JP2560513C - - Google Patents

Description

Description: BACKGROUND OF THE INVENTION [Field of Industrial Application] The present invention relates to an encoder capable of performing high-precision measurement using light diffraction and interference. [Prior Art] Conventionally, as an encoder of this type, for example, one disclosed in Japanese Patent Application Laid-Open No. 63-309816 is known, and specifically, has a configuration as shown in FIG. The coherent light emitted from the laser diode 60 is converted into a condensing lens 61
Generates a condensed light beam, and the light beam is split into two P and S-polarized light beams by the polarization beam splitter 62. The polarized lights are respectively reflected by the reflecting mirrors 63a and 63b, and reach the transmissive diffraction grating 64 at a predetermined equal incident angle. The diffraction grating 64 has a predetermined pitch along the measurement direction, and is provided movably along the measurement direction. Here, the condensing lens 61 is configured to condense each light beam passing through the polarizing beam splitter 62 and the reflecting mirrors 63a and 63b on the diffraction grating 64. Now, each light beam that has crossed the diffraction grating 64 with a predetermined equal incident angle is diffracted by the diffraction grating 64 in a direction orthogonal to the pitch direction of the diffraction grating 64 to correct a change in the diffraction angle. The light is converted into a parallel light beam by the lens 65. Here, the lens 65 for correcting the change in the diffraction angle is configured to cancel the influence of the change in the diffraction angle even when the diffraction angle changes due to the wavelength change of the output light due to the temperature change of the laser diode 60. It is functioning. That is, even when the diffraction angle changes, the lens 65 forms a parallel light beam while directing the two light beams illuminating the diffraction grating 64 in two predetermined directions in the same direction, so that the two parallel light beams always overlap. Have been. After that, the parallel light beam passing through the lens 65 is converted into circularly polarized light by passing through the 波長 wavelength plate 66, and is split into two by the beam splitter 67. Each split light beam is photoelectrically detected by the photodetectors 69a and 69b via the polarizing plates 68a and 68b, and is based on the output signals of the light receiving elements 69a and 69b according to the movement of the diffraction grating 64 in the measurement direction. Thus, the moving amount of the diffraction grating 64 can be detected based on the phase difference between the two output signals. Therefore, the encoder described above is a lens for correcting a change in the diffraction angle.
With the arrangement of 65, even when the diffraction angle changes, the photodetector can always detect the interference light, so that high-precision measurement can be performed. [Problem to be Solved by the Invention] In the above-described conventional technology, a change in the diffraction angle of the moving diffraction grating 64 due to a wavelength change due to a temperature change of the laser diode 60 is corrected by disposing a correction lens 65. However, it is cumbersome and difficult to adjust the focus and the optical axis of the correction lens 65 and the movable diffraction grating 64. As described above, this apparatus is advantageous in changing the diffraction angle with respect to the wavelength fluctuation of the light source, but the transmission type movable diffraction grating 64 is insufficiently corrected for the fluctuation in the vertical direction in FIG. is there. That is, due to this change, the emission directions of the two diffracted lights reaching the detectors 69a and 69b change, and the state of condensing the detection light via the correction lens 65 changes. The interference state on the detection surface changes greatly. As a result, the output signal detected by the detector may become unstable. In order to obtain two or more sinusoidal output signals having different phase differences and discriminate the moving direction of the diffraction grating 44, a plurality of optical components such as a polarizing beam splitter, a wave plate, a beam splitter, and a polarizing plate are used. doing. For this reason, not only the number of parts increases, but also the optical configuration becomes complicated, and further, it is difficult to adjust them. As a result, there is a possibility that not only an increase in the size of the device but also an increase in manufacturing costs may occur. Therefore, the present invention has been made in view of the above-described problems, and has a simple configuration and extremely easy adjustment in manufacturing, but has an interference with a wavelength variation of a light source due to a temperature change and a mechanical variation of a scale. It is an object of the present invention to provide an inexpensive diffractive interference encoder capable of performing highly accurate measurement in which a change in the light interference state is extremely small. [Means for Solving the Problems] In order to achieve the above object, the present invention, specifically, as shown in FIG. 2A, comprises first diffraction gratings arranged in parallel and having the same periodic pitch as each other. And a second diffraction grating (30, 40); and a parallel light beam supply means (5) for vertically irradiating the first and second diffraction gratings (30, 40) with a coherent parallel light beam L from the first diffraction grating (30) side. 1,2) and a first position separated by a distance between the first diffraction grating (30) and the second diffraction grating (40) due to the diffraction action of the first and second diffraction gratings (30, 40). In order to cause the diffracted lights intersecting at the first position and the second position to interfere with each other, at least one of each of the first diffraction gratings (30) is independently provided corresponding to the first position and the second position. A diffraction provided at a periodic pitch equal to and parallel to the first and second diffraction gratings (30, 40). A third diffraction grating having grating elements (31a, 31b), and a predetermined one of the diffraction interference lights generated for each diffraction grating element (31a, 31b) of the third diffraction grating. Light detecting means (5a, 5b) for photoelectrically detecting independently, and each of the diffraction grating elements (31a, 31b) of the third diffraction grating is provided with the light detecting means (5a, 5b).
