CN117130004B - Small dynamic range double-optical-comb combined distance measuring device and distance measuring method - Google Patents
Small dynamic range double-optical-comb combined distance measuring device and distance measuring method Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/06—Systems determining position data of a target
- G01S17/08—Systems determining position data of a target for measuring distance only
- G01S17/32—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
- G01S17/36—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated with phase comparison between the received signal and the contemporaneously transmitted signal
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K7/00—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements
- G01K7/16—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using resistive elements
- G01K7/22—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using resistive elements the element being a non-linear resistance, e.g. thermistor
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/481—Constructional features, e.g. arrangements of optical elements
- G01S7/4814—Constructional features, e.g. arrangements of optical elements of transmitters alone
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/481—Constructional features, e.g. arrangements of optical elements
- G01S7/4818—Constructional features, e.g. arrangements of optical elements using optical fibres
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/497—Means for monitoring or calibrating
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Abstract
The invention relates to a small dynamic range double-optical comb combined distance measuring device and a method, wherein the device comprises a double-optical comb light source, a first polarization maintaining optical fiber, a second polarization maintaining optical fiber, a distance measuring light path module, a third polarization maintaining optical fiber, a fourth polarization maintaining optical fiber and a signal detecting and processing module; the light source in the double-optical comb light source is locked by the heavy frequency, the offset frequency runs freely, the double-optical comb ranging light path is regulated, so that the time delay of the reference interference signal and the measuring interference signal in the initial state is basically 0, and the carrier wave phase difference noise sigma delta phi of the double-optical comb ranging system is generated at the moment c Minimum; by carrier phase difference delta phi of reference interference signal and measurement interference signal c Obtaining a distance value D c . Therefore, the invention realizes high-precision absolute ranging through stable carrier wave phase difference and realizes high-precision ranging through a double-optical-comb combined ranging method.
Description
Technical Field
The invention relates to a small dynamic range double-optical comb combined distance measuring device and a distance measuring method, and relates to the field of optical precision measurement.
Background
The double optical comb measuring system utilizes two optical frequency combs with small repetition frequency difference, and can generate 10 by heterodyne principle of comb teeth to comb teeth between the optical frequency combs 4 The heterodyne signals are used for coordination measurement, and unique comprehensive performance is provided in the aspects of high precision, high speed and large fuzzy range. Double optical comb system based on optical fiber optical comb, its phase noise fluctuation range reaches about 10 4 The radian enables time jitter to reach the level of about 100 nanoseconds, and limits the measurement accuracy of the double optical comb system.
The traditional double optical comb measuring system with small time jitter and stable phase requires additional noise compensation measures, such as a tight locking method and a post-processing correction method. These methods are often complex in system or cumbersome in processing steps. In practice, in many double optical comb applications, only the delay jitter and phase difference stability between the reference and measurement interference signals are of interest, and not the phase stability and time jitter itself. Research proves that the time delay jitter between the reference interference signal and the measurement interference signal is mainly influenced by the frequency noise of the light source and the intensity noise of the detector, and the shorter the pulse interval time between the reference signal and the measurement signal is, the smaller the time delay jitter between the interference signals caused by the frequency noise is. When the pulse interval time is small enough, the influence of frequency noise on the time delay between measurement and reference interference signals is negligible, at the moment, the time delay jitter is mainly influenced by system intensity noise, and the phase noise of each longitudinal mode of the optical frequency comb caused by the intensity noise is random noise, so that the time delay is calculated by adopting a phase frequency slope, and the distance measurement error caused by the time delay jitter is further amplified.
The double optical comb distance measurement has two classical methods, the first is a time-of-flight method, the measured distance can be obtained by referring to the interference signal and measuring the time delay deltat between the envelopes of the interference signal, the method is characterized in that the non-fuzzy range is large, the range can be generally up to the order of meters, the precision is generally in the order of micrometers to hundred nanometers, and the precision is mainly influenced by the time delay jitter between the interference signals. The second method is a carrier interferometry, the measured distance of which can be obtained by referring to the carrier phase difference between the interference signal and the measurement interference signal, the accuracy of the method is mainly influenced by the stability of the carrier phase difference of the interference signal, when the phase difference is stable, the accuracy can generally reach the nanometer level, but the non-fuzzy range is small, only half carrier wavelength is needed, and the accuracy is generally in the hundred nm level. The carrier wave phase difference of the double optical comb ranging system, which is affected by the phase noise of the double optical comb system and is not externally connected with a noise compensation device at presentIs far greater than 2 pi, and the carrier phase difference information cannot be usedCarrier interferometry is ineffective. Therefore, the double optical comb ranging method without the external noise compensation device only utilizes the flight time information to acquire the measured distance value through the flight time method, and the time jitter between interference signals greatly limits the measurement precision of the double optical comb system and limits the ranging application of the double optical comb system with high precision.
