Zero drift error suppression method and system for double-optical-comb absolute range finder

Abstract

The invention discloses a zero drift error suppression method and system for a double-optical-comb absolute range finder. The method comprises the steps that a semi-transparent and semi-reflective film is arranged on the end face of an optical fiber connected with an optical fiber collimating mirror to form a zero drift pattern; the measurement target pattern and the zero drift pattern are identified through amplitude triggering in the period of the measurement signal, the flight time difference between the measurement target pattern and the reference target pattern of the reference signal is solved to obtain two distance values, the two distance values represent a measurement target part and a zero drift part respectively, and the target measurement result can be corrected through the change trend. The zero drift pattern is generated through optical fiber coating, and common-path signal differential processing is utilized, so that the zero drift amount can be quantified in real time, the measurement result can be corrected, the distance measurement precision is remarkably improved, and the reliability of environment interference resistance of a distance measurement system is improved.

Description

Technical Field

[0001] This invention relates to the field of high-precision absolute ranging, and in particular to a method and system for suppressing zero-point drift error in a dual-comb absolute rangefinder. Background Technology

[0002] High-precision absolute ranging is a key technology in large-scale metrology, and it is of great significance to the development of high-end equipment manufacturing, geodesy, and GNSS navigation. Measurement applications cover both indoor and outdoor scenarios, with measurement scales ranging from a few meters to tens of meters and extending to hundreds of meters to kilometers, requiring ranging accuracy at the sub-millimeter and even micrometer levels. In high-precision measurement and geodesy, zero-point drift error is a common concern for absolute rangefinders, especially in scenarios with drastic environmental changes, such as temperature variations and mechanical vibrations. To achieve micrometer-level ranging accuracy, zero-point drift error is a significant source of error. To suppress the impact of zero-point error, the research and design phase typically involves improving the stability of the optical path's mechanical structure, which increases measurement costs. Other studies estimate the zero-point drift error caused by temperature and correct the measurement results through post-compensation. However, both methods affect the integration and flexibility of the ranging system.

[0003] The emergence of optical frequency combs ("optical combs") has provided new support for high-precision absolute ranging, especially dual-comb absolute ranging. It combines the advantages of traditional laser pulse time-of-flight methods, such as strong anti-interference capability and fast measurement speed, while simultaneously converting the pulse signal in the optical frequency domain into an interferogram signal in the radio frequency domain through optical sampling, greatly improving the resolution of the ranging system. Besides optimizing and improving ranging accuracy in dual-comb ranging systems, the mechanical structure and fiber optic components within the optical system will deform due to changes in the measurement environment. Simultaneously, mechanical vibrations during measurement will cause zero-point drift, affecting the ranging accuracy. For example, in dual-comb ranging systems, due to assembly, debugging, and high system integration, the influence of zero-point drift in the measurement optical path requires maintaining the stability of the optical path structure over a long period. In a relatively stable indoor environment, the zero-point drift error is typically a few micrometers to tens of micrometers. In the varied conditions of outdoor environments, the zero-point drift error can reach tens of micrometers to sub-micrometers, severely affecting ranging accuracy. Therefore, it is necessary to limit the zero-point drift error. Therefore, a method and system for suppressing zero-point drift error in a dual-comb absolute rangefinder are needed. Summary of the Invention

[0004] This application provides a method and system for suppressing zero-point drift error in a dual-comb absolute rangefinder.

[0005] This application provides a method for suppressing zero-point drift error in a dual-comb absolute rangefinder, including the following steps:

[0006] A semi-transparent, semi-reflective film is installed on the end face of the fiber connected to the fiber collimating lens to form a zero-point drift pattern;

[0007] During the period of the measurement signal, the target pattern and the zero drift pattern are identified by amplitude triggering. The time difference between the target pattern and the reference pattern of the reference signal is calculated to obtain two distance values, which represent the target part and the zero drift part in the measurement result, respectively. The target measurement result can be corrected by the trend of change.

[0008] Furthermore, the target measurement results can be corrected by observing the changing trend. Specifically, this includes creating a large zero drift by heating the fiber optic module and then correcting the measurement results by adjusting the change in the corresponding value of the coating surface to reduce the zero drift.

