System for measuring lens group mirror spacing and refractive index

Abstract

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

Description

Technical Field

[0001] This disclosure relates to the field of optical precision metrology, and in particular to a system for measuring the inter-mirror spacing and refractive index of a lens group. Background Technology

[0002] With the advancement of technology in various research fields, the performance requirements for optical systems are becoming increasingly stringent. This is especially true for high-precision optical systems such as lithography machine exposure lenses. Accurate and rapid measurement of the inter-mirror spacing of lens groups is crucial for improving the performance of optical systems and ensuring assembly quality. When measuring the inter-mirror spacing of lens groups using optical measurement methods, the lens refractive index must be referenced when calculating the lens's geometric thickness using the optical path value. However, the error between the nominal and actual values ​​of the lens refractive index can affect the accuracy of the measurement. Therefore, real-time measurement of the lens refractive index is necessary. Summary of the Invention

[0003] In view of this, this disclosure proposes a technical solution for a system for measuring the inter-mirror spacing and refractive index of a lens group.

[0004] According to one aspect of this disclosure, a system for measuring the interplanar spacing and refractive index of a lens group is provided. The system includes a light source device, a measuring device, a coupler, and a signal acquisition and processing device. The light source device is used 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 measuring device is used to incident the first optical frequency comb signal onto a reference optical path and a measuring optical path to generate a reflected light signal, and to reflect the reflected light signal back to the coupler. The reference optical path includes a second reflecting mirror, and the measuring optical path includes a first reflecting mirror and a lens group under test, wherein the lens group under test includes N lenses under test, where N is greater than or equal to 1. The coupler is used to interfere the second optical frequency comb signal with the reflected light signal to generate a measuring interference light signal. The signal acquisition and processing device is used to determine the interplanar spacing and / or refractive index of the lens group under test based on the measuring interference light signal.

[0005] In one possible implementation, the measuring device includes a circulator, a collimating lens, and a beam splitter;

[0006] The circulator is used to direct the first optical frequency comb signal onto the collimating lens; the collimating lens is used to direct the first optical frequency comb signal from the optical fiber into free space; the beam splitter is used to split the outgoing first optical frequency comb signal into a measurement signal and a reference signal, directing the measurement signal onto the measurement optical path and the reference signal onto the reference optical path.

[0007] In one possible implementation, the beam splitter is used to combine the measurement signal reflected by the lens group under test and the first reflector, and the reference signal reflected by the second reflector into the reflected light signal.

[0008] In one possible implementation, the collimating lens is used to couple the reflected light signal from free space into the optical fiber.

[0009] In one possible implementation, the circulator is used to allow the reflected light signal to be incident on the coupler.

[0010] In one possible implementation, the collimating lens includes a variable-focus collimating lens.

[0011] In one possible implementation, the signal acquisition and processing device includes a detector, a filtering 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 filtering 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 perform time-frequency analysis on the filtered measurement electrical signal to determine the inter-mirror spacing and / or refractive index of the lens group under test.

[0012] In one possible implementation, the step of performing time-frequency analysis on the filtered measurement electrical signal to determine the mirror spacing and / or refractive index of the lens group under test includes: analyzing the filtered measurement electrical signal to determine the phase frequency information of the measurement electrical signal; determining the optical path between adjacent mirrors in the lens group under test based on the phase frequency information; and determining the mirror spacing and / or refractive index of the lens group under test based on the optical path between adjacent mirrors.

[0013] In one possible implementation, determining the optical path between adjacent mirrors in the lens group under test based on the phase frequency information includes: determining the time difference between two adjacent measurement electrical signals based on the phase frequency information; determining the repetition frequency ratio between the measurement electrical signal and the first optical frequency comb signal based on a preset repetition frequency difference between the first optical frequency comb signal and the second optical frequency comb signal; and determining the optical path between adjacent mirrors in the lens group under test based on the time difference and the repetition frequency ratio.

[0014] In one possible implementation, determining the mirror spacing and / or refractive index of the lens group under test based on the optical path between the adjacent mirrors includes: determining the change in optical path between the first reflector and the second reflector before and after the lens group under test is placed in the system; and determining the mirror spacing and / or refractive index of the lens group under test based on the change in optical path and the optical path between the adjacent mirrors.

[0015] The lens group mirror spacing and refractive index measurement system of this disclosure uses a measuring device to incident a first optical frequency comb signal generated by a light source device onto a reference optical path and a measuring optical path to determine the reflected light signal. A coupler is used to interfere the reflected light signal and the second optical frequency comb signal to generate a measurement interference light signal. The measurement interference light signal is then analyzed in time and frequency by a signal acquisition and processing device, which can realize real-time, fast and accurate simultaneous measurement of the mirror spacing and refractive index of the lens group under test.

