JP2686323B2 - Focus error detection device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus error detection device for a recording medium in which information is recorded and reproduced optically, and particularly, a range in which a focus error signal linearly changes is wide, The present invention relates to a focus error detection device capable of stable and easy servo.

[Conventional technology]

FIG. 4 shows, for example, JP-A-57-18032 and JP-A-58.
FIG. 2 is a configuration diagram showing a general focus error detection device called a Foucault method described in Japanese Patent Publication No. 208946.

In the figure, 3 is a light emitting source such as a semiconductor laser that emits a recording / reproducing light beam as irradiation light E. Reference numeral 4 is a beam splitter which transmits the irradiation light E and reflects reflected light R from the recording medium 8. Reference numeral 5 is a collimator lens which collimates the irradiation light E and condenses the reflected light R. 6
Rotates the deflection surface of the parallel irradiation light E and reflected light R 1 /
A four-wave plate 7 is an objective lens that collects the irradiation light E that has passed through the quarter-wave plate 6 and makes the reflected light R parallel light.
Reference numeral 8 denotes a recording medium such as an optical disk, which is adapted to irradiate the condensed irradiation light E to optically record and reproduce information. Reference numeral 9 is an optical splitter that splits the reflected light R from the recording medium 8 incident through the objective lens 7, the quarter-wave plate 6, the collimator lens 5, and the beam splitter 4 into two reflected lights R1 and R2. . This light splitter 9 is also called a Foucault prism, and the two rectangular refracting surfaces 9a and 9b are obtuse ridge lines.
The three-dimensional shape is in contact with each other at 9c, and the ridgeline 9c is arranged so as to cross the optical axis A of the reflected light R vertically. Reference numeral 10 is a split type photodetector for detecting the split reflected lights R1 and R2, respectively, and is composed of two photodetection elements 11 to 14 arranged in a plane perpendicular to the optical axis A. Also, the photodetector elements 11 to 14
The respective light-receiving surfaces are arranged in a line in the arrow X direction perpendicular to both the optical axis A and the ridge line 9c, and are properly positioned in the optical axis A and the arrow X direction. Then, the pair of photodetector elements 11
And 12 constitute a two-divided photodetector for detecting one reflected light R1, and the other pair of photodetector elements 13 and 14 are the other reflected light R2.
It constitutes a two-division photodetector for detecting. Usually, each of these two-division photodetectors is composed of a two-division pin photodiode.

S1 to S4 are detection signals obtained by the photodetection elements 11 to 14, which are input to an arithmetic processing circuit (not shown) and used for focus servo and the like.

Next, the operation of the focus error detection device shown in FIG. 4 will be described with reference to FIGS.

Irradiation light E emitted from the light emitting source 3 when recording / reproducing information passes through the beam splitter 4, becomes parallel light by the collimator lens 5, the deflection surface is rotated by the 1/4 wavelength plate 6, and the objective lens 7 The recording medium 8 is condensed and irradiated.

Then, the reflected light R reflected by the recording medium 8 is condensed at a predetermined convergence angle via the objective lens 7, 1/4 wavelength plate 6 and collimator lens 5, reflected by the beam splitter 4, and further divided into light. The reflected light R1 and R2 are split by the device 9 and applied to the split photodetector 10.

At this time, when the focal point of the irradiation light E coincides with the recording medium 8, that is, when it is in focus, as shown in FIG.
R2 is focused on the split type photodetector 10, one reflected light R1 is condensed at the gap between the photodetection elements 11 and 12, that is, the center point of the separation band 10a, and the other reflected light R2 is the photodetection element. 13 and 14 dividers
Focused on 10b. When this state is viewed from the light receiving surface side, it is as shown in FIG. 6, and the respective reflected lights R1 and R2 are irradiated as focused spots P1 and P2 having a diameter of 20 to 30 μm. The focusing spots P1 and P2 each have an elliptical shape as described later.

If the distance between the recording medium 8 and the objective lens 7 is too short, the reflected lights R1 and R2 are irradiated near the separation bands 10a and 10b before focusing, as shown in FIG. Therefore, the reflected lights R1 and R2 are not detected by the photodetector 12 and the shaded area in FIG.
The light-receiving surfaces of 13 are irradiated as semicircular spots P3 and P4.

On the contrary, when the distance between the recording medium 8 and the objective lens 7 is too large, the reflected lights R1 and R2 are focused in front of the split type photodetector 10 as shown in FIG. Therefore, the reflected lights R1 and R2 are irradiated as semicircular spots P5 and P6 on the light receiving surfaces of the photodetectors 11 and 14 as shown in FIG.

