JP2552660B2 - Focus error detector - 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 recording / reproduction is optically performed, and particularly, a range in which a focus error signal linearly changes is wide, The present invention relates to a focus error detection device that enables stable and easy focus servo.

[Prior Art] FIG. 4 shows, for example, JP-A-57-18032 and JP-A-5-18032.
It is a block diagram which shows the general focus error detection apparatus called the Foucault method described in 8-208946.

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.

(4) is a beam splitter that transmits the irradiation light E and reflects the reflected light R from the recording medium (8) described later,
(5) is a collimator lens for converting the irradiation light E into parallel light and condensing the reflected light R, (6) is a 1/4 wavelength plate for rotating the plane of deflection of the parallel irradiation light E and reflected light R, (7 ) Is 1/4
The objective lens collects the irradiation light E transmitted through the wave plate (6) and makes the reflected light R parallel light.

Reference numeral (8) is 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 two reflections of 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). An optical splitter that splits the light into R1 and R2. This light splitter (9) is also called a Foucault prism and has a three-dimensional shape in which two rectangular refracting surfaces (9a) and (9b) are in contact at an obtuse ridgeline (9c), and the ridgeline (9c) is a reflected light R. Are arranged so as to vertically cross the optical axis A of.

(10) is a split type photodetector for detecting the split reflected lights R1 and R2, respectively, and is composed of four photodetection elements (11) to (14) arranged in a plane perpendicular to the optical axis A. Has been done. Further, the light receiving surfaces of the photodetectors (11) to (14) are arranged in a line in the arrow X direction perpendicular to both the optical axis A and the ridge line (9c), and in the optical axis A and the arrow X direction. It is properly positioned. The pair of photodetection elements (11) and (12) constitutes a two-divided photodetector that detects one reflected light R1, and the other pair of photodetection elements (13) and (14) is the other reflected light. It constitutes a two-division photodetector that detects R2. Usually, each of these two-divided photodetectors is composed of a two-divided pin photodiode, as will be described later.

S1 to S4 are detection signals obtained by the photodetection elements (11) to (14) and are input to an arithmetic processing circuit (not shown) to be used for focus servo or 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 a light emitting source (3) when recording and reproducing information passes through a beam splitter (4), becomes parallel light by a collimator lens (5), and is a quarter wavelength plate (6).
The deflecting surface is rotated by, the light is condensed by the objective lens (7), and the recording medium (8) is irradiated with the light.

Then, the reflected light R reflected by the recording medium (8) is condensed at a predetermined convergence angle via the objective lens (7), the quarter-wave plate (6) and the collimator lens (5), and the beam splitter The reflected light is reflected by (4), and is further divided into two reflected lights R1 and R2 by a light splitter (9), and the split light detector (10) is irradiated with the reflected light.

At this time, when the focus of the irradiation light E is coincident with the recording medium (8), that is, focused, the reflected lights R1 and R2 are focused on the split type photodetector (10) as shown in FIG. And one of the reflected light
R1 is the gap or separation zone (10) between the photodetectors (11) and (12).
The light reflected by the other reflected light R2 is focused on the center point of a)
It is focused on the separation zone (10b) of 3) and (14). When this state is viewed from the light receiving surface side, it becomes as shown in Fig. 6, and each reflected light 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.

Further, when the distance between the recording medium (8) and the objective lens (7) is too short, as shown in FIG. 7, the separation bands (10a) and (10b) are formed before the reflected lights R1 and R2 are focused. Is irradiated in the vicinity of. Therefore, the reflected lights R1 and R2 are irradiated as semicircular spots P3 and P4 on the light receiving surfaces of the photodetectors (12) and (13) as shown by the shaded areas in FIG.

On the contrary, when the distance between the recording medium (8) and the objective lens (7) is too long, 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 (1 that receives the reflected lights R1 and R2)
1) to (14) generate currents, that is, detection signals S1 to S4 corresponding to the respective received light amounts. The arithmetic processing circuit, based on these detection signals S1 to S4, detects the photodetection elements (11), (1
4) The sum of each detection signal S1 and S4 and the inner photodetector (1
2) The focus error signal F is obtained by the equation F = (S1 + S4)-(S2 + S3), where the difference between the detection signals S2 and S3 in (13) is calculated.

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 shown in FIGS. 7 and 8. When the distance between the recording medium (8) and the objective lens (7) is too short, the distance is negative. As shown in FIGS. 9 and 10, the distance between the recording medium (8) and the objective lens (7) is small. Positive if too far.

From the polarity and magnitude of the focus error signal F obtained in this way, the focus error amount W with respect to the appropriate distance between the recording medium (8) and the objective lens (7) is calculated, and a focus adjustment mechanism (not shown) is used. Control. For example, the objective lens (7) is moved in the optical axis direction of the irradiation light E, and the focus servo is performed until the focus error amount W, that is, 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, and shows the recording medium (8) and the objective lens (7). The distance between and is properly adjusted, and the reflected light R1 is irradiated as a focused spot P1.

