Patent Publication Number: US-2003231573-A1

Title: Optical pickup device

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an optical pickup for use in a hologram recording system or a hologram recording/reproducing system.  
       [0003] 2. Description of the Related Art  
       [0004] A system for recording information on a disk-shaped recording medium by using holograms is drawing much attention. This method records information on a recording medium as an interference pattern, and can be expected to realize high-density recording. For example, there are JP-A-2001-256654, JP-A-2001-273650, JP-A-2002-83431 and JP-A2002-123949.  
       [0005] In general, information is recorded on a plurality of concentric tracks or on a single helical track of the disk-shaped recording medium. In the case where information is to be recorded or reproduced on the disk-shaped recording medium, it is necessary to control focus and tracking.  
       [0006] In hologram recording, it is further necessary to secure recording power at a predetermined level or higher. In the case of the disk-shaped recording medium, information is recorded while the disk-shaped recording medium is being continuously rotated. Since the writing speed of information is proportional to the rotating speed of the disk-shaped recording medium, means for giving sufficient exposure energy in a short time is desired. Of course, an optical pickup and a driving system therefore need to be lightweight and compact and to be inexpensively manufacturable. Small mass serves to improve tracking performance.  
       [0007] For example, as means for increasing recording power without increasing the output power of a laser light source, it is possible to consider the idea of causing information light and reference light to follow up the movement of a track of the disk-shaped recording medium in a direction tangential to the track along the rotating direction of the disk-shaped recording medium, thereby reducing the relative speed difference between the disk-shaped recording medium and the information light and the reference light. The operation and the control of causing laser light to follow up the movement of a track in a direction tangential to the track will be hereinafter referred to as “track follow-up”.  
       [0008] In this track follow-up, a pickup itself is made to follow up the rotation of the disk-shaped recording medium so that a laser illumination position is fixed at the same position for a predetermined time. However, if the pickup itself is moved, there occurs the disadvantage that the response of servo control is inferior.  
       OBJECT AND SUMMARY OF THE INVENTION  
       [0009] The invention has been made to ameliorate the above-described disadvantage, and an object of the invention is to provide an optical pickup device capable of exhibiting a good response to focusing, tracking and track follow-up.  
       [0010] An optical pickup device according to the invention includes: a main optical system having a laser light source; an electrically driven, first beam deflector for deflecting laser light outputted from the main optical system; an electrically driven, second beam deflector for deflecting laser light deflected by the first beam deflector; an objective lens for focusing output light of the second beam deflector on a disk-shaped recording medium; and an objective lens driving device for driving the objective lens in opposite directions toward and away from the disk-shaped recording medium. One of the first and second beam deflectors deflects the laser beam to cause the laser beam to move on a recording layer of the disk-shaped recording medium in a radial direction of the disk-shaped recording medium, and the other of the first and second beam deflectors deflects the laser beam to cause the laser beam to move on the recording layer of the disk-shaped recording medium in a circumferential direction of the disk-shaped recording medium.  
       [0011] According to this construction, since track follow-up, tracking and focusing are driven by separate driving means, it is possible to select driving means each having a response suitable for a respective one of track follow-up, tracking and focusing. Accordingly, it is possible to easily obtain preferable characteristics for each of track follow-up, tracking and focusing.  
       [0012] The main optical system is made of, for example, an interference optical system.  
       [0013] Preferably, each of the first and second beam deflectors is made of a galvano-mirror. Accordingly, it is possible to obtain a sufficiently high-speed response.  
       [0014] An optical pickup device according to the invention further includes a relay optical system disposed between the first beam deflector and the second beam deflector and constructed to transfer the output light of the first beam deflector to the second beam deflector. Accordingly, it is possible to increase the degree of freedom of arrangement of the first and second beam deflectors.  
       [0015] Preferably, the first beam deflector is disposed at an object-side principal point of the relay optical system and the second beam deflector is disposed at an image-side principal point of the relay optical system. Accordingly, the laser beam enters the second beam deflector at the same position irrespective of the deflection of the laser beam by the first beam deflector.  
