CN116026244A - System for measuring lens group lens surface spacing and refractive index - Google Patents

Abstract

The present disclosure relates to a system for measuring lens group lens spacing and refractive index. The system comprises: the light source device is used for generating a first optical frequency comb signal and a second optical frequency comb signal, wherein the first optical frequency comb signal and the second optical frequency comb signal have a preset repetition frequency difference; the measuring device is used for enabling the first optical frequency comb signal to be incident into the reference light path and the measuring light path, generating a reflected optical signal and enabling the reflected optical signal to be reflected back to the coupler, wherein the reference light path comprises a second reflecting mirror, the measuring light path comprises a first reflecting mirror and a lens group to be measured, the lens group to be measured comprises N lenses to be measured, and N is greater than or equal to 1; the coupler is used for enabling the second optical frequency comb signal to interfere with the reflected optical signal and generating a measurement interference optical signal; and the signal acquisition processing device is used for determining the mirror surface distance and/or the refractive index of the lens group to be measured according to the interference light signal. By the method, the mirror surface distance and the refractive index of the lenses in the lens group can be measured in high precision and in real time.

Description

Measuring system for lens distance and refractive index

technical field

The disclosure relates to the technical field of optical precision metrology, and in particular to a measurement system for the distance between mirror surfaces of a lens group and the refractive index.

Background technique

With the development of technology in various research fields, the performance requirements for optical systems are getting higher and higher, especially for high-precision optical systems such as exposure objective lenses of lithography machines. Accurate and rapid measurement of the distance between lens groups is very important for improving optical systems. Performance and guaranteed assembly quality are critical. When using the optical measurement method to measure the distance between the mirrors of the lens group, because the optical path value is used to calculate the geometric thickness of the lens, the refractive index of the lens needs to be referred to, and the error between the nominal value and the real value of the refractive index of the lens will affect the accuracy of the measurement. Therefore, it is necessary to Real-time measurement of lens refractive index.

Contents of the invention

In view of this, the present disclosure proposes a technical solution of a measurement system for the distance between mirror surfaces of a lens group and the refractive index.

According to one aspect of the present disclosure, there is provided a measurement system for the distance between mirrors of a lens group and the refractive index, the system includes a light source device, a measurement device, a coupler and a signal acquisition and processing device; the light source device is used to generate a first An optical frequency comb signal and a second optical frequency comb signal, wherein the first optical frequency comb signal and the second optical frequency comb signal have a preset repetition frequency difference; the measuring device is used to make the first optical frequency comb signal An optical frequency comb signal is incident on the reference optical path and the measurement optical path to generate a reflected optical signal, and the reflected optical signal is returned to the coupler, wherein the reference optical path includes a second mirror, and the measurement optical path includes The first reflecting mirror and the lens group to be tested, the lens group to be tested includes N lenses to be tested, and N is greater than or equal to 1; the coupler is used to make the second optical frequency comb signal and the reflected optical signal Interfering to generate a measurement interference light signal; the signal acquisition and processing device is used to determine the mirror spacing and/or the refractive index of the lens group to be measured according to the measurement interference light signal.

In a possible implementation manner, the measuring device includes a circulator, a collimating mirror and a spectroscopic device;

The circulator is used to make the first optical frequency comb signal incident to the collimating mirror; the collimating mirror is used to make the first optical frequency comb signal exit from the optical fiber to free space; the The light splitting device is used to split the outgoing first optical frequency comb signal into a measurement signal and a reference signal, make the measurement signal incident on the measurement optical path, and make the reference signal incident on the reference optical path.

In a possible implementation manner, the spectroscopic device is used to make the measurement signal reflected by the lens group under test and the first mirror, and the reference signal reflected by the second mirror The signals are combined into the reflected light signal.

In a possible implementation manner, the collimating mirror is used to couple the reflected optical signal into an optical fiber from a free space.

In a possible implementation manner, the circulator is used to make the reflected optical signal incident to the coupler.

In a possible implementation manner, the collimator includes a variable-focus collimator.

In a possible implementation manner, the signal acquisition and processing device includes a detector, a filter device, a digital acquisition card and a computer processing unit; the detector is used to convert the measurement interference optical signal into a measurement electrical signal; The filter device is used to filter the measurement electrical signal; the digital acquisition card is used to save the filtered measurement electrical signal to the computer processing unit; the computer processing unit is used to The filtered measurement electrical signal is subjected to time-frequency analysis to determine the mirror spacing and/or the refractive index of the lens group to be tested.

In a possible implementation manner, the performing time-frequency analysis on the filtered measurement electrical signal to determine the mirror distance and/or the refractive index of the lens group to be tested includes: analyzing the filtered measurement electrical signal Analyze the electrical signal to determine the phase-frequency information of the measured electrical signal; determine the optical path between adjacent mirrors in the lens group to be tested according to the phase-frequency information; determine the optical path between adjacent mirrors according to the process to determine the mirror spacing and/or refractive index of the lens group to be tested.

