JPS6038736A - Optical head - Google Patents

Abstract

PURPOSE:To perform the tracking compensation by using photodetectors to detect the changes of an optical distribution in the direction orthogonal to the signal tracks of the 1st and 2nd luminous fluxes split by a splitter and at the same time to eliminate the effect of a tracking action on the automatic focus control. CONSTITUTION:The luminous flux reflected by a recording medium 16 goes back along its going path to transmit through a polarized beam splitter 13 and is divided into two parts by a splitter 22 to enter 2-split photodetectors 23 and 24 respectively. The splitter 22 consists of a reflecting part 19 and transmitting parts 18 and 18' of luminous flux. The optical distribution on the detector 23 is divided into upper and lower parts centering on a dividing line 21 which is coincident with a track direction T-T'. The optical distribution on the detector 24 has a capsule section form and divided by the line 21. The focusing state is detected from the difference (A+B)-(C+D) between output values of both detectors 23 and 24. Then a tracking signal is detected from the difference (A+C)-(B+D).

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to the structure of an optical head, and in particular, the present invention relates to the structure of an optical head, and in particular, the present invention relates to the structure of an optical head. The present invention relates to the structure of an optical head that focuses a spot.

[Prior art]

Specific examples of the recording medium include those on which video or audio signals are recorded in advance, such as video discs and digital audio discs, or DRAW discs that have a recording material layer that changes with light or thermal energy caused by light. recording media and magneto-optical recording media.

The former is a recording medium that only performs signal reproduction, whereas the latter is a recording medium that allows the user to perform both signal reproduction and signal recording. Further, the magneto-optical recording medium has the characteristics that signals can be erased and can be repeatedly recorded and reproduced.

When recording and reproducing signals by rotating (tracking) the recording medium and focusing a minute light spot on the recording medium, the minute spot is focused due to surface runout (vertical displacement) and lateral displacement of the recording body. The relative position between the optical head and the recording medium changes. As a result, the size of the spot on the recording medium fluctuates, or the irradiation position of the spot may shift from the signal track, making the extracted signal and the recorded pattern unclear.

To solve this problem, a photodetector (photoelectric conversion element) receives the reflected light from the recording medium, and detects changes in the relative position between the optical head and the recording medium based on changes in the amount or shape of the light beam. However, this is fed back as a focus error signal and a tracking error signal to the optical head or some of the components of the optical head to correct positional deviations caused by shake.

A conventional optical head having this type of correction means will be described below with reference to FIGS. 1 to 3.

In FIG. 1, 1 is a recording medium, 2 is a condensing lens,
This condensing lens is movable in the optical axis direction by a driving means 3. Reference numeral 4 denotes a rotating mirror for deflecting multiple beams of light. 5 is a quarter wavelength plate, 6 is a collimator lens,
Reference numeral 7 designates a polarized beam 2-noritter, 18 a light source such as a semiconductor laser, 9 a cylindrical lens, and 10 a four-split photodetector.

The light beam emitted from the light source 8 is polarized by the polarizing beam sinter 7.
After passing through, it becomes a parallel beam of light. Further, the polarization plane of the light beam emitted from the light source 8 is adjusted to be parallel to the plane of the paper in FIG. 1, and the polarization beam sinter 7 transmits the light beam having such a polarization plane with almost no loss. The parallel light flux becomes a circularly or elliptically polarized light flux by passing through the quarter-wave plate 5. This luminous flux is a beam that can be rotated around its axis. After being reflected by a turning mirror (rotating mirror) 4, the light is focused by a condensing lens 2 onto a minute spot on the recording medium 1.

On the other hand, the reflected light beam from the recording medium 1 reaches the /4 wavelength plate 5 via the condenser lens 2 and the focusing mirror 4. The light flux that has passed through the quarter-wave plate 5 has its polarization plane perpendicular to the direction of incidence, and most of the light flux is reflected by the polarizing beam splitter 7 and passes through the cylindrical lens 9 to be divided into four parts for detection. incident on the vessel lO.

The reflected light forms a light beam having an astigmatism distribution shape on the photodetector 10 by an astigmatism generating optical system including a collimator lens 6 and a cylindrical lens 9. Therefore,
The focus state of the spot on the recording medium 1 can be detected based on the distribution state.

Each element of the 4-split photodetector 10 is labeled A, B, and C.

If D, the focus of the spot is
The light distribution state on the photodetector depending on the state is shown in FIG. 2 (4).

