JPS59216108A - Phase plate - Google Patents

Abstract

PURPOSE:To obtain a phase plate which has relaxed work precision and maintains the functions even to divergent luminous flux by pasting two phase plates which have the same specific angle between plate surface normals and optical axes on specific condition. CONSTITUTION:Work is so performed that the plate surface normals of lambda/8 plates 4 and 5 having the same dimensions are at the angle beta to their optical axes 6 and 7, and those plates are set while their orientation flats 8 for optical axis display are symmetrical about pasted surfaces and adhered together in such a way that adhered surfaces of the lambda/8 plates 4 and 5 form a wedge with an angle delta with an interposed isotropic adhesive layer 9 which has the same refractive index with the lambda/8 plates 4 and 4, thus obtaining lambda/4. When this lambda/4 plate is applied to an ellipsometer, neither high-precision work nor assembly is required specially and a uniform quenching ratio is obtained even if a beam having a very large angle of divergency from a semiconductor laser, etc., is used without being collimated.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a phase plate that produces a desired phase difference between polarization components orthogonal to 71 of light that has passed through a crystal plate that has birefringence or birefringence and optical rotation. .

Conventionally, an ellipsometer having a configuration as shown in FIG. 1 has been used to precisely measure the thickness of an extremely thin 5i02 or other H2 oxide film (approximately 20X) formed on the surface of a high-density electronic memory disk. It is being but,
The quarter-wave plate (hereinafter referred to as a quarter-wave plate), which is a component of this, serves as the standard for film thickness measurement, and it is particularly important to improve its processing accuracy and reduce costs. .

In addition, for the optical pickup of a digital φ audio player or video φ disk player that uses three semiconductor lasers as a light source, it is recommended to use a zero-order mode input/fourth plate, taking into account the wavelength variations in such light sources. desirable.

However, in conventional quartz plates using crystal, it was common to cut out the plate so that the normal to the plate surface and the optical axis of the crystal were perpendicular to each other so that the light rays were incident perpendicularly to the plate surface. For example, the thickness of the zero-order mode λ/4 plate is approximately 177 cm for a He-Ne laser (input = 133281).
The length was 1 m, making processing extremely difficult.

In order to avoid the above-mentioned processing problems, for example, 19λ
A so-called kneeling house type phase plate has been put into practical use, in which a /4 plate and an 18λ/4 plate are bonded together so that their optical axes are perpendicular to each other to form a zero-order mode input/4.

On the other hand, Special Publication No. 52-4943 and Special Publication No. 52-48
As disclosed in Japanese Patent No. 48, a so-called bias kit type phase plate has been proposed which provides a required angle between the plate surface normal and the optical axis. According to this, the zero-order mode crystal λ/4
By setting the bias angle of the plate to, for example, 13°, the thickness of the plate becomes 340 p, m in the zero-order mode for a double He-Ne laser. Therefore, no processing problems arise.

However, in the above-mentioned bonded crystal phase plate, the difference in thickness between the two phase plates that gives the beam tadency of 806 mm is He
-Ne laser wavelength is 17.6 lI, so for example, the phase angle accuracy required in an ellipsometer is 9.
If 0″±1″, the accuracy of the plate thickness difference is ±0.2 p.
, m or less, and processing including adhesion was extremely difficult.

In addition, in order to obtain the above-mentioned phase angle accuracy in the bias cut method, it is only necessary to process the plate thickness to within ±4pm, so there is not much difficulty in processing, and there is also a bonding process. It is advantageous because it does not. However, the precision required when assembling this onto the optical path is difficult; for example, even if the incident beam has a divergence angle of 1 milliradian, the
There was a drawback that the function as a 4-board was degraded.

An object of the present invention is to eliminate the drawbacks of the conventional phase plate described above, and to avoid strict requirements regarding processing accuracy and assembly accuracy during assembly. It is an object of the present invention to provide a phase plate that can maintain its function even when

In order to achieve this object quickly, in the present invention, two phase plates are prepared in which the normal to the phase plate surface and the optical axis thereof form a predetermined angle, and the directions of the slow axis and fast axis are made to coincide, In addition, they are bonded together so that their optical axes are not parallel, and a desired phase difference is created between the polarized light components that intersect by 1μ in the image of the light beam that passes through them.

