CN111781607A - Forward and reverse tuning dispersion cancellation method and device based on laser frequency modulation continuous wave - Google Patents

Forward and reverse tuning dispersion cancellation method and device based on laser frequency modulation continuous wave Download PDF

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CN111781607A
CN111781607A CN202010788052.5A CN202010788052A CN111781607A CN 111781607 A CN111781607 A CN 111781607A CN 202010788052 A CN202010788052 A CN 202010788052A CN 111781607 A CN111781607 A CN 111781607A
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tuning
dispersion
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measurement signal
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CN111781607B (en
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许新科
孔明
赵军
王道档
刘维
刘璐
郭天太
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China Jiliang University
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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
    • 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
    • G01S17/34—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal
    • 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
    • G01S7/491—Details of non-pulse systems
    • 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 forward and reverse tuning dispersion cancellation method and device based on laser frequency modulation continuous wave; performing forward and reverse tuning through an external cavity tunable laser to obtain a forward tuned measurement signal and a reverse tuned measurement signal; respectively extracting the phases of the forward and reverse measurement signals, and performing phase expansion; the phases of the two signals are added to calculate the average, so that the offset of the dispersion phase is realized, and the measurement signal with the dispersion influence reduced is obtained; and performing ChirpZ conversion on the measurement signal to obtain the target distance for reducing the dispersion influence. The method can complete the dispersion compensation of the system by single measurement without pre-calibrating the dispersion coefficient of the device and circulating iterative compensation, obtain the target distance for reducing the dispersion influence and improve the stability and the measurement precision of the FMCW laser ranging device.

Description

Forward and reverse tuning dispersion cancellation method and device based on laser frequency modulation continuous wave
Technical Field
The invention relates to the field of optical measurement, in particular to a forward and reverse tuning dispersion cancellation method and device based on laser frequency modulation continuous waves.
Background
Since the 21 st century, the development of modern science and technology is fast, and the industrial revolution drives the continuous innovation of industrial technology, which puts higher requirements on high-precision large-size space measurement, and the laser has the advantages of good monochromaticity, good coherence, good directivity and the like, and is widely used in the field of industrial measurement. Common laser ranging methods include a pulse method, a phase method, an incremental interference method, an absolute interference method and the like, and the Frequency Modulated Continuous Wave (FMCW) laser ranging technology has the advantages of no blind zone ranging, high precision and high resolution, has important application value in the fields of large-size precision measurement and processing and manufacturing, and becomes a research hotspot in the field of laser absolute ranging. The FMCW laser ranging technology is that linearly modulated laser emitted by a laser and an echo of a target point generate beat frequency, and the frequency of the beat frequency is in direct proportion to the distance to be measured, so that the distance to be measured can be measured. For an FMCW laser ranging system, ideal linear tuning is difficult to achieve, frequency modulation nonlinearity is often shown, the frequency spectrum of a measuring signal is widened due to the effect, and further ranging errors are caused; the mode of constructing the optical fiber auxiliary interference synchronous measurement signal introduces an optical fiber structure, when the optical fiber structure is combined with a broadband tuning light source, a dispersion mismatch effect is generated, so that distortion of a target spectrum peak value profile and peak value position deviation are caused, a measurement value changes along with increase of a tuning bandwidth, measurement instability is caused, and measurement accuracy is influenced.
Therefore, it is necessary to provide a solution to reduce the influence of dispersion on the measurement result and further improve the accuracy of the FMCW laser ranging apparatus when the fiber-assisted interferometer is constructed to synchronize the measurement signal and eliminate the influence of beat frequency nonlinearity on the ranging result.
Disclosure of Invention
Aiming at the problems that an optical fiber structure is introduced when synchronous measurement signals of an optical fiber auxiliary interferometer are constructed and the influence of beat frequency nonlinearity on a distance measurement result is eliminated at present, and when the optical fiber structure is combined with a broadband tuning light source, a dispersion mismatch effect can be generated and the measurement precision is influenced, the invention provides a method and a device for compensating dispersion of the device based on forward and backward tuning dispersion of laser frequency modulation continuous waves.
