Abstract:
Position detection apparatus comprises: an incident unit that forms a parallel beam from a light beam of a light source and supplies the parallel beam to a beam splitter in a way that the separated beams, obtained from the parallel beam by the beam splitter, are respectively incident upon the reference surface and the measurement target surface at an angle; and a reflection unit that reflects the separated beams, which have been reflected by the reference surface and the measurement target surface and integrated to a light path by the beam splitter, to be supplied to the beam splitter as a parallel beam along the light path, and generates a signal corresponding to a phase difference of the separated beams by interfering a light beam that has been supplied from the reflection unit, separated by the beam splitter, reflected by the reference surface and the measurement target surface, and integrated again to a light path by the beam splitter.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a position detection apparatus and method for detecting an object&#39;s position variation without a contact by utilizing interference measurement.  
       BACKGROUND OF THE INVENTION  
       [0002]     The Michelson interferometer employing a laser is widely utilized in the field that requires nanometer measurement. An interferometer of this type needs to receive reflected light from a measurement-target object. For this purpose, mirrors are generally used. In the method utilizing mirrors, since optical alignment must be adjusted with extremely high precision, normally a cube corner reflector is employed for the measurement target so that interference measurement is not interrupted by deviation of the optical alignment. However, because a cube corner reflector is a prism, it cannot be attached to a microscopic location. In other words, if the measurement-target object is microscopic, position measurement employing an interferometer cannot be performed.  
         [0003]     In view of this, the applicant of this invention has proposed to focus light on a mirror serving as the measurement target as shown in  FIG. 8  so as to prevent a disturbance of the interference state caused by deviation of the mirror alignment (Japanese Patent Application Laid-Open (KOKAI) No. 2001-076325).  
         [0004]     Referring to  FIG. 8 , a divergent light beam outputted by a light source LD is converted to a moderately focused light beam BEAM, transmitted through a non-polarization beam splitter NBS, and separated into P polarization component and S polarization component by a polarization beam splitter PBS. More specifically, while the P polarization component is transmitted through the PBS and reflected by a reference mirror M 2 , the S polarization component is reflected by the PBS and reflected by a measurement target surface M 1 . Then these reflected light beams are combined at the polarization beam splitter PBS and reflected by the non-polarization beam splitter NBS. The light beam reflected by the non-polarization beam splitter NBS goes through a quarter wavelength plate QWP and converted to linearly polarized light, whose polarization orientation rotates in accordance with a variation of a phase difference that bases upon a variation of the path length difference in the separated two light beams. The linear polarized light is divided into four light beams by a division device GBS. Each of the beams is transmitted through four polarization devices PP 1 , PP 2 , PP 3  and PP 4  arranged in a way that each of the polarization orientations is shifted by 45° In this manner, the four light beams are converted to four signal beams, whose interference cycles have 90° phase difference to each other, and received by respective photoreceptive devices PD 1  PD 2 , PD 3  and PD 4 , then four periodic signals are outputted.  
         [0005]     In this conventional example, since light is focused on the measurement target, even if an alignment deviation (angle deviation) is generated, the wave surface of the reflected light does not change. Note that the center of the reflected light (optical axis) deviates. Since the wave surface does not change, the interference state between the reflected light and the reference light is stable.  
         [0006]     Since this method does not use a cube corner reflector, it can be used as a new compact displacement sensor that measures a microscopic out-of-plane displacement on a measurement-target surface, with the use of a semiconductor laser for an optical sensor head.  
         [0007]     However, since this method focuses light on a measurement-target surface, the spatial resolution becomes extremely high. Therefore, if a horizontal deviation or the like occurs on the measurement target object when measuring an out-of-plane displacement of the measurement-target surface, the superficial shape component of the measurement-target object is also detected, and the measurement may become unstable. Therefore, depending on the application, it is better to have a low spatial resolution in the direction of horizontal deviation on the surface for realizing stable out-of-plane displacement measurement. For this reason, widening the light irradiating area on the measurement target (mirror) has been desired.  
