Multi-lens group lens center thickness detection method and device based on low coherence detection

Abstract

This invention provides a method and apparatus for detecting the center thickness of a multi-lens group lens based on low-coherence detection, belonging to the field of modern optical precision measurement technology. It utilizes a moving guide rail to scan and construct equal-path low-coherence interference fringes to obtain the positions of the front and rear surfaces of each lens. A high-coherence Michelson interferometer is used for phase extraction to obtain the phase difference between the front and rear surfaces of the lens, thus determining the lens thickness. An optical switch is used to construct a switchable forward and reverse measurement structure, and an optical path compensation mechanism is introduced as needed, effectively solving the problem of weak or even missing reflection signals from the rear lenses due to the gradual attenuation of light energy during multi-lens group measurements. This invention employs a non-contact measurement method, which is fast and easy to operate. It is suitable for measuring the center thickness and optical spacing of single lenses and multi-lens groups, possessing good versatility and engineering application value, especially suitable for multi-lens group measurement scenarios with weak reflection signals or large equivalent optical path ranges.

Description

Technical Field

[0001] This invention relates to the field of modern optical precision measurement technology, specifically to a method and apparatus for detecting the center thickness of a multi-lens group lens based on low-coherence detection. Background Technology

[0002] In the field of modern optical system manufacturing and precision testing, the center thickness of a lens and the optical spacing between lenses in a multi-lens group are core parameters that directly affect the system's imaging quality, focal length accuracy, and optical path stability. Traditional measurement techniques, particularly contact methods, are prone to scratching the lens surface coating and damaging optical performance, and are inefficient, making them unsuitable for batch testing of multi-lens groups. Non-contact measurement methods, such as image-based and confocal methods, suffer from limited measurement accuracy and working distance limitations, failing to meet the testing requirements of high-precision multi-lens groups.

[0003] Low-coherence interferometry, with its advantages of non-contact operation, high precision, and the ability to perform absolute measurements, has become the mainstream technology for thickness measurement of optical components. (For example, Chinese invention patent CN 109855546 A (application number 201811603799.8) discloses a short-coherence interferometry system that uses a broadband SLD light source and balanced detection technology to measure the thickness of the lens center.) However, in multi-lens group measurement scenarios, existing low-coherence interferometry schemes still face significant technical bottlenecks: On the one hand, when there are many lenses in a multi-lens group, the measurement light needs to undergo multiple transmissions and reflections, resulting in a sharp attenuation of the reflected light energy on the surface of the rear lens. This causes the signal intensity at the weak reflection interface to be lower than the detector's recognition threshold, making it impossible to receive the reflected light and thus unable to form an effective interference signal. Consequently, it is impossible to measure the thickness of the rear lens and the spacing between lenses. On the other hand, the equivalent optical path range varies greatly among multi-lens groups of different specifications. Measurement systems with fixed optical path structures cannot simultaneously meet the measurement requirements of short optical path and small thickness as well as long optical path and large thickness, resulting in limited measurement range and insufficient versatility. In addition, the existing signal processing algorithms for low-coherence interferometry are relatively simple and are easily affected by light intensity interference caused by vibrations in the guide rail movement environment, making it difficult to achieve high-precision measurement and failing to meet the manufacturing and testing requirements of high-end optical systems.

[0004] Therefore, developing a low-coherence interferometric measurement method and device that can effectively solve the problem of weak reflection signal detection in multi-lens groups, meet the needs of large-range measurement, and have high measurement accuracy has become an urgent technical problem to be solved in the field of optical precision measurement. Summary of the Invention

[0005] Therefore, the present invention provides a method and apparatus for detecting the center thickness of a multi-lens group lens based on low coherence detection to solve the above problems. By introducing a switchable optical path structure, the interference signal intensity of the weak reflection interface is improved, and the measurable optical path range of the system is expanded.

[0006] This invention provides a method for detecting the center thickness of a multi-lens group lens in low-coherence detection, comprising the following steps:

[0007] Step 1: Connect all components and fiber optic patch cords according to the system flowchart, and connect to the PC. Turn on the low-coherence and high-coherence light sources, open the data acquisition software, and fill in the relevant parameters. Number the lens group consisting of n lenses along the direction of light propagation. Number the lenses closest to the forward incident port of the measurement light as lens 1 to lens n, and group the lenses to be tested. Measure lenses 1 to m when incident forward, and lenses n to n-m+1 when incident backward. Finally, integrate the two sets of data to obtain the thickness of all n lenses.

[0008] Step 2: Switch the 1×2 optical switch to allow the measurement light to be incident from the front of the lens group under test in order to measure the first group of lenses.

[0009] Step 3: Determine the equivalent optical path range of the first set of lenses. When the equivalent optical path is large, switch the 1×N optical switch (9) and the 1×N optical switch to introduce a suitable delayed optical path to ensure that the reference arm and the measuring arm meet the low coherence interference condition.

[0010] Step 4: After confirming that the system is in normal condition, press the motor controller switch. At this time, the standard plane mirror moves and the system enters the real-time data acquisition mode.

[0011] Step 5: After the plane mirror has finished moving, import the acquired raw signal into the system for processing.

[0012] Step 6: Perform DC component removal, motion smoothing, and amplitude normalization operations on the original signal.