Hereinafter, the present invention will be described in detail with reference to the IA aspect.

First, *as an example to help understand the invention, extinction ratio deterioration during optical rotation measurement of a conventionally known phase plate will be explained.

Now, as shown in FIG. 1, consider the case where an in/quarter plate is inserted between two orthogonal nicols. Here, 1 is a polarizing prism, 2 is an input/quarter plate, and 3 is an analyzing prism.

As is well known, quartz has both linear birefringence (hereinafter referred to as birefringence) and circular refraction (hereinafter referred to as optical rotation). In such a crystal, it is necessary to consider both the phase difference δ' per unit thickness caused only by birefringence and the phase difference 2ρ' per unit thickness caused only by optical rotation.

Now, on the Poincaré sphere shown in Figure 2, take a vector of size δ' in the direction of the neutral axis Of, C:s,
If a vector of magnitude 2ρ' is taken in the direction of R in the polar axis direction, the intrinsic polarizations X and Xo of this crystal are given at the point where their combined vector intersects the Poincaré sphere. If the angle between this composite vector and the Cs + Gf vector is θ, then θ=jan'(2ρ'/δ') For a normal phase plate, this value is 0.3 to 0.4 degrees (depending on the wavelength). On the other hand, in the case of the above-mentioned phase plate in which the bias angle β of the optical axis is 13', the bias angle β reaches 7 to 8 degrees.

In the input/4 plate, it is possible to see how the polarization changes due to the 6-rotation action with this composite vector as the rotation axis. When taking the extinction of the input/4 plate in Fig. 1,
Even if the linearly polarized light (P) emitted from the polarizing prism 1 passes through the input quarter plate 2, it must become linearly polarized light (Co) again. That is, the light is rotated by 80'' and converted from a point P on the equator (corresponding to linearly polarized light) of the Poincaré sphere to a point co on the equator, with the eigendeflection point XO as the center.

However, if the incident beam has a divergence angle, the magnitude of δ' changes, which causes the rotation axis center x to change.

changes to X, , X2, and the corresponding rotation points C, , C
In other words, as shown in Figure 3, when examining the extinction state of the beam emitted from the analyzer, it is found that although it is extinguished in the center, the beam in the periphery S is suppressed by the incidence of the analyzer prism (linearly polarized light). Since it is elliptically polarized light, it cannot be quenched.

As described above, it will be understood that when the incident beam is not collimated using the conventional phase plate, even if the processing accuracy of the phase plate is improved, it is difficult to obtain uniform extinction over the entire beam cross section.

In order to solve this problem, the present invention adopts the following configuration.

FIGS. 4(1) and 4(2) are a sectional view and a perspective view, respectively, showing an embodiment of a phase plate according to the present invention.

For example, when constructing a 1/4 plate, as shown in the same figure (1), processing is performed so that the surface normal of the 4/8 plates 4 and 5, which have the same dimensions, forms an angle β with the optical axes 6 and 7. Then, these two input/8 plates 4 and 5 are set so that the orientation flat 8 for displaying the optical axis is symmetrical to the bonding surface as shown in (2) in the same figure, so that both birefringence and optical rotation can be adjusted. The tangent M layer 9 has almost the same refractive index as both λ/8 plates.
Glue through. In this case, the adhesive surfaces of re-entrant/8 plates 4 and 5 generally form a wedge of angle δ. The wenge angle δ is derived from the bonding accuracy, but may be set intentionally for a specific purpose.

Now, in a phase plate having such a configuration, an equation for change in refractive index difference is given.

Here, nx, ny and Ilz are each X + 1
and the refractive index in two directions. In the case of crystal, the optical axis is 2
In the direction, nx = ny = 1.546. nz=1.555
is given by

Therefore, if the direction forming an angle β with the optical axis 6 is the optical path lO, and the refractive indices in the slow axis C5 direction and the fast/fast axis Cf direction are n and n', respectively, n/J''=ny by approximate calculation. e Ksin2/3 + ny
(2) nβ' = ny
(3) is given.

If the beam has an emission angle tl'1, Δβ, the optical path length change becomes effective at (Δβ)2, so this is ignored.