In FIG. 11, (1) is a two-divided pin photodiode that forms a pair of photo-detecting elements (11) and (12), and two-division pin photodiode that forms another pair of photo-detecting elements (13) and (14). A split photodetector (10) is configured with a pin photodiode (not shown). (2) is a photodetector (1
These are separation bands that divide the light-receiving surfaces of 1) and (12), and their width d is about 10 μm.

When focused as shown in FIG.
Due to the diffraction phenomenon when divided, the focused spot P1 of the reflected light R1 has an elliptical shape. Therefore, the diameter L of the focused spot P1 is about twice the diameter of the focused spot (circular shape not shown) of the reflected light R when not divided.

Generally, when the convergence angle of the incident light beam is α and the wavelength of the light source is λ, the focused spot diameter (Airy disk diameter) r 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. For example, in the case of reflected light R1 having a convergence angle α of about 2 ° and a wavelength λ of 0.78 μm with respect to the direction parallel to the dividing line of the separation band (2), the focused spot diameter L is about 55 μm from the formula
Becomes Usually, the value of the focused spot diameter L is about 40 to 60 μm, although it varies depending on the design specifications of the information recording / reproducing apparatus.

Next, the operation of the conventional focus error detection device using the two-divided pin photodiode (1) shown in FIG. 11 will be described with reference to FIGS. 4 to 10, 12, and 13. 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 divided into two.
The light-receiving surface of the split pin photodiode (1) is irradiated.

At this time, if the distance between the recording medium (8) and the objective lens (7) is proper, the width d of the separation band (2) as shown in FIG.
A focused spot P1 having a diameter L (≈40 to 60 μm) which is 4 to 6 times as large as (≈10 μm) is symmetrically applied 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 W, but the reflected light R1
Since the amount of light is constant regardless of the spot shape, the detection signals S1 and S2 obtained from the photodetectors (11) and (12) are
Varies with the movement of the reflected light R1, as shown in FIG.

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 photodetection element (11) is detected. )
The detection signal S1 of 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.

Then, when the focus error amount W 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 W is converted into the displacement of the distance between the recording medium (8) and the objective lens (7),
It is about ± 1.0 μm. Therefore, the focus servo can be performed only within this narrow range.

[Problems to be Solved by the Invention] As described above, in the conventional focus error detection device, the separation band (2) between the photodetection elements (11), (12) (and (13), (14)) is provided. Since the width d of the focus error signal is only about 10 μm, the focus error signal F fluctuates sensitively with respect to the displacement of the spot position of each reflected light R1 and R2 (that is, the detection sensitivity is high), and The range that changes linearly with respect to the amount W becomes narrow, and the offset signal in the focus error signal F must be lowered,
Further, there is a problem that the tracking range of the focus servo is narrowed.

The present invention has been made to solve the above problems, by rather lowering the detection sensitivity of the focus error signal, to widen the range in which the focus error signal linearly changes with respect to the focus error amount, An object of the present invention is to obtain a focus error detection device which facilitates pull-in and follow-up control of the focus servo and can perform the focus servo stably.

[Means for Solving the Problems] The focus error detection device according to the present invention reduces the detection sensitivity of the focus error signal and expands the range in which the focus error signal changes linearly. The separation band width of the container is set to be almost equal to or larger than the focused spot diameter of the reflected light, and the light detection sensitivity in the separated band is linear with respect to the direction perpendicular to the division line of the separated band. Is set so that the light detection sensitivity at the center of the separation band is about half of the light detection sensitivity on the light receiving surface of the light detection element.

[Operation] In the present invention, since the separation band is linear and has a light-receiving sensitivity distribution that is halved at the center, the fluctuation of the detection signal with respect to the displacement of the spot position of the reflected light is relatively small, and the focus error signal is the focus error. The range that changes linearly with the amount increases.

[Embodiment] 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 according to one embodiment of the present invention, and (11) and (12) are the same as those of the conventional device described above. Further, (20) and (21) correspond to the conventional two-divided pin photodiode (1) and the separation band (2), respectively. Further, other configurations not shown are shown in FIGS.
This is similar to the general focus error detection device shown in the figure.

Reference numeral (20) is a two-divided photodetector, that is, a two-divided pin photodiode that forms a pair of photodetection elements (11) and (12), and another pair of photodetection elements (13) and (14) is formed. A split type photodetector is configured with a split pin photodiode (not shown). Reference numeral (21) is a separation band that divides the light receiving surfaces of the photodetectors (11) and (12), and has a width D.
Is set to approximately 50 μm, that is, to be equal to or larger than the focused spot diameter L by the reflected light R1 (in the case of FIG. 1, almost equal).