In a possible implementation manner, the determining the optical distance between adjacent mirrors in the lens group to be tested according to the phase frequency information includes: determining two adjacent mirrors according to the phase frequency information The time difference between the measurement electrical signals; according to the preset repetition frequency difference between the first optical frequency comb signal and the second optical frequency comb signal, determine the difference between the measurement electrical signal and the first optical frequency comb signal The repetition frequency ratio between them; according to the time difference and the repetition frequency ratio, determine the optical distance between adjacent mirror surfaces in the lens group to be tested.

In a possible implementation manner, the determining the mirror distance and/or the refractive index of the lens group to be tested according to the optical distance between the adjacent mirror surfaces includes: determining the position of the lens to be tested in the system Measure the optical path change between the first mirror and the second mirror before and after the lens group; determine the lens group to be tested according to the optical path change and the optical distance between the adjacent mirrors The mirror spacing and/or refractive index of .

In the measurement system for lens group mirror spacing and refractive index of the embodiment of the present disclosure, the first optical frequency comb signal generated by the light source device is incident on the reference optical path and the measurement optical path through the measuring device, and the reflected optical signal is determined, and the reflected optical signal is made Interfering with the second optical frequency comb signal to generate a measurement interference light signal; time-frequency analysis is performed on the measurement interference light signal through the signal acquisition and processing device, which can realize real-time, fast and accurate simultaneous measurement of the mirror spacing and refraction of the lens group to be tested Rate.

Other features and aspects of the present disclosure will become apparent from the following detailed description of exemplary embodiments with reference to the accompanying drawings.

Description of drawings

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the disclosure and, together with the specification, serve to explain the principles of the disclosure.

FIG. 1 shows a schematic structural diagram of a measurement system for lens spacing and refractive index of a lens group according to an embodiment of the present disclosure;

Fig. 2 shows a schematic diagram of the positional relationship of interference signals corresponding to different reflective surfaces in the time domain according to an embodiment of the present disclosure;

FIG. 3 shows a schematic diagram of performing time-frequency analysis on electrical measurement signals according to an embodiment of the present disclosure;

Fig. 4 shows a schematic diagram of a phase-frequency curve and a phase difference corresponding curve of two radio frequency interference signals according to a disclosed embodiment;

Fig. 5 shows a schematic diagram of a dual-comb multi-heterodyne interference principle and the amplitude-frequency curve of the interference signal according to an embodiment of the present disclosure.

Detailed ways

Various exemplary embodiments, features, and aspects of the present disclosure will be described in detail below with reference to the accompanying drawings. The same reference numbers in the figures indicate functionally identical or similar elements. While various aspects of the embodiments are shown in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

The word "exemplary" is used exclusively herein to mean "serving as an example, embodiment, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as superior or better than other embodiments.

The term "and/or" in this article is just an association relationship describing associated objects, which means that there can be three relationships, for example, A and/or B can mean: A exists alone, A and B exist simultaneously, and there exists alone B these three situations. In addition, the term "at least one" herein means any one of a variety or any combination of at least two of the more, for example, including at least one of A, B, and C, which may mean including from A, Any one or more elements selected from the set formed by B and C.

In addition, in order to better illustrate the present disclosure, numerous specific details are given in the following specific implementation manners. It will be understood by those skilled in the art that the present disclosure may be practiced without some of the specific details. In some instances, methods, means, components and circuits that are well known to those skilled in the art have not been described in detail so as to obscure the gist of the present disclosure.

A common optical system consists of multiple lenses, and the geometric thickness of each lens determines its position along the optical axis in the optical system; the surface geometric distance between adjacent lenses is used to compensate for errors in the manufacturing process and to perform tolerances test. The geometric thickness of the lens and the surface geometric distance between adjacent lenses can be expressed by the mirror spacing of the lens group. Therefore, accurate and fast measurement of the mirror spacing of the lens group is crucial to improving the performance of the optical system and ensuring the assembly quality of the optical system, especially for high-precision optical systems such as exposure objectives of lithography machines.

In the prior art, a non-contact and non-destructive optical measurement method is usually used to measure the mirror distance of the lens group. However, since the direct measurement result of the optical measurement method is the optical distance, the optical measurement method in the prior art usually needs to calculate the geometry of the lens according to the nominal value of the refractive index of each lens in the lens group, combined with the measured optical distance. thickness. Since the nominal value of the refractive index of the lens is not the true refractive index under the current environmental conditions, the calculation of the geometric thickness of the lens through the nominal value of the refractive index will inevitably produce errors. Therefore, in order to accurately measure the mirror pitch of the lens group, it is necessary to measure the refractive index of the lens in real time.

In the prior art, methods such as confocal and low-coherence interferometry are commonly used to measure the mirror distance of the lens group. However, there is a mechanical scanning process in the measurement process of these methods, which is relatively sensitive to the vibration of the measurement environment. In addition, when the number of lenses in the lens group to be measured increases, the time for mechanical scanning will also increase, resulting in a limited measurement speed.

The disclosure provides a measurement system for the distance between the mirror surfaces of the lens group and the refractive index, which can be used to measure the distance between the mirror surfaces of the lens group and the refractive index. The measurement system provided by the present disclosure will be described in detail below.

Fig. 1 shows a schematic structural diagram of a measurement system for lens group mirror spacing and refractive index according to an embodiment of the present disclosure; as shown in Fig. 1, along the light propagation direction, the measurement system may include a light source device 101, a measurement device 102, A coupler 104 and a signal acquisition and processing device 105 .

