A method of dynamic absolute distance measurement by frequency-shift sinusoidal frequency modulation interferometry

Abstract

The application relates to a frequency-shifted sinusoidal frequency modulation interference dynamic absolute distance measurement method and relates to the technical field of target dynamic absolute distance measurement. The present application is to solve the problem that the current processing method for sinusoidal frequency modulation interference measurement needs to be equipped with a long optical fiber auxiliary interferometer, which not only increases the sampling pressure of the system, but also is easily affected by the environment and changes. A first coupler divides the light source signal into two beams and inputs the two beams into a measurement interferometer and an auxiliary interferometer respectively, cosine signals and sine signals of an acousto-optic modulator are used to sequentially perform mixing, low-pass filtering and arctangent calculation on measurement interference signals and auxiliary interference signals, and corresponding phases are obtained. Then, the phase of the measurement interference signal is filtered, the low-frequency part directly calculates the relative displacement of the target, the high-frequency part first calculates the envelope of the signal, and then the envelope of the auxiliary interferometer phase is calculated, and finally, the two envelopes are ratioed, so that the influence of modulation bandwidth fluctuation can be eliminated, and the absolute distance of the target can be calculated.

Description

Technical Field

[0001] This invention belongs to the field of target absolute distance measurement technology. Background Technology

[0002] Frequency-modulated continuous wave (FMCW) absolute range measurement technology originates from the field of traditional microwave radar technology. It modulates the frequency of the transmitted signal in time and obtains target information by measuring the beat frequency difference between the transmitted and received signals. At any given instant, by measuring the difference between the transmitted and received signal frequencies, not only the target distance can be measured, but also the target's radial velocity. Sinusoidal frequency modulation (SFM), as a type of FMCW, has advantages such as simple modulation, no measurement blind zone, and minimal nonlinear effects, making it a promising technology. However, in actual measurements, the beat signal of sinusoidal frequency modulation interferometry is a broadband signal that crosses zero frequency, meaning its beat frequency is no longer a constant. This implies that we cannot directly obtain the target's range information through spectrum analysis. Therefore, correctly and effectively processing the broadband beat signal is one of the crucial guarantees for achieving high-precision sinusoidal frequency modulation interferometry.

[0003] To address this issue, in 2011, Song Ningfang et al. from Beijing University of Aeronautics and Astronautics obtained a fixed beat frequency by performing Fourier transform on the beat signal segments and then averaging them, thus achieving the measurement of absolute distance. In 2018, Shi Chunzhao et al. from Tianjin University transformed sinusoidal frequency modulation into linear frequency modulation processing with more severe nonlinearity, and proposed a method based on optical frequency resampling to correct the nonlinearity, successfully converting the beat signal into a single-frequency signal and completing the measurement of target distance.

[0004] Currently, the main method for sinusoidal frequency-modulated interferometry is to solve for a constant beat frequency and then calculate the distance. However, the average segmentation method has too low measurement accuracy, while the optical frequency sampling method needs to satisfy the sampling theorem and requires a long fiber optic interferometer. In complex environments, the long fiber of the interferometer not only increases the sampling burden on the system but is also more susceptible to environmental changes, which is not conducive to generating a more ideal auxiliary interferometric signal. Summary of the Invention

[0005] This invention addresses the problem that current methods for sinusoidal frequency modulation interferometry require long fiber-optic auxiliary interferometers, which not only increases the sampling burden on the system but also makes it susceptible to environmental influences. The invention provides a dynamic absolute distance measurement method for frequency-shifting sinusoidal frequency modulation interferometry.