The invention provides a forward and reverse tuning dispersion cancellation method based on laser frequency modulation continuous wave, which comprises the following steps:
s1: the laser of the external cavity tunable laser for linear frequency modulation output is divided into two parts through the isolator and the first coupler: one part enters the main optical path to generate a measuring signal; one part of the optical fiber enters a first auxiliary optical path to generate an auxiliary interference signal, the auxiliary interference signal is used as a clock signal, and the equal-frequency resampling is carried out on the measurement signal to obtain a measurement signal containing the optical fiber dispersion influence;
s2: the external cavity tunable laser carries out forward tuning and reverse tuning, and samples the measurement signal according to the step S1 to obtain a forward tuned measurement signal and a reverse tuned measurement signal;
s3: respectively extracting the phases of the forward and reverse measurement signals, and performing phase expansion;
s4, the phases of the forward and reverse measurement signals are added to calculate the average, so as to realize the offset of the dispersion phase and obtain the measurement signal with the dispersion influence reduced;
s5: the chirp conversion is performed on the measurement signal in step S4, and the target distance with reduced chromatic dispersion influence can be obtained.
Preferably, in step S2, the external cavity tunable laser is tuned in forward and reverse directions with a certain wavelength as a starting point, and a corresponding measurement signal is obtained.
Preferably, the constant frequency resampling method for the measurement signal in step S1 is a zero-crossing point sampling method.
Preferably, in step S3, Hilbert transform is used to extract the phases of the measurement signals in forward and backward tuning respectively.
Preferably, the external cavity tunable laser chirp scheme in step S1 includes a triangular-wave chirp scheme.
The invention also provides a device for counteracting the forward and reverse tuned dispersion based on the laser frequency modulated continuous wave, which comprises an external cavity tuned laser (1), an isolator (2), a first coupler (3), a main light path (4), a first auxiliary light path (5), an optical fiber emergent end face (6), an optical transmitting/receiving system (7), a data acquisition card (8) and a signal processing system (9);
the main optical path (4) comprises a second coupler (41), an optical circulator (42), a first 3dB coupler (43) and a first detector (44); the first auxiliary optical path (5) comprises a third coupler (51), an optical fiber (52) with unequal arm length difference, a second 3dB coupler (53) and a second detector (54); optical signals are transmitted between the optical devices of the main optical path (4) and the first auxiliary optical path (5) through optical fibers;
the external cavity tuned laser (1) is used for carrying out linear frequency modulation, output light is divided into two paths of light after passing through the isolator (2) and the first coupler (3), wherein 99% of energy enters the main light path (4) and is divided into two portions of light after passing through the second coupler (41), one portion of light reaches a target after passing through the optical circulator (42), the optical fiber emergent end face (6) and the optical transmitting/receiving system (7), and light returning along the original path after being reflected by the surface of the target reaches the first detector (44) through the first 3dB coupler (43); another part of light directly reaches a first detector (44) after passing through the first 3dB coupler (43), and forms heterodyne interference with the target return light, and the part is a measurement signal; the measuring signal is detected by the first detector (44), converted into an electric signal and output, and recorded by the data acquisition card (8);
1% of the energy split off by the first coupler (3) enters the first auxiliary optical path (5); the light is divided into two parts of light with equal energy after passing through a third coupler (51), passes through an optical fiber (52) with unequal arm length difference and then passes through a second 3dB coupler (53), and a heterodyne interference signal is formed on a second detector (54), wherein the two parts of light are auxiliary interference signals; the auxiliary interference signal is detected by the second detector (54), converted into an electric signal and output to be recorded by the data acquisition card (8);
the auxiliary interference signal is used as a clock signal, the measurement signal is subjected to equal-frequency resampling to obtain a corrected measurement signal, and the corrected measurement signal is detected by the first detector (44), converted into an electric signal and recorded by the data acquisition card (8); the data acquisition card (8) transmits acquired data to the signal processing system (9), and the signal processing system (9) processes signals from the first detector (44) and the second detector (54) to obtain a measurement signal with beat frequency nonlinear correction; the measurement signal is a measurement signal containing dispersion influence;
forward tuning and backward tuning are carried out through the external cavity tuning laser (1) to obtain a forward tuning measurement signal and a backward tuning measurement signal; the two measurement signals are detected by the first detector (44), converted into electric signals and recorded by the data acquisition card (8); the data acquisition card (8) transmits acquired data to the signal processing system (9), the signal processing system (9) processes signals from the first detector (44), extracts phases of two measurement signals, adds the two phases to calculate and average the two phases to obtain measurement signal data for reducing dispersion influence, and performs ChirpZ conversion on the two signal data to obtain a target distance.
