Device for initializing an optical disc

A device capable of initializing an optical disc in high-speed and accurate manner includes a moving table (106) movable in the radius direction of a optical disc (101) to which first and second photo head moving sections (107) and (108) are fixed. A laser beam (131) emitted from a fixed section (105) is branched into two erasing beams through a beam splitter (136) and a reflection mirror (138), and the erasing beams are irradiated on the optical disc (101) from respective objective lenses (142) and (152). The erasing beam are of sizes that can erase a plurality of tracks simultaneously. The moving table (106) advances by a plurality of tracks to thereby initialize the plurality of tracks at once.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for initializing a rewritable 
disc such as a magneto-optical disc and a phase change magnetic disc. 
2. Description of the Related Art 
Researches have been made with rapid progress into an external storage unit 
that satisfies both two conditions of a high-speed access property of a 
magnetic disc for a computer and a large storage capacity of an optical 
disc. The coming-generation external storage unit will enable a high-speed 
random access at a high-speed transfer rate, and be large in capacity and 
excellent in the shelf life of the medium. The optical discs which satisfy 
the above conditions and are characterized by the durability of the medium 
due to non-contact are most promising as the coming-generation external 
storage unit. 
FIG. 9 shows the cross-sectional structure of a conventional 
magneto-optical optical disc for a magneto-optical medium. In the figure, 
a magneto-optical optical disc 300 is structured in such a manner that 
tracks are formed on a substrate 301 made of polycarbonate, etc., and 
pre-grooves 302 for tracking are formed in the tracks. A recording film 
303 is formed so as to be interposed between protective films 3041 and 
3042, and a reflection film 305 is formed on the upper protective film 
304, to bring the recording film 303 to an enhancement effect. 
FIG. 10 shows the principle of the structure of the device for initializing 
the magneto-optical optical disc shown in FIG. 9. This technique is 
disclosed in Japanese Patent Unexamined Publication No. Sho 
63(1988)-222350. The magneto-optical optical disc 300 is mounted on a 
magnet 311 and heated using a powerful light source 312 such as a Xenon 
lamp. A mirror 313 is used to convert a light beam emitted from the light 
source 312 into a parallel light beam and to irradiate it onto the 
magneto-optical optical disc 300. The magneto-optical optical disc 300 is 
heated by the irradiation of the light beam from the light source 312, and 
when its temperature rises to Curie temperature or higher, a next 
direction is set to a direction of an external magnet so that the entire 
magneto-optical optical disc 300 is initialized. 
FIG. 11 shows the principle of the structure of the device for initializing 
the optical disc which has been proposed to improve the efficiency of 
initialization. This technique is disclosed in Japanese Patent Unexamined 
Publication No. Hei 4(1992)-271019. In the case of conducting a normal 
initialization, a parallel light beam is made incident to an objective 
lens 351 from a laser light source or a powerful light source such as a 
Xenon lamp not shown. Then, an incident beam 352 is irradiated 
substantially vertically onto a phase change disc 353 and converged 
thereon. 
On the contrary, in order to improve the efficiency of initialization, an 
inclined incident beam 354 is used to be converged on the phase change 
disc 356 through an objective lens 355. .theta..sub.1 (=90 degrees) and 
.theta..sub.2 represent the inclinations of incident beams by the 
respective objective lenses 351 and 355. It should be noted that the phase 
change disc 356 is the same film formation structure as the 
magneto-optical optical disc 300 shown in FIG. 9, and the phase change 
disc 356 is different from the magneto-optical optical disc 300 in that 
the magneto-optical film of the recording film 303 (FIG. 9) is replaced by 
a phase change film. 
FIG. 12 shows the shapes of the normal incident beam and the obliquely 
inclined incident beam shown in FIG. 11, which have been irradiated on a 
disc surface. A spot 357 formed by the normal incident beam 352 (FIG. 11) 
is circular whereas a spot 358 formed by the inclined incident beam 354 
(FIG. 11) is oval. In FIG. 12, parallel lines on the phase change disc 356 
represent a track 359, respectively. 
FIG. 13 shows temperature rising characteristics of the normal incident 
beam and the inclined incident beam. A first curve 361 is obtained by the 
normal incident beam 352 in FIG. 11, and a second curve 362 is obtained by 
the inclined incident beam 354 in the figure. There are found the 
advantages that the incident beam 354 indicated by the second curve 362 is 
gentler in the rising slope of temperature than the incident beam 352, and 
the use of the phase change disc 356 (FIG. 11) allows the efficiency of 
erasing to improve. 