The light source device 101 is configured to generate a first optical frequency comb signal and a second optical frequency comb signal, wherein the first optical frequency comb signal and the second optical frequency comb signal have a preset repetition frequency difference;

The measurement device 102 is used to make the first optical frequency comb signal incident on the reference optical path and the measurement optical path, generate a reflected optical signal, and make the reflected optical signal reflect back to the coupler 104, wherein the reference optical path includes a second reflector 1032, and measure The optical path includes a first reflector 1031 and a lens group to be tested, and the lens group to be tested includes N lenses to be tested, where N is greater than or equal to 1;

A coupler 104, configured to interfere with the second optical frequency comb signal and the reflected optical signal to generate a measurement interference optical signal;

The signal acquisition and processing device 105 is configured to determine the mirror distance and/or the refractive index of the lens group to be tested according to the measured interference light signal.

Wherein, the light source device 101 may include at least two optical frequency comb generating devices, which are respectively used to generate the first optical frequency comb signal and the second optical frequency comb signal; the first optical frequency comb signal and the second optical frequency comb signal may also be generated in other ways. The optical frequency comb signal is not specifically limited in the present disclosure.

The repetition frequency of the first optical frequency comb signal can be determined as f _r1 , the repetition frequency of the second optical frequency comb signal can be determined as f _r2 , the first optical frequency comb signal and the second optical frequency comb signal are used to realize the dual optical comb multi- Foreign interference. The first optical frequency comb signal and the second optical frequency comb signal have a small preset repetition frequency difference _Δfr , and the frequency difference can be set according to actual usage requirements, which is not specifically limited in the present disclosure.

The lens group to be tested can be placed in the light emitting direction of the measuring device 102, and the upper limit of the number of lenses to be tested in the lens group to be tested is limited by the intensity of the first optical frequency comb signal. As long as the intensity of the first optical frequency comb signal is sufficient to collect the reflected light generated on each reflective surface, the number of lenses to be tested can be infinitely increased.

The lens to be tested may be any type of optical lens, for example, a concave lens or a convex lens, which is not specifically limited in the present disclosure.

The materials and specifications of each lens to be tested in the lens group to be tested may be the same or different, which is not specifically limited in the present disclosure.

The measurement device 102 will be described in detail later in conjunction with possible implementations of the present disclosure, and details are not repeated here.

The coupler 104 can make the second optical frequency comb signal coincide with the reflected optical signal, and then cause the second optical frequency comb signal to interfere with the reflected optical signal, realize the conversion from the optical frequency domain to the radio frequency domain, and generate measurement interference in the radio frequency domain light signal. The coupler 104 may be any type of coupler, for example, a fiber coupler or a space coupler, which is not specifically limited in the present disclosure.

The signal collection and processing device 105 will be described in detail later in combination with possible implementation manners of the present disclosure, and details are not repeated here.

The mirror spacing of the lens group to be tested means the distance between two adjacent mirrors in the lens group to be tested, which may include the geometric thickness of each lens to be tested in the lens group to be tested, and the distance between two adjacent lenses to be tested surface geometric distance. The refractive index of the lens group to be tested includes the refractive index of each lens to be tested in the lens group to be tested.

In the measurement system for lens group mirror spacing and refractive index of the embodiment of the present disclosure, the first optical frequency comb signal generated by the light source device is incident on the reference optical path and the measurement optical path through the measuring device, and the reflected optical signal is determined, and the reflected optical signal is made Interfering with the second optical frequency comb signal to generate a measurement interference light signal; time-frequency analysis is performed on the measurement interference light signal through the signal acquisition and processing device, which can realize fast real-time and accurate simultaneous measurement of the mirror spacing and refractive index of the lens group to be tested .

In a possible implementation, the measurement device 102 includes a circulator 1021, a collimating mirror 1022 and a light splitting device 1023; the circulator 1021 is used to make the first optical frequency comb signal incident to the collimating mirror 1022; the collimating mirror 1022 , used to make the first optical frequency comb signal exit from the optical fiber to free space; the optical splitter 1023 is used to split the emitted first optical frequency comb signal into a measurement signal and a reference signal, so that the measurement signal is incident on the measurement optical path, Make the reference signal incident on the reference optical path.

Taking the above-mentioned FIG. 1 as an example, as shown in FIG. 1 , the measurement device 102 may include a circulator 1021 , a collimating mirror 1022 and a spectroscopic device 1023 along the light propagation direction.

The circulator 1021 can be used to make the first optical frequency comb signal incident to the collimating mirror 1022 . For the circulator 1021, any type of circulator can be selected according to the system optical path setting in actual use, for example, a three-port circulator, which is not specifically limited in the present disclosure.

The collimating mirror 1022 can be used to make the first optical frequency comb signal output from the optical fiber to free space, and can adjust the focus state of the output first optical frequency comb signal. The first optical frequency comb signal emitted from the optical fiber to free space can be in a parallel state, or in a slightly convergent or divergent state. The collimating mirror 1022 may be any type of collimating mirror, for example, a transmissive collimating mirror, which is not specifically limited in the present disclosure. In a possible implementation manner, the collimating mirror 1022 includes a variable-focus collimating mirror. When the collimating mirror 1022 is a variable-focus collimating mirror, the focus of the outgoing light beam can be changed through the collimating mirror 1022, thereby changing the intensity of the reflected light signal of each surface, improving the signal-to-noise ratio of the reflected light signal, thereby improving the measurement accuracy and increasing the The number of lenses that can be measured.

