JPH06194189A - Optical encoder - Google Patents

Abstract

PURPOSE:To reduce the number of part items to realize a small size and a low cost. CONSTITUTION:A semiconductor laser 1 capable of being polarization-controlled in a TE mode and a TM mode is used. A double refraction plate 4 is arranged in an optical passage from a beam splitter 3 to a mirror 5. A refractive index of the double refraction plate 4 differs according to the polarization direction of a light beam. Namely, optical passage length difference of a transmissive light beam is different in the TE mode from in the TM mode. When a polarization mode of the semiconductor laser 1 is switched at a high speed between the TE and TM modes, light detection output can be obtained from a photo detector 11 while being affected by modulation of phase difference determined by the light passage length difference of the double refraction plate 4, and by taking out photo detection output in the TM mode and that in the TE mode separately, the movement direction can be judged in addition to a movement quantity of a diffraction grating scale 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical encoder such as a linear encoder or a rotary encoder which optically measures the amount of movement of an object to be measured.

[0002]

2. Description of the Related Art The optical configuration of a conventional optical encoder is shown in FIG. In this optical encoder, the moving amount of the diffraction grating scale 119 is calculated based on the diffracted light beam 12 of the light beams 112 and 118.
The interference of 0 is converted into a change in the light intensity and detected. For this reason, the polarization direction of the light beam 105 generated by the semiconductor laser 101 and the lens 102 is set to the direction of 45 degrees between the paper surface and the plane perpendicular to the paper surface by the half-wave plate 103, and the beam splitter having no polarization characteristic is set. It is divided into two at 104.

One light beam is a beam splitter 104.
After being transmitted, the polarized beam splitter 106 splits the polarized beam parallel to the paper surface (P wave) and the polarized beam perpendicular to the paper surface (S wave), and the P wave component is transmitted toward the mirror 111. On the other hand, the S-wave component is reflected by the polarization beam splitter 106, transmitted through the quarter-wave plate 107 to become circularly polarized light, reflected by the mirror 108, transmitted through the quarter-wave plate 107 again, and then P-wave polarized. Is incident on and transmitted through the polarization beam splitter 106, and is further ¼ with the mirror 110.
The wave plate 109 converts the S-wave polarization again, and the polarization beam splitter 106 reflects the S-wave component toward the mirror 111.

The other light beam reflected by the beam splitter 104 is split by the polarization beam splitter 113 into a polarized beam (P wave) parallel to the paper surface and a polarized beam (S wave) perpendicular to the paper surface, and a P wave component is obtained. Are transmitted toward the mirror 117. On the other hand, the S-wave component is reflected by the polarization beam splitter 113, passes through the quarter-wave plate 114 to become circularly polarized light, is reflected by the mirror 116, passes through the quarter-wave plate 114 again, and is P-wave polarized. Is incident on and transmitted through the polarization beam splitter 113, and is further ¼ with the mirror 130.
The wave plate 131 converts the S-wave polarization again into S-wave polarization, and the polarization beam splitter 113 reflects the S-wave component toward the mirror 117.

Here, the quarter-wave plate 114 and the mirror 11
6, a parallel plate 115 is inserted to adjust the optical path length, and the optical path length difference between the S wave component and the P wave component of the light beam through the polarization beam splitter 106 and the polarization beam splitter 113 are set. The difference in the optical path length difference between the S-wave component and the P-wave component of the transmitted light beam is set to be ¼ of the wavelength of the semiconductor laser 101.

Therefore, the phase difference between the interference fringes of the S wave component and the P wave component of the light beams 112 and 118 intersecting at the incident angles ± θ between the mirrors 111 and 117 is 90 degrees. Grating scale 119 for light beams 112 and 118
Of the diffracted light beam 120 of
After the S wave component and the P wave component are separated by the
The interference light intensity is detected at 23 and 124.

FIGS. 6A and 6B show the photodetectors 123 and 1 with respect to the movement amount x of the diffraction grating scale 119.
24 shows the output waveform from 24, L corresponds to a 1/2 scale pitch, and the phase difference δL of both photodetection outputs is 9
It will be 0 degrees. By comparing the outputs of both waveforms, it is possible to determine the moving direction of the diffraction grating scale 119 in addition to the moving amount.