         [0008]     Furthermore, in the above-described method, measurement is performed by focusing light on the measurement target. However, if there is a large out-of-plane displacement, the light focusing conditions are not satisfied, impairing the function of stabilizing the interference state to deal with an alignment deviation. For this reason, in general the measurable area has been limited to several tens of μm. Therefore, there have been demands for a method that enables easy alignment (enlarged allowance for an angle deviation) and that enlarges the measurable area of an out-of-plane displacement on the order of millimeter.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention has been proposed in view of the above-described conventional example, and has as its object to provide a position detection apparatus and method adopting a new system, which enlarges a light irradiating area on a measurement-target surface, which enlarges an out-of-plane displacement measurement area, and which does not receive influence of an alignment deviation.  
         [0010]     According to the present invention, the foregoing object is attained by providing a position detection apparatus comprising: 
        a beam splitter arranged to separate an incident parallel beam and to supply separated beams to a reference surface and a measurement target surface respectively;     an incident unit that forms a parallel beam from a light beam of a light source and supplies the parallel beam to the beam splitter so that the separated beams separated by the beam splitter are respectively incident upon the reference surface and the measurement target surface at an angle;     a reflection unit that reflects the separated beams, which have been reflected by the reference surface and the measurement target surface and integrated to a light path by the beam splitter, to be supplied to the beam splitter as a parallel beam along the light path; and     generation means that generates a signal corresponding to a phase difference of the separated beams by interfering a returning beam that has been supplied from the reflection unit, separated by the beam splitter, reflected by the reference surface and the measurement target surface, and integrated again to a light path by the beam splitter.        
 
         [0015]     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0017]      FIGS. 1A and 1B  are explanatory views of an optical system of a laser interferometer according to the first embodiment;  
         [0018]      FIGS. 2A and 2B  are explanatory views of a light path in a case where the measurement-target surface is slightly tilted in the optical system shown in  FIGS. 1A and 1B ;  
         [0019]      FIGS. 3A and 3B  are explanatory views of a light path in a case where the measurement target surface has a parallel out-of-plane displacement on the order of millimeter in the optical system shown in  FIGS. 1A and 1B ;  
         [0020]      FIGS. 4A and 4B  are explanatory views of an optical system of a laser interferometer according to the second embodiment;  
         [0021]      FIGS. 5A and 5B  are explanatory views of an optical system of a laser interferometer according to the third embodiment;  
         [0022]      FIGS. 6A and 6B  are explanatory views of an optical system of a laser interferometer according to the fourth embodiment;  
         [0023]      FIGS. 7A and 7B  are explanatory views of an optical system of a laser interferometer according to the fifth embodiment; and  
         [0024]      FIG. 8  is an explanatory view of an optical system of a conventional interference measurement method, in which light is focused on a measurement target surface. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.  
       First Embodiment  
       [0026]      FIGS. 1A and 1B  are explanatory views of an optical system of a laser interferometer according to the first embodiment. In  FIGS. 1A and 1B , linear polarized light divergent light from a semiconductor laser  101  is focused on a position P 1  on a focal plane of a lens  112  by a lens  111 . The linear polarized light diverging from the position P 1  is incident upon the lens  112 , and outputted as a parallel beam having a slightly angled optical axis. Note in this embodiment, “having a slightly angled optical axis” means that the optical axis direction of a parallel beam is set in a way that transmitted light and reflected light of a beam splitter are incident slightly at an angle (e.g., e.g., about 1° to 10°) with respect to the normal direction of the surface of a measurement target object  121  and the surface of a reference mirror  122 . The parallel beam exiting from the lens  112  is separated into two light beams, i.e., a reflected beam (S polarized light) and a transmitted beam (P polarized light) (hereinafter comprehensively called the separated beams), by polarization components of a polarization beam splitter  113 . While the reflected beam (S polarized light) irradiates a reference mirror  122 , the transmitted beam (P polarized light) irradiates a measurement target object (mirror)  121 . The reflected light beams of the respective mirrors go through the polarization beam splitter  113 , and are focused on the position P 2  on the focal plane of the lens  112 . Note that the position P 2  is slightly shifted from the position P 1 .  