Preferably, the external cavity tunable laser (1) is tuned in forward and reverse directions respectively with a certain wavelength as a starting point to obtain corresponding measurement signals.
Preferably, the constant frequency resampling method for the measurement signal is a zero crossing point sampling method.
Preferably, Hilbert transform is adopted to extract the phases of the measurement signals under forward and backward tuning respectively.
Preferably, the external cavity tunable laser chirp mode includes a triangular wave frequency modulation mode.
The invention has the beneficial effects that:
the method comprises the steps of performing forward tuning and reverse tuning through an external cavity tuning laser, extracting phases of measurement signals by adopting Hilbert transformation, adding the phases of the measurement signals and averaging the phases of the measurement signals to reduce the influence of color loss pairing measurement results in a measurement device, further obtaining the measurement signals with the reduced chromatic dispersion influence, and performing ChirpZ transformation on the measurement signals to obtain a target distance; by the method, the influence of frequency modulation nonlinearity on the measurement result is eliminated, the dispersion coefficient of the device does not need to be calibrated in advance, the cyclic iterative compensation is also not needed, the dispersion compensation of the device can be completed by single measurement, the measurement signal for reducing the dispersion influence is obtained, and the target distance for reducing the dispersion influence is further obtained; the precision of the FMCW laser ranging device is improved.
Drawings
FIG. 1 is a flow chart of a method for forward and backward tuning dispersion cancellation based on laser frequency modulated continuous waves;
FIG. 2 is a schematic diagram of the optical path structure of the apparatus of the present invention;
FIG. 3 is a frequency domain plot of a measured signal according to one embodiment;
FIG. 4 shows an embodiment of measuring ranging values for different segments of a signal;
FIG. 5 illustrates the residual phase of forward tuning in one embodiment;
FIG. 6 illustrates the residual phase of the back tuning of one embodiment;
FIG. 7 illustrates the residual phase after forward and backward tuning dispersion cancellation in accordance with one embodiment;
FIG. 8 illustrates a target distance peak profile before and after system dispersion cancellation in accordance with an exemplary embodiment;
in the figure: 1: an external cavity tuned laser; 2: an isolator; 3: a first coupler; 4: a main light path; 5: a first auxiliary optical path; 6: an optical fiber exit end face; 7: an optical transmission/reception system; 8: a data acquisition card; 9: a signal processing system; 41: a second coupler; 42: an optical circulator; 43: a first 3dB coupler; 44: a first detector; 51: a third coupler; 52: optical fibers of unequal arm length difference; 53: a second 3dB coupler; 54: a second detector;
Detailed Description
The present invention will be described in detail with reference to the specific embodiments shown in the drawings, which are not intended to limit the present invention, and structural, methodological, or functional changes made by those skilled in the art according to the specific embodiments are included in the scope of the present invention.
As shown in fig. 1, a method for canceling forward and backward tuned dispersion based on laser frequency modulated continuous wave includes the following steps:
s1: the laser of the external cavity tunable laser for linear frequency modulation output is divided into two parts through the isolator and the first coupler: one part enters the main optical path to generate a measuring signal; one part of the optical fiber enters a first auxiliary optical path to generate an auxiliary interference signal, the auxiliary interference signal is used as a clock signal, and the equal-frequency resampling is carried out on the measurement signal to obtain a measurement signal containing the optical fiber dispersion influence;
in a preferred embodiment, the manner of performing equal-frequency resampling on the measurement signal is a zero-crossing point sampling manner.