In order to give a relative phase difference between the detection signals of the diffraction interference light detected independently by the first and third diffraction gratings, the first and third diffraction gratings are provided at a relatively predetermined distance in the pitch direction. It detects a relative movement amount of the second diffraction grating (40) in the pitch direction with respect to (30, 31a, 31b). Further, the second diffraction grating (40) is formed of a reflection type diffraction grating, and the first and third diffraction gratings (30, 31a, 31b) are provided so as to be integrated on the same plane of the same member. Is also good. [Operation] As described above, according to the present invention, the first, second, and third diffraction gratings formed at the same periodic pitch in the same direction are arranged in parallel at equal intervals and in parallel with each other. Is formed so as to have at least two diffraction grating elements, and a parallel light beam is vertically irradiated from the first diffraction grating side, and is generated for each diffraction grating element of the third diffraction grating by the diffraction action of the first to third diffraction gratings. Only one predetermined diffraction interference light to be detected is independently detected by the photodetector. More specifically, as shown in FIG. 1A, the parallel light beam L is applied to the first diffraction grating 30 of the scale 3.
The by vertical irradiation, the theta _n direction -n-order diffracted light L (-n) is generated, in the vertical direction zero-order diffracted light L (0) is generated. Here, the right-sided nth-order diffracted light with respect to the 0th-order light is referred to as + nth-order diffracted light, and the left-handed one is referred to as -nth-order diffracted light. By this -n-order diffracted light L (-n) illuminates the second diffraction grating 40 of the reflective scale 4 at an incident angle theta _n, n order diffracted light L (-n, n) is generated in a vertical direction, Further, when the 0th-order diffracted light L (0) from the first diffraction grating 30 is perpendicularly incident on the second diffraction grating 40, -nth- _order diffracted light L (0, -n) is generated in the θn direction. N from the second diffraction grating 40
The first-order diffracted light L (n, n) and the zero-order diffracted light L (0, n) intersect on the third diffraction grating 31a of the scale 3, and the two diffracted lights are the diffraction grating elements 31a of the third diffraction grating ( Hereinafter, the third
This is referred to as a diffraction grating element 31a. ). Then, interfere with diffracted light each other to emit the same direction, for example, as shown in Figure 1A, the interference light I ₀ generated in a vertical direction is guided to a detector (not shown). Now, when the wavelength of the parallel light beam L diffracted at the diffraction order θ _n by the n-th order by the first diffraction grating 30 shifts, for example, the diffraction angle changes to θ _nH , but the first diffraction grating 30 has the same pitch as the first diffraction grating 30. Since the diffraction angles of the second diffraction grating 40 and the third diffraction grating element 31a similarly change to θ _nH , the interference light that interferes due to the diffraction action of the third diffraction grating element 31a simply crosses from I ₀ to I ₁ . Only by shifting, the interference state is always kept constant. Also, as shown in FIG. 1B, for example, when the second diffraction grating formed on the moving scale fluctuates downward (gap fluctuation) as indicated by a dotted line, the diffraction effect is obtained by the first to third diffraction gratings. The interference light simply shifts laterally from I ₀ to I ₂ , and the interference state is always kept constant. As described above, even when the diffraction angle changes due to the wavelength change or when the second diffraction grating or the like changes in the direction orthogonal to the pitch, the two detection light beams that interfere with the diffraction action of the first to third diffraction gratings. Can be made equal to each other, so that stable interference light whose interference state is always constant can be obtained. Further, in the present invention, each third diffraction grating element is shifted by a predetermined distance in the pitch direction so as to give a predetermined relative