JPS6139289Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an optical scale reading device that uses optical interference, and more particularly to an optical scale reading device that uses a phase modulation type diffraction grating in the scale to improve diffraction efficiency and the like.

2. Description of the Related Art Optical scale reading devices that utilize light interference are conventionally known. In this type of device, a reflection film is deposited in a grid pattern on a glass substrate as a scale, and an amplitude modulation type diffraction grating in which reflection surfaces and transmission surfaces are alternately arranged is used. Such amplitude modulation type diffraction gratings have the following drawbacks.

(1) Diffraction efficiency is low. (2) In order to cause two beams of light among the 0th order and ±1st order diffracted light to interfere, a stopper or the like is required to remove unnecessary diffracted light. Or, head and scale spacing performance is limited.

(3) In interference between ±1st-order diffracted lights, the resolution is 1/
2, which is advantageous, but interference is less likely to occur.

(4) It is difficult to create a dense grid to obtain high resolution.

The present invention has been made in view of these points, and has realized an optical scale reading device that eliminates the above-mentioned drawbacks by using a phase modulation type diffraction grating as a scale. Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, 1 is a coherent light source. For example, a semiconductor laser is used as the light source. L ₁ is a first lens that focuses the output light of the light source 1 . 2 is a cube prism that receives the light transmitted through the lens. 3 is a 1/4 wavelength plate that receives the light passing through the prism. L ₂ is a second lens that receives the light transmitted through the quarter-wave plate and focuses the light. 4 is a scale that receives the incident light that has passed through the lens. A phase modulation type diffraction grating is used as the scale (details will be described later). The light incident on the scale 4 causes multimode diffraction when reflected. This reflected diffraction light passes through the lens L ₂ and the quarter-wave plate 3 and is reflected by the cube prism 2.

5 is a light receiving means for receiving interference fringes generated by the reflected and diffracted light. The light receiving means includes light receiving elements d ₁ to
It is composed of d ₄ . P _A to P _D are amplifiers that receive the output of each light receiving element. 6 is a control circuit which receives the outputs of these amplifiers and outputs a signal OUT1 indicating the moving distance of the scale 4 and a signal OUT2 indicating the moving direction. The operation of the device configured as described above will be explained as follows.

Light emitted from a light source 1 is focused by a lens L ₁ and enters a cube prism 2 . As shown in Figure 2, the cube prism transmits all of the component γ whose polarization angle matches that of the prism for the incident α which has polarization components in all directions, but all of the polarization component β in the direction perpendicular to it is transmitted. It has the property of being reflective. When, for example, a semiconductor laser is used as the coherent light source 1, the light is 50 to 90% linearly polarized. Therefore, if the polarization angle of the semiconductor laser beam is made to match the polarization angle of the polarizing cube prism 2, all linearly polarized components of the incident light will be transmitted. This transmitted linearly polarized light is passed through a quarter wavelength plate 3. At this time, if the quarter wavelength plate 3 is rotated to match the polarization angle, the transmitted light can be made into circularly polarized light as shown in FIG.

This light is focused on the scale 4 via the lens _L2 and diffracted. FIG. 4 is a diagram showing the operation of the phase modulation type diffraction grating. 10 is a glass substrate. 11 is a resist whose surface is coated with Au or A. The depth h of the unevenness between the substrate 10 and the resist portion is set to be equal to 1/4. Here, λ is the wavelength of light. Since the phase difference is λ/2 due to the uneven portion shown in the figure, the 0th-order diffracted light is small, but in the ±1st-order diffraction angle directions, the phase difference becomes π and they become stronger. In this way, the unevenness depth h is determined. In the figure, f ₁ is +1st-order diffracted light, and f ₂ is -1st-order diffracted light. 12 is a scattered light intensity distribution.
In addition to the rectangular grating shown in the figure, the grating shape may be one using a triangular wave phase grating as shown in FIG. 5, one using a sine wave phase grating as shown in FIG. A dimensional phase grating is used. The triangular wave-like phase grating shown in FIG. 5 has a high resolution because the 0th order diffracted light is small and the ±1st order diffracted lights are used. The sinusoidal phase grating shown in FIG. 6 has the advantage of being easy to manufacture. Note that f ₃ in FIG. 6 indicates the 0th order diffracted light.

