CN117006961A - Device and method for measuring distance between continuous mirror surfaces on axis based on low-coherence light interference - Google Patents
Device and method for measuring distance between continuous mirror surfaces on axis based on low-coherence light interference Download PDFInfo
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Abstract
The invention discloses a device and a method for measuring the distance between continuous mirror surfaces on an axis based on low coherence light interference, belonging to the technical field of optical precision measurement; the invention provides a continuous mirror surface on-axis distance measuring device based on low coherence light interference, and designs a distance measuring method matched with the device based on the device. The invention has simple structure, low implementation cost and convenient operation, and can effectively improve the accuracy of mirror surface spacing measurement.
Description
Technical Field
The invention relates to the technical field of optical precision measurement, in particular to a device and a method for measuring the distance between continuous mirror surfaces on an axis based on low-coherence optical interference.
Background
In an optical vehicle or laboratory, to avoid mechanical damage to a measurement object, a non-contact physical measurement method is used to measure the distance between central axes of continuous mirrors, for example: the method comprises an image method, an image calibration method, an axial dispersion method, a confocal method, a differential confocal method, a low-coherence optical interferometry, a Fizeau (Fizeau) interferometry, a polarization interferometry and the like, and the method is used for obtaining the position information of the surface by using test light through specular reflection, so that the measurement of continuous mirror surface spacing is realized.
The low coherence optical interferometry is the optimal measurement method in the continuous mirror surface interval measurement method, and the measurement accuracy can reach 600nm by adopting more complex auxiliary facilities and spectrum or data processing technology. By improving auxiliary facilities and data processing methods, such as measuring the moving distance by using a high-precision grating ruler, and combining an effective data processing method, the measuring precision of the low-coherence interferometry can be improved to 200nm.
In view of the fact that the low coherence measurement method is limited to be improved in precision through complex auxiliary facilities and data processing, the optical mirror processing or continuous mirror assembly needs a non-contact mirror pitch measurement method which is low in implementation cost, simple and convenient to operate and particularly good in precision. In order to solve the above problems, the present invention provides a device and a method for measuring the distance between continuous mirror surfaces on axis based on low coherence light interference.
Disclosure of Invention
The invention aims to provide a device and a method for measuring the distance between continuous mirror surfaces on an axis based on low-coherence light interference so as to solve the technical problems in the background art.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
the device consists of a low-coherence light source, a micro objective, a small aperture diaphragm, an achromatic collimating objective, a first beam splitting prism, a pentagonal prism, a complementary wedge prism group, a plane reflecting mirror, a second beam splitting prism, a measured continuous mirror, an imaging lens and a CCD camera.
Based on the measuring device, a continuous mirror surface on-axis distance measuring method based on low coherence light interference is provided, and the method specifically comprises the following steps:
s1, finishing the integral arrangement and installation of the measuring device, firstly adjusting the low-coherence light source, the micro objective, the aperture diaphragm and the achromatic collimating objective to be coaxial, filtering stray light at a beam convergence point through the aperture diaphragm after a light beam emitted by the low-coherence light source passes through the micro objective, and then emitting parallel light through the achromatic collimating objective;
s2, adjusting the first beam splitting prism to enable the parallel light to vertically enter the first beam splitting prism, and separating a low-coherence reflected light beam and a transmitted light beam, wherein the reflected light beam travels above and is a reference light beam; the transmitted beam is the test beam;
s3, adjusting the pentagonal prism, the complementary wedge-shaped prism group, the plane reflector and the second beam splitting prism, so that the reference beam passes through the complementary wedge-shaped prism group after being reflected by the pentagonal prism, is reflected by the plane reflector and then passes through the second beam splitting prism in a transmission mode;
s4, adjusting the coaxial of the tested continuous mirror surface and the optical path where the test beam is located, enabling the test beam to pass through the second beam splitting prism in a transmission mode, enabling the test beam to enter the continuous mirror surface with the tested space, enabling each mirror surface of the continuous mirror surface with the tested space to sequentially reflect the test beam, enabling each reflected beam to return to the second beam splitting prism along the original path, and enabling the reflected beams to coaxially overlap with the reference beam after being reflected by the second beam splitting prism;
s5, adjusting the equivalent thickness of the complementary wedge-shaped prism group on the reference light path, at least enabling light rays in a large enough range taking an axis as a center in the cross section of the reference light beam to pass through, and adjusting the position of the measured continuous mirror surface along the axial direction until the situation that the test light beam reflected by the first mirror surface of the measured continuous mirror surface and the reference light beam are converged and overlapped and pass through the imaging lens at the same time is observed, and then an aplanatic interference fringe of low-coherence light is generated on the receiving surface of the CCD camera;
s6, recording position readings of the movable wedge prisms in the complementary wedge prism group along the direction of the inclined edges of the movable wedge prisms when the aplanatic interference fringes are observed in S5, continuously adjusting the equivalent thickness of the complementary wedge prism group on the reference light path, sequentially observing interference fringes of low coherent light generated by test light beams and reference light beams reflected by each subsequent mirror surface of the tested continuous mirror surface, and recording position readings of the movable wedge prisms in the complementary wedge prism group along the direction of the inclined edges of the movable wedge prisms when the observed aplanatic interference fringes are observed;
and S7, calculating the on-axis distance and the measurement error of the measured continuous mirror surface according to the position reading data obtained in the S6 and combining the interference principle of low-coherence light.
Preferably, the complementary wedge prism group comprises a first wedge prism and a second wedge prism, the first wedge prism and the second wedge prism are made of the same material and have the same wedge angle, the first wedge prism and the second wedge prism are placed in complementary positions on the same horizontal plane, the planes of hypotenuses are parallel to each other, a tiny space exists between the planes, and the second wedge prism is a movable wedge prism which can move in the horizontal plane along the hypotenuse direction to measure displacement.