phase difference to an output signal of the diffraction interference light photoelectrically detected. It is. Thus, a push-pull signal, a direction discrimination signal, and the like can be easily obtained electrically, so that highly accurate measurement can be performed. Further, according to the present invention, since a simple configuration capable of minimizing the number of components of the optical member can be realized, not only the adjustment in manufacturing is easy, but also a reduction in the weight and cost of the device can be achieved. . Embodiment FIG. 2A shows a reflection type encoder in which the diffraction grating 4 is formed by a reflection type diffraction grating. The first embodiment of the present invention will be described in detail with reference to FIG. 2A. Will be described. As shown in the figure, a transmission scale 3 and a reflection-type moving scale 4 opposed to and parallel to the transmission scale 3 are provided so as to be movable in the direction of the arrow (horizontal direction). On the transmission scale 3, a diffraction grating 30 as a first diffraction grating and diffraction grating elements 31a and 31b constituting another third diffraction grating independent of the diffraction grating 30 are formed on the same plane. On the reflection type moving scale 4, a diffraction grating 40 as a second diffraction grating is formed. Then, the diffraction gratings 30, 31 provided on each scale
a, 31b and 40 are formed at a periodic pitch in which the widths of the concave portions and the convex portions are equal. Above the diffraction grating 30 on the transmission scale 3, a laser diode 1 and a collimating lens 2 for supplying a parallel light beam are arranged. The diffraction grating elements 31a and 31b on the transmission scale 3 (hereinafter, diffraction gratings 31a and 31b)
b. ), The detectors 5a, 5b corresponding to the diffraction gratings 31a, 31b.
b are arranged respectively. First, a coherent (coherent) light beam supplied from, for example, a laser diode 1 as a light source is converted into a parallel light beam L by a collimator lens 2 and vertically incident on a diffraction grating 30 formed on a transmission scale 3. Then, when the parallel light beam passes through the diffraction grating 30, diffracted light is generated, and in the direction in which the parallel light beam passes, the 0th-order light L (0) is interposed between the 0th-order light L (0) and right and left θ In the direction, n-order light L (n) and -n-order light L (-n) are generated. Here, if the wavelength of the light beam supplied from the laser diode 1 is λ, the pitch of the diffraction grating 30 is P, and the order of the diffracted light generated by the diffraction grating 30 is n, the diffraction grating
Diffraction angle theta _n of ± n order diffracted light generated by 30 is given uniquely by the following equation. Here, n is an integer (n: 0, ± 1, ± 2 ...). At this time, the diffraction angle of the _nth-order diffracted light is θ _n , and the diffraction angle of the −nth- _order diffracted light is θ _−n , and a relationship of θ _n = −θ _−n is established. Therefore, the description will be made _{assuming that} the diffraction angles of the ± n-order diffracted lights are both θn. Now, consider the 0th order light L (0), the 1st order light L (1), and the -1st order light L (-1) generated from the diffraction grating 30. As shown in FIG. 2A, the zero-order light L (0) is perpendicularly incident on the diffraction grating 40 of the reflection type moving scale 4, while the first-order light L (1) and the minus first-order light L (-1) are generated. Are obliquely incident on the diffraction grating 40 of the reflection type moving scale 4 at incident angles θ opposite to each other. Then, the diffraction grating 40 on the reflection-type moving scale 4 diffracts the zero-order light L (0) again, and in the left and right θ directions, the primary light L (0,1) and the −1st-order light L (0, −). 1) is generated, and the primary light L (1) is diffracted again to generate a −1st-order light L (1, -1) in the vertical direction. At the same time, the diffraction grating 40 re-diffracts the -1st-order light L (-1) so that the first-order light L (-1,1) in the vertical direction.
Generate The diffracted light L (0,1) and the diffracted light L (1, -1) thus generated and the diffracted light L (0,1)
-1) and the diffracted light L (-1,1) are transmitted scale 3 and reflective moving scale 4 respectively.
