Abstract:
An interference measuring apparatus has an optical system for dividing a coherent light beam into two light beams, causing the divided two light beams to pass along discrete optical paths and making them into linearly polarized light beams orthogonal to each other and given modulation to the phase of the wave front of at least one of them, and again superposing the wave fronts of the linearly polarized light beams one upon the other, a light dividing member for dividing the light beams superposed one upon the other by the optical system into a plurality of light beams, a rock crystal plate formed with a predetermined level difference in conformity with the incidence positions of the plurality of light beams, a polarizing plate for taking out each light beam transmitted through the rock crystal plate with a 45° polarized component, and a plurality of light receiving elements for individually receiving the light beams taken out by the polarizing plate, a plurality of different interference phase signals being provided by the reception of lights of the light receiving elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an interference measuring apparatus and a grating interference-type encoder for generating a phase difference signal and highly accurately measuring a length and an angle in an industrial apparatus such as FA (factory automation). 
     2. Related Background Art 
     A laser interferometer or an incremental encoder has heretofore been utilized as a highly accurate positioning apparatus such as FA. These apparatuses convert positional deviation into a pulse train, and count the pulse number at this time to thereby detect relative positional deviation. At this time, it is also necessary to detect a direction of movement and therefore, usually two or more phase difference signals A phase and B phase are outputted and phase shift of 90° is given with a sine wave signal of a cycle being 360°. 
     In such a high resolving power incremental encoder and laser interferometer, there is known a method of disposing two detecting optical systems with their spatial positions deviated from each other to thereby generate phase difference signals of A phase and B phase. There is also known a method of causing polarized light beams orthogonal to each other to interfere with each other through a quarter wavelength plate, converting them into a linearly polarized light beam of which the polarization direction rotates correspondingly to the phase difference between the wave fronts of the two light beams, and then further dividing it into a plurality of light beams. Those light beams are caused to be transmitted through polarizing plates disposed with their polarization axes deviated in different directions to thereby generate a phase difference signal light beam. 
     FIG. 1 of the accompanying drawings shows a perspective view of a non-contact distance sensor of the conventional laser interference-type, and a laser beam L from a coherent light source  1  passes through a collimator lens  2  and a non-polarizing beam splitter  3  and is polarized on the polarizing surface  4   a  of a probe-like polarizing prism  4 . S-polarized light reflected by the polarizing surface  4   a  emerges from the probe-like polarizing prism  4  toward a slider  5 , is reflected by the surface  5   a  to be measured by the slider  5 , and again returns along the original optical path to the polarizing surface  4   a  of the probe-like polarizing prism  4 . 
     On the other hand, P-polarized light transmitted through the polarizing surface  4   a  is reflected by the upper reference mirror surface  4   b  of the probe-like polarizing prism  4  and likewise returns to the polarizing surface  4   a . These two polarized lights are re-combined on the polarizing surface  4   a , travel through the probe-like polarizing prism  4 , are reflected by a non-polarizing beam splitter  3 , pass through a quarter wavelength-plate  6  and an aperture in an aperture plate  7 , and are amplitude-divided by a four-division diffraction grating  8 . These amplitude-divided light beams pass through polarizing plates  9   a - 9   d , and are received by the four areas  10   a - 10   d  of a light-receiving element  10 . The minute displacement of the slider  5  is measured by an interference signal at this time. 
     However, in the above-described example of the prior art, the phase shift is given by arrangement or the like of the polarizing plates  9   a - 9   d  and therefore, there is a possibility that if there are an alignment error and manufacturing errors of the polarizing plates  9   a - 9   d , the phase difference signal is not stable. On the other hand, in the case of the interference between linearly polarized lights orthogonal to each other, a space is required for arrangement of optical parts such as the quarter wavelength plate  6  and the four polarizing plates  9   a - 9   d  and therefore, the apparatus becomes bulky and the assembly adjustment of all these is necessary. 
     SUMMARY OF THE INVENTION 
     In view of the above-described example of the prior art, it is an object of the present invention to provide a compact interference measuring apparatus easy to assemble for collectively detecting a plurality of stable phase difference signals. 
     It is another object of the present invention to provide an interference measuring apparatus such as a compact and highly accurate grating interference-type encoder. 
     Other objects of the present invention will become apparent from the following description of some embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an interference measuring apparatus according to the prior art. 
     FIG. 2 is a perspective view of an interference measuring apparatus according to a first embodiment of the present invention. 
     FIG. 3A is an illustration of the phase shift of a wave front by a phase difference rock crystal plate. 
     FIG. 3B is a perspective view of a push-pull optical system. 