Preferably, the calculation formula of the interval on the axis of the measured continuous mirror surface in S7 is:
2n i t i ＝(n p -n 0 )(x i+1 -x i )sinθ (1)
(1) Wherein n is i Representing the refractive index of the material between two adjacent mirrors of the measured continuous mirror; t is t i Representing the axial distance between two adjacent mirrors of the measured continuous mirror; n is n p Representing the refractive index of the glass material of the complementary wedge prism group; n is n 0 Representing the refractive index of air; x is x i+1 、x i Representing a position reading of the movable wedge prism in the direction of its hypotenuse corresponding to interference fringes of two adjacent low-coherence lights; θ represents the prism wedge angle of the complementary wedge prism group, i.e., the wedge angles of the first wedge prism and the second wedge prism;
and (3) rewriting the (1) to obtain a calculation formula of the distance between the adjacent two mirror surfaces on the shaft:
combining (2), obtaining a measurement error calculation formula of the distance between two adjacent mirror surfaces by a measurement error theory, wherein the calculation formula is as follows:
(3) Wherein Deltax is i+1 ，Δx i The second wedge prism, i.e. the movable wedge prism, of the complementary wedge prism set, is shown as the position measurement error when moving in the direction of its hypotenuse.
Compared with the prior art, the invention provides a device and a method for measuring the distance between continuous mirror surfaces on the axis based on low-coherence light interference, which have the following beneficial effects:
(1) The invention measures the distance between the continuous mirror surfaces on the axes in a non-contact and non-damage mode, which is suitable for an optical lens processing workshop and a lens assembly debugging workshop;
(2) The invention measures and positions by an aplanatic interference method of a low-coherence light source with a wide spectrum, has sensitive response and good accuracy, and the positioned interferogram data is suitable for automatic analysis and treatment;
(3) The complementary wedge-shaped prism group is simple and easy to manufacture and process, the prism wedge angle of the complementary wedge-shaped prism group can be designed according to the requirement of measurement precision, and the precision of measuring the distance on the continuous mirror surface shaft can be easily controlled within 20 nm;
(4) The invention adopts the one-way direction adjustment and the measurement of the optical path difference in the optical path, and the measurement precision is obviously superior to the precision of the two-way direction adjustment and the measurement;
(5) The measuring moving direction of the invention is the transverse movement in the direction close to being perpendicular to the optical axis, so that the longitudinal scanning of the coherent light beam to the position of the mirror surface to be measured is realized, namely the longitudinal scanning is changed into the transverse scanning, and the interferometer does not need to stretch or integrally move in the measuring process;
(6) The invention uses a low-precision displacement mechanism to obtain high-precision displacement adjustment, thereby realizing high-precision adjustment and measurement of optical path difference;
(7) The invention can realize the measurement of the large distance of the continuous mirror surface by using the optical path compensation method. The large-spacing measurement can adopt a mode of cascading a plurality of wedge-shaped prism groups so as to compensate the adjustment of the thickness of the single wedge-shaped prism group and the deficiency of the measurement range. The limit of the interval between the continuous mirrors is small, and the thickness range of measurement is large and can be from 1 μm to 100mm.
Drawings
FIG. 1 is a schematic diagram of the low coherence interferometry of continuous mirror spacing referred to in example 1 of the present invention;
FIG. 2 is a cross-sectional view showing the optical structure of a complementary wedge prism group for precisely adjusting and measuring optical path difference as mentioned in example 1 of the present invention;
FIG. 3 is a three-dimensional view of the optical structure of the complementary wedge prism set for precisely adjusting and measuring optical path difference as mentioned in example 1 of the present invention;
FIG. 4 is a schematic diagram showing steps of measuring the continuous mirror spacing of the low coherence interferometry of example 1 of the present invention.
FIG. 5 is a diagram of the optical path of the low coherence interferometry continuous mirror spacing referred to in example 4 of the present invention;
FIG. 6 is a diagram of the optical path of the low coherence interferometry continuous mirror spacing referred to in example 5 of the present invention;
FIG. 7 is a diagram of the optical path of the low coherence interferometry continuous mirror spacing referred to in example 6 of the present invention;
FIG. 8 is a diagram of the optical path of the low coherence interferometry continuous mirror spacing referred to in example 7 of the present invention;
the reference numerals in the figures illustrate:
1. a low coherence light source; 2. a microobjective; 3. a small aperture stop; 4. achromatic collimating objective; 5. a first beam splitting prism; 6. pentagonal prism; 7. a complementary wedge prism set; 701. a first wedge prism; 702. a second wedge prism; 8. a planar mirror; 9. a second beam splitting prism; 10. a measured continuous mirror surface; 11. an imaging lens; 12. a CCD camera; 13. a beam splitting wedge; 14. an optical parallel plate; 15. a linearly polarized light polarizer; 16. a 1/2 lambda plate; 17. a first wedge prism group; 18. a second wedge prism group; 19. a split-two fiber coupler; 20. a first optical fiber collimator; 21. a second fiber collimator; 22. a three-terminal optical fiber circulator; 23. a third fiber collimator; 24. two-in-one optical fiber coupler; 25. and a fourth optical fiber collimating mirror.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments.
Example 1:
the invention provides a continuous mirror surface on-axis distance measuring device based on low-coherence light interference, namely a light path structure of low-coherence light interference.