That is, This is because.

Also, the change in refractive index difference Δ due to the angle change Δβ in the direction of the light beam
(n/-ny) is Δ(n/g-ny) = ny * fist5in2β・Δβ
(4) is given by.

In the second phase plate whose optical axis direction changes by δ, the refractive index difference change Δ'(17y-ny) is Δ(1'W
ny) =ny @＊5in2(β+δ)・Δ
It is given by β (5).

Therefore, assuming that the thickness of the λ/8 plates 4 and 5 is t, the optical path length change Δt due to Δβ or δ becomes a term of the second or higher order of Δβ and δ as described above. Therefore, if this is ignored, the retardation r will be almost constant and the wavelength will change. For , it is given by .

The retardation phase R is is given by

On the other hand, when the beam is incident almost perpendicularly to the phase plate shown in FIGS. 4(1) and (2), the retardation change Δr and its phase change ΔR due to the divergence angle Δβ and the bonding angle δ are Δr= t(Δ(nρ−fly)−Δ
(n-1).

Therefore, in ΔF, the product of Δβ and δ becomes the main term of the first approximation, so it can be made negligibly small.

For example, β = 0.227 radians (13 degrees), Δβ =
When ΔR is calculated as 0.005 radian (17 minutes) and δ=0.003 radian (10 minutes), ΔR= 2. I x 10 = radian (0,012 degrees)
becomes.

In this case, even if the beam spread is 5 milliradians, the beam can be measured within a measurement error of 0.01 degrees, and the phase plate made by bonding dextrorotatory crystals has been explained.
This corresponds to a bias-cut/4-plate single plate with the same optical rotation and temperature characteristics, and if β is 13 degrees, the optical rotation power will be about 8 degrees.

On the other hand, the optical rotation power and temperature characteristics can be improved by a combination of a right-handed rotation/8 plate and a left-handed rotation/8 plate.

Now, when the above-mentioned synthetic phase plate is set in the orthogonal Nicol extinction system shown in FIG. 1 to obtain extinction, the change in the polarization state will be considered near aS on the Poincaré sphere.

That is, as shown in FIG. 5, the linearly polarized light point P is right-handed
The polarization state point B when passing through the plate 11 is at a position symmetrical to the point P with respect to the slow axis point C8, and the polarization state point B when passing through the plate 11 is symmetrical to the point P with respect to the slow axis point C8.
1, which corresponds to a point rotated by 45 degrees from point P in a counterclockwise direction around the intrinsic polarization axis point Xo.

Next, the polarization state point B is returned to the original point P by the left rotation/8 plate 12. That is, the unique polarization axis point of the left-handed /8 plate and 12 is located at a position symmetrical to Xo with respect to the equator.

This is because the slow axes Cs of the right-handed and left-handed crystal plates are bonded together so that they coincide with each other, and when rotated 456 counterclockwise around this Xo, point B becomes 0 point (i.e., point P), and the apparent optical rotation power disappears. However, in the general case 1, for example, when the incident beam polarization state is close to circularly polarized light, the optical rotation power cannot necessarily be corrected sufficiently and can only be effectively corrected by about 3/4. However, for example, when β = 13°, the combination of right-handed rotation is about 8 degrees, whereas the effective angle of right-handed rotation and left-handed rotation is 1 degree.
．． Since the optical rotation power is around 6 degrees, measurement accuracy is improved.

Also, since the refractive indices no and ne of ordinary and extraordinary rays and the refractive indices nL and n of left-rotated light and dextral-rotated light change due to temperature changes, the characteristic polarization axis point x0. The x0 coordinate and rotation angle will also change. However, since both of these changes are symmetrical to the equator, the temperature dependence of both can be canceled out, although not necessarily completely.

Therefore, when applying the 74 plate according to the present invention to an ellipsometer, a beam with a considerable divergence angle such as a semiconductor laser can be used without collimation without requiring special high-precision processing and assembly. It becomes possible to obtain a uniform extinction ratio.

