JP2011013170A - Refractive index measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure accurately and nondestructively a refractive index of a transparent object (specimen) such as glass or plastics.SOLUTION: By two probe optical systems 106, 108 utilizing a polarization maintaining fiber, an optical path length of light transmitted through the specimen 107 specified locally is calculated by utilizing an interference signal, and a geometric thickness of the part is calculated by measuring the position of the probe optical systems 106, 108, and the refractive index of the specimen 107 is determined from both calculated values. Hereby, highly-accurate and nondestructive refractive index measurement can be achieved.

Description

  The present invention relates to a refractive index measuring apparatus suitable for measuring the refractive index and refractive index distribution of glass and plastic lenses.

  In recent years, molded lenses have become widespread as optical elements used in digital copying and digital cameras.

  This molded lens is superior to a glass-polished lens in terms of manufacturability of an aspherical lens and is low in cost. On the other hand, depending on the molding conditions, it is unstable that the refractive index distribution inside the lens tends to be uneven. There are also aspects.

  The non-uniformity of the refractive index inside the lens has a great influence on the optical characteristics of the lens and may cause a deterioration in imaging performance.

  For this reason, it is necessary to measure the refractive index distribution with high accuracy in order to stabilize the quality of the molded lens.

  As a conventional technique for measuring the refractive index, there is a method of measuring and obtaining a declination by a minimum declination method or the like, and it is known that measurement can be performed with high accuracy.

  In addition, for specimens such as diamond and ore, immerse the specimen in a solution with a known refractive index, observe the Becke line appearing at the boundary line, and replace the solution until the line disappears. A method is known in which the refractive index of a test object is indirectly measured from the rate.

  On the other hand, a measurement method using an interferometer is also known.

  The one shown in FIG. 11 is a Mach-Zehnder interferometer as a whole, and uses one light beam branched from the laser 1 by the half mirror 2 as a plane wave reference light 3 that passes through the compensation plate 4 and uses the other. The inspection light 5 is used as the inspection light 5, and the test object 6 is immersed in the matching liquid of the immersion tank 7 whose refractive index is almost equal to the refractive index, and the interference fringes 8 generated in both the light beams are observed by the image sensor 9. Is.

  FIG. 12 shows a Michelson interferometer as a whole, which uses a low-coherent light source 10 to measure the intensity of interference light at the boundary between the transparent plate-like test object 11 and air. By observing with the photodetector 12, the refractive index and the thickness are simultaneously measured to obtain the refractive index.

  As these measuring apparatuses, there are those disclosed in Patent Documents 1 to 3.

  Patent Document 1 discloses an apparatus for measuring a refractive index distribution on the surface of a measurement object using a near-field microscope. In this Patent Document 1, an object is irradiated from the opposite side of the near-field probe to generate an evanescent wave, which modulates the distance between the surface of the object to be measured and the probe, and measures the intensity modulation of this light. The refractive index is measured.

  In addition, in order to measure the degree of polarization in a region smaller than the value of the light wavelength, a polarizing near-field probe with a sharpened tip of the polarization-preserving fiber and an elliptical or slit-shaped opening is used. is there.

  Patent Document 2 discloses a method for manufacturing a polarization near-field probe.

  In Patent Document 3, a measurement object having an optical rotation, such as an organic substance, is irradiated with near-field light linearly polarized from a polarization near-field probe, transmitted through the measurement object, and passed through an analyzer. Is measured by the spectroscope from the angle dependence of the analyzer. In Patent Document 3, conversely, the object to be measured is irradiated with linearly polarized light, and the transmitted light is collected by the polarized near-field probe, and the polarization plane selectivity of the probe itself is used. The optical rotation dispersion of the measurement object is measured. As used herein, a polarization-maintaining fiber is an optical fiber that preserves the polarization plane, and maintains a circularly polarized wave and a circularly polarized wave holding fiber that maintains a linearly polarized wave. Classified as fiber.

Japanese Patent Laid-Open No. 11-51863 JP 2002-162333 A JP 2006-300708 A

  However, in the measurement that calculates the optical thickness by measuring the peak of the interference signal, the contrast of the interference signal is reduced due to the phase disturbance caused by the wave guide in the optical fiber, which greatly affects the measurement accuracy of the refractive index. There are things to do.

  One of the major causes of the decrease in the contrast of the interference signal is a phase disturbance due to the wave guide in the optical fiber. In general, when a change in tension, temperature, vibration, etc. occurs in an optical fiber, coupling occurs between modes of light propagating in the optical fiber. As a result, the polarization plane is not preserved and the polarization characteristics are reduced. This is because it will occur. Also in single-mode fiber, due to the uneven distribution of internal stress at the time of manufacture, the polarization plane is not preserved in the same way as above, and as a result, the wave front is disturbed, resulting in a large phase disturbance. There is.

  When using a near-field probe for optical measurement, the near-field probe itself is manufactured by processing an optical fiber, so there are the above-mentioned problems, and the influence of the shape of the aperture on the polarization plane disturbance. Cannot be ignored.

  The refractive index microscope using the near-field microscope in Patent Document 1 can measure only the refractive index of the surface of the test object, and is not suitable for measuring a test object having a large thickness.

  In the measurement apparatus using the polarization near-field probe described in Patent Documents 2 and 3, the optical rotation dispersion of a measurement object having optical anisotropy can be measured, but the refractive index cannot be measured.

  Further, if this polarization near-field probe is simply used in a general probe optical system, the amount of light is reduced, and the measurement cannot be performed at all.

  The present invention has been made to solve the above-described problems, and aims to realize non-destructive and highly accurate refractive index measurement.

  In order to solve the above problems, a refractive index measuring device of the present invention includes a light source that emits linearly polarized coherent light, an optical member that separates the coherent light from the light source into inspection light and reference light, and the inspection light. A first probe optical system for condensing and irradiating the surface of the test object, and a second probe optical system for condensing the inspection light irradiated from the first probe optical system and transmitted through the test object; Driving means for coaxially adjusting the optical axis of the first probe optical system and the optical axis of the second probe optical system; and positions of the first probe optical system and the second probe optical system from a reference position A distance measuring means for measuring a distance as a difference between the reference light and a light receiving element for detecting an intensity distribution of interference light generated by combining the reference light and the inspection light collected by the second probe optical system, The test object is the first probe optical The position of each probe optical system when the probe optical system is focused on the surface of a flat transparent reference object having a known refractive index and thickness, and a stage fixed between the probe optical system and the second probe optical system. Recording in advance as a reference position of each probe optical system, measuring the total difference ΔD from the reference position of each probe optical system when the probe optical system is focused on the surface of the test object, The wavelength of the light source is set to a plurality of different wavelengths, and the interference fringe phase Δi of the interference light observed in the light receiving element is measured for each wavelength, and the sum of the differences ΔD and the phases of the interference fringes due to a plurality of different wavelengths are measured. An arithmetic unit that calculates the refractive index of the test object based on Δi.