Light emitted by the low-coherence light source 1 passes through the micro objective lens 2, stray light is filtered out from a light beam convergence point through the aperture diaphragm 3, parallel light is emitted through the achromatic collimating objective lens 4, light reflected by the first beam splitting prism 5 is a reference light beam, and transmitted light is a test light beam. The reference beam is firstly reflected twice by the pentagonal prism 6 during the traveling, then the optical path of the reference beam is regulated to be increased or reduced by the complementary wedge prism group 7, and then the reference beam is reflected by the plane mirror 8 and is transmitted by the second beam splitting prism 9. After the test light beam is transmitted through the second beam splitting prism 9, the test light beam is reflected by each mirror surface of the tested continuous mirror surface 10, and the test light beam reflected by each mirror surface is returned to the second beam splitting prism 9 along the original path and is reflected by the second beam splitting prism 9. The reference beam transmitted by the second beam splitting prism 9 and the test beam reflected by the second beam splitting prism 9 are converged and overlapped to travel on the same path, interference is generated under the condition of equal optical path length of the reference beam and the test beam, the interference fringes are received by the CCD camera 12 after passing through the imaging lens 11, and the interference fringes are received on the CCD receiving surface. The equivalent thickness of the complementary wedge prism group 7 is continuously adjusted, the reference beam and the test beam reflected by each mirror surface of the tested continuous mirror surface 10 are in equal optical path in sequence, and equal optical path interference fringes of low coherent light are sequentially generated, so that the on-axis position positioning of each mirror surface of the tested continuous mirror surface 10 and the measurement of the on-axis distance of each adjacent mirror surface are realized.
Based on the distance measuring device, the principle and the technical scheme for measuring the distance on the continuous mirror surface shaft are as follows:
referring to FIG. 1, a schematic diagram of the distance between successive mirror axes of a low coherence interferometry is shown. As shown in fig. 1, in the illustrated optical path structure, the low-coherence parallel light beam is split into two low-coherence light beams by the first beam splitting prism 5, one of which is a reference light beam and the other of which is a test light beam, and they are converged and overlapped by the second beam splitting prism 9 in preparation for generating low-coherence light interference. The equivalent thickness of the complementary wedge prism group 7 on the reference light path is adjusted to increase or decrease the optical path length of the reference light beam in the single-pass direction, and the low-coherence aplanatic interference fringes of the test light beam and the reference light beam reflected by each mirror surface in the measured continuous mirror surface are sequentially obtained.
Low coherence light interference principle:
the low coherence light with wider wavelength continuous distribution is used as a light source for generating interference, and two paths of light beams (a reference light beam and a test light beam) separated by an amplitude division method can generate stable interference fringes only under the condition of strict aplanatic length. Therefore, the low coherence light interference fringe is the basis for judging the strict equal optical path length of two paths of light. In fig. 1, the optical devices in the optical paths of the reference beam and the test beam are respectively adjusted to be coaxial, the reference beam and the test beam reflected by each mirror surface of the tested continuous mirror surface 10 are respectively transmitted and reflected by the second beam splitting prism 9, and then the two beams are converged and overlapped to be coaxial, and enter the CCD camera 12 after passing through the imaging lens 11. When the measured continuous mirror 10 moves from a position far away from the second beam splitter prism 9 along the axial direction, the aplanatic interference fringes of the low-coherence light generated on the receiving surface of the CCD camera 12 by the test light reflected by the first mirror of the measured continuous mirror 10 and the reference light are observed first. After that, the complementary wedge prism group 7 on the reference light path is continuously adjusted, and the equivalent thickness is increased, so that the test light beams reflected by each mirror surface of the tested continuous mirror surface 10 can be sequentially found, and the test light beams respectively interfere with the aplanatic interference fringes which are generated by the reference light beams on the receiving surface of the CCD camera 12 and generate low-coherence light.
Function of pentagonal prism 6:
the pentagonal prism 6 in the process of reference beam traveling reflects the reference beam secondarily, so as to ensure that the times of reflection of the reference beam and the test beam are both even times (or even times in the example), that is, the reference beam and the test beam after being split in amplitude from the first beam splitting prism 5 are ensured to be converged after passing through the second beam splitting prism 9 in a transmission and reflection mode respectively, the reference beam and the test beam separated at the same point can be overlapped in a one-to-one correspondence mode, which is a necessary condition for generating aplanatic interference in a low-coherence light splitting amplitude mode.
The optical path length adjusting principle of the complementary wedge prism group 7:
the structure of the complementary wedge prism group 7 on the reference beam path in fig. 1 is shown in fig. 2 and 3. Fig. 2 is a schematic diagram of the precise adjustment and measurement of the optical path length of the complementary wedge prism group 7. Fig. 3 is a structural three-dimensional view of the complementary wedge prism group 7. The complementary wedge prism group 7 is composed of a pair of first wedge prisms 701 and second wedge prisms 702 which are made of the same material, have the same wedge angle θ and are small, and can have the same length or different lengths. If the lengths of the two wedge prisms are unequal, the two wedge prisms are longer and are used for long-distance movement, namely the movable wedge prism, so that the optical path difference of the reference beam can be conveniently adjusted and measured in a large range. In fig. 2, it is shown that the second wedge prism 702 is longer than the first wedge prism 701, and the second wedge prism 702 is used for adjustment and displacement measurement. The two wedge prisms of the complementary wedge prism group 7 are placed in complementary positions on the same horizontal plane, the planes of the bevel edges are parallel to each other, and a small space is reserved between the planes, so that the optical structure can be regarded as an equivalent optical parallel plate as a whole. The second wedge prism 702 moves along the direction of the oblique side in the horizontal plane (the paper surface is shown in fig. 2), so that the thickness of the equivalent optical parallel plate is continuously changed, the optical path length of the incident parallel light vertical to the end surface is also changed, but the direction of the emergent light is unchanged, and the parallel light is not laterally moved, so that the arrangement and the debugging of subsequent optical devices are not affected.
