JPS6361936A - Double reflection measuring instrument - Google Patents

Abstract

PURPOSE:To reduce the quantity of measuring operation greatly, to shorten a measurement time, and to take a measurement even when a body to be measured is in operation by measuring the differential output of light which is transmitted through or reflected by a birefringent medium. CONSTITUTION:Laser light emitted by a semiconductor laser 5 is projected on a recording film 2 through a collimator lens 6, an objective 11, and cover glass 3 and reflected light from the film 2 is reflected by the 1st polarizing element 8. The reflected laser light is further passed through the 2nd polarizer 12 and incident on a photodetector 15 for a servo. Laser light containing a signal component by Kerr effect which is reflected by the 2nd polarizer 12 is converged by a condenser lens 16 and branched into two by a polarization beam splitter 17. Then they are made incident on the 1st and the 2nd photodetectors 18a and 18b and converted into electric signals respectively. Then the signal which are detected by the photodetectors 18a and 18b are amplified 21 and 22 and inputted to a differential circuit 23. Then the differential circuit 23 subtracts the in-phase components of the signals and the opposite- phase components are added and outputted 12.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an apparatus for easily measuring the amount of birefringence in a birefringent medium whose refractive index is not uniform.

[Conventional technology]

The conventional method for measuring birefringence is as described in Solid State Physics Vol. 12,
Discussed in NILlo (1977) pp. 581-587. According to this measurement method, a birefringent medium, which is an object to be measured, is irradiated with parallel light using a volarimeter (or ellipsometer), and birefringence is measured by the Senarmont method. In the Senarmont method, light guided from a light source is linearly polarized using a polarizer such as a Glan-Thompson prism, and the crystal axis of the medium is
By placing a birefringent medium so that the angle is 5 degrees (that is, the azimuth is 45 degrees) and irradiating the quarter-wave plate with the transmitted or reflected light, the phase difference of the birefringent medium can be Creates elliptically polarized light. Furthermore, this elliptically polarized light is irradiated onto another 1/4 wavelength plate, and then passed through an analyzer made of a Glan-Tomlin prism, etc., similar to the polarizer, and the analyzer is rotated around the optical axis to align it with the extinction position. The phase difference of the birefringent medium is measured from the angular difference between the rotation angle at this time and the rotation angle of the analyzer at the extinction position when there is no birefringence.

[Problem that the invention seeks to solve]

As mentioned above, in the conventional birefringence measurement method, it is necessary to irradiate the birefringent medium with light at an azimuth angle of 45 degrees. It was necessary to irradiate it with light. Furthermore, it is necessary to rotate the analyzer during measurement. For these reasons, conventional techniques not only take time to measure, but also cannot be measured unless the object to be measured is in a stationary state. For example, it is difficult to measure birefringent media that are rotating, such as the cover glass of an optical disk. It was possible.

The purpose of the present invention is to significantly reduce the amount of work involved in birefringence measurement, which was required in the prior art described above, to shorten the measurement time, and to provide birefringence measurement that can be performed even when the object to be measured is in an operating state. The goal is to provide equipment.

[Means for solving problems]

In order to achieve this objective, in the present invention, the light transmitted or reflected by a birefringent medium is split into two by a polarizing beam splitter, the split light is further photoelectrically converted, and the differential output is detected. A method was used to measure the amount of birefringence such as the phase difference and azimuth of the birefringent medium from the dynamic output value.

[Effect]

In the differential optical system described above, if the object to be measured does not have birefringence, the light transmitted or reflected by it is split into two by a polarizing beam splitter and the light amounts of each are compared. If there is birefringence, a difference in the amount of light will occur between the two branches of light. In other words, this light amount difference is caused by the phase difference, which is the amount of birefringence of the medium, and the angle between the polarization direction of the medium and the medium crystal axis, that is, the azimuth angle.In the present invention, this light amount difference is generated by photoelectric conversion. The amount of birefringence of the object to be measured is measured by detecting it using the output of the differential amplifier afterwards.

