JPH0550055B2 - - Google Patents

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 TECHNOLOGY AND PROBLEMS For example, a focus servo device as shown in FIG. 1 is conventionally known as a focus servo device for an optical disk playback device that plays back signals optically recorded on a disk.

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. For example, audio information is recorded on the recording surface 5 in the form of pits so that it can be read out optically.

Further, 6 indicates a focus actuator,
7 is a permanent magnet, 8 is a yoke, 9 is a control coil, and the above-mentioned objective lens 4 is a focus coil 9.
It can be attached integrally with. A focus servo signal is supplied to the control coil 9 from a drive circuit, which will be described later, and the objective lens 4 is driven in the tube axis direction 10 in accordance with the focus servo signal. As a result, even if the disk moves up and down, for example, the laser beam from the laser 1 passes through the beam splitter 2 and collimator lens 3 and is correctly focused on the recording surface 5 of the disk by the objective lens 4. be done.

Further, 11 is a concave lens, 12 is a cylindrical lens, 13 is a multi-division detector, and the recording surface 5
The laser beam reflected by the laser beam passes through an objective lens 4, a collimator lens 3, and a beam splitter 2, and then is supplied to a multi-division detector 13 via a concave lens 11 and a cylindrical lens 12. The signal from this detector 13 is supplied to a focus error detection circuit 14 to detect a focus error signal. This error signal is then supplied to the drive circuit 16 via the phase compensation circuit 15, and as described above, the drive circuit 16 supplies the focus coil 9 to the drive circuit 16.
A focus servo signal is supplied to.

The focus servo device as shown in FIG. 1 requires a focus actuator 6 for driving the objective lens 4 in the tube axis direction 10, resulting in a large optical system and high cost. . Further, 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. Furthermore, if the drive frequency of the focus actuator 6 matches the vibration frequency of a damper (not shown), for example, resonance will occur, making it impossible to obtain a high servo band, and furthermore, phase compensation to prevent resonance is troublesome. It is. Furthermore, the driving sound of the focus actuator 6 becomes noise and is bothersome.

OBJECTS OF THE INVENTION The present invention has been made in view of the above 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 hologram lens that converts spherical wave light generated from the light source into a spherical wave condensed onto a disk, and the hologram lens. A beam splitter that separates the light returned from the disk through the light source and the light generated from the light source, and the light flux of the return light from the disk separated by the beam splitter that passes through an optical element for detecting focus error. a focus error signal detection means for detecting a focus error signal based on the detection output from the light detection means; and a focus error signal detection means for detecting a focus error signal based on the detection output from the focus error signal detection means. and wavelength control means for changing the oscillation wavelength of the light source.

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, 17 is a semiconductor laser whose oscillation wavelength is variable, and 18 is its power supply. As the semiconductor laser 17, for example,
A method using ultrasonic waves as described in Japanese Patent No. 58-21888 is preferable because it is easy to use and has a wide variable range. That is, a comb-shaped electrode ultrasonic vibrator is provided on the GaAs 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 17 is supplied to a hologram 20 via a beam splitter 19.

This hologram 20 has a lens action that converts a spherical wave that diverges from one point into a spherical wave that converges at another point.

As shown in FIG. 3, such a hologram 20 is constructed by disposing an auxiliary hologram 22 and a thin film 20a such as dichromate gelatin, which is to become the hologram 20, on the optical path of an objective lens 21. It can be created by interfering on the thin film 20a a converging spherical wave with a numerical aperture of sin θ ₁ and a diverging spherical wave with a numerical aperture of sin θ ₂ produced by the objective lens 21. According to this, the thin film 20a becomes a hologram 20 having a lens function that converts a spherical wave with a numerical aperture of sin θ ₁ that diverges from point F ₁ into a spherical wave with a numerical aperture of sin θ ₂ that focuses on point F ₂ . The auxiliary hologram 22 is connected to the objective lens 23 as shown in FIG.
A diverging spherical wave with a numerical aperture of sinθ ₁ is generated by a thin film 22a.
It can be made by interfering with the top.

In this way, the hologram 20 has a lens action that converts a spherical wave that diverges from one point into a spherical wave that converges at another point, so that the laser beam from the semiconductor laser 17 that is supplied to the hologram 20 via the beam splitter 19 can be focused on the recording surface 5 of the disk.

By the way, since the hologram 20 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 device 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 a location of the hologram is θo and the outgoing angle (diffraction angle) is θi, then the pitch d of the interference fringes formed there is as follows.
The following formula holds true.

sinθi−sinθo=λ/d...(1) λ: Laser light oscillation wavelength If we vary this equation (1) by θi, we get dθi・cosθi=dλ/d...(2), which makes equation (1) When d is eliminated using
3) becomes.

If the radius of the hologram 20 is a, and θi and θo at the outermost circumference of the hologram 20 are θ^i and θ^o, then the focal length f
is given by f=acotθ^i...(4). Then, by varying this equation (4) with respect to θi, Δf/f=dθ^i/cosθi·sinθi (5) is obtained.

Substituting equation (3) into equation (5) and eliminating dθ^i, Δf/f=sinθ^i−sinθ^o/cos ² θi・sinθi・
dλ／λ＝Kdλ／λ……(6) K＝sinθ^i−sinθ^o／cos ² θi・sinθi＝sinθ^i−si
nθ^o/sinθi (1-sin ² θi) = NA ₂ - _{NA 1} /NA ₂ {1-(NA ₂ ) ² } NA ₁ , NA ₂ ; Numerical aperture of the incident wave and the output wave of the hologram 20, respectively. .

