Optical information processing apparatus

Abstract: An optical information processing apparatus which includes a substrate (101) of a transparent material, first and second optical waveguides (102,103) formed on opposite faces of said transparent substrate (101), a light introduc­ ing device for introducing reflected light from an informa­ tion writing medium into the first and second optical waveguides (102, 103), and a detecting device for detecting reproduction signal, tracking error signal and focusing error signal based on the reflected light introduced into the first and second optical waveguides (102, 103).

Description: The present invention generally relates to information processing apparatus in which a recording medium is irradiated with laser light, such as an optical disc device or the like. Following recent remarkable developments in the information oriented society, optical disc devices capable of recording and reproducing information in large capacity have actually been put into use. Such optical disc devices includes those for compact discs, video discs, additionally-writing type optical discs, rewritable magneto-optical discs, etc., micro-optical techniques have been used in an attempt to miniaturise and reduce the weight of the most important part of such optical disc devices i.e. the optical pick-up portion or optical head, in order to improve high speed access and stability. However, since conventional optical heads employ bulky optical elements (analyser, beam splitter, prism, lens or the like), there has been a limit to the reduction in size that has been possible. Various proposals have been made recently for reducing the size of the optical head. One of these relates to an optical head for a magneto-optical disc which utilizes a waveguide type differential detection device (as disclosed in Institute of Electronics, Information and Communication Engineers (IEICE) magazine OQE 86-177, written by Sunagawa et al.) The known optical head referred to above is schematically shown in Fig. 1. The waveguide type differential detection device of Fig. 1 includes a Pyrex (trade name) glass substrate 57, a planar waveguide layer 58 of #7059 glass formed on the substrate 57 by sputtering, a clad layer 59 of silicon nitride further formed by plasma CVD process, and trifocal focusing grating couplers 60, 61 and 62 further formed thereon through employment of an electron beam direct drawing method and subsequent etching of a resist layer disposed on the clad layer 59. The grating cycle is designed so that the focusing grating coupler 60 at the centre excites the TM (transverse magnetic) mode , while the focusing grating couplers 61 and 62 at the opposite sides excite the TE (transverse electric) mode . Light emitted from a semi-conductor laser source 51 is formed into parallel beams by a collimating lens 52, and applied to the focusing grating couplers 60, 61 and 62. The zero-order diffracted beam is transmitted through the above focusing grating couplers 60, 61 and 62, and collected on the magneto-optical disc D by a condenser lens 53. The reflected light is subjected to rotation in a polarizing direction at the electro-optical disc D as a result of the Kerr effect. It is then directed in the reverse direction through the condenser lens 53, and formed into TM and TE modes by the focusing grating couplers 60, 61 and 62. These are then led to the waveguide layer 58, and collected onto photo-detectors 63, 64, 65, 66 and 67 formed on the glass substrate 57. Thus, by calculating the differentiation of outputs of said photo-detectors 63, 64, 65, 66 and 67, the focusing error signal, tracking error signal and reproduction signal are obtained. Since the above waveguide type differential detection device is small in size and light in weight, with dimensions of about 5 x 12 mm², a very small optical head may be constructed including the semiconductor laser 51 and lenses 52 and 53. Therefore, the focusing and tracking functions can be executed by moving the entire optical head with an actuator (not shown) based on the above respective signals. Incidentally, the optical head employing the conventional waveguide type differential detection device as shown in Fig. 1 adopts a push-pull practice for the detection of the tracking error signal, in which deviation of the spot is detected by the intensity of ± primary diffracted light produced by the pit Dp or track Dt on the magneto-optical disc D. Accordingly, there is the problem that when the magneto-optical disc D is inclined, the optical axis of the diffracted light is shifted and produces a DC offset in the tracking error signal. This results in operational instability. Moreover, a similar problem to the above is encountered regarding the focusing error signal employing the same diffraction light. Apart from the above, there has also been recently proposed an integrated type optical head as described hereinbelow (as disclosed in Lecture No. 2P-L-15 of the Society of Applied Physics, fall in 1985, by Ura et al.) The integrated type optical head as shown in Fig. 2 includes a silicon substrate 31, a buffer layer 32 of SiO₂ formed on the surface of the substrate 31 by head oxidizing process, a flat plate