A High-Precision Peak Positioning Method for a Dual-Channel Low-Coherence Interference System

Abstract

The present invention relates to the technical field of dual-channel positioning, and specifically provides a high-precision peak positioning method for a dual-channel low-coherence interference system, which includes the following steps: parallel sampling of a low-coherence interference module and a long-coherence ranging module is performed through the dual-channel low-coherence interference system to obtain a low-coherence interference signal and a long-coherence ranging signal; for the low-coherence interference signal, the modulation method is used to calculate the envelope peak-to-peak sampling points; for the long-coherence ranging signal, the phase shift method is used to calculate the truncated phase of each long-coherence sampling point; for the long-coherence ranging signal, the phase unwrapping is used to solve the position of the peak sampling point; for the low-coherence interference signal, the phase shift method is used to calculate the phase difference between the envelope peak-to-peak sampling point and the actual peak of the envelope; the geometric position in the reference arm corresponding to the actual peak of the envelope peak is solved through phase compensation, achieving high-precision envelope peak-to-peak positioning and significantly reducing the positioning error of the traditional modulation method.

Description

Technical Field

[0001] The present invention relates to the technical field of dual-channel positioning, and particularly to a high-precision peak positioning method for a dual-channel low-coherence interference system. Background Technique

[0002] The low-coherence interference method is a high-precision and non-contact interference measurement technique, which is suitable for measuring the lens thickness and air gap size of an optical system. This method uses a broadband light source as the coherent light source. The coherence length of the light source is short, and interference peaks are generated only when the optical paths of the measurement light and the reference light are equal. Therefore, it has high spatial positioning accuracy.

[0003] The modulation method is a relatively simple and efficient existing method for realizing the peak-to-peak positioning of the envelope of low-coherence interference signals. However, there are two problems when calculating the peak-to-peak value of the envelope of interference signals by calculating the modulation degree, such as Figure 5 shown:

[0004] First, the modulation method has a positioning error of one sampling step. That is, the point with the maximum modulation degree is not necessarily the sampling point of the peak-to-peak value of the envelope, but may be a neighboring point of the sampling point of the peak-to-peak value of the envelope;

[0005] Second, the peak-to-peak value of the envelope of the low-coherence interference signal often does not exactly fall on the sampling point of the peak-to-peak value of the envelope, but is between the sampling point of the peak-to-peak value of the envelope and a neighboring low-coherence sampling point, which will introduce a positioning error of half a sampling step.

[0006] In view of this, the present invention provides a high-precision peak positioning method for a dual-channel low-coherence interference system. Summary of the Invention

[0007] The purpose of the present invention is to provide a high-precision peak positioning method for a dual-channel low-coherence interference system. This method introduces a phase-shifting method for phase compensation on the basis of the traditional modulation method to solve the problem of positioning error of the peak-to-peak value of the envelope.

[0008] To achieve the above purpose, the present invention provides the following technical solutions:

[0009] In a first aspect, the present invention provides a high-precision peak positioning method for a dual-channel low-coherence interference system, which is used for peak-to-peak positioning of interference signal envelopes and includes the following steps:

[0010] Step S1, parallel sampling is performed on a low-coherence interference module and a long-coherence ranging module through a dual-channel low-coherence interference system, so as to obtain a low-coherence interference signal and a long-coherence ranging signal;

[0011] Step S2, for the low-coherence interference signal, the modulation method is used to calculate the sampling point of the peak-to-peak value of the envelope;

[0012] Step S3: For the long coherent ranging signal, use the phase shift method to calculate the truncated phase of each long coherent sampling point.

[0013] Step S4: For the truncated phase of the long coherent sampling points, solve the position of the peak sampling point through phase unwrapping.

[0014] Step S5: Use the phase shift method in Step S4 to calculate the phase difference between the peak-to-peak sampling point of the envelope and the actual peak of the envelope.

[0015] Step S6: Solve the geometric position in the reference arm corresponding to the actual peak of the envelope through phase compensation.

[0016] As a preferred technical solution of the first aspect of the present invention, to solve the distance between each surface of the lens group or the size of the air gap based on the peak-to-peak position of the envelope, the following steps are included:

[0017] After determining the peak-to-peak position of the envelope, subtract the adjacent peak-to-peak positions of the envelope measured sequentially during the guide rail scanning process to obtain the geometric distance of the optical surfaces corresponding to these two peaks in the reference arm, and convert it to the optical path length according to the air group refractive index corresponding to the wavelength light.

[0018] In the low coherent interference signal, the optical path length is a multiple of the optical thickness between the corresponding optical surfaces of the adjacent peak-to-peak positions of the envelope in the measured mirror group, and the geometric thickness between the adjacent optical surfaces in the measured object is extracted.