         [0027]     In the neighborhood of the position P 2 , reflection coating  114  is provided. The light beam focused on the position P 2  is returned to the original light path by the reflection coating  114 . The light beam reflected by the reflection coating  114  exits from the lens  112  as a parallel beam having a slightly angled optical axis (parallel beam having a direction that is virtually parallel to the optical axis of the parallel beam incident upon the lens  112 ). Thereafter, the parallel beam is separated into two light beams by the polarization beam splitter  113 , irradiating the reflected beam (S polarized light) to the reference mirror  122  and the transmitted beam (P polarized light) to the measurement target object  121 . Then, the respective reflected light beams are again incident on the lens  112  as a light beam having the same light path through the polarization beam splitter  113 . The beam incident upon the lens  112  is focused on the position P 1  on the focal plane, and extracted to the light-source side. In this manner, the S polarized light beam reciprocates twice between the surface of the reference mirror  122  and the beam splitter  113 , and the P polarized light beam reciprocates twice between the surface of the measurement target object  121  and the beam splitter  113 .  
         [0028]     The light beam is extracted on the photoreceptive device side by a non-polarization beam splitter  102 . The extracted beam transmits through a quarter wavelength plate  103 , thereby being converted to linearly polarized light whose polarization orientation rotates in accordance with a variation of a phase difference, then through a condenser lens  104  and an aperture member  105 , it reaches a beam division device  106 , where the light beam is divided into three beams. Because the light beam incident upon a photoreceptive device  108  expands, the condenser lens  104  is provided so that the light beam is appropriately condensed and efficiently received by the photoreceptive device  108 . The aperture member  105  limits the area of the light beam at the time of dividing the interfering light beam into three beams, so as to assure three homogeneous beams. Without the aperture member  105 , divided three beams overlap when the original beam expands. Each of the three beams divided in the foregoing manner is incident upon respective photoreceptors of the tripartition photoreceptive device  108  through a polarization device array  107 , in which each polarization axis is shifted by 60°. As a result, detected are three interference signals U, V and W 1  whose phases are shifted by 120°, which base upon the out-of-plane displacement of the measurement target object  121 .  
         [0029]     Since the three interference signals U, V and W are obtained by employing the principle of interference measurement utilizing two reciprocative light paths, they are sine wave signals having a ¼ cycle of the light source wavelength. For instance, assuming that a laser diode having 0.8 μm wavelength is employed, sine wave signals having a cycle of 0.2 μm is obtained. By calculating the wave number, and further, by electrically dividing the signals with a well-known electric phase splitter, it is possible to detect a relative position deviation with a resolution on the order of nanometer.  
         [0030]     Alternatively, it may be so constructed that, instead of generating three types of interference signals having a 120° phase difference, four or two types of signals having 90° phase difference may be generated.  
         [0031]      FIGS. 2A and 2B  are explanatory views of a light path in a case where the measurement-target surface is slightly tilted in the optical system shown in  FIGS. 1A and 1B . Because of the inclination of the measurement target object  121 , the reflected beam (P polarized light) from the surface of the measurement target object  121  exits at an angle different from an ideal light path. As a result, the parallel beam incident upon the lens  112  enters a position P 3  on the focal plane, which is slightly shifted from the position P 2 . The beam of the P polarized light incident on the position P 3  returns to the original light path by the reflection coating  114  provided in the neighborhood of the positions P 2  and P 3 . In other words, by the effect of reciprocating the beam in the cat&#39;s eye optical system consisting of the lens  112  and the reflection coating  114 , the light beam is reflected in the same orientation as the normal light path, and outputted as a parallel beam from the lens  112 . Among the light beams, the beam of P polarized light is transmitted through the polarization beam splitter  113 , and reflected by the measurement target object  121 . Therefore, the light beam is again transmitted through the polarization beam splitter  113 , and focused on the position P 1  on the focal plane of the lens  112 , then outputted to the light-source side as divergent light.  
