JPH0312035A - Method for measuring phase difference of respective detection signals based on reflected light form one pair of sub-spots of optical pickup - Google Patents

Abstract

PURPOSE:To eliminate the influence of the phase change of sub-beams by utilizing the fact that the sub-beam E, F signals are reverse in polarity and deviate at a specified angle from each other and finding the average value of the DC output based on the phase difference between the respective E, F signals and the main beam M signal. CONSTITUTION:The output signal of an integrator 17 based on the phase difference between the electric signal M obtd. from the reflected light from the main spot and the electric signal E obtd. from the reflected light from one of a pair of the sub-spots and the output signal from an integrator 21 based on the phase difference between the signal M and the signal F obtd. from the reflected light from the other of the sub-spot are added 22 and the average output value (i) is obtd. in an average circuit 23. The sub-beam E and F signals are reverse in polarities and deviate by 180 deg. from each other and, therefore, the average output value i can eliminate the influence of the phase change of the sub-beams. The phase difference between the E signal and the F signal is, therefore, measured in the form of having no errors even if the position of the main spot deviates to the inner side of the sub-spot.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention provides detection based on reflected light from a sub-spot for tracking error detection in an optical pickup mounted on an optical disc player such as a DC (compact disc) player. It is related to the method of measuring the phase difference of signals, and in particular, by using the detection output from a pair of sub-spots and the detection output from the main spot, it is possible to measure the error-free position of each detection signal based on each reflected light from a pair of sub-spots. Concerning a method for measuring phase difference.

[Prior Art] When performing a tracking servo to make an optical pickup follow a track on a recording surface of a CD in a CD player, it is necessary to detect a tracking error signal, and one method for this has conventionally been to detect a tracking error signal using a three-beam system. law has been adopted.

This three-beam method uses a pair of dedicated beams (sub-beams) separate from the main beam for reading information to detect tracking errors. That is, laser light from a semiconductor laser (not shown) is divided in advance into a main beam and two sub-beams by a diffraction grating, and is irradiated onto the recording surface of the CD in an arrangement as shown in Figures 4A-B. . In Figures 4A-B, the symbol T is the track where the bits are arranged, the center M of the three spots is the main spot used for signal readout and focusing, and E and F on both sides are used for tracking error detection. It is a sub-spot where

In addition, in FIG. 4B, SP indicates the distance of a straight line connecting the centers of each sub-spot E and F.

Also, TP is the track T between each sub-spot E and F.
In the example of FIG. 4B, TP is set to 0.8 μm, which is half of the track pitch of 1.6 μm.

The reflected return lights from these three spots M, E, and F are received by a photodetector of the optical pickup, and bit information and tracking errors are detected. In this photodetector, a total of six photodiodes A' to F' as shown in FIG. 6 are arranged in a pattern as shown in the figure.

The tracking error signal is differentially extracted via operational amplifier 1 by receiving the reflected return light from sub-spots E and F by two diodes E' and F' at both ends of the figure. . Note that the photodiodes A' to D' are for detecting bit information.

Further, in the example shown in FIGS. 4A-B, a pair of sub-spots E
and F are set so that a straight line connecting them makes a predetermined angle θa with respect to the direction of the track T. This predetermined angle θa is the angle at which the sub-spots E and F are arranged so that the tracking error level is maximized. Also, when this arrangement angle θa is satisfied, the sub-spot E
Signal (E signal) detected from the return light (sub beam) of
The phase difference between the signal (F signal) detected from the return light (sub beam) of sub spot F is 1 as shown in FIG.
It becomes 80°. Note that the E signal and F signal and the signal CM signal detected from the return light (main beam) from the main spot M have a phase difference of 90°.

Therefore, when assembling the optical pickup, in order to maximize the tracking error level, the phase difference between the E signal and the F signal becomes 180°, and the pair of sub-spots E and F
It is necessary to position and adjust the arrangement angle of the diffraction grating so that the arrangement angle of the diffraction grating becomes θa. To do this, it is necessary to measure the amount of deviation ΔEF from the angle θa to the actual arrangement angle of the sub-spot. The amount of deviation ΔEF in this case is ΔEF = 180@-(Δθ/θa) x 180° = 18
0@-(Δθ/ 5in-'(TP/SP))X
It is determined by 180°.

In addition, the phase difference between the E signal and the F signal (E
-F phase difference), 1 of that E-F phase difference
The conventional method for measuring the deviation amount ΔEF with respect to 80' is to input the E signal and F signal into a phase comparator (for example, an AND gate), integrate the output, and calculate the integrated output as the phase difference between the E signal and F signal. There is a method (EF phase measurement method) in which the actual E-F phase difference is compared with a reference value when the reference value is 180°.
For example, the integrated output is the same when it is larger than 80'' by 101 and when it is smaller by 10', and there is a drawback that it is impossible to determine whether the amount of phase difference is positive or negative.