     FIG. 4 is a perspective view of a push-pull optical system according to a second embodiment of the present invention. 
     FIG. 5 is a perspective view of an interference measuring apparatus according to a third embodiment of the present invention. 
     FIG. 6 is a view for explaining production of 90° phase difference bright and dark signal of four phases. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will hereinafter be described in detail with respect to some embodiments thereof shown in FIGS. 2 to  6 . 
     FIG. 2 shows a perspective view of a first embodiment, and in a non-contact distance sensor  20 , a coherent light source  21  such as a laser diode, a collimator lens  22 , a non-polarizing beam splitter  23  and an optical type probe-like polarizing prism  24  are arranged in succession. The probe-like polarizing prism  24  has a polarizing beam splitter surface  24   a  and a reference mirror surface  24   b , and a slider side  25  which is a region to be measured is disposed on the front surface of the probe-like polarizing prism  24  in the direction of reflection of the polarizing surface  24   a.    
     An aperture plate  26 , a four-division diffraction grating plate  27  such as a hounds-tooth-checkered phase diffraction grating, a phase difference rock crystal plate  28  having four areas  28   a - 28   d  of different thicknesses on a substrate, a 45° azimuth polarizing plate  29  having an optic axis in an azimuth of 45° and a four-division light receiving element  30  having four light receiving areas  30   a - 30   d  are arranged in succession in the direction of reflection of the non-polarizing beam splitter  23 . 
     During measurement, a divergent light L from the coherent light source  21  is made into a substantially condensed light by the collimator lens  22 , passes through the non-polarizing beam splitter  23 , and is divided into a transmitted light and a reflected light on the polarizing beam splitter surface  24   a  of the probe-like polarizing prism  24 . S-polarized light reflected by the polarizing surface  24   a  emerges from the probe-like polarizing prism  24  toward the slider side  25 , and is reflected by the slider side  25 , and the reflected light passes along the original optical path and is returned to the polarizing surface  24   a  of the probe-like polarizing prism  24 . On the other hand, P-polarized light transmitted through the polarizing surface  24   a  is reflected by the reference mirror surface  24   b  in the probe-like polarizing prism  24 , and is likewise returned to the polarizing surface  24   a.    
     These two polarized lights are combined together on the polarizing surface  24   a  of the probe-like polarizing prism  24 , and become a bright-dark signal of an interference light beam by a polarizing interference optical system which will be described later. When the distance between the probe-like polarization prism  24  and the slider side  25  changes, each time the difference between the lengths of the forward and backward optical paths of the two light beams separated by the polarization surface  24   a  becomes integer times as great as the wavelength of the coherent light source  21 , the bright and dark thereof changes. That is, if a laser diode of a wavelength 0.78 μm is used as the coherent light source  21 , when the distance between it and the slider side  25  deviates by 0.39 μm, the light and shade change into a sine waveform by a cycle. 
     This change in the bright and dark is converted into an electrical signal by the light receiving element  30 , and if the distance between the prism  24  and the slider side  25  is preset to such a distance as will be the middle of the bright and dark, when the distance between the prism  24  and the slider side  25  changes minutely, the level of the electrical signal will change sensitively. Accordingly, by the utilization of this sine wave-like change in the level of the electrical signal and by the use of a conventional electric circuit of resolving power capable of dividing a sine wave of a wavelength 0.39 μm into several tens of phases, the change in the distance can be detected with resolving power of the order of 0.01 μm. Also, if there is a 90° phase difference bright-dark signal of two phases, a sine wave can be divided into several tens to several hundreds of phases by the use of a conventional electrical interpolation circuit and therefore, the change in the distance can be detected with higher resolving power of 0.001 μm. 
     FIG. 3A shows a method of generating a 90° phase difference bright-dark signal of four phases, and at the stage whereat the light emerges from the probe-like polarizing prism  24 , the wave fronts of the two light beams overlap each other, but the light beams are linearly polarized light beams of wave fronts orthogonal to each other and do not interfere with each other to become a bright-dark signal. These light beams are reflected by the non-polarizing beam splitter  23 , pass through the aperture in the aperture plate  26  and are amplitude-divided into four equivalent light beams including two linearly polarized light beams orthogonal to each other by the four-division diffraction grating plate  27 . At this time, these four light beams travel and separate with only their intensities reduced by the division, and enter the rock crystal plate  28  disposed in an appropriate space. 
