JPH09105647A - Optical apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical apparatus by which a polarization direction can be selected by passing the linearly polarized component turning accompanying change in the phase difference between luminous fluxes of linearly polarized light obtained by passing the multiplexed luminous flux of two linearly polarized luminous fluxes through a quarter-wave plate, through a polarizing plate. SOLUTION: Linearly polarized luminous fluxes LM1 , LM2 are reflected and transmitted respectively by a polarization beam splitter 3, and a multiplexed luminous flux is obtained, and the multiplexed luminous flux is transmitted through a quarter-wave plate 43 polarization axis is tilted by 45 deg. with reference to the polarization direction of one of one of the luminous fluxes LM1 , LM2 . Then, clockwise polarized light TM1 and counterclockwise polarized light TM2 are obtained, they are overlapped so as to obtain linearly polarized light TL which has a polarization face in a prescribed direction. A polarization direction at this time is changed accompanying the rotation of a radiation grating. Then, the polarized light TL is divided into two luminous fluxes TL1 , TL2 by a semitransparent mirror 10 so as to be introduced into polarizing plates 11, 11' which are used to select respectively prescribed polarization direction components. The polarizing plates 11, 11' are used to change the luminous fluxes TL1 , TL2 into components in a constant vibration direction, and only the components are received by photodetectors 12, 12'.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical device,
In particular, in order to extract a change in the intensity of interference light due to a mutual phase change between two linearly polarized light beams having orthogonal polarization directions at a predetermined phase, for example, displacement information (moving speed, moving direction, rotation speed, rotation) of a displaced object The present invention relates to an optical device that is optimal when applied to an optical encoder or the like that photoelectrically detects a direction.

[0002]

2. Description of the Related Art Conventionally, an optical device for extracting a change in the intensity of interference light due to a mutual phase change between two linearly polarized light beams having polarization directions orthogonal to each other at a predetermined phase is disclosed in, for example, Japanese Patent Publication No. 55-9641. No., JP-A-48-67
No. 59, for example. In this publication, two linearly polarized light beams having orthogonal polarization directions obtained by an interferometer are transmitted through an analyzer having an inclination of 45 ° to extract and interfere with each 45 ° component.

In such an optical device, in order to obtain interference light having a phase difference of 90 °, a form in which a quarter-wave plate having an optical axis parallel to one of two incident light beams and perpendicular to the other is transmitted. Was used.

[0004]

However, in such an optical device, there is a problem that the polarization direction component to be extracted is substantially limited to only the polarization direction component forming 45 ° for each linearly polarized light.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical device in which, when an interference light beam is formed from two linearly polarized light beams having orthogonal polarization directions, a polarization direction to be extracted can be freely selected.

[0006]

The optical device according to the present invention comprises (1)
-1) A quarter-wave plate provided at a position where a combined luminous flux of two linearly polarized luminous fluxes having orthogonal polarization azimuths passes with a polarization axis inclined by 45 ° with respect to any one of the polarization azimuths of the two luminous fluxes And a polarization direction selecting means for selecting a predetermined polarization direction component from the light beam passing through the quarter-wave plate.

In particular, it is characterized in that the polarization direction selecting means has a polarizing plate.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic view of an embodiment when the optical device of the present invention is applied to a rotary encoder.

In FIG. 1, reference numeral 1 denotes a coherent light source such as a laser, 2 denotes a collimator lens, and 3 denotes a polarizing beam splitter. The polarization axis of the linearly polarized light from the laser 1 is 45 °. Are located in 4 ₁ , 4
Reference numerals ₂ and 4 ₃ denote quarter-wave plates, respectively, for the linearly polarized light reflected and transmitted by the polarizing beam splitter 3.
Each polarization axis is arranged at 45 °. That is, 1/4-wave plate 4 ₁ polarization beam splitter 3
The polarization axis of the reflected light beam is 45 ° to the linear polarization direction
Is arranged such that, 1/4-wave plate 4 ₂ is disposed so that its polarization axis with respect to the linear polarization direction of the transmitted light beam of the polarization beam splitter 3 is 45 °. Also 1 /
4 wave plate 4 ₃ is arranged so that its polarization axis with respect to either the polarization direction of the transmitted or reflected light beam of the polarization beam splitter 3 is 45 °. 5 is a reflecting mirror.

