JPH02203426A - Focus servo device - Google Patents

Abstract

PURPOSE:To correct in a short time a focus error caused with switch of the output of a laser diode by using an acceleration circuit which moves an optical lens with sudden acceleration and sudden deceleration. CONSTITUTION:An optical lens 18 is moved with sudden acceleration and sudden deceleration in the distant direction from the surface of a medium when a low drive current is switched to a high drive current. At the same time, the lens 18 is moved with sudden acceleration and sudden deceleration in the closing direction to the surface of the medium when a high drive current is switched to a low drive current. Thus it is possible to correct in a short time a focus error due to the color aberration of an optical system including the lens 18 which is caused by the change of the oscillation wavelength with switch of the drive current of a laser diode.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a focus servo device used in write-once optical disk drive devices, rewritable optical disk drive devices, and the like.

BACKGROUND ART FIG. 3 shows the configuration of a conventional focus servo device.

In FIG. 3, 31 outputs a relatively low drive current to the laser diode 32 when reading data written on the medium, and outputs a relatively high drive current to the laser diode 32 when writing data on the medium. The laser diode 32, which is a read/write switching unit, emits a beam with an output corresponding to the drive current from the read/write switching unit 31, and the lens 36 focuses this beam and irradiates the medium surface with a spot light. .

33 is a focus error detection unit that outputs a focus error signal indicating the amount of deviation from the in-focus position of the spot light irradiated on the medium surface by the light reflected by the medium surface; 34;
is a focus servo control section that controls a mechanism 35 that drives the lens 36 based on this focus error signal.

Next, the operation of the conventional example described above, especially the operation of the focus servo control section 34, will be explained. In FIG. wake up

Therefore, the focus servo control section 34 outputs a drive signal to the lens drive mechanism 35 so that the focus error signal from the focus error detection section 33 becomes "0", that is, the spot light is focused on the medium surface. and lens 36
along its optical axis.

Problems to be Solved by the Invention However, in the above conventional focus servo device,
When the read/write switching section 31 switches the output of the laser diode 32, the oscillation wavelength of the laser diode 32 changes, so even if the focus is out of focus due to surface wobbling of the medium, the optical system including the lens 36 will be affected by this change in the oscillation wavelength. There is a problem in that a large defocus occurs due to chromatic aberration, and therefore it takes a long time to return to the in-focus position.

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, it is an object of the present invention to provide a focus servo device that can correct a focus shift in a short time when switching the output of a laser diode.

Means for Solving the Problems In order to achieve the above object, the present invention provides rapid acceleration and sudden acceleration in the direction of moving the optical lens away from the medium surface when switching from a relatively low drive current to a relatively high drive current. On the other hand, when switching from a relatively high drive current to a relatively low drive current, the optical lens is moved in a direction closer to the medium surface with rapid acceleration and rapid deceleration.

According to the above structure, the present invention allows the optical lens to be rapidly accelerated and decelerated even if the oscillation wavelength changes when the driving current of the laser diode is changed and a large focus shift occurs due to chromatic aberration of the optical system including the lens. Embodiments Hereinafter, the present invention will be described in detail with reference to the drawings. 1st, 1st
1 is a block diagram showing an embodiment of a focus servo device according to the present invention, and FIG. 2 is a timing chart showing main signals of the focus servo device of FIG. 1.

In FIG. 1, 11 outputs a relatively low drive current to the laser diode 12 when reading data written on the medium, and outputs a relatively high drive current to the laser diode 12 when writing data to the medium. The read/write switching section and the laser diode 12 are connected as shown in Figure 2 (
As shown in a), the read/write switching section 11 emits a beam of output according to the drive current, and the lens 18 condenses this beam to irradiate the medium surface with a spot light.

13 is caused by the light reflected by the medium surface in Figure 2 (c
), the focus error detection unit 14 outputs a focus error signal indicating the amount of deviation from the focus position of the spotlight irradiated on the medium surface, and the focus error detection unit 14 outputs the focus error signal from the focus error detection unit 13. so that it becomes 0.
That is, it is a focus servo control section that outputs a drive signal to the lens drive mechanism 35 via the changeover switch 16 so that the spot light is focused on the medium surface.

15, as shown in FIG. 2(b), when the read/write switching unit 11 switches from a relatively low drive current to a relatively high drive current, the lens 18 is rapidly accelerated and moved away from the medium surface. On the other hand, when the read/write switching unit 11 switches from a relatively high drive current to a relatively low drive current, the lens 18 is moved at a sudden acceleration and deceleration in a direction closer to the medium surface. The changeover switch 16 is an acceleration pulse generation section that generates pulses, and the changeover switch 16 selectively selects the drive signal from the focus servo control section 14 or the acceleration pulse from the acceleration pulse generation section 15 based on the switching signal from the read/write switching section 11. It is output to the lens drive mechanism 35.

Next, the operation of the above embodiment will be explained.

First, when the read/write switching unit 11 outputs a relatively low drive current to the laser diode 12 in order to read data written on the medium, the changeover switch 16
outputs a drive signal from the focus servo control section 14 to the lens drive mechanism 17.

In this case, the focus error signal is a signal 21 with relatively small fluctuations depending on the surface runout of the medium, as shown in FIG. 2(C), and therefore, as shown in FIG. 2(d),
The spot light is focused onto the media surface.

