An optical head including a beam splitter

Abstract: A beam splitter is constructed by a first prism member, a second prism member and a third prism member. The first prism is cemented to the second prism and the second prism is cemented to the third prism. At least one of the three prism members is of a shape in which its vertical angle is cut off.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates chiefly to an optical head including a beam splitter for use in an optical information recording-reproducing apparatus for optically or magneto-optically recording and/or reproducing information. Related Background Art In information recording-reproducing apparatuses for effecting the recording and/or reproduction of information by the use of light or light and magnetism, recording media in the form of disks, cards or tapes have heretofore been used. Among these optical or magneto-optical information recording media, there are widely known ones capable of recording and reproduction and ones capable of reproduction only. To record information on a medium capable of recording, a light beam modulated in accordance with recording information is condensed into the shape of a minute spot on said medium, and this spot is scanned along information tracks on said recording medium. Thereby, optically detectable information pit trains are formed on said recording medium. Also, to reproduce information from said recording medium, a light beam of such a degree of low constant power not enough to effect recording on said recording medium is condensed on said recording medium to thereby form a minute spot thereon, and the information pit train on the information tracks are scanned by this minute spot. As a result, the reflected light or transmitted light from said recording medium is separated through an optical system and is read out. The optical system used for the recording and/or reproduction of information on the above-described recording medium is equipped with an optical head which can be moved in the direction of the information tracks on the recording medium and a direction across that direction, and by the use of an objective lens provided on said optical head, a minute spot is formed on the recording medium from said light beam, and the information tracks on said recording medium can be scanned with the minute spot. Said objective lens is held on said optical head so as to be independently movable in the direction of the optical axis thereof (the focusing direction) and a direction orthogonal to both of said direction of the optical axis and the direction of the information tracks on the recording medium (the tracking direction). Usually, a resilient member is used for such holding of the objective lens, and the movement of said objective lens in the two directions is effected by the driving of an actuator utilizing a magnetic interaction. Among the above-mentioned optical information recording media, particularly the card-like optical information recording medium (hereinafter referred to as the optical card) is expected to have a big demand in the future as an information recording medium which is light in weight and convenient for carrying and has a relatively great capacity. The construction of such an optical card is generally as shown in Figures 1 and 2 of the accompanying drawings. That is, in Figure 1, the optical card 1 has a number of information tracks 2 parallel-arranged in L-F direction on the information recording surface thereof, and a home position 3 which is the reference position of the access to the information tracks 2 is provided on said information recording surface. The information tracks 2 are arranged as 2-1, 2-2, 2-3, ... in succession from the home position 3, and as shown in Figure 2, adjacent to these information tracks 2, tracks for tracking are provided in the order of 4-1, 4-2, 4-3 .... These tracks 4 for tracking are used as a guide for auto tracking (hereinafter referred to as AT) which controls the light beam spot so as not to deviate from a predetermined information track during the recording and/or reproduction of information. In the optical system, a servo system for this AT functions to detect the deviation of said light beam spot from the information track (AT error), to negatively feed back the detection signal to the actuator, to move said objective lens in the tracking direction (D direction in Figure 1) and to cause the light beam spot to follow a desired information track. On the other hand, auto focusing (hereinafter referred to as AF) is effected to make the light beam spot on the surface of the optical card into a suitable size (focus the light beam spot) as long as the information tracks are scanned by the light beam spot during the recording and/or reproduction of information. In the optical system, a servo system for this AF functions to detect the deviation of the light beam spot from the in-focus state (AF error), to negatively feed back the detection signal to the actuator for focusing, to move the objective lens in the focusing direction and to focus the light beam spot on the surface of the