JPS61269241A - Focus detecting device for optical head - Google Patents

Abstract

PURPOSE:To obtain the titled device which is low in its cost and small in size by executing a focus detection by utilizing an interference of each diffracted beam from an optical recording medium. CONSTITUTION:The titled device has the first-the fourth photodetectors 1-4 for outputting signals S1-S4, and an AC coupling circuit AC-couples a signal (S1+S2+S3+S4), a signal (S1+S2)-(S3+S4) and a signal (S2+S4)-(S1+S3), respectively. A zero cross detecting circuit 21 detects a zero cross point of the signal (S1+S2+S3+S4). Sampling circuits 25, 32 sample the signal (S2+S4)-(S1+S3), based on a detecting output of the zero cross detecting circuit 21. Multiplying circuits 22, 23 and 26 multiply the signal (S2+S4)-(S1+S3) by the polarity of the signal (S1+S2)-(S3+S4) in its time point. In this way, a result of this multiplication becomes an out-of-focus quantity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a focus detection device used to adjust the focus of an optical head.

[Summary of the invention]

The present invention provides a focus detection device for an optical head that performs focus detection using interference between diffracted beams from an optical recording medium while the optical system of the optical head is excited, thereby making the device cost-effective. This device is compact and capable of performing stable focus detection, and is also capable of determining the amount of out-of-focus with a good S/N ratio.

(Prior Art) There are various types of focus detection devices for optical heads, but the applicant of the present invention proposed a diffraction device that has a grating in a direction perpendicular to the track direction. We have previously proposed a device that performs focus detection using interference between 0th-order and ±1st-order diffraction beams, which are regarded as a grating.

[Problem that the invention seeks to solve]

However, the above-mentioned device is, in principle, affected by movement of the diffraction grating, that is, tracking loss. In other words, the focus error signal obtained from the above device is determined by the fact that the beam spot traverses the recording track of the optical recording medium (
(hereinafter referred to as traverse), and the presence of a focus error can only be shown in a state where the beam spot is tracked by a land or group.

Figures 4C and 4D show these states, with the fourth diagram showing the focus error signal in a tracked state, and Figure 4C showing the focus error signal in a state modulated by traverse. shown respectively.

On the other hand, when tracking control is not performed, traverse occurs due to disturbances such as eccentricity and vibration of the optical disk. As a result, when tracking control is not performed, the focus error signal is constantly modulated, making it impossible to perform focus detection.

[Means for solving problems]

The focus detection device for an optical head according to the present invention is located on the opposite side of the objective lens element 2 from the optical recording medium 16 and in the far field of the recording track of the optical recording medium 16 in a direction perpendicular to both the recording track and the optical axis. A first photodetector 1 is arranged adjacent to each other at a position separated by 1/2q from the optical axis and outputs a signal S, and a first photodetector 1 is arranged at a position further away from the optical axis to output a signal S, and a position closer to the optical axis. a second photodetector 2 disposed at and outputting a signal S2;
The first and second photodetectors 1.2 are adjacent to each other at positions separated by 1/2 q in the opposite direction from the optical axis, and are arranged closer to the optical axis to receive the signal S3. a third photodetector 3 that outputs an output S4; a fourth photodetector 4 that is disposed at a position further away from the optical axis and outputs an output S4; and a beam 1 that is incident on the optical recording medium 16.
an excitation mechanism that excites the optical system of the optical head 11 so that the spot No. 4 crosses one or more of the recording tracks; a signal (Sl+32+33+34); a signal (Sl+Sz);
−(Ss+S+) and signal (SZ+S4) −(SI
+53), and an AC coupling circuit that AC-couples each of the AC-coupled signals (5+ + St + S3 + s
4. ) Zero-cross detection circuit 2 detects the zero-cross point of
1 and the AC-coupled signal (Sz 1 S4)
a sampling circuit 2 that samples (Sl + 53) based on the detection output of the zero cross detection circuit 21;
5.32 and the signal sampled by this sampling circuit 25.32 (5g + so) (Sl
+33) to the signal at that time (SI+52)
Multiplication circuit 22.23 that multiplies the polarity of (Sz+Sn)
．． 26, respectively, and the result of this multiplication is used as the amount of out-of-focus.