The optical splitting device 1023 can be used to split the outgoing first optical frequency comb signal into two beams of measurement signal and reference signal, and make the measurement signal incident on the measurement optical path and the reference signal incident on the reference optical path. The light splitting device 1023 may be any type of light splitting device, for example, a beam splitter, which is not specifically limited in the present disclosure.

The measurement signal and reference signal generated by the optical splitting device 1023, the light directions of which are perpendicular to each other as shown in Figure 1, as long as the reflected light can be returned to the optical splitting device along the original path of the measurement optical path and the reference optical path, the measurement signal and reference signal The light direction of the signal depends on the light splitting device 1023, which is not specifically limited in the present disclosure.

The lens group to be tested can be placed between the first mirror 1031 and the spectroscopic device 1023 . For a specific way of placing the lens group to be tested, reference may be made to the way of placing lenses in the related art, for example, placing the lens group on a bracket, which is not specifically limited in the present disclosure.

After the measurement signal is incident on the lens group to be tested, reflected light can be generated on the front and rear mirror surfaces of each lens in the lens group to be tested and the first mirror 1031 . After the reference signal is incident on the second mirror 1032, reflected light can also be generated. All the reflected light can return to the spectroscopic device 1023 along the measurement optical path and the reference optical path.

In a possible implementation manner, the spectroscopic device 1023 is used to combine the measurement signal reflected by the lens group to be tested and the first reflector 1031 , and the reference signal reflected by the second reflector 1032 into a reflected light signal.

The measurement signal reflected by the lens group to be tested and the first reflector 1031 and the reference signal reflected by the second reflector 1032 return to the spectroscopic device 1023 in free space, and are combined into a reflected light signal by the spectroscopic device 1023 .

In a possible implementation manner, the collimating mirror 1022 is used to couple the reflected optical signal into the optical fiber from free space.

The reflected light signals combined by the optical splitting device 1021 are incident to the collimating mirror 1022, and the reflected light can be coupled into the optical fiber from free space through the collimating mirror 1022. The reflected optical signal may re-enter the circulator 1021 along the optical path. The repetition frequency of the reflected optical signal is the same as that of the first optical frequency comb signal.

In a possible implementation manner, the circulator 1021 is used to make the reflected optical signal incident to the coupler 104 .

The reflected optical signal can enter the coupler 104 through the circulator, and interfere with the second optical frequency comb signal as local oscillator light at the coupler 104 to generate a measurement interference optical signal.

Taking the aforementioned FIG. 1 as an example, as shown in FIG. 1 , the circulator 1021 may be a three-port circulator. The first optical frequency comb signal is incident on the port 1 of the circulator 1021, and is emitted to the collimating mirror 1022 from the port 2 of the circulator 1021; exits to the coupler 104.

In a possible implementation, the signal acquisition and processing device 105 includes a detector 1051, a filter device 1052, a digital acquisition card (not shown) and a computer processing unit 1053; the detector 1051 is used to convert the measurement interference light signal into Measuring electrical signals; filter device 1052, for filtering the measuring electrical signals; digital acquisition card, for saving the filtered measuring electrical signals to the computer processing unit; computer processing unit 1053, for filtering the filtered measuring electrical signals Carry out time-frequency analysis to determine the mirror spacing and/or refractive index of the lens group to be tested.

Taking the above-mentioned FIG. 1 as an example, as shown in FIG. 1 , the signal acquisition and processing device 105 includes a detector 1051 , a filter device 1052 , a digital acquisition card and a computer processing unit 1053 along the transmission direction of the measurement interference optical signal.

The detector 1051 can be used to collect the measurement interference light signal output by the coupler 104, and convert the measurement interference light signal from an optical signal in the radio frequency domain to a measurement electrical signal. The detector 1051 may be any type of detector used for collecting optical interference signals, which is not specifically limited in the present disclosure.

The type of the electrical signal to be measured depends on the detector 1051, and it is usually a radio frequency electrical signal, and may also be other types of electrical signals, which are not specifically limited in the present disclosure.

The filtering device 1052 can be used to filter the electrical measurement signal, such as high-pass filtering and low-pass filtering. The filter device 1052 may be any type of electrical filter device, such as a radio frequency electrical filter, which is not specifically limited in the present disclosure. The function of the filter is to filter out high-frequency noise, low-frequency DC signal, etc., so that the signal-to-noise ratio of the collected signal is better, and the measurement accuracy is higher.

In order to prevent the frequency spectrum aliasing of the dual optical comb system, to realize the accurate measurement of the spectral phase, and then ensure the accuracy and authenticity of the measured electrical signal for the computer processing unit 1053 to perform time-frequency analysis, the bandwidth of the filter device 1052 should be smaller than the first 1/2 of the repetition frequency of the optical frequency comb signal and/or the second optical frequency comb signal.