In a preferred embodiment, the external cavity tunable laser chirp comprises a triangular-wave chirp.
The measurement signal is sampled by the zero crossing point of the period amplitude of the auxiliary interference signal, beat frequency nonlinear correction of the measurement signal can be realized, the corrected measurement signal is changed into a sine signal, and the sampled measurement signal can be expressed as:
Figure BDA0002622740990000061
wherein，τairAnd τauxRespectively, the time delay of the measurement signal and the auxiliary interference signal. When the time delay of the auxiliary interference signal is known, the target optical path including the dispersion influence can be obtained through the FFT conversion of the sampling signal.
In practical application, since a broadband tuning light source is adopted, the influence of the fiber dispersion in the first auxiliary optical path on the measurement needs to be considered, and then the beat frequency formed by the auxiliary interference signal is as follows:
Figure BDA0002622740990000071
wherein the content of the first and second substances,
Figure BDA0002622740990000072
wherein R isauxRepresenting the fiber length of the first auxiliary optical path, μ is the slope of the linear tuning β1＝1/υgAnd upsilongRepresenting group velocity, dispersion coefficient β2＝dβ1/dω，O(ζ)nIs aboutnThe high order term of the error.
After the measurement signal is resampled as a clock by the auxiliary interference signal, the beat frequency of the measurement signal is derived as follows: the main light path is mainly carried out in air, and the dispersion coefficient of the air can be ignored. Thus, the ratio of the time delays of the measurement signal and the auxiliary interference signal can be expressed as equation (4):
Figure BDA0002622740990000073
due to Delta tauaux/τaux< 1, the higher order terms in equation (4) are negligible. The higher order term of formula (4) is represented by O (ζ)n。
Wherein
Figure BDA0002622740990000074
Where Δ τ isauxIndicating the effect of dispersion by optical fibresThe resulting delay variation of the auxiliary interference signal. Derived through the above process, the sampling signal is as follows:
if μ < 0, then,
Figure BDA0002622740990000081
if μ > 0, the ratio of the sum of the two,
Figure BDA0002622740990000082
wherein P isT、PLAnd ηHRespectively representing transmission light, local oscillator light power and heterodyne interference efficiency.
S2: the external cavity tunable laser carries out forward and reverse tuning to obtain a forward tuning measurement signal and a reverse tuning measurement signal;
in a preferred embodiment, the external cavity tunable laser is tuned in forward and reverse directions with a certain wavelength as a starting point, and corresponding measurement signals are obtained.
S3: respectively extracting the phases of the forward and reverse measurement interference signals, and performing phase expansion;
in a preferred scheme, Hilbert transform is adopted to extract the phases of the measurement signals under forward and reverse tuning respectively.
S4, the phases of the two signals are added to calculate the average, so as to realize the offset of the dispersion phase and obtain the measurement signal with the dispersion influence reduced;
the phases of the forward and reverse tuning measurement signals are added and averaged in steps S2-S4, and the process formula is as follows:
Figure BDA0002622740990000091
wherein
Figure BDA0002622740990000092
Reconstructing the measurement signal using equation (8) is:
Figure BDA0002622740990000093
s5: the chirp conversion is performed on the measurement signal in step S4, and the target distance with reduced chromatic dispersion influence can be obtained.
