JPH03292638A - Focus position detector - Google Patents

Abstract

PURPOSE:To accurately detect a signal by arranging a one-direction angle displacement generating member on the optical path through which the reflection light from an optical information recording medium is led to a signal detecting optical system. CONSTITUTION:In a signal detecting optical system 16, a prism 18 is arranged on the optical path where the reflection light from a magneto-optical disk 15 which is reflected by a beam splitter 12 passes a lambda/2 plate 17. This prism 18 functions to displace the reflection light at an angle only in one direction in accordance with defocusing. A focus error signal is detected in positions before and after a light spot P where light is condensed through a polarizing beam splitter 19 by condenser lenses 20 and 22. The beam sectional form is always a perfect circle in any position in the optical axis direction in the case of the in-focus state, and the focus error signal does not include the offset even in the case of the occurrence of positional deviation in the optical axis direction, and therefore, the signal is accurately detected.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a focus position detection device in an optical information recording/reproducing device or the like.

2. Description of the Related Art Conventionally, in optical information recording and reproducing devices, etc., general methods for determining a focus error signal for detecting a shift in the focal position of an objective lens include the Naifezi method, the astigmatism method,
These include the Foucault method and the beam size method. In this case, taking a typical astigmatism method as an example,
There is one disclosed in Publication No. 3. As shown in FIG. 7, light emitted from a laser light source (not shown) is passed through a beam splitter 1, focused by an objective lens 2, and irradiated onto the surface of an optical disk 3, and the reflected light is then The beam is then reflected by the beam splitter 1 and guided to the signal detection optical system 4, focused by the condenser lens 5, transmitted through the cylindrical lens 6, and detected by the photodetector 8.

As a result, each position a on the optical path where astigmatism occurs after passing through the cylindrical lens 6 as shown in FIG.

The cross-sectional shape of the beam at points b, c, d, and e is as shown in FIG. 9, and the beam becomes a perfect circle at position C and is deformed in the vertical or horizontal direction at positions before and after it. Therefore, by positioning the photodetector 8, which has a light-receiving surface divided into a plurality of parts, in advance at point C, which forms a perfect circle when the objective lens 2 is in focus, the beam can be Since the amount of light received changes as a result of the deformation, it is possible to detect a focus error signal.

Problems to be Solved by the Invention The methods for detecting a focus error signal as described above are all commonly used methods, but in these detection methods, the position of the photodetector 8 itself There is a problem in that due to changes in optical path length, wavelength, etc. due to deviation or temperature change, an offset is included in the detected focus error signal, making it impossible to perform accurate signal detection.

Means for Solving the Problem Therefore, in order to solve such problems, claim 1
In the described invention, information is recorded and reproduced by condensing light emitted from a laser light source with an objective lens and irradiating it onto an optical information recording medium, and a signal is detected from the reflected light from the optical information recording medium. In an optical information recording and reproducing device that reproduces information and detects focus error signals and the like by guiding the light to an optical system, the reflected light from the optical information recording medium is guided to the signal detection optical system on the optical path due to defocusing. A unidirectional angular displacement generating member that generates angular displacement of the reflected light in only one direction is provided, and the reflected light displaced in one direction by the unidirectional angular displacement generating member is passed through a condensing lens and condensed. A focus error signal detection means for detecting the difference in light amount between the positions before and after the point is provided.

In addition, in the invention as claimed in claim 2, information is recorded and reproduced by condensing the light emitted from the laser light source with an objective lens and irradiating it onto the optical information recording medium, and also recording and reproducing information from the optical information recording medium. In an optical information recording and reproducing device that guides reflected light to a signal detection optical system to reproduce information and detect focus error signals, etc., the reflected light from the optical information recording medium is on an optical path guided to the signal detection optical system. A unidirectional angular displacement generating member that generates an angular displacement of the reflected light due to defocus in only one direction is provided, and the optical path of the reflected light displaced in one direction by the unidirectional angular displacement generating member 1 is provided. A condenser lens is disposed on each optical path divided into two by the optical path separator, and a condenser lens is disposed on the optical path in front of the condensing point formed by one of the condenser lenses. A focus light-receiving element was disposed on the optical path behind the light-converging point formed by the other condenser lens.