optical waveguide layer 33 of glass formed on the buffer layer 32 by sputtering, a clad layer of silicon nitride further formed by plasma CVD (chemical vapor deposition) process, and a focusing grating coupler 34 and two sets of focusing grating beam splitters 35 and 36 further formed through adoption of the electron beam drawing method and etching technique after application of resist on the clad layer. The light emitted from a semiconductor laser source 41 is introduced from the side face, into the flat plate optical waveguide layer 33 on the silicon substrate 31 and reaches the two sets of focusing grating beam splitters 35 and 36 as it advances while expanding. The light path of the light transmitted through said beam splitters 35 and 36 is deflected upwardly above the silicon substrate 31 by the coupler 34, and is collected onto the pit Dp on the optical disc D. The light reflected from the optical disc D enters the flat plate optical waveguide layer 33 by way of the focusing grating coupler 34 and is diffracted by the beam splitters 35 and 36. The diffracted light is focused onto photo-detectors 37, 38, 39 and 40 formed on the silicon substrate 31. The recorded information of the above disc D is read based on the amount of the reflection light from the pit Dp. In other words, it may be detected as a sum S of the detection outputs of the photo-detectors 37, 38, 39 and 40. Meanwhile for the detection of the focusing error signal, the so-called Foucault method is employed, and the detection is effected in the manner as described below. When the optical disc D lies in front of the focusing point, the amount of light received by the outer photo-detectors 37 and 39 becomes larger than that received by the inner photo-detectors 38 and 40, due to the arrangement of the optical system. Conversely, when the optical disc D lies behind the focusing point, the amount of light received by the inner photo-detectors 38 and 40 becomes larger than that received by the outer photo-detectors 37 and 39. Based on the above fact, the sum of the outputs of the photo-detectors 37 and 39, and the sum of the outputs of the photo-detectors 38 and 40 are calculated, and the focusing error signal FE is derived from the difference between the above two sums of the outputs. Meanwhile, for the detection of the tracking error signal, the so-called push-pull practice is employed, and the amounts of light received by the photo-detectors 37 and 38, and those of the photo-detectors 39 and 40 are altered by the deviation in the tracking. Based on this fact, the sum of the outputs of the photo-detectors 39 and 40, and that of the outputs of the photo-detectors 37 and 38 are calculated, and from the difference between the above two output sums, the tracking error signal TE is derived. Owing to the fact that the above silicon substrate 31 is small in size and light in weight with dimensions of 5 x 12mm², a very compact optical head can be constructed including the semi-conductor laser source 41. Thus, by moving the whole optical head by an actuator (not shown) based on the focusing error signal and tracking error signal,the focusing and tracking functions may be effected. Furthermore, in order to alleviate factors such as aberration and the like, there has also conventionally been proposed a method in which light emitted from the focusing grating coupler 34 is formed into parallel light so as to be focused on the disc through a condensing lens. However, the conventional integrated type optical head as shown in Fig. 2 adopts the push-pull practice for detecting deviation of the spot based on the strength of ± primary diffraction light produced by the pit Dp on the optical disc D as a method for detecting the tracking error signal. Therefore, the optical axis of the diffracted light is shifted upon inclination of the optical disc D to produce DC offset in the tracking error signal , thus resulting in operational instability. Since the focusing error signal employs the same diffraction light, a similar problem is also present therein. Moreover, due to adoption of the end face coupling method for introducing the light emitted from the semi-conductor laser into the optical waveguide layer 33, the coupling efficiency is low, and there is such a disadvantage that the efficiency of the semi-conductor laser light is lowered, with only 10 to 20% of the emitted laser light becoming the waveguide light, while the remaining light ray is undesirably lost. Patent Abstracts of Japan, Unexamined Applications, P. Field, Vol. 10, No. 160, June 7, 1986, page 28 P465, Kokai No. 61-11 947 (Canon K.K.) discloses an optical head having a light splitter in the form of a diffraction grating bisected into two frame gratings which diffract the light reflected from an information recording surface in opposite directions towards different photo-detectors. Patent Abstracts of Japan, Unexamined Applications, P. Field, Vol. 11, No. 72, March 5, 1987, page 36 P 554, Kokai No 61-233 439 (Canon Inc) discloses an optical pickup device which includes two superposed optical dividers, each having a respective diffraction structure which separates a portion of the light reflected from a recording medium for transmission