The second wedge prism 702, i.e. the right wedge triangle deltaabc, after a certain distance in the direction of hypotenuse BA reaches the new position of the dashed wedge triangle deltaa ' B ' C ' in fig. 2. It can be seen that when the wedge triangle Δabc moves a distance along the hypotenuse direction to reach the new position where Δa 'B' C 'is located, the vertex a where the wedge angle is located moves to a', and the thickness increment of the equivalent parallel flat plate isIn the right triangle ΔA' AN, the apex angle is the wedge angle θ, and +.>Use->Representing the distance the wedge triangle deltaabc (i.e. the second wedge prism 702) moves in the direction of hypotenuse BA,/, is>The thickness increment of the equivalent optical parallel plate is represented by t=xsin θ.
The wedge angle θ of the wedge prism is small, where T is referred to as the longitudinal thickness increment and x is referred to as the approximate lateral displacement for ease of description. As can be seen from the formula t=xsin θ, since θ <90 °, sin θ <1, there is T < x. This illustrates that a large approximate lateral displacement x produces a small longitudinal thickness increment T in linear proportion. It follows that the large approximate lateral measurement error Δx is transmitted to the measurement error Δt of the longitudinal optical parallel plate thickness increment to be linearly reduced, thereby improving the measurement accuracy of the longitudinal equivalent optical parallel plate thickness. The smaller the value of the wedge angle θ, the higher the accuracy. From the concept of optical path, precise adjustment and measurement of the optical path are realized. Since the wedge angle θ of the complementary wedge prism group 7 can be redesigned and changed, the complementary wedge prism group with the corresponding wedge angle θ can be designed according to the precision requirement, thereby meeting the required precision requirement.
Measuring the distance between the continuous mirror surfaces on the axes, and:
referring to fig. 4, fig. 4 is a schematic diagram showing steps of measuring continuous mirror spacing by low coherence interferometry. As shown in fig. 4, the moving process of the movable long wedge prism in the complementary wedge prism group 7 is expressed by a dotted line. The broken line of turning on the complementary wedge prism group 7 indicates that the reflection test light beams of three continuous mirrors of the measured continuous mirror 10 are sequentially found in the moving process of the long wedge prism, and the reflection test light beams sequentially correspond to the positions when the reference light beams passing through the complementary wedge prism group 7 generate aplanatic interference, and specifically comprise the following contents:
the first step: a low-coherence light source 1, a microscope objective 2, a small aperture diaphragm 3, an achromatic collimating objective 4, a first beam splitter prism 5, a pentagonal prism 6, a complementary wedge prism group 7, a plane reflector 8, a second beam splitter prism 9, a measured continuous mirror 10, an imaging lens 11 and a CCD camera 12 are arranged. A low-coherence light source 1, a microscope objective 2, a small aperture diaphragm 3, an achromatic collimating objective 4 which are coaxial and emit parallel light are well adjusted. The parallel light is perpendicularly incident to the first beam splitter prism 5, wherein the transmitted light is a test beam and the reflected light is a reference beam. The test light beam is vertically incident into the second beam splitting prism 9, and the transmitted test light beam enters the tested continuous mirror surface 10, and the coaxiality of all optical devices on the test light path is adjusted. The test light beam is reflected by each mirror surface of the tested continuous mirror surface 10 and returns along the original path, and then is reflected by the second beam splitting prism 9 and enters the CCD camera 12 through the imaging lens 11, and the imaging lens 11 and the CCD camera 12 are adjusted to be coaxial on the test light path. The reference beam reflected by the first beam splitting prism 5 is reflected by the pentagonal prism 6, vertically passes through the complementary wedge prism group 7, is reflected by the plane reflecting mirror 8, vertically enters the second beam splitting prism 9, and is converged and overlapped with each test beam reflected by the second beam splitting prism 9 after being transmitted by the second beam splitting prism 9, and each optical device is adjusted to be coaxial on the reference light path. The overlapping reference beam and test beam pass through the imaging lens 11 and enter the CCD camera 12, and are coaxial.
And a second step of: the equivalent thickness of the complementary wedge prism group 7 on the reference beam is adjusted, i.e. the movable wedge prism of the complementary wedge prism group 7 is moved in the hypotenuse direction in the horizontal plane (fig. 2 shows the paper) at least to allow light rays to pass through a sufficiently large range centered on the axis in the cross section of the reference beam. The position of the measured continuous mirror 10 is adjusted in the axial direction on the test light path until the aplanatic interference fringes of the low-coherence light of the reflected test light beam of the first mirror of the measured continuous mirror 10 and the reference light beam are observed on the receiving surface of the CCD camera 12. The position reading x of the movable wedge prism in the complementary wedge prism group 7 at this time in the direction of its hypotenuse is recorded 1 。
And a third step of: continuously adjusting the equivalent thickness of the complementary wedge prism group 7 on the reference light path, and sequentially observing the reflection measurement of the test light beam by the second mirror surface, the third mirror surface and the subsequent mirror surfaces of the tested continuous mirror surface 10 respectivelyInterference fringes of low-coherence light generated by the test beam and the reference beam, and position readings x of movable wedge prisms in the complementary wedge prism group 7 along the hypotenuse direction of the movable wedge prisms when observing the interference fringes of each equal optical path 2 ，x 3 …。
Based on the above operation flow, the calculation flow of the measurement result of the mirror spacing on the axis of the measured continuous mirror 10 is as follows:
as known from the interference principle of low-coherence light, the complementary wedge prism group 7 in the reference light path is used as an optical parallel plate with equivalent variable thickness, and the increased thickness replaces air with corresponding thickness, so that the optical path of the reference light beam is increased, low-coherence light interference fringes are generated twice continuously, and the optical path increment of the reference light beam is equal to the optical path difference of the corresponding two adjacent mirror reflection test light beams of the measured continuous mirror 10. If the continuous mirror surface is the mirror surface of the coaxial lens group, the optical path difference of the reflected test light beam is the optical path difference of the on-axis light.