[0008]

However, according to such a conventional optical encoder, in order to obtain two signals whose phases are shifted by 90 degrees as detection signals (scale signals) of the amount of movement of the scales, they are orthogonal to each other. It is necessary to construct an interference optical system for light beams that pass through the same optical path with polarized light and have slightly different optical path lengths, and many optical components such as beam splitters, optical wave plates, and mirrors are required. However, it had the drawback of being large and expensive. Further, when the optical encoder is used for controlling the rotation speed of the rotating spindle motor, since the rotation direction is constant, it is not necessary to determine the direction, and the optical encoder only needs to obtain the scale signal of one phase. Even in such a case, many optical components such as a beam splitter and a mirror and their adjustments are required, which has a drawback that the device is large and expensive.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an optical encoder capable of reducing the number of parts, downsizing and cost reduction. It is in.

[0010]

In order to achieve such an object, the first invention of the present application (the invention according to claim 1) is
A light source whose polarization mode is selectively switchable between TE / TM modes, and a diffraction grating that moves along with the object to be measured at a position where the first and second interference light beams from the light source intersect. The scale, the birefringence means for making the optical path length of the passing light beam different between the TE mode and the TM mode in the generation optical path of the first interference light beam, and the interference light intensity of the diffracted light beam from the diffraction grating scale. And a light detecting means for detecting A second invention (the invention according to claim 2) of the first invention is the first invention, wherein the polarization mode switching means for alternately switching the polarization mode of the light source to the TE / TM mode and the detection output of the light detection means are The first and second sampling and holding means are provided with the first and second synchronization modes in synchronization with the switching of the polarization mode by the polarization mode switching means.
And a means for separating and extracting the detection output in the TE mode and the detection output in the TM mode while switching the sample hold by the sample hold means.

[0011]

Therefore, according to this invention, in the first invention, the first and second interference light beams from the light source intersect on the diffraction grating scale, and the interference light intensity of the diffraction light beam from the diffraction grating scale is increased. It is detected by the light detecting means. The optical path length of the passing light beam in the birefringent means arranged in the generation optical path of the first interference light beam differs depending on whether the polarization mode of the light source is the TE mode or the TM mode. Therefore, when the polarization mode of the light source is the TE mode and when the TM
There is a phase difference between the interference light intensities of the diffracted light beams detected by the light detection means when the mode is selected. Also, the second
According to the invention, when the detection output of the light detecting means is given to the first and second sample and hold means and the polarization mode of the light source is alternately switched to the TE / TM mode, the TE mode is synchronized with the switching of the polarization mode. The detection output in (1 phase scale signal) and the detection output in TM mode (2 phase scale signal) are separated and taken out.

[0012]

EXAMPLES The present invention will now be described in detail based on examples.

FIG. 1 is an optical configuration diagram showing a first embodiment of the present invention. In the figure, 1 indicates the polarization mode TE /
A semiconductor laser capable of selectively switching between TM modes,
1a and 1b are electrodes of this semiconductor laser 1, 2 is a lens,
3 is a beam splitter, 4 is a birefringent plate made of a parallel flat plate made of a material having a birefringent property such as quartz, calcite and titanium oxide, 5, 6 are mirrors, 7 and 8 are interference light beams, 9
Is a diffraction grating scale that moves with an object to be measured (not shown), 10 is a diffracted light beam, and 11 is a photodetector.

The semiconductor laser 1 capable of controlling the TE mode and TM mode polarization is disclosed in Japanese Patent Application No. 4-42313.
In the structure typified by the Japanese Patent No. (prior application), the polarization mode can be selectively controlled by controlling the current injection amount into the electrodes 1a and 1b. The description is omitted because it is described in the above-mentioned prior application.

The light beam generated by the semiconductor laser 1 and the lens 2 is split into two light beams by the beam splitter 3. One light beam is transmitted through the birefringent plate 4 and then reflected by the mirror 5 to be an interference light beam 7. The other light beam is reflected by the mirror 6 as it is to be an interference light beam 8. The interference light beams 7 and 8 intersect on the diffraction grating scale 9, and the diffracted light beam 10 from the diffraction grating scale 9 intersects.
Are provided to the photodetector 11. The photodetector 11 detects the interference light intensity of the diffracted light beam 10.

With the linear movement of the diffraction grating scale 9 in the x direction in the figure, the intensity of the interference light detected by the photodetector 11 is modulated into a sine wave, and the interference light is moved every time the diffraction grating scale 9 moves by one pitch. The intensity is modulated for two cycles.