According to the research conclusion, in a section with smaller time delay of the reference-measurement interference signal, not only the time jitter is smaller, but also the carrier phase difference tends to be stable, wherein the stability means that the time of flight measurement precision after short-time coherent averaging can reach lambda c And/4 or less, the peak-to-peak value of the carrier phase difference jitter is within 2pi. At this time, the half-carrier wavelength lambda is determined by using the measurement result of the flight time c And (3) carrying out integral multiple of/2, and then, transferring to a carrier phase difference measurement result to realize small-range, high-precision and high-speed absolute distance measurement. Compared with the traditional double-optical-comb measuring system with stable phase, the method can realize high-precision and high-speed absolute distance measurement by utilizing the flight time information and the carrier phase information at the same time without adding any noise compensation method. According to the method, by controlling the integral time of frequency noise, a phase stable double-optical-comb system without noise compensation is constructed, and absolute distance measurement of nanometer-scale precision in a millimeter-scale dynamic range is realized. The method has two limitations, namely, the repetition frequency of two optical frequency combs and the carrier envelope offset frequency are required to be locked, wherein the carrier envelope offset frequency is very complex to lock, and the purpose of locking the carrier envelope offset frequency is to accurately trace the source optical frequency. The second is that the reference interference signal and the measurement interference signal are received by a detector, and the basic requirement of the method for realizing the ranging is that the two interference signals are not overlapped, so that the reference mirror and the measurement mirror need to be provided with an initial optical path difference of about 2mm in an initial state. This results in the above method sacrificing a dynamic range of 2mm (the range of phase difference stable is only about 5 mm), and also sacrifices a range of the highest phase accuracy (carrier phase difference is the most stable in the case of approximately equal length of the reference arm and the measuring arm).
Disclosure of Invention
The present invention aims to solve at least one of the technical problems existing in the prior art. Therefore, in order to solve the above problems, the present invention aims to provide a small dynamic range double optical comb combined ranging device and a ranging method with stable phase difference.
In order to achieve the above object, the present invention has the technical scheme that:
in a first aspect, the present invention provides a small dynamic range double optical comb combined ranging method, including:
the double optical comb ranging light path is regulated to enable the time delay of the reference interference signal and the measuring interference signal to be basically 0 in the initial state, and at the moment, the carrier wave phase difference noise of the double optical comb ranging systemMinimum;
by carrier phase difference delta phi of reference interference signal and measurement interference signal c Obtaining a distance value D c ：
Wherein,for measuring carrier phase difference between reference interference signal and measurement interference signal lambda c Is the carrier wave wavelength in the air, N c And an integer period representing the half carrier wavelength is determined by using a ranging result of a time-of-flight method.
Further, the method also comprises the step of correcting the ranging result, comprising the following steps:
obtaining a distance value D c Then, according to the distance measurement error caused by temperature compensation temperature drift in the double-optical-comb distance measurement light path, the corrected distance measurement result is D c ′：
D c ′＝D c -kΔe
Wherein k is a temperature compensation coefficient, and deltae is the difference between the current temperature in the optical comb ranging light path and the initial temperature when the ranging is started.
In a second aspect, the invention also provides a small dynamic range double-optical-comb combined distance measuring device, which comprises a double-optical-comb light source, a first polarization maintaining optical fiber, a second polarization maintaining optical fiber, a distance measuring light path module, a third polarization maintaining optical fiber, a fourth polarization maintaining optical fiber and a signal detecting and processing module; wherein, the light source in the double-light comb light source is locked by heavy frequency and the frequency deviation runs freely;
the double-optical comb light source is characterized in that the double-optical comb light source is used for respectively inputting signal light and local oscillation light signals into the ranging light path module through the first polarization maintaining optical fiber and the second polarization maintaining optical fiber, the ranging light path module is used for outputting space light to a target mirror, the ranging light path module is used for respectively outputting output reference interference signals and measurement interference signals into the signal detection and processing module through the third polarization maintaining optical fiber and the fourth polarization maintaining optical fiber, and the signal detection and processing module is used for calculating a ranging result through carrier phase difference of the reference interference signals and the measurement interference signals.
Further, the ranging light path module comprises a first optical fiber beam splitter, a second optical fiber beam splitter, a first optical fiber coupler, a second optical fiber coupler, an optical fiber circulator, a first optical fiber filter, a second optical fiber filter and an optical fiber collimator;
the double-light comb light source outputs two paths of repetition frequencies f respectively through optical fibers r1 And f r2 The femtosecond laser beam is used as signal light and local oscillation light, and is divided into two paths through the first optical fiber beam splitter and the second optical fiber beam splitter respectively, wherein one path of the femtosecond laser beam is used as a reference interference signal by being detected by the signal detection and processing module through the third polarization maintaining optical fiber after being interfered by the combined light of the first optical fiber coupler and passing through the first optical fiber filter; the other path of light beam of the signal light is emitted to a target mirror after passing through the optical fiber circulator and the optical fiber collimator, and the return light of the target mirror is emitted through the other end of the optical fiber circulator after being coupled into the optical fiber through the optical fiber collimator, then is interfered with local oscillation light through the second optical fiber coupler, passes through the second optical fiber filter and is detected by the signal detection and processing module through the fourth polarization maintaining optical fiber to be used as a measurement interference signal.