[0016] Other features and aspects of this disclosure will become clear from the following detailed description of exemplary embodiments with reference to the accompanying drawings. Attached Figure Description

[0017] The accompanying drawings, which are included in and form part of this specification, illustrate exemplary embodiments, features, and aspects of this disclosure together with the specification and serve to explain the principles of this disclosure.

[0018] Figure 1 A schematic diagram of a lens group interplanar spacing and refractive index measurement system according to an embodiment of the present disclosure is shown.

[0019] Figure 2 A schematic diagram showing the positional relationship in the time domain of interference signals corresponding to different reflective surfaces according to an embodiment of the present disclosure;

[0020] Figure 3 A schematic diagram illustrating time-frequency analysis of a measured electrical signal according to an embodiment of the present disclosure is shown.

[0021] Figure 4 A schematic diagram showing the phase frequency curves and phase difference curves of two radio frequency interference signals according to a disclosed embodiment is provided.

[0022] Figure 5 The diagram illustrates a dual-comb multi-heterodyne interference principle and an interference signal amplitude-frequency curve according to an embodiment of the present disclosure. Detailed Implementation

[0023] Various exemplary embodiments, features, and aspects of this disclosure will now be described in detail with reference to the accompanying drawings. The same reference numerals in the drawings denote elements that have the same or similar functions. Although various aspects of the embodiments are shown in the drawings, they are not necessarily drawn to scale unless specifically indicated otherwise.

[0024] The term “exemplary” as used herein means “serving as an example, embodiment, or illustration.” Any embodiment illustrated herein as “exemplary” is not necessarily to be construed as superior to or better than other embodiments.

[0025] In this document, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A alone, A and B simultaneously, and B alone. Furthermore, the term "at least one" in this document means any combination of at least two of any one or more elements. For example, including at least one of A, B, and C can mean including any one or more elements selected from the set consisting of A, B, and C.

[0026] Furthermore, to better illustrate this disclosure, numerous specific details are set forth in the following detailed description. Those skilled in the art will understand that this disclosure can be practiced without certain specific details. In some instances, methods, means, components, and circuits well known to those skilled in the art have not been described in detail in order to highlight the main points of this disclosure.

[0027] Common optical systems consist of multiple lenses, each whose geometric thickness determines its position along the optical axis within the system. The surface geometric distance between adjacent lenses is used to compensate for manufacturing errors and for tolerance testing. The lens's geometric thickness and the surface geometric distance between adjacent lenses can be represented by the mirror spacing of the lens group. Therefore, accurate and rapid measurement of the mirror spacing of the lens group is crucial for improving the performance of optical systems and ensuring their assembly quality, especially for high-precision optical systems such as lithography machine exposure lenses.

[0028] In existing technologies, non-contact and non-destructive optical measurement methods are typically used to measure the inter-mirror distance of a lens group. However, since the direct measurement result of optical measurement methods is the optical path length, existing methods usually require calculating the geometric thickness of the lens based on the nominal refractive index of each lens in the lens group and the measured optical path length. Because the nominal refractive index of a lens is not the true refractive index under current environmental conditions, calculating the geometric thickness of the lens using the nominal refractive index inevitably introduces errors. Therefore, to accurately measure the inter-mirror distance of a lens group, it is necessary to measure the refractive index of the lenses in real time.

[0029] In existing technologies, confocal and low-coherence interferometry methods are commonly used to measure the inter-mirror spacing of lens groups. However, these methods involve a mechanical scanning process, which is sensitive to vibrations in the measurement environment. Furthermore, as the number of lenses in the lens group increases, the mechanical scanning time also increases, thus limiting the measurement speed.

[0030] This disclosure provides a system for measuring the inter-mirror spacing and refractive index of a lens group, which can be used to measure the inter-mirror spacing and refractive index of a lens group. The measurement system provided in this disclosure is described in detail below.

[0031] Figure 1 This diagram illustrates a structural schematic of a lens group interplanar spacing and refractive index measurement system according to an embodiment of the present disclosure; as shown. Figure 1 As shown, along the direction of light propagation, the measurement system may include a light source device 101, a measuring device 102, a coupler 104, and a signal acquisition and processing device 105.

[0032] The light source device 101 is used 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;

[0033] The measuring device 102 is used to incident a first optical frequency comb signal onto a reference optical path and a measuring optical path, generate a reflected optical signal, and cause the reflected optical signal to be reflected back to the coupler 104. The reference optical path includes a second reflector 1032, and the measuring optical path includes a first reflector 1031 and a lens group under test. The lens group under test includes N lenses under test, where N is greater than or equal to 1.

