JPH03144922A - Focus controller - Google Patents

Abstract

PURPOSE:To sufficiently shorten the access time by providing a feedback control section changing a wavelength of a laser beam so that a focus of the laser beam made incident on an optical recording medium is placed onto the recording face of an optical recording medium. CONSTITUTION:A laser beam from a semiconductor laser 11 is reflected in a horogram converging element 12 and focused therein and focused further by an objective lens 13 and made incident on an optical recording medium 1. The laser beam reflected in the optical recording medium 1 is reflected and focused in the horogram focus element 12 so as to generate astigmatism and the result is made incident on a focus error detection section 14. A focus detection section 14 inputs a detection output of a quadripartite photodiode 15 to an arithmetic circuit, where a focus error signal FE is calculated. A feedback control circuit 17 applies a focus error signal FE to the semiconductor device 11 as an electric input and the wavelength of the laser beam from the semiconductor laser 11 is varied so that the focus of the laser beam made incident on the optical recording medium 1 is placed onto the recording face of the optical recording medium 1.

Description

DETAILED DESCRIPTION OF THE INVENTION r. Industrial Application Field This invention relates to a focus control device for a system that records information on or reproduces information from an optical recording medium using a laser beam.

"Prior Art" In a system that records information on or reproduces information from an optical recording medium using a laser beam, the focus of the laser beam incident on the optical recording medium is always set to the recording surface of the optical recording medium. Conventionally, this focus control involves focusing the objective lens, which focuses the laser light from the laser light source and makes it incident on the optical recording medium, using the focus error signal output from the focus error detection section. This is done by moving it in the direction of the tip.

FIG. 6 shows an example of such a conventional focus control device, in which laser light from a semiconductor laser 21 is reflected and focused by a hologram focusing element 22, and is further focused by an objective lens 2.
3, the laser beam is reflected by the optical recording medium 1, passes through the objective lens 23, is reflected by the hologram focusing element 22, and is focused to produce astigmatism, resulting in a focus error. The light enters the four-segment photodiode 25 of the detection unit 24, and the focus error detection unit 24
The focus error signal FE obtained from the arithmetic circuit 26 of
is supplied to the drive circuit 27, and the output of the drive circuit 27 is supplied to the actuator 28 for focus control, so that the focus of the laser beam incident on the optical recording medium 1 is located beyond the recording surface of the optical recording medium 1. For example, when the focus error signal FE becomes positive, the objective lens 23 is moved in a direction away from the optical recording medium 1 so that the focus of the laser beam incident on the optical recording medium 1 is located on the recording surface of the optical recording medium 1. ,
Conversely, if the focus of the laser beam incident on the optical recording medium l is located in front of the recording surface of the optical recording medium l, a focus error signal F is generated.
For example, when E becomes negative, the objective lens 23 is moved toward the optical recording medium 1 so that the focal point of the laser beam incident on the optical recording medium 1 is located on the recording surface of the optical recording medium.

"Problem to be Solved by the Invention" However, in the conventional focus control device, an example of which is shown in FIG. As a result, the focal point of the laser beam incident on the optical recording medium 1 is positioned on the recording surface of the optical recording medium 1, so the focus control operation cannot be made faster than a certain level, causing a time delay in the control, and
When a track is accessed by moving the optical head or optical pickup in a predetermined direction on the optical recording medium 1, since the optical head or optical pickup cannot be made smaller and lighter to a certain extent because it has two actuators for focus control. There is a disadvantage that it is not possible to shorten the access time beyond a certain level.

Therefore, the present invention provides a focus control device for a system that records information on an optical recording medium using a laser beam or reproduces information from an optical recording medium, and uses electronic focus control to significantly speed up the focus control operation. This allows for almost no time delay in control, and by making the optical head or optical pickup smaller and lighter, the access time when accessing a track by moving the optical head or optical pickup in a predetermined direction of the optical recording medium can be reduced. This allows it to be sufficiently short.