(B) and (C). Here, in Figure 2 (4
) shows the front focus state, FIG. 2(B) shows the focused state, and FIG. 2(C) shows the back focus state.

Therefore, from the light amount of each element of the 4-split photodetector, (
Detecting the output of A+C') -(B+D) and determining whether the output value is negative, zero, or -positive to determine whether it is in the front bin state 2 in-focus state or in the rear focus state. By feeding this signal to the driving device 3 through a processing system having an appropriate gain, automatic focus control (autofocus) becomes possible.

Further, automatic tracking control (auto tracking) for correctly tracking the signal track can be performed by the following method.

That is, as shown in FIG. 2 Q)), the four-split photodetector 10
The dividing line is arranged along the track direction T-T' indicated by a broken line.

When a minute spot deviates from the signal track due to the eccentricity of the recording body 1, which is a rotating body, the intensity distribution of the light beam becomes biased, as shown in FIG. ) -(B+C') Track deviation can be detected as a change in the output value. This track deviation signal is similar to the above-mentioned automatic focus control,
After electrical processing, it is fed back to the drive system 3 of the bideting mirror 4, thereby enabling automatic tracking control (auto tracking).

In the conventional autofocus and autotracking explained above, when the focal length of the condensing lens 2 is f, the turning mirror 4 is rotated by θ degrees as shown in FIG. 3 (4). The spot on the recording medium 1 can be moved laterally by 2fθ. However, the reflected light beam is reflected by the condenser lens 2. When the light reaches the 4-split photodetector 10 via the pivoting mirror 4 and the cylindrical lens 9, the optical axis is shifted due to the lateral movement of the light spot, and the photodetector 10
The light distribution above moves from the dotted line circle to the broken line circle as shown in FIG. 3(B).

At this time, the change in the output value of (A10) - (B + C) is a minute amount if the light distribution is symmetrical to the origin) and if it moves along the dividing line, but in reality Since the optical system uses the cylindrical lens 9, the movement and shape of the light distribution are not ideal. Therefore, the movement of the light flux caused by the above-mentioned tracking operation must be suppressed to such an extent that it does not affect the focus signal, resulting in a problem that the tracking control range is limited.

That is, the conventional optical head described above has the disadvantage that when the bideting mirror 4 is shaken for tracking, the focus signal is also affected, making accurate automatic focus control difficult; Split photodetector 10 and 7
The positioning of the lindrical lens 9 in the generatrix direction must be performed strictly, which has the disadvantage that it takes a lot of time to assemble the optical head and increases manufacturing costs.

As a conventional method to eliminate such drawbacks, for example, a focus signal is obtained using the Foucault method (Naifezi method), and a photodetector is arranged conjugately with the light source (Fig. 3 (A)).
A method has been proposed to solve the problem of light flux deviation during tracking (location 11 in the center), but focus detection using this Foucault method involves a loss of light quantity because the light flux is blocked by a knife in the optical path. It has the disadvantage of becoming.

〔the purpose〕

An object of the present invention is to eliminate the drawbacks of the conventional optical head described above, to eliminate the influence of tracking operation on autofocus (automatic focus control), and to reduce manufacturing costs by simplifying the optical system. The object of the present invention is to provide a new optical head that is highly effective.

[Explanation of Examples]

The present invention will be described in detail below with reference to FIGS. 4 to 9.

As shown in FIG. 4 (4), the light beam emitted by the light source 12 such as a semiconductor laser is reflected by the polarizing beam splitter 13, passes through the 174 wavelength plate 14, and then passes through the condenser lens 15.
The light is focused onto the recording medium 16 as a minute spot.

The light beam that has been reflected multiple times by the recording medium 16 is returned to the condensing lens r.
15, 1/4 wavelength plate 14 and polarizing beam snoritter
13 and is split into two by the light splitter 22, and then reaches the respective two-split photodetectors 23 and 24.

Polarization plane of luminous flux and polarization beam slitters - 13 and 1/4
The immediate relationship with the wave plate 4 is the same as in the conventional optical head shown in FIG.

The light splitter 22, as shown in FIG. 4(B), consists of a light beam reflecting section 19 and transmitting sections 18 and 18'.

Reference numeral 20 in FIG. 4(B) indicates the distribution of light flux on the light splitter 22.