along that optical divider toward a common sensor. The present invention therefore aims to provide an improved optical information processing apparatus which can alleviate the above-mentioned problems. In accordance with a first aspect of the present invention, there is provided an optical information processing apparatus comprising means for irradiating a recording medium with laser light, a substrate, waveguide means formed on the substrate, coupling means for introducing light reflected from the recording medium into the waveguide means, and a plurality of photo-detectors arranged to detect light travelling in the waveguide means to produce detection signals for use in determining focusing and tracking error and reproducing information recorded on the recording medium characterised in that the waveguide means comprises first and second optical waveguides and in that the coupling means includes means for splitting light reflected from the recording medium into TE and TM modes for transmission in respective ones of the first and second optical waveguides, and wherein the plurality of photo-detectors are disposed in two sets, a first set, one of the first and second optical waveguides and a second set being located to receive light transmitted by the other of the first and second optical waveguides. In this aspect an improvement in S/N-ratio is obtained. The first and second sets of photo-detectors may each comprise a group of three photo-detectors for detection of focusing error, each group of three photo-detectors comprising first and second side photo-detectors, and a middle detector and means being provided for processing the detection signals from said groups of three photo-detectors to determine focusing error, said processing means comprising means for generating an intensity signal corresponding to the difference between the sum of the detection signals produced by the side photo-detectors and the detection signal produced by the middle photo-detector for each of the first and second sets of photo-detectors, and means for comparing the intensity signals so generated for each of the first and second sets of photo-detectors. In another aspect of the present invention, there is provided an optical information processing apparatus comprising means for irradiating a recording medium with laser light, a substrate, waveguide means formed on the substrate, coupling and focusing means for introducing light reflected from the recording medium into the waveguide means, and a plurality of photo-detectors for detecting light travelling in the waveguide means, and means responsive to the signals produced by the photo-detectors for use in determining focusing and tracking error and reproducing information recorded on the recording medium characterised in that the plurality of photo-detectors comprises a first photo-detector disposed to receive light beyond the focal point of the coupling and focusing means, and a second photo-detector disposed to receive light in front of the focal point of the coupling and focusing means. In this way focus error detection is improved. In a further aspect of the present invention there is provided an optical information processing apparatus comprising, means for irradiating a recording medium with laser light, a substrate, an optical waveguide formed on the substrate for guiding light reflected from the recording medium, and a plurality of photo-detectors for detecting light travelling in the optical waveguide for use in determining focusing and tracking error and reproducing information recording on the recording medium characterised in that the means for irradiating a recording medium with laser light, comprises means for irradiating the recording medium with a plurality of laser light spots, and wherein the optical waveguide transmits the reflected light from each laser spot to a respective one of the plurality of photo-detectors. In this way tracking error detection is improved. Some preferred embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figs. 1 and 2 are schematic perspective views showing arrangements in conventional optical information processing apparatuses (already referred to); Fig. 3 is a schematic perspective view showing the general construction of an optical information processing apparatus according to one preferred embodiment of the present invention; Figs. 4A, 4B and 4C are diagrams for explaining light spots on a magneto-optical disc as used for the apparatus of Fig. 3; Figs. 5A and 5B are diagrams showing state of incidence of reflected light upon three-division photo-detectors in the arrangement of Fig. 3, Fig. 6 is a graphical diagram for explaining differential output of the photo-detectors in the arrangement of Fig. 3, Fig. 7A is a view similar to Fig. 3, which particularly shows a modification thereof, Fig. 7B is a top plan view showing, on an enlarged scale, arrangement of the photo-detectors on a glass substrate in the modification of Fig. 7A, Fig. 8 is a view similar to Fig. 3, which particularly shows a further embodiment of the invention, Fig. 9 is an electrical block diagram of a signal detection circuit employed in the arrangement of Fig. 8, Figs. 10A, 10B and 10C are diagrams