Reference numerals 6 ₁ , 6 ₂ , 6 ₁ ′, 6 ₂ ′ denote cylindrical lenses, and 7, 7 ′ denote reflecting mirrors. Reference numeral 8 denotes a radiation grating that forms a scale in which, for example, grid patterns of a light transmitting portion and a reflecting portion are provided at equal angles on a disk, and 9 denotes a rotation axis of a rotating object to be tested (not shown). 10 is a beam splitter, 11, 1
1 'is a polarizing plate, which is arranged so that the polarization directions are at 45 ° to each other. 12, 12 'are light receiving elements.

The light beam emitted from the laser 1 is converted into a substantially parallel light beam by the collimator lens 2 and enters the polarization beam splitter 3. Polarizing beam splitter 3
Indicates that the direction is 4 with respect to the linear polarization direction of the laser 1.
It is arranged so as to be at an angle of 5 °, and divides a light beam from the laser 1 into a reflected light beam and a transmitted light beam having substantially equal light amounts. After passing through the divided two light beams each quarter-wave plate 4 ₁ and 4 _2, a circularly polarized light. Among them, the transmitted light beam via a cylindrical lens _61, is linear irradiance on position M ₁ on the radiation grid 8.

On the other hand, the reflected light flux passes through a reflecting mirror 5 and a cylindrical lens 6 ₁ ′ to a position M ₂ on a radiation grating 8.
The top is illuminated linearly. Here, the cylindrical lens 6
₁ , 6 ₁ ′ are arranged as necessary so as to linearly irradiate the light beam in a direction orthogonal to the radiation direction of the radiation grating 8.
By performing the linear irradiation in this manner, it is possible to reduce the pitch error of the lattice pattern of the translucent portion and the reflective portion corresponding to the irradiated portion of the light beam on the radiation grating 8. In addition, the radiation grating 8
Irradiation position M ₁ and M ₂ above are set such that the positional relationship between a substantially point-symmetrical with respect to the rotation center of the subject rotating object (not shown).

The luminous flux incident on the radiation grating 8 is
Is reflected and diffracted by the lattice pattern. Now, assuming that the pitch of the lattice pattern at the light beam irradiation position is p, the diffraction angle θ _m of the m-th order reflected diffracted light L, L ′ is expressed by the following equation (1).

Sin θ _m = mλ / p ‥‥‥ (1) where λ is the wavelength of the light beam.

Now, assuming that the radiation grating 8 is rotating at an angular velocity ω, the irradiation positions M ₁ ,
Assuming that the distance to M ₂ is r, the peripheral velocity at the irradiation positions M ₁ and M ₂ is v = rω.

[0016]

[Outside 1] Here, represents an inner product of vectors.

[0017] Then, the cylindrical lens 6 _2, 6
'Via the reflecting mirror 7, 7' ₂ re irradiating position M _1, M ₂ in. Then, the m-th order reflected diffracted light beam is diffracted again at the positions M ₁ and M ₂ , and returns to the original optical path. However, it undergoes the amount of Doppler shift represented by the expression (2) again at the time of re-diffraction. Therefore, the light beam that is re-diffracted at the position M ₁ and returns to the original optical path undergoes a Doppler shift of frequency 2Δf, and the light beam that is re-diffracted at the position M ₂ and returns to the original optical path is frequency −2.
It receives a Doppler shift of Δf.

[0018] Here, the light beam is again diffracted back to its original light path at the position M ₁ is 1/4 to 1 again through the wavelength plate 4 _2, its orientation is linearly polarized light LM ₁ next rotated 90 degrees from the incoming, The light is reflected by the polarization beam splitter 3. The light beam is again diffracted by the position M ₂ back to the original optical path is also 1/4 to 1 again through the wavelength plate 4 _1, its orientation is linearly polarized light LM ₂ next rotated 90 degrees from the incoming polarization beam splitter 3 Through. The two linearly polarized light beams re-diffracted at the positions M ₁ and M ₂ overlap and become a combined light beam. Two linearly polarized light beams LM that have passed through the polarizing beam splitter 3
₁ or LM ₂ is through one of the optical beam LM ₁ or quarter-wave plate 4 ₃ tilted 45 ° the polarization axis with respect to the polarization direction of the LM _2, is split by the beam splitter 10 into two beams predetermined polarization Light-receiving elements 12, 1 through polarizing plates 11, 11 'as polarization direction selecting means for selecting direction components
2 ′.

