JP3237458U - Interfering encoder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an interference type encoder capable of reducing an error caused by interference between stray light and interference light. An interference type encoder 1 combines a laser light source 120 that emits a laser beam 160, a light transmitting fiber 141 that propagates the laser light, and a laser beam from the light transmitting fiber with a measurement light 161 and a reference light 162. A polarized light that generates interference light 170 from the measured light diffracted by the polarizing beam splitter 103 that separates into the light, the reflecting unit 101 that reflects the reference light, and the diffracted lattice 201 that diffracts the measured light, and the reflected reference light. The child 104, a plurality of light receiving fibers 142, 144, 146 that propagate interference light and are arranged on the optical path, connectors 143, 145 that connect adjacent light receiving fibers on the optical path, and a light receiving fiber. It is provided with an error reducing unit 107 that reduces an error caused by interference between stray light generated by reflection of interference light and interference light at an end surface. [Selection diagram] Fig. 1

Description

The present invention relates to an interference type encoder.

In the interference type encoder, the laser light from the laser light source is sent to the encoder head, which is the interference meter optical system, by the light transmitting fiber (optical fiber), and the interference light from the encoder head is sent to the detector by the light receiving fiber. There is an encoder (see, for example, Patent Document 1).

Special Table 2014-507655

In order to solve the above problems, the present invention proposes the following means.
One aspect of the interference type encoder of the present invention is a laser light source that emits laser light, a light transmitting fiber that propagates the laser light, and the laser light from the light transmitting fiber as measurement light and reference light. A polarized beam splitter that separates the light into a light beam, a reflecting portion that reflects the reference light, and a polarizing plate that generates interference light from the measured light diffracted by the diffractive lattice that diffracts the measured light and the reflected reference light. A child, a plurality of light receiving fibers that propagate the interference light and are arranged side by side on the optical path of the interference light, a connector that connects the light receiving fibers adjacent to each other on the optical path, and the light receiving fiber. It is provided with an error reducing unit for reducing an error caused by the interference between the stray light generated by the reflection of the interference light on the end face and the interference light.

It is a schematic block diagram which shows an example of the interference type encoder of 1st Embodiment of this invention. It is a schematic block diagram which shows an example of the interference type encoder of 2nd Embodiment of this invention.

(First Embodiment)
Hereinafter, a mode for carrying out the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing an example of an interference type encoder, and shows a schematic configuration of an interference type encoder 1 including an optical fiber.
The interference type encoder 1 includes an encoder head 100, a laser light source 120, a detector 130, a light transmitting fiber 141, a plurality of light receiving fibers 142, 144, 146, a plurality of connectors 143, 145, and a depolarizing element. (Error reduction unit) 107 and a processor 150 are provided.

The laser light source 120 emits a single wavelength (frequency) and linearly polarized laser light (laser beam) 160. The coherence length of the laser beam 160 is longer than the optical path length difference between the measurement optical path and the reference optical path.
In the present embodiment, the light transmitting fiber 141 is a single-mode fiber of a type that maintains a plane of polarization. The light transmitting fiber 141 propagates the laser beam 160.

The encoder head 100 is an interferometer optical system. The encoder head 100 includes a first corner cube prism (reflection unit) 101, a second corner cube prism 102, a polarization beam splitter 103, a splitter 104, and lenses 105 and 106.
In the present embodiment, the interference type encoder 1 includes three light receiving fibers 142, 144, 146, and two connectors 143, 145.

The light receiving fibers 142, 144, 146 propagate the interference light 170 described later. The light receiving fibers 142, 144, and 146 are arranged side by side in this order on the optical path described later of the interference light 170. In the present embodiment, the light receiving fibers 142, 144, 146 are multimode fibers. Of the three light-receiving fibers 142, 144, 146, the light-receiving fiber closest to the splitter 104 is the light-receiving fiber 142.
The connectors 143 and 145 connect the light receiving fibers 142, 144 and 146 adjacent to each other on the optical path. Specifically, the connector 143 connects the light receiving fibers 142 and 144 to each other, and the connector 145 connects the light receiving fibers 144 and 146 to each other.
The number of light receiving fibers included in the interference type encoder 1 is not limited as long as it is a plurality, and may be two or four or more. The number of connectors included in the interference type encoder 1 may be one or three or more.