[0019] As a preferred technical solution of the first aspect of the present invention, the low coherent interference signal includes multiple envelope peaks, each envelope peak represents an optical surface respectively, and the optical surface is located through the peak sampling point of the envelope peak. The low coherent interference signal uses the modulation method to calculate the peak sampling point of the envelope peak, and the specific process is as follows:

[0020] For the low coherent interference signal, use the threshold method to filter the interference signal, and then perform interference partitioning on the filtered signal. Each independent envelope signal is divided into a region, and corresponds to an independent optical surface.

[0021] For the envelope signal corresponding to an optical surface, the process of determining the corresponding peak sampling point of the envelope peak is as follows: sequentially select the intensity values of each low coherent sampling point and its adjacent four low coherent sampling points in the interference envelope region, and calculate the modulation degree of the central low coherent sampling point.

[0022] The low coherent sampling point with the maximum modulation degree in the interference envelope region is the peak sampling point of the envelope peak of this region.

[0023] As a preferred technical solution of the first aspect of the present invention, to solve the position of the peak sampling point of each envelope peak based on the long coherent ranging signal, the following steps are included:

[0024] The truncated phase is restored to a continuous absolute phase through phase unwrapping, and the position of the peak sampling point is solved.

[0025] For a long coherent ranging signal, the specific process of calculating the truncated phase of each long coherent sampling point by the phase shift method is as follows:

[0026] For the long coherent ranging signal, taking each long coherent sampling point as the center, the intensity values of consecutive n long coherent sampling points are selected, and the truncated phase of this long coherent sampling point is calculated through the n-step phase shift formula.

[0027] As a preferred technical solution of the first aspect of the present invention, the acquisition logic of the position of the peak sampling point is:

[0028] Traverse the phase values of all long coherent sampling points and record the number of phase truncation points;

[0029] Based on the number of phase truncation points and the truncated phase of the peak sampling point, the absolute phase of the peak sampling point is calculated through phase superposition, and the absolute phase determines the position of the peak sampling point.

[0030] As a preferred technical solution of the first aspect of the present invention, for a low coherent interference signal, the specific process of calculating the phase difference between the peak-to-peak sampling point of the envelope and the actual peak of the envelope of the low coherent interference signal by the phase shift method is:

[0031] Taking the preliminarily located peak-to-peak sampling point of the envelope as the center, the intensity values of consecutive n low coherent sampling points are selected, and the phase difference between this peak sampling point and the actual peak is calculated through the n-step phase shift formula.

[0032] As a preferred technical solution of the first aspect of the present invention, after the phase difference between the peak-to-peak sampling point of the envelope and the actual peak of the envelope of the low coherent interference signal, the peak-to-peak positioning of the low coherent interference signal envelope is obtained through phase compensation, and the geometric position in the reference arm corresponding to the actual peak of the envelope is solved.

[0033] In a second aspect, the present invention provides a dual-channel low coherent interference system for the measurement data of the first aspect, including two parts: a low coherent interference module and a long coherent ranging module, and the two modules adopt parallel sampling;

[0034] The low coherent interference module uses a broadband light source as the coherent light source. The coherence length of this light source is short, and interference peaks can only be generated when the optical paths of the measurement light and the reference light are equal. Therefore, it has very good spatial positioning characteristics; this module mainly collects low coherent interference signals for subsequent processing;

[0035] The long coherent ranging module has a long coherence length and can generate stable long coherent interference signals within a large range; this module mainly collects long coherent ranging signals for real-time measurement of the displacement of the stepping motor;

[0036] The low-coherence interference module and the long-coherence ranging module perform parallel sampling.

[0037] As a preferred technical solution of the second aspect of the present invention, the specific process of the low-coherence interference module collecting the low-coherence interference signal is as follows:

[0038] The light emitted by the low-coherence light source is split by a beam splitter into a low-coherence measurement light beam and a low-coherence reference light beam;

[0039] The low-coherence measurement light passes through a collimating lens and is incident into the object to be measured, and is reflected on the various surfaces of the object to be measured. The reflected light passes through the collimating lens again and returns to the fiber optic coupler;

[0040] The low-coherence reference light passes through a collimating lens and is incident on the scanning mirror. After reflection, it passes through the collimating lens again and returns to the fiber optic coupler;

[0041] When the optical path difference between the low-coherence reference light and the low-coherence measurement light is less than the coherence length, the reflected light of the low-coherence measurement light and the reflected light of the low-coherence reference light are coherently superimposed in the fiber optic coupler to form a low-coherence interference signal.