Further toThe center wavelength and the bandwidth of the first optical fiber filter and the second optical fiber filter are consistent, wherein the center wavelength is approximately equal to the center wavelength of the double-optical comb light source, and the bandwidth is deltav comb Should be smaller than f r1 f r2 /2(Δf r ) To ensure that the aliasing phenomenon of the double optical comb frequency spectrum, delta f, does not occur r And is a frequency difference.
Further, the signal detection and processing module detects a bandwidth greater than f r2 To satisfy the nyquist sampling theorem.
Further, the optical fiber length requirement of the optical path is:
L 5 substantially equal to L 6 So that the initial phases introduced by the optical frequency comb are equal and L 3 Should be greater than L 1 +2L 2 +L 4 The difference L of 3 -(L 1 +2L 2 +L 4 ) Multiplying the refractive index of the fiber by the initial distance L from the incident end face of the fiber to the target mirror corresponds to twice the optical path length, where L 1 L is the distance between the first fiber splitter and the fiber circulator 2 L is the distance between the fiber optic circulator and the fiber optic collimator 3 L is the distance between the first fiber splitter and the first fiber coupler 4 L is the distance between the fiber circulator and the second fiber coupler 5 L is the distance between the first fiber coupler and the second fiber splitter 6 Is the distance from the second fiber splitter to the second fiber coupler.
Further, the optical fiber length requirement of the optical path is implemented in any one of the following forms, including:
first form: the initial distance L between the optical fiber collimator and the target mirror is finely adjusted, so that the time delay of the reference interference signal and the measurement interference signal is basically 0 in an initial state;
second form: at L 1 ，L 2 ，L 3 Or L 4 An adjustable optical fiber delay line is added to any section, and the time delay of the reference interference signal and the measurement interference signal in the initial state is basically 0 by adjusting the length of the optical fiber delay line.
Further, a thermistor is further arranged in the ranging light path module and used for sensing temperature change of the optical fiber light path and is connected to the signal detection and processing module through a cable.
Further, the method for determining the carrier frequency and the carrier phase difference of the optical frequency comb comprises the following steps:
determining the center wavelength lambda of the signal light after passing through the optical fiber filter by the wavelength measuring device c The corresponding frequency is taken as a carrier frequency value f c ；
The amplitude spectrum information of the sub-frequency comb is obtained by carrying out Fourier transform on the acquired reference interference signals, and the frequency value corresponding to the highest amplitude point is used as the carrier frequency f of the sub-frequency comb RF(c) At a frequency f RF(c) The corresponding phase difference of the reference and measurement interference signals is used as the carrier frequency f of the optical frequency comb c Corresponding phase difference
The invention adopts the technical proposal and has the following characteristics:
1. the invention relates to a low dynamic range high-precision ranging method realized by a double-optical-comb light source based on heavy frequency locking and offset frequency free running, which is characterized in that the integer multiple period of a carrier interference phase is determined by a time-of-flight method, high-precision absolute ranging is realized by a stable carrier phase difference, and the high-precision ranging is realized by a double-optical-comb combined ranging method.
2. The invention improves the compactness and flexibility of the double-optical-comb ranging optical path through the all-optical-fiber ranging optical path, and enables the reference interference signal and the measuring interference signal to be arranged near 0 in the initial state by the design of the optical fiber length and the optical path mode of respectively detecting the reference interference signal and the measuring interference signal by the two detectors, thereby ensuring the shortest integration time of frequency noise and realizing the maximum dynamic ranging range and high-stability output of phase difference.
3. According to the invention, the temperature in the optical path is monitored in real time through the thermistor, so that the influence on a ranging result caused by different lengths of the reference optical fiber and the measuring optical fiber is compensated, and a high-precision absolute distance value is obtained in the range.
4. The invention solves the problem that the reference interference signal and the measurement interference signal are overlapped in the time domain and cannot be measured by the way of separately detecting the reference interference signal and the measurement interference signal, and eliminates the ranging blind area.
In conclusion, the invention is simple and practical, the light source only needs to be locked in a repetition frequency mode, the offset frequency can run freely, and the phase difference stable double-optical-comb ranging device without noise compensation is constructed by adopting an all-fiber optical path mode to control the integral time of frequency noise, so that the invention is suitable for dynamic high-precision ranging application with smaller axial ranging range, such as fixed-length satellite antenna measurement, vibration measurement and the like.
Drawings
Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. Like parts are designated with like reference numerals throughout the drawings. In the drawings:
FIG. 1 is a diagram of a small dynamic range dual optical comb combination ranging apparatus according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a ranging light path module according to an embodiment of the present invention;
FIG. 3 is a mapping relationship between an optical frequency comb and a sub-frequency comb according to an embodiment of the present invention;
fig. 4 is a schematic diagram of a combination ranging scheme of a time-of-flight method and a carrier interferometry according to an embodiment of the present invention.
