Abstract:
In an optical head of an optical memory device which records, reproduces or erases information onto or from a recording medium by applying a laser beam to the recording medium, a beam splitter is disposed in an optical path to conduct a portion of the laser beam emitted from a laser device to a light detector. In accordance with a detected value from this light detector, the influence of a stray beam generated in an optical system is removed from a servo error signal.

Description:
This application is a continuation of application Ser. No. 839,197 filed on Mar. 13, 1986, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical memory device which records, reproduces or erases information onto or from a recording medium by applying a light beam such as a laser beam to the recording medium. More particularly, the present invention is concerned with an optical head in such an optical memory device. 
     2. Description of the Related Art 
     Extensive studies have recently been made about the potential use of an optical memory device as a high density storage, a high capacity storage having high speed accessing. As examples of such an optical memory device there are a memory device in which a row of fine pits are formed in a recording medium and the diffraction phenomenon of light beam at the pit portions is utilized for the reproduction of information, and a memory device in which bit-shaped areas having different refractive indices are formed on a recording medium and variations in reflectance or transmissivity are utilized for the reproduction of information. 
     Those optical memory devices are each provided with an optical head, in which there are performed a focus control and a tracking control for a radiated light beam relative to a recording medium by means of a servo control unit. FIG. 1 shows a construction of an optical system for this optical head, in which an isolator is formed by a polarization beam splitter 104 and a quarter-wave plate 105 to prevent a light beam reflected from a recording medium 101 from returning to a laser device 102 and to conduct the reflected beam efficiently to a servo signal detector or information signal detector 103. In this construction, the portion of the reflected beam remaining on the surfaces of such optical elements as the beam splitter 104 and quarter-wave plate 105 (such residual reflected beam will hereinafter be referred to as &#34;stray beam&#34;) mostly returns to the laser device 102, with only a very small quantity of light beam reaching the servo signal detector 103, which is negligible as disturbance influencing a servo signal in the case where the reflectance of a recording medium is high as in VD (Video Disc) or CD (Compact Disc). 
     However, where the reflectance of a recording medium is low, or in the case of an optical head in which it is difficult to introduce such an isolator as shown in FIG. 1, e.g. an optical head for an optomagnetic recording device, if the residual reflectance on the surface of an optical element cannot be suppressed below a certain level, a stray beam on the surface is added as offset to a servo signal, thus causing problems; for example, the control for the optical head becomes unstable or it becomes difficult to effect mounting and adjustment of a light detector for servo control. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     1. Objects of the Invention 
     The present invention has been accomplished in view of the above-mentioned circumstances and it is the object thereof to provide an optical head capable of eliminating the influence of a stray beam generated in an optical system thereby improving servo characteristics. 
     Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     2. Summary of the Invention 
     An optical head in an optical memory device according to the present invention used for recording, reproducing or erasing information by applying a laser beam to a recording medium is of a construction which includes a beam splitter and a light detector disposed in an optical path to conduct a portion of a laser beam emitted from a laser device to the light detector. Since a portion of the emitted laser beam is detected and the influence of a stray beam generated in an optical system is removed from a servo error signal in accordance with the detected value, there is no longer a deviation of a target value in servo control induced by a difference in reflectance of recording mediums or a difference in the quantity of light beam emitted from a laser device, thus permitting a stable control for the optical head. According to the present invention, therefore, it is possible to provide an optical head which is applicable to recording mediums having various optical characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only and thus are not limitative of the present invention and wherein: 
     FIG. 1 is a schematic view of an essential part of a conventional optical head; 
     FIG. 2 is a schematic view of an essential part of an optical head according to an embodiment of the present invention; 
     FIG. 3 is a schematic view showing a reflected information beam and a stray beam both present on a light detector used in the embodiment of the invention; 
     FIG. 4 is a graph showing a waveform of a focus error signal; 
     FIG. 5 is a graph showing a waveform of a tracking error signal; and 
     FIG. 6 is a block diagram showing a circuit configuration of a servo control system used in the embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 shows a schematic construction of an optical head for an opto-magnetic recording device, embodying the present invention, in which the reference numeral 1 denotes a magnetic recording medium having a vertical magnetic anisotropy. The optical head is provided with a semiconductor laser 2 which emits a predetermined laser beam, a collimator lens 3 which converts the emitted laser beam into a collimated beam, and a shaping prism 4 for converting the laser beam emitted from the semiconductor laser 2, which beam is elliptic, into a generally circular laser beam. 
     A polarization beam splitter 5 functions to improve the polarization of transmitted beam and further rotate the polarization direction of reflected beam thereby increasing an apparent magneto-optic rotation angle. It also serves to conduct a portion of the emitted laser beam to later-described light detectors 6, 11, 12 and 17. The light detector 6 receives a portion of the laser beam reflected by the polarization beam splitter 5 and it is somewhat inclined relative to an optical path so that the reflected beam from the surface of this light sensing element may not become a stray beam. The optical head is further provided with a total reflection prism 7 functioning to bend by 90° an optical path of the laser beam which has passed through the polarization beam splitter 5, and an objective lens 8 for forming a fine spot of the laser beam on the magnetic recording medium 1. The objective lens 8 is controlled by a later-described servo control system to adjust the focus and position of a light spot formed on an information track of the magnetic recording medium 1. 
     The optical head is also provided with a polarization beam splitter 9 which functions to increase the magneto-optic rotation angle relative to the reflected beam like the polarization beam splitter 5, a spot lens 10 for applying the reflected information beam from the polarization beam splitter 9 onto the light detectors 11 and 12 at predetermined size and shape, and a half-wave plate 13 which rotates the polarization direction of the information beam in a middle direction between S and P axes of a polarization beam splitter 14 serving as an analyzer. 
     Further provided are a spot lens 15 and a cylindrical lens 16 having a focal line inclined by 45° with respect to a tangential direction of the track on the magnetic recording medium 1. A composite element type light detector 17 is also provided which has four light sensing elements. Using these light sensing elements and under a synergistic effect of both the spot lens 15 and the cylindrical lens 16, the light detector 17 not only detects a change of a relative distance between the magnetic recording medium 1 and the objective lens 8 but also detects a positional deviation between the light spot and the information track on the magnetic recording medium 1. 
     FIG. 3 schematically shows the presence of an information beam &#34;a&#34; reflected from the magnetic recording medium and a stray beam &#34;b&#34; both on the composite element type light detector 17. A servo error signal or focus error signal F E  and a tracking error signal T E  are influenced by the stray beam. According to the construction of this embodiment, the focus error signal F E  and the tracking error signal T E  are obtained by an astigmatism method and a push-pull method, respectively. More specifically, the focus error signal F E  and the tracking error signal T E  are obtained from outputs SA, SB, SC and SD of the four light sensing elements A, B, C and D of the composite element type light detector 17, according to the following equations: 
     