         [0032]     Meanwhile, the reflected beam (S polarized light) from the reference mirror  122  follows the ideal light path, and ultimately is focused on the position P 1  on the focal plane of the lens  112 , then outputted to the light-source side as divergent light. In this manner, the light beam (P polarized light) that has gone through the surface of the measurement target object  121  and the light beam (S polarized light) that has gone through the reference mirror  122  ultimately exit from the focal plane of the lens  112 . As described above, by virtue of coinciding the exiting positions, the wave surfaces of the spherical waves of both light beams become uniform, resulting in a stable interference state. This effect also applies to the second to fifth embodiments which will follow.  
         [0033]     Although there are two different orientations in inclination of the surface of the measurement target object  121 , the only difference is the shifting direction of the focused light reflection position (position P 3 ) on the focal plane of the lens  112 . Therefore, basically the same effect can be achieved. As described above, since an interference optical system that is unsusceptible to an inclination of the measurement-target surface can be achieved, it is possible to perform position detection without arranging a cube corner reflector on the measurement target, realizing downsizing and easy manipulation of the optical system.  
         [0034]      FIGS. 3A and 3B  are explanatory views of a light path in a case where the measurement target surface has a parallel out-of-plane displacement on the order of millimeter in the optical system shown in  FIGS. 1A and 1B . Since the optical axis center of the reflected light (P polarized light) from the measurement target object  121  is parallelly deviated from the ideal light path, the reflected light beam reaches the position P 2  on the focal plane of the lens  112 . The position P 2  is determined based on the focal distance of the lens and an incident angle of the parallel beam incident upon the lens. In the case of  FIGS. 3A and 3B , these values are unchanged. The beam of P polarized light that has reached the position P 2  is reflected by the reflection coating  114  provided in the neighborhood of the position P 2 , and the parallel beam outputted from the lens  112  ends up following the light path that has the same orientation but is parallelly deviated from the original light path. The parallel beam of the P polarized light is transmitted through the polarization beam splitter  113 , and reflected by the surface of the measurement target object  121 . In this stage, the parallel beam (P polarized light) is reflected in the same orientation as the normal light path, by the effect of reciprocating the beam in the cat&#39;s eye optical system consisting of the lens  112  and the reflection coating  114 . Therefore, the parallel beam reflected by the measurement target object  121  is transmitted through the polarization beam splitter  111 , incident upon the lens  112  and focused on the position P 1 , then outputted to the light-source side as divergent light. Note that the separated beam, the S polarized light, obtained by the beam splitter  113  follows the ideal light path described in  FIGS. 1A and 1B , and is focused on the position P 1 , then outputted to the light-source side as divergent light. As described above, by virtue of coinciding the exiting positions, the wave surfaces of the spherical waves of both light beams become uniform, resulting in a stable interference state.  
         [0035]     Note that since the optical axis of the light beam is deviated in parallel, an eclipse may occur because of the optical effective diameter of the lens  112 . Furthermore, when the reflected light interferes with the reference light, the overlapping area of the wave surface becomes small because of the unmatched optical axis. For these reasons, the larger the out-of-plane displacement becomes, the more the signal amplitude which is obtained by receiving the interfering light beams with the photoreceptive device array is reduced. However, for instance, assuming that a light irradiation angle θ of the measurement target surface is 5° and the out-of-plane displacement is 1 mm, the optical axis deviation X is obtained as follows: 
 
 X= 4 mm×tan θ=0.34 mm 
 
 If the light beam diameter Φ is on the order of millimeter, about 50% overlaps of the interfering light beams are assured. Therefore, even if an out-of-plane displacement on the order of 1 mm is generated, measurement can be performed (note that the above values are a rough standard). 