However, the conventional device for initializing the optical disc as 
described above suffers from the problems stated below. 
(1) First, in case of the device for initializing the optical disc shown in 
FIG. 10, an area on which a light beam such as a laser beam is irradiated 
is large. Hence, a plastic disc substrate could take a risk which 
constitutes to rise up a temperature, and could be deformed. 
(2) Also, in the above conventional devices for initializing the optical 
disc, it is required that the set value of a power necessary for 
initializing the optical disc is obtained in accordance with the 
temperature rising conditions per a unit area. Hence, there arises such a 
problem that the initializing conditions cannot be always accurately 
calculated. 
(3) Further, in the device that conducts erasing by use of the inclined 
incident beam 354 shown in FIGS. 11 to 13, although the efficiency of 
erasing is enhanced, the spot 358 of the light beam is increased in 
diameter as shown in FIG. 12. This leads to such a problem that it is 
impossible to obtain a sufficient convergent light beam for erasing on 
each track so that tracking servo is unstabilized, thereby disenabling 
stabilized erasing operation. 
SUMMARY OF THE INVENTION 
The present invention has been made to eliminate the above problems with 
the conventional device, and therefore an object of the present invention 
is to provide a device for initializing an optical disc, which is capable 
of initializing a magneto-optical optical disc or a phase change optical 
disc stably and at a high speed. 
In order to solve the above problems, according to the present invention, 
there is provided a device for initializing an optical disc, comprising: a 
plurality of erasing beam irradiating means for irradiating a relatively 
large beam onto a plurality of tracks at different positions on the 
optical disc simultaneously to erase information written at once; moving 
means for moving the plurality of erasing beam irradiating means in the 
radius direction of the optical disc under a condition where erasing beams 
irradiated from the plurality of erasing beam irradiating means are 
irradiated onto the optical disc at given intervals; and multi-tracking 
control means for controlling the moving means to move the erasing beam 
irradiating means in the radius direction of the optical disc such that 
the erasing beam irradiating means erases a certain number of tracks, that 
are erased at once, every time the plurality of erasing beam irradiating 
means erase all of information in subject tracks of the optical disc. 
In the present invention, the plurality of the erasing beam irradiating 
means irradiate the relatively large spots such as oval spots with a 
larger diameter in the radius direction of the optical disc or circular 
spots relatively large in diameter, onto a plurality of positions of the 
optical disc, simultaneously. Then, every time the plurality of erasing 
beam irradiating means erase all of the information in the subject tracks 
of the optical disc, that is, every time the optical disc makes one 
rotation at the minimum, the number of tracks whose information can be 
erased at once by the respective erasing beam irradiating means, or a 
given plurality of tracks which are less than that number are jumped, and 
the moving means is stepped radically of the optical disc, to thereby use 
the plurality of erasing beams simultaneously. In addition, because the 
width of the beams is large, the magneto-optical optical disc or the phase 
change optical disc is initialized stably and at a high speed.

DETAILED DESCRIPTION OF THE EMBODIMENT 
Now, a description will be given in more detail of preferred embodiments of 
the present invention with reference to the accompanying drawings. 
FIG. 1 is a side view showing a main portion of a device for initializing 
an optical disc in accordance with an embodiment of the present invention. 
The device is so designed that an optical disc 101 consisting of a 
magneto-optical optical disc or a phase change optical disc is fitted to a 
rotating shaft of a disc motor 102 so that a disc surface disposed 
vertically is rotated as indicated by an arrow 103. An optical head 104 
for writing, reading or erasing data with respect to the optical disc 102 
is disposed below the optical disc 102. 
The optical head 104 includes a fixed section 105 which is fixed at a 
position apart from the disc motor 102 by a predetermined distance toward 
the right side of the figure, first and second optical head moving 
sections 107 and 108 mounted on a moving table 106 which is disposed 
movably in the radius direction of the optical disc 101, and a moving 
mechanism 109 for moving the moving table 106. In this figure, the moving 
table 106 is moved in the radius direction of the optical disc while the 
first and second optical head moving sections 107 and 108 are focussed on 
the optical disc 101, and thereby the device can initialize the entire 
surface of the optical disc. 