       [0016] Preferably, the main optical system includes an image sensor for converting reproducing light reproduced from the disk-shaped recording medium, into an electrical signal.  
       [0017] Various other objects, advantages and features of the present invention will become readily apparent to those of ordinary skill in the art, and the novel features will be particularly pointed out in the appended claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0018] The following detailed description, given by way of example and not intended to limit the present invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, wherein like reference numerals denote like elements and parts, in which:  
     [0019]FIG. 1 is a perspective view showing an embodiment of the invention;  
     [0020]FIG. 2 is a plan view showing an optical system of this embodiment;  
     [0021]FIG. 3 is a schematic block diagram showing the construction of this embodiment;  
     [0022]FIG. 4 is a timing chart showing the control operation of a galvano-mirror  54  during recording;  
     [0023]FIG. 5 is a perspective view showing either of a galvano-mirror  48  and the galvano-mirror  54 ;  
     [0024]FIG. 6 is an exploded perspective view of the galvano-mirror  48  or  54 ;  
     [0025]FIG. 7 is a cross-sectional view taken along line A-A of FIG. 5;  
     [0026]FIG. 8 is a cross-sectional view taken along line B-B of FIG. 5;  
     [0027]FIG. 9 is an explanatory view showing the optical functions of the galvano-mirror  54  and an objective lens  56 ; and  
     [0028]FIG. 10 is an explanatory view showing the optical functions of the galvano-mirror  48 , relay lenses  50  and  52 , the galvano-mirror  54  and the objective lens  56 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0029] A preferred embodiment of the invention will be described below in detail with reference to the accompanying drawings.  
     [0030]FIG. 1 is a perspective view showing the embodiment of the invention, FIG. 2 is a plan view showing the essential parts of an optical-pickup optical system of the embodiment, and FIG. 3 is a schematic block diagram showing the construction of the embodiment.  
     [0031] An optical pickup  10  of the embodiment is accommodated in a case  12 . A Mach-Zehnder interference optical system for recording and reproducing a hologram is disposed in the case  12 . The Mach-Zehnder interference optical system includes a polarizing beam splitter (PBS)  14 , half mirrors  16  and  18 , and a polarizing beam splitter  20 . Namely, one optical path is formed to lead from the PBS  14  to the PBS  20  via the half mirror  16 , while the other optical path is formed to lead from the PBS  14  to the PBS  20  via the half mirror  18 . A spatial optical modulator  22  is disposed on the former optical path, while a phase modulator  24  is disposed on the latter optical path. As will be described later in detail, information light for hologram recording propagates along the former optical path, while reference light for hologram recording and reference light for hologram reproduction propagate along the latter optical path.  
     [0032] Each of the spatial light modulator  22  and the phase modulator  24  is made of an element, such as a liquid crystal panel, which has a plurality of two-dimensionally distributed pixels and enables the transmission/non-transmission and the transmission phase of each of the pixels to be controlled from the outside. Each of the spatial light modulator  22  and the phase modulator  24  is controlled in various modes which differ for recording, servo control and reproduction.  
     [0033] During servo control, the spatial light modulator  22  is controlled so that all the pixels assume an optically non-transmitting state, while the phase modulator  24  is controlled so that the pixels are brought into phase with one another.  
     [0034] During recording, the spatial light modulator  22  is controlled so that each of the pixels assumes an optically transmitting or non-transmitting state according to whether information to be recorded is “0” or “1”. The phase modulator  24  is controlled to assume a predetermined modulation pattern in which the phase of light transmitted through each of the pixels assumes 0 degrees or 90 degrees with respect to a predetermined reference phase. The predetermined modulation pattern may be arbitrarily selected by a user, or may also be automatically determined according to predetermined conditions.  
     [0035] During reproduction, the spatial light modulator  22  is controlled so that all the pixels are brought into the optically non-transmitting state. The phase modulator  24  is controlled so that the phase of light transmitted through each of the pixels assumes a predetermined modulation pattern corresponding to the modulation pattern assumed during recording.  
     [0036] In terms of operation, it is apparent that the phase modulator  24  may also be disposed on the optical path which leads from the PBS  14  to the PBS  20  via the half mirror  16  and the spatial light modulator  22  may also be disposed on the optical path which leads from the PBS  14  to the PBS  20  via the half mirror  18 .  
