JPH07190712A - Interferometer - Google Patents

Abstract

PURPOSE:To measure the quantity of displacement of a measured object simply with high accuracy by providing a second laser beam source which is low in coherence while being different in wave length at the oscillating center, and making the position of the measured object a reference position when an interference signal due to reflection light from the measured object and a second reference object is outputted. CONSTITUTION:A reference position optical system is constituted by using a light source 2 with a wave length lambda2 at an oscillating center low in time coherence. Output beam from the light source 2 is synthesized with output beam from a light source 1 over its light path using a mirror 4 and a beam splitter 3 so as to allow the synthesized light to be incident on a polarization beam splitter 5, it is then divided into a referencing light path and a measuring light path, and reflection light from a reflecting mirror (a second reference object) 9 and reflection light from a moving mirror (measured object) 8 are coaxially synthesized with a measuring beam, so that detection beam is thereby extracted. After that, the position of the moving mirror 8 is made a reference point B0, when the difference in light path length between the reference light path and the measuring light path is zero, an interference signal outputted at that time is inputted into a processing circuit 26 so as to be converted into a pulse signal, so that it is thereby utilized as a zero point signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer which is applied to a length measuring instrument or the like which requires a high degree of displacement measurement accuracy.

[0002]

2. Description of the Related Art Conventionally, as a displacement measuring method using an interference fringe counting method, a measuring light from an object to be measured and a reference light are used.
An interferometer is known in which two interference output signals having a 90 ° phase difference in which the DC level is zero and the amplitudes are equal to each other are produced, and the displacement of the object to be measured is obtained from the mutual interference state of these signals.

FIG. 6 shows a Michelson interferometer for length measurement using a semiconductor laser as a light source as an example of such an interferometer. In FIG. 6, the coherent light emitted from the semiconductor laser light source 101 is a beam splitter 103.
And is branched into two by transmission or reflection. One of the lights reflected by the beam splitter 103 is
The light enters the reference optical path, is folded back by the reference fixed mirror 107, and returns to the beam splitter 103 again. Since this reflected light passes through the λ / 8 plate 105 twice when reciprocating between the reference fixed mirror 107 and the beam splitter 103, the two polarization components whose polarization directions are orthogonal to each other have a phase difference of 90 °. Will happen.

Further, the light transmitted through the beam splitter 103 travels along the optical path for length measurement, and the movable mirror 10 corresponding to the object to be measured.
It reaches 8 and is folded back here again to the beam splitter 1
Incident on 03. The lights from the two directions that have been split and returned again in this manner are reflected and transmitted by the beam splitter 103 to be emitted coaxially as a beam for length measurement.

This emission beam is incident on the polarization beam splitter 110, where the S-polarized component is reflected and the P-polarized component is transmitted to be separated into two light beams. The two light fluxes are respectively detected by the detector 111 as the sin component of the interference signal and the detector 11
At 3, the cos component of the interference signal is detected and output to the fringe counter 120. In the fringe counter 120, the displacement amount L of the movable mirror 108 for the object to be measured is obtained by integrating the interference fringes from the two input signals.

[0006]

However, in the interferometer according to the prior art as described above, there is no coaxial reference point on the optical path within the displacement measurable range of the movable mirror. Therefore, the measurement of displacement is relative,
It was inconvenient because the entire measurable range could not be used effectively. In particular, in the case of an interferometer using a semiconductor laser having a shorter coherence length than a gas laser as a light source, FIG.
As shown in (a), the movable range of the movable mirror, which corresponds to the measurable intensity range of the interference fringe signal (measurable optical path difference), is limited, which is not convenient.

Further, it is conceivable to provide a mechanical reference point setting means in the apparatus, but in this case, it is not possible to cope with environmental changes such as temperature and humidity around the optical system, fluctuations of air, etc. Highly accurate displacement measurement cannot be expected using such mechanical reference points.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide an interferometer which is easier to use than the prior art and which can easily measure the displacement amount of an object to be measured with higher accuracy. .