for explaining arrangement of light spots on an optical disc, Figs. 11A and 11B are diagrams showing state of incidence of reflected light into three-division photo-detectors in the arrangement of Fig. 8, and Fig. 12 is a graphical diagram for explaining differential output of the three-division photo-detector in the arrangement of Fig. 8. DETAILED DESCRIPTION OF THE INVENTION Before the description of the preferred embodiments proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. Referring now to the drawings, there is shown in Fig. 3, an optical information processing apparatus according to one preferred embodiment of the present invention, which includes a transparent substrate 101 of a glass material, e.g. Pyrex (trade name) glass having a thickness of 1mm, optical waveguide layers 102 and 103 of #7059 glass each 0.76µm in thickness and respectively formed on upper and lower faces of the glass substrate 101 by sputtering process, and silicon nitride clad layers 105 and 106 each 0.04 µm in thickness and further formed on the optical waveguide layers 102 and 103 by plasma CVD process. After formation of the above clad layers 105 and 106, a focusing grating coupler 107 for exciting TE mode is formed on the glass substrate 101, while another focusing grating coupler 108 for exciting TM mode is formed on the lower surface of said glass substrate 101 respectively. Positioning on the upper and lower faces of the substrate 101 may be effected at high accuracy through employment of a duplex mask aligner (not particularly shown). The focusing grating couplers 107 and 108 referred to above are manufactured by using a photomask in which a grating pattern prepared by the electron beam drawing process is transferred and also, so arranged that through utilization of the fact that effective refractive indexes of the optical waveguides are different between the TE mode and TM mode, the respective modes are selectively guided by slightly varying the grating cycles to each other. Light in three beams with a spot interval of 100µm emitted from a semi-conductor laser 110 disposed below the substrate 101 is formed into parallel light by a collimating lens 111 provided above said semi-conductor laser so as to be incident upon the focusing grating coupler 108. Subsequently, the zero-order diffraction light transmitted through the focusing grating coupler 108 further passes through the focusing grating coupler 107, and thereafter, collected onto the magneto-optical disc 114 by a condenser lens 112 in a state as illustrated in Fig. 4A, in which a main spot is denoted by Numeral 141, sub-spots are represented by Numerals 142 and 143, and a recording track is indicated by Numeral 116. The reflected light of the main spot 141 subjected to rotation in the polarizing direction by Kerr effect at the magneto-optical disc 114, passes through the condenser lens 112 in the reverse direction, and by the focusing grating couplers 107 and 108, the TE mode becomes the waveguide light of the waveguide layer 102, while the TM mode becomes that of the waveguide layer 103. Thus, light intensity thereof is detected by two sets of three-division photo-detectors 120 and 121 of amorphous silicon formed on the opposite faces of the glass substrate 101. The three-division photo-detector 120 referred to above is disposed in a position after a focal point of the focusing grating coupler 107, while the photo-detector 121 is provided in a position before a focal point of the focusing grating coupler 108 as shown. Reflected light is incident upon the three-division photo-detectors 120 and 121 in the state as illustrated in Figs. 5A and 5B respectively, in which lines b denote the case where the focal point of the light spot is located on the optical disc 114, lines a show the case where the focal point of the light spot is present before the optical disc 114, and lines c represent the case where the focal point of the light spot is in the inner side of the optical disc 114. The differences between the outputs of each of the central detectors 127 and 132, and the respective sums of the outputs of the detectors 126 and 128, and 131 and 133 on opposite sides of the respective central detectors in the above three-division photo-detectors 120 and 121, are respectively obtained as x and y shown in Fig. 3, and are formed in curves as plotted in Fig. 6 with respect to the focal point of the light spot. Since the focusing error signal z which is the differential outputs of x and y, is in the form of an S-shaped curve, it becomes possible to detect the position of the focal point. Meanwhile, reflected light from the two sub-spots 142 and 143 is incident upon the photo-detectors 124 and 125 for the TE mode, and also upon the photo-detectors 129 and 130 for the TM mode respectively by the focusing grating couplers 107 and 108. The recording track 116 formed on the magneto-optical disc 114 is different in thickness from the substrate 115 by 1/8 wavelength. Therefore, in the case where the main spot 141 is located at the centre of the recording track 116 as shown in Fig. 4A, the reflected light rays of the sub-spots 142 and 143 are of the same intensity, but when the main spot 141 is deviated from the