CN117006961B - Device and method for measuring distance between continuous mirror surfaces on axis based on low-coherence light interference - Google Patents

Abstract

The present invention discloses a device and method for measuring the on-axis spacing of continuous mirrors based on low-coherence light interference, and belongs to the field of optical precision measurement technology; the present invention proposes a device for measuring the on-axis spacing of continuous mirrors based on low-coherence light interference, and designs a spacing measurement method matching the device based on the device, uses a complementary wedge-shaped prism group equivalent to an optical parallel plate with adjustable thickness, accurately adjusts the optical path difference of the reference light beam participating in the interference in one-way direction, makes the test light beam longitudinally scan each mirror surface of the measured continuous mirror surface in sequence, measures the on-axis optical path difference between each continuous mirror surface, and obtains the on-axis spacing between each adjacent mirror surface in the measured continuous mirror surface. The present invention has a simple structure, low implementation cost, convenient operation, and can effectively improve the accuracy of the mirror spacing measurement.

Description

Continuous mirror on-axis spacing measurement device and method based on low-coherence light interference

Technical Field

The invention relates to the technical field of optical precision measurement, and in particular to a device and method for measuring the on-axis spacing of continuous mirrors based on low-coherence light interference.

Background Art

In optical workshops or laboratories, in order to avoid mechanical damage to the measured object, non-contact physical measurement methods are used to measure the spacing on the central axis of continuous mirrors, such as: image method, image calibration method, axial dispersion method, confocal method, differential confocal method, low-coherence light interferometry, Fizeau interferometry and polarization interferometry. They all use test light to obtain surface position information through mirror reflection, thereby realizing the measurement of continuous mirror spacing.

Low-coherence light interferometry is the best method for measuring the distance between continuous mirrors. It uses more complex auxiliary facilities and spectral or data processing technology, and its measurement accuracy can reach 600nm. By improving the auxiliary facilities and data processing methods, such as using a high-precision grating ruler to measure the moving distance, combined with effective data processing methods, the measurement accuracy of low-coherence light interferometry can be increased to 200nm.

In view of the fact that low-coherence measurement methods require complex auxiliary facilities and data processing to improve the accuracy to a limited extent, optical mirror processing or continuous mirror assembly requires a non-contact, low-cost, easy-to-operate, and especially highly accurate mirror spacing measurement method. In order to solve the above problems, the present invention proposes a continuous mirror axis spacing measurement device and method based on low-coherence light interference.

Summary of the invention

The object of the present invention is to provide a device and method for measuring the on-axis spacing of continuous mirrors based on low-coherence light interference to solve the technical problems raised in the background technology.

In order to achieve the above object, the present invention adopts the following technical solutions:

A device for measuring the on-axis spacing of continuous mirror surfaces based on low-coherence light interference comprises a low-coherence light source, a microscope objective lens, a pinhole diaphragm, an achromatic collimating objective lens, a first beam splitter prism, a pentagonal prism, a complementary wedge prism group, a plane reflector, a second beam splitter prism, a continuous mirror surface to be measured, an imaging lens, and a CCD camera.

Based on the above measurement device, a method for measuring the on-axis spacing of a continuous mirror based on low-coherence light interference is proposed, and the method specifically includes the following contents:

S1. Complete the overall arrangement and installation of the measuring device. First, adjust the low-coherence light source, the microscope objective lens, the pinhole diaphragm, and the achromatic collimating objective lens to be coaxial. The light beam emitted by the low-coherence light source passes through the microscope objective lens, and then the pinhole diaphragm is used to filter out stray light at the beam convergence point, and then the achromatic collimating objective lens emits parallel light.

S2, adjusting the first beam splitter prism so that the parallel light is incident vertically on the first beam splitter prism, and separating the low-coherence reflected light beam and the transmitted light beam, wherein the reflected light beam travels upward and is the reference light beam; and the transmitted light beam is the test light beam;

S3, adjusting the penta prism, the complementary wedge prism group, the plane reflector, and the second beam splitter prism so that the reference beam is reflected by the penta prism, passes through the complementary wedge prism group, is reflected by the plane reflector, and then passes through the second beam splitter prism in a transmission manner;

S4, adjusting the optical path of the continuous mirror to be tested and the test beam to be coaxial, the test beam passes through the second beam splitter prism in a transmission mode, and then enters the continuous mirror of the measured interval, each mirror of the continuous mirror of the measured interval reflects the test beam in sequence, and each reflected beam returns to the second beam splitter prism along the original path, and after being reflected by the second beam splitter prism, overlaps with the reference beam coaxially;

S5, adjusting the equivalent thickness of the complementary wedge prism group on the reference light path, so that at least a sufficiently large range of light with the axis as the center in the cross section of the reference beam passes through, and then adjusting the position of the continuous mirror surface to be measured along the axis direction, until it is observed that the test beam reflected by the first mirror surface of the continuous mirror surface to be measured and the reference beam meet and overlap and pass through the imaging lens together, and generate equal optical path interference fringes of low coherence light on the receiving surface of the CCD camera;

S6, recording the position reading of the movable wedge prism in the complementary wedge prism set along the hypotenuse direction when the equal optical path interference fringes are observed in S5, then continuously adjusting the equivalent thickness of the complementary wedge prism set on the reference light path, sequentially observing the interference fringes of the low-coherence light generated by the test light beam reflected by each subsequent mirror surface of the measured continuous mirror surface and the reference light beam, and recording the position reading of the movable wedge prism in the complementary wedge prism set along the hypotenuse direction when each equal optical path interference fringes is observed;

S7. Based on the position reading data obtained in S6 and in combination with the interference principle of low-coherence light, the on-axis spacing and measurement error of the measured continuous mirror are calculated.