The reflected diffracted light reflected and diffracted by the scale 4 is also circularly polarized light. This reflected and diffracted light is passed through a lens.
It is collected at L ₂ and passed through the 1/4 wavelength plate 3 again. The transmitted light, like the output light δ shown in FIG. 3, becomes linearly polarized light having a polarization angle of 90° different from the above-mentioned incident linearly polarized light. Therefore,
This time all the light is reflected. This reflected diffracted light forms interference fringes on the light receiving means 5 due to the ±1st order diffracted light. S ₁ and S ₂ in FIG. 1 indicate these interference fringes.
The light receiving elements d ₁ to _{d 4} arranged in the interference fringes S ₁ and S ₂ adjacent to each other generate electrical signals corresponding to the brightness and darkness of the light.

Suppose now that the scale 4 is moved in a certain direction while being irradiated with coherent light from the light source 1. At this time, the light input to the light receiving elements d ₁ to _{d 4} is
Periodic brightness and darkness are produced at twice the grid pitch of scale 4. As mentioned above, these light receiving elements are installed at positions that are out of phase by 90 degrees, so the outputs of these light receiving elements are
The result is a sine wave with a phase shift of 90 degrees. These outputs are input to subsequent amplifiers A to D, respectively. Amplifiers A to D amplify the input signal to an appropriate level and perform impedance conversion. Let the respective outputs of these amplifiers be P _A , P _B , P _C , and P _D .

The control circuit 6 receives these P _{A -PD} outputs and outputs a pulse corresponding to (P _A -P _C ) to one output terminal OUT1. The number of pulses output from the output terminal corresponds to the moving distance of the scale 4. In addition, other output terminals of the control circuit 6
From OUT2, a signal indicating the moving direction of the scale 4 is outputted using the phase difference between (P _A - P _C ) and (P _B - P _D ). For example, the output format of this signal is "1" when scale 4 moves to the right.
The movement to the left can be made to correspond to "0". Or it may be the other way around. By using both outputs from the output terminals OUT1 and OUT2, this device can be used as an optical scale reading device.

The device of the present invention described above has the following features by using a phase modulation type diffraction grating as a scale.

(1) Diffraction efficiency is high because optical losses such as transmission and absorption are small.

(2) Diffraction efficiency is high because only the required order of diffracted light can be extracted due to the grating structure. Therefore, S/N
ratio is improved.

(3) Since only the required order of diffracted light can be extracted as the light beam to be interfered with, a mechanism for blocking unnecessary diffracted light is not required. Therefore, the configuration of the read head is simplified and the operating range of the scale and read head spacing is expanded.

As explained in detail above, according to the present invention, a phase modulation type diffraction grating is used as a scale,
An optical scale reading device with improved diffraction efficiency and the like can be realized.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention. FIG. 2 is a diagram showing the operation of the Cube prism. FIG. 3 is a diagram showing the operation of a quarter-wave plate. 4 to 6 are diagrams showing the operation of the phase modulation type diffraction grating. 1...Coherent light source, 2...Cube prism, 3...1/4 wavelength plate, 4...Scale, 5...
Light receiving means, 6...control circuit, _L1 , _L2 ...lens, _d1 to _d4 ...light receiving element, A to D...amplifier, 1
0...Substrate, 11...Resist.

Claims (1)

[Scope of utility model registration request] A coherent light source is irradiated through an optical system onto a scale forming a diffraction grating, and when the irradiated light is reflected or transmitted, a specific mode of diffracted light among the multiple modes of diffracted light interferes with each other. In an optical scale reading device that receives the interference fringes generated by a light receiving element and extracts them as electrical signals at each certain phase, the phase difference of the reflected light or the phase difference of the transmitted light is alternately and regularly arranged as the scale. An optical scale reading device characterized by using a phase modulation type diffraction grating configured as follows.