[0042] As a preferred technical solution of the second aspect of the present invention, the specific process of the long-coherence ranging module collecting the long-coherence ranging signal is as follows:

[0043] The light emitted by the long-coherence light source is split by a beam splitter into a long-coherence measurement light beam and a long-coherence reference light beam,

[0044] The long-coherence measurement light is reflected by the scanning mirror and then reflected by the silver-plated layer on the back surface of the beam splitter plate to the photodetector;

[0045] The long-coherence reference light is reflected by the fixed mirror and then passes through the beam splitter plate and is incident on the photodetector;

[0046] The long-coherence reference light and the long-coherence measurement light are coherently superimposed in the photodetector to generate a long-coherence ranging signal.

[0047] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0048] On the one hand, most of the prior art is based on a single-channel low-coherence interference system. The single-channel system measures the displacement distance of the scanning mirror completely relying on the readings of the stepping motor or the electric displacement stage. However, affected by the displacement accuracy of the stepping motor and the electric displacement stage, the mechanical error introduced by it reaches the micron level. The dual-channel low-coherence interference system adopted by the peak positioning algorithm proposed by the present invention introduces a long-coherence ranging module and uses interference measurement technology to achieve high-precision measurement of the displacement distance of the scanning mirror, avoiding the mechanical error brought by the stepping motor and the electric displacement stage.

[0049] On the other hand, in order to achieve high-precision positioning of the peak-to-peak value of the interference signal envelope, the existing technologies generally adopt methods such as the modulation method and the Hilbert transform. These methods can only achieve the positioning of the sampling point of the peak-to-peak value of the envelope closest to the peak, but most of the time, the peak sampling point is not exactly located at the peak-to-peak value of the interference signal envelope, resulting in a positioning error of half a sampling step (in the order of hundreds of nanometers). Based on the existing modulation method, the present invention first roughly locates the position of the peak-to-peak value of the low-coherence interference signal envelope by the modulation method, then calculates the phase difference between the roughly located peak position and the actual peak of the envelope by the phase-shift method, and finally realizes high-precision positioning of the peak-to-peak value of the envelope through phase compensation, significantly reducing the positioning error of the existing technology. The positioning error of this method is in the order of nanometers. Description of the Drawings

[0050] Figure 1 It is a structural diagram of the dual-channel low-coherence interference measurement system of the present invention;

[0051] Figure 2 It is a flowchart of the envelope peak-to-peak value positioning algorithm of the present invention;

[0052] Figure 3 It is a data analysis diagram of the long-coherence interference signal of the present invention;

[0053] Figure 4 It is a low-coherence light interference signal diagram of the existing technology and the present invention; wherein: (a) is the overall interference diagram with four optical surfaces, and (b) is the partial enlarged view of the first envelope peak on the left;

[0054] Figure 5 It is a schematic diagram of the envelope peak-to-peak value positioning and positioning error achieved by the modulation method in the existing technology;

[0055] Figure 6 It is a schematic diagram of the principle of compensating the envelope peak-to-peak value positioning error by the n-step phase-shift method in the present invention. Detailed Embodiment

[0056] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0057] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "vertical", "upper", "lower", "horizontal", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention.

[0058] In the description of the present invention, it should also be noted that, unless otherwise clearly specified and defined, the terms "arranged", "installed", "connected", and "coupled" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0059] Embodiment 1

[0060] Please refer to Figure 1 , the present invention provides a technical solution: a dual-channel low-coherence interference system, including a low-coherence interference module and a long-coherence ranging module. During the measurement process, the two modules sample in parallel, so as to obtain the low-coherence interference signal and the long-coherence ranging signal for signal processing.

[0061] It should be noted that: the low-coherence interference module uses a broadband light source with a short coherence length, and only generates a low-coherence interference signal when the optical path difference between the measurement light and the reference light is close to zero. The spatial positioning of the optical surface is realized by positioning the peak-to-peak value of the low-coherence interference signal envelope; the long-coherence ranging module: uses a long-coherence light source, which can generate stable interference signals in a large range and is suitable for real-time measurement of the displacement of the scanning mirror.

[0062] The low-coherence module is responsible for accurately positioning the optical surface, and the long-coherence module is responsible for calibrating the mechanical displacement error. The combination of the two can cover the full-range measurement requirements from macroscopic displacement to microscopic positioning, solve the problem that the traditional single-channel system relies on the mechanical readings of the stepper motor or the displacement stage, directly measure the displacement by the interference method, and reduce the mechanical error to the nanometer level.

[0063] The two modules collect data simultaneously and at the same frequency to ensure the time synchronization of the data, avoid the timing error introduced by time-sharing sampling, and synchronously process the two signals through the data acquisition card to improve the overall efficiency and response speed of the system.