[0034] Coupler 104 is used to interfere with the second optical frequency comb signal and the reflected light signal to generate a measurement interference light signal;

[0035] The signal acquisition and processing device 105 is used to determine the inter-mirror spacing and / or refractive index of the lens group under test based on the measured interference light signal.

[0036] The light source device 101 may include at least two optical frequency comb generators, which are used to generate a first optical frequency comb signal and a second optical frequency comb signal, respectively; the first optical frequency comb signal and the second optical frequency comb signal may also be generated in other ways, and this disclosure does not specifically limit them.

[0037] 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 and second optical frequency comb signals are used to achieve dual-comb multiheterodyne interference. The first and second optical frequency comb signals have a small, preset repetition frequency difference Δf. r The frequency difference can be set according to actual usage requirements, and this disclosure does not impose specific limitations on it.

[0038] The lens group under test can be placed in the direction of the emitted light from the measuring device 102. The upper limit of the number of lenses under test in the lens group 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 at each reflecting surface, the number of lenses under test can be increased indefinitely.

[0039] The lens under test can be any type of optical lens, such as a concave lens or a convex lens, and this disclosure does not specifically limit it.

[0040] The materials and specifications of each lens in the lens group under test can be the same or different, and this disclosure does not make specific limitations in this regard.

[0041] The measuring device 102 will be described in detail later in conjunction with the possible implementations of this disclosure, and will not be repeated here.

[0042] Coupler 104 enables the second optical frequency comb signal to coincide with the reflected optical signal, thereby causing interference between the two signals and achieving a conversion from the optical frequency domain to the radio frequency domain, generating a measurement interference optical signal in the radio frequency domain. Coupler 104 can be any type of coupler, such as an optical fiber coupler or a spatial coupler, and this disclosure does not specifically limit it.

[0043] The signal acquisition and processing device 105 will be described in detail later in conjunction with the possible implementations of this disclosure, and will not be repeated here.

[0044] The mirror spacing of the lens group under test represents the distance between two adjacent mirrors in the lens group under test, and may include the geometric thickness of each lens under test in the lens group under test, as well as the surface geometric distance between two adjacent lenses under test. The refractive index of the lens group under test includes the refractive index of each lens under test in the lens group under test.

[0045] The lens group mirror spacing and refractive index measurement system of this disclosure uses a measuring device to incident a first optical frequency comb signal generated by a light source device onto a reference optical path and a measuring optical path to determine the reflected light signal. A coupler is used to interfere the reflected light signal and the second optical frequency comb signal to generate a measurement interference light signal. The measurement interference light signal is then analyzed in time and frequency by a signal acquisition and processing device, which enables rapid, real-time and accurate simultaneous measurement of the mirror spacing and refractive index of the lens group under test.

[0046] In one possible implementation, the measuring device 102 includes a circulator 1021, a collimating lens 1022, and a beam splitter 1023; the circulator 1021 is used to incident a first optical frequency comb signal onto the collimating lens 1022; the collimating lens 1022 is used to emit the first optical frequency comb signal from the optical fiber into free space; the beam 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 onto the measurement optical path and the reference signal is incident onto the reference optical path.

[0047] Based on the above Figure 1 For example, Figure 1 As shown, along the direction of light propagation, the measuring device 102 may include a circulator 1021, a collimating lens 1022, and a beam splitter 1023.

[0048] Circulator 1021 can be used to direct the first optical frequency comb signal onto collimating lens 1022. Circulator 1021 can be any type of circulator depending on the system optical path configuration during actual use, such as a three-port circulator; this disclosure does not specifically limit its use.

[0049] Collimating lens 1022 can be used to output a first optical frequency comb signal from an optical fiber into free space, and the focusing state of the output first optical frequency comb signal can be adjusted. The first optical frequency comb signal output from the optical fiber into free space can be in a parallel state, a slightly convergent state, or a divergent state. Collimating lens 1022 can be any type of collimating lens, such as a transmission collimating lens, and this disclosure does not specifically limit it. In one possible implementation, collimating lens 1022 includes a variable-focus collimating lens. When collimating lens 1022 is a variable-focus collimating lens, the focal point of the output beam can be changed by collimating lens 1022, thereby changing the intensity of the reflected light signal from each surface, improving the signal-to-noise ratio of the reflected light signal, thereby improving measurement accuracy and increasing the number of measurable lenses.