Further, another refractive index measuring apparatus of the present invention for solving the above-mentioned problems is a light source that emits low coherent light, an optical member that separates the low coherent light from the light source into inspection light and reference light, A first probe optical system for condensing and irradiating the inspection light onto the surface of the test object, a second probe optical system for condensing light transmitted through the test object by the inspection light, and the first probe Driving means for coaxially adjusting the optical axis of the optical system and the optical axis of the second probe optical system, and ranging with the positions of the first probe optical system and the second probe optical system as a difference from a reference position Measuring means, a light receiving element for detecting an intensity distribution of interference light generated by combining the reference light and the inspection light collected by the second probe optical system, and an optical path of the reference light A reference light mirror disposed on the reference light mirror, and the reference light mirror An optical path length adjusting means for adjusting the optical path length of the reference light by moving the reference light, a length measuring means for measuring the distance of the reference light mirror as a difference from a reference position of the reference light mirror, and the test object A stage for fixing an object between the first probe optical system and the second probe optical system, a polarization-preserving fiber for guiding the inspection light from the second probe optical system to the light receiving element, and a refractive index N0 And the position of each probe optical system when each probe optical system is focused on the surface of a flat transparent reference object having a known thickness D0 is recorded in advance as the reference position of each probe optical system, The position where the interference signal intensity at the maximum is recorded in advance as the reference position of the reference light mirror, and the reference light mirror is focused while focusing each probe optical system on the surface of the test object. Is adjusted to maximize the interference signal intensity at the light receiving element, and the difference from the reference position of the probe optical system is ΔD, and the difference from the reference position of the reference light mirror is ΔND. A calculation unit that calculates the refractive index N of the test object according to the following (Equation 1).
N = (2ΔND + N0 × D0) / (ΔD + D0) (Formula 1)

  In addition, another refractive index measuring apparatus of the present invention for solving the above-described problems includes a light source that emits coherent light including two different frequencies, and a first optical member that branches the coherent light from the light source into two light beams. A first counter that detects a phase of a first beat signal due to a wavelength difference in one of the two light beams, and two of the two light beams, inspection light and reference light, having different wavelengths. A second optical member for branching into two light beams, a first probe optical system for condensing and irradiating the inspection light on the surface of the test object, and condensing the light transmitted through the test object by the test light A second probe optical system; drive means for coaxially adjusting an optical axis of the first probe optical system and an optical axis of the second probe optical system; the first probe optical system and the second probe optical system; From the reference position A length measuring means for measuring distance as a minute; a second counter for detecting a phase of a second beat signal generated by combining the reference light and the inspection light collected by the second probe optical system; Focus each probe optical system on a surface of a flat transparent reference object having a known refractive index and thickness, and a stage for fixing the test object between the first probe optical system and the second probe optical system The position of each probe optical system in this case is recorded in advance as a reference position of each probe optical system, and the phases of the first and second beat signals observed in the first and second counters are observed. The distance between the first probe optical system and the second probe optical system is increased by displacing the first probe optical system and the second probe optical system in the optical axis direction, and at the same time, each probe optical system is applied to the surface of the object to be tested. Measuring the phase difference of the first and second beat signals observed in the first and second counters simultaneously with measuring the difference ΔD from the reference position of both probe optical systems, and summing the difference Based on ΔD, the phase change of the first and second beat signals when measuring the reference object, and the phase change of the first and second beat signals when measuring the test object, the refractive index of the test object is determined. And an arithmetic unit for calculating.

  According to the present invention, non-destructive and highly accurate refractive index measurement can be realized.

Configuration diagram of a refractive index measuring apparatus according to Embodiment 1 of the present invention (A) The figure which looked at the probe tip in the first embodiment from the top, (b) The perspective view of the probe tip in the first embodiment The figure which shows the irradiation by the probe in Embodiment 1 Flowchart in the first embodiment (A) The figure which shows the time of the reference | standard measurement for the principle explanation in the Embodiment 1 and (b) The figure which shows the time of the test object for the principle explanation in the Embodiment 1 The figure which shows the interference light intensity for the principle explanation in the first embodiment Configuration diagram of a refractive index measuring apparatus in Embodiment 2 of the present invention Flowchart in the second embodiment Configuration diagram of a refractive index measuring apparatus in Embodiment 3 of the present invention Flowchart in the third embodiment Diagram for explaining a conventional example Diagram for explaining a conventional example

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals, and description thereof is omitted as appropriate.

(Embodiment 1)
FIG. 1 is a conceptual configuration diagram of a refractive index measuring apparatus according to Embodiment 1 of the present invention. FIG. 4 is a flowchart of refractive index measurement in Embodiment 1 of the present invention.

  In FIG. 1, a light beam emitted from a light source 101 that emits low-coherent light typified by SLD is divided into inspection light and reference light by a beam splitter 102.

  The inspection light passes through the ND filter 103 and is then coupled to the polarization-maintaining fiber 105 by the objective lens 104. The inspection light propagated through the polarization plane preserving fiber 105 enters the probe optical system 106 and is irradiated to the object 107. The inspection light irradiated on the test object 107 passes through the test object 107 and is collected by the probe optical system 108. The inspection light collected by the probe optical system 108 propagates through the polarization plane preserving fiber 109, is enlarged by the lens 110, passes through the quarter-wave plate 111, passes through the beam splitter 112, and then receives the light receiving element 113. To reach.

  The probe optical systems 106 and 108 are arranged so that the optical axes of the respective optical systems are coaxial, and are displaced by the drive units 114 and 115 so as to maintain the coaxiality. 116 and 117 are measured and recorded.

  The probe optical systems 106 and 108 are provided with a mechanism that rotates about the optical axis. The probe optical systems 106 and 108 are rotated around the optical axis by the rotation driving units 118 and 119, and the rotation angles thereof are measured by the displacement sensors 120 and 121, respectively.

  The probe optical systems 106 and 108 include an optical element obtained by processing the tip portion of the polarization-maintaining fiber, which is connected through a part of the polarization-maintaining fibers 105 and 109 or a fiber coupler.

  When the rotation driving units 118 and 119 that rotate the probe optical systems 106 and 108 around the optical axis rotate, the polarization plane directions of the polarization plane preserving fibers 105 and 109 with respect to the polarization directions of the respective probe optical systems are changed. In order not to change, a mechanism for rotating both connecting portions is adopted.

  The test object 107 is fixed by a holder 122, and the holder 122 can displace the test object on a plane perpendicular to the optical axis formed by the probe optical systems 106 and 108 by the driving unit 123a.

  The probe optical systems 106 and 108 include focus determination mechanisms 123b and 123c that can focus the probe optical systems 106 and 108 on the surface of the test object 107 at a certain distance.

  The focus of the probe optical systems 106 and 108 on the surface of the test object 107 is measured by measuring a shear stress that is a mechanical interaction generated between the near-field probe of the probe optical systems 106 and 108 and the surface of the test object 107. The focus determination mechanism controls the drive units 114 and 115 so that the value becomes constant. Alternatively, it is possible to adopt a form in which focus determination is performed using an interference signal by interposing the focus optical systems 124 and 134 in the optical path of the inspection light.

