A heavy frequency self-calibration double optical frequency comb ranging system and method

Abstract

The application discloses a kind of heavy frequency self-calibration double optical frequency comb ranging systems and methods, the system includes double optical frequency comb light source module, interferometer ranging module and signal acquisition and data processing module.Double optical frequency comb light source module uses two optical frequency comb light sources with small heavy frequency difference.Interferometer ranging module converts the distance information to be measured into the flight time of signal optical frequency comb by Michelson interferometer, then uses double optical frequency comb interferometer to amplify the flight time from optical scale to radio frequency scale.Signal acquisition and data processing module obtains the heavy frequency size after undersampling by peak finding, intercepting, fast fourier transform, phase spectrum fitting and other operations to obtain the flight time in real time, to realize the self-calibration of the distance to be measured by periodic fast fourier transform and amplitude spectrum analysis.The application only uses double optical frequency comb interference signal itself and simple signal post-processing algorithm, without any external heavy frequency detection module and frequency counter, to realize accurate double optical frequency comb ranging.

Description

Technical Field

[0001] This invention mainly relates to the field of optical frequency comb ranging technology, specifically a repetition rate self-calibration dual optical frequency comb ranging system and method. Background Technology

[0002] Optical frequency combs, a significant invention of the 21st century, were awarded the 2005 Nobel Prize in Physics. Their unique advantages, such as wide spectral density, high spatial collimation, and precisely controllable comb teeth, have greatly propelled the development of precision measurement. To fully utilize the unique advantages of optical frequency combs, two combs with slightly different repetition frequencies can be combined to form a dual-frequency comb interferometer. This technology can rapidly and without scanning detect the amplitude / phase information encoded on the optical frequency comb based on multiheterodyne interferometry. This technology has successfully achieved rapid and accurate large-range absolute distance measurement, known as dual-frequency comb ranging, and is expected to be widely used in aerospace, defense, advanced manufacturing, and consumer electronics. However, this method requires precise repetition frequency control of the two optical frequency combs, or real-time repetition frequency calibration using additional repetition frequency detection modules and frequency counters, resulting in high system complexity and cost, limiting its application prospects in practical field testing. Therefore, it is necessary to explore simpler, lower-cost, and easier-to-implement dual-frequency comb ranging systems and methods. Summary of the Invention

[0003] The repetition rate self-calibration dual optical frequency comb ranging system and method provided by the present invention solves the technical problem that the existing technology of using dual optical frequency comb ranging requires the use of an additional repetition rate detection module and frequency counter.

[0004] To solve the above-mentioned technical problems, the repetition rate self-calibration dual optical frequency comb ranging system proposed in this invention includes a dual optical frequency comb light source module, a heterodyne interferometry and ranging module, and a signal acquisition and data processing module connected in sequence, wherein:

[0005] Dual optical frequency comb light source module, used to output signal optical frequency combs with different repetition frequencies and overlapping spectral ranges, and local oscillator optical frequency comb;

[0006] The heterodyne interferometry and ranging module is used to decompose the signal optical frequency comb into reference light and measurement light, and to synthesize the decomposed signal optical frequency comb and local oscillator optical frequency comb to obtain a dual optical frequency comb interference signal;

[0007] The signal acquisition and data processing module is used to acquire and process the dual optical frequency comb interference signal, obtain the real-time repetition rate estimate of the signal optical frequency comb and the local oscillator optical frequency comb, and obtain the measured value of the distance to be measured based on the real-time repetition rate estimate.

[0008] Furthermore, the heterodyne interferometry and ranging module includes a Michelson interferometer, a dual-frequency comb interferometer, and a photodetector, wherein:

[0009] The Michelson interferometer splits the signal light into a reference light and a measurement light using a beam splitter 1, and receives the reflected light from the reference light after passing through a reference distance and the measurement light after passing through the measured distance.

[0010] The dual-optical-frequency-comb interferometer combines the reflected light and the local oscillator frequency comb through beam splitter 2 to obtain the dual-optical-frequency-comb interference signal;

[0011] A photodetector is used to receive dual-frequency comb interference signals, which include self-heterodyne interference signals and mutual heterodyne interference signals.

[0012] Furthermore, the signal acquisition and data processing module includes a signal acquisition module and a signal processing module, wherein:

[0013] The signal acquisition module is used to acquire the dual-frequency comb interference signal output by the photodetector according to a preset sampling rate. The formula for calculating the preset sampling rate is as follows:

[0014]

[0015] The value of i ranges from 1 to 2, and when i = 1, it represents the signal optical frequency comb; when i = 2, it represents the local oscillator optical frequency comb. F s δ represents the sampling rate of the data acquisition card. (1) and δ (2) These represent the preset frequency differences between the repetition rate of the signal optical frequency comb and the local oscillator optical frequency comb and the preset sampling rate, respectively. and These represent the repetition frequencies of the signal optical frequency comb and the local oscillator optical frequency comb, respectively.

[0016] The signal processing module is used to process the dual optical frequency comb interference signal to obtain the real-time repetition rate estimate of the signal optical frequency comb and the local oscillator optical frequency comb, and to obtain the measured value of the distance to be measured based on the real-time repetition rate estimate.

[0017] Furthermore, the signal processing module includes a signal conditioning module, an undersampling repetition rate estimation module, a real-time repetition rate estimation module, and a distance acquisition module, wherein:

[0018] The signal conditioning module is used to perform fast Fourier transform on the dual optical frequency comb interference signal to obtain the interference amplitude spectrum, and to extract the self-heterodyne interference signal spectrum with a preset frequency difference from the interference amplitude spectrum. The self-heterodyne interference signal spectrum includes the self-heterodyne interference signal spectrum of the signal optical frequency comb and the local oscillator optical frequency comb.