Note that the reflecting section 19 is not limited to a total reflection mirror, and the reflecting section 19 and the transmitting section 18, 18' can be interchanged, and as will be understood from the following explanation, they can be interchanged. Similar effects can also be obtained by doing this. Furthermore, the symbol (arrow) in Fig. 4(B)
T-T' indicates the direction of the signal track on the recording medium 16.

The relationship between the luminous flux distribution on the two-split photodetectors 23 and 24, the dividing line, and the signal track direction is shown in FIG. 4(C).

).

That is, the light distribution on one of the two-split detectors 23 is as shown in FIG.
C) As shown in K, the transparent part 18°18' of the light splitter 22
Because it has passed through the area, it has an arcuate partial circular shape with upper and lower parts. As shown in the dividing line 21 or in the figure, the light receiving elements A and BKt are arranged so that the light distribution is aligned with the light receiving elements A and BKt, and in a direction that coincides with the direction of the signal track TT'.

The light distribution on the other two-split detector 24 has a capsule cross-sectional shape because it is the reflected light from the reflection section 19 of the light splitter 220, as shown in FIG. The relationship in direction T-T' is the same as in the case of FIG. 4(C) described above.

When the surface of the recording medium 16 and the condensing lens 15 are in focus, the light beam emitted by the light source 12 passes through the recording medium 16 and is divided into two beams for detection, as shown by solid lines in FIG. 4(A). Vessel 2
3. Reach 24.

In this focused state, the light distribution on the light splitter 22 is
As shown in FIG.
The value of A+B)-(C+D) can be calculated and the output value can be used as the signal level at the time of focusing.

If the recording medium 16 is shaken or the like, the symbol 16 in FIG. 4 (4)
When shifted to the position ', the reflected light flux remains in the state shown by the broken line, and the sight to the seven detectors 23 decreases, while the amount of light to the other detector 24 increases. Therefore, (A
The output value of +B)-(C+D) is decreased. On the other hand, recording body 16
is displaced in the direction opposite to the position indicated by the reference numeral 16', the output value of (A+B)-(C+D) increases. In this way, a focus error signal is obtained, and the focus state can be detected.

Next, regarding the operation of detecting the tracking signal, that is, the signal for correcting the deviation of the spot from the signal track by the optical head, as explained in FIGS. This will be explained with reference to the figures.

In each of FIGS. 5(4), (B) and (C), the two-split photodetectors 23 and 24 from the top, the light intensity distribution on these detectors, and the relative position between the light spot and the signal track on the recording medium The relationships are shown.

That is, Fig. 5 (4) shows the case where the spot rides on the signal track, Fig. 5 (B) shows the case where the spot shifts to the left, and Fig. 5 (C) shows the case where the spot shifts to the right. Depending on each case, the light intensity distribution will be as shown by the light intensity distribution on the middle detector in FIG.

Therefore, as is clear from the state shown in FIG. 5, the light receiving elements A of each of the two-split photodetectors 23 and 24.

Obtained according to the amount of light received at B, C', and D (A+C
) - (B0D) is the tracking signal, and the tracking error () is detected by the change in this value.A similar detection method for the above-mentioned focus signal is described in Japanese Patent Publication No. 53-43302. This method detects a focused state when the difference in the amount of light directed to two photodetectors is zero, and detects a focus error when this is not the case. Therefore, in the known technology, since the amount of light is divided equally between the two photodetectors, the tolerance for the area ratio of the reflective part and the non-reflective part of the light splitter is small, and therefore manufacturing is difficult. be. Another drawback is that the positional accuracy of the light splitter is extremely strict. Furthermore, this known technique does not perform any tracking signal detection, but merely detects the tracking signal.
Since the present invention only detects a waste signal, it is extremely insufficient as an optical head for an optical information recording medium compared to the embodiments described above.

According to the embodiment of the glue lS of the present invention explained above with reference to Figs. Since the direction is directly parallel to the tonk direction, the following effects can be obtained.

That is, when tracking is performed by moving the condensing lens 15 in a direction perpendicular to the track based on the tracking signal, a movement of the light beam occurs on the light splitter 22, but as shown in FIG. Even if you move to
The direction of the stripes forming the reflective surface 19 (direction perpendicular to the signal track direction T-T'), the signal track direction T
-T', the amount of light divided into each photodetector 23, 24 is changed by setting the position of the condenser lens 15 in the correcting direction (parallel to the optical axis) in the relationship as in the above embodiment. can be prevented.