centre of the recording track 116 as in Fig. 4B, the sub-spots 142 and 143 are made asymmetrical with respect to the centre of the recording track 116, thus producing a difference in the intensity of the reflecting light. Accordingly, the tracking error signal may be detected by taking the difference between the sum of the outputs of the photo-detectors 124 and 129, and the sum of the outputs of the photo-detectors 125 and 130. It is to be noted here that the recording track 116 may be either in the form of a line or pit. In the above embodiment, it is so arranged that the TE mode and TM mode of the reflected light from the magneto-optical disc 114 are respectively excited by the focusing grating couplers 107 and 108 so as to be introduced into the optical waveguides 102 and 103, and such TE mode and TM mode are respectively detected by the set of three-division photo-detector 120 and the photo-detectors 124 and 125, and by the set of three-division photo-detectors 121 and the photo-detectors 129 and 130 thereby to calculate the differentiation thereof, thus avoiding any offset being produced with respect to the inclination of the magneto-optical disc 114. Accordingly, stable focusing and tracking functions may be effected for writing, recording and reproduction of the magneto-optical disc device at high accuracy. It should be noted here that, in the foregoing embodiment, although the focal distances of the focusing grating couplers 107 and 108 are adapted to be equal to each other, and the three-division photo-detector 120 is disposed after the focal point, with the three-division photo-detector 121 being provided before said focal point, the arrangement may be modified, for example, in such a manner that, as shown in Fig. 7A, with the focal distances of the focusing grating couplers 107 and 108 being made different from each other, an Si substrate 180 on which two sets of three-division photo detectors 120' and 121', and photo-detectors 124' and 125' and 129' and 130' are formed as illustrated in Fig. 7B, may be disposed in the vicinity of the end face of the glass substrate 101. The three-division photo-detectors 120' and 121', and the photo-detectors 124', 125', 129', and 130' are constituted by a PIN diode array, and functioning principle thereof is the same as that in the foregoing embodiment. Moreover, it may be so modified that the above Si substrate 180 is spaced from the end face of the glass substrate 101, and each reflected light ray from the glass substrate 101 is adapted to be incident upon the above PIN diode array through a lens means. The magneto-optical device employed in the above embodiment may be replaced by other disc devices not utilizing the Kerr effect, e.g. optical disc devices for compact discs, video discs, or additionally-writing type discs, etc. Furthermore, in the foregoing embodiment, although the semi-conductor laser of three beam type is employed, it may be so modified that light emitted from one beam semi-conductor laser is divided into diffraction light of zero order and ± first order to be used for the purpose. Similarly, in the above embodiment, although the focusing grating couplers 107 and 108 are arranged to selectively guide the TE mode and TM mode by slightly altering the grating cycle to each other, the arrangement may also be so modified that the same grating pattern may be used respectively for the TE mode and TM mode by slightly changing the layer thickness of the optical waveguide layers 102 and 103. Meanwhile, the focusing grating coupler described as employed in the foregoing embodiment may be replaced by a grating coupler and a waveguide lens means. Moreover, in each of the above embodiments, either one of the photo-detector pairs 124/125 or 129/130 (Fig. 3), 124'/125', or 129'/130' (Fig. 7) may be omitted for simplification of the construction. As is seen from the foregoing description, the optical information processing apparatus according to the first embodiment of the present invention is so arranged that light reflected from the information writing medium is introduced by the light introducing means, into the first and second optical waveguides formed on the opposite sides of the transparent substrate, thereby to detect the reproduction signal, tracking error signal, and focusing error signal by detecting means based on the reflected light introduced into said optical waveguides, and therefore, it is possible to effect stable focusing function and tracking function even when the above information writing medium is inclined, while the optical head may be made compact in size at high performance. Furthermore, by the arrangement that the above detecting means includes the set of three-division photo detector disposed before the focal point of the reflected light introduced into said first optical waveguide, and another set of three-division photo-detector provided after the focal point of the reflected light introduced into the second optical waveguide, whereby the differentiation of the light receiving outputs between the central portion and opposite side portions is calculated with respect to each of the above two sets of photo-detectors so as to detect the focusing error signal through