[0019] The undersampled repetition rate estimation module is used to estimate the undersampled repetition rate of the optical frequency comb and the local oscillator frequency comb after undersampling based on the spectrum of the self-heterodyne interferometric signal. The specific calculation formula is as follows:

[0020]

[0021] Where i takes values ​​between 1 and 2, and i = 1 represents the signal optical frequency comb, and i = 2 represents the local oscillator optical frequency comb. and These are the undersampled repetition rate estimates for the signal optical frequency comb and the local oscillator optical frequency comb, respectively. and k represents the frequency points in the spectrum of the self-heterodyne interference signal of the signal optical frequency comb and the local oscillator optical frequency comb where the amplitude is greater than a preset threshold. (1) and k (2) These represent the number of frequency points in the self-heterodyne interference signal spectrum of the signal optical frequency comb and the local oscillator optical frequency comb, respectively, whose amplitudes are greater than a preset threshold. and They are respectively with and The corresponding amplitude;

[0022] The real-time repetition rate estimation module is used to obtain the real-time repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb based on the undersampled repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb. The specific calculation formula is as follows:

[0023]

[0024] Where i takes values ​​between 1 and 2, and i = 1 represents the signal optical frequency comb, and i = 2 represents the local oscillator optical frequency comb. and These represent the real-time frequency repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb, respectively. and F represents the undersampled repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb, respectively. s The preset sampling rate is α, where α is the sign coefficient. When -1 is taken, in Take +1 at the time;

[0025] The distance acquisition module is used to obtain the measured value of the distance to be measured based on the real-time repetition rate estimation values ​​of the signal optical frequency comb and the local oscillator optical frequency comb.

[0026] Furthermore, the signal conditioning module also includes an interference signal separation module, which further includes a cross-heterodyne interference signal bandwidth adjustment module and a center frequency adjustment module, wherein:

[0027] The mutual heterodyne interference signal bandwidth adjustment module is used to control the bandwidth of the mutual heterodyne interference signal by adjusting the repetition frequency difference between the signal optical frequency comb and the local oscillator optical frequency comb.

[0028] The center frequency adjustment module is used to set the center frequency of the heterodyne interference signal by adjusting the pump powers of the signal optical frequency comb and the local oscillator optical frequency comb, and the minimum frequency f min and the maximum frequency f max of the heterodyne interference signal need to satisfy the formula:

[0029] f min > max{|δ (1) |, |δ (2) |},

[0030]

[0031] where δ (1) and δ (2) respectively represent the preset frequency differences between the repetition frequencies of the signal optical frequency comb and the local oscillator optical frequency comb and the preset sampling rate, and respectively represent the repetition frequencies of the signal optical frequency comb and the local oscillator optical frequency comb, and F s represents the sampling rate of the acquisition card.

[0032] Furthermore, the distance acquisition module includes a peak finding operation module, a truncation module, a differential phase spectrum calculation module, and a distance calculation module, where:

[0033] The peak finding operation module is used to perform peak finding operations on the dual optical frequency comb interference signal, where the interference peak corresponding to the reference light is the reference peak, and the interference peak corresponding to the measurement light is the measurement peak;

[0034] The truncation module is used to perform truncation near the reference peak and the measurement peak using short-time windows respectively to obtain the reference frame and the measurement frame;

[0035] The differential phase spectrum calculation module is used to calculate the differential phase spectrum of the measurement frame and the reference frame through fast Fourier transform;

[0036] The distance calculation module calculates the distance to be measured using the following formula:

[0037]

[0038] where, is the measured value of the distance to be measured, L1 is the reference distance, L2 is the measured distance, β is the sign coefficient, taking -1 when L1 > L2 and taking +1 when L1 < L2, c is the speed of light in vacuum, and n g is the refractive index of the propagation medium, dΔφ / dΔf is the slope of the differential phase spectrum, and respectively represent the real-time repetition frequency estimation values of the signal optical frequency comb and the local oscillator optical frequency comb.

[0039] Furthermore, the distance calculation module can also calculate the distance to be measured using the following formula:

[0040]

[0041] in, The slope of the differential phase spectrum between two adjacent reference frames;

[0042] Furthermore, when the actual relative distance |L1-L2| exceeds the unambiguous distance, the distance calculation module also uses the following formula to calculate the distance to be measured:

[0043]

[0044] In the formula is the unambiguous distance, and n is an empirical parameter.

[0045] The repetition rate self-calibration dual-frequency comb ranging method provided by this invention includes:

[0046] Acquire signal optical frequency combs and local oscillator optical frequency combs with different repetition frequencies and overlapping spectral ranges;

[0047] The signal optical frequency comb is decomposed into reference light and measurement light, and the decomposed signal optical frequency comb and local oscillator optical frequency comb are synthesized to obtain a dual optical frequency comb interference signal;

[0048] The dual-frequency comb interference signal is acquired and processed to obtain the real-time repetition rate estimate of the signal frequency comb and the local oscillator frequency comb. Based on the real-time repetition rate estimate, the measured distance is obtained.

[0049] Furthermore, the dual-frequency comb interference signal is acquired and processed to obtain real-time repetition rate estimates for the signal frequency comb and the local oscillator frequency comb, including:

[0050] The dual-frequency comb interference signal output by the photodetector is acquired according to a preset sampling rate, where the preset sampling rate is calculated using the following formula:

[0051]

[0052] The value of i ranges from 1 to 2, and when i = 1, it represents the signal optical frequency comb; when i = 2, it represents the local oscillator optical frequency comb. F s δ represents the sampling rate of the data acquisition card. (1) and δ (2) These represent the preset frequency differences between the repetition rate of the signal optical frequency comb and the local oscillator optical frequency comb and the preset sampling rate, respectively. and These represent the repetition frequencies of the signal optical frequency comb and the local oscillator optical frequency comb, respectively.