  The focus optical system 124 is an optical system branched by a beam splitter 127 in the optical path from the inspection light passing through the test object 107 and reaching the light receiving element 113 by the polarization plane preserving fiber 109. The focus optical system 124 includes a light source 125 that emits coherent light. The light emitted from the light source 125 is guided to the beam splitter 127 by the half mirror 126 and divided into two light beams. One light beam travels straight to the mirror 129, is reflected, passes through the beam splitters 127 and 126, and is guided to the light receiving unit 128.

  The other light beam is guided to the probe optical system 108 by the beam splitter 127, and the light reflected by the surface of the test object 107 returns to the probe optical system 108, reflected by the beam splitter 127, and transmitted through the half mirror 126. Thereafter, the light reaches the light receiving unit 128.

  These two light beams interfere with each other at the light receiving unit 128 to obtain a focus signal. This focusing signal can be kept constant at the nanoscale by using, for example, the ratio of the intensity of incident light and reflected light as a determination signal and making the ratio constant.

  Furthermore, the shape of the test surface can be measured by finely displacing the mirror 129 in the optical axis direction, analyzing the interference fringes appearing on the light receiving unit 128 by phase modulation interferometry, and calculating the phase distribution. Focus can be made with high accuracy.

  The focus optical system 134 has the same configuration as the focus optical system 124 and is an optical system branched by the beam splitter 131 in the optical path until the inspection light reaches the surface of the test object 107. In the focus optical system 134, the low coherent light emitted from the light source 101 is branched into two light beams by the beam splitter 131.

  One light beam is reflected by the beam splitter 131, guided to the mirror 132, reflected, transmitted through the beam splitter 131, reflected by the mirror 133, and guided to the light receiving unit 130.

  The other light beam passes through the beam splitter 131, propagates through the polarization-maintaining fiber 105, is guided to the probe optical system 106, reaches the surface of the test object 107, is reflected on the surface, and is reflected on the beam splitter 131. Then, the light is reflected by the mirror 133 and guided to the light receiving unit 130.

  If the difference between the optical path lengths of these two light beams is less than or equal to the coherence length of the light source, both light beams interfere with each other in the light receiving unit 130, and if the intensity of this interference signal is set as a focus signal, the surface of the object 107 and the probe optical system 106 The interval between and can be kept constant.

  Further, in the light receiving unit 130, the distance between the probe and the surface can be controlled more accurately by combining the spectroscope and measuring the intensity of the interference signal for each wavelength.

  Further, in the focus optical system 134, as in the focus optical system 124, it is possible to obtain a focus signal by a similar method using another light source for focus.

  The reference light branched by the beam splitter 102 passes through the ND filter 135 that adjusts the amount of light, and then its optical path is turned back by the mirror 136 to reach the corner cube 137 that is the reference plane. This reflected light returns to the mirror 136. The light enters the beam splitter 112, is reflected, and reaches the light receiving element 113.

  The corner cube 137 can be displaced in the same direction as the optical axis by the drive unit 138, and the optical path length of the reference light can be adjusted by the amount of displacement. Further, the displacement amount of the corner cube 137 is measured and recorded by the length measuring device 139.

  In the beam splitter 112, the reference light and the inspection light overlap each other, and interference occurs when the difference in optical path length is within the range of the coherence length of the light emitted from the light source, and the interference signal is observed at the light receiving element 113.

  The interference light observed in the light receiving element 113 is converted into an electric signal by the A / D converter 143, and the interference intensity is determined by the interference signal calculation unit 144.

  The positional information of the probe optical systems 106 and 108 measured by the length measuring devices 116 and 117 is sent to the calculation unit 140 where the geometric thickness of the test object 107 is calculated.

  Further, the position information of the corner cube 137 measured by the length measuring device 139 is sent to the calculation unit 141 where the optical thickness of the test object 107 is calculated.

  Based on the geometric thickness value calculated by the calculation unit 140 and the optical thickness value calculated by the calculation unit 141, the refractive index calculation unit 142 calculates the refractive index.

  On the other hand, the optical rotation recording unit 145 records the rotation angles of the probe optical systems 106 and 108 from the rotational displacement sensors 120 and 121 and the light intensity in this state from the interference signal calculation unit 144.

  The optical rotation calculation unit 146 calculates the rotation angle of the probe optical systems 106 and 108 recorded by the optical rotation recording unit 145 and the interference signal intensity information, the refractive index calculated by the refractive index calculation unit 142, and the calculation unit 140. The optical rotation is calculated from the geometric thickness value.

  FIG. 2 shows the tip shape of the near-field probe used in the first embodiment. FIG. 2A is a top view of the tip of the near-field probe, and FIG. 2B is a perspective view of the tip of the near-field probe. This is a sharpened tip of a polarization-maintaining fiber.

  2 (a) and 2 (b), 21 is a clad, 22 is a sharpened portion of the core, and the surface is covered with gold or the like. Reference numeral 23 denotes an opening, which has an elliptical shape or a slit shape, and the axis of polarization of the fiber coincides with the direction of the long axis of the opening. Here, the optical fiber core itself has an elliptical shape.

  FIG. 3 is a diagram in which near-field light is irradiated from the probe optical system onto the surface of the test object.

  In FIG. 3, reference numeral 31 denotes incident light, which is light propagated from the light source 101 and propagates while maintaining the polarization plane 34 in the core 32 of the single-mode polarization plane preserving fiber 105. Reference numeral 33 denotes an optical fiber cladding. The probe tip 39 is a portion where the tip of the core of the optical fiber is sharpened, and is covered with an opaque film such as gold.

  At the sharpened probe tip 39 of the core of the optical fiber, there is an elliptical or slit-shaped opening 35 smaller than the wavelength order of the light, and near-field light 36 is generated. At this time, the near-field light is linearly polarized light in a predetermined vibration direction and stays in a range of the size of the opening 35 from the probe tip 39.

  In addition to the near-field light, there is also propagating light from the opening 35, which is linearly polarized light in a predetermined vibration direction. As the size of the opening 35 is smaller, the proportion of the near-field light is larger than that of the propagating light. Conversely, when the size of the opening 35 is increased toward the wavelength order, the amount of the near-field light decreases, The proportion increases.

  When the probe tip 39 is brought close to a region of a wavelength less than or equal to the wavelength of the test object 38, the propagation light is transmitted into the test object 38, and at the same time, the field created by the near-field light is disturbed to cause scattering. Together with the propagating light, it passes through the specimen as transmitted light 37. The polarization plane of the transmitted light 37 changes depending on the birefringence state of the glass material in the test object 38, but the polarization plane is maintained when there is no birefringence.

  Further, a part of the backscattered light of the near-field light or the reflected light of the propagating light from the surface of the test object 38 passes through the opening and the reverse path through the optical fiber. The backscattered light reflected from the surface of the test object follows the reverse path through the optical fiber. By detecting the intensity of this backscattered light, the distance between the probe tip and the surface of the test object 38 can be controlled in nano order.