As shown in fig. 2, the present invention further provides a device for forward and backward tuning dispersion cancellation based on laser frequency modulated continuous wave, which comprises an external cavity tuning laser 1, an isolator 2, a first coupler 3, a main optical path 4, a first auxiliary optical path 5, an optical fiber exit end face 6, an optical transmitting/receiving system 7, a data acquisition card 8, and a signal processing system 9;
the main optical path 4 includes a second coupler 41, an optical circulator 42, a first 3dB coupler 43, and a first detector 44; the first auxiliary optical path 5 comprises a third coupler 51, an optical fiber 52 with unequal arm length difference, a second 3dB coupler 53 and a second detector 54; optical signals are transmitted between the optical devices of the main optical path 4 and the first auxiliary optical path 5 through optical fibers;
the external cavity tuning laser 1 carries out linear frequency modulation, and a preferred scheme is adopted, wherein the linear frequency modulation mode comprises a triangular wave frequency modulation mode; the output light is divided into two paths of light after passing through the isolator 2 and the first coupler 3, wherein 99% of energy enters the main light path 4 and is divided into two portions of light after passing through the second coupler 41, one portion of light reaches a target after passing through the optical circulator 42, the optical fiber emergent end face 6 and the optical transmitting/receiving system 7, and the light returning along the original path after being reflected by the surface of the target reaches the first detector 44 through the first 3dB coupler 43; another part of the light directly reaches the first detector 44 after passing through the first 3dB coupler 43, and forms heterodyne interference with the target return light, and this part is a measurement signal; the measurement signal is detected by the first detector 44, converted into an electric signal and output, and recorded by the data acquisition card 8;
1% of the energy split by the first coupler 3 enters the first auxiliary optical path 5; the light which passes through the third coupler 51 and then is divided into two parts of light with equal energy passes through the optical fiber 52 with unequal arm length difference and then passes through the second 3dB coupler 53, and a heterodyne interference signal is formed on the second detector 54, and the part is an auxiliary interference signal; the auxiliary interference signal is detected by the second detector 54 and then converted into an electric signal, and the electric signal is output and recorded by the data acquisition card 8;
the auxiliary interference signal is used as a clock signal, the measurement signal is subjected to equal-frequency resampling to obtain a corrected measurement signal, and the corrected measurement signal is detected by the first detector 44, converted into an electric signal and recorded by the data acquisition card 8; the data acquisition card 8 transmits the acquired data to the signal processing system 9, and the signal processing system 9 processes the signals from the first detector 44 and the second detector 54 to obtain a measurement signal with beat frequency nonlinear correction; the measurement signal is a measurement signal containing dispersion influence;
forward tuning and backward tuning are carried out through the external cavity tuning laser 1 to obtain a forward tuning measurement signal and a backward tuning measurement signal; the two measurement signals are detected by the first detector 44 and then converted into electric signals which are recorded by the data acquisition card 8; the data acquisition card 8 transmits the acquired data to the signal processing system 9, the signal processing system 9 processes the signal from the first detector 44, extracts the phases of the two measurement signals, adds the two phases to calculate the average to obtain the measurement signal data for reducing the influence of chromatic dispersion, and performs ChirpZ conversion on the two signal data to obtain the target distance for reducing the influence of chromatic dispersion.
In a preferred embodiment, the external cavity tunable laser 1 is tuned in forward and reverse directions with a certain wavelength as a starting point, and corresponding measurement signals are obtained.
In a preferred embodiment, the manner of performing equal-frequency resampling on the measurement signal is a zero-crossing point sampling manner.
In a preferred scheme, Hilbert transform is adopted to extract the phases of the measurement signals under forward and reverse tuning respectively.
A specific embodiment describes a forward and reverse tuning dispersion cancellation method based on laser frequency modulation continuous wave in detail;
the external cavity frequency modulation laser is set to a triangular wave tuning mode. And respectively tuning from 1552nm to 1542nm (corresponding to forward tuning), tuning from 1552nm to 1562nm (corresponding to backward tuning), setting the output power of the external cavity frequency modulation laser to 1.5mW, setting the tuning speed to 100nm/s, setting the first auxiliary optical path to 220m, and placing the target on the air-floating optical platform. The first detector collects a measurement signal formed between the returned target and the local oscillator light, the second detector collects a beat frequency interference signal of the first auxiliary optical path, the beat frequency interference signal is used as a sampling clock to carry out beat frequency nonlinear correction on the measurement signal, the measurement signal after nonlinear correction is subjected to frequency spectrum transformation, and a frequency domain diagram is shown in fig. 3. It should be noted that the chirp transform can be used for the subdivision of the frequency spectrum, and the effect is equivalent to zero-padding fourier transform, but the algorithm efficiency is higher.
Where the optical path length of the fiber exit end face is 4.524271m, the spectral peak formed by the target is shown in fig. 3, where the distance between the target and the fiber exit end face is the fraction of free space. And taking the emergent end face of the optical fiber as a measurement starting point, the target distance is mainly the free space part distance.