     This rock crystal plate  28  is used as an optical phase plate, and has two optic axes, i.e., f-axis and s-axis, and since there is a difference in refractive index between these axes, a linearly polarized wave component in which an electric field component in the electromagnetic wave of light has entered in parallelism to the f-axis emerges with its phase advanced as compared with a linearly polarized wave component in which the electric field component has entered in parallelism to the s-axis. The phase difference Γ (deg) at this time is represented by the following expression when the wavelength of the coherent light source  21  is defined as λ, the refractive index of the rock crystal for an abnormal ray of light is defined as ne, the refractive index of the rock crystal for a normal ray of light is defined as no, and the thickness of the rock crystal plate  28  is fined as t. 
     
       
         Γ=(360/λ)·(ne−no)t 
       
     
     The refractive index ne for an extraordinary ray of light corresponds to the linear polarized wave substantially parallel to the s-axis, and the refractive index no for an ordinary ray of light corresponds to the linearly polarized light beam parallel to the f-axis. The typical values of the refractive index of rock crystal are ne=1.5477 and no=1.5387 and therefore, t satisfying the phase difference Γ=90 (deg) can be found as follows by substantially λ=0.78 μm, ne=1.5477 and no=1.5387. 
      t=λ/{4(ne−no)}=21.67 μm 
     In the present embodiment, the rock crystal plate  28 , as shown in FIG. 3B, is disposed so that the f-axis which is an optic axis may be the direction of the S-polarized wave, and is provided with level differences by the etching process so as to assume four different thicknesses of four-square configuration, and the four divided light beams enter respective areas  28   a - 28   d  and emerge from the back surfaces thereof. The area  28   a  is a portion which is not worked so as to have a level difference and has the original thickness of the rock crystal substrate, and the area  28   b  has its portion corresponding to Δt=21.67 μm etching-processed by hydrofluoric acid so that the phase of the wave front of only the S-polarized component may be delayed by 90° as compared with the area  28   a.    
     The area  28   c  has its portion corresponding to 2·Δt etching-processed so that the phase of the wave front of only the S-polarized component may be delayed by 180° as compared with the area  28   a , and the area  28   d  has its portion corresponding to 3·Δt etching-processed so that the phase of the wave front of only the S-polarized component may be delayed by 270° as compared with the area  28   a . These three level differences are achieved by effecting the etching of the level difference of 2·Δt once in the areas  28   c  and  28   d , and effecting the etching of the level difference of Δt once in the areas  28   a  and  28   d.    
     Light beams (P, S) transmitted through the area  28   a  are relatively advanced in the phase of the wave front of the S-polarized wave among the P- and S-polarized waves by the original thickness of the rock crystal plate  28  and the disposition of the optic axis thereof, and the amount of this advance is defined as ΔΦ. As regards the light beams (P, S) transmitted through the area  28   b , the advance of the phase of the wave front of the S-polarized wave is returned by 90° and the amount of phase advance is ΔΦ−90° because the rock crystal plate  28  has become thinner by Δt than the original thickness thereof by etching. 
     As regards the light beams (P, S) transmitted through the area  28   c , the advance of the phase of the wave front of the S-polarized wave is returned by 180° and the amount of phase advance is ΔΦ−180° because the rock crystal plate  28  has become thinner by 2·Δt than the original thickness thereof by etching. As regards the light beams (P, S) transmitted through the area  28   d , the advance of the phase of the wave front of the S-polarized wave is returned by 270° and the amount of phase advance is ΔΦ−270° because the rock crystal plate  28  has become thinner by 3·Δt than the original thickness thereof by etching. 
     The light beams (P, S) transmitted through these four areas  28   a - 28   d  has its 45° polarized component extracted by the polarizing plate  29  having its optic axis in the azimuth of 45°, and these P- and S- polarized waves are converted into 45° linearly polarized waves while having phase information and, therefore, cause interference therebetween and become bright-dark signal light beams. At this time, the phase-difference between the light beams P and S which have emerged from the respective areas  28   a - 28   d  deviates each integer times λ/4 and therefore, the timing (phase) of the bright and dark of the interference light deviates each quarter cycle. Accordingly, these interference light beams enter the four light receiving elements  30   a - 30   d , whereby four phase difference signals are obtained at a time. These four interference light beams have a phase shift at each quarter cycle and, therefore, have the A+ phase, B+ phase, A− phase and B− phase of the 90° phase difference signals, which are four-phase push-pull signals. 
     In the present embodiment, the four-division diffraction grating  27 , the phase difference rock crystal plate  28 , the 45° azimuth polarizing plate  29  and the four-division light receiving elements  30   a - 30   d  are simply disposed in an orderly way and therefore, the work of disposing a plurality of minute polarizing plates or the like becomes unnecessary, and a very simple and compact apparatus can be realized. Particularly, it can be easily realized by processing the diffraction grating  27 , the rock crystal plate  28  and the polarizing plate  29  for each large substrate by a method similar to the semiconductor process, and finally cutting them. 