Let the refractive index of air be n 0 The refractive index of the glass material between adjacent ones of the measured continuous mirrors 10 is n i Mirror spacing on axis t i The refractive index of the glass material of the complementary wedge prism group 7 is n p In the process that the wedge angle of the prism of the wedge prism group is theta and the movable wedge prism moves along the direction of the hypotenuse, the position readings of two adjacent stripes are respectively x i ，x i+1 . The optical path difference of the test light beams reflected by two adjacent mirrors of the measured continuous mirror surface 10 is 2n i t i The reference beam is increased in optical path length in the single-pass direction due to the movement of the movable wedge prism of the wedge prism group by an amount (n) p -n 0 )(x i+1 -x i ) sin theta, based on the aplanatic interference condition of low-coherence light, has
2n i t i ＝(n p -n 0 )(x i+1 -x i )sinθ
Obtaining the distance t between the mirror surfaces on the shaft i Is that
Its measurement error deltat i Is that
Δx in the above i ，Δx i+1 Is the measurement error when the movable wedge prism of the complementary wedge prism group 7 moves in the hypotenuse direction.
Example 2:
based on the content of example 1, the above-mentioned measured continuous mirror surface on-axis pitch measurement accuracy was analyzed:
the refractive index of the glass material is generally between 1.4 and 1.7, the refractive index of air is about 1, the term value containing the refractive index and the constant 1/2 in the error calculation formula is about 1/6, and the prism wedge angle theta < <90 DEG in the wedge-shaped prism group can be known that sin theta < <1, and the measurement error transmission generated as a result is linearly reduced. The error calculation formula shows that the error of the distance on the mirror axis of the measuring surface caused by the displacement error measured by the measuring ruler driving the movable wedge prism to move in the complementary wedge prism group 7 is greatly reduced. In addition, smaller measurement errors can be obtained by designing smaller prism wedge angles θ. Obviously, under the condition that other conditions are the same, the measurement error value of the distance on the mirror surface axis caused by the optical path difference of the single-pass direction adjustment and the measurement reference beam is 1/2 of that of the double-pass bidirectional adjustment and the measurement, and the measurement precision is doubled.
Example 3:
based on the embodiment 1-2, the measurement precision of the continuous mirror surface on-axis spacing measuring device and method based on low coherence light interference provided by the invention is verified by a specific example:
assuming that the accuracy of the movement measurement rule of the movable wedge prism in the complementary wedge prism group 7 is + -1 μm, x is i And x i+1 The sum of errors of (2) μm. A certain LED low coherence light is used as a light source with its center wavelength λ=680 nm. The refractive indices of the glass materials K9 and QK2 for red light 656.27nm are 1.51390 and 1.47590, respectively. The wavelengths of 680nm and 656.27nm are not very different, and can be approximately considered as the aboveThe refractive index of the glass material is also 680nm for red light.
Table 1 shows an example of measurement accuracy analysis. The method is characterized in that the prism material of the complementary wedge-shaped prism group 7 is K9, the materials between two mirror surfaces of the measured continuous mirror surface 10 are respectively K9 and QK2, and the measurement precision is realized when the wedge angle value of the edge angle of the complementary wedge-shaped prism group 7 is respectively 10 degrees, 5 degrees and 3 degrees.
TABLE 1 measurement accuracy analysis example (refractive index of air at ordinary temperature and pressure is n) 0 ＝1.000273)
As can be seen from table 1, the smaller the prism wedge angle of the complementary wedge prism group 7, the higher the measurement accuracy of the continuous mirror pitch. The wedge angle design of the wedge prism can be changed, so that the measurement precision of the continuous mirror surface distance is within the range meeting the precision requirement, for example, the measurement precision is required to be within 20nm, and when the prism material of the complementary wedge prism group 7 and the material of the measured continuous mirror surface 10 are both K9, the wedge angle theta is smaller than 3 degrees, so that the requirement is met.
Example 4:
based on embodiment 1, an apparatus and a method for obtaining a low coherence light interference fringe with high contrast are provided:
when the low-coherence light beam splitting mode generates a reference beam and a test beam which participate in interference, the first beam splitting prism is a common cubic beam splitting prism (BS) which plays a role in splitting amplitude, and the intensities of the separated reference beam and test beam are 5:5, namely the intensities of the two beams are the same. However, after the test beam enters the measured continuous mirror 10, the test beam is reflected by each mirror of the measured continuous mirror 10 in turn, and transmitted by each mirror in turn, and finally, the test beam reflected by each mirror interferes with the reference beam to generate low coherence light, so that the intensity of the test beam reflected by each mirror is gradually reduced compared with that of the reference beam, and especially, the intensity of the test beam reflected by the later mirror is weaker. According to the optical interference principle, when the intensity (or amplitude) of two beams of light participating in interference is 1:1, the contrast of interference fringes is the best; the greater their intensity (or amplitude) phase difference, the poorer the contrast of the interference fringes, and even the less the contrast of the interference fringes that appear, is insufficient for observation and measurement analysis. In order to solve the above problem, the reference beam may be subjected to an intensity attenuation process to attenuate the intensity to an appropriate relative value. One method is to have the planar mirror 8 of fig. 1 be replaced with a split wedge 13, such as the split wedge 13 of fig. 5. The beam splitting wedge 13 reflects the reference beam on the one hand and transmits the reference beam on the other hand, and after transmitting a large part of the light intensity, the beam splitting wedge 13 with a suitable transmission and reflection ratio (T: R) can be selected so that the reference beam is attenuated to a suitable intensity, for example, T: r=9:1.