The depolarization element 107 functions as an error reducing unit that reduces the error caused by the interference between the stray light generated by the interference light 170 reflected at the end faces of the light receiving fibers 142, 144, 146 and the interference light 170. The depolarization element 107 is, for example, the optical element described in US Pat. No. 6,599,834. Specifically, in the depolarization element 107, a birefringence member such as a liquid crystal display is arranged so that the retardation of the birefringence is different for each section in which the effective diameter through which the interference light 170 passes is subdivided. It is an element. In the depolarization element 107, the input of linearly polarized light has a different polarization direction of the output for each section, so that the polarization direction is pseudo-random when viewed as a whole of the interference light 170.
Since the depolarizing element 107 is preferably arranged in a portion where the interference light 170 is parallel light, it is preferably arranged on the optical path between the light receiving fiber 142 and the polarizing element 104.

The encoder scale 200 used together with the interference type encoder 1 includes a diffraction grid 201 that diffracts the measurement light 161 described later of the laser light 160. The interference type encoder 1 calculates the amount of movement of the encoder scale 200 with respect to the encoder head 100.

The laser beam 160 from the laser light source 120 is propagated to the encoder head 100 by the light transmitting fiber 141. The laser beam 160 is converted into a parallel light flux by the lens 105 and is incident on the polarization beam splitter 103. At this time, the light transmission fiber which is a polarization plane holding fiber so that the polarization direction of the laser beam 160 is approximately 45 degrees with respect to the direction of P polarization with respect to the polarization separation surface 103a of the polarization beam splitter 103 and the direction of S polarization. The azimuth angle of the ejection end of 141 is adjusted. The laser beam 160 from the light transmitting fiber 141 incident on the polarizing beam splitter 103 is polarized and separated (separated) into the measurement light 161 and the reference light 162 on the polarization separation surface 103a of the polarization beam splitter 103. Here, "polarization separation" means that the laser beam 160 is separated into a P-polarization component and an S-polarization component.
The measurement light 161 of the S polarization component is reflected by the polarization separation surface 103a of the polarization beam splitter 103 and heads toward the encoder scale 200. On the other hand, the reference light 162 of the P polarization component passes through the polarization separation surface 103a and is incident on the first corner cube prism 101. The first corner cube prism 101 reflects the reference light 162 toward the polarizing beam splitter 103.

The measurement light 161 is diffracted by the diffractive grid 201 of the encoder scale 200. The diffracted measurement light 161 is turned around by the second corner cube prism 102, and is diffracted again by the diffracted lattice 201. The measurement light 161 that has been split twice by the diffraction grid 201 is incident on the polarization beam splitter 103.

The measurement light 161 diffracted by the diffraction lattice 201 and returned to the polarizing beam splitter 103 and the reference light 162 reflected by the first corner cube prism 101 are emitted from the polarizing beam splitter 103 as interference light 170 which is output light. Will be done. The splitter 104 generates interference light 170 from the measurement light 161 and the reference light 162.
The interference light 170 propagates through the light receiving fibers 142, 144, 146 connected by the connectors 143, 145.

The interference light 170 is detected in the detector 130 by, for example, photoelectric conversion. The detection signal is transmitted from the detector 130 to the processor 150. The processor 150 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. The processor 150 calculates the movement amount of the encoder scale 200 based on the detection signal from the detector 130.
The detector 130 and the processor 150 generate heat when performing detection and calculation. In order to dispose the detector 130 and the processor 150 at positions away from the encoder head 100 so as not to be affected by this heat generation, the light receiving fibers 142, 144, 146 are connected by connectors 143, 145 and extended.

