Method for suppressing echo side frequency band of optical fiber phased array coherent laser radar

Abstract

The invention discloses a method for suppressing an optical fiber phased array coherent laser radar echo side frequency band, and belongs to the technical field of laser radars. The invention aims to solve the problem that the frequency measurement error is obviously increased due to the large amplitude of a sideband signal. According to the method, short-time Fourier transform is carried out on echo signals, and echo side frequency band suppression is carried out by controlling the ratio of time delay between the starting time of a window function corresponding to the short-time Fourier transform and the starting time of echo signal light beam scanning to a joint work period to approach to 0; in the process, by measuring the frequency shift width of the time-frequency analysis result, the time delay between the starting time of the window function corresponding to the short-time Fourier transform and the starting time of the echo signal light beam scanning is indirectly controlled. The method is used for suppressing the echo side frequency band of the optical fiber phased array coherent laser radar.

Description

Technical Field

[0001] The invention belongs to the technical field of laser radars and relates to a method for suppressing sidebands in an echo of a fiber phased array laser radar. Background Art

[0002] Fiber phased array has excellent performances such as all-solid-state structure, high power output, fast scanning and precise pointing. It is an effective way to solve the weaknesses of current fiber coherent laser radar, such as short range and slow scanning speed of scanner. Fiber phased array achieves high-speed scanning by high-frequency phase modulation of each sub-beam in the array. However, these phase modulation signals will cause multiple sidebands to be generated near the main frequency band of the optical signal. The frequency modulation law of these sidebands is exactly the same as that of the main frequency band. When the amplitude of the sideband signal is equivalent to that of the main frequency band, the error of frequency measurement will be significantly increased. Therefore, it is urgent to propose a method to suppress the sideband signal to improve the accuracy of frequency measurement when performing spectrum analysis on the echo signal of fiber phased array coherent laser radar. Summary of the invention

[0003] In order to solve the problem that the large amplitude of the sideband signal will significantly increase the error of frequency measurement, the present invention proposes a method for suppressing the echo sideband of the fiber phased array coherent laser radar.

[0004] A method for suppressing echo sidebands of a fiber phased array coherent laser radar comprises the following steps:

[0005] Perform short-time Fourier transform on the echo signal and suppress the echo sideband by controlling dtp to approach 0, where dtp = dt / (T P +T S ), T P , T S is the phase locking duration and beam scanning duration within the corresponding cycle to complete a beam scanning and phase locking process; dt is the delay between the start time of the window function corresponding to the short-time Fourier transform and the start time of the echo signal beam scanning.

[0006] Furthermore, the method of controlling dtp to approach 0 is as follows:

[0007] By controlling the delay between the start time of the window function corresponding to the short-time Fourier transform and the start time of the echo signal beam scanning, the dtp is controlled to approach 0.

[0008] Furthermore, the delay between the start time of the window function corresponding to the short-time Fourier transform and the start time of the echo signal beam scanning is controlled as follows:

[0009] The delay between the start time of the window function corresponding to the short-time Fourier transform and the start time of the echo signal beam scanning is indirectly controlled by measuring the frequency shift width of the time-frequency analysis result.

[0010] Furthermore, by measuring the frequency shift width of the time-frequency analysis result, the delay between the start time of the window function corresponding to the short-time Fourier transform and the start time of the echo signal beam scanning is indirectly controlled to ensure that the overall frequency shift width of the time-frequency distribution is minimized.

[0011] Furthermore, in the process of performing short-time Fourier transform on the echo signal, the sliding period T of the window function is SF Matches the joint working period T of the echo signal, that is, T = T SF The combined working cycle T is the corresponding cycle for completing one beam scanning and phase locking process.

[0012] Furthermore, the fiber optic phased array coherent laser radar needs to set the parameters of the fiber optic phased array coherent laser radar echo signal in advance. In the process of setting the parameters of the fiber optic phased array coherent laser radar echo signal, the modulation depth m of the scanning phase modulation signal is set to 1.8411.

[0013] Furthermore, in the process of setting the parameters of the fiber phased array coherent laser radar echo signal, it is also necessary to set the signal main frequency f0 and the scanning frequency f RF , joint working period T, and the signal-to-noise ratio SNR of the echo signal light; T is the joint working period, that is, the period corresponding to completing one beam scanning and phase locking process.