The digital acquisition card can be used to save the filtered measured electrical signal to the computer processing unit 1053. The parameters of the digital acquisition card should satisfy the Nyquist sampling theorem, that is, the sampling frequency of the digital acquisition card should be greater than the frequency of the measured electrical signal 2 times.

The computer processing unit 1053 can be used to perform time-frequency analysis on the filtered measurement electrical signal to determine the mirror spacing and/or refractive index of the lens group to be tested. The computer processing unit 1053 may be any electronic device or system capable of performing a process of determining the mirror distance and/or the refractive index of the lens group to be tested, which is not specifically limited in the present disclosure.

Exemplarily, the computer processing unit 1053 may be a user equipment (User Equipment, UE), a mobile device, a user terminal, a terminal, a handheld device, a computing device, or a vehicle-mounted device, etc. Exemplarily, examples of some terminals are: a display, a smart Mobile phone or portable device, mobile phone (Mobile Phone), tablet computer, laptop computer, handheld computer, mobile Internet device (Mobile Internet device, MID), wearable device, virtual reality (Virtual Reality, VR) device, augmented reality (Augmentedreality, AR ) equipment, wireless terminals in Industrial Control, wireless terminals in Selfdriving, wireless terminals in Remote Medical Surgery, wireless terminals in Smart Grid, transportation safety Wireless terminals in (Transportation Safety), wireless terminals in Smart City, wireless terminals in Smart Home, wireless terminals in Internet of Vehicles, etc. For example, the server may be a local server or a cloud server.

The process of time-frequency analysis of the filtered measurement electrical signal by the computer processing unit 1053 to determine the mirror distance and/or refractive index of the lens group to be tested will be described in detail later in the following in conjunction with possible implementations of the present disclosure. Here I won't go into details.

In a possible implementation manner, time-frequency analysis is performed on the filtered measurement electrical signal to determine the mirror distance and/or refractive index of the lens group to be tested, including: analyzing the filtered measurement electrical signal to determine the measurement electrical Phase-frequency information of the signal; according to the phase-frequency information, determine the optical distance between adjacent mirrors in the lens group to be tested; determine the mirror spacing and/or refractive index of the lens group to be tested according to the optical distance between adjacent mirrors.

After the reflected optical signal is injected into the coupler 104, it is asynchronously optically sampled by the second optical frequency comb signal to generate 2N+2 measurement interference optical signals, which are converted from the optical frequency domain to the radio frequency domain. The measured interference light signals correspond to the reflective surfaces of each lens mirror (two for each lens), the reflective surfaces of the first reflector 1031 and the second reflector 1032 . The positional relationship of each measured interference optical signal in the time domain can reflect the physical positional relationship of the corresponding reflective surface in space.

Fig. 2 shows a schematic diagram of a positional relationship of interference signals corresponding to different reflective surfaces in the time domain according to an embodiment of the present disclosure. As shown in Fig. 2, it is taken as an example to place a lens group to be tested including 3 lenses to be tested in the system. The measurement interference light signals S1 to S8 can represent the 8 measurement interference light signals collected after the lens group to be tested is placed in the system, and S1 to S8 are arranged in the time domain position according to the time sequence of their corresponding reflected light generation. Wherein, the measurement interference light signal S1 corresponds to the reflection surface of the second reflector 1032, the measurement interference light signal S2 corresponds to the reflection surface of the first mirror surface of the lens under test 201, and the measurement interference light signal S3 corresponds to the reflection surface of the second mirror surface of the lens under test 201. For the reflective surface, the measurement interference light signal S4 corresponds to the reflection surface of the first mirror surface of the lens under test 202, the measurement interference light signal S5 corresponds to the reflection surface of the second mirror surface of the lens under test 202, and the measurement interference light signal S6 corresponds to the reflection surface of the second mirror surface of the lens under test 203. For the reflective surface of one mirror, the measured interference light signal S7 corresponds to the reflective surface of the second mirror of the lens under test 203 , and the measured interference light signal S8 corresponds to the reflective surface of the first reflective mirror 1031 . By measuring the positional relationship of the interference light signals S1 to S8 in the time domain, the spatial physical relationship of the reflective surface of each mirror in the lens group to be tested can be reflected.

When the lens group to be tested is not placed in the system, the measurement interference signals S1 and S8' corresponding to the second reflector and the first reflector can be determined; then the lens group to be tested is placed in the system, and the second reflector, the first reflector can be determined A mirror and the measurement interference signals S1 to S8 corresponding to the two reflection surfaces of each lens to be tested in the lens group to be tested; combined with the positional relationship of the measurement interference signals in the time domain during the two processes, the lens to be tested can be determined Set the mirror spacing and index of refraction.

Similarly, the measured electrical signal converted and determined by the detector 1051 also has the above characteristics. By analyzing the filtered measurement electrical signal, the phase frequency information of the measurement electrical signal can be determined.