[0013] Step 7: Use MATLAB's findpeaks function to filter significant peaks and exclude noisy peaks.

[0014] Step 8: Using the interference peaks confirmed in the previous step as the center, construct a local analysis window in their neighborhood; perform first-order difference on the local signal and square it to obtain the gradient energy distribution; calculate the precise position of the interference peaks using the improved centroid method based on the gradient energy distribution.

[0015] Step 9: According to the order of the interference peaks in the signal, assign adjacent interference peaks to the front and rear surfaces of the same lens, respectively. Extract the high coherence channel signal from the interval corresponding to the interference signals of the front and rear surfaces of the 1st to mth lenses and preprocess it. The specific method is as follows: perform a fast Fourier transform on the extracted signal and apply a low-pass filter in the frequency domain; perform an inverse transform on the filtered signal and smooth it to obtain a stable interference fringe signal for each lens peak pair.

[0016] Step 10: Based on the stripe signal obtained in the above steps, perform extreme point detection, count the number of peaks and valleys in each interval, and take the average value as the integer part of the stripe; take local intervals of length π in the corresponding phase intervals centered at the start and end of the stripe signal, perform Hilbert transform on each, calculate the instantaneous phase, and obtain the edge phase difference; calculate the stripe residual correction term based on the phase difference.

[0017] Step 11: Based on the obtained integer part of the fringe and the residual correction term, obtain the final number of fringe, and then calculate the thickness of the lens being measured.

[0018] Step 12: Switch the 1×2 optical switch to make the measurement light incident from the opposite direction of the lens group under test to measure the thickness of the other half of the lens group, and repeat steps 3 to 11.

[0019] Furthermore, in step 8, an improved centroid method is used, employing the gradient energy obtained by first-order difference and squaring of the local signal as weights, to accurately locate the interference peak position. The calculation formula is as follows:

[0020]

[0021] in, This represents the local gradient energy after removing the mean.

[0022] Furthermore, in step 10, the number of integer stripes can be calculated using the following formula. :

[0023]

[0024] in Let be the total number of peaks in the fringe signal of the k-th lens. Let be the total number of valleys in the fringe signal of the k-th lens.

[0025] Further, in step 10, the fringe residual correction term is obtained based on the Hilbert transform, specifically including: at the start and end points of the fringe signal, a local analysis window of length π is constructed in the high coherence channel signal, centered on the corresponding endpoints; the Hilbert transform is performed on the fringe signal within the local analysis window to obtain the instantaneous phase distribution; a first index point is selected within the local window at the start point. Second index point Select the third index point within the local window at the termination end. and the fourth index point . Corresponding to the start and end points of the stripe signal. correspond The first extreme value position after that. Corresponding to the end point of the stripe signal. In the corresponding window The first extreme position before.

[0026] The following formula can be used to calculate the phase difference between two feature points in an edge region. ,

[0027]

[0028]

[0029] in These are the phase values ​​of the four index points of the fringe signal of the k-th lens.

[0030] The phase difference can be converted into the equivalent number of fringes using the following formula.

[0031]

[0032] Furthermore, in step 11, the thickness of the lens under test can be derived based on the following formula: number of interference fringes + phase correction, combined with optical principles. The integer number of fringes is obtained by using the integer part of the fringes and the residual correction term. ,

[0033]

[0034] in Let the thickness be the thickness of the k-th lens. Let be the number of fringes of the k-th lens. Let be the equivalent fringe number of the k-th lens. The wavelength of a low-coherence light source. Let be the group refractive index of the k-th lens at the corresponding wavelength.

[0035] This invention also discloses a multi-lens group lens center thickness detection device with low coherence detection. The device includes: a high- and low-coherence light source, a high-coherence light source, a first isolator, a second isolator, a first fiber coupler, a second fiber coupler, a first circulator, a wavelength division multiplexer, a first 1×N switch, a delay fiber, a second 1×N switch, a first collimator, a guide rail, a Faraday rotator, a movable standard plane mirror, a data acquisition unit, a photodetector, a balanced photodetector, a third fiber coupler, a second circulator, a 1×2 switch, a second collimator, a lens group under test, and a third collimator.

[0036] Furthermore, a 1×2 optical switch is installed in the measuring arm, with one end connected to the second circulator and the other end connected to the second collimator and the third collimator respectively. It is used to selectively switch the measuring light from the forward or reverse incident direction of the lens group under test, so as to realize the accurate measurement of the thickness of the entire lens and the spacing between the lenses, and effectively avoid the problem of signal attenuation from the reflection of the rear lens.

[0037] Furthermore, a 1×N optical switch, a time-delay fiber, and a 1×N optical switch are set in the reference arm, with one end connected to the wavelength division multiplexer and the other end connected to the collimator. This is used to switch the optical path according to the equivalent optical path range of the lens group under test. By introducing a time-delay fiber to compensate for the optical path of the reference arm, the optical path of the reference arm and the measurement arm are matched to meet the low coherence interference condition and break through the range limitation of the fixed optical path structure.

[0038] Furthermore, the center wavelength of the low-coherence light source is 1310 nm, and the center wavelength of the high-coherence light source is 1550 nm.

[0039] Furthermore, the three fiber couplers employ different splitting ratios to match the intensity of the measurement light and the reference light, thereby improving the contrast of the interference signals. Specifically, the first fiber coupler has a coupling ratio of 99:1, the second fiber coupler has a coupling ratio of 90:10, and the third fiber coupler has a coupling ratio of 50:50.