[0050] The beam splitter 1023 can be used to split the emitted first optical frequency comb signal into two beams: a measurement signal and a reference signal, and to direct the measurement signal into the measurement optical path and the reference signal into the reference optical path. The beam splitter 1023 can be any type of beam splitter, such as a beam splitter mirror, and this disclosure does not specifically limit it.

[0051] The direction of the light rays generated by the beam splitter 1023, such as the measurement signal and the reference signal, is as follows: Figure 1 Apart from being perpendicular to each other, as long as the reflected light can be reflected back to the beam splitter along the original paths of the measurement optical path and the reference optical path, the direction of the light rays of the measurement signal and the reference signal depends on the beam splitter 1023, and this disclosure does not make specific limitations on this.

[0052] The lens group under test can be placed between the first reflecting mirror 1031 and the beam splitter 1023. The specific method of placing the lens group under test can refer to the lens placement method in related technologies, such as placing the lens group on a support. This disclosure does not make specific limitations in this regard.

[0053] After the measurement signal is incident on the lens group under test, reflected light can be generated at the front and rear mirrors of each lens in the lens group and at the first reflecting mirror 1031. After the reference signal is incident on the second reflecting mirror 1032, reflected light can also be generated. All reflected light can be reflected back to the beam splitter 1023 along the measurement optical path and the reference optical path.

[0054] In one possible implementation, the beam splitter 1023 is used to combine the measurement signal reflected by the lens group under test and the first reflector 1031, and the reference signal reflected by the second reflector 1032 into a reflected light signal.

[0055] The measurement signal reflected by the lens group under test and the first reflector 1031, and the reference signal reflected by the second reflector 1032, are reflected back to the beam splitter 1023 in free space and combined into a reflected light signal by the beam splitter 1023.

[0056] In one possible implementation, the collimating lens 1022 is used to couple the reflected light signal from free space into the optical fiber.

[0057] The reflected light signal, combined by the beam splitter 1021, is incident on the collimating lens 1022, which couples the reflected light from free space into the optical fiber. The reflected light signal can then be re-injected into the circulator 1021 along the optical path. The repetition frequency of the reflected light signal is the same as that of the first optical frequency comb signal.

[0058] In one possible implementation, circulator 1021 is used to direct the reflected light signal to coupler 104.

[0059] The reflected light signal can be incident on the coupler 104 after passing through the circulator, and interfere with the second optical frequency comb signal, which is the local oscillator light, at the coupler 104 to generate a measurement interference light signal.

[0060] Based on the above Figure 1 For example, Figure 1 As shown, the circulator 1021 can be a three-port circulator. The first optical frequency comb signal is incident on port 1 of the circulator 1021 and exits from port 2 of the circulator 1021 to the collimating lens 1022; the reflected optical signal is incident on port 2 of the circulator 1021 and exits from port 3 of the circulator 1021 to the coupler 104.

[0061] In one 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 optical signal into a measurement electrical signal; the filter device 1052 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 1053 is used to perform time-frequency analysis on the filtered measurement electrical signal to determine the interplanar spacing and / or refractive index of the lens group under test.

[0062] Based on the above Figure 1 For example, Figure 1 As shown, along the transmission direction of the measurement interference light signal, 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.

[0063] Detector 1051 can be used to acquire the measurement interference optical signal output by coupler 104 and convert the measurement interference optical signal from an optical signal in the radio frequency domain into a measurement electrical signal. Detector 1051 can be any type of detector used to acquire optical interference signals, and this disclosure does not specifically limit it.

[0064] The type of electrical signal being measured depends on the detector 1051. It is usually a radio frequency signal, but it can also be other types of electrical signals. This disclosure does not specifically limit this type of signal.

[0065] The filter device 1052 can be used to filter the measured electrical signal, such as through high-pass filtering and low-pass filtering. The filter device 1052 can be any type of electrical filter device, such as a radio frequency filter; this disclosure does not specifically limit it. The function of the filter is to remove high-frequency noise, low-frequency DC signals, etc., resulting in a better signal-to-noise ratio of the acquired signal, thereby improving measurement accuracy.

[0066] To prevent spectral aliasing in the dual optical comb system and to achieve accurate measurement of the spectral phase, thereby ensuring the accuracy and authenticity of the measurement electrical signals used for time-frequency analysis by the computer processing unit 1053, the bandwidth of the filter device 1052 is less than 1 / 2 of the repetition frequency of the first optical comb signal and / or the second optical comb signal.

[0067] The digital acquisition card can be used to save the filtered measurement 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 twice the frequency of the measurement electrical signal.