Detailed Description
It is to be understood that the terminology used herein is for the purpose of describing particular example embodiments only, and is not intended to be limiting. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "includes," "including," and "having" are inclusive and therefore specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order described or illustrated, unless an order of performance is explicitly stated. It should also be appreciated that additional or alternative steps may be used.
Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as "first," "second," and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.
For ease of description, spatially relative terms, such as "inner," "outer," "lower," "upper," and the like, may be used herein to describe one element or feature's relationship to another element or feature as illustrated in the figures. Such spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures.
The invention provides a small dynamic range double-optical-comb combined distance measuring device and a distance measuring method, which are based on double-optical-comb light sources with heavy frequency locking and free running offset frequency to realize high-precision distance measurement. The small dynamic range in this embodiment refers to a range where the carrier phase difference is stable, which can be obtained by analyzing the jitter of the carrier phase difference, and the stable carrier phase difference range requires that the random jitter of the carrier phase difference is less than 2pi, and for an optical fiber double optical comb ranging system with a repetition frequency of 50MHz, the stable range is usually in centimeter level.
Double optical comb ranging is based on the principle of spectral phase difference measurement,and the distance is calculated through the spectrum phase difference obtained after the multi-longitudinal-mode heterodyne of the two optical frequency combs. The spectrum phase noise is essentially caused by the frequency noise of the two optical frequency combs, and is transferred to the sub-frequency combs in a double-optical-comb heterodyne mode, so that the frequency noise of the two optical frequency combs is combined into a whole. The repetition frequency (repetition frequency) of the two optical frequency combs is locked to a radio frequency reference, and the carrier envelope offset frequency (offset frequency) runs freely, wherein the noise of the repetition frequency is represented by a coefficient i 1 And i 2 Amplification, i 1 And i 2 Is of the order of magnitude greater, about 10 calculated from the optical frequency 6 Order of magnitude, therefore, i can be ignored 1 –i 2 The effect of this coefficient component on frequency noise. Frequency noise sigma f of sub-frequency comb longitudinal mode RF (k) Can be expressed approximately as a heavy frequency difference noise sigma delta f r Is added with the noise sigma delta f of the difference between the two optical frequency comb offset frequencies ceo ：
σf RF (k)＝i 1 σf r1 -i 2 σf r2 +σΔf ceo ≈i 1 σΔf r +σΔf ceo (1)
Here, the repetition frequency of the optical frequency comb is determined by the cavity length, the repetition frequency noise is mainly caused by the instability of the cavity length, the offset frequency of the optical frequency comb is determined by the intra-cavity dispersion, and the influence of the pumping current and the temperature is mainly caused.
Comb frequency noise sigma f according to sub-frequency RF (k) In consideration of amplification of the delay Δt between interference signals with respect to the delay Δτ between pulses in the time domain, carrier phase difference noise of the sub-frequency combAnd the corresponding optical frequency comb carrier (with the serial number i c ) Phase difference noise->The carrier frequency noise, which can be expressed as a sub-frequency comb, integrates over Δt time:
wherein t is M -t R Representing the integration time, equal to the time delay of the reference interference signal and the measured interference signal.
Because the time delay of the reference and measurement interference signals is calculated by the phase frequency slope, the noise of the offset frequency can be eliminated, and the comb teeth of the sub-frequency comb are delta f r The time delay stability (σΔt) between the reference and measured interference signals can be expressed as:
as can be seen from equations (2) and (3), when the heavy and offset noise are fixed, the carrier phase difference noise is positively correlated with the integration time, i.e., the time delay Δt between the interference signals. The time delay stability is irrelevant to the offset noise, and is positively correlated with the time delay delta t at a certain time of the heavy frequency offset noise.
According to the measurement principle of the double optical combs, besides frequency noise, intensity noise also affects delay stability and carrier phase difference noise, when delta t is smaller, sigma delta t is mainly affected by the intensity noise, and the range error of the time-of-flight method is far larger than that of the carrier interferometry. Therefore, if only the time-of-flight method is used for distance measurement, σΔt due to intensity noise may limit the measurement accuracy of the double optical comb system. To obtain higher accuracy, ranging results combined with carrier interferometry are necessary. However, ifIf the random jitter exceeds 2 pi, the information of the carrier phase difference is invalid, and the carrier interferometry is not applicable.
From equation (2), it can be found that when the time delay t of the interference signal is referenced and measured M –t R When approaching 0, it is apparent that the phase difference noise of the sub-frequency combAlso tends toNear 0. The invention thus provides for the control of the integration time of equation (2), i.e. the time delay t of the reference and measurement interference signal M -t R So that->Random jitter of (a) is always less than 2 pi, and corresponding delay interval + -delta t max For the stable carrier phase difference interval, the dynamic distance range + -delta D corresponding to the interval max In the method, a relatively stable carrier phase difference signal and a time delay signal can be obtained without adding any noise compensation method, and high-precision double-optical-comb combined ranging is realized. It should be noted here that for different parameters and different kinds of double optical comb systems, the stability interval of carrier phase difference ± Δt max Corresponding dynamic distance + -DeltaD max The analysis may be performed on a case-by-case basis.
