fluxes ℓ₁ and ℓ₂ almost completely overlapped with each other on the segment dd' provides an even light intensity distribution as indicated by 6. Thus, the laser light flux is divided and synthesized by the dividing prism 1 and the synthesizing prism 2 so as to convert the Gaussian distribution into the non-Gaussian distribution. In order realize the basic idea as mentioned above, in one embodiment of this invention as shown in Fig. 3, a quadrangular pyramid prism 8 is used as a light flux dividing prism and another quadrangular pyramid prism 9 is used as a light flux synthesizing prism. A light flux L₀ incident to the quadrangular pyramid prism 8 provides, as indicated by 12, a light intensity distribution on a plane 7 where the light intensity is stronger at its centeral part and attenuates towards its periphery (i.e. two-dimentional Gaussian distribution). When this light flux L₀ is incident on the quadrangular pyramid prism 8, it is divided into four light flux parts, which are refracted, travel and incident on the light flux synthesizing prism 9, respectively. This light flux synthesizing prism 9 serves to refract the four-divided light flux parts so as to be overlapped again and to synthesize them into one light flux on a plane 11. The light intensity distribution on the plane 11 provides, as indicated by 14, an even quadrangular prism shape. Incidentally, the light intensity distribution on a halfway plane 10 is, as indicated by 13, one where the light intensity at its central part is weaker than that in its peripheral part since the four-divided light fluxes are not completely overlapped. Although in this embodiment, quadrangular prisms are used as a light flux dividing/synthesizing prism, a polyangular pyramid such as a pentangular pyramid and a hexangular pyramid can be used in a like manner. In this case, the resultant light intensity distribution provides a polyangular pyramid prism such as a pentangular prism or a hexangular prism. Fig. 4 shows another embodiment of this invention. In this embodiment, a cone prism is used as a light flux dividing/synthesizing prism. A light flux L₀ incident to a conical dividing prism 16 provides a two-dimentional Gaussian light intensity distribution on a plane 15 as shown by 20. When this light flux is incident on the dividing prism 16, it is divided and refracted anywhere on the incident plane, and is incident on a conical synthesizing prism 17. The function of this conical prism is easy to understand by considering it to be a plyangular pyramid prism with an unlimited number of apexes on its bottom. Now, the light fluxes refracted at the synthesizing prism 17 are overlapped with each other in any direction on a plane 19, and provides a column shape light intensity distribution there as shown by 22. Also, on a plane 18 they provide a light intensity distribution with a weaker intensity at its central part as shown by 21 since they are not entirely overlapped there. An elliptical cone prism may be used as a dividing/synthesizing prism. Explanation will be made on the advantages of the light intensity distribution of a light flux being not a Gaussian distribution but an even distribution within the flux. When a light flux having a Gaussian distribution as shown by 23 in Fig. 5a is irradiated onto an optical recording medium (optical disk) 31, it provides a Gaussian temperature distribution of the optical recording material formed on the optical recording medium, as shown by 24, where the temperature is the highest at its center and attenuates towards its periphery and a great temperature difference exists. On the other hand, the light flux having an even light intensity distribution as shown by 26 in Fig. 5b provides, within the flux, an even temperature distribution of the optical recording material formed on the optical recording medium 31 as shown in 27 in Fig. 5b. Meanwhile, as mentioned above, in order to erase the information recorded on an optical recording material or newly write information, it is necessary to increase the temperature of the material to the erasing temperature (range between T₁ and T₂ shown in Fig. 6) or the recording temperature (range between T₂ and T₃ shown in Fig. 6) which is specific to the material. To this end, forming an even light intensity distribution by using light flux dividing and synthesizing prisms provides, within the flux, a fixed or even temperature distribution of the optical recording material formed on the optical disk 31, as shown by 32 in Fig. 6, thus enabling to set the same erasing temperature (between T₁ and T₂) or writing temperature at a part 33 between the points X₅ and X₆ on the optical disk. Thus, a fixed temperature between T₁ and T₂ can be set at any portion of an information recording track 30. Therefore, by maintaining this fixed temperature for a certain period, the information recorded on the information recording track 30 can be simultaneously erased. In this case, it is not necessary to gradually decrease the light flux intensity so that an additional advantage of greatly reducing the time required for erasing is provided. Fig. 7 shows an embodiment of this invention wherein an optical flux dividing prism 38 and an optical flux synthesizing prism 39 are incorporated in an optical system of the optical