 
         [0036]     As described above, the first embodiment employs a coherent light source to radiate a parallel beam, having a beam diameter on the order of millimeter, to a measurement target surface slightly at an angle with respect to the normal of the measurement target surface. The reflected beam is received by the cat&#39;s eye optical system, then the reflected beam from the cat&#39;s eye optical system is again irradiated back to the measurement target surface, and interference between the reflected light and reference light is obtained. Therefore, the embodiment has an advantage in that the measurement area of an out-of-plane displacement can be enlarged on the order of millimeter, and that it is unsusceptible to deviation in the optical alignment.  
       Second Embodiment  
       [0037]     In the first embodiment, the lens  112  and the mirror  114  on the focal plane of the lens are employed to constitute the cat&#39;s eye optical system. In the second embodiment, a so-called ¼-pitch gradient index rod lens having reflection coating on its end surface constitutes the cat&#39;s eye optical system, thereby achieving the similar effects as the first embodiment.  
         [0038]      FIGS. 4A and 4B  are explanatory views of an optical system of a laser interferometer according to the second embodiment. Linear polarized light divergent light from the semiconductor laser  101  transmits through a lens  211  and a wedge prism  212  (prism  212  is a device for changing the orientation of the main beam), and is incident upon the position P 21  on the end surface of the so-called ¼-pitch gradient index rod lens  213  as a focused beam having a slightly angled optical axis. In a case of employing the gradient index rod lens  213  as a cat&#39;s eye device, it is necessary to shift the incident angle of the light beam upon the cat&#39;s eye device and to shift the focus position of the light beam from the center as shown in  FIGS. 4A and 4B . By virtue of this configuration, the reflection position can be shifted from the incident position, and the exiting light from the gradient index rod lens  213  can be spatially separated from the light incident upon the gradient index rod lens  213 . The light beam incident upon the gradient index rod lens  213  is outputted as a parallel beam from the other surface of the lens  213 , and separated into two light beams by polarization components of a polarization beam splitter  214 . While the reflected beam (S polarized light) obtained by the polarization beam splitter  214  irradiates a reference mirror  222 , the transmitted beam (P polarized light) irradiates the surface (mirror) of a measurement target object  221 . The respective reflected light beams go through the polarization beam splitter  214 , and are focused on the position P 22  on the focal plane of the gradient index rod lens  213 .  
         [0039]     By reflection coating  215  provided in the neighborhood of the position P 22 , the focused light beam is returned to the original light path, then exits from the gradient index rod lens  213  as a parallel beam, and is separated again into two light beams by the polarization beam splitter  214 . Then, the reflected beam (S polarized light) irradiates the reference mirror  222  and the transmitted beam (P polarized light) irradiates the measurement target object  221 . The respective reflected light beams are again incident on the gradient index rod lens  213  through the polarization beam splitter  214 . As a result, a divergent beam can be extracted from the position P 21  on the end surface of the gradient index rod lens  213 . In this manner, the S polarized light beam reciprocates twice between the surface of the reference mirror  222  and the polarization beam splitter  214 , and the P polarized light beam reciprocates twice between the surface of the measurement target object  221  and the polarization beam splitter  214 .  
         [0040]     The structure for obtaining interference signals from the light beam diverged from the position P 21  is similar to the first embodiment. More specifically, the light beam diverged from the position P 21  is extracted on the photoreceptive device side by the non-polarization beam splitter  102 . The extracted beam transmits through the quarter wavelength plate  103 , thereby being converted to linearly polarized light whose polarization orientation rotates in accordance with a variation of a phase difference. The light beam is divided into three beams by the beam division device  106 . Each of the three beams is incident upon respective photoreceptors of the tripartition photoreceptive device  108  through the polarization device array  107 , in which each polarization axis is shifted by 60°. As a result, detected are three interference signals, whose phases are shifted by 120°, which base upon the out-of-plane displacement of the measurement target object  221 .  