(However, q is the standardized pitch of the recording track expressed as NA-Q/λ when the radius of the aperture of the objective lens 12 is normalized to 1, where NA is the radius of the aperture of the objective lens 12. (Numerical aperture, Q is the actual pitch of the recording track, and λ is the wavelength of the beam 14.) [Operation] In the focus detection device for an optical head according to the present invention, the optical system of the optical head 11 is excited, This excitation causes the beam I
The spot No. 4 crosses the recording track of the optical recording medium 16, and the signal formed by the interference of the diffracted beams from the optical recording medium 16 is detected in synchronization with the above-mentioned crossing, and the amount of defocusing is determined by this detection. Desired.

〔Example〕

Hereinafter, an embodiment of the present invention applied to an optical head for an optical disc will be described with reference to FIGS. 1 to 4.

FIG. 1 shows the circuit system in this embodiment, FIG. 2 shows the photodetectors 1 to 4 in this embodiment and beam spots 5 to 7 on these photodetectors 1 to 4, and FIG. 3 shows the circuit system in this embodiment. Although the applied optical heads are shown, the optical head shown in FIG. 3 will be explained first.

Objective lens 1 used in the optical head 11 shown in FIG.
2 may have either a finite magnification or an infinite magnification. As shown in FIG. 3A, when the objective lens 12 has a finite magnification, the beam 14 emitted from the light source 13 such as a laser diode passes through the beam splinter 15 and the objective lens 12, and then reaches the optical disk 16. The image is formed on top.

The beam 14 reflected by the optical disk 16 passes through the objective lens 12 and the beam splitter 15 and enters the photodetector 17 having the photodetectors 1 to 4 described above.

Further, when the objective lens 12 has infinite magnification, the optical head 11 has a collimator lens 18 between the light source 13 and the beam splinter 15, as shown in FIG. 3B.

That is, the photodetector 17 may receive a convergent beam as shown in FIG. 3A, or a parallel beam as shown in FIG. 3B. Further, as shown clearly in FIG. 3A, the photodetector 17 may be placed at a position other than the conjugate point in the far field.

Then, either the objective lens 12, the tracking mirror (not shown), or the entire optical system 11 is excited by an exciting mechanism (not shown). For this purpose, one or more recording tracks (not shown) of the optical disc I6 are provided.
The spot of the beam 14 traverses, and a traverse occurs.

Then, when the output signals of photodetectors 1 to 4 are set to 31 to S4, respectively, the signal (S+ +32+
33+34), (SI+Sri (S3+54) and (S2+54) - (Sl +53)) are obtained.

By the way, when the recording tracks of the optical disk 16 are configured by DC groups or focuses, the optical disk 16 can be treated as a diffraction grating having gratings in the radial direction and equally spaced.

For this reason, beam spots 5 to 7 are formed on the photodetector 17 by the 0th-order and ±1st-order diffracted beams as shown in FIG.

The X-axis corresponding to the direction along the recording track and the Y-axis corresponding to the radial direction of the optical disk 16 constitute an x-y coordinate, and the radius of the aperture of the objective lens 12 is normalized to 1 to form the beam spot 5. When the center of is located at the origin of the x-y coordinate, the centers of the beam spots 6 and 7 are located at points ±l/q on the y-axis, respectively.

However, q is the radius of the aperture of the objective lens 12 as described above.
is the standardized pitch of the recording track expressed as NA-Q/λ, where NA is the numerical aperture of the objective lens 12, Q is the actual pitch of the recording track,
λ is the wavelength of beam 14.

In other words, beam spots 5 and 6, and beam spots 5 and 7 are
They overlap with y=±1/2q as the center line. As a result, the 0th-order diffracted beam and the +1st-order diffracted beam interfere with each other, and the 0th-order diffracted beam and the 11th-order diffracted beam interfere with each other, forming interference fringes.

