Patent Publication Number: US-2007103694-A1

Title: Interferometry system

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an interferometry system for detecting the amount of displacement of a measurement target in a non-contact manner.  
      2. Description of the Related Art  
       FIG. 4  illustrates a first conventional interferometry system. A beam L of laser light emitted from a semiconductor laser light source  1  is collimated into a parallel beam by a collimating lens  2 . The parallel beam is separated into a measurement beam La and a reference beam Lb by a polarizing beam splitter  3 . The measurement beam La passes through a quarter-wave plate  4   a  and is then converged by a condenser lens  5 . The converged measurement beam La is reflected by a measurement target S.  
      The reference beam Lb reflected from the polarizing beam splitter  3  passes through a quarter-wave plate  4   b  and then reflected by a reference mirror  6 .  
      The beam La reflected from the measurement target S and the beam Lb reflected from the reference mirror  6  pass through the quarter-wave plate  4   a  and the quarter-wave plate  4   b , respectively, again. Then, the reference beam Lb passes through the polarizing beam splitter  3 , the measurement beam La is reflected by the polarizing beam splitter  3 , and the beams La and Lb are formed into a combined beam Lc. The combined beam Lc enters a quarter-wave plate  4   c.    
      For the combined beam Lc, only polarization information of the light of the measurement beam La reflected from the measurement target is modulated. Therefore, a beam that has passed through the quarter-wave plate  4   c  is rotating linearly polarized light.  
      Then, the combined beam Lc enters a non-polarizing beam-splitter  7  and is separated into measurement beams Ld and Le. The measurement beams Ld and Le pass through polarizing plates  8   a  and  8   b , respectively. The polarizing plates  8   a  and  8   b  are disposed so that their respective optic axes are inclined 45° with respect to each other, so that sinusoidal signals A and B which are 90° out of phase with respect to each other, as shown in  FIG. 5 , are obtained in photoelectric sensors  9   a  and  9   b , respectively.  
      The polarization direction of the combined beam Lc is rotated by displacement of the measurement target S. As a result, in response to displacement of the measurement target S, one period of a sinusoidal signal is obtained at a displacement of λ/2. This conventional displacement gauge outputs incremental sinusoidal signals. However, only relative positions after measurement begins can be obtained because there is no information on the reference (start) positions.  
       FIG. 6  illustrates a second conventional interferometry system, which improves upon the first conventional interferometry system. A beam having a wavelength of λ1 is emitted from a semiconductor laser light source  1   a . A beam having a wavelength of λ2 is emitted from a semiconductor laser light source  1   b . The beams are collimated into parallel beams by corresponding collimating lenses  2   a  and  2   b . Then, a combined beam Lc in which the measurement beam La and reference beam Lb are combined is generated by using a polarizing beam splitter  3   b , as in the case of  FIG. 4 .  
      The combined beam Lc enters a non-polarizing beam-splitter  10  and is separated into measurement beams Ld and Le. The beam Ld passes through a band-pass filter  11   a  which allows light having a wavelength of λ1 to pass therethrough. The transmitted beam holds information indicating a wavelength of λ1 alone. The beam Ld with a wavelength of λ1 passes through a quarter-wave plate  12   a , thus becoming linearly polarized light. For information on polarization, the polarization direction rotates on the basis of changes in the position of the measurement target S. The rotating linearly polarized light beam Ld is separated by a non-polarizing beam-splitter  13   a . The transmitted light and the reflected light pass through a polarizing plate  14   a  and a polarizing plate  14   b , respectively, thus becoming intensity signals. The intensity signals enter corresponding photoelectric sensors  15   a  and  15   b  and converted into electric signals. At this time, each of the photoelectric sensors  15   a  and  15   b  outputs one period of a sinusoidal signal for a displacement of (½)λ1 of the measurement target S.  
      The beam Le, which has passed through the non-polarizing beam-splitter  10 , passes through a band-pass filter lie which allows light having a wavelength of λ2 to pass therethrough. The transmitted light holds information indicating a wavelength of λ2 alone. The transmitted light passes through a quarter-wave plate  12   e  and then is separated by a non-polarizing beam-splitter  13   f . The separated beams pass through corresponding polarizing plates  14   e  and  14   f  and enter corresponding photoelectric sensors  15   e  and  15   f . In this case, each of the photoelectric sensors  15   e  and  15   f  outputs one period of a sinusoidal signal for a displacement of (½)λ2 of the measurement target S.  