     [0037] Servo-control laser light and hologram-recording reproducing laser light enter the PBS  14 . Namely, a servo-control laser  26 , a collimator lens  28  and a quarter wavelength plate  30  are disposed on the side of one entrance surface of the PBS  14 . A hologram recording/reproducing laser  32  and a collimator lens  34  are disposed on the side of another entrance surface of the PBS  14 .  
     [0038] A condenser lens  36  and an image sensor  38  for receiving hologram-recording reproducing light are disposed on the outward side of the half mirror  16 . A condenser lens  40 , a cylindrical lens  42  and a light receiver  44  for receiving (reflected light of) servo-control laser light are disposed on the outward side of the half mirror  18 . Each of galvano-mirrors  48  and  54  functions as a beam deflector or a light deflector.  
     [0039] An optical rotary plate  46 , a tracking galvano-mirror  48 , relay lenses  50  and  52 , a follow-up galvano-mirror  54  for causing a laser beam to follow up a track on a recording disk  60  in a direction tangential to the track during recording, and an objective lens  56  are disposed between the recording disk  60  and the interference optical system including the PBS&#39;s  14  and  20  and the half mirrors  16  and  18 .  
     [0040] he optical rotary plate  46  has a form divided into a right optical rotary plate  46 R and a left optical rotary plate  46 L. The right optical rotary plate  46 R rotates entering polarized light in the clockwise direction by 45 degrees, while the left optical rotary plate  46 L rotates entering polarized light in the counterclockwise direction by 45 degrees.  
     [0041] The tracking galvano-mirror  48  is used for moving the laser beam at high speed in the radial direction of the recording disk  60  and positioning the laser beam on a desired track.  
     [0042] The follow-up galvano-mirror  54  is used for moving the laser beam along the track of the recording disk  60  in the rotating direction thereof during recording. Accordingly, the rotating speed of the recording disk  60  can be made relatively slow, whereby recording light of far higher power can be applied to the recording disk  60 . In the case where the speed at which the laser beam is moved by the follow-up galvano-mirror  54  coincides with the linear velocity of the recording disk  60  on the disk medium surface of the recording disk  60 , the recording disk  60  is placed into a state equivalent to the state of being temporarily stopped during recording, whereby it is possible to realize high-power recording with long-time exposure.  
     [0043] Referring to FIGS. 1 and 2, letting the X-Y plane denote a plane parallel to the disk medium surface of the recording disk  60  and the X-axis denote an axis perpendicular to the disk medium surface of the recording disk  60 , the tracking galvano-mirror  48  can oscillate about the Z-axis, whereby reflected light can be scanned in the X-axis direction in the X-Y plane. The follow-up galvano-mirror  54  can oscillate about the X-axis, whereby reflected light can be scanned in the Y-axis direction in the X-Y plane.  
     [0044] The objective lens  56  can be moved in opposite directions toward and away from the recording disk  60  by a direct driving type of actuator, thereby controlling focus.  
     [0045] A spindle motor  62  rotates the recording disk  60 . The case  12  of the optical pickup  10  can be moved in the X-axis direction, i.e., in the radial direction of the recording disk  60 , by guide shafts  64  and  66 . Each of the guide shafts  64  and  66  is made of, for example, a lead screw (ball thread). A thread motor  68  causes the guide shaft  66  to rotate about the axis thereof, thereby moving the case  12  in opposite directions along the X-axis.  
     [0046] A spindle servo device  70  drives the spindle motor  62  in accordance with a control signal supplied from a control device  72 , to control the rotating speed of the recording disk  60  at a predetermined value. A thread servo device  74  rotates the thread motor  68  in a desired rotating direction at a desired speed in accordance with an instruction given by the control device  72 . A follow-up servo device  76  oscillates the follow-up galvano-mirror  54  at predetermined timing in accordance with an instruction given by the control device  72  and the output of the light receiver  44 . A track servo device  78  oscillates the tracking galvano-mirror  48  at predetermined timing in accordance with the output of the light receiver  44 , and positions the laser beam on a specified track. A focus servo device  80  positions the objective lens  56  in the Z-axis direction in accordance with the output of the light receiver  44  so that the focus of the objective lens  56  coincides with the recording layer of the recording disk  60 . Incidentally, the timing of oscillation of each of the galvano-mirrors is shown in FIG. 4 which will be described later.  