Furthermore, the phase plate according to the present invention can naturally be applied to an optical pickup section of a digital audio player or a video disc player configured as shown in FIG. Generally, these optical pickup sections pass the beam emitted from the semiconductor laser 13 through a collimating lens 14.
The beam is made into a parallel light beam, passed through an optical isolator 16 with a polarizing film 15 interposed therebetween, makes the beam into a specific polarization state, passes through a λ/4 plate 17 and becomes circularly polarized light, and then passes through a focusing lens 18 to the surface of the disk 19. The reflected light is modulated according to the size of the bit and is passed through the λ/4 plate 17 again so that the polarization state is perpendicular to that at the time of incidence, so that it is completely reflected on the polarizing film 15 surface.
The light is made to enter the sensor 21 via the pattern shaping lens 2o.

In an optical pickup having such a configuration, since the beam emitted by the semiconductor laser 13 has a considerable divergence angle, it is difficult to achieve sufficient collimation, and the input/quarter plate 17 cannot fully perform its function. As a result, the current situation is that light returns to the semiconductor laser 13. Therefore, the emission state of the semiconductor laser I3 becomes unstable, making it difficult to obtain good image and sound quality, and it is also extremely difficult to design the polarizing film 15 in order to reduce the above-mentioned return light as much as possible. There were some problems.

However, if the phase plate according to the present invention is applied, it becomes extremely insensitive even to beams with a large divergence angle, so depending on the specifications of the optical pickup, it is possible to eliminate the beam/L remating optical system.

Moreover, it is obvious that the design of the polarizing film 15 becomes easy and that the cost can be significantly reduced.

Since the present invention is constructed as described above, the following effects can be obtained.

■ Changes in retardation due to changes in beam incidence angle
Since the phase plate cancels out, it maintains its function as a phase plate even for beams that do not match the incident beam on the optical axis of the device or have a large divergence angle. Therefore, even for the beam as described above, good extinction can be obtained over the entire required cross section of the beam.

(2) When obtaining a certain retardation phase angle, the smaller the angle between the normal to the phase plate surface and the optical axis k, the thicker the phase plate can be.

Therefore, the manufacturing accuracy of the thickness becomes the same as the phase angle accuracy of the phase plate, and the yield in manufacturing the phase plate is improved.

■ When applied to an ellipsometer, etc., multiple reflections within the phase plate can be removed off the optical axis of the device by providing the required wedge angle between the plate surfaces of the bonded phase plates. Extinction ratio deterioration due to the influence of interfering light flux due to such multiple reflections can be significantly improved.

■ By bonding dextrorotatory and levorotary phase plates, a phase plate without optical rotation can be obtained. Furthermore, phase plate characteristics with small temperature characteristics can be obtained.

- Phase plates used in the ellipsometer or optical pickup described above generally do not require substantial consideration of the influence of optical rotation, so it is possible to combine dextrorotatory and levorotary phase plates. be. Therefore, a highly functional phase plate can be obtained at low cost using phase plates from the same lot.

[Brief explanation of the drawing]

Figure 3 is a diagram illustrating the display of optical rotation of His Holiness, and Figure 3 is a diagram illustrating deterioration of extinction ratio due to beam broadening, based on the combination of the fourth plate. FIG. 6 is a diagram illustrating the optical rotation characteristics of the input/quarter plate, and is a block diagram showing the optical pickup section of a digital ψ disk player to which the phase plate according to the present invention is applied. l...Polarizing prism, 2...In/4 plates, 3...Analysis prism, 4.5...In/8 plates, 6.7...Optical axis, 8...Optical axis Orientation flat for display, 9...adhesive layer, 10...optical path, 11...right-handed rotation/8 plates, 12...left-handed rotation/8 plates. CS: W phase axis, Cf: fast axis, 13: semiconductor laser, 14: collimator I/lens, 15: polarizing film. 16... Optical isolator. 17...Enter/4 plate, 18...Focal adjustment lens, 18...Disc, 20...Pattern shaping lens, 21...Sensor. Patent applicant: Toyo Tsushinki Co., Ltd.
Applicants Japan Broadcasting

Claims (1)

[Claims] 1) Two phase plates whose angles between the plate surface normal and the optical axis have the same predetermined value are made so that their slow axes and fast axes coincide, and the optical axes are not parallel to each other. A phase plate characterized by being pasted together.