  At this time, a shear stress is generated between the probe tip 39 and the surface of the test object 38, and the distance between the probe tip and the test object surface can be controlled by monitoring this force.

  Note that the probe optical system 108 preferably has the same configuration as the probe optical system 106. However, it is desirable that the probe optical system 108 has a larger opening than the probe optical system 106 because it is necessary to collect weak transmitted light emitted from the test object 107.

  For the same reason, it is desirable that the aspect ratio is smaller in the elliptical or slit-like opening of the probe optical system 108 than in the probe optical system 106. In the shape of the aperture of the probe optical system 108, the larger the aspect ratio is, the larger the extinction ratio of the polarized light of the outgoing propagation light and the near-field light is. There is. Further, although the amount of light increases as the size of the opening increases, there is a demerit that the amount of generated near-field light decreases and the positional accuracy deteriorates in proportion to the size of the opening.

  In view of this, with respect to the aperture of the probe optical system 106, the aspect ratio that is the ratio of the major axis to the minor axis is 3: 1 to 10: 1, and the size in the minor axis direction is the center wavelength of the light source as λ. A size of λ / 5 to λ / 2 is desirable.

  As for the aperture of the probe optical system 108, the aspect ratio, which is the ratio of the major axis to the minor axis, is 2: 1 to 8: 1, and the size in the minor axis direction is λ as the center wavelength of the light source. A size of λ / 5 to λ is desirable.

  The length measuring devices 123b and 123c for measuring the displacement amount of the probe optical systems 106 and 108 and the displacement amount of the reference plane are preferably laser length measuring devices and can be measured with nano-order accuracy. Thereby, the geometric thickness of the test object can be measured with nano-order accuracy.

  By using a near-field probe for both the probe optical systems 106 and 108, the distance between the surface of the test object 107 and the probe optical systems 106 and 108 can be controlled in nano order, and the aperture is small. There is an advantage that the start point and the end point of the light beam passing through can be specified.

  Here, a method for measuring the distance between the lens surfaces in the lens barrel by the surface distance measuring apparatus having the above configuration will be described with reference to the flowchart of FIG.

  (Step S0) In the initial state, the probe optical system 106 and the probe optical system 108 are arranged so that the optical axes of the respective optical systems are coaxial, and are in the initial position.

  (Step S1) The reference object 150 which is a plate on a uniform plane having a known thickness (D0) and refractive index (N0) and no birefringence is held in the holder 122 as a reference object. The stage 123a is installed so as to be between the probe optical systems 106 and 108. At this time, the parallelism of the reference object 150 is adjusted to be perpendicular to the optical axis formed by the probe optical system 106 and the probe optical system 108. This adjustment method will be described later.

  (Step S2) Each of the probe optical systems 106 and 108 is focused on the surface of the reference object 150 by a focusing mechanism in the optical system. The focusing method is performed by moving the probe optical systems 106 and 108 on the same axis by the driving units 114 and 115 in accordance with the focus determination signals of the length measuring devices 123b and 123c which are focus determination mechanisms. This focus determination signal is a light intensity signal and an interference signal by the focus optical systems 124 and 134, or a mechanical shear stress generated when the probe optical systems 106 and 108 approach the surface of the reference object 150.

  It should be noted that it is desirable to focus the probe optical systems 106 and 108 individually, not simultaneously. This is because the light emitted from each probe optical system passes through the reference object 150 (test object) and results in a noise component during focusing.

  (Step S <b> 3) In the interference signal calculation unit 144, the amount of light emitted from the probe optical system 106, transmitted through the reference object 150, and collected by the probe optical system 108 is measured. At this stage, the interference signal calculation unit 144 has not yet adjusted so that the optical path lengths of the reference light and the inspection light are smaller than the coherence length, so that no interference signal appears, and only the light quantity is measured. Is done. Even when an interference signal appears, only the amount of light is returned as a value.

  (Step S4) The probe optical system 108 is slightly rotated around the optical axis. What is being performed here is alignment with respect to the rotation direction of the probe optical systems 106 and 108.

  (Step S5) The optical axis rotation angle of the probe optical system 108 is measured. The rotation angle is measured by the rotation angle sensors 120 and 121.

  (Step S6) The light intensity in the interference signal calculation unit 144 in the state of Step S5 is measured to determine whether or not the light amount is maximum.

  The probe optical systems 106 and 108 are made of a polarization-maintaining fiber, and further, the opening at the tip thereof is elliptical or slit-shaped, so that it plays the role of an analyzer, and the light that is emitted or collected and propagated is Limited to light polarized in the long axis direction. Now, since the reference object is uniform without birefringence, the measured light quantity is maximized when the major axes of the apertures of both probe optical systems are aligned, and the light quantity is minimized. This is when the major axis is shifted 90 degrees. The maximum and minimum arrangements are measured in steps S4 to S6, and the arrangement with the maximum amount of light is finally returned.

  In step S4 to S6, the probe optical system 106 did not move and only the probe optical system 108 was rotated. However, in step S6, the probe optical system 106 is rotated after recognizing the arrangement that provides the maximum light amount. S4 to S6 may be repeated. This makes it possible to analyze factors other than the reference object that is the object to be measured, for example, the mechanical strain of the fiber constituting the measurement system, the focus accuracy on the surface of the object to be measured, and the influence of rotation.

  From step S4 to S6, the angular arrangement that maximizes the amount of light is set as R10 for the probe optical system 106 and R20 for the probe optical system 108, which are the initial positions and recorded in the optical rotation recording unit 145.

  (Step S7) The interference signal calculator 144 evaluates the strength of the interference signal.

  (Step S8) The corner cube 137, which is the reference plane, is displaced so that the interference signal in step S7 is maximized. Here, the interference signal is maximized when the optical path differences between the inspection light and the reference light coincide.

  (Step S9) The position of the corner cube 137 when the interference signal is determined to be maximum is measured by the length measuring device 139 and recorded (ZR0). This value is the reference value of the length measuring device 139. At this time, the direction from the measuring instrument to the corner cube is set to a positive value.

  (Step S10) The positions of the probe optical systems 106 and 108 are measured by the length measuring devices 116 and 117, respectively (Z10 and Z20). This value is a reference value for the length measuring devices 116 and 117. At this time, on the optical axis (z axis) formed by both probe optical systems, the direction from each length measuring device to each probe optical system is set to a positive value.

  Further, the relative position (XY axis) between the holder 122 of the test object and both probe optical systems is measured by another measuring means.

  (Step S11) The probe optical systems 106 and 108 are spaced apart by the drive units 114 and 115 in accordance with the instructions of the measurer, and the reference object 150 is removed from the holder.

  (Step S12) The object 107 to be measured is held on the holder 122, and is placed by the stage 123 so as to be between the probe optical systems 106 and 108.

  (Step S13) The probe optical systems 106 and 108 are focused on the surface of the specimen 107 in the same manner as in step S2. Which portion of the test object 107 is measured for the refractive index is determined by the focus position, and the focus position is controlled by the stage 123a.