Effect: Therefore, the light obtained by passing the light displaced in one direction by the unidirectional angular displacement generating member through the condensing lens has a cross-sectional shape of the beam at any position along the optical axis when it is focused. Since it is always a perfect circle, it is completely unaffected by deviations in the optical axis direction, thereby making it possible to eliminate offsets from being included in the focus error signal due to deviations in the optical axis direction.

Embodiment An embodiment of the present invention will be described with reference to FIGS. 1 to 5. First, the overall configuration of an optical information recording/reproducing apparatus including a focal position detection device will be described based on FIG. Light emitted from a semiconductor laser 9 as a laser light source is collimated by a coupling lens 10, then beam-shaped by a beam shaping prism 11, transmitted through a beam splitter 12, deflected by a deflection prism 13, and then passed through an objective lens. The light is focused by the light beam 14 and irradiated onto the surface of a magneto-optical disk 15 serving as an optical information recording medium, thereby recording information. Further, the reflected light from the magneto-optical disk 15 passes through the objective lens 14 and the deflection prism 13 sequentially, is reflected by the beam splitter 12, and is guided into the signal detection optical system 16.

Within this signal detection optical system 16, a beam splitter 12
The reflected light from the magneto-optical disk 15 is λ
/2 On the optical path that passed through the plate 17.

A prism 18 is provided as a unidirectional angular displacement generating member. This prism 18 has the function of generating angular displacement of reflected light in only one direction due to defocus. A polarizing beam splitter 19 as an optical path separating member is disposed on the optical path displaced in one direction by the prism 18. A condensing lens 20 is disposed on the optical path reflected by this polarizing beam splitter 19, and a focus light receiving element 21 is disposed on the optical path in front of the condensing point P formed by this condensing lens 20. ing. In addition, a condenser lens 22 is placed on the optical path that has passed through the polarizing beam splitter 19.
A focus light receiving element 23 is disposed on the optical path behind the condensing point P formed by the condensing lens 22 . Further, an aperture 24 is provided on the optical path between the prism 18 and the polarizing beam splitter 19. In this case, the polarizing beam splitter 19, the condenser lens 20.22, the focus light receiving element 21.23, and the aperture 24 constitute a focus error signal detection means 25.

In this configuration, the function of the prism 18 will be described step by step. Now, as shown in FIG. 2, if there is a prism 18 with an apex angle ψ, this prism 18 has a beam shaping effect with a magnification of n: refractive index, as is well known.

At this time, the angle also changes at the same time, and θ and 0 in Figure 2
2, there is θ1. When θ2 is small, θ, ″: MO
3...(3) The following relationship holds true.

Now, as shown in FIG. 2, a case will be considered in which the position of the light spot irradiated onto the surface of the magneto-optical disk 15 by the objective lens 14 is shifted, that is, a so-called focus shift occurs. When this focus shift occurs, an angular shift of a certain angle θ occurs in the return optical path. If θ is very small, the relationship θ:kr holds true, where rl is the distance of the light ray from the optical axis 26. After this light beam passes through the prism 18, it becomes (3
It is clear from the above that θ in the equation ( ) = Mθ, but since the beam is shaped at the same time, the prism 18
The distance from the optical axis 26 after passing through is r, #
r, ...(4). That is, r=r,
At the position, the angle θ is equal to Mθ.

On the other hand, the direction in which the prism 18 does not have a beam shaping effect (
(perpendicular to the plane of the paper), the angle θ and the distance r from the optical axis 26 remain unchanged.

That is, the angle θ is at the position r=r, so r=r,=
At the position r, /M, θa=θ, /M.

Therefore, when comparing the ray angles in both directions after passing through the prism 18 at equal distances from the optical axis 26, θ□:θa = M knee == M''θ1:01, that is, the prism 18 is substantially M8 This will have the effect of expanding the angle twice as much.

