JPH0384744A - Focus error detector - Google Patents

Abstract

PURPOSE:To make the focus servo stable and easy by setting the width of a separating zone larger than a focal spot diameter when a focus of a reflected light is coincident with a recording medium and it to be less than the spread width of a depletion layer of a PIN photodiode. CONSTITUTION:The separate strip width D of a 2-split photodetector 20 of a focus error detector is selected to be larger than a focus spot diameter L of a reflected light irradiating in the vicinity of the separating zone and less than a spread width W of the depletion layer of the 2-split photodetector 20. Thus, the fluctuation in the detection signal with respect to the displacement of the spot location of the reflected light is decreased comparatively and a range of the focus error signal changing linearly with respect to the focus error is expanded. Thus, the focus servo is implemented stably and easily.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a focus error detection device for a recording medium in which information is recorded and reproduced optically, and in particular, the range in which the focus error signal changes linearly is wide, and the focus The present invention relates to a focus error detection device that allows stable and easy servo control.

[Conventional technology]

FIG. 4 is a configuration diagram showing a general focus error detection device called a Foucault type, which is described in, for example, Japanese Patent Laid-Open No. 57-18032 and Japanese Patent Laid-Open No. 58-208946.

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 irradiated light E and reflects the reflected light R from the recording medium 8; 5 is a collimator lens that converts the irradiated light E into parallel light and condenses the reflected light R; 6 is the parallel irradiated light 1/ to rotate the polarization plane of E and reflected light R
A four-wave plate 7 is an objective lens that condenses the irradiated light E transmitted through the quarter-wave plate 6 and converts the reflected light R into parallel light. Reference numeral 8 denotes a recording medium such as an optical disk, on which information is optically recorded and reproduced by being irradiated with the focused irradiation light E. 9 is an objective lens 7.1/4 wavelength plate 6.
The reflected light R from the recording medium 8 incident through the collimator lens 5 and the beam splitter 4 is divided into two reflected lights 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 touch at an obtuse ridgeline 9C, and the ridgeline 9C perpendicularly crosses the optical axis A of the reflected light R. It is located. A divided photodetector 10 detects the divided reflected lights R1 and R2, respectively, and is composed of two photodetecting elements 11 to 14 arranged in a plane perpendicular to the optical axis A.

Further, the light receiving surfaces of the photodetecting elements 11 to 14 are arranged in a line in the direction of the arrow X perpendicular to both the optical axis A and the ridge line 9C, and are properly positioned in the direction of the optical axis A and the arrow X.

The pair of photodetecting elements 11 and 12 constitute a two-split photodetector that detects one reflected light R1, and the other pair of photodetecting elements 13 and 14 constitutes a two-split photodetector that detects the other reflected light R2.
Usually, these two-split photodetectors are each composed of a two-split pin photodiode.

31 to S4 are detection signals obtained by each of the photodetecting elements 11 to 14, which are input to an arithmetic processing circuit (not shown);
It is now used in focus servos, etc.

Next, the operation of the focus error detection device shown in FIG. 4 will be described with reference to FIGS. 5 to 10.

When recording and reproducing information, the irradiation light E emitted from the light emission R3 passes through the beam splitter 4, becomes parallel light by the collimator lens 5, has its polarization plane rotated by the quarter-wave plate 6, and is focused by the objective lens 7. The light is emitted and irradiated onto the recording medium 8.

Then, the reflected light R reflected by the recording medium 8 is condensed at a predetermined convergence angle via the objective lens 7, quarter wavelength plate 6 and collimator lens 5, reflected by the beam splitter 4, and further split into light. The reflected light is divided into two reflected lights R1 and R2 by the device 9 and is irradiated onto the split type photodetector 10.

At this time, if the focus of the irradiated light E matches the recording medium 8, that is, it is focused, the reflected lights R1 and R2 are focused on the split type photodetector 10, as shown in FIG. reflected light R
1 is the gap between the photodetecting elements 11 and 12, that is, the separation band 10a
The other reflected light R2 is focused on the center point of the photodetector element 1.
The light is focused on separation zones 10b of Nos. 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 is a focused spot PI with a diameter of 20 to 30 μm.