Thus, the frequency 2Δf and the frequency −2Δ
Since the two light beams having undergone the Doppler shift of f overlap each other, the frequency of the output signal of the light receiving elements 12 and 12 ′ is 2
Δf − (− 2Δf) = 4Δf. That is, the frequency F of the output signal of the light receiving elements 12, 12 'is F = 4.DELTA.f = 4.
rωsin θ _m / λ, and from the expression of the diffraction condition of the expression (1), F = 4 mrω / p. If the total number of lattice patterns of the radiation grating 8 is N and the equiangular pitch is Δφ, p = rΔ
From φ, Δφ = 2π / N, F = 2mNω / π ‥‥‥ (3) Now, assuming that the wave number of the output signal of the light receiving element during the time Δt is n and the rotation angle of the radiation grating 8 during Δt is θ, then n = 2mNθ / π よ り from n = FΔt and θ = ωΔt. ‥‥ (4), and by counting the wave number n of the output signal waveform of the light receiving element, the rotation angle θ of the radiation grating 8 can be obtained by the equation (4).

When detecting the rotation angle, it is more preferable that the rotation direction can be detected. Therefore, in the present embodiment, as is known in a conventional photoelectric rotary encoder or the like, a plurality of light receiving elements are prepared and arranged so that the phase of each signal is shifted by 90 degrees, and about 90 degrees due to rotation. A method of extracting a signal indicating the rotation direction from the phase difference signal is used.

In this embodiment, two light receiving elements 1
A phase shift of 90 ° is provided between the output signals obtained by the polarization beam splitter 3 and 1/12 ′.
And creating a combination of the quarter wave plate 4 ₃ and the polarizing plate 11, 11 '.

FIG. 6 shows the components 4 ₃ , 10, 1 constituting the optical device of the present invention after the polarization beam splitter 3 in FIG.
It is a schematic diagram which shows the optical action of 1,11 '.

That is, as shown in FIG. 6, the linearly polarized light beams LM ₁ and LM _{2 which} are re-diffracted at the positions M ₁ and M ₂ on the radiation grating 8 and return to the original optical path are reflected and transmitted by the polarization beam splitter 3, respectively. Are overlapped and become a combined luminous flux, 2
A light beam LM _1, circular polarization TM ₁ and counterclockwise circularly polarizing TM ₂ clockwise by transmitting a polarization axis of 45 ° 1/4 wavelength plate 4 ₃ inclined with respect to either the polarization direction of the LM ₂ Become.

When the two circularly polarized light beams TM ₁ and TM ₂ overlap, a linearly polarized light TL having a plane of polarization in a predetermined direction is obtained as a result of partially canceling out the polarized light components.

At this time, the polarization direction of the linearly polarized light TL changes as the radiation grating 8 rotates. And this linearly polarized light T
L is split into _two light beams TL ₁ and TL ₂ by a half mirror 10 and is guided to polarizing plates 11 and 11 ′ as polarization direction selecting means for selecting predetermined polarization direction components.

The two split light beams TL ₁ and TL ₂
Are only components in a certain vibration direction via the polarizing plates 11 and 11 ′, and only this component is received by the light receiving elements 12 and 12 ′. At this time, light and dark signals S ₁ and S ₂ accompanying the rotation of the radiation grating 8 are obtained from the light receiving elements 12 and 12 ′ for the first time.

In this embodiment, two polarizing plates 11, 11 'are used.
Are shifted from each other by 45 °, whereby light and dark signals S ₁ and S ₂ whose phases are shifted by 90 ° from each other are obtained from the light receiving elements 12 and 12 ′.

Then, as shown in FIG.
The waveforms of the output signals 2, 12 'are shaped, and after detecting the rotation direction, they are integrated in a counter to determine the rotation angle.

FIG. 3 is an explanatory view of the radiation grating 8 of FIG. 1, the irradiation positions M ₁ and M _{2 of the} two light beams on the radiation grating 8 and the rotation center of the rotating object to be measured.