On the other hand, if the beam 14 incident on the optical disc 16 is out of focus from the optical disc 16, the optical disc 16
Distortion occurs in the wavefront of the beam 14 reflected by the beam. Therefore, the interference fringes between the diffracted beams move according to the amount of defocus of the beam 14 incident on the optical disk 16.

As a result, as shown in FIG.
= 1/2q, and photodetectors 3 and 4
If they are placed adjacent to each other at the position y = -1/2 q, the already mentioned signals (S, +s, +s, +s4), (S++Sz
) (S3+St) and (S2+S4) −(s+
+ss) are approximately shown as follows.

λq ・−〜−−−〜・・・−−−−−・−・・■ However, d is the width of photodetectors 1 to 4 in the y-axis direction, and φ is the width of the 0th-order diffracted beam and the ±1st-order diffracted beam. The phase difference of each spectrum with the diffracted beam, ■ is the amount of tracking deviation, -2° is the wavefront aberration coefficient of defocus, A and B are the respective spectra of the 0th-order diffraction beam and the ±1st-order diffraction beam, and Incoming beam 1
This is a constant determined by the intensity distribution of 4. Therefore 0~0
In the equation, the term with ■ represents the modulation due to traverse, Hz. The term with represents the amount of defocus.

The signal represented by the formula 0 to 0 is input to an AC coupling circuit (not shown) in order to remove the DC component. Figure 4 A-
C shows the relationship between the signal of the above-mentioned 0 to 0 formula, which is subjected to traverse modulation and AC coupled while changing the focus state, and time. These signals are then input to terminals A to C of the circuit system shown in FIG. 1, respectively.

Assuming that the spot of the beam 14 is located at the boundary between the group and the land of the optical disk 16 at time L1, the signal shown in FIG. 4A is generated at t=1. to zero cross. Therefore, the zero-cross detection circuit 21 sends a signal to the gate circuit 22 in response to this zero-cross.

Further, a signal corresponding to the polarity of the signal shown in FIG. 4B is also sent from the comparison circuit 23 to the gate circuit 22. 1=1
．． Now, since the signal shown in FIG. 4B is negative, the signal is sent from the pulse shaping circuit 24 to the sampling circuit 25.

As a result, 1=1. Then, the positive signal shown in FIG. 4C is inverted by the NOT circuit 26 and then sampled. Therefore, the signal taken out from the terminal through the hold circuit 27 and the low-pass circuit 28 is negative.

Next, let t2 be the time when the spot of the beam 14 comes to the boundary adjacent to the boundary between the group and the land where it was located at time t. At this time L=t2, the signal shown in FIG. 4A also crosses zero, but the signal shown in FIG. 4B is positive at that time. For this purpose, a signal is now sent from the pulse shaping circuit 31 to the sampling circuit 32.

As a result, 1=1. Then, the negative signal shown in FIG. 4C is sampled as is. Therefore, the signal taken out from the terminal also remains negative.

In this way, even though the optical system is excited, a signal with continuous polarity is obtained from the terminal as shown in the fourth diagram. However, this signal is nothing but a focus error signal indicating the amount of defocus.

On the other hand, one of the signals shown in FIG. 4A or B is inputted from terminal B'. When the signal shown in FIG. 4B is input, at point E, which is the output side of the full-wave rectifier circuit 33,
A signal shown in FIG. E is obtained, and at point F, which is the output side of the envelope detection circuit 34, a signal shown by the curve in FIG. 4F is obtained.

Furthermore, since the comparator circuit 35 is provided with a dark value shown by the straight line in FIG. 4F, the signal shown in FIG. 4G is obtained at point G, which is the output side of the comparator circuit 35. Furthermore, a zero-crossing detection circuit 36 detects a zero-crossing point of the focus error signal. Therefore, a switch signal for performing focus control is obtained from the terminal H on the output side of the AND circuit 37.