recording and reproducing device. In this figure, the laser light flux radiated from a semiconductor laser 36 is collimated by a collimator lens 37, incident on the light flux dividing prism 38 and divided there. The light fluxes resulting from the division are synthesized through overlapping by the light flux synthesizing prism 39, which results in an even light intensity of the light flux. The evened light flux passes through a polarized-light beam splitter 40 and a 1/4 wavelength plate 41 to provide a circular-polarized light flux. This light flux is converged into a minute spot by a projecting lens 43 and this spot is irradiated onto the optical disk 31. The optical recording medium formed on the optical disk 31 absorbs this light and is heated. Then, suitably adjusting the light intensity of the laser light flux enables to record, erase or reproduce information. Generally, when recording information on the optical disk 31, the optical recording medium is heated and abruptly cooled, and when erasing the information, it is heated to its erasing temperature and maintained at the temperature for a certain period. When reproducing the information, a weak light is irradiated onto the optical disk 31, and the reflected light is detected to read out the recorded information. Since the information has been recorded with differences of the reflecting index, detecting the intensity of the reflected light allows to read out the recorded information. Further, when recording, erasing or reproducing information in the above-mentioned manner, the spot light must be irradiated onto the optical disk 31 so as to be focused there. To this end, it is necessary to move the projecting lens 43 before and behind by an actuator 42 to provide the focused light. Signals for this focusing are produced as follows. The reflected light from the optical disk passes the 1/4 wavelength plate 41 through the projecting lens 43 in the direction opposite to the direction mentioned above. Then, the reflected laser light flux is converted from the circular-polarized light to the linear-polarized light which is incident on the polarized beam splitter 40. The direction of the linear polarization, however, is opposite to the above case so that the light flux cannot pass the polarized-light beam splitter 40 but is vertically reflected, passes lenses 44, 45 and is incident on a four-division sensor 46. The light flux incident on the four-division sensor 46 has different shapes according to the degree of out-of-focus of the light spot irradiated onto the optical disk 31 so that the output signal from the four-division sensor 46 also varies according to the degree of out-of-focus. Then, this output signal is applied to an actuator driving signal production source 47 to produce a signal for actuating the actuator 42. Fig. 8 shows another embodiment of this invention. This embodiment is different from the embodiment as shown in Fig. 7 in that the light flux dividing prism 38 and the light flux synthesizing prism 39 are placed between the polarized-light beam splitter 40 and the 1/4 wavelength plate 41. Fig. 9 shows still another embodiment of this invention. This embodiment is different from the embodiment as shown in Fig. 8 in that there is employed an optical component 48 with the light flux dividing prism 38, the light flux synthesizing prism 39 and the projecting prism 43 integrally formed. Fig. 10 shows still another embodiment of this invention. This embodiment is different from the embodiment as shown in Fig. 9 in that there is employed an optical component 49 with the polarized-light beam splitter 40, the light flux dividing prism 38 and the light flux synthesizing prism 39 integrally formed. Fig. 11 shows yet a further embodiment of this invention. This embodiment is different from the embodiment as shown in Fig. 10 in that there is employed a light-flux dividing and synthesizing prism 50 with the light flux dividing prism 38 and the light flux synthesizing prism 39 integrally formed. Fig. 12 shows how a minute spot light provides a condition of heating-and-abrupt cooling, and of heating-and-gradual cooling on an optical recording medium. Fig. 12 (a) is a side view showing how minute spot lights L₁ and L₂ of a substantially square and rectangular shape, respectively, converged from a laser light is irradiated onto a recording medium travelling leftwards as shown by an arrow; Fig. 12 (b) shows the irradiated spot lights on a track 53; and Fig. 12 (c) shows a temperature distribution at the parts of the recording thin film where the minute spot lights are irradiated, wherein the abscissa indicates a distance and the ordinate indicates the temperature of the recording thin film. In the figure, 52 designates a substrate on which the recording medium is formed and 51 designates a protection film. When the recording thin film is locally irradiated with the minute spot light L₁ with the intensity strengthened, the pertinent part has an increasing temperature while it is irradiated and heated. After the irradiation of light is terminated, however, the generated heat is swiftly absorbed and diffused into the recording thin film, and the substrate and protection film adjacent thereto, thus establishing a condition of abrupt cooling. On the other hand, when the