Further, even if a light beam is generated on each photodetector 23, 24, as long as the light beam does not jump out of the light receiving surface, no effect will be caused on the focus signal. This provides an optical head structure that can accurately and easily perform both autofocus and automatic signal track following control (autotracking).

FIG. 7(4) to FIG. 7(C) are views showing modified embodiments of the optical head according to the present invention.

In this embodiment, polarizing beam splitters 31, 1
The /4 wavelength plate 32 and the light splitter (light splitting element) 35 are integrated and have a structure that is extremely advantageous for downsizing the optical head.

That is, the light beam from the light source 30 is transmitted to the polarizing beam splitter 3.
1, and after passing through a quarter-wave plate 32, it is focused by a ray lens 33 onto a recording medium 34 as a minute spot. The reflected light flux from the recording medium passes through the condensing lens 33 and the quarter-wave plate 32 again, and then passes through the polarizing beam splitter 3.
Transmit 1. A light splitting element 35 such as a prism is provided on the output side end face of the polarizing beam splitter 31, and a part of the light beam changes its traveling direction, and the remaining light beam is in the original traveling direction. The light is incident on one photodetector (four-division photodetector) 36 formed integrally.

The 4-split photodetector 36 has 4 parts as shown in FIG. 7C).
It is composed of light receiving surfaces (light receiving elements) A, B, C, and D, and is divided into four parts by a vertical dividing line along the track direction T-T' and a horizontal dividing line as shown in the figure. Light splitter 35
has a transmitting part and a reflecting part similar to those in FIG. 4(B), so the relative positional relationship of the distribution of light flux on the photodetector 36 (hatched part) to each dividing line is as shown in the figure. be.

According to the embodiment shown in FIG. 7, in addition to obtaining the same effects as in the embodiments shown in FIGS. are integrated as shown in Fig. 7 (B), and as shown in Fig. 4 (4).
) Since the photodetectors 36 corresponding to the two 2-split photodetectors 23 and 24 in 414 are integrated into a 4-split photodetector, it is possible to significantly reduce the size of the optical head. .

An example of an electrical processing system for obtaining the information signal, autofocus signal, and autotracking signal in each of the embodiments described above will be described with reference to FIG. 8.

In FIG. 8, a multi-information signal, an autofocus signal, and an autotracking signal are obtained by processing the electrical signals obtained from the husbands of the light receiving elements A, B, C, and D as follows.

That is, the information signal S is obtained as a high frequency component by amplifying the (A+B) and (C+D) signals by the summing amplifier 40 and passing through the frequency separator 41.

The autofocus signal AF is (A+B) and (C+D
) by the differential amplifier 42 and the low frequency component signal passed through the frequency separator 41.

Auto tracking signal A'lf:, (A+D) and (
B + C) signals and the difference between them is taken by the differential amplifier 44)
, is obtained as a signal divided by the signal of the low frequency signal component that has passed through the frequency separator 41 in the divider 45.

In the electrical processing system for obtaining each of the above-mentioned signals, most of the noise components due to variations in the light amount of the light source or uneven reflection of the recording medium are included in the low frequency components that pass through the frequency separator 41.

Therefore, this noise component included in the autofocus signal AF and autotracking signal ATK can be removed by performing division by the low frequency component in the respective dividers 43 and 45.

In addition, as a method of processing without performing division in each of the dividers 43 and 45, in the case of autofocus, the light is divided into equal amounts of light, and in the case of autotracking, the method shown in FIG. 7 (
C) It is possible to make the light distribution at K symmetrical with respect to the vertical dividing line. However, such a processing method requires high and strict requirements for the dimensional accuracy or positional accuracy of each element, so it cannot be said to be a preferable processing method.

Furthermore, the tracking signal AT is (A-B) or (D-C
) can also be obtained.

The polarizing beam series shown in FIG. 7 (B)
In the integrated structure of a 11/4 wavelength plate and a light splitter, the integrated part of the polarizing beam splitter and the light splitter is:
It can also be implemented using various polarization structures as shown in FIGS. 9(A) to 9(2).

That is, in the case of FIG. 9(A), the polarizing beam splitter
A reflecting mirror 51 is provided on a part of one surface 52 of the mirror 50, and light is divided by this reflecting mirror 51.

FIG. 9(B) shows one surface 52 of the polarizing beam slitter 50.
A blazed diffraction grating 53 is provided to perform light splitting using the diffraction effect.