At this time, the photodetector element 11 that receives the reflected lights R1 and R2
To 14 are currents corresponding to the respective received light amounts, that is, the detection signal S1
~ S4 is generated. The arithmetic processing circuit uses these detection signals S1
Based on S4 to S4, each detection signal S of the outer photodetector elements 11 and 14
The sum of 1 and S4 and the detection signals S2 of the inner photodetector elements 12 and 13
The focus error signal F is obtained by the equation F = (S1 + S4)-(S2 + S3) (1) which calculates the difference between the sum and the sum of S3.

The focus error signal F is zero when the distance between the recording medium 8 and the objective lens 7 is proper as shown in FIGS. 5 and 6, and is the recording medium as shown in FIGS. 7 and 8. 8
When the distance between the objective lens 7 and the objective lens 7 is too short, it is negative, and when the distance between the recording medium 8 and the objective lens 7 is too far, as shown in FIGS. 9 and 10, it is positive.

Based on the polarity and the magnitude of the focus error signal F thus obtained, the focus error amount with respect to the appropriate distance between the recording medium 8 and the objective lens 7 is calculated, and the focus adjustment mechanism (not shown) is controlled. For example, by moving the objective lens 7 in the optical axis direction of the irradiation light E, the focus error amount Z,
That is, the focus servo is performed until the focus error signal F becomes zero.

FIG. 11 is an explanatory view showing a part of the light receiving surface of the conventional split type photodetector 10 used in the focus error detecting device of FIG. 4, in which the distance between the recording medium 8 and the objective lens 7 is properly adjusted. The reflected light R1 is irradiated as a focused spot P1.

In FIG. 11, reference numeral 1 denotes a two-divided pin photodiode that forms a pair of photodetection elements 11 and 12, and is divided together with a two-divided pin photodiode (not shown) that forms another pair of photodetection elements 13 and 14. The photo detector 10 is configured. Reference numeral 2 denotes a separation band that divides the light receiving surfaces of the photodetection elements 11 and 12, and has a width d of about 10 μm.

When focused as shown in FIG. 11, the focused spot of the reflected light R1 is caused by the diffraction phenomenon when the light is split into two by the light splitter 9.
P1 has an elliptical shape. Therefore, the diameter L of the focused spot P1
Is approximately twice the diameter of the focused spot (circular shape not shown) of the reflected light R when not divided.

In general, when the convergence angle of the incident light beam is α and the wavelength of the light source is λ, the focused spot diameter r (Airy disk diameter) is represented by r = 1.22λ / sinα. Further, the focused spot diameter L of the split reflected light R1 is expressed as L≈2r = 2.44λ / sinα (2). For example, in the case of reflected light R1 having a convergence angle α of about 2 ° with respect to a direction parallel to the dividing line of the separation band 2 and a wavelength λ of 0.78 μm, the focused spot diameter L is about 50 μ from the equation (2).
m. Usually, the value of the focus spot diameter L is about 40 to 50 μm, although it varies depending on the design specifications of the information recording / reproducing apparatus.

Next, with reference to FIGS. 4 to 10, 12, and 13, the operation of the conventional focus error detecting device using the two-divided pin photodiode 1 of FIG. 11 will be described.
Here, description will be made focusing on the two-divided pin photodiode 1 that receives one of the reflected lights R1.

Similarly to the above, the reflected light R generated by irradiating the recording medium 8 with the irradiation light E is divided into two, and one reflected light R1 is applied to the light receiving surface of the two-divided pin photodiode 1.

At this time, if the distance between the recording medium 8 and the objective lens 7 is proper, the width d (≈10 μm) of the separation band 2 as shown in FIG.
A focused spot P1 having a diameter L (.apprxeq.40 to 60 .mu.m) which is 4 to 6 times as large as the above is symmetrically irradiated to each light receiving surface of the photodetection elements 11 and 12.

When a focus error occurs here, the irradiation position of the reflected light R1 moves in the direction of the arrow X, and at the same time, the irradiation area spreads to become a semicircular spot P3 or P5. At this time, the spot diameter is almost proportional to the focus error amount Z, but the reflected light R1
Since the amount of light is constant regardless of the shape of the spot, the detection signals S1 and S2 obtained from the photodetection elements 11 and 12 fluctuate as shown in FIG. 12 as the reflected light R1 moves.

That is, when the distance between the recording medium 8 and the objective lens 7 is too short, the detection signal S2 of the photodetection element 12 becomes large (see FIG. 8), and when it is too far, the detection signal S1 of the photodetection element 11 becomes large. . At the same time, since the detection signals S3 and S4 are obtained from the other two-divided pin photodiode (not shown), the focus error signal F is obtained from these detection signals S1 to S4 based on the equation (1).