In this embodiment, the moving mechanism 109 is made up of internal threads 
113 and 114 which are threaded inside of projections 111 and 112 so as be 
transverse to the projections 111 and 112 disposed on the bottom of the 
moving table 106, a bar-shaped member 116 having an external thread 
screwed with the internal threads 113 and 114, a pair of bearings 117 and 
118 that support both ends of the bar-shaped member 116, a first spur gear 
121 fixed to a base side of the bar-shaped member 116, a second spur gear 
122 engaged with the first spur gear 121, and a drive motor 123 fixed to 
the second spur gear 122 at its rotating shaft. The bar-shaped member 116 
is disposed along a radius direction of the optical disc 101, and the 
drive motor 123 is made up of, for example, a stepping motor or a DC 
(direct current) motor. Upon rotating the drive motor 123 forwardly or 
reversely, the moving table 106 is allowed to move in a direction denoted 
by an arrow 125 which is the radius direction of the optical disc 101. 
The fixed section 105 is so designed that a first laser beam 131 is emitted 
therefrom in parallel to the axis of the moving table 106. A laser pen 133 
containing a laser 132 therein is disposed on the optical head fixing 
section 105, and a laser beam emitted from the laser pen 133 is collimated 
by a collimator lens 134 into a first laser beam 131. In this embodiment, 
the first laser beam 131 is 830 nm in wavelength .lambda..sub.1 and 50 mW 
in its output. 
The first optical head moving section 107 has a beam splitter 136 in the 
vicinity of its bottom, which splits the first laser beam 131 into a beam 
obtained by deflecting the first laser beam 131 by 90 degrees and a beam 
straightly progressing. The former beam is focused on a first focusing 
position 137 of the optical disc 101 as will be described later and 
follows the track with the movement of the moving table 106 in a state 
where focusing servo is conducted. The latter beam is made incident to the 
second optical head moving section 108 and deflected by 90 degrees by a 
reflection mirror 138 which is disposed in the vicinity of its bottom. 
Similarly, the latter beam is focused on a second focusing position 139 of 
the optical disc 101 and follows the track with the movement of the moving 
table 106 in the state where focusing servo is conducted. 
The first laser beam 131 deflected by 90 degrees by the first optical head 
moving section 107 goes straightly through a dichroic mirror 141 and is 
made incident to an objective lens 142 so as to be focused on the first 
focusing position 137. The objective lens 142 is assembled with a 
two-axial actuator 143 which will be described later. A first hologram 
optical head unit 144 is disposed on the first optical head moving section 
107. The first hologram optical head unit 144 allows a second laser beam 
146 emitted therefrom to be deflected by 90 degrees by the dichroic mirror 
141 and made incident to the objective lens 142 so as to be focused nearly 
on the first focusing position 137. Then, a focusing error signal and a 
tracking error signal are detected, as will be described later, from a 
light reflected from the first focusing position 137. The second laser 
beam 146 as used is 780 nm in wavelength .lambda..sub.2 which is different 
from the wavelength .lambda..sub.1 of the first laser beam 131. 
The first laser beam 131 deflected by 90 degrees by the second optical head 
moving section 108 goes straightly through a dichroic mirror 151 and is 
made incident to an objective lens 152 so as to be converged on the second 
focusing position 139. The objective lens 152 is assembled with a 
two-axial actuator 153. A second hologram optical head unit 154 is 
disposed on the second optical head moving section 108. The second 
hologram optical head unit 154 allows a third laser beam 156 emitted 
therefrom to be deflected by 90 degrees by the dichroic mirror 151 and 
made incident to the objective lens 152 so as to be focused nearly on the 
second focusing position 139. Then, a focusing error signal and a tracking 
error signal are detected, as was described above, from a light reflected 
from the second focusing position 139. The third laser beam 156 as used is 
780 nm in wavelength .lambda..sub.2 which is different from the wavelength 
.lambda..sub.1 of the first laser beam 131. 
It should be noted that the output of the second or third laser beam 146 or 
156 outputted from the first and second hologram optical head units 144 
and 154 is used for erasing and also branched into two outputs with the 
result that it is remarkably lower than the output of the first laser beam 
131 used in the respective optical head moving sections 107 and 108. 
FIG. 2 is a diagram showing the main portions of the optical system 
including the first and second optical head moving sections. In the 
figure, the laser beam of the first wavelength .lambda..sub.1 and the 
laser beam of the second wavelength .lambda..sub.2 outputted from the 
first optical head moving section 107 are focused on a predetermined track 
161 on the optical disc 101, respectively, and a laser beam of the first 
wavelength .lambda..sub.1 and a laser beam of the second wavelength 
.lambda..sub.1 outputted from the second optical head moving section 108 
are focused on another track 162 inside of the track 161 on the optical 
disc 101, respectively. 