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

Now, under the conditions of NA ₁ = 0.1, NA ₂ = 0.47, that is, K = 1.01, and f = 7.8 mm, λ = 780 nm, and if the change Δλ in the wavelength λ of the laser light is 10 nm, the change Δf in the focal length f is It becomes 0.1mm.

By the way, the above-mentioned ultrasonic laser 17
When the order of Black diffraction is m, the ultrasonic frequency is fc, the refractive index of the medium is n, and the speed of sound is V, if the ultrasonic frequency fc is changed by Δfc, the change Δλ in the wavelength λ of the laser light is Δλ=−2nV/m・Δfc/fc ² ...(7).

Now, V=5000m/sec, n=3, fc=38GHz,
When Δfc=3.4GHz, the change Δλ in the wavelength λ of the laser light is 70 nm. As a result, under the above-mentioned conditions, for example, the change in focal length f becomes 0.7 mm, which is sufficient to absorb the fluctuation of the disk.

As described above, by changing the wavelength λ of the laser beam from the laser 17, the hologram 20
The focal length f of the hologram 20 can be changed, and in this example, the wavelength λ of the laser beam is changed based on the focus error signal, and the hologram 20
The focal length f of the laser beam 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 passes through the hologram 20 and the beam splitter 19, and then passes through the conical lens 24 provided on the output side end face of the beam splitter 19 to two photodetectors. are supplied to photodetectors 25 arranged concentrically. Then, the difference between the photodetectors on the inner and outer peripheries is calculated by the arithmetic unit 26, and the difference signal is used as the focus error signal S _F. The method for obtaining the focus error signal _SF is not limited to this, and other conventionally known methods may be used.

Note that the sum of the photodetectors on the inner and outer peripheries of the photodetector 25 is calculated by the calculator 27, and the sum signal is used as the information signal.
Becomes S _I.

The focus error signal S _F from the arithmetic unit 26 is supplied to the wavelength control circuit 28 . A high frequency signal corresponding to the focus error signal S _F is generated from this wavelength control circuit 28, and this is supplied to the semiconductor laser 17 described above, so that the ultrasonic frequency fc is changed in accordance with the focus error signal S _F. It will be done.

Therefore, the wavelength λ of the laser beam from the laser 17 is changed in accordance with the focus error signal S _F , and as a result, the focal length f of the hologram 20 is changed, so that the laser beam is correctly focused on the recording surface 5 of the disk. be made to do.

According to the focus servo device of this example described above, since it does not have a movable part, there are disadvantages of having a movable part as in the conventional example, namely, the optical system is large and the cost is high; Disadvantages such as heavy weight and high power consumption are eliminated.
For example, a pick-up using a focus servo device like the one in this example is 10 mm (diameter) x 20 mm (length).
It can be made into a compact shape of ~10mm x 30mm.

In the example shown in FIG. 6, a pick-up 29 using a focus servo device like the example shown in FIG. It has been done. In FIG. 6, parts corresponding to those in FIG. 2 are designated by the same reference numerals.

In the figure, 31 is a magnetic circuit for driving segments, 32 is a driving coil, and 33 is a shaft.

Further, 34 is a photodetector to which laser light reflected by the recording surface 5 of the disk is supplied. The detection signal Sd from the photodetector 34 is supplied to a focus error detection circuit 35, and a focus error signal S _F is obtained by a conventionally known method. This focus error signal S _F is then supplied to the wavelength control circuit 28, and focus servo similar to the above-mentioned example in FIG. 2 is performed. In addition, a detection signal from the photodetector 34
Sd is supplied to the tracking error detection circuit 36, and a tracking error signal S _T
is obtained. Then, this tracking error signal S _T is supplied to the drive coil 32 via the phase compensation circuit 37 and the drive circuit 38, and tracking servo is performed. Further, the detection signal Sd from the photodetector 34 is supplied to an information signal detection circuit 39, and an information signal S _I is obtained.

In the case of the example shown in FIG. 6, since the pickup 29 can be made compact as described above, the power consumption of the segment motor 30 is low and its design is easy.

In the case of this example in FIG. 6, the recording surface 5 of the disk
The upper pit may be less than 1 μm x 1 μm 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 large and expensive. becomes,
Disadvantages such as heavy moving parts and 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, Figs. 3 to 5 are diagrams for explaining the same, and Fig. 6 1 is a perspective view showing an example of a pickup drive device to which the present invention is applied; FIG. 5 is a recording surface of the disk, 17 is a semiconductor laser, 19 is a beam splitter, 20 is a hologram, 24 is a conical lens, 25 is a photodetector, 26 and 27 are respective computing units, 28 is a wavelength control circuit, and S _F is The focus error signal, S _I , is an information signal.

Claims (1)

[Claims] 1. A light source with a variable oscillation wavelength, a hologram lens that converts spherical wave light generated from the light source into a spherical wave condensed onto a disk, and light from the disk via the hologram lens. a beam splitter that separates the returned light from the disk and the light generated from the light source; and a beam splitter that receives the light flux from the disk separated by the beam splitter that passes through a focus error detection optical element. a light detection means; a focus error signal detection means for detecting a focus error signal based on a detection output from the light detection means; and a focus error signal detection means for detecting a focus error signal based on the focus error signal supplied from the focus error signal detection means. A focus servo device comprising wavelength control means for changing the oscillation wavelength of a light source.