[0053] A fast Fourier transform is performed on the dual optical frequency comb interference signal to obtain the interference amplitude spectrum. The self-heterodyne interference signal spectrum with a preset frequency difference is then extracted from the interference amplitude spectrum. The self-heterodyne interference signal spectrum includes the self-heterodyne interference signal spectra of the signal optical frequency comb and the local oscillator optical frequency comb.

[0054] The undersampled repetition rate estimates of the optical frequency comb and local oscillator frequency comb after undersampling are estimated based on the spectrum of the self-heterodyne interferometric signal. The specific calculation formula is as follows:

[0055]

[0056] Where i takes values ​​between 1 and 2, and i = 1 represents the signal optical frequency comb, and i = 2 represents the local oscillator optical frequency comb. and These are the undersampled repetition rate estimates for the signal optical frequency comb and the local oscillator optical frequency comb, respectively. and k represents the frequency points in the spectrum of the self-heterodyne interference signal of the signal optical frequency comb and the local oscillator optical frequency comb where the amplitude is greater than a preset threshold. (1) and k (2) These represent the number of frequency points in the self-heterodyne interference signal spectrum of the signal optical frequency comb and the local oscillator optical frequency comb, respectively, whose amplitudes are greater than a preset threshold. and They are respectively with and The corresponding amplitude;

[0057] Based on the undersampled repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb after undersampling, the real-time repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb are obtained in real time. The specific calculation formula is as follows:

[0058]

[0059] Where i takes values ​​between 1 and 2, and i = 1 represents the signal optical frequency comb, and i = 2 represents the local oscillator optical frequency comb. and These represent the real-time frequency repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb, respectively. and F represents the undersampled repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb, respectively. s The preset sampling rate is α, where α is the sign coefficient. When -1 is taken, in Take +1.

[0060] Furthermore, the measured distance is obtained based on the real-time repetition rate estimation values ​​of the signal optical frequency comb and the local oscillator optical frequency comb, including:

[0061] Peak finding operation is performed on the dual optical frequency comb interference signal, where the interference peak corresponding to the reference light is the reference peak, and the interference peak corresponding to the measurement light is the measurement peak;

[0062] Short-time time windows are respectively used to intercept near the reference peak and the measurement peak to obtain the reference frame and the measurement frame;

[0063] The differential phase spectrum of the measurement frame and the reference frame is calculated by fast Fourier transform;

[0064] The distance to be measured is calculated, and the specific formula is:

[0065]

[0066] where is the measured value of the distance to be measured, L1 is the reference distance, L2 is the measured distance, β is the sign coefficient, taking -1 when L1 > L2 and taking +1 when L1 < L2, c is the speed of light in vacuum, n g is the refractive index of the propagation medium, dΔφ / dΔf is the slope of the differential phase spectrum, and respectively represent the real-time repetition frequency estimation values of the signal optical frequency comb and the local oscillator optical frequency comb.

[0067] The present invention proposes a repetition frequency self-calibration dual optical frequency comb ranging system and method. The system includes a dual optical frequency comb light source module, an interferometer ranging module, and a signal acquisition and data processing module. The dual optical frequency comb light source module uses two optical frequency comb light sources with a small repetition frequency difference. The two optical frequency combs can be in a free-running state without any frequency locking. The interferometer ranging module converts the distance information to be measured into the flight time of the signal optical frequency comb through a Michelson interferometer, and then uses the asynchronous optical sampling effect of the dual optical frequency comb interferometer to magnify the flight time from the optical scale to the radio frequency scale, so as to be detected with high precision by a photodetector. The signal acquisition and data processing module obtains the flight time in real time through operations such as peak finding, interception, fast Fourier transform, and phase spectrum fitting, and then obtains the magnitude of the under-sampled repetition frequency through periodic fast Fourier transform and amplitude spectrum analysis to achieve self-calibration of the measured distance. The present invention only uses the dual optical frequency comb interference signal itself and simple signal post-processing algorithms, and can achieve accurate dual optical frequency comb ranging without any external repetition frequency detection module and frequency counter. This method can be used to achieve absolute distance measurement with low complexity, large range, and high precision. Description of the Drawings

[0068] Figure 1 is a schematic structural diagram of the dual optical frequency comb ranging system according to Embodiment 3 of the present invention;

[0069] Figure 2 is a schematic principle diagram of the dual optical frequency comb ranging method according to Embodiment 3 of the present invention;

[0070] Figure 3 Figures (a) and (b) are schematic diagrams illustrating the principle of the undersampling process and scale calibration operation of the sampling rate being less than the repetition rate of the optical frequency comb and greater than the repetition rate of the optical frequency comb, respectively, in Embodiment 3 of the present invention.

[0071] Figure 4 This is a flowchart of the data processing algorithm in Embodiment 3 of the present invention. Detailed Implementation

[0072] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0073] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but the present invention can be implemented in many different ways as defined and covered by the claims.

[0074] Example 1

[0075] The repetition rate self-calibration dual-frequency comb ranging system provided in Embodiment 1 of the present invention includes a dual-frequency comb light source module, a heterodyne interferometry and ranging module, and a signal acquisition and data processing module connected in sequence, wherein:

[0076] Dual optical frequency comb light source module, used to output signal optical frequency combs with different repetition frequencies and overlapping spectral ranges, and local oscillator optical frequency comb;

[0077] The heterodyne interferometry and ranging module is used to decompose the signal optical frequency comb into reference light and measurement light, and to synthesize the decomposed signal optical frequency comb and local oscillator optical frequency comb to obtain a dual optical frequency comb interference signal;

[0078] The signal acquisition and data processing module is used to acquire and process the dual optical frequency comb interference signal, obtain the real-time repetition rate estimate of the signal optical frequency comb and the local oscillator optical frequency comb, and obtain the measured value of the distance to be measured based on the real-time repetition rate estimate.