Preferably, the complementary wedge-shaped prism group includes a first wedge-shaped prism and a second wedge-shaped prism, the first wedge-shaped prism and the second wedge-shaped prism are made of the same material and have the same wedge angle, the first wedge-shaped prism and the second wedge-shaped prism are placed in complementary positions on the same horizontal plane, the surfaces where the hypotenuses are located are parallel to each other, and there is a small gap between the surfaces, and the second wedge-shaped prism is a movable wedge-shaped prism that can move along the hypotenuse direction in the horizontal plane to measure displacement.

Preferably, the calculation formula for the measured on-axis spacing of the continuous mirror surfaces in S7 is:

2n _i t _i =(n _p -n ₀ )(x _i+1 -x _i )sinθ (1)

(1) In the formula, n _i represents the refractive index of the material between two adjacent continuous mirror surfaces to be measured; _ti represents the on-axis distance between two adjacent continuous mirror surfaces to be measured; n _p represents the refractive index of the glass material of the complementary wedge prism group; n ₀ represents the refractive index of air; x _i+1 and x _i represent the position readings of the movable wedge prism in the hypotenuse direction corresponding to the interference fringes of two adjacent low-coherence lights; θ represents the prism wedge angle of the complementary wedge prism group, that is, the wedge angle of the first wedge prism and the second wedge prism;

Rewriting formula (1) yields the following formula for calculating the distance between two adjacent mirror axes:

Combined with formula (2), the measurement error calculation formula for the distance between two adjacent mirror axes can be obtained from the measurement error theory:

In the formula (3), Δxi ₊₁ and _Δxi represent the position measurement error of the second wedge prism of the complementary wedge prism set, that is, the movable wedge prism, when it moves along the hypotenuse direction.

Compared with the prior art, the present invention provides a continuous mirror axis spacing measurement device and method based on low-coherence light interference, which has the following beneficial effects:

(1) The present invention measures the distance between continuous mirror axes in a non-contact and non-destructive manner, and is applicable to both optical lens processing workshops and lens assembly and debugging workshops;

(2) The present invention uses an equal optical path interference method with a low coherence light source with a wide spectrum to measure positioning, which has a sensitive response and good accuracy, and the interference pattern data of positioning is suitable for automated analysis and processing;

(3) The complementary wedge-shaped prism set of the present invention is easy to manufacture and process, and the prism wedge angle of the complementary wedge-shaped prism set can be designed according to the requirements of measurement accuracy. The accuracy of measuring the on-axis spacing of continuous mirror surfaces can be easily controlled within 20 nm.

(4) The present invention uses a one-way direction adjustment and measurement of the optical path difference in the optical path, and the measurement accuracy is significantly better than the accuracy of the two-way direction adjustment and measurement;

(5) The measuring movement direction of the present invention is a lateral movement nearly perpendicular to the optical axis direction, so that the coherent light beam can scan the position of the measured mirror in the longitudinal direction, that is, the longitudinal scanning is changed to the lateral scanning, and the interferometer does not need to be extended or moved as a whole during the measurement process;

(6) The present invention uses a low-precision displacement mechanism to obtain high-precision displacement adjustment, thereby achieving high-precision adjustment and measurement of optical path difference;

(7) The optical path compensation method of the present invention can realize the measurement of large spacing between continuous mirrors. Large spacing measurement can be achieved by cascading multiple wedge prism groups to compensate for the insufficient thickness adjustment and measurement range of a single wedge prism group. The measurement of the spacing between continuous mirrors is less restricted and the measurement thickness range is large, ranging from 1 μm to 100 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the principle of measuring the distance between continuous mirror surfaces by low coherence light interference mentioned in Example 1 of the present invention;

FIG2 is a cross-sectional view of the optical structure of a complementary wedge-shaped prism assembly for precise adjustment and measurement of optical path difference mentioned in Example 1 of the present invention;

FIG3 is a three-dimensional view of the optical structure of a complementary wedge-shaped prism assembly for precise adjustment and measurement of optical path difference mentioned in Example 1 of the present invention;

FIG. 4 is a schematic diagram of the steps of measuring the continuous mirror spacing by low coherence light interference mentioned in Example 1 of the present invention.