Then, when the focus error amount Z is calculated by the Foucault method described above, a curve as shown in FIG. 13 is obtained.
As is clear from FIG. 13, the range in which the focus error signal F changes in proportion to the focus error amount Z is approximately ± 1.0 when converted into the displacement of the distance between the recording medium 8 and the objective lens 7.
μm. Therefore, the focus servo can be performed only within this narrow range.

[Problems to be solved by the invention]

The conventional focus error detection device is configured as described above, and since the width of the separation band 2 between the photodetection elements 11 and 12 (and 13 and 14) is only about 10 μm, the focus error signal F is The range in which the focus error signal changes linearly with respect to the focus error amount is narrowed because the spot position of each reflected light fluctuates sensitively, and the offset signal in the focus error signal F must be lowered. There is a problem that the tracking range of the focus servo is also narrowed.

The present invention has been made to solve the above problems, and widens the range in which the focus error signal linearly changes with respect to the focus error amount, thereby facilitating pull-in and tracking control of the focus servo. It is an object of the present invention to obtain a focus error detection device capable of stably performing

[Means for solving the problem]

The present invention relates to an optical splitter that splits the reflected light from a recording medium into two, and a pair of two-split photodetectors that are arranged in the vicinity of the respective condensing positions of the reflected light split by the splitter. And a focus error signal and a focus error based on a detection signal output from each photodetector In the focus error detection device for obtaining the amount, the width D of each separation band of the two-division photodetector reduces the detection sensitivity of the focus error signal and expands the range in which the focus error signal linearly changes. Is set to be equal to or larger than the focused spot diameter L of the reflected light or larger than the focused spot diameter L of the reflected light and equal to or smaller than the spread width of the depletion layer of the two-division photodetector. In the center of Detection sensitivity is characterized in that it is set to be approximately half of the light detection sensitivity of the light receiving surface of the light detection element.

[Action]

In the present invention, the width D of each separation band of the two-division photodetector is substantially equal to or larger than the focused spot diameter L of the reflected light, or more than the focused spot diameter L of the reflected light, and the two-divided light detection is performed. The width of the depletion layer of the detector is set to be less than or equal to the width of the depletion layer. The range in which the error signal changes linearly (linear zone) becomes wider. As a result, it is possible to obtain a focus error detection device that can perform focus servo stably and easily.

〔Example〕

 An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is an explanatory view showing a two-divided photodetector of a focus error detecting device according to an embodiment of the present invention. In the drawing, 11 and 12 are the same as those of the above-mentioned conventional device. Two
Reference numerals 0 and 21 correspond to the conventional two-divided pin photodiode 1 and the separation band 2, respectively. Further, other configurations not shown are the same as those of the general focus error detection device shown in FIGS. 4 to 10.

Here, 20 forms a pair of photodetector elements 11 and 12 2
It is a split photodetector, that is, a split photodetector, and constitutes a split photodetector together with a split pin photodiode (not shown) forming another pair of photodetection elements 13 and 14. Reference numeral 21 is a separation band that divides the light receiving surfaces of the photodetection elements 11 and 12.

In the present invention, the width D is about 50 μm, that is, the reflected light
Since the focus spot diameter L by R1 is equal to or larger than (in the case of FIG. 1, almost equal) the depletion layer spread width W of the split type photodetector is set to be equal to or smaller than the second width within the width D of the separation band 21. As shown in the figure, the detection signals S1 and S2 change linearly at a gentle angle with respect to the displacement in the arrow X direction, and at the center of the dividing line of the separation band 21, the detection signals S1 and S2 are different from each other. Photo detector
It outputs about half of the maximum photocurrent of 11,12.

Next, the operation of the embodiment of the present invention shown in FIG. 1 will be described with reference to the detection signal characteristic diagram of FIG. 2, the focus error signal characteristic diagram of FIG. 3 and FIGS. To do.

As shown in FIG. 1, the focused spot P1 of the reflected light R1 is irradiated on the central portion of the separation zone 21, and the width D of the separation zone 21 is not less than the focused spot diameter L and not more than the width W of the depletion layer. Therefore, the reflected light R1 is hardly received by the photodetection elements 11 and 12, and even if the spot position is displaced in the arrow X direction due to the focus error, the area irradiated to the separation band 21 is large. The area irradiated to the light detection element 11 or 12 is relatively small. Then, within the width D of the separation band 21, the detection signals S1 and S2 linearly change at a gentle angle with respect to the displacement in the arrow X direction, as shown in FIG. Further, at the center of the dividing line of the separation band 21, each detection signal S1
The sensitivity distribution is set so that S2 and S2 output about half of the maximum photocurrent.