The focusing and tracking servo operations by the first and second optical 
head moving sections 107 and 108 are conducted by driving the two-axial 
objective lens actuators 143 and 153 in response to the focusing error 
signal and the tracking error signal detected by the corresponding first 
or second hologram optical head unit 144 or 154, which passes through a 
servo amplifier 164 or a phase compensating circuit 165 (In FIG. 2, only 
the first optical head moving section 107 is shown.). This operation will 
be described in more detail below. 
The first optical head moving section 107 allows the first laser beam 131 
of the wavelength .lambda..sub.1 to be made incident to the objective lens 
142 through the beam splitter 136 and the dichroic mirror 141 in the 
stated order, and to be focused on the first focusing position 137 on the 
track 161. On the contrary, a divergent light beam of the wavelength 
.lambda..sub.2 emitted from a laser not shown within the first hologram 
optical head unit 144 is converted into a parallel light beam by a 
collimator lens 171 and emitted as the second laser beam 146. Then, the 
second laser beam 146 is deflected by 90 degrees by the dichroic mirror 
141 and made incident to the objective lens 142 so as to be focused on a 
focusing position 174 on the optical disc 101. The laser beam of the 
wavelength .lambda..sub.2 is reflected by the surface of the disc and 
reversely progressed so as to be made incident to the collimator lens 171 
and focused on a multi-divided sensor not shown within the first hologram 
optical head unit 144. Then, a focusing error signal 177 and a tracking 
error signal 178 are detected by the output of the sensor and a head 
amplifier not shown. 
The focusing error signal 177 and the tracking error signal 178 are 
inputted to the servo amplifier 164 and the phase compensating circuit 165 
after being amplified by amplifiers 181 and 182, and outputs are inputted 
to the two-axial objective lens actuator 143. In response to the outputs, 
the two-axial objective lens actuator 143 drives the objective lens 142 in 
a focusing direction and a tracking direction. 
The same is applied to the second optical head moving section 108, and the 
first laser beam 131 that has passed through the beam splitter 136 is made 
incident to the objective lens 152 through the reflection mirror 138 and 
the dichroic mirror 151 in the stated order, and focused on the first 
focusing position 139 on the track 162. On the contrary, a divergent light 
beam of the wavelength .lambda..sub.2 emitted from a laser not shown 
within the second hologram optical head unit 154 is converted into a 
parallel light beam by a collimator lens 191 and emitted as the second 
laser beam 156. Then, the second laser beam 156 is deflected by 90 degrees 
by the dichroic mirror 151 and made incident to the objective lens 152 so 
as to be focused on a focusing position 194 on the optical disc 101. The 
laser beam of the wavelength .lambda..sub.2 is reflected by the surface of 
the disc and reversely progressed so as to be made incident to the 
collimator lens 191 and converged on a multi-divided sensor not shown 
within the second hologram optical head unit 154. Then, a focusing error 
signal and a tracking error signal are detected by the output of the 
sensor and a head amplifier not shown, and inputted to the two-axial 
objective lens actuator 153 through a servo amplifier and a phase 
compensating circuit not shown, respectively. The two-axial objective lens 
actuator 153 drives the objective lens 152 in the focusing direction and 
the tracking direction in response to those signals. 
FIG. 3 is a top view showing the two-axial objective lens actuator, and 
FIG. 4 is an explored view showing a main portion of the two-axial 
objective lens actuator. Since the two-axial objective lens actuators 143 
and 153 shown in FIG. 2 are entirely identical in structure with each 
other, the two-axial objective lens actuator 143 in the first optical head 
moving section 107 will be described in this specification. 
The two-axial objective lens actuator 143 includes an objective lens holder 
202 which is pentagonal in plan shape and has a square opening 201 
disposed in its center portion. A focus coil 204 to one surface of which a 
pair of tracking coils 203 are fitted is stuck to an inner wall of the 
opening portion 202, and the objective lens 142 is fitted to a cylindrical 
hole 206 which is opened in the vicinity of a substantially V-shaped 
projected portion of the pentagon. The objective lens holder 202 thus 
structured is fitted into a magnetic circuit 209 consisting of a magnetic 
yoke 208 which is U-shaped, where electromagnetic drive is conducted. 