In the double optical comb system, the time delay delta t between the reference interference signal and the measurement interference signal is amplified by f relative to the time delay delta tau of the reference pulse and the measurement pulse r1 /Δf r Multiple times. Theoretically, if the difference Δf of the repetition frequency r When Δt is unchanged, the range of Δτ can be increased, so that the dynamic ranging range of the method can be increased simultaneously. It should be noted that to satisfy the nyquist sampling theorem, Δf when the spectral bandwidth range is unchanged r Is limited. Therefore, the repetition frequency difference is increased, and the repetition frequency of the optical frequency comb needs to be synchronously increased. Further, referring to equation (2), carrier phase difference noiseCan be expressed as i c σΔf r Integration over Δt time, integer period i with constant optical frequency c Will decrease with increasing frequency of emphasis. Therefore, the carrier phase difference noise corresponding to the high-repetition-frequency laser is +.>The dynamic ranging range where the carrier phase difference is stable can be further increased as it is reduced.
Based on the above thought, the invention provides a small dynamic range double-optical comb combined distance measuring device and a method, wherein the device comprises a double-optical comb light source, a first polarization maintaining optical fiber, a second polarization maintaining optical fiber, a distance measuring light path module, a third polarization maintaining optical fiber, a fourth polarization maintaining optical fiber and a signal detecting and processing module; the light source in the double-optical comb light source is locked in a heavy frequency mode, the offset frequency runs freely, the double-optical comb ranging light path is adjusted, so that the time delay of a reference interference signal and a measuring interference signal in an initial state is basically 0, and at the moment, the carrier wave phase difference noise of the double-optical comb ranging systemMinimum; carrier phase difference by reference interference signal and measurement interference signal +.>Obtaining a distance value D c . Therefore, the invention realizes high-precision absolute ranging through stable carrier wave phase difference and realizes high-precision ranging through a double-optical-comb combined ranging method.
Exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. While exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention may be embodied in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
Embodiment one: as shown in FIG. 1, the small dynamic range double-optical-comb combined distance measuring device provided by the embodiment comprises a double-optical-comb light source 1, a polarization maintaining optical fiber 2 and a polarization maintaining optical fiber
3. The device comprises a ranging light path module 4, a polarization maintaining optical fiber 5, a polarization maintaining optical fiber 6 and a signal detection and processing module 7, wherein a light source in the double-light comb light source 1 is locked by a heavy frequency and the polarization is free to run. The double-optical comb light source 1 respectively inputs signal light and local oscillation light signals for the ranging light path module 4 through the polarization maintaining optical fiber 2 and the polarization maintaining optical fiber 3, and the ranging light path module 4 outputs space light to the target mirror 8. The reference interference signal and the measurement interference signal are respectively output to enter a signal detection and processing module 7 for sampling processing through a polarization maintaining optical fiber 5 and a polarization maintaining optical fiber 6, and a double-optical-comb ranging result is calculated.
In a preferred embodiment, as shown in FIG. 2, ranging light path module 4 includes a fiber optic splitter 41, a fiber optic splitter 42, a fiber optic coupler 43, a fiber optic coupler 44, a fiber optic circulator 45, a fiber optic filter 46, a fiber optic filter 47, and a fiber optic collimator 48. The double-optical comb light source 1 outputs two paths of repetition frequencies f respectively through optical fibers r1 And f r2 The femtosecond laser beam of (2) is used as signal light and local oscillation light, and is divided into two paths through an optical fiber beam splitter 41 and an optical fiber beam splitter 42, wherein one path of the femtosecond laser beam is combined through an optical fiber coupler 43 to generate interference, passes through an optical fiber filter 46 and is detected by a signal detection and processing module 7 through a polarization maintaining fiber 5 to be used as a reference interference signal. The other path of light beam of the signal light is emitted to the target mirror 8 after passing through the optical fiber circulator 45 and the optical fiber collimator 48, the return light of the target mirror 8 is coupled into the optical fiber through the optical fiber collimator 48 and is emitted from the other end of the optical fiber circulator 45, then the return light is interfered with the local oscillation light through the optical fiber coupler 44, and the interference signal is detected by the signal detection and processing module 7 through the polarization maintaining optical fiber 6 after passing through the optical fiber filter 47, so as to be used as a measurement interference signal.
Further, the center wavelength and bandwidth of the optical fiber filters 46 and 47 are kept consistent, wherein the center wavelength should be approximately equal to the center wavelength of the dual-optical comb light source, bandwidth Deltav comb Should be smaller than f r1 f r2 /2(Δf r ) So as to ensure that the double optical comb frequency spectrum aliasing phenomenon does not occur.
Further, the signal detection and processing module 7 detects a bandwidth greater than f r2 To satisfy the nyquist sampling theorem. Before the collected interference signal is processed, the signal is required to be processed by 0-f r2 And 2, the low-pass filtering of the optical fiber/optical fiber composite fiber ensures the effectiveness of double optical comb measurement.