It becomes P2 and is irradiated. In addition, each focusing spot PI,
P2 has an elliptical shape as described later.

Furthermore, 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 each separation strip 10a and LOb before being focused, as shown in FIG. Therefore, the reflected lights R1 and R2 are projected into semicircular spots P3.
It becomes P4 and is irradiated.

Conversely, if the distance between the recording medium 8 and the objective lens 7 is too fast, the reflected lights R1 and R2 will be focused in front of the segmented photodetector 10, as shown in FIG.

Therefore, the reflected lights R1 and R2 are projected onto the respective light-receiving surfaces of the photodetecting elements 11 and 14 at semicircular spots P5, as shown in FIG.
It becomes P6 and is irradiated.

At this time, the photodetecting elements 11 to 14 that have received each of the reflected lights R1 and R2 generate currents, that is, detection signals S1 to S4, according to the respective amounts of received light. The arithmetic processing circuit detects the outer photodetecting element 11 based on these detection signals 51 to 34.
．． The focus error signal F is calculated using the following formula (1) which calculates the difference between the sum of the detection signals S1 and S4 of the inner photodetectors 12 and 14 and the sum of the detection signals S2 and S3 of the inner photodetectors 12 and 13.

This focus error signal F is zero when the distance between the recording medium 8 and the objective lens 7 is appropriate as shown in FIGS. 5 and 6, and is zero when the distance between the recording medium 8 and the objective lens 7 is appropriate as shown in FIGS. If the distance between the recording medium 8 and the objective lens 7 is too short, the value will be negative, and if the distance between the recording medium 8 and the objective lens 7 is too long, as shown in FIGS. 9 and 10, the value will be positive.

From the polarity and magnitude of the focus error signal F thus obtained, a focus error amount for the appropriate distance between the recording medium 8 and the objective lens 7 is calculated, and a 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 is 2. That is, focus servo is performed until the focus error signal F becomes zero.

FIG. 11 is an explanatory diagram showing a part of the light receiving surface of the conventional split type photodetector 10 used in the focus error detection device of FIG. 4, and the distance between the recording medium 8 and the objective lens 7 is adjusted appropriately. This shows a state in which the reflected light R1 becomes a focused spot P1 and is irradiated.

In FIG. 11, l is a pair of photodetecting elements 11 and 12.
This is a two-split pin photodiode forming a split-pin photodiode, and constitutes a split-type photodetector 10 together with a two-split pin photodiode (not shown) forming another pair of photodetecting elements 13 and 14. Reference numeral 2 denotes a separation band that divides the light-receiving surfaces of the photodetecting elements 11 and 12, and its width d is about 10 μm.

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

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 r (Airy disk diameter) is expressed as r=1.22λ/sin α. Also, the diameter of the focused spot of the divided reflected light R1 is L#2 r-2,44λ/sin a ”・
(2) 0 For example, the convergence angle α in the direction parallel to the dividing line of the separation band 2 is approximately 2°, and the wavelength λ is 0.7
In the case of reflected light R1 of 8 μm, the focused spot diameter is (2)
From the formula, it is approximately 50IIm.0 Normally, the value of the focused spot diameter varies depending on the design specifications of the information recording/reproducing device, but it is 4
It is about 0 to 50 μm.

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

Similarly to the above, the reflected light R generated when the recording medium 8 is irradiated with the irradiation light E is divided into two parts, and one of the reflected lights R1 is irradiated onto the light receiving surface of the two-split pin photodiode l.

At this time, if the distance between the recording medium 8 and the objective lens 7 is appropriate, the width d of the separation zone 2 (''= 1
A focused spot P1 having a diameter L (#40 to 60 μm) that is 4 to 6 times larger than 0 am> is symmetrically irradiated onto each light receiving surface of the photodetecting elements 1 and 12.

If 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 expands to become a semicircular spot (P3 or P5).

At this time, the spot diameter is approximately proportional to the focus error amount Z, but the amount of reflected light R1 is constant regardless of the shape of the spot, so the detection signals 31 and 32 obtained from each photodetector element 11 and 12 are as follows. As the reflected light R1 moves, it changes as shown in FIG.