[0009] On the other hand, a zero-point drift error suppression system for a dual-comb absolute rangefinder, used to implement the aforementioned method for suppressing zero-point drift error in a dual-comb absolute rangefinder, includes a first optical comb, a second optical comb, an optical fiber collimator, a first coupler, a second coupler, a third coupler, a fourth coupler, a signal acquisition unit, and an optical fiber circulator. The output end of the first optical comb is connected to the input end of the first coupler, the first output end of the first coupler is connected to the input end of the optical fiber circulator, the adapter end of the optical fiber circulator is connected to the input end of the optical fiber collimator, a measurement target is provided at the optical path output end of the optical fiber collimator, and a semi-transparent, semi-reflective film is provided on the fiber end face connected to the optical fiber collimator. The feedback end of the fiber optic circulator is connected to the first input end of the fourth coupler. The second output end of the first coupler is connected to the first input end of the third coupler. The output end of the second optical comb is connected to the input end of the second coupler. The first input end of the second coupler is connected to the second input end of the third coupler. The second input end of the second coupler is connected to the second input end of the fourth coupler. The output ends of the third coupler and the fourth coupler are respectively connected to the input end of the signal acquisition unit. The output end of the signal acquisition unit is connected to the calculation module to acquire the measurement target part and the zero drift part in the ranging result.

[0010] Furthermore, the signal acquisition unit includes a photodetector, a first narrowband filter, a second narrowband filter, a photodetector, a first bandpass filter, and a second bandpass filter. The output terminal of the third coupler is connected to the input terminal of the first narrowband filter, the output terminal of the fourth coupler is connected to the input terminal of the second narrowband filter, the output terminal of the first narrowband filter is connected to the input terminal of the photodetector, the output terminal of the photodetector is connected to the input terminal of the first bandpass filter, and the output terminal of the first bandpass filter is connected to the signal acquisition unit.

[0011] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages:

[0012] This invention generates an additional reference pattern through coating and utilizes common-path signal differential processing to quantify zero drift and correct measurement results in real time, significantly improving ranging accuracy. It is widely applicable in absolute rangefinders based on the time-of-flight method, which is beneficial for system compactness and simplification, and is of great significance for improving the accuracy and stability of such rangefinders. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the optical path for the zero-point drift error suppression method of the dual-comb absolute rangefinder provided in the embodiments of this application.

[0014] Figure 2 The flowchart shows the algorithm for the zero-point drift error suppression method of the dual-comb absolute rangefinder provided in the embodiments of this application.

[0015] Figure 3 A single-cycle signal waveform diagram of the zero-point drift error suppression method for the dual-comb absolute rangefinder provided in this application embodiment;

[0016] Figure 4 A correlation verification diagram showing the original change trend between the measurement target portion and the zero-point drift portion of the zero-point drift error suppression method for the dual-comb absolute rangefinder provided in this application embodiment;

[0017] Figure 5 The graph shows the variation of the zero drift suppression effect after moving average filtering of two measurement results of the zero drift error suppression method for the dual-comb absolute rangefinder provided in the embodiments of this application. Detailed Implementation

[0018] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0019] like Figure 1As shown, the present invention includes a first optical comb, a second optical comb, and a collimator. It is characterized by further including a first coupler, a second coupler, a third coupler, a fourth coupler, a signal acquisition unit, and a fiber optic circulator. The output end of the first optical comb is connected to the input end of the first coupler. The first output end of the first coupler is connected to the input end of the circulator. The adapter end of the fiber optic circulator is connected to the input end of the fiber optic collimator. A measurement target is provided at the optical path output end of the fiber optic collimator. A semi-transparent, semi-reflective film is provided on the fiber end face connected to the fiber collimator. The second output end of the first coupler is connected to the first input end of the third coupler. The output end of the second optical comb is connected to the input end of the second coupler. The first input end of the second coupler is connected to the second input end of the third coupler. The second input end of the second coupler is connected to the second input end of the fourth coupler. The output ends of the third coupler and the fourth coupler are respectively connected to the input end of the signal acquisition unit. The output end of the signal acquisition unit is connected to a calculation module to acquire the measurement target portion and the zero-point drift portion of the ranging result.

[0020] The signal processing unit includes a photodetector, a first narrowband filter, a second narrowband filter, a photodetector, a first bandpass filter, and a second bandpass filter. The output of the third coupler is connected to the input of the first narrowband filter, the output of the fourth coupler is connected to the input of the second narrowband filter, the output of the first narrowband filter is connected to the input of the photodetector, the output of the photodetector is connected to the input of the first bandpass filter, and the output of the first bandpass filter is connected to the signal acquisition unit.