[0068] The computer processing unit 1053 can be used to perform time-frequency analysis on the filtered measurement electrical signal to determine the interplanar spacing and / or refractive index of the lens group under test. The computer processing unit 1053 can be any electronic device or system capable of performing the corresponding process of determining the interplanar spacing and / or refractive index of the lens group under test; this disclosure does not specifically limit it in this regard.

[0069] For example, the computer processing unit 1053 can be user equipment (UE), mobile device, user terminal, terminal, handheld device, computing device, or vehicle-mounted device, etc. Examples of terminals include: displays, smartphones or portable devices, mobile phones, tablets, laptops, PDAs, mobile internet devices (MIDs), wearable devices, virtual reality (VR) devices, augmented reality (AR) devices, wireless terminals in industrial control, wireless terminals in self-driving, wireless terminals in remote medical surgery, wireless terminals in smart grids, wireless terminals in transportation safety, wireless terminals in smart cities, wireless terminals in smart homes, and wireless terminals in vehicle-to-everything (V2X) networks, etc. For example, the server can be a local server or a cloud server.

[0070] The following sections will describe in detail the process by which the computer processing unit 1053 performs time-frequency analysis on the filtered measurement electrical signal to determine the inter-mirror spacing and / or refractive index of the lens group under test, in conjunction with possible implementations of this disclosure. These details will not be elaborated upon here.

[0071] In one possible implementation, time-frequency analysis is performed on the filtered measurement electrical signal to determine the mirror spacing and / or refractive index of the lens group under test, including: analyzing the filtered measurement electrical signal to determine the phase frequency information of the measurement electrical signal; determining the optical path between adjacent mirrors in the lens group under test based on the phase frequency information; and determining the mirror spacing and / or refractive index of the lens group under test based on the optical path between adjacent mirrors.

[0072] After the reflected light signal enters the coupler 104, it is asynchronously optically sampled by the second optical frequency comb signal, generating 2N+2 measurement interference light signals, which are then converted from the optical frequency domain to the radio frequency domain. The measurement interference light signals correspond to the reflecting surfaces of each lens (two for each lens), the first reflecting mirror 1031, and the second reflecting mirror 1032, respectively. The positional relationship of each measurement interference light signal in the time domain reflects the physical spatial position of the corresponding reflecting surface.

[0073] Figure 2 This diagram illustrates the positional relationship in the time domain of interference signals corresponding to different reflective surfaces according to an embodiment of the present disclosure. For example... Figure 2 As shown, taking a lens group consisting of three lenses under test placed in the system as an example, the measured interference light signals S1 to S8 represent the eight measured interference light signals collected after the lens group under test is placed in the system. S1 to S8 are arranged in the time domain according to the time sequence of the corresponding reflected light generation. Among them, the measured interference light signal S1 corresponds to the reflecting surface of the second reflecting mirror 1032, the measured interference light signal S2 corresponds to the reflecting surface of the first mirror of the lens under test 201, the measured interference light signal S3 corresponds to the reflecting surface of the second mirror of the lens under test 201, the measured interference light signal S4 corresponds to the reflecting surface of the first mirror of the lens under test 202, the measured interference light signal S5 corresponds to the reflecting surface of the second mirror of the lens under test 202, the measured interference light signal S6 corresponds to the reflecting surface of the first mirror of the lens under test 203, the measured interference light signal S7 corresponds to the reflecting surface of the second mirror of the lens under test 203, and the measured interference light signal S8 corresponds to the reflecting surface of the first reflecting 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 reflecting surfaces of each mirror in the lens group under test can be reflected.

[0074] When the lens group under test is not placed in the system, the measurement interference signals S1 and S8' corresponding to the second and first reflecting mirrors can be determined. Then, when the lens group under test is placed in the system, the measurement interference signals S1 to S8 corresponding to the two reflecting surfaces of the second and first reflecting mirrors and each lens under test in the lens group can be determined. Combining the positional relationship of the measurement interference signals in the time domain in the two processes, the mirror spacing and refractive index of the lens group under test can be determined.

[0075] Similarly, the measurement signal determined by detector 1051 also possesses the above characteristics. By analyzing the filtered measurement signal, the phase frequency information of the measurement signal can be determined.