Because the wavelength distribution of the low-coherence light is wider, the transmission optical devices of the reference beam and the test beam separated in the low-coherence light amplitude division mode on the respective optical paths can generate dispersion on the passed light beams, and the different dispersion degrees can also influence the contrast ratio of interference fringes. If the degree of dispersion of the two beams is larger, the contrast of interference fringes is lower, which is unfavorable for observation and measurement analysis. In order to make the dispersion degree of the reference beam and the test beam as consistent as possible, the glass path and the air path of the reference beam transmitted in the optical path of the reference beam are respectively equal to or nearly equal to the corresponding path of the test beam in the optical path of the test beam. When the optical path is arranged, the geometrical path of the reference beam is equivalent to that of the test beam, and the reference beam has a path in the optical device glass which is approximately (2+1.414) D-d= 2.414D more than that of the test beam reflected by the first mirror surface of the tested continuous mirror surface 10 due to the twice reflection of the pentagonal prism 6, wherein D is the maximum light transmission aperture of the first beam splitter prism 5 and the second beam splitter prism 9, and is also the maximum light transmission aperture of the pentagonal prism 6. In addition, the optical path of the reference beam is provided with a complementary wedge prism group for adjusting and measuring the optical path difference, and the complementary wedge prism group has an initial equivalent thickness. To solve the problem of unequal dispersion of the reference beam and the test beam, an optical parallel plate with proper dispersion property and refractive index material can be placed in the optical path of the test beam, so that the test beam can vertically pass through. The thickness is about the initial equivalent thickness of the complementary wedge prism set plus 2.414D. Such as the optical parallel plate 14 in fig. 5.
Example 5:
based on embodiment 1, an apparatus and a method two for obtaining a low coherence light interference fringe of high contrast:
in order to obtain good contrast of low-coherence light interference fringes, the light intensity ratio of the initial reference beam to the test beam can be reduced, and a polarized light interference method can be adopted. Fig. 6 is a device diagram of the present embodiment. Based on the device of example 1, the ordinary cubic beam splitter prism (BS) as the first beam splitter prism 5 is replaced by a polarizing beam splitter Prism (PBS), while the second beam splitter prism 9 is still an ordinary cubic beam splitter prism (BS), a linear polarization polarizer 15 is placed in front of the first beam splitter prism 5 (PBS), a 1/2 lambda plate 16 is placed on the optical path of the test beam after the first beam splitter prism 5 (PBS), and an optical parallel plate 14 for balancing dispersion is also placed.
Based on the device after the above change, the principle of reducing the light intensity ratio of the initial reference beam with respect to the test beam is as follows:
after passing through the linear polarization polarizer 15, the low-coherence parallel light beam generates linear polarization light, which can be mathematically decomposed into linear polarization light having a vibration electric vector parallel to the incident surface of the first beam splitting prism 5 (PBS) and linear polarization light having a vibration electric vector perpendicular to the incident surface. The first beam splitting prism 5 is a polarization beam splitting Prism (PBS) that transmits only linearly polarized light whose vibration electric vector is parallel to the incident plane, and reflects only linearly polarized light whose vibration electric vector is perpendicular to the incident plane. Rotating the linearly polarized light polarizer 15 about the axis of the parallel light beam changes the amplitude ratio of the linearly polarized light transmitted by the first beam splitting prism 5 and the linearly polarized light reflected, that is, changes the light intensity ratio of the transmitted linearly polarized light as the test light beam and the linearly polarized light reflected as the reference light beam. Rotating the linear polarizer 15 to a proper angular position, the ratio of the intensity of the reference beam to the intensity of the test beam can satisfy the condition that the contrast of the subsequent interference fringes is optimal. The 1/2 lambda plate 16 on the optical path of the test beam plays a role in rotating the vibration direction of the linearly polarized light (electric vector direction), and the 1/2 lambda plate 16 is rotated to a proper angle position around the axis of the test beam, so that the vibration direction of the linearly polarized light (electric vector direction) as the test beam is rotated by 90 DEG, and thus the vibration direction of the linearly polarized light of the subsequent test beam is the same as that of the linearly polarized light of the reference beam, and interference can be generated in the future.
Example 6:
based on example 4, an apparatus and method for extending the continuous mirror pitch measurement range:
fig. 7 is a schematic diagram of an apparatus for expanding the measurement range of the continuous mirror pitch, which is based on embodiment 4, an identical complementary wedge prism group is added on the optical path of the reference beam, and two wedge prism groups are identical, forming cascaded complementary wedge prism groups 17 and 18, so that the measurement range of the mirror pitch can be doubled, and although the measurement error is 2 times of that of a single complementary wedge prism group 7, the measurement error can be partially reduced by reducing the design of the wedge angle of the prisms, thereby meeting the requirement of measurement accuracy.
Example 7:
based on example 1, apparatus and method for interferometry of continuous specular on-axis spacing with low coherence light of optical fibers and fiber devices:
fig. 8 is a schematic diagram of an apparatus for interferometry of continuous specular on-axis spacing with low coherence light in fiber and fiber optics based on example 1. The device comprises a low-coherence light source 1, a split two-fiber coupler 19, a first fiber collimating lens 20, a complementary wedge prism group 7, a plane reflecting mirror 8, a second fiber collimating lens 21, a three-terminal fiber ring 22, a third fiber collimating lens 23, a measured continuous mirror 10, a two-in-one fiber coupler 24, a fourth fiber collimating lens 25, an imaging lens 11, a CCD camera 12 and optical fibers on the light path.