[0064] The peak-to-peak value of the envelope of the low-coherence interference signal may not exactly fall on the envelope peak-to-peak value sampling point, and there is an error of half a sampling step in the traditional modulation method. Through the phase compensation of the long-coherence module and combining with the phase-shifting method, the positioning error is reduced to the nanometer level; in the measurement of multi-layer optical surfaces (such as lens groups), it is necessary to obtain the surface position and spacing at the same time. The dual-channel design locates the surface through the low-coherence module and calibrates the displacement through the long-coherence module, and finally accurately calculates the geometric thickness or air gap, realizing high-precision positioning and measurement at the nanometer level.

[0065] It should be noted that parallel sampling refers to using a data acquisition card to collect data from the low-coherence interference module and the long-coherence ranging module simultaneously and at the same frequency; parallel sampling enables each low-coherence sampling point in the low-coherence interference signal to have a unique corresponding long-coherence sampling point that is collected at the same time in the long-coherence ranging signal; similarly, each long-coherence sampling point in the long-coherence ranging signal has a unique corresponding low-coherence sampling point that is collected at the same time in the low-coherence interference signal; such a corresponding relationship enables more information about this group of corresponding low-coherence sampling points and long-coherence sampling points to be calculated from multiple angles, greatly improving the calculation accuracy.

[0066] The low-coherence interference module adopts a fiber-optic structure and is used to measure the low-coherence interference signal on the surface of the object to be measured;

[0067] Specifically, in the low-coherence interference module, the low-coherence light emitted by the low-coherence light source is divided into a low-coherence measurement light beam and a low-coherence reference light beam by a fiber-optic coupler. The low-coherence measurement light is incident into the object to be measured through a collimating lens, is reflected on each surface of the object to be measured, and the reflected light is incident into the fiber-optic coupler through the collimating lens. The low-coherence reference light is incident on the scanning mirror through the collimating lens, and after being reflected by the scanning mirror, it is incident into the fiber-optic coupler again through the collimating lens. During the measurement process, the scanning mirror traverses the entire translation guide rail, and the optical path of the low-coherence reference light changes as the scanning mirror moves. When the optical path difference between the low-coherence reference light and the low-coherence measurement light reflected from a certain surface of the object to be measured is less than the coherence length, the reflected light of the low-coherence measurement light and the reflected light of the low-coherence reference light are coherently superimposed in the fiber-optic coupler to form a low-coherence interference signal. The low-coherence interference signal is collected by a photodetector, captured by a data acquisition card, and finally input into a computer for signal processing.

[0068] The long-coherence ranging module adopts a typical Michelson interferometer optical path structure and is used to measure the long-coherence ranging signal corresponding to the displacement of the stepping motor in real time to ensure the geometric position accuracy of the reference arm.

[0069] Specifically, in the long-coherence ranging module, the laser emitted by the long-coherence laser is divided into a long-coherence reference light and a long-coherence measurement light by a beam splitter. The long-coherence measurement light is reflected by the scanning mirror and then reflected by the silver coating on the back surface of the beam splitter into the photodetector. The long-coherence reference light is reflected by the fixed mirror and then transmitted through the beam splitter into the photodetector. The reflected light of the long-coherence reference light and the reflected light of the long-coherence measurement light are coherently superimposed in the photodetector to generate a long-coherence ranging signal, which is captured by a data acquisition card and finally input into a computer for signal processing.

[0070] Embodiment 2

[0071] Please refer to Figure 2, the present invention provides a technical solution: a high-precision peak positioning method for a dual-channel low-coherence interference system, which is used for the positioning of an optical surface and the measurement of the lens thickness and air gap. Each optical surface corresponds to a low-coherence interference signal that presents in the form of an envelope peak in the interference system. Therefore, by positioning the highest point of the envelope peak presented by the optical surface, the position of the optical surface can be accurately located. The method includes the following steps:

[0072] Step S1, parallel sampling is performed on the low-coherence interference module and the long-coherence ranging module through the dual-channel low-coherence interference system described in Embodiment 1, so as to obtain a low-coherence interference signal and a long-coherence ranging signal;

[0073] It should be noted that: the low-coherence interference signal is composed of one or more envelope peaks (as shown in Figure 4 (a)), and each envelope peak corresponds to an optical surface. The light emitted by the low-coherence light source has a certain wavelength range and its intensity presents a Gaussian distribution. Therefore, the measured interference signal is the superposition of the interference signals generated by the light of each wavelength emitted by the light source at a certain optical path difference (as shown in Figure 4 (b)). The intensity of the interference signal generated by the light emitted by the low-coherence light source at the optical path difference is:

[0074] (4)

[0075] Wherein, represents the spectral density distribution of the light source, and the wavelength range is .