Fig. 3 shows a schematic diagram of performing time-frequency analysis on a measurement electrical signal according to an embodiment of the present disclosure. As shown in Figure 3, by performing Fourier transform on the measured electrical signal in Figure 3(a), the phase-frequency information in Figure 3(c) and the amplitude-frequency information in Figure 3(b) corresponding to the measured electrical signal can be obtained ( That is, the phase-frequency curve and the amplitude-frequency curve). According to the phase-frequency information corresponding to each measurement electrical signal, the optical distance between adjacent mirror surfaces in the lens group to be tested can be determined. The optical path can represent the actual travel distance of light, and the optical path between adjacent mirrors can be determined by the mirror spacing and refractive index of the lens group to be tested. Therefore, by determining the optical distance between adjacent mirrors in the lens group to be tested, the distance between the mirrors and/or the refractive index of the lens group to be tested can be determined.

The following will combine the possible ways of the present disclosure to determine the optical distance between adjacent mirrors in the lens group to be tested according to the phase frequency information; and determine the mirror spacing of the lens group to be tested according to the optical distance between adjacent mirrors and/or the two processes of the refractive index are described in detail, and will not be repeated here.

In a possible implementation manner, determining the optical distance between adjacent mirrors in the lens group to be tested according to the phase-frequency information includes: determining the time difference between two adjacent measurement electrical signals according to the phase-frequency information; The preset repetition frequency difference between the first optical frequency comb signal and the second optical frequency comb signal determines the repetition frequency ratio between the measured electrical signal and the first optical frequency comb signal; determines the lens group to be tested according to the time difference and the repetition frequency ratio The optical path between adjacent mirrors in .

The phase-frequency information determined by Fourier transform analysis, in which the phase is wrapped phase, that is, the phase is truncated in the range of [-π, π] or [0, 2π], and phase unwrapping (also called phase Expand) to obtain continuous phase-frequency information.

Taking the above-mentioned FIG. 3 as an example, the continuous phase-frequency information in FIG. 3( d ) can be determined by unwrapping the phase-frequency information in FIG. 3( c ).

Fig. 4 shows a schematic diagram of a phase-frequency curve and a corresponding phase difference curve of two radio frequency interference signals according to a disclosed embodiment. As shown in FIG. 4 , by making a difference between the phase values corresponding to each frequency in the phase-frequency curves after phase expansion of any two radio frequency interference signals, the corresponding curve of the phase difference can be determined. By determining the slope value of the curve corresponding to the phase difference, the time difference between the corresponding two radio frequency interference signals can be determined. The time difference between any two RF interference signals can be expressed as the following formula (1):

表示一个射频干涉信号的相位值曲线，

表示另一个射频干涉信号的相位值曲线，

表示相位差对应的曲线，

表示相位差对应的曲线的斜率值，Δt表示任意两个射频干涉信号之间的时间差。in,

Represents the phase value curve of a radio frequency interference signal,

Represents the phase value curve of another radio frequency interference signal,

represents the curve corresponding to the phase difference,

Indicates the slope value of the curve corresponding to the phase difference, and Δt indicates the time difference between any two radio frequency interference signals.

Correspondingly, the time difference between any two measured electrical signals can also be expressed by the above formula (1).

Fig. 5 shows a schematic diagram of a dual-comb multi-heterodyne interference principle and the amplitude-frequency curve of the interference signal according to an embodiment of the present disclosure. As shown in FIG. 5 , since the reflected optical signal and the second optical frequency comb signal have a small preset repetition frequency difference _Δfr , the two optical comb signals will generate a certain time slip every time a time period passes. According to the principle of dual-comb multi-heterodyne interference, the information in the optical frequency domain will be mapped to the radio frequency domain, and the interval between the radio frequency interference signal and the first optical frequency comb signal is reduced by Δf _r /f _r1 , while the corresponding time domain information cycle , that is, the period of the radio frequency interference signal is amplified by f _r1 /Δf _r . Therefore, the scaling factor from the optical frequency domain to the radio frequency domain can be determined, representing the repetition frequency ratio Δf _r /f _r1 between the radio frequency interference signal and the first optical frequency comb signal.

Correspondingly, the repetition frequency ratio between the measurement electrical signal and the first optical frequency comb signal is Δf _r /f _r1 .

According to the time difference between two adjacent measurement electrical signals in the time domain position, and the repetition frequency ratio between the measurement electrical signal and the first optical frequency comb signal, the distance between the corresponding two adjacent mirrors in the lens group to be tested can be determined light path between. In combination with the above, the optical distance between adjacent mirrors can also be determined through the mirror spacing and refractive index of the lens group to be tested. Therefore, the optical path between two adjacent mirrors can be expressed as the following formula (2):

L _i =2·n(d _i )·d _i =c·Δt·Δf _r /f _r1 (2)

Among them, n(di) is the refractive index of the corresponding medium, di is the mirror distance between two adjacent mirrors, and c is the speed of light in vacuum.

Taking the aforementioned FIG. 2 as an example, as shown in FIG. 2 , the lens group to be tested includes a lens to be tested 201 , a lens to be tested 202 and a lens to be tested 203 .

