JPH0250534B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus servo device, and particularly to a focus servo device in an optical information reading device.

In an optical information reading device, information is read by irradiating an information reading beam onto a recording medium on which information is recorded (hereinafter referred to as a disk) and demodulating the return beam that has been modulated on the recording surface of the disk. It is done. Such a reading device is provided with a so-called focus servo device to accurately focus the irradiation beam onto the information recording surface of the disk.

Conventionally, various types of focus servo devices have been proposed, such as a system using a combination of a cylindrical lens and a four-part detector, or a so-called critical angle system, but all of these systems cut the return beam after passing through the recording surface of the disk. A focus signal is derived by detecting the difference in the amount of light, and based on this focus signal, an optical system for irradiating the disk with an information reading beam is driven in the direction of its optical axis. However, for example, in the case of a method using a combination of a cylindrical lens and a 4-split detector, accurate focus servo cannot be performed because the influence when the spot light of the irradiation beam crosses the recording track of the disk is superimposed on the focus signal as noise. It turns out. To avoid this, a method has been proposed in which a cylindrical lens is tilted at 45 degrees to the track. Also, in the critical angle method described above, the direction of division is perpendicular to the track to avoid the effects of crossing the track.

In addition, there are distortion factors in the optical system, such as aberrations in the objective lens and non-uniformity in the beam distribution.This distortion in the optical system causes the intensity distribution of the beam on the detector to become non-uniform, which is superimposed on the focus signal as noise. The disadvantage is that the focus servo cannot be performed accurately because the

The present invention has been made in view of the above-mentioned points, and provides a focus servo device that eliminates the effects of spot light crossing a track and the effects of optical system distortion and enables highly accurate focus servo. The purpose is to

A focus servo device according to the present invention splits a return beam that has passed through a recording medium using a half mirror, sends the split first beam to a first light receiving element, and sends a first split beam to a second light receiving element.
The beam is reflected by a total reflection mirror and then incident on a second light receiving element, and a focus error signal is derived from the difference in output between the first and second light receiving elements. are integrally provided, and at least a portion of the first or second beam is made incident on the second or first light receiving element when the optical system deviates from a predetermined position with respect to the recording medium.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, a beam emitted from a light source 1 such as a semiconductor laser passes through a collimator lens 2, a beam splitter 3, and a λ/4 plate 4, and is focused by an objective lens 5 very close to the information recording surface of a disk 6. Ru. The return beam reflected by the recording surface of the disk 6 travels backward along the same optical path as the incident beam, is bent at a right angle by the beam splitter 3, and enters the triangular prism 7. triangular prism 7
The part 8 is a half mirror, and the part 9 is a total reflection mirror. Of the returned beams, the first beam reflected by the half mirror 8 is incident on the first light receiving element A, and the second beam transmitted through the half mirror 8 is reflected by the total reflection mirror 9 and transmitted to the second light receiving element. incident on B. Light receiving elements A and B are integrally provided.

Here, when the objective lens 5 is at a predetermined position relative to the disk 6 in the direction of its optical axis, that is, when it is out of focus, as shown in FIG.
The first beam enters the light receiving element A in a condensed state,
The second beam enters the light receiving element B in a diffused state,
Furthermore, the positional relationship between the half mirror 8, the total reflection mirror 9, and the light receiving elements A and B is determined so that the beam shapes in each light receiving element are equal and, for example, each beam is adjacent to each other across the boundary line of each light receiving element. Then, the outputs of the light receiving elements A and B are compared by a differential amplifier 10, and the difference signal (A-B) is derived as a focus error signal.

In this configuration, when the focus is off (FIG. 2), the beam shapes at the light receiving elements A and B are equal as shown in FIG. B) becomes zero. When the objective lens 5 leaves the disk 6, the returning beam converges slightly as shown in FIG. The beam is diffused and a portion of it is incident on the light receiving element A. At this time, each beam shape is shown in FIG. 5b. As a result, the amount of light received by the light receiving element A increases and the amount of light received by the light receiving element B decreases, so that the differential output (A-B) increases. As the objective lens 5 moves further away, the amount of light received by the light receiving element A further increases as shown in FIG. 5a.

On the other hand, when the objective lens 5 approaches the disk 6,
As shown in Figure 4, the return beam is slightly diffused, so the first beam is incident on photodetector A and part of it is also incident on photodetector B, and the second beam is received in a condensed state. incident on element B. The shapes of each beam at this time are shown in FIG. 5d. As a result, the amount of light received by the light receiving element A decreases and the amount of light received by the light receiving element B increases, so that the differential output (A-B) decreases. As the objective lens 5 approaches further, the amount of light received by the light receiving element B further increases as shown in FIG. 5e. FIG. 6 shows changes in the differential output (A-B) as the position of the objective lens 5 with respect to the disk 6 changes.

As described above, according to the present invention, no matter what kind of distribution the information reading beam has, the beams with the same distribution are incident on the two light receiving elements, and the difference signal is extracted as a focus signal. Since the noise components are completely canceled out, highly accurate focus servo can be performed without noise caused by cross-track effects or distortion of the optical system. Furthermore, since the entire amount of light of the return beam is made incident on the light receiving element, efficiency and S/N ratio can be improved. Furthermore, since the two light-receiving elements can be integrated, variations in temperature characteristics, etc. of the two elements can be minimized, thereby enabling more accurate focus servo. Furthermore, in this embodiment, almost the same effect can be obtained even if the light receiving elements are arranged at positions such that when the focus servo is off, each beam is irradiated to two light receiving elements and does not completely overlap. .

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention, and FIG.
Figures 4 to 4 are diagrams showing the behavior of the return beam as the relative positions of the objective lens and the disk change, Figure 5 is a diagram showing changes in the beam shape at photodetectors A and B, and Figure 6 is FIG. 6 is a diagram showing a change in differential output (A-B) with respect to a change in relative position between an objective lens and a disk. Explanation of symbols of main parts 1...Light source, 3...Beam splitter, 5...Objective lens, 6...Disk, 8
... Half mirror, 9... Total reflection mirror, A, B... Light receiving element.

Claims (1)

[Claims] 1 A focus servo device that controls an optical system that irradiates an information reading beam onto a recording medium to maintain it at a predetermined position with respect to the recording medium in the direction of its optical axis, which divides the return beam that has passed through the recording medium. a first light-receiving element that receives a first beam from the half mirror; a total reflection mirror that reflects a second beam from the half mirror; and a second beam reflected by the total reflection mirror. a second light-receiving element that receives two beams, and means for deriving a focus error signal by comparing each output of the first and second light-receiving elements,
The first and second light receiving elements are integrally provided, and when at least the optical system is deviated from the predetermined position, a portion of the first or second beam is directed to the second or first light receiving element. A focus servo device characterized in that it is made to be incident.