Here, the interference type encoder 1 not provided with the depolarization element 107 will be referred to as a conventional interference type encoder below. In the conventional interference type encoder, it is necessary to polish the end faces of the light receiving fibers 142, 144, 146. At this time, a processed alteration layer is generated on the polished surface, and the refractive index is slightly different between the unpolished surface (bulk portion) of the light receiving fibers 142, 144, 146 and the processed altered layer close to the polished surface. There is. Therefore, about 1% of reflection (stray light) is generated at the boundary between the processed alteration layer and the bulk portion, and the interference light 170 reflected by the connector 145 is reflected again by the connector 143 and reaches the detector 130. , Interferes with most of the interference light 170 transmitted through the connectors 143 and 145.
When the temperature of the light receiving fiber 144 sandwiched between the connectors 143 and 145 changes or vibration is applied from the outside, the phase of the interference light 170 reciprocating between the connectors 143 and 145 changes and fluctuates. An error occurs. Moreover, since temperature changes and external vibrations are low-frequency fluctuations, there is a problem that they cannot be averaged within the measurement time.

Further, when foreign matter such as dust is sandwiched between the connectors 143 and 145, a minute gap is generated between the end faces of the light receiving fibers 142, 144 and 146 to be connected, and there is also a problem that the reflected stray light increases due to Fresnel reflection. there were. Therefore, it is necessary to clean this end face every time the connection work is performed. However, when the size of the foreign matter is about several micrometers, it is difficult to surely confirm whether or not the foreign matter has been removed.

On the other hand, the interference type encoder 1 of the present embodiment includes the depolarization element 107. Therefore, in the light receiving fibers 142, 144, 146, it is possible to reduce the error caused by the interference between the stray light and the interference light 170.
The error reducing unit is the depolarization element 107. Interference light 170 whose polarization is randomized by the depolarization element 107 is incident on the light receiving fiber 142. Therefore, the contrast (interfering property) of the interference between the stray light and the interfering light 170 is lowered, and the error caused by the interference between the stray light and the interfering light 170 can be reduced.

(Second Embodiment)
Next, the second embodiment of the present invention will be described with reference to FIG. 2, but the same parts as those of the embodiment are designated by the same reference numerals, the description thereof will be omitted, and only the differences will be described.
As shown in FIG. 2, the interference type encoder 2 of the present embodiment includes an oscillator (error reduction unit) 179 instead of the depolarization element 107 of the interference type encoder 1 of the first embodiment. The oscillator 179 functions as an error reducing unit, similarly to the depolarizing element 107. The oscillator 179 includes a vibrator 180 and a driver 181.
For example, the vibrator 180 is attached to a protective jacket of the light receiving fiber 144. The oscillator 179 vibrates the light receiving fiber 144 in a direction intersecting the optical path. The driver 181 is connected to the vibrator 180 and excites and vibrates the vibrator 180.
The oscillator 179 may vibrate at least one of the light receiving fibers 142, 144, 146 in a direction intersecting the optical path.

The interference type encoder 2 of this embodiment includes an oscillator 179. As a result, the phase of the interference light 170 reciprocating in the light receiving fiber 144 is modulated by the frequency of the vibrator 180 (oscillator 179), and the frequency of the error is shifted to the frequency region of the vibrator 180. Therefore, by averaging within the measurement time, it is possible to reduce the error caused by the interference between the stray light and the interference light 170.

Here, the measurement error E of the displacement of the diffraction grid 201 with respect to the encoder head 100 due to the reciprocating stray light generated in the light receiving fiber 144 can be expressed as in the equation (1).
Here, A is a coefficient determined depending on the reflection at the connectors 143 and 145 and the alignment state where the original measurement light 161 and the reference light 162 enter the light receiving fiber 142. φ (t) is a phase fluctuation due to a disturbance that fluctuates depending on the time t.

When the vibration of the frequency f is applied to the measurement error E expressed as in the equation (1) by the vibrator 180 as described above, the phase modulation of the sine wave is added with the amplitude as B. Therefore, the measurement error E is expressed as in Eq. (2).

From Eq. (2), it can be seen that when an amplitude B having a magnitude of approximately π or more and a frequency f sufficiently larger than the frequency determined by the reciprocal of the measurement time are given, the phase of the outer sine function changes randomly. .. Therefore, the error can be substantially reduced by averaging within the measurement time.