optical card. Therefore, in the optical system, the light beam applied to the optical card is divided into three light beams, which are made into three light beam spots S1, S2 and S3, respectively, on the optical card, as shown in Figure 2. Tracking control for the information tracks is effected by the use of the light beam spots S1 and S3, and focusing control, the production of information pits during recording and the reading-out of the information pits during reproduction are effected by the use of the light beam spot S2. In the respective information tracks shown in Figure 2, reference characters 6-1, 6-2, and 7-1, 7-2 designate preformed left address portions and right address portions, and by these address portions being read out during the recording and/or reproduction of information, discrimination between the information tracks is effected. Also, in Figure 2, reference characters 5-1 and 5-2 denote predetermined information recorded on a data portion. As is well known, there are broadly two kinds of optical information recording systems using the above-described optical system. One of them is a single-light-source system which effects recording and reproduction by one and the same light source, and the other is a two-light-source system which effects recording and reproduction by two different light sources. The two-light-source system, as compared with the single-light-source system, is said to have the advantages that the reproducing light is not deteriorated and the system is suited for the realization of a higher speed. The optical system of the conventional two-light-source system, as known e.g. from EP-A-468 685, is of a construction as shown in Figure 3 of the accompanying drawings in which recording light and reproducing light are supplied from discrete light sources, whereby the deterioration of the reproducing light is prevented and high-speed recording is made possible. In Fig. 3, a semiconductor laser 21 as a light source emits light of a wavelength of 780 nm and another semiconductor laser 22 emits light of a wavelength of 830 nm. In Figure 3, reference numerals 23 and 24 designate collimator lenses, reference numeral 25 denotes a diffraction grating for dividing a light beam, reference numeral 26 designates a dichroic prism designed to transmit therethrough light of 780 nm which is a P-polarized component and reflect light of 830 nm, and reference numeral 27 denotes a polarizing beam splitter (a beam shaping prism). Reference numeral 29 designates a quarter wavelength plate, reference numeral 30 denotes an objective lens, reference numeral 32 designates a light intercepting plate, reference numeral 33 denotes a toric lens, and reference numeral 34 designates a photodetector. Light beams emitted from the semiconductor lasers 21 and 22 are divergent light beams and therefore, pass through the collimator lenses 23 and 24, whereby they are converted into parallel light beams. The light of 780 nm from the semiconductor laser 21 further enters the diffraction grating 25, and is divided into three effective light beams (0-order diffracted light and ±1st-order diffracted light) by the function of this diffraction grating. The light beam of 780 nm and the light beam of 830 nm from the semiconductor laser 22 enter the dichroic prism 26 having a necessary spectral characteristic, and the light of 780 nm which is P-polarized light is transmitted through the dichroic prism and the light of 830 nm is reflected by the dichroic prism, and the two light beams in their combined state emerge from the dichroic prism 26. The light beam passed through this dichroic prism 26 enters the polarizing beam splitter 27 having the light splitting function. The polarizing beam splitter 27 has such a spectral characteristic that transmits P-polarized light and reflects S-polarized light, but the light beams of two wavelengths are P-polarized light components and are therefore transmitted through this polarizing beam splitter. The polarizing beam splitter 27 is three-point-supported relative to an optical head housing (not shown) by abutting portions P 1 , P 2 and P 3 . The polarizing beam splitter 27 is composed of three prism members. These light beams of two wavelengths are then converted into circularly polarized lights when they are transmitted through the quarter wavelength plate 29, and are converged on the optical card 1 which is the information recording medium by the objective lens 30. The light beam of 780 nm, in the form of three minute beam spots S1 (+1st-order diffracted light), S2 (0-order diffracted light) and S3 (-lst-order diffracted light), is applied onto the optical card 1, and S2 is used as reproducing light and a light signal for AF control, and S1 and S3 are used as light signals for AT tracking. Also, the light beam of 830 nm, in the form of a minute beam spot S2 (0-order diffracted light), is applied onto the optical card 1 and is used as a recording light. In JP-A-63120574, there is described an optical element composed of three prism elements. Each of the prisms is cemented to a common base plate and is also provided with