Intersect at different positions on the scale 3 that are separated by the distance (gap) between. Diffraction gratings 31a and 31b are provided independently of the diffraction grating 30 corresponding to the respective intersection positions. The diffraction gratings 31a and 31b are arranged in the moving direction of the reflection type moving scale 4 as shown in FIG. 2B. , For example, are separated by a predetermined distance so as to cause a relative shift of 1/4 pitch. Then, the pair of the diffraction grating 31a, the diffracted light respectively intersect on 31b, the respective diffraction grating 31a, is diffracted respectively by 31b, the interference light in the direction in which the orders of the diffracted light is generated (I _A, I _B , I _{N1 to} I _N4 ) are generated. In the present embodiment, and it detects the direction of arrangement of the grating as indicated by the solid line two interference light generated in a direction perpendicular to the (pitch _{direction) (I A, I B)} . The interference light I _A is the diffraction effect of the diffraction grating 31a, the diffracted light L (0, -1) generated in the direction orthogonal to the array direction of the grating of the diffracted light L (-1 generated in the primary light and the transmission direction while generated by the 0-order light of 1), the interference light I _B is the diffraction effect of the diffraction grating 31b,
It is generated by the -1st-order light of the diffracted light L (0,1) generated in the direction orthogonal to the arrangement direction of the grating and the 0th-order light of the diffracted light L (1, -1) generated in the transmission direction. And these interference light (I _A, I _B), the diffraction grating 31a, a pair of detectors 5a disposed corresponding to 31b, is photoelectrically detected by 5b, the movement amount of the reflection type moving scale 4 Two corresponding sinusoidal output signals A and B are obtained. At this time, the relative phase difference between the two output signals A and B is determined by the diffraction grating 31a,
Since 31b is provided so as to cause a relative shift of 1/4 pitch in the moving direction of the second diffraction grating, the state is shifted by 90 °. Therefore, it is possible to detect the discrimination and the amount of movement of the moving direction of the reflective moving scale 4 based on the two output signals A and B in which the phases are relatively shifted. Here, when the moving scale 4 having the second diffraction grating 40 is configured as a reflection type as in this embodiment, the parallel light beam incident on the transmission scale 3 and the interference light emitted from the transmission scale 3 to be detected are detected. For separation, the transmission scale 3 and the moving scale 4
It is desirable that the gap g (gap) satisfy the following condition. Here, n: the order of the n-th order diffracted light for detection diffracted by the first diffraction grating 31 of the scale 3. φ: parallel light beam diameter. λ: wavelength of a parallel light beam. P: pitch of the first diffraction grating 31 of the scale 3. Now, as shown in FIG. 2A, in the detectors (5A, 5B), the diffracted lights generated in a direction orthogonal to the arrangement direction (pitch direction) of the diffraction gratings (31a, 31b) on the scale 3 are perpendicular to each other. Although the interference light is detected, as described above, interference light ( _IN1 , _IN2 , _IN3 , _IN4 ) as shown by the dotted lines is also generated in the right and left directions with respect to the pitch direction. However, when these interference lights are incident on the detectors 5a and 5b, the phases of the output signals A and B photoelectrically converted by the detectors 5a and 5b greatly jump due to a change in the diffraction angle caused by a gap change or a wavelength shift. Precise position detection may be difficult. Therefore, in the present embodiment, the arrangement direction of the gratings of the diffraction gratings 31a and 31b on the scale 3 (
Detector 5a so that only interference light generated in the orthogonal direction (pitch direction) can be detected.
, 5b are arranged apart from the diffraction gratings 31a, 31b. Thus, stable output signals A and B with high reliability can be obtained. In order to shield the interference light which becomes noise, a stop having a predetermined opening shape, a cylindrical light blocking member, or the like may be provided. Thus, not only the predetermined interference light can be reliably extracted, but also the distance between the detector and the diffraction gratings 31a and 31b of the transmission scale 3 can be reduced, so that the apparatus can be easily made compact. This can be applied to the embodiments described below. According to the above configuration, even if a change in the diffraction angle and a change in the gap due to the change in the wavelength of the laser diode 1 occur, two sets of light beams that travel on four different optical paths and interfere with each other due to the diffraction action of the three diffraction gratings having the same pitch. Since the optical path lengths of the diffracted lights for detection are all equal and the diffraction angles of these diffracted lights are all equal, the output that is photoelectrically detected by each detector simply by laterally shifting the detected interference light The phase of the signal is kept constant without change. Note that the signal processing can be performed by a method applied to the photoelectric encoder. For example, as a signal processing system for performing signal processing, as shown in FIG.