[0079] The repetition rate self-calibration dual-optical frequency comb ranging system of the present invention includes a dual-optical frequency comb light source module, an interferometer ranging module, and a signal acquisition and data processing module. The dual-optical frequency comb light source module uses two optical frequency combs with a small repetition rate difference. Both optical frequency combs can operate freely without any frequency locking. The interferometer ranging module converts the distance to be measured into the time of flight of the signal optical frequency comb using a Michelson interferometer. Then, utilizing the asynchronous optical sampling effect of the dual-optical frequency comb interferometer, the time of flight is amplified from the optical scale to the radio frequency scale, allowing for high-precision detection by a photodetector. The signal acquisition and data processing module acquires the time of flight in real time through operations such as peak finding, truncation, fast Fourier transform, and phase spectrum fitting. It then obtains the repetition rate after undersampling through periodic fast Fourier transform and amplitude spectrum analysis to achieve self-calibration of the measured distance. This invention uses only the dual optical frequency comb interference signal itself and a simple signal post-processing algorithm to achieve accurate dual optical frequency comb ranging without any external repetition frequency detection module and frequency counter. This method can be used to achieve low-complexity, large-range, and high-precision absolute distance measurement.

[0080] Example 2

[0081] The main objective of this invention is to overcome the shortcomings of existing technologies and provide a dual-optical frequency comb ranging system and method based on repetition rate self-calibration. This method eliminates the need for additional repetition rate detection modules and frequency counters, achieving high-precision and high-accuracy absolute distance measurement using only a freely operating dual-optical frequency comb light source.

[0082] The repetition rate self-calibration dual-comb ranging system provided in Embodiment 2 of this invention includes two optical comb outputs with a small repetition rate difference and overlapping spectral ranges. The dual-comb light source can operate freely without repetition rate control or detection. The signal optical comb is split into reference light and measurement light by beam splitter 1. After passing through the reference distance L1 and the measured distance L2 respectively, the light returns along the same path, is recombined, and then recombined with the local oscillator optical comb by beam splitter 2. The light is then received by the photodetector to obtain the dual-comb interference signal. Due to the square law characteristic of the photodetector, the interference signal simultaneously contains both self-heterodyne interference signal and mutual heterodyne interference signal. The self-heterodyne interference signal generated by the detector is generated by intermodal beat frequency and manifests as the repetition rate signal of the two optical combs (the higher harmonics of the repetition rate can be ignored due to the filtering effect of the detector). To achieve high-fidelity acquisition of the repetition rate signal, the sampling rate F of the signal acquisition card is set to... s Repetition frequency of optical frequency comb Having a tiny frequency difference When i=1, it represents the signal optical frequency comb; when i=2, it represents the local oscillator optical frequency comb. Under the undersampling effect, the self-heterodyne interference signal of the optical frequency comb is transformed into a frequency of |δ (i)|sine wave. To ensure the separation of the self-heterodyne and cross-heterodyne signals, the bandwidth of the cross-heterodyne signal is controlled by adjusting the repetition frequency difference between the two optical frequency combs, and the center frequency of the cross-heterodyne signal is controlled by adjusting the pump power of one of the optical frequency combs, so that the minimum frequency f of the cross-heterodyne signal is minimized. min and maximum frequency f max The following relationship must be satisfied:

[0083] f min >max{|δ (1) |,|δ (2) |} (1)

[0084]

[0085] In the formula |δ (1) | and |δ (2) These represent the self-heterodyne frequencies of the signal optical frequency comb and the local oscillator optical frequency comb under undersampling conditions. In this case, the acquired interference signal can be decomposed into a low-frequency self-heterodyne interference signal and a high-frequency mutual heterodyne interference signal.

[0086] To achieve accurate repetition rate calibration, a fast Fourier transform is performed on the interference signal sequence containing multiple interference peaks to obtain the amplitude spectrum, and then the repetition rate is determined at the |δ| of interest. (i) Extract the desired self-heterodyne interference signal spectrum near the specified frequency. Calculate the optical frequency comb repetition rate of the undersampled signal using the following formula:

[0087]

[0088] In the formula f k For frequency points in the spectrum where the amplitude is greater than a preset threshold, a k f k The corresponding amplitude, This represents the estimated repetition rate of the optical frequency comb after undersampling;

[0089] Optionally, the location of the maximum value in the spectrum of the self-heterodyne interference signal is calculated to characterize the location of the undersampled repetition frequency;

[0090] Optionally, a digital bandpass filter is designed based on spectral characteristics to recover an independent self-heterodyne interference signal in the time domain, and the undersampled repetition frequency value is estimated by period measurement;

[0091] Optionally, a digital bandpass filter is designed based on spectral characteristics to recover an independent self-heterodyne interference signal in the time domain, and the undersampled repetition frequency value is estimated by real-time phase measurement and linear fitting.

[0092] The physical resolution of the above undersampling frequency repetition estimation method depends on the time window length of the interferometric signal used for estimation, while the computational resolution depends on the length of the signal undergoing Fourier transform. When the time window length of the interferometric signal is fixed, zero-padding can improve the estimation accuracy.

[0093] Based on the set sampling rate and undersampled repetition frequency, the real-time optical frequency comb repetition frequency is recovered using the following formula:

[0094]

[0095] In the formula The repetition rate estimate of the optical frequency comb is represented by α, where α is the sign coefficient. When -1 is taken, in Add 1 to the value. Without any prior knowledge, this can be achieved by fine-tuning F. s Observe the change in the magnitude of the downsampling repetition frequency. If the two change in the same direction, then α = -1; if the two change in opposite directions, then α = +1.