a three-point support on the baseplate. The objective of this structure is to avoid deformation of the prisms under thermal stress. US-A-3 788 733 describes an optical system comprising a fixed prism and a movable prism, the movable prism being supported at three points for rotation about two mutually perpendicular axes. The prism is mounted on a ball joint at one apex, and has two adjusting screws bearing on a face opposite the apex. Tension elements attached to the other apices of the prism urge it into contact with the adjusting screws. The system provides a refracting prism for directing a light beam in a predetermined and adjustable direction. EP-A-0 067 405 shows a structure for an erecting prism comprising two prism elements cemented together, with one of the prism elements clamped between a resilient plunger and a concave locating surface against which two edges of the prism bear. In this case, the positions of the light beam spots on the optical card 1 are similar to those in Figure 2, that is, the light beam spots S1 and S3 are positioned on adjacent tracks 4 for tracking and the light beam spot S2 is positioned on the information track 2 between those tracks 4. Also, the positional relation between the light beam spot S2 of 780 nm and the light beam spot S2 of 830 nm is free in principle and here, these two light beam spots are positionally coincident with each other. Thus, the reflected light from the light beam spots formed on the optical card 1 passes through the objective lens 30 and becomes a substantially parallel light beam, and is again transmitted through the quarter wavelength plate 29 and becomes a light beam having its direction of polarization rotated by 90° from that during the incidence. Therefore, the reflected light enters the polarizing beam splitter 27 as an S-polarized light beam. However, the polarizing beam-splitter 27 has the characteristic of reflecting S-polarized light, as previously described. Here, the reflected light from the light spot of 830 nm is reflected and directed to the light intercepting plate 32. Also, the reflected light from the light spot of 780 nm is converged by the toric lens 33 and enters the photodetector 34. The photodetector 34 is comprised, for example, of two light receiving elements and a four-division light receiving element, and a signal for tracking control is provided by the former and a signal for focusing control and reproduction is provided by the latter. What poses a problem here is that in the aforedescribed information recording-reproducing apparatus for optically or magneto-optically effecting the recording and/or reproduction of information, to accomplish stable recording and/or reproduction or AT/AF control, high manufacturing and assembling accuracy of optical parts in the optical system is required. Particularly, in the aforedescribed polarizing beam splitter 27 constructed by a combination of a plurality of optical parts (glass-molded articles), the cementing accuracy of the optical parts, in addition to the fixing accuracy, must be high. Therefore, there are known the following two methods in the cementing work for the polarizing beam splitter. One method is a method as shown in Figure 4 of the accompanying drawings wherein when three prism members A, B and C are to be superposed one upon another so as to have the light splitting function on the cemented surfaces thereof, the sides A 1 , B 1 and C 1 thereof opposite to the location in which the vertical angles of the respective prism members concentrate are abutted against a reference plane X and in that state, the prism members are cemented together. The other method is a method as shown in Figure 5 of the accompanying drawings wherein the vertical angles of the respective prism members are concentrated at a point Y and the prism members are cemented together. However, where the prism members are cemented together by any of such methods, if there exist the working errors of the respective prism members, the dimensional error during the abutting of the prism members, the irregularity of the thicknesses of an adhesive agent, etc., there will occur an angular error attributable to the shift of each prism member or the mutual pressing among the prism members. That is, if as shown in Figure 6 of the accompanying drawings, a working error exists in the prism member B and the prism member B is smaller than the design value, a gap E will be created between the prism members A and B. In order to eliminate such gap E, it would occur to mind to shift each prism member as shown in Figure 7 of the accompanying drawings, concentrate the vertical angles of the respective prism members at a point Y and cement the prism members together. In such case, however, the prism member A would be inclined. This would lead to the deviation of the emergence position and the deviation of the emergence angle of the beam from a constituent member as a polarizing beam splitter, and would finally bring about a fluctuation to the AT/AF control signal. Description will now be made of a problem arising when the polarizing beam splitter 27 in which