In order to study the dispersion mismatch effect of the measuring device, a measured signal formed by forward and backward tuning of an external cavity tuning laser is divided into five segments of sub-signals, then chirp transformation is performed on each segment of sub-signal, and a corresponding distance is calculated, and the result is shown in fig. 4. It can be seen that in the case of forward tuning, the range value of the target increases linearly with increasing tuning range. In the case of back-tuning, the range values decrease linearly as the tuning range increases. This is due to dispersion mismatch, and as the tuning range increases, the target spectral peak shifts.
And extracting the phase of the measured signal under the forward and backward tuning conditions by using Hilbert transform, and then expanding the phase of the measured signal and performing linear fitting. As can be seen from fig. 5, the residual phase is a curve with an upward opening in the case of forward tuning, indicating that the beat frequency is not a constant value, but varies with the tuning range. In the back-tuning case, the residual phase after linear fitting to the measured signal phase is shown in fig. 6, and it can be seen that the residual phase is an open-down curve. The phase after the forward and backward tuned dispersion cancels out is shown in fig. 7, and the result shows that the residual phase fluctuates around zero, which indicates that the dispersion effect of the measurement system has cancelled out.
And extracting the whole measurement signal after dispersion cancellation by using ChirpZ transformation. The distance distribution of the target peak before and after dispersion cancellation is shown in fig. 8, and it can be seen that there is a certain degree of distortion in the forward or reverse tuned target peak, which is caused by system dispersion, and the target peak after dispersion cancellation has less distortion. The advantage of this method is that no estimation of the dispersion coefficient is required, nor is iterative compensation required. The measurement is completed by single compensation, and the dispersion compensation efficiency is improved.
The method comprises the steps of performing forward tuning and reverse tuning through an external cavity tuning laser, extracting phases of interference signals to be measured by adopting Hilbert transformation, adding the phases of the interference signals to be averaged to reduce dispersion mismatch influence in a measuring device, further obtaining the measuring signals with the reduced dispersion influence, performing ChirpZ transformation on the signals to obtain target distances, completing dispersion compensation of the device by a single measurement without calibrating dispersion coefficients of the device in advance and circulating iterative compensation, and obtaining the measuring signals with the reduced dispersion influence to obtain the target distances with the reduced dispersion influence; the precision of the FMCW laser ranging system is improved.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (10)

1. A forward and reverse tuning dispersion cancellation method based on laser frequency modulation continuous wave is characterized in that: the method comprises the following steps:
s1: the laser of the external cavity tunable laser for linear frequency modulation output is divided into two parts through the isolator and the first coupler: one part enters the main optical path to generate a measuring signal; one part of the optical fiber enters a first auxiliary optical path to generate an auxiliary interference signal, the auxiliary interference signal is used as a clock signal, and the equal-frequency resampling is carried out on the measurement signal to obtain a measurement signal containing the optical fiber dispersion influence;
s2: the external cavity tunable laser carries out forward tuning and reverse tuning, and samples the measurement signal according to the step S1 to obtain a forward tuned measurement signal and a reverse tuned measurement signal;
s3: respectively extracting the phases of the forward and reverse measurement signals, and performing phase expansion;
s4, the phases of the forward and reverse measurement signals are added to calculate the average, so as to realize the offset of the dispersion phase and obtain the measurement signal with the dispersion influence reduced;
s5: the chirp conversion is performed on the measurement signal in step S4, and the target distance with reduced chromatic dispersion influence can be obtained.
2. The method for cancellation of forward and backward tuned dispersion based on laser frequency modulated continuous wave according to claim 1, wherein: in step S2, the external cavity tunable laser is tuned forward and backward with a certain wavelength as a starting point to obtain corresponding measurement signals.
3. The method for cancellation of forward and backward tuned dispersion based on laser frequency modulated continuous wave according to claim 1, wherein: the manner of performing the equal-frequency resampling on the measurement signal in step S1 is a zero-crossing point sampling manner.
