JPH01155223A - Method for measuring optical anisotropy of transparent disk - Google Patents

Abstract

PURPOSE:To continuously measure phase difference and an optical axis angle by optical anisotropy, by scanning a disk by circular polarization laser beam and receiving two sets of polarized wave components of transmitted beam through two detectors of required deflected direction angles. CONSTITUTION:A transparent disk 1 moving in the direction of a radius (r) while rotated by circular polarization laser beam due to a laser beam source 21, a half mirror 23, a lambda/4 plate 24 and a converging lens 25 and transmitted beam is divided into two by the half mirror 23' of an anisotropy measuring apparatus 4A. The optical axis angle due to optical anisotropy of the disk 1 is rotated by phi by detectors 5a, 5b having an deflection direction angle of pi/2 and the divided beams become deflected wave components right-angled to each other and having phase difference to be received and output voltages I1, I2 are generated from beam detectors 27a, 27b to continuously measure the phase difference and the optical axis phi on the basis of formula I, II (I is reference detection voltage in a case having no optical anisotropy when a detector having deflection direction angle pi/4 is used and is the angle formed with respect to an X-axis by one of two detectors). The same result is obtained even when reflected light is used.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for measuring optical anisotropy of a substrate of a transparent optical disk, and more particularly, to a method for measuring optical anisotropy of a substrate of a transparent optical disk, and more specifically, a method for measuring optical anisotropy of a substrate of a transparent optical disk. The present invention relates to a method of measuring the distribution of the resulting phase difference angle δ and the anisotropic optical axis angle φ.

[Prior Art] Optical disks or magneto-optical disks, which have recently been rapidly developing as information recording media, record information data at high density on a large number of concentric tracks set on the circumference of a disk substrate. be.

FIG. 3 is a schematic diagram of the configuration of an example of an optical system for reading information from an optical disc. The optical device 2 projects a laser beam onto the track 1a of the rotating optical disc 1, and receives a received signal of the reflected light. The signal processing circuit 3 processes the signal and outputs an information signal. The optical device 2 can be used directly as a light source! ! A polarized laser light source 21 is used, and the laser beam passes through a projection lens 22 and a half mirror 23, is converted into circularly polarized light by a λ/4 plate, 2 metal plates, and is focused by a focusing lens 25 and projected onto a disk l. Ru. Here, the substrate of the optical disk is made of transparent glass, and tracks and information are recorded on the lower surface (hereinafter simply referred to as the back surface) 1c of the disk in the figure. The laser beam passes through the upper surface (hereinafter simply referred to as the front surface) lb, and the reflected light whose light intensity has changed depending on the information on the back surface passes through the surface again, passes through the focusing lens, λ/4 plate, and half mirror, and is focused by the light receiving lens 26. The light is emitted and received by the light receiver 27.

Regarding the phase change of the polarization plane by the above-mentioned λ/4 plate,
It's well known, but I'll explain it. In Fig. 3, a linearly polarized laser beam (A) from a light source is divided into two polarized waves E x on the x and y axes that are perpendicular to each other and have the same phase, as shown in the appended complex coordinate vector diagram (A). =E y=E
0sinωt is synthesized and the direction is π/with respect to the X axis.
It vibrates as REQSinωt which forms 4. When this passes through the λ/4 plate, a phase difference of π/2 occurs in the image light wave, and the composite wave Ec becomes E as shown in the vector diagram (+7).
c=E □sin (ωt+πlO, resulting in a circularly polarized light wave that rotates in the θ direction with ωt.

Generally, when a transparent body has optical anisotropy, incident light is split into two polarized waves, and since the phase speed of the light differs depending on the polarization direction, a phase difference occurs between the polarized waves. This anisotropy varies depending on the material and the pressing force during manufacturing, but since optical disks or magneto-optical disks are manufactured by pressing the material, anisotropy is likely to occur, as shown in Figure 4 ( In a), when the laser beam T is projected obliquely onto the optical disc 1, a part of it is reflected by the surface 1b and becomes reflected light R, but most of it is transmitted and has anisotropy. When the light is birefringent, it is divided into a normal wave [0] and an extraordinary wave [E] whose polarization directions are perpendicular to each other.