That is, if the distance between the recording medium 8 and the objective lens 7 is too short, the detection signal S2 of the photodetecting element 12 becomes large (eighth
(see figure), if it is too fast, the detection signal 3 of the photodetecting element 11
1 becomes larger. At the same time, since detection signals S3 and S4 are obtained from the other two-split pin photodiode (not shown), a focus error signal F is determined from these detection signals 51 to S4 based on equation (1).

Then, the focus error amount Z is determined by the Foucault method mentioned above.
When calculated, 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 when converted to the displacement of the distance between the recording medium 8 and the objective lens 7.
．． It is 0am. Therefore, focus servo can be performed only within this narrow range.

[Problem to be solved by the invention]

The conventional focus error detection device is configured as described above, and each photodetector element 11, 12 (and 13, 14)
Since the width of the separation band 2 between the two is only about 1101I, the focus error signal F fluctuates sensitively to the displacement of the spot position of each reflected light, and the focus error signal changes linearly with the focus error amount. Therefore, the offset signal in the focus error signal F must be lowered, and the follow-up range of the focus servo also becomes narrower.

This invention was made to solve the above-mentioned problems, and it widens the range in which the focus error signal changes linearly with respect to the amount of focus error, facilitates focus servo pull-in and follow-up control, and improves focus servo control. An object of the present invention is to obtain a focus error detection device that can stably perform the following.

[Means to solve the problem]

The focus error detection device according to the present invention sets the separation band width of the two-split photodetector to be larger than the focused spot diameter of the reflected light irradiated near the separation band and smaller than the spread width of the depletion layer of the two-split photodetector. This is the setting.

[Effect]

In this invention, the width of the separation band of the two-split photodetector of the focus error detection device is set to be greater than or equal to the focused spot diameter of the reflected light irradiated near the separation band and less than or equal to the spread width of the depletion layer of the two-segment photodetector. Since it is set to , the fluctuation of the detection signal with respect to the displacement of the spot position of the reflected light can be made relatively small, and the range in which the focus error signal changes linearly with respect to the focus error amount is expanded.

〔Example〕

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

FIG. 1 is an explanatory diagram showing a two-split photodetector of a focus error detection device according to an embodiment of the present invention, and in the figure, numerals 11 and 12 are similar to those in the conventional device described above.

20 and 21 correspond to the conventional two-split pin photodiode 11 and the separation band 2, respectively. Furthermore, other configurations not shown are similar to the general focus error detection device shown in FIGS. 4 to 10.

Here, 20 is a two-split photodetector, that is, a two-split pin photodiode, forming a pair of photodetecting elements 11 and 12, and a two-split pin photodiode forming another pair of photodetecting elements 13 and 14.
Together with a split pin photodiode (not shown), it constitutes a split photodetector. 21 is the photodetector element 11 and 1
This is a separation band that divides each of the two light-receiving surfaces.

In the present invention, the width is set to be approximately 50 μm, that is, greater than or equal to the focal spot diameter of the reflected light R1 (in the case of FIG. 1, approximately equal) and less than or equal to the spread width W of the depletion layer of the split photodetector. Therefore, within the width of the separation strip 21, the second
As shown in the figure, the detection signals S1 and S2 change linearly at a gentle angle with respect to the displacement in the direction of the arrow
At the center of the dividing line of 1, each detection signal S1 and S2
outputs approximately half of the maximum photocurrent of each photodetector element 11, 12.

Next, the operation of the embodiment of the present invention shown in FIG. 1 will be explained with reference to the detection signal characteristic diagram in FIG. 2, the focus error signal characteristic diagram in FIG. 3, and FIGS. 4 to 10. do.

As shown in FIG. 1, the focusing spot 1-Pi for the reflected light R1 is
is irradiated onto the center of the separation zone 21, and the width of the separation zone 21 is greater than the diameter of the focused spot and less than the spread width W of the depletion layer. Even if light is no longer received and the spot position is displaced in the direction of arrow X due to a focus error, the area irradiated to the separation strip 21 will be large and the area irradiated to the photodetector element 11 or 12 will be relatively small. . And the separation strip 21
As shown in FIG. 2, the detection signal S1
and S2 change linearly at a gentle angle with respect to displacement in the direction of arrow X. Furthermore, at the center of the dividing line of the separation band 21, the sensitivity distribution is set so that each detection signal S1 and S2 outputs approximately half of the maximum photocurrent.