  (Steps S14 to S17) are similar operations from Steps S3 to S6, and are alignments of the probe optical systems 106 and 108. If the test object 107 has no birefringence, the same arrangement as in step S6 is when the light amount is maximum, and the same is true for the case where the light amount is minimum. However, when birefringence exists in the test object, the plane of polarization rotates, and the value varies depending on the amount of birefringence and the axial direction.

  From step S14 to step S17, the angular arrangement that maximizes the amount of light is R11 for the probe optical system 106 and R21 for the probe optical system 108, which is recorded in the optical rotation recording unit 145 and returned to the arrangement of step S18.

  (Step S18) The interference signal calculation unit 144 evaluates the intensity of the interference signal.

  (Step S19) The corner cube 137 is displaced so that the interference signal in step S18 is maximized.

  (Step S20) The position of the corner cube 137 when the interference signal is determined to be maximum is measured by the length measuring device 139 and recorded (ZR1).

  (Step S21) The positions of the probe optical system 106 and the probe optical system 108 are measured by the length measuring device 116 and the length measuring device 117, respectively (Z11, Z21).

(Step S22) The geometric thickness D at this measurement position is calculated by the following (Formula 2).
D = − (Z11 + Z21) + (Z10 + Z20) + D0 (Formula 2)

  This is because the probe optical systems 106 and 108 always maintain a certain distance from the surface of the test object 107, so that the difference in position between the probe optical systems 106 and 108 when the reference object 150 is measured and the test object 107 is measured. This is because this corresponds to the difference in geometric thickness between the test object 107 and the reference object 150. This can be understood by comparing the figure when measuring the reference object of FIG. 5A and the figure when measuring the test object of FIG. 5B.

(Step S23) The geometric thickness ND at this measurement position is calculated by the following (Equation 3).
ND = −2 × (ZR1−ZR0) + N0 × D0 (Formula 3)

  The light source 101 is a low-coherent light source typified by SLD, and an interference signal is generated when the optical path length difference between the reference light and the inspection light is equal to or less than the coherent length. Can be considered to be zero. Therefore, the difference in the position of the corner cube, which is the reference plane at the time of measuring the reference object and the measurement of the test object, corresponds to the difference between the optical path lengths of the test object and the reference object. These are understood by comparing FIG. 5 (a) when measuring the reference object and FIG. 5 (b) when measuring the test object.

  In addition, FIG. 6 shows a graph of the intensity of interference light in the image sensor. In FIG. 6, the horizontal axis represents the amount of movement of the reference plane, and the vertical axis represents the intensity of the interference light. The interference light exists only when the optical path difference between the inspection light and the reference light is less than or equal to the coherence lens length of the light source. The half width of the intensity is almost equal to the half width of the wavelength included in the light source.

  In (Equation 4), the optical path length value is twice the difference in the position of the corner cube, because of the folding optical system, the movement of the corner cube is equivalent to half the displacement of the optical path difference of the reference light. It is to do.

(Step S24) Using the geometric thickness and optical path length obtained in steps S22 and S23, the local refractive index N at the measurement point is obtained as in the following (Expression 4).
N = ND / D (Formula 4)

This (Expression 5) is expressed as the following (Expression 5) when the total difference from the reference position of the probe optical system 106 is replaced with ΔD, and the difference from the reference position of the reference mirror is replaced with ΔND. .
N = (2ΔND + N0 × D0) / (ΔD + D0) (Formula 5)

  Steps S25 to S28 are a flow for measuring the optical rotation of the test object 107.

  (Step S25) Similar to step S15, the probe optical system 108 rotates by a set angle about the optical axis.

  (Step S26) The optical axis rotation angle (R2i) is measured. The rotation angle is measured by the rotation angle sensors 120 and 121, and the position information and the light intensity are recorded in the optical rotation recording unit 145.

  (Step S27) The light intensity of the interference signal calculation unit 144 in the state of step S26 is measured.

  (Step S28) The optical rotation calculation unit 146 calculates the local optical rotation based on the rotation angle information and the light intensity information accumulated in the optical rotation recording unit 145 and the local refractive index calculated in Step S24. be able to.

Now, it is assumed that the test object 107 is a crystalline substance having a uniaxial optical axis. The refractive index for the normal light gland is No, the refractive index for the extraordinary ray is Ne, the angle between the major axis of the probe optical system 106 and the principal axis of the crystal of the test object 107 is θ, the thickness of the test object 107 is D, the wavelength Is set to λ, a phase difference Δ expressed by the following (formula 6) is generated after transmission.
Δ = 2πD (Ne−N0) / λ (Expression 6)

At this time, the intensity (I) of the transmitted light is expressed by the following (formula 7).
I = I ₀ sin ² (2θ) sin ² (Δ / 2) (Expression 7)

  Thus, when birefringence exists in the test object 107, the transmitted light intensity varies with angle. By plotting the intensity change against the rotation angle, an indicator of the presence of local birefringence can be created.

  (Step S29) The probe optical systems 106 and 108 are out of focus with respect to the test object 107, and the test object 107 is moved to a part different from the previously measured site by the stage 123a.

  (Step S30) Thereafter, Steps S11 to S26 are repeated to obtain local refractive indexes and optical rotations at different sites, and based on these, XY distributions of refractive indexes and optical rotations are created.

  The amount of displacement in the optical axis direction of each probe optical system is obtained by reading the value of the length measuring device at each location, and the orientation of the reference object is adjusted by the holder so that the value is constant at multiple locations. If adjusted, the parallelism of the axis of the reference object and the probe optical system coincides. Therefore, the adjustment of the parallelism between the flat reference object and the axis of the probe optical system can be adjusted using this relationship. However, in this case, the reference object is a completely parallel object.

  Further, in step S2, the probe optical system is focused, and the positions where the displacement amount of the probe optical system is measured are set at a plurality of locations on the XY plane, and the measured numerical values may be compared.

  As described above, in the first embodiment, the refractive index of the test object 107 is calculated by obtaining the difference from the reference object whose refractive index and thickness are known.

  Hereinafter, a specific configuration of the constituent elements constituting the first embodiment and a more desirable configuration will be described.

  As a specific example of the low coherence lens light source 101, a super luminescent diode (SLD), a light emitted from a white light source (halogen lamp or xenon lamp) using a monochromator only in a specific wavelength region, a laser light emitting diode, or the like is used. be able to. The wavelength is not particularly limited and can be used from ultraviolet to infrared light. However, the shorter the coherence length, the more desirable, specifically about 10 μm. The wavelength of the light source is the transmittance of the test object. The short wavelength side is desirable to the extent that does not decrease. The coherence length of the light source affects the positional accuracy of the reference plane. By combining with the phase shift interferometry, the interference intensity signal width can be measured with an accuracy of about 10 μm and the positional accuracy of the reference plane can be 1 μm or less.

  As the image pickup element of the light receiving element 113, it is desirable to use a detector having a high sensitivity such as a photomultiplier tube or an APD in order to increase the intensity of the interference signal.