Therefore, in the characteristic of the focus error signal F with respect to the focus error amount Z, as shown by the solid line in FIG. 3, the inclination of the portion having the oblique straight line becomes gentle as compared with the conventional characteristic shown by the broken line. At the same time, the range that changes linearly is the recording medium 8.
± 3.0μm converted to the displacement of the distance between the objective lens 7 and
And become wider.

Focus servo can be performed stably and easily by using the focus error amount Z having a wide range of linear change.

In the above embodiment, the case where the width D of the separation band 21 is 50 μm has been described, but the reflected light R is changed in accordance with various specifications changes.
Wavelength λ, the convergence angle α, and the physical constants of the light-receiving surface of the split-type photodetector are different, the width D of the separation band 21 is equal to or larger than the focusing spot diameter L, and the width W of the depletion layer of the split-type photodetector is W. The following value, that is, from the above formula (2), may be any value that satisfies W ≧ D ≧ 2.44λ / sinα.

Further, in the above embodiment, the case where the recording medium 8 is an optical disk has been described as an example, but it is needless to say that the present invention can be applied to a focus error detection device such as an autofocus camera and has the same effect.

〔The invention's effect〕

As is clear from the above description, according to the present invention, the width D of each separation band of the two-division photodetector is substantially equal to the focused spot diameter L of the reflected light or is equal to or larger than the focused spot diameter L of the reflected light. , And the width of the depletion layer of the two-division photodetector is set to be equal to or less than that, and in addition, the photodetection sensitivity at the center of the separation band is set to be about half of the photodetection sensitivity on the light receiving surface of the photodetection element. Since the setting is made, the range in which the focus error signal linearly changes (linear zone) is widened, and as a result, it is possible to obtain a focus error detection device that can perform focus servo stably and easily.

[Brief description of the drawings]

FIG. 1 is a diagram showing a two-division photodetector in a focus error detection device according to an embodiment of the present invention, FIG. 2 is a characteristic diagram of a detection signal obtained by the two-division photodetector of FIG. 1, and FIG. Is a characteristic diagram of a focus error signal and a focus error amount based on the detection signal of FIG. 2, FIG. 4 is a configuration diagram showing a general focus error detection device, and FIG. 5 is a division type when reflected light is focused. FIG. 6 is an explanatory view showing a photodetector portion, FIG. 6 is an explanatory view showing a light receiving surface of the split type photodetector of FIG. 5, and FIG. 7 is a split type light when reflected light is irradiated before focusing. FIG. 8 is an explanatory view showing a detector portion, FIG. 8 is an explanatory view showing a light receiving surface of the split type photodetector of FIG. 7, and FIG. 9 is a split type photodetection in the case of focusing before reflected light is irradiated. Explanatory diagram showing the container part,
FIG. 10 is an explanatory view showing the light receiving surface of the split type photodetector of FIG. 9,
FIG. 11 is an explanatory view showing a light receiving surface of a conventional two-division photodetector,
FIG. 12 is a characteristic diagram of a detection signal by the two-division photodetector of FIG. 11, and FIG. 13 is a characteristic diagram of a focus error signal and a focus error amount based on the detection signal of FIG. 8: Recording medium, 20: 2-pin pin photodiode,
21 ... Separation band, D ... Separation band width, L ... Focus spot diameter, W ... Depletion layer spread width, F ... Focus error signal, Z ... Focus error amount, R, R1, R2 ...... Reflected light, P1,
P2: Focus spot. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

(57) [Claims] 1. A light splitter for splitting a reflected light from a recording medium into two, and a pair of two-split light detections arranged in the vicinity of respective condensing positions of the reflected light split by the splitter. And a focus error signal and a focus based on a detection signal output from each photodetector, the reflected light being irradiated in the vicinity of the separation band between the photodetectors of the pair of two-split photodetectors. In a focus error detection device that obtains an error amount, the width D of each separation band of the two-division photodetector reduces the detection sensitivity of the focus error signal and expands the range in which the focus error signal changes linearly. Therefore, it is set to be equal to or larger than the focused spot diameter L of the reflected light or equal to or larger than the focused spot diameter L of the reflected light and equal to or smaller than the spread width of the depletion layer of the two-divided photodetector. In the center of the belt The focus error detection device is characterized in that the light detection sensitivity is set to about half of the light detection sensitivity on the light receiving surface of the light detection element.