FIG5 is a light path diagram of low coherence light interference measurement of continuous mirror spacing mentioned in Example 4 of the present invention;

FIG6 is a light path diagram of low coherence light interference measurement of continuous mirror spacing mentioned in Example 5 of the present invention;

FIG7 is a light path diagram of low coherence light interference measurement of continuous mirror spacing mentioned in Example 6 of the present invention;

FIG8 is a light path diagram of low coherence light interference measurement of continuous mirror spacing mentioned in Example 7 of the present invention;

Description of the numbers in the figure:

1. Low coherence light source; 2. Microscope objective lens; 3. Pinhole diaphragm; 4. Achromatic collimating objective lens; 5. First beam splitter prism; 6. Penta prism; 7. Complementary wedge prism group; 701. First wedge prism; 702. Second wedge prism; 8. Plane reflector; 9. Second beam splitter prism; 10. Measured continuous mirror; 11. Imaging lens; 12. CCD camera; 13. Beam splitter wedge; 14. Optical parallel plate; 15. Linear polarized light polarizer; 16. 1/2λ wave plate; 17. First wedge prism group; 18. Second wedge prism group; 19. One-to-two fiber coupler; 20. First fiber collimator; 21. Second fiber collimator; 22. Three-end fiber circulator; 23. Third fiber collimator; 24. Two-in-one fiber coupler; 25. Fourth fiber collimator.

DETAILED DESCRIPTION

The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

Embodiment 1:

The present invention proposes a device for measuring the on-axis spacing of continuous mirrors based on low-coherence light interference, that is, an optical path structure of low-coherence light interference, which uses a complementary wedge-shaped prism group equivalent to an optical parallel plate with adjustable thickness to precisely adjust the single-path optical path difference of a reference light beam participating in the interference, so that the test light beam longitudinally scans each mirror surface of the continuous mirror surface to be measured, measures the on-axis optical path difference between each adjacent mirror surface, and obtains the on-axis spacing of each adjacent mirror surface in the measured continuous mirror surface.

The light emitted by the low-coherence light source 1 passes through the microscope objective 2, and then passes through the pinhole diaphragm 3 at the beam convergence point to filter out stray light, and then passes through the achromatic collimating objective 4 to emit parallel light. The light reflected by the first beam splitter prism 5 is the reference beam, and the transmitted light is the test beam. During the travel of the reference beam, it is first reflected twice by the pentagonal prism 6, and then adjusted by the complementary wedge prism group 7 to increase or decrease its optical path, and then reflected by the plane reflector 8, and then transmitted by the second beam splitter prism 9. After the test beam is transmitted by the second beam splitter prism 9, it passes through each mirror surface of the continuous mirror surface 10 to be tested and is also reflected by each mirror surface. The test beam reflected by each mirror surface returns to the second beam splitter prism 9 along the original path and is reflected by the second beam splitter prism 9. The reference beam transmitted by the second beam splitter prism 9 and the test beam reflected by the second beam splitter prism 9 meet and overlap and travel on the same path, and generate interference under the condition that the reference beam and the test beam are equal in optical path. After passing through the imaging lens 11, they are received by the CCD camera 12, and interference fringes are received on the CCD receiving surface. The equivalent thickness of the complementary wedge prism group 7 is continuously adjusted, and the reference beam is equal in optical path with the test beam reflected by each mirror surface of the measured continuous mirror surface 10 in turn, and equal in optical path interference fringes of low coherence light are generated in turn, so as to realize the on-axis position positioning of each mirror surface of the measured continuous mirror surface 10 and the measurement of the on-axis spacing between each adjacent mirror surface.

Based on the above spacing measurement device, the measurement principle and technical solution of the continuous mirror axis spacing are as follows:

Please refer to FIG1, which is a schematic diagram of the principle of measuring the on-axis spacing of continuous mirrors by low-coherence light interference. As shown in FIG1, in the optical path structure shown in the figure, the low-coherence parallel light beam is separated into two low-coherence light beams by the first beam splitter prism 5, one of which is a reference beam and the other is a test beam, which are combined and overlapped by the second beam splitter prism 9 to generate 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 of the reference beam in the single-path direction, and the low-coherence equal-path interference fringes of the test beam and the reference beam reflected by each mirror in the measured continuous mirror are obtained in turn.

Principle of low coherence light interference:

Low-coherence light with a wide wavelength continuous distribution is used as the light source for generating interference. The two light beams (reference beam and test beam) separated by the amplitude division method can only generate stable interference fringes under the condition that they are strictly equal in optical path. Therefore, the low-coherence light interference fringes are the basis for judging whether the two light beams are strictly equal in optical path. In Figure 1, the optical devices in the reference beam and the test beam optical paths are adjusted to be coaxial. After the reference beam and the test beam reflected by each mirror surface of the continuous mirror surface 10 to be measured are transmitted and reflected by the second beam splitter prism 9 respectively, the two beams converge and overlap and remain coaxial, and then enter the CCD camera 12 after passing through the imaging lens 11. When the continuous mirror surface 10 to be measured moves axially from a long distance to the second beam splitter prism 9, the equal optical path interference fringes of low-coherence light generated by the test light reflected by the first mirror surface of the continuous mirror surface 10 to be measured and the reference light are first observed on the receiving surface of the CCD camera 12. Thereafter, the complementary wedge prism group 7 on the reference light path is continuously adjusted to increase its equivalent thickness, so that the test light beams reflected by each mirror surface of the continuous mirror surface 10 to be tested can be found in turn, and the equal optical path interference fringes of low coherence light generated by the reference light beam on the receiving surface of the CCD camera 12 can be respectively found.

