JPS6066337A - Focus servo device - Google Patents

Abstract

PURPOSE:To remove defects such as a large optical system, high production cost, a heavy movable part, and high power consumption by feeding back a focus error signal to a wavelength control means of a variable wavelength light source. CONSTITUTION:Laser light from a semiconductor laser 17 provided with a power source 18 is supplied to a hologram 20 through a beam splitter (BS) 19 and a laser beam focused in accordance with the wavelength of the laser light is made incident on the recording surface 5 of a disc. The reflected light is reflected by the BS19 and made incident on a photodetector 25 in which two photodetectors are concentrically arranged through a conical lens 24. The difference of signals from the BS19 is catched by an operator 26 and inputted to a wavelength control circuit 28 as a focus error signal SF and a high frequency signal is supplied to the semiconductor laser 17, so that the wavelength of the laser light is changed and the laser light is correctly focused on the recording surface 5. Since the movable part is removed, a defect due to the movable part is removed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a focus servo device suitable for application to, for example, an optical disc playback device.

Background Art and its Problems For example, a focus servo device as shown in FIG. 1 is conventionally known as a focus servo device for an optical disc playback device that plays back signals optically recorded on a disc.

In the figure, (1) is a semiconductor laser, (2) is a beam splitter, (3) is a collimator lens, (4) is an objective lens, and (5) is a recording surface of a disk. On this recording surface (51), for example, audio information is recorded in the form of bits so that it can be read out optically.

In addition, (6) indicates a focus actuator, and (7
) is a water-immersed magnet, (8) is a yoke, (9) is a control coil, and the above-mentioned objective lens (4) is a focus coil (
9) can be integrally attached. A focus servo signal is supplied to this tlilJ control coil (9) from a drive circuit to be described later, and the objective lens +41 is driven in the tube axis direction 0 (11) according to the focus servo signal. Even if there is movement, the laser beam from the laser beam (II) is correctly focused on the recording surface (5) of the disk by the objective lens (4) via the beam splitter (2-input collimator lens + 31). In addition, Qll is a concave lens, u2! is a cylindrical lens, (13 is a multi-segment detector, and the laser beam reflected on the recording surface (5) is transmitted through the objective lens (4), the collimator lens (3), After passing through the beam splitter (2),
It is supplied to a multi-division detector (13) via a concave lens (ID) and a cylindrical lens (121).The signal from this detector (t3) is supplied to a focus error detection circuit and a focus error signal is detected.And , this error signal is supplied to the drive circuit (1G) via the phase 11ii compensation circuit (15), and as described above, the focus servo signal is coupled from this drive circuit (16) to the focus coil (9). .

The focus servo device shown in FIG. 1 requires a focus actuator (6) that drives the objective lens (4) in the tube axis direction (IQI), resulting in a large optical system and high cost. Furthermore, since the movable parts such as the objective lens (4) and the focus coil (9) weigh several grams or more, power consumption for driving is large.

In addition, if the dynamic frequency of the focus actuator (6) hits the oscillation station wave number of a damper (not shown), resonance will occur, making it impossible to set the servo band to a high level, and furthermore, there is no way to prevent resonance. phase compensation is troublesome.

Furthermore, the drive sound of the focus actuator (6) becomes noise and is bothersome.

OBJECTS OF THE INVENTION The present invention has been made in view of these points, and is intended to eliminate the above-mentioned drawbacks.

Summary of the Invention In order to achieve the above object, the present invention provides a light source with a variable oscillation wavelength, a wavelength control means for changing the oscillation wavelength, and a hologram lens for condensing the light generated from the light source onto a disk. and a focus error detection means, and the focus servo is performed by changing the oscillation wavelength by feeding back a focus error signal from the focus error detection means to the wavelength control means. It is something to do.

Therefore, since it does not have a movable part, the disadvantages caused by having a movable part as in the conventional example are eliminated.

Embodiment Hereinafter, an embodiment of the present invention will be described with reference to FIG.

In the figure, 0η is a semiconductor laser whose oscillation wavelength is variable, and (18! is its power supply).