Therefore, the present inventor studied a method for measuring the E-F phase difference that can determine whether the amount of deviation from the actual E-F phase difference of 180° is positive or negative. Although the following is not a publicly known technique, it is a technique studied by the present inventor, and its outline is as follows.

Main spot M and sub spot E (or F) are CD
an integral output based on a phase comparison of the M signal and the E (or F) signal generated when crossing the upper track T;
The integrated output based on the M signal is determined, and based on these, E
- Amount of deviation ΔEF for 180° phase difference between F signals
By determining (MS phase measurement method), it is possible to determine whether the above ΔEF is positive or negative with respect to 180°. That is,
According to this method, as shown in FIG. 7, the detection signal based on the return light (main beam) from the main spot M is
The signal is applied to a comparator 3 via a preamplifier 2, subjected to waveform shaping, and sent to an integrating circuit 4. Further, a detection signal based on the return light (sub beam) from sub spot E or F is given to comparator 6 via preamplifier 5 and waveform shaped. The output of the comparator 6 is then applied to the next stage gate (AND circuit) 7 together with the output of the comparator 3 for phase comparison. The output of this gate 7 indicates that both the comparators 3 and 6 are high.
Therefore, when the outputs of the comparators 3 and 6 are in the same phase, the gate output has the maximum width, and when the outputs are in opposite phases, the gate output has the minimum width. This gate output is sent to the next stage integrating circuit 8, and the integrated output i
is now available.

The deviation amount ΔEF of the actual phase difference between the E signal and the F signal with respect to 180° is calculated from the output d from the integrating circuit 4 and the integrated output i, as follows: ΔEF=180°−(i/d)×180@×2 It is expressed as As a result, the E-F phase difference (or E
It becomes possible to measure the deviation amount ΔEF) with respect to 180° of the −F phase difference.

[Problem to be solved by the invention]

However, in the above considered technology, the conventional EF
A problem that did not occur with the phase measurement method has arisen: an error occurs in the measured value of the E-F phase difference (or deviation amount ΔEF) due to the difference in phase characteristics between the main beam preamplifier and the sub-beam preamplifier. .

That is, since the light amount of the sub beam is smaller than that of the main beam, the gain of the sub beam preamplifier is set higher than that of the main beam preamplifier to correct this. Therefore, as shown in FIG. 7, the feedback resistance for the amplifier's feedback is as large as 820Ω, whereas the feedback resistance Ra of the preamplifier 2 is 22Ω. It has become.

Therefore, the frequency band of the sub-beam preamplifier is
The phase delay of the sub-beam due to the difference in phase characteristics between the two preamplifiers, which is narrower and has a longer delay time than that for the main beam, is the cause of the error. This will be explained with reference to the timing chart of FIG.

FIG. 8(a) shows the output of the comparator 3 of FIG. The phase of the output of the comparator 6 in FIG. 7 is determined by the difference in the phase characteristics of the two preamplifiers 2.5, as shown in FIG.
) is delayed by a timing of Δt compared to the ideal phase with no delay (shown in FIG. 8(b)). Therefore, the output from gate 7 in FIG. 7 is also different from the output without phase lag in FIG. 8(b) as shown in FIG.
Gate output with the output in figure (a) (shown in figure 8 (d))
The pulse width is thicker (has become) by Δt compared to .Dotted lines m and n shown in FIGS. 8(d) and (e) indicate the integrated output of each gate output. When two integral outputs m and n are compared, an error of ΔV occurs as shown in FIG. 8(f), which causes E-
An error will also occur in the measured value of the F phase difference.

Further, such errors, which do not pose a problem in the conventional EF phase measurement method but become a new problem in the MS phase measurement method, also occur due to positional deviation of the main spot with respect to the sub-spot. That is, the track T on the CD has a curvature of radius R as shown in FIG. This may cause an error.

That is, in Fig. 9, the angle θ in the figure from the distance SP/2 between spots M and E (or F) and the radius R is θ = 5 in-' ((SP/21/R) and the track T and Since tanθ = (SP/2)/(R-ΔX), the distance ΔX from the center of the main spot M is 8X"R-(SP/2)/tanθ. Also, Δθ in the figure is , tanΔθ=Δx/(SP/2), so Δθ=tan-'(Δx/(SP/21),
Then, converting this Δθ into the E-F phase difference ΔEF, ΔEF= (Δθ/5in-'(T P/S P)
) x 180°.

According to the above formula, errors as shown in the table below will occur depending on the conditions for R (mm 2 ) and SP (μm).