     [0047] The user can specify an operation mode such as a recording mode or a reproduction mode for the control device  72  through a manipulation device  82  including a manipulation panel and manipulation switches. The manipulation device  82  may also be a separate computer.  
     [0048] In the embodiment, one track of the recording disk  60  is divided into a plurality of areas, and the whole (or only the leading part) of each of the areas serves to activate the track servo device  78  or the focus servo device  80 . During recording, the data part of each of the areas serves to activate the follow-up servo device  76 .  
     [0049] The propagation path of the output light of the servo-control laser  26  will be described before in brief. As described previously, during servo control, the control device  72  controls the spatial light modulator  22  so that all the pixels thereof assume the optically non-transmitting state, and controls the phase modulator  24  so that the pixels thereof are brought into phase with one another.  
     [0050] The servo-control laser  26  outputs linearly polarized red laser light. The output light of the servo-control laser  26  is collimated into a parallel laser beam by the collimator lens  28  and the parallel laser beam is circularly polarized by the quarter wavelength plate  30 , and the obtained circularly polarized light enters the PBS  14 . The PBS  14  divides the entering light into two light beams, the one of which is applied to the spatial light modulator  22  and the other of which is applied to the phase modulator  24 . The spatial light modulator  22  intercepts the entering light, while the phase modulator  24  outputs the entering light without modification. Accordingly, the output light of the servo-control laser  26  enters the half mirror  18  and is half-reflected to the PBS  20  by the half mirror  18 . The PBS  20  supplies the laser light from the half mirror  18  to the optical rotary plate  46 . The laser light is already circularly polarized and, therefore, the optical rotary plate  46  does not at all influence servo-control laser light.  
     [0051] The laser light which has entered the optical rotary plate  46  is reflected by the tracking galvano-mirror  48 , relayed by the relay lenses  50  and  52 , reflected by the follow-up galvano-mirror  54 , focused onto the recording disk  60  by the objective lens  56 , and reflected by the recording disk  60 . The servo-control laser light reflected by the recording disk  60  enters the light receiver  44  via the objective lens  56 , the follow-up galvano-mirror  54 , the relay lenses  52  and  50 , the tracking galvano-mirror  48 , the PBS  20 , the half mirror  18 , the condenser lens  40  and the cylindrical lens  42 . The light receiver  44  converts the entering light into an electrical signal, and applies the electrical signal to the control device  72 , the follow-up servo device  76 , the track servo device  78  and the focus servo device  80 .  
     [0052] The propagation path of the output light of the hologram recording/reproducing laser  32  during recording will be described below in brief As described previously, during recording, the control device  72  controls the spatial light modulator  22  so that each of the pixels assumes an optically transmitting or non-transmitting state according to whether information to be recorded has a binary value of “0” or “1”, and also controls the phase modulator  24  so that the phase modulator  24  assumes the predetermined modulation pattern in which the phase of light transmitted through each of the pixels assumes 0 degrees or 90 degrees with respect to the predetermined reference phase.  
     [0053] The hologram recording/reproducing laser  32  outputs linearly polarized green laser light which is inclined by 45 degrees to the plane of polarization of transmitted light (or reflected polarized light) of the polarizing beam splitter  14 . The output light of the hologram recording/reproducing laser  32  is collimated into a parallel laser beam by the collimator lens  34 , and the parallel laser beam enters the PBS  14 . The entering light includes an S-polarized component and a P-polarized component each having an equal light intensity by the PBS  14 . The one of the components (for example, the S-polarized component) enters the spatial light modulator  22 , while the other (for example, the P-polarized component) enters the phase modulator  24 .  