, 5b, a preamplifier 10 for amplifying the output signals A and B, a waveform shaping circuit 11 for shaping each amplified signal into a pulse, and a direction discriminating circuit 12 for converting each pulse signal into a binary pulse signal. The measurement value can be obtained by providing a counter circuit 13 for counting the pulse signal via the direction discriminating circuit and a display unit 14 for displaying the measurement result by counting the pulse signal in a desired format. Next, FIG. 4A is a view showing a second embodiment of the present invention, and members having the same functions as those in FIG. 1 are denoted by the same reference numerals. The difference from the first embodiment described above is that on the transmission scale 3, three third diffraction gratings for obtaining four interference lights are formed in parallel at right and left sides of the first diffraction grating 30. And four detectors 5a to 5b are provided corresponding to the respective diffraction gratings 31a to 31b. FIG. 4B is a plan view of the transmission scale 3. P is the pitch of the lattice, m
Is an integer, the diffraction gratings 31a and 31b are (m + ／) in the measurement direction as shown in the figure.
) Are arranged at a distance of P. A diffraction grating 31a, a diffraction grating 31c arranged in parallel with the diffraction grating 31a,
In order that the diffraction grating 31b and the diffraction grating 31d arranged in parallel with each other are displaced by 1/4 pitch in the measurement direction, the diffraction grating 31c and the diffraction grating 31d are shifted by (m + 3/4) P in the measurement direction. They are arranged at a distance. Therefore, four output signals A to D photoelectrically detected by the detectors 5a to 5d are four interference light beams emitted vertically from the diffraction gratings 31a to 31d on the scale 3.
Are shifted by 90 ° from each other. Now, the output signal C of the interference light from the diffraction grating 31c which is photoelectrically detected by the detector 5c.
Is the reference signal and the phase is 0 °, the relative phase difference between this reference signal and the output signal obtained from each detector is the output signal A of the detector 5A corresponding to the diffraction grating 31a.
Is 90 °, the output signal B of the detector 5B corresponding to the diffraction grating 31b is 180 °, and the output signal D of the detector 5d corresponding to the diffraction grating 31d is 270 °. For this reason, if the four output signals are arbitrarily combined with each other and subjected to signal processing, the movement amount and the movement direction of the reflection-type moving scale 4 can be detected. For example, in combination with the output signals of A and D and the output signals of B and C, the amplitude of one of the signals in each set is electrically inverted. That is, the detectors 40A and 40A
D, the detectors 40B and 40C are electrically connected in anti-parallel to generate two push-pull signals. Then, the relative phase difference between the two push-pull signals becomes 90 °, and it is preferable to detect the position and discriminate the moving direction of the scale based on the two signals. Thereby, even if the scale 3 and the movable scale 6 fall, the reading error of the detection signal can be minimized. With such a configuration in which the detection error can be compensated electrically, the manufacturing tolerance of the diffraction grating formed on each scale can be reduced. Next, a third embodiment will be described. FIG. 5 shows the configuration of the third embodiment. Members having the same functions as those in FIG. 1 are denoted by the same reference numerals. The encoder according to the present embodiment has a transmission type diffraction grating on a transparent member moving scale 4.
40 is provided to make it a transmission type. Each of the diffraction gratings 40, 30, 31a, 31b on each of the scales 4, 3A, 3B is also provided with an amplitude grating in which transmission areas (or reflection areas) and light-shielding areas are alternately formed at the same pitch. I have. Then, the gap between the transmission type moving scale 4 and the second transmission scale 3B is made equal to the gap between the first transmission scale 3A and the transmission type moving scale 6.