According to this method, there is no need to find the crystal axis and make the light incident on it at an azimuth angle of 45 degrees, which was required in the prior art, and it is also unnecessary to rotate the analyzer to find the extinction position during measurement. There is no need to do any additional work. Therefore, by using the birefringence measurement method of the present invention, the amount of measurement work and time can be significantly reduced, and furthermore, measurement can be performed even when the object to be measured is in an operating state.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a magneto-optical reproducing device according to an embodiment of the present invention, and FIG. 2 shows a side view of a portion of the magneto-optical reproducing device in FIG. The magneto-optical reproducing device has a differential optical system for detecting magneto-optical signals, and its configuration itself has the birefringence correction function of the present invention.

In these figures, reference numeral 1 denotes a magneto-optical disk having a light-reflecting magnetic recording film 2 and a cover glass 3, and is rotated by a motor 4. To reproduce the recorded signal, laser light emitted from a semiconductor laser 5, which is a linearly polarized light source, is collimated by a collimating lens 6, shaped into a substantially circular light intensity distribution by a beam shaping prism 7, and then processed by a beam splitter. The first polarizer 8 reflects part of the light and transmits the rest. Of these, the transmitted laser beam further passes through a wavelength plate 9 that is an optical rotation plate or a phase plate and a mirror 1o, is condensed by an objective lens 11, and is transmitted through a cover glass 3 and irradiated onto a recording film 2. At this time, the plane of polarization of the laser beam irradiated onto the recording film 2 changes depending on the orientation state of the magnetic moment in the recorded portion and the unrecorded portion of the recording film 2 due to the Kerr effect. (That is, Kerr rotation occurs.) Further, the reflected light from the recording film 2 passes through the cover glass 3 again, and then returns to parallel light at the objective lens 11.
The first polarizer 8 passes the laser beam in the polarization direction (
For example, S-polarized light) is almost entirely reflected, and polarized light components having a phase difference of 1° (for example, P-polarized light) are partially reflected and the rest are transmitted. The S-polarized light and the P-polarized reflected laser light are further processed by a second polarizer 12, which is a beam splitter.Similar to the polarizer 8, almost all of the S-polarized light is reflected, a portion of the P-polarized light is reflected, and the rest is Transparent. The laser beam transmitted here is condensed by a convex lens 13, and is condensed by a cylindrical lens 1.
4 and enters the servo photodetector 15.

This servo photodetector 15 is divided into a plurality of parts depending on the detection method, and processes their output signals 26 to obtain a focus error signal and a track error signal (detection method is omitted). The objective lens is controlled so that the laser beam is accurately positioned on the object. On the other hand, the second
The laser light including the signal component due to the Kerr effect reflected by the polarizer 12 is focused by the convex lens 16, enters the polarizing beam splitter 17, and is split into two. One of the two branched laser beams enters the first photodetector 18α,
The other light enters the second photodetector 18b and is converted into an electrical signal. In this embodiment, since a polarizing beam splitter is used as a means for splitting this laser beam into two, each of these two-branched laser beams passes through an analyzer.

Therefore, the analyzer 17 and the first photodetector 18α constitute a first photoelectric conversion system 19, and the analyzer 17 and the second photodetector 18h constitute a second photoelectric conversion system 20. I will do it. Furthermore, these photodetectors 18α.

The signal detected at 18h is sent to preamplifiers 21 and 2.
2 and input to the differential circuit. In the differential circuit n, the in-phase components of each signal input by the preamplifier 21 or 22 are subtracted, and at the same time, the opposite-phase components are added, so the amplitude of the reproduced signal is doubled and output. Ru.

Next, this differential output signal will be explained in more detail.

FIG. 3 is an explanatory diagram showing signals detected by the photoelectric conversions 19 and 20 of FIG. 1. In this figure, ]01 and 102 are the P and S polarization directions of the laser beam, and 10
3 and 104 indicate the transmission axes of the analyzer 17 in FIG. As shown in the figure, when the recording film 2 is irradiated with a laser beam, the reflected laser beam causes rotation of the plane of polarization by an angle θ and a partial rotation in the recorded portion and the unrecorded portion.