     [0054] The spatial light modulator  22  enables or disables the entering light to be transmitted through each of the pixels in accordance with information to be recorded, whereby information light for carrying the information to be recorded is generated. Half of the information light is transmitted through the half mirror  16 , while the other half is reflected by the half mirror  16  and enters the optical rotary plate  46  through the PBS  20 .  
     [0055] In the meantime, the phase modulator  24  modulates the phase of the P-polarized component received from the PBS  14 , in accordance with a modulation pattern set by the control device  72 , whereby reference light for hologram recording is generated. Half of this reference light is transmitted through the half mirror  18 , while the other half is reflected by the half mirror  18  and is also reflected by the PBS  20  and enters the optical rotary plate  46 .  
     [0056] At the time point when the information light enters the optical rotary plate  46 , the information light includes the S-polarized component, while the reference light includes the P-polarized component. The optical rotary plate  46  is divided into the right optical rotary plate  46 R which rotates the plane of polarization of entering light in the clockwise direction by 45 degrees, and the left optical rotary plate  46 L which rotates the plane of polarization of entering light in the counterclockwise direction by 45 degrees. Accordingly, the information light is separated into two mutually perpendicular polarized components which are rotated by 45 degrees with respect to each other. The reference light is also similar to the information light. The information light transmitted through the right optical rotary plate  46 R of the optical rotary plate  46  and the reference light transmitted through the left optical rotary plate  46 L of the optical rotary plate  46  include the components polarized in the same direction and are capable of interfering with each other. Similarly, the information light transmitted through the left optical rotary plate  46 L and the reference light transmitted through the right optical rotary plate  46 R include the components polarized in the same direction and are capable of interfering with each other.  
     [0057] The information light and the reference light which have been transmitted through the optical rotary plate  46  are reflected by the tracking galvano-mirror  48 , relayed by the relay lenses  50  and  52 , reflected by the follow-up galvano-mirror  54 , and focused onto the recording disk  60  by the objective lens  56 . In this manner, an interference pattern formed by the information light and the reference light is recorded on the recording disk  60 .  
     [0058] During recording on the recording disk  60 , the control device  72  controls the follow-up galvano-mirror  54  through the follow-up servo device  76 , and moves the spots of the information light and the reference light on the recording disk  60  instantaneously by a short time in the direction tangential to the track of the recording disk  60 . Accordingly, the spots of the information light and the reference light can be located at the same position on the recording disk  60  for a longer time, whereby larger light power can be applied to the recording disk  60 . In other words, the output power of the hologram recording/reproducing laser  32  can be reduced.  
     [0059] The propagation path of reproducing reference light for reproducing information from the recording disk  60  during reproduction and the propagation path of reproducing information light for carrying reproduced information during reproduction will be described below in brief. As described previously, during reproduction, the control device  72  controls the spatial light modulator  22  so that all the pixels are brought into the optically non-transmitting state, and controls the phase modulator  24  so that the phase of light transmitted through each of the pixels assumes a modulation pattern axisymmetric with respect to the modulation pattern assumed during recording.  
     [0060] Similarly to the case of recording, the hologram recording/reproducing laser  32  outputs linearly polarized green laser light which is inclined by 45 degrees to the plane of polarization of transmitted light (or reflected light) of the polarizing beam splitter  14 . The output light of the hologram recording/reproducing laser  32  is collimated into a parallel laser beam by the collimator lens  34 , and the parallel laser beam enters the PBS  14 . The PBS  14  divides the entering light into an S-polarized component and a P-polarized component each having an equal light intensity, and applies one of the components (for example, the S-polarized component) to the spatial light modulator  22  and the other (for example, the P-polarized component) to the phase modulator  24 .  
     [0061] The spatial light modulator  22  disables the transmission of the S-polarized component received from the PBS  14 . In the meantime, the phase modulator  24  modulates the phase of the P-polarized component received from the PBS  14 , in accordance with the modulation pattern axisymmetric with respect to the modulation pattern assumed during recording, whereby reproducing reference light is generated. Half of the reproducing reference light is transmitted through the half mirror  18 , while the other half is reflected by the half mirror  18  and is also reflected by the PBS  20  and enters the optical rotary plate  46 .  
