JPS63103927A - Mueller matrix measuring instrument - Google Patents

Abstract

PURPOSE:To measure optical characteristics of a disk such as a substrate completely by preparing 16 light quantity data in total at the same irradiation position under specific detection conditions and finding the Mueller matrix at the irradiation position by a arithmetic operation based upon the data. CONSTITUTION:A signal processor 33 reads incident light quantity data on incident light beams Pd1-Pd4 out of a memory when the angles of rotation of a polarizer 13 and a lambda/4 plate 14 are set to specific conditions, and specific arithmetic operation is carried out to find the Stokes parameter Pc1 of reflected light Pc. Similarly, respective incident light quantity data when said angles of rotation are set to specific states are read out of the memory successively to find respective Stokes parameters Pc2, Pc4, and Pc5 of the reflected light Pc at this time. Then, arithmetic operation based upon those respective Stokes parameters is carried out to find the Mueller matrix at the irradiation position A1. Similarly, respective incident light quantity data matrixes corresponding to respective irradiation positions A2-An-1 are found. Their measurement results are displayed on a display 3.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an apparatus for measuring the Mueller matrix of an optical element, and more particularly to an apparatus for measuring optical characteristics of a sub-substrate 1 for an optical disk.

(Prior art) Conventionally, an ellipsometer (ellipsometer) has been used to measure the optical characteristics of optical disk substrates, etc.
Or a similar device was used.

In these devices, the measured disk is irradiated with parallel light beams of known polarization, and the transmitted or reflected light is passed through, for example, a rotating analyzer to measure its polarization state. The optical characteristics of the substrate of the measurement disk were determined.

(Problems to be Solved by the Invention) However, in actual use when the disk to be measured is used as a recording medium, the incident light on the substrate is not a parallel beam but a focused beam. Therefore, it is extremely troublesome to determine the influence of the optical characteristics of the substrate on the actual reproduced signal from the measurement results using an ellipsometer.

Furthermore, since the parallel beam diameter of an ellipsometer cannot be made as small as the actual track width formed on the disk, it has been difficult to determine the effect that changes in optical properties in minute areas have on actual reproduced signals. . In addition, the ellipsometer has a disadvantage in that its shape is large in order to improve the accuracy of the optical axis, and that adjustments are required when mounting an object to be measured, and these adjustments take time.

(Means for solving the problem) A means for irradiating an optical disc with focused laser irradiation light, a polarization setting means for selectively polarizing the irradiation light into four types of polarization, and a light reflected from a reflective surface of the optical disc. On the basis of the,
a tracking control means for controlling the tracking of the laser irradiation light; a focus control means for controlling the focus of the irradiation point based on the reflected light; a polarization component dividing means for dividing the reflected light into four polarization components; and a light amount of the polarization components. It consists of 4 light quantity detection means for detecting the 4 light quantity detection means, a storage means for storing the light quantity data detected by the 4 light quantity detection means, and a calculation means for calculating the Mueller matrix of the optical disk based on this light quantity data. The means performs calculations based on the 16 light quantity data stored in the storage means for each of the four types of polarized laser irradiation light selected by the polarization characteristic setting means at the same irradiation position on the optical disc.

(Operation) A total of 16 pieces of light amount data are prepared at the same irradiation position under predetermined detection conditions, and the Mueller matrix at this irradiation position is determined by calculation based on these light amount data.

(Example) When measuring the Mueller matrix of an optical element to be measured, the characteristics of the optical element of the measurement system are expressed using the Mueller matrix,
This is done by expressing the emitted or detected light using Stokes parameters. In this case, in order to measure the Mueller matrix of the optical element to be measured, the optical element to be measured is sequentially irradiated with four types of predetermined polarized light, and the Stokes parameters of each transmitted light or reflected light are measured. There is a way to do it. Furthermore, in order to obtain this Stokes parameter, a polarizer and a λ/4 plate are placed on the optical path of the transmitted light or reflected light to the photodetector, and the transmitted light that passes through these under four types of arrangement conditions is calculated. In other words, it is necessary to detect the amount of reflected light using a photodetector.

The principle will be explained below using FIG. 4.