This phenomenon is the Kerr effect, and this rotation angle θ is called the Kerr rotation angle. The amount of light P incident on the photoelectric conversion systems 19 and 20 is an AC light amount component PAC which is a high frequency magneto-optical signal.
It can be classified into DC light amount component PDC, which includes the servo signal and the birefringence component to be measured, and +11 and PDC, respectively.
It is expressed by the formula +21.

P, tc=(1(-〇x)'(+θK))°η”di
, k (11Pnc −2(I(−〇x )+1
(+θI))°η”disk f2) Here, η: From the incidence on the disk to the photodetectors 18a, 18
Light utilization rate up to b (excluding dichroism of polarizers 8 and 12) Pdisk ' Amount of light incident on disk 1 (F) Also, I in the above equation can be expressed as (
It is expressed by equation 31.

1 =: (E□)2+(Ey)”
+31 Here, E: the amplitude of the S-polarized light detected by the analyzer 17, Ey, the amplitude of the P-polarized light detected by the two-dimensional photon 7, and E:
When linearly polarized laser light is expressed by Stokes parameters and each polarization element is combined by a Mueller matrix, (4)
It can be expressed as the formula.

Here, as shown in FIG.
If we want to detect the amplitude E at the transmission axis 103 rotated by 04 from 02, we can obtain the following: Then, θ: force - rotation angle (degrees) θ0: rotation angle of analyzer 17) δ: phase difference (ds) α: azimuth angle (ds) R5: dichroism of polarizers 8 and 12 ( S polarized light reflectance) Rp
: (P polarized light reflectance) This (4)
Solving the equation, (3j, from equation (4), I(±θ
, ) can be expressed as follows.

I(-θK) = IEyo(-θz)l” + l Ey(
-θK) 12 niichi momme, blood θ month, tassel δ, k part brass, ko θa-
L;)”+(ccs brass・blood δ(times θ.・ratio 2α・is−
mfjecas2c1・1p)P (511(+θ)""xc+θ,)I"+1%(-button 12=(asf
)x−outθ,・! δ・lRp −5tncrsθa−
A)”+(External・Blood δ(circumstance θ.・Spider・4-60.・1
Add・~))” +s+Therefore, substituting equations (51, +61 into equations fi+, +21 and finding picPDC, PAC(θ,)=idn2f)x−ydn2θ.・(2
) δ・～・８・η・Pd1ik f71Pr
x(θ=z) = (Ca5t) r bite θ.・(2) δ・
7%)” + (mfJx・■θ4・＾) 2+(■niichiketsu δ (Cxsθ.・5 stones 2α・l←shiθ.・−2α・＾)
)2] *η・Pd1sk
It becomes i81. The amount of DC light incident on the photoelectric conversion system 19 and the top is determined by the analyzer rotation angle θ4.
(θ6 is normally 45 degrees) and π-06, and the respective amounts of incident light are expressed by equations (8) and (8t).

Pnc (π-θ.) = C (Ok-inside θ4・■δ・Sho)2
+(Blood 0K・Tessel θ.・West) 2+(casax-output δ(-
Times θ4・ldn distribution・小咄θα・可瀉・蚉)）2〕*η
・Pd1sk
t8 Therefore, the light amount difference ΔP of the rain detection system is expressed by equation (9).

dゝPDc(π-Ca)-PDc(Ca)-k・Blood 2θ
White 6, West, dδ, 5 stones 4α, η, Pd1sk
(91 Furthermore, when the light intensity difference ΔP in equation (9) is photoelectrically converted,
A DC light amount component is detected as a differential voltage output shown in the al formula.

DC-DC(π-θ,) Ix:(θ7)-cxyn
x-51n2θ, -4-Q・7δ・*Gi11・Pdt
J & K-Ral where, K: Photoelectric conversion efficiency of photodetectors 18c and 18h (R: amplifier gain resistance (Ω) α [In the formula, when θz=Q, 13r=1 θ.=45 degrees In this case, self 2θ = 1. Also, the amount of light received by the photodetectors 18a and 18b Pdt
t is ~・box・η・Pd1rk=Pdet, and from these, equation 111 can be rewritten as equation α1'.