[0009]

In order to achieve the above object, in an interferometer according to a first aspect of the present invention, an output beam from a first laser light source is split into a reference optical path and a measurement optical path. A measurement interference optical system for coaxially synthesizing reflected light from the first reference object on the reference optical path and reflected light from the object to be measured on the measurement optical path to extract a measurement beam. A second laser light source of low coherence having an oscillation center wavelength different from that of the first laser light source;
The output beam from the laser light source is branched into the reference optical path and the measurement optical path, and the reflected light from the second reference object and the reflected light from the measured object on the reference optical path are measured. A detection optical system for extracting the detection beam by synthesizing coaxially with the working beam, and detecting an interference signal due to the reflected light from the DUT and the reflected light from the second reference object from the detection beam. A reference position detection optical system having a detection unit that uses the position of the object to be measured when the interference signal is output as a reference position.

Further, in the interferometer according to the invention described in claim 2, in the interferometer according to claim 1, the first reference object is the first beam of the output beam from the first laser light source. The direction of incidence on the reference object is substantially parallel to the measurement optical path, and the distance from the branch point corresponds to the detectable intensity range of the interference fringe signal from the measurement beam from the branch point. The distance is substantially equal to the distance to the center of the movable range of the measured object on the measuring optical path.

[0011]

In the present invention, the output beam from the first laser light source is split into the reference optical path and the measurement optical path in the measuring interference optical system, and the reflected light from the first reference object on the reference optical path is split. And the reflected light from the object to be measured on the optical path for measurement are combined on the same axis to extract the beam for measurement, detect an interference output signal having a phase difference, and displace the object to be measured from these mutual interference states. In the interferometer for determining the, a second laser light source having a low coherence having an oscillation center wavelength different from that of the first laser light source, and an output beam from the second laser light source is branched into a reference optical path and a measurement optical path. And a reference position detecting optical system for extracting the detection beam by synthesizing the reflected light from the second reference object and the reflected light from the measured object on the reference optical path coaxially with the measuring beam. Is.

In the above configuration, the interference optical system with low coherence used as the second laser light source is shown in FIG.
As shown in, the interference signal is obtained only when the optical path lengths of the reference optical path and the measurement optical path are substantially equal. The coherence length lc is expressed by λ / Δλ, where λ is the central wavelength and Δλ is the spectral width of the light source. Therefore, by selecting the spectral width of the second light source and adjusting the optical path length, it is possible to arbitrarily set a very narrow specific position where the interference signal can be output.

In the present invention, since the measuring optical path of the detecting optical system using the second laser light source is coaxial with the measuring optical path of the measuring optical system using the first laser light source, The position is a reference point in the movable mirror displacement amount measurable range,
For example, it can be set to the zero point. Therefore, in the interferometer according to the present invention, the reference point can always be confirmed within the displacement amount measurement range, and the displacement can be measured from the reference state set for each displacement amount measurement. Utilized measurement can be done easily. Moreover, since this reference point is set coaxially with the measurement optical path along which the test object moves, even if environmental changes occur in the optical system, they are under the same influence and can be handled. Accurate displacement measurement is possible.

In the present invention, the first reference object is
The incident direction of the output beam from the first laser light source to the first reference object is substantially parallel to the measurement optical path, and the distance from the branch point is the interference from the measurement beam from the branch point. Since the distance to the center of the movable range of the object to be measured on the optical path for measurement corresponding to the detectable intensity range of the fringe signal is arranged so as to be substantially the same, as shown in FIG. The displacement amount measurable range (-L to L) is expanded to twice the displacement amount measurable range using a conventional semiconductor laser as a light source, and even a relatively large displacement amount can be easily measured. At this time, according to the above configuration, the first
The reference object can be placed near the measurement optical path at a position where conditions such as air fluctuations are almost the same.

In the present invention, the output laser from the first laser light source and the output laser from the second laser light source after branching follow the same optical path for reference, but these lasers are the oscillation centers of each other. Since the wavelengths are different, it is possible to separate them by using a wavelength selection means such as a dichroic prism and make them incident on the respective reference objects.

[0016]

EXAMPLES The present invention will be described below with reference to examples. FIG. 1 is a block diagram showing an interferometer for length measurement according to a first embodiment of the present invention. The present embodiment is applied to a laser displacement meter using a Michelson interferometer, and has a frequency stabilizing laser light source using a semiconductor laser as the first light source 1 for measurement. Interferometer optical system and super as the second light source 2.
It is composed of a reference position detection optical system using a low coherence light source having a relatively high power of a luminescent diode or a multimode oscillation type semiconductor laser.