In the present embodiment, two points M ₁ , approximately point-symmetric with respect to the center of the radiation
And the center of the radiation grid 8 by the M ₂ and the measurement point, and reduce the effect of eccentricity of the rotation center of the test rotor.

That is, it is mechanically difficult to completely match the center of the radiation grating 8 with the center of rotation, and eccentricity between the two is inevitable. For example, as shown in FIG. 3, when the amount of eccentricity is a between the center O of the radiation grating 8 and the center of rotation O ′, the measurement point M ₁ at a distance r from the center of rotation is used. The Doppler frequency shift changes from r / (r + a) to r / (ra) as compared to when there is no eccentricity.

On the other hand, at this time, the frequency shift at the measuring point M ₂ which is point-symmetric with respect to the position M ₁ with respect to the center of rotation is opposite to the change at the position M ₁ from r / r−a. r / r
Since changes + to a, by measuring two points located M ₁ and M ₂ at the same time, and reduce the effect of eccentricity.

In the present embodiment, for example, a conventionally used index scale type photoelectric rotary encoder corresponds to the above-mentioned expression (4).
Since the relationship between the wave number n of the output signal from the light receiving element, the total number N of the main scales, and the rotation angle θ is n = Nθ / 2π ‥‥‥ (5), the rotation angle Δθ per one wave number is obtained. Is Δθ = 2π / N (radian) ‥‥‥ (6). On the other hand, in the present embodiment, from equation (4), Δθ = π / 2 mN (radian) ‥‥‥ (7).

Therefore, according to the present embodiment, the rotation angle can be detected with an accuracy of 4 m times higher than that of the conventional example even if the scale having the same division number is used.

In the conventional photoelectric rotary encoder, the distance between the light transmitting portion and the light shielding portion is limited to about 10 μm in consideration of the influence of light diffraction.

Now, as the rotation angle detection accuracy, for example, 3
In order to obtain 0 seconds, the conventional example requires N = 360 × 60 × 60/30 = 43,200 as the number of divisions of the main scale from the above equation (6). Therefore, if the interval between the light transmitting part and the light shielding part at the outermost periphery of the main scale is 10 μm, the diameter of the main scale needs to be 0.01 mm × 43,200 / π = 137.5 mm.

However, according to the present embodiment, in order to obtain the same rotation angle detection accuracy as in the conventional example, the division number of the radiation grating may be 1/4 m. When m = 1 using ± 1st-order diffracted light, a radiation grating 8 for obtaining a rotation angle detection accuracy of 30 seconds.
May be 43,200 / 4 = 10,800. In this embodiment, if the laser diffracted light is used, the distance between the light transmitting portion and the reflecting portion may be small. For example, if the distance is 4 μm, the diameter of the radiation grating is 0.004 mm × 10,800 / π = 13.75 mm. It will be good. That is, according to the present embodiment, the shape that can obtain the same rotation angle detection accuracy as that of the conventional index scale type photoelectric rotary encoder is 1
A size of / 10 or less is sufficient. Accordingly, the load on the rotating object to be tested is much smaller than in the conventional example, and accurate measurement can be performed.

FIG. 4 is a schematic view of a part of another embodiment when the optical device of the present invention is applied to a rotary encoder, and shows a portion where a light beam enters the radiation grating 8 of FIG. In the figure, the numbers assigned to the respective elements indicate the same elements as those shown in FIG. Position M of radiation grating 8
± m-order transmitted diffracted light of the light beam incident on ₁ and M ₂ is again incident on the radiation grating 8 via the cylindrical lenses 6 ₂ and 6 ₂ ′ and the reflecting mirrors 7 and 7 ′, and is returned to the original optical path. Thus, the same effect as in the embodiment shown in FIG. 1 is obtained.

FIG. 5 is a schematic view of another embodiment showing the vicinity of the radiation grating 8 when the optical device of the present invention is applied to a rotary encoder.