[0076] Figure 4 The peak value of the envelope peak in the interference signal appears at the position corresponding to zero optical path difference , and as the optical path difference increases, the intensity of the envelope signal gradually decreases. The coherence length of the observable envelope signal can be expressed by Equation (5):

[0077] (5)

[0078] Wherein, is the central wavelength, is the spectral width of the wavelength.

[0079] It should be further noted that: the actually collected low-coherence interference signals and long-coherence ranging signals often contain noise signals. Since the intensity of the noise signals is relatively small compared to the envelope signals in the low-coherence interference signals, the threshold method can be used to filter out the noise signals in the low-coherence interference signals. And the influence of the noise signals in the long-coherence interference signals on the subsequent calculation results is very small. For the filtered low-coherence interference signals, the low-coherence interference envelope peaks corresponding to each optical surface are divided into independent regions, and then for each low-coherence interference envelope region, the geometric positions in the reference arm corresponding to the peak values of the envelope are solved respectively using steps S2 to S6, that is, the geometric positions in the reference arm corresponding to a certain optical surface are solved.

[0080] Step S2, for the low-coherence interference signals, calculate the sampling points of the peak values of the envelope using the modulation method;

[0081] Specifically, by collecting the intensity values of five consecutive low-coherence sampling points within the interference envelope region ; calculate the intensity values of the corresponding low-coherence sampling points for the modulation :

[0082] (6)

[0083] The scanning mirror in the low-coherence interference measurement system traverses the entire translation guide rail and calculates the modulation of all low-coherence sampling points within the interference envelope region through formula (6), and the low-coherence sampling point corresponding to the maximum modulation value is the sampling point of the peak value of the envelope .

[0084] Step S3, for the long-coherence ranging signals, calculate the truncated phase of each long-coherence sampling point using the phase-shift method;

[0085] It should be noted that: after locating the sampling points of the peak values of the envelope, it is necessary to solve the positions of each sampling point of the peak value of the envelope in order to calculate the distance between each sampling point of the peak value of the envelope, that is, the optical distance between the optical surfaces. The process of solving the positions of each sampling point of the peak value of the envelope requires the use of long-coherence ranging signals, which can be specifically divided into two steps:

[0086] First, it is necessary to calculate the truncated phase of each long-coherence sampling point in the long-coherence ranging signals;

[0087] Then, phase unwrapping is performed to restore the truncated phase into a continuous absolute phase, so as to be converted into distance information that can represent the positions of the sampling points of the peak values of the envelope.

[0088] Specifically, read the intensity values of 7 consecutive long-coherence sampling points centered on the long-coherence sampling point to be measured in the long-coherence ranging signals , and calculate the phase of the long coherent sampling point to be measured by equation (7): :

[0089] (7)

[0090] Where: The inverse tangent is a four-quadrant inverse tangent, and its value range is or , the truncated phase of each long coherent sampling point in the long coherent ranging signal is solved by equation (7).

[0091] Step S4, for the long coherent ranging signal, the position of the peak sampling point is solved by phase unwrapping;

[0092] Specifically, the phase extracted by equation (7) is truncated and is called the wrapped phase or truncated phase, such as Figure 3 As shown by the dotted line; it needs to be restored to a continuous state, such as Figure 3 As shown by the solid line, the recovered phase is called continuous phase or absolute phase, and this process is called phase unwrapping.

[0093] More specifically, for long coherent ranging signals, the specific process of solving the peak sampling point position by phase unwrapping is: traverse all phase values, use (The initial value is 0) Record the number of phase truncation points. Calculate the phase difference between the next point and the previous point. If the phase difference is less than ,but value ,on the contrary The value remains unchanged. At this time, the absolute phase of the latter point for:

[0094] (8)

[0095] Thus, the geometric position in the reference arm corresponding to the peak sampling point is calculated :

[0096] (9)

[0097] in, is the phase interval between two adjacent long coherence sampling points in the long coherence interferometer module, is the sampling step length.