         [0041]     As described above, by virtue of employing a gradient index rod lens as a cat&#39;s eye optical device, the second embodiment achieves an effect of realizing a small and stable optical system.  
       Third Embodiment  
       [0042]      FIGS. 5A and 5B  are explanatory views of an optical system of a laser interferometer according to the third embodiment.  
         [0043]     As shown in  FIGS. 5A and 5B , linear polarized light divergent light from the semiconductor laser  101  transmits through a collimator lens  102 , and a parallel beam having a slightly angled optical axis is generated and outputted. The parallel beam is separated into two light beams by polarization components of a polarization beam splitter  312 . While the reflected beam (S polarized light) irradiates a reference mirror  322 , the transmitted beam (P polarized light) irradiates the surface (mirror) of a measurement target object  321 . The respective reflected light beams are incident on a so-called ¼-pitch gradient index rod lens  313  through the polarization beam splitter  312 . In the gradient index rod lens  313 , the incident light beam is focused on the position P 32  on the end surface, and through a reflection coating  314  provided in the neighborhood of the position P 32 , returned to the original light path. In this manner, the parallel beam exits from the gradient index rod lens  313 , and is separated again into two light beams by the polarization beam splitter  312 . Then, the reflected beam (S polarized light) irradiates the reference mirror  322  and the transmitted beam (P polarized light) irradiates the surface of the measurement target object  321 . The respective reflected light beams are extracted to the light source side through the polarization beam splitter  312 . As a result, the separated S polarized light beam reciprocates twice between the surface of the reference mirror  322  and the beam splitter  312 , and the separated P polarized light beam reciprocates twice between the surface of the measurement target object  321  and the beam splitter  312 .  
         [0044]     From the light beam extracted on the light-source side in the above-described manner, interference signals are acquired by the structure similar to that of the first embodiment. More specifically, the light beam is extracted on the photoreceptive device side by the non-polarization beam splitter  102 . The extracted beam transmits through the quarter wavelength plate  103 , thereby being converted to linearly polarized light whose polarization orientation rotates in accordance with a variation of a phase difference. The light beam is divided into three beams by the beam division device GBS  106 . Each of the three beams is incident upon respective photoreceptors of the tripartition photoreceptive device  108  through the polarization device array  107 , in which each polarization axis is shifted by 60°. In this manner, detected are three interference signals, whose phases are shifted by 120°, which base upon the out-of-plane displacement of the surface of the measurement target object  321 .  
         [0045]     Note that the gradient index rod lens  313  and the reflection coating  314  employed in the third embodiment are equivalent to the cat&#39;s eye function. They may be substituted by the lens shown in  FIGS. 1A and 1B  and the reflection coating arranged on the focal plane of the lens.  
       Fourth Embodiment  
       [0046]      FIGS. 6A and 6B  are explanatory views of an optical system of a laser interferometer according to the fourth embodiment.  
         [0047]     In the fourth embodiment, linear polarized light divergent light from the semiconductor laser  101  is incident upon a polarization preserving optical fiber  402  through a lens  411  in a way that the polarization axis of the optical fiber matches the polarization surface of the light beam. The other end surface of the optical fiber is arranged at the position P 41  on the focal plane of a lens  412 , for outputting the divergent light. The divergent light is converted to a parallel beam by the lens  412 , and outputted as a parallel beam having a slightly angled optical axis. The parallel beam is separated into two light beams having respective polarization components by a polarization beam splitter  413 . Among the two light beams, the reflected beam (S polarized light) irradiates a reference mirror  422 , and the transmitted beam (P polarized light) irradiates the surface (mirror) of a measurement target object  421 .  