[0006] A method for measuring dynamic absolute distance using frequency-shifting sinusoidal frequency-modulated interferometry, specifically:

[0007] The optical signal emitted from the swept frequency light source 1 is incident on the first coupler 2. The first coupler 2 splits the incident optical signal into two beams, which are then input to a measuring interferometer and an auxiliary interferometer, respectively. Both the measuring interferometer and the auxiliary interferometer are equipped with acousto-optic modulators, ensuring that the modulation frequencies of the two signals from the measuring interferometer and the two signals from the auxiliary interferometer do not overlap. The driving signal of the acousto-optic modulator is a cosine signal E. rQ (t), for the cosine signal E rQ (t) Perform Hilbert transform to obtain the sinusoidal signal E rI (t),

[0008] Using the cosine signal E rQ (t) and the sinusoidal signal E rI (t) The measurement interference signal E output by the measurement interferometer m (t) The mixing, low-pass filtering, and arctangent calculation are performed sequentially to obtain the measurement interference signal E. m Phase of (t)

[0009] Using the cosine signal E rQ (t) and the sinusoidal signal E rI (t) The auxiliary interference signal E output by the auxiliary interferometer f (t) The auxiliary interference signal E is obtained by sequentially performing frequency mixing, low-pass filtering, and arctangent calculation. f Phase of (t)

[0010] phase High-pass filtering is performed on the high-frequency terms to obtain the high-pass filtering result. phase The low-frequency term in the image is low-pass filtered to obtain the low-pass filter result.

[0011] Using low-pass filtering results The amplitude ΔR(t) of the target under test is calculated using the following formula:

[0012]

[0013] Where c is the speed of light, and ω0 is the optical frequency of the light signal emitted by the sweep frequency light source 1;

[0014] To each and Unwrap the envelope and obtain the following: Envelope Z m and Envelope Z f ,

[0015] Z m and Z fThe absolute distance R of the measured object is obtained by comparison m .

[0016] Further, the above-mentioned measurement interferometer comprises a second coupler 3, a circulator 4, a measurement acousto-optic modulator 7, a fourth coupler 8 and a measurement balanced detector 9,

[0017] The one light signal output by the first coupler 2 is incident to the second coupler 3, and the second coupler 3 divides the input light signal into two beams, one of which is focused to the measured object 6 after passing through the circulator 4, and the light signal reflected by the measured object 6 is input to the fourth coupler 8 after passing through the circulator 4,

[0018] The other light signal output by the second coupler 3 is incident to the measurement acousto-optic modulator 7, the measurement acousto-optic modulator 7 shifts the frequency of the input signal, the light signal modulated by the measurement acousto-optic modulator 7 is incident to the fourth coupler 8, the fourth coupler 8 divides the input light signal into two light signals with a phase difference of 180° and then the light signals are incident to the measurement balanced detector 9, and the measurement balanced detector 9 outputs a measurement interference signal E m (t).

[0019] Further, the above-mentioned auxiliary interferometer comprises a third coupler 10, an auxiliary acousto-optic modulator 11, a fifth coupler 12 and an auxiliary balanced detector 13,

[0020] The one light signal output by the first coupler 2 is incident to the third coupler 10, and the third coupler 10 divides the input light signal into two beams, one of which is incident to the fifth coupler 12, and the other is incident to the fifth coupler 12 after being modulated by the auxiliary acousto-optic modulator 11, there is an optical path difference between the two light signals between the third coupler 10 and the fifth coupler 12, the fifth coupler 12 divides the input light signal into two light signals with a phase difference of 180° and then the light signals are incident to the auxiliary balanced detector 13, and the auxiliary balanced detector 13 outputs an auxiliary interference signal E f (t).

[0021] Further, the expression of the above-mentioned measurement interference signal E m (t) is:

[0022]

[0023] The expression of the above-mentioned auxiliary interference signal E f (t) is:

[0024]

[0025] Where Δω is the modulation bandwidth, ω m is the modulation frequency, and R mwhere c is the speed of light, t is the time, ω0is the optical frequency of the emitted light signal of the swept light source 1, ΔR(t) is the amplitude ΔR(t) of the measured object 6, f AOM is the acousto-optic modulation frequency shift parameter, R f is the arm length difference of the auxiliary interferometer.

[0026] Further, the expression of the cosine signal E rQ (t) is:

[0027] E rQ (t) = cos [2πf AOM t],

[0028] The expression of the sine signal E rI (t) is:

[0029] E rI (t) = sin [2πf AOM t].