At the same time, as shown in figure (b), the polarization directions X, Y of the incident wave
When the directions [0] and [E] are at an angle φ with respect to the axis, the vector of the incident wave is rotated by the angle φ. When this is detected by an analyzer with a different polarization direction, the amplitude decreases.

Note that 01 and n2 shown in the figure are refractive indices with respect to the polarization direction.

Due to the difference in phase velocity between the polarized light waves [01 and [E] waves, a phase difference δ occurs between the two waveforms when passing through the optical disk. However, although the figure shows the case where the laser beam is incident obliquely on the disk surface, the same occurs even when the laser beam is perpendicular to the disk surface, and when the laser beam is reflected from the back surface and travels back and forth, the phase difference becomes twice as large.

Next, FIG. 4(c) shows the case of a magneto-optical disk.
In this case, the laser beam to be projected is a direct light wave (corresponding to the λ/4 plate 2 and metal plate 4 removed in Figure 3), and the polarization direction of the laser beam and the analyzer used in the receiver are used. The polarization direction of is set to a direction P1 perpendicular to the radius r of the disk. On the other hand, when the anisotropic optical axis of the disk forms an angle φ as shown in P2 in the figure, the amplitude of the laser beam decreases in proportion to cosφ for the above-mentioned reason.

Figures 5 (a), (b), (c), and (d) are diagrams showing the intensity of the laser beam that caused the above phase difference δ corresponding to the vector diagram in Figure 1. (a) is the composite vector when the polarized light Ex advances the phase angle δ with respect to Ey in the circularly polarized light (17) in Fig. 1. Its locus becomes an ellipse, and the minimum value ( short axis), 3π/4
The maximum value (long axis) is taken at .

Figure (b) shows the position (A) where the laser beam has passed through the λ/4 plate and half mirror in Figure 1. It should normally be a straight line, but due to the phase difference δ it is still elliptical. It becomes a trajectory. Note that in Figure 1 (a) and (b), δ=
40°, and the anisotropic optical axis angle Φ with respect to the X and Y axes.
This is based on the calculated value when is set to a specific value.

Now, although the above vector diagram is for the amplitude Eo of the polarized light wave, what is detected by the light receiver is the light intensity, and it is necessary to take the square of the amplitude value for the intensity. In the following, the square of the amplitude E is expressed as the intensity I, and FIG.
A) shows a vector diagram of the light intensity for the elliptically polarized light wave (displayed in polar coordinates 100).If there were no anisotropy, the composite vector would rotate in a circular orbit of Io, but due to the phase difference δ, the intensity would change along the major axis I. o(1+sin δ) to short axis Io(1-s
It becomes an elliptical orbit up to inδ). However, its orbital period is half the original, or twice as fast. Figure 5 (
d) is a vector diagram of the light intensity for the elliptically polarized light wave of IN(b). If there was no anisotropy, it would be a straight line of 2■0, but
Due to the phase difference δ, the major axis I o (1+cos δ) and the minor axis I.

It becomes an ellipse of (1-cos δ). The principle of detecting information on an optical disc is based on the change in the light reception level of one reflected wave, so the light reception level changes due to such an ellipse (
When the maximum value decreases and the minimum value increases) and the anisotropy distribution on the optical disk is not uniform, the rotation of the optical disk causes the received light level to fluctuate, causing noise. Furthermore, in the case of a magneto-optical disk, level fluctuations due to changes in the anisotropic optical axis described above are added to the noise. Therefore,
Although it is necessary to provide an optical disk substrate without anisotropy, it is also necessary to inspect it after manufacturing.