[0040] Furthermore, the movable standard plane mirror is set on a movable guide rail or displacement platform. The movement of the movable standard plane mirror along the optical axis is used to change the optical path of the reference arm in order to match the optical path of each reflection interface of the lens group under test, thereby obtaining the corresponding interference signal.

[0041] Furthermore, based on the position difference of the interference signals corresponding to the reflected light from the front and rear surfaces of each lens in the lens group under test, and in combination with the refractive index parameters of each lens under test, the center thickness of each lens under test can be calculated.

[0042] The present invention has the following advantages over the prior art:

[0043] 1. This invention solves the problem of weak reflection signals through bidirectional detection. By constructing a dual detection optical path with forward and reverse directions using a 1×2 optical switch, the incident direction of the measurement light can be flexibly switched according to the structural characteristics of the multi-lens group. When there are many lenses, the bidirectional detection is used to measure the center thickness of the lens in the corresponding direction. The results from the two directions are combined to obtain a complete measurement of the center thickness of the multi-lens group. This effectively avoids the problem of weak or even missing interference signal energy caused by the energy attenuation of reflected light from the rear lens in traditional unidirectional detection. It ensures that stable interference signals can be formed on the front and rear surfaces of all lenses in the multi-lens group, and achieves accurate measurement of the thickness of the entire lens and the spacing between lenses.

[0044] 2. This invention features a switchable delayed optical path to extend the measurement range. By adding a 1×N optical switch before the movable reference plane mirror and using multiple delayed optical paths with different delays, optical path compensation is introduced as needed according to the equivalent optical path range of the lens group under test. This ensures that the optical path of the reference arm and the optical path of the measurement arm always meet the low coherence interference condition, breaking through the range limitation of the fixed optical path structure. This enables full-coverage measurement of a large range of multi-lens groups, significantly improving the system's versatility and adaptability to various application scenarios.

[0045] 3. The high-precision algorithm of this invention ensures sub-micron level measurement accuracy. It adopts a combined algorithm of "using Hilbert transform phase recovery to obtain residual terms and using fringe counting to obtain integer terms". The phase information of the interference signal is accurately extracted through Hilbert transform, and the phase residual caused by non-integer fringes is compensated. Combined with fringe counting, high-precision calculation of optical path difference is achieved, which effectively improves measurement accuracy and meets the precision detection requirements of high-end optical systems. Attached Figure Description

[0046] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0047] Figure 1 This is a schematic diagram of the structure of a method and apparatus for detecting the center thickness of a multi-lens group lens with low coherence detection according to the present invention.

[0048] Figure 2 This is the original waveform of the low-coherence signal in the multi-lens group lens center thickness detection method of the present invention. Figure 1 .

[0049] Figure 3 This is the original waveform of the high-coherence signal in a multi-lens group lens center thickness detection method for low-coherence detection according to the present invention. Figure 2 .

[0050] Figure 4 This is an envelope waveform diagram obtained after preprocessing of a low-coherence signal in a multi-lens group lens center thickness detection method of the present invention.

[0051] Figure 5 This is a diagram showing the results of the improved centroid method for accurately locating interference peaks during the forward measurement of a multi-lens group lens center thickness detection method for low coherence detection according to the present invention.

[0052] Figure 6This diagram illustrates the improved centroid method for accurately locating interference peaks during reverse measurement in a multi-lens group lens center thickness detection method for low coherence detection according to the present invention.

[0053] Figure 7 This is a graph showing the peak and trough detection results of a high-coherence smoothed signal in a multi-lens group lens center thickness detection method for low-coherence detection according to the present invention.

[0054] Figure 8 This is a comparison diagram of the phase expansion of the residual term using Hilbert transform in a low-coherence detection method for multi-lens group lens center thickness according to the present invention.

[0055] Explanation of reference numerals in the attached figures:

[0056] 1. High coherence light source; 2. Low coherence light source; 3. First isolator; 4. Second isolator; 5. First fiber optic coupler; 5-1. First fiber optic coupler port one; 5-2. First fiber optic coupler port two; 5-3. First fiber optic coupler port three; 6. Second fiber optic coupler; 6-1. Second fiber optic coupler port one; 6-2. Second fiber optic coupler port two; 6-3. Second fiber optic coupler port three; 6-4. Second fiber optic coupler port four; 7. First circulator; 7-1. First circulator port one; 7-2. First circulator port two; 7-3. First circulator port three; 8. Wavelength division multiplexer; 8-1. Wavelength division multiplexer port one; 8-2. Wavelength division multiplexer port two; 8-3. Wavelength division multiplexer port three; 9. First 1×N switch; 10. Delay fiber; 11. Second 1×N switch; 12. ×N switch; 13. First collimator; 14. Guide rail; 15. Faraday rotator; 16. Movable standard plane mirror; 17. Data acquisition unit; 18. Photodetector; 19. Balanced photodetector; 10. Third fiber optic coupler; 19-1. Port 1 of the third fiber optic coupler; 19-2. Port 2 of the third fiber optic coupler; 19-3. Port 3 of the third fiber optic coupler; 19-4. Port 4 of the third fiber optic coupler; 20. Second circulator; 20-1. Port 1 of the second circulator; 20-2. Port 2 of the second circulator; 20-3. Port 3 of the second circulator; 21. 1×2 switch; 21-1. Port 1 of the 1×2 switch; 21-2. Port 2 of the 1×2 switch; 21-3. Port 3 of the 1×2 switch; 22. Second collimator; 23. Lens group under test; 24. Third collimator. Detailed Implementation