As described above, in this embodiment, the optical system of the optical head 11 is forcibly excited, and the amount of defocus is determined from the signal obtained in this excited state. Therefore, even when tracking control is not being performed, focus detection can be performed without being affected by eccentricity of the optical disk 16 or disturbances.

Note that traverse also occurs due to eccentricity of the optical disk 16, disturbance, etc., but this traverse has a low frequency.

Therefore, unless excitation is performed as in this embodiment, a tracking error signal with a smooth waveform cannot be obtained.

By the way, in the above embodiment, each of the photodetectors 1 to 4 is made up of a single photodetection element, but each of the photodetectors 1 to 4 is made up of a plurality of photodetection elements arranged in the X-axis direction. may have been done.

Further, in the above embodiment, the present invention is applied to the optical head 11 for the optical disc 16, but the present invention is applied to the optical head 11 for the optical disc 16.
The present invention can also be applied to optical heads for optical recording media other than No. 6.

〔Effect of the invention〕

The focus detection device for an optical head according to the present invention performs focus detection using interference between diffracted beams from an optical recording medium, so it is low cost and compact.

Furthermore, since the amount of defocus is determined from the signal obtained in the excitation state, stable focus detection that allows focus control can be performed even in a state where tracking control is not performed.

In addition, since the detection is performed in synchronization with the beam spot crossing the recording track of the optical recording medium,
It is possible to determine the amount of defocus with a good S/N ratio.

[Brief explanation of the drawing]

Figures 1 to 4 show one embodiment of the present invention.
The figure is a block diagram of the circuit system, Figure 2 is a front view of the photodetector,
FIG. 3 is a side view of the optical head, and FIG. 4 is a graph of signals at each point in the circuit system shown in FIG. 1. In addition, in the symbols used in the drawings, L2, 3.4...- Photodetector It---------- One optical head 12----------- ---・--・-・Objective lens 14
−−−〜−・−・−・−・−Beam 16・−・−−−
−・・・・・・～・−Optical disc 21−−m−−・
...−・−・−・−Zero cross detection times IY! 22・-
−1−−−−・−−−−−・・・・−Gate circuit 23−
・−−−−〜・−・−・−・・−・・−Comparison circuit 25・
−・−−−−−・−−−m−−・−Sampling circuit 2
6--------------------NOT circuit 32.---111--This is a sampling circuit.

Claims (1)

[Claims] On the opposite side of the objective lens from the optical recording medium and in the far field of the recording track of this optical recording medium, in a direction perpendicular to both this recording track and the optical axis, by 1/2 q from this optical axis. A first photodetector is arranged adjacent to each other at a distance and is further away from the optical axis and outputs a signal S_1, and a first photodetector is arranged closer to the optical axis and outputs a signal S_2. 2 photodetectors are adjacent to each other at a position spaced apart by 1/2q from the optical axis in a direction opposite to the first and second photodetectors, and are arranged at a position closer to the optical axis. a third photodetector that outputs a signal S_3 and a fourth photodetector disposed at a position further away from the optical axis and outputs an output S_4; and a spot of the beam incident on the optical recording medium is one. an excitation mechanism that excites the optical system of the optical head so as to traverse the recording tracks of more than one record;
1+S_2)-(S_3+S_4) and signal (S_2+
an AC coupling circuit for AC coupling the husband and wife of S_4)-(S_1+S_3);
A zero-crossing detection circuit detects the zero-crossing point of
a sampling circuit that samples the signal (1+S_3) based on the detection output of the zero-cross detection circuit;
A focus detection system for an optical head, comprising a multiplication circuit that multiplies S_2+S_4)-(S_1+S_3) by the polarity of the signal (S_1+S_2)-(S_3+S_4) at that time, and the multiplication result is used as the amount of out-of-focus. Device. (However, q is the standardized pitch of the recording track expressed as NA・Q/λ when the radius of the aperture of the objective lens is normalized to 1, where NA is the aperture of the objective lens. number, Q is the actual pitch of the recording track, λ
is the wavelength of the beam. )