      The periods of sinusoidal signals A and B obtained by this structure are acquired in response to a displacement of the measurement target S. As illustrated in  FIGS. 7A and 7B , the periods are λ1/2 and λ2/2, respectively. The phase difference of the two kinds of signals is zero when the optical path length of the reference beam Lb coincides with that of the measurement beam La. The phase difference is produced in accordance with the difference of the optical path length. Observing the phase difference allows measurement of absolute positions of the measurement target S.  
      In the conventional systems described above, absolute positions can be measured at a predetermined constant distance. For example, when practical wavelengths, e.g., 650 nm and 655 nm, are selected, the phase shift between adjacent sinusoidal signals is no larger than one hundredth of the period of the original sinusoidal signal.  
      For detection with sufficient accuracy, significantly precise stability of about one five-hundredth of the period is required.  
      In the case of a structure like the conventional system shown in  FIG. 6 , it is necessary to set the optical axes and the positions of the two semiconductor laser light sources  1   a  and  1   b  with precision. However, it is difficult to maintain stability of a phase difference at or below the one five-hundredth of the period (=1.3 nm) because of effects of variations in the optical axes, minute inclination of the target, and other factors.  
      In addition, since the semiconductor laser light sources  1   a  and  1   b  are disposed in the vicinity of an optical system, the optical system is radiated with a large amount of heat and affected by the heat. This causes unstable phase differences.  
     SUMMARY OF THE INVENTION  
      The present invention provides a stable high-precision interferometry system that uses an optical fiber.  
      According to a first aspect of the present invention, an interferometry system includes a combiner configured to combine a first beam having a first wavelength and a second beam having a second wavelength into a third beam, a separation and combination optical member configured to separate the incident third beam into a reference beam and a measurement beam and to combine the reference beam reflected from a reference mirror and the measurement beam reflected from a measurement target into a fourth beam, an optical separator configured to separate the fourth beam into a fifth beam and a sixth beam, an optical filter configured to allow a beam having the first wavelength from the fifth beam to pass therethrough, an optical filter configured to allow a beam having the second wavelength from the sixth beam to pass therethrough, a photoelectric sensor configured to receive the transmitted beam having the first wavelength and to convert the beam into an electric signal, a photoelectric sensor configured to receive the transmitted beam having the second wavelength and to convert the beam into an electric signal, and a calculation unit configured to receive the electric signals and to calculate the amount of displacement of the measurement target. In the interferometry system, the third beam is emitted from a first optical fiber.  
      One of the major technical features of an interferometry system according to at least one exemplary embodiment of the present invention is to emit a reference beam and a measurement beam from an output end face of the same optical fiber toward a light transmission member in an interferometry system, such as the one described above.  
      The interferometry system according to at least one exemplary embodiment of the present invention can measure absolute positions more precisely by emitting a beam having a plurality of wavelengths as measurement and reference beams from an output end face of the same optical fiber.  
      The interferometry system can also measure a plurality of wavelengths with a simpler configuration by guiding a beam having a plurality of wavelengths emitted from output end faces of different optical fibers into the same optical fiber via a combiner.  
      In addition, the interferometry system can also improve the optical-fiber stability and perform measurement more precisely by polarizing a beam from an output end face of the same optical fiber by using a polarizing element.  
      Moreover, if using a polarization maintaining fiber as the optical fibers for guiding beams into the combiner, the interferometry system can also perform measurement more precisely.  
      Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a configuration of an interferometry system according to a first exemplary embodiment of the present invention.  
       FIGS. 2A and 2B  are waveform diagrams of output signals of the interferometry system according to the first exemplary embodiment.  
       FIG. 3  illustrates a configuration of an interferometry system according to a second exemplary embodiment of the present invention.  
       FIG. 4  illustrates a configuration of a first conventional interferometer.  
       FIG. 5  is a waveform diagram of an output signal in the first conventional interferometer.  
       FIG. 6  illustrates a configuration of a second conventional interferometer.  
       FIGS. 7A and 7B  are waveform diagrams of an output signal in the second conventional interferometer. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      The present invention is described on the basis of exemplary embodiments illustrated in FIGS.  1  to  3 .  