[0096] After obtaining the accurate optical frequency comb repetition rate, dual-frequency comb time-of-flight ranging can be performed. First, peak finding is performed on the acquired dual-frequency comb interference signals. A short-time window is used to capture the area near each interference peak. The short-time interference signal corresponding to the reference light is used as the reference frame, and the short-time interference signal corresponding to the measurement light is used as the measurement frame. The differential phase spectrum of the measurement frame and the reference frame is calculated using Fast Fourier Transform, and the distance to be measured is calculated using the following formula:

[0097]

[0098] In the formula, β is a sign coefficient, which takes the value of -1 when L1>L2 and +1 when L1>L2, c is the speed of light in vacuum, and n g Let dΔφ / dΔf be the refractive index of the propagation medium, and dΔφ / dΔf be the slope of the differential phase spectrum. The repetition rate estimate of the optical frequency comb representing the signal. This represents the estimated repetition rate of the local oscillator optical frequency comb. This is the measured value of the distance to be measured.

[0099] Alternatively, the distance to be measured can be calculated using the following formula:

[0100]

[0101] In the formula, dΔφ′ / dΔf is the slope of the differential phase spectrum between two adjacent reference frames. In this case, only the repetition rate of the signal optical frequency comb needs to be calibrated, and the magnitude of the repetition rate of the local oscillator optical frequency comb does not need to be considered.

[0102] Because dual-frequency comb ranging has an inherent unambiguous distance limitation, when the actual relative distance |L1-L2| exceeds the unambiguous distance, the following formula needs to be used for deambiguation:

[0103]

[0104] In the formula L1 represents the unambiguous distance, and L2 represents the distance measurement result after deambiguation. The magnitude of the integer n can be determined through prior knowledge or other auxiliary ranging methods.

[0105] Example 3

[0106] Reference Figure 1 The repetition rate self-calibration dual optical frequency comb ranging system provided in Embodiment 3 of the present invention includes a dual optical frequency comb light source module, a heterodyne interferometry and ranging module, and a signal acquisition and data processing module. The dual optical frequency comb light source module includes two optical frequency comb light sources with different repetition frequencies and overlapping spectral ranges. The two optical frequency combs can be in a free-running state, and the optional generation methods include two independent free-running optical frequency combs, two free-running optical frequency combs sharing a common environment, and dual optical frequency comb output generated simultaneously using a single laser resonator.

[0107] The heterodyne interferometry and ranging module includes a Michelson interferometer and a dual-frequency comb interferometer. The signal frequency comb first passes through the Michelson interferometer, with the absolute distances between the reference arm and the measurement arm being L1 and L2, respectively. At this point, the signal frequency comb will be decomposed into two identical carrier envelope pulse sequences, corresponding to the reference light and the measurement light, respectively, with a time delay Δτ between them:

[0108] Δτ=2n g |L1-L2| / c (8)

[0109] Where n g Let be the refractive index of the light propagation medium, and c be the speed of light in a vacuum.

[0110] The signal optical frequency comb separated by the Michelson interferometer is combined with the local oscillator optical frequency comb and enters the dual-frequency-comb interferometer. After the light is combined, a filter is used to constrain the spectral range of the frequency combs to conform to the bandpass sampling theorem. Due to the small repetition rate difference between the two frequency combs, an asynchronous optical sampling process is formed in the time domain. Figure 2 As shown, the resulting dual-frequency comb interferometric signal also includes two sets of carrier envelope pulse sequences, corresponding to the reference light and the measurement light, respectively, with the time delay Δt between them amplified as follows:

[0111] Δt=k·Δτ (9)

[0112] The scaling factor k can be expressed as:

[0113]

[0114] Among them, frequency difference and These represent the repetition frequencies of the signal optical frequency comb and the local oscillator optical frequency comb, respectively. Since the two optical frequency combs are in a free-running state, their repetition frequencies are affected by factors such as ambient temperature fluctuations, vibration, and pump noise, resulting in fluctuations. Consequently, the scaling factor is not an ideal constant but a variable that needs to be calibrated.

[0115] To address this frequency repetition calibration issue, traditional methods require specialized detectors to receive the outputs of each of the two optical frequency combs, followed by accurate frequency measurement using a frequency counter. This results in high complexity and cost for dual-frequency comb ranging systems, making them unsuitable for large-scale field applications. In reality, the dual-frequency comb interference signal contains not only the mutual heterodyne interference signal formed by the beat frequencies of the two optical frequency combs, but also the self-heterodyne interference signals of each optical frequency comb. For example... Figure 3 As shown, the self-heterodyne interference signal originally included the repetition patterns of the two optical frequency combs. and Once detected by the detector, data is collected using a signal acquisition card. The sampling rate F of the acquisition card should be set appropriately. s This makes it repetitive with the optical frequency comb. Having a tiny frequency difference When i=1, it represents the signal optical frequency comb; when i=2, it represents the local oscillator optical frequency comb. Under the undersampling effect, the self-heterodyne interference signal of the optical frequency comb is transformed into a frequency of |δ (1) | and |δ (2) |sine wave.

[0116] To avoid overlap between the mutual heterodyne and self-heterodyne interference signals, the bandwidth of the mutual heterodyne interference signal is controlled by adjusting the repetition frequency difference between the two optical frequency combs. The center frequency of the mutual heterodyne interference signal is then set by adjusting the pump power of the two optical frequency combs. This ensures that the mutual heterodyne and self-heterodyne interference signals are separated in the amplitude spectrum, avoiding information ambiguity caused by overlap. The minimum frequency f of the mutual heterodyne signal... min and maximum frequency f max Formulas (1) and (2) must be satisfied.