     [0062] The optical rotary plate  46  rotates the polarization plane of half of the reproducing reference light received from the PBS  20 , in the clockwise direction by 45 degrees, and the polarization plane of the other half in the counter clockwise direction by 45 degrees, thereby generating two mutually perpendicular polarized components. These two polarized components are reflected by the tracking galvano-mirror  48 , relayed by the relay lenses  50  and  52 , reflected by the follow-up galvano-mirror  54 , and focused onto the recording disk  60  by the objective lens  56 .  
     [0063] Since the reproducing reference light is made incident on the interference pattern recorded on the recording disk  60 , reproducing information light corresponding to the information light generated during recording is generated, and enters the PBS  20  via the objective lens  56 , the follow-up galvano-mirror  54 , the relay lenses  52  and  50 , the tracking galvano-mirror  48  and the optical rotary plate  46 . Part of the reproducing reference light is reflected by the recording disk  60 , and, similar to the reproducing information light, enters the PBS  20  via the objective lens  56 , the follow-up galvano-mirror  54 , the relay lenses  52  and  50 , the tracking galvano-mirror  48  and the optical rotary plate  46 .  
     [0064] The reproducing information light enters the PBS  20  to be S-polarized light by being transmitted through the optical rotary plate  46 . On the other hand, returned light of the reproducing reference light is demodulated into P-polarized light by the optical rotary plate  46  and enters the PBS  20 . The PBS  20  supplies the reproducing information light to the half mirror  16  and the returned light of the reproducing reference light to the half mirror  18 . In other words, the reproducing information light and the reproducing reference light are separated from each other. The reproducing information light transmitted through the PBS  20  enters the half mirror  16 , and half of the reproducing information light is transmitted through the half mirror  16  and is made to enter the image sensor  38  by the condenser lens  36 . An image of the interference pattern recorded on the recording disk  60  is formed on the image pickup surface of the image sensor  38  by the condenser lens  36 . The image sensor  38  converts into an electrical signal the reproducing information light that has recorded information two-dimensionally in a light-beam cross section. Each pixel of an image signal outputted from the image sensor  38  represents recorded information.  
     [0065] By using the optical rotary plate  46  divided into the right optical rotary plate  46 R and the left optical rotary plate  46 L, it is possible to prevent the reproducing reference light from entering the image sensor  38 , i.e., it is possible to reproduce information with high SNR.  
     [0066] During reproduction as well, similarly to the case of recording, the control device  72  may also control the follow-up galvano-mirror  54  to move the reproducing reference light on the recording disk  60  instantaneously in the direction tangential to the track of the recording disk  60 . Accordingly, the optical power of the reproducing reference light can be reduced, and the CNR (code/noise ratio) of the reproducing information light can be improved.  
     [0067]FIG. 4 is a timing chart showing the control operation of the follow-up galvano-mirror  54  during recording. The horizontal axis represents time, while the vertical axis represents displacement d occurring in the direction tangential to the track. This example shows the case where the recording disk  60  is rotated at 100 rpm. In hologram recording, information is intermittently recorded on the recording disk  60 . Since a large amount of information can be contained in one spot, a large amount of information can be recorded and reproduced even if information is not continuously recorded. Accordingly, since the follow-up operation of the follow-up galvano-mirror  54  becomes intermittent, the return operation of returning a beam position which was moved during the previous follow-up operation needs to be performed in preparation for the next follow-up operation. In the example shown in FIG. 4, the speed of each follow-up operation is 418.7 mm/sec. The duration of each follow-up operation is 16μ seconds, and the period thereof is 437μ seconds (about 2.3 kHz). These numerical values can be satisfactorily realized with a galvano-mirror.  
     [0068] In a structure which moves an objective lens itself laterally in the direction of a track on a recording disk, its structure resonance frequency is 17 kHz, whereas the structure resonance frequency of the galvano-mirror is near 80 kHz at which high-speed response can be realized.  
     [0069]FIG. 5 is a perspective view showing either of the galvano-mirrors  48  and  54 , and FIG. 6 is an exploded perspective view of the galvano-mirror  48  or  54  shown in FIG. 5. FIG. 7 is a cross-sectional view taken along line A-A of FIG. 5, and FIG. 8 is a cross-sectional view taken along line B-B of FIG. 5.  