``Means for Solving rURB'' In this invention, a semiconductor laser whose output laser beam wavelength can be changed by electrical input is used as a laser light source, and the laser beam from this semiconductor laser is focused by a hologram focusing element to generate light. The laser beam is made incident on the recording medium and reflected by the optical recording medium, and is focused by the hologram condensing element and made incident on the focus error detection section, and the focus error signal output from this focus error detection section is converted into the focus error signal outputted from the focus error detection section. A hologram focusing element is supplied as the electrical input to the semiconductor laser so that the focus of the laser light incident on the optical recording medium is located on the recording surface of the optical recording medium. The focal length of the holographic focusing element changes depending on the wavelength of the light focused by the hologram focusing element. Therefore, in the focus control device of the present invention configured as described above, the focal length of the hologram focusing element is changed by changing the wavelength of the laser beam from the semiconductor laser by the focus error signal, and the focal length of the hologram focusing element is changed. When the focal point of the laser beam incident on the optical recording medium is located beyond the recording surface of the optical recording medium and the focus error signal becomes, for example, positive, the focal length of the hologram focusing element is shortened and the focal length of the laser beam incident on the optical recording medium is shortened. The wavelength of the laser beam from the semiconductor laser is changed so that the focal point is located on the recording surface of the optical recording medium, and conversely, the focal point of the laser beam incident on the optical recording medium is positioned in front of the recording surface of the optical recording medium. For example, when the focus error signal becomes negative, the focal length of the hologram focusing element is lengthened so that the focal point of the laser beam incident on the optical recording medium is located on the recording surface of the optical recording medium. The focal point of the laser beam incident on the medium is always located on the recording surface of the optical recording medium.

In the focus control device of the present invention, since the focus control is electronic as described above, the focus control operation can be made extremely fast, with almost no time delay occurring in the control, and the focus control Since no actuator is required and the optical head or optical pickup can be made smaller and lighter, the access time when accessing a track can be sufficiently shortened by moving the optical head or optical pickup in a predetermined direction of the optical recording medium. .

“Embodiment” FIG. 1 shows an example of a focus control device of the present invention, and the focus control device in this example includes a semiconductor laser 11
, a hologram condensing element 12 , an objective lens 13 , a focus error detection section 14 , and a feedback control section 17 .

As the semiconductor laser 11, a device such as a DFB (distributed feedback type) semiconductor laser, which can change the wavelength of the output laser light by electrical input, is used.

In this example, the hologram focusing element 12 reflects the laser beam from the semiconductor laser 11, focuses it and makes it incident on the optical recording medium 1, and also reflects the laser beam reflected by the optical recording medium l, and The beam is focused so as to produce astigmatism and is made incident on the focus error detection section 14, and has the functions of a reflection ξ mirror, a beam splitter, and a cylindrical lens.

The objective lens 13 is not necessary in principle, but when the focal length of the hologram condenser 12 is long and the laser beam from the semiconductor laser 11 is focused only by the hologram condenser 12, the objective lens 13 is not necessary. Since it is necessary to lengthen the optical path length to the optical recording medium 1, the laser beam focused by the hologram focusing element 12 is further focused to shorten the optical path length from the hologram focusing element 12 to the optical recording medium l. established for the purpose of

Therefore, the laser beam from the semiconductor laser 11 is reflected and focused by the hologram focusing element 12, further focused by the objective lens 13, and enters the optical recording medium 1, and the laser beam reflected by the optical recording medium 1 passes through the objective lens 13, is reflected by the hologram condensing element 12, is focused to produce astigmatism, and enters the focus error detection section 14.

The focus error detection unit 14 includes a hologram condensing element 1
The laser beam from the optical recording medium 1 is reflected by 2 and focused to produce astigmatism, and enters the 4-split photodiode 15, and as shown in FIG. The sum of the outputs A and C of the two photodiodes 15a and 15c located diagonally on one side of the photodiode 15 and the outputs B and D of the two photodiodes 15b and 15d located on the other diagonal direction. The difference from the sum is calculated, and the arithmetic circuit 16 calculates FE-(A+C)-(B
+D) is obtained.

In this way, the calculation circuit 16 of the focus error detection section 14
The polarity and magnitude of the focus error signal FE obtained from the optical recording medium 1 change depending on the direction and magnitude of the deviation of the focal point of the laser beam incident on the optical recording medium 1 from the recording surface of the optical recording medium 1. That is, when the focal point of the laser beam incident on the optical recording medium 1 is located on the recording surface of the optical recording medium 1,
The laser beam incident on the focus error detection unit 14 is
As shown in the figure, there are four photodiodes 15a, 15b, and 15c on the quadrant photodiode 15.