     First Exemplary Embodiment  
       FIG. 1  illustrates a configuration of an interferometry system according to a first exemplary embodiment. Lenses  22  and  22 ′ are disposed along the optical axes of semiconductor laser light sources  21  and  21 ′, respectively. The input end faces of polarization maintaining fibers  23  and  23 ′ are disposed at the converging points of the lenses  22  and  22 ′, respectively. The output end face of the two polarization maintaining fibers  23  and  23 ′ are connected to a combiner  24 . The output end face of the combiner  24  is connected to an input end face of another optical fiber  25 .  
      A collimating lens  26 , a polarizing beam splitter  27 , a quarter-wave plate  28   a , a condenser lens  29 , and a measurement target S are arranged along the optical axis of the output end face of the optical fiber  25 .  
      A quarter-wave plate  28   b  and a reference mirror  30  are arranged along the direction of reflection of the polarizing beam splitter  27 .  
      A non-polarizing beam-splitter  31  is disposed in the direction of reflection of the reference mirror  30 , i.e., in a direction in which a beam reflected from the reference mirror  30  passes through the polarizing beam splitter  27 .  
      A band-pass filter  32   c  which allows a beam having a wavelength of λ1 to pass therethrough, a quarter-wave plate  33   c , a non-polarizing beam-splitter  34   c , a polarizing plate  35   c , and a photoelectric sensor  36   c  are arranged along the direction of reflection of the non-polarizing beam-splitter  31 .  
      A polarizing plate  35   d  and a photoelectric sensor  36   d  are disposed along the direction of reflection of the non-polarizing beam-splitter  34   c.    
      A band-pass filter  32   e  which allows a beam having a wavelength of λ2 to pass therethrough, a quarter-wave plate  33   e , a non-polarizing beam-splitter  34   e , a polarizing plate  35   e , and a photoelectric sensor  36   e  are arranged along the direction of transmission of the non-polarizing beam-splitter  31 .  
      A polarizing plate  35   f  and a photoelectric sensor  36   f  are disposed along the direction of reflection of the non-polarizing beam-splitter  34   e.    
      A laser measurement beam La with a wavelength of λ1 emitted from the semiconductor laser light source  21  is converged by the lens  22  and then guided into the input end face of the polarization maintaining fiber  23 . A laser reference beam Lb with a wavelength of λ2 emitted from the semiconductor laser light source  21 ′ is converged by the lens  22 ′ and then guided into the input end face of the polarization maintaining fiber  23 ′. The emitted beams from the polarization maintaining fibers  23  and  23 ′ are guided into the optical fiber  25  via the combiner  24 .  
      A beam in which wavelengths of λ1 and λ2 coexist is emitted from the output end face of the optical fiber  25 . Then, the emitted beam is separated into P and S waves by the plane of the polarizing beam splitter  27 . The beam that has passed through the polarizing beam splitter  27  passes through the quarter-wave plate  28   a  as the measurement beam La. The measurement beam La is converged by the condenser lens  29  to form a converged beam. The measurement target S is radiated with the converged beam.  
      The beam reflected from the polarizing beam splitter  27  is defined as the reference beam Lb. The reference beam Lb passes through the quarter-wave plate  28   b  and then reflected by the reference mirror  30 .  
      The measurement beam La that has reached the measurement target S is reflected by the measurement target S. Then, the measurement beam La returns an optical path along which the measurement beam La traveled and is reflected by the polarizing beam splitter  27 .  
      The reference beam Lb reflected from the reference mirror  30  returns an optical path along which the reference beam Lb traveled and then passes through the polarizing beam splitter  27 . Then, the reference beam Lb and the measurement beam La are combined into a combined beam Lc. The combined beam Lc then enters the non-polarizing beam-splitter  31  and is separated. Light reflected from the non-polarizing beam-splitter  31  is defined as a beam Ld, and transmitted light is defined as a beam Le.  
      When the power of the condenser lens  29  is set such that the converging point of the measurement beam La on the measurement target S is equal in wave-optics optical path length to the reference mirror  30 , which reflects the reference beam Lb, the interferometry system can achieve optimal performance.  
      In other words, both the light reflected from the measurement target S and the light reflected from the reference mirror  30  are combined as parallel light.  