As the semiconductor laser aη, for example, Japanese Patent Application Laid-Open No. 58-21
The method using ultrasonic waves as described in Publication No. 888 is easy and variable fi! It is suitable because of its large area. That is, a comb-shaped electrode ultrasonic vibrator is provided on the QaAs layer, and a high frequency signal is applied to this to change the ultrasonic frequency, thereby changing the oscillation wavelength of the laser.

Laser light from this semiconductor laser Q7) is supplied to the hologram (20) via a beam splitter a3.

This hologram @ has a lens action that converts a spherical wave that diverges from one point into a spherical wave that converges at another point.

Such a hologram (2Ql), as shown in Fig. 3, has an auxiliary hologram @ and a thin film (20a) of dichlumate gelatin, etc., which is to become the hologram (A), placed on the optical path of the objective lens (21). , auxiliary hologram (22)
A focused spherical wave with a numerical aperture of sinθl and an objective lens (
21) A diverging spherical wave with a numerical aperture of sin θ2 is formed by a thin Bt
It can be made by interfering on J (20a). According to this, R, the membrane (20a) is at point F! A hologram (2) has a lens action that converts a spherical wave with a numerical aperture of sin θ1 that diverges from the point F2 into a spherical wave with a numerical aperture of sin θ2 that converges on a point F2. As shown in the figure, the numerical aperture of the objective lens c!31 is generated by causing spherical waves to interfere on the i8 film (22a).

In this way, the hologram (2) has a lens effect that converts a spherical wave diverging from one point into a spherical wave converging at another point, so the semiconductor laser (2) supplied to the L-c hologram (201) via the beam splitter a9. 17) can be focused on the recording surface (5) of the disk.

By the way, since the hologram (2o) has a phenomenon called diffraction, when the wavelength of the laser beam changes, the focal position changes as shown by the broken line in FIG. 2. The focus servo arrangement of the present invention utilizes this principle.

The principle will be explained in detail below.

That is, as shown in Fig. 5, if the incident angle at the location of the hologram is 00 and the exit angle (diffraction angle) is θi, the following equation holds true between the pitch d of the interference fringes formed there. .

sin θ1-sin θO-λ/d --- (1) λ;
Laser light oscillation wavelength When this equation (1) is varied by θi, dθ1-cosθ1=dλ/d...(2),
When d is eliminated using the +11 formula, it becomes.

01.01 at the outermost circumference of the a1 hologram (20), which is the radius of the hologram 0()). θO is 9. , 9o, the focal length f is given by f = a cot9i -00, (41. Then, by varying this +41 formula by θi, the following is obtained.

By substituting equation (3) into equation (5) and canceling dGi, NA2-NAI NAI , NA2 ; respectively become the numerical apertures of the hologram (2) of the incident wave and the output wave of the force.

Therefore, it can be seen that when the wavelength λ of the laser beam changes, the focal length f also changes.

Now, NA, 1 = Q, 1, NAz = 0.47
That is, = 1.01, and f"" 7.8 ta,
λ = 7801 m] Condition "'C-, laser If the change Δλ in the wavelength λ of the light is lQnm, the focal length is 9flH
The change Δf in f is 0.1 core.

By the way, in the above-mentioned ultrasonic laser (17), the order of Bragg diffraction is m1, the ultrasonic frequency is fc,
When the refractive index of the eight bodies is n and the speed of sound is V, when the ultrasonic frequency fc is changed by ΔfC, the change Δλ in the wavelength λ of the laser light is as follows.

Now, ■=5000mAeC1n-3. fC=38GH2
, Δfc=3.4 GHz, the change Δλ in the wavelength λ of the laser light is 7Q nm. As a result, under the above-mentioned conditions, for example, the change Δf in the focal length rr'r(f is 0.7 km, and a focal length change Δf sufficient to absorb the fluctuation of the disk is obtained.

As mentioned above, by changing the wavelength λ of the laser light from the laser (tη), the focal length f of the hologram 0@
In this example, the wavelength λ of the laser light is changed based on the focus error signal,
Then, the focal length f of the hologram (20) is changed so that the laser beam is properly focused on the recording surface (5) of the disk.

That is, the laser beam reflected by the recording surface (5) of the disk is passed through the hologram (A) and the beam splitter (19), and then passes through the beam splitter a! The two photodetectors are supplied to the photodetector (251) arranged concentrically through a conical lens (24) provided on the light-emitting side end surface of the photodetector 1.Then, the difference between the seven photodetectors on the inner and outer circumferences is The difference signal is taken by the arithmetic unit (2e) and is the focus error signal S.
It is considered F. The method to obtain the focus error signal SF is as follows:
The method is not limited to this, and other conventionally known methods may be used.