When the materials of the three lenses to be tested are the same, it can be determined that the refractive indices n of the three lenses to be tested are equal, and the refractive index of air can be determined as a known fixed value n ₀ according to the measurement environment. At this moment, the light is incident from the lens under test 201 until it exits from the lens under test 203, and the optical path passed can be expressed as the following formula (3):

Among them, L ₁ , L ₂ , L ₃ , L ₄ , and L ₅ respectively represent the optical path difference between two beams of reflected light generated by each beam passing through adjacent mirrors, Δt _3-2 , Δt _4-3 , Δt _5-4 , Δt _6-5 , Δt _7-6 respectively represent the time difference between every two adjacent measurement electrical signals in the time domain position, d ₁ , d ₂ , d ₃ , d ₄ , d ₅ represent the lens group The mirror pitch, that is, d ₁ is the thickness of the first lens, d ₂ is the distance between the adjacent surfaces of the first and second lenses, d ₃ is the thickness of the second lens, d ₄ is the second and third lenses The distance between adjacent surfaces, d ₅ is the thickness of the third lens.

When the materials of the three lenses to be tested are different, the refractive indices of the three lenses to be tested can be determined to be n ₁ , n ₂ and n ₃ respectively. At this moment, the light is incident from the lens under test 201 until it exits from the lens under test 203, and the optical path passed can be expressed as the following formula (4):

In a possible implementation, determining the mirror distance and/or refractive index of the lens group to be tested according to the optical distance between adjacent mirror surfaces includes: determining the first mirror 1031 before and after the lens group to be tested is placed in the system The optical path change between the second reflecting mirror 1032; according to the optical path change and the optical path between adjacent mirrors, determine the mirror spacing and/or refractive index of the lens group to be tested.

Placing the lens group to be tested in the system will cause the medium between the equivalent common optical path of the first reflector 1031 and the second reflector 1032 to change, and the medium change will cause the refractive index of the corresponding position to be different from that of when the system is not placed with the lens group to be tested different, which in turn causes the optical distance between the first reflector 1031 and the second reflector 1032 to change. According to the optical path change and the optical path between adjacent mirrors, the mirror distance and/or the refractive index of the lens group to be tested can be determined.

Taking the above-mentioned Fig. 2 as an example, as shown in Fig. 2, after the lens group to be tested is placed in the system, the measurement electrical signal S8 corresponding to the first reflector 1031 is compared to that before the lens group to be tested is placed in the system, the first reflector 1031 The position of the corresponding measurement electrical signal S8' in the time domain has changed. The change of the optical path between the first reflector 1031 and the second reflector 1032 can be reflected by the position change of the measurement electrical signal corresponding to the first reflector 1031 in the time domain.

When the materials of the three lenses to be tested are the same, the three lenses to be tested can be put into the system at the same time. The medium between the first mirror 1031 and the second mirror 1032 changes after the lens group to be tested is placed, and the medium including the lens to be tested 201, the lens to be tested 202 and the lens to be tested 203 is changed from air to the lens to be tested Corresponding to the material, the medium change can be expressed as a refractive index change (nn ₀ ). Therefore, the optical path change ΔL can be expressed as the following formula (5):

ΔL=c·(Δt _8-1 −Δt _8′-1 )·Δf _r /f _r1 =2(nn ₀ )(d ₁ +d ₃ +d ₅ ) (5)

Among them, Δt _8-1 represents the time difference between the measurement electrical signals corresponding to the first reflector 1031 and the second reflector 1032 after the lens group to be measured is placed in the system, and Δt _8'-1 represents the lens group to be measured is placed in the system Before, the time difference between the electrical signals corresponding to the first reflector 1031 and the second reflector 1032 is measured.

According to the above formula (3) and formula (5), the refractive index of the lens to be tested can be determined through the mathematical relationship between the optical path and the reflecting surface. The refractive index n of the lens to be tested can be expressed as the following formula (6):

Further, according to the above formula (3), formula (5) and formula (6), through the mathematical relationship between the optical path and the reflection surface, the mirror spacing d ₁ , d ₂ , d ₃ , d ₄ , d ₅ .

When the materials of the three lenses to be tested are different, the three lenses to be tested can be added to the system in sequence, and it is respectively determined that after the lens to be tested 201 is placed in the system, after the lenses to be tested 201 and 202 are placed in the system, and the system The optical path changes of the lenses 201, 202, and 203 to be tested are put in.

Putting the lens under test 201 into the system, the optical path change ΔL ₁ can be expressed as the following formula (7):

ΔL ₁ =2(n ₁ -n ₀ )d ₁ (7)

Continuing to put the lens to be tested 202 into the system, the optical path change _ΔL can be expressed as the following formula (8):

ΔL ₂ =2(n ₂ -n ₀ )d ₃ (8)

Continuing to put the lens under test 203 into the system, the optical path change _ΔL3 can be expressed as the following formula (9):

ΔL ₃ =2(n ₃ -n ₀ )d ₅ (9)

According to the above formula (4), and formulas (7) to (9), through the mathematical relationship between the optical path and the reflection surface, the refractive indices of the three lenses to be tested can be determined as n ₁ , n ₂ and n ₃ . The refractive index _n1 of the lens to be measured 201 can be expressed as the following formula (10):

The refractive index _n of the lens to be measured 202 can be expressed as the following formula (11):

The refractive index _n of the lens to be measured 203 can be expressed as the following formula (12):

Further, according to the above formula (4), and formula (7) to formula (12), through the mathematical relationship between the optical path and the reflection surface, the mirror spacing d ₁ , d ₂ , d ₃ of the lens group to be tested can be determined , d ₄ , d ₅ .