When a circular spot SO having an area equal to 15d is formed, the focus error signal FE becomes zero, and the focus of the laser beam incident on the optical recording medium 1 is located beyond the recording surface of the optical recording medium l. As shown in FIG.
Oval spot sp with the diagonal direction of 5c being the long sleeve direction
When the focus error signal FB becomes positive and the focal point of the laser beam incident on the optical recording medium 1 is located in front of the recording surface of the optical recording medium 1, the laser beam incident on the focus error detection section 14 becomes positive. As shown in FIG. 5, a photodiode 15b is placed on the four-part photodiode 15,
Oval-shaped subop with the long ring direction in the diagonal direction of 15d)S
q is formed, and the focus error signal FB becomes negative.

The feedback control section 17 can be constituted by an amplifier circuit 18, and supplies the focus error signal FE output from the focus error detection section 14 as the electrical input to the semiconductor laser 11 with a predetermined polarity and level to perform optical recording. The wavelength of the laser beam from the semiconductor laser 11 is changed so that the focal point of the laser beam incident on the medium 1 is located on the recording surface of the optical recording medium 1.

That is, the focal length of the hologram focusing element 12 changes depending on the wavelength of the laser beam from the semiconductor laser 11,
The focus of the laser beam incident on the optical recording medium l is the optical recording medium l.
When the focus error signal FE becomes positive as described above, the focal length of the hologram focusing element 12 is shortened by changing the wavelength of the laser beam from the semiconductor laser 11, and the focus error signal FE becomes positive as described above. The focal point of the laser beam incident on the recording medium 1 is positioned on the recording surface of the optical recording medium 1, and conversely, the focal point of the laser beam incident on the optical recording medium l is positioned in front of the recording surface of the optical recording medium 1. When the focus error signal FE becomes negative as described above, the wavelength of the laser beam from the semiconductor laser 11 is changed to lengthen the focal length of the hologram condensing element 12 and the laser beam is incident on the optical recording medium 1. The focus of the laser beam is positioned on the recording surface of the optical recording medium 1. Therefore, the focus of the laser beam incident on the optical recording medium 1 is always located on the recording surface of the optical recording medium 1.

Note that by changing the wavelength of the laser beam from the semiconductor laser 11 in this way, the size of the spot of the laser beam incident on the optical recording medium 1 on the recording surface of the optical recording medium 1 changes. When the focal point of the laser beam incident on the optical recording medium 1 is located on the recording surface of the optical recording medium 1, the spot becomes circular, so there is no particular problem.

As mentioned above, since the focus control is electronic, the focus control operation can be made extremely fast, with almost no time delay in control, and there is no need for an actuator for focus control, making it possible to operate the focus control using an optical head or optical pickup. Since the optical head or the optical pickup can be made smaller and lighter, the access time when accessing a track can be sufficiently shortened by moving the optical head or optical pickup in a predetermined direction of the optical recording medium l.

"Effects of the Invention" As described above, according to the present invention, the focus control operation can be significantly speeded up by electronic focus control, there is almost no time delay in control, and the optical head or optical pickup can be By being smaller and lighter, the access time when accessing a track by moving the optical head or optical pickup in a predetermined direction of the optical recording medium can be sufficiently shortened.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of the focus control device of the present invention, FIG. 2 is a diagram showing the configuration of its focus error detection section, and FIGS. 3, 4, and 5 are diagrams showing respective focus states. FIG. 6 is a diagram showing an example of a conventional focus control device.

Claims (1)

[Claims] (1) A semiconductor laser that can change the wavelength of the output laser light by electrical input; The laser light from this semiconductor laser is focused to enter an optical recording medium, and the laser light reflected by this optical recording medium is focused. a hologram focusing element; a focus error detection section into which the laser light from the optical recording medium focused by the hologram focusing element is incident; and a focus error signal output from the focus error detection section to the semiconductor laser. a feedback control unit that is supplied as an input and changes the wavelength of the laser beam from the semiconductor laser so that the focal point of the laser beam incident on the optical recording medium is located on the recording surface of the optical recording medium; Focus control device.