         [0048]     The respective reflected light beams go through the polarization beam splitter  413 , are focused on the position P 42  on the focal plane of the lens  412 , and returned to the original light path by a reflection coating  414  provided in the neighborhood of the position P 42 . In this manner, the parallel beam exits again from the lens  412 . The parallel beam is separated into two light beams by the polarization beam splitter  413 . The reflected beam (S polarized light) irradiates the reference mirror  422  and the transmitted beam (P polarized light) irradiates the surface (mirror) of the measurement target object  421 . The respective reflected light beams are incident upon the lens  412  through the polarization beam splitter  413 , and head toward the position P 41  on the focal plane of the lens  412 . As a result, the S polarized light reciprocates twice between the surface of the reference mirror  422  and the beam splitter  413 , and the P polarized light reciprocates twice between the surface of the measurement target object  421  and the beam splitter  413 .  
         [0049]     The light beam is extracted on the photoreceptive device side by the non-polarization beam splitter  102 , which is placed before the position P 41  where there is the end surface of the polarization preserving optical fiber  402 . Thereafter, interference signals are generated by the structure similar to the first embodiment. More specifically, the extracted beam transmits through the quarter wavelength plate  103 , thereby being converted to linearly polarized light whose polarization orientation rotates in accordance with a variation of a phase difference. The beam of linear polarized light is divided into three beams by the beam division device  106 . Each of the three beams is incident upon respective photoreceptors of the tripartition photoreceptive device  108  through the polarization device array  107 , in which each polarization axis is shifted by 60°. As a result, detected are three interference signals, whose phases are shifted by 120°, which base upon the out-of-plane displacement of the measurement target object surface (surface of the measurement target object  421 ). By virtue of isolating the light source from the detection head unit using the optical fiber, it is possible to readily downsize the detection head for position detection, and is effective in stabilizing the interference state because the influence of light source heat generation is avoided.  
       Fifth Embodiment  
       [0050]      FIGS. 7A and 7B  are explanatory views of an optical system of a laser interferometer according to the fifth embodiment. By improving the first or fourth embodiment, the fifth embodiment is constructed in a way that the forward light path from the light source side is spatially isolated from the returning light path heading to the photoreceptive device side.  
         [0051]     Linear polarized light divergent light from the semiconductor laser  101  is incident upon the polarization preserving optical fiber  402  through the lens  411  in a way that the polarization axis of the optical fiber matches the polarization surface of the light beam. The other end surface of the optical fiber  402  is arranged on the focal plane of the lens  412  slightly at an angle, for outputting the divergent beam. The divergent beam is converted to a parallel beam by the lens  412 , and outputted as a parallel beam having a slightly angled optical axis. The parallel beam is separated into two light beams having respective polarization components by the polarization beam splitter  413 . The reflected beam (S polarized light) irradiates the surface of the reference mirror  422 , and the transmitted beam (P polarized light) irradiates the surface of the measurement target object  421 . The respective reflected light beams are incident upon the lens  412  through the polarization beam splitter  413 , and focused on the position P 52  on the focal plane of the lens  412 . The beam focused on the position P 52  returns to a light path of a next area by the reflection coating  414  provided in the neighborhood of the position P 52 , and exits again from the lens  412  as a parallel beam.  
         [0052]     The exited parallel beam is separated into two light beams by the polarization beam splitter  413 . The reflected beam (S polarized light) irradiates the surface of the reference mirror  422  and the transmitted beam (P polarized light) irradiates the surface (mirror) of the measurement target object  421 . The respective reflected light beams are incident upon the lens  412  through the polarization beam splitter  413 , focused on the position P 51  on the focal plane of the lens  412 , and outputted to the optical fiber side. In this manner, the S polarized light beam reciprocates twice between the surface of the reference mirror  422  and the beam splitter  413 , and the P polarized light beam reciprocates twice between the surface of the measurement target object  421  and the beam splitter  413 .  