[0014] Furthermore, before suppressing the echo sideband by controlling the dtp to approach 0, it is necessary to set the Ts / T value of the phased array system in advance. The Ts / T value is determined based on the Ts / T variation curve and the signal-to-noise ratio SNR value corresponding to the fiber phased array coherent lidar echo.

[0015] Furthermore, the echo signal main frequency f0 is set to 70 MHz, and the scanning frequency f RF The combined operating period T is 5ms, and the signal-to-noise ratio SNR is 10dB.

[0016] Furthermore, in the process of determining the Ts / T value according to the Ts / T variation curve and the signal-to-noise ratio SNR value corresponding to the fiber phased array coherent laser radar echo, when dtp=0, the Ts / T value is 12%.

[0017] The above technical solution has the following advantages and positive effects:

[0018] The present invention is based on the basic principle of scanning-phase-locked joint phase modulation of fiber phased array coherent laser radar, constructs a sideband model of the echo signal, and can suppress the amplitude of the sideband to the noise level of the echo signal by controlling the ratio of the scanning duration in each joint phase modulation cycle, thereby improving the accuracy of frequency estimation of the fiber phased array coherent laser radar.

[0019] In addition, the method proposed in the present invention does not require the addition of additional optical devices in the fiber phased array coherent lidar system, thus laying a technical foundation for its practical application. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a flow chart of an embodiment of the present invention;

[0021] Figure 2 1 is a schematic diagram of an echo signal model and a short-time Fourier transform window function waveform in an embodiment of the present invention;

[0022] Figure 3 is a graph showing the sideband suppression ratio changing with percentage in an embodiment of the present invention;

[0023] Figure 4 3 is a spectrum diagram of echo signals before and after suppression obtained experimentally in an embodiment of the present invention. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0025] Specific implementation method 1: Combination Figure 1 To explain this embodiment,

[0026] A method for suppressing sidebands in an optical fiber phased array laser radar echo according to an embodiment of the present invention includes a processing process in a modeling phase and a processing process in a suppression phase;

[0027] The modeling stage refers to modeling the fiber phased array coherent lidar echo signal to obtain the sideband amplitude expression. The specific operations are as follows:

[0028] Step 1: Based on the phase modulation amount of the array beam by the fiber phased array, the echo signal expression is obtained.

[0029] The phase modulation process of the optical beam by the optical fiber phased array includes the phase locking process and the beam scanning process.

[0030] Figure 2The characteristics of the phase modulation signal are described. The phase modulation signal of the phase locking process is to compensate for the phase difference between each sub-optical path and provide the initial phase condition for scanning. The phase modulation signal of the scanning process is to achieve the scanning effect by deflecting the light beam back and forth within a certain angle range through continuous oscillation phase modulation. Because external vibration and internal temperature drift can easily cause additional phase jitter, phase locking needs to be performed at regular intervals.

[0031] The time required to complete a beam scanning and phase locking process is set as the joint working period T, and the phase locking duration is T P , the beam scanning duration is T S Within one T, the echo signal received by the fiber phased array coherent laser detection system can be expressed in sections as follows:

[0032]

[0033] Among them, A k represents the amplitude of the echo signal after the photoelectric conversion of the kth beam; f0 represents the main frequency of the echo signal; t represents the time t; f D (t) represents the frequency signal to be measured. represents the phase of the kth beam after phase compensation by the electro-optic modulator; V πk represents the half-wave voltage of the k-channel electro-optic phase modulator; V RFk represents the amplitude of the k-th modulation signal; f RF represents the frequency of the modulating signal; t S0 Indicates the start time of the scan modulation signal.

[0034] Within a T, depending on the division method, the phase locking duration may appear first and then the beam scanning duration, or the beam scanning duration may appear first and then the phase locking duration, so it does not affect the form of formula (1).

[0035] For the echo signal described by formula (1), the useful frequency signal f is extracted by the time-frequency analysis method of short-time Fourier transform processing. D (t).

[0036] For the echo signal I k The short-time Fourier transform of (t) is:

[0037]

[0038] Among them, ω represents the angular frequency; w(t) represents the window function, and τ represents the integral variable form of time.