In FIG. 5, reference numerals 13 and 13 'denote corner cube reflecting mirrors, respectively. Positions M ₁ and M of radiation grating 8
± m order reflection of the light beam incident on the ₂ (or may be transmitted as shown in FIG.) Diffracted light, the cylindrical lens 6
The light is returned to the original optical path by the corner cube reflecting mirror via ₂ and 6 ₂ ′, and the same effect as that of the embodiment shown in FIG. 1 is obtained. When the center of the radiation grating 8 and the rotation center of the rotating object to be measured are eccentric, the interval between the light transmitting portion and the reflecting portion of the radiation grating 8 changes depending on the location, and therefore the diffraction angles of the diffracted lights L and L ′. Will change.

In the embodiment shown in FIG. 1, a semiconductor laser is most preferable as the light source 1 in terms of its small size, low cost, and high output.

However, it is known that the oscillation wavelength of a semiconductor laser changes due to a change in ambient temperature. In the embodiment of FIG. 1, when the wavelength of the light source 1 changes, the diffraction angles of the diffracted lights L and L 'change. At this time, 7 and 7 'in FIG.
When the plane mirror is used as described above, the reflection mirror is in a so-called tilt state, and a large number of interference fringes are generated on the front surface of the light receiving elements 12, 12 ', and the S / N of the output signal of the light receiving elements 12, 12' is reduced. May be. Therefore, as shown in FIG. 5, by using the corner cube reflecting mirrors 13 and 13 'as reflecting mirrors, even if the diffraction angles of the diffracted lights L and L' diffracted by the radiation grating 8 change for the above reason, the corner cube reflection Mirror 1
The luminous flux reflected by 3 and 13 'can be returned to the original optical path. This prevents the occurrence of many interference fringes on the front surfaces of the light receiving elements 12 and 12 ', thereby improving the S / N ratio of the output signals of the light receiving elements 12 and 12'.

In each embodiment in which the present invention described above is applied to a rotary encoder, two m-order diffracted lights are used. However, even ± m-order diffracted lights have two different orders. Diffracted light may be used.

In each embodiment in which the present invention is applied to a rotary encoder, the eccentricity error between the center of the radiation grating and the rotation center of the rotating object to be tested can be ignored. Only one light receiving element may be provided.

[0045]

As described above, according to the present invention, a combined luminous flux of two linearly polarized luminous fluxes having orthogonal polarization directions is converted into reverse circularly polarized light by passing through a quarter wavelength plate. The linearly polarized light component obtained by the combined state of the light rotates with the change in the phase difference between the orthogonal linearly polarized light beams,
The rotating linearly polarized light component is configured to select a predetermined polarization direction component using, for example, an analyzer,
The polarization direction to be extracted can be freely selected by rotating the selection direction of the polarization direction selection means, for example, the polarization axis in the case of an analyzer. That is, it is possible to realize an optical device in which the polarization direction to be extracted can be freely selected.

[Brief description of the drawings]

FIG. 1 is a schematic view of an embodiment when the present invention is applied to a rotary encoder.

FIG. 2 is an explanatory diagram of a vector display of a light beam incident on the radiation grating and a reflected diffraction light in FIG.

FIG. 3 is an explanatory diagram showing eccentricity between the center of the radiation grating and the center of rotation in FIG. 1;

FIG. 4 is an explanatory view of a part of another embodiment when the present invention is applied to a rotary encoder.

FIG. 5 is an explanatory diagram of a part of another embodiment when the present invention is applied to a rotary encoder.

FIG. 6 is an explanatory view of a part of FIG. 1;

[Explanation of symbols]

REFERENCE SIGNS LIST 1 light source 2 collimator lens 3 polarizing beam splitter 4 ₁ , 4 ₂ , 4 ₃ 1/4 wavelength plate 5, 7, 7 ′ reflecting mirror 6 ₁ , 6 ₂ , 6 ₂ ′ cylindrical lens 8 radiation grating 9 rotating object to be tested Axis of rotation 10 beam splitter 11, 11 'polarizing plate 12, 12' light receiving element 13, 13 'corner cube reflecting mirror

Claims (2)

[Claims] 1. A quarter which is provided at a position where a combined light beam of two linearly polarized light beams having orthogonal polarization directions passes with a polarization axis inclined by 45 ° with respect to any one of the polarization directions of the two light beams. An optical apparatus comprising: a wave plate; and a polarization direction selecting unit for selecting a predetermined polarization direction component from a light beam passing through the quarter-wave plate. 2. The optical device according to claim 1, wherein said polarization direction selecting means has a polarizing plate.