ΔV(, c=m'δ・tkn411・Pd, 1・K−R
-δ・Blood4α・'tht (1G'
CVdet = Pc1at, ni-R) i.e. H'
From the equation, the differential output signal iDc represents that the phase difference δ and the azimuth α act in a direction to reduce the photoelectric conversion value Vdtt of the amount of received light when there is no birefringence. Furthermore, as shown in equation 11[', it can be seen that the measurement method of the present invention is capable of measurement regardless of the presence or absence of a magneto-optical signal. Therefore, if you measure Vdgt using a material without birefringence, such as a glass substrate, and then measure the target birefringent medium, even if the disk is rotating, the differential output IDC will phase difference δ
, the azimuth α can be determined.

Furthermore, the fluctuation frequency of birefringence is low, and by passing the differential signal through a low-pass filter, the birefringence component can be extracted from a mixture of magneto-optical signals, servo signals, and the like.

Furthermore, if the servo signal component has a lower frequency, a bandpass filter may be used.

Next, a method for measuring the phase difference δ and the azimuth α will be explained using FIGS. 4 to 9. These figures show a measurement method when the wavelength plate 9 in FIG. 1 is used as an optical rotation plate.

Figure 4 shows the disk main cover glass 3 in Figure 1.
This shows the polarization direction of the laser beam irradiated on the
110 is the crystal axis of the cover glass 3, and 111 to 113 are the incident polarization directions of linearly polarized laser light. As shown in this figure, when the polarization direction 111 of the laser beam and the crystal axis 110 of the cover glass 3 do not match and are in an angular relationship of azimuth α (positive in the counterclockwise direction), the wave plate 9 can be further converted into an actuator. When the laser beams are input at azimuth angles (α+6] and (α-6), and the differential output is reproduced, steps α and 2 in Fig. 5 (1) are obtained.
4b output waveform is obtained. Therefore, when focusing on the phase difference δ and azimuth α at a certain point on the cover glass 3, the differential output voltages υ1 and ν at time t when the laser beam illuminates the measurement point while the disk 1 is rotating, Measure. Specifically, the differential output may be monitored in waveform using a storage oscilloscope or the like with, for example, a still playback state so that the laser beam irradiates the same location even when the disc 1 is rotating.

By irradiating the same place with laser light in this way, the phase difference δ is constant as shown in equation 007, and the differential output is changed by changing the azimuth (α + b) and (α - b). Since the measurement is performed, the values ν1 and v2 at the two points shown in FIG. 5 (I[) are measured. These values are obtained from the member's formula: vl-=dlJ-self4(α-h) ・Vctet
(Ll)υ, -δ・Ratio 4(α+b)・Vdt
t (6), and further (b)
, From equation (6), ;η":D= sfn only
Mδ, 7dtt Therefore, the azimuth angle α is expressed by equation (6).

(L-"tm-'(-work"' ・tm4 h)
e334 Q-ν1 In this equation, the given azimuth angle is a known quantity, I'
S sυ! Since is a measured value, the azimuth angle α can be determined from these values. Furthermore, since α has been obtained, the phase difference δ can be determined by substituting this value into the α ring formula or equation (6).

Furthermore, in the above embodiment, the wavelength plate 9 is used as an optical rotation plate,
We have described a method of measuring the phase difference and azimuth of a birefringent medium by giving a known amount of optical rotation angle lζ.
The same measurement can be made by using the phase plate as a phase plate and giving a known amount of phase difference.

As described above, according to the measurement method of the present invention, it is not necessary to find the azimuth angle for each measurement point and adjust the optical system, or to rotate the analyzer to find the extinction position.
The amount of measurement work and time can be significantly reduced.