[0039] For a given time t, the product of the window function and the echo signal is kThe FFT (Fourier transform) of (τ)w(τ-t) is the spectrum of the signal at that moment.

[0040] The window function slides on the time axis, such as Figure 2 As shown by the dashed line in the middle, the time interval of each slide does not exceed the window function period. This time interval corresponds to the period of the short-time Fourier transform, defined as T SF .

[0041] The suppression stage refers to suppressing the sideband signal amplitude to below the noise amplitude by changing the value of Ts / T based on the period matching condition. The specific operations are as follows:

[0042] Step 2: Set the parameters of the fiber phased array coherent lidar echo signal, including the signal main frequency f0, scanning frequency f RF , the combined duty cycle T, the signal-to-noise ratio SNR of the echo signal light and the modulation depth of the scanning phase modulation signal

[0043]

[0044] In this embodiment, the echo signal main frequency is set to 70 MHz, the scanning frequency is set to 100 kHz, and the combined working period is set to 5 ms. These values ​​can also be set to other values ​​according to actual needs.

[0045] The signal-to-noise ratio (SNR) of the echo signal light determines the amplitude of the noise. In this embodiment, a typical signal-to-noise ratio (SNR) of the optical fiber coherent detection system under laboratory conditions is selected as SNR=10 dB, and other values ​​may also be set according to actual needs.

[0046] The amplitude of the nth-order sideband is proportional to the Bessel function of the first kind, that is, F(f0+nf RF )∝J n (m), according to the properties of the first kind of Bessel function, when m is 1.8411, J n (m) reaches its maximum value at the 1st-order sideband.

[0047] Step 3: Echo signal I k (t) Perform short-time Fourier transform, analyze the change law of sideband amplitude with Ts / T, and obtain the optimal Ts / T ratio.

[0048] The following two conditions must be met when performing short-time Fourier transform on the echo signal:

[0049] ① Set the sliding period of the window function T SF Matches the joint working period T of the echo signal, that is, T = T SF . In this way, the signal spectrum after short-time Fourier transform processing is uniform in the time dimension;

[0050] ② Match the start time of the window function with the start time of the echo signal beam scanning, that is, the delay dt between the two is 0.

[0051] Based on the above two conditions, combined with formula (1) and formula (2), it can be deduced that the amplitude of the sideband is:

[0052]

[0053] Where * represents the convolution operator, F and W represent the echo signal I and k (t) and the window function w(t) at t∈[t S0 ,t S0 +T SF ] correspond to the Fourier transform spectra in each of them, where the intensity value of F at the n-order sideband obeys the first-kind Bessel function distribution.

[0054] In order to quantitatively analyze the relationship between the mainband power amplitude and the sideband power amplitude, define the variable suppression ratio:

[0055]

[0056] Among them, the main frequency band amplitude A mainfrequency-max and the maximum amplitude of the sideband A sideband-max All are normalized amplitudes.

[0057] Figure 3 The curve of the sideband suppression ratio changing with Ts / T in the example is given, where the suppression ratio of the sideband decreases with the increase of Ts / T, indicating that the amplitude of the sideband is gradually increasing. The dotted line represents the suppression ratio curve of the noise amplitude when SNR=10dB, and the fluctuation range is small. dtp=dt / (T P +T S )=dt / T, when dtp=0, when Ts / T increases to 12%, the amplitude of the sideband is close to the noise amplitude, and when Ts / T continues to increase, the amplitude of the sideband will exceed the noise amplitude. Therefore, in this embodiment, the optimal value of Ts / T is 12%.

[0058] Step 4: Match the scanning start time of the echo signal with the start time of the short-time Fourier transform window function, that is, set dtp=0, so that the sideband of the actual echo signal is completely suppressed.

[0059] If dtp≠0, as Figure 3 As shown in , if the phased array system sets Ts / T to 12%, the amplitude of the sideband is still greater than the noise amplitude, and the sideband is still not completely suppressed.

[0060] In practical applications, it is impossible to directly match the scanning start time of the echo signal with the start time of the window function. The two times can be matched indirectly by measuring the frequency shift width of the time-frequency analysis result. Within one T, the start time of the short-time Fourier transform in the echo signal is changed. When the frequency shift width of the overall time-frequency distribution is the smallest, it means that the match has been achieved and the sideband can be completely suppressed.