[0030] Further, the phase φ m (t) of the measurement interference signal E

[0031]

[0032] The phase φ f (t) of the auxiliary interference signal E

[0033]

[0034] where LPF[] denotes a low-pass filter.

[0035] Further, the high-pass filter result H

[0036]

[0037] The low-pass filter result L

[0038]

[0039] where HPF[] denotes a high-pass filter, and LPF[] denotes a low-pass filter.

[0040] Further, the envelope Z of the above-mentioned m and the envelope Z f of the above-mentioned have the following expressions, respectively:

[0041]

[0042]

[0043] Further, the above-mentioned absolute distance R of the target is obtained by comparing Z m and Z f . m :

[0044]

[0045] Further, the above-mentioned absolute distance R of the target is obtained by comparing Z m and Z f . m :

[0046] The dynamic absolute distance measurement method of the frequency-shifted sinusoidal frequency modulation interference disclosed in the application uses an acousto-optic modulator to shift the beat signals of the measurement interferometer and the auxiliary interferometer away from the baseband, and then uses the acousto-optic frequency-shifted signals to demodulate the phase of the beat signals, filters the phase of the beat signals of the measurement interferometer, directly solves the relative displacement of the target from the low-frequency part, and solves the envelope from the high-frequency part, i.e., the phase generation carrier, and combines the envelope of the phase of the beat signals of the auxiliary interferometer to eliminate the influence of the modulation bandwidth fluctuation, and obtains the absolute distance information of the target.

[0047] 1. The method based on the acousto-optic frequency shift realizes the quadrature demodulation of the phase of the beat signals of the sinusoidal frequency modulation interference, successfully demodulates the phase of the beat signals with high precision, and simultaneously completes the measurement of the absolute distance and the relative motion of the target, and the influence of the modulation bandwidth fluctuation can be avoided by using the short auxiliary interferometer, the sampling pressure of the system is relieved, there is no dispersion mismatching, and the portability of the system is provided.

[0048] 2. The filtering method of reconstructing the signal by using the Fourier transform-interference frequency zeroing-inverse Fourier transform avoids the influence of the signal envelope caused by the digital filter in the filtering process, and provides higher measurement precision for the system.

[0049] 3. The envelope of the phase generation carrier in the phase of the beat signals of the measurement interferometer is used to solve the absolute distance of the target, and the influence caused by the modulation bandwidth fluctuation is eliminated by the auxiliary interferometer, and the measurement precision of the system is improved. BRIEF DESCRIPTION OF DRAWINGS

[0050] Figure 1 It is a schematic diagram of an optical path structure;

[0051] Figure 2 It is a signal frequency curve diagram of the measurement path and the auxiliary path;

[0052] Figure 3 It is a signal frequency difference curve diagram of the measurement path and the reference path;

[0053] Figure 4are frequency spectrum diagrams of the frequency-shifted beat signals, (a) represents before frequency shifting, and (b) represents after frequency shifting;

[0054] Figure 5 are schematic diagrams of phase demodulation errors, (a) represents an auxiliary interferometer, and (b) represents a measurement interferometer;

[0055] Figure 6 are schematic diagrams of distance information of a target, (a) represents a vibration track, and (b) represents a motion track. DETAILED DESCRIPTION

[0056] The technical solutions in the embodiments of the present application will be clearly and completely described with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application. It should be noted that, in the case of no conflict, the embodiments in the present application and the features in the embodiments can be combined with each other.