Here, the polarization mode of the semiconductor laser 1 is set to T
When switching between the E / TM modes, the optical path from the beam splitter 3 to the mirror 5 (optical path for generating the interference beam 7)
The optical path length of can be changed electrically. this is,
This is because the birefringent plate 4 is a parallel plate made of a material having birefringence such as quartz, calcite and titanium oxide, and the refractive index varies depending on the polarization direction of the light beam.

That is, the wavelength of the laser is λ, and the birefringent plate 4
, And the thickness of the birefringent plate 4 is T, the optical path length difference δφ due to the difference in polarization direction is T / (λ
• Given by δn). Here, if the constant of the birefringent plate 4 is set so that δφ = 1/4, when the polarization mode of the semiconductor laser 1 is switched between the TE / TM modes,
The intensity of the interference light of the diffracted light beam 10 in the TE mode and the intensity of the interference light of the diffracted light beam 10 in the TM mode are ¼.
The cycle shifts.

Therefore, when the polarization mode of the semiconductor laser 1 is switched between the TE mode and the TM mode at a cycle sufficiently shorter than the modulation cycle of the photodetection output of the photodetector 11 due to the movement of the diffraction grating scale 9, birefringence occurs. While receiving the modulation of the phase difference δφ set by the optical path length difference on the plate 4, the photodetector 11
The detection output of the scale position can be obtained more. This will be described more specifically with reference to FIGS. 2 and 3.

FIG. 2A shows the photodetection output of the photodetector 11,
2B is a TE / TM mode control signal to the semiconductor laser 1, FIG. 2C is a photodetection output in TE mode (first-phase scale signal), and FIG. 2D is a light in TM mode. The detection output (second-phase scale signal) is shown. FIG. 3 shows a signal processing circuit for obtaining the two-phase scale signals shown in FIGS. 2 (c) and 2 (d).

In FIG. 3, 40 is a photodetection output from the photodetector 11 (FIG. 2A), 41 is a TE / TM mode control signal (FIG. 2B), and 42 is photodetection in the TE mode. Output (FIG. 2C), 43 is photodetection output in the TM mode (FIG. 2D), 51 is an amplifier for amplifying and outputting the photodetection output 40, 52 is an inverter, and 53 and 54 are sample and hold circuits. Is.

In FIG. 1, a polarization mode switching means (not shown) is used to change the polarization mode of the semiconductor laser 1 to TE / TM.
When the mode is alternately switched, the photodetection output 40 from the photodetector 11 is modulated in a sine wave shape according to the movement amount of the diffraction grating scale 9 while being modulated by the optical path length difference δφ due to polarization.

Here, δφ = 1/4 is set, and the amplifier 5
The photodetection output 40 amplified by 1 is applied to the sample and hold circuits 53 and 54, and the TE / TM mode control signal 41 is supplied.
When the signals are output by switching the polarities of both the sample and hold in synchronism with the above, the photodetection output 43 in the TM mode and the photodetection output 42 in the TE mode can be separated and taken out.

The photodetection output 42 in the TE mode and the photodetection output 43 in the TM mode are signals that are 90 degrees out of phase with each other, and by comparing their magnitude relations, the movement amount of the diffraction grating scale 9 is compared. In addition, the moving direction can be determined.

FIG. 4 is an optical configuration diagram showing a second embodiment of the present invention. In the figure, 21 indicates the polarization mode of TE
, A semiconductor laser that can be selectively switched between TM modes, 22 and 23 are waveguides, 24 is a birefringent waveguide, 25,
26 is a lens, 27 and 28 are interference light beams, 29 is a diffraction grating scale, 30 is a diffracted light beam, 31 is a lens, 3
2 is a photodetector.

The waveguides 22 and 23 are made of a material such as quartz, PMMA, or polyimide, have a refractive index guide layer, and are formed at both ends of the semiconductor laser 21 by bending the optical path. The birefringent waveguide 24 is inserted in a part of the waveguide 23, that is, in the generation optical path of the interference beam 28. The birefringence in the birefringent waveguide 24 may be obtained by applying stress to a material having a photoelastic effect such as polyimide or polycarbonate, or by using an optically anisotropic material such as quartz or titanium oxide. You may get it. The lenses 25 and 26 arranged at the end portions of the waveguides 22 and 23 are mixed with silicon oxide or silicon nitride, and have a refractive index distribution in the vertical direction.
It has an aspherical shape in the horizontal direction. Further, the lens 31 arranged on the front surface of the photodetector 32 is also formed similarly to the lenses 25 and 26.