Therefore, the characteristic of the focus error signal F with respect to the focus error amount Z has a gentle slope in the portion having an oblique straight line, as shown by the solid line in FIG. 3, compared to the conventional characteristic shown by the broken line. At the same time, the range of linear change is approximately ±3.0 μm in terms of displacement of the distance between the recording medium 8 and the objective lens 7.
It becomes wider.

In this way, focus servo can be stably and easily performed using the focus error amount Z that varies linearly over a wide range.

In the above embodiment, the case where the width of the separation band 21 is 50 μm was explained, but due to various specification changes, the wavelength λ of the reflected light R, the convergence angle α, and the physics of the light-receiving surface of the split type photodetector Since the constants are different, the width of the separation band 21 is larger than the focal spot diameter and smaller than the spread width W of the depletion layer of the split type photodetector, that is, from the above equation (2), W≧D≧2,44λ/ Any value that satisfies 5intx is sufficient.

Further, in the above embodiment, the case where the recording medium 8 is an optical disk has been described as an example, but it goes without saying that the present invention can also be applied to a focus error detection device such as an autofocus camera, and the same effect can be obtained.

〔Effect of the invention〕

As described above, according to the present invention, the separation band width of the two-segment photodetector is set to be greater than or equal to the focal spot diameter of the reflected light irradiated near the separation band and less than or equal to the spread width of the depletion layer of the one-segment type photodetector. Since the setting is set to It can be made relatively small, the range in which the focus error signal changes linearly with respect to the focus error amount is expanded, and the focus servo can be performed stably and easily.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a two-split 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-split photodetector of FIG. 1, and FIG. is a characteristic diagram of the focus error signal and focus error amount based on the detection signal in Figure 2, Figure 4 is a configuration diagram showing a general focus error detection device, and Figure 5 is a split type when reflected light is focused. An explanatory diagram showing the photodetector part, Figure 6 is an explanatory diagram showing the light receiving surface of the split type photodetector in Figure 5, and Figure 7 is an explanatory diagram showing the split type light when the reflected light is irradiated before it is focused. An explanatory diagram showing the detector part, Fig. 8 is an explanatory diagram showing the light-receiving surface of the split-type photodetector in Fig. 7, and Fig. 9 shows split-type photodetection when the reflected light is focused before being irradiated. FIG. 10 is an explanatory diagram showing the light-receiving surface of the split type photodetector of FIG. 9. FIG. 11 is an explanatory diagram showing the light-receiving surface of the conventional two-split photodetector. 11 is a characteristic diagram of a detection signal from the two-split photodetector of FIG. 11, and FIG. 13 is a characteristic diagram of a focus error signal and focus error amount based on the detection signal of FIG. 12. 8... Recording medium, 20... 2-split pin photodiode, 21... Separation band, D... Width of separation band, L...
・Focused spot diameter, W...expansion width of depletion layer, F...
・Focus error signal, Z...Focus error amount, R
, R1°R2... Reflected light, PI, R2... Focusing spot. Note that the same reference numerals in the figures indicate the same or equivalent parts. Figure Figure 12): i da zii Figure 3 Figure Figure Figure Figure Figure Figure Figure So 2J゛nonnw/7nozu)7 ti! Figure 10 αzz, carp bruise 2iW Figure 11 Figure 12 Figure 2 each Figure 13

Claims (1)

[Claims] (1) A pin photodiode pair consisting of two photodetecting elements that receive reflected light from a recording medium divided into two and output a current as a detection signal according to the amount of received light, and a separation band sandwiched between them. In a focus error detection device having a two-split photodetector, the width of the separation band is equal to or larger than the focused spot diameter when the focus of the reflected light coincides with the recording medium, and A focus error detection device characterized in that the width is set to be less than the spread width of a depletion layer of a diode.