[0057] DETAILED DESCRIPTION Figures 1 to 6 The present embodiment is specifically described. The dynamic absolute distance measurement method of frequency-shifted sinusoidal frequency modulation interference in the present embodiment is specifically as follows:

[0058] The light signal emitted by the sweep light source 1 is incident to the first coupler 2, the first coupler 2 divides the incident light signal into two light signals and inputs the two light signals to the measurement interferometer and the auxiliary interferometer respectively, the measurement interferometer and the auxiliary interferometer are both provided with an acousto-optic modulator, so that the modulation frequencies of the two signals of the measurement interferometer and the modulation frequencies of the two signals of the auxiliary interferometer are both non-crossing, and the driving signal of the acousto-optic modulator is a cosine signal E rQ (t) = cos [2πf AOM t], the Hilbert transform is performed on the cosine signal E rQ (t) to obtain a sine signal E rI (t) = sin [2πf AOM t]. The frequency spectrum comparison of the frequency-shifted beat signals is as shown in FIG. 2. Figure 4

[0059] Specifically, the specific operation modes of the measurement interferometer and the auxiliary interferometer are as follows:

[0060] ​The measuring interferometer comprises a second coupler 3, a circulator 4, a measuring acousto-optic modulator 7, a fourth coupler 8 and a measuring balanced detector 9. One of the light signals output by the first coupler 2 is incident on the second coupler 3, which divides the input light signal into two beams. One of the light signals output by the second coupler 3 is focused by a focusing lens 5 to the measured target 6 after passing through the circulator 4, and the light signal reflected by the measured target 6 is input to the fourth coupler 8 through the circulator 4. The other light signal output by the second coupler 3 is incident on the measuring acousto-optic modulator 7, which shifts the frequency of the input signal. The light signal modulated by the measuring acousto-optic modulator 7 is incident on the fourth coupler 8, which divides the input light signal into two light signals with a phase difference of 180° and then inputs the two light signals to the measuring balanced detector 9. The measuring balanced detector 9 outputs a measuring interference signal E m (t). The expression of the measuring interference signal E m (t) is as follows:

[0061]

[0062] The auxiliary interferometer comprises a third coupler 10, an auxiliary acousto-optic modulator 11, a fifth coupler 12 and an auxiliary balanced detector 13. One of the light signals output by the first coupler 2 is incident on the third coupler 10, which divides the input light signal into two beams. One of the two light signals is incident on the fifth coupler 12, and the other light signal is modulated by the auxiliary acousto-optic modulator 11 and then incident on the fifth coupler 12. There is an optical path difference between the two light signals between the third coupler 10 and the fifth coupler 12. The fifth coupler 12 divides the input light signal into two light signals with a phase difference of 180° and then inputs the two light signals to the auxiliary balanced detector 13. The auxiliary balanced detector 13 outputs an auxiliary interference signal E f (t). The expression of the auxiliary interference signal E f (t) is as follows:

[0063]

[0064] where Δω is a modulation bandwidth, ω m is a modulation frequency, R m is an absolute distance of the measured target 6, c is a light speed, t is a time, ω0 is a light frequency of the light signal emitted by the swept-frequency light source 1, ΔR(t) is an amplitude ΔR(t) of the measured target 6, f AOM is an acousto-optic modulation frequency shift parameter, and the frequency shift parameters of the measuring acousto-optic modulator 7 and the auxiliary acousto-optic modulator 11 are both f AOM , and R f is a difference in arm length of the auxiliary interferometer.

[0065] The cosine signal E rQ(t) and the sine signal E rI (t) the measurement interference signal E outputted by the measurement interferometer m (t) sequentially mixes, low-pass filters LPF[] and arctangent calculates to obtain the measurement interference signal E m the phase of the measurement interference signal E

[0066] using the cosine signal E rQ (t) and the sine signal E rI (t) the auxiliary interference signal E outputted by the auxiliary interferometer f (t) sequentially mixes, low-pass filters LPF[] and arctangent calculates to obtain the auxiliary interference signal E f the phase of the auxiliary interference signal E

[0067] The above demodulation results are shown as Figure 5

[0068] high-pass filter the high-frequency term in the phase to obtain a high-pass filtering result

[0069]

[0070] wherein, HPF[] represents high-pass filtering.