Therefore, when such a ray enters the condensing lens 14, when it is focused, the cross-sectional shape of the beam after passing through the condensing lens 14 is always a perfect circle 27, as shown in FIG. 5(b). Furthermore, when a defocus occurs, the cross-sectional shape of the beam becomes an ellipse 28, as shown in FIGS. 5(a) and 5(c). In this case, the beam of the ellipse 28 has its long axis and short axis at the front bin (a).
, the position to be reversed at the rear bin (c), for example, A in FIG.
.

Position B is created. Therefore, as shown in FIG. 4, the light-receiving surfaces a,, a□, a, are divided into four at the position A.

A focus light receiving element 21 having a diameter of a4 is arranged, and
The light-receiving surface divided into four at the B position b l lb,,
b,, By arranging the focus light receiving element 23 having b4, at the position of the front bin in FIG. 4(a),
b, 10b4, and at the position of the focused point in Figure 4(b), a, +a, = a, +a4 b, 10s, = b, 10b4, and the position of the rear bin in Figure 4(c). So, a, +a, > a, +a.

b, tensu, <b, tenb4 becomes.

As a result, the values of each focus error signal detected by the focus light receiving elements 21 and 23 are changed to Fa and F.
If b, F a = (a, 10 as) (at 10 am)
Fb=(b, 10s,)-(b, 10sb4).

Calculating these Fa and FbO differences, F○=Fa-Fb=(a++as)-(am+aj-(b,+b,)+(
b, +b, )... (5) From this, the value of the focus error signal FO to be finally determined can be known.

As described above, by detecting the focus error signal Fo at the positions before and after the condensing point P condensed by the condenser lens 20.22 using the prism 18 that generates angular displacement in only one direction, the fifth As shown in Figure (b), when the beam is focused, the cross-sectional shape of the beam is always a perfect circle at any position in the forward axis direction, so even if a positional shift occurs in the optical axis direction, the focus error signal Since Fo does not include an offset, it becomes possible to perform accurate signal detection.

It should be noted that detection of the magneto-optical signal can be performed by a conventionally commonly used detection method, but it is also possible to detect it by using the focus light receiving elements 21 and 23 in this embodiment as they are. That is, in the configuration of FIG. 1, when detecting the magneto-optical signal Mo, Mo=(a, +a, 10a, 10a4)-(b, +b, +b
, +b4) (6).

Next, a modification of the present invention will be described. In the embodiment described above, in order to detect the focus error signal Fo, the aperture 24 is placed in front of the condenser lens 20, 22 as shown in FIG. 1, so that the cross-sectional shape of the beam becomes a perfect circle. However, here, signal detection is performed without using such an aperture 24. That is,
As shown in FIG. 6, by adopting a configuration in which no aperture 24 is placed between the prism 18 and the condensing lens 20, the beam shaping action of the prism 18 allows the beam cross section at the time of focusing (b) to be The shape will be an ellipse. However, since the ellipticity at the time of focusing is always constant, the dividing line inclination of the focus light receiving element 21.23 is
By setting Y=m in advance, the same effect as in the embodiment described above can be obtained.

Note that in this embodiment, the condenser lenses 20, 22 . teeth.

Although it is arranged after the polarizing beam splitter 19, it is not limited to this arrangement; for example, it is possible to replace these parts and use a common condenser lens. Further, when magneto-optical detection is not performed, a half prism can be used instead of the polarizing beam splitter 19, and in this case, the λ/2 plate is also not required.

Effects of the Invention The invention according to claim 1 provides a unidirectional angle that causes an angular displacement of the reflected light in only one direction due to a defocus on the optical path where the reflected light from the optical information recording medium is guided to the signal detection optical system. Focus error signal detection means that includes a displacement generating member, and detects a difference in light amount between positions before and after the condensing point by passing reflected light displaced in one direction by the unidirectional angular displacement generating member through a condensing lens. , the light obtained by passing the light displaced in one direction by the unidirectional angular displacement generating member through the condensing lens has a cross-sectional shape of the beam at which point in the optical axis direction when focusing. Since the position is always a perfect circle, it is completely unaffected by deviations in the optical axis direction, and as a result, the focus error signal does not include an offset amount due to deviations in the optical axis direction, allowing accurate signal detection. This makes it possible to do the following.