The reference beam and the test beam are generated by a one-to-two optical fiber coupler 19 with an intensity distribution ratio of 1:9, one path passing through the complementary wedge prism group 7 is the reference beam (intensity ratio is 1/10), and the other path passing through the three-terminal optical fiber circulator 22 is the test beam (intensity ratio is 9/10). The reference beam is firstly converted into a parallel beam by the first optical fiber collimating mirror 20, then vertically passes through the complementary wedge-shaped prism group 7, is then reflected by the plane reflecting mirror 8, enters the optical fiber by the second optical fiber collimating mirror 21 to continue transmission, and finally enters the input end of the two-in-one optical fiber coupler 24. The test light beam firstly enters the three-end optical fiber circulator 22, light output by the corresponding output end passes through a section of optical fiber, then is converted into parallel light beams by the third optical fiber collimator 23, the parallel light beams coaxially enter the tested continuous mirror surface 10, the parallel light beams are reflected by each mirror surface of the tested continuous mirror surface 10, the reflected light of each mirror surface returns along the original path, the reflected light returns to the three-end optical fiber circulator 22 after being coupled by the third optical fiber collimator 23, the light is output by the corresponding output end of the three-end optical fiber circulator 22, and finally enters the input end of the two-in-one optical fiber coupler 24. The reference beam and the test beam enter the two-in-one optical fiber coupler 24 together, after being coupled and overlapped, the reference beam enters the fourth optical fiber collimating lens 25 from the output end of the optical fiber coupler, the collimated beam enters the CCD camera 12 after passing through the imaging lens 11, and interference fringes are received on the receiving surface of the CCD camera 12. And selecting proper lengths of the optical fibers according to the equal optical path interference condition of low-coherence light interference, namely the principle that the optical paths of the reference beam and the test beam are equal, and arranging the optical paths of the reference beam and the test beam. The purpose of the plane mirror 8 here is to have the reference beam reflected once, since the test beam involved in interference is reflected once by a certain mirror of the successive mirror 10 under test, so that the number of reflections of the low-coherence reference beam and the test beam is the same, which is one of the requirements for the generation of interference fringes.
The method for measuring the continuous mirror spacing is as follows:
the first step: adjusting the first optical fiber collimator lens 20 to emit parallel light beams; the plane reflector 8 and the second optical fiber collimating mirror 21 are adjusted to be coaxial with the first optical fiber collimating mirror 20, and the parallel light beams are coupled into the subsequent optical fibers; the complementary wedge prism group 7 is arranged to make the parallel beam vertically incident and pass through, and the equivalent thickness is adjusted to make the light beam pass through in a large enough range with the axis as the center in the cross section of the reference beam. Adjusting a third optical fiber collimating mirror 23 to emit parallel light beams; the measured continuous mirror surface 10 is adjusted to be coaxial with the third fiber collimator lens 23. Adjusting a fourth optical fiber collimating mirror 25 to emit parallel light beams; the adjusting imaging lens 11 is coaxial with the fourth fiber collimator lens 25 and the CCD camera 12 is also coaxial with them.
And a second step of: the position of the measured continuous mirror 10 is adjusted in the axial direction on the test light path until the aplanatic interference fringes of the low-coherence light of the reflected test light beam of the first mirror of the measured continuous mirror 10 and the reference light beam are observed on the receiving surface of the CCD camera 12. The position reading x of the movable wedge prism in the complementary wedge prism group 7 at this time in the direction of its hypotenuse is recorded 1 。
And a third step of: continuously adjusting equivalent thickness of the complementary wedge prism group 7 on the reference light path, sequentially observing interference fringes of low-coherence light generated by test light beams respectively reflected by the second mirror surface, the third mirror surface and the subsequent mirror surfaces of the tested continuous mirror surface 10 and the reference light beam, and recording position readings x of movable wedge prisms in the complementary wedge prism group 7 along the hypotenuse direction of the movable wedge prisms when the observed interference fringes are observed 2 ，x 3 …。
Based on the above operation flow, the calculation flow of the measurement result of the distance between the axes of the measured continuous mirror surfaces is as follows:
as known from the interference principle of low-coherence light, the complementary wedge prism group 7 in the reference light path is used as an optical parallel plate with equivalent variable thickness, and the increased thickness replaces air with corresponding thickness, so that the optical path of the reference light beam is increased, low-coherence light interference fringes are generated twice continuously, and the optical path increment of the reference light beam is equal to the optical path difference of the corresponding two adjacent mirror reflection test light beams of the measured continuous mirror 10. If the continuous mirror surface is the mirror surface of the coaxial lens group, the optical path difference of the reflected test light beam is the optical path difference of the on-axis light.
Let the refractive index of air be n 0 The refractive index of the glass material between adjacent ones of the measured continuous mirrors 10 is n i Mirror spacing on axis t i The refractive index of the glass material of the complementary wedge prism group 7 is n p In the process that the wedge angle of the wedge prism group is theta and the movable wedge prism moves along the hypotenuse direction, the positions of adjacent stripes are readRespectively x i ，x i+1 . The optical path difference of the test light beams reflected by two adjacent mirrors of the measured continuous mirror surface 10 is 2n i t i The reference beam is increased in optical path length in the single-pass direction due to the movement of the movable wedge prism of the wedge prism group by an amount (n) p -n 0 )(x i+1 -x i ) sin theta, based on the interference condition of the aplanatic of the low-coherence light, has
2n i t i ＝(n p -n 0 )(x i+1 -x i )sinθ
Obtaining the distance t between the mirror surfaces on the shaft i Is that
Its measurement error deltat i Is that
Δx in the above i ，Δx i+1 Is the position measurement error when the movable wedge prism of the complementary wedge prism group 7 moves in the direction of the prism hypotenuse.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should make equivalent substitutions or modifications according to the technical scheme of the present invention and the inventive concept thereof, and should be covered by the scope of the present invention.