[0021] Based on the dual-comb ranging optical path, the measurement pulse corresponding pattern and the reference pulse corresponding pattern are separated. An end-face coating with approximately 10% reflectivity is added to the measuring arm near the collimator, generating another independent pulse signal within a single cycle of the measurement path. This pulse signal, along with the original target measurement pulse signal, is reflected back through a fiber optic circulator and then combined with the local oscillator comb, producing two corresponding measurement path patterns, independent of the reference path pattern generated by directly combining the probe comb with the local oscillator comb. Similar to the single pattern generated by the original target, the two measurement path patterns can be used to calculate the time-of-flight difference within a single cycle with the reference pattern signal, thus obtaining the distance information for the two targets. The difference between the two patterns can eliminate the common-mode error of zero drift.

[0022] 70% of the light from the first optical comb enters port 1 of the fiber optic circulator through the first coupler, and is then output from port 2. After passing through an optical collimator, it is directed towards the measurement target for practical measurement scenarios such as distance measurement. 30% of the light enters the subsequent optical path through the first coupler, and together with the signal from the second optical comb, it passes through a series of couplers (such as the second, third, and fourth couplers) and filters (the first and second narrowband filters). After being converted by a photodetector, it passes through the first and second bandpass filters and enters the solution module for signal processing and analysis. After passing through the second coupler, the signal from the second optical comb is distributed to the third and fourth couplers and other components, where it merges with the 30% signal from the first optical comb. After passing through a narrowband filter, a photodetector, and a bandpass filter, it enters the solution module, which is mainly used for solution processing. In a dual-channel dual-comb ranging optical path, the two largest pattern signals acquired by the measurement path within a single cycle represent the target pattern and the zero-point drift pattern, respectively. There may also be smaller patterns formed due to secondary reflections, which are discarded. The flight time difference between the two largest patterns and the reference target pattern is calculated separately to obtain two measurement results, which represent the target part and the zero-point drift part of the measured distance, respectively.

[0023] The online calculation software continuously collects and calculates two values ​​within a single period at intervals, denoted as the measurement decimal and the zero drift decimal, respectively. The two measurement results show strong consistency in their changes over the time scale. To visually demonstrate the impact of the zero drift value, the results are compared and corrected using a 0.1s (approximately 500 measurement periods) moving average. This method can reduce the zero drift to the 10% level.

[0024] like Figure 2 As shown, based on the data from the signal acquisition unit, the system first synchronously acquires the target measurement pattern and the zero-point drift pattern, and distinguishes between the two through amplitude triggering. Both the target measurement portion and the zero-point drift portion undergo moving average preprocessing to smooth the data. The zero-point drift data, after moving average, is used to calibrate the zero point, generating a fixed zero-drift reference value. In the real-time processing stage, the system performs moving average processing on the real-time measurement portion and the real-time zero-drift portion respectively; subsequently, the real-time measurement value is first subtracted from the real-time zero-drift component, and then subtracted from the calibrated fixed zero-drift value. Through these two subtraction operations, the final output is the accurate result after zero-point drift suppression.

[0025] like Figure 3As shown, this scheme achieves accurate distance measurement by comparing the target measurement pattern with the zero-point drift pattern. The specific process is as follows: Simultaneously acquire signals from both the target measurement portion and the zero-point drift portion within the same sampling interval; the zero-point drift portion characterizes the inherent drift characteristics of the system, while the target measurement portion contains the distance information to be measured; a reference target pattern is specifically set as the comparison object. By quantifying the offset characteristics of the target measurement pattern relative to the reference target pattern, and combining this with compensation and correction of the zero-point drift portion, the accurate distance value is finally calculated.

[0026] like Figure 4 As shown, the original fluctuations of the calculated target distance (measured decimal) and the zero-drift reference value (zero-drift decimal) during continuous measurement were compared. The changes in the decimal are consistent with the changes in the actual distance value. The horizontal axis represents the measurement point number (2000 points in total, corresponding to approximately 0.4 seconds). The curves of the two are highly consistent, indicating that they are both affected by the same environmental factor (such as slow temperature changes). Zero drift is a common-mode error. The fluctuation range of the measured decimal is consistent with the fluctuation range of the zero-drift decimal. The slight difference stems from the different paths of the two. The target signal includes the target distance, while the reference signal only reflects the internal drift of the optical path. This indicates that the impact of zero drift error on the measurement results is systematic, that is, both have the same drift component and trend. Through subsequent zero-drift quantization correction or differential processing, this common-mode error can be eliminated, and the influence of the zero-point drift can be suppressed.