comparison of the above two differentiations, the focusing function can be made still more stable. Reference is further made to Fig. 8 showing an optical information processing device according to a second embodiment of the present invention. In Fig. 8, the processing device includes a transparent substrate 201 of Pyrex glass having a thickness of 1mm, a planar plate optical waveguide layer 202 of #7059 glass 0.76µm in thickness and formed on the glass substrate 201 by sputtering process, and a silicon nitride clad layer further formed on the optical waveguide layer 202 by plasma CVD process. After further application of a resist layer thereon, a focusing grating coupler 203 and a grating beam splitter 205 are formed through utilization of the electron beam direct drawing method and etching technique. Furthermore, on the glass substrate 201, two sets of three-division photo-detectors 206 and 207 and photo-detectors 208 and 209 are formed through vapor deposition. Light in three beams with a spot interval of 100µm emitted from a semi-conductor laser 211 disposed below the substrate 201 is formed into parallel light by a collimating lens 212 provided above said semi-conductor laser so as to be incident upon the focusing grating coupler 203. Subsequently, the zero-order diffraction light transmitted through the focusing grating coupler 203 forms a main spot 221 and sub-spots 222 and 223 on the optical disc 214 by a condenser lens 213 in the state as illustrated in Figs. 10A, 10B and 10C. The reflected light rays of the above respective spots pass through the condenser lens 213 in the reverse direction so as to be the guide light of the optical waveguide layer 202 by the focusing grating coupler 203, and advance towards the photo-detectors 208, 209 and 262 as the focusing light. The reflected light from the main spot 221, i.e.the light ray advancing to the photo-detector 262 is incident upon the three-division photo-detectors 261, 262 and 263 disposed before the focusing point. Meanwhile, half of the light ray changed in its optical path by the grating beam splitter 205 formed in the course of the light path of the ray directed to the photo-detector 262, is incident upon the three-division photo-detectors 271, and 273 disposed after the focusing point thereof. In the case where the optical disc 214 is present at the focal point of the main spot 221, the reflected light rays from the optical disc 214 are incident upon the three-division photo-detectors 261, 262, and 263, and 271, 272 and 273 as shown by lines b in Figs. 11A and 11B. Meanwhile, when the optical disc 214 is located behind the focal point of the main spot 221, the reflected light rays are incident as shown by lines a in Figs 11A and 11B, while when the optical disc 214 is located before the focal point of the main spot 221, the reflected light rays are incident as shown by lines C in Figs. 11A and 11B. Accordingly, the differential outputs between the central portions 262 and 272, and opposite side portions 261, 263 and 271 and 273 of the three-division photo-detectors 206 and 207 as shown by x and y in a block diagram of the signal detection circuit in Fig. 9 are respectively represented by curves as shown in Fig. 12 with respect to the disc position. The focusing error signal FE which is the differential output of x and y referred to above is represented by the S-shaped curve as shown by z in Fig. 12, thus making it possible to detect the focal point position. Also, the tracking error signal is detected as follows. The pit 215 (Fig. 8) formed on the disc 214 is different in thickness from the substrate 216 by λ/4 (where λ: wavelength). Therefore, in the case where the main spot 221 is located at the center of the track as shown in Fig. 10A, the reflected light rays from the two sub-spots 222 and 223 are of the same intensity, but when the main spot 221 is deviated from the center of the track as in Figs. 10B and 10C, the two sub-spots 222 and 223 become asymmetrical with respect to the center of the track for different reflection light density. Accordingly, by taking the difference between the outputs of the photo-detectors 208 and 209 as shown in Fig. 9, the tracking error signal TE may be detected. It is to be noted here that the recording signal S is selected as shown in Fig. 9 by receiving the reflected light from the main spot 221. As described above, since the incident light from the semi-conductor laser 211 is arranged to be projected onto the optical disc 214 without being introduced into the optical waveguide layer 202, efficiency for utilizing the semi-conductor laser light may be improved. Moreover, owing to the arrangement that the reflected light from the optical disc 214 is introduced into the optical waveguide layer 202 by the grating coupler 203, and the difference between the light receiving outputs derived from the central portion and the opposite side portions of the light ray altered in its light path by the grating beam splitter 205 formed in the course of the reflected light, is calculated with respect to each of the three-division photo-detector 206 disposed before the focal point of the reflected light, and the three-division photo-detector 207 disposed behind the