In response to the above, conventionally, a method has been used in which the phase difference due to anisotropy is partially measured with respect to an optical disk in a stationary state. This will be explained with reference to FIGS. 6(a) and 6(b). In FIG. 6(a), a laser beam is projected onto the optical disk 1 or magneto-optical disk 1' to be inspected, and the transmitted light is passed through an analyzer 5. The light is received by the light receiver 27. The analyzer is rotated around the optical axis to measure the maximum value I a+ax and the minimum value I win of the light receiving voltage shown in FIG. These voltage values are shown in Figure 5 (C
), in the case of a linearly polarized light wave, corresponds to the major and minor axes of the ellipse shown in FIG. 5(d), and the phase angle δ is determined by the following equation.

sin δ=(Imax −1 ain)/(Imax +
tmin) = 14) However, this method not only requires a mechanism for rotating the analyzer;
It takes a considerable amount of time to measure one location. On the other hand, the anisotropy of an optical disk or magneto-optical disk is distributed arbitrarily over the entire surface of the disk, and it is impossible to quickly measure the anisotropy distributed over the entire surface using the above method. close. Furthermore, in cases where the level of the received light signal varies depending on the angle of the anisotropic optical axis, as in the case of a magneto-optical disk, the optical axis cannot be measured using the conventional methods described above.

To deal with these problems, a method is needed to continuously and quickly measure the phase difference δ and the optical axis angle φ with respect to a rotating disk.

[Object of the Invention] The present invention has been made in view of the above-mentioned circumstances. The purpose is to provide a method for continuous and rapid measurement.

[Means for Solving the Problems] The present invention is a method for measuring optical anisotropy over the entire surface of a transparent optical disk or magneto-optical disk (hereinafter simply referred to as a disk) to be inspected, in which the disk is rotated by a spindle and the radius A circularly polarized laser beam is projected onto it and scanned.

When the laser beam passes through the disk, the groove direction is rotated due to optical anisotropy, and a phase difference is generated between mutually perpendicular polarized wave components. The laser beam transmitted through the disk or the laser beam reflected from the back surface of the disk is
The light is divided by a half mirror, passed through two analyzers whose polarization directions make appropriate angles to each other, and received by respective light receivers.

The angle φ of the anisotropic optical axis on the entire surface of the disc is determined by the reference voltage obtained from this output voltage and the output voltage.
It measures the distribution of the phase difference δ.

Reference voltage above ■. is the output voltage 1. of the two light receivers relative to the disk to be inspected, assuming that the angle formed by the polarization directions of the two analyzers is π/2. / and 12', the calculation formula: % formula % (1) or the two obtained by performing the above measurement on a standard disk without anisotropy with the angle of the analyzer set to π/4 It is calculated by equation (1) using the output voltages 11/ and 12/ of the photoreceiver.

Next, the method for determining the angle φ of the anisotropic optical axis and the phase difference δ between the polarized waves is to set the angle formed by the two analyzers to π/4, and use the reference voltage I obtained above. and the output voltage 11 of the optical receiver with respect to the disk to be inspected. I2, the following calculation formula: % formula % (2) This peak δ is increased by the following formula to find the angle φ.

5in2(φ-θ) = (Io-211)/I(,
sin δ - (3) where θ is a known angle that one of the two analyzers makes with the X axis.

[Operation] In the above-described method for measuring optical anisotropy of a transparent disk according to the present invention, the disk to be inspected is rotated, moved in the radial direction, and the entire surface is scanned with a circularly polarized laser beam. The beam is split by half mirrors on each side, either for the transmitted beam that passes through the top and bottom surfaces of the disk and exits to the bottom, or for the reflected beam that passes through the front surface of the disk, reflects off the back surface, and returns upward. , each 2
The light is received by a pair of analyzer and light receiver. When the transparent disk has optical anisotropy, the polarization direction of the laser beam is rotated, and a phase difference occurs between the polarization wave components perpendicular to each other.