[0057] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0058] Example 1

[0059] Reference Figures 1 to 8 This embodiment provides a method for detecting the center thickness of a multi-lens group lens with low coherence detection, including the following steps:

[0060] Step 1: Connect all components and fiber optic patch cords according to the system flowchart, and connect to the PC. Turn on the low-coherence light source 1 and the high-coherence light source 2, open the data acquisition software, and fill in the relevant parameters. Number the lens group consisting of n lenses along the direction of light propagation. Number the lenses closest to the forward incident port of the measurement light as lens 1 to lens n, and group the lenses to be tested. Measure lenses 1 to m when incident forward, and lenses n to n-m+1 when incident backward. Finally, integrate the two sets of data to obtain the thickness of all n lenses.

[0061] Step 2: Switch the 1×2 optical switch 21 to make the measurement light enter from the front of the lens group 23 under test to measure the first group of lenses.

[0062] Step 3: Determine the equivalent optical path range of the first set of lenses. When the equivalent optical path is large, switch 1×N optical switch 9 and 1×N optical switch 11 to introduce a suitable delayed optical path. When the equivalent optical path is small, make the measurement light propagate along the main optical path to ensure that the reference arm and the measurement arm meet the low coherence interference condition.

[0063] Step 4: After confirming that the system is in normal condition, press the motor controller switch. At this time, the standard plane mirror moves and the system enters the real-time data acquisition mode.

[0064] Step 5: After the plane mirror has finished moving, import the acquired raw signal into the system for processing.

[0065] Step 6: Perform DC component removal, motion smoothing, and amplitude normalization operations on the original signal.

[0066] Step 7: Use MATLAB's findpeaks function to filter out significant peaks and exclude noisy peaks.

[0067] Step 8: Using the interference peaks identified in the previous step as centers, construct local analysis windows within their neighborhoods; perform first-order differences on the local signals and square them to obtain the gradient energy distribution; based on the gradient energy distribution, calculate the precise location of the interference peaks using the improved centroid method. The calculation formula is as follows:

[0068]

[0069] in, This represents the local gradient energy after removing the mean.

[0070] Step 9: According to the order of the interference peaks in the signal, assign adjacent interference peaks to the front and rear surfaces of the same lens, respectively. Extract the high coherence channel signal from the interval corresponding to the interference signals of the front and rear surfaces of the 1st to mth lenses and preprocess it. The specific method is as follows: perform a fast Fourier transform on the extracted signal and apply a low-pass filter in the frequency domain; perform an inverse transform on the filtered signal and smooth it to obtain a stable interference fringe signal for each lens peak pair.

[0071] Step 10: Based on the stripe signal obtained in the above steps, perform extreme point detection, count the number of peaks and valleys in each interval, and take the average value as the integer part of the stripe; take local intervals of length π in the corresponding phase intervals centered at the start and end of the stripe signal, perform Hilbert transform on each, calculate the instantaneous phase, and obtain the edge phase difference; calculate the stripe residual correction term based on the phase difference.

[0072] The number of integer stripes can be calculated using the following formula. :

[0073]

[0074] in Let be the total number of peaks in the fringe signal of the k-th lens. Let be the total number of valleys in the fringe signal of the k-th lens.

[0075] The fringe residual correction term is obtained based on the Hilbert transform, specifically including: at the start and end points of the fringe signal, a local analysis window of length π is constructed in the high-coherence channel signal, centered on the corresponding endpoints; the fringe signal within the local analysis window is subjected to the Hilbert transform to obtain the instantaneous phase distribution; a first index point is selected within the local window at the start point. Second index point Select the third index point within the local window at the termination end. and the fourth index point . Corresponding to the start and end points of the stripe signal. correspond The first extreme value position after that. Corresponding to the end point of the stripe signal. In the corresponding window The first extreme position before.

[0076] The following formula can be used to calculate the phase difference between two feature points in an edge region. ,

[0077]

[0078]

[0079] in These are the phase values ​​of the four index points of the fringe signal of the k-th lens.

[0080] The phase difference can be converted into the equivalent number of fringes using the following formula.

[0081]

[0082] Step 11: Based on the obtained integer part of the fringe and the residual correction term, calculate the thickness of the lens under test using the interference fringe counting method.

[0083] The thickness of the object being measured can be derived using the following formula, which is based on the number of interference fringes plus phase correction and optical principles. The integer number of fringes can be obtained by using the integer part of the fringes and the residual correction term. ,

[0084]

[0085] in Let the thickness be the thickness of the k-th lens. Let be the number of fringes of the k-th lens. Let be the equivalent fringe number of the k-th lens. The wavelength of a low-coherence light source. Let be the group refractive index of the k-th lens at the corresponding wavelength.

[0086] Step 12: Switch the 1×2 optical switch 21 so that the measurement light is incident from the reverse direction of the lens group 23 to measure the thickness of the other half of the lens group, and repeat steps 3 to 11.