[0117] The detailed flowchart of the signal acquisition and data processing module is as follows: Figure 4 As shown, a fast Fourier transform is performed on an interference signal sequence containing multiple interference peaks to obtain the amplitude spectrum, and then... (i)Extract the desired self-heterodyne interference signal spectrum near the specified frequency. The resolution of the spectrum needs to be significantly better than the repetition frequency difference to avoid overlap of the repetition modes of the two optical frequency combs. Therefore, a sufficiently long dual-frequency comb interference signal needs to be extracted to ensure physical resolution, and zero-padding is used to improve the computational resolution of the Fourier transform, ensuring the accuracy of the subsequent repetition frequency estimation algorithm.

[0118] After obtaining the amplitude spectrum of the self-heterodyne interferometer signal with sufficient resolution, the repetition rate of the undersampled optical frequency comb is calculated using the following formula:

[0119]

[0120] The value of i ranges from 1 to 2, where i = 1 represents the signal optical frequency comb and i = 2 represents the local oscillator optical frequency comb. and These are the undersampled repetition rate estimates for the signal optical frequency comb and the local oscillator optical frequency comb, respectively. and k represents the frequency points in the spectrum of the self-heterodyne interference signal of the signal optical frequency comb and the local oscillator optical frequency comb where the amplitude is greater than a preset threshold. (1) and k (2) These represent the number of frequency points in the self-heterodyne interference signal spectrum of the signal optical frequency comb and the local oscillator optical frequency comb, respectively, whose amplitudes are greater than a preset threshold. and They are respectively with and The corresponding amplitude. Other real-time frequency estimation methods can also be used to estimate the optical frequency comb repetition rate (RFR). For example, the location of the undersampled RFR can be characterized by calculating the maximum value of the self-heterodyne interference signal spectrum, or a digital bandpass filter can be designed based on spectral characteristics to recover the independent self-heterodyne interference signal in the time domain, and the undersampled RFR value can be estimated by fitting the period measurement or real-time phase measurement. Based on the set sampling rate and the undersampled RFR, the real-time optical frequency comb RFR is recovered using the following formula.

[0121]

[0122] In the formula, α is the sign coefficient. When -1 is taken, in Take +1 at time. This represents the estimated repetition rate of the signal / local oscillator optical frequency comb. Without any prior knowledge, this can be achieved by fine-tuning F. s Observe the change in the magnitude of the downsampling repetition frequency. If the two change in the same direction, then α = -1; if the two change in opposite directions, then α = +1.

[0123] Once the accurate repetition rate is obtained, it lays the foundation for the calibration of the dual optical frequency comb ranging method. Next, it is necessary to calculate the time of flight of the dual optical frequency comb interference signal. Perform peak finding on the collected dual optical frequency comb interference signal. The interference peak corresponding to the reference light is the reference peak, and the interference peak corresponding to the measurement light is the measurement peak. Use short-time windows to intercept near the reference peak and the measurement peak respectively to obtain the reference frame and the measurement frame. Calculate the differential phase spectrum of the measurement frame and the reference frame through fast Fourier transform, and calculate the distance to be measured using the following formula:

[0124]

[0125] where β is the sign coefficient, taking -1 when L1 > L2 and taking +1 when L1 < L2, c is the speed of light in vacuum, n g is the refractive index of the propagation medium, dΔφ / dΔf is the slope of the differential phase spectrum, and respectively represent the estimated repetition rate values of the signal optical frequency comb and the local oscillator optical frequency comb. is the measured value of the distance to be measured.

[0126] Preferably, in order to further reduce the workload of the calibration process, the distance to be measured can be calculated using the following formula:

[0127]

[0128] where dΔφ′ / dΔf is the slope of the differential phase spectrum between two adjacent reference frames. In this case, only the repetition rate of the signal optical frequency comb needs to be calibrated, without considering the change of the repetition rate of the local oscillator optical frequency comb.

[0129] When the actual relative distance |L1 - L2| exceeds the unambiguous distance, the actual distance to be measured can be calculated by the following formula:

[0130]

[0131] where is the unambiguous distance, and the value of the integer n can be judged through prior knowledge or other auxiliary ranging means.

[0132] The repetition rate self-calibration dual-optical-comb ranging system of this embodiment includes a dual-optical-comb light source module, an interferometer ranging module, and a signal acquisition and data processing module. The dual-optical-comb light source module uses two optical-comb light sources with a small repetition rate difference. Both optical-combs can operate freely without any frequency locking. The interferometer ranging module converts the distance to be measured into the time-of-flight of the signal optical-comb using a Michelson interferometer. Then, utilizing the asynchronous optical sampling effect of the dual-optical-comb interferometer, the time-of-flight is amplified from the optical scale to the radio frequency scale, allowing for high-precision detection by a photodetector. The signal acquisition and data processing module acquires the time-of-flight in real time through operations such as peak finding, truncation, fast Fourier transform, and phase spectrum fitting. It then obtains the repetition rate after undersampling through periodic fast Fourier transform and amplitude spectrum analysis to achieve self-calibration of the measured distance. This invention uses only the dual optical frequency comb interference signal itself and a simple signal post-processing algorithm to achieve accurate dual optical frequency comb ranging without any external repetition frequency detection module and frequency counter. This method can be used to achieve low-complexity, large-range, and high-precision absolute distance measurement.