an angular error attributable to the shift of each prism member or the mutual pressing among the prism members has occurred as described above is supported. If as shown in Figures 8 and 9 of the accompanying drawings, abutting portions P 1 , P 2 and P 3 as shown are adopted to three-point-support the polarizing beam splitter 27 in which such angular error has occurred on an optical head housing, actual transmitted light (indicated by solid line in Figure 8) will be shifted relative to the ideal optical axis (indicated by dotted line in Figure 8) or will have an angular error. SUMMARY OF THE INVENTION The present invention has been made with taking consideration of the above-noted circumstances and intends to provide an optical head wherein a beam splitter may be mounted accurately when the beam splitter comprises a number of prism members cemented together. For this purpose, according to the present invention, there is provided optical head comprising: means for applying a light beam on to a recording medium; a beam splitter for splitting a light beam from the recording medium, the beam splitter comprising a first prism member and a second prism member cemented thereto at a cemented surface; and detecting means for detecting the light beam split by the beam splitter; characterised in that the beam splitter is mounted in the optical head by a three point support (P 1 P 2 P 3 ), and wherein the three point support (P 1 P 2 P 3 ) engages only the first prism member, and engages two surfaces of the first prism member. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying Figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plan view of an optical card. Figure 2 is a fragmentary enlarged view of the optical card shown in Figure 1. Figure 3 shows the construction of an optical head optical system of the conventional two-light-source type. Figure 4 is a side view showing the assembled structure of a prior-art polarizing beam splitter. Figure 5 is a side view showing the assembled structure of another prior-art polarizing beam splitter. Figures 6 and 7 are views for illustrating a problem arising when a working error exists in prism members constituting a polarizing beam splitter. Figure 8 is a side view showing the mounted structure of a prior-art polarizing beam splitter. Figure 9 is a side view showing the mounted structure of another prior-art polarizing beam splitter. Figure 10 shows the construction of an embodiment of an optical information recording-reproducing apparatus using a polarizing beam splitter comprising three prism members. Figure 11 is a side view showing the assembled structure of a polarizing beam splitter. Figure 12 is a side view showing the assembled structure of another polarizing beam splitter. Figure 13 is a side view showing the assembled structure of still another polarizing beam splitter. Figure 14 is a side view showing the mounted structure of a polarizing beam splitter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Some embodiments of an optical information recording-reproducing apparatus using a polarizing beam splitter mounted in an optical head by a three point support according to the present invention will hereinafter be described in detail with reference to Figures 10 to 13. In these figures, the same portions as those in the construction of the prior art are given the same reference characters and need not be described. Reference numeral 41 designates a polarizing beam splitter (a beam shaping prism). Particularly in this embodiment, it has the polarizing splitting function and in addition, the beam shaping function and the wavelength dividing function. That is, the polarizing beam splitter 41 is of structure comprising three prism members A, B and C cemented together, and the cemented surfaces (joined surfaces) a and b of the semiconductor laser side thereof have respective unique spectral characteristics. The surface a has a spectral characteristic of transmitting P-polarized light and reflecting S-polarized light, and the surface b has a spectral characteristic of transmitting P-polarized light and reflecting S-polarized light of a waveband of 780 nm. Also, the polarizing beam splitter 41 is formed of at least two kinds of glass materials and has an achromatizing action. Accordingly, the polarizing beam splitter can correct chromatic aberration by the achromatizing action and suppress the influence of the wavelength fluctuation of laser light. In Figure 10, reference numeral 42 denotes a light intercepting plate which is a stopper for absorbing light of a wavelength of 830 nm reflected by the surface a . The information recording and reproducing operations in this optical system are similar to those in the prior art and therefore need not be described. The polarizing beam splitter 41 adopted herein is assembled in the following manner. That is, the polarizing beam splitter 41 is of such a shape that as shown in Figure 11, one vertical angle of one of the prism members constituting it, for example, the prism member A in this embodiment, is cut off (flattened) (at a location whereat the vertical