On the other hand, the light is received with the polarization direction of two analyzers set at an appropriate angle, a reference voltage is calculated from the output voltage of each light receiver, and the angle φ of the optical axis of the optical anisotropy is calculated from this and the output voltage. Then, the phase difference δ generated in the polarized light wave can be calculated.

This is performed at appropriate sampling intervals as the disk rotates, and the entire surface of the disk is measured.

The calculation formulas <1) to (3) above are all simple formulas derived by selecting the angle between the polarization directions of the two analyzers as π/2 or π/4 through theoretical study. ,
While the disk is rotating, the output voltages 1 and 12 of the optical receiver obtained by measurement are sampled at regular intervals, and the phase difference δ- and optical axis angle φ can be quickly calculated for each sampling. be.

Now, in order to calculate the target phase difference δ and optical axis angle φ from the output voltage of the optical receiver obtained by measurement, the reference voltage I
. is necessary. This is calculated by equation (1) from the output voltage for the disk under test or the output voltage for a standard disk without anisotropy. However, in this case, the angular interval of the analyzer is π/
2, and for standard discs it should be π/4. This difference in the angular spacing of the analyzer is
This is due to the fact that the test disk has anisotropy, whereas the standard disk does not.6 Next, as mentioned in the prior art section, optical disks generate circularly polarized waves, and magneto-optical disks generate circularly polarized waves. (Linearly polarized light waves are used for the second pair, but since the anisotropy of the disk is an inherent characteristic of the disk itself, there is no problem in measuring in either polarization direction, but here we will use circularly polarized light waves.) The reason for this is that the calculation formulas are simplified by setting the angle between the polarization directions of the two analyzers and the circularly polarized laser beam to π/2 or π/4. ) is an important point of this invention.

[Example] First, the phase difference δ due to optical anisotropy, which is the basis of this invention
The basis of the calculation formula for determining the angle φ of the optical axis will be explained.

In FIG. 1, the orthogonal X and Y axes are arbitrarily fixed to the measuring device. In contrast, the disk rotates,
The coordinates of the measurement point onto which the laser beam is projected are fixed at that position corresponding to the X and Y axes. At this position, the circularly polarized laser beam is split into a normal wave [01] and an extraordinary wave [E] that are perpendicular to each other along the anisotropic optical axis of the disk. That is, the angle φ that the direction of [01°[11] makes with the X and Y axes is the optical axis angle φ herein.

Now, the circularly polarized laser beam is shown in the vector diagram (t
Following F).

Ex=E, )cosωt, Ey=Egsinωt
-.-=<5''), both polarization direction components move by an angle φ and a phase difference δ occurs, so that both image light waves Ex' and Ey' transmitted through the disk are as follows.

Ex’ = Eo cos(ωt+φ+δ).

Ey' ”Eo 5in(ωt+φ) ・−
----(6) Next, in Fig. 1 for these,
When the polarized light wave in (6) is detected by an analyzer whose polarization plane is in the direction of ATl at an angle θ with respect to the X axis, the combined output is F AT = Ex ' cos (φ - θ) + EV '
cos(π/2+φ−θ) −−−−−−(7)
becomes. Equation (7) is rearranged with respect to cos(ωt), and both sides are squared. Since the light intensity is the square of the amplitude E, let this be I, and E. I. The following equation is obtained.

EAT = I = I o [1-sin2(φ-
θ) sin δ1xcos(ωt+φ−γ)・・・・・・
-(8) Here, γ in the eos term at the end of the above equation is a function of φ and θ. This term changes over time with the angular frequency ω of the light, but since the measurement time is long enough for this term, when it is integrated over time and the average value is taken, it becomes a constant coefficient 1/2. Therefore, the force to be measured is I = I o/2
・[1-sin2(φ-θ)sin δ] ・−・−・(
9) can be applied.

In the above case, the problem is that the light intensity changes depending on the information bits recorded on the disk to be inspected. However, since the density of pits is high, if the range of measurement points is widened to some extent, the light intensity can be averaged and equation (9) can be applied. Of course, this problem does not exist in the case of a material disc on which no information is recorded.