Namely, the optically axial direction of the objective lens holder 202 is 
a focusing (F) direction, and a direction orthogonal to the focusing 
direction and right-angled to a straight line connecting the center of the 
objective lens holder 202 and the cylindrical hole 206 is a tracking (T) 
direction. 
An end portion of the objective lens holder 202 on an opposite side of a 
side where the hole 206 is defined is projected so as to form a projection 
211. The projection 211 opposes to a light emitting diode 215 disposed on 
a wall portion 214 standing on one end of an actuator base 212 to which 
the bottom of the magnetic yoke 208 is fixed. The wall portion 214 may be 
integrally molded with the actuator base 212. A pair of photo sensors 216 
are disposed on both sides of the light emitting diode 215 in the wall 
portion 214. A state in which a light beam emitted from the light emitting 
diode 215 is received by those photo sensors 216 depends on the position 
of the projection 211. Namely, the movement in the radical direction of 
the optical disc 101 (FIGS. 1 and 2), that is, in the tracking (T) 
direction by the tracking operation is detected by the output of those 
paired photo sensors 216. 
FIG. 5 is a graph representing a relation between the movement amount (mm) 
of the objective lens holder in the radius direction of the optical disc 
and the output voltage (V) of the paired optical sensors. In this example, 
one output of the paired photo sensors 216 shown in FIG. 3 is in a 
positive direction whereas the other output is in a negative direction to 
provide a differential output. A voltage slightly lower than a peak of a 
curve 221 representing the output voltage in the positive direction is set 
as an upper limit voltage V.sub.1, and similarly a voltage slightly higher 
than a peak of a curve 221 representing the output voltage in the negative 
direction is set as a lower limit voltage V.sub.1. With such setting, the 
moving range is limited so that the objective lens 202 is moved in the 
voltage range which is smaller than the absolute values of the voltages 
V.sub.1 and V.sub.2. 
FIG. 6 is a graph representing a relation between a time elapse and the 
positional displacement of the objective lens, as well as the positional 
change of the objective lens holder when the optical head moves from the 
inner periphery toward the output periphery. When the optical head 104 
shown in FIG. 1 moves from the inner periphery of the optical disc 101 
(FIGS. 1 and 2) toward the outer periphery thereof, the objective lens 
holders 202 installed in the two-axial objective lens actuators 143 and 
153 shown in FIG. 2, respectively, are repeatedly moved between its upper 
limit L.sub.U1 and its lower limit L.sub.D1, and while the objective 
lenses 142 and 152 are moving between the upper limit L.sub.U2 and the 
lower limit L.sub.D2 within a range limited by the above voltages V.sub.1 
and V.sub.2, their positions gradually move from the inner periphery of 
the optical disc 101 toward the outer periphery thereof. Namely, in moving 
the objective lenses 142 and 152, the moving ranges of the two-axial 
objective lens actuators 143 and 153 are detected by the pair of sensors 
216, and when their output voltage exceeds the upper limit voltage V.sub.1 
shown in FIG. 5, the drive motor 123 is controlled in such a manner that 
the moving table 106, that is, the actuator base 212 moves under control. 
When a plurality of optical head moving sections 107 and 108 are fitted to 
one moving table 106 so as to be moved integrally as in this embodiment, a 
signal for movement of the moving table 106 can be produced on the basis 
of a tracking error signal for one optical head moving section 107 or 108. 
FIG. 7 shows a state in which the device of this embodiment is erasing 
information in the track on the optical disc. In this example, a laser of 
the gain waveguide type is used without using a laser of the refractive 
waveguide type. Then, a focused beam 231 is formed which has a long axis 
in the radius direction of the optical disc 101 so that 20 or more tracks 
can be erased at once. 
FIG. 8 is a diagram showing the shape of the focused beam of 830 nm 
wavelength and the output distribution. The length of the focused beam 231 
is 50 .mu.m or longer in the radius direction of the optical disc 101. For 
that reason, 20 or more tracks 241 can be erased by a predetermined power 
at once. Assuming that the number of the tracks 241 which can be erased at 
once is 20 tracks, 19 tracks can be jumped in the radius direction every 
time the optical disc 101 (FIG. 7) makes one rotation. This operation can 
be realized by control of the movement in the radius direction of the 
optical disc made by the first hologram optical head unit 144 (FIG. 2) 
that enables accurate tracking operation by the dichroic mirror 141 (FIG. 
2). The timing setting of tracking for jumping 19 tracks in the radius 
direction can be made, for example, by using a pulse timing outputted from 
an encoder detecting the rotation of a disc motor. 