[0071] low-pass filter the low-frequency term in the phase to obtain a low-pass filtering result

[0072]

[0073] using the low-pass filtering result According to the following formula, the amplitude ΔR(t) of the measured target is calculated:

[0074]

[0075] wherein, c is the speed of light, ω0 is the light frequency of the light signal emitted by the swept-frequency light source 1;

[0076] respectively envelope and , and respectively obtain the envelope Z of m and of f :

[0077]

[0078] ​

[0079] Z m and Z f , the absolute distance R m of the measured target is obtained.

[0080]

[0081] The simulation results are shown in Figure 6 , where Figure 6 (a) is the vibration trajectory of the target, and Figure 6 (b) is the motion trajectory of the target.

[0082] The present embodiment first adds an acousto-optic modulator with the same parameter to the reference path of the measurement interferometer and the auxiliary interferometer, so that the modulation frequencies of the reference path and the measurement path do not overlap, as shown in Figure 2 , and the frequency difference is no longer zero, as shown in Figure 3 , so as to achieve the purpose of moving the interference signal away from zero frequency. Then, two standard orthogonal signals are constructed by the acousto-optic modulator and the Hilbert transform, and then the phase of the measurement interferometer and the auxiliary interferometer signal is extracted with high precision based on orthogonal demodulation. Then, the phase of the measurement interferometer is filtered, and the low-frequency part can be directly solved to obtain the relative displacement of the target. The high-frequency part needs to solve the envelope of the signal first, and then solve the envelope of the auxiliary interferometer phase. Finally, the two envelopes are compared, and the influence of the modulation bandwidth fluctuation can be eliminated, and the absolute distance of the target is solved. Through the above processing, the dynamic absolute distance measurement of the sinusoidal frequency modulation interference can be realized. Compared with the existing method, the present embodiment has the advantages of simple structure, no measurement blind area, small nonlinear influence, real-time trajectory tracking of dynamic target, online traceability, etc.

[0083] Although the present application is described herein with reference to particular embodiments, it is to be understood that these examples are merely illustrative of principles and applications of the present application. It should therefore be understood that numerous modifications can be made to the illustrative embodiments and that other arrangements can be devised without departing from the spirit and scope of the present application as defined by the appended claims. It should be understood that the features described in connection with one embodiment can be used in conjunction with other embodiments described herein. It should be understood that the features described in connection with one embodiment can be used in conjunction with other embodiments described herein.

Claims

1. A method of frequency-shifted sinusoidal frequency-modulated interferometric dynamic absolute distance measurement, characterized in that, Specifically, The light signal emitted by the swept light source (1) is incident to the first coupler (2), the first coupler (2) divides the incident light signal into two light signals and inputs them into the measurement interferometer and the auxiliary interferometer respectively, the measurement interferometer and the auxiliary interferometer are both provided with an acousto-optic modulator, so that the modulation frequency of the two signals of the measurement interferometer and the modulation frequency of the two signals of the auxiliary interferometer do not cross, the driving signal of the acousto-optic modulator is a cosine signal E rQ (t), the Hilbert transform of the cosine signal E rQ (t) is a sine signal E rI (t). The cosine signal E rQ (t) is mixed with the cosine signal E rI (t) and the sine signal E m (t) is mixed, low pass filtered and arctangent calculated in sequence to obtain the phase of the measurement interference signal E m (t) The cosine signal E rQ (t) is mixed with the auxiliary interferometer output rI (t) to obtain the auxiliary interference signal E f (t) is sequentially mixed, low-pass filtered, and arctangent calculated to obtain the phase of the auxiliary interference signal E f (t) high pass filtering the high frequency terms in the phase results in a high pass filtered result low pass filtering the low frequency terms in the phase results in a low pass filtered result Utilizing the low pass filtering result The amplitude ΔR(t) of the measured target is calculated according to the following formula: Wherein, c is the speed of light, ω0 is the light frequency of the swept light source (1) emitted light signal; The envelopes Z and are respectively unwrapped, and the envelopes Z of m and are respectively obtained, f ​ Z m and Z f to obtain the absolute distance R m of the measured target.