         F.sub.E =(SA+SC)-(SB+SD)                                   (1) 
    
     
         T.sub.E =(SC+SD)-(SA+SB)                                   (2) 
    
     When the stray beam &#34;b&#34; is positioned at such a spot as shown in FIG. 3, the focus error signal F E  and the tracking error signal T E  each have a certain amount of offset as shown in FIGS. 4 and 5. 
     Where the servo error signals F E  and T E  have offsets, there usually arise such problems as an unstable lead-in of servo operation and a constant deviation of a target point in servo control. As to the latter, it is apparent that in the case where the quantity of light reflected from a recording medium or the quantity of light of transmission varies upon recording or erasing, or in the case of using a recording medium having a different reflectance, a target value in servo control is varied by a change in the quantity of reflected light. 
     In this connection, a servo control system is used in this embodiment. The servo control unit overcomes the above problem by using a monitor output of the light detector 6 which receives a portion of the emitted laser beam. FIG. 6 illustrates a construction of this servo control system, whereby the offset of each of the servo error signals F E  and T E  caused by the stray beam is cancelled. The outputs SA, SB, SC and SD of the four light sensing elements of the composite element type light detector 17 are fed to non-inverted input terminals of four operational amplifiers AA, AB, AC and AD. On the other hand, an output SE of the light detector 6 is fed to four gain adjusters GA, GB, GC and GD, which in turn provides output signals to inverted input terminals of the operational amplifiers AA, AB, AC and AD. Output signals from the operational amplifiers AA, AB, AC and AD are fed to an operation circuit OC, which in turn provides an output signal to a drive unit (not shown) for the objective lens 8 through an amplifier (not shown). 
     The gain adjusters GA, GB, GC and GD are so set as to cancel according to the output SE of the light detector 6 outputs of levels proportional to the intensities of stray beams incident on the light sensing elements of the composite element type light detector 17, from the outputs SA, SB, SC and SD of the same detector. The operational amplifiers AA, AB, AC and AD add or subtract the outputs SA, SB, SC and SD of the detector 17 with respect to the outputs of the gain adjusters GA, GB, GC and GD and provide the results of the operation to the operation circuit OC. The operation circuit OC performs the operations of the foregoing equations (1) and (2) and outputs an offset-free servo error signal. And the objective lens 8 is controlled in accordance with this servo error signal. 
     The gain setting in the gain adjusters GA, GB, GC and GD may be performed so that the outputs of the light sensing elements of the composite element type light detector 17 become zero under emission of a predetermined laser beam in the absence of the recording medium 1. And it is performed for each individual optical head. 
     The invention being thus described, it will be obvious that the same may be varied in many ways Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.