focal point, thereby to detect the focusing error signal by comparing the above two difference values, there is no possibility to produce offset with respect to the inclination of the optical disc 214, and thus, stable a focusing function may be effected. Furthermore, since it is so arranged that the main spot 221 and sub-spots 222 and 223 are formed on the optical disc 214, and the reflected light rays therefrom are introduced into the optical waveguide layer 202 for subsequent reception by the photo-detectors 208 and 209 being determined thereby to detect the tracking error signal, no offset is produced with respect to the inclination of the optical disc 214. Therefore, stable tracking function can also be effected. It should be noted here that, in the foregoing embodiment, although the three beam semi-conductor laser is employed, this may be replaced by a one beam semi-conductor laser whose emitted light ray is divided into diffraction light of zero order and ± first order through a diffraction grating. As is clear from the above description, the optical information processing apparatus according to the second embodiment of the present invention includes the optical waveguide formed on the substrate, the light introducing means for introducing light reflected from the recording medium into said optical waveguide to form two collected light rays, the set of three-division photo-detectors disposed before the focusing point of the reflected light introduced into said optical waveguide, and another set of three-division photo-detector disposed after the focusing point of the reflected light introduced into said optical waveguide whereby in each of said two sets of three-division photo-detectors, the difference between the light receiving output at its central portion and the sum of the light receiving outputs at its opposite sides is calculated and a focusing error signal is derived by comparing the difference values obtained from said two sets of three-division photo-detectors, and therefore, stable focusing function may be effected without forming undesirable offset phenomenon. Furthermore, since the optical information processing apparatus according to the modification of the second embodiment includes the optical waveguide formed on the substrate, means for irradiating a plurality of laser light spots onto the recording medium so as to introduce the reflected light into the optical waveguide, and the set of photo-detectors for subjecting to photo-electric conversion the reflected light introduced into said optical waveguide for each reflected light corresponding to each spot of the plurality of the laser light spots, so as to correspond to the light intensity thereof, whereby detection of tracking error signal is effected through comparison of light receiving outputs of the photo-detectors, stable tracking function may also be achieved without giving rise to offset phenomenon. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art.

Claims: An optical information processing apparatus comprising means for irradiating a recording medium with laser light (110), a substrate (101), waveguide means (102, 103) formed on the substrate, coupling means (107, 108) for introducing light reflected from the recording medium into the waveguide means, and a plurality of photo-detectors (120, 124, 125, 121, 129, 130; 120', 124', 125', 121', 129', 130') arranged to detect light travelling in the waveguide means (102, 103) to produce detection signals for use in determining focusing and tracking error and reproducing information recorded on the recording medium characterised in that the waveguide means comprises first and second optical waveguides (102, 103), and in that the coupling means includes means (107, 108) for splitting light reflected from the recording medium into TE transverse electric and TM transverse magnetic modes for transmission in respective ones of the first and second optical waveguides, and wherein said plurality of photo-detectors are disposed in two sets, a first set (120, 124, 125; 120', 124', 125') being located to receive light transmitted by one (102) of the first and second optical waveguides and a second set (121, 129, 130; 121', 129', 130') being located to receive light transmitted by the other (103) of the first and second optical waveguides. An optical information processing apparatus according to claim 1 wherein the first and second sets of photo-detectors each comprises a group of three photo-detectors (126, 127, 128, 131, 132, 133; 126', 127', 128', 131', 132', 133') for detection of focusing error. An optical information processing apparatus according to claim 2 wherein each group of three photo-detectors comprises first and second side photo-detectors (126/128, 131/133; 126'/128', 131'/133') and a middle photo-detector (127, 132; 127', 132'), and wherein there is provided means for processing the detection signals from said groups of three photo-detectors to determine focusing error, said processing means comprising means for generating an intensity signal corresponding the difference between the sum of the detection signals produced by the side photo-detectors and the detection signal produced by the middle photo-detector