[0076] Figure 3 This diagram illustrates a time-frequency analysis of a measured electrical signal according to an embodiment of the present disclosure. Figure 3 As shown, by analyzing Figure 3(a) By performing a Fourier transform on the measured electrical signal, the corresponding value of the measured electrical signal can be obtained. Figure 3 (c) phase frequency information and Figure 3 (b) Amplitude-frequency information (i.e., phase-frequency curve and amplitude-frequency curve). Based on the phase-frequency information corresponding to each measured electrical signal, the optical path between adjacent mirrors in the lens group under test can be determined. The optical path represents the actual distance light travels, and the optical path between adjacent mirrors can be determined by the mirror spacing and refractive index of the lens group under test. Therefore, by determining the optical path between adjacent mirrors in the lens group under test, the mirror spacing and / or refractive index of the lens group under test can be determined.

[0077] The following sections will describe in detail the two processes of determining the optical path between adjacent mirrors in the lens group under test based on phase frequency information, and determining the mirror spacing and / or refractive index of the lens group under test based on the optical path between adjacent mirrors, in conjunction with the possible methods disclosed herein. These will not be elaborated upon here.

[0078] In one possible implementation, determining the optical path between adjacent mirrors in the lens group under test based on phase frequency information includes: determining the time difference between two adjacent measurement electrical signals based on phase frequency information; determining the repetition frequency ratio between the measurement electrical signal and the first optical frequency comb signal based on the preset repetition frequency difference between the first optical frequency comb signal and the second optical frequency comb signal; and determining the optical path between adjacent mirrors in the lens group under test based on the time difference and the repetition frequency ratio.

[0079] The phase frequency information determined by Fourier transform analysis contains a wrapped phase, which means that the phase is truncated within the range of [-π, π] or [0, 2π]. Phase unwrapping (also known as phase expansion) is required to obtain continuous phase frequency information.

[0080] Based on the above Figure 3 For example, through the analysis of Figure 3 (c) Phase frequency information can be de-wrapped to determine the phase frequency information. Figure 3 (d) Continuous phase frequency information.

[0081] Figure 4 A schematic diagram showing the phase frequency curves and phase difference curves of two radio frequency interference signals according to a disclosed embodiment is provided. Figure 4 As shown, by subtracting the phase value corresponding to each frequency in the phase-frequency curves of any two radio frequency interference signals after phase expansion, the corresponding curve of phase difference can be determined. By determining the slope of the corresponding curve of phase difference, the time difference between the two corresponding radio frequency interference signals can be determined. The time difference between any two radio frequency interference signals can be expressed as the following formula (1):

[0082]

[0083] in, The curve representing the phase value of a radio frequency interference signal. The curve representing the phase value of another radio frequency interference signal. The curve representing the phase difference, The slope of the curve corresponding to the phase difference is represented by Δt, which represents the time difference between any two radio frequency interference signals.

[0084] Correspondingly, the time difference between any two measured electrical signals can also be expressed by the above formula (1).

[0085] Figure 5 This diagram illustrates a dual-comb multi-heterodyne interference principle and an amplitude-frequency curve of the interference signal according to an embodiment of the present disclosure. Figure 5 As shown, due to the small preset repetition frequency difference Δf between the reflected light signal and the second optical frequency comb signal... r Therefore, after each time period, the two optical comb signals will experience a certain time shift. According to the principle of dual-comb multi-heterodyne interference, information from 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 will be reduced by Δf. r / f r1 The corresponding time-domain information period, i.e., 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 between the radio frequency interference signal and the first optical frequency comb signal. r / f r1 .

[0086] Correspondingly, the ratio of the repetition frequency between the measured electrical signal and the first optical frequency comb signal is Δf. r / f r1 .

[0087] Based on the time difference between two adjacent measured electrical signals in the time domain and the repetition frequency ratio between the measured electrical signal and the first optical frequency comb signal, the optical path between two adjacent mirrors in the lens group under test can be determined. Combined with the above, the optical path between adjacent mirrors can also be determined using the mirror spacing and refractive index of the lens group under test. Therefore, the optical path between two adjacent mirrors can be expressed as the following formula (2):

[0088] L i =2·n(d i )·d i =c·Δt·Δf r / f r1 (2)

[0089] Where n(di) is the refractive index of the corresponding medium, di is the mirror spacing between two adjacent mirrors, and c is the speed of light in a vacuum.

[0090] Based on the above Figure 2 For example, Figure 2 As shown, the lens group under test includes lens 201, lens 202 and lens 203.