(Third Embodiment)
Next, the third embodiment of the present invention will be described, but the same parts as those in the above embodiment are designated by the same reference numerals, the description thereof will be omitted, and only the differences will be described.
In the interference type encoder of the first embodiment, the polarization plane holding fiber is used for the light transmission fiber 141 as in the conventional interference type encoder. The polarization plane holding fiber is characterized in that it can transmit linearly polarized light that is stable in time.

In the conventional interference type encoder, when the laser beam 160 is incident on the light transmitting fiber 141 from the laser light source 120, the linearly polarized light is incident on the slow axis of the light transmitting fiber 141 which is a polarization plane holding fiber. It's normal. However, due to the alignment accuracy, some light is inevitably incident on the fast axis in the orthogonal polarization direction and propagates. Moreover, when the temperature or the bent state of the light transmitting fiber 141 changes, the retardation between the fast axis and the slow axis changes. The laser beam 160 propagating along the fast axis interferes with each other, causing an error.

On the other hand, in the interference type encoder of the present embodiment, the light transmission fiber 141 of the conventional interference type encoder is a single polarization single transverse mode polarizing fiber. In the single-polarized single-transverse-mode polarized fiber, only the slow-axis light propagates by increasing the attenuation of the fast-axis light. Since the light transmitting fiber 141 is a single polarized single transverse mode polarized fiber, the fast axis light is not generated in the first place, so that an error does not occur in the light transmitting fiber 141.

Further, in the interference type encoder of the third embodiment, the light transmitting fiber 141 may be a photonic crystal fiber. The photonic crystal fiber, like the plane-holding fiber, is birefringent and has polarization directions of a slow axis and a fast axis that are orthogonal to each other. The photonic crystal fiber is different from a normal polarization plane holding fiber in that the sensitivity of the retardation change between the slow axis and the fast axis to the temperature and bending stress of the optical fiber is small. In some cases, the retardation change with respect to the temperature change was reported to be about 1/30. Therefore, the phase does not change even if the fast-axis propagating light is mixed. Therefore, the error of the interference signal is substantially small and can be set to a negligible level.

Although the first to third embodiments of the present invention have been described in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and the configuration does not deviate from the gist of the present invention. Changes, combinations, deletions, etc. of are also included.
For example, in the first to third embodiments, the interference encoder may not include the second corner cube prism 102, the lenses 105, 106, the detector 130, and the processor 150.

1, 2 Interference encoder 101 1st corner cube prism (reflection part)
103 Polarizing beam splitter 104 Polarizer 107 Depolarizing element (error reduction unit)
120 Laser light source 141 Light transmission fiber 142, 144, 146 Light receiving fiber 143, 145 Connector 160 Laser light 161 Measurement light 162 Reference light 170 Interference light 179 Transducer (error reduction unit)
201 Diffraction grid

Claims (5)

A laser light source that emits laser light and
The light transmitting fiber propagating the laser beam and
A polarization beam splitter that separates the laser beam from the light transmission fiber into measurement light and reference light.
A reflector that reflects the reference light,
A modulator that generates interference light from the measurement light diffracted by the diffraction lattice that diffracts the measurement light and the reflected reference light, and
A plurality of light receiving fibers that propagate the interference light and are arranged side by side on the optical path of the interference light.
A connector for connecting the light receiving fibers adjacent to each other on the optical path,
An error reducing unit that reduces the error caused by the interference between the stray light generated by the reflection of the interference light at the end face of the light receiving fiber and the interference light.
Interfering encoder. The error reducing unit is a depolarizing element arranged on the optical path between the light receiving fiber closest to the polarizing element and the polarizing element among the plurality of light receiving fiber pieces. Interference type encoder described in. The interference type encoder according to claim 1, wherein the error reducing unit is an oscillator that vibrates at least one of the plurality of light receiving optical fibers in a direction intersecting the optical path. The interference type encoder according to any one of claims 1 to 3, wherein the light transmitting fiber is a single polarized single transverse mode polarized fiber. The interference type encoder according to any one of claims 1 to 3, wherein the light transmitting fiber is a photonic crystal fiber.

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