In the width D of the separation band (21), as shown in FIG. 2, the detection signals S1 and S2 change linearly with respect to the displacement in the arrow X direction, and At the center of the dividing line, each detection signal S1 and S2 is detected by each photodetector element (11),
It outputs about half of the maximum photocurrent of (12). That is, the detector sensitivity in the separation band (21) changes linearly with respect to the direction perpendicular to the dividing line of the separation band (21),
The sensitivity of each light receiving surface at the center of the separation band (21) is set to be about half the sensitivity on the light receiving surface.

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, when the focused spot P1 of the reflected light R1 is irradiated to the central portion of the separation band (21), the width D of the separation band (21) is equal to or larger than the focused spot diameter L, so that each light The reflected light R1 is hardly received by the detection elements (11) and (12).

Further, even if a spot error occurs and the spot position is displaced in the direction of the arrow X, the area irradiated to the separation band (21) is large, and the area irradiated to the photodetector element (11) or (12) is relatively large. small.

Accordingly, as shown by the solid line in FIG. 3, the characteristic of the focus error signal F with respect to the focus error amount W has a gentler slope in the portion having linearity as compared with the conventional characteristic shown by the broken line. At the same time, the linearly changing range is widened to about ± 3.0 μm in terms of displacement of the distance between the recording medium (8) and the objective lens (7).

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

In the above embodiment, the width D of the separation band (21) is 50 μm, but the reflected light R is changed due to various specification changes.
Since the wavelength λ and the convergence angle α are different, the width D of the separation band (21) is equal to or larger than the focused spot diameter L, that is, a value satisfying D ≧ 2.44λ / sinα from the above equation. Even when the separation band width D is substantially equal to the focused spot diameter L, it is clear that the same effect as described above can be obtained. Therefore, the separation band width D may be set to a value that satisfies D≈2.44λ / sinα.

Further, although the case where the recording medium (8) is an optical disk is taken as an example, it is needless to say that it can be applied to a focus error detection device such as an autofocus camera and the same effect can be obtained.

As described above, according to the present invention, in order to reduce the detection sensitivity of the focus error signal and expand the range in which the focus error signal changes linearly, the separation band width of the two-division photodetector is increased. Is set to be approximately equal to or larger than the focused spot diameter of the reflected light, and the light detection sensitivity in the separation band is changed linearly with respect to the direction perpendicular to the division line of the separation band. Set the light detection sensitivity at the center of the separation band,
The focus error signal is set to be about half of the light detection sensitivity on the light receiving surface of the light detection element, and the fluctuation of the detection signal with respect to the displacement of the spot position of the reflected light is made relatively small. There is an effect that the range that changes linearly is expanded, and a focus error detection device that can perform focus servo stably and easily is obtained.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a two-division photodetector in 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. 3 is FIG. FIG. 4 is a characteristic diagram of a focus error signal and a focus error amount based on the detection signal of FIG. 4, FIG. 4 is a configuration diagram showing a general focus error detection device, and FIG. 5 is a split type photodetector portion when reflected light is focused. FIG. 6 is an explanatory view showing the light receiving surface of the split type photodetector of FIG. 5, and FIG. 7 shows the split type photodetector portion when reflected light is irradiated before focusing. Explanatory drawing which shows, 8th
FIG. 9 is an explanatory view showing the light receiving surface of the split type photodetector of FIG. 7, FIG. 9 is an explanatory view showing the split type photodetector portion when focusing is performed before irradiation of reflected light, and FIG. Is an explanatory view showing the light-receiving surface of the split-type photodetector of FIG. 9, FIG. 11 is an explanatory view showing the light-receiving surface of a conventional two-split photodetector, and FIG. 12 is the two-split detector of FIG. FIG. 13 is a characteristic diagram of the detection signal, and FIG. 13 is a characteristic diagram of the focus error signal and the focus error amount based on the detection signal of FIG. (8) …… Recording medium, (11), (12) …… Photodetector (20) …… Divided pin photodiode (21) …… Separator, R, R1, R2 …… Reflected light D …… Width of separation band, P1, P2 ... Focused spot L ... Focused spot diameter F ... Focus error signal W ... Focus error amount In the drawings, the same reference numerals indicate the same or corresponding portions.

Claims (2)

(57) [Claims] 1. A light splitter that splits the reflected light from a recording medium into two, and a pair of two-split photodetectors arranged in the vicinity of the respective focusing positions of the split reflected light, Each of the reflected lights is irradiated in the vicinity of the separation band between the photodetecting elements of the pair of two-divided photodetectors, and the focus error signal and the focus error amount are calculated based on the detection signal output from each of the photodetecting elements. In the obtained focus error detection device, 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. It is set to be substantially equal to or larger than the focused spot diameter L of the reflected light, and is set to be equal to or larger than the focused spot diameter L, and the light detection sensitivity in the separation band is linear with respect to the direction perpendicular to the division line of the separation band. Change The focus error detection device is characterized in that the light detection sensitivity at the center of the separation band is set to be about half of the light detection sensitivity on the light receiving surface of the light detection element. 2. The two-divided photodetector is a two-divided pin photo-diode.
A focus error detection device according to the item.