[0133] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A frequency repetition rate self-calibration dual optical frequency comb ranging system, characterized in that, The system includes a dual-frequency comb light source module, a heterodyne interferometry and ranging module, and a signal acquisition and data processing module connected in sequence, wherein: The dual optical frequency comb light source module is used to output signal optical frequency combs with different repetition frequencies and overlapping spectral ranges, as well as local oscillator optical frequency combs. The heterodyne interferometry and ranging module is used to decompose the signal optical frequency comb into reference light and measurement light, and to synthesize the decomposed signal optical frequency comb and local oscillator optical frequency comb to obtain a dual optical frequency comb interference signal. The signal acquisition and data processing module is used to acquire and process the dual optical frequency comb interference signal to obtain real-time repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb, and to obtain the measured value of the distance to be measured based on the real-time repetition rate estimates. The signal acquisition and data processing module includes a signal acquisition module and a signal processing module, wherein: The signal acquisition module is used to acquire the dual-frequency comb interference signal output by the photodetector according to a preset sampling rate, wherein the preset sampling rate is calculated using the following formula: ， in The value range of is 1 and 2, and Time represents the optical frequency comb of the signal. The time represents the local oscillator optical frequency comb. This represents the sampling rate of the data acquisition card. and These represent the preset frequency differences between the repetition rate of the signal optical frequency comb and the local oscillator optical frequency comb and the preset sampling rate, respectively. and These represent the repetition frequencies of the signal optical frequency comb and the local oscillator optical frequency comb, respectively. The signal processing module includes a signal conditioning module, an undersampled repetition rate estimation module, a real-time repetition rate estimation module, and a distance acquisition module. The undersampled repetition rate estimation module is used to estimate the undersampled repetition rate of the signal optical frequency comb and the local oscillator optical frequency comb based on the spectrum of the self-heterodyne interferometric signal. The specific calculation formula is as follows: , in, The value range of is 1 and 2, and Time represents the optical frequency comb of the signal. The time represents the local oscillator optical frequency comb. and These are the undersampled repetition rate estimates for the signal optical frequency comb and the local oscillator optical frequency comb, respectively. and These are the frequency points in the spectrum of the self-heterodyne interference signal of the signal optical frequency comb and the local oscillator optical frequency comb, respectively, where the amplitude is greater than a preset threshold. and These represent the number of frequency points in the self-heterodyne interference signal spectrum of the signal optical frequency comb and the local oscillator optical frequency comb, respectively, whose amplitudes are greater than a preset threshold. and They are respectively with and The corresponding amplitude.

2. The repetition rate self-calibration dual optical frequency comb ranging system according to claim 1, characterized in that, The heterodyne interferometry and ranging module includes a Michelson interferometer, a dual-frequency comb interferometer, and a photodetector, wherein: The Michelson interferometer splits the signal light into a reference light and a measurement light by a beam splitter 1, and receives the reflected light from the reference light after passing a reference distance and the measurement light after passing a measured distance. The dual-optical frequency comb interferometer combines the reflected light and the local oscillator frequency comb through the beam splitter 2 to obtain a dual-optical frequency comb interference signal. The photodetector is used to receive the dual-frequency comb interference signal, and the dual-frequency comb interference signal includes a self-heterodyne interference signal and a mutual heterodyne interference signal.

3. The frequency repetition rate self-calibration dual optical frequency comb ranging system according to claim 1 or 2, characterized in that, The signal processing module is used to process the dual optical frequency comb interference signal to obtain the real-time repetition rate estimate of the signal optical frequency comb and the local oscillator optical frequency comb, and to obtain the measured value of the distance to be measured based on the real-time repetition rate estimate.

4. The repetition rate self-calibration dual optical frequency comb ranging system according to claim 3, characterized in that, The signal conditioning module is used to perform a fast Fourier transform on the dual optical frequency comb interference signal to obtain the interference amplitude spectrum, and to extract the self-heterodyne interference signal spectrum with a preset frequency difference from the interference amplitude spectrum. The self-heterodyne interference signal spectrum includes the self-heterodyne interference signal spectrum of the signal optical frequency comb and the local oscillator optical frequency comb. The real-time repetition rate estimation module is used to obtain the real-time repetition rate estimation values ​​of the signal optical frequency comb and the local oscillator optical frequency comb in real time based on the undersampled repetition rate estimation values ​​of the signal optical frequency comb and the local oscillator optical frequency comb. The specific calculation formula is as follows: ， in, The value range of is 1 and 2, and Time represents the optical frequency comb of the signal. The time represents the local oscillator optical frequency comb. and These represent the real-time frequency repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb, respectively. and These are the undersampled repetition rate estimates for the signal optical frequency comb and the local oscillator optical frequency comb, respectively. For the preset sampling rate, For the sign coefficient, in Time to take ,exist Time to take ; The distance acquisition module is used to obtain the measured value of the distance to be measured based on the real-time repetition rate estimation values ​​of the signal optical frequency comb and the local oscillator optical frequency comb.

5. The repetition rate self-calibration dual optical frequency comb ranging system according to claim 4, characterized in that, The signal conditioning module further includes an interference signal separation module, and the interference signal separation module includes a cross-heterodyne interference signal bandwidth adjustment module and a center frequency adjustment module, wherein: The mutual heterodyne interference signal bandwidth adjustment module is used to control the bandwidth of the mutual heterodyne interference signal by adjusting the repetition frequency difference between the signal optical frequency comb and the local oscillator optical frequency comb. The center frequency adjustment module is used to set the center frequency of the mutual heterodyne interference signal and the minimum frequency of the mutual heterodyne interference signal by adjusting the pump power of the signal optical frequency comb and the local oscillator optical frequency comb. and maximum frequency The formula must be satisfied: ， ， in, and These represent the preset frequency differences between the repetition rate of the signal optical frequency comb and the local oscillator optical frequency comb and the preset sampling rate, respectively. and These represent the repetition frequencies of the signal optical frequency comb and the local oscillator optical frequency comb, respectively. This represents the sampling rate of the data acquisition card.