Furthermore, birefringence can be measured even when the optical disk 1, which is the object to be measured, is rotating. Another great feature is
The conventional measurement method used the above-mentioned volarimeter, etc., and usually transmitted parallel light through a cover glass and measured the amount of birefringence at this time, but according to the measurement method of the present invention, the amount of birefringence of the magneto-optical disk was measured. By using the optical head used for reproduction, the laser beam focused by the objective lens 11 is incident on the cover glass 3 and reflected, which not only makes it possible to measure birefringence in actual use conditions, but also makes it possible to measure birefringence on the recording film. It becomes possible to measure pit orders that are narrowed down to the diffraction limit, and measurement accuracy can be greatly improved.

Next, the wave plate 9 and the actuator will be specifically explained.

6 to 9 show an example of the wave plate 9 that produces the optical rotation described in FIGS. 1 to 5, and its actuator. As shown in FIG. 6, this wavelength plate 9 is a half-wave plate in which a crystal axis 120 is orthogonal to the optical axis 41.

In this case, when the half-wave plate 9 is rotated by an angle θ in a plane perpendicular to the optical axis 41 and around the optical axis 41, an angle β is created between the input polarization direction 121 and the output polarization direction 124.
produces optical rotation.

FIG. 7 shows the relationship between the rotation angle θ and the optical rotation angle β, and as the half-wave plate 9 rotates, the optical rotation angle β changes following the straight line y.

Next, the actuator 29 of the optical rotation plate 9 of this embodiment will be explained using FIG. 80 and FIG. 9.

Fig. 8 shows a specific example of this actuator conversion, and Fig. 9 is a diagram showing the actuator conversion from the A side in Fig. 8, and the same numbers are the same. It is something.

51α, 51b include permanent magnets 5oα, sob inside, and are both fixed iron cores made of magnetic material such as iron, 5
Coils 2α and 52b are wound around the fixed and fixed iron cores 51a and 51b, respectively, and are fixed to a rotating member to which the optical polarization plate 9 is attached. Reference numeral 54 denotes a support portion for supporting the rotating member, for example, a bearing or the like is used.

Because of this configuration, the fixed iron core 51a.

A magnetic field is generated in the coils 52a and 52b, and an optical rotation plate drive circuit n generates a magnetic field in the coils 52a and 52b.
Since current flows according to each control signal, the rotating member 53 rotates in the direction of the arrow θ. Therefore, the optical rotation plate 9 attached to the rotating member frame is rotationally controlled in the θ direction by the signal current, and as a result, the laser beam passing through the optical rotation plate 9 is given an optical rotation angle α. Further, as in the above embodiment, in order to provide a known amount of azimuth angle, without using the wave plate 9,
By moving the entire optical head, the polarization direction 112 in FIG.
, 113.

Next, the wave plate 9 that generates a phase difference and the actuator decoy used therein will be explained.

10 to 12 show, for example, a 172-wave plate 9 whose crystal axis 120 is parallel to the optical axis 41, and FIGS. 13 to 1.
FIG. 5 shows polarization characteristics when a half-wave plate 9 in which the crystal axis 120 intersects obliquely with the optical axis 41 is used as a phase plate. In these figures, the same parts are denoted by the same numbers as in FIGS. 1 to 9.

The 172 wavelength plate 9 shown in FIG. 10 is perpendicular to the optical axis 41, that is, when θ=0° in FIG. 11, the phase difference δ=0° in FIG. 12, but the P polarization direction 122 or S polarization is When the angle θ is tilted in the direction 123, the phase difference δ changes as shown at 42 in FIG. 12 or as shown in FIG. Further, the 1/2 wavelength plate 9 shown in FIG. 13 has an inclination θ of θ as shown in FIG.
By changing from 1 to θ, the phase difference δ changes as shown at 42 or 0 in FIG. 15.

Although a 1/2 wavelength plate is used as an example of the phase plate here, other phase control means may be used, such as a Babinet corrector, a Soleil corrector, or a phase plate such as an acousto-optic element that simply uses the photoelastic effect. A similar effect can be obtained by using .

Next, the effect of the actuator for driving the phase plate 9 will be explained.