     FIG. 4 shows a perspective view of a push-pull optical system according to a second embodiment, and a rock crystal plate  31  in this embodiment is formed with a level difference by the etching process so as to have two different thicknesses in a horizontal direction, and is disposed so that the f-axis which is an optic axis may be the direction of S-polarized wave. This rock crystal plate  31  is designed such that each two of four divided light beams enter areas  31   a  and  31   b , respectively, and are transmitted from the back surface thereof, and the area  31   a  is a portion which is not worked into a level difference, and has the original thickness of a rock crystal substrate. Also, the area  31   b , as compared with the area  31   a , is worked into a level difference so that only the S-polarized component may be delayed in the phase of a λ/4 wave front, and this level difference is achieved by effecting the etching of a level difference corresponding to λ/2 once in the area  31   b . Also, a polarizing plate  32  in the present embodiment comprises a polarizing plate  32   a  having an optic axis in the azimuth of 45° and a polarizing plate  32   b  having an optical axis in the azimuth of 135°, the polarizing plates  32   a  and  32   b  being disposed in a vertical direction. 
     Of the light beams (P, S) divided in the four-division diffraction grating plate  27 , the left light beam passes through the area  31   a  of the rock crystal plate  31  and the right light beam passes through the area  31   b  of the rock crystal plate  31 . At this time, the light beams (P, S) transmitted through the area  31   a  are relatively advanced in the phase of the wave front of the S-polarized wave between P- and S-polarized waves by the original thickness of the rock crystal plate  31  and the disposition of the optical axis thereof, and this amount of advance is defined as ΔΦ. As regards the light beams (P, S) transmitted through the area  31   b , the advance of the phase of the wave front of the S-polarized wave is returned by λ/4 and the amount of phase advance is ΔΦ−λ/4 because the rock crystal plate  31  has become thinner by Δt than the original thickness thereof by etching. 
     Of the light beams (P, S) transmitted through these two areas  31   a  and  31   b , the upper two light beams pass through the polarizing plate  32   a  in the azimuth of 45°, and the lower two light beams pass through the polarizing plate  32   b  in the azimuth of 135°, and a 45° polarized component or a 135° polarized component is extracted, but at this time, the P- and S-polarized waves are converted into 45° or 135° linearly polarized waves while having phase information, and cause interference and become a bright-dark signal light beam. 
     The wave front phase between the light beams P and S which have emerged from the two areas  31   a  and  31   b  deviates by λ/4 and therefore, the timing (phase) of the bright and dark of the interference light beam of the light beam transmitted through the polarizing plate  32   a  is such that the light beam transmitted through the area  31   b  advances by a quarter cycle as compared with the light beam transmitted through the area  31   a . Also, the timing (phase) of the bright and dark of the interference light beam of the light beam transmitted through the polarizing plate  32   b  is such that the light beam transmitted through the area  31   b  advances by a quarter cycle as compared with the light beam passed through the area  31   a . Here, the bright and dark phase of the light beam transmitted through the polarizing plate  32   a  and the bright and dark phase of the light beam transmitted through the polarizing plate  32   b  deviate by 180° from each other and, therefore, the phase of the four bright and dark light beams become 0°, 90°, 180° and 270°, and become a four-phase push-pull signal as in the first embodiment. 
     In the present embodiment, the four-division diffraction grating  27 , the phase difference rock crystal plate  31 , the 45° azimuth polarizing plate  32   a , the 135° azimuth polarizing plate  32   b  and the four-division light receiving elements  30   a - 30   d  are simply disposed in an orderly way, whereby a very simple and compact construction can be realized. Particularly, the rock crystal plate  31  can be worked by an etching process, and the polarizing plate  32   a  and the polarizing plate  32   b  are stuck together adjacent to each other, whereby assembly adjustment can be effected relatively simply. 
     The present embodiment can be applied to a grating interference-type encoder in which coherent light beams divided into two are applied to diffraction gratings moved relative to each other, and diffracted lights of different order numbers are taken out to thereby generate an interference phase signal. It can also be applied to other popular grating interference-type encoders and interference measuring apparatuses. 
     FIG. 5 shows a perspective view of a third embodiment. In a non-contact sensor  20 , as in the case of FIG. 2, a coherent light source  21  such as a laser diode, a collimator lens  22 , a non-polarizing beam splitter  23  and an optical-type probe-like polarizing prism  24  are disposed in succession, and the probe-like polarizing prism  24  has a polarizing beam splitter surface  24   a  and a reference mirror surface  24   b , and a slider side  25  which is a non-measuring region is disposed on the front surface of the probe-like polarizing prism  24  in the direction of reflection of the polarizing surface  24   a.    