Error in EF phase difference The present invention is intended to solve the above-mentioned problems, and by using the detection output from a pair of sub-spots and the detection output from the main spot, it is possible to detect the reflection from a pair of sub-spots. It is an object of the present invention to provide a method for measuring the phase difference of each detection output based on light without error.

[Means for Solving the Problems 1] A method for measuring the phase difference of each detection signal based on reflected light from a pair of sub-spots in an optical pickup according to the present invention is to divide a laser beam from a light source into three beams using a diffraction grating. A main spot and a pair of sub-spots are formed on the recording surface of the optical disc by dividing, and information recorded on the track is read based on the light reflected from the main spot, and based on the light reflected from the pair of sub-spots. (a) DC output based on the phase difference between the electrical signal obtained from the reflected light from the main spot and the electrical signal obtained from the reflected light from one of the pair of sub-spots. and detecting a DC output based on a phase difference between an electrical signal obtained from the reflected light from the main spot and an electrical signal obtained from the reflected light from the other of the pair of sub-spots; Take the average value of the two DC outputs detected in step (a), and (c) obtain the above (a)! At the same time as or before and after each of steps 3 and (b), detect a DC output based on an electrical signal obtained from the reflected light from the main spot, and (d) detect the DC output obtained in step (b) above. Average value and above (c)
The method is characterized in that the phase difference of the electric signal based on the reflected light from the pair of sub-spots is extracted from the DC output obtained in the step.

[Function] The signals obtained from the pair of sub-spots are inputted with opposite polarities and shifted by 180° from each other.

Therefore, if the phase of the sub-beams changes due to factors such as the difference in the phase characteristics of the preamplifiers mentioned above or the positional shift of the main spot with respect to the bus spot, the phase difference between the E signal obtained from one sub-spot and the M signal obtained from the main spot will be The DC output based on the F signal and the M signal obtained from the other sub-spot decreases, but the DC output based on the phase difference between the F signal and the M signal obtained from the other sub-spot changes upward by the same proportion. Therefore, by using the two DC outputs, the phase change of the sub-beam due to the above factors can be canceled out, resulting in an error-free E.
It becomes possible to measure the phase difference between the signal and the F signal.

〔Example〕

The present invention will be described in detail below based on the drawings.

FIG. 1 is a circuit diagram showing a measuring device used in a method for measuring a phase difference between an E signal and an F signal according to an embodiment of the present invention;
FIG. 2 is a timing chart for explaining the operation of the measuring device shown in FIG. 1, and FIG. 3 is a graph showing the relationship between the meter output voltage and the phase difference of the E-F signal measured by this embodiment.

In FIG. 1, reference numeral 11 is a preamplifier into which the main beam detection signal from the main spot M is input, 12 is a comparator that shapes the waveform of the output from this preamplifier 11, and 13 is a comparator that integrates the output from this comparator 12. This is an integrating circuit that outputs a DC output d. Also code 14
15 is a comparator that shapes the waveform of the output from this preamplifier; 16 is the output of this comparator 15 and the comparator 12; 17 is an integrating circuit that integrates the output of this gate 16 (AND circuit) to which the output of gate 16 is input. Further, reference numeral I8 denotes a preamplifier into which an electric signal based on the sub-beam from the other sub-spot F of the pair of sub-spots is input, 19 a comparator for waveform shaping the output from this preamplifier 18, and 20 this comparator 19. and the output of the comparator 12
21 is an integrating circuit that integrates the output of this gate 20 (AND circuit). Further, reference numeral 22 is an adder circuit that adds the DC output from this integrating circuit 21 and the DC output from the integrating circuit 17, and reference numeral 23 is a dividing circuit that obtains 172 DC outputs i of the DC output of this adding circuit 22. .

Next, the operation will be explained with reference to the time chart shown in FIG.

(a) of FIG. 2 shows the output waveform of the comparator 12,
(b) and (C) show the output waveforms of comparators 15 and 19, respectively.

In (b) and (C), the broken line shows the waveform when there is no phase lag, and the solid line shows the waveform when phase lag occurs due to the above-mentioned factors. In the latter case, the same amount of phase delay occurs in each output of each comparator 15 and 19.

Next, FIG. 2(d) shows the gate output when the output of each comparator 12 and 15 passes through the next stage gate (-16), and FIG. 2(e) shows the gate output when the output of each comparator 12 and 19 passes through the next stage gate The gate output when passing through the gate 20 is shown. In addition, in (d) and (e), the waveforms indicated by broken lines indicate the gate outputs when there is no phase lag in the outputs from the comparators 15 and 19, and the waveforms indicated by solid lines indicate the gate outputs when there is a phase lag. There is. Also (d)
In and (e), waveforms EDC and FDC shown by dashed-dotted lines are DC outputs when the gate outputs shown by solid lines (when there is the above-mentioned phase delay) pass through integration circuits I7 and 2I.