In the interference optical system for measurement, the first light source 1
The output beam from is incident on the polarization beam splitter 5, where it is separated into polarization components orthogonal to each other by transmission and reflection and used as reference light and length measurement light, respectively. In the present embodiment, the P-polarized component is used for the length measuring light (transmitted light) and the S-polarized component is used for the reference light (reflected light), but these selections are arbitrary.

The length-measuring light that has passed through the polarization beam splitter 5 and exits travels along the measurement optical path and is reflected by the movable mirror 8 (corner cube) for the object under measurement on the optical path. Moving mirror 8
Is composed of a folding mirror having two reflecting portions with an opening angle of 90 degrees, it reflects the length measuring light so as to be reflected in a different path exactly in the opposite direction to the incident direction, and then again returns to the polarization beam splitter. Lead to 5.

The reference light reflected by the polarization beam splitter 5 enters the reference optical path and is bent by a dichroic prism 6 which has a reflecting action on the light of wavelength λ ₁ , and a fixed mirror (co-reflector) is used. It is incident on the nave 7).
Here, the incident direction to the fixed mirror 7 is configured to be parallel to the measurement optical path. The fixed mirror 7 is composed of a folding mirror similarly to the movable mirror 8, reflects the reference light so as to be reflected by another path in the direction opposite to the incident direction, and guides it to the polarization beam splitter 5 again via the dichroic prism 6.

In the polarization beam splitter 5, the length-measuring light and the reference light, which are respectively reflected, are coaxially combined and emitted as an output beam for measurement. In this embodiment, the polarization selection performance of the polarization beam splitter 5 (here, S polarization is reflected and P polarization is transmitted) is used to guide the length measurement light and the reference light to the same position on the polarization splitting surface. The reference light of the S-polarized component reflected here and the measuring light of the P-polarized component that is transmitted are combined. The measurement beam from the polarization beam splitter 5 passes through the beam splitter 10 and then
Optical filter 11 that transmits only the light wave component of wavelength λ ₁
Is incident on, is transmitted, and is guided to a detector composed of a 90 ° phase difference DC interference optical system.

The 90 ° phase difference DC interferometer outputs four optical signals having a 90 ° phase difference from the orthogonal polarization components. The photodetectors 21, 22, 23 and 24 photoelectrically output these optical signals. It is composed of a processing circuit 26 for converting and counting the interference fringe signal and a display 27.

In the detector, the measurement beam as inspection light composed of polarization components orthogonal to each other is a λ / 2 plate 12
As a result, the polarization axis is inclined by 45 ° with respect to the incident surface of the beam splitter 13, and the light is incident. The beam splitter 13 splits the light in two directions with equal intensity by reflection and transmission. The reflected component enters the polarization beam splitter 14.

The output beam incident on the polarization beam splitter 14 is guided in a state where the polarization plane is inclined by 45 degrees with respect to the polarization separation plane. Therefore, the length measuring light and the reference light are transmitted and reflected components, respectively. And separated. Then, the transmitted light and the reflected light separated here are polarized and interfere with each other to be transmitted interference light and reflected interference light, and are guided to the detector 21 and the detector 22. And each detector 21, 22 is
It is composed of a variable-gain photoelectric converter, which detects the interference light and photoelectrically converts it, and outputs a detection signal to the processing circuit 26. Since the polarization directions of the transmitted interference light and the reflected interference light are orthogonal to each other, the interference intensities are in opposite phases and are 0 ° and 180 °.
An interference signal with a phase difference of ° can be obtained.

On the other hand, the transmitted light from the beam splitter 13 has a phase of 90 ° with respect to the reflected light by the λ / 4 plate 16.
After being rotated by an angle, it enters the polarization beam splitter 17. The transmitted light and the reflected light that are polarized and separated by the polarization beam splitter 17 are guided to the detector 23 and the detector 24 as transmitted interference light and reflected interference light, respectively. These detectors detect the respective interference lights, photoelectrically convert the interference lights, and output detection signals to the processing circuit 26. Here, interference signals having a phase difference of 90 ° and 270 ° are obtained between the transmitted interference light and the reflected interference light, respectively.