  With the above configuration, the local refractive index of the test object is obtained by measuring the difference from the refractive index of the reference object, and the accuracy is the precision of the last four digits or less.

  The refractive index in the first embodiment is a group refractive index in consideration of wavelength dispersion, and is not a single-wavelength refractive index (phase refractive index).

(Embodiment 2)
FIG. 7 is a conceptual configuration diagram of a refractive index measuring apparatus according to Embodiment 2 of the present invention. In FIG. 7, the constituent elements having the same numbers as those in FIG. 1 function in the same manner as in the first embodiment. Further, the focusing method of the probe optical systems 106 and 108 with respect to the surface of the object 107 is the same as in the first embodiment. The configuration of the tip of the probe optical system is the same as that of the first embodiment.

  In FIG. 7, reference numeral 201 denotes a light source that emits coherent light and can change the wavelength. Reference numeral 202 denotes a controller that changes the wavelength emitted from the light source 201. Reference numeral 204 denotes a calculation unit that calculates the phase difference of interference fringes observed by the light receiving element 113.

  Light emitted from the light source 201 at a certain wavelength λ0 is divided into inspection light and reference light by the beam splitter 102 as in the first embodiment. In the first embodiment, the light source 101 has a low coherent emission wavelength range such as an SLD. In the second embodiment, the light source 101 is a coherent single wavelength light source 201. As in the first embodiment, the inspection light and the reference light are combined at the light receiving element 113 and output an interference signal. In the second embodiment, since the light source 201 is coherent light, the light receiving element 113 always outputs an interference signal.

Now, if the optical path length to the light receiving element of the inspection light is L1 (λ) and the optical path length to the light receiving element of the reference light is L2 (λ), the intensity I of the interference light in the light receiving element is It is expressed by equations (8) and (9).
I = I0 [1 + γcosφ] (Equation 8)
φ = 2π / λ ((L2 (λ) −L1 (λ)) (Equation 9)

  In (Expression 8) and (Expression 9), φ is the phase of the interference light. I0 and γ are proportional constants.

  The optical path length of the inspection light is divided into an optical path length Lt (λ) that passes through the test object and a portion Lo (λ) other than that.

  Now, when the wavelength λ of the light source is changed minutely, the phase of the interference light changes depending on the wavelength according to the above equation. When the wavelength is changed, if the medium has dispersion, the refractive index also changes. However, assuming that it is negligibly small, the phase change of the interference light due to the wavelength change is inversely proportional to the optical path difference. From this, the optical path length difference can be obtained by observing the phase change of the interference light caused by the wavelength change.

  Further, since the optical path length L1 of the inspection light is divided into a portion (Lt) that changes depending on the test object having an unknown refractive index and a portion (Lo) that does not change, the test object is divided into the refractive index and the refractive index. By using a reference object having a known thickness, both can be separated, and the optical thickness of the test object, that is, the optical path transmitted through the test object can be calculated.

Now, assuming that the wavelength change Δλ is small, the phase change of the interference fringes due to the wavelength change is expressed as (Equation 10).
Δφ (λ) = - 2π ( L2-Lt-Lo) Δλ / λ 2 ... ( Equation 10)

When the reference object having the optical thickness ND0 and the test object having the optical thickness ND are inserted, the phase change of the interference fringes due to the wavelength change Δλ is expressed as (Expression 11) and (Expression 12), respectively.
Δφ (λ) 0 = -2π ( L2-Lt-ND0) Δλ / λ 2 ... ( Equation 11)
Δφ (λ) 1 = −2π (L2−Lt−ND) Δλ / λ ² (Equation 12)

  By looking at the difference in the change rate of the phase, the optical thickness of the test object can be obtained as the difference from the optical thickness of the reference object.

Actually, there is chromatic dispersion in the refractive index of the medium, and this is expressed as an equation using the wavelength as a variable in the following (Mechanical Dispersion Formula) of (Equation 12).
N ² −1 = A1λ ² / (λ ² −B 1) + A ² λ ² / (λ ² −B ² ) + A 3 λ ² / (λ ² −B 3) (Equation 13)

  In this case, the optical path difference can be calculated by fitting with a plurality of wavelengths using the coefficient as an unknown variable.

  FIG. 8 is a flowchart of the refractive index measurement in the second embodiment.

  (Steps S207 to S209) and (Steps S218 to S220) are steps for measuring the above-described optical path length difference, which is different from the first embodiment.

  As the imaging device, in order to increase the intensity of the interference signal, it is desirable to use a detector having high sensitivity such as a photomultiplier tube or APD.

  The refractive index measured in the second embodiment is a refractive index at a single wavelength and a phase refractive index.

  In the second embodiment, the optical thickness is calculated using the amount of displacement due to the wavelength of the phase of the beat signal due to interference. Therefore, the optical waveguide must be preserved so that the phase of the light is not disturbed as much as possible by the waveguide, and the fiber optical system is required to be composed of all polarization-preserving fibers, and the probe optical system. The same applies to.

(Embodiment 3)
FIG. 9 is a conceptual configuration diagram of a refractive index measuring device according to Embodiment 3 of the present invention. FIG. 10 is a flowchart of refractive index measurement in the third embodiment of the present invention. 9, components having the same numbers as in FIG. 1 is for the same function as in the first embodiment. The focus of the probe optical system 106 and the probe optical system 108 on the surface of the test object is the same as in the first embodiment. The structure of the probe optical system is the same as that of the first embodiment.

  In FIG. 9, reference numeral 301 denotes a light source that emits two coherent lights having slightly different frequencies. Reference numeral 302 denotes a light source frequency control unit. 303 and 306 are beat signal counters for detecting the phase of the beat signal. Reference numeral 305 denotes a polarization beam splitter, which divides the light path into two slightly different optical paths having different polarization directions. Reference numerals 307 and 310 denote polarization plane preserving fibers. Reference numeral 308 denotes an objective lens that couples the reference light to the polarization plane preserving fiber 307. Reference numeral 304 denotes an optical connector that combines the polarization-maintaining fiber 307 and the fiber 109 into the fiber 310. Reference numeral 309 denotes an ND filter. Reference numeral 311 denotes an arithmetic unit that calculates the optical path length from the difference between the beat signal counters 303 and 306.

  In the first embodiment, the light source is a low coherent light source such as an SLD and has a wide emission wavelength range, but in the second embodiment, coherent light such as a Zeeman laser is emitted with slightly different frequencies. The light source 301 to be used. Alternatively, an optical path in which this frequency is slightly shifted may be created using an acousto-optic element.

  Lights of different frequencies can be divided into two by a polarizing beam splitter 305, which is an optical element having different polarization characteristics and utilizing the difference in polarization characteristics.

  The light emitted from the light source 301 is divided into two light beams by the beam splitter 102.

  One light beam is guided to the beat signal counter 303, and the phase of the beat signal is observed (beat signal 1).

  The other light beam is further divided into two light beams having different frequencies by the polarization beam splitter 305, one of which is inspection light that passes through the test object 107 and one of which is reference light.