As shown in FIGS. 2(d) and (e), due to the phase delay, the pulse width of the gate output on the subspot F side becomes narrow (and conversely, the pulse width of the gate output on the subspot F side becomes narrower). When these gate outputs are integrated separately, the integrated output E on the sub-spot F side becomes wider than when there is no phase delay (when there is no error).
DC is decreasing and the F side integral output FDC is increasing. Therefore, by finding the average value i (= (EDC+FDC)/2) of these two integral outputs EDC and FDC, an error-free integral output can be obtained. The amount of deviation ΔEF for 180° of (E-F phase difference) is: ΔEF ; 180° - (i/d) x 180°X2 = 18
0' - [((EDC+FDC)/2)/d] X 3
60° allows it to be determined without error.

In addition, FIG. 3 is a graph showing an example of actual measurement according to this embodiment.
The horizontal axis shows the phase difference between the E signal and the F signal (E-F phase difference), and the vertical axis shows the meter output voltage for phase display.
corresponds to the E-F phase difference of 180°.

As described above, according to this embodiment, each gate 16.
The E-F phase difference can be measured without error from the average value of each DC output obtained by integrating the 20 outputs and the integrated output of the main signal.

[Effect] As described above, according to the present invention, by utilizing the fact that the polarities of the E signal and the F signal are opposite polarities and deviate from each other by 180 degrees,
The influence of the phase change of the sub-beam (or main beam) is eliminated by finding the average value of the DC output based on the phase difference between the E signal and the M signal and the DC output based on the phase difference between the F signal and the M signal. That's what I do. Therefore, even if there is a phase delay of the sub-beam due to a difference in phase characteristics between the main beam preamplifier and the sub-beam preamplifier, or the main spot position shifts to the inside of the sub-spot, the phase difference between the E signal and the F signal can be corrected as an error. It becomes possible to measure without Therefore, the positioning adjustment of the diffraction grating can be performed in a short time and with high precision when assembling the optical pickup.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a measuring device used in a method for measuring a phase difference between an E signal and an F signal according to an embodiment of the present invention;
Fig. 2 is a timing chart for explaining the operation of the measuring device shown in Fig. 1, Fig. 3 is a graph showing the relationship between the meter output voltage and the E-F phase difference measured by this embodiment, and Fig. 4A-B The figure shows the tracks on the recording surface of a CD, the main spot, and a pair of sub-spots, Fig. 5 is a waveform diagram showing electrical signals based on the return light from the main spot and a pair of sub-spots, and Fig. 6 shows the optical signals. A diagram showing the photodiode pattern of the photodetector of the pickup, FIG. 7 is a circuit diagram showing a measuring device for measuring the phase difference of the E-F signal considered by the present inventor, and FIG. 8 is a diagram showing the photodiode pattern of the photodetector of the pickup. Timing chart for explaining the operation of the measuring device, No. 9
The figure is a diagram for explaining an error during E-F phase difference measurement that occurs because the position of the main spot shifts to the inside of the sub-spot. 11.14.18...Preamplifier, 12.15°19
...Comparator, 13.17.21... Integrating circuit, 16.20... Gate, 22... Adding circuit, 23
...Division circuit. X-He-Ishiya Chihikisen A Figure 4B Figure 5 Figure Ov Figure 8

Claims (1)

[Claims] 1. A laser beam from a light source is divided into three beams by a diffraction grating to form a main spot and a pair of sub-spots on the recording surface of an optical disk, and based on the reflected light from the main spot. In an optical pickup that reads information recorded on a track by using the main spot and detects a tracking error based on the reflected light from the pair of sub spots, (a) an electric signal obtained from the reflected light from the main spot and the pair of sub spots are detected; A DC output is detected based on the phase difference between the electrical signal obtained from the reflected light from one of the sub-spots, and the electrical signal obtained from the reflected light from the main spot and the reflected light from the other of the pair of sub-spots. Detect the DC output based on the phase difference with the electrical signal obtained from (B) Take the average value of the two DC outputs detected in the step (A) above, (C) Steps (A) and (B) above. At the same time or before and after each step, detect the DC output based on the electric signal obtained from the reflected light from the main spot, and (d) compare the average value obtained in the step (b) with the above (h). )
Each detection based on the reflected light from the pair of sub-spots in the optical pickup is characterized in that the phase difference of each electric signal based on the reflected light from the pair of sub-spots is extracted from the DC output obtained in the step of How to measure the phase difference of a signal