JPS63228422A - Focus detecting device - Google Patents

Abstract

PURPOSE:To attain the focus control conveniently and with a high sensitivity by inclining the central line of the focus side interface of a lens located at a focus side to the optical axis of other lens, forming an approximately linear diffracted pattern at a dividing light detecting device and removing a focus signal with the differential output. CONSTITUTION:When the distance of an objective lens and a light disk surface is made near or far, a linear diffracted pattern 30 is moved in the rectangular direction (X direction) to the linear direction of the diffracted pattern 30 at the position of a two-divided photodetector. For example, the diffracting pattern 30 located at the center at the time of focusing, when an objective lens is made far from the optical disk surface, is dislocated in a +X direction, and when the objective lens is made near to the optical disk surface, is dislocated in the reverse -X direction from the center. Further, when the lens is made near to the optical disk, the diffracted pattern itself is composed of one linear shape, deformed, divided into several pieces and the whole is further dislocated in the -X direction. Consequently, by removing the differential output of the two-divided light detecting device, the dislocation from the time of the focusing can be detected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Fields] The present invention relates to an optical head that writes information on an optical disk or reads information recorded on an optical disk using a light beam emitted from a light emitting element. Regarding a detection device for performing focus servo of an objective lens, more specifically, a detection lens group and a split photodetector are arranged in an optical path reflected from an optical disk surface and returned,
This relates to a focus detection device in which a linear diffraction pattern is formed by tilting at least the focal-side interface of the lens located closest to the focal point, and a differential output is extracted from the linear diffraction pattern using a split photodetector to obtain a focus signal. be.

[Prior Art] An optical head using a semiconductor laser is generally used to write information on an optical disk or read information from an optical disk. In an optical head, a laser beam is focused by an objective lens and irradiated onto a predetermined position on an optical disk.
In order to write or read information correctly, it must be controlled so that the irradiated light spot is precisely focused on the signal surface of the optical disk. For this reason, focus servo is performed on the objective lens of the optical head.

Various methods have been devised for the detection VT position for focus servo, and a typical example is the astigmatism method. In this method, a condensing lens, a cylindrical lens that has a convergence effect in only one direction, and a 4-split photodetector are placed in the optical path that is reflected back from the optical disk to provide an astigmatic imaging effect. It is something that has been set. Here, the four-split photodetector is provided at a position such that the beam shape at the photodetector becomes a perfect circle when the laser beam is accurately focused on the signal plane by the objective lens. Therefore, when the objective lens moves away from or approaches the optical disk, the cylindrical lens acts to form an oval beam shape that is perpendicular to each other. Therefore, the focus error signal can be detected by adding together the diagonal outputs of the four-split photodetector and then taking the difference between them.

[Problems to be Solved by the Invention] However, in the prior art as described above, a cylindrical lens is required in addition to the condenser lens, the number of parts is large, and the optical path length is long. In addition, the detection unit required high positioning accuracy for each optical component.

An object of the present invention is to eliminate the drawbacks of the prior art as described above, simplify the configuration of the photodetector, shorten the optical path length, and reduce the size of the photodetector. It is an object of the present invention to provide a focus detection device that does not require high alignment accuracy and can reduce costs as a whole.

[Means for Solving the Problems] The present invention, which can achieve the above objects, uses a detection lens in which a plurality of lenses are combined in the optical path reflected from the optical disk surface by the optical head and returned. This is a focus detection device having a configuration in which a group and a split photodetector are arranged.

Here, the detection lens group is a combination of multiple lenses for aberration correction, etc., and the number of focal-side lenses is small (in both cases, the center line of the focal-side interface is It is tilted with respect to the optical axis of the other lenses, and a differential output is extracted by the aforementioned split photodetector.

The simplest structure is to tilt only the lens located on the focal side of the detection lens group with respect to the optical axis, but it is possible to use a special shaped lens in which only the focal side interface of the focal side lens is tilted. may be combined.

The installation position of the split photodetector may be at approximately the focal point of the detection lens group, or may be at a position shifted therefrom.