In a preferred embodiment, the dual-optical comb light source 1 may be a single-cavity dual-comb laser with better cross-correlation, so as to reduce the influence of the noise of the heavy frequency difference and improve the dynamic ranging range and the ranging accuracy. It should be noted that, in this embodiment, the type of the dual-optical comb light source used is not limited, and may be selected according to actual needs.
In a preferred embodiment, the optical path takes the form of an all-fiber optical path, in which the design of the optical fiber length of the optical path first ensures L 5 Substantially equal to L 6 The primary phase introduced by the optical frequency comb 2 is equal, the length error can be ensured by the optical fiber fusion process, L 3 Should be greater than L 1 +2L 2 +L 4 The difference L of 3 -(L 1 +2L 2 +L 4 ) Multiplying the refractive index of the fiber by the initial distance L from the incident end face of the fiber to the target mirror corresponds to twice the optical path length, where L 1 L is the distance between the fiber optic splitter 41 and the fiber optic circulator 45 2 L is the distance between the fiber optic circulator 45 and the fiber optic collimator 48 3 L is the distance between the fiber splitter 41 and the fiber coupler 43 4 L is the distance between the fiber optic circulator 45 and the fiber optic coupler 44 5 L is the distance between the fiber coupler 43 and the fiber splitter 42 6 Which is the distance between the fiber splitter 42 and the fiber coupler 44.
Specifically, the implementation manner is implemented in the following manner, and includes:
the first is that the length of the optical fiber can be roughly distributed by the initial distance L between the optical fiber collimator 48 and the target mirror 8, and the time delay of the reference interference signal and the measurement interference signal in the initial state can be basically 0 by fine adjustment L, so that the maximum range for realizing high-precision dynamic distance measurement, namely + -delta D, is ensured max 。
The second is at L 1 ，L 2 ，L 3 Or L 4 An adjustable optical fiber delay line is added to any section, and when the initial distance L between the optical fiber collimator 48 and the target mirror 8 is difficult to adjust, the time delay of the reference interference signal and the measurement interference signal in the initial state is basically 0 by adjusting the length of the optical fiber delay line.
In a preferred embodiment, a thermistor 9 is further arranged in the ranging light path module 4 and is used for monitoring the internal temperature of the ranging light path module and compensating the ranging error caused by unequal arms of the optical fiber light path according to the thermal expansion coefficient of the optical fiber; the thermistor 9 is arranged at the central position inside the ranging light path module and is used for sensing the temperature change of the optical fiber light path and is connected to the signal detection and processing module 7 through the cable 10 to monitor the temperature change of the optical fiber device.
In a preferred embodiment, since the dual-optical comb light source 1 used in the present invention is only locked to the repetition frequency and the offset frequency is free to run, the relationship between the sub-frequency comb frequency and the optical frequency comb frequency cannot be determined through one-to-one mapping, and the frequency value of the carrier wave cannot be determined.
The method for determining the carrier frequency and calculating the carrier phase difference comprises the following steps:
(1) Determination of the center wavelength lambda of the signal light in FIG. 3 (a) after passing through the optical fiber band-pass filter by the wavelength measuring device c The corresponding frequency is taken as a carrier frequency value f c ；
(2) The amplitude spectrum information of the sub-frequency comb, namely, fig. 3 (b), is obtained by performing Fourier transform on the acquired reference interference signal, and the frequency value corresponding to the highest amplitude point is taken as the carrier frequency f of the sub-frequency comb RF(c) 。
(3) At a frequency f RF(c) The corresponding phase difference of the reference and measurement interference signals is used as the carrier frequency f of the optical frequency comb c Corresponding phase difference
Here, it is assumed that the carrier wavelength is 1.5 μm, ranging round trip is considered in a ranging range of ±7.5mm, the integer period of the corresponding carrier is 10000 at maximum, the positioning accuracy of the wavelength measurement device can be generally better than 0.01nm, and the corresponding carrier interferometry ranging error is 100nm, so that the accuracy requirements of most measurement scenes can be met, and therefore, the method for determining the carrier frequency and calculating the carrier phase difference is feasible.
Embodiment two: the embodiment also provides a small dynamic range double optical comb combined ranging method with stable phase difference, which determines an integer multiple period of carrier interference phase by a time-of-flight method and realizes high-precision absolute ranging by stable carrier phase difference, and the method comprises the following steps:
S1、before implementation, the time delay of the reference interference signal and the measurement interference signal in the initial state is basically 0 by adjusting the light path, and at the moment, the carrier phase difference noise of the double-optical-comb ranging systemMinimum, dynamic range is maximum.
S2, rough measurement is carried out based on a ranging principle of a time-of-flight method.