The function of Penta Prism 6:

The purpose of the secondary reflection of the reference beam by the pentagonal prism 6 during its travel is to ensure that the reference beam and the test beam are reflected an even number of times (or an odd number of times, an even number in this case). That is to say, the reference beam and the test beam, which have been amplitude-split by the first beam-splitting prism 5, can overlap one by one with the reference light and the test light separated from the same point after they meet after passing through the second beam-splitting prism 9 in transmission and reflection respectively. This is a necessary condition for producing equal optical path interference by the low coherent light amplitude-splitting method.

Principle of adjusting optical path of complementary wedge prism group 7:

The structure of the complementary wedge prism group 7 on the reference optical path in FIG1 is shown in FIG2 and FIG3. FIG2 is a schematic diagram of the complementary wedge prism group 7 for precise adjustment and measurement of the optical path. FIG3 is a three-dimensional view of the structure 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 made of the same material and with the same and small wedge angle θ. They can be of the same length or of different lengths. If the two wedge prisms are of different lengths, the longer one is used for long-distance movement, which is called a movable wedge prism, which is convenient for adjusting and measuring the optical path difference of the reference beam in a large range. FIG2 shows 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, and the surfaces where the hypotenuses are located are parallel to each other, with a small gap between the surfaces. Such an optical structure can be regarded as an equivalent optical parallel plate as a whole. The second wedge-shaped prism 702 moves in the horizontal plane (the paper plane shown in FIG. 2 ) in the direction of the hypotenuse, which causes the thickness of this equivalent optical parallel plate to change continuously, and the optical path of the incident parallel light perpendicular to the end face also changes accordingly, but the direction of the outgoing light remains unchanged, and there will be no lateral shift for the parallel light, which does not affect the subsequent arrangement and debugging of the optical devices.

The second wedge prism 702, i.e. the wedge triangle ΔABC on the right, moves a distance along the hypotenuse BA direction and reaches the new position of the dotted wedge triangle ΔA'B'C' in FIG2. It can be seen that when the wedge triangle ΔABC moves a distance along the hypotenuse direction and reaches the new position of ΔA'B'C', the vertex A where the wedge angle is located moves to A', and the thickness increment of the equivalent parallel plate is In the right triangle ΔA'AN, the vertex angle is the wedge angle θ, so use represents the distance that the wedge triangle ΔABC (i.e., the second wedge prism 702) moves along the hypotenuse BA, Represents the thickness increment of the equivalent optical parallel plate, then T = xsinθ.

The wedge angle θ of the wedge prism is very small. For the convenience of description, T is referred to as the longitudinal thickness increment and x is referred to as the approximate lateral displacement. From the formula T=xsinθ, it can be seen that since θ<90°, sinθ<1, so T<x. This shows that a large approximate lateral displacement x produces a small longitudinal thickness increment T in linear proportion. It can be seen that the measurement error ΔT transmitted to the longitudinal optical parallel plate thickness increment by the large approximate lateral measurement error Δx is 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 optical path, precise adjustment and measurement of the optical path is achieved. 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 accuracy requirements to meet the required accuracy requirements.

The measurement process and steps of the continuous mirror axis spacing:

Please refer to FIG4, which is a schematic diagram of the steps of measuring the spacing between continuous mirror surfaces by low coherence light interference. As shown in FIG4, the moving process of the movable long wedge prism in the complementary wedge prism group 7 is expressed by a dotted line. The turning dotted line on the complementary wedge prism group 7 indicates that the reflected test beams of the three consecutive mirror surfaces of the measured continuous mirror surface 10 are found in turn during the movement of the long wedge prism, and the corresponding positions when the reflected test beams successively generate equal optical path interference with the reference beam passing through the complementary wedge prism group 7, specifically including the following contents:

Step 1: Arrange the low-coherence light source 1, microscope objective 2, pinhole diaphragm 3, achromatic collimator objective 4, first beam splitter prism 5, pentagonal prism 6, complementary wedge prism group 7, plane reflector 8, second beam splitter prism 9, continuous mirror to be tested 10, imaging lens 11 and CCD camera 12. Adjust the low-coherence light source 1, microscope objective 2, pinhole diaphragm 3, achromatic collimator objective 4, they are coaxial and emit parallel light. The parallel light is vertically incident on the first beam splitter prism 5, of which the transmitted light is the test beam and the reflected light is the reference beam. The test beam is then vertically incident on the second beam splitter prism 9, and the transmitted test beam enters the continuous mirror to be tested 10, and the optical devices on the test light path are adjusted to be coaxial. After being reflected by each of the continuous mirrors 10 to be tested, the test beam returns along the original path, and then is reflected by the second beam splitter prism 9 and then passes through the imaging lens 11 to enter the CCD camera 12. The imaging lens 11 and the CCD camera 12 are adjusted to be coaxial on the test optical path. The reference beam reflected by the first beam splitter prism 5 is reflected by the pentagonal prism 6, passes vertically through the complementary wedge prism group 7, and then is reflected by the plane reflector 8. It is then vertically incident on the second beam splitter prism 9, and after being transmitted through the second beam splitter prism 9, it meets and overlaps with each of the test beams reflected by the second beam splitter prism 9, and each of the optical components is adjusted to be coaxial on the reference optical path. At this time, the overlapping reference beam and the test beam pass through the imaging lens 11 and enter the CCD camera 12, and are also coaxial.