The three scales are arranged in parallel and at equal intervals. Here, the present embodiment is basically the same as that of the above-described first embodiment, and a detailed description thereof will be omitted. In this embodiment, as in the second embodiment, two pairs of diffraction gratings for generating interference light for detection may be provided, and four detectors may be provided correspondingly. good. Incidentally, in each embodiment of the present invention, for example, as shown in FIG. 2A, a third diffraction grating 31a, interference light I _A to emit perpendicularly from 31b, but detects the I _B, outside the diagonal direction Since the interference lights I _N1 and I _N4 generated at the time have the movement information of the movement scale, they can be detected by the detector. At this time, the interference state of the interference lights I _N1 and I _N4 is always constant even if the diffraction angle and the gap change due to the wavelength change of the light source. According to this configuration, even when the diameter of the collimating lens 2 becomes large, a compact shape can be ensured. However, the illumination efficiency is high, In order to ensure a more stable output signal, as described in the respective embodiments, the interference light I _A to emit vertically from the third diffraction grating, it is desirable to detect the I _B . In each of the embodiments described above, in order to prevent return light, an optical isolator including, for example, a polarization beam splitter and a λ / 4 plate is provided between the optical path of the collimator lens 2 and the first diffraction grating. May be arranged. In each embodiment of the present invention, the first to third diffraction gratings are unified with a phase grating or an amplitude grating, but these may be used in combination as appropriate. Further, according to the present invention, a stable output signal of interference light can always be reliably obtained even in a state where the diameter of a light beam for illuminating each diffraction grating is reduced. In each embodiment of the present invention, the scale having the second diffraction grating is moved. However, the scale having the second diffraction grating may be fixed and the scale having the first and third diffraction gratings may be integrally moved. [Effects of the Invention] As described above, according to the present invention, despite the simple configuration with a very small number of components, the adverse effects of the interference light for detection due to the wavelength variation of the light source due to the temperature change and the gap variation of the scale are removed. This makes it possible to achieve a high-performance diffraction interference type encoder which is very easy to adjust in manufacturing and can always obtain a stable and predetermined output signal. As a result, not only can the manufacturing cost be reduced, but also the size and weight can be reduced. When the second diffraction grating is of a reflection type, the illumination system, the detection system, the first and third
Since the diffraction grating can be incorporated in one housing, further cost reduction and downsizing of the device can be expected. Furthermore, as in the first and second embodiments, the first diffraction grating and the third diffraction grating are integrated with each other while the second diffraction grating is of a reflection type, so that the number of parts can be extremely reduced. Since the number of adjustment parts in manufacturing can be significantly reduced, a significant cost reduction can be expected.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a diagram showing a state when a light source wavelength fluctuates in the encoder of the present invention. FIG. 1B is a diagram showing a state when a gap fluctuates in the encoder of the present invention. FIG. 2A is a diagram showing a schematic configuration of a first embodiment according to the present invention.
FIG. 2B is a plan view of the transmission scale 3 of FIG. 2A. FIG. 3 is a block diagram showing a signal processing system of the first embodiment. FIG. 4A is a perspective view showing the configuration of the second embodiment according to the present invention. FIG. 4B is a plan view of the transmission scale 3 of FIG. 4A. Fifth
FIG. 7 is a diagram showing a schematic configuration of a third embodiment according to the present invention. FIG. 6 is a schematic configuration diagram of a conventional encoder. [Description of Signs of Main Parts] 3, 3A, 3B, 4 Scale 31 First diffraction grating 40 Second diffraction grating 31a, 31b, 31c, 31d Third diffraction grating 5a, 5b, 5c, 5d Detector (light Detection means)

Claims (1)

Claims: 1) First and second diffraction gratings arranged in parallel and having the same periodic pitch as each other, and coherent from the first diffraction grating side to the first and second diffraction gratings. Parallel light beam supply means for vertically irradiating a parallel light beam; first and second positions separated by a distance between the first and second diffraction gratings by a diffraction effect of the first and second diffraction gratings In order to cause the diffracted lights intersecting with each other to interfere with each other, at least one of the first diffraction gratings is provided independently corresponding to the first position and the second position, and the first and second diffraction gratings are provided. A third diffraction grating having diffraction grating elements formed at a periodic pitch equal to and in the same direction as the second diffraction grating; and a diffraction interference light generated for each diffraction grating element of the third diffraction grating. Of one given diffraction interference light Light detecting means for photoelectrically detecting only the respective diffraction grating elements, wherein each of the diffraction grating elements of the third diffraction grating is relatively positioned between the detection signals of the diffraction interference light independently received by the light detecting means. A predetermined distance in the pitch direction so as to provide a large phase difference, and detects a relative movement amount of the second diffraction grating in the pitch direction with respect to the first and third diffraction gratings. A diffraction interference encoder. 2. The diffraction interference according to claim 1, wherein the second diffraction grating is formed by a reflection type diffraction grating, and the first and third diffraction gratings are provided on the same plane. Type encoder.