The polarization directions of the two analyzers are now AT in Figure 1.

When AT2 forms an angle θ2-θl=π/4 with this, the light intensity transmitted through each analyzer is 1. , I2 is I
r = Io/2 ・[1-sin2(φ-θ1) s
inδ1゜r 2 = Io/2- (1-cos2(
φ−θ1) sin δI−・−(IQ) Calculation formula (2) for sin δ and 5in2(φ−θ) from both equations,
(3) Gain.

5in2δ” ((It-1o/2) 10(h-1o/
2) ]/I o /4- (2).

5io2(φ-θ+)=(Io-21t)/I,
) sin δ - (3) What should be noted here is that when the angle of the analyzer (θ2 - θ1) is arbitrary, equation (1o)
is sin δ.

This results in a somewhat complicated trigonometric equation of sin 2 (φ-61) and sin 2 (φ-θ2), which is not suitable in terms of time for calculation processing in real time for each sampling.

However, by setting this angle to π/4, the simple formula (1
1) ) is derived. In addition, as a method for determining the reference voltage IO described below, the same applies when calculating by formula (1) assuming the angular interval of the analyzer is π/2, in short, the angular interval of the analyzer is π/4 or π /2 is equation (9),
This invention focuses on this singular point in the solution of (10).

Next, it is necessary to find the reference voltage 1o necessary for calculating each of the above equations, and this will be explained.

There are two ways to determine the reference voltage Io, one of which is to calculate the angle of the two analyzers in the above equation (9) by (θ2-01
)=π/2, the second term in [ ] has the opposite sign 5i
n2 (φ-θ1) sin δ, and formula (1) can be obtained by summing I,' and I2'.

Io = I,'+I2'...
(The second way to find the reference voltage IO is to measure using a separate disk without anisotropy as a standard. In this case, if the angle of the analyzer is π/4, there is no anisotropy, so the polarization remains a circle and becomes sin δ=O, so the second term in square brackets [ ] in equation (10) disappears and can still be calculated using equation (1).

FIG. 2 shows the configuration of an embodiment of the method for measuring anisotropy of a transparent disk according to the present invention.
The direct illumination laser beam is made into an appropriate diameter by an expander 28, reflected by a half mirror 23, and becomes circularly polarized by a λ/4 plate 24.

The light is then focused by a focusing lens 25 and irradiated onto the disk 1 to be inspected. When measuring by light transmitted through the disc,
An anisotropy measuring device 4A is provided below the disk 1 to be inspected. In addition, in the case of using reflected light from the disk, an anisotropy measuring device 4B is provided above the disk to be inspected.
A and 4B have the same configuration. However, as shown in the diagram, the reflected light becomes linearly polarized light again from the λ/4 plate 2 and the metal plate 5, so the λ/4
It is necessary to change between the plate 2 sheet metal 5 kiss the expander 28 and the half mirror 23. In addition, in the case of reflected light, since the laser beam reciprocates, the phase difference δ is doubled as described above.

The disk 1 to be inspected rotates in the θ direction and moves stepwise or continuously in the direction of the radius r.

The laser beam transmitted or reflected by the disk is focused by a focusing lens 25' or 25. In either case, the laser beam is split by a half mirror 23' and each passes through the analyzers 5a and 5b to the receiver 2.
The light is received by 7a and 27b. The angle formed by the polarization direction of each analyzer is set to π/4 as explained in FIG. ), (3). Since the calculation formula is simple, it can be easily executed in real time with appropriate sampling intervals.

Next, the reference voltage I. When determining
and l, / or by measuring with a non-anisotropic disk as a standard, using an angular spacing of π/4■1
', l, / using equation (1). In this case, although not shown, it is also possible to separately provide analyzers with angular intervals of π/2 and π/4.

