JP2600783B2 - Optical device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION (Industrial application field) The present invention relates to an optical device, and in particular, a phase variation is added to a variation of a physical quantity such as a position, a rotation angle, and a density of a measurement object by the measurement object. Optics that convert into a rotation of the azimuth of linearly polarized light by superposition of coherent light beams, and detect the amount of change of the physical quantity of the measured object and the direction of the change by detecting the rotation amount and direction of the polarization azimuth. It concerns the device.

(Prior Art) Conventionally, a change in a physical quantity such as a position, a rotation angle, and the like of an object to be measured is caused by irradiating light to the object to be measured, and two light beams emitted from the object to be measured or one light beam and another reference. There is a method of superimposing light beams on each other, synthesizing linearly polarized light that rotates according to the phase difference between the two light beams, and detecting and obtaining the rotation amount and direction of the polarization direction at this time. Among these methods, for example, the following method has been conventionally proposed as a method of converting the interference fringes into light and dark and detecting the rotation direction of the amount of polarization.

The two light beams to be superimposed are linearly polarized light whose polarization planes are orthogonal to each other, and the two light beams are transmitted through a phase plate arranged to have anisotropy in a direction inclined by 45 degrees with respect to the polarization direction of both light beams, Both light beams are converted into circularly polarized light rotating in opposite directions. Then, the two light beams are converted into a single linearly polarized light by overlapping. At this time, the polarization direction of the linearly polarized light rotates according to the phase difference between the first two light beams to be superimposed. By transmitting the polarization direction of the linearly polarized light through a polarizing plate having a polarization axis in an appropriate direction, it can be converted into a bright and dark sinusoidal intensity signal.

By appropriately selecting the direction of the polarization axis of the polarizing plate, the phase of the change in brightness can be arbitrarily changed.

For example, when this light beam is divided into two and detected through polarizing plates whose polarization axes are shifted from each other by 45 degrees, the light and dark phases are mutually different.
90 degrees off. Conventionally, a beam splitter has been used as a light splitting unit for splitting light into two lights, reflected light and transmitted light. A metal or dielectric substance is deposited on the light splitting surface of the beam splitter in a thickness on the order of wavelength, and the amount of transmitted light and reflected light is adjusted by the type, thickness, configuration, etc. of the deposited substance. For example, if the transmitted light amount is T, the reflected light amount is R, and the polarized light component is P and S, respectively, the non-polarized beam splitter satisfies T _P = αR _P T _S = αR _S , and the light intensity ratio between the transmitted light and the reflected light. Is always 1
Becomes

On the other hand, it is difficult to reduce the phase difference between the PS polarization components as viewed from the light splitting surface in the light reflected by the light splitting surface to zero, and in many cases, it fluctuates more sensitively depending on the wavelength and the angle of incidence on the reflecting surface. I do.

For example, in a non-polarizing beam splitter having a distribution ratio of 1 at a wavelength of 780 nm, the spectral characteristics of the transmitted light amount and the reflected light amount are as shown in FIG.

The spectral characteristics of the transmitted light phase and the reflected light phase are the fourth.
As shown in the figure.

If a phase difference occurs between the PS polarization components in the reflected light, when the light incident on the beam splitter is linearly polarized light having an azimuth of 45 degrees with respect to the light splitting plane, the light is converted into elliptically polarized light by the beam splitter. As a result, of the two light beams obtained by the non-polarizing beam splitter, the reflected light beam having a large phase difference between the PS polarized components is observed through a polarizing plate having a polarization direction of 45 degrees that transmits the same amount of the PS polarized component. In this case, the amount of light is lost even if the linearly polarized light incident on the beam splitter and the polarization direction of the polarizer match, and light leaks from the polarizer even if the incident linearly polarized light and the polarization direction of the polarizer are orthogonal. The contrast of light and dark, which changes with the rotation of the plane of polarization of the linearly polarized light incident on the beam splitter, is reduced.For example, when the phase difference between the P-S polarized light components becomes 90 degrees, the light becomes circularly polarized light. Even through a polarizing plate, the brightness cannot be observed.