Distance to be measured D TOF The time delay Δt between the reference interference signal and the measured interference signal envelope can be obtained:
wherein v is g Representing the pulse group velocity, the time delay Δt can be calculated from the phase-frequency slope of the sub-frequency comb. The method has the characteristic of large non-fuzzy range, and the measurement accuracy can reach micron level. The non-ambiguous range is the furthest distance that the last transmitted pulse was received by the detector before the next transmitted pulse is sent out, and its value can be expressed as L pp /2＝v g /2f r1 。
S3, the distance value D is obtained through a carrier interferometry c By reference to the carrier phase difference of the interference signal and the measurement interference signalAnd (3) obtaining:
wherein,for measuring carrier phase difference between reference interference signal and measurement interference signal lambda c Is the carrier wave wavelength in the air, obtained by a wavelength measuring device, N c Representing half carrier wavelength (lambda) c Integer period of/2). At the position ofIn actual measurement, N c Unknown, ranging result D using time-of-flight method TOF To determine the integer multiple period N of the carrier wavelength c ：
In lambda, lambda c Indicating the carrier wavelength in the air, obtained by a wavelength measuring device, N c Representing half carrier wavelength (lambda) c Integer period of/2). As shown in fig. 4, when the ranging accuracy of the time-of-flight method is better than lambda c With/4, the result can uniquely determine lambda c Integer period N of/2 c The time-of-flight method is directly connected with the carrier interferometry, and the distance measurement with large non-fuzzy range and high precision is realized by combining the ranging mode.
Further, a distance value D is obtained c Then, compensating the distance measurement error caused by temperature drift according to the measured temperature of the thermistor, wherein the corrected distance measurement result is D c ′：
D c ′＝D c -kΔe (6)
Wherein k is a temperature compensation coefficient, namely a distance variable quantity of each 1 ℃ of temperature change, and delta e is a difference between a current temperature obtained by the thermistor and an initial temperature obtained by the thermistor when the ranging is started for the first time, so as to avoid a ranging error caused by temperature change in the whole measuring process.
It should be noted that in the actual measurement process, the distance value of the reference point is usually measured through the above steps in the initial state, and then the reference distance should be subtracted from the distance value after the measurement is started to obtain the absolute distance value relative to the reference point. The maximum deviation from the reference point should be less than + -DeltaD max I.e. to ensure the validity of the carrier interferometry.
In this specification, each embodiment is described in a progressive manner, and identical and similar parts of each embodiment are all referred to each other, and each embodiment mainly describes differences from other embodiments. In the description of the present specification, reference to the terms "one preferred embodiment," "further," "specifically," "in the present embodiment," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the embodiments of the present specification. In this specification, schematic representations of the above terms are not necessarily directed to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, the different embodiments or examples described in this specification and the features of the different embodiments or examples may be combined and combined by those skilled in the art without contradiction.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and are not limiting; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims (8)
1. The combined distance measuring method of the double optical combs with small dynamic range is characterized by comprising the following steps:
the optical fiber distance measuring device is provided with a distance measuring optical path module, wherein the distance measuring optical path module comprises a first optical fiber beam splitter, a second optical fiber beam splitter, a first optical fiber coupler, a second optical fiber coupler, an optical fiber circulator, a first optical fiber filter, a second optical fiber filter and an optical fiber collimator, and the optical fiber length requirement of an optical path is as follows: l (L) 5 Substantially equal to L 6 So that the initial phases introduced by the optical frequency comb are equal and L 3 Should be greater than L 1 +2L 2 +L 4 The difference L of 3 -(L 1 +2L 2 +L 4 ) Multiplying the refractive index of the fiber by the initial distance L from the incident end face of the fiber to the target mirror corresponds to twice the optical path length, where L 1 L is the distance between the first fiber splitter and the fiber circulator 2 L is the distance between the fiber optic circulator and the fiber optic collimator 3 L is the distance between the first fiber splitter and the first fiber coupler 4 L is the distance between the fiber circulator and the second fiber coupler 5 L is the distance between the first fiber coupler and the second fiber splitter 6 A distance from the second fiber splitter to the second fiber coupler;
the double optical comb ranging light path is regulated to enable the time delay of the reference interference signal and the measuring interference signal to be basically 0 in the initial state, and at the moment, the carrier wave phase difference noise of the double optical comb ranging systemMinimum;
by carrier phase difference of reference interference signal and measurement interference signalObtaining a distance value D c ：
Wherein,for measuring carrier phase difference between reference interference signal and measurement interference signal lambda c Is the carrier wave wavelength in the air, N c And an integer period representing the half carrier wavelength is determined by using a ranging result of a time-of-flight method.
2. The small dynamic range double optical comb combination ranging method of claim 1, further comprising the step of correcting a ranging result, comprising:
obtaining a distance value D c Then, according to the distance measurement error caused by temperature compensation temperature drift in the double-optical-comb distance measurement light path, correctingThe ranging result of (2) is D c ′：
D c ′＝D c -kΔe
Wherein k is a temperature compensation coefficient, and deltae is the difference between the current temperature in the optical comb ranging light path and the initial temperature when the ranging is started.