[0027] like Figure 5 As shown, a significant zero drift was induced by artificially heating the fiber optic module. The difference between the two results was observed, and the measurement results were corrected by the change in the corresponding result of the coating surface. The corrected measurement decimal = real-time measurement decimal - real-time zero drift decimal + initial zero drift decimal. The robustness of the method under strong interference was tested by artificially heating the fiber optic module to simulate extreme environmental disturbances. Two sets of comparative experiments were included. Local heating increased the fiber temperature by 5°C, introducing a 96μm zero drift. By differentially correcting the reference signal of the coating surface, the zero drift was reduced to 12μm, with a suppression amplitude of 87.5%.

[0028] Even under strong interference scenarios, where the zero-drift error increases by an order of magnitude, the common-path signal differential mechanism can still linearly cancel the drift component in the measured value by real-time monitoring of the coating surface reference value. This demonstrates the adaptability of the technical solution to complex environments. In distance information calculation, the zero-point drift pattern and the zero-point drift reference pattern are identified by amplitude within one cycle of the measurement signal. The time-of-flight difference is calculated between these two patterns and the reference pattern of the reference signal, resulting in two distance values ​​representing the target measurement result and the zero-point drift reference result, respectively. The former can be corrected by observing the trend of the latter. This method can suppress the influence of zero-point drift error through common-path signal differential measurement. This method is widely applicable in absolute rangefinders based on the time-of-flight method, which is beneficial for system compactness and simplification and has significant implications for improving the stability of such rangefinders.

[0029] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for suppressing zero-point drift error in a dual-comb absolute rangefinder, characterized in that, Includes the following steps: A semi-transparent, semi-reflective film is installed on the end face of the fiber connected to the fiber collimating lens to form a zero-point drift pattern; During the period of the measurement signal, the measurement target pattern and the zero drift pattern are identified by amplitude. The flight time difference is calculated with the reference target pattern of the reference signal to obtain two distance values ​​representing the measurement target part and the zero drift part, respectively. The result of the target measurement at the corresponding time can be corrected by subtracting the change amount from the change amount based on the changing trend and amount of the reference result.

2. The method for suppressing zero-point drift error in a dual-comb absolute rangefinder according to claim 1, characterized in that, The results of the measurement target can be corrected by observing the changing trend. For example, if the temperature change of the external working environment of the fiber optic module causes a large zero drift, the measurement result can be corrected by observing the change value of the corresponding coating surface to reduce the zero drift.

3. A zero-point drift error suppression system for a dual-comb absolute rangefinder, used to implement the zero-point drift error suppression method for a dual-comb absolute rangefinder as described in any one of claims 1-2, comprising a first optical comb, a second optical comb, and an optical fiber collimating lens, characterized in that, It also includes a first coupler, a second coupler, a third coupler, a fourth coupler, a signal acquisition unit, and an optical fiber circulator. The output end of the first optical comb is connected to the input end of the first coupler. The first output end of the first coupler is connected to the input end of the optical fiber circulator. The adapter end of the circulator is connected to the input end of the optical fiber collimator. A measurement target is provided at the optical path output end of the optical fiber collimator. A semi-transparent and semi-reflective film is provided on the end face of the optical fiber connected to the optical fiber collimator. The feedback end of the optical fiber circulator is connected to the first input end of the fourth coupler. The second output end of the first coupler is connected to the first input end of the third coupler. The output end of the second optical comb is connected to the input end of the second coupler. The first input end of the second coupler is connected to the second input end of the third coupler. The second input end of the second coupler is connected to the second input end of the fourth coupler. The output ends of the third coupler and the fourth coupler are respectively connected to the input end of the signal acquisition unit. The output end of the signal acquisition unit is connected to the calculation module to acquire the measurement target part and the zero-point drift part in the measurement result.

4. The zero-point drift error suppression system for a dual-comb absolute rangefinder according to claim 3, characterized in that, The signal processing unit includes a photodetector, a first narrowband filter, a second narrowband filter, a photodetector, a first bandpass filter, and a second bandpass filter. The output of the third coupler is connected to the input of the first narrowband filter, the output of the fourth coupler is connected to the input of the second narrowband filter, the output of the first narrowband filter is connected to the input of the photodetector, the output of the photodetector is connected to the input of the first bandpass filter, and the output of the first bandpass filter is connected to the calculation module.

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