FIG. 16 shows an example of this actuator effect, and FIG. 17 shows the AA closed cross section of FIG. 16. In each figure, the same numbers indicate the same parts, 55 is the housing of the entire actuator made of non-magnetic material such as plastic, and 51 and 53 are both iron cores made of magnetic material such as iron. Yes, 51
53 is a fixed core that includes a permanent magnet therein, and 53 is a movable core that rotates in the direction of an arrow 116 by a support member 54. 52 is a coil wound around the movable iron core;
56 is a member for fixing the phase plate 9 to the movable iron core group.

With this configuration, a magnetic field as shown by the broken line arrow 117 in the figure is generated in the fixed core 51 and the movable core 53, and since a current is passed through the coil 52, the movable core 53 is Rotate in 116 directions. Therefore, the phase plate 9 is tilt-controlled in the 0 direction by the signal current, and as a result, a phase difference is imparted to the laser light passing through the phase plate 9.

Next, FIGS. 18 and 19 show other embodiments of the present invention. These examples show the minimum configuration required for birefringence measurement according to the present invention, and FIG. 18 shows a measurement method using one-way light, and FIG. 19 shows a measurement method using reciprocating light.

In these figures, 30 is a birefringent medium which is an object to be measured. The light emitted from the laser 5 as a diverging light is converted into parallel light by the collimating lens 6, passes through the wavelength plate 9, and further passes through the birefringent medium garden as parallel light. In FIG. 18, the light enters the differential optical system as it is, but in FIG. 19, it is reflected by the mirror 31, returns to the original optical path again, is reflected by the beam splitter 32, and enters the differential optical system.

The measurement principle and method are the same as in the first embodiment described above. The measurement system of this example is closest to a measurement system using a conventional volarimeter etc. in that parallel light is transmitted through a birefringent medium, but as described above, the measurement method of the present invention
Not only can the measurement work and time be shortened and the dynamic analysis of the object to be measured can be performed, but the device itself can also be significantly simplified and lowered in cost.

〔Effect of the invention〕

As explained in detail above, according to the present invention, the differential output of light transmitted or reflected by a birefringent medium is measured, and the phase difference and azimuth angle of the birefringent medium are determined from this. Regarding the birefringence I11 standard, the amount of work and measurement time can be significantly reduced, and measurement can be performed even when the birefringence medium as the object to be measured is in an operating state.

Furthermore, the present invention is effective in simplifying and reducing the cost of the measuring device. ``When measuring birefringence of optical discs, etc., the measurement accuracy can be significantly improved because the measurement can be performed using the optical head that is actually used. You can make significant improvements.

[Brief explanation of drawings]

1 to 5 are explanatory diagrams of a birefringence measurement method and apparatus showing an embodiment of the present invention, FIGS. 6 to 17 are illustrations of the operation of a wave plate and its actuator, and FIGS. FIG. 19 is an explanatory diagram showing another embodiment of the present invention. 1... Optical disc 3... Cover glass 5.
...Semiconductor laser 9...Wave plates 15, 18
a, 18 b...-Photodetector ■ Foil Toe Whisper 31 Figure 4 Figure S Figure 8 Figure 97 1O (21 Figure 1I Figure 12 Σ Figure 13 Figure 14 Figure 1S Illustration 18 n 19 huh? n

Claims (1)

[Claims] 1. A light source, a means for condensing light from the light source onto a birefringent medium, a polarizing beam splitter for splitting transmitted light or reflected light from the birefringent medium into two, and a polarizing beam splitter for splitting the transmitted light or reflected light from the birefringent medium into two; 1. A birefringence measuring device comprising: two photoelectric converters each receiving light from the birefringent medium; and measuring the amount of birefringence of the birefringent medium based on the difference in output of each of the photoelectric converters. 2. In the apparatus according to claim 1, an optical rotation plate or a phase plate is provided in the optical path, and a differential output at two or more points is determined by providing a known amount of optical rotation angle or phase difference;
A birefringence measurement device characterized by measuring the amount of birefringence of the birefringence medium using these differential output values and the given known quantity, ie, the optical rotation angle or the phase difference.