Step 2: Adjust the equivalent thickness of the complementary wedge prism set 7 on the reference optical path, that is, move the movable wedge prism of the complementary wedge prism set 7 along the hypotenuse direction in the horizontal plane (Figure 2 shows the paper plane) to at least allow the light to pass through a sufficiently large range centered on the axis in the cross section of the reference beam. In the test optical path, adjust the position of the continuous mirror surface 10 to be tested along the axial direction until the equal optical path interference fringes of the reflected test beam of the first mirror surface of the continuous mirror surface 10 to be tested and the low coherence light of the reference beam are observed on the receiving surface of the CCD camera 12. Record the position reading x ₁ of the movable wedge prism in the complementary wedge prism set 7 along its hypotenuse direction at this time.

Step 3: Continuously adjust the equivalent thickness of the complementary wedge prism group 7 on the reference light path, and observe in turn the interference fringes of low-coherence light generated by the test beam reflected by the second mirror surface, the third mirror surface and subsequent mirror surfaces of the measured continuous mirror surface 10 and the reference beam, and record the position readings x ₂ , x ₃ , etc. of the movable wedge prism in the complementary wedge prism group 7 along its hypotenuse direction when observing each equal optical path interference fringes.

Based on the above operation process, the calculation process of the measurement results of the mirror spacing on the 10 axes of the measured continuous mirror is as follows:

According to the interference principle of low-coherence light, the complementary wedge-shaped prism group 7 in the reference light path is an equivalent optical parallel plate of variable thickness. The increase in thickness replaces the corresponding thickness of air, causing the increase in the optical path of the reference beam. The low-coherence light interference fringes generated twice in succession, the optical path increment of the reference beam should be equal to the optical path difference of the test beam reflected by the two adjacent mirrors of the corresponding continuous mirror 10. If the continuous mirror is a mirror of a coaxial lens group, the optical path difference of the reflected test beam refers to the optical path difference of the light on the axis.

Assume that the refractive index of air is n ₀ , the refractive index of the glass material between the adjacent mirror surfaces of the continuous mirror surface 10 to be tested is n _i , the distance between the mirror surfaces on the axis is t _i , the refractive index of the glass material of the complementary wedge prism group 7 is n _p , the wedge angle of the prism of the wedge prism group is θ, and the two adjacent fringe position readings during the movement of the movable wedge prism along the hypotenuse direction are _xi and _xi+1 respectively. The optical path difference of the test light beam reflected by the two adjacent mirror surfaces of the continuous mirror surface 10 to be tested is 2n _i t _i . The optical path of the reference light beam increases in the single-path direction due to the movement of the movable wedge prism of the wedge prism group, and the increase is (n _p -n ₀ )( _xi+1 -xi ₎ sinθ. According to the equal optical path interference condition of low coherent light, we have

2n _i t _i =(n _p -n ₀ )(x _i+1 -x _i )sinθ

The on-axis mirror spacing _ti is obtained as

Its measurement error Δt _i is

In the above formula, _Δxi and Δxi ₊₁ are the measurement errors when the movable wedge prism of the complementary wedge prism set 7 moves along the hypotenuse direction.

Embodiment 2:

Based on the content of Example 1, the measurement accuracy of the above-mentioned continuous mirror axis spacing is analyzed:

The refractive index of general glass materials is between 1.4 and 1.7, and the refractive index of air is about 1. In the above error calculation formula, the value of the term containing the refractive index and the constant 1/2 is about 1/6. Since the prism wedge angle θ in the wedge prism group is << 90°, it can be known that sinθ << 1, and the resulting measurement error transmission is linearly reduced. The error calculation formula shows that the error of the distance between the measuring mirror axes caused by the displacement error measured by the measuring ruler that drives the movable wedge prism to move in the complementary wedge prism group 7 is greatly reduced linearly. In addition, a smaller prism wedge angle θ can be designed to obtain a smaller measurement error. Obviously, under the same other conditions, the measurement error value of the distance between the mirror axes caused by the optical path difference of the single-way direction adjustment and measurement reference beam is 1/2 of that in the two-way bidirectional adjustment and measurement, and the measurement accuracy is doubled.

Embodiment 3:

Based on Examples 1-2, the measurement accuracy of the continuous mirror axis spacing measurement device and method based on low-coherence light interference proposed by the present invention is verified by using specific examples:

Assuming that the movable wedge prism in the complementary wedge prism group 7 has a moving measuring scale accuracy of ±1 μm, the sum of the errors of _xi and _xi+1 is ±2 μm. A certain LED low-coherence light is used as the light source, and its central wavelength λ=680 nm. The refractive indices of glass materials K9 and QK2 for red light 656.27 nm are 1.51390 and 1.47590 respectively. The wavelengths of red light 680 nm and red light 656.27 nm are not much different, and it can be approximately considered that the refractive index of the above glass materials is also the refractive index for red light 680 nm.