In addition, the phase difference between the PS polarization components changes sensitively with respect to the wavelength fluctuation of the light source and the error of the incident angle. For example, when a light source such as a semiconductor laser is used, the oscillation wavelength fluctuates on the order of about 10 nm with respect to variations in the center wavelength of individual lasers or changes in temperature. As a result, good contrast between light and dark is detected. It becomes very difficult.

As shown in FIG. 4, it can be seen that this tendency is remarkable in the light flux generally reflected on the split surface of the non-polarizing beam splitter.

Also, when an element such as a total reflection surface that generates a phase difference between the PS polarization components is inserted in a path for transmitting a rotating linearly polarized light beam before and after the light splitting means, that is, 45 degrees with respect to the reflection surface. The linearly polarized light having the azimuth of degree is similarly converted into elliptically polarized light, and the same problem as described above occurs.

In an encoder or the like that also needs to detect the direction of the displacement of the position and rotation, the two outputs are A-phase and B-phase, and if the reflected light is used among the two light beams obtained by the non-polarizing beam splitter, only the A-phase is used. However, there is a problem in that the amplitude component changes in response to a change in the environmental temperature, and the measurement accuracy and the like deteriorate.

(Problems to be Solved by the Invention) The present invention converts a change in a physical quantity of an object to be measured into a rotation of a polarization direction of light based on the object to be measured, and detects the rotation of the polarization direction through a light splitting unit. When the variation amount and the variation direction of the object to be measured are obtained by, for example, even if the wavelength of the light incident on the object to be measured fluctuates, the light and dark signals of the interference fringes can be obtained by appropriately setting the reflecting surface and the light dividing means. It is an object of the present invention to provide an optical device that can always perform high-precision detection without causing fluctuation or variation of the amplitude component of the optical device.

(Means for Solving the Problems) In an optical device for detecting the polarization direction of a light beam, a first reflecting member for reflecting a light beam whose polarization direction rotates is provided,
A second reflecting member that reflects light reflected as a P-polarized component by the reflecting member as an S-polarized component or reflects light reflected as an S-polarized component by the first reflecting member as a P-polarized component; A polarization element for extracting only a component of a specific polarization direction from a light beam reflected from a member, and a detection unit for detecting light emitted from the polarization element.

(Embodiment) FIG. 1 is a schematic diagram of a first embodiment when the present invention is applied to a four-phase output type rotary encoder.

In FIG. 1, reference numeral 1 denotes a light source, which comprises, for example, a semiconductor laser or the like. Reference numeral 2 denotes a collimator lens that converts a light beam from the light source 1 into a parallel light beam. Reference numeral 3 denotes a prism having a polarizing beam splitter surface 4 as a light splitting surface inside. 5a and 5b are folding prisms, which are provided in a part of the prism 3. 6 is a radial diffraction grating disk,
It is attached to a rotating object to be measured (not shown). 7a, 7b
Is a quarter wavelength plate, and 8a and 8b are reflection members, which are composed of, for example, a reflection mirror or a cat's eye optical system. 9 is a quarter-wave plate, 10 is a first non-polarizing beam splitter, 11 and 12 are second and third non-polarizing beam splitters, 13a to 13d are polarizing plates, 14a to 14d, respectively.
Are detection elements for each of the four phases.

In this embodiment, the light beam from the light source 1 is converted into a parallel light beam by the collimator lens 2 and is incident on the incident surface 3a of the prism 3.
After being reflected by the reflection surface 3b, the light is guided to the polarization beam splitter surface 4 and divided into two linearly polarized light beams, for example, a horizontal direction and a vertical direction, of the reflected light 1a and the transmitted light 1b having the same amount of light.

Among them, for example, reflected light 1a of linearly polarized light in the horizontal direction with respect to the polarizing beam splitter surface 4 is reflected by the reflecting surface 3b and the reflecting surface of the folding prism 5, and then is incident on the surface of the diffraction grating disk 6.