In this state, when the read/write switching unit 11 switches to a relatively high drive current and outputs it to the laser diode 12 in order to write data to the medium, the laser diode 12
Since the oscillation wavelength of is slightly shifted to a longer wavelength,
As shown in FIG. 2(d), the spot light is focused at a position far from the medium surface, resulting in a large focus shift.

In this case, the focus error signal becomes a signal 22 with relatively large fluctuations at a negative level depending on the chromatic aberration of the lens 18, as shown in FIG. 2(c).

Therefore, when the changeover switch 16 is switched to output the acceleration pulse from the acceleration pulse generation section 15 to the lens drive mechanism 35 in accordance with the switching signal from the read/write switching section 11, the lens 18 is suddenly accelerated and moved away from the medium surface. It moves with rapid deceleration, and therefore, as shown in FIG. 2(d), the spot light is focused on the medium surface in a short time (focus error signal 23).

As an example, the shift width of the oscillation wavelength of the laser diode 12 is 1μ, and the peak value of the acceleration pulse is
Then, the time t required for movement is = 0.2 x 1
0-3 (seconds).

After this time t, when the changeover switch 16 is switched to output the drive signal from the focus servo control unit 4 to the lens drive mechanism 17, the focus error signal is output to the medium as shown in FIG. 2(c). The signal 24 has a relatively small fluctuation according to the surface runout of the
), the spot light is focused on the medium surface.

On the other hand, in this state, when the read/write switching unit 11 switches to a relatively low drive current and outputs it to the laser diode 12 in order to read the data written on the medium, the oscillation wavelength of the laser diode 12 returns to the original short wavelength. Because of this shift, the spot light is focused at a position close to the medium surface, resulting in a large focus shift, as shown in FIG. 2(d).

In this case, the focus error signal becomes a signal 25 with relatively large fluctuations at a positive level depending on the chromatic aberration of the lens 18, as shown in FIG. 2(c).

Therefore, when the changeover switch 16 is switched to output the acceleration pulse from the acceleration pulse generation section 15 to the lens drive mechanism 35 in accordance with the switching signal from the read/write switching section 11, the lens 18 is suddenly accelerated and moved in the direction closer to the medium surface. It moves at a rapid deceleration, and therefore, the spot light is focused on the medium surface in a short time, as shown in FIG. 2(d). (Focus error signal 26.27).

Therefore, according to the above embodiment, when the read/write switching unit 11 switches from a relatively low drive current to a relatively high drive current, the acceleration pulse generator 15
is moved away from the medium surface at a rapid acceleration and deceleration, and on the other hand, when the read/write switching unit 11 switches from a relatively high drive current to a relatively low drive current, the acceleration pulse generator 15 moves the lens 18 away from the medium surface. Since an acceleration pulse is generated to move the medium from the medium surface at a rapid acceleration and deceleration, a large defocus occurs due to a change in the oscillation wavelength of the laser diode 32 and chromatic aberration of the optical system including the lens 18. However, it can be fixed in a short time.

In addition, focus deviation when switching between reading and writing can be corrected in a short time, reducing the waiting time until focusing.
Media can be accessed at high speed.

Furthermore, since the present invention corrects the focus shift caused by the optical system using the control system, optical glass with low wavelength dispersion is not required, and therefore, the cost of optical pickup can be reduced.

DETAILED DESCRIPTION OF THE INVENTION As described in detail, the present invention moves the optical lens away from the medium surface with rapid acceleration and rapid deceleration when switching from a relatively low drive current to a relatively high drive current; On the other hand, when switching from a relatively high drive current to a relatively low drive current, the optical lens is moved in the direction closer to the medium surface with rapid acceleration and deceleration. Even if a large focus shift occurs due to chromatic aberration in the optical system including the lens, it can be corrected in a short time.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of a focus servo device according to the present invention, FIG. 2 is a timing chart showing main signals of the focus servo device of FIG. 1, and FIG. 3 is a block diagram showing a conventional focus servo device. FIG. 2 is a block diagram showing the device. 11... Read/write switching section, 12... Laser diode, 13... Focus error detection section, 14... Focus servo control section, 15... Acceleration pulse generation section,
16... Selector switch, 17... Lens drive mechanism,
18. Lens.

Claims (2)

[Claims] (1) A laser diode for irradiating a spot light onto a medium through an optical lens, a drive circuit that selectively supplies a relatively low drive current and a relatively high drive current to the laser diode, and the drive circuit When switching from a relatively low drive current to a relatively high drive current, the optical lens is moved away from the medium surface at a rapid acceleration and deceleration, and on the other hand, when the drive current is switched from a relatively high drive current to a relatively low drive current. and an acceleration circuit that generates a signal for moving the optical lens in a direction closer to the medium surface with rapid acceleration and deceleration when switching is performed. (2) A circuit that detects a focus error signal indicating the amount of deviation from the focused position of the spot light irradiated onto the medium by the laser diode, and a circuit that detects a focus error signal that indicates the amount of deviation from the focused position of the spot light irradiated onto the medium by the laser diode, and a circuit that causes the spot light to be focused on the medium based on the focus error signal. A focus servo circuit outputs a signal for driving a lens, and when the drive circuit constantly outputs a relatively low drive current or a relatively high drive current, selects and outputs a signal from the focus servo circuit. The focus servo device according to claim 1, further comprising means for selecting and outputting the signal of the acceleration circuit for a predetermined time after the drive circuit switches the drive current.