[Effects of the Invention] As is clear from the above description, in the method for measuring optical anisotropy of a transparent disk according to the present invention, the angular interval between two analyzers is calculated by π by theoretical analysis of a circularly polarized laser beam. /2 and π/4, the phase difference δ and optical axis angle φ can be calculated using a simpler calculation formula than the output voltage of the two photodetectors. It is possible to quickly measure the anisotropy characteristics of the entire surface of a magnetic disk, and it is possible to inspect both whether or not information is recorded on the disk to be inspected. As described above, the method according to the present invention makes it possible to solve the drawback that the conventional anisotropy inspection device measures only a portion of a stationary disk and cannot measure the optical axis angle, and its effectiveness is extremely large. There is something.

[Brief explanation of the drawing]

FIG. 1 is a coordinate diagram illustrating the principle of the method for measuring anisotropy of a transparent disk according to the present invention, FIG. 2 is a configuration diagram of an embodiment of the method for measuring anisotropy of a transparent disk according to the present invention, and FIG. Figure 4 shows the configuration of the optical disc information reading optical device and a vector diagram of the polarized light wave of the laser beam in each part.
Figures (a), (b), and (c) are explanatory diagrams of the effects of anisotropy on optical disks and magneto-optical disks, and Figure 5 (a)
) to (d) are vector diagrams of polarized light waves for FIGS. 4(a) to (C), and FIGS. 6(a) to (b) are explanatory diagrams of the conventional anisotropy measurement method. - Optical disk, 1'... magneto-optical disk, 2... optical device, 21... laser light source, 22-
...Light projection lens, 23...Half mirror-12
4...λ/4 plate, 25... Focusing lens, 26... Light receiving lens, 27... Light receiver. 2... Expander, 3... Signal processing device, 4
A, 4 B... Anisotropy measuring device, 5... Analyzer.

Claims (3)

[Claims] (1) A circularly polarized laser beam is projected onto a transparent optical disk or magneto-optical disk (hereinafter referred to simply as a disk) to be inspected, which rotates with a spindle and moves in the radial direction, and scans it. By transmitting through the disk, the polarization direction is rotated in the direction of the anisotropic optical axis due to optical anisotropy, and the laser beam, which has a phase difference between mutually perpendicular polarized wave components, is transmitted to the front or back side of the disk. Split by a half mirror, each of the split laser beams is received by two light receivers through two analyzers whose polarization directions make an appropriate angle to each other and transmit polarized wave components corresponding to this angle, The method is characterized in that the distribution of the angle φ of the anisotropic optical axis and the phase difference δ over the entire surface of the disk is measured using the output voltage of the photodetector and a reference voltage obtained from the output voltage.
Method for measuring optical anisotropy of transparent disks. (2) In the above, the angle formed by the polarization directions of the two analyzers is π/2, and the reference voltage I_0 is used, and the output voltages I_1' and I_2' of the two optical receivers for the optical disk to be inspected are used. The following calculation formula: I ＿0= I ＿1′+ I ＿2′・・・・・・・・・(
1), or by using the output voltages I_1' and I_2' of the two photodetectors obtained for a standard disk without anisotropy, assuming the angle formed by the analyzer as π/4. A method for measuring optical anisotropy of a transparent disk according to claim 1, which is calculated using equation (1). (3) In the above, assuming that the angle formed by the polarization directions of the two analyzers is π/4, and using the above reference voltage I_0 and the above output voltages I_1 and I_2 for the disk to be inspected, the following calculation formulas are used: sin＾2δ=[( I ＿1− I ＿0
/2)^2+( I ＿2− I ＿0/2)＾2]/ I ＿
0^2/4...(2) sin2(φ-θ)=( I
_0-2 I _1)/I _0 sin δ... (3) (where θ in equation (3) is a known angle that one of the two analyzers makes with the X axis), the angle δ of the above phase difference is calculated. , and the angle φ of the anisotropic optical axis, the method for measuring optical anisotropy of a transparent disk according to claim 1 or 2, wherein the angle φ of the anisotropic optical axis is calculated.