Claims (3)
1. The method is characterized by being realized by a continuous mirror surface on-axis spacing measuring device based on low-coherence light interference, the device comprises a low-coherence light source (1), a microscope objective (2), a small aperture diaphragm (3), an achromatic collimating objective (4), a first beam splitting prism (5), a pentagonal prism (6), a complementary wedge prism group (7), a plane reflecting mirror (8), a second beam splitting prism (9), a measured continuous mirror surface (10), an imaging lens (11) and a CCD camera (12), and the method specifically comprises the following steps based on the measuring device:
s1, finishing the whole arrangement and installation of the measuring device, firstly adjusting a low-coherence light source (1), a micro objective lens (2), a small aperture diaphragm (3) and an achromatic collimating objective lens (4) to be coaxial, filtering stray light at a beam convergence point through the small aperture diaphragm (3) after a light beam emitted by the low-coherence light source (1) passes through the micro objective lens (2), and then emitting parallel light through the achromatic collimating objective lens (4);
s2, adjusting the first beam splitting prism (5) so that the parallel light vertically enters the first beam splitting prism (5) to separate a low-coherence reflected light beam and a transmitted light beam, wherein the reflected light beam travels above and is a reference light beam; the transmitted beam is the test beam;
s3, adjusting the pentagonal prism (6), the complementary wedge-shaped prism group (7), the plane reflecting mirror (8) and the second beam splitting prism (9) so that the reference beam passes through the complementary wedge-shaped prism group (7), is reflected by the plane reflecting mirror (8) and then passes through the second beam splitting prism (9) in a transmission mode after being reflected by the pentagonal prism (6);
s4, adjusting the coaxial of the tested continuous mirror surface (10) and the optical path where the test light beam is located, enabling the test light beam to pass through the second beam splitting prism (9) in a transmission mode, enabling the test light beam to enter the tested continuous mirror surface (10) at a tested interval, enabling all the mirror surfaces of the tested continuous mirror surface (10) at the tested interval to sequentially reflect the test light beam, enabling all the reflected light beams to return to the second beam splitting prism (9) along the original path, and enabling the reflected light beams to coaxially overlap with the reference light beam after being reflected by the second beam splitting prism (9);
s5, adjusting the equivalent thickness of the complementary wedge-shaped prism group (7) on the reference light path, at least enabling light rays in a large enough range taking an axis as a center in the cross section of the reference light beam to pass through, and adjusting the position of the measured continuous mirror surface (10) along the axial direction until the fact that the test light beam reflected by the first mirror surface of the measured continuous mirror surface (10) and the reference light beam are converged and overlapped and pass through the imaging lens (11) together is observed, and then an aplanatic interference fringe of low-coherence light is generated on the receiving surface of the CCD camera (12);
s6, recording position readings of the movable wedge prisms in the complementary wedge prism group (7) along the hypotenuse direction of the movable wedge prisms when the aplanatic interference fringes are observed in S5, continuously adjusting the equivalent thickness of the complementary wedge prism group (7) on the reference light path, sequentially observing interference fringes of low-coherence light generated by test light beams and reference light beams reflected by each subsequent mirror surface of the detected continuous mirror surface (10), and recording position readings of the movable wedge prisms in the complementary wedge prism group (7) along the hypotenuse direction of the movable wedge prisms when the observed aplanatic interference fringes are observed;
and S7, calculating the on-axis distance and measurement error of the measured continuous mirror surface (10) according to the position reading data obtained in the S6 and by combining the interference principle of low-coherence light.
2. The method for measuring the distance between continuous mirror surfaces on an axis based on low coherence light interference according to claim 1, wherein the complementary wedge prism group (7) comprises a first wedge prism (701) and a second wedge prism (702), the first wedge prism (701) and the second wedge prism (702) are made of the same material and have the same wedge angle, the first wedge prism (701) and the second wedge prism (702) are placed in complementary positions on the same horizontal plane, the planes of the bevel edges are parallel to each other, a small interval exists between the planes, and the second wedge prism (702) is a movable wedge prism capable of moving in the horizontal plane along the bevel edge direction of the movable wedge prism to measure displacement.
3. The method for measuring the distance between the axes of the continuous mirrors based on the interference of low coherence light according to claim 2, wherein the calculation formula of the distance between the axes of the continuous mirrors to be measured in S7 is:
2n i t i ＝(n p -n 0 )(x i+1 -x i )sinθ (1)
(1) Wherein n is i Representing the refractive index of the material between two adjacent mirrors of the measured continuous mirror; t is t i Representing the axial distance between two adjacent mirrors of the measured continuous mirror; n is n p A refractive index of a glass material representing the complementary wedge prism group (7); n is n 0 Representing the refractive index of air; x is x i+1 、x i Representation corresponds to two adjacent times of low coherenceThe position of the movable wedge prism of the interference fringe of light in the direction of the hypotenuse thereof is read; θ represents the prism wedge angle of the complementary wedge prism group (7), i.e., the wedge angles of the first wedge prism (701) and the second wedge prism (702);
and (3) rewriting the (1) to obtain a calculation formula of the distance between the adjacent two mirror surfaces on the shaft:
combining (2), obtaining a measurement error calculation formula of the distance between two adjacent mirror surfaces by a measurement error theory, wherein the calculation formula is as follows:
(3) Wherein Deltax is i+1 ，Δx i A second wedge prism (702) representing a complementary wedge prism group (7), i.e. a position measurement error when moving in the direction of its hypotenuse.
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