  The reference light enters the ND filter 309 and is then coupled to the fiber 307 by the objective lens 308.

  The reference light propagating through the fiber 307 is combined with the inspection light transmitted through the test object from the fiber 109 by the connector 304 to cause interference, and is transmitted through the fiber 310 and guided to the beat signal counter 306 (beat signal 2).

  Now, when light having two slightly different frequencies Δω are combined, a beat signal that oscillates with a frequency difference in time is generated, and the direct observation of this is called heterodyne detection.

As in the second embodiment, when the optical path length of the inspection light from the light source is L1 and the optical path length of the reference light is L2, the beat signal intensity observed by the beat signal counter 306 is (Equation 14) and (Equation). 15) is expressed as follows.
I = I0 (1 + γcos (Δω (t−φ / Δω)) (Expression 14)
φ = 2π / λ ((L1-L2) (Equation 15)

  In the second embodiment, the optical thickness of the test object 107 is obtained by changing the wavelength.

  In the third embodiment, based on the time phase of the beat signal of the light source 301 observed by the beat signal counter 303, the time phase of the beat signal observed by the interference between the inspection light and the reference light by the beat signal counter 306 is used. The optical thickness of the test object 107 is measured.

  Since the beat signal is generated only after interference occurs, the heterodyne detection method is a mechanism that is less susceptible to noise in principle.

  Let T be the difference between the phase of the beat signal I 0 observed by the beat signal counter 306 and the phase of the beat signal counter 303 when the test object 107 is placed.

  Similar to the second embodiment, the optical path length L1 of the inspection light is divided into a portion (Lt) that varies depending on the test object having an unknown refractive index and a portion (Lo) that does not vary.

At this time, the intensity of the beat signal observed by the beat signal counter 306 is expressed by the following (Expression 16) and (Expression 17).
I = I01 (1 + γ1cos (Δω (t−φo1 / Δω))) (Expression 16)
φo1 = 2π / λ ((Lt + Lo−L2)) + φ0 (Expression 17)

  However, in (Expression 17) and (Expression 18), φ0 is the initial phase of the light source.

Further, since the intensity of the beat signal observed by the beat signal counter 303 is the same in the optical path of light having different frequencies for producing the beat signal up to the detector (counter), the intensity of the beat signal is expressed by the following equation (18). ), (Equation 19).
I = I02 (1 + γ2cos (Δω (t−φoo / Δω))) (Expression 18)
φo2 = φ0 (Equation 19)

Therefore, the phase time difference T is expressed by the following (Equation 20).
T = 2π / λ · (Lt + Lo−L2) / Δω (Equation 20)

The phase time difference when the measurement object is the reference object 150 is T1, the phase time difference when the measurement object 107 is T2, the optical thickness of the reference object 150 is ND0, and the optical thickness of the test object 107 is ND. They are represented by the following (formula 21) and (formula 22), respectively.
T1 = 2π / λ · (Lt + ND0−L2) / Δω (Formula 21)
T2 = 2π / λ · (Lt + ND−L2) / Δω (Formula 22)

From this, the difference ΔT between the reference object and the phase time is expressed by the following (Equation 23).
ΔT = T2−T1 = 2π / λ (ND−ND0) / Δω (Equation 23)

  The difference between T1 and T2 corresponds to the difference between the optical path length transmitted through the reference object 150 and the optical path length transmitted through the test object 107. Therefore, the optical thickness ND of the test object 107 can be obtained in the form of a difference from the optical thickness of the reference object 150.

  FIG. 9 is a flowchart of the refractive index measurement in the third embodiment.

  The physical thickness of the test object 107 is measured by the same method as in the first embodiment or the second embodiment.

  In addition, in the beat signal counters 303 and 306, in order to cause interference between two lights having different polarization directions, an optical element that converts a polarization direction and a polarization type, such as a quarter-wave plate, is included in the counter.

  Also, the refractive index of the medium changes due to the influence of dispersion when the wavelength is different. In this embodiment, the difference between the two frequencies used in the heterodyne detection is about 1 KHz, and the change in the refractive index can be ignored. It is a sufficiently small range.

  In order to perform the exact measurement, it is necessary to fit and calculate the coefficient part using the dispersion formula used in the second embodiment.

  In the third embodiment, the optical thickness is calculated using the time difference of the phase of the beat signal of the heterodyne interference.

  Therefore, the optical waveguide must be preserved so that the phase of the light is not disturbed as much as possible, and the optical system of the fiber is required to be composed of a polarization-maintaining fiber.

  In the third embodiment, the reference light is coupled from the air to the polarization preserving fiber 307 by the lens 308, and is combined with the test light propagated from the polarization preserving fiber 109 by the connector 304 to preserve the polarization plane. It takes the form of propagating through the fiber 310 and reaching the beat signal counter 306.

  (Steps S <b> 307 to S <b> 309) and (Steps S <b> 318 to S <b> 320) are steps for measuring the above-described optical path length difference that is different from the first embodiment.

  Polarization plane preserving fibers 307 and 310 may be in the form of aerial waveguides as in the second embodiment.

  Further, in the third embodiment, the polarization plane preserving fiber 109 plays a role of collecting light transmitted from the test object. This is combined by the polarization plane fiber 109 at the polarization plane preserving fiber 307 and the connector 304, irradiates the reference light onto the surface of the test object, condenses the reflected light from the surface and the transmitted light of the inspection light, and It may be in the form of being guided to 306.

  Further, since the light transmitted through the test object as the inspection light is very weak, it is desirable to provide a mechanism for multiplying the signal before the beat signal counter 306 detects the beat signal.

  In the third embodiment, the light source 301 that emits two slightly different frequencies, such as a Zeeman laser, is used, but the same method can be used even if a light source having a plurality of stable frequencies, such as an optical comb laser, is used. Can be used to measure the refractive index.

  In the third embodiment, the measured refractive index is a phase refractive index.

  By using the refractive index measuring apparatus according to the present invention, the refractive index of an object having an arbitrary shape can be measured accurately and easily using light interference regardless of the refractive index range of the measured object. be able to. Thereby, a refractive index distribution measuring device for a molded lens used for a camera or the like can be provided.