recording thin film is locally irradiated with the minute spot light L₂ of a rectangular shape, longitudinally converged in the travelling direction of the recording medium, a temperature distribution more extended in the travelling direction than in the spot light L₁ is provided as shown in Fig. 12 (c) so that the time while the pertinent part is heated is lengthened, and the part is more gradually cooled than in the spot light L₁. Thus, if the minute spot light L₁ with a substantially square shape is locally irradiated onto the travelling recording thin film and its intensity is changed with time according to the information to be written, the condition of heating-and-abruptly cooling is established on the pertinent part so that pits can be written. On the other hand, if the minute spot light L₂ with a rectangular shape extended in the travelling direction is locally irradiated onto the travelling recording thin film and its intensity is changed, the condition of heating-and-gradually cooling is established so that the pits written can be erased. Incidentally, a reproduction signal is provided by the above substantially square minute spot light L₁. Both the substantially square minute spot light L₁ and rectangular minute spot light L₂ are formed by a convergence lens 43. More specifically, if both laser beams, a₁ of a substantially square shape and a₂ of a rectangular shape extended in the direction of the track 53, are irradiated within an effective diameter (aperture) of the convergence lens 43 and their optical axes are set to be not parallel, both the substantially square minute spot light L₁ and rectangular minute spot light L₂ with a distance apart from each other at their center are provided. This distance ℓ is determined by an angle ϑ formed by the above optical axes along the travelling direction of the recording medium, and the smaller ϑ provides the smaller ℓ so that the minute spot lights L₁ and L₂ approach each other. Fig. 13 shows a further embodiment of this invention. In this figure, 36 denotes a semiconductor laser having substantially equal light beam spreading angles in a horizonal direction and vertical direction for its light emitting surface; and 37 denotes a light collecting lens which collects the light beam from the semiconductor laser 36 to provide a substantially circular parallel light beam that is incident on a quadrangular pyramid prism 38. This prism 38 divides the substantially circular laser beam into four light flux portions that are incident on a prism 39. The prism 39 superposes or synthesizes the four light flux portions to provide an even light intensity distribution of the laser beam (In this figure, an optical path for dividing and synthesizing or superposition of the light flux is simplified). The synthesized laser beam passes a beam splitter (half prism) 60, a polarized-light beam splitter 40 and a λ/4 plate 41 and is incident on a projecting lens 43. This projecting lens 43 converges the incident square light beam a₁ to provide a substantially square minute spot light L₁ on a guide track 64 as shown in Fig. 13. Numeral 42 denotes an actuator for driving the projecting lens 43, which actuator drives the projecting lens 43 in a direction vertical to an optical disk in accordance with the plane movement of the optical disk to perform a known focusing control, and drives it in a direction vertical to the guide track 64 to perform a tracking control for the guide track 64 having eccentricity. If desired, the actuator 42 also drives the projecting lens 43 in a direction of the tangential lens 43 in a direction of the tangential line of the guide track to perform a time axis correction control. Numeral 56 denotes a semiconductor laser having a larger light beam spreading angle in a vertical direction for the light emitting plane than in a horizonal direction therefor; and 57 denotes a light collecting lens which collects the light beams to provide a substantially parallel light beam. This substantially parallel light beam passes quadrangular pyramid prisms 58, 59, is divided and synthesized to provide a light beam a₂ having an even light intensity distribution. This light beam a₂ is reflected at the beam splitter 60, travels on the optical path substantially the same as that of the light beam a₁, and is incident on the projecting lens 43. Thus, a minute spot light L₂ having a rectangular shape and a longitudinal direction coincident with the direction of the track is formed on the track which is irradiated with the minute spot light L₁. The light beam reflected at the optical disk is incident on the polarized-light beam splitter via the projecting lens 43 and the λ/4 plate 41. Since the light beam has passed on the λ/4 plate twice at the incidence and refection, it rotates 90° in its polarization and is now reflected at the polarized-light beam splitter 40. Numeral 44 denotes a single lens which converts reflected light beams b₁ and b₂ into a converged light. 