In the embodiment of the present disclosure, the first optical frequency comb signal generated by the light source device is incident on the reference optical path and the measurement optical path through the measuring device, the reflected optical signal is determined, and the reflected optical signal and the second optical frequency comb signal are interfered by a coupler , to generate the measurement interference light signal; through the signal acquisition and processing device to perform time-frequency analysis on the measurement interference light signal, without prior knowledge of the lens, it can realize fast real-time and accurate simultaneous measurement of the mirror spacing and refractive index of the lens group to be tested . In addition, the measurement process will not be affected by the mechanical vibration of the system, and increasing the number of measured lenses will not increase the measurement time.

It should be noted that although FIG. 1 is used as an example to introduce the measurement system of the distance between the lens groups and the refractive index as above, those skilled in the art can understand that the present disclosure should not be limited thereto. In fact, the user can flexibly set the specific structure of the measurement system for the distance between the mirrors of the lens group and the refractive index according to personal preferences and/or actual application scenarios, as long as the measurement of the distance between the mirrors of the lens group and/or the refractive index based on the above process can be realized. Can.

Having described various embodiments of the present disclosure above, the foregoing description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and alterations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principle of each embodiment, practical application or technical improvement in the market, or to enable other ordinary skilled in the art to understand each embodiment disclosed herein.

Claims (10)

1. The system is characterized by comprising a light source device, a measuring device, a coupler and a signal acquisition and processing device;

the light source device is used for generating a first optical frequency comb signal and a second optical frequency comb signal, wherein the first optical frequency comb signal and the second optical frequency comb signal have a preset repetition frequency difference;

the measuring device is used for enabling the first optical frequency comb signal to be incident into a reference light path and a measuring light path, generating a reflected light signal and enabling the reflected light signal to be reflected back to the coupler, wherein the reference light path comprises a second reflecting mirror, the measuring light path comprises a first reflecting mirror and a lens group to be measured, and the lens group to be measured comprises N lenses to be measured, and N is greater than or equal to 1;

the coupler is used for enabling the second optical frequency comb signal to interfere with the reflected optical signal to generate a measurement interference optical signal;

the signal acquisition processing device is used for determining the mirror surface distance and/or the refractive index of the lens group to be tested according to the interference light signal.

2. The system of claim 1 or 2, wherein the measuring device comprises a circulator, a collimator mirror and a spectroscopic device;

the circulator is used for enabling the first optical frequency comb signal to be incident to the collimating mirror;

the collimating mirror is used for enabling the first optical frequency comb signal to be emitted from the optical fiber to a free space;

the beam splitter is configured to split the outgoing first optical frequency comb signal into a measurement signal and a reference signal, so that the measurement signal is incident on the measurement light path, and the reference signal is incident on the reference light path.

3. The system of claim 2, wherein the light splitting device is configured to combine the measurement signal reflected by the lens group under test and the first mirror and the reference signal reflected by the second mirror into the reflected light signal.

4. The system of claim 2, wherein the collimating mirror is configured to couple the reflected light signal from free space into an optical fiber.

5. The system of claim 2, wherein the circulator is configured to cause the reflected optical signal to be incident on the coupler.

6. The system of claim 2, wherein the collimating mirror comprises a variable focus collimating mirror.

7. The system of claim 1 or 2, wherein the signal acquisition processing device comprises a detector, a filter device, a digital acquisition card, and a computer processing unit;

the detector is used for converting the measurement interference optical signal into a measurement electrical signal;

the filter device is used for filtering the measurement electric signal;

the digital acquisition card is used for storing the filtered measurement electric signals to the computer processing unit;

and the computer processing unit is used for carrying out time-frequency analysis on the filtered measurement electric signals and determining the mirror surface distance and/or the refractive index of the lens group to be measured.

8. The system of claim 7, wherein the time-frequency analysis of the filtered measurement electrical signal to determine the mirror pitch and/or refractive index of the lens group under test comprises:

analyzing the filtered measurement electric signals and determining phase frequency information of the measurement electric signals;

determining the optical path between adjacent mirror surfaces in the lens group to be detected according to the phase frequency information;

and determining the mirror surface distance and/or the refractive index of the lens group to be tested according to the optical path between the adjacent mirror surfaces.

9. The system of claim 8, wherein determining the optical path between adjacent mirrors in the lens group under test based on the phase frequency information comprises:

determining the time difference between two adjacent measurement electric signals according to the phase frequency information;

determining a repetition frequency ratio between the measurement electric signal and the first optical frequency comb signal according to a repetition frequency difference preset by the first optical frequency comb signal and the second optical frequency comb signal;

and determining the optical path between the adjacent mirror surfaces in the lens group to be tested according to the time difference and the repetition frequency ratio.

10. The system of claim 8, wherein determining the mirror pitch and/or refractive index of the lens group under test based on the optical path between the adjacent mirrors comprises:

determining the optical path change between the first reflecting mirror and the second reflecting mirror before and after the lens group to be measured is placed in the system;

and determining the mirror surface distance and/or the refractive index of the lens group to be tested according to the optical path change and the optical path between the adjacent mirror surfaces.

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