Then, the diffracted light from the diffraction grating disc 6, for example, the first-order diffracted light is converted into circularly polarized light through the quarter wavelength plate 7a and reflected in the same optical path by the reflecting member 8a as circularly polarized light in the opposite direction. The light is passed, and its polarization direction is changed to 90 degrees with respect to the light beam on the outward path to be linearly polarized light.

Next, the light is guided into the prism 3 via the diffraction grating disk 6 and the folded prism 5a, reflected by the reflection surface 3b, and passed through the polarization beam splitter surface 4. After being reflected by the reflection surface 3c, the light is emitted from the emission surface 3d.

On the other hand, transmitted light of linearly polarized light in a direction perpendicular to the polarization beam splitter surface 4 that has passed through the polarization beam splitter surface 4
1b is reflected by the reflecting surface 3c, reflected by the reflecting surface of the folded prism 5b, then incident on the diffraction grating disc 6, and diffracted light from the diffraction grating disc 6, for example, -1st-order diffracted light, is a quarter wavelength plate. Linearly polarized light having a circular polarization through 7b, reflected in the same optical path as reverse circularly polarized light by the reflection member 8b, passed through the quarter-wave plate 7b, and changed its polarization direction by 90 degrees with respect to the light beam on the outward path. And

Next, the light is introduced into the prism 3 via the diffraction grating disk 6 and the folded prism 5b, and is reflected in the order of the reflection surface 3c and the polarization beam splitter surface 4. After being reflected again by the reflection surface 3c, the light is emitted from the emission surface 3d.

At this time, the two light beams emitted from the exit surface 3d of the prism 3 overlap, pass through the quarter-wave plate 9, and are converted into linearly polarized light (a light beam whose polarization direction rotates). At this time, the direction of the linearly polarized light rotates with the rotation of the diffraction grating disk 6, and rotates twice for one pitch of the radiation grating.

The linearly polarized light beam is converted into a first non-polarized beam splitter 10.
The light is split into two light beams, a reflected light 15a and a transmitted light 15b, by a first light splitting surface (first reflecting member) 10a. Among them, the reflected light 15a has a phase δ between the PS polarization components as viewed from the light dividing surface 10. Therefore, the second light splitting surface (second reflecting member) 11a of the second non-polarizing beam splitter 11 is changed to the first non-polarizing beam splitter 1
0 in the first light splitting surface 10a, the relationship between the P-polarized light component and the S-polarized light component is reversed.
The light acting as the polarized light component is caused to act as the S polarized light component on the second light splitting surface 11a.

Conversely, light acting as an S-polarized light component on the first light splitting surface 10a is caused to act as a P-polarized light component on the second light splitting surface 11a. As a result, the phase difference between the polarized light components of the light reflected by the first light splitting surface 10a and the light reflected by the second light splitting surface 11a is offset. Then, the light at this time is detected by a detecting element (detecting means) 14a via a polarizing plate (polarizing element) 13a having a polarizing axis directed in an arbitrary direction, thereby obtaining a bright and dark signal with good contrast.

Further, since the light that has passed through the second light splitting surface 11a does not cancel out the phase difference, the detection element 1 passes through the polarizing plate 13b whose polarization axis is directed to the direction in which only the P-polarized component or the S-polarized component passes.
Detected in 4b.

On the other hand, the first light splitting surface of the first non-polarizing beam splitter 10
The light 15b that has passed through 10a is split into two by a third light splitting surface 12a of the third non-polarizing beam splitter 12.

The light 15b that has passed through the first light splitting surface 10a has a slight phase shift δ
Has the _0, the light beam passes through the third beam splitting surface 12a is a phase difference [delta] ₀ because the relationship is reversed between the P-S-polarized light component is canceled out. Therefore, even if this light beam is received by the detection element 14c via the polarizing plate 13c whose polarization axis is directed in an arbitrary direction, a bright and dark signal with good contrast is always obtained.