The linearly polarized laser light emitted from the laser 1 is made into parallel light via the collimator lens 2, and becomes circularly polarized light Pa by passing through the λ/4 plate 3. 4.5 are respectively predetermined rotation angles θ1 . By appropriately selecting these rotation angles θ and θ2 using a polarizer and a λ/4 plate having an angle of θ2, the optical element 6 to be measured is irradiated with the passing circularly polarized light Pa as irradiation light pb having a desired polarization. The output light Pc transmitted or reflected from the optical element 6 to be measured has a rotation angle of 01 and a rotation angle of 01, respectively, with respect to the reference plane.
After passing through the λ/4 plate 7 having the angle θ and the analyzer 8, the amount of light is detected as incident light Pd on the photodetector 9.

In the above configuration, first, a method for determining the Stokes parameter of the output light Pc from the optical element 6 to be measured will be explained.

Assume that the Stokes parameters of the output light Pc and the input light Pd are Pc and Pd, respectively, and the Mueller matrices of the λ/4 plate 7 and the analyzer 8 are M7 and M8, respectively.

Pd=M8・M7・Pc−(1).

The amount of light detected by the photodetector 9 is the amount indicating the first component of the Stokes parameter Pd of the incident light Pd, and the Stokes parameter of the incident light Pd under each condition shown below is expressed as Pd.
1, Pd2, Pd3, and Pd4, and let these first components be Pdl, Pd2, Pd3, and Pd4, respectively, then the rotation angles of the λ/4 plate 7 and the analyzer 8 are 01=0, θ. =0
In this case, equation (1) is Pd1= (Pc1~PC4) - (2)
becomes.

Similarly 0. When =0, θ, =π/2, pa2=
CPc1 Pc, )--- (3).

Similarly 0. =0. When 04=π/4, Pd4=
-(Pc, +Pc4) (5)

When Pc1 to PC4 are calculated from the above equations (2) to (5), the following is obtained. Therefore, in order to obtain the Stokes parameter Pc of the output light Pc from the optical element 6 to be measured, the λ/4 plate 7,
It can be determined by setting each rotation angle of the analyzer 8 to the four conditions described above and measuring the amount of incident light Pd under each setting.

Next, a method for obtaining the Mueller matrix of the optical element 6 to be measured will be explained.

The Mueller matrix of the optical element 6 to be measured is M6. Irradiation light pb
If the Stokes parameter of is pb, then Pc=:M6・
Pb...(7)

The irradiation light pb has a desired Stokes parameter by appropriately selecting the rotation angles θ□ and θ2 of the polarizer 4 and the λ/4 plate 5 as shown below. The Stokes parameters of the output light Pc obtained by irradiating the optical element 6 with these irradiation lights pb are Pal, Pc2, and Pc3, respectively.
, Pc4, the above equation (7) becomes as follows.

The rotation angles of the polarizer 4 and the λ/4 plate 5 are respectively θ, = 0,
When θ2=0, the irradiated light pb becomes horizontally linearly polarized light, and equation (7) similarly shows that when θ,=π/2 and θ2=0, the irradiated light pb
becomes vertically linearly polarized light, and equation (7) similarly becomes 01=π/4
．． When 0□=π/4, the irradiation light pb becomes linearly polarized light of π/4, and equation (7) similarly becomes θ, = 0, θ2 = π/4.
In this case, the irradiation light pb becomes right-handed circularly polarized light, and the equation (7) becomes as follows.

In addition, each Stokes parameter of the emitted light in the above formula PC1~
Pc4 can be determined by the method described above.

By sequentially irradiating the optical element 6 with the four types of polarized irradiation light pb and determining the Stokes parameters Pcl to Pc4 of the output light Pc from the optical element 6, the Muller Matrix can be measured.

The present invention is based on the above-mentioned principle, and one embodiment thereof will be described below with reference to FIG.

Laser light output from a laser 10 placed at a predetermined position inside the pickup, indicated by a broken line, is collimated by a collimator lens 11 and then passed through a λ/4 plate 2 to become circularly polarized light Pa. Become.

This circularly polarized light Pa has a rotation angle of 01 with respect to the reference plane.
, the irradiation light pb of the desired polarization passes through the polarizer 13 and the λ/4 plate 14 held in the pickup so that θ6 can be changed.
After that, it further passes through the spectroscopic mirrors 15 and 15□ and reaches the total reflection mirror 16.