[0087] This invention also provides a multi-lens group lens center thickness detection device with low coherence detection, comprising a low-coherence light source 1 and a high-coherence light source 2 connected to a first fiber optic coupler port 5-3 on a first fiber optic coupler 5 and a second fiber optic coupler port 6-1 on a second fiber optic coupler 6 via a first isolator 3 and a second isolator 4, respectively; the first fiber optic coupler port 5-1 and the first fiber optic coupler port 5-2 on the first fiber optic coupler 5 are respectively connected to a first circulator port 7-1 on a first circulator 7 via optical fibers. 1. The second circulator port 20-1 on the second circulator 20 is connected; the first circulator port 7-2 on the first circulator 7 and the second fiber optic coupler port 6-4 on the second fiber optic coupler 6 are respectively connected to the wavelength division multiplexer port 8-2 and the wavelength division multiplexer port 8-1 on the wavelength division multiplexer 8 via optical fibers; the wavelength division multiplexer port 8-3 is connected to the first 1×N switch 9, and is sequentially connected to the movable guide rail 13 via the delay fiber group 10, the second 1×N switch 11, and the first collimator 12. Standard plane mirror 15; the second circulator port 20-2 of the second circulator 20 is connected to the first 1×2 switch port 21-1, the second 1×2 switch port 21-2 and the third 1×2 switch port 21-3 are respectively connected to the second collimator 22 and the third collimator 24, and point towards the lens group 23 under test; the second fiber optic coupler port 6-2 and the second fiber optic coupler port 6-3 are respectively connected to the Faraday rotator mirror 14 and the photodetector 17 via optical fibers; the first circulator 7 on the first circulator... Circulator port 3 7-3 and second circulator port 3 20-3 on the second circulator 20 are respectively connected to third fiber optic coupler port 2 19-2 and third fiber optic coupler port 4 19-4 on the third fiber optic coupler 19 via optical fibers. Third fiber optic coupler port 19-1 and third fiber optic coupler port 3 19-3 on the third fiber optic coupler 19 are connected to the balanced detector 18. Photodetector 17 and balanced photodetector 18 are connected to data acquisition card 16 for synchronous acquisition and analysis of signals.

[0088] In this embodiment, the working center wavelength of the low-coherence light source is 1310 nm. The selected high-coherence light source has a working center wavelength of 1550 nm. The coupling ratio of fiber coupler 6 is 90:1, the coupling ratio of fiber coupler 5 is 99:1, and the coupling ratio of fiber coupler 19 is 50:50. A 1×2 switch 21 is installed in the measurement arm to switch between the forward and reverse measurement optical paths of the lens group under test. When dealing with a large number of lenses, the bidirectional detection is used to measure the center thickness of the lenses in the corresponding directions, and the results from the two directions are combined to obtain a complete multi-lens center thickness measurement. The 1×N switch is configured in conjunction with the delay optical path to introduce additional optical paths as needed under different measurement conditions. When the equivalent optical path of the lens system under test is large, the optical path between the reference arm and the measurement arm is matched through a suitable delay optical path. To match the intensity of the measurement light and the reference light, three fiber couplers with different splitting ratios are used to perform splitting and interference respectively. According to the theory of optical coherence, this splitting ratio matches the intensity of the measurement light and the reference light, which is beneficial for improving the contrast of the interference signal.

[0089] The system optical path is as follows:

[0090] A high-coherence light source 2 emits laser light, which is unidirectionally isolated by a second isolator 4 before entering a second beam splitter 6. The second beam splitter 6 splits the incident low-coherence laser light into two optical signals. One signal, a weak-power beam, enters the output of the second beam splitter 6, is directed to a Faraday rotator 14, and then returns as a reference beam. The other signal, a measurement beam, passes through a wavelength division multiplexer 8 and a collimator 12 before illuminating a movable standard plane mirror 15. The beam is reflected back to the fiber coupler by the movable standard plane mirror. The two beams superimpose to generate an interference signal, which is then collected by a photodetector 17. The laser light emitted from the low-coherence light source 1 is first unidirectionally isolated by a first isolator 3 before entering a first beam splitter 5. The first beam splitter 5 splits the incident laser light into two optical signals. One signal enters the output of the first beam splitter 5 into a first circulator 7, and the other signal enters the output of the first beam splitter 5 into a second circulator 20. The laser light entering the first circulator 7 is directionally output from its port and enters the wavelength division multiplexer 8. A 1×N switch 9 and a 1×N switch 11 are installed in the reference arm. One end of each switch is connected to the main optical path of the reference arm, and the other end is connected to the delay optical path. When the equivalent optical path length of the lens or lens system under test is small, the measurement light directly enters the main measurement optical path through the 1×N optical switch 9. When the equivalent optical path length of the lens group under test is large, the measurement light enters the delay optical path by switching the first 1×N switch 9 and the second 1×N optical switch 11, thereby increasing the optical path length of the reference arm. The beam combined by the wavelength division multiplexer 8 continues to propagate along the reference arm and passes through the first collimator 12. The reference light is reflected by the movable standard plane mirror 15, and the reflected light returns along the original incident optical path. The reflected light returns and passes through the first collimator 12 again, then enters the wavelength division multiplexer 8, and is output from the port of the wavelength division multiplexer 8. It then enters the first circulator 7, is output from the port of the first circulator 7, and enters the fiber coupler 19 as the reference arm return optical signal. Simultaneously, another laser beam split from the fiber coupler 5 enters the second circulator 20 and is output from the port of the second circulator 20. By switching the 1×2 optical switch 21, the laser beam can be selectively incident from the forward or reverse direction of the lens group 23 under test, collimated by the collimator 22 or collimator 24, and then emitted onto the surface of the lens group 23 under test. After reflection at the surface of the lens group under test, the beam returns along the original optical path, passes through the collimator, re-enters the second circulator 20, and is output from the port of the second circulator 20, forming the measurement arm return optical signal. The return optical signal from the measuring arm and the return optical signal from the reference arm simultaneously enter the fiber coupler 19, coupling and splitting the two incident lights into two interference optical signals of equal intensity, which are then output to the two input terminals of the balanced photodetector 18. The balanced photodetector 18 performs differential detection processing on the two input interference optical signals, converting the optical signals into electrical signals and effectively suppressing common-mode noise.The processed electrical signal is output to the data acquisition card 16 and transmitted to the computer for acquisition, display and analysis.