[0098] Step S5, for the low coherence interference signal, using the phase shift method to calculate the phase difference between the envelope peak peak sampling point and the envelope peak actual peak;

[0099] It should be noted that: the acquired low-coherence interference signal is in the form of discrete low-coherence sampling points, but the actual low-coherence interference signal is continuous; it is obvious that there is a certain error in using the peak-to-peak sampling points of discrete low-coherence sampling points to characterize the peak value of the actual continuous low-coherence interference signal, especially when the peak value of the actual continuous low-coherence interference signal does not exactly fall on the discrete low-coherence sampling points; therefore, the error (characterized by the phase difference here) between the obtained peak-to-peak sampling point and the actual peak value of the low-coherence interference signal envelope is obtained, and this error is compensated back to achieve a higher-precision positioning of the peak-to-peak value of the low-coherence interference signal envelope;

[0100] Specifically, select the intensity values corresponding to the seven consecutive low-coherence sampling points (including the peak sampling point) closest to the peak sampling point in the low-coherence interference signal (the intensity value corresponding to the peak sampling point ), as Figure 6 shown, calculate the phase difference between the peak sampling point and the actual peak position of the envelope peak through Equation (10) :

[0101] (10)

[0102] In the formula: the arctangent mentioned is the four-quadrant arctangent, and its value range is or .

[0103] Step S6, solve the geometric position in the reference arm corresponding to the actual peak value of the envelope peak through phase compensation.

[0104] It should be noted that: in the actual signal processing process, since the peak-to-peak sampling point of the envelope peak often does not exactly fall on the peak-to-peak value of the envelope peak, there is a small phase difference between the above-mentioned obtained peak sampling point and the actual peak value of the envelope peak. Using the phase shift method to calculate the phase difference between the peak-to-peak sampling point of the envelope peak and the actual peak value of the envelope peak, in this step, the phase difference measured by the phase shift method is used to perform phase compensation on the position of the peak-to-peak sampling point of the envelope peak, so as to achieve precise positioning of the peak-to-peak value of the envelope peak and determine the geometric position in the reference arm corresponding to the peak-to-peak value of the envelope peak.

[0105] Specifically, calculate the geometric position in the reference arm corresponding to the peak-to-peak value of the envelope peak through Equation (11) :

[0106] (11)

[0107] Among them, is the wavelength of the light emitted by the low-coherence light source.

[0108] Embodiment 3

[0109] Based on Embodiment 2, this embodiment further illustrates that after the peak positioning method realizes high-precision peak positioning, it is necessary to convert the envelope peak-to-peak positioning result into the geometric distance between the two optical surfaces of the object to be measured, so as to achieve precise measurement of the lens thickness and air gap size. The specific implementation method is as follows:

[0110] For each envelope region corresponding to an optical surface, steps S2 to S6 are respectively adopted to obtain the geometric position in the reference arm corresponding to the envelope peak-to-peak value. After that, it is also necessary to calculate the geometric distance between two adjacent optical surfaces.

[0111] Specifically, after determining the envelope peak-to-peak position, the two envelope peak-to-peak positions successively measured during the guide rail scanning 、 are subtracted to obtain the geometric distance between the optical surfaces corresponding to these two peaks in the reference arm :

[0112] (1)

[0113] And the geometric distance is converted into the optical path length :

[0114] (2)

[0115] Wherein, is the air group refractive index corresponding to the light with wavelength .

[0116] Due to the equal optical path characteristic of low-coherence interference, the optical path length is equal to the optical thickness between the surfaces corresponding to the adjacent envelope peak-to-peak positions in the measured mirror group. Therefore, the geometric thickness between adjacent surfaces in the object to be measured can be expressed by the following formula:

[0117] (3)

[0118] Wherein, is the material group refractive index. Thus, the geometric thickness between adjacent surfaces of the object to be measured is calculated.

[0119] It should be further noted that in steps S3 and S5, the phase shift method is respectively used for the long-coherence ranging signal collected by the long-coherence ranging module and the low-coherence interference signal collected by the low-coherence interference module. The use of the phase shift method needs to satisfy that the phase interval between adjacent long-coherence / low-coherence sampling points is If this condition is not met, it will introduce errors to the calculation results. In an example of a preferred dual-channel low-coherence interference system used in Embodiment 1, since the light source wavelengths used in the low-coherence interference module and the long-coherence ranging module are different and they sample in parallel, it is obvious that the phase shift method usage condition where the phase interval between adjacent long-coherence / low-coherence sampling points is cannot be satisfied in both the low-coherence interference module and the long-coherence ranging module, which brings errors to the measurement of lens thickness and gap.

[0120] To make the phase interval corresponding to adjacent long-coherence / low-coherence sampling points as close as possible to , so that the overall positioning error of the system is minimized, the sampling step of the stepper motor can be set as:

[0121] (13)

[0122] where is the light wavelength emitted by the low-coherence light source, and is the light wavelength emitted by the long-coherence light source.