Table 1 is an example of measurement accuracy analysis. It is the measurement accuracy when the prism material of the complementary wedge prism set 7 is K9, the materials between the two mirror surfaces of the measured continuous mirror surface 10 are K9 and QK2 respectively, and the wedge angle values of the complementary wedge prism set 7 are 10°, 5° and 3° respectively.

Table 1 An example of measurement accuracy analysis (the refractive index of air at normal temperature and pressure is n ₀ = 1.000273)

It can be seen from Table 1 that the smaller the prism wedge angle of the complementary wedge prism set 7 is, the higher the measurement accuracy of the continuous mirror surface spacing is. The wedge angle design of the wedge prism can be changed to make the measurement accuracy of the continuous mirror surface spacing within the range that meets the accuracy requirements. For example, when the measurement accuracy is required to be within 20nm, and the prism material of the complementary wedge prism set 7 and the material of the measured continuous mirror surface 10 are both K9, the wedge angle θ is less than 3° to meet the requirements.

Embodiment 4:

Based on Example 1, a device and method for obtaining high-contrast low-coherence light interference fringes:

When the reference beam and the test beam that participate in the interference are generated by the low-coherence light amplitude splitting method, the first beam splitter is an ordinary cubic beam splitter (BS), which plays the role of amplitude splitting. The intensity of the separated reference beam and the test beam is 5:5, that is, the intensity of the two beams is the same. However, after the test beam enters the continuous mirror 10 to be measured, it is reflected by each mirror of the continuous mirror 10 to be measured in sequence on the one hand, and is also transmitted by each mirror in sequence on the other hand. Finally, the test beam reflected by each mirror generates low-coherence light interference with the reference beam. It can be seen that the intensity of the test beam reflected by each mirror will gradually weaken compared with the reference beam, especially the intensity of the test beam reflected by the mirror further back is weaker. According to the principle of optical interference, when the intensity (or amplitude) of the two beams participating in the interference is 1:1, the contrast of the interference fringes is the best; when their intensity (or amplitude) differs more, the contrast of the interference fringes is worse, and even the contrast of the interference fringes is too small to be used for observation and measurement analysis. In order to solve the above problem, the reference beam can be subjected to intensity attenuation processing to make its intensity attenuate to an appropriate relative value. One method is to replace the plane reflector 8 in FIG1 with a beam splitter wedge 13, such as the beam splitter wedge 13 in FIG5. The beam splitter wedge 13 reflects the reference beam on the one hand, and transmits the reference beam on the other hand. After transmitting most of the light intensity, it is equivalent to attenuating the reference beam. A beam splitter wedge 13 with a suitable transmission and reflection ratio (T:R) can be selected to attenuate the reference beam to a suitable intensity, such as T:R=9:1.

Since the wavelength distribution of low-coherence light is relatively wide, the reference beam and the test beam separated by the amplitude division method of low-coherence light will produce dispersion on the light beams passing through the transmission optical devices in their respective optical paths. The different degrees of dispersion will also affect the contrast of the interference fringes. If the difference in the degree of dispersion of the two light beams is greater, the contrast of the interference fringes will be lower, which is not conducive to observation and measurement analysis. In the present invention, the main thing that produces dispersion for low-coherence light is a transparent glass optical device. In order to make the dispersion degree of the reference beam and the test beam consistent as much as possible, the glass distance and air distance that the reference beam passes through in its optical path can be made equal or nearly equal to the corresponding distance of the test beam in its optical path. When arranging the optical path, the geometrical distance of the reference beam is equal to the geometrical distance of the test beam. At this time, the reference beam has been reflected twice by the pentagonal prism 6, so its distance in the optical device glass is about (2+1.414)D-D=2.414D longer than the distance of the test beam reflected by the first mirror of the continuous mirror 10 to be measured in the optical device glass, where D is the maximum aperture of the first beam splitter prism 5 and the second beam splitter prism 9, and is also the maximum aperture of the pentagonal prism 6. In addition, there is a complementary wedge prism group for adjusting and measuring the optical path difference in the optical path of the reference beam, which has an initial equivalent thickness. In order to solve the problem of unequal dispersion of the reference beam and the test beam, an optical parallel plate with suitable dispersion properties and refractive index materials can be placed in the optical path of the test beam to allow the test beam to pass vertically. Its thickness is about the initial equivalent thickness of the complementary wedge prism group plus 2.414D. As shown in the optical parallel plate 14 in Figure 5.