[0091] The principle of this invention is as follows: A movable standard plane mirror 15 moves along a guide rail driven by a motor. When the equal optical path condition is met, it interferes with the light reflected from the front and rear surfaces of each lens in the lens group 23 under test. The computer collects data through a data acquisition card 16 and records the position of the movable standard plane mirror 15 when interference occurs. Based on the recorded position readings, the center thickness of each lens can be calculated using a built-in algorithm in the software.

[0092] The measurement principle of this invention is based on low-coherence interference theory. When the optical path of the reference arm is equal to or nearly equal to the optical path of the corresponding reflective interface of the lens under test in the measuring arm, a significant interference signal will be generated at the system output. By locating the position where the interference signal appears, accurate measurement of the front and rear surfaces of the lens under test can be achieved. During the measurement process, the movable standard plane mirror 15 moves precisely linearly along the guide rail under the drive of a motor, thereby continuously changing the optical path of the reference arm. When the optical path of the reference arm satisfies the equal optical path condition with the optical path of the reflected light from the front and rear surfaces of the corresponding lenses in the lens group 23 under test, the system generates interference signals respectively. The computer acquires the corresponding interference signals through the data acquisition card 16 and records the displacement position of the movable standard plane mirror 15 when interference occurs. Based on the difference in displacement corresponding to the two interference signals, the center thickness of the lens can be calculated through the built-in algorithm in the software. For multi-lens groups or lens systems with a large optical path range, this invention introduces a switchable delayed optical path in the reference arm. The optical signal is selectively introduced into the delayed optical path by the first 1×N switch 9 and the second 1×N switch 11, thereby increasing the optical path length of the reference arm. This allows the optical path of the reference arm to cover lens systems with long optical paths or wide measurement ranges, ensuring that the equal optical path matching requirements for low coherence interference are met under different measurement conditions, thus improving the system's adaptability and measurement stability. Furthermore, a forward and reverse switchable measurement optical path structure is set in the measurement arm. By switching the 1×2 optical switch 21, the measurement light can be selectively incident from the front or back of the lens group 23 under test, realizing forward or reverse measurement of the lens group under test. When dealing with a large number of lenses, bidirectional detection is used to measure the center thickness of the lenses in the corresponding direction. The results from the two directions are combined to obtain a complete multi-lens center thickness measurement. This effectively avoids the problem of weak or even missing interference signal energy caused by the energy attenuation of reflected light from the rear lens in traditional unidirectional detection, ensuring that stable interference signals can be formed on the front and rear surfaces of all lenses in the multi-lens group, achieving accurate measurement of the entire lens thickness and the inter-lens spacing.

[0093] Example 2

[0094] To verify the feasibility and accuracy of the low-coherence interferometric measurement system and method described in this invention in actual measurement, the center thickness of the lens group under test was measured and verified using the system of this invention.

[0095] In this embodiment, the measurement system structure is as follows: Figure 1 As shown, the center wavelength of the low-coherence light source is 1310 nm, and the center wavelength of the high-coherence light source is 1550 nm. A standard plane mirror 15 is mounted on a movable guide rail 13 and moves linearly at a constant speed along the optical axis under motor drive. The sampling rate of the system's data acquisition card 16 is set to 10.4 MSa / s. The sample to be tested is a lens group 23 consisting of 20 lenses, with a refractive index of n=1.5168. In the experiment, the lens group is numbered along the direction of light propagation, with the lenses closest to the forward incident port of the light being numbered sequentially from lens 1 to lens 20.

[0096] During the forward measurement process, the measurement light is introduced from the forward incident end of the lens group by switching the 1×2 switch 21, and the lenses numbered 1 to 15 are measured. When it is determined that the equivalent optical path range of this part of the lens group is large, a suitable delayed optical path is introduced by switching the 1×N switch to ensure that the reference arm and the measurement arm always meet the low coherence interference condition. After confirming that the system is in normal condition, the motor controller is started to make the standard plane mirror move at a constant speed along the guide rail, and the system enters the real-time data acquisition mode. After the plane mirror finishes moving, the acquired raw interference signal is imported into the system for subsequent processing. The acquired raw signal is sequentially processed by removing DC components, motion smoothing, and amplitude normalization to improve the signal-to-noise ratio and suppress noise interference. Figure 4 The image shows the envelope waveform after preprocessing the original low-coherence signal. Then, MATLAB's `findpeaks` function is used to filter the peaks of the low-coherence interference signal, obtaining the approximate positions of each significant interference peak. A local analysis window is constructed within the neighborhood of each selected interference peak, and the local signal is subjected to first-order difference and squaring to obtain the gradient energy distribution. Based on this, an improved centroid method is used to accurately calculate the interference peak positions, thereby achieving high-precision positioning of the interference signals on the front and rear surfaces of the lens. Figure 5 The smooth waveform of the low-coherence interference signal and the peak position determined by the improved centroid method are given.