[0123] At this time, the phase interval corresponding to adjacent low-coherence sampling points in the low-coherence interference system is:

[0124] (14)

[0125] The phase interval corresponding to adjacent long-coherence sampling points in the long-coherence ranging system is:

[0126] (15)

[0127] The stepper motor and the high-precision electric displacement stage used in this embodiment have a certain mechanical precision. Especially during the long-distance displacement process, the direct reading of the distance will generate a large uncertainty, which does not meet the requirements of high-precision measurement; in the traditional low-coherence interference measurement system, a long-coherence ranging module is added. Through the Michelson interferometer structure, the actual displacement of the stepper motor is measured, thereby avoiding the measurement error caused by the mechanical precision of the hardware itself and realizing the improvement of measurement accuracy;

[0128] Based on the traditional interference signal envelope peak-to-peak positioning algorithm, considering the positioning accuracy of the modulation method itself and the situation where the envelope peak-to-peak value exactly falls between the low-coherence sampling points, the traditional algorithm is optimized through the phase shift method to ensure that the positioning error of the extracted envelope peak-to-peak value is at the 1 nm level. However, there are still the following deficiencies:

[0129] To avoid the measurement error caused by the mechanical precision of the stepper motor, the present invention introduces a long-coherence ranging module into the traditional low-coherence interference module, and both modules adopt the phase-shift method. However, since the light source wavelengths used in the two modules are different, it cannot meet the condition that the phase interval between two adjacent long-coherence / low-coherence sampling points in the phase-shift method is , which introduces a new error. However, the envelope peak-to-peak positioning error caused by this error is at the 1nm level, which is much smaller than the error caused by the mechanical precision of the stepper motor.

[0130] Those of ordinary skill in the art can realize that the units and algorithm steps of each example described in combination with the embodiments disclosed in the present invention can be implemented by electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are executed in a hardware or software manner depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of the present invention.

[0131] Those skilled in the art can clearly understand that for the convenience and brevity of description, the specific working processes of the above-described systems, devices, and units can refer to the corresponding processes in the foregoing method embodiments, and will not be described herein again.

[0132] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A high-precision peak positioning method for a dual-channel low-coherence interference system, characterized in that: For the peak-to-peak localization of the interference signal envelope, it includes the following steps: Step S1: Parallel sampling is performed on the low-coherence interference module and the long-coherence ranging module through a dual-channel low-coherence interference system, thereby obtaining a low-coherence interference signal and a long-coherence ranging signal; Step S2: For the low-coherence interference signal, the modulation method is used to calculate the peak-to-peak sampling points of the envelope; Step S3: For the long-coherence ranging signal, taking the long-coherence sampling points to be measured as the center, the intensity values of consecutive long-coherence sampling points are selected, and the truncated phase of each long-coherence sampling point is calculated using the phase shift method; Step S4: For the truncated phase of the long-coherence sampling points, the position of the peak sampling point is solved through phase unwrapping. The acquisition logic of the position of the peak sampling point is as follows: Traverse the phase values of all long-coherence sampling points and record the number of phase truncation points; Based on the number of phase truncation points and the truncated phase of the peak sampling point, the absolute phase of the peak sampling point is calculated through phase superposition, and the absolute phase determines the position of the peak sampling point; Step S5: Use the phase shift method in Step S4 to calculate the phase difference between the peak-to-peak sampling points of the envelope and the actual peak of the envelope; Step S6: Solve the geometric position in the reference arm corresponding to the actual peak of the envelope through phase compensation.

2. The high-precision peak positioning method for a dual-channel low-coherence interference system according to claim 1, characterized in that: Based on the peak-to-peak position of the envelope to solve the distance between each surface of the lens group or the size of the air gap, it includes the following steps: After determining the peak-to-peak position of the envelope, subtract the adjacent peak-to-peak positions of the envelope measured successively during the guide rail scanning process to obtain the geometric distance of the optical surface corresponding to these two peaks in the reference arm, and convert it to the optical path length according to the air group refractive index corresponding to the wavelength of light; In the low-coherence interference signal, the optical path length is a multiple of the optical thickness between the corresponding optical surfaces of the distance between the adjacent peak-to-peak positions of the envelope in the measured mirror group, and the geometric thickness between the adjacent optical surfaces in the measured object is extracted.

3. A high-precision peak positioning method for a dual-channel low-coherence interference system according to claim 1, characterized in that: The low-coherence interference signal includes multiple envelope peaks, and each envelope peak represents an optical surface respectively. The optical surface is located through the peak sampling points of the peak value of the envelope. The modulation method is used to calculate the peak-to-peak sampling points of the envelope for the low-coherence interference signal. The specific process is as follows: For the low-coherence interference signal, the interference signal is filtered using the threshold method, and then the filtered signal is divided into interference zones. Each independent envelope signal is divided into a zone and corresponds to an independent optical surface; For the envelope signal corresponding to an optical surface, the process of determining the corresponding peak-to-peak sampling points of the envelope is as follows: Successively select the intensity values of each low-coherence sampling point and its adjacent four low-coherence sampling points within the interference envelope region, and calculate the modulation degree of the central low-coherence sampling point; The low-coherence sampling point with the maximum modulation degree within the interference envelope region is the peak-to-peak sampling point of this region.