[0091] When the three lenses under test are made of the same material, it can be determined that the refractive indices n of the three lenses under test are equal, and the air refractive index can be determined as a known fixed value n0 according to the measurement environment. At this time, the optical path traversed by the light from the lens under test 201 to the lens under test 203 can be expressed by the following formula (3):

[0092]

[0093] Where L1, L2, L3, L4, and L5 represent the optical path difference between two reflected beams generated by each beam of light passing through adjacent mirrors, Δt 3-2 , Δt 4-3 Δt 5-4 , Δt 6-5 , Δt 7-6 d1, d2, d3, d4, and d5 represent the time difference between any two adjacent measured electrical signals in the time domain, respectively. d1, d2, d3, d4, and d5 represent the mirror spacing of the lens group, that is, d1 is the thickness of the first lens, d2 is the distance between adjacent surfaces of the first and second lenses, d3 is the thickness of the second lens, d4 is the distance between adjacent surfaces of the second and third lenses, and d5 is the thickness of the third lens.

[0094] When the materials of the three lenses under test are different, the refractive indices of the three lenses under test can be determined to be n1, n2 and n3 respectively. At this time, the optical path traversed by the light ray from the lens under test 201 to the lens under test 203 can be expressed by the following formula (4):

[0095]

[0096] In one possible implementation, determining the mirror spacing and / or refractive index of the lens group under test based on the optical path between adjacent mirrors includes: determining the change in optical path between the first reflecting mirror 1031 and the second reflecting mirror 1032 before and after the lens group under test is placed in the system; and determining the mirror spacing and / or refractive index of the lens group under test based on the change in optical path and the optical path between adjacent mirrors.

[0097] Placing the lens group under test in the system will change the medium between the first reflecting mirror 1031 and the second reflecting mirror 1032 in their equivalent common optical path. This change in medium results in a different refractive index at the corresponding location compared to when the system is not equipped with the lens group under test, which in turn causes a change in the optical path between the first reflecting mirror 1031 and the second reflecting mirror 1032. Based on the change in optical path and the optical path between adjacent mirrors, the mirror spacing and / or refractive index of the lens group under test can be determined.

[0098] Based on the above Figure 2 For example, Figure 2 As shown, after the lens group under test is placed in the system, the position of the measurement electrical signal S8 corresponding to the first reflecting mirror 1031 in the time domain changes compared to the measurement electrical signal S8' corresponding to the first reflecting mirror 1031 before the lens group under test is placed in the system. The change in the position of the measurement electrical signal corresponding to the first reflecting mirror 1031 in the time domain can reflect the change in optical path between the first reflecting mirror 1031 and the second reflecting mirror 1032.

[0099] When the materials of the three lenses under test are the same, the three lenses under test can be placed into the system simultaneously. After the lens group under test is placed, the change in the medium between the first reflecting mirror 1031 and the second reflecting mirror 1032, including the change in the medium at the locations of the lenses under test 201, 202 and 203 from air to the material corresponding to the lenses under test, can be expressed as a change in refractive index (n-n0). Therefore, the change in optical path ΔL can be expressed as the following formula (5):

[0100] ΔL=c·(Δt 8-1 -Δt 8′-1 )·Δf r / f r1 =2(n-n0)(d1+d3+d5) (5)

[0101] Where, Δt 8-1 Δt represents the time difference between the measurement electrical signals corresponding to the first reflecting mirror 1031 and the second reflecting mirror 1032 after the lens group under test is placed in the system. 8’-1 This indicates the time difference between the measurement electrical signals corresponding to the first reflecting mirror 1031 and the second reflecting mirror 1032 before the lens group under test is placed in the system.

[0102] Based on the above formulas (3) and (5), the refractive index of the lens under test can be determined through the mathematical relationship between the optical path and the reflecting surface. The refractive index n of the lens under test can be expressed as the following formula (6):

[0103]

[0104] Furthermore, based on the above formulas (3), (5) and (6), the mirror spacing d1, d2, d3, d4 and d5 of the lens group to be tested can be determined through the mathematical relationship between the optical path and the reflecting surface.

[0105] 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 the optical path changes after lens 201 is placed in the system, after lenses 201 and 202 are placed in the system, and after lenses 201, 202 and 203 are placed in the system can be determined respectively.

[0106] When the lens 201 to be tested is placed into the system, the optical path change ΔL1 can be expressed as the following formula (7):

[0107] ΔL1=2(n1-n0)d1 (7)

[0108] Continuing to place the lens 202 under test into the system, the optical path change ΔL2 can be expressed as the following formula (8):

[0109] ΔL2=2(n2-n0)d3 (8)

[0110] Continuing to place the lens 203 under test into the system, the optical path change ΔL3 can be expressed as the following formula (9):

[0111] ΔL3=2(n3-n0)d5 (9)

[0112] Based on the above formula (4), and formulas (7) to (9), the refractive indices of the three lenses under test can be determined as n1, n2, and n3 through the mathematical relationship between the optical path and the reflecting surface. The refractive index n1 of the lens under test 201 can be expressed as the following formula (10):

[0113]

[0114] The refractive index n2 of the lens 202 under test can be expressed by the following formula (11):

[0115]

[0116] The refractive index n3 of the lens 203 under test can be expressed by the following formula (12):

[0117]

[0118] Furthermore, based on the above formula (4), and formulas (7) to (12), the mirror spacing d1, d2, d3, d4, and d5 of the lens group to be tested can be determined through the mathematical relationship between the optical path and the reflecting surface.