21, 33 Clad 22, 32 Core 23, 35 Aperture 31 Incident light 34 Polarization plane 36 Near field light 37 Transmitted light 38, 107 Test object 29 Sharpened portion 33 Stress applying material 39 Probe tip 101, 125, 201, 301 Light source 102, 112, 127, 131 Beam splitter 103, 135, 309 ND filter 104, 308 Objective lens 105, 109, 307, 310 Polarization plane preserving fiber 106, 108 Probe optical system 110 Lens 111 1/4 wavelength plate 113 Light receiving element 114 115, 123a, 138 Drive unit 116, 117 Length measuring device 118, 119 Rotation drive unit 120, 121 Displacement sensor 122 Holder 123b, 123c, 139 Length measuring device 124, 134 Focus optical system 126 Half mirror 128, 130 Light receiving unit 1 29, 132, 133, 136 Mirror 137 Corner cube 140, 141, 203 Calculation unit 142 Refractive index calculation unit 143 A / D converter 144 Interference signal calculation unit 145 Optical rotation recording unit 146 Optical rotation calculation unit 150 Reference object 202 Controller 302 Control unit 303, 306 Beat signal counter 304 Connector 305 Polarizing beam splitter 311 Calculation unit

Claims (12)

A light source that emits linearly polarized coherent light;
An optical member for separating coherent light from the light source into inspection light and reference light;
A first probe optical system for condensing and irradiating the inspection light on the surface of the object;
A second probe optical system that collects the inspection light irradiated from the first probe optical system and transmitted through the test object;
Driving means for coaxially adjusting the optical axis of the first probe optical system and the optical axis of the second probe optical system;
A length measuring means for measuring the distance of the positions of the first probe optical system and the second probe optical system as a difference from a reference position;
A light receiving element that detects an intensity distribution of interference light generated by combining the reference light and the inspection light collected by the second probe optical system;
A stage for fixing the test object between the first probe optical system and the second probe optical system;
The position of each probe optical system when each probe optical system is focused on the surface of a flat transparent reference object having a known refractive index and thickness is recorded in advance as the reference position of each probe optical system, Measure the total difference ΔD from the reference position of each probe optical system when the probe optical system is focused on the surface of the test object, set the wavelength of the light source to a plurality of different wavelengths, for each wavelength An operation for measuring a phase Δi of interference fringes observed in the light receiving element and calculating a refractive index of the test object based on the sum ΔD of the differences and the phases Δi of the interference fringes with a plurality of different wavelengths. And a refractive index measuring device. A light source that emits low coherent light;
An optical member for separating low-coherent light from the light source into inspection light and reference light;
A first probe optical system for focusing and irradiating the inspection light on the surface of the object;
A second probe optical system for condensing the light transmitted through the test object by the inspection light;
Driving means for coaxially adjusting the optical axis of the first probe optical system and the optical axis of the second probe optical system;
A length measuring means for measuring the distance of the positions of the first probe optical system and the second probe optical system as a difference from a reference position;
A light receiving element that detects an intensity distribution of interference light generated by combining the reference light and the inspection light collected by the second probe optical system;
A reference light mirror disposed in the optical path of the reference light;
An optical path length adjusting means for adjusting an optical path length of the reference light by moving the reference light mirror;
A length measuring means for ranging the movement amount of the reference light mirror as a difference from a reference position of the reference mirror,
A stage for fixing the test object between the first probe optical system and the second probe optical system;
A polarization maintaining fiber that guides the inspection light from the second probe optical system to the light receiving element;
The position of each probe optical system when the probe optical system is focused on the surface of a flat transparent reference object having a known refractive index N0 and thickness D0 is recorded in advance as the reference position of each probe optical system, The position at which the interference signal intensity at the light receiving element is maximized is recorded in advance as a reference position of the reference light mirror, and the reference light mirror is moved while focusing each probe optical system on the surface of the test object. When the total difference from the reference position of the probe optical system when adjusted so that the interference signal intensity at the light receiving element is maximized is ΔD, and the difference from the reference position of the reference light mirror is ΔND, A refractive index measuring apparatus comprising: an arithmetic unit that calculates a refractive index N of the test object according to (Equation 1) below.
N = (2ΔND + N0 × D0) / (ΔD + D0) (Formula 1) A light source that emits coherent light including two different frequencies;
A first optical member that splits coherent light from the light source into two light fluxes;
A first counter for detecting a phase of a first beat signal due to a wavelength difference in one of the two light beams;
A second optical member that splits the other of the two light beams into two light beams of inspection light and reference light having different wavelengths;
A first probe optical system for focusing and irradiating the inspection light on the surface of the object;
A second probe optical system for condensing the light transmitted through the test object by the inspection light;
Driving means for coaxially adjusting the optical axis of the first probe optical system and the optical axis of the second probe optical system;
A length measuring means for measuring the distance of the positions of the first probe optical system and the second probe optical system as a difference from a reference position;
A second counter for detecting a phase of a second beat signal generated by combining the reference light and the inspection light collected by the second probe optical system;
A stage for fixing the test object between the first probe optical system and the second probe optical system;
The position of each probe optical system when the probe optical system is focused on the surface of a flat transparent reference object having a known refractive index and thickness is recorded in advance as the reference position of each probe optical system, and The phases of the first and second beat signals observed in the first and second counters are observed and recorded in advance, and the interval between the first probe optical system and the second probe optical system is set in the direction of the optical axis. The first and second counters are simultaneously measured by focusing each probe optical system on the surface of the object and measuring the total difference ΔD from the reference position of both probe optical systems. Measuring the phase of the first and second beat signals observed in step, and calculating the total ΔD of the difference and the phase change of the first and second beat signals when measuring the reference object A refractive index measuring apparatus comprising: an arithmetic unit that calculates a refractive index of the test object based on phase changes of the first and second beat signals at the time of the test object measurement. 3. The refractive index measuring apparatus according to claim 1, wherein at least one of the first probe optical system and the second probe optical system is a single-mode polarization-maintaining fiber having a polished tip. The tip of the single-mode polarization-maintaining fiber used in at least one of the first probe optical system and the second probe optical system is exposed only within a range smaller than the wavelength of the irradiation light of the core portion. , the refractive index measuring apparatus according to claim 4, wherein the other portions are covered with a coating layer. The exposed portion of at least one tip of the first probe optical system and the second probe optical system is rotationally asymmetric and has a slit shape or an ellipse shape. Refractive index measuring device. A moving mechanism that moves the optical axis of the first probe optical system and the optical axis of the second probe optical system in an interlocked state while being in a coaxial state;
The first probe optical system and the second probe optical system scan the surface of the test object, measure the local refractive index, and integrate the two to determine the two-dimensional refractive index of the test object. The refractive index measuring device according to claim 1, further comprising a measuring unit that measures the distribution. The light source of the focusing mechanism of the first probe optical system and the second probe optical system, and the light source of the inspection light and the reference light are branched from the same light source. 7. The refractive index measuring apparatus according to any one of 7 above. The refractive index measurement apparatus according to claim 1, wherein at least one of the first probe optical system and the second probe optical system includes a mechanism that rotates about an optical axis thereof. At least one of the first probe optical system or the second probe optical system based on the light quantity of the inspection light measured while the first probe optical system or the second probe optical system is rotated about its optical axis. The refractive index measuring apparatus according to claim 9, further comprising means for adjusting two rotation angles. The refractive index measuring apparatus according to claim 1, wherein the first probe optical system and the second probe optical system include a mechanism that rotates in association with the optical axis thereof. The birefringence of the test object is determined based on the amount of the inspection light measured while the first probe optical system and the second probe optical system are rotated around the optical axis and the rotation angle. The refractive index measuring apparatus according to claim 11, further comprising a measuring unit that measures the distribution of the refractive index.

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