FIG. 9(C) shows one surface 52 of the polarizing beam sinter 50.
A holographic open element 54 is provided to perform light division. In this case, a fine-pitch diffraction grating 55 is provided on the end face of the light splitter 54 to make the diffraction angle of the diffracted light sufficiently large, thereby causing total reflection at the interface between the splitter 54 and the air. The structure can be such that the diffracted light is emitted from the end face.

FIG. 9 0)) is one side 52 of the polarized vinyl solitter 50.
A light condensing element 56 such as a half lens or a half Fresnel lens is provided in the lens, and the light is split by this element.

Even by employing the structure illustrated in FIGS. 9(A) to 9(D) above, substantially the same effects and effects as in the embodiments shown in FIGS. 7(A) to 7(C) can be obtained. can be achieved.

In the optical system according to the embodiment of the present invention shown in FIG. 7(5), a collimator lens is provided between the light source 30 and the polarizing beam sinter 31 in order to convert the diverging light from the light source 30 into parallel light. A structure in which another condensing lens is provided between the polarizing beam slittor 31 and the photodetector 36;
Alternatively, a structure in which a collimator lens is provided between the 1/4 wavelength plate 32 and the condenser lens 33 can be adopted, and the same effect as in the case of FIG. 7 can also be achieved with these structures.

According to each of the embodiments described above with reference to FIGS. 4 to 9, an optical head with an extremely simple structure that can easily and accurately perform both autofocus and autotracking can be obtained.

In addition, compared to conventional optical heads, there is no need to use the zero method (a method in which the servo target value is set to a state where the difference in light intensity is zero) for the hope (automatic control) system, and the cylindrical lens can be omitted. The positional accuracy and dimensional accuracy of each element can be relaxed, and manufacturing costs can be reduced.

In addition, a light splitter (light splitter 22135, 5.1°53.5
Since the structure of 4.56) is made into a striped shape as shown by the reflecting portion 19 in FIG. 4(B), the influence of the tracking operation on the focus signal can be eliminated.

Furthermore, in each of the embodiments shown in FIGS. 7 and 9, a light splitter (light splitting element) is integrally provided on one end face of the light beam sinter, so the optical system can be housed in a small space. It is possible to achieve miniaturization of the optical head.

〔effect〕

As is clear from the above description, according to the present invention, it is possible to obtain an optical head that has a simple structure and can easily perform both automatic focus control (autofocus) and automatic tracking control (autotracking), and can actually rotate accurately. .

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram illustrating the optical system of a conventional optical head, Fig. 2 is an explanatory diagram showing a four-split photodetector and various states of light distribution, and Fig. 3 is an explanatory diagram illustrating the optical system of a conventional optical head. FIG. 4 is an explanatory diagram showing an embodiment of the optical head of the present invention. FIG. 5 is an explanatory diagram showing the state of light distribution due to tracking deviation in the photodetector of FIG. 4. FIG. is an explanatory diagram illustrating the movement of the light beam in the light splitter of FIG. 4,
FIG. 7 is an explanatory diagram showing another embodiment of the optical head of the present invention;
FIG. 8 is a block diagram showing an example of an electrical processing system for obtaining each injected signal using the optical head of the present invention, and FIG. 9 shows various modified structures of the optical splitter used in the optical head of FIG. It is an explanatory diagram to illustrate. 16.34... Recording body, 15.33... Condensing lens, 22.35, 51, 53, 54.56... Light splitter, 23.24, 36... Photodetector, A ,B,C,D・
... Light receiving element of photodetector, T - T' ... Signal track direction, S ... Information signal, AF ... Autofocus signal, A.T. ... Auto tracking signal. Figure 1 Figure 2 (A) CB) (C) Figure 3 (B) Figure 4 Time (B) (C) (D) Figure 5 Figure 6 ↑ Figure 7 ↓ and Figure 8

Claims (1)

[Claims] (1) A recording medium is irradiated with light focused by a condensing lens, and a beam reflected by the recording medium is split into a first beam and a second beam by a light splitter, and the first beam and The second light beam is received by a photodetector, and the distance between the recording medium and the condensing lens is corrected based on the difference between the electric signals, and the light beam in the direction orthogonal to the signal track of the first light beam and the second light beam is generated. An optical head characterized in that a change in distribution is detected by the photodetector and tracking correction is performed using the electrical signal.