      The beam Ld passes through the band-pass filter  32   c , which transmits light having a wavelength of λ1, but does not transmit light having a wavelength of λ2. The transmitted light holds information indicating a wavelength of λ1 alone. The beam with a wavelength of λ1 passes through the quarter-wave plate  33   c , thus becoming linearly polarized light. For the beam that has passed through the quarter-wave plate  33   c , its polarization direction rotates on the basis of displacement of the measurement target S. The rotating linearly polarized light beam Ld is separated by the non-polarizing beam-splitter  34   c.    
      The transmitted light and the reflected light pass through the polarizing plate  35   c  and the polarizing plate  35   d , respectively, thus becoming intensity signals. The intensity signals enter the corresponding photoelectric sensors  36   c  and  36   d  and are converted into electric signals.  
      Each of the electric signals is one period of a sinusoidal signal for a displacement of (½)λ1 of the measurement target S.  
      The polarizing plates  35   c  and  35   d  are disposed such that their respective polarization axes are inclined 45° with respect to each other. As illustrated in  FIG. 2A , sinusoidal signals from the photoelectric sensors  36   c  and  36   d  are signals having A and B phases which are 90° out of phase with respect to each other.  
      The beam Le that has passed through the non-polarizing beam-splitter  31  passes through the band-pass filter  32   e , which transmits light having a wavelength of λ2, but does not transmit light having a wavelength of λ1. The transmitted light holds information indicating a wavelength of λ2 alone.  
      The beam Le passes through the quarter-wave plate  33   e  and is then separated into two components by the non-polarizing beam-splitter  34   e . The separated beams pass through the corresponding polarizing plates  35   e  and  35   f  and then enter the corresponding photoelectric sensors  36   e  and  36   f.    
      Each of the photoelectric sensors  36   e  and  36   f  outputs one period of a sinusoidal signal for a displacement of (½)λ2 of the measurement target S.  
      Since the polarizing plates  35   e  and  35   f  are disposed such that their respective polarization axes are inclined 45° with respect to each other, the obtained electric signals are sinusoidal signals having A and B phases which are 90° out of phase with respect to each other, as illustrated in  FIG. 2B .  
      Signal processing circuits  37   c ,  37   d ,  37   e , and  37   f  process signals from the photoelectric sensors  36   c ,  36   d ,  36   e , and  36   f.    
      A processing circuit (central processing unit (CPU))  38  receives signals processed in the signal processing circuits  37   c  to  37   f , and calculates the amount of displacement.  
      It is to be noted that the beams having wavelengths of λ1 and λ2 from the two semiconductor laser light sources  21  and  21 ′ are able to be treated as a complete point source at the output end face of the optical fiber  25 .  
      It is also to be noted that the two beams having two wavelengths λ1 and λ2 are present at the same spatial point at the location of the point source.  
      In other words, in the case where the interferometry system is structured as a displacement gauge, the output end faces of the two beams move identically in an optical system in the interferometry system in response to displacement and inclination of a target. As a result, no phase shift occurs between the two beams.  
      For example, when 650 nm and 655 nm are selected, as in the case of conventional systems, a high degree of precision for approximately one five-hundredth of the period (1.3 nm) needed to identify the phase difference between adjacent sinusoidal signals can be obtained.  
      In addition, since the semiconductor laser light sources  21  and  21 ′, which generate a large amount of heat, are remote from the optical system by virtue of the optical fibers  23 ,  23 ′, and  25 , a main heat source is not present in the vicinity of the optical system. Therefore, this exemplary embodiment is useful in terms of the stability of the optical system.  
     Second Exemplary Embodiment  
       FIG. 3  illustrates a configuration of an interferometry system according to a second exemplary embodiment.  
      In this exemplary embodiment, a polarizing plate  39  is inserted immediately after the collimating lens  26 . In  FIG. 3 , the same reference numerals as in  FIG. 1  represent components having the same functions as in  FIG. 1 .  
      Generally, polarization maintaining fibers do not have a function of maintaining polarization for optical components perpendicular to polarization planes. Therefore, light emitted from polarization maintaining fibers have instability of polarization.  
      However, since the provision of the polarizing plate  39  having a high extinction ratio enables the displacement-gauge optical system to obtain a more stable beam, a displacement gauge with higher stability and higher precision than the first exemplary embodiment can be obtained.  
      While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions.  
      This application claims the benefit of Japanese Application No. 2005-324349 filed Nov. 9, 2005, which is hereby incorporated by reference herein in its entirety.