[0061] Figure 4 The sideband suppression effect of the echo signal in the experiment is given, where the thick solid line is the echo signal spectrum without phase modulation, which is used as a reference; the thin solid line is the spectrum of the fiber phased array echo signal before suppression, with multiple sidebands appearing; the dotted line is the spectrum of the echo signal after suppression, with the sidebands completely disappearing, and the spectrum width is the same as that of the reference signal.

[0062] The above calculation examples of the present invention are only used to explain the calculation model and calculation process of the present invention in detail, and are not intended to limit the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for suppressing the echo sideband of a fiber phased array coherent laser radar, characterized in that: The following steps are involved: Perform short-time Fourier transform on the echo signal and suppress the echo sideband by controlling dtp to approach 0, where dtp = dt / (T P +T S ), T P , T S is the phase locking duration and beam scanning duration within the corresponding cycle to complete a beam scanning and phase locking process; dt is the delay between the start time of the window function corresponding to the short-time Fourier transform and the start time of the echo signal beam scanning.

2. The method for suppressing the echo sideband of the optical fiber phased array coherent laser radar according to claim 1, characterized in that: The way to control dtp to approach 0 is as follows: By controlling the delay between the start time of the window function corresponding to the short-time Fourier transform and the start time of the echo signal beam scanning, the dtp is controlled to approach 0.

3. The method for suppressing the echo sideband of the optical fiber phased array coherent laser radar according to claim 2, characterized in that: The method of controlling the delay between the start time of the window function corresponding to the short-time Fourier transform and the start time of the echo signal beam scanning is as follows: The delay between the start time of the window function corresponding to the short-time Fourier transform and the start time of the echo signal beam scanning is indirectly controlled by measuring the frequency shift width of the time-frequency analysis result.

4. The method for suppressing the echo sideband of the optical fiber phased array coherent laser radar according to claim 3, characterized in that: By measuring the frequency shift width of the time-frequency analysis result, the delay between the start time of the window function corresponding to the short-time Fourier transform and the start time of the echo signal beam scanning is indirectly controlled to ensure that the overall frequency shift width of the time-frequency distribution is minimized.

5. The method for suppressing the echo sideband of the optical fiber phased array coherent laser radar according to claim 1, characterized in that: In the process of short-time Fourier transform of the echo signal, the sliding period T of the window function is SF Matches the joint working period T of the echo signal, that is, T = T SF The combined working cycle T is the corresponding cycle for completing one beam scanning and phase locking process.

6. A method for suppressing echo sidebands of a fiber phased array coherent laser radar according to any one of claims 1 to 5, characterized in that: The fiber optic phased array coherent laser radar needs to set the parameters of the fiber optic phased array coherent laser radar echo signal in advance. In the process of setting the parameters of the fiber optic phased array coherent laser radar echo signal, the modulation depth m of the scanning phase modulation signal is set to 1.8411.

7. The method for suppressing the echo sideband of the optical fiber phased array coherent laser radar according to claim 6, characterized in that: In the process of setting the parameters of the fiber phased array coherent laser radar echo signal, it is also necessary to set the signal main frequency f0 and scanning frequency f RF , joint working period T, and the signal-to-noise ratio SNR of the echo signal light; T is the joint working period, that is, the period corresponding to completing one beam scanning and phase locking process.

8. The method for suppressing the echo sideband of the optical fiber phased array coherent laser radar according to claim 7, characterized in that: Before suppressing the echo sideband by controlling dtp to approach 0, it is necessary to set the Ts / T value of the phased array system in advance. The Ts / T value is determined according to the Ts / T variation curve and the signal-to-noise ratio SNR value corresponding to the fiber phased array coherent lidar echo.

9. The method for suppressing the echo sideband of the optical fiber phased array coherent laser radar according to claim 8, characterized in that: Set the echo signal main frequency f0 to 70MHz and the scanning frequency f RF The combined operating period T is 5ms, and the signal-to-noise ratio SNR is 10dB.

10. The method for suppressing the echo sideband of the optical fiber phased array coherent laser radar according to claim 9, characterized in that: In the process of determining the Ts / T value according to the Ts / T variation curve and the signal-to-noise ratio SNR value corresponding to the fiber phased array coherent laser radar echo, when dtp=0, the Ts / T value is 12%.

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