The irradiation light reflected by the total reflection mirror 16 is focused by the objective lens 17, and then is irradiated as actual irradiation light Pb' onto a reflective surface 25□, which is similar to an optical disk on which tracks are formed.

The objective lens 17 is installed in the pickup so as to be movable in the tracking direction and the focus direction by drive currents flowing through a tracking coil 23 and a focusing coil 24 arranged in the vicinity thereof.

The reflected light Pc reflected by this reflective surface 251 passes through the objective lens 17 again and becomes parallel light, and then the total reflection mirror 1
6 and reaches the spectroscopic mirror 15□. Reflective surface 251
As shown in FIG. 3, the light beam is covered with a substrate 254 made of polycarbonate or the like, and optical effects such as birefringence that occur when light is reflected mainly occur when it passes through this substrate.

The reflected light reflected by the spectroscopic mirror 15□ is the spectroscopic mirror 1
53.15 The reflected light to be measured P c
, , Pc2, Pc, , Pc4 pass through the optical element groups 18a, 18b, 18c, and 18d, respectively.

These optical element groups each consist of a λ/4 plate and an analyzer, each set at a desired rotation angle with respect to the reference plane, and after optically processing the reflected light to be measured that passes through it, each photodetector 19a
, 19b, 19c, 19d are incident lights Pd□, P
d2, Pd3. Pd4 is emitted respectively.

On the other hand, the reflected light that has passed through the spectroscopic mirror 15□ is reflected by the spectroscopic mirror 15□, and then from this reflected light to the reflective surface 25□.
The light enters a control information detector 20 that detects tracking and focusing information of the upper irradiation point.

Reference numerals 21 and 22 denote a polarizer rotation drive device and a λ/4 plate rotation drive device provided in the pickup, respectively, and drive output from the rotation drive circuit 27 based on the rotation angle command signal S4 from the signal processing device 33. Polarizer 13 according to the signal,
and the λ/4 plate 14 are set at desired rotation angles θ and θ6, respectively.

Reference numeral 26 denotes a spindle motor which rotates the optical disc at a desired rotation speed by a drive means (not shown).

The light amount signals detected by each photodetector 19a to 19d are A
After being converted into a digital signal by the /D converter 32, the signal is input to a signal processing device 33, where various arithmetic operations are performed. Further, this signal processing bag rn33 inputs a rotation information signal S5 from the spindle motor 26 to detect rotation information similar to that of an optical disk, and also inputs a position signal SG of a potentiometer 36 that detects the moving position of the pickup 35 for irradiation. The track position of the point is detected, and a track jump command signal S8. Each rotation angle θ5 of the polarizer 13 and the λ/4 plate 14 inside the pickup. θ6
A rotation angle command signal S4 for setting the rotation angle is outputted.

The pickup control circuit 28 receives tracking and focusing information signals output from the control information detector 2o, and outputs a tracking control signal S and a focus control signal S2 to perform these controls.

The objective lens drive circuit 31 inputs the focus control signal S2 and the high frequency signal of the tracking control signal S1 selected by the filter 29, and applies each drive current based on these signals to the focusing coil 24.
Each signal is output to the tracking coil 23 to drive the objective lens 17. The pickup drive circuit 30 also inputs the low frequency signal of the tracking control signal S1 selected by the filter 29, and drives a pickup drive means (not shown) based on this signal to move the entire pickup in the track direction.

Furthermore, the pickup control circuit 28 includes a signal processing device 33
A tracking control signal S□ based on a track jump command signal S from the irradiation point is output, thereby enabling track jump movement of the irradiation point.

These series of focus control, tracking control, and track jump control are known techniques, and detailed explanations thereof will be omitted.

The operation of the apparatus of the present invention having the above configuration will be explained.

In addition, the relationship between the irradiation light pb, which is the output light of the λ/4 plate 14, and the actual irradiation light Pb', which is the output light of the objective lens 17, and the reflected light Pc from the reflective surface 25□, which is similar to an optical disk, and each optical element group. Reflected light to be measured PC1 which is the incident light of 18a to 18d
The relationship of PC4 differs depending on the optical characteristics of the spectroscopic mirror, objective lens 17, and total reflection mirror 16 interposed therebetween, but for the sake of simplicity, Pb=
It is assumed that Pb', Pc=Pc1=Pc,=Pc,=Pc4.