Further, the invention according to claim 2 provides a unidirectional angular displacement that causes an angular displacement of the reflected light in only one direction due to a defocus on the optical path on which the reflected light from the optical information recording medium is guided to the signal detection optical system. A generating member is disposed, and an optical path separating member is disposed on the optical path of the reflected light displaced in one direction by the unidirectional angular displacement generating member, and the beam is focused on each optical path divided into two by the optical path separating member. A focus light receiving element is arranged on the optical path in front of the condensing point formed by one of the optical lenses, and a focus light receiving element is disposed on the optical path in front of the condensing point formed by the other condensing lens. Since the focus light receiving element is arranged on the optical path, the light obtained by passing the light displaced in one direction by the unidirectional angular displacement generating member through the condensing lens has a beam cross-sectional shape when focused. Since it is always a perfect circle at any position in the optical axis direction, it is completely unaffected by deviations in the optical axis direction, and as a result, the focus error signal does not include an offset amount due to deviations in the optical axis direction. This eliminates the problem, which makes it possible to perform accurate signal detection.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram showing an embodiment of the present invention, Figs. 2 and 3 are explanatory diagrams describing the function of the prism, and Fig. 4 shows a configuration in which focus light-receiving elements are arranged before and after the focal point. FIG. 5 is an explanatory diagram showing the cross-sectional shape of the beam at the positions before and after the condensing point. FIG. 6 is a modification of the present invention, and shows the shape of the converging point when no aperture is used. An explanatory diagram showing the cross-sectional shape of the beam at the front and rear positions, Fig. 7 is a configuration diagram showing a conventional example, Fig. 8 is a perspective view showing the state of the signal detection optical system, and Fig. 9 shows an astigmatic point on the optical path. FIG. 3 is an explanatory diagram showing how the beam cross-sectional shape changes when aberrations occur. 9... Laser light source, 14... Objective lens, 15...
・Optical information recording medium, 16... Signal detection optical system, 18.
... Unidirectional angular displacement generating member, 19... Optical path separation member, 20... Condensing lens, 21... Focus light receiving element, 22... Focusing lens, 23... Focus light receiving element, 25 ...Focus error signal detection optical system, P
・・・Focusing point applicant Rico Co., Ltd. 33 Figure J Figure, % is Figure (A side) (B@) -A Figure 3 8 Figure

Claims (1)

[Claims] 1. Information is recorded and reproduced by condensing the light emitted from a laser light source with an objective lens and irradiating it onto an optical information recording medium, and the reflected light from the optical information recording medium is In an optical information recording and reproducing device that guides the optical information to the signal detection optical system and reproduces information and detects focus error signals, etc., the reflected light from the optical information recording medium is misfocused on the optical path guided to the signal detection optical system. A unidirectional angular displacement generating member is disposed to generate an angular displacement of the reflected light in only one direction, and the reflected light displaced in one direction by the unidirectional angular displacement generating member is passed through a condensing lens. 1. A focus position detection device comprising focus error signal detection means for detecting a difference in light amount between positions before and after the focal point. 2. The light emitted from the laser light source is focused by an objective lens and irradiated onto an optical information recording medium to record and reproduce information, and the reflected light from the optical information recording medium is sent to a signal detection optical system. In an optical information recording and reproducing device that reproduces guidance information and detects focus error signals, etc., the reflected light from the optical information recording medium is guided to the signal detection optical system on the optical path where the reflected light due to defocus is detected. A unidirectional angular displacement generating member that generates angular displacement in only one direction is disposed, an optical path separation member is disposed on the optical path of the reflected light displaced in one direction by the unidirectional angular displacement generating member, A condenser lens is disposed on each optical path divided into two by the optical path separation member, and a focus light receiving element is disposed on the optical path in front of the condensing point formed by one of the condensing lenses,
A focus position detection device characterized in that a focus light receiving element is disposed on an optical path behind a condensing point formed by the other condensing lens.