Embodiment 5:

Based on Example 1, a second device and method for obtaining high-contrast low-coherence light interference fringes:

In order to obtain a good contrast of low-coherence light interference fringes and reduce the light intensity ratio of the initial reference beam to the test beam, polarized light interference can also be used. Figure 6 is a device diagram of this embodiment. It is based on the device of Example 1, and the ordinary cubic beam splitter prism (BS) used as the first beam splitter prism 5 is replaced with a polarization beam splitter prism (PBS), while the second beam splitter prism 9 is still an ordinary cubic beam splitter prism (BS), a linear polarized light polarizer 15 is placed before the first beam splitter prism 5 (PBS), a 1/2λ wave plate 16 is placed in 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 above modified device, the principle of reducing the ratio of the initial reference beam intensity to the test beam intensity is as follows:

After the low-coherence parallel light beam passes through the linear polarized light polarizer 15, linear polarized light is generated. This linear polarized light can be mathematically decomposed into linear polarized light with a vibrating electric vector parallel to the incident plane of the first beam splitter prism 5 (PBS) and linear polarized light with a vibrating electric vector perpendicular to the incident plane. The first beam splitter prism 5 is a polarization beam splitter prism (PBS), which transmits only linear polarized light with a vibrating electric vector parallel to the incident plane, and reflects only linear polarized light with a vibrating electric vector perpendicular to the incident plane. Rotating the linear polarized light polarizer 15 around the axis of the parallel light beam will change the amplitude ratio of the linear polarized light transmitted and reflected by the first beam splitter prism 5, that is, change the intensity ratio of the transmitted linear polarized light as the test beam and the reflected linear polarized light as the reference beam. By rotating the linear polarized light polarizer 15 to an appropriate angle, the intensity ratio of the reference beam to the test beam can satisfy the contrast of the subsequent interference fringes to reach the optimal state. The 1/2λ wave plate 16 on the optical path of the test beam plays the role of rotating the vibration direction (electric vector direction) of the linearly polarized light. The 1/2λ wave plate 16 is rotated around the axis of the test beam to an appropriate angle position so that the vibration direction (electric vector direction) of the linearly polarized light serving as the test beam is rotated 90°. In this way, the vibration direction of the linearly polarized light of the subsequent test beam is the same as the vibration direction of the linearly polarized light of the reference beam, which facilitates interference in the future.

Embodiment 6:

Based on Example 4, a device and method for extending the continuous mirror spacing measurement range:

FIG7 is a schematic diagram of a device for extending the continuous mirror spacing measurement range. It is based on Example 4, and adds a same complementary wedge prism group to the optical path of the reference beam. The two wedge prism groups are exactly the same, forming cascaded complementary wedge prism groups 17 and 18. In this way, the measurement range of the mirror spacing can be doubled. Although the measurement error is twice that of a single complementary wedge prism group 7, the measurement error can be partially reduced by reducing the design of the prism wedge angle to meet the measurement accuracy requirements.

Embodiment 7:

Based on Example 1, a device and method for measuring the on-axis spacing of continuous mirrors by low-coherence light interference of optical fibers and optical fiber devices:

Fig. 8 is a schematic diagram of a device for measuring the on-axis spacing of continuous mirror surfaces by low-coherence light interference of optical fibers and optical fiber devices based on Example 1. The device comprises a low-coherence light source 1, a one-to-two optical fiber coupler 19, a first optical fiber collimator 20, a complementary wedge-shaped prism set 7, a plane reflector 8, a second optical fiber collimator 21, a three-end optical fiber circulator 22, a third optical fiber collimator 23, a continuous mirror surface 10 to be measured, a two-in-one optical fiber coupler 24, a fourth optical fiber collimator 25, an imaging lens 11, a CCD camera 12, and optical fibers on the optical path.

The reference beam and the test beam are generated by a one-to-two fiber coupler 19 with an intensity distribution ratio of 1:9. The upper path that passes through the complementary wedge-shaped prism group 7 is the reference beam (with an intensity ratio of 1/10), and the lower path that will pass through the three-end fiber circulator 22 is the test beam (with an intensity ratio of 9/10). The reference beam is first converted into a parallel beam by the first fiber collimator 20, then passes vertically through the complementary wedge-shaped prism group 7, and then is reflected by the plane reflector 8, and then passes through the second fiber collimator 21 to enter the optical fiber for further transmission, and finally enters the input end of the two-in-one fiber coupler 24. The test beam first enters the three-end fiber circulator 22, and the light output from the corresponding output end passes through a section of optical fiber, and is then converted into a parallel beam by the third fiber collimator 23. The parallel beam coaxially enters the continuous mirror 10 to be tested, and is reflected by each mirror of the continuous mirror 10 to be tested. The reflected light of each mirror returns along the original path, and is coupled by the third fiber collimator 23 and returns to the three-end fiber circulator 22, and is output from the corresponding output end of the three-end fiber circulator 22, and finally enters the input end of the two-in-one fiber coupler 24. The reference beam and the test beam enter the two-in-one fiber coupler 24 together, and after coupling and overlapping, enter the fourth fiber collimator 25 from its output end. The collimated beam passes through the imaging lens 11 and enters the CCD camera 12, and the interference fringes are received on the receiving surface of the CCD camera 12. According to the equal optical path interference condition of low coherence light interference, that is, the principle that the optical path of the reference beam and the test beam is equal, the length of each section of the optical fiber is selected appropriately, and the optical path of the reference beam and the test beam is arranged. The function of the plane reflector 8 here is to reflect the reference beam once, because the test beam participating in the interference is reflected once by a certain mirror surface of the continuous mirror surface 10 being tested, so that the low-coherence reference beam and the test beam are reflected the same number of times, which is one of the necessary conditions for generating interference fringes.