These setting conditions can be achieved by inserting an optical element having appropriate optical characteristics at a predetermined position in the optical path for correction. Further, by performing arithmetic processing that takes into account the difference in these characteristics, it is possible to equivalently perform correction to 4, but a description of these correction methods will be omitted.

The Stokes parameter Pc of the reflected light Pc is as described above (
1) Calculate using the formula.

In this case, the Mueller matrices of the λ/4 plates and analyzers of each of the optical element groups 18a to 18d correspond to M7 and M8 in the equation. Then, the rotation angles of the λ/4 plate and analyzer of the optical element group 18a are set to Olo, those of the optical element group 18b are set to 0 and π/2, and those of the optical element group 18c are set to π/2.
4, π/4, and that of the optical element group 18d as 0, π
/4, the photodetectors 19a-
The respective amounts of incident light Pd~Pd detected at 19d correspond to Pdl~Pd4 in the above equations (2)~(5). Therefore.

The Stokes parameter Pc of the reflected light Pc is given by (6) above.
It is obtained by calculating the formula.

On the other hand, the Mueller matrix of the reflective surface 25□ of the optical disc is
This is carried out by a method using the above-mentioned equation (7).

In this case, the Mueller matrix at the irradiation position of the reflecting surface 25□ corresponds to M6 in the equation. Then, each rotation angle of the polarizer 13 and the λ/4 plate 14 is 06.06: 1) θ5=0, θ6=0 2) 0. =π/2.0G=0 3) θ5=λ/4, θ6=λ/4 4) 05=O10, =λ/4 These states are sequentially set. Let Pcl to Pc4 be the Stokes parameters of the reflected light Pc obtained by irradiating the same irradiation position on the reflective surface with the irradiation lights Pb to Pb4 obtained when θ6 and θ6 are set to 1) to 4). , these correspond to the respective Stokes parameters PC1 to Pc4 in equations (8) to (11) described above.

Next, an actual measurement example will be explained.

A spiral track 25□ is formed in a predetermined area of the reflective surface 25 of the optical disc, and FIG. 2 shows the circumference of the spiral track 25.

The signal processing device 33 outputs a rotation information signal S5, a position signal S,
are input to detect positional information such as the track position and rotation angle of the irradiation point that is tracking-controlled along the track 25□, and this irradiation point is determined as the irradiation position A.
. When the rotation angle command signal S4 is reached, the rotation angle of the pickup Kawauchi polarizer 13 and the λ/4 plate 14 is set to 05.
．． 06 are set to 0G=O1θ6=0, respectively.

Then, when the irradiation point reaches each irradiation position A□ to An-1 corresponding to a preset rotation angle, the incident light amount data of the photodetectors 19a to 19d detected at each of these positions is It is sequentially stored along with the information in the memory within the signal processing device. Then, when reaching An at the same rotation angle as the irradiation position A0, the track jump command signal S,
Output , jump the irradiation point by one track, and press A again. Return to At the same time, a rotation angle command signal S is output to each rotation angle θ9 of the polarizer 13 and the λ/4 plate 14 in the pickup 1. θ6 is 0. =π/2. θ6=O
Set to . Then, when the irradiation point reaches each of the irradiation positions A0 to An-1 again, the incident light amount data of the photodetectors 19a to 19d detected at those positions is stored together with the position information. Similarly, the rotation angles θ6 and θ of the polarizer and the λ/4 plate are respectively set as θ5=π/4 and θs=π/
Incident light amount data at each irradiation position A1 to An-1 when set to each state of 4 and θ, =O1θ6zπ/4 are stored together with position information.

By performing the above measurements, the memory in the signal processing device 33 stores the rotation angle θ of the polarizer 13 and the λ/4 plate 14 for each irradiation when the rotation angle 06 is set to the conditions 1) to 4) above. Incident light amount data at positions A to An-1 are stored together with position information.

After performing the above measurements, the signal processing device 33 further performs arithmetic processing based on the stored incident light amount data,
A Mueller matrix for each of the irradiation positions A1 to An-1 on the optical disc is calculated.