[0119] In this embodiment, a first optical frequency comb signal generated by a light source device is incident on a reference optical path and a measurement optical path using a measuring device to determine the reflected light signal. A coupler is then used to interfere the reflected light signal with the second optical frequency comb signal, generating a measurement interference light signal. A signal acquisition and processing device performs time-frequency analysis on the measurement interference light signal. Without prior knowledge of the lenses, rapid, real-time, and accurate simultaneous measurement of the interplanar spacing and refractive index of the lens group under test can be achieved. Furthermore, the measurement process is not affected by system mechanical vibration, and increasing the number of lenses being measured does not increase the measurement time.

[0120] It should be noted that, although... Figure 1 The above example illustrates a system for measuring the interplanar distance and refractive index of a lens group; however, those skilled in the art will understand that this disclosure is not limited thereto. In fact, users can flexibly configure the specific structure of the system for measuring the interplanar distance and refractive index of a lens group according to their personal preferences and / or actual application scenarios, as long as it can achieve the measurement of the interplanar distance and / or refractive index of the lens group based on the above process.

[0121] The various embodiments of this disclosure have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A system for measuring the inter-mirror spacing and refractive index of a lens group, characterized in that, The system includes a light source device, a measuring device, a coupler, and a signal acquisition and processing device; The light source device is used 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 measuring device is used to incident the first optical frequency comb signal onto the reference optical path and the measuring optical path to generate a reflected optical signal, and to reflect the reflected optical signal back to the coupler. The reference optical path includes a second reflector, the measuring optical path includes a first reflector and a lens group under test, the lens group under test includes N lenses under test, where N is greater than or equal to 1, and the reflected optical signal includes: the reflected light generated by the first optical frequency comb signal on the front and rear mirror surfaces of the first reflector, the second reflector, and each lens in the lens group under test. The coupler is used to interfere with the second optical frequency comb signal and the reflected light signal to generate a measurement interference light signal, wherein the positional relationship of the measurement interference light signal in the time domain is used to reflect the physical relationship in space of the reflecting surfaces of each mirror in the lens group under test; The signal acquisition and processing device is used to determine the interplane spacing and / or refractive index of the lens group under test based on the measured interference light signal. The signal acquisition and processing device includes a detector, a filtering 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 filtering device is used to filter the measured 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 for: The filtered measurement electrical signal is analyzed to determine the phase frequency information of the measurement electrical signal; Based on the phase frequency information, the optical path between adjacent mirrors in the lens group under test is determined; Determine the change in optical path between the first and second reflecting mirrors before and after the lens group under test is placed in the system; The inter-mirror spacing and / or refractive index of the lens group under test are determined based on the optical path change and the optical path between adjacent mirrors.

2. The system according to claim 1, characterized in that, The measuring device includes a circulator, a collimating lens, and a beam splitter; The circulator is used to direct the first optical frequency comb signal onto the collimating lens; The collimating lens is used to allow the first optical frequency comb signal to exit from the optical fiber into free space; The beam splitter 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 and the reference signal is incident on the reference optical path.

3. The system according to claim 2, characterized in that, The beam splitter is used to combine the measurement signal reflected by the lens group under test and the first reflector, and the reference signal reflected by the second reflector into the reflected light signal.

4. The system according to claim 2, characterized in that, The collimating lens is used to couple the reflected light signal from free space into the optical fiber.

5. The system according to claim 2, characterized in that, The circulator is used to direct the reflected light signal to the coupler.

6. The system according to claim 2, characterized in that, The collimating lens includes a variable focus collimating lens.

7. The system according to claim 1, characterized in that, Determining the optical path between adjacent mirrors in the lens group under test based on the phase frequency information includes: Based on the phase frequency information, the time difference between two adjacent measured electrical signals is determined; The repetition frequency ratio between the measured electrical signal and the first optical frequency comb signal is determined based on the preset repetition frequency difference between the first optical frequency comb signal and the second optical frequency comb signal. The optical path between adjacent mirrors in the lens group under test is determined based on the time difference and the repetition frequency ratio.

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