The method for measuring the distance between consecutive mirrors is as follows:

Step 1: Adjust the first fiber collimator 20 to emit a parallel light beam; adjust the plane reflector 8 and the second fiber collimator 21 to make them coaxial with the first fiber collimator 20, and couple the parallel light beam into the subsequent optical fiber; insert the complementary wedge prism group 7 to allow the parallel light beam to pass vertically, and adjust its equivalent thickness to at least allow the light to pass within a sufficiently large range centered on the axis in the cross section of the reference beam. Adjust the third fiber collimator 23 to emit a parallel light beam; adjust the measured continuous mirror 10 to be coaxial with the third fiber collimator 23. Adjust the fourth fiber collimator 25 to emit a parallel light beam; adjust the imaging lens 11 to be coaxial with the fourth fiber collimator 25, and the CCD camera 12 is also coaxial with them.

Step 2: In the test optical path, adjust the position of the continuous mirror 10 to be tested along the axial direction until the equal optical path interference fringes of the low coherence light of the reflected test beam and the reference beam of the first mirror surface of the continuous mirror 10 to be tested are observed on the receiving surface of the CCD camera 12. Record the position reading x ₁ of the movable wedge prism in the complementary wedge prism set 7 along its hypotenuse direction at this time.

Step 3: Continuously adjust the equivalent thickness of the complementary wedge prism group 7 on the reference light path, and observe in turn the interference fringes of low-coherence light generated by the test beam reflected by the second mirror surface, the third mirror surface and subsequent mirror surfaces of the measured continuous mirror surface 10 and the reference beam, and record the position readings x ₂ , x ₃ , etc. of the movable wedge prism in the complementary wedge prism group 7 along its hypotenuse direction when observing each equal optical path interference fringes.

Based on the above operation process, the calculation process of the measurement results of the measured continuous mirror axis spacing is as follows:

According to the interference principle of low-coherence light, the complementary wedge-shaped prism group 7 in the reference light path is an equivalent optical parallel plate of variable thickness. The increase in thickness replaces the corresponding thickness of air, causing the increase in the optical path of the reference beam. The low-coherence light interference fringes generated twice in succession, the optical path increment of the reference beam should be equal to the optical path difference of the test beam reflected by the two adjacent mirrors of the corresponding continuous mirror 10. If the continuous mirror is a mirror of a coaxial lens group, the optical path difference of the reflected test beam refers to the optical path difference of the light on the axis.

Assume that the refractive index of air is n ₀ , the refractive index of the glass material between the adjacent mirror surfaces of the continuous mirror surface 10 to be tested is n _i , the distance between the mirror surfaces on the axis is t _i , the refractive index of the glass material of the complementary wedge prism group 7 is n _p , the wedge angle of the prism of the wedge prism group is θ, and the two adjacent fringe position readings during the movement of the movable wedge prism along the hypotenuse direction are _xi and _xi+1 respectively. The optical path difference of the test light beam reflected by the two adjacent mirror surfaces of the continuous mirror surface 10 to be tested is 2n _i t _i . The optical path of the reference light beam increases in the single-path direction due to the movement of the movable wedge prism of the wedge prism group, and the increase is (n _p -n ₀ )( _xi+1 -xi ₎ sinθ. According to the interference condition of equal optical path of low coherent light, we have

2n _i t _i =(n _p -n ₀ )(x _i+1 -x _i )sinθ

The on-axis mirror spacing _ti is obtained as

Its measurement error Δt _i is

In the above formula, _Δxi and Δxi ₊₁ are position measurement errors when the movable wedge prism of the complementary wedge prism set 7 moves along the hypotenuse direction of the prism.

The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims (2)

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;

s7, according to the position reading data obtained in the S6, the on-axis distance and the measurement error of the measured continuous mirror surface (10) are calculated by combining the interference principle of low-coherence light;

In the method, 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 materials 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 tiny interval exists between the planes, and the second wedge prism (702) is a movable wedge prism which can move in the horizontal plane along the bevel edge direction to measure displacement.

2. The method for measuring the distance between the axes of the continuous mirrors based on the interference of low coherence light according to claim 1, wherein the calculation formula of the distance between the axes of the continuous mirrors to be measured in S7 is:

2n_it_i=(n_p-n₀)(x_i+1-x_i)sinθ(1)

(1) Wherein n _i represents the refractive index of the material between two adjacent mirrors of the measured continuous mirror; t _i represents the on-axis distance between two adjacent mirrors of the measured continuous mirror; n _p denotes the refractive index of the glass material of the complementary wedge prism group (7); n ₀ represents the refractive index of air; x _i+1、x_i represents the position reading of the movable wedge prism in the direction of its hypotenuse corresponding to the interference fringes of two adjacent low-coherence lights; 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) In the method, in the process of the invention, 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.