For example, the Mueller matrix at the irradiation position A□ can be calculated as follows.

The signal processing device 33 controls each rotation angle θ1 . When θ6 is set to the condition 1), the incident light amount data of the incident lights Pd1 to Pd is retrieved from the memory, and the equation (6) is calculated to obtain the Stokes parameter Pcl of the reflected light Pc.

Similarly, each of the above rotation angles θ5 and θ6 are set to 2) to 4) above.
Each incident light amount data when set to each state is sequentially pulled out from the memory, and each Stokes parameter Pc2, Pc3 . Find each Pc4. Next, based on each of these Stokes parameters, the above (
12) Calculate the equation to find the Mueller matrix M at the irradiation position A1.

In the same way, the incident light amount data corresponding to each irradiation position A2 to An-1 is sequentially pulled out from the memory, and the above (6), (
12) Calculate the equations to find Mueller matrices M2 to Mn-1 at these positions.

34 is a display that displays these measurement results;
Various display methods are being considered, such as digital display of these measurement results together with track position and rotation angle information, and graphic display of the optical disk surface in color based on the position information and measurement results. However, detailed explanations of these will be omitted.

In the above embodiment, a method of measuring the Mueller matrix at each irradiation position over one track round was shown, but the measurement order is not limited to this. Irradiating the same irradiation position on the track with irradiation light having the four types of polarization characteristics described above,
By determining the amount of light detected by the special photodetector, it becomes possible to calculate the Mueller matrix, so it goes without saying that various methods can be considered for determining the amount of light data.

In addition, a polarizer 1 rotatably held within the pickup
3 is not limited to this.

For example, in place of the λ/4 plate 12 and the polarizer 13, a Faraday element whose polarization plane is rotated by voltage may be used, and various other embodiments can be considered.

(Effects of the Invention) According to the present invention, it is possible to completely measure the optical characteristics of a disk such as a substrate while the optical disk is actually used as a recording medium. Therefore, by analyzing the measurement results, it is possible to contribute to the investigation of various causes of errors that occur during recording and reproduction using optical discs. Also,
It can be used for various inspections such as checking the performance of substrates and checking whether optical discs are good or bad.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an embodiment of the present invention. Figures 2 and 3
FIG. 4 shows diagrams for explaining the present invention, respectively. 10...Laser, 11-...Collimator lens, 1
2.14...λ/4 plate, 13...Polarizer, 153~
155... Spectroscopic mirror, 16... Total reflection mirror, 1
7... Objective lens, 18a-18d... Optical element group, 19a-19d... Photodetector, 20... Control information detector, 21... Polarizer rotation drive device, 22... λ
/4 temporary rotation drive device, 25-optical disk, 26... spindle motor, 27... rotation drive circuit, 28...
Pickup control circuit, 29...filter, 30...
- Pickup drive circuit, 31... Objective lens drive circuit, 32... A/D converter, 33... Signal processing device, 34... Display, 35-... Pickup,
36... Potentiometer.

Claims (1)

[Claims] 1) An apparatus for measuring the Mueller matrix of an optical disc, comprising means for irradiating the optical disc with focused laser irradiation light, and polarization setting means for selectively polarizing the irradiation light into four types of polarization. a tracking control means for controlling the tracking of the irradiation point of the laser irradiation light based on the reflected light from the optical disk; a focus control means for controlling the focus of the irradiation point based on the reflected light; polarization component dividing means for dividing light into four polarization components; four light amount detection means for detecting the light amount of each of the polarization components; storage means for storing light amount data detected by the four light amount detection means; a calculation means for calculating the Mueller matrix of the optical disc based on the light amount data; Correspondingly, the Mueller matrix measuring device is characterized in that the calculation is performed based on the 16 pieces of light amount data stored in the storage means. 2) The polarization component splitting means includes splitting mirrors 15_3, 15_4, and 15_5 that split the reflected light into four optical paths;
Optical element groups 18a and 18 are respectively arranged on the four optical paths and detect different polarization components from the reflected light.
18. The Mueller matrix measuring device according to claim 1, characterized in that it is comprised of: b, 18c, and 18d.