6. The repetition rate self-calibration dual optical frequency comb ranging system according to claim 5, characterized in that, The distance acquisition module includes a peak finding module, a truncation module, a differential phase spectrum calculation module, and a distance calculation module, wherein: The peak finding module is used to perform peak finding operation on the dual-optical frequency comb interference signal, wherein the interference peak corresponding to the reference light is the reference peak, and the interference peak corresponding to the measurement light is the measurement peak. The interception module is used to intercept near the reference peak and the measurement peak using short time windows to obtain the reference frame and the measurement frame, respectively. The differential phase spectrum calculation module is used to calculate the differential phase spectrum of the measurement frame and the reference frame through fast Fourier transform. The distance calculation module uses the following formula to calculate the distance to be measured: ， in, The measured value of the distance to be measured. For reference distance, The distance being measured, For the sign coefficient, in Time to take ,exist Time to take , The speed of light in a vacuum. For the refractive index of the propagation medium, The slope of the differential phase spectrum. and These represent the real-time repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb, respectively.

7. The frequency repetition rate self-calibration dual optical frequency comb ranging system according to claim 6, characterized in that, The distance calculation module can also calculate the distance to be measured using the following formula: ， in, The slope of the differential phase spectrum between two adjacent reference frames; And when the actual relative distance When the distance exceeds the unambiguous distance, the distance calculation module also uses the following formula to calculate the distance to be measured: ， In the formula For unambiguous distance, These are empirical parameters.

8. A method for ranging using a frequency repetition rate self-calibration dual optical frequency comb ranging system according to any one of claims 1-7, characterized in that, The method includes: Acquire signal optical frequency combs and local oscillator optical frequency combs with different repetition frequencies and overlapping spectral ranges; The signal optical frequency comb is decomposed into reference light and measurement light, and the decomposed signal optical frequency comb and local oscillator optical frequency comb are synthesized to obtain a dual optical frequency comb interference signal; The dual optical frequency comb interference signal is acquired and processed to obtain the real-time repetition rate estimate of the signal optical frequency comb and the local oscillator optical frequency comb, and the measured value of the distance to be measured is obtained based on the real-time repetition rate estimate.

9. The repetition rate self-calibration dual-frequency comb ranging method according to claim 8, characterized in that, The acquisition and processing of the dual-frequency comb interference signal to obtain real-time repetition rate estimates for the signal frequency comb and the local oscillator frequency comb includes: The dual-frequency comb interference signal output by the photodetector is acquired according to a preset sampling rate, wherein the preset sampling rate is calculated using the following formula: ， in The value range of is 1 and 2, and Time represents the optical frequency comb of the signal. The time represents the local oscillator optical frequency comb. This represents the sampling rate of the data acquisition card. and These represent the preset frequency differences between the repetition rate of the signal optical frequency comb and the local oscillator optical frequency comb and the preset sampling rate, respectively. and These represent the repetition frequencies of the signal optical frequency comb and the local oscillator optical frequency comb, respectively. A fast Fourier transform is performed on the dual optical frequency comb interference signal to obtain the interference amplitude spectrum, and a self-heterodyne interference signal spectrum with a preset frequency difference is extracted from the interference amplitude spectrum. The self-heterodyne interference signal spectrum includes the self-heterodyne interference signal spectrum of the signal optical frequency comb and the local oscillator optical frequency comb. The undersampled repetition rate estimates of the optical frequency comb and local oscillator frequency comb after undersampling are estimated based on the spectrum of the self-heterodyne interferometric signal. The specific calculation formula is as follows: , in, The value range of is 1 and 2, and Time represents the optical frequency comb of the signal. The time represents the local oscillator optical frequency comb. and These are the undersampled repetition rate estimates for the signal optical frequency comb and the local oscillator optical frequency comb, respectively. and These are the frequency points in the spectrum of the self-heterodyne interference signal of the signal optical frequency comb and the local oscillator optical frequency comb, respectively, where the amplitude is greater than a preset threshold. and These represent the number of frequency points in the self-heterodyne interference signal spectrum of the signal optical frequency comb and the local oscillator optical frequency comb, respectively, whose amplitudes are greater than a preset threshold. and They are respectively with and The corresponding amplitude; Based on the undersampled repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb after undersampling, the real-time repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb are obtained in real time. The specific calculation formula is as follows: ， in, The value range of is 1 and 2, and Time represents the optical frequency comb of the signal. The time represents the local oscillator optical frequency comb. and These represent the real-time frequency repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb, respectively. and These are the undersampled repetition rate estimates for the signal optical frequency comb and the local oscillator optical frequency comb, respectively. For the preset sampling rate, For the sign coefficient, in Time to take ,exist Time to take .

10. The repetition rate self-calibration dual-frequency comb ranging method according to claim 9, characterized in that, The measured distance is obtained based on the real-time repetition rate estimation values ​​of the signal optical frequency comb and the local oscillator optical frequency comb, including: Peak finding is performed on the dual-optical frequency comb interference signal, where the interference peak corresponding to the reference light is the reference peak, and the interference peak corresponding to the measurement light is the measurement peak. Short time windows are used to capture the reference peak and the measurement peak respectively to obtain the reference frame and the measurement frame; The differential phase spectrum of the measurement frame and the reference frame is calculated using Fast Fourier Transform; The specific formula for calculating the distance to be measured is as follows: ， in, The measured value of the distance to be measured. For reference distance, The distance being measured, For the sign coefficient, in Time to take ,exist Time to take , The speed of light in a vacuum. For the refractive index of the propagation medium, The slope of the differential phase spectrum. and These represent the real-time repetition rate estimates of the signal optical frequency comb and the local oscillator optical frequency comb, respectively.

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