On the other hand, the reference position detecting optical system has a second light source 2 having an oscillation center wavelength λ _{2 having} a wide oscillation spectrum width, that is, a temporal coherence lowered, such as a super luminescent diode. It is configured by using. Here, the interference output is obtained only when the optical path length difference between the reference optical path and the measurement optical path is zero. Therefore, here, the position of the movable mirror 8 at this time is set as the reference point B ₀ and set as the zero point of the displacement measurable range. That is, the interference signal output when the optical path length difference between both optical paths is zero due to the movement of the movable mirror 8 in the measurement optical path is input to the zero-point pulse generator, and the zero-point pulse signal is output.

In such a reference position detecting optical system,
The output beam from the second light source 2 is combined with the output beam from the first light source 1 on the same optical path by the mirror 4 and the beam splitter 3, and then enters the polarization beam splitter 5.

The S-polarized light component reflected by the polarization beam splitter 5 travels as a reference light to the reference optical path and is transmitted through a dichroic prism 6 which transmits light of wavelength λ ₂ and a reflecting mirror 9.
Incident on. The reflecting mirror 9 is composed of a folding mirror, reflects the reference light so as to be reflected by another path in the direction opposite to the incident direction, and guides it to the polarization beam splitter 5 again via the dichroic prism 6.

The P-polarized component measuring light that has passed through the polarization beam splitter 5 travels along the optical path for measurement and is incident on the movable mirror 8, where it is reflected so as to be reflected by another path in the opposite direction to the incident direction, and again the polarized beam is emitted. It is guided to the splitter 5.

The two linearly polarized light components are reflected and transmitted by the polarization beam splitter 5, respectively, and are combined, and then reflected by the beam splitter 10 to transmit only the light wave component of wavelength λ ₂ through the optical filter 19. Through. Further, it becomes an interference signal through the analyzer 20 having an azimuth angle of 45 °, enters the detector 25, is photoelectrically converted therein, and is output to the processing circuit 26.

FIG. 2 shows an outline of the electric processing system of this embodiment. In the processing circuit 26, each detector 2
The four-phase signals photoelectrically converted by 1, 22, 23, and 24 are
After being appropriately amplified by the amplifier 28, the phase difference is 18
Signals different by 0 ° are guided to the differentiator 29, and phase difference 0 ° -1
It is converted into a differential signal having opposite phases of 80 ° and 90 ° -270 °, and two electric signals having phases different by 90 ° are obtained.

This electric signal is input to the fringe counter 30 to calculate and integrate the interference fringe signal generated as the movable mirror 8 moves. From this result, the distance or the displacement amount is calculated and displayed on the display 27. Further, the displacement information can be transmitted to an external calculator or the like via the external interface.

On the other hand, the interference signal from the reference position detection optical system is appropriately amplified by the amplifier 28, then converted into a pulse signal by the zero-point pulse generator 31, and the fringe counter 3
Used for zero signal to zero.

In the first embodiment, as shown in the interference fringe signal intensity (visibility) characteristic of the interferometer as shown in FIG. 5B, the fixed mirror of the reference optical path is changed to the reference optical path. Since the distance from the branch to the measuring optical path is arranged to be almost the same as the distance from the branch point to the middle point of the displacement measurable range, the measurable distance is doubled from the conventional one. ,
Larger displacements can be easily measured. Further, in the length measuring machine of the present embodiment, the zero point position is always detected or each time the displacement is measured, so that the measuring operation is easier and more accurate than the conventional apparatus. Furthermore, by bending the reference optical path and arranging it so that it is close to the measurement optical path (both optical paths are parallel), environmental conditions such as air sway are almost the same, and highly accurate displacement measurement values are obtained. can get.

In the present invention, a method of forming interference light between the reference light and the length measuring light, a method of synthesizing the reference light and the length measuring light coaxially, and a method having a 90 ° phase difference. The interference fringe measuring method for obtaining the displacement of the object to be measured from the mutual interference state of the two interference output signals is not limited to the method shown in the present embodiment, and a method other than the method shown here is also applicable to the present invention. Can be applied.

Next, as a second embodiment, FIG. 3 shows the interferometer for length measurement according to the present invention, which is constructed as a closed system and made into a device. This device includes a measuring interference optical system 4 including a light source, a light-transmitting / receiving optical system, and a detector at one end of a case 40.
1, a reference fixed mirror at a position apart from this by a predetermined distance,
Further, the measuring movable mirror 8 is housed in a state that it is mounted on a fixed plate 45 on a guide rail 44 installed in the longitudinal direction in the case.