In addition, the photodetector layers 1 on the inner and outer peripheries of the photodetector (c) are taken by the arithmetic unit (27), and the sum signal is sent to the information signal number SI.
becomes.

The focus error signal SF from the arithmetic unit C26) is supplied to the wavelength control circuit (c). And this wavelength control circuit c! A high frequency signal corresponding to the focus error signal 8F is generated from the barrel, and this is supplied to the above-mentioned semiconductor laser surround, and the ultrasonic frequency ifc is generated as the focus error signal S.
It can be changed in response to F.

Therefore, the wavelength λ of the laser beam from the laser (17) is changed in accordance with the focus error signal SF, and as a result, the focal length f of the hologram (2Q) is changed, and the laser beam is directed onto the recording surface (5) of the disk. Made to focus correctly.

According to the focus servo device of this example described above, since it does not have a movable part, it suffers from the disadvantages of having a movable part as in the conventional example, namely, the optical system becomes thick (and the cost is high). The drawbacks such as heavy moving parts (high power consumption etc.) are eliminated.For example, a pickup using a 7 orcus servo device like this example has LOvs (ii: diameter)
It can be made into a compact shape of approximately 20ffi (length) to 10yi x 30m.

In the example in Figure 6, a pickup (c) using a focus servo device like the example in Figure 2 is attached to the tip of a segment motor, and tracking servo and feeding are realized by this segment motor (30). This is how it was done. In FIG. 6, parts corresponding to those in FIG. 2 are designated by the same reference numerals.

In the figure, CD is a magnetic circuit for driving segments, (
32 is a driving coil, and C331 is a shaft.

Further, 0yu is a photodetector to which laser light reflected by the recording surface (5) of the disk is supplied. This photodetector (3
The detection signal Sd from 4) is sent to the focus error detection circuit (
The focus error signal SF is obtained by a conventionally known method. This focus error signal SF is then supplied to the wavelength control circuit (28), and focus servo similar to the above-mentioned example in FIG. 2 is performed. Further, the detection signal Sd from the photodetector (34) is supplied to a tracking error detection circuit (3G), and a tracking error signal ST is obtained by a conventionally known method. Then, this tracking error signal ST is supplied to the control coil G2 via the phase compensation circuit (371, 1 drive circuit 08), and tracking servo is performed.

Further, the detection signal Sd from the photodetector (34) is supplied to the information signal detection circuit, which controls the information signal Si.

In the first case of this example in Fig. 6, as mentioned above, the pickup Q
Since ω can be made into a compact shape, the power consumption of the segment motor C301 is low, and the segment motor C301 can be easily separated.

In the case of the example shown in FIG. 6, the pits on the recording surface (5) of the disk may be 1 μm x 1 μm or less, and the track pitch may be 1.5 μm.

Effects of the Invention As is clear from the embodiments described above, the present invention does not have a movable part, so it does not have the disadvantages of having a movable part as in the conventional example, that is, the optical system is thick (and Disadvantages such as high cost and heavy moving parts (high power consumption) are eliminated.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an example of a conventional focus servo device, Fig. 2 is a block diagram showing an embodiment of the present invention, and Figs.
The figures are for explanation, and FIG. 6 is a perspective view showing an example of a pickup drive device to which the present invention is applied. (5) is the recording surface of the disk, aυ is the ten fathom laser, a
3 is a beam splitter, (20) is a hologram, (Incorporated)
is a conical lens, (c) is a photodetector, (26) and (27)
) are arithmetic units, (c) is a wavelength control circuit, SF is a focus error signal, and Si is an information signal. 7,゛) Agent Sada Ito・,4′ゞ,″

Claims (1)

[Claims] a light source with a variable oscillation wavelength; a wavelength control means for changing the oscillation wavelength; a hologram lens for condensing light generated from the light source onto a disk;
A focus servo device comprising a focus error m1ll, and performing focus servo by changing the oscillation wavelength by feeding back a focus error signal from the focus niller detection means to the wavelength control means. .