[0097] Based on the order of the interference peaks in the signal, adjacent interference peaks are assigned to the front and rear surfaces of the same lens, respectively. For lenses 1 to 15, interference signals from the high-coherence channels are extracted from the corresponding interference peak intervals on the front and rear surfaces and preprocessed: first, a Fast Fourier Transform is performed on the extracted signals, followed by a low-pass filter in the frequency domain; then, an inverse transform and smoothing process is performed on the filtered signals to obtain stable interference fringe signals. Extreme point detection is performed on the processed fringe signals, and the number of peaks and troughs in each interval is counted, with the average value taken as the integer part of the fringe count. Figure 7 The results of peak and trough detection for a highly coherent smoothed signal are presented. Simultaneously, local intervals of length π are selected within the corresponding phase intervals, centered at the start and end points of the fringe signal, and Hilbert transforms are performed to extract the instantaneous phase. The phase difference is then calculated to obtain the fringe residual correction term. Figure 8 The diagram illustrates the Hilbert transform expansion phase results and the corresponding phase difference calculation near the start and end points of the interference signal. Based on the integer part of the fringes and the residual correction term obtained in the above steps, the center thicknesses of lenses 1 to m are calculated, and the results are shown in Table 1 below.

[0098] Table 1

[0099] Lens Code 1 2 3 4 5 6 7 8 Lens thickness 4.025746 4.106015 4.167816 4.096104 4.022626 4.060262 4.170873 4.024193 Lens Code 9 10 11 12 13 14 15 Lens thickness 4.158345 4.097088 4.129893 4.090127 4.093332 4.099059 4.094997

[0100] Subsequently, by switching the 1×2 switch, the measurement light entered from the reverse incident end of the lens group, and the lenses numbered 20 to 6 were measured. The above signal acquisition and data processing steps were repeated. The thickness detection results of the lenses numbered 20 to 6 are shown in Table 2 below:

[0101] Table 2

[0102] Lens Code 20 19 18 17 16 15 14 13 Lens thickness 4.022126 4.104926 4.169319 4.096873 4.023901 4.094997 4.099059 4.093332 Lens Code 12 11 10 9 8 7 6 Lens thickness 4.090127 4.129893 4.097088 4.158345 4.024193 4.170873 4.060262

[0103] Under both forward and reverse measurement conditions, the system can accurately detect low-coherence interference signals corresponding to the front and rear surfaces of each lens, verifying the measurement reliability of the method of the present invention under different incident directions.

[0104] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for detecting the center thickness of a multi-lens group lens based on low-coherence detection, characterized in that, include: Step 1: Connect each device and fiber optic patch cord according to the system flowchart and connect to the PC. Turn on the low coherence light source (1) and the high coherence light source (2), open the data acquisition software and fill in the relevant parameters. Number the lens group consisting of n lenses along the direction of light propagation. Number the lenses closest to the forward incident port of the measurement light as the 1st to the nth lens and group the lenses to be tested. Measure the 1st to mth lenses when incident forward and measure the nth to n-m+1th lenses when incident backward. Finally, integrate the two sets of data to obtain the thickness of all n lenses. Step 2: Switch the 1×2 optical switch (21) to make the measurement light enter from the front of the lens group (23) to measure the first group of lenses; Step 3: Determine the equivalent optical path range of the first set of lenses. When the equivalent optical path is large, switch the 1×N optical switch (9) and the 1×N optical switch (11) to introduce a suitable delayed optical path to ensure that the reference arm and the measuring arm meet the low coherence interference condition. Step 4: After confirming that the system is in normal condition, press the motor controller switch. At this time, the standard plane mirror moves and the system enters the real-time data acquisition mode. Step 5: After the plane mirror has finished moving, import the acquired raw signal into the system for processing; Step 6: Perform DC component removal, motion smoothing, and amplitude normalization operations on the original signal. Step 7: Use MATLAB's findpeaks function to filter out significant peaks and exclude noisy peaks. Step 8: Using the interference peaks confirmed in the previous step as the center, construct a local analysis window in their neighborhood; perform first-order difference on the local signal and square it to obtain the gradient energy distribution; calculate the precise position of the interference peaks using the improved centroid method based on the gradient energy distribution. Step 9: According to the order of the interference peaks in the signal, assign adjacent interference peaks to the front and rear surfaces of the same lens, respectively. Extract the high coherence channel signal from the interval corresponding to the interference signals of the front and rear surfaces of the 1st to mth lenses and preprocess it. The specific method is as follows: perform a fast Fourier transform on the extracted signal and apply a low-pass filter in the frequency domain; perform an inverse transform on the filtered signal and smooth it to obtain a stable interference fringe signal for each lens corresponding to the peak pair. Step 10: Based on the stripe signal obtained in the above steps, perform extreme point detection, count the number of peaks and valleys in each interval, and take the average value as the integer part of the stripe; take local intervals of length π in the corresponding phase intervals centered at the start and end of the stripe signal, perform Hilbert transform on each, calculate the instantaneous phase, and obtain the edge phase difference; calculate the stripe residual correction term based on the phase difference. Step 11: Based on the obtained integer part of the fringe and the residual correction term, obtain the final number of fringe, and then calculate the thickness of the lens being measured; Step 12: Switch the 1×2 optical switch (21) to make the measurement light incident from the reverse side of the lens group (23) to measure the thickness of the other half of the lens group, and repeat steps 3 to 11.