4. A high-precision peak positioning method for a dual-channel low-coherence interference system according to claim 3, characterized in that: Based on the long-coherence ranging signal to solve the position of each peak-to-peak sampling point of the envelope, it includes the following steps: Restore the truncated phase to a continuous absolute phase through phase unwrapping and solve the position of the peak sampling point; For the long-coherence ranging signal, the specific process of calculating the truncated phase of each long-coherence sampling point using the phase shift method is as follows: For the long-coherent ranging signal, centered on each long-coherent sampling point, select the intensity values of consecutive n long-coherent sampling points, and calculate the truncated phase of this long-coherent sampling point through the n-step phase-shift formula.

5. A high-precision peak positioning method for a dual-channel low-coherence interference system according to claim 4, characterized in that: For the low-coherent interference signal, the specific process of calculating the phase difference between the peak-to-peak sampling point of the envelope and the actual peak of the envelope using the phase-shift method is as follows: Centered on the preliminarily located peak-to-peak sampling point of the envelope, select the intensity values of consecutive n low-coherent sampling points, and calculate the phase difference between this peak sampling point and the actual peak through the n-step phase-shift formula.

6. The high-precision peak positioning method for a dual-channel low-coherence interference system according to claim 5, wherein: After obtaining the phase difference between the peak-to-peak sampling point of the envelope and the actual peak of the envelope of the low-coherent interference signal, perform phase compensation to obtain the peak-to-peak positioning of the envelope of the low-coherent interference signal, and solve the geometric position in the reference arm corresponding to the actual peak of the envelope.

7. A dual-channel low-coherence interference system for providing measurement data for the high-precision peak positioning method of the dual-channel low-coherence interference system described in claims 1-6, characterized in that: It includes two parts: a low-coherent interference module and a long-coherent ranging module, and the two modules perform parallel sampling; The low-coherent interference module uses a broad-spectrum light source as the coherent light source. This light source has a short coherence length and can only generate an interference peak when the optical paths of the measurement light and the reference light are equal. Therefore, it has very good spatial positioning characteristics; this module mainly collects low-coherent interference signals for subsequent processing; The long-coherent ranging module has a long coherence length. Centered on the long-coherent sampling point to be measured, select the intensity values of consecutive long-coherent sampling points, and use the phase-shift method to calculate the truncated phase of each long-coherent sampling point; restore the truncated phase to a continuous absolute phase through phase unwrapping, solve the position of the peak sampling point and determine the geometric position in the reference arm; collect long-coherent ranging signals for real-time measurement of the displacement of the stepping motor; The low-coherent interference module and the long-coherent ranging module perform parallel sampling.

8. A dual-channel low-coherence interference system according to claim 7, characterized in that: The specific process of the low-coherent interference module collecting low-coherent interference signals is as follows: The light emitted by the low-coherent light source is divided by a beam splitter into a low-coherent measurement light beam and a low-coherent reference light beam; The low-coherent measurement light passes through a collimating lens and is incident into the object to be measured, and is reflected on the surfaces of the object to be measured. The reflected light passes through the collimating lens again and returns to the fiber optic coupler; The low-coherent reference light passes through a collimating lens and is incident on the scanning mirror, and after reflection, passes through the collimating lens again and returns to the fiber optic coupler; When the optical path difference between the low-coherent reference light and the low-coherent measurement light is less than the coherence length, the reflected light of the low-coherent measurement light and the reflected light of the low-coherent reference light are coherently superimposed in the fiber optic coupler to form a low-coherent interference signal.

9. A dual-channel low-coherence interference system according to claim 7, characterized in that: The specific process of the long-coherent ranging module collecting long-coherent ranging signals is as follows: The light emitted by the long-coherent light source is divided by a beam splitter into a long-coherent measurement light beam and a long-coherent reference light beam, The long-coherent measurement light is reflected by the scanning mirror and then reflected by the silver coating on the back surface of the beam splitter plate into the photodetector; The long-coherent reference light is reflected by the fixed mirror and then passes through the beam splitter plate and is incident into the photodetector; The long-coherent reference light and the long-coherent measurement light are coherently superimposed in the photodetector to generate a long-coherent ranging signal.

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