2. A frequency-shifted sinusoidal frequency-modulation interferometric dynamic absolute distance measurement method according to claim 1, characterized in that, The measurement interferometer comprises a second coupler (3), a circulator (4), a measurement acousto-optic modulator (7), a fourth coupler (8) and a measurement balanced detector (9), One of the light signals output by the first coupler (2) is incident to the second coupler (3), and the second coupler (3) divides the input light signal into two beams, one of which is focused to the measured target (6) after passing through the circulator (4), and the light signal reflected by the measured target (6) is input to the fourth coupler (8) after passing through the circulator (4), The other beam of light signals outputted by the second coupler (3) is incident to a measuring acousto-optic modulator (7) which shifts the frequency of the input signal, and the light signal modulated by the measuring acousto-optic modulator (7) is incident to a fourth coupler (8), which divides the input light signal into two beams of light signals with a phase difference of 180° and then the light signals are incident to a measuring balanced detector (9), which outputs a measuring interference signal E m (t).

3. A frequency-shifted sinusoidal frequency-modulation interferometric dynamic absolute distance measurement method according to claim 2, characterized in that, The auxiliary interferometer comprises a third coupler (10), an auxiliary acousto-optic modulator (11), a fifth coupler (12) and an auxiliary balanced detector (13), The light signal outputted by the first coupler (2) is incident to the third coupler (10), the third coupler (10) divides the inputted light signal into two beams, one of which is incident to the fifth coupler (12), and the other is modulated by the auxiliary acousto-optic modulator (11) and then incident to the fifth coupler (12), there is an optical path difference between the two light signals between the third coupler (10) and the fifth coupler (12), the fifth coupler (12) divides the inputted light signal into two beams of light signals with a phase difference of 180° and then is incident to the auxiliary balanced detector (13), the auxiliary balanced detector (13) outputs an auxiliary interference signal E f (t).

4. A method of frequency-shift sinusoidal frequency-modulation interferometric dynamic absolute distance measurement according to claim 1, characterized in that, The measured interference signal E m The expression of (t) is: The auxiliary interference signal E f The expression of (t) is: where Δω is the modulation bandwidth, ω m is the modulation frequency, R m is the absolute distance of the measured target (6), c is the speed of light, t is the time, ω0 is the light frequency of the emitted light signal of the swept light source (1), ΔR(t) is the amplitude ΔR(t) of the measured target (6), f AOM is the acousto-optic modulation frequency shift parameter, R f is the arm length difference of the auxiliary interferometer.

5. A frequency-shifted sinusoidal frequency-modulation interferometric dynamic absolute distance measurement method according to claim 4, characterized in that, The cosine signal E rQ The expression for (t) is: E rQ (t) = cos[2πf AOM t], The sinusoidal signal E rI The expression of (t) is: E rI (t) = sin[2πf AOM t].

6. A frequency-shifted sinusoidal frequency-modulation interferometric dynamic absolute distance measurement method according to claim 5, characterized in that The measured interference signal E is obtained using the equation m the phase of (t) The auxiliary interference signal E is obtained using the formula f the phase of (t) Wherein, LPF[] represents low-pass filtering.

7. A frequency-shifted sinusoidal frequency-modulation interferometric dynamic absolute distance measurement method according to claim 5, characterized in that, A high pass filtering result is obtained according to the following formula A low pass filtering result is obtained according to the following formula Wherein, HPF[] represents high-pass filtering, and LPF[] represents low-pass filtering.

8. A frequency-shifted sinusoidal frequency-modulation interferometric dynamic absolute distance measurement method according to claim 5, characterized in that, the envelope Z of m and the envelope Z of f the expressions are as follows, respectively:

9. The frequency-shifted sinusoidal frequency modulation interference dynamic absolute distance measurement method according to claim 8, characterized in that, Z m and Z f to obtain the absolute distance R m of the measured target.

10. A frequency-shift sinusoidal frequency-modulation interferometric dynamic absolute distance measurement method according to claim 2, characterized in that, One of the light signals output by the second coupler (3) is focused to the measured target (6) through the focusing lens (5) after passing through the circulator (4).

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