2. The method for detecting the center thickness of a multi-lens group lens based on low-coherence detection according to claim 1, characterized in that, In step 8, an improved centroid method is used as the weight to accurately locate the interference peak position by employing the gradient energy obtained by first-order difference and squaring the local signal. The calculation formula is as follows: in, This represents the local gradient energy after removing the mean.

3. The method for detecting the center thickness of a multi-lens group lens based on low-coherence detection according to claim 2, characterized in that, In step 10, the number of integer stripes is calculated according to the following formula. : ； in Let be the total number of peaks in the fringe signal of the k-th lens. Let be the total number of valleys in the fringe signal of the k-th lens.

4. The method for detecting the center thickness of a multi-lens group lens based on low-coherence detection according to claim 3, characterized in that, In step 10, the fringe residual correction term is obtained based on the Hilbert transform, specifically including: at the start and end points of the fringe signal, a local analysis window of length π is constructed in the high-coherence channel signal, centered on the corresponding endpoints; the fringe signal within the local analysis window is subjected to the Hilbert transform to obtain the instantaneous phase distribution; and a first index point is selected within the local window at the start point. Second index point Select the third index point within the local window at the termination end. and the fourth index point Corresponding to the start and end points of the stripe signal. correspond The first extreme value position after; Corresponding to the end point of the stripe signal. In the corresponding window The first extreme position before; The phase difference between the two endpoints of the edge region and the adjacent extreme values ​​is calculated using the following formula. , ； ； in These are the phase values ​​of the four index points of the fringe signal of the k-th lens; The phase difference is converted into the equivalent number of fringes using the following formula. 。 5. The method for detecting the center thickness of a multi-lens group lens based on low-coherence detection according to claim 4, characterized in that, In step 11, the thickness of the lens under test can be derived based on the number of interference fringes plus phase correction, combined with optical principles, according to the following formula. The integer number of fringes is obtained by using the integer part of the fringes and the residual correction term. , ； in Let the thickness be the thickness of the k-th lens. Let be the number of fringes of the k-th lens. Let be the equivalent fringe number of the k-th lens. The wavelength of a low-coherence light source. Let be the group refractive index of the k-th lens at the corresponding wavelength.

6. A multi-lens group lens center thickness detection device for low-coherence detection, characterized in that, include: Low-coherence light source (1) and high-coherence light source (2) are connected to the first fiber coupler (5) and the second fiber coupler (6) respectively through the first isolator (3) and the second isolator (4); the first fiber coupler (5) is connected to the first circulator (7) and the second circulator (20) through optical fibers; the first circulator (7) and the second fiber coupler (6) are connected to the wavelength division multiplexer (8) through optical fibers, the wavelength division multiplexer (8) is connected to the first 1×N switch (9), and sequentially points to the movable standard plate on the guide rail (13) through the delay fiber group (10), the second 1×N switch (11) and the first collimator (12). A face mirror (15); a second circulator (20) is connected to a second collimator (22) and a third collimator (24) respectively via a 1×2 switch (21), and points towards the lens group under test (23); a second fiber coupler (6) is connected to a Faraday rotator (14) and a photodetector (17) via optical fibers; a first circulator (7) and a second circulator (20) are connected to a third fiber coupler (19) via optical fibers, and the third fiber coupler (19) is connected to a balanced detector (18); the photodetector (17) and the balanced photodetector (18) are connected to a data acquisition card (16) for synchronous acquisition and analysis of signals.

7. The multi-lens group lens center thickness detection device for low coherence detection according to claim 6, characterized in that, The 1×2 optical switch (21) is located in the measuring arm and is used to selectively switch the measuring light from the lens group (23) under test to be incident in the forward or reverse direction.

8. The multi-lens group lens center thickness detection device for low coherence detection according to claim 7, characterized in that, The 1×N optical switch (9), the delay fiber (10), and the 1×N optical switch are set in the reference arm to switch the optical path according to the equivalent optical path range of the lens group (23) under test, and to compensate the optical path of the reference arm by introducing multiple sets of delay fibers (10).

9. The multi-lens group lens center thickness detection device for low coherence detection according to claim 8, characterized in that, The center wavelength of the low coherence light source (1) is 1310nm, and the center wavelength of the high coherence light source (2) is 1550nm.

10. The multi-lens group lens center thickness detection device for low coherence detection according to claim 9, characterized in that, The coupling ratio of the first fiber coupler (5) is 99:1, the coupling ratio of the second fiber coupler (6) is 90:10, and the coupling ratio of the third fiber coupler (19) is 50:50.

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