for each of the first and second sets of photo-detectors, and means for comparing the intensity signals so generated for each of the first and second sets of photo-detectors. An optical information processing apparatus comprising means for irradiating a recording medium with laser light (110; 211), a substrate (101; 201), waveguide means (102, 103; 202) formed on the substrate, coupling and focusing means (107, 108; 203) for introducing light reflected from the recording medium into the waveguide means, and a plurality of photo-detectors (120, 124, 125, 121, 129, 130; 120', 124', 125', 121', 129', 130'; 206, 208, 209, 207) for detecting light travelling in the waveguide means (102,103; 202), and means responsive to the signals produced by the photo-detectors for use in determining focusing and tracking error and reproducing information recorded on the recording medium characterised in that the plurality of photo-detectors comprises a first photo-detector (120; 120'; 207) being disposed to receive light beyond the focal point of the coupling and focusing means, and a second photo-detector (121; 121'; 206) disposed to receive light in front of the focal point of the coupling and focusing means. An optical information processing apparatus according to claim 4 wherein each of the first and second photo-detectors comprises first and second side photo-detectors (126/128, 131/133; 126'/128', 131'/133'; 261/263; 271/273) and a middle photo-detector (127, 132; 127', 132'; 262, 272), and wherein the means for determining focusing error comprises means for generating an intensity signal corresponding to the difference between the sum of the signals produced by the side detectors and the signal produced by the middle photo-detector for each of the first and second photo-detectors, and means for comparing the intensity signals so generated for each of the first and second photo-detectors. An optical information processing apparatus according to claim 4 or claim 5 wherein the waveguide means is an optical waveguide and wherein the coupling and focusing means comprises a focusing grating coupler (203) and a grating beam splitter (205) for directing first and second beams to respective ones (206, 207) of the first and second photo-detectors. An optical information processing apparatus according to claim 4 or claim 5 wherein the waveguide means comprises first and second optical waveguides (102, 103) and wherein the coupling and focusing means comprises first and second focusing grating couplers (107, 108) for transmitting first and second beams to respective ones of the first and second photo-detectors (206, 207) along respective ones of the first and second waveguides. An optical information processing apparatus according to claim 7 wherein the focal lengths of the first and second grating couplers (107, 108) are substantially the same. An optical information processing apparatus according to claim 7 wherein the focal lengths of the first and second grating couplers (107, 108) differ. An optical information processing apparatus comprising, means for irradiating a recording medium with laser light (110; 211), a substrate (101; 201), an optical waveguide (102,103; 202) formed on the substrate for guiding light reflected from the recording medium, and a plurality of photo-detectors (120, 124, 125, 121, 129, 130; 120', 124', 125', 121', 129', 130'; 206, 208, 209, 207) for detecting light travelling in the optical waveguide (102,103; 202) for use in determining focusing and tracking error and reproducing information recorded on the recording medium characterised in that the means for irradiating a recording medium with laser light (110; 211), comprises means for irradiating the recording medium with a plurality of laser light spots (110; 211), and wherein the optical waveguide (102,103; 202) transmits the reflected light from each laser spot (141, 142, 143; 221, 222, 223) to a respective one of the plurality of photo-detectors (120, 124, 125, 121, 129, 130; 120', 124', 125', 121', 129', 130'; 206, 208, 209, 207). An optical information processing apparatus according to claim 10 wherein the means for irradiating the recording medium with a plurality of laser light spots is a semi-conductor laser of the three beam type. An optical information processing apparatus according to claim 10 wherein the means for irradiating the recording medium with a plurality of laser light spots comprises a single beam semi-conductor laser and means for diffracting the laser light into a zero order and +/- first order beams. An optical information processing apparatus according to any one of claims 4 to 12 wherein the means for reproducing signals recorded on the recording medium comprises means responsive to polarization effects in the reflected light. An optical information processing apparatus according to any one of claims 4 to 12 wherein the means for reproducing signals recorded on the recording medium comprises means responsive to the quantity of light reflected from the recording medium.

Inventors: YAMAMOTO OSAMU, YAMAMOTO SABURO, YOSHIDA TOSHIHIKO, YAMAMOTO, OSAMU, YAMAMOTO, SABURO, YOSHIDA, TOSHIHIKO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G11B7
IPC codes: G11B7