As was described above, according to the present invention, since 
relatively large spots such as oval spots with a larger diameter in the 
radius direction of the optical disc or circular spots relatively large in 
diameter are irradiated onto a plurality of positions of the optical disc 
simultaneously, and the erasing beams are jumped by a predetermined number 
of tracks in the radius direction of the optical disc for conducting an 
erasing operation, the magnet-optical optical disc or the phase change 
optical disc can be initialized stably and at a high speed. 
According to the second aspect of the present invention, the plurality of 
erasing beam irradiating means comprises a common and single light source 
of the refractive waveguide type or the gain waveguide type which is 
positionally fixed for irradiating the erasing beam; and beam branching 
means disposed on the moving means for branching a laser beam irradiated 
from the laser light source to irradiate branched laser beams onto the 
optical disc radically at given intervals. Namely, in the second aspect of 
the present invention, the laser light source of the plurality of erasing 
beam irradiating means is used commonly for downsizing the device and 
lowering the costs. 
According to the third aspect of the present invention, each of the erasing 
beam irradiating means comprises an objective lens through which the 
erasing beam is irradiated onto the optical disc, and a two-axial 
objective lens actuator that moves the objective lens in the radius 
direction of the optical disc and in the optical-axial direction of the 
objective lens. Also, the multi-tracking control means allows a beam 
different in wavelength from the erasing beam to be irradiated onto the 
optical disc through the objective lens, on the base of which a focusing 
error signal relating to a focal point of the erasing beam that has passed 
through the objective lens, and a tracking error signal relating to the 
tracks of the optical disc are obtained, and advances the moving means in 
the radius direction of the optical disc by a plurality of tracks, the 
maximum number of which is the number of tracks whose information data are 
erased at once by the erasing beam irradiating means, while adjusting 
focusing operation and tracking operation. Namely, in the third aspect of 
the present invention, the objective lens is so designed as to be movable 
in the predetermined direction by the two-axial objective actuator, and 
the beam different in wavelength from the erasing beam is irradiated onto 
the optical disc through the objective lens. Using the tracking error 
signal obtained by irradiation of the beam on the optical disc, the moving 
means is advanced in the radius direction of the optical disc by a 
plurality of tracks, the maximum number of which is the number of tracks 
whose information data are erased at once by the erasing beam irradiating 
means. With thus making the beam different in wavelength from the erasing 
beam, a false detection of a signal can be prevented, thereby being 
capable of enhancing the reliability of the device. 
According to the fourth aspect added to the third aspect of the present 
invention, in the respective two-axial objective lens actuators, movable 
ranges of the objective lens in the radius direction of the optical disc 
are set and the moving means moves the plurality of erasing beam 
irradiating means in the radius direction of the optical disc when the 
moving amount of the objective lens in the two-axial objective lens 
actuator exceeds the movable ranges. Thus, the control of the radius 
direction movement of the objective lens in the two-axial objective lens 
actuator is conducted in associated with the movement of the plurality of 
erasing beam irradiating means. With this arrangement, more accurate and 
more rapid control can be performed. 
According to the fifth aspect added to the third or fourth aspect of the 
present invention, the objective lens actuator comprises; a holder for 
holding the objective lens; a reflector shaped in an projection or the 
like which moves in association with the movement of the holder; light 
emitting means such as a light emitting diode which is disposed opposite 
to the reflector for irradiating a light onto the reflector; a pair of 
photo sensors such as photodiodes which are disposed on both sides of the 
light emitting means for receiving the light reflected from the reflector, 
respectively; differential output means such as a differential amplifier 
for taking a difference between outputs of those two photo sensors; and 
detecting means for detecting the movement of the holder in the radius 
direction of the optical disc on the basis of an output of the 
differential output means. Using a phenomenon that a light incident to one 
of the photo sensors increases and a light incident to the other decreases 
with the movement of the holder, the position of the objective lens is 
specified by a simple mechanism. 
According to a sixth aspect added to the fifth aspect of the present 
invention, the multi-tracking control means is so designed that a 
predetermined number of tracks are jumped at once when the erasing beam 
irradiating means is moved in radius direction; the objective lens 
actuator is jumped toward a subject track, and at the time of this 
movement, the movement amount of the holder is detected, and when the 
movement of the holder exceeds an upper limit of a predetermined 
displacement, the erasing beam irradiating means is moved. With this 
associated action, the movement with a high accuracy can be realized.