The fixed mirror (not shown) is fixedly arranged at the midpoint of the measurable range, and the movable mirror 8 moves in the longitudinal direction inside the case by the movement of the fixed plate 45 mounted on the guide rail. It is possible to move on the measurement optical path. The laser beam is isolated from the outside by the rubber seal 43, and is constructed so as not to be affected by dust, oil, chips and the like.
A mounting bracket 46 is used to connect the movable mirror 48 and the object to be measured. The display 47 is connected to the measuring interference optical system 41 from the end of the case 40, and counts and integrates the interference fringe signals by the output from the detector to calculate the displacement amount.

In the apparatus having the above construction, the measuring interference optical system and the reference position detecting optical meter have the same functions and actions as those shown in the first embodiment. In this way, the deviceized interferometer makes it possible to easily measure the displacement amount or distance of the object to be inspected, anytime, anywhere.

In the above embodiment, the reflecting mirror 9 for detecting the reference position is provided separately from the fixed mirror 7 for reference, and the reference position is set as the zero point at the end of the branch point of the displacement amount measurable region. However, the fixed mirror 7 may also be used as the reflecting mirror 9 for detecting the reference position. In this case, the reference point is the midpoint of the displacement amount measurable region.

[0039]

As described above, according to the present invention, the displacement amount measurable range and the absolute reference position which are larger than those of the conventional ones are used, and the displacement amount can be measured with higher accuracy and easily. Can be done. Further, according to the configuration of the present invention, it is possible to realize an interferometer capable of easily measuring the displacement amount, distance, etc. of the object to be measured.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an interferometer for length measurement according to a first embodiment of the present invention.

FIG. 2 is a block diagram illustrating an electrical processing system of the interferometer shown in the first embodiment.

FIG. 3 is a schematic configuration diagram showing an interferometer device according to a second embodiment of the present invention.

FIG. 4 is a diagram showing an interference output with respect to an optical path length difference of an interferometer using a low coherence light source.

FIG. 5 is a diagram showing the relationship between the optical path difference (displacement amount measurable range length) and the interference fringe signal intensity of an interferometer using a semiconductor laser as a light source, and FIG. in the case of,
(B) shows the case of the interferometer according to the present invention.

FIG. 6 is a schematic configuration diagram showing an interferometer using a semiconductor laser according to a conventional technique.

[Explanation of symbols]

1, 101: frequency stabilizing laser light source (first light source) 2: low coherence light source (second light source) 3, 10, 13, 103: beam splitter 4: mirror 5, 14 , 17, 110: Polarizing beam splitter 6: Dichroic prism 7, 9, 107: Fixed mirror (corner tube) 8, 48, 108: Moving mirror (corner tube) 11, 19: Optical filter 12: λ / 2 plate 15, 18: Prism 16: λ / 4 plate 20: Analyzer 21, 22, 23, 24, 25, 111, 113: Photodetector 26: Processing circuit 27, 47 : Display 28: Amplifier 29: Differencer 30, 120: Fringe counter 31: Zero pulse generator 40: Case 41: Interfering optical system for measurement 42: Heat sink 43: Rubber seal 44: Guide rail 45: Fixing plate 46 : Mounting gold 105: λ / 8 plate

Claims (2)

[Claims] 1. An output beam from a first laser light source is branched into a reference optical path and a measurement optical path, and reflected light from a first reference object on the reference optical path and on the measurement optical path. A measuring interference optical system for coaxially synthesizing reflected light from an object to be measured to extract a measuring beam, and a second laser light source of low coherence having an oscillation center wavelength different from that of the first laser light source. An output beam from the second laser light source is branched into a reference optical path and a measurement optical path, and reflected light from a second reference object on the reference optical path and reflected light from the measured object. And a detection optical system for coaxially synthesizing the measurement beam with the measurement beam to extract a detection beam, and interference of reflected light from the object to be measured and reflected light from the second reference object from the detection beam. When a signal is detected and the interference signal is output Serial interferometer, characterized in that it and a reference position detecting optical system having a detection means as a reference position a position of the object to be measured. 2. The first reference object is such that an incident direction of an output beam from a first laser light source on the first reference object is substantially parallel to the measurement optical path, and the branch point. Is substantially equal to the distance from the branch point to the center of the movable range on the measurement optical path of the DUT corresponding to the detectable intensity range of the interference fringe signal from the measurement beam. The interferometer according to claim 1, wherein the interferometer is arranged so that