     [0070] A mirror  112  is fixed to a holder  110 . The holder  110  is rotatably supported by pins  114   a  and  114   b  of a leaf spring  114 . The leaf spring  114  itself is fixed to a support portion (not shown) of the case  12 . A coil  116  is fixed to the bottom of the holder  110 . Yokes  118  and  120  are disposed at two opposite positions across the coil  116 , and magnets  122  and  124  are respectively fixed to the yokes  118  and  120 .  
     [0071] When electric current is made to flow through the coil  116 , the mirror  112  is rotated about the pins  114   a  and  114   b  against the leaf spring  114 . The rotating direction and the rotating speed of the mirror  112  can be controlled by adjusting the magnitude and the polarity of electric current flowing through the coil  116 .  
     [0072] In this example, a mechanism for making a recording beam to follow up each individual track is provided as a mechanism separate from a focusing mechanism and a tracking mechanism, whereby a sufficient follow-up speed can be realized.  
     [0073] As shown in FIG. 9, in the case where the center of rotation of the follow-up galvano-mirror  54  is placed at the rear focus position of the objective lens  56  and the follow-up galvano-mirror  54  is rotated in synchronism with the rotation of the recording disk  60 , the laser beam (information light and recording reference light during recording or reproducing reference light during reproduction) scanned by the follow-up galvano-mirror  54  follows up the movement of a target track of the recording disk  60 . Accordingly, during recording, since the power of recording light becomes large, the output power of a light source can be reduced. During reproduction, since the image pickup time of reproducing light becomes long, SNR is improved. By oscillating the follow-up galvano-mirror  54  at the rear focus position (entrance pupil) of the objective lens  56 , it is possible to realize a so-called telecentric optical system which causes light incident on the recording disk  60  to make parallel displacement. Accordingly, it is possible to realize stable recording and reproduction irrespective of the position of the beam being scanned.  
     [0074] As shown in FIG. 10, in the case where the tracking galvano-mirror  48  is placed at the rear principal point of the relay lens  50  and the follow-up galvano-mirror  54  is placed at the front principal point of the relay lens  52 , the galvano-mirrors  48  and  54  are respectively placed at conjugate positions. In FIG. 10, f1 denotes the focal length of the relay lens  50 , f2 denotes the focal length of the relay lens  52 , and f3 denotes the focal length of the objective lens  56 . In the optical system shown in FIG. 10, by oscillating the tracking galvano-mirror  48  about the Z-axis, it is possible to cause the laser beam to make parallel displacement in the radial direction of the recording disk  60 . Consequently, the laser beam can be moved at high speed in the radial and circumferential directions of the recording disk  60  by the galvano-mirrors  48  and  54 .  
     [0075] In this example, since a focusing lens actuator for driving an objective lens for the purpose of focusing is provided separately from a track follow-up actuator, the mass of the objective lens may be selected to take account of only the follow-up characteristics of the focusing lens actuator, whereby a large (heavy) objective lens can be used. In other words, it is possible to increase the aperture of the objective lens and it is also possible to increase the amount of information to be recorded, whereby it is very advantageously possible to achieve high density and high transfer rate.  
     [0076] The track follow-up galvano-mirror  54  can be controlled in a frequency band of as high as 20-30 kHz. Accordingly, it is possible to achieve a great reduction in the amount of laser light to be outputted as well as a high transfer rate.  
     [0077] The galvano-mirror  48  may also be used for track follow-up, and the galvano-mirror  54  may also be used for tracking. However, in this case, the oscillation axis of each of the galvano-mirrors  48  and  54  needs to be modified.  
     [0078] As can be readily understood from the foregoing description, according to the invention, it is possible to realize a track follow-up mechanism of high-speed response which causes information light and reproducing reference light to follow up the movement of a track of a recording medium in a direction tangential to the track. Accordingly, it is possible to substantially reduce the output power of a light source.  
     [0079] It is intended that the appended claims be interpreted as including the embodiments described herein, the alternatives mentioned above, and all equivalents thereto.