3. The small dynamic range double-optical comb combined distance measuring device is characterized by comprising a double-optical comb light source, a first polarization maintaining optical fiber, a second polarization maintaining optical fiber, a distance measuring light path module, a third polarization maintaining optical fiber, a fourth polarization maintaining optical fiber and a signal detecting and processing module; wherein, the light source in the double-light comb light source is locked by heavy frequency and the frequency deviation runs freely;
the double-optical comb light source respectively inputs signal light and local oscillation light signals for the ranging light path module through the first polarization maintaining optical fiber and the second polarization maintaining optical fiber, the ranging light path module is used for outputting space light to a target mirror, the ranging light path module respectively outputs and enters the output reference interference signal and the output measuring interference signal into the signal detection and processing module through the third polarization maintaining optical fiber and the fourth polarization maintaining optical fiber, and the signal detection and processing module calculates a ranging result through carrier phase difference of the reference interference signal and the measuring interference signal;
the ranging light path module comprises a first optical fiber beam splitter, a second optical fiber beam splitter, a first optical fiber coupler, a second optical fiber coupler, an optical fiber circulator, a first optical fiber filter, a second optical fiber filter and an optical fiber collimator; the double-light comb light source outputs two paths of repetition frequencies f respectively through optical fibers r1 And f r2 The femtosecond laser beam is used as signal light and local oscillation light, and is divided into two paths through the first optical fiber beam splitter and the second optical fiber beam splitter respectively, wherein one path of the femtosecond laser beam is used as a reference interference signal by being detected by the signal detection and processing module through the third polarization maintaining optical fiber after being interfered by the combined light of the first optical fiber coupler and passing through the first optical fiber filter; the other path of light beam of the signal light is emitted to a target mirror after passing through the optical fiber circulator and the optical fiber collimator, and the return light of the target mirror is coupled into the optical fiber through the optical fiber collimator and then passes through the optical fiber ringThe other end of the shaper emits light, then the light interferes with local oscillation light through the second optical fiber coupler, and the light is detected by the signal detection and processing module through a fourth polarization maintaining optical fiber after passing through the second optical fiber filter to be used as a measurement interference signal;
the optical fiber length requirement of the optical path is: l (L) 5 Substantially equal to L 6 So that the initial phases introduced by the optical frequency comb are equal and L 3 Should be greater than L 1 +2L 2 +L 4 The difference L of 3 -(L 1 +2L 2 +L 4 ) Multiplying the refractive index of the fiber by the initial distance L from the incident end face of the fiber to the target mirror corresponds to twice the optical path length, where L 1 L is the distance between the first fiber splitter and the fiber circulator 2 L is the distance between the fiber optic circulator and the fiber optic collimator 3 L is the distance between the first fiber splitter and the first fiber coupler 4 L is the distance between the fiber circulator and the second fiber coupler 5 L is the distance between the first fiber coupler and the second fiber splitter 6 Is the distance from the second fiber splitter to the second fiber coupler.
4. The small dynamic range double optical comb combination ranging apparatus as claimed in claim 3, wherein the center wavelength and bandwidth of the first and second optical filters are kept identical, wherein the center wavelength should be approximately equal to the center wavelength of the double optical comb light source, bandwidth Δv comb Should be smaller than f r1 f r2 /2(Δf r ) To ensure that the aliasing phenomenon of the double optical comb frequency spectrum, delta f, does not occur r And is a frequency difference.
5. A small dynamic range double optical comb combined ranging apparatus as defined in claim 3, wherein the signal detection and processing module detects a bandwidth greater than f r2 To satisfy the nyquist sampling theorem.
6. A small dynamic range double optical comb combination ranging apparatus as claimed in claim 3, wherein the optical fiber length requirement of the optical path is achieved in any one of the following forms, comprising:
first form: the initial distance L between the optical fiber collimator and the target mirror is finely adjusted, so that the time delay of the reference interference signal and the measurement interference signal is basically 0 in an initial state;
second form: at L 1 ，L 2 ，L 3 Or L 4 An adjustable optical fiber delay line is added to any section, and the time delay of the reference interference signal and the measurement interference signal in the initial state is basically 0 by adjusting the length of the optical fiber delay line.
7. The small dynamic range double optical comb combined distance measuring device according to claim 3, wherein a thermistor is further arranged in the distance measuring optical path module and used for sensing temperature change of an optical fiber optical path and is connected to the signal detecting and processing module through a cable.
8. A small dynamic range dual optical comb combination ranging apparatus as defined in claim 3 wherein the means for determining the carrier frequency and carrier phase difference of the optical frequency comb comprises:
determining the center wavelength lambda of the signal light after passing through the optical fiber filter by the wavelength measuring device c The corresponding frequency is taken as a carrier frequency value f c ；
The amplitude spectrum information of the sub-frequency comb is obtained by carrying out Fourier transform on the acquired reference interference signals, and the frequency value corresponding to the highest amplitude point is used as the carrier frequency f of the sub-frequency comb RF(c) At a frequency f RF(c) The corresponding phase difference of the reference and measurement interference signals is used as the carrier frequency f of the optical frequency comb c Corresponding phase difference
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