4. The method for cancellation of forward and backward tuned dispersion based on laser frequency modulated continuous wave according to claim 1, wherein: in step S3, Hilbert transform is used to extract the phases of the measurement signals under forward and backward tuning, respectively.
5. The method for cancellation of forward and backward tuned dispersion based on laser frequency modulated continuous wave according to claim 1, wherein: in step S1, the external cavity tunable laser chirp mode includes a triangular wave chirp mode.
6. A device based on forward and reverse tuning dispersion cancellation of laser frequency modulation continuous wave is characterized in that: the device comprises an external cavity tuning laser (1), an isolator (2), a first coupler (3), a main light path (4), a first auxiliary light path (5), an optical fiber emergent end face (6), an optical transmitting/receiving system (7), a data acquisition card (8) and a signal processing system (9);
the main optical path (4) comprises a second coupler (41), an optical circulator (42), a first 3dB coupler (43) and a first detector (44); the first auxiliary optical path (5) comprises a third coupler (51), an optical fiber (52) with unequal arm length difference, a second 3dB coupler (53) and a second detector (54); optical signals are transmitted between the optical devices of the main optical path (4) and the first auxiliary optical path (5) through optical fibers;
the external cavity tuned laser (1) is used for carrying out linear frequency modulation, output light is divided into two paths of light after passing through the isolator (2) and the first coupler (3), wherein 99% of energy enters the main light path (4) and is divided into two portions of light after passing through the second coupler (41), one portion of light reaches a target after passing through the optical circulator (42), the optical fiber emergent end face (6) and the optical transmitting/receiving system (7), and light returning along the original path after being reflected by the surface of the target reaches the first detector (44) through the first 3dB coupler (43); another part of light directly reaches a first detector (44) after passing through the first 3dB coupler (43), and forms heterodyne interference with the target return light, and the part is a measurement signal; the measuring signal is detected by the first detector (44), converted into an electric signal and output, and recorded by the data acquisition card (8);
1% of the energy split off by the first coupler (3) enters the first auxiliary optical path (5); the light is divided into two parts of light with equal energy after passing through a third coupler (51), passes through an optical fiber (52) with unequal arm length difference and then passes through a second 3dB coupler (53), and a heterodyne interference signal is formed on a second detector (54), wherein the two parts of light are auxiliary interference signals; the auxiliary interference signal is detected by the second detector (54), converted into an electric signal and output to be recorded by the data acquisition card (8);
the auxiliary interference signal is used as a clock signal, the measurement signal is subjected to equal-frequency resampling to obtain a corrected measurement signal, and the corrected measurement signal is detected by the first detector (44), converted into an electric signal and recorded by the data acquisition card (8); the data acquisition card (8) transmits acquired data to the signal processing system (9), and the signal processing system (9) processes signals from the first detector (44) and the second detector (54) to obtain a measurement signal with beat frequency nonlinear correction; the measurement signal is a measurement signal containing dispersion influence;
forward tuning and backward tuning are carried out through the external cavity tuning laser (1) to obtain a forward tuning measurement signal and a backward tuning measurement signal; the two measurement signals are detected by the first detector (44), converted into electric signals and recorded by the data acquisition card (8); the data acquisition card (8) transmits acquired data to the signal processing system (9), the signal processing system (9) processes signals from the first detector (44), extracts phases of two measurement signals, adds the two phases to calculate and average the two phases to obtain measurement signal data for reducing dispersion influence, and performs ChirpZ conversion on the two signal data to obtain a target distance.
7. The apparatus according to claim 6, wherein the apparatus comprises: the external cavity tunable laser (1) respectively carries out forward and reverse tuning by taking a certain wavelength as a starting point to obtain corresponding measurement signals.
8. The apparatus according to claim 6, wherein the apparatus comprises: the manner of performing equal-frequency resampling on the measurement signal is a zero-crossing point sampling manner.
9. The apparatus according to claim 6, wherein the apparatus comprises: and respectively extracting the phases of the measurement signals under forward and reverse tuning by adopting Hilbert transform.
10. The apparatus according to claim 6, wherein the apparatus comprises: the external cavity tunable laser linear frequency modulation mode comprises a triangular wave frequency modulation mode.
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