         [0053]     In the fifth embodiment, the light beam in the returning path is spatially extracted by a partial reflection beam splitter  511 , which is placed before the position P 51  where there is the end surface of the polarization preserving optical fiber in the forward path. The extracted beam transmits through the quarter wavelength plate  103 , thereby being converted to linearly polarized light whose polarization orientation rotates in accordance with a variation of a phase difference. The linearly polarized light is divided into three beams by the beam division device  106 . Each of the three beams is incident upon respective photoreceptors of the tripartition photoreceptive device  108  through the polarization device array  107 , in which each polarization axis is shifted by 60°. As a result, detected are three interference signals, whose phases are shifted by 120°, which base upon the out-of-plane displacement of the surface of the measurement target object  421 . Note that the partial reflection beam splitter  511  may be substituted by a micro-prism.  
         [0054]     The measurable area of an out-of-plane displacement is determined only by coherence of a light source in a case of a Michelson interferometer irradiating a parallel beam. For the purpose of downsizing, a surface-emitting laser diode is desirable as a light source (semiconductor laser  101 ). However, a surface-emitting laser diode may cause a transverse mode variation of returning light, which may generate such phenomenon that the plane of polarization moves. For this reason, a measure for preventing the light returning to the surface-emitting laser diode is necessary. In the fifth embodiment, since the light beam heading toward the light source side can be cut off by the partial reflection beam splitter  511 , it is advantageous in terms of the aforementioned aspect. Note in a case of realizing the first to fourth embodiments, it is preferable to provide an isolator or the like for eliminating the returning light.  
         [0055]     Note that in each of the above-described embodiments, the non-polarization beam splitter  102  is adopted by the first to fourth embodiments and the partial reflection beam splitter  511  is adopted by the fifth embodiment as a device extracting, to the photoreceptive-device side, interference light beams that have reciprocated twice between the reference mirror and the polarization beam splitter as well as between the measurement target mirror and the polarization beam splitter. However, the partial reference beam splitter  511  may be employed in place of the non-polarization beam splitter  102 , or the non-polarization beam splitter  102  may be employed in place of the partial reflection beam splitter  511 . Alternatively, a micro-prism may be employed in place of these devices.  
         [0056]     According to the above-described embodiments, the following effects can be attained. 
    (1) Since parallel light irradiation on the order of millimeter is employed, it is unsusceptible to the superficial shape or microscopic scratch on the measurement target surface.     (2) By utilizing parallel light irradiation on the order of millimeter in combination with a light source having an excellent coherence, it is possible to realize a measurement area on the order of millimeter.     (3) Since two-reciprocative path (two-reciprocative interference measurement optical system) employing a cat&#39;s eye device is realized, it is possible to achieve a high resolution (sine wave signals having a ¼ cycle of the light source wavelength can be outputted).     (4) By virtue of the two-reciprocative interference measurement optical system employing a cat&#39;s eye device, it is possible to realize stable measurement even if there is an alignment deviation on the measurement target surface.     (5) According to the fifth embodiment in particular, since the forward light path is spatially isolated from the returning light path, it is possible to eliminate the light returning to the light source; thus, more stable measurement is possible without utilizing an isolator or the like.     (6) By virtue of eliminating the use of a cube corner reflector arranged on a measurement target portion and an internal cube corner reflector for a reference beam (first to fifth embodiments), and also by commonly utilizing the cat&#39;s eye lens and the collimator lens (third embodiment), it is possible to realize a downsized optical system.     (7) By virtue of reducing the portion exposed to the air, stable measurement can be realized (the portion exposed to the air is a light path from the polarization beam splitter to the measurement target surface, where the light beam passes in the air. When the air in this portion vibrates, a fluctuation is caused in the interference signals. Therefore, the shorter the exposed portion, the more stable the measurement.)    
 
         [0064]     According to the present invention, it is possible to provide a position detection apparatus and method adopting a new system, which enlarges a light irradiating area on a measurement-target surface, which enlarges an out-of-plane displacement measurement area, and which does not receive influence of an alignment deviation.  
         [0065]     The present invention is not limited to the above embodiment and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention, the following claims are made.  
       CLAIM OF PRIORITY  
       [0066]     This application claims priority from Japanese Patent Application No. 2004-302063 filed on Oct. 15, 2004, which is hereby incorporated by reference herein.