Also, since the light reflected by the third light splitting surface 12a does not cancel out the phase difference, it is detected by the detecting element 14d via the polarizing plate 13d whose polarizing axis is directed so that only the P-polarized component or the S-polarized component passes. ing.

In this embodiment, the polarizing plate may be formed of another optical member as long as it is an element for defining the polarization direction.

In this embodiment, the rotation angle and the displacement direction of the object to be measured attached to the diffraction grating disk 6 are detected using the four-phase output signals obtained from the detection elements 14a to 14d.

FIG. 2 is a schematic view of an embodiment when the present invention is applied to a two-phase rotary encoder.

In this embodiment, the operation from the light source 1 to the prism 3, the emission from the exit surface 3d and the passage through the quarter-wave plate 9 is exactly the same as in the first embodiment shown in FIG.

Linearly polarized light passing through the quarter-wave plate 9 (polarization direction of light beams to rotate) are reflected deviated [delta] ₁ of phase between P-S-polarized light component by the reflecting surface (first reflecting member) 17 in FIG. . This light beam is split into two by the non-polarizing beam splitter light splitting surface 10, and the reflected light beam is added with a phase shift δ ₂ between PS and PS. The polarization plate 13 arranged so as to coincide with the direction of the P or S polarized light component does not cancel even if
The signal is detected by the detection element 14a for the A phase via a. on the other hand,
The light beam that has passed through the light splitting surface 10 is reflected by the reflection surface 17 and the reflection surface (second reflection member) 18 of the same quality, and the relationship between the reflection surface 17 and the PS polarization plane is reversed. Phase shift-
caused a [delta] _1, the phase shift between the summed P-S is almost cancel 0. Even if an arbitrary polarization component is taken out by a polarization element, a light signal with good contrast can be obtained from this light flux. The detection is performed by the B-phase detecting element (detecting means) 14b. With this configuration, the phase difference between sinusoidal signals obtained from the A and B phases is 9
0 degrees.

The present invention can be applied not only to the rotary encoder but also to a linear encoder. Furthermore, the present invention can be similarly applied to an optical measuring instrument utilizing a change in optical path length, a change in phase, and the like of two interference lights of linearly polarized light orthogonal to each other.

(Effects of the Invention) According to the present invention, when a change in a physical quantity of an object to be measured is converted into a rotation of a polarization direction of a linearly polarized light beam, the reflection in the light splitting unit among a plurality of light beams split by the light splitting unit is determined. By detecting the light through at least one other reflecting surface such that the phase shift between the PS polarization components generated on the surface cancels out, for example, even if the oscillation wavelength from the light source changes, the contrast of the interference signal changes. Thus, an optical device that can always perform high-precision detection with good balance between output signals can be achieved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a first embodiment when the present invention is applied to a four-phase rotary encoder, and FIG. 2 is a schematic diagram of a second embodiment when the present invention is applied to a two-phase rotary encoder. FIGS. 3 and 4 are illustrations of spectral characteristics of reflected light and transmitted light in a conventional non-polarizing beam splitter. In the figure, 1 is a light source, 2 is a collimator lens, 3 is a prism, 4 is a beam splitter surface, 5a and 5b are folding prisms, 6 is a diffraction grating disk, 7a, 7b and 9 are 1/4 wavelength plates, 8a , 8b are reflective members, 10, 11, 12 are non-polarizing beam splitters, 13a
13d is a polarizing plate, and 14a to 14d are detection elements.

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tetsu Ishii 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-63-263452 (JP, A) JP-A-62 -200219 (JP, A) JP-A-61-66926 (JP, A)

Claims (1)

(57) [Claims] 1. An optical device for detecting a polarization direction of a light beam, wherein the first reflection member reflects a light beam having a rotating polarization direction, and light reflected as a P-polarization component by the first reflection member is converted to an S-polarization component. A second reflecting member for reflecting light reflected by the first reflecting member as an S-polarized light component as a P-polarized light component, and a polarized light for extracting only a component of a specific polarization direction from a light beam reflected from the second reflecting member; An optical device comprising: an element; and detection means for detecting light emitted from the polarizing element.