61 denotes a knife edge which is arranged in the neighborhood of the focusing positions of the reflected light beams b₁ and b₂ so as to intercept only the reflected light beam b₂ from the semiconductor laser 56. Numeral 62 denotes a light detector separated into four parts, for example, which gives control signals for focus control and tracking control and reproduced signals in the known manner. Numerals 54 and 55 denote driving circuits for driving the semiconductor lasers 36 and 56, respectively which are intensity-modulated in accordance with input signals S₁ and S₂. The arrangement of Fig. 13 provides two light spots placed in proximity to each other on the same guide track, i.e., one having a substantially square shape and the other having a rectangular shape extented in the longitudinal direction of the guide track; these light spots, tracking on the guide track on the optical recording disk, permit to individually establish both conditions of heating-and-abrupt cooling and heating-and-gradual cooling. In Fig. 13, both light beams are irradiated in the following manner. Fig. 13 shows the state where the focus control has beam initiated or "pulled-in". In this state, the reflected lights b₁ and b₂ can be separated from each other by the knife edge 61 since they are focused on different positions. Unless the focus control is not initiated or "pulled-in", however, the reflected lights b₁ and b₂ to be focused by the lens 44 are not focused in the neighborhood of the knife edge 61. This causes the reflected light b₂ to be irradiated onto the light detector 62, which influences so badly on the focus control pull-in operation as not to permit its stabilized "pulled-in" operation. Also even after the focus control is initiated, the drifting light of the reflected light b₂ may enter the light detector 62, which exerts bad influence on the focus control. In such reasons, at least until the focus control is initiated, or while reproduced signals are being fetched from the optical disk, light is not emitted from the semiconductor laser 56 for providing the rectangular minute spot light L₂, and so the light beam a₂ is not irradiated. As for the distance between the minute spot lights L₁ and L₂, they can be approached each other so near as to be thermally influenced from each other since they are formed by the same convergence lens. Therefore, the irradiation of both minute spot lights L₁ and L₂ at the time of heating-and-gradual cooling as shown in Fig. 14 (a) provides a longer temperature distribution in the travelling direction of the track, enhancing the effect of gradual cooling. Also when both minute light spots L₁ and L₂ are irradiated with the intensity of L₂ weakened at the time of heating-and-abrupt cooling as shown in Fig. 14 (b), the minute light spot L₂ provides two advantageous effects: of preheating which reduces the light intensity of L₁ required for recording, and of stabilizing the recording condition by once performing the complete erasing to place the recording material in an entirely uniform crystal state for example. Incidentally, the light intensity of each minute spot is set at an optimum value in accordance with the recording erasing or reproducing condition, which is obtained by the intensity-modulating by the semiconductor laser driving devices 54 and 55. As for the time sequence of providing both minute spots L₁ and L₂, when L₂ precedes L₁ the recording can be performed while the erasing is being performed, and when L₁ precedes L₂, the section to be erased can be determined by the signals reproduced using L₁ and so L₂ can be irradiated onto the section. In accordance with this invention, there is provided an even light intensity contribution of the light spot irradiated onto an optical recording medium so that an even temperature distribution of the recording medium is also provided, enabling to perform a reversible change of the optical characteristics thereof at a high speed. Therefore, this invention provides an advantageous effect of real-time erasing, recording and reproducing.

Claims: An optical recording and reproducing device, in which a light beam emanating from a light source (36; 56), such as a laser, is converged into a minute light spot and irradiated onto a recording medium (31; 63) to reversibly record signals as changes in the reflection or transmission factor by using a state transition in a heating-and-cooling cycle in the recording medium, including means (38, 39; 48; 49; 50; 58, 59) disposed in the light path between the light source (36; 56) and the recording medium (31; 63) for rendering the intensity distribution of the light beam substantially uniform, characterised in that said means includes a beam dividing prism (38; 58) and a beam synthesising prism (39; 59), the two prisms being so disposed that the beam impinges on the apex of the beam dividing prism (38; 58) and exits from the apex of the beam synthesising prism (39; 59). The device of claim 1, wherein said beam dividing and synthesising prisms (38, 39; 58, 59) are pyramid or cone prisms. The device of claim 1 or 2, wherein said beam dividing and synthesising prisms (48; 49; 50) are integrally formed. The device of claim 1 or 2, wherein at least one of said beam dividing and synthesising prisms is integrally formed with an optical component (48; 49) included in the optical system of the device.

Inventors: ITO, TETSUO, KOYANAGI, HIROAKI, MIYAMOTO, NORIFUMI, NIHEI, HIDEKI, SASAKI, HIROSHI, SATO, YOSHIO, SHIMADA, SATOSHI, TSUBOI, NOBUYOSHI
Applicants: HITACHI, LTD.
Application year: 1985
Main classification: G11B7
IPC codes: G11B7