[Function] When the center line of at least the focal-side interface of the focal-side lens in the detection lens group is tilted with respect to the optical axis of the other lenses, the split photodetector provided almost at the focal point position of the detection lens group A linear diffraction pattern is obtained. This diffraction pattern moves in a direction perpendicular to the linear direction of the diffraction pattern in accordance with the position of the objective lens relative to the optical disk surface, that is, as the objective lens approaches or moves away from the optical disk.

The present invention makes use of such a phenomenon, and detects a focus error signal by extracting differential outputs using divided photodetectors.

In this case, the photodetector may be placed in such a way that the linear diffraction pattern exactly matches the divided portions of the split photodetector, but in order to adjust the detection sensitivity, for example, the linear diffraction pattern It is also effective to tilt the division direction with respect to the diffraction pattern.

Furthermore, if the split photodetector is placed behind the focal point of the detection lens, several linear diffraction patterns will occur, but the light intensity will change depending on the distance between the objective lens and the optical disk surface, resulting in a diffraction pattern. still moves in a direction perpendicular to the straight line direction.

Therefore, even if this is used, focus detection can be performed in the same way, and strict precision is not required for assembling each optical component, resulting in good workability.

[Example] FIG. 1 is an explanatory diagram showing the basic configuration of a focus detection device according to the present invention. Here, in order to simplify the explanation, illustration of the trunk detection system is omitted.

Light from the semiconductor laser 10 passes through a collimator lens 12 and a beam splinter 14 toward an objective lens 16, where it is condensed and irradiated onto the signal surface of an optical disk 18. The reflected light from the optical disk 18 passes through the objective lens 16 and returns to the beam splinter 14, where the optical path is bent at right angles and a plurality of disks (three in this example) are used to correct aberrations.
The light is condensed through a detection lens group 20 that is a combination of lenses, and is directed toward a two-split photodetector 22.

As is clear from FIG. 1, the feature of the present invention is that the center line P of at least the focal-side interface 24a of the focal-side lens 24 of the detection lens group 20 is tilted with respect to the optical axis Q of the other lenses. Another point is that the configuration is such that differential outputs are taken out from the split photodetector (in this embodiment, the two-split photodetector 22).

In this embodiment, the entire focal-side lens 24 is tilted by an angle θ with respect to the optical axis Q of the other lenses. Further, each output of the two-split photodetector 22 is applied to a differential amplifier 26, and is configured to be differentially amplified there.

The optimal inclination angle θ of the focal-side lens 24 varies depending on the refractive index, curvature, structure, etc. of the lens, but for example, it was experimentally performed using a combination lens with a numerical aperture of 0.5, which is commonly used in objective lenses. According to the results, by setting the angle to about 10 degrees and installing the two-split photodetector 22 at approximately the focal point of the detection lens group 20, it is possible to obtain a single, almost linear, good beam as shown in FIG. A diffraction pattern 30 was obtained.

FIG. 2A shows a diffraction pattern 30 generated in the two-split photodetector 22 provided almost at the focal point of the detection lens group.
In Figure B, the solid line shows the light intensity distribution of the diffraction pattern when the objective lens is in focus, and the broken line shows the light intensity distribution when the objective lens is too close to the optical disk.

FIG. 3 shows this result in more detail. When the distance between the objective lens and the optical disk surface is brought closer or farther apart, this linear diffraction pattern 30 moves in the direction perpendicular to the linear direction (X direction) of the diffraction pattern 30 at the position of the two-split photodetector. For example, the diffraction pattern 30 that was centered at the time of focusing shifts in the direction (+40 μm) when the objective lens moves away from the optical disk surface, and conversely shifts away from the center when the objective lens approaches the optical disk surface (−40 μm). When the diffraction pattern approaches the optical disk further (-80 μm), the diffraction pattern itself is considerably deformed from a single straight line and split into several lines, and the entire diffraction pattern is further shifted in the -X direction.Therefore, 2 By extracting the differential outputs of the split photodetectors, it is possible to detect deviations from the in-focus state.