angles of the prism members concentrate when the three prism members A, B and C are superposed one upon another to construct the polarizing beam splitter). Then, the prism members B and C are first cemented together with a necessary adhesive agent interposed therebetween so that the vertical angles thereof may coincide with each other at a point Y. Subsequently, the flattened portion of the prism member A is located near the point Y, and the prism member A is cemented onto the prism member B with a necessary adhesive agent interposed therebetween. The prism member A is then slidden in Z direction so that the spacing L between the incidence position (the prism member A side) and the emergence position (the prism member C side) of a beam in the beam shaping prism 41 may be a predetermined value. At a stage whereat the spacing L has been adjusted thereby, the cementing of the prism members A and B is completed. If this is done, during the adjustment of the spacing, the vertical angles of the three prism members do not press one another at the point Y and the parallel shift of a beam and an angular error do not occur. Thus, the light beam always passes through the ideal optical axis of the polarizing beam splitter 41. The size of the flattened portion of the prism member A may be a suitable size if it is within a range in which the light beam is not eclipsed during the use. In Figure 12, the construction of a polarizing beam splitter in which not only the vertical angle of the prism member A but also the vertical angle of the prism member C are cut off (flattened) is shown as another embodiment of the polarizing beam splitter 41. A merit similar to that of the above-described embodiment is also obtained by this construction. Of course, the vertical angles of all prism members may be cut off (flattened). Also, as shown in Figure 13, if the spacing L is compensated for, the prism members A, B and C may be superposed one upon another in such a manner that the cemented surfaces a and b are position-adjusted to each other so that the sides of the prism members A, B and C which are opposed to the vertical angles may be adjusted to the reference plane X for abutting. Figure 14 illustrates a beam splitter having a three point support according to the present invention. When the polarizing beam splitter 41 is to be mounted on an optical head housing (not shown), it is advantageous in accurately holding of the optical axis to secure a right-angled prism C to the optical head housing at three points P 1 , P 2 and P 3 on the two surfaces of the prism which define the right-angled portion of the prism therebetween, as shown in Figure 14. Thus, the polarizing beam splitter is fixed by being abutted against the optical head housing by means of only one right-angledprism of the three prisms and therefore, when fixing it, the cementing error of the prisms and the cumulated working error of each prism can be eliminated. Also in Figure 14, a beam splitter using three prism members is shown as an example, but in a beam splitter comprised of two or more prism members superposed one upon another, three-point support may be effected on only one of the prism members, as described above, whereby the mounting of the beam splitter onto the optical head housing can be accomplished accurately. As described above, by adopting a construction in which three-point support is effected on a single prism member, the mounting of the beam splitter onto an optical head housing can be accomplished accurately and thus, the parallel shift and angular error which have been encountered in the conventional mounting means can be avoided.

Claims: An optical head comprising: means (21, 22) for applying a light beam on to a recording medium (1); a beam splitter (41) for splitting a light beam from the recording medium (1), the beam splitter (41) comprising a first prism member (C) and a second prism member (B) cemented thereto at a cemented plane (b); and detecting means (34) for detecting the light beam split by the beam splitter (41); characterised in that the beam splitter (41) is mounted in the optical head by a three point support (P 1 P 2 P 3 ), and wherein the three point support (P 1 P 2 P 3 ) engages only the first prism member (C), and engages two surfaces of the first prism member (C). An optical head according to claim 1 wherein a light beam reflected by the cemented surface (b) between the first (C) and second (B) prism members is incident on the detecting means (34). An optical head according to claim 1 or claim 2 wherein the first prism member (C) is a right triangular prism. An optical head according to claim 3 wherein the cemented surface (b) of the first prism member (C) is its hypotenuse face, and the three-point support engages (P 1 P 2 P 3 ) the other two faces of the first prism member (C). An optical head according to any preceding claim wherein a third prism member (A) is cemented to the second prism member (B).

Inventors: INOUE MASATO, INOUE, MASATO, Inoue, Masato, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: G11B7
IPC codes: G02B5, G02B7, G11B7, G11B11, G02B27