     A quarter wavelength plate  41 , an aperture plate  42 , a four-division diffraction grating plate  43  such as a hounds-tooth-checkered phase diffraction grating, a polarizing plate mask  44  having four gratings  44   a - 44   d  formed on a substrate with their arrangement azimuths shifted by 45°, and a four-division light receiving element  45  having four areas  45   a - 45   d  are arranged in succession in the direction of reflection of the non-polarizing beam splitter  23 . 
     During measurement, a divergent light L from the coherent light source  21 , as in the case of FIG. 2, is divided into a transmitted light and a reflected light on the polarizing beam splitter surface  24   a  of the probe-like polarizing prism  24 . S-polarized light reflected by the polarizing surface  24   a  emerges from the probe-like polarizing prism  24  toward the slider side  25 , is reflected by the slider side  25 , and is returned to the polarizing surface  24   a  of the probe-like polarizing prism  24 . On the other hand, P-polarized light transmitted through the polarizing surface  24   a  is likewise returned to the polarizing surface  24   a.    
     These two polarized lights are combined together on the polarizing surface  24   a  of the probe-like polarizing prism  24 , and become the bright-dark signal of an interference light beam by a polarizing interference optical system which will be described later. When the distance between the probe-like polarizing prism  24  and the slider side  25  changes, the difference between the lengths of the forward and backward optical paths of the two light beams separated by the polarizing surface  24   a  becomes integer times as great as the wavelength of the coherent light source  21 , whereafter the bright and dark thereof change. 
     This change in the bright and dark is converted into an electrical signal by the light receiving element  45 , and as in the case of FIG. 2, any change in the distance can be detected with high resolving power. 
     FIG. 6 shows a method of producing the 90° phase difference bright-dark signal of four phases described above, and at the stage whereat the two light beams emerge from the probe-like polarizing prism  24 , the two light beams are linearly polarized light beams of which the wave fronts overlap each other, but are orthogonal to each other, and do not interfere with each other to become a bright-dark signal. These are reflected by the non-polarizing beam splitter  23 , are transmitted through the quarter wavelength plate  41  and are converted into oppositely directed circularly polarized lights, and their vector-combined wave fronts become a linearly polarized light. Also, the direction of the polarized wave front of the linearly polarized light depends on the phase difference between the two light beams, and when the phase shifts by 360° between the two light beams, the polarized wave front rotates by 180°. 
     This linearly polarized light beam of which the polarized wave front rotates passes through an aperture in the aperture plate  42  and is amplitude divided into equivalent linearly polarized light beams by the four-division diffraction grating plate  43 . At this time, these four light beams separate from one another with the signal in a state in which only the intensities thereof are reduced by the division, and enter the polarizing plate mask  44  disposed in an appropriate space. 
     This polarizing plate mask  44  functions as an optical polarizing plate and can therefore be freely designed when it is formed in advance on a glass plate by the photolithographic process or the like. In the present embodiment, in order to obtain a four-phase bright-dark signal, there is required an interference signal of which the bright and dark phases shift by 90° each and therefore, metal grating lines are formed with their arrangement azimuths shifted by 45° from one another in each of areas through which the four light beams are transmitted. Also, the pitch of the grating lines is made sufficiently smaller than the wavelength of the coherent light source. 
     When the rotating linearly polarized light is orthogonal to the grating lines and when it coincides with the grating lines, the transmitted light becomes maximum and becomes minimum, respectively, and therefore, phase difference signals differing in the timing of bright and dark from one another are obtained in the four areas. 
     In the present embodiment, the quarter wavelength plate  41 , the aperture plate  42 , the four-division diffraction grating plate  43 , the polarizing plate mask  44  and the four-division light receiving elements  45   a - 45   d  are simply disposed in an orderly way and therefore, the apparatus can be very simply and compactly constructed and the phase difference is settled highly accurately. 
     The present embodiment can be applied to a grating interference-type encoder in which a coherent light beam divided into two is applied to diffraction gratings moved relative to each other and diffracted lights of different orders are taken out to thereby produce an interference signal. It can also be applied to other popular grating interference-type encoders and interference measuring apparatuses. 
     As described above, the above-described interference-type measuring apparatus and grating interference-type encoder can realize an optical system very simply and compactly, and can collectively detect a plurality of stable phase difference signals and can effect highly accurate measurement. 
     Also, the above-described optical apparatus can realize such a measuring apparatus.