The light beams emitted from the waveguides 22 and 23 pass through lenses 25 and 26 into interference beams 27 and 28, which are irradiated while being collimated, and intersect on the diffraction grating scale 29. The diffracted light beam 30 from the diffraction grating scale 29 is focused on the photodetector 32 by the lens 31. The photo detector 32 detects the interference light intensity of the diffracted light beam 30.

Here, the birefringence waveguide 24, like the birefringence plate 4 shown in the first embodiment, has a different refractive index depending on the polarization direction of the light beam, and the passing light beam of the TE mode and the TM mode. If the optical path length difference is set to 1/4 wavelength, the phase difference of the photodetection output of the photodetector 32 when the diffraction grating scale 29 moves can be set to 90 degrees.

Therefore, the polarization mode of the semiconductor laser 21 is changed to TE / T at a period sufficiently shorter than the modulation period of the photodetection output of the photodetector 32 due to the movement of the diffraction grating scale 29.
When switching between the M modes, it is possible to obtain a scale position detection output from the photodetector 32 while undergoing modulation with a phase difference of 90 degrees set by the optical path length difference in the birefringent waveguide 24.

Extraction of the photodetection output in the TE mode and the photodetection output in the TM mode, that is, the extraction of the two-phase scale signal is performed in the same manner as in the first embodiment described with reference to FIGS. 2 and 3. And the T-mode optical detection output with a phase difference of 90 degrees with respect to each other.
Diffraction grating scale 2 based on photodetection output in M mode
In addition to the movement amount of 9, the movement direction can be determined.

[0031]

As is apparent from the above description, according to the present invention, many light beams such as a polarization beam splitter and an optical wave plate which are necessary for obtaining a two-phase scale signal in the conventional optical encoder are used. A diffracted light beam is generated by a light source capable of selectively switching its polarization mode between TE and TM modes and a birefringence means for making the optical path length of the passing light beam different between TE mode and TM mode, without the need for parts and adjustments thereof. It becomes possible to easily configure an optical system capable of obtaining a two-phase scale signal by detecting the interference light intensity of the beam,
It becomes possible to promote miniaturization and price reduction.

[Brief description of drawings]

FIG. 1 is an optical configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a signal waveform diagram of each part for explaining the operation of the signal processing circuit shown in FIG.

FIG. 3 is a diagram showing a signal processing circuit for obtaining a two-phase scale signal in the first embodiment.

FIG. 4 is an optical configuration diagram showing a second embodiment of the present invention.

FIG. 5 is an optical configuration diagram of a conventional optical encoder.

FIG. 6 is a diagram showing an output signal of each photodetector at the time of moving a diffraction grating scale in a conventional optical encoder.

[Description of Reference Signs] 1 semiconductor laser 2 lens 3 beam splitter 4 birefringent plate 5,6 mirror 7,8 interference light beam 9 diffraction grating scale 10 diffraction light beam 11 photodetector 40 photodetection output 41 TE / TM mode control signal 42 Optical detection output in TE mode 43 Optical detection output in TM mode 53, 54 Sample and hold circuit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Renji Sawada 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. An optical encoder for optically measuring the amount of movement of an object to be measured, a light source capable of selectively switching its polarization mode between TE / TM modes, and first and second light sources from this light source. A diffraction grating scale that moves along with the object to be measured at a position where the interference light beams intersect with each other, and a passing light beam in the TE mode and the TM mode in the generation optical path of the first interference light beam. An optical encoder comprising: a birefringent means for making the optical path lengths of the two different, and a light detecting means for detecting the interference light intensity of the diffracted light beam from the diffraction grating scale. 2. The polarization mode switching means for alternately switching the polarization mode of the light source to the TE / TM mode, and the detection output of the light detecting means to the first and second sample and hold means, and While switching the sample and hold in the first and second sample and hold means in synchronization with the switching of the polarization mode by the mode switching means,
An optical encoder provided with means for separating and extracting a detection output in the TE mode and a detection output in the TM mode.