FIG. 4 is a plot of the differential output voltages obtained while changing the objective lens from being very far away from the optical disk surface to being extremely close to it. When actually performing focus control, the servo operation is performed using the range shown by the solid line near the time of focus. In this range, a nearly linear relationship is obtained, indicating that overall good focus control is possible.

Now, FIG. 5 is an explanatory diagram showing another example of a group of detection lenses that can be used in the present invention. In this embodiment, the focal-side lens 24 is a lens having a special shape in which only the focal-side interface 24a has its center line P inclined with respect to the optical axis Q of the other lenses. Even when such a lens is used, the same result as when tilting the entire focal-side lens can be obtained, and a desired diffraction pattern can be obtained.

FIG. 6 shows another embodiment of the invention.

In this embodiment, the two-split photodetector 22 has a detection lens group 20.
The diffraction pattern is placed approximately at the focal point of the diffraction pattern, but it is placed at a slight angle with respect to the resulting approximately straight diffraction pattern. In this way, sensitivity can be adjusted by tilting the direction of the two divisions with respect to the diffraction pattern.

FIGS. 7A and 7B show still another embodiment of the present invention.

This is the case where the two-split photodetector 22 is installed in front of the focal point of the detection lens group 20. In this case, multiple almost linear diffraction patterns appear, but if the differential output voltage of the left and right photodetectors is adjusted in advance so that it becomes exactly 0 when the objective lens is focused, the objective lens can be focused. If the light is not focused, the light intensity of the diffraction pattern will change and a differential output signal will appear.
Based on this, it becomes possible to apply focus servo. Note that in Figure B, the broken line represents the change in light intensity when the objective lens approaches the optical disk.

Furthermore, if a four-split photodetector is used instead of the two-split photodetector described above, it is also possible to detect trunk errors using the bush-pull method from the reflected light from the pregrouped optical disks.

[Effects of the Invention] As described above, the present invention reduces the number of lenses located on the focus side of the plurality of detection lens groups (by tilting the center line of the interface on the focus side with respect to the optical axis of the other lenses). A linear diffraction pattern is formed on a split photodetector, and a focus signal is extracted by its differential output, and a combination lens used as a detection lens group for the purpose of aberration correction, such as that used in objective lenses, etc. The optical path length of the detection unit is shortened, allowing for miniaturization, and the positioning accuracy of each optical component does not need to be so precise.
This has an excellent effect of easily performing focus control with high sensitivity.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing the basic configuration of the focus detection device according to the present invention, Fig. 2A is an explanatory diagram of the diffraction pattern obtained thereby, B is a graph showing the light intensity distribution, and Fig. 3 is an objective An explanatory diagram showing the distance between the lens and the optical disk and the change in the diffraction pattern, Fig. 4 is a graph showing the relationship between the position of the objective lens and the differential output from the two-split photodetector, and Fig. 5 is used in the present invention. FIG. 6 is an explanatory diagram showing another example of the arrangement of the two-split photodetector according to the present invention, and FIG. An explanatory diagram of the diffraction pattern, B is a graph showing its light intensity distribution. DESCRIPTION OF SYMBOLS 10... Semiconductor laser, 12... Collimator lens, 14... Beam splitter, 16... Objective lens, 18... Optical disk, 2o... Detection lens group, 2
2...Two-split photodetector, 24...Focal side lens, 2
4a...Focal side interface, 26...Differential amplifier. Patent Applicant: Fuji Electrochemical Co., Ltd. Representative: Minoru Shigemi Figure 1 Figure 2 A 9 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] 1. A detection lens group consisting of a combination of multiple lenses and a split photodetector are arranged in the optical path reflected from the optical disk surface by the optical head and returned, and the detection lens group has a substantially linear shape. At least the center line of the focal-side interface of the focal-side lens is tilted with respect to the optical axis of the other lenses so that a diffraction pattern is formed, and a differential output is extracted from the split photodetector. Features a focus detection device.